hollow-structured mesoporous materials: chemical synthesis, functionalization and applications

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Hollow-Structured Mesoporous Materials: Chemical Synthesis, Functionalization and Applications

Yongsheng Li * and Jianlin Shi *

1. Introduction

The past 20 years have witnessed great advances regarding the synthesis, characterization, modifi cation/functionalization and application of mesoporous materials since the discovery of M41S-type ones by Mobil researchers. [ 1 ] Particular charac-

teristics of mesoporous materials such as extraordinarily large specifi c surface area and pore volume, well-defi ned ordered mesostructure, tunable pore size, varieties of the framework, and so on, have attracted great attention worldwide and opened up broad spaces for their applications in various fi elds, such as catalysis, adsorp-tion and separation, drug storage and delivery, nanofabrication, etc. [ 2 ] Amongst the various architectures of mesoporous materials, hollow-structured mesoporous materials (HMMs), which integrate hollow interior or voids with mesoporous shells of various dimensions into one nano-structure, have attracted even more atten-tion owing to their outstanding features of low density, the extensive presence of mesoporous channels on the shell, and the resultant high permeability, etc. [ 3 ] The hollow void within the mesoporous spheres can be used as a nanoreactor for loading catalytically active species for cata-lytic reactions, or as a nanocontainer for drug storage and delivery for biomedical applications after suitable surface modifi -cations. On the other hand, mesoporous shells with a well-tuned thickness within the nanometer scale of HMMs are highly

favorable as the pathway for the mass transfer of reactants and products into/out of the voids, mostly for liquid-phase reac-tions, or for the loading and release of drugs or other guest molecules for drug delivery. As the pore channels are much shortened, and thus the diffusion blockage become less signifi -cant, the inner surface of the mesopores in the shell structure can be utilized more effi ciently when these hollow spheres are used as catalyst supports. [ 4 ] Therefore, HMMs have dem-onstrated great potential in the fi elds of guest encapsulation, controlled drug release and delivery, confi ned-space catalysis, storage, adsorption and separation, and so on. [ 3 ]

The fi rst example of hollow mesoporous materials could track back to the porous lamellar silica with a vesicular mor-phology ( Figure 1 a,b), which was synthesized by Pinnavaia et al. in 1996. [ 5 ] The approach was based on the hydrolysis and cross-linking of a neutral inorganic alkoxide precursor in the interlayered regions of multilamellar vesicles formed from neu-tral bora-amphiphile surfactant molecules containing two polar head groups linked by a hydrophobic alkyl chain. Unlike other surfactant-templating methods, this approach produced porous vesicle-like lamellar silicas (designated MSU-V), which possess

Hollow-structured mesoporous materials (HMMs), as a kind of mesoporous material with unique morphology, have been of great interest in the past decade because of the subtle combination of the hollow architecture with the mesoporous nanostructure. Benefi tting from the merits of low density, large void space, large specifi c surface area, and, especially, the good biocompat-ibility, HMMs present promising application prospects in various fi elds, such as adsorption and storage, confi ned catalysis when catalytically active species are incorporated in the core and/or shell, controlled drug release, targeted drug delivery, and simultaneous diagnosis and therapy of cancers when the surface and/or core of the HMMs are functionalized with functional ligands and/or nanoparticles, and so on. In this review, recent progress in the design, synthesis, functionalization, and applications of hollow mesoporous materials are discussed. Two main synthetic strategies, soft-templating and hard-templating routes, are broadly sorted and described in detail. Progress in the main application aspects of HMMs, such as adsorption and storage, catalysis, and biomedicine, are also discussed in detail in this article, in terms of the unique features of the combined large void space in the core and the mesoporous network in the shell. Functionalization of the core and pore/outer surfaces with functional organic groups and/or nanoparticles, and their performance, are summarized in this article. Finally, an outlook of their prospects and challenges in terms of their controlled synthesis and scaled

application is presented.

DOI: 10.1002/adma.201305319

Prof. Y. Li, Prof. J. Shi Lab of Low-Dimensional Materials ChemistrySchool of Materials Science and EngineeringKey Laboratory for Ultrafi ne Materials of Ministry of EducationEast China University of Science and Technology 130 Meilong Road , Shanghai 200237 , China E-mail: [email protected]; [email protected] Prof. J. Shi The State Key Laboratory of High Performance Ceramics and Superfi ne MicrostructuresShanghai Institute of CeramicsChinese Academy of Sciences 1295 Dingxi Road , Shanghai 200050 , China

Adv. Mater. 2014, DOI: 10.1002/adma.201305319

http://doi.wiley.com/10.1002/adma.201305319

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high specifi c surface area and pore volume. More importantly, the step of separate pillaring became unnecessary in creating porosity into a lamellar host structure. Almost at the same time, Mou et al. [ 6 ] designed another route for synthesizing hollow tubular MCM-41 (Figure 1 c,d) based on the sequen-tial separation of the self-organization of template silicates and the subsequent condensation of silicates. By delaying the formation of the rigid structure of the silicates, hierarchically ordered “tubules-within-a-tubule” of MCM-41 were obtained through a liquid-crystal phase-transformation mechanism. In the same year, Schüth et al. [ 7 ] reported the synthesis of hollow mesoporous spheres via interfacial reactions in oil-in-water emulsion, which was created by adding tetraethyl orthosilicate (TEOS) dissolved in an organic solvent (such as n-hexane, ben-zene, toluene, mesitylene, and others) to an acidic solution con-taining surfactant cetyltrimethylammonium bromide (CTAB) under stirring.

Following these, diverse morphologies of hollow mesoporous materials, such as spheres, [ 8 ] vesicles, [ 9 ] helicoids, [ 10 ] fi bers, [ 11 ] rattles, [ 12 ] and so on, have been fabricated by using a variety of chemical routes. The most intensively investigated hollow mesoporous materials are silica-based owing to the special features of silica such as controllable sol-gel processing, easy functionalization, and facile regulation of silica frameworks. However, the relatively low acidity and stability of silica-based hollow mesoporous materials have greatly limited their applica-tions in varied fi elds. [ 13 ] In order to enhance the diversity and to facilitate their practical applications, various components, such as heteroatom-doped silicas (Ti, Zr), [ 14 ] metal oxides (TiO 2 , ZrO 2 , CeO 2 , ZnO), [ 15 ] metals (Pd), [ 16 ] polymers (PCL, PZS), [ 17 ] carbon [ 18 ] and others [ 19 ] have been designed and prepared. The applications of HMMs have also been widely investigated in many fi elds [ 20 ] especially as catalyst supports in the fi eld of catalysis, in which the hollow cores provide big enough spaces for locating the active species, and the mesoporous shell layer

Yongsheng Li received his B.Sc. (1994) from Zhengzhou Institute of Technology, and obtained his Ph.D. (2001) from Dalian University of Technology. As a post-doc-toral research fellow, he has worked in Shanghai Institute of Ceramics, CAS and Institut de Recherches sur la Catalyse et l’environnement de Lyon, CNRS, France, respectively.

He is now a professor in East China University of Science and Technology. His recent research interest focuses on the design and controlled synthesis of porous materials and hybrid nanocomposites and their biomedical and catalytic applications.

Jianlin Shi received his Bachelor degree from Nanjing University of Technology in 1983, and obtained his Ph.D. degree in 1989 at Shanghai Institute of Ceramics, Chinese Academy of Sciences. Since then, he has been working at the same institute. His cur-rent research mainly focuses the structural design and synthesis of mesoporous

materials and mesostructured nanocomposites, and the catalytic and biomedical performances of the mate-rials for applications in environmental protection and nanomedicine.

offers a short diffusion pathway for both reactants and products and the location for anchoring active species. On the other hand, the large encapsulation capacity of the hollow cores and the easy functionalization of the mesoporous shells of HMMs create excellent opportunities for exploring their application potentials in biomedical fi eld, including drug loading and con-trolled release for serious disease chemotherapy, molecular bioimaging, and theranostics for simultaneous diagnosis and therapy. [ 21 ]

Several recent articles have reviewed the research progress on conventional mesoporous materials without the special hollow structure. [ 22 ] Most recently, several reviews [ 3,20 ] have dis-cussed, rather briefl y, the progress in some specifi c aspects of the chemical syntheses and applications of hollow mesoporous silicas. Alternatively in this review, we make efforts to give a comprehensive overview of the varioussynthetic strategies for hollow-structured materials, both with silica and non-siliceous components, structural regulations and modifi cations, and their potential applications mainly in the fi elds of catalysis, adsorption and drug delivery. In the fi rst part, the design and synthesis of various kinds of hollow mesoporous materials via

Figure 1. Proposed formation mechenisms (a and c) and TEM (B) and SEM (D) images of the fi rst HMM sample. (1) Mixed lamellar-hexagonal membrane phase; (2) Membrane curvature formation by acidifi cation; (3) Membrane bending into tubule through neutralization; (4) The mem-brane consists of a hexagonalarray of cylindrical micelles. a,b) Repro-duced with permission. [ 5 ] Copyright 1996, American Association for the Advancement of Science. c,d) Reproduced with permission. [ 6 ] Copyright 1996, American Association for the Advancement of Science.


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such as extraction or calcination for hollow structure formation when the templates are the additional organics to the HMM precursors. In the case of using precursor molecule droplets themselves as the core template, which is also called “self-tem-plating” (route 2), these molecules will be later utilized as the building units of the hollow structures, and therefore are grad-ually consumed in the following construction/formation of the mesostructured shells; thus, a core template removal process, such as calcination or extraction, becomes unnecessary. [ 24 ]

2.1.1. Emulsion Templating Approach

It is well known that emulsion chemistry deals with multiphase liquid systems in which one isolated heterogeneous liquid phase (e.g., organic droplets) is dispersed in the other liquid matrix (e.g., water). [ 25 ] Usually polymeric particles can be syn-thesized by polymerizing the organic droplets and later sepa-rating them from the liquid matrix. If inorganic components are included in the isolated liquid droplets, macro- or nanoscale inorganic particles with or without porous structures, such as silica and other metal oxides, can be synthesized; these inherit the topology and/or morphology of the droplets. With the oil-in-water/water-in-oil emulsion chemistry, hollow-structured calcium carbonate and mesostructured silicas have been fabricated. [ 9a , 19c ] The basic idea is to use sol-gel processing to deposit inorganic materials, such as silica or metal oxides at the interface between the droplets and media in a dispersed emul-sion, where the favorable features of the oil droplets being both highly deformable and easily removable are employed. [ 25 ]

soft-templating or hard-templating routes will be summarized in detail; in the second, the applications of hollow mesoporous mate-rials will be overviewed in detail mainly in the following areas: catalysis, storage, adsorption and separation, and drug delivery/biomedical applications; the fi nal part is a summary and outlook for hollow mesoporous materials. To try to facilitate the understanding of each synthetic strategy and the applications of HMMs, we present a number of schematic illustrations in the following main text.

2. Design and Chemical Synthesis of HMMs

To create hollow voids during the mesostruc-ture construction, a number of techniques have been developed so that hollow inte-riors and mesoporous shells can be com-bined together well. In general, the routes for fabricating hollow mesoporous materials can be sorted as soft-templating and hard-templating approaches, according to the types of templates employed for the crea-tion of the hollow interiors. The soft-tem-plating route refers to the direct generation of both mesopores and the hollow structure almost simultaneously via the self-assembly between precursor molecules and organic surfactant tem-plates/other organic additives. Herein, the core templates are usually droplets of “soft” precursor molecules, surfactants, or some organic additives, and will be consumed or removed in the later procedures of the HMMs formation. In the meantime, the mesopore channels are created by the surfactant templates/micelles after being removed in the later procedures. As for the hard-templating route, some specially prepared rigid solid particles are employed as “hard” core templates, and sacrifi ced after the formation of the mesoporous shell on the core for the construction of a hollow structure within mesoporous particles, in which mesopores are usually similarly generated by the self-assembly among the precursors and surfactant micelles. [ 23 ]

2.1. Soft-Templating Route

Scheme 1 illustrates the general strategy of the soft-templating methodologies for preparing hollow mesoporous materials with detailed processing steps. The construction of HMMs starts from the transient formation of the core templates, which may take place almost simultaneously with the subsequent generation of mesoporosity on the shell. The core templates can be heterogeneous but nevertheless fl exible liquid particles in an aqueous liquid matrix, such as emulsion droplets, vesi-cles, or gas bubbles, which usually originate from, but are not limited to, precursor molecules, templates, or some additives originally added for constructing the mesostructures (route 1). Usually the core template should be removed by various routes

Scheme 1. Schematic illustration of the soft-templating route for preparing HMMs. Route 1: heterogenerous soft-templating route; Route 2: self-generated soft-templating route. Step (1–1): nucleation of the precursor/surfactant mixture at the interface between the heterog-enous core and liquid medium containing surfactant and precursor molecules before self-assembly; step (1–2): shell growth by the precursor hydrolysis around the core template and the self-assembly with surfactants leading to the mesostructure formation in the shell; step (1–3): removal of both core and mesopore templates via calcination or extraction. Step (2–1): nuclea-tion of the precursor/surfactant mixture at the interface between the precursor core and liquid medium containing surfactant molecules before self-assembly; step (2–2): shell growth by the self-assembly between the surfactants and the precursors leading to the gradual consumption of core template molecules and the synchroneous mesotructure formation in the shell; step (2–3): surfactant as mesopore template removal via calcination or extraction.


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fers from the reported HMMs with hexago-nally or circumferentially arranged mesopore channels in the shells. On the basis of this, mesoporous zeolite with a hollow spherical/ellipsoidal capsule structure was further obtained under the similar process. [ 29 ]

In order to promote the formation of the emulsion system or stabilize the emul-sion droplets, some organic additives (co-solvents), such as N,N-dimethylformide (DMF), [ 30 ] n-octane [ 31 ] or dodecyl amine (DDA), [ 32 ] have been used to cooperate with the TEOS. It has been shown that DMF

added in the solution as a bridging co-solvent can both promote the dispersion of TEOS in the aqueous solution to form small droplets, and speed up the hydrolysis of TEOS at the inter-face. [ 30 ] In this way, ordered mesostructure was formed at the interface of the water-TEOS droplets, and hollow mesoporous silica spheres (Figure 2 c) were fi nally obtained once the TEOS droplets were consumed. Furthermore, the pore structure, wall thickness, and the sphere dimension can be well tuned by means of varying the amount of DMF.

It is widely believed that surfactant molecules are responsible for the construction of mesostructures in preparing mesoporous materials by self-assembling with precursors of the shell com-ponents. Interestingly, it has been found that surfactants them-selves can also serve as core templates in certain cases for forming the hollow voids. Chen et al. [ 33 ] successfully prepared hollow mesoporous silica spheres with varied morphologies by utilizing the sodium salt of the anionic surfactant N-lauroylsar-cosine (Sar-Na) as a template. With the addition of hydrochloric acid, a part of the Sar anions was converted into Sar-H, which acts as droplets owing to the amphiphilic property. Then, the condensation between the added silica precursors (3-amino-propyltrimethoxysilane (APTMS) and TEOS) and the emulsion system took place, resulting in hollow mesostructured spheres. Differently from the previously reported emulsion-templating routes, the anionic surfactant Sar-Na was used as both a sur-factant template for mesopores and an oil phase (droplets) for interior hollow-core formation after acidifi cation ( Figure 3 a,b). Afterwards, Han et al. [ 34 ] further verifi ed the dual functions of emulsion and micelle formation of the anionic surfactants. They employed a variety of anionic surfactants, including pal-mitic acid (C 16 AA), N-acyl- L -phenylalanine (C 18 Phe), N-palmi-toyl- L -alaine (C 16 AlaA) and oleic acid (OA) as templates and 3-aminopropyl-triethoxysilane (APTES) and TEOS as silica sources to obtain silica hollow spheres with mesoporous shells. It was found that ethanol, which acts as a co-surfactant, plays a key role in the porosity generation of the shell.

Similarly, cationic surfactants have also been demonstrated to have the dual functions of forming emulsion and micelles. Accordingly, a micellar aggregate templating route has been developed by Shi et al. [ 35 ] to synthesize hollow mesoporous silica nanospheres (Figure 3 c,d) with tunable sizes of both the sphere diameter and the shell thickness by utilizing (−)-N-dodecyl-N-methylephedrinium bromide (DMEB) as the core template. It is known that DMEB can easily form small micellar aggregates in water at a relatively low critical aggregation concentration, so that

TEOS is commonly used as the silica source in preparing mesoporous silica materials. More importantly, it can also serve as the “void” template via forming an emulsion in aqueous solution/sol for fabricating hollow mesoporous materials. Inspired by the idea of Mou et al. [ 6,26 ] for preparing hollow mesoporous materials by a delayed neutralization procedure, hollow microspheres of silica with ordered mesoporous walls ( Figure 2 a) were synthesized by Mann et al. [ 8 ] by a simple pro-cess involving dilution and neutralization of an aqueous reac-tion mixture of TEOS/CTAB under ambient conditions. The critical step was demonstrated to be the formation of TEOS emulsion droplets in the reaction mixture resulting from the dilution. Therefore, it is of utmost importance to control the reaction conditions precisely, including the stirring rate, induc-tion time, and the time delays between the dilution and neu-tralization steps during the synthesis, so that the formation of TEOS droplets and the interfacial reaction could take place successively and independently. [ 15c , 27 ] Noticeably, the mesopore channels in the shells of these materials, though with ordered porous structures sometimes, are usually neither stable enough nor penetrating across the shells.

Later, hollow spheres of mesoporous aluminosilicate with a highly ordered three-dimensional and consequently pen-etrating pore network (Figure 2 b) were successfully developed by Shi et al. [ 28 ] by employing the self-templating approach. In the synthesis, the precursor for synthesizing zeolite ZSM-5 was used as the starting material, which was proposed to form an oil(TEOS)-in-water emulsion in solvent. The parameters, such as the temperature at which the precursor was prepared, the mixing order of the reactants and the aging duration, were found to be important for the formation of the emulsion, which affects the fi nal hollow particle structure. In this approach, emulsion was formed by adding TEOS to the aqueous solu-tion containing Al 2 (SO 4 ) 3 ·18H 2 O under vigorous stirring, and was further stabilized via the quick hydrolysis of the outer layer TEOS molecules around the TEOS droplets in strong basic con-ditions. This is different from that of mesostructured silica ves-icles, which is created through minimizing the surface energy by hydrogen bonding between electrically neutral gemini sur-factants and silica precursors. [ 5 ] The TEOS droplets as the core templates were later used/consumed in the generation of the mesostructured shell via their assembly with surfactant mol-ecules (CTAB), consequently leaving hollow voids. Accord-ingly, the resultant materials possess ordered 3D penetrating mesopores in the shell, allowing guest molecules to diffuse

Figure 2. SEM (a) and TEM (b,c) images of hollow mesoporous silicas templated by TEOS formed emulsion droplets. a) Reproduced with permission. [ 8 ] Copyright 2001, Royal Scoiety of Chemistry; b) Reproduced with permission. [ 27 ] Copyright 2010, Elsevier; c) Reproduced with permission. [ 28a ] Copyright 2003, ACS Publications.


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ordered mesoporous channels were penetrating across the shell. Furthermore, much higher capability in storing guest molecules, in comparison with conventional MCM-41, has been proved for these hollow mesoporous silica spheres, and more than half of the guest molecules were located in the hollow cores.

Interestingly, based on the above mechanism, the fi rst example of hollow non-siliceous oxide spheres, lead titanate, was obtained via an oil-water interface templating route at the micrometer scale by using acetylacetone and 1-butanol as organic reagents and amphiphilic dodecylamine as the sur-factant, through a neutral supramolecular templating route. [ 19c ]

In the system involving non-ionic surfactants, organic compounds also play a role in controlling the morphology of the fi nal mesoporous materials. Li et al. [ 40 ] used kerosene to assist sorbitan monooleate (C 24 H 44 O 6 , Span 80) in creating a water-in-oil emulsion. Following the sol-gel process of TEOS, stable hollow silica microspheres were prepared. With the same process, hollow mesoporous titanium-silica bicompo-nent microspheres with excellent catalytic performance in the epoxidation of cyclohexene by tert-butylhydroperoxide were successfully fabricated. [ 14a , 41 ] TMB is widely used to expand the pores of mesoporous materials, it can also form emulsion droplets in water. [ 42 ] Under stabilization by a triblock copolymer (EO 76 -PO 29 -EO 76 ), silica hollow spheres with multilamellar or worm-like porous shell structures and uniform sizes could be obtained. [ 43 ] Based on this, Guo et al. [ 44 ] recently prepared hollow spheres with mesoporous channels perpendicularly arranging on the shells, and the hollow core was generated by the removable template of a benzene-in-water emulsion.

With a similar aqueous emulsion co-assembly approach, followed by hydrothermal treatment, a series of mesoporous carbon and carbon-silica nanocomposite vesicles has been syn-thesized. It was proposed that the silicate oligomer and soluble resol precursors would interact and co-assemble with the tri-block copolymer EO 97 -PO 69 -EO 97 (F127) template through hydrogen bonding. Then, uniform lamellar mesostructure was formed via the co-assembly on the oil/water emulsion interface, which were generated by adding TMB as a co-solvent. [ 9b ]

Although the above emulsion templating approaches have been widely used in preparing hollow spherical porous silica or other non-siliceous materials, researchers have been trying to fi nd nontoxic substitutes to replace the commonly used environment-unfriendly and water-immiscible organic additives to generate emulsion systems. Supercritical carbon dioxide (scCO 2 ), which is nontoxic and non-fl ammable, and can offer several attractive fea-tures such as high diffusivity, low viscosity, high biocompatibility, and non-cohesiveness, has been demonstrated to have promising potential in the preparation of hollow mesoporous materials ( Figure 4 ). [ 45 ] Mokaya et al. [ 46 ] found that hollow silica spheres of large-sized mesopores on the shell could be synthesized through a CO 2 -in-water emulsion templating route using PEO-PPO-PEO as a template under supercritical fl uid conditions. The mesoporo-sity and morphology of the hollow silica spheres could be tuned by varying the operating CO 2 pressure. Moreover, compressed CO 2 gas fl ow could improve the monodisperity and uniformity of the hollow silica spheres. [ 15i , 47 ]

Table 1 lists the physico-chemical properties of several repre-sentative HMMs prepared via emulsion routes. It can be found

it could be used as a “dual” template to assist the construction of the hollow core and the mesostructure simultaneously. The structure of the micellar aggregates can be further stabilized by the addition of carboxyethylsilanetriol sodium salt (CSS), owing to the electrostatic interaction between CSS and DMEB. Under basic conditions, TEOS hydrolysis is rather quick compared with that under neutral conditions, and the assembly between con-densed TEOS and DMEB around the micellar aggregates occurs in the meantime, which subsequently results in the formation of hollow mesoprous silica spheres after template removal. By adjusting the pH value of the precursor, the condensation rate of TEOS and the growth rate of the mesostructured shell can be altered. Accordingly, the sphere diameter and the thickness of the shell and even the morphology of the resultant product can be tuned. In view of employing the micelle aggregates as void templates, the diameter of the fi nal mesoporous silica spheres can be easily controlled to be under 100 nm, which is especially favorable for targeted drug-delivery applications.

Apart from organic silica sources and surfactants, the cooperative principle between organic compounds and sur-factants has been frequently used to assist the formation of the emulsion system or stabilize the emulsion droplets. [ 10,36 ] He et al. [ 37 ] attempted to control the morphology and structure of mesoporous silica by using various auxiliary compounds, including sodium bis(2-ethylhexyl) sulfosuccinate, ethyl ether, and dodecanethiol as a co-solvent or co-template, which was found to be helpful in forming a stable microemulsion via incorporation with CTAB. By changing the mass ratio of dode-canethiol (C 12 -SH)/CTAB, mesoporous silicas with different morphologies, such as solid spheres, hierarchical spheres, and hollow-structured spheres were obtained. Furthermore, the diameter of the hollow mesoporous spheres could be tuned and decreased to less than 100 nm by adding different amounts of trimethylbenzene (TMB) to the solutions. [ 38 ] Utilizing the aggregation property between poly(vinylpyrrolidone) (PVP) and CTAB mixture, Zhu et al. [ 39 ] successfully prepared hollow mesoporous silica spheres with a uniform size and morphology at room temperature. It was found that most of the hexagonally

Figure 3. TEM images of hollow mesoporous silica spheres prepared via anionic (a,b) and cationic (c,d) surfactant emulsion routes. a,b) Repro-duced with permission. [ 33 ] Copyright 2006 Wiley; c,d) Reproduced with permission. [ 35 ] 2008 Royal Scoiety of Chemistry.


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vesicles, and liquid-crystal phases, are formed in surfactant solutions under different conditions. [ 49 ] As shown in Scheme 2 , vesicles are the self-organized structures from surfactants and have fragile bilayer shells and are therefore subject to dis-solving and collapsing when the solution properties, such as concentration and temperature, signifi cantly change. Inorganic components can be introduced into the shells of the surfactant vesicles, which will result in organic/inorganic hybrid vesi-cles or even inorganic vesicles if the organics can be removed without destroying the vesicle structure. When inorganic components such as silica are used as building units for con-structing solid vesicle structures, the resulting materials are highly stable and more-biocompatible replacements for the original surfactant components, which will be useful for bio-medical applications as, for example, drug-delivery vehicles or for other applications such as high-capacity gas adsorption. [ 50 ]

Rankin et al. [ 50,51 ] reported a new type of vesicle-like hollow silica with single-walled and ordered mesoporous shells, which was generated through the co-assembly between silica and a vesicle template of fl uorinated surfactant. By changing the stirring/shearing rate during synthesis, they showed that elon-gated silica particles with multiple hollow chambers were pre-pared. By mixing the fl uorocarbon surfactant (FC 4 ) with CTAB or F127, periodic mesoporous organosilica hollow spheres with

that highly ordered HMMs can be obtained if the emulsions are created in the presence of a silica source (TEOS), however, the synthesized HMMs show a broadened distribution in par-ticle size, [ 7 , 8 , 28a ] and, the dimension and uniformity of TEOS-created emulsion droplets are highly dependent on various experimental parameters, such as stirring rate, temperature, and additives. In comparison, HMMs of smaller particle sizes and disordered or wormhole-like mesostructured shells are usually obtained where surfactants act as both emulsion drop-lets and templates for mesostructures, [ 33 , 35 , 37b ] but the disper-sivity of HMMs can be improved with the increasing particle size. [ 8,26,40,48 ] In this case the self-assembly properties (e.g., aggregate or vesicle formation) infl uence the emulsion prop-erty directly, resulting in relatively small emulsion droplets and fi nal smaller particle sizes where surfactants are employed for producing emulsions. Nevertheless, in both cases, the proper-ties of resultant HMMs are largely dependent upon the nature of the emulsion droplets, and the uniformity and dispersity are rather hard to get under fi ne control.

2.1.2 Vesicle Templating Route

Due to the diverse chemistry of surfactants, various shapes and morphologies of surfactant aggregates, including micelles,

Table 1. Properties of representative HMMs prepared via emulsion routes.

Emulsion Additives Framework Dispersivity a) Particle size [µm]

Uniformity a) Periodicity a) Mesostructure Pore orientation

Shell thickness [nm]

Ref.

TEOS - SiO 2 h 1.0 m h P6mm Parallel to

shell surface

20 S. Mann[ 8 ]

TEOS - aluminosilicate - 0.6 m h Ia-3d 3D 200 J. L. Shi[ 28a ]

TEOS mesitylene SiO 2 - 1–100 - h P6mm, p63/

mmc, pm3n

radially F. Schüth[ 7 ]

TEOS butanol aluminosilicate h 2.5–7.6 h h Radially/

latitudinally

C. Y. Mou[ 26,48 ]

Sar - SiO 2 m 3 - - Wormhole-like - - T. Chen[ 33 ]

DMEB - SiO 2 m 0.1 h m - 15 J. L. Shi[ 35 ]

CTAB C 12 -SH SiO 2 m <0.1 m/h m - - 20 J. He[ 37b ]

Span 80 kerosene SiO 2 h 30–50 m - P6mm - 4 Z. Wang[ 40 ]

CO 2 - SiO 2 m 0.1–0.5 m m P6mm radially 35–40 T. Chen[ 45b ]

a) “h”- high, “m”- medium, “-“ poor or not specifi ed.

Figure 4. Schematic diagram (left) and TEM images (right, a, b) of hollow mesoporous silica spheres prepared via emulsion technology assisted by CO 2 gas bubbles. Reproduced with permission. [ 45b ] Copyright 2009, ACS Publications.


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found that the larger pore diameter in the cores could be tuned by changing the number of PS block units in PS- b -PAA in a certain range. However, on increasing the number of PS block units to 260, hollow mesoporous silica spheres (Figure 6 c,f) were obtained. [ 54 ] It has been suggested that such an evolution of the particle structure can be most probably attributed to the micelle morphology transformation from rod-like to vesicle or other kinds of large-sized composite micelles. Based on the co-assembly between CTAB-coated rod-like aggregates and silicate oligomers, [ 55 ] mesostructures of different morphologies were obtained by using PS- b -PAA with different numbers of PS block units. Following the concept of a composite soft-tem-plating approach, Zhao et al. later also reported the synthesis of dual-mesoporous silica materials with large mesopores (ca. 20 nm) packed in an Fm-3m structure and small worm-like mesopores (ca. 2.5 nm) homogeneously distributed in the large pore walls by using poly(ethylene oxide)- block -poly(methyl methacrylate) (PEO- b -PMMA) and alkyl trimethylammonium bromide (C n TAB) surfactants as co-templates through the solvent evaporation induced aggregating assembly (EIAA) approach. [ 56 ]

On the basis of vesicle templating technology, mesoporous Ce-doped Pd nanospheres with hollow chambers and enhanced catalytic effi ciency were synthesized through a vesicle template formed by tetrabutylphosphonium bromide (Bu 4 PBr). [ 16b ] Owing to the piling-up of Pd nanoparticles, the outer shell presented a poorly ordered mesoporous structure. However, no signifi cant infl uence on the self-assembly of the hollow Pd nanospheres by the addition of Ce(NO 3 ) 3 was found, and Ce 2 O 3 species were distributed uniformly on the outer shells. The as-prepared catalyst exhibited enhanced activity and selectivity to cyclohexanone in the liquid-phase hydrogena-tion of phenol, largely due to the promotion effects from the unique structure.

From the above description, it can be clearly seen that the soft-templating methodology for preparing hollow mesoporous materials, which is usually a one-pot or one-step process in mixed solutions, is relatively simple and the periodicity of the resultant materials can be well controlled during soft-tem-plating. However, precise control over the reaction conditions is generally not easy, though very critical for the hollow-struc-ture construction, owing to the fact that soft templates are highly sensitive to the reaction environment such as concen-trations, temperature, stirring, and even the order of precursor additions. Consequently, hollow mesoporous materials syn-thesized via the soft-templating route are usually not uniform

a tunable wall thickness or a multishelled mesoporous struc-ture ( Figure 5 ) were synthesized via a vesicle and liquid-crystal “dual-templating” approach. It was proposed that composite vesicles could be generated via the possible interaction between the cationic FC 4 and the negatively charged silica species when FC 4 and CTAB were used as co-templates. Then, these com-posite vesicles function as the “nuclei” for the further growth of mesoporous shells, in which the liquid-crystal templating pro-cess determined the fi nal shell structure. [ 52 ]

In addition to fl uorocarbon surfactants, the mixture of CTAB, triblock copolymer EO 20 PO 68 EO 20 (P123), and sodium dodecyl sulfate (SDS) can generate thermodynamically stable and neutral surfactant-coated vesicles, which have been used to obtain hollow silica spheres with mesoporous shells after the fast silicifi cation in a dilute silicate solution at a neutral pH value. [ 49 ] Recently, core–shell structured dual-mesoporous silica spheres (DMSS) ( Figure 6 a,b,d,e), which possess smaller pores (2.0 nm) in the shell and larger tunable pores (12.8–18.5 nm) in the core, were successfully synthesized by Shi et al. [ 53 ] by utilizing amphiphilic block copolymer polystyrene- block -poly (acrylic acid) (PS- b -PAA) and CTAB as co-templates. It was

Scheme 2. Schematic illustration of the vesicle templating route for synthesizing HMMs. Step 1: nucleation of inorganic component precursor in the bilayer vesicles; step 2: shell growth by the precursor hydrolysis and the self-assembly with surfactants; step 3: template removal via calcination or extraction.

Figure 5. SEM and TEM images of multishelled hollow mesoporous silica nanospheres prepared via a vesicle templating route. Reproduced with permission. [ 52c ] Copyright 2010, Royal Scoiety of Chemistry.


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based upon their chemical properties: polymer latex, silica, and carbon spheres.

2.2.2. Polymer Latex as Core Template

Compared with liquid emulsions, PS latex is of well-defi ned and tunable particle size and readily available. Latex spheres have been usually employed as pore templates for fabricating macroporous materials, or macro-/mesoporous materials via a dual-templating approach in combination with surfactants, as mesopore templates. [ 57 ] Xia et al. [ 58 ] have successfully pre-pared hollow silica spheres with latex particles as a hard tem-plate, but the shells of the spheres were microporous. With this approach, the hollow-core diameter can be easily adjusted by selecting latexes of different sizes, while the shell thickness can be tuned by varying the ratio of silica-to-latex during the fabri-cation process. [ 59 ]

In early 2001, Qiu et al. [ 60 ] fi rstly used negatively charged col-loidal polystyrene (PS) beads to assist the self-assembly between inorganic silica and surfactant micelles in a clear solution at pH > 12, to obtain hollow structured mesoporous silica spheres. It was demonstrated that the surfactant directed the assembly of the silica species into a mesoporous 2D hexagonal arrangement, whereas the PS beads played a role in forming the hollow core in the interior of the particulate materials. After the removal of the PS beads and surfactant molecules via high-temperature calcination, hollow silica spheres with mesostructured shells were obtained. The modifi cation of the PS beads with other functional molecules, such as acrylic acid [ 61 ] or polystyrene- co -poly(4-vinylpyridine) (PS- co -PVP), [ 62 ] will facilitate the assembly of a mesoporous shell around the beads due to the introduction of a favorable chemical environment for electrostatic attraction, or a pendent catalyst on the latex surface to initiate the sol–gel

enough in size and/or morphology. As a result, aggregation among the synthesized HMM particles are frequently found, leading to diffi culties in regulating their dispersity and sus-pensibility in solutions, which are, unfortunately, extremely critical in, for example, biomedical applications. To address these issues, therefore almost at the same time, approaches of using a hard-core template combined with a surfactant co-template were also vigorously investigated, which may offer a more-reliable dual-templating approach for the synthesis of hollow or hierarchically structured mesoporous silica nanoparticles.

2.2. Hard-Templating Route

2.2.1. General Approach

Generally, solid rigid particles are employed in hard-templating routes as the core template; these can be easily removed through calcination, dissolution, or etching after the formation of mesoporous shells around the cores. Scheme 3 illustrates the hard-templating routes for synthesizing siliceous and non-siliceous HMMs. In these routes, hard-template synthesis is a necessity, and special attention is usually required for the pur-pose of dimension and morphology control of the fi nal HMMs. After forming mesoporous shells around the template cores, removing them via calcination, etching, or extraction is another necessary step. Therefore, the hard-templating route involves multi-steps, whereas the properties of the resultant HMMs, such as the sizes of the void space and the entire sphere, and the monodispersivity, are usually tunable based on those of the hard templates employed. There are mainly three types of most frequently used hard templates, which are classifi ed

Figure 6. TEM images of dual-mesoporous and hollow mesoporous silica spheres prepared via using double surfactants as co-templates. a,b,d,e) Repro-duced with permission. [ 53 ] Copyright 2010, ACS Publications; c,f) Reproduced with permission. [ 54 ] Copyright 2013, Royal Scoiety of Chemistry.


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the possibility of forming solid mesoporous particles, as well as fusing them together was signifi cantly reduced. The resultant HMSs possess smooth, uniform, and ordered mesoporous silica shells. They found that both the particle size and the shell thickness of HMSs could be fi nely tuned by changing the amounts of latex and silica source. It has also been demon-strated that the synthetic conditions, including the concentra-tion of surfactant, the volume fraction of organic solvent, and others, determine the size, periodicity, and monodisperity of

process of TEOS. However, the mesoporous shells are usually disordered. In order to improve the periodicity of the mesopore channel arrangement in the shells of the hollow spheres tem-plated by the dual PS latex/surfactant templating route, Rankin et al. [ 59 ] employed an unmodifi ed polystyrene latex and CTAB in a concentrated aqueous ammonia solution to promote aggrega-tion among small particles (CTAB/silica aggregates) and large particles (Sukuca-coated latex), so that mesoporous shells of an ordered pore structure around the hard cores could be gen-erated. The shell thickness and core diameter could be tuned independently; however, irreversible particle aggregation would take place during the synthesis, leading to micrometric and dis-ordered aggregates, which will thus give rise to diffi culties in applications demanding a high dispersion of particles, in the cases of, for example, inorganic fi llers in rubbers and blood-injectable drug-delivery vehicles.

By precisely controlling the experimental parameters, such as the charging properties of the PS-sphere surface, the weight ratio of TEOS to PS, the precursor components, monodisperse hollow silica spheres with ordered mesoporous channels on the shells, which are perpendicular to the core surface, were synthesized for the fi rst time. [ 63 ] The particle diameter (less than 200 nm), the shell thickness, and the mesopore orienta-tion could be tuned independently. Recently, a facile and scal-able methodology was reported by Giannelis et al. [ 64 ] to synthe-size monodisperse hollow mesoporous silica (HMS) capsules ( Figure 7 a,b) with a relatively concentrated latex template. It was verifi ed that the hydrolysis rate of TEOS could be precisely controlled by tuning the ratio of ethanol to water under weak basic conditions. Together with the high latex concentration,

Figure 7. TEM images of representative hollow mesoporous silica nanospheres prepared via PS-templating route. a,b) Reproduced with permission. [ 64 ] Copyright 2010, ACS Publications; c,d) Reproduced with permission. [ 65 ] Copyright 2010, ACS Publications.

Scheme 3. Schematic illustrations of hard-templating routes: (1) Heterogeneous hard-templating and (2,3) homogeneous hard-templating for syn-thesizing hollow-structured mesoporous materials (route 2: structural difference-based selective etching for synthesizing hollow mesoporous silicas; route 3: etching and calcination for hollow mesoporous non-silica synthesis). Step (1–1, 2–1, 3–1): formation of HMMs around the solid core template; step (1–2): removal of the templates from both the core and mesopores via calcination, etching, or extraction procedures; step (2–2): removal of SiO 2 cores via structural difference-based selective etching; (step 3–2, 2–3): removal of surfactants from the mesopores via calcination or extraction; step (3–3): fi lling the mesopores with non-siliceous components; step (3–4): removal of SiO 2 core via etching by HF or NaOH.


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were generated owing to the aluminum incorporation into the silicate framework. Then, with the introduction of phenol and formaldehyde into the mesopores of SCMS aluminosilicate, further carbonization took place, which yielded the silica core/carbon-aluminosilicate shell nanocomposites. The dissolution of the silica core and aluminosilicate shell (i.e., SCMS tem-plate) using either NaOH- or HF-solution generated HCMS carbon capsules. In this approach, the dimension of the hollow core and the mesoporous shell thickness of the HCSM carbon capsules can be well tuned by employing SCMS silica sphere templates of appropriate core diameter and corresponding shell thickness. Later, gold nanoparticles contained in hollow spherical carbon/polymer capsules were fabricated by using core-shelled composite silica spheres composed of sub-microm-eter-sized Au-NP-containing solid cores and a mesoporous shell as a hard core template. [ 18a , 73a,b ] The above-mentioned synthetic methodology for preparing hollow mesoporous carbon spheres includes multiple synthetic steps: 1) preparation of core-shell structured SCMS templates by coating mesoporous silica layers on the silica core; 2) infi ltration into the mesopores of calcined

the fi nal products (Figure 7 c,d). [ 65 ] Furthermore, monodisperse and uniform hollow mesoporous organosilica nanospheres can be prepared by using PS latex spheres as the hard core tem-plate and 1,2-bis(trimethoxysilyl)ethane (BTME) as the organo-silica source through an NH 4 OH-catalyzed sol–gel process. The resultant products have been demonstrated to be good supports for creating pH-responsive supramolecular nanovalve systems that can be alternatively triggered by acid or base for the con-trolled release of the entrapped guest molecules. [ 66 ]

Not only the silica-based hollow mesoporous spheres, but also non-silica based hollow mesoporous spheres can be facilely fabricated by employing PS latex as a hard template. [ 67 ] To pro-mote the condensation of the non-siliceous precursors onto the surface of PS spheres, it is necessary to modify the surface with various functional groups, such as –NH 2 , –COOH and –OH. Tang et al. [ 68 ] found that magnetite could be easily coated on PS spheres with their surface being carboxyl-functionalized to obtain mesoporous magnetite hollow spheres. By plasma treat-ment, it is easy to introduce hydroxyl groups onto monodis-perse PS spheres, so that titanium (IV) isopropoxide or tetra-n-butoxygermane could condense with hydroxyl groups to form TiO 2 or GeO 2 coating around the PS spheres, which will be turned into hollow spherical particles upon the core dissolution with tetrahydrofuran (THF). [ 69 ]

Moreover, cross-linked poly(methacrylic acid) (PMAA) spheres can also be employed as hard cores to fabricate micro-capsules with hollow core and mesoporous shell structure. After the treatment by ionic liquid salts, the surface of PMAA was modifi ed by adsorbed double layers, which promoted the subsequent deposition of inorganic salts, so that mesoporous SiO 2 , Al 2 O 3 , and TiO 2 microcapsules with different cavity sizes were fabricated successfully. After loading and entrapping chiral catalysts inside the silica microcapsules, the resultant heterogeneous catalysts presented high activity and enantiose-lectivity in the synthesis of chiral β-blockers. [ 15d , 70 ]

2.2.3. SiO 2 Spheres as Hard Core Templates

The synthesis of monodisperse silica spheres with different sizes has been widely investigated, and the Stöber method has been verifi ed to be one of the most effi cient routes for obtaining uniform silica spheres for decades. [ 71 ] With or without surface functionalization, silica spheres have been demonstrated to be a suitable candidate for being utilized as a hard core template. By combining the Stober approach, the Giesche growth process, and the Kaiser approach, core-shelled monodisperse silica spheres on the nanometer scale, com-posed of a non-porous solid silica core and a thin mesoporous silica shell (SCMS) were obtained by the simultaneous sol–gel polymerization of TEOS and octadecyltrimethoxysilane (C 18 TMS) on the previously prepared nonporous silica spheres, followed by the removal of the organic groups. [ 72 ] On the basis of this, Heyon et al. employed silica/aluminosilicate spheres with SCMS structures as template materials and in situ polym-erized phenol-resin or poly(divinylbenzene) as the carbon source, as shown in Figure 8 a, to fabricate carbon capsules with a hollow macroporous core/mesoporous shell (HCMS). It was demonstrated that strong acidic catalytic sites, which are helpful for the polymerization of phenol and formaldehyde,

Figure 8. Schematic diagram (a), SEM (b) and TEM (inset of b) images of hollow mesoporous carbon spheres prepared via SiO 2 -template route. Reproduced with permission. [ 18a ] Copyright 2002, Wiley.


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precursor. [ 75 ] It was demonstrated that the aluminum species incorporated into the wall of the HMASs played a key role in the forma-tion of the HMCSs, which could both deter-mine the hollow morphology of HMASs, and generate acidic catalytic sites to catalyze the polymerization of the carbon precursor introduced into the pore channels during the replicating process. Consequently, the rep-licating process was simplifi ed and the con-ventional catalyst loading step or additional acidic catalysis was no longer needed.

Although it is convenient to fabricate hollow mesoporous carbon spheres based on

the SCMS hard templating route, there are very few reports on the synthesis of hollow mesoporous silica spheres by the same route. [ 76 ] Recently, Shi et al. developed a novel strategy, “struc-tural difference-based selective etching”, to construct mono-disperse hollow/rattle-type mesoporous silica spheres. Such a strategy features taking advantage of the structure differences, rather than traditional compositional differences, between the core and the shell of a silica core/mesoporous silica shell struc-ture to create hollow voids ( Figure 10 ). [ 77 ] It was found that the silicate condensation degree in the meosopore framework of the shell layer is signifi cantly higher than that in the solid silica core, which was generated by the self-assembly between C 18 TMS and TEOS, and the Stöber method. Consequently it is possible to employ an appropriate etching agent, Na 2 CO 3 solu-tion, to selectively remove the hard solid core, while keeping the outer mesoporous shell mostly intact. On the basis of this, highly disperse hollow mesoporous silica spheres with control-lable particle/pore sizes could be synthesized, which showed high loading capacity (1222 mg g −1 ) for an anticancer drug (doxorubicin). [ 78 ] Comparatively, this strategy is rather simple, controllable, and scalable. [ 77,78 ] Very recently, Zheng et al. [ 79 ] further simplifi ed the silica-based etching strategy. They intro-duced cationic surfactant of CTAB into the system of Na 2 CO 3 solution to etch the solid spheres, and found that high-quality hollow mesoporous silica spheres with either a wormhole-like or an oriented mesoporous shell could be facilely prepared. It was proposed that the cationic surfactant plays a critical role in the formation of HMSS from solid silica spheres: 1) as a soft

(i.e., surfactant-removed) SCMS with carbon precursors, such as the mixture of phenol-formaldehyde, furfuryl alcohol, and glucose; 3) polymerization of the precursor within the mesopores in the shell, 4) in situ carbonization of the polymer, and 5) removal of the silica core and silica framework from the shell. [ 73 ] Nevertheless, as can be clearly seen in Figure 8 b, [ 18a ] the incorporation of the carbon precursor into mesoporous channels of SCMS through infi ltration process will most prob-ably result in sphere aggregation.

Interestingly, Ikeda et al. successfully fabricated hollow mesoporous carbon spheres via a hydrothermal treatment route rather than high-temperature calcination to carbonize the carbon precursor, which effectively prevented aggregation among the carbon particles. Selective deposition of the pre-cursor into the pore systems of the template was achieved by employing an effective electrostatic attraction between the solid template and the carbon precursor. The template was positively charged by amino-modifi cation so that its mesopore surface was then uniformly covered by negatively charged polysaccha-ride, which resulted from the hydrothermal treatment of glu-cose, leading to the formation of isolated hollow mesoporous carbon spheres that inversely replicated the templates. [ 74 ]

Moreover, we have found that hollow mesoporous carbon spheres (HMCSs) with highly ordered, 3D cubic mesostruc-tured mesopore networks in the shells ( Figure 9 ) could be directly replicated from hollow mesoporous aluminosili-cate spheres (HMASs) by employing a simple incipient-wet-ness impregnation route with furfuryl alcohol as a carbon

Figure 10. Schematic diagram (left) and TEM images (right) of hollow mesoporous spheres prepared via “structural difference-based selective etching” approach (Route A: in Na 2 CO 3 solution and route B: in ammonia solution). Reproduced with permission. [ 77 ] Copyright 2010, ACS Publications.

Figure 9. SEM and TEM images of hollow mesoporous aluminosilicate spheres prepared by the nanocasting method. Reproduced with permission. [ 75 ] Copyright 2008, Elsevier.


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could function as both the carbon precursor and a promoter to accelerate carbon deposition during the CVD process. There-fore, it was not necessary to remove the surfactant from the mesopore network and then to introduce additional carbon pre-cursors into it (i.e., only one CVD process was needed for the production of carbon-fi lled silica spheres). After the removal of silica with HF treatment, hollow mesoporous carbon spheres were obtained. More importantly, graphitic mesoporous carbon spheres with hollow structure were produced by using ethylene as the carbon precursor during the CVD process. [ 85 ]

2.2.4. Carbon Spheres as Hard Core Templates

Monodisperse carbon spheres are another kind of inter-esting sacrifi cial template. They can be easily prepared from the aqueous solutions of glucose and polysaccharedies under hydrothermal treating conditions. [ 86 ] The as prepared carbon spheres are found to possess abundant functional groups such as –OH and –C=O on the surface, inherited from the precur-sors, which provided favorable chemical environment for adsorbing other precursors and/or nanoparticles in the fol-lowing processes. Based on this, a facile route was proposed by Zhu et al. to prepare hollow mesoporous silica spheres and rattle-type Fe 3 O 4 @SiO 2 hollow mesoprous spheres with large void spaces by using the colloidal carbon spheres as the templates ( Figure 11 ). [ 87 ] The fabrication involved the one-pot hydrothermal synthesis of colloidal carbon spheres adsorbed with iron precursors, the simultaneous sol-gel polymerization of TEOS and C 18 TMS to deposite the organosilicate-incor-porated silica shells on the colloidal carbon spheres, and the removal of the carbon templates and the organic groups of C 18 -TMS by heat treatment, leaving iron species within the core as Fe 2 O 3 nanoparticles, and fi nally the reduction of Fe 2 O 3 core

template to direct the construction of the mesoporous structure in the shell, 2) as an assistant to accelerate the etching of sSiO 2 (i.e., hard-template), 3) as a stabilizer to protect the silicate-CTAB shell from alkaline etching. Based on the same approach, aqueous colloidal hollow periodic mesoporous oganosilicas (HPMOs) with tailored compositions and nanostructures were further synthesized; [ 80 ] they exhibit unique biological behaviors both in vitro and in vivo.

Another simplifi ed and representative approach for the fabrication of hollow mesoporous materials by nanocasting route was reported by Fuertes et al. [ 18b , 81 ] This approach is to impregnate sub-micrometer-sized SCMS with a preceramic polymer-polycarbomethylsilane (PCMS). After the pyrolysis and following removal of the silica template, mesoporous silicon oxycabide capsules were obtained. Further calcination of the mesoporous silicon oxycabide capsules gave rise to the cap-sules with silica framework. It should be noted that, the organic carbon chains (–(CH 2 ) 17 –CH 3 ) in the organosilicon compound (C 18 TMS) used in the synthesis SCMS silica spheres functioned as both the carbon precursor and mesoporogen agent. There-fore, the synthetic procedure for obtaining uniform and non-aggregated spherical carbon capsules was signifi cantly simpli-fi ed. To achieve this, sulphuric acid was chosen as a catalyst to convert the organic moiety into carbon with a considerably increased carbon yield via dehydration and sulphonation reac-tions. The advantage of this approach is that neither the incor-poration step of polymeric carbon precursors nor an infi ltration step is required, thereby ensuring the facile synthesis of iso-lated hollow mesoporous carbon particles. [ 18b , 82 ] With an incip-ient wetness impregnation technology, controlled amounts of a variety of inorganic precursors, such as Fe, Co, Ni, and Cr could be incorporated into the hollow core of the carbon capsules, resulting in core/shell structured nanocomposites with the inorganic cores confi ned within the mesoporous carbon cap-sules. A rather unique characteristic of these core/shell struc-tured nanocomposites is that, though the inner hollow core can be almost completely fi lled by the nanoparticles, the mesopore channels on the carbon shell remain open and few nanoparticle depositions can be found in the shell. This feature makes these nanocomposites promising candidates in many fi elds, such as high-performance catalyst supports, materials for energy storage application (i.e., Li-ion batteries), or advanced adsor-bents with novel functionalities (magnetism, etc.). [ 82 ]

Liquid impregnation is known to have several drawbacks, including shrinkage of the silica network during synthesis and the formation of additional (micro)porosity during car-bonization. [ 83 ] In addition to the liquid impregnation technique, chemical vapor deposition (CVD) was also adopted by Xia et al. to nanocast hollow mesoporous carbon and other spheres with heterogeneous components. [ 84 ] The carbon sources are usually styrene, acetonitrile, benzene or ethylene. It has been demonstrated that hollow mesoporous carbon spheres con-taining highly ordered CMK-3 can be successfully fabricated by employing conventional SBA-15 as the template through CVD route, during which the pyrolysis/carbonization temper-ature of styrene is crucial to the formation of hollow carbon spheres. [ 82 , 84a ] Recently, Chen et al. presented a rather simple and controllable CVD nanocasting method to prepare highly ordered hollow carbon spheres with graphitic shell structure.

Figure 11. SEM and TEM images of rattle-type Fe 3 O 4 @SiO 2 hollow mesoporous spheres by using colloidal carbon spheres as core templates. Reproduced with permission. [ 87 ] Copyright 2009, ACS Publications.


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benefi ted from the high-quality hard templates, which are fea-tured with uniform sizes, regular morphologies, high disper-sions and easy surface modifi cations. Though an extra step is needed for the preparation of the hard templates, it is just the separate syntheses of the templates and the mesoporous shells in different media that makes the preparation of monodis-persed and uniform HMMS possible, if compared to those by the soft-templating routes. However, it is also worth noting that the periodicity of the mesopore arrangement in these resultant HMMs is usually poor or sometimes hardly controllable, which is probably due to the fact that C 18 TMS rather than CTAB was more frequently employed to create the mesoporous shell.

2.2.5. Other Hard Core Templates

In addition to the above-mentioned hard templates, some other inorganic or metal oxides can also be used as hard core tem-plates. [ 91 ] In particular, hematite has been verifi ed to be a useful hard template to fabricate hollow silica particles. The synthetic strategy involves coating an organic-incorporated silicate shell with a desired thickness by the co-polymerization of TEOS and C 18 TMS mixture on a pre-formed hematite particle of desired size and shape, subsequent removal of the organic groups by calcination to form hematite core-mesoporous silica shell com-posite particles (as-HMPs), and fi nally etching away of the cores with hydrochloric acid solution. With this strategy, uniform and well-dispersed hollow mesoporous silica particles ( Figure 12 a) could be facilely synthesized. Moreover, the HMS shape, such

into Fe 3 O 4 nanoparticles under hydrogen atmosphere. On the other hand, owing to the abundant –OH groups on the surface of the carbon nanospheres, Pd nanoparticles (ca. 5 nm) were expected to anchor on the outer surface of them with uniform distribution. Upon forming mesoporous silica layer on the Pd/C spheres and the following removal of the hard carbon cores and CTAB surfactant, Pd nanoparticles residing inside the hollow spheres were obtained, which exhibited extremely high activity for Suzuki cross-coupling reactions. [ 88 ] Interest-ingly, C-doped hollow TiO 2 mesoporous microspheres with superior visible light photocatalytic activity for the degradation of toluene have also been achieved by choosing carbonaceous polysaccharide microspheres as hard cores via rapid combus-tion process. [ 89 ] Moreover, hollow spheres of crystalline porous metal oxides, such as γ-Al 2 O 3 , MgO-Al 2 O 3 and binary MgTiO 3 with relatively high specifi c surface area have been prepared by using hollow mesoporous carbon spheres as hard templates. [ 90 ] The metal oxides were fabricated within the pore channels of the carbon templates after the following removal of the carbon shell.

Table 2 summarizes the physico-chemical properties of HMMs prepared via hard-templating routes. With solid polymer, SiO 2 or carbon spheres as core templates, it has been found that highly dispersed HMMs with uniform particle sizes and tunable and controllable hollow core dimensions and shell thicknesses can be readily prepared. Moreover, the framework components of HMMs can be varied from SiO 2 to the non-siliceous, such as carbon components. These templated HMMs

Figure 12. TEM images of hollow mesoporous silicas with various morphologies by using hematite as core templates. Reproduced with permission. [ 91 ] Copyright 2009, Royal Society of Chemistry.

Table 2. Properties of some representative HMMs prepared via hard core templating routes

Template Framework Dispersivity Hollow core diameter [nm]

Shell thickness [nm]

Uniformity Periodicity Ref.

PS SiO 2 low 62–138 320–800 + + Y. Wei[ 60 ]

PS SiO 2 high 60–90 5–40 + + B. Charleux[ 63 ]

PS SiO 2 high 140–1500 30–60 + - E. Giannelis[ 64 ]

SiO 2 @m SiO 2 Carbon high 220 60 + T. Hyeon[ 18a ]

SiO 2 Carbon high 168 27–49 + - M. Matsumura[ 74 ]

SiO 2 SiO 2 high 35–470 5–100 + - J. L. Shi[ 77,79 ]

SiO 2 @m SiO 2 SiO 2 /silicon

oxycarbide

high 270–320 40 + - A. B. Fuertes[ 81 ]

C SiO 2 high 700 100 + - S. Kaskel[ 87 ]


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cores (Figure 12 b,c). Moreover, hollow structured phenylene-bridged periodic mesoporous organosilica (PMO) spheres with a uniform particle size of 100–200 nm could also be obtained by using α-Fe 2 O 3 as a hard template and 1,4-Bis(triethoxysilyl)benzene (BTEB) as silica source, respectively. [ 91,92 ] By adopting hydroxyapatite nanoparticles as an etching-removable core material, hollow mesoporous silica could be prepared as well. Interestingly, the morphology of HMS can be well controlled and tailored according to the nature of HA template. [ 93 ]

It is of great interest to fabricate organic-inorganic hybrid or even pure organic hollow mesoporous sphere materials, which are expected to possess improved dispersivity, biodegradability and other properties that pure inorganic materials do not have. Huang et al. developed a facile strategy for preparing hollow mesoporous submicrospheres of poly[(cyclotriphosphazene- co -4,4′-sulfonayldiphenol)] (PSZ) by choosing CaCO 3 spheres as templates. The fabrication includes the polycondensation between hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulfo-nyldiphenol (BPS) via a very simple precipitation polymerization procedure on the hard core, and the removal of templates. This method needs a single ultrasonic radiation treatment at room temperature. The as-synthesized hollow mesoporous sub-micrometer-sized spheres possess excellent biocompatibility and dispersity in both aqueous and organic media. Moreover, these crossliniked polyphosphazene hollow mesoporous sub-micrometric spheres manifested a relatively high drug storage capacity of 380 mg per g doxorubicin hydrochloride, and extremely sustained release property (up to 15 d). [ 17b ]

2.3. Aerosel-Templating and Template-Free Routes

In addition to the common soft- or hard- templates employed for HMMs syntheses, alternatively, aerosols may be also created during synthesis by, for example, evaporation, thermal spraying and salt decomposition, and can thus be used as the template for HMMs preparation. For example, evaporation-induced self assembly (EISA) process of amphiphilic molecules in droplets containing alkoxysilane precursors, which has been shown to be a versatile approach for synthesizing ordered spherical mesoporous or mesostructured silica particles or mesoporous fi lms, can generate aerosols, and under the assistance of the aerosol, hierarchical or hollow mesoporous materials could be thus fabricated. [ 94 ] On this basis, Linden et al. and Sanchez and co-workers reported the aerosol-based syntheses of core-shell structured mesoporous nanoshperes with bimodal porosity and large-pore amorphous mesostructured aluminosilicates by using a mixture of two surfactants as structure-directing agents, [ 95a-c ] which presents a highly effective pathway for the fabrication of HMMs. Afterwards, Ward et al. demonstrated that surfactant-directed synthesis of mesoporous silica could be combined with polymer poly(acrylic acid) phase separation during evaporating droplets in a one step synthesis, leading to hierarchical porosity after subsequent calcination. [ 95d ] Owing to the macroscopic phase separation of the sorbitan monooleate surfactant (Span 80) during aerosol assisted spraying, hollow spheres of phenolic resin/silica composite could be obtained. Both the size and number of the hollow cavity were found to

be tunable by changing the using amount of Span 80 to a cer-tain extent. [ 96 ] Recently, Brinker et al. reported that macroscopic phase separation driven by controlled salt ((NH 4 ) 2 SO 4 ) nuclea-tion and surfactant-directed self-assembly within aerosol drop-lets can produce hollow spherical silica particles with ordered mesoporous shells in a simple process. Salt decomposition cre-ated a spherical void in the particle interior and catalyzed silica condensation which stabilized the hollow particle. [ 97 ] Further-more, with an aerosol-spraying approach, hollow mesoporous spherical BiFeO 3 with enhanced activity and durability, could be designed and prepared. [ 19d ] Template-free hydrothermal syn-thesis, during which Ostwald ripening plays a crucial role, was also adopted to prepare CeO 2 , or ZnO-based hollow mesoporous spheres. [ 15c , 15f ] As neither surfactants nor templates were used in the reaction system, the mesoporous shell was built from the aggregation of nanoparticles, while the void core was a result of the solid evacuation driven by an Ostwald ripening process.

Generally speaking, though few examples of template-free syn-thesis of HMMs have been reported, a template is usually nec-essary in fabricating hollow-structured materials, which can be either externally introduced (solid particles for hard-templating) or self-generated (liquid droplets/micelles for soft-templating). As summarized above, soft-templating (including seldom used aerosol template) is usually a one-pot or one-step process which can yield HMMs with ordered mesoporous structure in the shell; unfortunately the particle dimension, morphology and dis-persity are usually hard to get under fi ne control. Alternatively, when employing silica core-mesoporous silica shell (SCMS) or hollow mesoporous silica templates for the replication of hollow mesoporous non-siliceous particles, [ 18a , 73a , 73b ] the aggregation is frequently unavoidable during the hard-templating process due to the undesired but sometimes inevitable deposition of the precursor on the outer surface of the template particles accom-panying the precursor deposition into the pore channels of the hard template, which will bind with each other and fi nally result in aggregation of the templated particles. One key to prevent the aggregation during hard-templating is to minimize the pre-cursor deposition on the outer surface by either casting all pre-cursors into the pore system by external pressure or vacuumi-zation, or by removing the deposited precursor from the outer surface without affecting those within the pore system. [ 107c ] However, more frequently, if one uses solid nanoparticle (e.g., PS Latex, silica) as the core template, and the porous structured shells are generated by the self-assembly between the mesoporo-gens and precursors on the surface of the solid core, usually the aggregation can be easily prevented. [ 62,63,75,82,87 ] Therefore hard-templating strategy is becoming more prevailing due to the fact that monodispersed HMMs with highly controllable hollow core diameter, shell thickness, and the mesoporosity in the shell as well, can be obtained with the hard-templating, which is highly important or even crucial in the applications in, for example, catalysis, and especially in biomedical fi elds.

3. Functionalization and Applications of HMMs

Owing to the special core-shell structure, HMMs can be func-tionalized in the hollow core, on the inner and outer surface of the shell, or in the mesopore channels in the shell. As the


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strategies of functionalization are similar to those for the con-ventional mesoporous materials, we will not discuss these spe-cifi cally, but will describe them in detail in the following section along with their applications.

Hollow mesoporous materials combine both macroporous and mesoporous features into one unit, where the hollow cavity can function as a storage reservoir or a microreactor, and the permeable mesoporous shell allows the guest species to dif-fuse through the mesopore channels between the macropo-rous core and the exterior of the spheres, and in addition, the mesopore channels in the shell layer can also be used to either incorporate, adsorb, or immobilize guest substances. There-fore, HMMs are believed to have great application potential in a variety of fi elds, such as energy storage, separation, sensors, catalysis and drug storage and delivery. [ 98 ]

3.1. Catalysis

Mesoporous materials have shown attractive performance in catalysis fi eld. [ 99 ] Hollow mesoporous materials have an inter-connected bimodal pore system composed of a hollow core and a mesoporous shell and have much higher pore volume com-pared to the conventional mesoporous materials. Therefore, they are potentially important as a catalyst support, in which the hollow core and sometimes the mesopores in the shell as well can be used for loading/dispersing catalyst nanoparticles/species, meanwhile the open mesopore network in the shell connecting the hollow core in the interior and the exterior can provide a free highway network around the active catalyst for the free diffusions of reactant molecules accessing active sites and products leaving the catalysts. Based on the uniform hier-archical nanostructure of hollow mesoporous materials, active catalyst nanoparticles can locate either in the mesopore net-work on the shell or within the hollow core. [ 100 ]

Scheme 4 illustrates the advantages of catalytic nanoparti-cles-encapsulated HMMs in catalysis: the catalyst is well pro-tected from the environment and the catalytic particles are well separated from each other as well, thus no catalytic particle growth and/or aggregation will happen during the catalytic reactions especially at elevated temperatures. In the meantime, the mesopore channels function as the gate for shape-selective catalysis where only the reactant/product molecules of smaller than the pore size can diffuse in/out of the composite system.

3.1.1. Catalytically Active Nanoparticles Encapsulated into the Cores of HMMs

Nanometer-sized noble metal particles have shown great potentials as advanced catalysts owing to their large sur-face area and size-dependent properties different from bulk metals. [ 101 ] However, the diffi culties in the recovery of the fi ne particulate catalysts from the reaction mixture, as well as the instability of the nanometer-sized particles under dif-ferent conditions, such as high pressure or high temperature, have strongly hindered their scalable applications. One of the most straightforward approaches to circumvent these prob-lems is to immobilize the metal nanoparticles on/in solid sup-ports. [ 102 ] It has been proved that “rattle-type” nanostructured

catalysts (Pt@ hm C) by encapsulating Pt nanoparticles into the hollow core of HCMS can work as a robust and reusable het-erogeneous catalyst for a number of reactions, such as hydro-genation, where the carbon shell functions as a barrier to prevent the possible coalescence of Pt nanoparticles between each other and also provide a void space for organic trans-forming on the surface of the ligand-free Pt nanoparticle. [ 103a ] Compared with the original Pt-PVP, a commercial Pt catalyst supported on activated carbon (Pt/AC), and Pt@SiO 2 - m SiO 2 , remarkable activity for hydrogenation of nitrobenzene has

Scheme 4. A comparison of the catalytic processes between using cata-lyst nanoparticles-loaded HMMs (1) and conventional nanoparticlate catalysts (2). No catalytic species aggregation will happen among those loaded in HMMs during catalytic reaction, while it will most probably do in traditional nanoparticulate catalysts.

Figure 13. Liquid-phase hydrogenation of nitrobenzene into aniline by various Pt catalysts at 303 K. Reproduced with permission. [ 103a ] Copyright 2006, Wiley.


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single metal nanoparticles, multiple dispersed inorganic oxide nanoparticles (TiO 2 and Fe 2 O 3 ) could also be encapsulated into the hollow cores of hollow mesoporous silica capsules (HMSC) with incipient wetness technique. A large amount of TiO 2 -encapsulated silica capsules photo-catalyzed much faster decomposition of methyl orange, especially at the initial reac-tion stage, than that by other mesoporous silica-based TiO 2 /SiO 2 composites. [ 100a ]

Recently, Zheng et al. successfully prepared hollow mesoporous aluminosilicate spheres (HMASs) with pore channels prepen-dicular to the surface simply by treating the solid silica spheres in a hot alkaline solution of sodium aluminate and CTAB. [ 104a ] It is demonstrated that the highly permeable prependicular pore channels could effectively prevent the catalytically active Au nanoparticles incorporated into the cores from aggregation in the reduction reaction of 4-nitrophenol. Forthermore, owing to the accessible acidity introduced by Al incorporation in the frame-work of the shells, the york-shell structured HMASs with Pd nanopartilces in the cores presented high catalytic performance and recyclability in the one-pot two-step reaction involving an initial acid catalysis and subsequent catalytic hydrogenation for desired benzimidazole derivatives. [ 104a ] On the other hand, by employing polystyrene spheres as both carriers and templates, Pd or Au nanocrystal-embedded hollow mesoporous TiO 2 and ZrO 2 microspheres were facilely synthesized by Wang et al. [ 104b ] Owing to the unique core-shell structure, Pd nanocube-embedded ZrO 2 microspheres exhibited a much higher catalytic activity and reaction rate, and a superior recyclability, in com-parision with the commercial Pd/C catalyst, in the reduction of 4-nitrophenol ( Figure 15 ). More interestingly, in addition to the advantages such as fast diffusion of reactants and products, and inhibited growth of the confi ned active nanoparticles within the mesopore channels in the shells, a synergistic catalytic effect between the encapsulated Pd nanoparticles and the mesoporous CeO 2 shells was believed to speed up the charge transfer and effectively inhibit the catalyst poisoning, so that the reduction of 4-nitrophenol could be signifi cantly accelerated. [ 104c ]

been achieved ( Figure 13 ): nitrobenzene was completely con-verted into aniline on Pt@ hm C, as compared to only a por-tion of nitrobenzene convertion on Pt-PVP and Pt/AC, and almost no nitrobenzene convertion on Pt@SiO 2 - m SiO 2 . These indicate that the presence of the hollow void in the parti-cles Pt@ hm C plays a crucial role in enhancing the catalytic activity of Pt@ hm C. On the other hand, though Pt-PVP exhib-ited higher activity than Pt/AC in the fi rst run, it cannot be recovered effi ciently for further reactions. Comparatively, the Pt@ hm C catalyst could be simply recovered by centrifuga-tion and recycled for further reactions. Moreover, in contrast to Pt-PVP and Pt/AC catalysts, Pt@ hm C also showed higher catalytic activity for other reactions, such as the hydrogena-tion of primary, secondary and cyclic olefi ns, as presented in Table 3 . These demonstrate the importance of the hollow core in maintaining the activity and stability of noble nanoparticles encapsulated into it. Based on the same idea, core-shell struc-tured Pt@mesoporous silica confi guration (Pt@ m SiO 2 ) was designed and prepared. The outer layer isolated the catalyti-cally active nanoparticles from each other and prevented them from sintering during catalytic processes at elevated tem-peratures. Furthermore, it is considered that the synergistic effects at the metal-support interfaces may be maximized, which are very important for their catalytic performances. Consequently, the Pt@ m SiO 2 catalyst could maintain its core-shell structure up to 750 °C, so that high-temperature CO oxidation could take place and high catalytic activity could be obtained. In comparison, it was not possible for bare Pt nano-particles to stand with such a high tempereature due to their sever deformation or aggregation ( Figure 14 ). [ 103b ] Apart from

Figure 14. CO oxidation activity of TTAB-capped Pt and Pt@mSiO 2 nano-particles (inset: TEM image of Pt@mSiO 2 nanoparticles after calcination at 550 °C). Reproduced with permission. [ 103b ] Copyright 2009, Nature Publishing Group.

Table 3. Hydrogenation of various olefi ns by pt@hmC, Pt/AC and Pt-PVP catalysts. Reproduced with permission.[ 103a ] Copyright 2006, Wiley.

Substrate Catalyst Product t b) [h]

Conv. c) [%]

Pt@ hm C 2 >99

Pt-PVP 2 91

Pt/AC 2 7

Pt@ hm C 2 96

Pt-PVP 2 70

Pt/AC 2 16

Pt@ hm C 1 91

Pt-PVP 1 42

Pt/AC 1 3

Pt@ hm C 15 72

Pt-PVP 15 46

Pt/AC 15 16

a) All reactions were carried out with 0.1 µmol of catalyst (Pt) and 0.5 mmol of substrate under H 2 (0.2 MPa in absolute pressure) at 348 K; b) Reaction time; c) Conversion of substrate.


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resulting from the large surface area and mesopore volumes and the 3D interconnected hierarchical nanoporous structure of HCMS. [ 107a,b ] Furthermore, the possibility of HCMS as a cathode catalyst support in H 2 -fueled PEMFC was evidenced by loading Pt (20 wt%) in HCMS, which exhibited an excel-lent catalytic activity to oxygen reduction reaction (ORR) com-parable to the state-of-the-art cathode catalyst. [ 107c ] In order to further improve the stability of the electrocatalysts, Schüth et al. [ 107d ] prepared hollow graphitic carbon spheres for loading Pt nanoparticles in the mesopores of the shells. Testing results showed that the electrochemical stability was enhanced while the high activity was retained, which are benefi ted from the mesoporous carbon network. These prove that the mesoporous shell of HCMS could effectively maintaine or enhance the cata-lytic activity/stability of the active nanoparticles by reducing their detachment and agglometration due to the confi ment and good separation of them in the mesopore system. In addi-tion, by using glycine as both carbon and nitrogen precursors, nitrogen-doped HMCSs with broken graphene in the wall were

3.1.2. Catalyticaly Active Components Loaded into the Shells of HMMs

In as early as 2004, Yu et al. found that carbon capsules with hollow core and mesoporous shell (HCMS) were excellent sup-ports for electrode catalysis in the direct methanol fuel cell (DMFC) ( Figure 16 ). [ 105 ] After loading Pt 50 -Ru 50 catalyst onto the mesoporous shell, such a HCMS presented much higher catalytical activity for methanol oxidation than the commer-cial E-TEK catalyst by about 80%, which was attributed solely to the excellent structure properties (i.e., high specifi c surface area and well-interconnected bimodal porosities of HMCS.) [ 105 ] Later, researchers further demonstrated that Pt/Ru- or Pt-loaded HCMS, as anode catalysts, also exhibited signifi cantly enhanced electrocatalytic activities in direct formic acid fuel cell (DFAFC) [ 106 ] or proton-exchange membrane fuel cell (PEMFC), which were probably due to the combined contributions by the uniformly dispersed catalyst nanoparticles and fast mass transport network around the active catalyst nanoparticles,

Figure 16. Polarization and power density plots at 30 °C (a) and 60 °C (b) for various hollow mesoporous carbon-supported Pt 50 Ru 50 (60 wt%) anode catalysts. Reproduced with permission. [ 105 ] Copyright 2004, Royal Society of Chemistry.

Figure 15. a) TEM image of the ZrO 2 microsphere after fi ve cycles for 4-nitrophenol reduction reaction. b) Plot of C(t)/C(0) against the reac-tion time in fi ve successive cycles of the reduction reaction with the Pd nanocube-embedded hollow mesoporous ZrO 2 microspheres as the cata-lyst. Reproduced with permission. [ 104b ] Copyright 2013, Wiley.


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shell thicknesses (0, 25, 50 to 100 nm) were prepared and the electrochemical capacitance and ionic transport performance were examined. It was found that the shell thickness affected the overall porosity and relative porosities of the shell, core and interstitial regions ( Figure 17 ). A performance-structure relationship is established from the electrochemical tests on a set of hierarchical structures with the stepwise increased thick-nesses of the mesoporous shell. At low scan rates and low cur-rents, capacitance depends on the surface area which increases with the increase in the mesoporous shell thickness. At high scan rates and high current loadings, ion transport become hin-dered in thicker shells. [ 108 ]

3.1.3. Enzyme-Encapsulated HMMs for Building Biosensors

Owing to the rigid structure, large interior space inside the shell and the isolated catalytic environment, HMMs can be employed as an excellent support to immobilize enzymes for estab-lishing biosensors. [ 109a,b ] For example, a novel rigid artifi cial

prepared. [ 107e ] Owing to the absence of active metals, such a material exhibited excellent methanol tolerantce in the ORR in alkaline solution, though the ORR activity was inferior to the commercial Pt/C catalyst. Very recently, a novel sulfur-impreg-nated hollow mesoporous TiO 2 spheres were obtained and studied as cathode material in Li-S batteries. [ 107f ] With a 68 wt% loading amount of S, an intriguing capacity retentation of 71% and a high coulombic effi ciency of 93% over 100 cycles, at a 1C rate, were achieved. By using hollow mesoporous organosilica spheres as support, MacMillan catalyst (H- Ph PMO-Mac) could be easily grafted by a co-condensation process and a “click chemistry” post-modifi cation. Due to the hydrophobic proper-ties of PMO surface and the accessible mesopores in the shell, the mass-transport process was accelerated, which endowed the H- Ph PMO-Mac catalyst with higher catalytic activity than solid (non-hollow) Ph PMO-Mac catalyst in asymmetric Diels–Alder reactions ( Table 4 ). [ 92 ]

In order to understand the effect of shell thickness on the cat-alytic activity, hollow mesoporous carbon spheres with different

Figure 17. CV curves (a,b) of various carbons recorded at 5 mV s −1 and 200 5 mV s −1 ; Charge–discharge curves (c) of C-CS0, C-CS50, C-CS80 and C-CS150 obtained at a constant current density of 10 A g −1 . Reproduced with permission. [ 108 ] Copyright 2011, Royal Society of Chemistry.

Table 4. Comparison of catalytic performances of the MacMillan catalyst on different ph PMO supports. a) Reproduced with permission.[ 92 ] Copyright 2011, Wiley.

PhCHO

CHO

Ph

PhCHO

+ +Cat. (20 mol%), TFA

Solvent, RT

Entry Catalyst t [h]

Solvent Yield c) [%]

endo/exo d) endo ee e) [%]

exo ee e) [%]

1 1 24 CH 3 CN/H 2 O b) 97 1:1.1 95 92

2 H- Ph PMO-Mac 24 CH 3 CN/H 2 O b) 58 1:1.1 89 86

3 Ph PMO-Mac 24 CH 3 CN/H 2 O b) 42 1:1.1 87 85

4 H- Ph PMO-Mac-G 24 CH 3 CN/H 2 O b) 46 1:1.0 72 69

5 Ph PMO-Mac-G 24 CH 3 CN/H 2 O b) 38 1:1.0 70 68

6 1 24 CH 3 CN/H 2 O b) 80 1:1.4 93 91

7 H- Ph PMO-Mac 12 H 2 b 98 1:1.1 81 81

8 Ph PMO-Mac 12 H 2 O 84 1:1.1 79 78

9 H- Ph PMO-Mac-G 12 H 2 O 86 1:1.2 63 62

10 Ph PMO-Mac-G 12 H 2 O 80 1:1.1 59 62

a) TFA = trifl uoroacetic acid.; b) CH3CN/H2O, 95:5(v/v); c) Yield of the isolated product; d) Determined by 1 H NMR spectroscopy; e) Determined by HPLC.


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performance and rate capability, as demonstrated by Yu et al. in storing electrochemical hydrogen or Li by using hollow core mesoporous shell carbon (HCMSC) as supports. It has been found that the charge capacity of HCMSC is up to 586 mAh/g, corresponding to 2.17% hydrogen uptake, in 6 M KOH at a dis-chage rate of 25 mA g −1 . Furthermore, excellent cycling capacity retainability and rate capability were obtained ( Figure 19 ). [ 110a,b ] Alternatively, the test of Li storage was perfomed by choosing HCMSC as an electrode material for electrochemical double layer capacitor. A very high specifi c capacitance of 162 F g −1 at 0.3 A g −1 curret density in a more practical two electrode symmetric system was achieved by employing organic electro-lytee. At an elevated current density of 1.0 A g −1 , the specifi c capacitance maintained as 148 F g −1 , about 91% of the initial capacitance at 0.3 A g −1 , which is about 2 times higher than the commercial activated carbon. Moreover, very high cyclic performance with about 88% retention of the initial capaci-tance value after up to 2000 charge–discharge cycles at 1.0 A g −1 was obtained. [ 110c ] Additionally, hollow mesoporous TiO 2 microspheres were investigated as an anode material for lith-ium-ion batteries. A capacity as high as 200 mAh/g, excellent cycle life and rate capability were achieved. Endowed with decreased crystallite size and enlarged specifi c surface area, hollow mesoporous TiO 2 spheres showed satisfactory electrical contacts and tolerance to the strain occurred during the charge–discharge process, thus almost free lithium-ion diffusion and superior electrochemical performance were achieved. [ 111a,b ] Recently, Lou et al. developed a facile hard-templating strategy for the synthesis of hollow Li 4 Ti 5 O 12 spheres for the use as anode materials for high-rate lithium-ion batteries (LIBs), which exhibits a remarkable rate capability up to 20C and long-term capacity retention for over 300 cycles. [ 111c ]

3.3. Adsorption and Separation

HMMs have exhibited much more advantages in mass trans-port compared with conventional mesoporous materials due to their large pore/cavity volumes and spherical morphology, which are anticipated to be of great importance in the fi eld of adsorption and separation. [ 112 ]

Bilirubin is a pathogenic substance and one of the common metabolites of hemoglobin, and is released into blood due to

superoxide dismutase (SOD) has been constructed by immobilizing Mn-(bis(salicylaldehyde)-3,4-diaminobenzoic acid) (Mn-CSalen), a well-known active center for the design of arti-fi cial superoxide dismutase (SOD), into HMMs. Thanks to the rigid structure, free diffusion pathways for product molecules, biological fi tness and resistance to extreme environmental con-ditions offered by HMMs, this novel rigid artifi cial SOD can be easily immobilized as a receptor and be combined with various measuring methods to establish biomimetic sensors ( Figure 18 ). More importantly, this novel rigid artifi cial SOD possesses higher activity than the unsupported active centers. [ 109c ]

3.2. Electrochemical Energy Storage

With the unique hollow core-shell structural features, HMMs have shown great application potentials in mass storage and dif-fusion, especially in electrochemical energy storage. Specifi cally, the macro-hollow core can serve as an effi cient mass storage and buffer reservoir, which greatly facilitates the reduction of the volume change during the charge–discharge cycling espe-cially at high rates. On the other hand, the mesoporous shell around the hollow core would shorten the mass diffusion path and accelerate the mass diffusion. [ 110 ] Consequently, HMMs show ultra-high mass storage capacity and excellent cycling

Figure 19. A) Galvanostatic charge–discharge curves at 100 mA h g −1 ; B) Cycling performance and coulombic effi ciency at a specifi c current of 100 mA h g −1 ; C) Rate performances at different current densities from 100 to 1000 mA h g −1 and then back to 100 mA h g −1 using commercial graphite, CMK-3 and HCMSC capsules as anode. Reproduced with permission. [ 110b ] Copyright 2011, Royal Society of Chemistry.

Figure 18. Schematic representation of the artifi cial SOD. Reproduced with permission. [ 109c ] Copyright 2011, Elsevier.


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guest molecules such as bilirubin and the much shorter chan-nels in the thin mesoporous shell is benefi cial for the bilirubin access to all mesoporous channels of HMCSs; the hydropho-bicity of HMCSs prefers much higher adsorption capacity of the hydrophobic bilirubin. [ 115a ] Furthermore, when loaded with magnetic hematite into the cores, HMCSs become magneti-cally separable absorbents while the higher adsorption capa-bility can be roughly maintained. [ 115b ] Moreover rattle-type mag-netic mesoporous carbon spheres (RTMMCSs) with a hollow magnetite core, perpendicularly aligned mesopore channels in the shell and a cavity in between were demonstrated to be useful as a magnetically separatable and reusable absorbent for fast, convenient and highly effi cient removal of microcys-tins, which was mainly attributed to the high surface area of the numerous accessible mesopores, as well as the presence of cavities between the core and shell. [ 116 ]

Similarly, hollow mesoporous silica spheres could be used to remove organic molecules from aqueous solution after certain modifi cations. [ 117 ] Zhou et al. reported that hexade-cyltrimethylammonium bromide (HDTMAB)-immobilized hollow mesoporous silica spheres could be used to effi ciently adsorb perfl uorooctane sulfonate (PFOS) even at low solution pH and ionic strength values through hydrophobic interaction, so that more than 99% PFOS can be removed from water due to the strong adsorption affi nity of the modifi ed spheres with PFOS. [ 117b ] Similarly, after hydrophobic modifi cation, hollow porous silica nanospheres could be applied to quickly remove 4-Nonylphenol from water with enhanced saturation adsorp-tion amounts. [ 117c ]

3.4. Biomedical Applications

Due to the high surface-to-volume ratio, high pore volume and also the ease of modifi cation of the outer surface, hollow mesoporous silica nanospheres (HMSNs) have attracted increasing attention for the encapsulation and delivery of chemical drugs in biomedical applications to meet the require-ments of themo-therapeutic drug vehicles in terms of its high biocompatibility, biodegradability, high loading effi ciency and controlled drug release properties. Especially, sophisticatedly engineered multifunctional HMSMs with diverse functions may accomplish (e.g., molecularly/magnetically targeted drug delivery, simultaneous multi-modality bioimaging and thera-peutic capability, in vivo stimuli-responsive drug release/therapy, multicomponent synergistic therapy, etc.). The distinctive func-tions, unique structures, convenience in multifunctionaliza-tion and the related excellent biomedical performances endow the multifunctionalized HMSNs with much more application potentials than traditional HMMs, and have attracted inten-sive attentions among chemists, material scientists, biologists, pharmaceutical companies and even doctors. [ 118 ]

3.4.1. Drug Carriers

It has been demonstrated that the drug (such as aspirin or i-buprofen) storage capacity of HMSNs is remarkably higher than that of conventional mesoporous silicas, MCM-41 and

the normal or abnormal destruction of red blood cells. [ 113 ] It is necessary to separate the excessive bilirubin from blood, which is believed to the main cause of a characteristic form of crip-pling known as athetoid cerebral palsy or even death. [ 114 ] A recent report shows that hollow mesoporous carbon spheres present considerably higher bilirubin adsorption capacity (304 mg g −1 ) and rate (bilurubin concentration from 250 mg L −1 down to 0.3 mg L −1 accomplished in 4 min) in PBS solution, as compared with commercial activated carbon ( Figure 20 & Table 5 ). More importantly, HMCSs show relatively high bilirubin adsorption selectivity against albumin at normal albumin concentration and negligible hemolytic activity. It is believed that the excellent adsorption performance of HMCSs is related to the structural features and the surface property:

Figure 20. Bilirubin equilibrium adsorption isotherms of active carbon (a), CMK-3 and HCMSs (b). Reproduced with permission. [ 115a ] Copyright 2009, Royal Scoiety of Chemistry.

Table 5. Comparison of the samples on the adsorption capacity of Bili-rubin. Reproduced with permission.[ 115 ] Copyright 2009, Royal Society of Chemistry.

Sample Initial conc. [mg L −1 ]

Balanced conc. [mg L −1 ]

Balanced period [min]

Adsorption capacity [mg g −1 ]

Activated carbon 250 110.3 300 70

CMK-3 250 0.3 10 198

HMCSs 250 0.3 4 304


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groups ( Figure 21 ). [ 120 ] However, as most of above mentioned HMSNs have a poor dispersity and stability in aqueous solu-tion due to the strong aggregation among particles, which would greatly prevent them from being used as carriers in drug delivery systems due to the impossibility of these aggregated HMSNs in circulating in blood stream and subsequently in reaching lesions by the enhanced permeability and retention (EPR) effect, therefore it is of great importance to substantially improve its dispersity and stability in aqueous solutions for pos-sible applications in drug delivery systems for enhanced thera-peutic effi cacy. Many studies have shown that the PEGylation of nanoparticles is one of the most effi cient ways to enhance the blood circulation and EPR effect. [ 121 ] Recently, PEGylated HMSNs (HMSNs-PEG) have been successfully fabricated by covalently grafting poly(oxyethylene)bis(amine) (PEG2000-NH 2 ) on HMSNs-NH 2 with p-phenylene diisothiocyanate (DITC) as a cross linker. The stability and dispersity of HMSNs were found to be signifi cantly improved by the introduction of PEG barriers ( Figure 22 ). Furthermore, compared to HMSNs, much lower in vitro cytotoxicity to HeLa and NIH3T3 cells up to a concentra-tion of 150 µg mg −1 , and near 2 times higher uptake amounts were obtained on HMSNs-PEG. With doxorubicin (DOX) as a model drug, DOX-loaded HMSNs-PEG exhibited noticeably higher cytotoxicity than that of DOX-loaded HMSNs against Hela and NIH3T3 cells ( Figure 23 ). [ 121 ]

Based on the unique hollow and mesoporous structure, HMSNs can serve as an alternative to traditional liposomes, where the large hollow interior functions as a reservoir for storing hydrophobic agents, while the mesoporous silica shell with controllable thickness, hydrophilic inner/outer surface, tunable pore size can guarantee effi cient encapsulation of hydrophilic agents. [ 78a ] This has been successfully demonstrated by the simultaneous co-loading and co-delivery of hydrophobic camptothecin (CPT) and hydrophilic DOX anticancer drugs, which brought forth the enhanced chemotherapeutic effect against DOX-resistant MCM-7/ADR cancer cells.

Scheme 5 gives an illustration of drug delivery using HMMs. Drug molecules are mostly loaded in the hollow cavities, and the drug-loaded HMMs can pass through the vasculature and uptaken by the cells via EPR and other targeting effects. Drug molecules will release in the cell plasma and diffuse into the nucleus to damage the DNA double chain, meanwhile the HMMs will either degrade into small fragments and/or molecules

MCM-48 with sustained-release properties due to the pres-ence of the large hollow voids in the interior and/or the nano-sized mesopore channels in the shell. [ 119 ] With HMSNs as the drug carrier, as given in Table 6 , the loading amount of ibu-profen could reach as high as 969 mg g −1 . More importantly, the release rate of the loaded ibuprofen molecules could be tuned by modifying the pore surface with functional –NH 2

Figure 22. CLSM images of Hela (a and b) and NIH3T3 (c and d) cells after 4 h incubation with RBITC/HMS and RBITC/HMS-PEG nanoparticles. Reproduced with permission. [ 121 ] Copyright 2011, Elsevier.

Figure 21. The release percentages of ibuprofen from as-prepared HMSC and modifi ed HMSCs systems. Reproduced with permission. [ 120 ] Copy-right 2011, Elsevier.

Table 6. The structure parameters and ibuprofen storage capacities of as-prepared HMSC and modifi ed HMSCs after the interaction with the hexane solutions of ibuprofen. Reproduced with permission.[ 120 ] Copy-right 2005, Elsevier.

Samples S BET [m 2 g −1 ]

V p [cm 3 g −1 )

IBU a) [mg g −1 ]

HMSC-IBU 590 0.596 969

N(0.3)-HMSC-IBU 478 0.439 768

N(0.6)-HMSC-IBU 353 0.319 681

N(1.0)-HMSC-IBU 242 0.195 569

NN(0.3)-HMSC-IBU 445 0.405 742

NN(0.6)-HMSC-IBU 326 0.295 659

NN(1.0)-HMSC-IBU 165 0.123 534

NNN(0.3)-HMSC-IBU 406 0.411 709

NNN(0.6)-HMSC-IBU 347 0.274 615

NNN(1.0)-HMSC-IBU 174 0.104 523

a) Calculated from UV absorbance analysis according to Lambert–Beer’s law.


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effi cient cellular uptake of DOX and the faster intracellular drug release the DMSNs would exhibit, and consequently, the quicker intracellular drug accumulation and stronger MDR-reversal effects were resulted. Based on these, the MDR-over-coming mechanism was proposed to be due to the effi cient cel-lular uptake, P-gp inhibition and ATP depletion.

Recent strategies for drug encapsulations are mostly focused on loading drugs and genes on the pre-synthesized HMSNs. The main reason is that incompatible techniques/procedures (e.g., strong chemical corrosion, high-temperature calcination or the use of organic solvent or reactive metals) were usually employed in synthesizing HMSNs, which may lead to the deactivation of therapeutic molecules when being loaded in HMSNs during the carrier synthesis. Alternatively, Yu et al have designed a distinctive approach of “preloading” for drug encapsulation, which was accomplished by encapsulating bio-medicines during the synthesis of the host HMSNs. Specifi -cally, doxorubicin (DOX) preloaded porous CaCO 3 nanospheres were chosen as hard templates to prepare hollow mesoporous silica@DOX nanospheres. The key step was to remove the CaCO 3 core template via mild erosion with ethylic acid solu-tion (PH = 4), which could enable the preloaded DOX to be reserved. With this approach, it is more convenient to utilize the interior voids to load cargoes and protect them within the hollow structured materials from external attacks. Moreover, enhanced therapeutic effects of DOX@HMSNs over the free drug itself were exhibited towards tumor cells. [ 124 ]

In addition to the high drug storage capability and stimuli-responsive release properties of HMSNs, the biological effects including biocompatibility, biodistribution and clearance are the other major concerns from the viewpoint of clinical applica-tions. It has been verifi ed by several studies that the biocom-patibility of HMSNs on a variety of cell types in vitro is fairly high. [ 66,125 ] Recently, Shi et al. have demonstrated the low in vitro cytotoxicity, [ 125a ] in vivo blood circulation, biodistribution

for the later excretions via urine and feces, or directly excreted via feces. [ 22c ]

By an effi cient surfactant-directed alkaline-etching strategy based on a reversible alkoxide dissolution/recondensation chemical process, mesopores in the shell of HMSNs could be tuned from 3.2 to larger than 10 nm, which provides opportu-nities of loading large entities such as biomolecules (siRNA) and nanoparticles (Fe 3 O 4 NPs). It was showed that the resulted large-sized and surface-functionalized HMSNs could effec-tively encapsulate siRNA molecules and high siRNA transfec-tion effi ciency could be achieved. [ 122 ] The Fe 3 O 4 NPs-loaded HMSNs exhibited superparamagnetic properties, and their use as T 2 -weighted MRI contrast agent was demonstrated by giving a signifi cant signal-decreasing effect. Especially, the biological roles of HMSNs with different shell-pore sizes were evaluated in killing multidrug-resistant (MDR) cancer cells. [ 123 ] It was revealed that drug release and the MDR-overcoming behaviors of DOX-loaded HMSNs (DMSNs) were pore-size-dependent, and substantial contribution of DMSNs to the anticancer activity against MCF-7/ADR cells was demonstrated. It was found that the larger the pore size of DMSNs was, the more

Scheme 5. Schematic illustration of HMMs as a drug/gene delivery system. Step 1: drug-loaded HMMs passing through vascular wall and uptaken by the tumor cells via EPR and other targeting effects; step 2: endocytosis of drug-loaded HMMs by cell; step 3: drug release from HMMs; step 4: self-degration or exocytosis of HMMs.

Figure 23. Cell viabilities against free DOX, DOX-loaded HMS and HMS-PEG nanoparticles at a DOX concentration of 2 µg/ml. Reproduced with permission. [ 121 ] Copyright 2011, Elsevier.


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the sustained-release patterns of loaded IBU molecules were monitored for the system of PAH/PSS coated HMS loaded with hydrophobic ibuprofen (IBU-HMS@PEM) at varied pH values from weak basicity to strong acidity, and the release amount from which reached around 80% in 48 h at pH 1.4, as com-pared to 10% over the same period at pH 8.0. These demon-strated that pH-responsive controlled drug-release pattern could be achieved in the IBU-HMS@PEM system by changing the pH value of the release medium. Moreover, it has been found that the drug releasing rate from IBU-HMS@PEM system could be well tuned by changing the salt concentration of the release medium. Obviously, such a system possesses the fea-tures of both high storage capacity for either hydrophobic [ 126b ] or hydrophilic [ 127 ] drugs and the stimuli-responsive controlled drug release, which may fi nd possible biomedical application as potential drug delivery systems. [ 128 ] On the other hand, covalent bonding could also be used to construct controlled drug release systems. Jin et al. proved that curcumin molecules, which were selectively attached to the inside surface of HMSNs by covalent bonding, could be effectively released by the cleavage of the amide bond by the selected base. [ 129 ] Thus, it is believed that this system, based on the amide bond, would be very promising for fabricating custom-made controlled-delivery devices trig-gered by specifi c target molecules.

Enzymes are known to possess many advantages (e.g., they are not biologically disruptive, can function under mild condi-tions, and are highly selective). Thus, they have been recently employed for triggering a responsive-controlled drug release from a gated HMSNs-based delivery system. [ 130 ] By combining the drug-loaded HMSNs with enzyme degradable poly(L-lysine) (PLL) polymer (positively charged) and the following gene (neg-atively charged) loading via electrostatic interaction, an enzyme-triggered drug and gene co-delivery system was developed. In vitro release results showed that the fl uorescein and cytosine-phosphodiester-guanine oligodeoxynucleotide (CpG ODN) loaded HMSNs/PLL particles exhibited an enzyme-triggered controlled release pattern of fl uorescein and CpG ODN

and clearance behaviors, and their size effects, of MSNs without hollow interior, especially after PEGylation. [ 125b ] It was showed that most MSNs were distributed in RES organs such as liver and spleen, as many other nanoparticles did, and could be mostly excreted via, for example, urine, in one month of vein-injection due to the degradation of MSNs into small siliceous acid molecules. [ 66 , 125c ] No signifi cant pathological injury to varied organs and death of mice tested were found. Not surprisingly, Tang et al. [ 125d ] recently reported the high bio-compatibility of HMSNs by using rattle-type HMSNs as drug carriers. Systematic studies on mortality, clinical features, pathological examinations as well as blood biochemical indexes demonstrated the low in vivo toxicity of HMSNs ( Figure 24 ). It was found that, similar to the situations of MSNs, HMSNs accumulated mainly in mononuclear phagocytic cells in liver or spleen and could be excreted from the body in or beyond 4 weeks. However, liver injury caused by HMSNs at high doses was observed. Therefore, more extensive and long-term toxicity evaluations are needed to confi rm these results.

Compared with the sustained-release system, the stimuli-responsive controlled-release system is more favored for achieving the site-selective and on-demand drug release pattern to effectively enhance the therapeutic effi cacy. In order to achieve this, it is necessary to create nanovalves, which are designed to perform valve-like functions to encapsulate and release drug molecules within the mesopores on demand, on the surface and/or at the pore openings of hollow mesoporous materials. In the meantime, the incorporated nanovalves can be manipu-lated by external stimuli including redox, light, enzyme, com-petitive binding, and pH activation. Amongst these, pH-respon-sive activation represents a feasible and convenient approach to provide a specifi c control through the perturbation of protons into the drug-loaded mesopore channels by readily available acidic and/or basic micro-environments because monitoring the pH value’s changing in solution is so simple and swift. [ 126 ] It is well known that the properties and structure of polyelec-trolyte multilayers (PEM) formed by sodium polystyrene sul-fonate (PSS)/polycation poly(allylamine hydrochloride) (PAH), are sensitive to the external conditions, including pH value and specifi c ion concentration of the surrounding medium. Therefore, by coating HMSNs with PSS/PAH multilayers via electrostatic interaction, a pH stimuli-responsive controlled drug-release system was fabricated. [ 121 ] As shown in Figure 25 ,

Figure 25. Cumulative drug release from the two systems in release media of different pH values. �: pH 1.4 from IBU-HMS, �: pH 1.4 from IBU-HMS@PEM, �: pH 8.0 from IBU-HMS, �: pH 8.0 from IBU-HMS@PEM. Reproduced with permission. [ 126b ] Copyright 2005, Wiley.

Figure 24. ICP-OES analysis result of silicon levels in liver, spleen, lung, kidney and brain of animals treated with MHSNs. Reproduced with permission. [ 125d ] Copyright 2011, Elsevier.


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labeled using electroporation with signifi cantly intensifi ed T 1 -weighted MR images in vitro. Moreover, it was found that the MRI signal could be durably available for over 14 days for monitoring the intracranial grafting process of HMnO@mSiO 2 -labeled MSCs. [ 137 ] Recently, Shi et al. [ 138 ] developed a facile strategy to fabricate hollow nanostructures with magnetic and mesoporous double-shell (HMMNSs) through coating an organosilicate-incorporated silica-shell on β-FeOOH nanorod core followed by in-situ decomposition and reduction of β-FeOOH. Due to the presence of magnetic Fe 3 O 4 nanocapsule as the inner shell converted from β-FeOOH , the MR signal in mouse liver and spleen decreased signifi cantly in 30 min post-intravenous administration, indicating HMMNSs a promising candidate as MRI contrast agent. Moreover, after modifi cation of HMMNSs with rhodamine B isothiocyanate (RBITC) and poly(ethylene glycol) (PEG), HMMNS-R/P showed both high loading capacity for water-insoluble anticancer drugs (docetaxel or camptothecin) and enhanced cytotoxicity in comparison with the corresponding free drugs. These confi rm that the synthe-sized HMMNS-R/P is a potential candidate for simultaneous bioimaging and drug delivery.

High-intensity focused ultrasound (HIFU) has been widely applied for the non-invasive surgery clinically due to their unique merits of high therapeutic effi ciency and low side-effects to patients. Typically, the ultrasound waves generated by outer transducers can be focused on the in vivo malignant tissues due to the intrinsic tissue-penetrating nature of ultrasound. The introduced ultrasonic energy can induce the mechanical, thermal, and cavitation effects to destroy the abnormal cells and tissues. However, the relatively low therapeutic effi ciency of HIFU on some deep tissues results in very long treatment durations of up to 10 h, which needs to be greatly enhanced to substantially shorten the treatment durations to less than one hour or even minutes. The clinically adopted method is to increase the therapeutic power to enhance the energy deposi-tion. The increased ultrasound power, however, can damage the normal tissues in the acoustic propagation channels. To overcome these problems, monodispersed hollow mesoporous silica nanocapsules (HMSNs) were designed and used as the carriers to load perfl uorohexane (PFH) therein to change the acoustic microenvironment of tissues and enhance the HIFU therapeutic effi cacy accordingly. [ 139 ] PFH is a biocompatible compound with the phase transition temperature of ca. 56 °C, which can be gasifi ed by the HIFU hyperthermal effect. The ex vivo results showed that MSNC-PFH could induce the substan-tially larger ablated tissue volume than either MSNC or phos-phate buffered saline (PBS). The in vivo assessment further demonstrated the high synergistic effect of MSNC-PFH. [ 139a ] Furthermore, multifunctional mesoporous composite nano-capsules for highly effi cient MRI-guided HIFU cancer sur-gery were constructed by dispersing manganese oxide species within the mesopores for effi cient T 1 -weighted MRI, together with the encapsulation of PFH molecules within large hollow cavities and their synchronous delivery to targets for active HIFU therapy ( Figure 26 ). It was demonstrated that the precise location on the targeted tumor site and greatly enhanced syner-gistic therapeutic effect could be achieved under the assistance of such a system. [ 139b ]

simultaneously in the α-chymotrypsin solution. Moreover, the release rates of fl uorescein and CpG ODN could be effi ciently tuned by adjusting the enzyme concentration. [ 131 ]

Moreover, the targeted drug delivery is another important issue for anticancer drug delivery system because most of the commonly used anticancer drugs have serious side-effects owing to their unspecifi c actions on healthy cells and tissues. Moreover, therapeutic effi cacy could be signifi cantly enhanced by the targeted drug delivery. Therefore, great effort has been made in designing the targeting anticancer drug delivery vehi-cles based on HMSNs. One alternative approach is to prepare magnetic hollow mesoporous silica spheres as targeting vehi-cles for delivering anticancer drugs to cancer tissues under an external magnetic fi eld. [ 87,132 ] These magnetic hollow mesoporous silica spheres could not only reach the targeted organs or tissues at accelerated rate, [ 133 ] but also exhibited enhanced cellulose tissue penetration behavior under applied external magnetic fi eld, which is promising for delivery appli-cations to plant cells. [ 134 ] The magnetic targeting protocol can be further combined with chemical donator-receptor type of tar-geting, such as the specifi c mutual recognition between folic acid (FA) grafted on HMSNs and intrinsic folate receptor (FR) on the membrane of certain kinds of cancer cells. [ 135 ] Signifi -cant magnetic/FA dual targeting effects, though not very dis-tinguished, can be observed based on the FA/PEG-grafted and magnetic NPs-loaded HMSNs.

3.4.2. Multifunctionalized Bioimaging and Therapeutic Agents

The most recent research interests in HMSNs have revealed a signifi cant trend of transforming them from single function to double functions, and even to multifunctions. Integrating various functions into HMSNs can produce remarkable multifunctional nanocarriers with 3F (fi nding-, fi ghting- and following) capabili-ties. The designed HMSNs could be effi ciently delivered into the lesion locations (fi nding), killing the abnormal cells (fi ghting) and monitoring the evolution of the diseases (following). Espe-cially, the integration of fl uorescent and magnetic functions could yield bimodal bioimaging probes, which combine the merits of high sensitivity of fl uorescent imaging, the noninvasive and high spatial resolution of MR imaging as well as for real-time moni-toring the evolution of diseases into one unit. Furthermore, the introduction of bifunctional materials into mesoporous silica will make the construction of multifunctional platforms possible for simultaneous bimodal bioimaging and drug delivery.

For instance, by encapsulating diverse functional inorganic nanostructures (such as Fe 3 O 4 magnetic nanoparticles, Au nanocrystals, upconversion nanoparticles and quantum dots) into the cavities or the mesopores of HMSNs, multifunctional HMSNs can be fabricated, which are potential nanotheranostic platforms for bioimaging diagnosis and simultaneous therapy for lesion sites such as cancers. [ 136 ] For the labeling and mag-netic resonance imaging (MRI) tracking of adipose-derived mesenchymal stem cells (MSCs), mesoporous silica-coated hollow manganese oxide nanoparticles as positive T 1 contrast agents were fabricated by Kim et al. [ 137 ] Contributed by the free access of water molecules to the magnetic core through the mesoporous silica shell, an effective longitudinal (R1) relaxation enhancement of water protons of 0.99 (mM −1 s −1 )


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core-shell structured UCNP@HMSNs are found to be capable of regulating the release of anticancer drug doxorubicin from the pore channels of mesoporous silica shells by making use of the photoisomerization effect of trans to cis isomers of azobenzene, which was modifi ed in the pore surface of the silica shells, under the irradiation by the upconverted UV–vis light by the UCNP cores. During this process, the azobenzene moiety acts as a molecular stirrer driven by the UV–vis light and the UCNP cores upconvert the external NIR radiation to the UV–vis light. This is an interesting example of using the upconversion properties of UCNPs not only in the fl uorescent bioimaging, but also in the drug release control by the in situ upconverted UV–vis light from NIR irradiation by the UCNP cores. [ 141 ]

4. Conclusions and Outlook

Over the past decade, hollow-structured mesoporous materials have been extensively developed toward the control over the hollow mesostructures, morphology and dimensions, multi-functionalization and the applications in the storage, adsorption and separation, confi ned catalysis and biomedical applications as drug carriers and theranostic agents. They are powerful tools to study the catalytic activity enhancement of confi ned catalysts, targeted delivery of drug molecules with largely enhanced thera-peutic effi cacy, simultaneous imaging and therapy on cancers, etc. This review summarizes the major progress of HMMs in term of the chemical synthesis and, more importantly, the appli-cations in biomedical and catalysis fi elds. The synthesis strategies for hollow mesoporous materials have been broadly catego-rized into two groups: i) soft-templating and ii) hard-templating approaches. The synthetic process via soft-templating approach is relatively simple and the periodicity of the resultant materials can be well controlled. However, the precise control over the

Most recently, a type of multi-functionalized thernostics, Gd-doped upconversion nanoparticles (UCNPs)-functionalized HMSNs, has been constructured by successively coating dense and mesoporous silica layers on UCNPs followed by the etching-away of the middle dense silica layer. The Gd-doped UCNPs are able to emit visible and UV light under the irradiation of extrnal NIR and act as T 1 -weighted contrast agent in magnetic reso-nance imaging as well due to the doping of Gd ions ( Figure 27 a). More importantly, such a composite was used to deliver a kind of anticancer drug CDDP to tumors for synergetic chemo-/radio-therapy by CDDP-radiosensitization and magnetic/luminescent dual-modal imaging. This CDDP-loaded UCNP-functionalized composites showed more effective in vitro radiasensitization effect that free CDDP as a radiosensitizer, and unambiguously enhanced radiotherapy effi cacy in vivo via synergetic chemo-/radiotherapy (Figure 27 b,c), along with the simultaneous dual-modal imaging functions. [ 140 ] More interestingly, the similar

Figure 27. a) Schematic illustration of radiosensitization by CDDP-loaded UCNP-HMSNs nanotheranostics. b) Tumor growth curves of Hela tumor xenografts following the treatments with different modes, control groups received PBS. c) Relative tumor volumes of mice in different treatment groups in half a month of post-injection. Reproduced with permission. [ 140 ] Copyright 2013, ACS Publications.

Figure 26. a) Technical principal of MRI-guided HIFU for the surgery of hepatic neoplasm in rabbits. b) In vivo coagulated necrotic-tumor volume by MRI-guided HIFU exposure under the irradiation power of 150 W cm −2 and duration of 5 s in rabbit liver tumors after different agents were applied through the ear vein (inset: digital pictures of tumor tissue after HIFU exposure). Reproduced with permission. [ 139b ] Copyright 2011, Wiley.


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Figure 28 outlined, but not limited to, some of basic requirements to HMMs for their practical cataytic and biomedial applications in future, and the possible tasks and/or mission researches are facing in promoting future fundamental researches and applications as well. For example, it is highly desirable to get facile control over the monodisperity and dimensions of HMMs with simple soft-templating approach, and also, monodispersed HMMs of under 100 nm, especially under 50 nm in particle diameter, meanwhile of larger than 5 nm in pore diameters, are still unavailable in lit-erature reports, which is important for further enhanced intracel-luar uptake, prolonged blood circulation and intranuclear drug/RNA delivery. From the viewpoint of future clinic applications, the currently available strategies, or emerging new strategies for HMMs synthesis, need to be scaled up to produce HMMs with commercial-scale quantities.

Challenges also remain in the biocompatibility enhancement of HMMs by fi nding more suitable biocompatible/biodegrad-able organic molecules for surface modifi cation, in addition to the commonly adopted methodology of PEGylation, and/or more biocompatible/biodegradable framework composition such as organic-inorganic constitutes. Especially the long term in vivo toxicity information of HMMS in not only mice or rabbit, also large animals like dogs or pigs, is still unavailable which may cost much longer time period and higher expense to carry out than the short term toxicity measurements on small animals.

The successful engineering of hollow mesoporous materials would be of particular interest for the development of multi-functional nanospheres as diagnostic and therapeutic agents on cancers, which is under quick development currently, and var-ious types of combinations among different bioimaging modali-ties (optical, magnetic resonance, X-ray CT, etc) and therapeutic protocols (chemical, radiation, thermal, acoustic, etc.) have been very recently reported and more examples are expected. However, multifunctionalization also brings about diffi culties in the careful control over the morphology, size and size dis-tribution, dispersivity of composite particles during synthesis, and also importantly, the bio-cytotoxicity assessment, both in vitro and in vivo, will become much more complicated because of the multi-components involved and unknown interactions between the components and their biological effect in vivo.

reaction conditions is usually not easy owing to the fact that the structure features and/or chemical properties of soft templates are rather sensitive to the reaction environments. Moreover, hollow mesoporous materials synthesized via soft-templating route are usually non-uniform in size and morphology, and poly-dispersed and sometimes highly aggregated. Compared with soft-templating, hard-templating is more effective in synthe-sizing mesoporous particles with defi ned and highly tunable par-ticle size, controllable morphology and good monodispersivity, though the synthetic process usually involves multi-steps, and the resultant HMMs frequently show less ordered pore arrange-ment in their shells as compared to those by soft-templating routes. The successes in synthesizing hollow mesoporous mate-rials have provided great opportunities to tune their mechanical, chemical and other properties, which are benefi cial to both fun-damental researches and practical applications. These advances have in turn catalyzed the quick expansion of the application lists in catalysis, storage, separation and drug delivery.

Different from the conventional mesoporous materials, HMMs are mainly featured with the presence of large hollow core in the particle interior and usually spherical morphology. Table 7 summarizes the roles of the hollow core, mesopore net-work in the shell and the shell layer when applied in the fi elds of catalysis, adsorption and biomedicine. One can see that it is the hollow cores that endow the materials with special and very useful characters distinguishing from common mesoporous materials. Especially, catalytically active nanoparticles encap-sulated into the core or loaded in the mesoporous channel are prevented from self-aggregation during reactions, so that the catalytic acitivity is signifi cantly enhanced. On the other hand, in the construction of nanotheranostics by multifunctionalizing the HMMs, both the core, mesopore channel and outer shell surface can be effectively employed for loading/grafting/linking functional nanoparticles and/or ligands for the multimode therapy, and single or even multimode bioimaging diagnosis, in addition to those for the single purpose of drug delivery.

However, there is still great challenges in preparing high-quality hollow mesoporous materials (e.g., uniformity, composi-tion, monodispersity, controlled particle size, adjustable mesopore size and shell thickness) with facile and controllable approaches.

Table 7. Corresponding roles of the hollow core, mesopore network and the mesoporous shell in HMMs when applied in catalysis, adsorption, drug delivery, and theranostics.

Application The hollow core Mesopore network in the shell The shell layer

Catalysis Encapsulation of Catalytic

nanoparticles

Pathways for the catalysts loading, reactant

and product molecules’ diffusion in/out;

Shape-selective catalysis

Separation, stablization and protection of catalytic species form

the environment and self-aggregation

Adsorption/storage Space for holding guest species

with high capacity

Pathways for the storage of guests Framework supporting the loaded guests

Drug delivery Encapsulation of drug mol-

ecules with enhanced capacity

Pathways for the drug loading and sus-

tained release from the core; Encapsulation

of the other type of drugs

Protection of the loaded drugs from the environmental attack;

Surface modifi cation for enhanced biocompatibility, dispersity

and/or molecular targeting, and controlled/sustained drug

releases

Theranostics Space for loading drugs and

functional nanoparticles such

as magnetite

Pathways for the drug loading and

sustained release from the core; Grafting

of functional species/nanoparticles for

bioimaging

Protection of the loaded drugs; Surface modifi cation for

enhanced biocompatibility, dispersity and/or molecular

targeting, and controlled drug release; Surface nanoparticle

linkage for multi-mode therapy and imaging


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In all, many aspects of HMMs should be carefully designed and modulated to meet the requirements of varied applica-tions. Morphologically, the shape, size and size distribution of HMMs, cavity size and mesoporous size are very important in determining their applicability in biomedicines, and also the diffusion of molecules in catlaysis; Compositionally, the inert and biodegradable compositions such as silica, silica based organic/inorgainc hybrides or polymeric frameworks are the prerequisites for biomedical applications, in contrast catalyti-cally active and highly thermal/hydrothermal stability of the HMMs are greatly favored for the applications in catalysis, adsorption/separation, and/or chemical sensing. Detailed and special attentions should be paid to the unique compositional/morphological/structural requirements for HMMs aiming at every specifi c application.

Acknowledgements The authors gratefully acknowledge fi nancial support by the 973 Program (Grant Nos. 2012CB933602 and 2013CB933200), the NSFC (Grant Nos. 51172070, 51132009, 51202068); the Program for New Century Excellent

In the respect of catalytic application of HMMS, both the high catalytic performance and superior stability are necessary. One solution to this requirement is to prepare HMMs with highly crystalline framework, which may endow the hollow mesoporous materials with catalytic activity on their framework and enhanced thermal and hydrothermal stability. In fact, HMMs themselves can be made catalytically active by the framework composition modulation (e.g., transient metal oxide instead of inert silica or carbon) and crystallization meanwhile keeping the mesopore network open, thus the guest catalytic component loading might become unnecessary. However, such intrinsically catalytically active HMMs of well-defi ned and tunable mesoporosity have been rarely reported. Furthermore, additional catalysts can be equally loaded into the HMMs of catalytically active framework in either hollow cores or in the mesoporous channels, some kinds of synergetic catalytic effects between the introduced cata-lytic species and original active sites on the HMMs framework may be generated. With the possible synergetic effect and the special hollow and penetrating mesoporous structure of HMMs, extraordinary catalytic performance, such as greatly enhanced catalytic activity, high reactant conversions and product selectivi-ties, and favorable reusability, can be in expectation.

Figure 28. Current and future research status of HMMs.

Defined hollow, meso� �

�

�

�

�

�

�

�

�

�

�

�

�

-structures

and tunable dimensions in cavity

and mesopore sizes;

Active guest loading in mespores

and cavities;

Framework crystallization;

High stability and reusability.

Defined hollow, meso-structures

and tunable dimensions in cavity

and mesopore sizes;

Monodispersity & suspensibility;

Nontoxicity & Biocompatibility;

Biodegradability & Excretions;

Multifunctionality.

Catalytic Biomedical

-crystallized HMMs;

Non-siliceous HMMs with

crystalline framework;

Homogeneous loading of active

guests in mesopores and/or

cavities;

Synergistic catalytic effects

between loaded guest molecules

and frameworks

Monodispersed HMMs of under

100 nm in particle diameter and

larger than 5 nm in pore

diameters;

Novel strategies for easy scale-

up;

Suitable molecules for

modification or frameworks for

enhanced biodegradability;

Easy multifunctionalization;

Long term in vivo toxicity,

distribution and excretion.

Basic requirements

for applications

Possible tasks/

mission to be

done/accomplished

in future

HMMs

�

�

�

�

Framework


28



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IEW

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Talents in University (NCET-10–0379); the Shu Guang project (Grant No. 11SG30); and the Fundamental Research Funds for the Central Universities (Grant Nos. WD1114002 and WD1124010).

Received: October 27, 2013 Revised: December 18, 2013

Published online:


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hollow-structured mesoporous materials: chemical synthesis, functionalization and applications

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