Morphology modulation and application of Au(I)–thiolate nanostructures

Abstract

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.

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Introduction

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

Au(I)–thiolate CPs have long been investigated for their unique structure characteristics and Au(I)–Au(I) aurophilic interaction.^9–13 For example, Au(I)–thiomalate (Myochrysine) featured double-helical geometry in the solid state.¹⁴ Lee group has synthesized luminescent Au(I)–alkanethiolates which have a remarkably high degree of conformational order and a well-developed lamellar structure by mixing gold salts with excess n-alkanethiols in the tetrahydrofuran.⁹ These lamellar structures are proposed to comprise parallel slabs of strongly connected Au ions and S atoms, with the substituents on S extending from both sides of each slab. Inter-slab stacking by inter-ligand interactions allows the three dimensional extending of the structure. One of the most intriguing properties of Au(I)–thiolate CP materials is their transformation to Au(0) species. Carboxylic acid and ester functionalized gold nanoparticles (average diameter of 8.7 ± 1.7 nm) have been obtained by heating the bulky Au(I)–alkanethiolate lamellar structures.¹⁰ GSH-capped Au nanoparticles of various sizes, in the 2–6 nm size regime, have been prepared by controlling the pH of the Au(I)–glutathione polymers.¹⁵ Obviously, the nature of the Au(I) precursors, such as morphology and ligands, can greatly affect the size and surface properties of the resulted Au particles. Therefore, the properties of Au(I) precursors is a key in forming Au particles with desirable properties. However, the obtained Au(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 assemblies is seldom. Our group has recently synthesized water-soluble nanosized Au(I)–3-mercaptopropionic acid (MPA) lamellar structures in aqueous solution by adopting pH sensitive MPA ligands.^16,17 Here, versatile morphologies of Au(I)–MPA quasi-rectangular nanosheets, quasi-square nanosheets, nanobelts, nanostrings and nanochips, were successfully achieved in microemulsion by controlling the molar ratio of water to surfactant (w value) and tuning the pH values of the reactants. Taking advantages of the structural characteristics of the obtained Au(I)–MPA nanostructures, they were used as ideal precursors for the preparation of Au particles and photoluminescent Au clusters by solvothermal method (Scheme 1).

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

Experimental section

Materials

All chemicals were commercially available and used without further purification. Hydrogen tetrachloroaurate trihydrate (HAuCl₄·3H₂O, AR grade) was purchased from Shenyang Jinke Reagent Company, 3-mercaptopropionic acid (MPA ≥ 99%) from Alfa Aesar Company. Sodium 3-mercaptopropionate (MPA-Na) aqueous solutions with different pH values were prepared by mixing different amount of NaOH with MPA. First, MPA (0.212 g, 0.002 mol) was dissolved in a small amount of purified water. 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 volumetric flasks and brought to volume by purified water. The pH values of 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 pH meter. Cetyltrimethylammonium bromide (CTAB) was obtained from Sinopharm Chemical Reagent Limited Corporation, cyclohexane was obtained from Tianjin Tiantai Fine Chemical Co., Ltd, and n-pentanol was obtained from Shantou Xilong Chemical Co. Company. Water was purified by a Mili-Q system.

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

HAuCl₄ aqueous solution (0.2 mL, 0.05 M) was added to 20 mL water in an Erlenmeyer flask and heated to boil on a hot plate, then MPA-Na aqueous solution (0.8 mL, 0.05 M) with pH value of 5.7 was injected, generating white precipitates immediately. The products were purified by centrifugation to remove the residues before characterization.

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 mixing solution was stirring for 30 min. Typically, HAuCl₄ (0.2 mL, 0.05 M) aqueous solutions and MPA-Na (0.2 mL, 0.2 M) with different pH values (5.7, 9.2, 10.3, 11.4 and 12.7) were added to 10 mL of the above microemulsion solutions, respectively. After substantial stirring, the two optically transparent microemulsion solutions were mixed and stirred for another 15 min (As the pH values of the mixture of HAuCl₄ and MPA-Na microemulsion can hardly be measured by the pH meters in our lab, their approximate pH values can be inferred from the pH of the mixtures of HAuCl₄ and MPA-Na aqueous solutions with the same volume ratio, consuming the organic phase has little effect on pH values of the system. When the pH values of the MPA-Na are 5.7 to 9.2, 10.3, 11.4 and 12.7, the pH value for the HAuCl₄ and MPA-Na mixture are 2.6, 3.9, 4.4, 6.7 and 11.8 respectively). The resulting microemulsion solution was then refluxed in a water bath set at 82 °C for 30 min and M-Au(I)–MPA nanostructures formed. After the reaction was completed, the resulting microemulsion was cooled to room temperature naturally, then the products (pH values of MPA-Na are 5.7 and 9.2) were collected by centrifuging, washed with absolute ethanol for two times and then distilled water for two times. At last, the precipitations were dispersed in water. When the pH values of the MPA-Na solutions increased to 10.3, 11.4 and 12.7, no further purification was made for the nanostructures in microemulsion.

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 transferred into the Teflon-lined vessel and heated in a oven at 150 °C for 2 h without stirring. After the reactions were completed, the resulted microemulsions were cooled to room temperature naturally. The products were collected by centrifuging, washed with absolute ethanol and distilled water several times. At last, the products were dispersed in water for characterization. For the preparation of red and blue photoluminescent Au clusters, the M-Au(I)–MPA nanochips microemulsion were transferred into the Teflon-lined vessel and heated in a oven at 150 °C for 2 h and 5 h without stirring respectively. For the photoluminescent Au clusters in microemulsion, no further purification 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) spectra were measured using a Shimadzu RF-5301 PC spectrophotometer with 1 cm light path cuvettes. Samples were subjected to spectroscopic measurements without purification. Transmission Electron Microscopy (TEM) was performed on JEOL, JEM-2010 electron microscope with an energy-dispersive X-ray spectroscopy (EDX) operating at 200 kV, samples were dropped on carbon-coated copper grids for measurement. Scanning Electron Microscopy (SEM) was carried out on the field emission microscope (JEOL, JSM 6700F) operating at an acceleration voltage of 200 kV, samples were dropped on silicon wafer for measurement. X-ray photoelectron spectroscopy (XPS) measurements were carried out at 15 kV and 17 mA, using a Thermo ESCALAB 250 spectrometer with a twin-anode Al Kα (1486.6 eV) X-ray source. X-ray diffraction (XRD) data was collected with a Rigaku D-Max 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). Tube voltage and current were 50 kV and 200 mA, respectively; scan range (2θ) was from 3.0° to 70.0°, scan step was 0.02°, dry powder samples were deposited in monocrystal Si sample holder for measurement. Atomic force microscopy (AFM) images were recorded in the tapping mode with a Nanoscope IIIa scanning probe microscope from Digital Instruments under ambient conditions, samples were electrostatically adsorbed on hydrophilic silica wafers for measurement.

Results and discussion

Au(I)–MPA nanostructures: from aqueous solution to microemulsion (M-Au(I)–MPA nanostructures)

Au(I)–thiolate CPs are usually prepared by reaction of thiol ligands with HAuCl₄. For better control of their assembly, water soluble functional ligand MPA was chosen here. Once MPA-Na ligand was added to HAuCl₄ aqueous solution, Au(I)–thiolate CPs generated and further assembled to bulky precipitate with lamellar structures immediately. The formed lamellar structure is similar with that prepared from alkanethiols in literatures, whose structure is shown in Fig. 1.^9–13 In this paper, keeping the reactants and their ratio unchanged, the reaction of HAuCl₄ and MPA-Na was transferred to cationic CTAB–isooctane–1-hexanol–water microemulsion system. As we know, the water phase inside the micelle can serve as a reactor for the formation and assembly of nanostructures, in which the size and morphology of the resulted products can be regulated.^18–20 To our excitement, unlike the bulky assemblies of Au(I)–MPA CPs in water (Fig. 2a), nanosized quasi-rectangular sheets with highly regular morphology were achieved in microemulsion with w value of 12 (M-Au(I)–MPA nanostructures) (Fig. 2b). Obviously, the reverse microemulsion indeed has spatially constraint function on the size and morphology of products in our system. Then, detailed comparison of Au(I)–MPA assemblies from aqueous solution and microemulsion were carried out. XRD analyses show they both have lamellar structures from the equal spaced diffraction peaks between 3° and 20° corresponding to (010), (020) and (030) facets.^9–11 The interlayer distances of the M-Au(I)–MPA nanostructure are calculated to be 12.05 Å, which is about the length of two MPA ligands bridged by hydrogen (H) bonds between the carboxylic acids. Meanwhile, their interlayer distance is smaller than that of Au(I)–MPA bulky precipitates (12.99 Å) from the (010) diffraction peaks. The relatively regular arrangement of M-Au(I)–MPA nanostructures may account for their smaller interlayer distance. XPS analyses show that the molar ratio of Au to S for M-Au(I)–MPA nanostructures is near 1.0. The electron binding energy of Au 4f_7/2 for M-Au(I)–MPA nanostructures 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 indicates strong Au(I)–Au(I) interaction exist in the assemblies. Meanwhile, similar to nanosized Au(I)–MPA lamellar structures prepared in aqueous solution, the M-Au(I)–MPA nanostructures also feature well-distinguishable UV-vis absorption originating from ligand-to-metal charge transfer (LMCT) at 391 nm and metal-centered charge transfer (MCCT) transitions at 350 nm.¹⁶ XRD, XPS and UV-vis absorption prove the bulky precipitates in aqueous solution and M-Au(I)–MPA nanostructures share similar composition and lamellar structures (Fig. 2c–f).¹⁶

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

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

The assembly process of the M-Au(I)–MPA quasi-rectangular nanosheets was further monitored with UV-vis spectroscopy, and the assembled intermediates were subjected to TEM characterization to reveal the assembly mechanism (Fig. 3). Once the two microemulsion solutions of HAuCl₄ and MPA-Na were mixed at room temperature, the absorption of AuCl₄⁻ disappeared immediately, indicating Au(III) ions were reduced to Au(I) species during the interdroplet collision (Fig. S1†).²³ The absorption of the mixed microemulsion kept unchanged thereafter. TEM image shows 5–25 nm nanospheres at this stage, which are within the typical dimensions of microemulsion droplets (Fig. 3b). The above results suggest that the microemulsion acts as a “template” to generate Au(I)–MPA CPs inside, and no further assembly occurs at room temperature.^18–20

Fig. 3 UV-vis spectroscopic monitoring of the assembly process of quasi-rectangular M-Au(I)–MPA nanosheets (a). TEM images of intermediate 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.

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

The assembled intermediates at 6 min and 8 min reveal that their assembly underwent a nanosphere-to-nanostring-to-irregular-nanosheet-to-rectangular-nanosheet process (Fig. 3c and d), which is consistent with our previous finding.¹⁶ As the lengths of nanostrings are much larger than the dimension of microemulsion droplets, coalescence and agglomeration of micelles must happen at boiling temperature.^18–20 In addition, molecular propagation driven by hydrogen bonding interaction is faster along the axis of the Au(I)⋯Au(I) chain than in the lateral directions, leading to the formation of string structures firstly.⁶ Then, the nanostrings align with each other to facilitate the further coordination of S to Au in a nearby chain concurrently, transforming the aligned strings into thin nanosheets. Based on the above results, the structural transformation process of the assembly intermediates are proposed as shown in Fig. S2.†

Although Au(I)–MPA nanosheets in aqueous solution have been obtained by tuning the pH value of the assembly system in our previous work,¹⁶ the nanosheets have ill-defined morphologies, presumably as a result of the rapid assembly speed and the absence of structure-director. In this microemulsion method, the assembly speed can be greatly slowed down by confining the reactants inside the micelles and the anisotropic fusion between droplets directs the assembly of Au(I)–MPA CPs. Here, we assume that the fusioned microemulsion droplets feature cylinder shape at w = 12, which further lead to the formation of well-defined quasi-rectangular nanostructures.^18,19

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

As we know, the water contents in microemulsion (w values) are highly related with the fusion rate, mass-exchange and coalescence direction between droplets, which greatly affect the morphology of the products.^18–20 We therefore adjusted the w values of M-Au(I)–MPA assembly systems. TEM images prove that w value can surely modulate the morphology of M-Au(I)–MPA nanostructures. When the w value increased from 12 to 20 and 40 with other reaction conditions unchanged, the products changed from quasi-rectangular nanosheets to quasi-square nanosheets, mixture of big irregular nanosheets and short nanostrings respectively (Fig. 4a–c). XRD analyses and UV-vis absorption spectra show that the assemblies at all these w values have lamellar structures (Fig. S3–S6†). AFM measurement of the typical M-Au(I)–MPA quasi-square nanosheets shows that their thicknesses are around 10–25 nm (Fig. S7†). From the XRD study, the height of a single layer of M-Au(I)–MPA lamellar structures is about 1.2 nm, indicating that the layer numbers of the quasi-square nanosheets are 8–20. As reported by literature, (i) the fusion and mass exchange rates between droplets increase greatly with w value.¹⁹ (ii) The spherical droplets fuse to form short cylinder droplets at lower w value with water-enriched domains locating at both ends, and nearly spherical droplets are formed at higher w value with water-enriched domains equilibrating at every direction. (iii) Further exchange or coalescence between droplets mainly happen at the water-enriched domains, which finally direct the morphology of products.^18–20,24 Obviously, the morphology of droplets have effect on the morphology of the M-Au(I)–MPA nanostructures. The shapes of M-Au(I)–MPA nanostructures changed from quasi-rectangular to quasi-square with the increase of w values from 12 to 20. With the further increase of water content, the microemulsion droplets are unstable. At this condition, size divergence of micelle droplets may happen and subsequently cause the generation of big nanosheets and nanostrings at the same time. In addition, except for the spatially constraint effect of microemulsion, the crystal growth mechanisms also greatly affect the morphology of the M-Au(I)–MPA nanostructures. As shown in Fig. 3, the assembly of the M-Au(I)–MPA nanostructures involve two stages, the first one is the assembly of Au(I)–MPA CP into string-shaped intermediates, the second one is the aggregation and fusion of the string-shaped intermediates into final structures. Because the assembly rate along Au–S slab by inter-string coordination, aurophilic interaction is faster than stacking of Au–S slabs by H bond, the generated structures are thin-sheets (Fig. 4a–c), rather than cubic or rodlike structures as reported in literature.¹⁹ Therefore, we conclude that both the template effect of the microemulsion and the crystal growth feature decide the morphology characteristics in our system.

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 = 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 chips are generated by TEM electron bombardment, but do not exist in the original samples.¹⁵

Here, pH sensitive MPA ligand is incorporated into the assembly system. Except the w value of the microemulsion, pH value of reactants is another important parameter for the morphology control of M-Au(I)–MPA nanostructures. At lower pH values, hydrogen bonding interaction forms between the protonated carboxylate groups (Fig. 5a). Then the hydrogen bonding interaction can greatly promote the formation of Au(I)–Au(I) interaction and further Au–S coordination, and finally lead to the assembly of Au(I)–MPA CPs into lamellar structures. And at higher pH values, the electrostatic repulsion between the deprotonated carboxylate groups is strong, which would weaken the Au(I)–Au(I) interaction and make the further coordination of Au to S and the assembly of Au(I)–MPA CPs unfavourable (Fig. 5b and c). As we expected, keeping the w value of microemulsion unchanged, the dimension of the assemblies decreased when the pH values of the reactants (MPA-Na) were increased from 5.7 to 9.2, 10.3 and 12.7 from TEM images (Fig. 4d–f), and the shapes of the assemblies changed from quasi-rectangular nanosheets to nanobelts to nanostrings and finally nanochips. The above results prove that increased pH value can hinder the assembly of Au(I)–MPA polymers, resulting in unassembled or partially assembled nanostructures as their products. The transformation process is further confirmed by UV-vis absorption and XPS analyses (Fig. S8†). The absorption pair representing for the lamellar structure blue shifted and their intensity diminished, meanwhile, the Au(I) 4f_7/2 binding energies shifted from 84.51 to 84.77 eV gradually, both suggesting that the interaction modes in these structures change to less aurophilic and Au–S coordination feature.^25,26 Further, the photoluminescence 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 and nanostrings have photoluminescence properties. However, when the pH value of MPA-Na reactant reached 12.7, almost no emission was observed for the M-Au(I)–MPA nanochips due to the absence of Au(I)–Au(I) aurophilic interaction (Fig. S9b†). The emission change of M-Au(I)–MPA nanostructures is accordant with that of UV-vis absorption and XPS analysis. Although the presence of NaOH may induce the transition of spherical micelles into more elongated, elliptical ones by reducing the repulsion of charged surfactant “heads” as reported in literature,^27,28 here, the increasing of pH values result in the decrease in the three-dimensional size of the products. It is more likely that the smaller sizes are resulted from the interligand repulsions, rather than from the morphological change of the micelle.

Fig. 5 Change in interligand interactions from attractive to repulsive by increasing the pH values of MPA-Na reactants and the decrease in resulted Au(I)–Au(I) interactions from (a) to (c). α₁ and α₂ are the Au–S–Au angles at low and high pH, respectively, α₁ < α₂. The Au(I)–Au(I) interaction is shown by the dashed line.

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

Au particle in micro/nano scale is an attractive research topic due to their size-dependent physical and chemical properties.^29,30 In particular, Au particles with large size (diameters > 200 nm) can be applied in biosensor field due to their highly sensitive localized surface plasmon resonance.³¹ Until now, fabrication techniques such as chemical synthesis,^32,33 electron beam lithography,³⁴ and nanosphere lithography³⁵ have been adopted for the size and morphology control of Au nanoparticles. Highly regular metal features on substrates can be achieved by electron beam lithography or nanosphere lithography. However, colloid particles in solution are the preferred starting materials for further manipulation due to their high flexibility. The colloidal Au particles are usually prepared from the chemical reduction (typical reductants are NaBH₄, sodium citrate and ascorbic acid etc.) of Au³⁺ salts. However, only Au particles with diameters ranging from several to hundreds of nanometers can be obtained. Large particles (diameters > 200 nm) can hardly be prepared by chemical synthesis. As reported by literature, Au(I) can be reduced to Au(0) by the thiol ligands during thermal decomposition process. The redox reaction is expected to generate Au atoms and dithiol compounds.^15,36–39 Taking advantages of the controllable morphology of M-Au(I)–MPA assemblies, they are supposed to be ideal precursors for the preparation of Au particles with different sizes.

Here, M-Au(I)–MPA quasi-square nanosheets, nanostrings and nanochips were chosen as precursors for the formation of Au(0) species. When M-Au(I)–MPA quasi-square nanosheets and nanostrings were heated at 150 °C in stainless Teflon-lined autoclave for 2 h, Au particles with average diameters of about 520 nm and 250 nm were obtained respectively (Fig. 6). EDX analysis confirms the presence of Au in these samples (Fig. S10†). The sizes of the obtained Au particles are much larger than that of their corresponding precursors. It is deduced that aggregation between the resulted Au nanoparticles happens during the heat treatments. Although elaborate size control of the Au particles is not achieved at the present stage, our method may be of general utility for the synthesis of large diameter Au particles.

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

Au clusters have also attracted much attention for their unique role in bridging the “missing link” between atomic and nanoparticle behavior.⁴⁰ Recent study has focused on their quantum electronic properties, including photoluminescence, sensing and so on.^41–46 For the preparation of photoluminescence Au clusters, so far, different chemical approaches, such as direct reduction of metal ions and etching-based strategy, have been exploited. Precise control of the experimental conditions is always needed. Here, to our delight, red (607 nm) and blue (421 nm) photoluminescent Au clusters were achieved when non-photoluminescent Au(I)–MPA nanochips in microemulsion were exposed to thermal decomposition at 150 °C in stainless Teflon-lined autoclave for 2 h and 5 h, respectively (Fig. 7 and S9b†).⁴⁷ The quantum yields for the red and blue photoluminescent Au clusters are 0.3% and 16.4%. No discernible surface plasmon band was observed in their UV-vis absorption spectra, indicating the small size of these clusters.^48,49 Consistent with the emission spectra, the red photoluminescent cluster have bigger size than the blue one from the UV-vis absorption band edge. We tried to prove this deduction by TEM analysis. As shown in Fig. S11,† particles with average diameters of about 2 nm were obtained for the red photoluminescent samples. Although clear lattice fringes were observed in the high-resolution TEM image of the blue photoluminescent Au clusters, the clusters aggregated with each other seriously.⁵⁰ This phenomenon is very typical for metal clusters with small size.⁵¹ It is supposed that the size reduction with reaction time is resulted from the ligand etching process.⁵² Then the decreased size accounts for the blue shift of the photoluminescence with increased heating time.⁵³ Compared with the methods reported in literature, our synthetic route has the following advantages: (i) two kinds of photoluminescent Au clusters are achieved by simply controlling the reaction time; (ii) this work provides a new strategy towards the fabrication of Au clusters by the self-redox of Au(I) precursors under heating condition without further reactants. Subsequently, this method would provide new modulation approaches (such as ligands of Au(I) precursors, reaction temperature, reaction time) for the further engineering of the properties of Au clusters. Further improvement of their performance is still needed.

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

Here, the sizes of the obtained Au nanoparticles decrease with the reduced size of M-Au(I)–MPA precursors. There are two possible reasons: (i) due to the stable three dimensional rigid structures of nanosheets, it's more difficult to thermal decompose the nanosheets than nanostrings and nanochips. Upon heating, the larger Au(I)–MPA precursors (nanosheets) lead to the formation of fewer nuclei for thermal reduction to Au nanoparticles.¹⁵ During the growth process, Au(I) is preferentially reduced onto the surface of the existing Au(0) species rather than independently forming a new particle. Fewer nuclei which have more Au ions in close proximity result in larger Au nanoparticles. Conversely, smaller size of Au(I) precursors favour the formation of more nuclei, hence leading to smaller nanoparticles. (ii) As shown before, the size of M-Au(I)–MPA nanostructures decreases with the increase of pH value of the reaction system. Due to the increased electrostatic repulsion between the deprotonated carboxylate groups on their surface, the smaller sized M-Au(I)–MPA nanostructures are not inclined to get close to each other. Then it can prevent the further growth and aggregation between the generated Au particles. This strategy provides an effective way of tailoring the size and properties of the Au particles based on the structure of the Au(I)–thiolate precursors.

Conclusion

The morphology modulation of M-Au(I)–MPA nanostructures was realized by exploiting the confinement and direction effect of microemulsion, together with tuning pH value of the reactants. A broad range of morphologies, such as quasi-rectangular nanosheets, quasi-square nanosheets, nanobelts, nanostrings and nanochips, were achieved. Further, Au particles with diameters of hundreds of nanometers and photoluminescent Au clusters were obtained using different M-Au(I)–MPA nanostructures as precursors under solvothermal conditions. Our work illustrates the structure and morphology versatilities of the Au(I)–MPA assemblies in nanoscale, which can be extended to other CP systems and further promote their application.

Acknowledgements

We thank NSFC (51001020, 21072025) for financial support, Kaiwen Chang for the supply of red fluorescent polymer-blend dots for quantum yield measurement.

Notes and references

1. A. M. Spokoyny, D. Kim, A. Sumrein and C. A. Mirkin, Chem. Soc. Rev., 2009, 38, 1218–1227.
2. S. Jung and M. Oh, Angew. Chem., Int. Ed., 2008, 47, 2049–2051.
3. W. Lin, W. J. Rieter and K. M. Taylor, Angew. Chem., Int. Ed., 2009, 48, 650–658.
4. M. Oh and C. A. Mirkin, Nature, 2005, 438, 651–654.
5. W. J. Rieter, K. M. Taylor, H. An, W. Lin and W. Lin, J. Am. Chem. Soc., 2006, 128, 9024–9025.
6. W. Lu, S. S. Y. Chui, K. M. Ng and C. M. Che, Angew. Chem., Int. Ed., 2008, 47, 4568–4572.
7. Y.-M. Jeon, G. S. Armatas, D. Kim, M. G. Kanatzidis and C. A. Mirkin, Small, 2009, 5, 46–50.
8. Y.-M. Jeon, J. Heo and C. A. Mirkin, J. Am. Chem. Soc., 2007, 129, 7480–7481.
9. S. H. Cha, J. U. Kim, K. H. Kim and J. C. Lee, Chem. Mater., 2007, 19, 6297–6303.
10. S. H. Cha, K. H. Kim, J. U. Kim, W. K. Lee and J. C. Lee, J. Phys. Chem. C, 2008, 112, 13862–13868.
11. A. Parikh, S. Gillmor, J. Beers, K. Beardmore, R. Cutts and B. Swanson, J. Phys. Chem. B, 1999, 103, 2850–2861.
12. J. M. Forward, D. Bohmann, J. P. Fackler Jr and R. J. Staples, Inorg. Chem., 1995, 34, 6330–6336.
13. V. W.-W. Yam, E. C.-C. Cheng and K.-K. Cheung, Angew. Chem., Int. Ed., 1999, 38, 197–199.
14. R. Bau, J. Am. Chem. Soc., 1998, 120, 9380–9381.
15. R. P. Briñas, M. Hu, L. Qian, E. S. Lymar and J. F. Hainfeld, J. Am. Chem. Soc., 2008, 130, 975–982.
16. H. Nie, M. Li, Y. Hao, X. Wang and S. X.-A. Zhang, Chem. Sci., 2013, 4, 1852–1857.
17. H. Nie, M. Li, Y. Hao, X. Wang, S. Gao, B. Yang, M. Gu, L. Sun and S. X.-A. Zhang, J. Colloid Interface Sci., 2014, 434, 104–112.
18. J.-C. Lin, J. T. Dipre and M. Z. Yates, Chem. Mater., 2003, 15, 2764–2773.
19. M. Cao, X. Wu, X. He and C. Hu, Langmuir, 2005, 21, 6093–6096.
20. P. K. Dutta and D. Robins, Langmuir, 1991, 7, 1048–1050.
21. Y. Negishi, K. Nobusada and T. Tsukuda, J. Am. Chem. Soc., 2005, 127, 5261–5270.
22. C. Zhou, C. Sun, M. Yu, Y. Qin, J. Wang, M. Kim and J. Zheng, J. Phys. Chem. C, 2010, 114, 7727–7732.
23. S. P. Moulik and B. K. Paul, Adv. Colloid Interface Sci., 1998, 78, 99–195.
24. D. E. Zhang, X. M. Ni, H. G. Zheng, Y. Li, X. J. Zhang and Z. P. Yang, Mater. Lett., 2005, 59, 2011–2014.
25. M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz and R. Jin, J. Am. Chem. Soc., 2008, 130, 5883–5885.
26. R. L. White-Morris, M. M. Olmstead and A. L. Balch, J. Am. Chem. Soc., 2003, 125, 1033–1040.
27. V. Uskoković, M. Drofenik and I. Ban, J. Magn. Magn. Mater., 2004, 284, 294–302.
28. F. Garcia Sanchez and C. Carnero Ruiz, J. Lumin., 1996, 69, 179–186.
29. S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 4212–4217.
30. B. D. Chithrani, A. A. Ghazani and W. C. Chan, Nano Lett., 2006, 6, 662–668.
31. F. Liu, M. M.-K. Wong, S.-K. Chiu, H. Lin, J. C. Ho and S. W. Pang, Biosens. Bioelectron., 2014, 55, 141–148.
32. N. R. Jana, L. Gearheart and C. J. Murphy, Langmuir, 2001, 17, 6782–6786.
33. K. R. Brown, D. G. Walter and M. J. Natan, Chem. Mater., 2000, 12, 306–313.
34. P. M. Mendes, S. Jacke, K. Critchley, J. Plaza, Y. Chen, K. Nikitin, R. E. Palmer, J. A. Preece, S. D. Evans and D. Fitzmaurice, Langmuir, 2004, 20, 3766–3768.
35. B. J. Y. Tan, C. H. Sow, T. S. Koh, K. C. Chin, A. T. S. Wee and C. K. Ong, J. Phys. Chem. B, 2005, 109, 11100–11109.
36. S. J. Lee, S. W. Han and K. Kim, Chem. Commun., 2002, 442–443.
37. K. Abe, T. Hanada, Y. Yoshida, N. Tanigaki, H. Takiguchi, H. Nagasawa, M. Nakamoto, T. Yamaguchi and K. Yase, Thin Solid Films, 1998, 327–329, 524–527.
38. S. Gomez, K. Philippot, V. Colliére, B. Chaudret, F. Senocq and P. Lecante, Chem. Commun., 2000, 1945–1946.
39. M. K. Corbierre and R. B. Lennox, Chem. Mater., 2005, 17, 5691–5696.
40. M. Zhu, E. Lanni, N. Garg, M. E. Bier and R. Jin, J. Am. Chem. Soc., 2008, 130, 1138–1139.
41. Y. Yang and S. Chen, Nano Lett., 2003, 3, 75–79.
42. R. Jin, S. Egusa and N. F. Scherer, J. Am. Chem. Soc., 2004, 126, 9900–9901.
43. J. Zheng, J. T. Petty and R. M. Dickson, J. Am. Chem. Soc., 2003, 125, 7780–7781.
44. B. K. Teo, X. Shi and H. Zhang, J. Am. Chem. Soc., 1992, 114, 2743–2745.
45. Y. Gu, Q. Wen, Y. Kuang, L. Tang and J. Jiang, RSC Adv., 2014, 4, 13753–13756.
46. Z.-X. Wang, C.-L. Zheng and S.-N. Ding, RSC Adv., 2014, 4, 9825–9829.
47. Y. Bao, C. Zhong, D. M. Vu, J. P. Temirov, R. B. Dyer and J. S. Martinez, J. Phys. Chem. C, 2007, 111, 12194–12198.
48. Z. Wang, W. Cai and J. Sui, ChemPhysChem, 2009, 10, 2012–2015.
49. C. Zhou, C. Sun, M. Yu, Y. Qin, J. Wang, M. Kim and J. Zheng, J. Phys. Chem. C, 2010, 114, 7727–7732.
50. Z.-M. Cui, Z. Chen, L.-Y. Jiang, W.-G. Song and L. Jiang, Mater. Lett., 2011, 65, 82–84.
51. T. K. Misra and C.-Y. Liu, J. Colloid Interface Sci., 2008, 326, 411–419.
52. X. Yuan, B. Zhang, Z. T. Luo, Q. F. Yao, D. T. Leong, N. Yan and J. P. Xie, Angew. Chem., Int. Ed., 2014, 53, 4623–4627.
53. C.-C. Huang, Z. Yang, K.-H. Lee and H.-T. Chang, Angew. Chem., Int. Ed., 2007, 46, 6824–6828.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06500j

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