Redox-Triggered Interfacial Reconstruction as an Analytical Transduction Mechanism for Optical Detection of Arsine with Liquid Crystals

Abstract

Arsine (AsH3) is widely used in semiconductor manufacturing yet poses severe health risks, motivating sensitive on-site monitoring for personnel safety. We report a liquid crystal (LC)-based optical sensor that translates arsine-triggered surface chemistry into an orientational transition. The sensing cell consists of nematic 4-cyano-4′-pentylbiphenyl (5CB) doped with 0.1 wt % 4′-hexyl-[1,1′-biphenyl]-4-carboxylic acid (HBCA) on an HAuCl4-derived reactive gold-coated substrate. In the absence of arsine, HBCA preferentially adsorbs at the coating and enforces homeotropic LC alignment (dark under crossed polarizers). Exposure to AsH3 initiates a redox-triggered interfacial reconstruction process in which AsH3 reduces surface Au(III)–Clx species to Au(0) and promotes nanoscale restructuring, collectively disrupting HBCA–Au interactions. The resulting anchoring change induces a homeotropic-to-nonhomeotropic (planar/hybrid) LC transition with a clear dark-to-bright optical readout. Mechanistic evidence from in situ Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and density functional theory (DFT) calculations supports the proposed pathway. The sensor exhibits high selectivity toward AsH3 over common solvent vapors and achieves a practical optical detection limit of 10 ppb. This work introduces interfacial redox-driven reconstruction as a programmable analytical transduction mechanism for gas sensing and provides a broadly applicable design principle for translating reactive interfacial chemistry into LC-amplified optical outputs.