PROCESS FOR PRODUCING STABILISED METAL NANOPARTICLES

The present invention relates to a process for producing metal nanoparticles, the metal colloid solution obtained as an intermediate in said process and the high metal content stabilised metal nanoparticles obtained as the final product of said process.

Metal nanoparticles have many different applications in areas such as decoration, catalysis, optoelectronics and biotechnology. Various techniques are known for their formation including chemical reduction of metal salts and electrochemical methods. Metal salts previously used in the production of metal nanoparticles have included chloroauric acid (Aslam et al., J. Mat. Chern. , 2004, 14, 1795 and Osterloh et al., Chem. Mat, 2004, 16 (13) 276), silver acetate (Nakamoto et al., Kagaku Kogyo, 2004, 55(12) 943 and Osterloh et al., Chem. Mat, 2004, 16 (13) 276) and silver nitrate (Nakamoto et al., Shikizai Kyokaishi, 2005, 78(5) 221).

Stabilisers, such as ligands, polymers and surfactants are often used in an effort to reduce nanoparticle agglomeration. One example of the use of ligands to stabilise the surface of nanoparticles is gold nanoparticles stabilised with thiols, as formed by the Brust method (Brust et al., J. Chem. Soc. Commun., 1994, p. 801). More recently stabilised nanoparticles have been made using long chain alkylamines in place of thiols in various methods including a one-pot aqueous synthesis (Aslam et al., J. Mat Chem., 2004, 14, 1795) and a two -phase aqueous/organic synthesis in the presence of NaBH ₄ (Leff et al., Langmuir, 1996, 12, 4723). The former method results in nanoparticles of greater than or equal to 9 nm in diameter, whilst the latter method has been shown to result in gold chloride ions being present at the surface of the nanoparticles (Kumar et al. Langmuir, 2003, 19, 6277).

We have discovered a new process that uses metal salts to make high metal content stabilised metal nanoparticles. This process does not use sulphur containing precursors and therefore does not lead to the generation of sulphurous gases during

application of the nanoparticles. Unlike most methods that do not use sulphur containing precursors our method produces a stable metal colJoid solution as an intermediate of the process allowing the solution to be stored for long periods of time, e.g. several months, and/or transported away from the site of manufacture to a different site for application, if necessary. Additionally, this process does not use halide containing precursors and therefore does not produce insoluble products. We have found that halides are restrictive to producing a wide range of nanoparticles due to insolubility problems, most likely resulting from side reactions causing decomposition to insoluble metallic products.

According to one aspect, the invention provides a process for making high metal content stabilised metal nanoparticles, which process comprising decomposing at least one metal acetylide in the presence of a first solvent under reducing conditions to yield a metal colloid solution and then recovering metal nanoparticles as a precipitate by either:

(a) evaporating the first solvent; or

(b) adding a second solvent to the first solvent.

The use of metal acetylides according to the above process results in near quantitative yields of stable derivatised amine nanoparticles with different types of amines.

Herein the term "high metal content" means that the metal content of the metal nanoparticles is greater than or equal to 65 wt%, for example 75 wt%.

Commonly the at least one metal acetylide is decomposed by carrying out the reaction at a temperature of from 70 ⁰ C to 200 ⁰ C. Alternatively, the at least one metal acetylide can be decomposed using a chemical reductant, by cathodic reduction or an electrochemical reductant, or by exposure to electromagnetic radiation, e.g. UV or visible light.

The process described above uses at least one metal acetylide. The at least one metal in the at least one metal acetylide can be selected from the platinum group metals and the coinage metals. Herein the platinum group metals comprise the metals ruthenium, rhodium, palladium, osmium, iridium and platinum, whilst coinage metals

comprise the metals silver, gold and copper. Commonly the at least one metal acetylide used in the present invention will comprise one or more of silver, gold and copper.

The inventors have found that the copper compounds are air sensitive, therefore when the process involves the use of at least one copper acetylide the present invention should be carried out under an inert atmosphere. Reactions not involving the use of at least one copper acetylide may be carried out in air.

The at least one acetylide group in the at least one metal acetylide used in the process of the invention can be selected from the list consisting of 1-dodecyne, 1-decyne, 1-nonyne, 1-octyne, 3-methyl-octyn-3-ol, l-octyn-3-ol, 1-ethynyl cyclohexanol, 10-undecynoic acid, 1-ethynyl cyclohexyl acetate and dehydrolinalool.

In one embodiment of the invention the first solvent comprises a substantially non water-miscible solvent, for example one or more selected from the group consisting of xylene, Shellsol (a C9 aromatic hydrocarbon mixture available from Shell chemicals), toluene, mesitylene, triethylamme, dioxane, cyclohexanone, 4-methyl-2-pentanone, cyclohexanol, dimethylacetamide and dimethylformamide. In an alternative embodiment the first solvent is water, thereby offering an aqueous route to the production of aqueous metal colloids.

If used, the second solvent may comprise a solvent with slight organic character, for example one or more selected from the group consisting of methanol, ethanol, iso-propanol and acetone. Alternatively the second solvent may comprise acetonitrile or a short-to-medium chain hydrocarbon solvent such as hexane.

According to another aspect of the invention, a metal colloid solution may be obtained as an intermediate in the process described above. Such a solution may absorb UV light in the wavelength range of from 510 to 540 iim for gold containing solutions, from 395 to 425 nm for silver containing solutions and from 555 to 585 nm for copper containing solutions.

One advantage of this intermediate metal colloid solution is that it can remain stable for a period of 3 months or more, e.g. 6 months in storage. Another advantage is that the intermediate metal colloid solution is concentrated, typically containing from 5% to 70% metal by weight. Both of these properties mean that the intermediate may readily be transported thereby enabling production of high metal content stabilised metal nanoparticles as and when needed, either at the same site or at a different site from where the metal colloid solution was produced.

Another aspect of the invention embodies the high metal content stabilised metal nanoparticles obtained by the process of the present invention. The nanoparticles can range in diameter from 2 to 10 nm, commonly from 2 to 6 nm.

In order that the invention may be more fully understood the following Examples are provided by way of illustration only:

EXAMPLE l Preparation of Gold Dodecyne

9.0 g (0.085 moles) of ethyl 2-hydroxy ethyl sulphide, in 40 ml of ethanol, was stirred with 20.0 g (0.041 moles, based on gold) of chloroauric acid solution, in 40 ml of water and 80 ml of ethanol, until virtually colourless (approx. 10 minutes). 6.7 g (0.040 moles) of 1-dodecyne in 40 ml of ethanol was added to the solution, followed by 100 ml of ethanol to ensure complete solubility of the 1-dodecyne. 22.0 g (0.268 moles) of sodium acetate dissolved in 60 ml of water and 50 ml of ethanol was then stirred into the solution to produce a precipitate of gold dodecyne. The precipitate was white with a lemon yellow tinge. The precipitate was stirred for an hour with approx. 100 ml of water added towards the end of the stirring time to ensure complete precipitation of the product, which is slightly soluble in methanol. The precipitate was collected by vacuum filtration then washed with approx. 500 ml of water and approx. 150 ml of methanol before being air-dried to produce a quantitative yield of white coloured product.

Gold dodecyne is a stable compound that did not discolour after several months of storage in a refrigerator.

EXAMPLE 2 Reaction of Gold Dodecyne

10.6g of gold dodecyne was dissolved in 90 ml of warm xylene to yield a clear yellow solution. This solution was then filtered to remove any residual gold metal and re-heated. At a temperature of 70 ⁰ C a light brown solution formed, becoming slightly pink in colour by 75 ⁰ C before darkening in colour to form a clear dark brown -black solution by 120 °C and finally a bright red concentrated colloid at 140 ⁰ C. This colloid remained concentrated and bright red on cooling to room temperature. Crude nanoparticles were collected by allowing the solution to evaporate to form a paste which was then dissolved in the minimum amount of triethylamine to form a concentrated red solution, vacuum filtered and re-precipitated by addition of approx. 300 ml of methanol before being air-dried to produce dark brown coloured nanoparticles.

EXAMPLE S Preparation of Silver Dodecyne

15.0 g (0.088 moles) of silver nitrate was dissolved in 300 ml of water and 150 ml of methanol was then added to the solution. Approx. 12 ml of concentrated ammonia was added dropwise to the rapidly stirred silver solution to precipitate and re-dissolve silver oxide, creating an ammoniacal silver nitrate solution.

15.0 g (0.090 moles) of 1 -dodecyne in 70 ml of methanol was then added to the ammoniacal solution and stirred for 30 minutes to form a whitish oil and some solid. Once the stirring stopped, the oil and solid settled allowing for half the colourless supernatant solution to be decanted out. This was replaced with methanol and the stirring recommenced upon which the white oil soon transformed into a fluffy white solid. This solid was stirred for an hour before being collected by vacuum filtration then washed with approx. 600 ml of water and approx. 150 ml of methanol before being air-dried to produce a quantitative yield of bright white coloured product.

(Alternatively, 200 ml of additional methanol can be added to the settled oily solution described above without prior liquid removal to achieve the same result.)

Silver dodecyne is a stable compound that did not discolour after several months of storage in a refrigerator, whilst showing good stability in light in comparison to many silver compounds.

EXAMPLE 4 Reaction of Silver Dodecyne

14.0 g of silver dodecyne was dissolved in 120 ml of toluene to form a colourless solution with stirring. The solution was refluxed at a temperature of 88 ⁰ C for up to 3 hours to form an orange colloid. The solution was allowed to cool, then filtered and added to 400 ml of slowly stirred methanol then allowed to stand for an hour. The excess clear dirty orange liquid was decanted off to leave a black-purple oil The oil was purified by dissolution in the minimum amount of triethylamine and re-precipitation by addition of approx. 300 ml of methanol before being air-dried to produce blue/purple coloured nanoparticles.

EXAMPLE S Preparation of Gold Octyn-3-ol

This gold acetylide was prepared using the same method as Example 1 except that l-octyn-3-ol was used in place of dodecyne. Gold l-octyne-3-ol formed in high yield (90%) as a white coloured, light sensitive powder that remained stable after 2-3 days storage in the dark.

EXAMPLE 6 Reaction of Gold Octyn-3-ol

3.0 g of gold l-octyn-3-ol was dissolved in 60 ml of warm cyclohexanone to form an orange solution at 60 ⁰ C. When heated to 100 ⁰ C the solution blackened, becoming dark red in colour before forming a red colloid above 110 ⁰ C (stable up to a temperature of 140 ⁰ C). The solution was cooled in an ice bath, and the cooled

solution poured into excess methanol (approx. 300 ml) to form a precipitate of black nanoparticles. The nanoparticles were washed with 2 x 25 ml of methanol before being air-dried to produce metallic brown coloured product.

EXAMPLE 7 Preparation of Gold 3-Methyl-Octyn-3-ol

This gold acetylide was prepared using the same method as Example 1 except that 3-methyl-l-octyn-3-ol was used in place of dodecyne. Gold 3-methyl-octyn-3-oI formed in high yield, predominantly in the form of an orange oil with approx. 10% yield of a lemon yellow coloured powder. Both the oil and the powder remained stable after 2-3 days storage in a refrigerator.

EXAMPLE 8 Reaction of Gold 3-Methyl-Octyne-3-ol

The oil from Example 7 was mixed with 80 ml of triethylamine and 20 ml of xylene then heated over a water bath to dissolve the oil, which formed a clear orange solution. The solution quickly darkened to an opaque brown colour by 80 ⁰ C, before forming a red colloid after heating to 100 ⁰ C for approx. 50 mins. The solution was filtered on cooling and the contents poured into excess methanol (approx. 250 ml) to form a precipitate of black powder. The nanoparticles were washed with 2 x 25 ml of methanol before being air-dried to produce a free-flowing, non-tacky brown nanoparticulate powder.

EXAMPLE 9 Preparation of Gold Ethynyl Cyclohexanol

9.0 g (0.085 moles) of ethyl 2-hydroxy ethyl sulphide, in 20 ml of ethanol, was stirred with 20.0 g (0.041 moles, based on gold) of chloroauric acid solution in 40 ml of water and 80 ml of ethanol until a colourless solution was obtained. Approximately 10 g of 1 -ethynyl cyclohexanol was melted in an oven and 5.25g (0.042 moles) poured into 20 ml of ethanol. This solution was added to the gold

precursor solution. 20.0 g (0.243 moles) of sodium acetate dissolved in 60 ml of water and 40 ml of ethanol was then stirred into the mixed solution which caused an immediate colour change from colourless to yellow, followed by precipitate formation after a few minutes. The solution was stirred for 40 minutes at room temperature, during which time a bright yellow precipitate formed. The precipitate was collected by vacuum filtration then washed with approx. 500 ml water and approx. 150 ml methanol before being air-dried to produce a quantitative yield of bright lemon yellow coloured product.

Gold ethynyl cyclohexanol is a stable compound that did not discolour after 2-3 months of storage in the dark (even when not refrigerated).

EXAMPLE 10 Reaction of Gold Ethynyl Cyclohexanol

6.0 g of gold ethynyl cyclohexanol was dissolved in 120 ml of warm cyclohexanone to form an orange solution at 70 ⁰ C. When heated to 100 ⁰ C the solution blackened, becoming orange in colour at 120 °C before forming a red colloid above 140 ⁰ C (stable up to a temperature of 150 ⁰ C). The solution was cooled in an ice bath, and the cooled solution poured into excess methanol (approx. 300 ml) to form a precipitate of black nanoparticles. The nanoparticles were washed with 2 x 25 ml of methanol before being air-dried to produce a black coloured product.

EXAMPLE 11a Preparation of Gold Dodecyne-Ethynyl Cyclohexanol (50:50) Mixed Compound

9.0 g (0.085 moles) of ethyl 2-hydroxy ethyl sulphide, in 10 ml of ethanol, was stirred with 20.Og (0.041 moles, based on gold) of chloroauric acid solution, in 40 ml of water and 80 ml of ethanol, until virtually colourless (approx. 5 minutes). 50 ml of ethanol was then added to ensure the subsequent addition of dodecyne would totally solubilise.

3.6 g (0.020 moles) of 1- dodecyne and 2.7 g (0.020 moles) of 1 -ethynyl cyclohexanol Ln 10 ml of ethanol was added to the solution, followed by 20.0 g (0.244 moles) of sodium acetate dissolved in 60ml of water and 50ml of ethanol was

then stirred into the solution to produce a lemon yellow precipitate of gold dodecyne/ ethynyl cyclohexanol (50:50). The precipitate was stirred for 40 mins, then collected by vacuum filtration, and washed with approx. 200 ml water and approx. 80 ml methanol before being air-dried to produce a quantitative yield of lemon yellow coloured product.

Gold dodecyne -ethynyl cyclohexanol (50:50) is a stable compound that did not discolour after 2-3 months of storage in the dark and only discoloured slightly when stored in the light for a day.

EXAMPLE lib Preparation of Gold Dodecyne-Ethynyl Cyclohexanol (25:75) Mixed Compound

This gold acetylide was prepared using the same method as Example 9a except that 1.8 g (0.010 moles) of 1-dodecyne and 4.1 g (0.030 moles) of 1 -ethynyl cyclohexanol were used. The compound is brilliant yellow in colour and shows improved light stability in comparison to gold dodecyne and the 50:50 mixed compound prepared in Example 9a.

EXAMPLE lie Preparation of Gold Dodecyne-Dehydrolinalool (50:50) Mixed Compound

This gold acetylide was prepared using the same method as Example 9a except that 3.7 g (0.020 moles) of dehydrolinalool was used in place of the 1 -ethynyl cyclohexanol. The compound is light tan in colour. The stability of this product has not yet been determined.

EXAMPLE 12a Reaction of Gold Dodecyne-Ethynyl Cyclohexanol (50:50) Mixed Compound

2.5 g of gold dodecyne-ethynyl cyclohexanol (50:50) was dissolved in 40 ml of warm xylene to form a ginger-orange coloured solution. When heated to 90 ⁰ C the solution darkened, before forming a red colloid at 130 ⁰ C. The solution was cooled in an ice bath, and the cooled solution poured into excess methanol (approx. 150 ml) to

form a precipitate of black nanoparticles. The nanoparticles were washed with

2 x 25 ml of methanol before being air-dried to produce a slightly sticky black coloured product

EXAMPLE 12b Reaction of Gold Dodecyne-Ethynyl Cyclohexanol (25:75) Mixed Compound

This gold acetylide was reacted using the same method as Example 12a except that gold dodecyne-ethynyl cyclohexanol (25:75) was used in place of the gold dodecyne-ethynyl cyclohexanol (50:50).

EXAMPLE 12c Reaction of Gold Dodecyne-Dehydrolinalool (50:50) Mixed Compound

11.5 g of gold dodecyne-dehydrolinalool (50:50) was dissolved in 55 ml of Shellsol to form a clear brown solution. When heated to 70 ⁰ C the solution darkened, before forming a red colloid at 120 ⁰ C. The solution was cooled in an ice bath, and some of the solvent allowed to evaporate over a period of two days before being adding excess methanol (approx. 150 ml) to form a precipitate of black nanoparticles. The nanoparticles were washed with 2 x 25 ml of methanol before being air-dried to produce a slightly sticky black coloured product.

EXAMPLE 13 Preparation of Gold 10-Undecynoic Acid

1.20 g (0.011 moles) of ethyl 2-hydroxy ethyl sulphide, in 5 ml of ethanol, was stirred with 3.0 g (0.006 moles, based on gold) of chloroauric acid solution in 6 ml of water and 12 ml of ethanol until a colourless solution was obtained. 1.23g (0.007 moles) of 10-undecynoic acid in 10 ml of ethanol was added to the solution, followed by dropwise addition of 3.0 g (0.037 moles) of sodium acetate dissolved in 10 ml of water and 5 ml of ethanol. Initially a pink colloid formed but towards the end of acetate addition, and at a PH of 7-7.5, a white precipitate formed. The precipitate was stirred for 30 minutes then collected by vacuum filtration before being

washed with approx. 30 ml water and approx. 20 ml methanol and air -dried to produce a quantitative yield of a white coloured, light sensitive product.

Gold 10-undecynoic acid remained stable after 2-3 days storage in the dark.

EXAMPLE 14a Reaction of Gold 10-Undecynoic Acid in Dimethylacetamide

0.5 g of gold 10-undecynoic acid was dissolved with warming in 20 ml of dimethylacetamide and then heated to approx. 100-120 ⁰ C at which temperature a red colloid formed. The solvents were removed using a rotary evaporator and the resultant oil allowed to evaporate further over several days to form a red solid.

EXAMPLE 14b Reaction of 10-Undecynoic Acid in Water

0.5 g of gold 10-undecynoic acid was dissolved in 30 ml of warm water, which was then heated to boiling point to produce a bright red stable colloid. The water was allowed to evaporate overnight in a fume hood to form a bright red solid. The excess ligand was removed by washing with a warm organic solvent in which the nanoparticles are not soluble.

EXAMPLE 15 Preparation of Gold 1-Decyne

This gold acetylide was prepared using the same method as Example 1 except that 1-decyne was used in place of 1-dodecyne. Gold decyne formed in quantitative yield as a yellow coloured powder that remained stable after being stored for a week in the dark.

EXAMPLE 16

Reaction of Gold Decyne

5.0 g of gold decyne was dissolved in 60 ml of triethyleamine to yield an orange solution. This solution was then filtered to remove residual gold metal and reheated. When heated the solution changed colour from orange to orange -black before forming a red colloid at 120 ⁰ C. When allowed to cool a black precipitate of nanoparticles formed. These nanoparticles were washed with 2 x 25 ml of methanol before being air-dried to produce a black coloured product.

It is worth noting that the metal acetylides may be prepared and decomposed using electrosynthetic methods in addition to the chemical methods described above.

Elemental Analysis Results of Acetylides:

Elemental Analysis Results of NanopartJcles: