Enhancing gold nanoparticle immobilization on thiolated silica: utilizing neutral ligands to achieve maximum surface coverage for improved SERS substrates

Using a silane-thiol linker enables the direct immobilization of gold nanoparticles (AuNP) on a silica substrate, providing benefits such as cost-effectiveness, improved stability, and easy scalability for surface-enhanced Raman spectroscopy (SERS) substrate fabrication. However, achieving sufficient surface coverage has proven to be a persistent challenge. Consequently, the immobilization process heavily relies on electrostatic interactions, which may attain the desired surface coverage but often result in suboptimal adhesion and reduced durability. To tackle these challenges, our study delved into the previously unexplored realm of the surface charge of the thiol terminal, emphasizing its pivotal significance for the first time. In this exploration, we identified repulsive surface charges between citrate-stabilized AuNP and a silane-thiol linker. This discovery prompted us to replace a common AuNP stabilizer, citrate, with a neutrally charged ligand (polyvinylpyrrolidone). By implementing optimal washing procedures following the substitution, we successfully alleviated the repulsive environment, thereby enhancing the immobilization process. As a consequence of this modification, we achieved successful AuNP immobilization, surpassing the jamming limit for the first time. This limit signifies the maximum surface coverage achievable through random sequential adsorption. This innovative development holds significant promise for improving the overall performance and effectiveness of SERS substrates in various applications. This study achieved maximum surface coverage and high chemical stability of SERS substrates by replacing citrate with a neutral ligand and controlling the ligand thickness, addressing poor immobilization of citrate-AuNP on thiolated silica.