FIELD OF THE INVENTION

The present invention relates to a process for preparing anisotropic metal nanoparticles and to anisotropic metal nanoparticles thus obtained.

BACKGROUND

Today there is a great interest in the production of anisotropic metal nanoparticles with different morphologies; the production of nanofibers being one of the most important due to their application potentials in the preparation of nanocomposites based on non- metal materials (ceramics, polymers, glasses, etc.) in order to render metal properties to these materials. Applications such as new antistatic nanocomposites, nanocomposites for shielding against electromagnetic radiation, nanocomposites and nanocomposite liquids for heat transfer, etc. make this a topic of great importance in recent technology.

In addition, it is important to obtain the desired metal properties by introducing the lowest possible amount of metal particles in the non-metal matrices both for the cost and for not deteriorating the intrinsic properties of the matrices themselves. Due to the fact that the anisotropic geometries, such as the cylindrical-shaped fibers, allow achieving percolation with very low concentration thresholds, obtaining simple and scalable methods which allow controlling the size and shape of the nanoparticles is a current challenge of extraordinary importance. At the same time, for some important applications, like transparent conductive films (TCFs), diameters less than approximately 50nm, to reduce the scattering and haze, is of great importance.

Due to the high importance of Silver Nanofibers (Ag NF) for the manufacturing of TCFs, new methods must be developed for their preparation that meet the following requirements: high aspect ratios (> 200-300), small NF diameters (< 50nm), low synthesis temperatures (<140°C), ambient pressure and air conditions (i.e. no controlled atmosphere, e.g. oxygen or nitrogen).

From the art solution methods, the polyol method is the most popular (see for example the review paper from X. Li et al, Cryst. Res. Technol. 201 1 , 5, 427). It was firstly reported by Ducamp-Sanguesa et al (J. Solid State Chem. 1992, 100, 272) and later by Sun et al. (Chem. Mater. 2002, 14, 4736; Nano Lett. 2002, 2, 165) and it is based in the reduction of AgN0 ₃ in the presence of Pt seeds and polyvinylpyrrolidone (PVP) as protecting agent to avoid aggregation. During the last decade, the method was deeply studied and improved. For example, it is possible to obtain Ag Nanowires (Ag NW) without the addition of external crystal seeds (self-seeding or seeding-less process) (C.X. Kan et al. J. Phys. D 2008, 41 , 155304). In all these procedures, the temperature is a critical parameter: temperatures above 140°C are required in order to obtain a large amount of Ag nanowires (NW) or Ag NF; otherwise, other Ag types of nanoparticles are obtained.

Other protecting/capping agents different from PVP have also been reported for the synthesis of Ag NW even at low temperatures, but they do not meet some of the previous requirements. For example, using some of protecting/capping agents like cetyltrimethylammonium bromide (CTAB, N.R. Jana et al, Chem. Commun. 2001 , 617), Vitamin C (Y. Liu et al, Mater. Res. Bull. 2005, 40, 1796), Vitamin B2 (M.N. Nadagouda et al, J. Nanomater. 2008, 782358), dodecyl benzene sulfonic acid (DBS, G.J. Zhou et al, J. Cryst. Growth 2006, 289, 255), tetrabutyl ammoninum bromide (TBAB, S.H. Kim et al., J. Alloys Compd. 2007, 433, 261 ), sodium dodecylsulfonate (SDBS, L. Fan et al, Cryst. Growth Des. 2008, 8, 2150) and polyvinyl butyral (PVB, P.M. Chang et al, Acta Chim. Sin. 2009, 67, 523), AgNW / AgNF with small and not scalable amounts, with low crystallinity or with low aspect ratios or large diameters were reported.

US 2013/0255444 A1 describes a process for producing silver nanowires which comprises a polymer obtained by polymerizing polymerizable monomers containing monomers of a N-substituted (meth)acrylamide that reacts with a silver compound in a polyol at a temperature from 25 °C to 180 °C under nitrogen atmosphere.

JP 2013194290 discloses a process to obtain copper nanowires which comprises the use of a copolymer of polyethyleneimine and polyethylene glycol.

Linear and branched polyethyleneimine have been described in the synthesis of non- anisotropic silver nanoparticles (Ag-NPs) (Signori et al., Lagmuir, 2010, 26(22), 17772- 17779; and Santos et al., The Journal of Physical Chemistry C, 2012, 1 16, 4594-4604, Shin et al., Bull. Korean Chem Soc, 201 1 , 32, 7, 2469-2472; and Qu et al., The Journal of Physical Chemistry C, 2013, 1 17, 3548-3555). In these documents, the ethyleneimine to silver cation molar ratio is below 9.

BRIEF DESCRIPTION OF THE INVENTION The inventors of the present invention have found that by using polyethylenimine (PEI) as capping/reducing agent and controlling the reaction conditions, it is possible to obtain and control the formation of nanofibers, which fulfils all the previous requirements for the production of silver nanofibers for the manufacturing of TCF; the nanofibers have aspect ratios above 300 and diameters below 50 nm; the process can be performed at low temperatures, i.e. below 140 °C, at atmospheric pressure and in air conditions, i.e. without the need of inert atmospheres, therefore improving the scale-up processes.

Therefore, one aspect of the present invention relates to a process for preparing anisotropic metal nanoparticles comprising the step of reducing the transition metal cation of a salt to oxidation state zero in the presence of a solvent and a polyalkyleneimine or a copolymer where one of the copolymer units is selected from a polyalkyleneimine and the other unit is selected from the group consisting of alkides, polyesters, polyvinyl alcohol, polyvinyl acetate, polyacrylamides, polyacrylic acid and polyisocyanates, wherein the alkyleneimine to metal cation molar ratio is above 10.

Another aspect of this invention refers to anisotropic metal nanoparticles obtained by the process of the invention.

Another further aspect refers to the use of a polyalkyleneimine or a copolymer where one of the copolymer units is selected from a polyalkyleneimine and the other unit is selected from the group consisting of alkides, polyesters, polyvinyl alcohol, polyvinyl acetate, polyacrylamides, polyacrylic acid and polyisocyanates for preparing anisotropic metal nanoparticles.

These aspects and preferred embodiments thereof are additionally also defined in the claims.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. Ag nanofibers with an aspect ratio (r) equal to approximately 350 obtained in Example 1 .

Figure 2. Ag nanofibers with an aspect ratio (r) equal to approximately 1000 obtained in Example 2.

Figure 3. Ag nanofibers with an aspect ratio (r) equal to approximately 550 obtained in Example 3. Figure 4. Ag nanofibers with an aspect ratio (r) equal to approximately 590 obtained in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, the following terms have the meaning detailed below.

"Anisotropic metal nanoparticle" of the present invention refers to metal nanoparticles having the shape selected from the group consisting of nanofibers, nanotriangles, nanostars, nanodiscs, nanocubes, nanotetrahedrons or nanoprisms, preferable anisotropic metal nanoparticle in the present invention are nanofibers.

Nanofibers are nanoparticles having elongated shape in one direction; they can be found as well in the literature as nanocylinders, nanorods, nanowires or nanotubes, all these names are encompassed in the present invention for the term nanofiber. Particularly, the term nanofiber in the present invention relates to nanoparticles (nanocylinders, nanorods, nanowires or nanotubes) that have a diameter less than 200 nm and a length along their major axis from 0.2 to 1000 μηη.

The diameter of the nanofibers is preferably less than 150 nm, particularly desirably less than 100 nm and even more preferably less than 75 nm. Preferred embodiments have diameters between 10 and 70 nm, between 15 and 60 nm, and more preferably between 18 and 53 nm. The diameter is an arithmetic mean of the respective diameters of 100 silver nanofibers which may be obtained by observation with a scanning electron microscope.

The length along the major axis of the nanofibers of the invention is preferably between 1 and 800 μηι, preferably between 3 and 400 μηι, preferably between 5 and 200 μηι, preferably between 7 and 100 μηη.

The nanofibers of the present invention may be defined by their aspect ratio. "Aspect ratio" or "ratio" in the present invention refers to the following relation:

(length along the major axis)/(the diameter)

In a preferred embodiment the nanofibers of the present invention present an aspect ratio ranging from 300 to 10000. In other embodiments the nanofibers of the present invention present an aspect ratio ranging from 300 to 8000, from 300 to 6000, from 300 to 4000, from 300 to 3000, from 310 to 2000, from 320 to 1500, or from 330 to 1000. The other anisotropic nanoparticles of the present invention are nanodiscs, nanotriangles, nanosquares, nanostars, nanocubes, nanotetrahedrons and nanoprisms. Therefore, isotropic nanoparticles such as nanospheres are not encompassed in the present invention. The anisotropic nanoparticles of the invention are characterized in that one of the dimensions is less than 1000 nm. In some preferred embodiments, the smallest dimension is less than 500 nm, preferably is less than 250 nm, less than 100nm or even less than 50 nm.

The metal of the anisotropic metal nanoparticles is selected from a transition metal, preferably a transition metal selected from the groups 10 and 1 1 , preferably a transition metal selected from the group consisting of silver, gold, copper, palladium, platinum and nickel, more preferably a transition metal selected from the group consisting of silver, gold and copper, more preferably a transition metal selected from silver and gold, and most preferably the anisotropic metal nanoparticles are anisotropic silver nanoparticles.

In a preferred embodiment the anisotropic metal nanoparticles are silver, gold, copper, palladium, platinum or nickel nanofibers. In a more preferred embodiment the anisotropic metal nanoparticles are silver nanofibers.

In another preferred embodiment, the process of the present invention comprises the step of reducing a transition metal cation of a salt to oxidation state zero in the presence of a solvent and a polyalkyleneimine and/or a copolymer where one of the copolymer units is selected from a polyalkyleneimine.

In one embodiment of this process the solvent and/or the polyalkyleneimine and/or a copolymer where one of the copolymer units is selected from a polyalkyleneimine act as a reducing agent of the transition metal cation.

In a preferred embodiment, all the metal atoms of the anisotropic nanoparticle are in oxidation state zero.

The expression "the step of reducing" is the same as "reduction step" and in the present invention means that in that step a cation is reduced to oxidation state zero.

The "transition metal cation" is preferably a cation of a transition metal selected from the group consisting of silver, gold, copper, palladium, platinum and nickel, more preferably a cation of a transition metal selected from the group consisting of silver, gold and copper, preferably a cation of a transition metal selected from silver and gold, and more preferably the transition metal cation is a silver cation. The expression "transition metal cation of a salt" refers to salts that comprise the metal cation. The anion of the salt is not relevant for the present invention. Suitable anions include inorganic and organic anions. They are normally polyatomic oxyanions of non- metals. Non-limiting examples of anions of the metallic salt are nitrates, nitrites, oxides, oxalates, borates (including fluoroborates, pyrazolylborates, etc.), carbonates, phosphates, sulfates, chlorates, acetates, citrates and halides (e.g., fluorides, chlorides, bromides and iodides), azides, sulfonates, carboxylates (such as, e.g., formates, acetates, propionates, oxalates and citrates), substituted carboxylates (including halogenocarboxylates such as, e.g., trifluoroacetates, hydroxycarboxylates, aminocarboxylates, etc.) and salts and acids wherein the transition metal is part of the anion (such as, e.g., hexachloroplatinates, tetrachloroaurate, tungstates and the corresponding acids) as well as combinations of any two or more of the foregoing. A preferred anion is nitrate.

Therefore, possible transition metal salts useful in the present invention are silver nitrate, silver chloride, silver sulfate, silver sulfamate, silver chlorate, and silver perchlorate; gold nitrate, gold chloride, gold sulfate, gold sulfamate, gold chlorate, and gold perchlorate; copper nitrate, copper chloride, copper sulfate, copper sulfamate, copper chlorate, and copper perchlorate; and salts of organic acids such as silver acetate and silver lactate; gold acetate and gold lactate; copper acetate and copper lactate; and the correspondent nickel, palladium and platinum salts. In a preferred embodiment the silver cation is obtained from using one of the following silver salts: silver nitrate, silver nitrite, silver oxide, silver fluoride, silver hydrogen fluoride, silver carbonate, silver oxalate, silver azide, silver tetrafluoroborate, silver acetate, silver propionate, silver butanoate, silver ethylbutanoate, silver pivalate, silver cyclohexanebutanoate, silver ethylhexanoate, silver neodecanoate, silver decanoate, silver trifluoroacetate, silver pentafluoropropionate, silver heptafluorobutyrate, silver trichloroacetate, silver 6,6,7,7,8,8,8 heptafluoro-2,2-dimethyl-3,5-octanedioate, silver lactate, silver citrate, silver glycolate, silver glyconate, silver benzoate, silver salicylate, silver phenylacetate, silver nitrophenylacetate, silver dinitrophenylacetate, silver difluorophenylacetate, silver 2-fluoro-5-nitrobenzoate, silver acetylacetonate, silver hexafluoroacetylacetouate, silver trifluoroacetylacetonate, silver tosylate, silver triflate, silver trispyrazolylborate, silver tris(dimethylpyrazolyl)borate, silver beta-diketonate olefin complexes and silver cyclopentadienides as well as combinations of any two or more of the foregoing. In a more preferred embodiment the silver salt is selected from the group consisting of silver nitrate, silver nitrite, silver oxide, silver fluoride, silver hydrogen fluoride, silver carbonate, silver oxalate, silver azide, silver tetrafluoroborate. silver acetate, silver propionate, silver butanoate, silver ethylbutanoate or silver pivalate as well as combinations of any two or more of the foregoing.

In a most preferred embodiment the silver salt is a silver salt of inorganic acid, more preferably silver nitrate.

A "solvent" must be present during the reducing step. The solvent is preferably a polar solvent. Preferably the solvent is selected from the group consisting of aliphatic glycols, aliphatic, cycloaliphatic and aromatic alcohols, ether alcohols, aminoalcohols, esters, ethers, sulfoxides, ionic liquids, water and mixtures thereof. In a preferred embodiment the aliphatic, cycloaliphatic and aromatic alcohols are selected from methanol, ethanol, propanol, isopropanol, isobutanol, isopentanol, butanol, pentanol, cyclopentanol, hexanol, cyclohexanol, octanol, decanol, isodecanol, undecanol, dodecanol, tetradecanol, hexadecanol, benzyl alcohol, butyl carbitol and the terpineols. In a preferred embodiment the ether alcohols are selected from the monoalkyi ethers of diols such as, e.g., the Ci _-6 monoalkyi ethers of Ci _-6 alkanediols and polyetherdiols derived therefrom, preferably selected from the monomethyl, monoethyl, monopropyl and mono butyl ethers of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1 ,3-propanediol, and 1 ,4-butanediol such as, e.g., 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol and 2-butoxyethanol. In a preferred embodiment the aminoalcohols are selected from ethanolamine, amides such as, e.g., dimethylformamide, dimethylacetamide 2-pyrrolidone and N- methylpyrrolidone. In a preferred embodiment the ethers are selected from tetrahydrofuran and tetrahydropyran. In a preferred embodiment the esters are selected from ethyl acetate and ethyl formate. In a preferred embodiment the sulfoxide is dimethylsulfoxide. In a preferred embodiment the ionic liquids are selected from [BMIm][MeS04] (also called 1 -butyl-3-methylimidazolium methylsulfate or methanesulfonate)), 1 ,3-dimethylimidazolium 1 ,1 ,1 -trifluoro-N- [(trifluoromethyl)sulfonyl]methanesulfonamide, 1 -butyl-1 -methylpyrrolidinium 1 ,1 ,1 - trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide; 1 -butyl-3-methylimidazolium 1 ,1 ,1 -trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide; 1 -butyl-3- methylimidazolium bis(perfluoroethylsulfonyl)imide; 1 -butyl-3-methylimidazolium dicyanamide; 1 -butyl-3-methylimidazolium hexafluorophosphate; 1 -n-butyl-3- methylimidazolium hexafluorophosphate; 1 -butyl-3-methylimidazolium tetrafluoroborate; 1 -n-butyl-3-methylimidazolium tetrafluoroborate; 1 -butyl-3- methylimidazolium triflate; 1 -butyl-3-methylimidazolium trifluoroacetate; 1 - butylpyridinium 1 ,1 ,1 -trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide; 1 -ethyl- 3-methylimidazolium dicyanamide; 1 -ethyl-3-methylimidazolium ethyl sulphate; 1 -ethyl- 3-methylimidazolium tetrafluoroborate; 1 -hexyl-3-methylimidazolium 1 ,1 ,1 -trifluoro-N- [(trifluoromethyl)sulfonyl]methanesulfonamide; 1 -hexyl-3-methylimidazolium hexafluorophosphate; 1 -hexyl-3-methylimidazolium tetrafluoroborate; and 1 -methyl-1 - propylpyrrolidinium 1 ,1 ,1 -trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide. Preferably the ionic liquid is [BMIm][MeS04].

In a preferred embodiment the solvent is selected from the group consisting of ethylene glycol, propylene glycol, glycerol, water and mixtures thereof. In a particular embodiment the solvent is a mixture of ethylene glycol and water.

In one embodiment the solvent acts at the same time as solvent and as reducing agent. In a preferred embodiment of this invention the solvents that act at the same time as solvent and as reducing agent are selected from aliphatic glycols, aromatic alcohols, polyols, ketones, amides, amines, esters, ionic liquids and mixtures thereof. In one embodiment the aliphatic alcohols, polyols and/or glycols that act at the same time as solvent and as reducing agent are selected from methanol, ethanol, 1 - propanol, 2-propanol, 1 -butanol, 1 -pentanol, 2-pentanol, tert-butyl alcohol, tert-amyl alcohol, and cyclohexanol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, tetra-ethylene glycol, polyethylene glycol that is liquid at the reaction temperature, such as for example, polyethylene glycol 300, 1 ,2-propanediol, di- propylene glycol, 1 ,2-butanediol, 1 ,3-butanediol, 2,3-butanediol, 1 ,4-butanediol and glycerol, preferably is selected from ethylene glycol, propylene glycol, glycerol. In a preferred embodiment the aromatic alcohol that acts at the same time as solvent and as reducing agent is benzyl alcohol.. In another embodiment the ketone is selected from 3-hydroxybutanone, 2,3-butanedione and methyl isobutylketone. In another embodiment the amide is selected from Ν,Ν-dimethylformamide (DMF) and formamide. In another embodiment the amine is oleylamine. In another embodiment the ester is (-)- ethyl-L-lactate. In another embodiment the ionic liquid is [BMIm][MeS04].

In a reducing step it is essential the presence of a reducing agent. In this invention, said reducing agent is the polyalkyleneimine and/or a copolymer where one of the copolymer units is selected from a polyalkyleneimine. However, in a preferred embodiment of the present invention there is at least a further reducing agent in the reaction media of the reduction step.

In a preferred embodiment the further reducing agent is selected from the group consisting of polyalkyleneimine, a copolymer where one of the copolymer units is selected from a polyalkylenimine; an organic reducing agent, in particular an organic reducing agent selected from the group consisting of ascorbic acid, oxalic acid, formic acid, diethyl 1 ,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate, tributylstannane, tributyltin hydride, trichlorosilane, triethylphosphine, trimethylphoshpine, triphenylphosphine, triphenylphosphite, triethylsilane, tris(trimethylsilyl)silane; an inorganic reducing agent; in particular an inorganic reducing agent selected from the group consisting of: sodium borohydride, hydrazine, lithium and aluminium hydride, hydroxylamine, sodium hypophosphite, Li, Na, and K metals, hydrogen, ammonia, tetrahydroborates, diborane, diisdobutylaluminium hydride, sulfite compounds, phosphite compounds, compounds containing the Sc ³⁺ ion, compounds containing the Ti ³⁺ ion, compounds containing the Mn ³⁺ ion, compounds containing the Sn ²⁺ ion, compounds containing the Fe ²⁺ ion, such as iron (II) acetylacetonate, and carbon; a solvent that acts both as solvent and as reducing agent selected from the list commented above; sugars and sugar alcohols; and combinations thereof.

Therefore, the reducing agent is a polyalkyeneimine and/or a copolymer where one of the copolymer units is selected from a polyalkyleneimine which, as commented, is present in the reaction media of the present invention. However, in one embodiment of the invention further reducing agents may be present as listed above.

Polyalkyleneimine refers to a polymer having a repeating unit composed of an amine group and an alkyl spacer. In a preferred embodiment the polyalkyleneimine is a substituted or unsubstituted, linear, branched or dendrimeric polyalkyleneimine selected from the group consisting of: polyethyleneimine, polypropyleneimine, polybutyleneimine polypentyleneimine, polyhexyleneimine, polyheptyleneimine and polyoctyleneimine. In a preferred embodiment the polyalkyleneimine is a substituted or unsubstituted, linear, branched or dendrimeric polyethyleneimine (PEI) or polypropyleneimine. In a more preferred embodiment the polyalkyleneimine is a substituted or unsubstituted, linear, branched or dendrimeric polyethyleneimine (PEI). In a more preferred embodiment is unsubstituted branched polyethyleneimine. Unsubstituted polyalkyleneimines do not present substituents nor in the alkyl neither in the amino group.

Substituted polyalkyleneimines present at least one substituent in the alkyl and/or in the amino group. The substituents are selected from: alkyl selected from methyl, ethyl, propyl and butyl;-OH; and hydroxyalkyl selected from hydroxymethyl, hydroxyethyl, hydroxypropyl and hydroxybutyl.

Linear polyalkyleneimines contain all secondary amines, in contrast to branched polyalkyleneimines which contain primary, secondary and tertiary amino groups. Totally branched polyalkyleneimines are named dendrimeric polyalkyleneimines.

In a preferred embodiment the polyalkyleneimine has an average molecular weight in a range of 800 to 1000000, of 1200 to 800000, of 1800 to 500000, of 2000 to 250000, of 3000 to 100000, of 4000 to 75000, of 5000 to 50000, of 6000 to 30000, preferably of 8000 to 28000.

A "copolymer where one of the copolymer units is selected from a polyalkyleneimine" is selected from random copolymers, block copolymers and graft copolymers wherein one unit the polyalkyleneimine is as defined above, preferably polyethyleneimine, and the other unit may be selected from alkides, polyesters, polyvinyl alcohol, polyvinyl acetate, polyacrylamides, polyacrylic acid and polyisocyanates.

The "alkyleneimine to metal cation molar ratio" refers to the molar ratio between alkyleneimine monomers of the polyalkyleneimine or copolymer of polyalkyleneimine and the metal cation. The molar amount of the alkyleneimine is calculated by dividing the mass (in grams) of the polyalkyleneimine used by the molecular weight of the alkyleneimine monomer unit. In the embodiment where a copolymer of a polyalkyleneimine is used, the molar amount of the alkyleneimine is calculated by dividing the mass (in grams) relative to the polyalkyleneimine portion of the copolymer by the molecular weight of the alkyleneimine monomer unit.

In a preferred embodiment where PEI is the polyalkyleneimine, the "ethyleneimine to metal cation molar ratio" is calculated dividing the mass (in grams) of PEI used by the molecular weight of the monomer unit, in this case -(CH2-CH2-NH)-, i.e. 43.04 Da. In a preferred embodiment, silver is the metal cation (Ag ⁺ ) and the moles of the silver cation are equivalent to the moles of silver in the salt. In a preferred embodiment the alkyleneimine to metal cation molar ratio is in the range between above 10 and 1000, preferably between 12 and 500, preferably between 15 and 100, preferably between 20 and 50, and more preferably around 25.

In a preferred embodiment the ethyleneimine to Ag ⁺ molar ratio is in the range between above 10 and 1000, preferably between 12 and 500, preferably between 15 and 100, preferably between 20 and 50, and more preferably around 25.

In a preferred embodiment in the process of the invention the reduction step reaction temperature should preferably be below 140 °C, preferably below 130 °C, preferably below 120 °C, and more preferably below 1 10 °C. In a particular embodiment, the reaction temperature should preferably range from room temperature, or 25 °C, to 140 °C, from 50-1 10 °C, from 60 °C-105 °C.

In a preferred embodiment a reducing catalyst is present in the reaction media of the reduction step. Preferably said catalyst is a halide ion selected from fluoride, chloride and bromide, more preferably chloride. The chloride ions employed by the present invention may be formed by dissolving inorganic salts or organic salts in the polar solvent of the reaction media. Specific examples of the salts from which chloride ions are formed may include: alkaline metal chlorides such as lithium chloride, sodium chloride, and potassium chloride; alkaline earth metal chlorides such as magnesium chloride and calcium chloride; earth metal chlorides such as aluminum chloride; chlorides of zinc group metals such as zinc chloride; chlorides of carbon group metals such as tin chloride; chlorides of transition metals such as manganese chloride, iron chloride, cobalt chloride, and zirconium oxychloride; amine hydrochlorides such as ammonia hydrochloride, which may also be called ammonium chloride, hydrazine hydrochloride, methylamine hydrochloride, dimethylamine hydrochloride, triethylamine hydrochloride, ethylamine hydrochloride, diethylamine hydrochloride, triethylamine hydrochloride, propylamine hydrochloride, dipropylamine hydrochloride, tripropylamine hydrochloride, butylamine hydrochloride, dibutylamine hydrochloride, tributylamine hydrochloride, pentylamine hydrochloride, hexylamine hydrochloride, ethanolamine hydrochloride, diethanolamine hydrochloride, triethanolamine hydrochloride, dimethylethanolamine hydrochloride, methyldiethanolamine hydrochloride, cyclohexylamine hydrochloride, ethylenediamine hydrochloride, diethylenetetramine hydrochloride, triethylenepentamine hydrochloride, anilinium chloride, toluidine hydrochloride, glucosamine hydrochloride, and acetamidine hydrochloride; amino acid hydrochlorides such as alanine hydrochloride, arginine hydrochloride, lysine hydrochloride, cysteine hydrochloride, glutamic acid hydrochloride, ornithine hydrochloride, and cystine dihydrochloride; and phosphonium chlorides such as tetrabutylphosphonium chloride, methoxy-methyl triphenylphosphonium chloride, and benzyltriphenyl-phosphonium chloride. Among them, lithium chloride, sodium chloride, zirconitllll oxychloride, methoxymethyl triphenylphosphonium chloride, and ammonium chloride are desirable. In a preferred embodiment, the inorganic salt containing the reducing catalyst in the reaction media of the reduction step is sodium chloride.

The "reaction media" refers to the physical environment that encompasses all the appropriate conditions for starting the reduction step.

In a particular embodiment a viscosity enhancer is also present in the reaction media of the reduction step. Viscosity enhancers useful for the present invention are selected from the group consisting of natural gums such as AGAR, acacia, tragacanth, sodium alginate, alkali-soluble latex, karaya, guar gum, etc; cellulose derivatives such as carboxymethyl cellulose, sodium carboxymethylcellulose, carboxymethyl guar, carboxymethyl hydroxypropyl guar, carboxymethylhydroxyethyl cellulose, sodium carboxymethyl hydroxyethylcellulose, methylcarboxymethyl cellulose, carboxymethyl starch, sodium alginate, alkali-soluble latex, and combinations thereof; microcrystalline cellulose; chitosan; synthetic polymers such as anionic acrylamide copolymer, amphoteric acrylamide copolymer, polyacrylic acid, acrylic acid copolymer, polyvynil pyrrolideone, polyvinyl alcohol and combinations thereof; clays such as magnesium aluminum silicate, bentonite, attapulgite; triethyl phosphate, carboxymethyl starch; and mixtures thereof.

In a particular embodiment an atomic quantum cluster (AQC) is also present in the reaction media of the reduction step, the AQC are known in the art as particles consisting in a material formed exclusively by zero-oxidation-state transition metal atoms with less than 200 metal atoms and with a size of less than 2 nm.

The zero-oxidation-state transition metal atoms of the AQCs present in the invention are selected from Au, Ag, Co, Cu, Pt, Fe, Cr, Pd, Ni, Rh, Pb and combinations thereof. Preferably the transition metal atoms are selected from Cu, Ag, Au, Pt, Pd, Ni and combinations thereof and more preferably are selected from Cu, Au and Ag zero- oxidation-state atoms.

In one embodiment the AQCs are formed by between 2 and 55 zero-oxidation-state transition metal atoms. In another embodiment, the AQCs consist of between 2 to 27 zero-valent transition metal atoms. In a further embodiment the AQCs consist of between 2 to 15 zero-valent transition metal atoms. In another further embodiment the AQCs consist of between 2 to 5 zero-valent transition metal atoms.

In another embodiment the mean size of the AQCs is between 0.3 nm and 1 .2 nm, in a particular embodiment the size is less than 1 nm. In a preferable embodiment they have an approximate size between 0.3 nm and 0.9 nm, and in another embodiment between 0.3 nm and 0.5 nm.

In another embodiment tetrabutylammonium bromide is also present in the reaction media. Tetrabutylammonium bromide improves the process for preparing anisotropic metal nanoparticles. Without being bound to any particular theory, tetrabutylammonium bromide improves the process of the present invention by helping to stabilize the initial seeds that are formed in the reaction media, resulting in a more homogeneus size distribution. Anisotropic metal nanoparticles are also formed when tetrabutylammonium bromide is not present in the reaction media, however said nanoparticles show a higher heterogeneity on their diameter and length.

In a further embodiment the invention relates to a process for producing anisotropic metal nanoparticles comprising allowing a polyalkyleneimine and/or a copolymer where one of the copolymer units is selected from a polyalkyleneimine to react with a transition metal cation of a salt in the presence of a polar solvent, a reducing catalyst, and a reducing agent.

In a particular embodiment, another capping agent may be present in the reaction media. The further capping agent is select from the group consisting of:

(a) monoethylenically unsaturated carboxylic acids of from about 3 to about 8 carbon atoms and salts thereof, such as, for example, acrylic acid, methacrylic acid, dimethylacrylic acid, ethacrylic acid, maleic acid, citraconic acid, methylenemalonic acid, allylacetic acid, vinylacetic acid, crotonic acid, fumaric acid, mesaconic acid and itaconic acid; the monomers of group (a) can be used either in the form of the free carboxylic acids or in partially or completely neutralized form; for the neutralization alkali metal bases, alkaline earth metal bases, ammonia or amines, e.g., sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, magnesium oxide, calcium hydroxide, calcium oxide, ammonia, triethylamine, methanolamine, diethanolamine, triethanolamine, morpholine, diethylenetriamine or tetraethylenepentamine may, for example, be used; (b) the esters, amides, anhydrides and nitriles of the carboxylic acids stated under (a) such as, e.g., methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl acrylate, hydroxyethyl acrylate, 2- or 3-hydroxypropyl acrylate, 2- or 4- hydroxybutyl acrylate, hydroxyethyl methacrylate, 2- or 3-hydroxypropyl methacrylate, hydroxyisobutyl acrylate, hydroxyisobutyl methacrylate, monomethyl maleate, dimethyl maleate, monoethyl maleate, diethyl maleate, maleic anhydride, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, acrylamide, methacrylamide; N,N-dimethylacrylamide, N- tert-butylacrylamide, acrylonitrile, methacrylonitrile, 2-dimethylaminoethyl acrylate, 2- dimethylaminoethyl methacrylate, 2-diethylaminoethyl acrylate, 2-diethylaminoethyl methacrylate and the salts of the last-mentioned monomers with carboxylic acids or mineral acids;

(c) acrylamidoglycolic acid, vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate and acrylamidomethylpropanesulfonic acid and monomers containing phosphonic acid groups, such as, e.g., vinyl phosphate, allyl phosphate and acrylamidomethylpropanephosphonic acid; and esters, amides and anhydrides of these acids;

(d) N-vinyllactams such as, e.g., N-vinylpyrrolidone, N-vinyl-2-piperidone and N- vinylcaprolactam;

(e) vinyl acetal, vinyl butyral, vinyl alcohol and ethers and esters thereof (such as, e.g., vinyl acetate, vinyl propionate and methylvinylether), allyl alcohol and ethers and esters thereof, N-vinylimidazole, N-vinyl-2-methylimidazoline, and the hydroxystyrenes; and

(f) mixtures thereof.

The nanoparticles obtained by the process of the invention are attached to at least one polyalkyleneimine molecule or to a copolymer where one of the copolymer units is selected from a polyalkyleneimine. Preferably the polyalkyleneimine is polyethyleneimine (PEI).

The following examples are merely illustrative of certain embodiments of the invention and cannot be considered as restricting it in any way.

EXAMPLES

Example 1. Synthesis of Ag nanofibers with an aspect ratio (r) equal to approximately 350 (T129) To a 0.5L round bottom flask with mechanical stirring was added ethylene glycol (221 g) at 90 °C, branched PEI (MW:25000) (2.4 g), Cu AQCs (1 x10 ^"3 mg), 12mM NaCI solution (2.8ml_), tetrabutylammonium bromide (50μΙ_) (concentration^ 97, 5g/L) and AgN0 ₃ (0.75g) dissolved in ethylene glycol (27.6 g). The reaction is constantly stirred for 67 hours. Ag nanofibers with average diameter of 43±14 nm and average length of 16±6 μηη are obtained as shown in Figure 1. The ethyleneimine (55.76 mmol of monomeric unit) to silver cation (4.41 mmol) molar ratio ([PEI]/[Ag ⁺ ]) is equal to 12.63.

Example 2. Synthesis of Ag nanofibers with an aspect ratio (r) equal to approximately 1000 (T130)

To a 0.5L round bottom flask with mechanical stirring was added ethylene glycol (221 g) at 100 °C, branched PEI (MW:25000) (4.8 g), Cu AQCs (1x10 ^"3 mg), 12mM NaCI solution (2.8ml_), tetrabutylammonium bromide (100 μΙ_) (197,5g/L) and AgNO ₃ (0.75g) dissolved in ethylene glycol (27.6 g). The reaction is constantly stirred for 71 hours. Ag nanofibers with average diameter of 22±8 nm and average length of 22±9 μηη are obtained as shown in Figure 2. The ethyleneimine (1 1 1.50 mmol of monomeric unit) to silver cation (4.41 mmol) molar ratio ([PEI]/[Ag ⁺ ]) is equal to 25.26.

Example 3. Synthesis of Ag nanofibers in a mixture of ethylene glycol and water

To a 0.5L round bottom flask with mechanical stirring were added 200 mL of a mixture of 50% in volume of ethylene glycol and water at 70 °C, and branched PEI (MW:25000) (4.8 g) dissolved in 25 mL of a mixture of 50% in volume of ethylene glycol and water. Cu AQCs (1 x10 ^"3 mg), 12mM NaCI solution (2.8mL), tetrabutylammonium bromide (Ι ΟΟμΙ.) (197,5g/L) and AgN0 ₃ (0.75g) dissolved in 25 mL of a mixture of 50% in volume of ethylene glycol and water were subsequently added. The reaction was constantly stirred for 24 hours. Ag nanofibers with average diameter of 51 ±17 nm and average length of 28±13 μηη are obtained as shown in Figure 3. The ethyleneimine (1 1 1.50 mmol of monomeric unit) to silver cation (4.41 mmol) molar ratio ([PEI]/[Ag ⁺ ]) is equal to 25.26.

Example 4. Synthesis of Ag nanofibers without AQCs

To a 0.5L round bottom flask with mechanical stirring was added ethylene glycol (221 g) at 90 °C, branched PEI (MW:25000) (4.8 g) dissolved in 27.6 g of ethylene glcol, 12mM NaCI solution (2.8ml_), tetrabutylammonium bromide (100 μΙ_) (197,5g/L) and AgN0 ₃ (0.75 g) dissolved in ethylene glycol (27.6 g). The reaction is constantly stirred for 45 hours. Ag nanofibers with average diameter of 22±7 nm and average length of 13±6 μηη are obtained as shown in Figure 4. The ethyleneimine (1 1 1.50 mmol of monomeric unit) to silver cation (4.41 mmol) molar ratio ([PEI]/[Ag ⁺ ]) is equal to 25.26.