Gold nanoparticles–polyoxometalates composites: a comprehensive overview from synthesis and characterization to applications

Gold nanoparticles (AuNPs) have attracted attention for their unique electronic and optical properties and their wide range of applications. The development of hybrid nanoparticles by combining AuNPs with other molecules or materials is a powerful way of introducing new functions and extending their use. In particular, gold nanoparticles can be associated to organometallic complexes, biological molecules, coordination polymers or polyoxometalates (POMs), the latter being a group of soluble anionic metal oxides composed of transition metals in high oxidation states. Inorganic POMs come in a wide variety of shapes and sizes and are valued especially for their biological and catalytic properties. They are particularly interesting for the synthesis of AuNPs composites, as they can stabilize nanoparticles through electrostatic or covalent interactions and steric effects without the presence of additional surfactants, reduce gold cations by photochemical, electrochemical, or chemical reduction, or directly when used in their reduced form, and confer properties to the resulting systems. POMs can also be covalently combined with organic molecules of interest, which can in turn contribute to the functionalities of the gold nanoparticles. This review focuses on gold nanoparticles directly coated with POMs and covers their synthesis and characterization. Particular attention is also paid to their applications, including (photo)(electro)catalysis, biomedical applications (antibacterial, anticancer and anti-Alzheimer's) and their use as sensors.