Self-Functional Off-Stoichiometry Polymeric Materials: Potential for Tunable Plasmonic Application

Abstract

Self-functional polymeric materials are of paramount importance in emerging lab-on-a-chip device development that finds various optical sensing applications. The unique surface functional characteristics make this polymer an ideal choice for the development of plasmonic sensing platforms. In this regard, a systematic experimental study was conducted to develop an off-stoichiometry thiol–ene–epoxy (OSTE+) platform by adjusting the stoichiometric ratio of the tetra-thiol moiety. The correlations between elastic moduli and optical properties of these flexible polymer thin films were investigated by comparing them to the molar concentrations of thiol, ene, and epoxy monomers. The unique tunable surface functional characteristics of these OSTE+ thin films were utilized for gold-nanoparticle-immobilized plasmonic surface development. The collective plasmonic resonance peak of the nanoparticles was modulated by optimizing the concentration of thiol groups (−SH). Changes in the chemical composition of −SH groups were correlated to the surface density of nanoparticles using X-ray photoelectron spectroscopy (XPS). These XPS measurements show covalent interactions between spherical gold nanoparticles and the available thiols that formed metal–thiol bonds (Au–S). The experimental observations of self-functional properties and the effect of the thiol-excess stoichiometry were correlated to the changes in binding energies of Au–S bonds due to nanoparticle interactions. This tunable plasmonic study of Au–S bonding with the sulfur or sulfide-mediated heteroatoms on the OSTE+ polymer surface has an extensive device innovation feasibility for light-manipulating sensing applications. Ultimately, this controllable thiol functionality of off-stoichiometry polymer thin films has enormous potential for the development of a single-polymer-based biocompatible surface for lab-on-chip devices.