Layer-by-layer chitosan and gold nanoparticles as surface-enhanced Raman scattering substrate

The layer-by-layer (LbL) method demonstrates significant versatility for constructing substrates with nanoscale-thick layers and diverse compositions. This technique is based on alternate deposition of species that interact electrostatically. Studies have demonstrated that this technique holds promise for use as a surface-enhanced Raman scattering (SERS) substrate, enabling the detection of pesticides at low concentrations and finding applications in the biological field. In this study, a simple methodology based on the layer-by-layer (LbL) method was developed to construct SERS substrates. Substrates with varying numbers of bilayers (1, 5, or 10) were built on glass slides. The positive layer was composed of the natural polysaccharide chitosan. In contrast, the negative layer consists of gold nanoparticles (AuNPs) with a negative charge on their surface, achieved using sodium citrate as a reducing agent. The SERS substrates were characterized by UV–VIS-NIR spectroscopy and atomic force microscopy (AFM). They were tested as SERS substrates by utilizing thiophenol (TP) at a concentration of 1.0 × 10^–3 mol L⁻¹. The distribution of the SERS signal was monitored through the 417 cm⁻¹ peak, which is attributed to the CH stretching mode characteristic of TP. The substrates exhibited a more significant enhancement with an increase in the number of bilayers. They have proven highly promising for future applications, such as diagnostic evaluation of diseases, detection of pesticide molecules, biological molecules, and various other applications.