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

## TL;DR

This study explores how different protective groups improve the stability and water solubility of a chemical used in surface modification, making it more versatile and eco-friendly.

## Contribution

The novel use of acetyl and Boc protective groups enhances MPS water solubility and surface modification efficiency.

## Key findings

- Ac-MPS and Boc-MPS show increased water solubility compared to MPS.
- Ac-MPS modification is more effective on glass and plastic substrates in aqueous solutions.
- Ac-MPS anchored AuNPs on plastic substrates show higher refractive index sensitivity than on glass.

## 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.