(Invited) Unique Size-Dependent Electrochemistry of Metal Nanoparticles in Aqueous and Hydrogel Environments

Metal nanoparticles (NPs) exhibit unique size-dependent physical and chemical properties, providing many potential applications including sensing, catalysis, optoelectronics, medical diagnostics and disease therapeutics, among others. Here we describe interesting size-dependent metal oxidation, metal reduction, and electrocatalytic properties for Au and other metal nanoparticles that are coated with relatively weak-binding stabilizers, which is necessary for effective electron transfer at the metal NP surface. Metal oxidation potentials decrease dramatically negative with decreasing NP size, especially below 4 nm down to ~1 nm in radius. Electrocatalytic seeded-growth kinetics also depend strongly on the metal NP size (seed size). The thermodynamics of other oxidation/reduction reactions, such as galvanic replacement, also depend dramatically on metal NP size, resulting in unique alloy nanostructures with improved electrocatalytic reactivity and stability. Size-dependent electrophoretic deposition of metal nanoparticles to form thin films also results from size-dependent electrocatalytic properties, allowing metal NP separations based on electrocatalytic activity. These oxidation, reduction, galvanic replacement, and electrocatalytic properties change dramatically when the metal NPs are coated with strong ligand stabilizers or when placed in a viscous hydrogel environment. The effect of the ligand and environment will be described here. A better understanding of size-dependent reactivity of metal NPs as a function of the type of stabilizer and environment is important to realize their potential for tuning the properties to optimize them for a specific application.