# Polyoxotungstates-stabilized gold nanoparticles: how to transfer from an aqueous suspension to CH3CN?

Jean-Baptiste Harlé, Élise Péruse, Jimika Hassanaly, Dalil Brouri, Anna

Proust, Audrey Denicourt-Novicky, Alain Roucoux, Anne Gauvin-Bialecki,

Richard Villanneau

► To cite this version:

Jean-Baptiste Harlé, Élise Péruse, Jimika Hassanaly, Dalil Brouri, Anna Proust, et al. Polyoxotungstates-stabilized gold nanoparticles: how to transfer from an aqueous suspension to CH3CN?. European Journal of Inorganic Chemistry, 2025, 28 (22), pp.e202500196. 10.1002/ejic.202500196 . hal-05078866

HAL Id: hal-05078866

https://hal.sorbonne-universite.fr/hal-05078866v1

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# Polyoxotungstates-stabilized gold nanoparticles: how to transfer from an aqueous suspension to CH₃CN

Jean-Baptiste Harlé,ª Elise Péruse,ᵇ Jimika Hassanaly,ᵇ Dalil Brouri,ᶜ Anna Proust,ᵇ Audrey Denicourt-Nowicki,ᵈ Alain Roucoux,ᵈ Anne Gauvin-Bialecki,ª Richard Villanneauᵇ,\*

a: Chembiopro, Laboratoire de Chimie et de Biotechnologie des Produits Naturels – UR, Faculté des Sciences et Technologies, Université de La Réunion, Campus du Moufia, 15 avenue René Cassin, 97744 Saint-Denis cédex 9, France.

b: Institut Parisien de Chimie Moléculaire, CNRS UMR 8232, Sorbonne Université, CNRS, Campus Pierre et Marie Curie, 4 Place Jussieu, F-75005 Paris, France. E-mail: richard.villanneau@sorbonne-universite.fr.

c: Laboratoire de Réactivité de Surface, CNRS UMR 7197, Sorbonne Université, CNRS, Campus Pierre et Marie Curie, 4 Place Jussieu, F-75005 Paris, France.

d: Université de Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226, Rennes F-35000, France.

## Abstract.

The present study aimed to evaluate available methods for preparing suspensions of gold nanoparticles stabilized by polyoxometalates (POMs@AuNPs) in CH₃CN, a solvent commonly used in oxidation reactions. POMs@AuNPs were successfully synthesized in CH₃CN using three different approaches: (i) in situ generation of nanoparticles via chemical reduction of the HAuCl₄ precursor with various reducing agents in CH₃CN in the presence of POMs; (ii) formation of POMs@AuNPs by reducing HAuCl₄ with NaBH₄ in the presence of [PW₁₁O₃₉]⁷⁻ or [AsW₉O₃₃]⁹⁻ in aqueous solution, followed by transfer into CH₃CN after centrifugation; and (iii) transfer of aqueous suspensions of Au⁰ nanoparticles, obtained by reducing HAuCl₄ with NaBH₄ and subsequent centrifugation, into CH₃CN, followed by the addition of soluble organic salts of [PW₁₂O₄₀]³⁻ or [AsW₉O₃₃{PO(CH₂)₂CO₂H}₂]⁵⁻. These strategies, inspired by literature, were used to assess their strengths and limitations regarding translation to CH₃CN. The resulting suspensions were characterized by UV-Vis spectroscopy, HR-TEM, EDS analysis, DLS, and zeta potential measurements. The most stable suspensions were obtained by transferring an aqueous suspension initially stabilized with a mixture of citrate ions and tannic acid into acetonitrile. Subsequent exchange with polyoxometalates in CH₃CN resulted in negligible changes in nanoparticle size or suspension stability.

<figure><img src="image_2.png" /></figure>
# 1. Introduction

Polyoxometalates (POMs) are discrete polymetallic structures formed by the assembly of oxo ligands and metal ions in their highest oxidation states (typically $W^{VI}$, $Mo^{VI}$, $V^V$, or $Nb^V$).[1] They are therefore oxoclusters of the ${M_xO_y}^{p-}$ type, but can also incorporate heteroatoms from the p-block (most often P, Si, As, Sb, Al, B...), leading to the subclass known as heteropolyoxometalates or heteropolyanions. Additionally, the $d^0$ electronic configuration of the metal centers confers high stability under oxidative conditions. Consequently, their most well-documented application is in catalytic oxidation processes. Their lacunary forms (obtained by the removal of one or more oxometallic centers) can also be functionalized with various groups derived from main-group elements (such as organophosphonyl or organosilyl groups), resulting in the formation of polyoxometalate hybrids.[2] Furthermore, these compounds and their transition metal-substituted derivatives have demonstrated excellent catalytic properties in oxidation, particularly in the presence of green oxidants such as hydrogen peroxide ($H_2O_2$).[3-4] By modulating the nature of their counterions, their solubility in water (generally associated with protons or alkaline ions) and/or organic solvents can be adjusted. In the latter case, the most commonly used counterions are tetraalkylammonium and, more rarely, phosphonium cations.

Another particularly interesting property relates to their use as stabilizing agents for metal nanoparticles ($M^0NPs$). Their negative charge and nanometric size enable them to efficiently stabilize $M^0NPs$ through a combination of electrostatic and steric effects, known as electro-steric stabilization.[5] Consequently, the preparation of metal nanoparticles stabilized by POMs (referred to as POMs@M0NPs) has been extensively documented in the literature for around thirty years. Several main strategies can be broadly identified. In the first strategy, metal nanoparticles were initially generated using conventional methods in the presence of classical stabilizing agents such as citrate ions. POMs were then introduced in a second step into the medium, where an exchange reaction occurred between POMs and the primary stabilizing agent.[6] The second approach is a one-pot method, in which POMs are added simultaneously with the metal source, making POMs the sole stabilizing agent present in the medium.[7-8] Nevertheless, due to their intrinsic properties and stability under a wide range of experimental conditions, alternative strategies have also demonstrated the versatility of POMs in stabilizing $M^0NPs$. For example, the use of reduced derivatives of POMs and metal salts of noble metals has shown that POMs can act both as metal cation reductants and stabilizing agents in aqueous media.[9] Alternatively, metallic NPs have been generated using previously photo-excited POMs in the presence of a sacrificial organic donor, with POMs functioning both as reductants to produce $M^0NPs$ and as oxidants for the organic sacrificial agent.[10]

Finally, a noteworthy strategy developed by R. G. Finke and colleagues deserves mention due to the meticulous work involved. These authors favored the use of organometallic derivatives of POMs containing noble metal-based fragments ({{RhCp*}²⁺, {Ir(COD)}⁺, {Ru(η6-arene)}²⁺...), in which POMs served as ligands for the organometallic cation.[11] Since POM@NPs were generated by reduction with $H_2$ in an organic medium (including acetone and acetonitrile), the presence of associated tetraalkylammonium counterions provided additional stabilization, leading to structures described as “Polyoxoanion- and tetrabutylammonium-stabilized nanoclusters.”[11] Notably, this work established a classification of POMs based on their stabilizing power, highlighting the roles of charge and nucleophilicity of the POMs in nanoparticle stabilization.
To our knowledge, with the notable exception of Finke's group, most studies reported in the literature have been conducted in aqueous media.[12] Very few works have addressed the generation of POMs@NPs suspensions in organic media (either in mixed CH3CN/toluene solvents or pure toluene).[13] Nonetheless, there is significant interest in obtaining such dual systems in organic solvents like nitriles, especially for catalytic applications involving the transformation of organic substrates, as demonstrated by Finke[11c] and Suzuki.[13b]

Our interest was based on the fact that gold Au⁰NPs have the ability to activate molecular dioxygen via adsorption processes or dissociative adsorption of O₂ or H₂, which are widely described in the literature. They therefore present significant potential for the selective epoxidation of alkenes (particularly for the epoxidation of terpenes, in which some of us are involved),[14] ideally without the use of additional polluting stoichiometric reagents such as chromates, permanganates, perchlorates, or organic peracids. As previously mentioned, most synthesis methods for gold nanoparticles stabilized by POMs (referred to as POMs@AuNPs) reported in the literature were carried out in aqueous media, in which alkenes are generally insoluble. The aim of this work is therefore to rationalize the generation of POM@AuNPs systems in CH₃CN (used here as a model organic solvent of interest for oxidation reactions involving polyoxometalates) through the implementation of some of the most classic procedures for stabilizing metallic NPs. To the best of our knowledge, this is the only study to compare different synthesis strategies in a non-aqueous medium. The criteria we used to evaluate the effectiveness of each method were the ease of obtaining and the stability of the POM@AuNPs suspensions, as well as the reproducibility of the preparation. These criteria were assessed systematically using a set of physicochemical techniques, including UV-Visible spectroscopy, HR-TEM, X-EDS analyses, DLS, and Zeta potential measurements. Additionally, one of our main concerns was to highlight the proximity of the POMs to the surface of the AuNPs in systems prepared in CH₃CN.

## 2. Experimental Section

### 2.1 Materials and methods.

DLS (Dynamic Light Scattering) and zeta potential measurement were performed on Malvern ZetaSizer Ultra and DelsaNano analysers. IR spectra (for the characterization of the POMs) were obtained from KBr pellets on a Jasco FT/IR-4100 spectrometer or by Attenuated Total Reflection Infra-Red spectroscopy, using a Tensor R27 FTIR equipped with a ZnSe crystal with resolution better than 1 cm⁻¹. The ¹H NMR solution spectra were recorded in 5 mm o.d. tubes on Bruker Avance II 300 or 400 spectrometers equipped with a QNP probe head. Chemical shifts were referenced to tetramethylsilane (TMS) for ¹H and to an external reference (H₃PO₄ solution 85%) for ³¹P. UV-Visible spectrophotometry measurements were performed on an Agilent Cary 5000 spectrometer. HR-TEM and TEM analyses were realized on a microscope operating at 200 kV with a resolution of 0.18 nm (JEOL JEM 2011 UHR) equipped with an EDX system (PGT IMIX-PC). Samples were deposited on Cu grids covered with an amorphous carbon film. The determination of the average diameters of AuNPs and POMs@AuNPs were based on the count of particles (counted on 180 to 250 NPs) on TEM micrographs, with Image J2 (Fiji) software. Raman spectra were recorded on solid samples on a Kaiser Optical Systems HL5R spectrometer equipped with a near-IR diode laser (785 nm). The samples were obtained after centrifugation of the corresponding suspensions in CH₃CN.
The synthesis of the different polyoxometalates (see figure 1) used during this study was
performed according to the literature.[15]

<figure><img src="image_3.png" /><figcaption>Figure 1. Structure of the different polyoxometalates used in the study (W polyhedra in blue, P polyhedra in green, As atom in prune, C atoms in black, O atoms in red).</figcaption></figure>

**2.2 Preparation of the suspensions in aqueous medium.**

In all experiments, the glassware used was previously washed with aqua regia.

**2.2.1 One-pot strategies**

A solution of HAuCl₄·3H₂O (0.006 g, 1.56*10⁻² mmol) in 2 mL of ultrapure water was prepared at room temperature and a solution of Na₈H[B-α-AsW₉O₃₃]·24H₂O (0.224 g, 7.8*10⁻² mmol, 5 eq.) or K₇[PW₁₁O₃₉]·xH₂O (0.10 g, 3.12*10⁻² mmol, for 2 eq.; 0.25 g, 7.8*10⁻² mmol, for 5 eq.; 0.40g, 1.25*10⁻¹ mmol, for 8 eq.) in 7 mL of ultrapure water was added. Then a solution of NaBH₄ (corresponding to 0.0017 g, 4.68*10⁻² mmol, for 3 eq.; 0.006g, 0.156 mmol, for 10 eq.) in 1 mL of ultrapure water was added dropwise to the previous one under stirring (total volume = 10 mL). [Au] = 1.56 mmol·L⁻¹.

Weinstock's variant of the one-pot method (adapted from ref [16]) A solution of HAuCl₄·3H₂O (0.017 g, 4.3*10⁻² mmol) and K₇[PW₁₁O₃₉]·xH₂O (0.16 g, 5*10⁻² mmol; 1.16 eq.) in 20 mL of ultrapure water was prepared and cooled in an ice bath (0°C). A NaBH₄ solution (corresponding to 0.0014 g, 3.75*10⁻² mmol; 0.87 eq.) in 0.5 mL of ultrapure water was then added dropwise. The final solution was stored cold (4°C) before characterizations. [Au] = 2.1 mmol·L⁻¹.

**2.2.2. Intermediate stabilization strategy using excess of BH₄⁻ (Astruc's method).**

The experiment was carried out in a 250 mL Schlenk flask under argon. First, HAuCl₄·3H₂O (0.005 g, 0.013 mmol) was added to 100 mL of ultrapure water, and this solution was deoxygenated under argon flow for 15 min. An aqueous solution of NaBH₄ (0.0045 g, 0.12 mmol, ≈10 eq.) was prepared in 1 mL ultrapure water in a 25 mL Schlenk, withdrawn under argon flow. The latter was added to the HAuCl₄ solution under vigorous stirring, instantly giving a characteristic red-brown coloration; [Au] = 0.13 mmol·L⁻¹.

In a second step, a solution of K₇[PW₁₁O₃₉]·xH₂O or Na₈H[B-α-AsW₉O₃₃]·24H₂O (0.066 mmol, 5 eq.) in 5 mL of water was added to the previous solution and characterized without further modification.

**2.2.3. Intermediate stabilization strategy using a surfactant (HEA16Br)**

The experiment was carried out in a 25 mL flask under an argon atmosphere. First, HAuCl₄·3H₂O (0.006 g, 0.016 mmol) and HEA16Br (HEA: N,N-dimethyl-N-cetyl-N-hydroxyethylammonium (see scheme 1), 0.003 g, 0.008 mmol, 0.5 eq.) were dissolved in 9 mL ultrapure water, and the resulting solution was deoxygenated by argon bubbling for 15 min. In parallel, 1 mL of NaBH₄ solution (0.019 g, 0.039 mmol, 2.5 eq.) was prepared under argon flow
and added to the mixture under vigorous stirring (for 3 hours). A red-brown coloration was instantly observed, leading to an Au⁰ suspension with [Au] = 1.6 mmol.L⁻¹.

In a second step, a solution of K₇[PW₁₁O₃₉].xH₂O (0.200 g, 0.062 mmol, 4 eq.) in 5 mL of water was added to the previous solution and characterized without further modification.

### 2.2.4 Synthesis of AuNPs using citrate salts and tannic acid (Turkevich variant)

In a 250 mL flask, tannic acid (see scheme 1, 0.0004 g, 0.25 10⁻⁶ mol, 0.01 eq.) and sodium citrate (0.085 g, 0.33 mmol, 13 eq.) were dissolved in 150 mL ultrapure water. A solution of potassium carbonate was prepared (1mL, 0.021 g, 0.15 mmol) and added to the mixture to maintain the pH of the solution at 7-8. The solution was heated to 70°C for 10 min. In parallel, in a 25mL Schlenk, HAuCl₄·3H₂O (0.01 g, 0.025 mmol) was dissolved in 1 mL ultrapure water, then heated to 70°C for 10 min before being added to the medium with vigorous stirring. Red coloration was observed 2 min after addition. Heating and stirring were maintained for 15 min. [Au] = 0.16 mmol.L⁻¹.

### 2.2.5 Extrapolation of the Astruc's method to CH₃CN.

Extrapolation of the Astruc's method in acetonitrile was achieved by reaction of HAuCl₄·3H₂O and tetrabutylammonium borohydride (NBu₄BH₄). In details, HAuCl₄·3H₂O (0.0017 g, 4.4·10⁻³ mmol) was dissolved in 30 mL of CH₃CN. In parallel, NBu₄BH₄ (0.011 g, 2.2·10⁻² mmol, 5 eq. or 4.4·10⁻³ mmol, 1 eq.) was dissolved in 1 mL, and the resulting solution was added to the previous one. A red-violet coloration was immediately observed. Then, (NBu₄)₃NaH[AsW₉O₃₃{PO(CH₂)₂CO₂H}₂] (0.027 g, 8.8·10⁻³ mmol, 2 eq.) or (NBu₄)₃[PW₁₂O₄₀] (0.032 g, 8.8·10⁻³ mmol, 2 eq.) was introduced once the suspension had stabilized. [Au] = 0.15 mmol.L⁻¹.

### 2.2.6 Transfer of [PW₁₁O₃₉]⁷⁻@AuNPs into CH₃CN

A sample (10 mL) of aqueous [PW₁₁O₃₉]⁷⁻@AuNPs suspension obtained either using HEA16Br stabilization protocol or tannic acid Turkevich variant was centrifugated (14000 rpm, 1 hour). The supernatant was removed, the solid residue was redispersed in 5 mL of degassed CH₃CN under argon and the sample was sonicated for 5 minutes, leading to a homogeneous violet suspension.

## 3. Results and discussion

Firstly, as previously mentioned, the choice of acetonitrile as the solvent in this study was motivated by its widespread use in catalytic processes for the epoxidation of alkenes, particularly in the presence of POM-type catalysts.[⁴,¹⁴b] It is noteworthy that, very recently, the trivacant [SiW₉O₃₄]¹⁰⁻ species was successfully employed by Suzuki and co-workers to stabilize AuNPs in toluene, in combination with tetraoctylammonium bromide.[¹³b] Beyond the common use of nitriles in POM-catalyzed epoxidation reactions of olefins, we believed that selecting more polar organic solvents than toluene—used in Suzuki's study—likely reduces the need for long-chain surface-active cations.

Furthermore, in this work, four different polyoxometalate species were chosen based on their solubility in water and/or organic media. In aqueous media, we used two species with differing stability under the experimental conditions. The first compound, Na₈H[B-α-AsW₉O₃₃]·24H₂O, was known to be intrinsically less stable at the pH values employed, compared to the [SiW₉O₃₄]¹⁰⁻ species used by Suzuki et al.[¹³b] Our aim was to assess whether the stability of the POM associated with the nanoparticles significantly influenced the overall stability of the final suspensions. The second water-soluble POM used was K₇[PW₁₁O₃₉]·xH₂O,
which exhibits a much broader pH stability range, even in weakly basic media. We are aware
that [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup> is somewhat less stable than its aluminum derivative, [AlW<sub>11</sub>O<sub>39</sub>]<sup>9-</sup>, which is
more negatively charged and has been frequently employed by the group of Weinstock.[5c]
However, [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup> offers the significant advantage of containing a phosphorus nucleus that
can be easily monitored by <sup>31</sup>P NMR. Consequently, as with the other two POMs in this study,
spectroscopic monitoring of their integrity during the various synthesis steps was
systematically performed.

The behavior of two compounds soluble only in organic media — (NBu₄)₃NaH[AsW₉O₃₃{PO(CH₂)₂CO₂H}₂] and (NBu₄)₃[PW₁₂O₄₀]— was also examined throughout this work. Both contain phosphorus nuclei, allowing characterization by <sup>31</sup>P NMR. Of particular interest for us was the ability of the former derivative to stabilize AuNPs, given that this hybrid POM had previously demonstrated excellent performance in the selective epoxidation of alkenes.[14b] This aligns with the original goal of the study: to combine the capacity of Au⁰NPs to activate molecular oxygen into peroxides and/or superoxide with selected POMs known for their catalytic activity in alkene oxidation.

Finally, [PW<sub>12</sub>O<sub>40</sub>]<sup>3-</sup> was used as a "benchmark" molecule, owing to its well-documented use in the literature in aqueous solutions—particularly in its acidic form—for stabilizing various metal nanoparticles.[10b-e,17] Its stability in organic solvents such as CH<sub>3</sub>CN, as a tetrabutylammonium salt, is also well established, and it is frequently employed as a precursor for active species in H<sub>2</sub>O<sub>2</sub>-based oxidation processes.[18]

**3.1 Preliminary studies in aqueous medium: comparison of different strategies for the stabilization of POMs@AuNPs from alkali salts of POMs**

Four methods for synthesizing gold nanoparticles stabilized by POMs in aqueous medium were evaluated during this work. Several parameters were considered when preparing the nanoparticles suspensions, including the order of reagent addition and the choice of reducing agent. Regarding this point, NaBH₄ proved to be the most effective reducing agent, and its use was preferred over other reagents such as H₂ and hydrazine. The methods described below were carried out at room temperature unless otherwise specified. A common feature of all these procedures was that the nanoparticles were initially obtained by reducing the precursor [HAuCl₄]·3H₂O with a reducing agent, in combination with different protective agents. However, some variations were introduced, as the addition of POMs was either performed simultaneously with the reductant or in a second step after the reduction stage. Although initially promising, the approach of using reduced POMs to generate nanoparticles proved to be relatively complex to implement in a reproducible manner. Similarly, the photoactivation strategy was discarded because it requires the addition of sacrificial reductants such as alcohols, which is incompatible with the subsequent use of the suspensions in clean oxidation processes, our ultimate goal.

**3.1.1 One-pot strategies**

The “one-pot” approach involved adding all reagents simultaneously to the medium. This method was initially performed at room temperature, using either commercial HAuCl₄ or Au(OH)₃.[19] When NaBH₄ was used as the reducing agent—regardless of the conditions or the POMs tested (notably K₇[PW₁₁O₃₉]·xH₂O and Na₈H[B-α-AsW₉O₃₃]·24H₂O)—only the sedimentation of non-stabilized aggregates was observed. A simple visual check to determine the time at which aggregates appeared allowed us to identify suspensions that were not stable. Additionally, a bluish coloration of the solution, characteristic of partial and localized reduction of the POMs (likely due to the presence of intervalence bands in their reduced
forms) generally appeared before fading rapidly. For the [PW₁₁O₃₉]⁷⁻ anion, ³¹P NMR spectra were recorded before and after NaBH₄ addition. The presence of reduced species derived from [PW₁₁O₃₉]⁷⁻ was difficult to detect with this technique, as paramagnetic species require special conditions. However, new peaks appeared at 6.6 ppm (corresponding to free phosphate anions) and at -13.4 ppm (assigned to [PW₁₂O₄₀]³⁻), in addition to the original [PW₁₁O₃₉]⁷⁻ signal at -10.9 ppm. These results clearly indicate partial degradation of [PW₁₁O₃₉]⁷⁻ under these highly reducing conditions. Notably, these are the only experimental conditions during the study under which partial degradation was observed and quantified by NMR. It is likely that, beyond the basic hydrolysis of NaBH₄, the local excess of reducing agent contributed to the decomposition of the POM in this particular setup.

A variant of this one-pot method was tested in aqueous medium by adapting the protocol described by Weinstock and colleagues,[¹⁶] but with the reaction maintained at 0°C during the addition of the reducing agent. It is worth noting that in the original work by Weinstock, the [AlW₁₁O₃₉]⁹⁻ anion, claimed to be particularly effective for stabilizing Au⁰NPs in water, was used. However, its catalytic properties in oxidation reactions were less clear than those of its phosphorus analogue.[²⁰] Under these conditions, stabilized nanoparticles were formed, especially in the presence of [PW₁₁O₃₉]⁷⁻. Conversely, no characterization was performed on the sample with [B-α-AsW₉O₃₃]⁹⁻, as the suspension formed initially did not remain stable over time.

In the presence of [PW₁₁O₃₉]⁷⁻, addition of NaBH₄ resulted in a red-violet coloration, indicating that the POMs were not reduced under these conditions. This visual observation was confirmed by ³¹P NMR spectroscopy (after dilution in D₂O), which showed a single signal at -10.8 ppm, attributable to [PW₁₁O₃₉]⁷⁻. The UV-Visible spectrum (see Figure S1 in the ESI) displayed an absorption band with a maximum at 534 nm, characteristic of surface plasmon resonance (SPR). Zeta potential measurements yielded an average value of -64.2 mV, consistent with the presence of a negatively charged stabilizing agent such as POMs, as demonstrated elsewhere.

Particle size was determined using both DLS and TEM (see Figure 2 and Table 1 for values). The average sizes obtained, 27.7 nm (DLS) and 33.5 nm (TEM), are relatively close but larger than typical literature values (~5.4 ± 1.2 nm), likely due to differences in stabilizing agents (e.g., [AlW₁₁O₃₉]⁹⁻ versus [PW₁₁O₃₉]⁷⁻) used in Weinstock's seminal work.[¹⁶] Notably, large aggregates (>200 nm), composed solely of gold (as confirmed by X-EDS analysis, see Figure 2 right), were observed, which tend to overestimate particle size in DLS measurements (based on the number of particles). These findings are consistent with the UV-Vis maximum at 534 nm, compared to the [AlW₁₁O₃₉]⁹⁻-stabilized AuNP system (λmax = 526 nm, with an average diameter of 13.8 nm), also described by Weinstock et al.[²¹]

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>
<caption>Figure 2. Selected TEM micrographs of [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup>@AuNP obtained by the Weinstock's one-pot variant in water (see paragraph "2.2.1 One-pot strategies" for details)</caption>

<table><caption>Table 1. Comparison of nanoparticle sizes measured by TEM by the different approaches in water</caption><thead><tr><th>Diameter (nm)</th><th>One-pot</th><th>Weinstock's one-pot variant</th><th>Astruc's method</th><th>HEA16Br strategy</th><th>Turkevitch strategy</th><th>Turkevitch-tannic acid variant</th></tr></thead><tbody><tr><td>AuNPs</td><td>/</td><td>/</td><td>4.3 ± 1.0</td><td>9.3 ± 6.6</td><td>28.6 ± 5.9</td><td>7.1 ± 0.9 nm</td></tr><tr><td>[PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup>@AuNPs</td><td>aggregates</td><td>33.5</td><td>4.3 ± 0.7</td><td>8.1 ± 3.3</td><td>26.3 ± 6.5</td><td>7.5 ± 1.3 nm</td></tr></tbody></table>

The method used here was straightforward and reproducible. Furthermore, the suspension appears to be stable over time (no supplementary visible sedimentation) providing that the suspensions were conserved at +4°C, but the TEM study showed very large particles made up solely of gold (aggregation of isolated gold particles with each other), so there is a multimodal distribution of sizes.

### 3.1.2 Intermediate stabilization strategy using excess of BH₄⁻ (Astruc's method).

The second method, adapted from the work of Astruc and colleagues,[22] involves an initial stabilization of the nanoparticles by an excess of NaBH₄ (10 equivalents) in water. To perform this, a solution of HAuCl₄·3H₂O was prepared in an aqueous medium, to which the NaBH₄ solution was added. In a second step, either K₇[PW₁₁O₃₉]·xH₂O or Na₈H[B-α-AsW₉O₃₃]·24H₂O (5 equivalents) were introduced once the initial solution had stabilized. As before, all characterizations were carried out on suspensions of [PW₁₁O₃₉]<sup>7-</sup>@AuNPs, since the use of [B-α-AsW₉O₃₃]<sup>9-</sup> led to the rapid formation of aggregates, likely due to its high instability in solution. In each case, the suspensions were characterized before and after the addition of the POMs using UV-Visible spectroscopy, ³¹P NMR in D₂O, DLS, zeta potential measurements, and TEM. Prior to the addition of the POMs, the UV-visible spectrum displayed a plasmon band with a λmax of 519 nm (see ESI, figure S3).

<figure><img src="image_6.png" /></figure>
**Figure 3.** Astruc's method: (top) TEM micrographs of a suspension of AuNPs (using NaBH₄ solution) and (bottom) of the corresponding suspension of [PW₁₁O₃₉]⁷⁻@AuNPs (see paragraph "2.2.2. Intermediate stabilization strategy using excess of BH₄⁻ (Astruc's method)" for details). The image on the bottom right was obtained after exposure to an X-ray beam for ten seconds to increase the contrast associated with the presence of POMs.

TEM analyses (figure 3, top) revealed a high degree of homogeneity in the AuNP samples, with an average particle size of 4.3 ± 1.0 nm (see Table 1 and ESI figure S4). Despite a somewhat surprisingly lower absolute mean size of 2.1 nm observed by DLS, this confirms that this synthesis method produces nanoparticles that are both smaller and more homogeneous than in the previous example. Addition of a K₇[PW₁₁O₃₉]·xH₂O solution did not induce significant changes in the suspension's characteristics. The TEM micrographs of these suspensions (figure 3, bottom) showed that the presence of POMs did not fundamentally alter the size or morphology of the Au⁰ NPs, which remained at approximately 4.3 ± 0.7 nm (see Table 1). Moreover, this synthesis route yielded fairly well-crystallized nanoparticles, as evidenced by the observation of atomic planes on the particles (bottom right micrograph). This image was obtained after brief exposure to the X-ray source, which enhanced contrast and revealed the POMs at the nanoparticle surface, something not visible with classical TEM due to contrast limitations. Prolonged X-ray exposure allowed the POMs to be "charged" with electrons, increasing their visibility on the deposit grid. Under these conditions, the POMs appeared homogeneously distributed across the grid, and close inspection confirmed their presence at the nanoparticle level. This was further supported by zeta potential measurements before and after POM addition. In both cases, the potential was negative, but the value after initial stabilization with BH₄⁻ (or hydrolyzed species derived from it)[23] was less negative (-25.5 mV) compared to that of [PW₁₁O₃₉]⁷⁻@AuNPs (-60.1 mV), indicating a stronger stabilizing contribution from the POMs. Additionally, ³¹P NMR analysis of the suspension showed only the peak at -10.9 ppm, corresponding to [PW₁₁O₃₉]⁷⁻, confirming that the POMs showed a relative stability in solution under these conditions.

### 3.1.3 Intermediate stabilization strategy using a surfactant (HEA16Br)

The third approach involved reducing the gold precursor with NaBH₄ in an aqueous medium in the presence of a surfactant, N,N-dimethyl-N-cetyl-N-(hydroxyethyl)ammonium bromide (referred to as HEA16Br, see Scheme 1), used as a growth-directing agent. The POMs were added in a second step. HEA16Br is a quaternary ammonium salt composed of a long C16 apolar chain, two methyl groups, and a hydroxyethyl functional group, which confers it a certain solubility in polar media such as water,[24] similar to other nanoparticle stabilizers containing hydroxyl groups.[25] Literature indicates that surfactants generally have a stabilizing effect on nanoparticles (including gold) and aid in controlling their nucleation and growth processes.[24] They interact with the nanoparticle surface, which is partially negatively charged due to species such as {Au⁰Cl₂}²⁻. The presence of the surfactant on the surface thus limits aggregation of AuNPs through electrostatic repulsion and steric hindrance. An influence on the morphology of the nanoparticles was anticipated in this case.

<figure><img src="image_7.png" /><figcaption>Scheme 1: representation of HEA16Br (left), (monodeprotonated) citrate ions (medium) and tannic acid (right).</figcaption></figure>
Characterizations were performed on suspensions obtained from HAuCl₄·3H₂O, initially prepared with various equivalents of HEA16Br. However, the mixture was optimized with 0.5 equivalent of surfactant per gold atom, this ratio being used in the following sections. UV-Visible spectroscopic analysis of the resulting suspension showed a maximum absorbance at λmax = 520 nm (see Figure 4). Exposure to air for 72 hours did not affect the stability or the absorbance of the suspensions, as confirmed by UV-Visible measurements.

<figure><img src="image_8.png" /></figure>

Figure 4. UV-vis spectra of AuNPs suspensions synthesized in the presence of HEA16Br (0.5 eq. vs HAuCl₄·3H₂O, see "2.2.3. Intermediate stabilization strategy using a surfactant (HEA16Br)" for details). In black: aqueous suspension of AuNPs immediately after reduction by NaBH₄; in dark blue: aqueous suspension of AuNPs stabilized by HEA16Br after 72h; in medium blue: supernatant of the AuNPs stabilized by HEA16Br after centrifugation (14000 RPM, 1h); in light blue: suspension of AuNPs stabilized by HEA16Br after redispersion in CH₃CN; in violet: [AsW₉O₃₃{P(O)CH₂CH₂CO₂H}₂]@AuNPs in CH₃CN obtained after redispersion of the AuNPs stabilized by HEA16Br followed by addition of (NBu₄)₃NaH[AsW₅O₃₃{P(O)CH₂CH₂CO₂H}₂] (see paragraph "3.2.3 Transfer of Au⁰ nanoparticles into CH₃CN followed by addition of POMs" for details).

The mean particle size was determined by TEM counting, yielding a value of 9.3 ± 6.6 nm (see Figures 5 and S5 in the ESI for the corresponding histogram). This size correlates with the absorption maximum at λmax. Therefore, this strategy produces nanoparticles that are larger in aqueous solution compared to the previous approach, but with less homogeneity (as indicated by the large standard deviation, see Figure S5) and a wider variety of morphologies, including triangles, pentagons, and even hexagons (Figure 5, left).
<figure><img src="image_9.png" /><figcaption>Figure 5. surfactant (HEA16Br) method: (left) TEM micrographs of an aqueous suspension of AuNPs with 0.5 eq. of HEA16Br and (right) of the corresponding suspension of [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup>@AuNPs (see 2.2.3. Intermediate stabilization strategy using a surfactant (HEA16Br)” for details).</figcaption></figure>

DLS studies confirmed this trend, as the average particle size measured by this technique was 17 nm, significantly larger than the size obtained from TEM micrographs. A solution of K<sub>7</sub>[PW<sub>11</sub>O<sub>39</sub>]-xH<sub>2</sub>O (4 equivalents per gold atom) was added in a second step to the initial aqueous suspension. First, <sup>31</sup>P NMR analysis of the resulting suspensions in the presence of HEA16Br (see Figure S6) showed that the predominant species was still [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup>, accounting for over 90%, although a second peak appeared at -11.83 ppm (not assigned to [PW<sub>12</sub>O<sub>40</sub>]<sup>3-</sup>), suggesting some partial evolution of the POMs under these conditions. However, longer-term monitoring revealed no further changes in the NMR spectra. TEM observations of these suspensions are shown in Figure 5 (right). The average size of the [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup>@AuNPs was 8.1 ± 3.3 nm. Nonetheless, the samples remained very heterogeneous after POM addition. Zeta potential measurements showed a positive value of +50.3 mV in the absence of POMs, indicating stabilization of the AuNPs by the HEA16<sup>+</sup> cations. The zeta potential also exceeded ±30 mV, confirming the stability of the suspension. In the presence of POMs, the zeta potential reversed sign to -55.6 mV, confirming the significant influence of the negatively charged [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup> anions near the nanoparticles. Additionally, the absolute value of the zeta potential remained above 30 mV, indicating that the suspension was stable both with and without POMs.

### 3.1.4 Intermediate stabilization strategy using citrate ions (Turkevich method)

The so-called Turkevich method<sup>[26]</sup> involved the prior stabilization of AuNPs (from HAuCl<sub>4</sub>·3H<sub>2</sub>O) using citrate ions (20 eq.), which served both as reducing agents (E° = -0.18 V/NHE) and stabilizing agents. In the absence of POMs, diameter measurements obtained by TEM micrograph counting yielded an average size of 23.3 ± 4.5 nm (see ESI figures S7), consistent with the λ<sub>max</sub> = 531 nm observed in UV/visible spectroscopy (ESI figure S8), indicating the formation of relatively large nanoparticles. The addition of K<sub>7</sub>[PW<sub>11</sub>O<sub>39</sub>]-xH<sub>2</sub>O did not significantly alter the nanoparticle size, as confirmed by both TEM counting and UV-visible measurements (ESI Figure S8), suggesting good stability of these suspensions. Zeta potential measurements showed negative charges around the nanoparticles both in the presence (-47.8 mV) and absence (-39.3 mV) of POMs, in accordance with literature reports.<sup>[27]</sup> Notably, the use of the [B-α-AsW<sub>9</sub>O<sub>33</sub>]<sup>9-</sup> anion resulted in stable suspensions over time, which was the only case observed during our study (ESI figure S7).

Despite the stability and relatively homogeneous particle sizes, the observed diameters remained somewhat disappointing (see figure 6). To better combine homogeneity, suspension
stability, and suitable nanoparticle size, the initial protocol was modified by synthesizing nanoparticles in the presence of tannic acid. Literature reports that the combined use of citrate and tannic acid as co-reducers and co-stabilizers generally enables the production of stable suspensions with tunable size and morphology of AuNPs.[28]

<figure><img src="image_10.png" /><figcaption>Figure 6. micrograph of a suspension of citrate-stabilized AuNPs (left) and of a suspension of citrate + tannic acid-stabilized AuNPs (right); see paragraph "2.2.4 Synthesis of AuNPs using citrate salts and tannic acid (Turkevich variant)" for details.</figcaption></figure>

The UV/visible spectrum of the suspension in the absence of POMs showed a maximum absorbance at 513 nm in aqueous solution. Correspondingly, TEM micrographs revealed smaller, highly homogeneous nanoparticles with a diameter of 7.1 ± 0.9 nm (see Figures 6 and S9 in the ESI for the histogram). After addition of K<sub>7</sub>[PW<sub>11</sub>O<sub>39</sub>]·xH<sub>2</sub>O, no significant change in size or homogeneity was detected by TEM (7.5 ± 1.3 nm) or UV/visible spectroscopy. Furthermore, no variation in the UV/visible spectrum was observed over time, indicating low sensitivity to air exposure. This stability suggests that these suspensions are well-suited for use in organic media, as they can be maintained for much longer periods compared to AuNPs synthesized via other strategies such as Astruc's method.

## 3.2 Stabilization of POM-decorated AuNPs in organic medium

Three strategies were conducted in order to generate/stabilize POMs@AuNPs in CH<sub>3</sub>CN as organic medium which can be summarized as follows:

- *in situ* generation of the nanoparticles by chemical reduction of the Au<sup>III</sup> precursor in acetonitrile in the presence of POMs;

- preliminary generation of POMs-decorated Au<sup>0</sup> nanoparticles in aqueous solution followed by transfer into acetonitrile; (see figure 7)

- preliminary generation of Au<sup>0</sup> nanoparticles in aqueous solution followed by transfer into acetonitrile, then stabilized by POMs (see figure 7).

<figure><img src="image_11.png" /></figure>
Figure 7. Schematic representation of two strategies for stabilizing POMs@AuNPs into CH3CN using POMs soluble in organic medium (POMs = [AsW9O33{PO(CH2)2CO2H}2]5-, left) or in water (POMs = [PW11O39]7-, right).

### 3.2.1 Attempted one-pot reduction in the presence of polyoxometalates in CH3CN.

Due to the formation of well-defined nanoparticles in terms of size and distribution, this strategy was only applied as an extrapolation of Astruc's method to organic media. It was indeed incompatible with Turkevich's approach, which requires the use of reducing agents (sodium citrate) that are not soluble in organic solvents. Stability tests of AuNPs in CH3CN were conducted using tetrabutylammonium borohydride (NBu4BH4) as the reductant of HAuCl4·3H2O (1 or 5 eq. per gold atom), with a solution of POMs added in a second step. Two different POMs (2 eq. per gold atom) were used: (NBu4)3NaH[AsW9O33{PO(CH2)2CO2H}2] and (NBu4)3[PW12O40]. In all cases, a red-violet coloration was observed before adding POMs. This coloration remained unchanged after POM addition when using 1 equivalent of NBu4BH4. However, with 5 equivalents, the suspension immediately turned blue-grey, indicating partial reduction of the POMs due to the excess reductant. Notably, this coloration quickly disappeared upon exposure to air, and the initial red-violet color was restored.

It is worth noting that this initial red-violet coloration disappeared after 24 hours in the absence of POMs, whereas it persisted for several weeks in the presence of both POMs. TEM characterization of the suspensions prepared with 1 equivalent of NBu4BH4 was performed before (AuNPs) and after ([PW12O40]3-@AuNPs) the addition of POMs (see Figures 8 and S10 in the ESI for the corresponding size histograms).

<figure><img src="image_12.png" /><figcaption>Figure 8: TEM micrographs for AuNPs (left) and [PW12O40]3-@AuNPs (right) obtained in CH3CN after reduction by 1 eq. of NBu4BH4, (see paragraph "2.2.5 Extrapolation of the Astruc's method to CH3CN" for details).</figcaption></figure>

The formation of gold nanoparticles with a consistent average size was observed both in the absence (21.1 ± 7.3 nm) and presence (21.0 ± 7.5 nm) of [PW12O40]3-. Therefore, the addition of POMs had no significant influence on the suspension.

This method in CH3CN appeared promising, as it produced stable suspensions without the need for centrifugation. The study also highlighted a stabilizing effect of POMs on the suspensions, which were initially unstable before POM addition. However, optimizing the number of POM equivalents is undoubtedly necessary to accurately assess their stabilizing role in an organic environment.

Nevertheless, the main drawback was the relatively large size of the AuNPs and the significant dispersity in their size, which seemed incompatible with subsequent catalytic applications. Finally, two other reducing agents (H2 and hydrazine) were tested but did not yield stable suspensions, so their use was not further pursued.
### 3.2.2 Transfer of POMs-decorated Au⁰ nanoparticles from water into CH₃CN

In this case, the transfer of two different [PW₁₁O₃₉]⁷⁻@AuNP suspensions (obtained via the Turkevich variant with tannic acid or using the HEA16Br surfactant, see paragraphs 3.1.3 and 3.1.4) was performed by centrifugation of the aqueous suspensions (14,000 rpm, 1 hour) followed by redispersion in CH₃CN. In both cases, the transfer protocols were duplicated to ensure the best possible comparison.

Analysis of the supernatant after centrifugation by UV-visible spectroscopy allowed us to quantify the amounts of transferred nanoparticles by subtracting the residual absorbance. This also confirmed the homogeneity of the transferred nanoparticle suspensions, as no significant shifts in wavelength were observed compared to the original suspensions.

After transfer into CH₃CN, stable violet suspensions of [PW₁₁O₃₉]⁷⁻@AuNPs were obtained in both cases and characterized by UV-visible spectroscopy, showing λmax = 540 nm for the samples with HEA16Br and λmax = 546 nm for those prepared via the Turkevich variant (see ESI figure S11). It is important to note that, under these conditions, the correlation between the λmax value and particle size is not particularly relevant, since the suspensions were formed in an organic medium that differs significantly from the initial aqueous suspension.

Finally, TEM and X-EDS analyses were performed to gather additional information on the composition of these systems (see figure 9).

<figure><img src="image_13.png" /><figcaption>Figure 9. TEM micrographs of [α-PW₁₁O₃₉]⁷⁻@AuNPs obtained in CH₃CN from tannic acid Turkevich variant (left) and from HEA16Br surfactant strategy (right). The corresponding X-EDS spectra are displayed in ESI figure S12.</figcaption></figure>

As expected, the average diameters of the [PW₁₁O₃₉]⁷⁻@AuNP nanoparticles after redispersion in CH₃CN observed by TEM were close to those obtained in the corresponding raw aqueous suspensions, as shown in the histograms in the ESI (figures S5, S9, S11, and Table 1):

- Tannic acid Turkevich variant in water: average diameter = 7.5 ± 1.3 nm.

- Tannic acid Turkevich variant after redispersion in CH₃CN: 5.87 ± 0.83 nm.

- HEA16Br surfactant strategy in water: average diameter = 8.1 ± 3.3 nm.

- HEA16Br surfactant strategy after redispersion in CH₃CN: 9.40 ± 3.38 nm.

In our opinion, the differences observed are on the order of the uncertainties associated with nanoparticle counting or centrifugation procedures. The same trends were observed throughout the transfer into CH₃CN: significant heterogeneity was observed in the case of the HEA16Br strategy, whereas a certain degree of size and shape homogeneity was noted in the other case.

X-EDS analyses proved particularly informative for each of the samples dispersed in acetonitrile. Indeed, tungsten quantification in the case of AuNPs obtained via the tannic acid
Turkevich variant revealed a near absence of $[PW_{11}O_{39}]^{7-}$ (Au/W ≈ 23) around the nanoparticles after dispersion in the organic medium. Conversely, tungsten was detected in the case of the HEA16Br strategy, indicating a much more efficient transfer of $[PW_{11}O_{39}]^{7-}$ units. An average Au/W ratio of approximately 5 was observed, confirming the substantial presence of POMs around the nanoparticles. Additionally, X-EDS analysis of the interstitial medium in this sample indicated the presence of a certain quantity of POMs outside the zones where the nanoparticles were observed. We therefore concluded that, as expected, the presence of positively charged HEA16⁺ cations facilitated a much larger transfer of POMs into the organic medium.

As a result of this strategy, the presence of POMs from the aqueous suspension could be demonstrated after their transfer to acetonitrile in these two systems, despite the insolubility of the POM salts used. The presence of HEA16⁺ cations as (co-)stabilizers undoubtedly played a role in their significant proximity to the nanoparticles. We can probably conclude that these cations acted as phase transfer agents for the POMs in this case. However, the influence of these cations on the stabilization of the AuNPs in the protocol requiring citrate ions and tannic acid raises questions. In this scenario, there was undoubtedly competition between the POMs and citrate anions. Therefore, we adopted a third strategy involving the prior transfer of AuNPs from the aqueous suspension to acetonitrile prior to the addition of POMs.

### 3.2.3 Transfer of Au�0 nanoparticles into CH3CN followed by addition of POMs

Several systems were selected based on the different synthetic methods proposed in the previous sections. In all cases, the protocols were duplicated to ensure the most accurate comparison possible. Suspensions of AuNPs (prepared as described in sections 3.1.2 to 3.1.4) were centrifuged (at 14,000 rpm for 1 hour) and redispersed in acetonitrile. In a second step, POMs (specifically $(NBu_4)_3[PW_{12}O_{40}]$ or $(NBu_4)_3NaH[AsW_9O_{33}\{P(O)CH_2CH_2CO_2H\}_2]$) were added as tetrabutylammonium salts to the various organic suspensions. Each sample was then analyzed using UV-visible spectroscopy, TEM, and X-EDS. The amounts of POMs (fixed at 0.1 equivalents per Au atom) were calculated based on the quantities of AuNPs transferred to the organic medium, determined from the differences in absorption measured by UV-visible spectroscopy before and after centrifugation of the aqueous suspensions. It is noteworthy that the POM/Au ratio was fixed at this specific value of 0.1 because higher POM concentrations led to the suspension collapsing in the short to medium term.

As a matter of fact, the addition of POMs, within the limits of this ratio, appeared to have a significant impact on the visual appearance of the suspensions. Indeed, the suspensions did not appear perfectly clear after redispersion in CH3CN in the absence of POMs, which was confirmed by their UV-visible spectra (see ESI figure S13). For example, figure 4 compares the UV-visible spectra of the HEA16⁺-stabilized AuNPs suspension at each stage of the transfer process. The spectrum of the AuNPs suspension obtained immediately after dispersion into CH3CN showed a residual absorbance that remained very high at longer wavelengths, characteristic of poor redispersion efficiency (estimated $\lambda_{max} \approx 545$ nm). Conversely, the addition of 0.1 eq. of POMs (either $[PW_{12}O_{40}]^{3-}$ or $[AsW_9O_{33}\{P(O)CH_2CH_2CO_2H\}_2]^{5-}$) to the organic medium immediately resulted in a more transparent appearance of the samples and a well-defined UV-visible spectrum (see figure 4). It is noteworthy that, despite the modification in the spectral shape, the maximum absorption wavelength remained unchanged.
This last remark is of particular importance because TEM studies performed in parallel showed no change in the size or morphology of the AuNPs following the addition of POMs. Continuing with the example of systems using HEA16Br, the measured diameters and the appearance of the samples obtained in acetonitrile (Figure 9), both in the absence and presence of POMs, are quite comparable. Thus, the average diameter of the AuNPs after redispersion in acetonitrile was 10.4 ± 4.2 nm (see corresponding histogram in ESI, Figure S14), which is close to what was observed in water (9.3 ± 6.6 nm) in paragraph 3.1.3. After the addition of 0.1 eq.? of $(NBu_4)_3NaH[AsW_9O_{33}(P(O)CH_2CH_2CO_2H)_2]$, the measured average diameter was 7.9 ± 3.1 nm, remaining within the same range considering the standard deviation of the distribution.

<figure><img src="image_14.png" /><figcaption>Figure 9. TEM micrographs of AuNPs stabilized by HEA16Br prepared in water (see paragraph "2.2.3. Intermediate stabilization strategy using a surfactant (HEA16Br)" for details), centrifugated then transferred into CH₃CN before (left) and after (right) addition of 0.1 eq. of $(NBu_4)_3NaH[AsW_9O_{33}(P(O)CH_2CH_2CO_2H)_2]$ per Au atom.</figcaption></figure>

In the present example, X-EDS spectra (recorded on an area containing only nanoparticles) indicated an Au/W ratio of ca. 1.7. No Au atoms were detected in the interstitial space. Comparing their respective Au/W ratio, the number of POMs around the AuNPs was, as expected, therefore much greater than for the strategy that involved transferring the POMs@AuNPs directly from the aqueous phase into acetonitrile (Au/W ratio = 5, see paragraph 3.2.2).

A Raman spectroscopy study was also conducted to confirm the presence of POMs around the nanoparticles in samples prepared via both strategies (Figure 10). These samples were obtained either i) after the initial addition of $K_7[PW_{11}O_{39}]·xH_2O$ to the AuNPs suspensions in water, followed by centrifugation and redispersion in CH₃CN, or ii) after centrifugation of the aqueous AuNPs suspensions, redispersion in CH₃CN, and subsequent addition of $(NBu_4)_3NaH[AsW_9O_{33}(P(O)CH_2CH_2CO_2H)_2]$ to the resulting suspension.

The Raman spectra were quite similar and characterized by bands attributed to the POMs—particularly those assigned to the W=Ot bands in the range [950–1030 cm⁻¹]—as well as bands corresponding to the HEA16⁺ cations. Although the amount of [PW₁₁O₃₉]⁻ units detected around the AuNPs was lower in concentration in the first strategy, as indicated by X-
EDS, these units could still be easily observed using Raman spectroscopy due to its high sensitivity to POM bands, as demonstrated in our previous work.[8]

<figure><img src="image_15.png" /></figure>

Figure 10. Comparison of the Raman spectra of POMS@AuNPs obtained by the HEA16Br surfactant strategy and transferred in CH<sub>3</sub>CN: green lines: initial preparation in aqueous solution followed by addition of K<sub>7</sub>[PW<sub>11</sub>O<sub>39</sub>].xH<sub>2</sub>O in water, then centrifugation and redispersion in CH<sub>3</sub>CN; black lines: initial preparation in aqueous solution followed by centrifugation, redispersion in CH<sub>3</sub>CN then addition of 0.1 eq. of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>] per Au atom. The wavenumbers values in blue were attributed to HEA16<sup>+</sup> cation, those in black to the POMs (including NBu<sub>4</sub><sup>+</sup> ions). The bands with an asterisk were attributed to remaining CH<sub>3</sub>CN.

The stability of the resulting suspensions was monitored by UV-visible spectroscopy over a period of 42 hours. Figures 11.A, B, and C illustrate the evolution of the stability of suspensions prepared using the Astruc method with respectively 0.01, 0.1, and 1 equivalent of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>] per Au atom. In all cases, a significant decrease in absorbance was observed over time, indicating a progressive loss of stability in these suspensions. However, this instability was less pronounced at 0.1 equivalents of POMs per Au atom, which explains why this ratio was chosen for the other samples as previously mentioned. This ratio appears to be optimal for maintaining moderate stability of the suspensions, regardless of the synthetic strategy employed, as similar trends were observed with the Turkevich tannic acid variant (Figure 11.D) and the HEA16Br strategy (Figure 11.E). For all these samples, comparable results were obtained using (NBu<sub>4</sub>)<sub>3</sub>[PW<sub>12</sub>O<sub>40</sub>].
<figure><img src="image_16.png" /><figcaption>**Figure 11.** UV-visible analysis of the stability of the different samples obtained after centrifugation, redispersion of aqueous AuNPs suspensions in CH<sub>3</sub>CN and subsequent addition of POMs as a function of time. **A:** AuNps obtained by the Astruc method with 0.01 eq./Au atoms of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>]; **B:** the same with 0.1 eq./Au atoms of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>]; **C:** the same with 1 eq./Au atoms of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>]; **D:** AuNps obtained by the Tannic acid Turkevich variant with 0.1 eq./Au atoms of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>]; **E:** AuNps obtained by the HEA16Br surfactant strategy with 0.1 eq./Au atoms of (NBu<sub>4</sub>)<sub>3</sub>NaH[AsW<sub>9</sub>O<sub>33</sub>{P(O)CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H}<sub>2</sub>].</figcaption></figure>

**4. Conclusion**

The aim of this work was to perform a (non-exhaustive) screening of available methods for preparing stable suspensions of gold nanoparticles stabilized by polyoxometalates in an organic medium, specifically CH<sub>3</sub>CN, a solvent commonly used in catalytic oxidation reactions. Several strategies, adapted from the literature, were implemented in parallel to evaluate their respective strengths and weaknesses for potential transfer to CH<sub>3</sub>CN. However, comparing these synthetic methods proved to be challenging, as each exhibited its own specific features. For instance, the amounts of POMs observed around the AuNPs could not be directly compared when POMs were introduced at the beginning versus at the end of the protocol. Establishing a universal criterion for the efficiency of each stabilization strategy in an organic medium is undoubtedly complex. The first criterion involved assessing the stability of POMs@AuNPs suspensions, initially in aqueous media and subsequently in CH<sub>3</sub>CN, using various physicochemical techniques. The second criterion was based on observations and analyses by TEM, including the construction of size histograms to evaluate both the average NPs diameter (which is a key parameter for catalytic applications) and the homogeneity of the samples.

Thus, POMs@AuNPs in CH<sub>3</sub>CN were successfully prepared using three different methods: (i) in situ generation of the nanoparticles by chemical reduction of the Au<sup>3+</sup> precursor with NBu<sub>4</sub>BH<sub>4</sub> in CH<sub>3</sub>CN in the presence of POMs; (ii) formation of POM-decorated Au<sup>0</sup> nanoparticles in aqueous solution followed by transfer into CH<sub>3</sub>CN after centrifugation; and
(iii) transfer of aqueous Auº nanoparticle suspensions into CH3CN after centrifugation, followed by the addition of soluble organic salts of POMs. All three strategies resulted in stable suspensions, with observed differences mainly related to the size and morphology of the AuNPs. At this stage of the study, the most satisfactory preparation method—considering both the small size of the AuNPs and the stability of the suspensions over time—appears to be the Turkevich method in the presence of tannic acid. Notably, except for the first (one-pot) strategy, all AuNPs were initially obtained in water, then transferred into CH3CN after centrifugation and redispersion. These two steps (centrifugation and redispersion) had no significant visible impact on their geometrical characteristics, as confirmed by TEM analysis.

The most significant contribution of POMs in this study was their role as stabilizing agents for suspensions in acetonitrile. This approach led to an improvement in both the immediate and long-term stability of the suspensions after transfer to CH3CN, achieved through controlled addition of POMs soluble in organic media. This enhancement was further validated by UV-visible spectroscopy. Finally, it is worth highlighting the evidence of transfer of water-soluble salts of POMs, such as K7[PW11O39].xH2O, into organic media after centrifugation of [PW11O39]7⁻@AuNPs suspensions prepared in aqueous media. This transfer was supported by HR-TEM and X-EDS studies.

From our perspective, this fundamental study represents an essential step toward developing bifunctional catalytic systems capable of performing controlled aerobic oxidation of non-water-soluble organic substrates.

**5. Acknowledgements.**

This study benefited from the support of the FEDER project TER PENOX funding by the European Regional Development Funds GURDTI 2020-0969-0022947, (EU-Région Réunion-French State national counterpart) for the post-doctoral fellowship of Dr Jean-Baptiste Harlé, and for financial reward for Mrs Elise Péruse and Jimika Hassanaly. The authors want also to acknowledge the French Centre National de la Recherche Scientifique (CNRS), the Université de la Réunion and Sorbonne Université.

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