Size-Dependent Shifts of Plasmon Resonance in Silver Nanoparticle Films Using Controlled Dissolution: Monitoring the Onset of Surface Screening Effects

A study of the size dependent blue- and red-shift of the plasmon band of silver nanoparticle films in aqueous solution is reported. A detection scheme, where the particle size is continuously decreased by means of controlled dissolution, while measuring the plasmon band position by UVvis absorption spectroscopy is used. Both blue- and red-shifts of the peak position are observed, depending on the presence of electron donors and/or acceptors in the solution, respectively. A great increase in plasmon shifts for smaller particle sizes (R < 5 nm vs R > 10 nm) is demonstrated, which we ascribe to a transition from an extrinsic regime for the larger particles, where shifts of the plasmon frequency are related to changes in the dielectric environment, while the dielectric function of the metal is constant, to an intrinsic regime for the smaller particles. For this intrinsic regime, operative for small particles, nonlocality of the dielectric constant has to be considered. The experimental data are fitted with a recently published theoretical model that takes dynamical surface screening and quantum size effects into account as well as by a Drude model with a corrected effective volume, resulting in a corrected electron density. The reported results have potential for developing nanosensors based on small nanoparticles below 5 nm in size by using their intrinsic response to adsorbed analytes. This detection scheme suggests a potential increase in the sensitivity of up to 3X, particularly when redox processes play a role, such as for the detection of reactive oxygen species.