The approach of combining different mechanisms of antibacterial action by designing hybrid nanomaterials provides a new paradigm in the fight against resistant bacteria. Here, we present a new method for the synthesis of silver nanoparticles enveloped in the biopolymer chitosan. The method aims at the production of bionanocomposites with enhanced antibacterial properties. We find that chitosan and silver nanoparticles act synergistically against two strains of Gram-positive Staphylococcus aureus (S. aureus). As a result the bionanocomposites exhibit higher antibacterial activity than any component acting alone. The minimum inhibitory (MIC) and minimum bactericidal (MBC) concentrations of the chitosan-silver nanoparticles synthesized at 0 °C were found to be lower than those reported for other types of silver nanoparticles. Atomic force microscopy (AFM) revealed dramatic changes in morphology of S. aureus cells due to disruption of bacterial cell wall integrity after incubation with chitosan-silver nanoparticles. Finally, we demonstrate that silver nanoparticles can be used not only as antibacterial agents but also as excellent plasmonic substrates to identify bacteria and monitor the induced biochemical changes in the bacterial cell wall via surface enhanced Raman scattering (SERS) spectroscopy.

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.

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.