Plasmonic sensing

Plasmonic sensors typically rely on detection of changes in the refractive index of the surrounding medium. Here, an alternative approach is reported based on electrical surface screening and controlled dissolution of ultrasmall silver nanoparticles (NPs; R < 5 nm) that can result in a great increase in the sensitivity, as compared to traditional refractive index sensing. Here, a detection scheme based on controlled dissolution is reported. Monitoring the plasmon band, while the particles are continuously decreased in size, enables the investigation of size-related effects on the same fixed particle ensemble, thereby overcoming typical variations due to differences between samples. For small particle sizes, charge displacement/transfer processes at the particle surface can change the material properties and morphology of the NPs dramatically, resulting in, e.g., a decrease of and a blue shift in the plasmon band. This is demonstrated by using the strong nucleophiles, cyanide and cysteamine, as ligands. The “dissolution paths” in terms of peak wavelength and amplitude shifts differ significantly between different types of analytes, which are suggested as a means to obtain selectivity of the detection that cannot be obtained by traditional refractive index sensing, without the use of bioprobes. A simple modified Drude model is used to account for shifts in the plasmon band position due to electrical charging. Here, a screening parameter is introduced in the expression for the free electron density of the NPs in order to calculate a size-dependent electron density, due to electrical surface screening.