Different Protecting Groups to Cap Mercapto Propyl Silatrane Affect Water Solubility and Surface Modification Efficiency

Abstract

Surface modification is an important field and widely applied to biosensors, biomaterials, and semiconductors. Mercapto propyl trimethoxyl silane (MPTMS) is a most common material applied to surface modification in biosensor chips. However, MPTMS is moisture sensitive, slow to modify reaction rates with substrate surfaces, and unstable due to thiol groups, which restrict the expansibility of MPTMS. Previously, we synthesized mercapto propyl silatrane (MPS) to improve moisture sensitivity and increase reactivity with substrate surfaces. Despite these improvements, MPS still requires a high-polarity organic solvent environment and the thiol groups remain susceptible to oxidation by oxygen. The utility of mercaptan-functionalized films critically depends on their stability under ambient conditions. As global environmental awareness increases, developing stable and environmentally friendly silane molecules has become increasingly important. In this report, we explored different protective groups (acetyl- (Ac-), di-tert-butyl carboxyl- (Boc-), and triphenylmethyl- (trityl-)) to cap the mercapto group of MPS, enhancing the stability of the thiol group. We characterized the Boc-MPS, Ac-MPS, and trityl-MPS modifications on substrates using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Additionally, we tracked the kinetic rate of capping MPS modification on substrates by using gold nanoparticles (AuNPs). Our results indicated that all protective groups successfully inhibited the oxidation of the thiol groups. Notably, some protective groups (Ac- and Boc-) increase the water solubility. By tracking the rate of Ac-MPS modification on glass surfaces using AuNPs, we observed that Ac-MPS demonstrated a higher surface modification effectiveness than MPS. Furthermore, the water solubility of Ac-MPS allowed for modification on both glass and plastic substrates in aqueous solutions, significantly broadening the application scope of Ac-MPS for various substrate modifications. In this study, we also tested the LSPR refractive index sensitivity of mercapto propyl trimethoxysilane (MPTMS) and Ac-MPS anchored AuNPs on a glass substrate. The results revealed that glass substrates with Ac-MPS anchored AuNPs exhibited greater sensitivity compared with MPTMS coatings. Interestingly, the refractive index test indicated that plastic substrates with Ac-MPS anchored AuNPs showed better sensitivity than glass substrates. This work presents a novel approach to enhancing the stability, water solubility, and surface coating efficiency of MPS through structural extension. We believe that this method can be widely applied to silatrane extensions, potentially eliminating the need for organic solvents in sol–gel surface modification systems. This work significantly expands the applications of silatrane in green chemistry and sustainable development.