Polyoxotungstate-Stabilized Gold Nanoparticles: How to Transfer from an Aqueous Suspension to CH3CN

The present study aims 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 are 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, are used to assess their strengths and limitations regarding translation to CH₃CN. The resulting suspensions are characterized by UV-Vis spectroscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy analysis, dynamic light scattering, and zeta potential measurements. The most stable suspensions are 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 results in negligible changes in nanoparticle size or suspension stability.