FIELD OF THE ART

The present invention relates to a process for producing atomic quantum clusters (AQCs).

STATE OF THE ART

The high catalytic activity of metal clusters (AQCs) of few atoms when compared with isolated atoms or nanoparticles is well known in the state of the art [A. Corma et al., Nature Chemistry, vol. 5, p. 775-781 , 2013]. Particularly, due to the potential applications of the atomic quantum clusters (AQCs) in the field of biosensors [Peyser, L.A.; Vinson, A.E.; Bartko, A.P.; Dickson, R.M., Science, 2001 , 291 ,103], electrocatalysis [Boyen H-G. et al. Science 2002,297, 1533], magnetism, photoluminescence or catalysis [Nano eng. Hydrogels for cell eng., 2012, Springer Netherlands, Ed. Bhushan, Bharat, pp. 2639-2648], the development of easy synthesis methods for producing AQCs in quantities which can be increased to large scale has arisen a great interest.

There are several methods for synthesizing stable AQCs that have been developed in the last years. In particular, there are two main approaches to the synthesis of metal clusters by soft chemical methods: i) top-down approaches by etching small nanoparticles with an excess of strong binding ligands; and ii) bottom-up approaches using strong binding ligands to inhibit the growth usually employing strong reducing agents [Nano eng. Hydrogels for cell eng., 2012, Springer Netherlands, Ed. Bhushan, Bharat, pp. 2639-2648]. However, the use of ligands usually required in both approaches may hinder some of the important properties of the AQCs, such as catalysis.

EP1914196 A1 (2008) from Universidad de Santiago reports a kinetic controlled method for producing stable AQCs that does not need the use of strong binding ligands or capping agents, wherein a metal salt or metal ion is reduced by a reducer, simultaneously maintaining a small rate constant and a low concentration of reagents. However, this method produces very low amounts of naked clusters (clusters without ligands) in the order of micromolar concentrations. In addition, the massive formation of nanoparticles when the reaction rate increases, due to the lack of ligands, impedes the massive production of naked AQCs by this method.

Therefore, despite the reported methods, there is still a need in the art for a new simple and scalable method for producing AQCs in high concentrations and with a high yield.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a scalable process for producing AQCs in the absence of ligands (naked AQCs) with an improved yield. The inventors of the present invention have developed a new process taking into account the different stability of AQCs and nanoparticles under oxidizing conditions. Thus, in a first aspect the invention is directed to a process for producing atomic quantum clusters (AQCs) comprising the following steps: a) providing a mixture comprising:

- a starting atomic quantum cluster in a picomolar to micromolar concentration,

- a metal salt,

- a polar solvent,

- a hole scavenger having a standard electrode potential lower than the higher occupied molecular orbital (HOMO) of the starting atomic quantum cluster,

wherein said metal salt and said hole scavenger are soluble in said polar solvent and do not react with each other;

wherein the number of equivalents of said hole scavenger is higher than the number of equivalents of the metal salt in the mixture;

b) applying a promoter to the mixture of step (a), wherein the promoter is a light radiation having energy equal or larger than the HOMO-LUMO gap of the starting atomic quantum cluster of the mixture of step (a); and

c) adding an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt; wherein the oxidant can be either added in the mixture of step (a), and/or added to said mixture during and/or after applying said promoter in step (b).

Further, in a second aspect the invention refers to a mixture comprising: atomic quantum cluster,

a metal salt,

an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt,

a hole scavenger having a standard electrode potential lower than the HOMO orbital of the atomic quantum cluster, and

a polar solvent, wherein the metal salt and the hole scavenger are both soluble in the mixture and do not react with each other, and

wherein the number of equivalents of hole scavenger in the mixture are higher than the number of equivalents of metal salt in the mixture.

FIGURES

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with the written description, serve to explain the principles of the invention. In the drawings:

Figure 1 : ESI-Mass spectra of the AQCs resulting from the process of the invention (Example 1 ).

Figure 2: UV-VIS spectra of the reaction mixture of example 1 at different times.

Figure 3: UV-VIS spectra of the reaction mixture of example 2 at different times.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a new and easy process for producing atomic quantum clusters with a high yield. Particularly, the process of the present invention allows obtaining naked AQCs in solution, in the absence of ligands and with yields of the order of 40%.

The process of the invention is a process for producing atomic quantum clusters (AQCs) comprising the following steps: a) providing a mixture of comprising;

- a starting atomic quantum cluster in a picomolar to micromolar concentration,

- a metal salt,

- a polar solvent, - a hole scavenger having a standard electrode potential lower than the higher occupied molecular orbital (HOMO) of the starting atomic quantum cluster, wherein said metal salt and said hole scavenger are soluble in said polar solvent and do not react with each other;

wherein the number of equivalents of said hole scavenger is higher than the number of equivalents of the metal salt in the mixture,

b) applying a promoter to the mixture of step a), wherein the promoter is a light radiation having energy equal or larger than the HOMO-LUMO gap of the starting atomic quantum cluster of the mixture of step a); and

c) adding an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt;

wherein the oxidant can be either added in the mixture of step a), and/or added to said mixture during and/or after applying said promoter in step b).

In a particular embodiment the oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt is in the mixture of step (a).

The term“clusters” refers to nanometric/sub-nanometric species consisting of well- defined structures of metal atoms with sizes below approximately 1-2 nm. Due to quantum effects, clusters present discrete energy levels and an increasing band gap as the size of the AQCs decreases

The term "atomic quantum cluster",“naked atomic quantum cluster” or "AQC" means, in accordance with the present invention, a group of two or more zero-valent transition metal atoms in the absence of any ligands. Thus the process of the invention is a process for producing atomic quantum clusters (AQCs) without ligands, i.e.: naked AQCs.

The atomic quantum clusters (AQCs) are reported in ES2277531 B2 and W02007/017550.

The atomic quantum clusters are also known as "metal quantum clusters" in the state of the art. The AQCs consist of identical (mononuclear clusters) or different (heteronuclear clusters) transition metals. The term“metal” in the context of the present invention refers to the elements of the periodic table known as“metal”, particularly “transition metal”, but it does not refer to the electrical behavior of said elements. The confinement of electrodes in the AQCs originates the quantum separation of the energy levels producing important changes in the properties of these materials, as reported in EP1914196A1. Thus, the metal atoms in the AQCs have a semiconductor-like or even insulating-like behavior.

The AQCs are represented as M _n , wherein M represents any zero-valent transition metal, and n represents the number of atoms. The number of atoms in the AQCs is less than 100 atoms, being the size of the AQCs of less than 1 to 2 nm.

The term“starting atomic quantum clusters” refers to the atomic quantum clusters that initiate the process of the invention. Moreover, the starting atomic quantum clusters act as catalysts in the process of the invention. In a particular embodiment the starting AQCs are formed by transition metals selected from: platinum (Pt), gold (Au), rhodium (Rh), iridium (Ir), palladium (Pd), ruthenium (Ru), osmium (Os), silver (Ag), copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), titanium (Ti), vanadium (V), chrome (Cr) or their bi and multimetal combinations. Preferably the metals of the AQCs are selected from Au, Ag, Cu, Pd and Pt or their bimetal combinations. More preferably the metals of the starting AQCs are selected from Au and Ag or their bimetal combinations; even more preferably the metals of the starting AQCs are Ag.

Suitable starting atomic quantum clusters include any AQC available in the market or obtained in the laboratory by methods known in the art. Moreover, some metal salts available in the market can already contain small amounts of AQCs, which can act as starting AQCs (Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001 ,291 , 103-106). However, a strict control of the amount of clusters present in the metal salt is recommended in order to get reproducible results.

According to the invention, the mixture provided in step a) contains starting atomic quantum cluster in a picomolar (1 x1 O ¹² M) to micromolar concentration (1 x1 O ⁶ M). In a preferred embodiment, the mixture of step a) of the process of the invention contains starting atomic quantum clusters in a concentration comprised between 1 x1 O ¹⁰ M to 1 x10 ⁷ M, preferably, between 1 x1 O ⁹ M and 1 x1 O ⁸ M, more preferably in a nanomolar concentration.

In the context of the present invention the term “metal salt” refers to a compound composed of a metallic cation (positively charged ions) and an anion (negative ion) so that the resulting net charge in the metal salt is zero. In a particular embodiment, the metal salt is the limiting reactant in the process of the present invention as understood in the art. In a particular embodiment the metal of the metal salt is selected from silver, platinum, palladium, gold, copper, iridium, rhodium, ruthenium, nickel, iron, cobalt, or their bi and multimetal combinations. Preferably the metal of the metal salt is selected from Au, Ag, Cu, Pd and Pt or their bimetal combinations; more preferably is Ag, Cu, Pd and Pt; even more preferably is Ag.

In a particular embodiment the metal of the metal salt and the metal of the starting AQCs is the same metal or is a different metal; preferably is a different metal. In another particular embodiment the metal of the metal salt and the metal of the starting AQCs is a different metal and therefore the metal of the produced AQCs is the same as the metal of the metal salt.

In a particular embodiment the metal of the metal salt and the metal of the starting AQCs is the same metal; preferably silver.

In one particular embodiment the metal salt is a silver salt, preferably a silver salt selected from silver bromate, bromite, chlorate, perchlorate, chlorite, fluoride, nitrate, nitrite, acetate, permanganate and mixtures thereof; preferably nitrate.

The metal salt and the hole scavenger of the reaction mixture of the process of the invention are soluble in the polar solvent and do not react with each other.

The mixture of step a) of the process of the invention further comprises a polar solvent wherein the metal salt and the hole scavenger are soluble. In a preferred embodiment, the polar solvent is selected from water, acetonitrile, chloroform, dichloromethane, acetic acid, toluene and mixtures thereof.

The term“hole scavenger” refers in the context of the invention to a sacrificial agent that is oxidized by the holes generated from the excitation of the starting AQCs. The hole scavenger in the process of the invention has a standard electrode potential lower than the HOMO orbital (higher occupied molecular orbital) of the starting AQCs, so that the hole scavenger provides an electron with a standard electrode potential sufficient to fill the hole generated in the starting AQCs upon applying the promoter to the reaction mixture.

The term “standard electrode potential” is well known in the state of the art and represents the measure of individual potential of a reversible electrode at standard state, i.e.: with solutes at an effective concentration of I moldrrf ³ , gases at a pressure of 1 atm. and at 25°C. The standard electrode potential is generally represented by E°. The standard electrode potential is also called reduction potential, since the higher the value of the standard electrode potentials, the easier it is for the element to be reduced (accept electrons); and thus, they are better oxidizing agents.

An approximate estimation of the HOMO orbital energy of the AQCs, EHOMO, can be done as was previously reported (J. Calvo, J. Rivas and M. A. Lopez-Quintela, in Synthesis of Subnanometric Nanoparticles, Encyclopedia of Nanotechnology , ed. B. Bharat., Springer Verlag, Dordrecht, 2012, 2639-2648; N. Vilar-Vidal, J. Rivas and M. A. Lopez-Quintela, ACS Catalysis , 2012, 2, 1693-1697) from the energy of the HOMO- LUMO gap (E _g ) of the AQCs, which can be either calculated by the Jellium model or experimentally by UV-vis absorption spectroscopy, and the Fermi level of the corresponding AQCs (EF), which in turns can be approximated by the Fermi level of the corresponding metal: EHOMO = - EF - ½ E _g . A more precise estimation of the AQC EHOMO can be made by ultraviolet photoelectron spectroscopy. The hole scavenger does not react with the metal salt in the process of the invention. Moreover, the hole scavenger, as well as the metal salt, is soluble in the reaction mixture of the process of the invention.

In a particular embodiment the hole scavenger is selected from a linear or branched alcohol having between 2 and 6 carbon atoms. Preferably, the hole scavenger is ethanol, propan-1-ol, isopropanol, butan-1-ol, butan-2-ol, isobutanol, 1 ,1-dimethyl- ethanol, pentan-1-ol, pentan-2-ol, pentan-3-ol, 2-methylbutan-1-ol, 3-methylbutan-1-ol, 3-methylbutan-2-ol, 2,2-dimethylpropan-1-ol, hexan-1-ol, hexan-2-ol, hexan-3-ol, 2- methylpentan-1-ol, 3-methylpentan-1-ol, 4-methylpentan-1-ol, 2-methylpentan-2-ol, 3- methylpentan-2-ol, 4-methylpentan-2-ol, 2-methylpentan-3-ol, 3-methylpentan-3-ol, 2,2- dimethylbutan-1-ol, 3,3-dimethylbutan-1-ol, 2,3-dimethylbutan-2-ol, 3,3-dimethylbutan- 2-ol, 2-ethylbutan-1-ol and mixtures thereof. In another particular embodiment the hole scavenger is selected from hydroquinone, iodide salt, oxalic acid, acetic acid, formic acid, sodium formate, sulfite and mixtures thereof.

Other suitable hole scavengers in the context of the invention include glycerol, vinylalcohol, polyvinylalcohol, alcohol amines such as triethanol amine, and mixtures thereof.

The number of equivalents of the hole scavenger in the mixture of step a) of the process of the invention is higher than the number of equivalents of the metal salt. The term“number of equivalents” refers to the number of moles of an ion in a solution multiplied by the valence of that ion.

In a particular embodiment, the mixture of step a) comprises: - between 1x10 ¹² M and 1x10 ⁶ M of atomic quantum clusters, preferably between 1x10 ¹⁰ M to 1x10 ⁷ M, more preferably between 1x10 ⁹ M and 1x10 ⁸ M,

- between 0.1 mM and 1 M of metal salt, preferably between 0.5 mM and 0.5 M, preferably between 1 mM and 0.05M,more preferably 10mM,

- between 1 mM and 10M of the oxidant, preferably between 10 mM and 1 M , more preferably 50mM,

- between 1 % v/v and 90%v/v of hole scavenger, preferably between 10%v/v and 60%, more preferably 40 %v/v, and

- between 10 % v/v and 99%v/v of polar solvent, preferably between 40% v/v and 90%, more preferably 60%v/v.

Moreover, the mixture of step a) may comprise an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt.; preferably over the standard electrode potential of the metal ion of the metal salt.

In a particular embodiment the oxidant in the process of the invention is selected from nitric acid, hydrogen peroxide, permanganate, perchlorate, ozone, persulfate, hypochlorite, chlorite, hypobromite, bromite, perchromate and mixtures thereof. Preferably, the oxidant in the process of the invention is selected from nitric acid or hydrogen peroxide.

In a more particular embodiment the mixture of step (a) consists of: a starting atomic quantum cluster in a picomolar to micromolar concentration, a metal salt, a polar solvent, a hole scavenger having a standard electrode potential lower than the higher occupied molecular orbital (HOMO) of the starting atomic quantum cluster, and an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt; wherein said metal salt and said hole scavenger are soluble in said polar solvent and do not react with each other; and wherein the number of equivalents of said hole scavenger is higher than the number of equivalents of the metal salt in the mixture.

According to step b) a promoter is applied to the mixture of step a), wherein the promoter is a light radiation having energy equal or larger than the HOMO-LUMO gap of the starting atomic quantum cluster of the mixture of step a).

The term“promoter” in the process of the invention refers to a light radiation having a wavelength shorter or equal than the excitation wavelength of the starting atomic quantum cluster; that is energy equal or higher than the energy of the HOMO-LUMO gap (higher occupied molecular orbital-lower unoccupied molecular orbital gap) of the starting AQCs.

As described in the European patent application EP1 1382196 and in EP113823751 , an approximate estimation of the AQCs excitation wavelengths can be determined experimentally by UV-vis absorption spectroscopy or theoretically by the Jellium model (see for example J.Calvo et al., Encyclopedia of Nanotechnology, Ed. by B. Bhushan, Springer Verlag, 2011 ).

In a preferred embodiment the promoter is a light radiation having a wavelength in the UV, visible and/or near IR range. Preferably, the promoter is a light radiation having a wavelength comprised between 200 nm and 800 nm, preferably between 350 and 750 nm, more preferably between 400 and 700 nm, even more preferably between 500 and 600 nm, and an intensity comprised between 0.01 milliwatts/cm ² and 10 watts/cm ² , preferably between 0.2 and 0.8 milliwatts/cm ² , even more preferably between 0.4 and 0.6 milliwatts/cm ² . In a more preferred embodiment the promoter is a light radiation from a lamp of about 1 miliwatts/cm ² and a wavelength of 250 nm.

The photocatalytic activity of the starting AQCs depends on their ability to absorb light from the promoter and create electron-hole pairs (excitons), i.e. induce charge separation by creating charge carriers (electrons and holes), which can later enable photocatalytic processes, e.g. reduction-oxidation (redox) reactions, by transferring the charge carriers to the charge acceptors, i.e. electron acceptor or hole acceptor.

Without being bound to any theory the inventors believe that the promoter produces the excitation of the starting AQCs in the reaction mixture generating an exciton (electron - hole pair) in the starting AQCs. This hole oxidizes the hole scavenger in the reaction mixture, while the electron reduces the metal cation of the metal salt to produce fresh AQCs. The reaction generally proceeds fast, mainly due to the presence of the starting AQCs acting as catalysts in the reduction of the metal ion. After the formation of the first fresh AQCs, the reaction proceeds further to the formation of nanoparticles. But, the oxidant in the reaction mixture having a standard electrode potential over the standard electrode potential of the metal ion (reduction standard potential), oxidizes the metal nanoparticles to metal ions producing the dissolution of the metal nanoparticles and the subsequently formation of metal salt, thus initiating again the process for producing more fresh AQCs and more nanoparticles. Due to the high stability of clusters in the presence of the oxidant, their concentration increases with time in the process of the invention, whereas the less stable species in the reaction mixture including metal ions and nanoparticles are continuously reduced or oxidized.

In a particular embodiment the reaction time of the process of the invention is comprised between 0.1 and 60 hours, preferably between 1.5 and 10 hours, even more preferably 3 hours.

The term“metal nanoparticle” refers in the context of the invention to any particle of bulk metal having dimensions in the nanoscale. Typical metal nanoparticles have dimensions from two to several tens of nanometers. Nanoparticles usually present a core-shell structure with a core of bulk metal surrounded by a shell of disordered atoms.

The process of the present invention comprises a step (c) of adding an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt; wherein the oxidant can be either added in the mixture of step (a), and/or added during and/or after applying said promoter in step (b). According to the process of the invention the oxidant can be either present in the mixture of step a), and/or added to said mixture during and/or after applying said promoter in step b). Thus, in a particular embodiment the oxidant is present in the mixture of step a) of the process of the invention and is further added to the mixture during the application of the promoter. In another particular embodiment the oxidant is present in the mixture of step a) of the process of the invention and is further added to the mixture during and after applying the promoter. In another particular embodiment the oxidant is present in the mixture of step a) of the process of the invention and is further added to the mixture after applying the promoter; preferably immediately after applying the promoter. In another particular embodiment, the oxidant is added to the mixture of step a) during the application of the promoter. In another particular embodiment, the oxidant is added to the mixture of step a) during and after applying the promoter. In another particular embodiment, the oxidant is added to the mixture of step a) after applying the promoter. In another particular embodiment, the oxidant is added to the mixture of step a).

The oxidant in the process of the invention having a standard electrode potential over the standard electrode potential of the metal of the metal salt, is able to oxidize the metal nanoparticles produced by the reduction of the metal ions of the metal salt. Thus, for example if the metal of the metal salt is silver, the oxidant of the process of the invention has a standard electrode potential over the standard electrode potential of silver, namely over + 0.80 V. Moreover, if the amount of oxidant is higher than the amount of metal salt in the reaction mixture, the yield of the process increases. Thus, in a particular embodiment, the amount of oxidant in the mixture of the process of the invention is higher than the amount of metal salt.

Moreover, while the AQCs of the reaction mixture are stable under the presence of a strong oxidant, i.e.: they conserve the number atoms and their properties, the metal nanoparticles are oxidized by the presence of the oxidant. The stability of the several AQCs has already been reported in the state of the art (Ag ₃ , Ag ₅ , Agg, Cus (S. Huseyinova, J. Blanco, F. G. Requejo, J. Ramallo-Lopez, M.C. Blanco, D. Buceta and M. A. Lopez-Quintela. J. Phys.Chem.C, 2016, 120, 15902-15908; J.M. Blanco, Electrochemical synthesis of Ag Atomic Quantum Clusters, University of Santiago de Compostela, 2017), and is associated with their large HOMO-LUMO gap (S. Huseyinova et al. J. Phys. Chem. C, 2016, 120, 15902-15908).

By contrast to the starting AQCs, the term“fresh AQCs” refers to the AQCs produced by the process of the invention. Advantageously, the process of the invention allows obtaining AQCs in a high yield; preferably“fresh AQCs in a high yield”. In a particular embodiment, the invention relates to the process wherein atomic quantum clusters are produced with a yield of above 10%, preferably above 20 %, more preferably around 40%. In a preferred embodiment the atomic quantum clusters are produced with a yield of 60%, preferably, above 80%, even more preferably of 100%. In a particular embodiment all metal in the reaction mixture is finally converted in AQCs, thus the atomic quantum clusters are produced with a yield of 100%. In a particular embodiment, the invention relates to a process wherein atomic quantum clusters are produced in at least milligram scale. The conditions of the process of the invention can be optimized by routine work in the laboratory.

In a preferred embodiment, the process of the present invention leads to a mixture comprising atomic quantum clusters; wherein said atomic quantum clusters are in a higher amount that in step (a).

In a preferred embodiment, the process of the present invention leads to a mixture comprising fresh atomic quantum clusters; preferably wherein said fresh atomic quantum clusters are different from the starting atomic quantum clusters of step (a); more preferably wherein said fresh atomic quantum clusters are produced with a yield of above 10%; preferably of above 20%; more preferably of around 40%.

In a preferred embodiment, the process of the present invention leads to a mixture comprising fresh atomic quantum clusters; wherein the amount of fresh atomic quantum clusters is increased with the reaction time.

In a preferred embodiment, the process of the present invention comprises a reaction mixture; wherein said reaction mixture is generated after adding the oxidant of step (cl and applying the promoter of step (b); preferably said reaction mixture comprises fresh atomic quantum clusters; more preferably in said reaction mixture fresh atomic quantum clusters are generated over reaction time.

In a particular embodiment, the invention relates to a process wherein atomic quantum clusters are produced in a concentration higher than the concentration of the starting atomic quantum cluster of step (a); preferably in a concentration higher than a micromolar concentration.

In a preferred embodiment, the process of the present invention leads to a mixture comprising atomic quantum clusters; wherein said atomic quantum clusters are in a concentration higher that the concentration of the atomic quantum clusters of step (a); preferably in a concentration higher than a micromolar concentration.

In a preferred embodiment, the process of the present invention leads to a mixture comprising fresh atomic quantum clusters; wherein said fresh atomic quantum clusters are in a higher amount that the starting atomic quantum clusters in step (a); preferably in a concentration higher than a micromolar concentration. In a particular embodiment, the atomic quantum clusters in step (a) are catalyst.

In a preferred embodiment, the process of the invention is a process for producing atomic quantum clusters (AQCs) comprises the following steps: a) providing a mixture comprising:

- a starting atomic quantum cluster in a picomolar to micromolar concentration,

- a metal salt,

- a polar solvent,

- a hole scavenger having a standard electrode potential lower than the higher occupied molecular orbital (HOMO) of the starting atomic quantum cluster, and wherein said metal salt and said hole scavenger are soluble in said polar solvent and do not react with each other;

wherein the number of equivalents of said hole scavenger is higher than the number of equivalents of the metal salt in the mixture,

b) applying a promoter to the mixture of step a), wherein the promoter is a light radiation having energy equal or larger than the HOMO-LUMO gap of the starting atomic quantum cluster of the mixture of step a); and

c) adding an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt of step (a); wherein said oxidant is added either to the mixture of step a), and/or added to the mixture during and/or after applying said promoter in step b); and wherein atomic quantum clusters are produced; preferably fresh atomic quantum clusters are produced; more preferably fresh atomic quantum clusters are produced with a yield of above 10%; preferably of above 20%; more preferably of around 40%.

In a more preferred embodiment, the amount of AQCs is increased by the process of the present invention; more preferably the amount of the fresh AQCs is increased by the process of the present invention.

In a particular embodiment the metal of fresh AQCs is the same or different from the metal of starting AQCs in step (a); preferably the same; more preferably is silver.

In a more particular embodiment, the metal of fresh AQCs is different from the metal of starting AQCs in step (a).

In a more preferred embodiment, the yield of AQCs is increased by the process of the present invention; preferably the yield of fresh AQCs is increased.

In the context of the present invention the term“yield” is understood as the percentage yield calculated from the amount of the obtained desired product and from the theoretical yield which is calculated by a stoichiometric calculation based on the number of moles of the limiting reactant as known in the art. In addition, the calculation of the theoretical yield assumes that only one reaction occurs and that the limiting reactant reacts completely. Preferably, the metal salt of the present invention is the limiting reactant to calculate the yield of the present invention. More preferably, in the present invention when all metal in the reaction mixture is finally converted in AQCs, the atomic quantum clusters are produced with a yield of 100%; in particular, when all the metal of the metal salt of the present invention is converted in fresh AQCs, the atomic quantum clusters are produced with a yield of 100%. Also, in a particular embodiment, when the metal of the starting metal AQCs is the same that the metal of the metal salt and of the produced metal AQCs of the present invention, said starting atomic quantum clusters are not taken into account when calculating the yield of the process of the present invention or are in such small amount that they do not affect significantly to said calculation; preferably they are not taken into account when calculating the yield.

In a particular embodiment, the yield of the present invention is calculated as a percentage yield calculated from the amount of the obtained moles of AQCs and the theoretical yield calculated from the number of moles of the limiting reactant; preferably wherein the calculation of the theoretical yield assumes that the limiting reactant reacts completely and only reacts in one reaction.

In a more particular embodiment, the yield of the present invention is calculated as a percentage yield by dividing the amount of the obtained moles of metal of the metal AQCs by the theoretical yield which is calculated by a stoichiometric calculation based on the number of moles of the metal of the metal salt of the present invention; wherein the calculation of the theoretical yield assumes that only one reaction occurs and that the metal salt of the present invention reacts completely.

In an even more particular embodiment the yield of the present invention is calculated as a percentage yield by dividing the amount of the obtained moles of the metal of the metal AQCs by the theoretical yield which is calculated by a stoichiometric calculation based on the number of moles of the metal of the metal salt of the present invention; wherein the calculation of the theoretical yield assumes that only one reaction occurs and that the metal salt of the present invention reacts completely.

In an even more particular embodiment the yield of the present invention is calculated as a percentage yield by dividing the amount of the obtained moles of the metal of the metal AQCs by the theoretical yield which is calculated by a stoichiometric calculation based on the number of moles of the metal of the metal salt of the present invention; wherein the calculation of the theoretical yield assumes that only one reaction occurs and that the metal salt of the present invention reacts completely;

preferably when the metal of the starting AQCs is the same as the metal of the metal of the metal salt, the initial moles of the starting AQCs are not taken into account in the yield calculation; preferably the initial moles of the starting AQCs are not taken into account in the calculation of the obtained moles of the metal of the fresh metal AQCs; more preferably the initial moles of the starting AQCs are subtracted from the total obtained moles of the metal of the metal AQCs to calculate the metal moles of the fresh AQCs;

more preferably when the metal of the starting AQCs is the same as the metal of the metal salt, the initial moles of the starting AQCs are not taken into account in the yield calculation; preferably the initial moles of the starting AQCs are not taken into account in the calculation of the obtained moles of the metal of the AQCs;

even more preferably the initial moles of the starting AQCs are subtracted from the total obtained moles of the metal of the metal AQCs.

The AQCs resulting from the process of the invention can be identified by Electrospray ionization (ESI) mass spectrometry. Figure 1 shows the ESI-mass spectrometry of Ag AQCs resulting from the process of the invention. The detected peaks are identified as the following Ag AQCs: Ag ₂ (230), Ag ₃ (401 ), Ag ₅ (570 and 786), Ag ₇ (912 and 1081 ), Agg (1248).

In a preferred embodiment, the metal atoms of the AQCs resulting from the process of the invention are selected from platinum (Pt), gold (Au), rhodium (Rh), iridium (Ir), palladium (Pd), ruthenium (Ru), osmium (Os), silver (Ag), copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), titanium (Ti), vanadium (V), chrome (Cr) or their bi and multimetal combinations. Preferably the metals of the AQCs are selected from Au, Ag, Cu, Pd and Pt or their bimetal combinations.

Further, the process of the invention allows producing AQCs of different number of metallic atoms by optimizing the conditions of the process such as the concentration and type of metal salt, the concentration of photocatalytic AQCs, the concentration and type of hole scavenger, and the wavelength of the promoter. In a particular embodiment the AQCs resulting from the process of the invention have a number of metal atoms comprised between 2 and 50. In a preferred embodiment, the AQCs produced by the process of the invention are composed of less than 30 metal atoms (M _n , n<30), preferably 15 metal atoms (M _n , n<15), even more preferably the present AQCs are formed by between 2 and 10 metal atoms (M _n , 2<n<10). In a particular embodiment, the mean size of the AQCs produced by the method of the invention is between 0.3 and 1.5 nm, preferably the mean size is less than 1 nm, more preferably between about 0.3 and 0.9 nm.

Moreover, the concentrations of AQCs in the solution may be measured by UV-VIS spectroscopy. Thus, for example, figure 2 shows the UV-VIS spectra of the reaction of the process of the invention at different times. After 5 hours and before addition of oxidant, the figure shows a plasmon band at around 420 nm associated to the presence of nanoparticles; and a band at aprox. 280 nm associated to the presence of clusters. By contrast, after 5 hours and after the addition of oxidant only the band of clusters remains.

In another aspect, the invention also relates to a mixture or composition comprising: atomic quantum cluster,

a metal salt,

an oxidant having a standard electrode potential over the standard electrode potential of the metal of the metal salt,

a hole scavenger having a standard electrode potential lower than the HOMO orbital of the atomic quantum cluster, and

a polar solvent, wherein the metal salt and the hole scavenger are both soluble in the mixture and do not react with each other, and wherein the number of equivalents of hole scavenger in the mixture are higher than the number of equivalents of metal salt in the mixture.

In a particular embodiment, the atomic quantum cluster of the mixture of the present invention is starting atomic quantum cluster; preferably in a in a picomolar to micromolar concentration.

In a particular embodiment, the invention relates to the mixture or composition of step a) comprising:

at least an atomic quantum cluster,

a metal salt,

optionally an oxidant having a standard electrode potential over the standard electrode potential of said metal ion,

a hole scavenger having a standard electrode potential lower than the HOMO orbital of the at least atomic quantum cluster, and a polar solvent, wherein the metal salt and the hole scavenger are both soluble in the mixture and do not react with each other, and wherein the number of equivalents of hole scavenger in the mixture are higher than the number of equivalents of metal salt in the mixture. In a preferred embodiment, the mixture or composition of step a) comprises:

- between 1x1 O ¹² M and 1x1 O ⁶ M of atomic quantum clusters, preferably between 1x1 O ¹⁰ M and 1x10 ⁷ M, more preferably between 1x1 O ⁹ M and 1x1 O ⁸ M,

- between 0.1 mM and 1 M of metal salt, preferably between 0.5 mM and 0.5M, preferably between 1 mM and 0.05M,more preferably about 10mM,

- between 1 mM and 10M of the oxidant, preferably between 10 mM and 1 M , more preferably about 50mM,

- between 1 % v/v and 90%v/v of the hole scavenger, preferably between 10%v/v and 60%, more preferably about 40 %v/v, and

- between 10 % v/v and 99%v/v of polar solvent, preferably between 40% v/v and 90%, more preferably about 60%v/v.

The above volume percentages have been calculated assuming that the AQcs, the metal salt, and the oxidant do not add volume to the mixture. Moreover, in the remote case they did, the sum of volume of the polar solvent and of the hole scavenger would be adjusted to 100% maintaining their relationship and taking into account the eventual volume added by the other components.

In the mixture or composition of step a) of the process of the invention, the AQCs present in the mixture correspond to the starting AQCs that initiate the process of the invention.

In another embodiment, the invention relates to the mixture or composition resulting from the process of the invention, preferably comprising:

- between 1x10 ⁵ M and 1 M of atomic quantum clusters, preferably between 1x1 O ³ M and 0.1 M, preferably about 5mM,

- between 0 and 0.9M of metal salt, preferably about 5mM,

- between 0 M and 5M of the oxidant,

- between 0 % v/v and 80%v/v of the hole scavenger, preferably between 30%v/v and 50% v/v, and - between 20% v/v and 100%v/v of polar solvent, preferably between 50% v/v and 70% v/v.

In another particular embodiment, the invention relates to the mixture or composition resulting from the process of the invention, preferably comprising:

- between 1x10 ⁵ M and 1 M of atomic quantum clusters, preferably between 1x1 O ³ M and 0.1 M, preferably about 5mM,

- between 0.01 and 0.9M of metal salt, preferably about 5mM,

- between 0.01 M and 5M of the oxidant,

- between 0.01 % v/v and 80%v/v of the hole scavenger, preferably between 30%v/v and 50% v/v, and

- between 20% v/v and 100%v/v of polar solvent, preferably between 50% v/v and 70% v/v.

The above volume percentages have been calculated assuming that the AQcs, the metal salt, and the oxidant do not add volume to the mixture. Moreover, in the remote case they did, the sum of volume of the polar solvent and of the hole scavenger would be adjusted to 100% maintaining their relationship and taking into account the eventual volume added by the other components.

In a preferred embodiment, said mixture comprises 100% v/v of polar solvent and between 1x10 ⁵ M and 1 M of AQCs, assuming that the AQcs does not add volume to the mixture, and that in the remote case they did, the volume of the polar solvent would be adjusted to 100% maintaining their relationship.

Atomic quantum clusters in the mixture of the invention include any AQC available in the market or obtained in the laboratory. In a preferred embodiment the AQCs of the mixture are formed by transition metals selected from: platinum (Pt), gold (Au), rhodium (Rh), iridium (Ir), palladium (Pd), ruthenium (Ru), osmium (Os), silver (Ag), copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), titanium (Ti), vanadium (V), chrome (Cr) and their bi and multimetal combinations. More preferably the metals of the AQCs are selected from Au, Ag, Cu, Pd and Pt or their bimetal combinations, even more preferably the metals of the AQCs are selected from Au and Ag or their bimetal combinations.

In a preferred embodiment the metal of the metal salt in the mixture or composition is selected from silver, platinum, palladium, gold, copper, iridium, rhodium, ruthenium, nickel, iron, cobalt, or their bi and multimetal combinations. Preferably the metal of the metal salt is selected from Au, Ag, Cu, Pd and Pt or their bimetal combinations. In a preferred embodiment the metal of the metal salt and the metal or the starting AQCs in the mixture of the invention is the same metal or is a different metal. In a more preferred embodiment the metal salt is a silver salt, preferably a silver salt selected from silver bromate, bromite, chlorate, perchlorate, chlorite, fluoride, nitrate, nitrite, acetate, permanganate and mixtures thereof.

The hole scavenger, as well as the metal salt, is soluble in the mixture of the invention. Moreover the hole scavenger does not react with the metal salt in the mixture of the invention.

In a preferred embodiment, the hole scavenger is selected from a linear or branched alcohol having between 2 and 6 carbon atoms. More preferably, the hole scavenger is selected from ethanol, propan-1-ol, isopropanol, butan-1-ol, butan-2-ol, isobutanol, 1 ,1- dimethyl-ethanol, pentan-1-ol, pentan-2-ol, pentan-3-ol, 2-methylbutan-1-ol, 3- methylbutan-1-ol, 3-methylbutan-2-ol, 2,2-dimethylpropan-1-ol, hexan-1-ol, hexan-2-ol, hexan-3-ol, 2-methylpentan-1-ol, 3-methylpentan-1-ol, 4-methylpentan-1-ol, 2- methylpentan-2-ol, 3-methylpentan-2-ol, 4-methylpentan-2-ol, 2-methylpentan-3-ol, 3- methylpentan-3-ol, 2,2-dimethylbutan-1-ol, 3,3-dimethylbutan-1-ol, 2,3-dimethylbutan- 2-ol, 3,3-dimethylbutan-2-ol, 2-ethylbutan-1-ol and mixtures thereof. In another preferred embodiment the hole scavenger is selected from hydroquinone, iodide salt, oxalic acid, acetic acid, formic acid, sodium formate, sulfite and mixtures thereof. Other suitable hole scavengers include glicerol, vinyl alcohol, polyvinyl alcohol, alcohol amines such as triethanol amine, and mixtures thereof.

In a preferred embodiment, the oxidant in the mixture of the invention is selected from nitric acid, hydrogen peroxide, permanganate, perchlorate, ozone, persulfate, hypochlorite, chlorite, hypobromite, bromite, perchromate and mixtures thereof, even more preferably, nitric acid or hydrogen peroxide.

In a preferred embodiment, the polar solvent is selected from water, acetonitrile, chloroform, dichloromethane, acetic acid, toluene and mixtures thereof.

The invention also relates to a mixture or composition obtainable by the process of the invention, preferably comprising:

- between 1x10 ⁵ M and 1 M of atomic quantum clusters, preferably between 1x10 ³ M and 0.1 M, preferably about 5 mM,

- between 0 and 0.9M of metal salt, preferably about 5 mM,

- between 0 M and 5 M of the oxidant, - between 0 % v/v and 80%v/v of the hole scavenger, preferably between 30%v/v and 50%, and

- between 20% v/v and 100%v/v of polar solvent, preferably between 50% v/v and 70% v/v,

wherein the metal salt and the hole scavenger are both soluble in the mixture and do not react with each other, and wherein the number of equivalents of hole scavenger in the mixture are higher than the number of equivalents of metal salt in the mixture.

The above volume percentages have been calculated assuming that the AQcs, the metal salt, and the oxidant do not add volume to the mixture. Moreover, in the remote case they did, the sum of volume of the polar solvent and of the hole scavenger would be adjusted to 100% maintaining their relationship and taking into account the eventual volume added by the other components.

EXAMPLES Example 1

750ml_ of H2O Milli-Q, 750ml_ of 2-propanol (hole scavenger), 1.2 g of AgNC>3 -already containing approx. 0.3 micrograms Ag AQCs- (0.5g/L of Ag) are added in a beaker of 2L. Then the sample is irradiated with a lamp of « 1 miliWatts/cm ² and with a wavelength of 250nm, under continuous stirring, during 5h. During this time 1 ml_ of HNO3 (65% v/v) -in a large excess to the silver salt- is added after 30 minutes of starting the irradiation and 0.5ml_ after 5h of irradiation. The final concentration of Ag ⁺ remaining in the solution (measured by an ion selective electrode) is 0.3g/L. The rest (0.2g/L) corresponds to naked AQCs, which are the only stable species under the used strong oxidative conditions.

Clusters have been identified by ESI-mass spectrometry (see figure 1 ). The observed peaks in the ESI-Mass spectra correspond to the following Ag AQCs: Ag ₂ (230), Ag ₃ (401 ), Ag ₅ (570 and 786), Ag ₉ (1248).

The concentration of clusters was checked by UV-Vis spectroscopy (figure 2) taking into account that the extinction coefficient of Ag AQCs is of the order of B =1000 M ¹ cm ^-1 (J. Neissa, C. Perez-Arnaiz, V. Porto, N. Busto, E. Borrajo, J. M. Leal, M. A. Lopez-Quintela, B. Garcia and F. Dominguez, Chem. Sci., 2015, 6, 6717-6724; D. Buceta, N. Busto, G. Barone, J. M. Leal, F. Dominguez, L. J. Giovanetti, F. G. Requejo, B. Garcia and M. A. Lopez-Quintela. Angew. Chem. Int. Ed. Engl. 2015, 54(26):7612- 6). Figure 2 shows the uv-vis spectra of the reaction at different times: A) initial (O’) - solid line-, B) after 5 hours (300’) of reaction (before addition of oxidant)-dotted line-, and C) after 5 hours (300’) of reaction and after addition of oxidant -dashed line-. After 5 hours of reaction (figure 2), it can be seen the Ag plasmon band, at around 420nm, indicating the presence of Ag nanoparticles. Also, the band at 275 nm, due to clusters, can be clearly seen. However, after 5 hours of reaction, and after the addition of oxidant only the band of clusters remains, indicating that clusters are stable under the presence of the oxidant, but nanoparticles are oxidized (figure 2). Moreover, figure 2 shows that the final absorbance at 275nm (using a cuvette of 1 cm), associated with AQCs (see the previous references), is « 0.45, from which one can obtain a concentration of Ag AQCs (considering an average cluster size of 5 atoms) of: 0.45/1000M ¹ cnr ¹ x 1 cm=0.45mM « 0.24 g/L. This value agrees very well with the calculated previously although it contains some uncertainties in value of e and in the average cluster size.

Example 2

1350ml_ of H2O Milli-Q, 150ml_ of 2-propanol (hole scavenger), 1.2 g of AgNOs - already containing approx. 0.3 micrograms Ag AQCs- (0.5g/L of Ag), 1 ml_ HNO ₃ (65% v/v) -in a large excess to the silver salt- are added in a beaker of 2L. Then the sample is irradiated with a lamp of « 1 miliWatt/cm ² and with a wavelength of 250nm, under continuous stirring, during 5h. Figure 3 shows that the final absorbance at 275nm (using a cuvette of 1 cm), associated with AQCs (see the previous example), is 0.15, from which one can obtain a concentration of Ag AQCs of: 0.15/1000M ¹ cnr ¹ x 1 cm=0.15mM « 80mg/L. The concentration of Ag AQCs in this example is smaller than in the previous example because the concentration of hole scavenger is also smaller.