Solution-phase, dual LSPR-SERS plasmonic sensors of high sensitivity and stability based on chitosan-coated anisotropic silver nanoparticles

There is a need to design highly sensitive plasmonic sensors which impart a good biocompatibility and optical stability to detect low levels of analytes in biological media. In this study we report the formation of chitosan-coated silver nanoparticles of triangular shape in solution by synergistic action of chitosan and trisodium citrate in the presence of silver seeds and ascorbic acid. It has been revealed that these anisotropic silver nanoparticles entrapped in biopolymeric shells are particularly stable and can be successfully used as versatile plasmonic substrates for molecular sensing in solution. In particular, the binding of the probe molecule monolayer (para-aminothiophenol, p-ATP) at the surface of individual chitosan-coated silver nanoparticles was demonstrated both by localized surface plasmon resonance (LSPR) shifts and surface-enhanced Raman scattering (SERS) spectra. While the LSPR-shift assay is operational for signaling molecular binding events, the SERS allows identifying the probe molecules and elucidating its orientation on the metal surface. The proof of concept for biosensing applications and dual functionality of plasmonic platform are evaluated through the combined LSPR-SERS detection of significant biological molecules, adenine. The potential of chitosan-silver nanostructures to extend the standard approach of LSPR sensing by integrating SERS measurements and operate as dual plasmonic sensors would be very attractive for investigation of analytes in biological fluids.