WO2009108125A1

WO2009108125A1 - Gold nanoparticles

Info

Publication number: WO2009108125A1
Application number: PCT/SG2009/000063
Authority: WO
Inventors: Yugen Zhang; Jackie Y. Ying; Lan Zhao
Original assignee: Agency For Science, Technology And Research
Priority date: 2008-02-28
Filing date: 2009-02-23
Publication date: 2009-09-03

Abstract

Gold nanoparticles are prepared by combining a gold salt, an N,N'-disubstituted imidazolium salt and a thiol.

Description

Gold nanoparticies Technical Field

The present invention relates to gold nanoparticies and processes for making them. Background of the Invention Gold nanoparticies, particularly with dimensions of under about 8 run with narrow size distribution, are useful in a broad range of applications. Although gold nanoparticies are commercially available and numerous preparative methods have been reported, a simple, robust and easily scalable synthesis for gold nanoparticies of dimensions of less than about 5 nm would be of great practical value. Methods that have been published to date suffer from numerous disadvantages, including low efficiency, long reaction times, difficulty in controlling particle size and difficulty in scale up.

Object of the Invention It is the object of the present invention to at least partially satisfy the above need.

Summary of the Invention In a first aspect of the invention there is provided a process for preparing gold nanoparticies comprising combining a gold salt, an N.N^'-disubstituted imidazolium salt and a thiol, under conditions whereby said gold salt. N.N^'-disubstituted imidazolium salt and thiol interact to form the gold nanoparticies.

The following options may be used in conjunction with the first aspect, either individually or in any suitable combination.

The gold salt may be a gold (III) salt. It may be a gold halide. The gold halide may be gold (III) chloride.

Each nitrogen substituent on the N.N^"-disubstituted imidazolium salt may be, independenth', alkyl, benzyl or aryl, each, independently, being optionally substituted. Each nitrogen substituent on the N.N^'-disubstituted imidazolium salt may be. independently, selected from the group consisting of benzyl, methyl, n-butyl, isopropyl and 2,6-diisopropylphenyl.

The N.N'-disubstituted imidazolium salt may be an N.N^'-disubstituted imidazolium halide or an N.N'-disubstituted imidazolium tetrafluoroborate. In particular, it may be an N.N'-dibenzylimidazolium halide.

The thiol may be a Cl to Cl 6 alkanethiol. It may be a long chain alkanethiol. It may be a ClO to Cl 6 alkanethiol. It may be dodecanethiol.

The process may be conducted in a solvent. The solvent may be chloroform, methanol, toluene or it may be a mixture of any two or more of these. The process may additionally comprise agitating the gold salt, N.N^'-disubstituted imidazolium salt and thiol during and/or after said combining. The agitating may- comprise stirring, mixing, shaking, sonicating or otherwise agitating.

The process may additionally comprise separating the gold nanoparticles. It may comprise isolating the gold nanoparticles. It may comprise at least partially separating the gold nanoparticles from unreacted reagents and/or from intermediate species and byproducts formed in the process. It may comprise at least partially removing the solvent

(in the event that a solvent is used).

In an embodiment there is provided a process for preparing gold nanoparticles comprising combining a gold (III) halidε. an N.N^'-disubstituted imidazolium salt and a long chain alkanethiol. whereby said gold salt N.N^"-disubstituted imidazolium salt and thiol interact to form the gold nanoparticles.

In another embodiment there is provided a process for preparing gold nanoparticles comprising combining a gold (III) halidε. an N.N^'-dibenzylimidazolium salt and a ClO to Cl 6 alkanethiol in a solvent, whereby said gold salt. N.N^"- dibenzylimidazolium salt and thiol interact at room temperature to form the gold nanoparticles in less than about 1 minute.

In a second aspect of the invention there is provided a dispersion of gold nanoparticles, said nanoparticles having a mean panicle diameter of less than about 5nm. The following options may be used in conjunction with the second aspect, either individually or in any suitable combination.

The dispersion ma}' be made by the process of the first aspect. The dispersion may be substantiall}' monodispersed. The nanoparticles may be substantially spherical. The gold nanoparticles may have the thiol on at least a portion of the surfaces thereof.

In an embodiment there is provided a dispersion of gold nanoparticles made by the process of the first aspect, said nanoparticles having a mean particle diameter of less than about 5nm and being substantially monodispersed. In a third aspect of the invention there is provided us of the dispersion of the second aspect in a catalytic, imaging, photonic, electronic or energy application.

In a fourth aspect of the invention there is provided a process for preparing gold nanoparticles comprising combining a gold salt and an N-heterocyclic carbene (NHC) under conditions whereby said gold salt and NHC interact to form the gold nanoparticles. The gold salt and the NHC may be combined in the presence of a thiol. The NHC may be a polymeric NHC. The NHC may be a stable NHC. The process may comprise generating the NHC in situ. The step of generating the NHC in situ may comprise combining a gold salt, an N.N^'-disubstituted imidazolium salt and a thiol under conditions whereby the N.N^'-disubstituted imidazolium salt is converted to the NHC.

Brief Description of the Drawings

A preferred embodiment of the present invention will now be described, by way of an example only, with reference to the accompanying drawings wherein: Figure 1 shows TEM images of gold particles. Small nanoparticles derived (a) with 1 in chloroform, and (b) with 2 in chloroform, (c. d) Aggregated small nanoparticles derived with 3 in chloroform, (e) Large particles derived with 1 in chloroform at a low thiol/ Au ratio of 1 : 1.

Detailed Description of the Preferred Embodiments The invention relates to the synthesis of ultrafinε gold nanoparticles mediated by imidazolium salts. The present specification describes a simple and robust protocol for the synthesis of stable, ultrafme gold nanoparticles under mild conditions. The reaction may^¬ be conducted at room temperature, and ma}_' be completed in a short time, for example in seconds. It makes use of imidazolium salts and does not require strong reducing agents. It is easy, inexpensive, robust and easy to scale up. Only ultrafme gold nanoparticles are obtained and the particles are highly stable and well dispersed.

The invention relates to preparation of gold nanoparticles by combining a gold salt, an N.N^'-disubstituted imidazolium salt and a thiol. No two of these three reagents without the third reagent will produce the gold nanoparticles. Thus any two may be initially combined and the third then added. In one example, the gold salt is initially combined with the thiol, and the imidazolium salt is then added, however other orders of addition may be used equally as effectively.

The process is commonly conducted in a solvent. The solvent ma)-" comprise a halogenated solvent, e.g. a chlorinated solvent. It may comprise methylene chloride and/or chloroform. It may be chloroform. It may comprise a short chain alcohol, e.g. methanol, ethanol, n-propanol, isopropanol, n-butanol or some other suitable alcohol. It may comprise an aromatic solvent, for example benzene, toluene, xylene, chlorobenzene. dichlorobenzene etc. It may comprise a mixture of any two or more of the above. The solvent may be added at any convenient stage of the reaction. In an example, the thiol may be dissolved in the solvent and the gold salt mixed with the resulting solution, optionally accompanied by stirring, shaking or other agitation to promote efficient mixing. The imidazolium salt may then be added to the resulting mixture, again optionally accompanied by stirring, shaking or other agitation to promote efficient mixing. Other orders of mixing may be used, for example the gold salt may be mixed with, or dissolved in, or suspended in, or dispersed in, the solvent, and the thiol may be added. The resulting mixture or solution or suspension or dispersion may then be treated with the imidazolium salt.

The ratio of gold salt to solvent may be about 0.1 to lOmmol/ 100ml, or about 0.1 to 5, 0.1 to 2. 0.1 to 1, 0.1 to 0.5, 0.5 to 10, 1 to 10, 5 to 10, 0.5 to 5, 0.5 to 2 or 1 to 2mmol/100ml, e.g. about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8. 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5. 6, 7, 8, 9 or lOmmol/lOOml.

The thiol may be used in a molar excess over the gold salt. The ratio of thiol to gold salt may be about 1 to about 5 (i.e. about 1 :1 to about 5:1). or about 1 to 3. 2 to 4, 2 to 3 or 1.5 to 2.5 on a molar basis, e.g. about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5. In some instances the gold salt may be used in a molar excess over the gold salt. In such cases, the ratio of thiol to gold salt may be about 1 to about 0.2 (i.e. about 1 :1 to about 1 :5), or about 1 to 0.5, 1 to 0.8, 0.8 to 0.2, 0.5 to 0.2 or 0.8 to 0.5. e.g. about 1, 0.9, 0.8, 0.7, 0.6, 0.5. 0.4, 0.3 or 0.2. The imidazolium salt may be used in a molar excess over the gold salt. It may be used in a molar excess over the thiol. The ratio of imidazolium salt to gold salt may be about 2 to about 8 (i.e. about 2:1 to about 8:1). or about 2 to 5. 5 to 8 or 3 to 5 on a molar basis, e.g. about 2, 2.5, 3, 3.5, 4. 4.5, 5, 5.5, 6, 6.5, 7, 7.5 or 8. The ratio of imidazolium salt to thiol may be about 1 to about 5 (i.e. about 1 :1 to about 5:1), or about 1 to 3. 2 to 4. 2 to 3 or 1.5 to 2.5 on a molar basis, e.g. about 1. 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5.

The gold salt may be a gold (I) salt or a gold (III) salt. It may be a gold halide, e.g. chloride, bromide or iodide. The gold halide may be for example gold (III) chloride or gold (III) bromide. The gold salt may be a hydrated gold salt. Thus the gold (III) chloride, for example, may be gold (III) chloride hydrate. It may be AuCIjOH₂O. In some cases a mixture of gold salts may be used.

Each nitrogen substituent on the N.N'-disubstituted imidazolium salt may be,

alkyl, benzyl or aryl. The imidazolium salt may be symmetrically substituted (i.e. the substituents on the nitrogen atoms may be the same) or it may be asymmetrically substituted (i.e. the substituents on the nitrogen atoms ma}' be different). Suitable alkyl groups include Cl to C6 straight chain alkyl groups, C3 to C6 branched chain alkyl groups and C3 to C6 cyclic alkyl groups. Suitable aryl groups include phenyl. 2,6-disubstituted phenyl groups (where the substituents are any of the alkyl groups described above), fused aryl groups and heteroaryl groups (pyridyl, thiophenyl, thiopheneyl etc.). Any of the above may also be optionally substituted. The imidazolium salt itself may be optionally substituted on either C4 or C5 or both, for example with one or two alkyl groups as described above. The imidazolium ring may be fused through C4 and C5 to an aryl or cycloalkyl ring. The aryl ring may be a phenyl ring, whereby the imidazolium ring would comprise a portion of a benzimidazole ring system, and the N.N^"-disubstituted imidazolium salt would be an N.N'-disubstituted benzimidazolium salt. The fused phenyl ring may also be optionally substituted, for example with alkyl groups such as those described above. In some cases a mixture of imidazolium salts (each as described above) may be used.

The imidazolium salt may be an N.N^'-dibenzylimidazolium salt. In this case the resulting gold nanoparticles may be unaggregatεd. In the case where the imidazolium salt is not an N.N'-dibenzylirnidazolium salt, the resulting gold nanoparticles may be agereεated.

The N.N'-disubstituted imidazolium salt may be an N.N'-disubstituted imidazolium halide. e.g. chloride, bromide or iodide. It may be an N.N^'-disubstituted imidazolium tetrafluoroborate. It may be some other salt. For example, it may be an N.N^'- dibenzylimidazolium chloride.

It is thought that in the reaction described herein, the imidazolium salt is converted in situ to an N-heterocyclic carbene (NHC). Accordingly, the reaction may be conducted using a stable NHC in place of an imidazolium salt. Thus there is provided herein a process for preparing gold nanoparticles comprising combining a gold salt and an NHC. ^■ and optionally a thiol, under conditions whereby said gold salt and NHC interact to form the gold nanoparticles. The NHC may be a stable NHC. It may be a polymeric NHC.

The thiol may be a Cl to C16 alkanethiol. It may be Cl to ClO, Cl to C6, C6 to C16. ClO to C16₅ C8 to C12 or ClO to C12, e.g. CL C2_; C3. C4, C5_; C6₅ C7, C8. C9. ClO, Cl L C12, C13, C14, C15 or C16. It may be a long chain alkanethiol. The carbon chain of the alkanethiol may be linear. It may be branched. It may be dodecanethiol. It may be 1 -dodecanethiol. As the alkyl chain may be derived from natural sources, the thiol may comprise a number of different chain lengths. In this case the chain length described above may be the mean chain length or the predominant chain length and there may be a distribution of chain lengths surrounding that chain length. For example a Cl 2 alkanethiol may have chain lengths ranging from C6 to Cl 6. with the more C12 than any other chain length.

The reaction need not be conducted under a special atmosphere. It may be conducted in air. or it may be conducted under nitrogen, argon, carbon dioxide or a mixture of any two or more of these, or under some other desired atmosphere. It may be conducted at room temperature. It may be conducted at about 0 to about 5O⁰C. or about 0 to 30, 0 to 20, 0 to 10, 10 to 50, 20 to 50, 30 to 50 or 10 to 3O⁰C, e.g. about 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 or 5O⁰C. The time required for the reaction (from the time at which all three reagents are combined) may depend on the nature of the reagents, in particular on the nature of the imidazolium salt and/or on the presence, nature and concentration of base. It may depend on the temperature at which the reaction is conducted. It may take for example from about 1 second to about 10 days, or about 1 second to 1 day or about 1 second to 1 hour, 1 second to 1 minute, 10 seconds to 1 minute, 1 minute to 10 days, 1 to 10 days, 1 minute to 1 hour or 1 to 10 minutes, e.g. about 1. 2, 3, 4, 5, 6, 1, 8, 9, 10, 20, 30_; 40 or 50 seconds, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40 or 50 minutes, I ₅ 2, 3. 4, 5, 6, 12 or 18 hours or 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 days. The completion of the reaction may be determined by the conversion of the reaction mixture (i.e. the initially formed mixture of thiol, gold salt and imidazolium salt) to a substantially colourless liquid.

The reaction may be conducted in the absence of strong reducing agents. It may be conducted in the absence of reducing agents with a reduction potential greater than that of the thiol, or with a reduction potential greater than that of the imidazolium salt, or with a reduction potential greater than both the thiol and the imidazolium salt together. It may be conducted in the absence of borane. It may be conducted in the absence of borohydride. It may be conducted in the absence of borane. It may be conducted in the absence of borohydride. It may be conducted in the absence of boron-containing reducing agents.

The reaction may be conducted in the presence of a base. It ma)- be conducted in the presence of a strong base, e.g. t-butoxidε. It may be conducted in the absence of strong base. Commonly the presence of a base will accelerate the reaction. The base may be added together with the imidazolium salt. The ratio of base to imidazolium salt may be about 0.5 to about 2 (i.e. about 0.5 :1 to about 2:1) or about 0.5 to 1. 1 to 2, 0.8 to 1.5, 0.8 to 1. 1 to 1.5. 1 to 1.2 or 0.8 to 1.2. e.s. about 0.5. 0.6. 0.7. 0.8. 0.9. 1. 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7. 1.8, 1.9 or 2. The base may be added in a solvent, e.g. an organic solvent such as dichloromethane.

The process described above may be readily scalable. Thus it may be suitable for making batches of dispersion comprising about lOOmg of gold nanoparticles. or about Ig. about 1Og, about lOOg or about lkg. It may be suitable for making dispersions comprising more than lkg of gold nanoparticles. It may be adapted to a continuous process. In an example of a continuous process, the gold salt may be continuously added to a stream of the thiol dissolved in the solvent, optionally with baffles or other means to promote efficient mixing. The imidazolium salt may then be added to the resulting mixed stream. again optionally with baffles or other means to promote efficient mixing. The continuously produced product stream would then contain the gold nanoparticles. By use of the appropriate reaction temperature and imidazolium salt, a high flow rate with high conversion may be obtained.

The process of the invention provides a dispersion of gold nanoparticles. The nanoparticles ma}' have a mean panicle diameter of less than about 5nm, or less than about 5. 3 or 2nm, or of about 0.5 to about 5nm. or of about 0.5 to 2. 0.5 to 1. 1 to 5, 2 to 5. 2 to 3 or 1 to 2nm. e.g. about 0.5. L 1.5. 2. 2.5. 3. 3.5. 4. 4.5 or 5nm. The nanoparticles may be substantially monodispersed. The}' may have a narrow polydispersity. The weight average particle size of the nanoparticles divided by the number average particle size of the nanoparticles may bε less than about 2. or less than about 1.9, 1.8. 1.7. 1.6. 1.5. 1.4. 1.3, 1.2 or 1.1 or may be about 1 to about 2. or about 1 to 1.8, 1 to 1.6. 1 to 1.4, 1 to 1.2 or 1 to 1.1, e.g. about 1. 1.05, 1.1. 1.2. 1.3, 1.4. 1.5, 1.6. 1.7, 1.8, 1.9 or 2. The nanoparticles may be substantial!}' spherical. They ma}' be some other shape, e.g. irregular, ovoid, elliptoid. oblate spherical, polyhedral etc. The dispersion may be clear. It may be slightly hazy. It may be colourless. The nanoparticles of the dispersion may be unaggregated. They may be aggregated. They may be partially aggregated. The particle size and distributions described above refer specifically to the nanoparticles themselves, and not necessarily to the aggregates. The shape, size and distribution of any aggregates formed may depend on the number of particles in the aggregates, which may depend, inter alia. on the solvent used and the reaction conditions used. In the case of aggregated nanoparticles. the mean aggregation number may be about 2 to about 1000, or about 2 to 200, 2 to 100, 2 to 50. 2 to 20. 2 to 10. 10 to 1000, 100 to 1000, 500 to 1000, 10 to 100, 100 to 500 or 10 to 50, e.g. about 2. 3, 4, 5, 6, 1, 8. 9, 10, 20. 30, 40, 50, 60, 70, 80. 90, 100. 200. 300. 400. 500. 600. 700. 800. 900 or 1000. The dispersion may comprise a solvent. The solvent may comprise a halogenated solvent, e.g. a chlorinated solvent. It may comprise methylene chloride and/or chloroform. It may be chloroform. It may comprise a short chain alcohol, e.g. methanol, ethanol. n-propanol, isopropanol, n-butanol or some other suitable alcohol. It may comprise an aromatic solvent, for example benzene, toluene, xylene, chlorobenzene, dichlorobenzene etc. It may comprise, or may be, a mixture of any two or more of the above.

Imidazolium salts (IMSs) are well known as room-temperature ionic liquids (RTILs) that can be used as electrolytes or green solvents because of their low vapour pressure and high chemical stability. IMSs may also be used as precursors for carbenes such as bisimidazolidine, with important applications in organic synthesis. They have been directly used as ligand precursors to coordinate with metals under heating. However. IMSs are seldom directly employed in materials synthesis. Herein, it is demonstrated that IMSs and thiols may be used together as mild reducing agents for the production of ultrafine gold nanoparticles.

Gold cations can form stable N-heterocyclic carbene-coordinated complexes in dry systems. In the present work. Au^"1 or Au^TJ was found to be easily reduced to gold nanoparticles in an imidazolium/thiol system. Unlike commonly used reduction processes involving borane or borohydridε. no strong reducing reagent was used in the present work. The gold nanoparticles were produced under very mild and robust conditions with remarkable efficiency. This simple synthesis for stable, ultrafine gold nanoparticles could be easily scaled up. and involved IMSs for the first time in the preparation of metal particles.

Various conditions have been examined in the synthesis protocol using imidazolium salt 1 (below). The ultrafine gold nanoparticles could be obtained in different solvent systems with or without base (potassium terf-butyloxide). However, the gold nanoparticles have a tendency to aggregate and precipitate from a methano I/water or basic solution. In contrast, use of pure CHCl₃ solvent led to a very stable and clear solution of ultrafine gold nanoparticles. The size of gold nanoparticles obtained by the process was not affected substantial!}' by changes in the reaction temperature between 20⁰C and 50⁰C. but no reaction occurred at < 0⁰C. Larger particles would be formed when low thiol/ Au ratios (< 1) were used (see Figure l(e)).

Different IMSs have been investigated. It was found that all IMSs could promote gold reduction with different activities. The reaction times for different IMSs under the standard conditions were in the following order: L 2 (seconds) < 3. 5 (about 5 min) < 4 (about 20 min) « 6 (days). This order suggested that N-heterocyclic carbene (NHC) derivatives with stronger electron donation ability and less steric hindrance would be more active for this reaction. However, it was also found that the benzyl substitution of IMS (i.e. 1 and 2) was important for deriving stable gold nanoparticle solutions. Finer gold nanoparticles (1-2 nm) were synthesized with 1 compared to 2 (< 3 run). When IMSs 3-5 were employed, aggregation of small particles to form cloudy suspensions were observed (Figure l(c)). Interesting!}", catalytic amounts of IMSs (1) (10 mol% of AuCl₃) could also promote the reduction of gold, and gold sol instead of clear solution was obtained in such cases.

To understand the synthesis mechanism, several control experiments were conducted. No reaction was observed in imidazolium/thiol system without AuCl;,. Au^"1" could form a stable solution with thiol or IMS. However, as a third component was added to each of these two solutions. Au^J~ would be reduced. Large amounts of 1- benzylimidazole. benzyl chloride (or bromide) and bisdodecanedisulfide were detected in the system with 1. but no similar compounds other than disulfide were found in the system with 3. 4 or 5. Based on these results, the following synthesis mechanism was proposed. Scheme 1

In Scheme 1, L may be Cl or Br. Also. Au is shown in certain of the structures as bonded to RSH. It will be understood that the bonding in fact links the sulfur atom of the thiol to Au, i.e. there is an S-Au bond in these structures. Amyes and coworkers (T. L. Amyes. S. T. Diver, J. P. Richard. F. M. Rivas, K.

Toth, J. Am. Chem. Soc, 2004. 126, 4366) have reported the carbon acid pKa of a series of imidazolium cations in aqueous solution. The equilibrium between imidazolium, carbene and bis-imidazolidine has also been studied. Scheme 1 proposed that the carbene derivative of 1 coordinated and reduced gold cation. This was followed by the decomposition to benzyl chloride (or bromide) and an intermediate A through a radical pathway. The thiol further reduced Au^J~ to Au⁰ and released benzylimidazolε. which played a very important role as a ligand generated in situ in coordination with and in protection of Au⁰ to form stable, ultrafine gold nanoparticles. This explained that slightly larger gold particles were formed with 2. due to the bulky benzylbenzoimidazole, which was less efficient in coordinating with and protecting gold nanoparticles. It also suggested a way to control the size of gold particles by the synthesis approach. In Scheme 2. the carbene derivative of 3 acted as a catalyst to promote the thiol reduction of Au^~J to Au⁰. Unlike the carbene with N-benzyl substitution, the carbene with N-alkyl or aryl substitution was much more stable, and no decomposition was observed. Since no efficient protection ligand for Au⁰ was generated in this reaction system, aggregated gold nanoparticles were produced.

Scheme 2

In conclusion, a simple and robust protocol for the synthesis of stable, ultrafine gold nanoparticles has been established using IMSs under mild conditions. The mechanism of this new protocol has also been examined. Example

AU solvents were used as received from commercial suppliers (HPLC grade). Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 electron microscope (200 kV). All syntheses were conducted in air by mixing AuCl₃ and thiol in the solvent, e.g. chloroform, methanol, toluene, or mixture of chloroform and methanol. The IMS solution (in the same solvent as the Au^J~ solution) was then added to the gold salt solution under stirring. The stirring was continued until the reduction was complete. Reaction products (e.g. benzylimidazole. benzyl chloride, and bisdodecanedisulfide) were analyzed by gas chromatography-mass spectrometry (GC- MS) (Shimadzu GCMS QP2010). and confirmed with nuclear magnetic resonance (NMR) spectrometry (Bruker AV -400 instrument).

Typically. 0.2 mmol Of AuCl₃-SH₂O was mixed with 0.4 mol of dodecanethiol in 20 ml of CHCl₃ to form a clear yellowish-brown solution. 0.6 mmol of IMS (13- dibenzylimidazolium bromide (I)) in 10 ml of CHCl₃ was added to the gold solution under stirring at room temperature. The solution would turn colorless in seconds to minutes. The small gold nanoparticles were well dissolved or dispersed in the solvent to form a clear solution or dispersion. TEM image (Figure 1) shows that uniform gold nanoparticles (1-2 run in size) were obtained.

Claims

Claims:

1. A process for preparing gold nanoparticles comprising combining a gold salt, an NJSP-disubstituted imidazolium salt and a thiol, under conditions whereby said gold salt. N.N'-disubstituted imidazolium salt and thiol interact to form the gold nanoparticles.

2. The process of claim 1 wherein the gold salt is a gold (III) salt.

3. The process of claim 1 or claim 2 wherein the gold salt is a gold halide.

4. The process of claim 3 wherein the gold halide is gold (III) chloride.

5. The process of any one of claims 1 to 4 wherein each nitrogen substituent on the N.N'-disubstituted imidazole is, independently, alkyl, benzyl or aryl. 6. The process of claim 5 wherein each nitrogen substituent on the N.N'- disubstituted imidazolium salt is. independently, selected from the group consisting of benzyl, methyl, n-butyl. isopropyl and 2,

6-diisopropylphenyl.

7. The process of any one of claims 1 to 6 wherein the N.N'-disubstituted imidazolium salt is an N,N^"-disubstituted imidazolium halide or an N.N'-disubstituted imidazolium tetrafluoroborate.

8. The process of claim 7 wherein the N.N'-disubstituted imidazolium salt is an N.N'-dibenzylimidazolium halide.

9. The process of any one of claims 1 to 8 wherein the thiol is a Cl to Cl 6 alkanethiol.

10. The process of claim 9 wherein the thiol is dodecanethiol.

11. The process of any one of claims 1 to 10 wherein the process is conducted in a solvent.

12. The process of claim 11 wherein the solvent is chloroform, methanol, toluene or a mixture of any two or more of these.

13. The process of any one of claims 1 to 1 1 comprising agitating the gold salt.

N.N^'-disubstituted imidazolium salt and thiol during and/or after said combining.

14. The process of any one of claims 1 to 13 additionally comprising separating the gold nanoparticles.

15. A dispersion of gold nanoparticles prepared by the process of any one of claims 1 to 14. said nanoparticles having a mean particle diameter of less than about 5nm.

16. The dispersion of claim 15 wherein the nanoparticles are substantially monodispersed.

17. The dispersion of 15 or claim 16 wherein the nanoparticles are substantially spherical.

18. Use of the dispersion of any one of claims 15 to 17 in a catalytic, imaging, photonic, electronic or energy application.

19. A process for preparing gold nanoparticles comprising combining a gold salt and an N-heterocyclic carbene (NHC) under conditions whereby said gold salt and NHC interact to form the gold nanoparticles.

20. The process of claim 19 wherein the gold salt and the NHC are combined in the presence of a thiol.

21. The process of claim 19 or claim 20 wherein the NHC is a polymeric NHC.