Silane‐Modified Mesoporous Silica Nanocoatings for Selective Mid‐Infrared Waveguide‐Based Gas Sensing

Abstract

This paper reports the use of silane monolayers of different polarities to enable recognition and selective interfacial enrichment of nonpolar versus polar gases for spectroscopic mid‐infrared (MIR) on‐chip sensing. However, detection of gases using on‐chip waveguides (WGs) is challenging because of the low concentrations of gas molecules within the evanescent field surrounding the WGs. To solve this issue, high‐surface‐area, precisely‐controlled‐thickness nanocoatings are introduced whose surface modification with strategically selected silanes enables control of preferential adsorption and concentration of polar versus nonpolar gas analytes. Conformal nanocoatings are constructed using layer‐by‐layer deposition of mesoporous silica nanoparticles (MSNs) at substrate withdrawal speeds controlled within the capillary regime. Surface modification of the WG‐immobilized MSNs with hydrophobic triethoxy(octyl)silane enables, for the first time, enhanced sensitivity and selectivity towards nonpolar methane gas as compared to its polar counterpart acetone. On the other hand, modification of the nanocoatings with (3‐aminopropyl)triethoxysilane leads to preferential binding of polar acetone vapors. This work demonstrates the capability of interfacial modification for controlling the selection and enhancement of the spectroscopic features of gas analytes with different polarities. The developed interfacial assemblies can enable diverse spectroscopic gas sensing platforms for applications in health diagnostics, environmental monitoring, and process control.