Comparative study of gold and silver based nanobiosensors

Due to their particular optical properties, resulting from the surface plasmon resonance (SPR) phenomenon, silver and gold nanoparticles (NPs) can be used as robust tools for optical biosensing [1]. Optical SPR biosensors are able to measure complex formation in real time. Indeed, the SPR absorption spectrum band of the NPs is sensitive to the shape, size, inter-particle distance and composition of the NP as well as the dielectric properties of the surrounding medium [2]. Due to the sensitivity of SPR to the local dielectric environment, plasmonic NPs can act as transducers that convert small changes in the local refractive index and the inter-particle distance into spectral shifts and broadening in the absorption spectra bands [3].
In our study, the prototypical biocytin-avidin interaction was used to study gradual changes with time in the absorption spectra bands of biotinylated 10 nm silver and gold NPs as a function of added avidin. After avidin addition, a SPR red-shift and a broadening of the SPR bands were observed with both NPs. These optical changes evolved with time and reached their final values after around 45 min for each system. The maximum SPR red-shifts were equal to 25 nm and 15 nm for silver NPs and gold NPs, respectively. The detection limit, described as the lowest concentration for clear identification of wavelength shift due to biomolecular recognition, is determined to be 4 nM for both silver and gold NPs. The specificity of the biocytin-avidin biosensors was checked by replacing avidin by BSA. When BSA was added, we observed a SPR band shift which was smaller than the detection limit of 3 nm, attesting the biosensor selectivity. Our work demonstrates the superiority of Ag over Au NPs for the elaboration of biosensors based on SPR.