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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 LSPRshift 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.

The integration of metal-enhanced fluorescence (MEF) and surfaceenhanced Raman scattering (SERS) onto single plasmonic nanoparticles is highly desirable to boost up accuracy and sensitivity in spectroscopic investigations. In this work, we demonstrate the feasibility of performing both MEF and SERS analysis on gold nanorods (GNRs) decorated with a unique molecular probe, here Rose Bengal (RB) fluorophore. While the cetyltrimethylammonium bromide layer on GNRs captures RB molecules preventing fluorescence quenching, the selective excitation of the two surface plasmon resonances of GNRs can trigger either the enhancement of fluorescence emission or Raman scattering. Both fluorescence enhancement mechanisms, i.e. increase of excitation rate and increase of quantum efficiency, promote an average 2.2-fold enhancement. For excitation with 785 nm laser line an average SERS enhancement of 1.7 × 10 5 is estimated and compared with the electromagnetic enhancement provided by finite-difference time-domain (FDTD) simulations. An extended dual-modal response of RB−GNRs conjugate is demonstrated by resonant excitation of both RB and GNRs with 532 nm laser line, which allows the surface-enhanced resonance Raman scattering (SERRS) and MEF signals to be detected simultaneously. The reporting abilities in SERS are demonstrated by conveying the Raman fingerprints of two molecular species concomitantly adsorbed on the surface of GNRs.

A great number of works have focused their research on the synthesis, design and optical properties of gold nanoparticles for potential biological applications (bioimaging, biosensing). For this kind of application, sharp gold nanostructures appear to exhibit the more interesting features since their surface plasmon bands are very sensitive to the surrounding medium. In this paper, a complete study of PEGylated gold nanostars and PEGylated bipyramidal-like nanostructures is presented. The nanoparticles are prepared in high yield and their surfaces are covered with a biocompatible polymer. The photophysical properties of gold bipyramids and nanostars, in suspension, are correlated with the optical response of single and isolated objects. The resulting spectra of isolated gold nanoparticles are subsequently correlated to their geometrical structure by transmission electron microscopy. Finally, the PEGylated gold nanoparticles were incubated with melanoma B16-F10 cells. Dark-field microscopy showed that the biocompatible gold nanoparticles were easily internalized and most of them localized within the cells.

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