Au(I)–thiolate nanostructures fabricated by chemical exfoliation and their transformation to gold nanoparticle assemblies
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Journal of Colloid and Interface Science 434 (2014) 104112Contents lists available at ScienceDirect
Journal of Colloid and Interface Science
www.elsevier .com/locate / jc isAu(I)thiolate nanostructures fabricated by chemical exfoliation andtheir transformation to gold nanoparticle assemblieshttp://dx.doi.org/10.1016/j.jcis.2014.08.0030021-9797/ 2014 Elsevier Inc. All rights reserved.
Corresponding authors. Fax: +86 0431 85153812.E-mail addresses: email@example.com (M. Li), firstname.lastname@example.org (S.X.-A.
Zhang).Hui Nie, Minjie Li , Yajiao Hao, Xudong Wang, Sheng Gao, Bingjie Yang, Mengdi Gu, Linlin Sun,Sean Xiao-An Zhang State Key Lab of Supramolecular Structure and Materials, Jilin University, Changchun 130000, PR China
a r t i c l e i n f o a b s t r a c tArticle history:Received 26 May 2014Accepted 2 August 2014Available online 12 August 2014
Keywords:Au(I)thiolate coordination polymerNanosheetNanostringGold nanoparticle assembliesChemical exfoliationThermal decompositionChemical exfoliation method was applied to transform bulky assemblies of Au(I)3-mercaptopropionate(MPA) coordination polymer (CP) to nanosheets and nanostrings using sodium citrate as an exfoliator.The exfoliation process and the structural characteristics of the Au(I)MPA nanosheets and nanostringswere fully investigated by transmission electron microscopy, atomic force microscopy, UVvis absorptionspectroscopy, X-ray photoelectron spectroscopy and so on. As the structural rigidity and stability of theobtained Au(I)MPA nanosheets, they are ideal precursors for fabrication of water soluble gold nanopar-ticle assemblies through progressive pyrolysis. This work provides a significant strategy toward the mor-phology regulation of CP nanostructures and will inspire further development of this research area.
2014 Elsevier Inc. All rights reserved.1. Introduction
Coordination polymers (CPs) are an important class of self-assembly functional materials [1,2]. Among these materials, coor-dination polymer particles (CPPs) have attracted growing interestdue to their highly structural tailorability . The size- and mor-phology-dependent properties of CPPs promise a wide scope ofapplications including catalysis [7,8], imaging , drug delivery, and so on . Thus, developing CPPs with diversified mor-phologies are highly important. Until now, CPP materials are typi-cally prepared by bottom-up methods from ligands and metal ions.For examples, Mirkin et al. have prepared chemically tailorablemetalmetalloligand colloidal nanospheres by solvent-inducedprecipitation in 2005 . Lin et al. have used the water-in-oilmicroemulsion strategy to fabricate Gd(III), Gd(III)/doped-Eu(III)and Gd(III)/doped-Tb(III) CP nanorods, and then evaluated themas MRI contrast agents . However, there is very little informationabout how and why one CPP morphology or shape forms asopposed to another. Therefore, morphology control of CPPs in apredictable manner is still a great challenge and a major obstaclewhich hampers its further development. Chemical exfoliation rep-resents a typical top-down method to downsize the architecturesand fabricate nanomaterials with diverse morphologies [13,14].However, it has not been applied to prepare CPPs yet.
Metal(I)thiolate CP materials, such as Au(I)thiolate and Ag(I)thiolate CPs, have long been investigated for their unique lamellarstructures and metallophilic interactions . Metal(I)thio-late lamellar structures are comprised of parallel slabs of stronglyconnected metal ions and S atoms, with the substituents on Sextending from both sides of each slab. And the stacking of layersthen involves weak interlayer interactions, such as Van der Waalsforce and hydrogen bonding between the ligands. Such lamellarmaterials have exhibited a versatile range of applications from pre-cursor of nanoparticles to the recent development as sensors andemitters . However, the lamellar structures are usually inmacroscopic scale, resulting in poor dispersibility and thus limitedapplications. As the anisotropic interactions in metal(I)thiolatelamellar structures, they are deduced as ideal objects for chemicalexfoliation and precursors for diverse metal(I)thiolate nanostruc-tures with unique optic or electronic properties.
Here, we report our success on control of the size and morphol-ogy of CPPs from bulky Au(I)-3-mercaptopropionate (MPA) lamel-lar assemblies by post chemical exfoliation. Au(I)MPA nanosheetswith tunable size and thickness, as well as nanostrings wereobtained by using different amount of exfoliation agent. Here,sodium citrate (CANa) is found to be a good choice for exfoliationagent, which acts as not only a decomposable pH tuner, but also asurfactant to enhance the dispersibility of the products (Scheme 1).The structural attributes of the obtained nanosheets and
Scheme 1. Illustration of the chemical exfoliation process of bulky Au(I)MPA lamellar structures (a) to nanosheets (b) and nanostrings (c); molecular-level structures ofAu(I)MPA lamellar structure in macroscopic scope and nanosheets (d) and nanostrings (e). R represents for CH2CH2COOH (or Na).
H. Nie et al. / Journal of Colloid and Interface Science 434 (2014) 104112 105nanostrings, including composition, size and thickness, were fullycharacterized. Additionally, the transformation of these nanostruc-tures is dominated by adjusting the multiple interactions in sys-tem, such as hydrogen bond interaction, Au(I)Au(I) interactionand AuS coordinate interaction. The chemical exfoliation processwas illustrated in molecular level based on UVvis spectral finger-printstructure relationship. Taking advantages of the high watersolubility, structural rigidity and stability of Au(I)MPA nano-sheets, they were uniformly dispersed in PEG matrix and thenapplied as ideal precursors for fabrication of water soluble Aunanoparticles assemblies via progressive pyrolysis.2. Experimental
All reagents were used as received. Hydrogen tetrachloroau-rate(III) trihydrate (HAuCl43H2O, AR grade) was purchased fromShenyang Jinke Reagent Company. 3-Mercaptopropionic acid(MPA P 99%) was obtained from Alfa Aesor Company. Citric acid(CA P 99.5%) was purchased from Tianjin Bodi Reagent Company.Sodium hydroxide (NaOH P 96%) and hydrochloric acid(wt% = 3638%) was obtained from Beijing Reagent Company.Sodium 3-mercaptopropionate (MPANa) was obtained by 1:1 Mratio neutralization of MPA with NaOH. Sodium citrate (CANa)was obtained by 1:3 M ratio neutralization of citric acid withNaOH. Polyethylene glycol (PEG) 20,000 (molecular weight:17,00022,000) was obtained from Tianjin Guangfu Fine ChemicalResearch Institute. Water was purified by a Mili-Q system.
2.2. Preparation and purification methods
All glass flasks and burettes were cleaned thoroughly with aquaregia before use.
Preparation of bulky Au(I)MPA lamellar structures: HAuCl4 inaqueous solution (0.2 mL, 0.05 M) was added to 20 mL water inan Erlenmeyer flask and heated to boil on a hot plate, then MPANa aqueous solution (0.8 mL, 0.05 M) was injected, generatingwhite precipitates immediately. The mixture was allowed to boilfor another 5 min to complete the reaction. The product wasallowed to cool to ambient temperature and then purified by cen-trifugation to remove the residues before characterizations.Preparation of Au(I)MPA nanosheets and nanostrings: Firstbulky Au(I)MPA lamellar structure was prepared as before. Thendifferent amounts of CANa (0.05 mol/L) were added to the aboveboiling solution. The mixtures were allowed to boil for another5 min to complete the reaction. The products were in colloidalsolutions. While MPANa to Au molar ratio was fixed at 4:1,blue-white solutions were obtained with CANa to Au molar ratiosranging from 2 to 15. The above solutions with CANa:Au molarratio ranging from 5 to 15 were purified by centrifugation toremove the residues. Then Au(I)MPA nanosheets were obtained.When the molar ratio of CANa:Au increased to 20 and 100, color-less solutions were obtained. These solutions were then purified bydialysis against water (Union Carbide, MWCO 1 kDa) for two days.When the CANa:Au molar ratio is 20, Au(I)MPA nanostringswere obtained. However, no regular structure was observed whenthe molar ratio of CANa:Au increased to 100.
Au nanoparticles prepared by electron beam irradiation:Au(I)MPA nanosheets and nanostrings solutions were droppedonto carbon-coated copper grids. Then the sample loaded coppergrids were irradiated by the electron beam in the TEM machine.The accelerating voltage is 200 kV.
Au nanoparticles prepared by thermal reduction: First, theAu(I)MPA nanosheets and nanostrings solutions were coated ontocarbon-coated copper grids. Then the sample loaded copper gridswere placed in vacuum oven and heated to 180 C for 2 h undervacuum conditions.
Au nanoparticle assemblies prepared by pyrolysis: First, theAu(I)MPA nanosheets (CANa:Au = 5:1) aqueous solution(0.006 mL, 0.00625 g/mL) was mixed with PEG (40 wt%, molecularweight 20,000) aqueous solution by stirring. The above mixture(0.16 mL) was then dropped on a 2.5 cm 2.5 cm glass slide, andfollowed by solvent evaporation to obtain the film. The glass slideswere cleaned by Piranha solution before use. The resulting com-posite film was heated at the desired temperature (180 C) usinga digital hot plate (C-MAG HS 10 IKAMAG Hot Plate Stirrer) withdifferent intervals to study their UVvis absorption and morphol-ogy evolution.2.3. Characterization
UVvis absorption spectra were measured using a ShimadzuUVvis 2550 spectrophotometer (wavelength resolution: 0.1 nm)
Fig. 1. Dynamic diameters of the nanosized Au(I)MPA structures with differentCANa to Au molar ratios (a); UVvis absorption spectra of products prepared withdifferent CANa to Au molar ratios (b).
Fig. 2. SEM image of Au(I)MPA bulky lamellar structures (a); TEM images of Au(I)MPA
106 H. Nie et al. / Journal of Colloid and Interface Science 434 (2014) 104112with 1 cm light path cuvettes, utilizing Mili-Q water as reference.The scan step was set as 1 nm. X-ray photoelectron spectroscopy(XPS) measurements were carried out at 15 kV and 17 mA, usinga Thermo ESCALAB 250 spectrometer with a twin-anode Al Ka(1486.6 eV) X-ray source. Transmission electron microscopy(TEM) images were obtained using a JEOL-2010 electronmicroscope operating at 200 kV. Samples were deposited onto car-bon-coated copper grids for measurements. Scanning electronmicroscopy (SEM) images were obtained using a JEOL JSM 6700Felectron microscope operating at 200 kV. Samples were depositedonto silicon wafers for measurements. Atomic force microscopy(AFM) images were recorded in the tapping mode with a Nano-scope IIIa scanning probe microscope from Digital Instrumentsunder ambient conditions. X-ray diffraction (XRD) data were col-lected with a Rigaku D-Max 2550 diffractometer with Cu Ka radi-ation (k = 1.5418 ). Tube voltage and current were 50 kV and200 mA, respectively; scan range (2h) was from 3.0 to 70.0, scanstep was 0.02. Dynamic light scattering (DLS) data were measuredby a Malvern Zetasizer Nano ZS instrument, which are equippedwith a 4 mW HeNe 633 nm laser and a back-scattering detector(175). The Dispersion Technology Software (DTS) (V5.30) wasused for data collection and analysis. Samples ([Au] = 0.005 M)were equilibrated (typically 2 min) to 25 C before measurement.3. Results and discussion
3.1. Morphological control of Au(I)MPA CPPs
Typically, Au(I)MPA bulky lamellar structures were preparedby adding MPANa to boiling HAuCl4 solution (MPANa:Au = 4:1)and the products were white precipitates. The obtained lamellarstructures are highly pH sensitive due to the COO group inMPA ligand. By adding base to the system, the carboxylate groupsare deprotonated and the hydrogen bonding interaction convertnanostructures with CANa to Au molar ratios of 5 (b), 15 (c) and 20 (d) respectively.
Fig. 3. XRD patterns of Au(I)MPA assemblies with/without CANa.
H. Nie et al. / Journal of Colloid and Interface Science 434 (2014) 104112 107to the static repulsive interaction. In our previous work, it hasbeen proved that the static repulsion between the ligands isstrong enough to break the Au(I)Au(I) bonds by increasing theAuAu distance. And this change would finally affect the Au(I)S coordination modes, which can be probed from UVvis spectra. CANa is found to be a good choice, which acts as not only aFig. 4. AFM images (ac) and height analysis (df) of Au(I)MPA nanosheets withbase, but also a surfactant to enhance the dispersibility of theproduct.
It is found that the precipitates disappeared and stable colloidsolution can be obtained when CANa:Au ratio reached 2. Bychanging molar ratio of CANa:Au from 2 to 20, the sizes of theassemblies can be continuously reduced from about 220 nm to40 nm as shown in DLS analysis (Fig. 1a). Then, well-resolvedUVvis absorption spectra can be obtained for these colloid solu-tions. Based on the spectra-structure relationship we have learnedbefore, nanosized Au(I)MPA lamellar structures with strong auro-philic interactions show typical sharp paired absorption peaks at391 and 350 nm . The first transition absorption at391 nm corresponds to the ligand-to-metal charge transfer(LMCT), while the second transition band is assigned to metal-centered charge transfer (MCCT) modified with Au(I)Au(I)aurophilic interaction . Meanwhile, the LMCT and MCCTtransition energies of Au(I)MPA structures in UVvis spectra arevery sensitive to the coordination environments . Increase ineither Au(I)Au(I) bond strength or bond numbers would lead toa gradual red-shift of LMCT and MCCT transitions [21,23,25,26].As shown in Fig. 1b, when the ratio of CANa:Au was less than15, the obtained solutions had absorption peaks at 391 and350 nm, which are the characteristic absorption of lamellar struc-tures. These two absorption peaks blue-shifted and diminished inintensity with the increasing amount of CANa, indicating theCANa to Au ratio of 2 (a), 5 (b) and 15 (c) respectively (scale bar = 500 nm).
Fig. 5. XPS analyses for Au(I)MPA nanostructures with different CANa to Aumolar ratios, (a) and (b) are Au 4f and S 2p binding energies.
Fig. 6. Photoemission spectra of Au(I)MPA nanostructures with different CANa toAu molar ratios.
108 H. Nie et al. / Journal of Colloid and Interface Science 434 (2014) 104112increase of AuAu distance and structural transformation. As theratio of CANa:Au was 100, no distinct absorption in UVvis spec-trum was observed. Then we can conclude that no Au(I)Au(I)interaction existed in that structure.
The exfoliation process of the bulky Au(I)MPA precipitates asthe addition of CANa is confirmed by scanning electron micro-scope (SEM) and tr...