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Morphology mod

State Key Lab of Supramolecular Structure a

P. R. China. E-mail: liminjie@jlu.edu.cn;

85153812; Tel: +86-0431-85153811

† Electronic supplementary informa10.1039/c4ra06500j

Cite this: RSC Adv., 2014, 4, 50521

Received 1st July 2014Accepted 24th September 2014

DOI: 10.1039/c4ra06500j

www.rsc.org/advances

This journal is © The Royal Society of C

ulation and application of Au(I)–thiolate nanostructures†

Hui Nie, Minjie Li,* Yajiao Hao, Xudong Wang, Sheng Gao, Peng Wang, Bo Juand Sean Xiao-An Zhang*

Controlled synthesis of Au(I)–3-mercaptopropionic acid (MPA) nanostructures with diverse morphologies,

such as quasi-rectangular nanosheets, quasi-square nanosheets, nanobelts, nanostrings and nanochips,

were successfully achieved. Regulating the morphology of Au(I)–MPA nanostructures was realized by

reverse microemulsion, which not only has a confinement effect on the size but also directs their

assembly into different morphologies. In addition, adjustment of the electrostatic interaction between

ligands induces consecutive responses in Au(I)–Au(I) interaction and Au–S coordination, and also results

in distinct morphology transformation. Taking advantage of the structural characteristics of the obtained

Au(I)–MPA nanostructures, they are used as ideal precursors for the preparation of Au particles and

photoluminescent Au clusters. This work not only provides effective strategies for the morphology

regulation of coordination polymer nanostructures but also extends their application.

Introduction

Supramolecular nanoarchitectures self-assembled from coor-dination polymers (CP) have attracted much attention due totheir highly structural tailorability.1–3 Until now, CP nano-structures with diverse morphologies have been fabricated,including nanospheres,4 nanorods,5 nanowheels6 and so on.More importantly, the morphology and size are key factors thataffect their chemical properties.7 Typically, the morphology ofCP assemblies depends on the coordination chemistry of themetal nodes, which can be tuned by controlling the coordina-tion environment. For example, Mirkin group has discoveredthat judicious choice of the solvent can be used to drive theNi(II)–salen amorphous spherical particles into rod-shapedcrystalline structures.8 Che et al. have reported a counterion-induced process that results in the conversion of initiallyformed nanowires into wheel-like structures.6 In addition,microemulsion technique represents an useful method formorphology control due to their template or direction effects.Gd(III)/doped-Eu(III) and Gd(III)/doped-Tb(III) CP nanorods withdifferent aspect ratios have been achieved by Lin et al. throughthe water-in-oil microemulsion method.5 Combination of thesetwo strategies in one system will allow broader structure andmorphology control.

Au(I)–thiolate CPs have long been investigated for theirunique structure characteristics and Au(I)–Au(I) aurophilic

nd Materials, Jilin University, Changchun,

seanzhang@jlu.edu.cn; Fax: +86-0431-

tion (ESI) available. See DOI:

hemistry 2014

interaction.9–13 For example, Au(I)–thiomalate (Myochrysine)featured double-helical geometry in the solid state.14 Lee grouphas synthesized luminescent Au(I)–alkanethiolates which have aremarkably high degree of conformational order and a well-developed lamellar structure by mixing gold salts with excessn-alkanethiols in the tetrahydrofuran.9 These lamellar struc-tures are proposed to comprise parallel slabs of strongly con-nected Au ions and S atoms, with the substituents on Sextending from both sides of each slab. Inter-slab stacking byinter-ligand interactions allows the three dimensional extend-ing of the structure. One of the most intriguing properties ofAu(I)–thiolate CP materials is their transformation to Au(0)species. Carboxylic acid and ester functionalized gold nano-particles (average diameter of 8.7 � 1.7 nm) have been obtainedby heating the bulky Au(I)–alkanethiolate lamellar structures.10

GSH-capped Au nanoparticles of various sizes, in the 2–6 nmsize regime, have been prepared by controlling the pH of theAu(I)–glutathione polymers.15 Obviously, the nature of the Au(I)precursors, such as morphology and ligands, can greatly affectthe size and surface properties of the resulted Au particles.Therefore, the properties of Au(I) precursors is a key in formingAu particles with desirable properties. However, the obtainedAu(I)–thiolate CP structures are usually in macroscopic sizes,which lead to poor dispersibility and limit their applications.Further morphology control of the Au(I)–thiolate CP assembliesis seldom. Our group has recently synthesized water-solublenanosized Au(I)–3-mercaptopropionic acid (MPA) lamellarstructures in aqueous solution by adopting pH sensitive MPAligands.16,17 Here, versatile morphologies of Au(I)–MPA quasi-rectangular nanosheets, quasi-square nanosheets, nanobelts,nanostrings and nanochips, were successfully achieved in

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microemulsion by controlling the molar ratio of water tosurfactant (w value) and tuning the pH values of the reactants.Taking advantages of the structural characteristics of theobtained Au(I)–MPA nanostructures, they were used as idealprecursors for the preparation of Au particles and photo-luminescent Au clusters by solvothermal method (Scheme 1).

Experimental sectionMaterials

All chemicals were commercially available and used withoutfurther purication. Hydrogen tetrachloroaurate trihydrate(HAuCl4$3H2O, AR grade) was purchased from Shenyang JinkeReagent Company, 3-mercaptopropionic acid (MPA $ 99%)from Alfa Aesar Company. Sodium 3-mercaptopropionate (MPA-Na) aqueous solutions with different pH values were preparedby mixing different amount of NaOH with MPA. First, MPA(0.212 g, 0.002 mol) was dissolved in a small amount of puriedwater. Then different amounts of NaOH (0.08, 0.10, 0.12, 0.16,0.20 g) were added to the MPA aqueous solution respectively.The obtained mixtures were transferred to 10 mL volumetricasks and brought to volume by puried water. The pH valuesof the obtained MPA-Na solutions were measured to be 5.7, 9.2,10.3, 11.4 and 12.7 respectively with a Sartorius PB-21 pHmeter.Cetyltrimethylammonium bromide (CTAB) was obtained fromSinopharm Chemical Reagent Limited Corporation, cyclo-hexane was obtained from Tianjin Tiantai Fine Chemical Co.,Ltd, and n-pentanol was obtained from Shantou Xilong Chem-ical Co. Company. Water was puried by a Mili-Q system.

Preparation of bulky Au(I)–MPA lamellar structures inaqueous solution

HAuCl4 aqueous solution (0.2 mL, 0.05 M) was added to 20 mLwater in an Erlenmeyer ask and heated to boil on a hot plate,then MPA-Na aqueous solution (0.8 mL, 0.05 M) with pH valueof 5.7 was injected, generating white precipitates immediately.The products were puried by centrifugation to remove theresidues before characterization.

Scheme 1 Microemulsion-assisted synthesis and application ofdiverse Au(I)–MPA nanostructures.

50522 | RSC Adv., 2014, 4, 50521–50528

Preparation of Au(I)–MPA nanostructures in microemulsion(M-Au(I)–MPA nanostructures)

First, the quaternary microemulsion system, CTAB–water–cyclohexane–1-pentanol, was prepared by dissolving CTAB (1 g)in cyclohexane (30 mL) and 1-pentanol (1.5 mL). The mixingsolution was stirring for 30 min. Typically, HAuCl4 (0.2 mL,0.05 M) aqueous solutions and MPA-Na (0.2 mL, 0.2 M) withdifferent pH values (5.7, 9.2, 10.3, 11.4 and 12.7) were added to10 mL of the above microemulsion solutions, respectively. Aersubstantial stirring, the two optically transparent micro-emulsion solutions were mixed and stirred for another 15 min(As the pH values of the mixture of HAuCl4 and MPA-Namicroemulsion can hardly be measured by the pH meters inour lab, their approximate pH values can be inferred from thepH of the mixtures of HAuCl4 and MPA-Na aqueous solutionswith the same volume ratio, consuming the organic phase haslittle effect on pH values of the system. When the pH values ofthe MPA-Na are 5.7 to 9.2, 10.3, 11.4 and 12.7, the pH value forthe HAuCl4 and MPA-Na mixture are 2.6, 3.9, 4.4, 6.7 and 11.8respectively). The resulting microemulsion solution was thenreuxed in a water bath set at 82 �C for 30min andM-Au(I)–MPAnanostructures formed. Aer the reaction was completed, theresulting microemulsion was cooled to room temperaturenaturally, then the products (pH values of MPA-Na are 5.7 and9.2) were collected by centrifuging, washed with absoluteethanol for two times and then distilled water for two times. Atlast, the precipitations were dispersed in water. When the pHvalues of the MPA-Na solutions increased to 10.3, 11.4 and 12.7,no further purication was made for the nanostructures inmicroemulsion.

Preparation of Au particles and photoluminescent Au clusters

For the preparation of Au particles, the M-Au(I)–MPA quasi-square nanosheets or nanostrings microemulsions were trans-ferred into the Teon-lined vessel and heated in a oven at 150 �Cfor 2 h without stirring. Aer the reactions were completed, theresulted microemulsions were cooled to room temperaturenaturally. The products were collected by centrifuging, washedwith absolute ethanol and distilled water several times. At last,the products were dispersed in water for characterization. Forthe preparation of red and blue photoluminescent Au clusters,the M-Au(I)–MPA nanochips microemulsion were transferredinto the Teon-lined vessel and heated in a oven at 150 �C for2 h and 5 h without stirring respectively. For the photo-luminescent Au clusters in microemulsion, no further puri-cation was carried out for their relatively small size.

Characterization

UV-vis absorption spectra were measured using a Shimadzu UV-vis 2550 spectrophotometer (wavelength resolution: 0.1 nm)with 1 cm light path cuvettes. Photoluminescence (PL) spectrawere measured using a Shimadzu RF-5301 PC spectrophotom-eter with 1 cm light path cuvettes. Samples were subjected tospectroscopic measurements without purication. Trans-mission Electron Microscopy (TEM) was performed on JEOL,

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JEM-2010 electron microscope with an energy-dispersive X-rayspectroscopy (EDX) operating at 200 kV, samples were drop-ped on carbon-coated copper grids for measurement. ScanningElectron Microscopy (SEM) was carried out on the eld emis-sion microscope (JEOL, JSM 6700F) operating at an accelerationvoltage of 200 kV, samples were dropped on silicon wafer formeasurement. X-ray photoelectron spectroscopy (XPS)measurements were carried out at 15 kV and 17 mA, using aThermo ESCALAB 250 spectrometer with a twin-anode Al Ka(1486.6 eV) X-ray source. X-ray diffraction (XRD) data wascollected with a Rigaku D-Max 2550 diffractometer with Cu Karadiation (l ¼ 1.5418 A). Tube voltage and current were 50 kVand 200 mA, respectively; scan range (2q) was from 3.0� to 70.0�,scan step was 0.02�, dry powder samples were deposited inmonocrystal Si sample holder for measurement. Atomic forcemicroscopy (AFM) images were recorded in the tapping modewith a Nanoscope IIIa scanning probe microscope from DigitalInstruments under ambient conditions, samples were electro-statically adsorbed on hydrophilic silica wafers formeasurement.

Fig. 2 (a) SEM image of Au(I)–MPA bulky precipitates synthesized inaqueous solution; (b) TEM image of M-Au(I)–MPA nanostructures; (c)XRD patterns of Au(I)–MPA bulky precipitates and M-Au(I)–MPAnanostructures. The small peaks from 25–60� are from the periodicarrangements S, Au and ligands. No Au(0) fcc patterns show up; (d) UV-vis absorption spectrum of M-Au(I)–MPA nanostructures; (e) XPSanalyses for elements existing in Au(I)–MPA bulky precipitates and M-Au(I)–MPA nanostructures; (f) Au 4f binding energies for Au(I)–MPAbulky precipitates and M-Au(I)–MPA nanostructures.

Results and discussionAu(I)–MPA nanostructures: from aqueous solution tomicroemulsion (M-Au(I)–MPA nanostructures)

Au(I)–thiolate CPs are usually prepared by reaction of thiolligands with HAuCl4. For better control of their assembly, watersoluble functional ligand MPA was chosen here. Once MPA-Naligand was added to HAuCl4 aqueous solution, Au(I)–thiolateCPs generated and further assembled to bulky precipitate withlamellar structures immediately. The formed lamellar structureis similar with that prepared from alkanethiols in literatures,whose structure is shown in Fig. 1.9–13 In this paper, keeping thereactants and their ratio unchanged, the reaction of HAuCl4 andMPA-Na was transferred to cationic CTAB–isooctane–1-hex-anol–water microemulsion system. As we know, the water phaseinside the micelle can serve as a reactor for the formation andassembly of nanostructures, in which the size and morphologyof the resulted products can be regulated.18–20 To our excite-ment, unlike the bulky assemblies of Au(I)–MPA CPs in water(Fig. 2a), nanosized quasi-rectangular sheets with highly regularmorphology were achieved in microemulsion with w value of 12(M-Au(I)–MPA nanostructures) (Fig. 2b). Obviously, the reverse

Fig. 1 Illustration of the reaction pathway of Au(I)–MPA CPs and theirassembly into lamellar structures.

This journal is © The Royal Society of Chemistry 2014

microemulsion indeed has spatially constraint function on thesize and morphology of products in our system. Then, detailedcomparison of Au(I)–MPA assemblies from aqueous solutionand microemulsion were carried out. XRD analyses show theyboth have lamellar structures from the equal spaced diffractionpeaks between 3� and 20� corresponding to (010), (020) and(030) facets.9–11 The interlayer distances of the M-Au(I)–MPAnanostructure are calculated to be 12.05 A, which is about thelength of two MPA ligands bridged by hydrogen (H) bondsbetween the carboxylic acids. Meanwhile, their interlayerdistance is smaller than that of Au(I)–MPA bulky precipitates(12.99 A) from the (010) diffraction peaks. The relatively regulararrangement of M-Au(I)–MPA nanostructures may account fortheir smaller interlayer distance. XPS analyses show that themolar ratio of Au to S for M-Au(I)–MPA nanostructures is near1.0. The electron binding energy of Au 4f7/2 for M-Au(I)–MPAnanostructures is 84.51, lying between that reported for Au(0)(83.9–84.0 eV) and Au(I) ions (85.0–86.0 eV),21,22 which indicatesstrong Au(I)–Au(I) interaction exist in the assemblies. Mean-while, similar to nanosized Au(I)–MPA lamellar structuresprepared in aqueous solution, the M-Au(I)–MPA nanostructuresalso feature well-distinguishable UV-vis absorption originatingfrom ligand-to-metal charge transfer (LMCT) at 391 nm and

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metal-centered charge transfer (MCCT) transitions at 350 nm.16

XRD, XPS and UV-vis absorption prove the bulky precipitates inaqueous solution and M-Au(I)–MPA nanostructures sharesimilar composition and lamellar structures (Fig. 2c–f).16

The assembly process of the M-Au(I)–MPA quasi-rectangularnanosheets was further monitored with UV-vis spectroscopy,and the assembled intermediates were subjected to TEM char-acterization to reveal the assemblymechanism (Fig. 3). Once thetwo microemulsion solutions of HAuCl4 and MPA-Na weremixed at room temperature, the absorption of AuCl4

� dis-appeared immediately, indicating Au(III) ions were reduced toAu(I) species during the interdroplet collision (Fig. S1†).23 Theabsorption of the mixed microemulsion kept unchangedthereaer. TEM image shows 5–25 nm nanospheres at thisstage, which are within the typical dimensions of micro-emulsion droplets (Fig. 3b). The above results suggest that themicroemulsion acts as a “template” to generate Au(I)–MPA CPsinside, and no further assembly occurs at roomtemperature.18–20

When the microemulsion system was brought to boiling, thepaired peaks at 391 and 350 nm, which represent the generationof lamellar nanosheets, increase in intensity with time and stopgrowing in half an hour (Fig. 3a).16 Meanwhile, the color of thetransparent microemulsion changed from colorless to blue-white.

The assembled intermediates at 6 min and 8 min reveal thattheir assembly underwent a nanosphere-to-nanostring-to-irregular-nanosheet-to-rectangular-nanosheet process (Fig. 3cand d), which is consistent with our previous nding.16 As thelengths of nanostrings are much larger than the dimension ofmicroemulsion droplets, coalescence and agglomeration ofmicelles must happen at boiling temperature.18–20 In addition,molecular propagation driven by hydrogen bonding interactionis faster along the axis of the Au(I)/Au(I) chain than in thelateral directions, leading to the formation of string structures

Fig. 3 UV-vis spectroscopic monitoring of the assembly process ofquasi-rectangular M-Au(I)–MPA nanosheets (a). TEM images of inter-mediate products taken at room temperature (b), boiling for 6 min (c)and 8 min (d), respectively during the assembly process of quasi-rectangular M-Au(I)–MPA nanosheets.

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rstly.6 Then, the nanostrings align with each other to facilitatethe further coordination of S to Au in a nearby chain concur-rently, transforming the aligned strings into thin nanosheets.Based on the above results, the structural transformationprocess of the assembly intermediates are proposed as shown inFig. S2.†

Although Au(I)–MPA nanosheets in aqueous solution havebeen obtained by tuning the pH value of the assembly system inour previous work,16 the nanosheets have ill-dened morphol-ogies, presumably as a result of the rapid assembly speed andthe absence of structure-director. In this microemulsionmethod, the assembly speed can be greatly slowed down byconning the reactants inside the micelles and the anisotropicfusion between droplets directs the assembly of Au(I)–MPA CPs.Here, we assume that the fusioned microemulsion dropletsfeature cylinder shape at w ¼ 12, which further lead to theformation of well-dened quasi-rectangular nanostructures.18,19

Morphology control of M-Au(I)–MPA nanostructures

As we know, the water contents in microemulsion (w values) arehighly related with the fusion rate, mass-exchange and coales-cence direction between droplets, which greatly affect themorphology of the products.18–20 We therefore adjusted the wvalues of M-Au(I)–MPA assembly systems. TEM images provethat w value can surely modulate the morphology of M-Au(I)–MPA nanostructures. When the w value increased from 12 to 20and 40 with other reaction conditions unchanged, the productschanged from quasi-rectangular nanosheets to quasi-squarenanosheets, mixture of big irregular nanosheets and shortnanostrings respectively (Fig. 4a–c). XRD analyses and UV-visabsorption spectra show that the assemblies at all these wvalues have lamellar structures (Fig. S3–S6†). AFM measure-ment of the typical M-Au(I)–MPA quasi-square nanosheetsshows that their thicknesses are around 10–25 nm (Fig. S7†).From the XRD study, the height of a single layer of M-Au(I)–MPAlamellar structures is about 1.2 nm, indicating that the layernumbers of the quasi-square nanosheets are 8–20. As reportedby literature, (i) the fusion and mass exchange rates betweendroplets increase greatly with w value.19 (ii) The sphericaldroplets fuse to form short cylinder droplets at lower w valuewith water-enriched domains locating at both ends, and nearlyspherical droplets are formed at higher w value with water-enriched domains equilibrating at every direction. (iii) Furtherexchange or coalescence between droplets mainly happen at thewater-enriched domains, which nally direct the morphology ofproducts.18–20,24 Obviously, the morphology of droplets haveeffect on the morphology of the M-Au(I)–MPA nanostructures.The shapes of M-Au(I)–MPA nanostructures changed fromquasi-rectangular to quasi-square with the increase of w valuesfrom 12 to 20. With the further increase of water content, themicroemulsion droplets are unstable. At this condition, sizedivergence of micelle droplets may happen and subsequentlycause the generation of big nanosheets and nanostrings at thesame time. In addition, except for the spatially constraint effectof microemulsion, the crystal growth mechanisms also greatlyaffect the morphology of the M-Au(I)–MPA nanostructures. As

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Fig. 4 TEM images of M-Au(I)–MPA nanostructures prepared at (a–c)w values of 12, 20 and 40 with MPA-Na (pH ¼ 5.7) as reactants; (d–f)pH values of 9.2, 10.3 and 12.7 (MPA-Na reactants) with w value of 12.The nanodots, which are evenly distributed along the strings and chipsare generated by TEM electron bombardment, but do not exist in theoriginal samples.15

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shown in Fig. 3, the assembly of the M-Au(I)–MPA nano-structures involve two stages, the rst one is the assembly ofAu(I)–MPA CP into string-shaped intermediates, the second oneis the aggregation and fusion of the string-shaped intermedi-ates into nal structures. Because the assembly rate along Au–Sslab by inter-string coordination, aurophilic interaction is fasterthan stacking of Au–S slabs by H bond, the generated structuresare thin-sheets (Fig. 4a–c), rather than cubic or rodlike struc-tures as reported in literature.19 Therefore, we conclude thatboth the template effect of the microemulsion and the crystalgrowth feature decide the morphology characteristics in oursystem.

Here, pH sensitive MPA ligand is incorporated into theassembly system. Except the w value of the microemulsion, pHvalue of reactants is another important parameter for themorphology control of M-Au(I)–MPA nanostructures. At lowerpH values, hydrogen bonding interaction forms between theprotonated carboxylate groups (Fig. 5a). Then the hydrogenbonding interaction can greatly promote the formation of Au(I)–Au(I) interaction and further Au–S coordination, and nally leadto the assembly of Au(I)–MPA CPs into lamellar structures. Andat higher pH values, the electrostatic repulsion between thedeprotonated carboxylate groups is strong, which would weakenthe Au(I)–Au(I) interaction and make the further coordination of

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Au to S and the assembly of Au(I)–MPA CPs unfavourable(Fig. 5b and c). As we expected, keeping the w value of micro-emulsion unchanged, the dimension of the assembliesdecreased when the pH values of the reactants (MPA-Na) wereincreased from 5.7 to 9.2, 10.3 and 12.7 from TEM images(Fig. 4d–f), and the shapes of the assemblies changed fromquasi-rectangular nanosheets to nanobelts to nanostrings andnally nanochips. The above results prove that increased pHvalue can hinder the assembly of Au(I)–MPA polymers, resultingin unassembled or partially assembled nanostructures as theirproducts. The transformation process is further conrmed byUV-vis absorption and XPS analyses (Fig. S8†). The absorptionpair representing for the lamellar structure blue shied andtheir intensity diminished, meanwhile, the Au(I) 4f7/2 bindingenergies shied from 84.51 to 84.77 eV gradually, both sug-gesting that the interaction modes in these structures change toless aurophilic and Au–S coordination feature.25,26 Further, thephotoluminescence property is also an indicator for the Au(I)–Au(I) aurophilic interaction in M-Au(I)–MPA nanoassemblies.9,13

As shown in Fig. S9a,†M-Au(I)–MPA nanosheets, nanobelts andnanostrings have photoluminescence properties. However,when the pH value of MPA-Na reactant reached 12.7, almost noemission was observed for the M-Au(I)–MPA nanochips due tothe absence of Au(I)–Au(I) aurophilic interaction (Fig. S9b†). Theemission change of M-Au(I)–MPA nanostructures is accordantwith that of UV-vis absorption and XPS analysis. Although thepresence of NaOH may induce the transition of sphericalmicelles into more elongated, elliptical ones by reducing therepulsion of charged surfactant “heads” as reported in litera-ture,27,28 here, the increasing of pH values result in the decreasein the three-dimensional size of the products. It is more likelythat the smaller sizes are resulted from the interligand repul-sions, rather than from the morphological change of themicelle.

Au particles and photoluminescent Au clusters prepared fromM-Au(I)–MPA nanostructures

Au particle in micro/nano scale is an attractive research topicdue to their size-dependent physical and chemical proper-ties.29,30 In particular, Au particles with large size (diameters >200 nm) can be applied in biosensor eld due to their highlysensitive localized surface plasmon resonance.31 Until now,fabrication techniques such as chemical synthesis,32,33 electronbeam lithography,34 and nanosphere lithography35 have beenadopted for the size and morphology control of Au nano-particles. Highly regular metal features on substrates can beachieved by electron beam lithography or nanosphere lithog-raphy. However, colloid particles in solution are the preferredstarting materials for further manipulation due to their highexibility. The colloidal Au particles are usually prepared fromthe chemical reduction (typical reductants are NaBH4, sodiumcitrate and ascorbic acid etc.) of Au3+ salts. However, only Auparticles with diameters ranging from several to hundreds ofnanometers can be obtained. Large particles (diameters >200 nm) can hardly be prepared by chemical synthesis. Asreported by literature, Au(I) can be reduced to Au(0) by the thiol

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Fig. 5 Change in interligand interactions from attractive to repulsive by increasing the pH values of MPA-Na reactants and the decrease inresulted Au(I)–Au(I) interactions from (a) to (c). a1 and a2 are the Au–S–Au angles at low and high pH, respectively, a1 < a2. The Au(I)–Au(I)interaction is shown by the dashed line.

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ligands during thermal decomposition process. The redoxreaction is expected to generate Au atoms and dithiolcompounds.15,36–39 Taking advantages of the controllablemorphology of M-Au(I)–MPA assemblies, they are supposed tobe ideal precursors for the preparation of Au particles withdifferent sizes.

Here, M-Au(I)–MPA quasi-square nanosheets, nanostringsand nanochips were chosen as precursors for the formation ofAu(0) species. When M-Au(I)–MPA quasi-square nanosheets andnanostrings were heated at 150 �C in stainless Teon-linedautoclave for 2 h, Au particles with average diameters of about520 nm and 250 nm were obtained respectively (Fig. 6). EDXanalysis conrms the presence of Au in these samples(Fig. S10†). The sizes of the obtained Au particles are muchlarger than that of their corresponding precursors. It is deducedthat aggregation between the resulted Au nanoparticleshappens during the heat treatments. Although elaborate sizecontrol of the Au particles is not achieved at the present stage,our method may be of general utility for the synthesis of largediameter Au particles.

Au clusters have also attracted much attention for theirunique role in bridging the “missing link” between atomic andnanoparticle behavior.40 Recent study has focused on theirquantum electronic properties, including photoluminescence,sensing and so on.41–46 For the preparation of photo-luminescence Au clusters, so far, different chemicalapproaches, such as direct reduction of metal ions and etching-

Fig. 6 SEM image of Au particles prepared from M-Au(I)–MPA quasi-square nanosheets (a) TEM image of Au particles prepared fromnanostrings (b).

50526 | RSC Adv., 2014, 4, 50521–50528

based strategy, have been exploited. Precise control of theexperimental conditions is always needed. Here, to our delight,red (607 nm) and blue (421 nm) photoluminescent Au clusterswere achieved when non-photoluminescent Au(I)–MPA nano-chips in microemulsion were exposed to thermal decomposi-tion at 150 �C in stainless Teon-lined autoclave for 2 h and 5 h,respectively (Fig. 7 and S9b†).47 The quantum yields for the redand blue photoluminescent Au clusters are 0.3% and 16.4%. Nodiscernible surface plasmon band was observed in their UV-visabsorption spectra, indicating the small size of these clus-ters.48,49 Consistent with the emission spectra, the red

Fig. 7 UV-vis absorption spectra, PL spectra and excitation spectra ofred (a) and blue (b) photoluminescent Au clusters ([Au] ¼ 5 � 10�4 M,excitation and emission slit widths are 5 and 5). Inset: the photos of Aucluster solutions under daylight and UV irradiation (365 nm).

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photoluminescent cluster have bigger size than the blue onefrom the UV-vis absorption band edge. We tried to prove thisdeduction by TEM analysis. As shown in Fig. S11,† particles withaverage diameters of about 2 nm were obtained for the redphotoluminescent samples. Although clear lattice fringes wereobserved in the high-resolution TEM image of the blue photo-luminescent Au clusters, the clusters aggregated with eachother seriously.50 This phenomenon is very typical for metalclusters with small size.51 It is supposed that the size reductionwith reaction time is resulted from the ligand etching process.52

Then the decreased size accounts for the blue shi of thephotoluminescence with increased heating time.53 Comparedwith the methods reported in literature, our synthetic route hasthe following advantages: (i) two kinds of photoluminescent Auclusters are achieved by simply controlling the reaction time; (ii)this work provides a new strategy towards the fabrication of Auclusters by the self-redox of Au(I) precursors under heatingcondition without further reactants. Subsequently, this methodwould provide new modulation approaches (such as ligands ofAu(I) precursors, reaction temperature, reaction time) for thefurther engineering of the properties of Au clusters. Furtherimprovement of their performance is still needed.

Here, the sizes of the obtained Au nanoparticles decreasewith the reduced size of M-Au(I)–MPA precursors. There are twopossible reasons: (i) due to the stable three dimensional rigidstructures of nanosheets, it's more difficult to thermal decom-pose the nanosheets than nanostrings and nanochips. Uponheating, the larger Au(I)–MPA precursors (nanosheets) lead tothe formation of fewer nuclei for thermal reduction to Aunanoparticles.15 During the growth process, Au(I) is preferen-tially reduced onto the surface of the existing Au(0) speciesrather than independently forming a new particle. Fewer nucleiwhich have more Au ions in close proximity result in larger Aunanoparticles. Conversely, smaller size of Au(I) precursorsfavour the formation of more nuclei, hence leading to smallernanoparticles. (ii) As shown before, the size of M-Au(I)–MPAnanostructures decreases with the increase of pH value of thereaction system. Due to the increased electrostatic repulsionbetween the deprotonated carboxylate groups on their surface,the smaller sized M-Au(I)–MPA nanostructures are not inclinedto get close to each other. Then it can prevent the further growthand aggregation between the generated Au particles. Thisstrategy provides an effective way of tailoring the size andproperties of the Au particles based on the structure of theAu(I)–thiolate precursors.

Conclusion

The morphology modulation of M-Au(I)–MPA nanostructureswas realized by exploiting the connement and direction effectof microemulsion, together with tuning pH value of the reac-tants. A broad range of morphologies, such as quasi-rectangularnanosheets, quasi-square nanosheets, nanobelts, nanostringsand nanochips, were achieved. Further, Au particles withdiameters of hundreds of nanometers and photoluminescentAu clusters were obtained using different M-Au(I)–MPA nano-structures as precursors under solvothermal conditions. Our

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work illustrates the structure and morphology versatilities ofthe Au(I)–MPA assemblies in nanoscale, which can be extendedto other CP systems and further promote their application.

Acknowledgements

We thank NSFC (51001020, 21072025) for nancial support,Kaiwen Chang for the supply of red uorescent polymer-blenddots for quantum yield measurement.

Notes and references

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