Surface Functionalization for Selective Mid-Infrared On-Chip Sensing

Abstract

We report on the development of conformal nanoparticle (NP)-based coatings, with flexible chemical modification, applied to the surface of mid-infrared (MIR) waveguide (WG)-based sensors, to simultaneously achieve sensitivity and selectivity enhancement of gaseous analytes. Traditional organic polymeric coatings are incompatible with MIR waveguide sensing because of the spectral overlap and the limited ability of hosting analyte molecules close to the evanescent field (EF) due to the lack of inherent porosity. On the other hand, commonly designed inorganic 3D porous coatings cannot be seamlessly integrated within the WGs due to the inability to conform to theWG within thin films of controlled thickness. Here, we construct conformal nanocoatings on the WGs that can be deposited at a nanometer-scale-controlled thickness. The coatings have high surface area readily available for adsorption and concentration of the analytes in the vicinity of the EF for improved sensitivity, as well as variable surface chemistry for improved selectivity. The nanocoatings were deposited on the WGs using our newly developed hybrid deposition technique incorporating a controlled substrate withdrawal speed within the layer-by-layer (LbL) method. Two types of LbL systems were explored: NP/NP and polymer/NP. The NP/NP system consisted of spherical ZnO₂ and SiO₂ NPs with diameters of 4.2 nm and 20 nm, respectively. The polymer/NP system was composed of branched polyethylenimine and mesoporous silica NPs of a~150-nm diameter and was calcinated prior to use. We investigated the effect of deposition conditions, such as withdrawal speed and solution pH, on the thickness, porosity and morphology of the coatings. The substrate withdrawal speed was a critical factor, where uniform coatings were only achieved in the convective regime (≤ 0.001 cm/s). In addition to enhanced detection sensitivity, these coatings improved the selectivity of the WGs due to the preferential interaction of polar gas molecules with the polar surfaces of the metal oxide NP-based coatings. Moreover, silane functionalization of such inorganic coatings was successfully achieved to enhance the selectivity of gas molecules with different polarities. Using LbL-coated MIR WGs, we demonstrated selective and reversible sensing of methane, acetone and ethanol using the C-H stretching vibrational bands of these analytes.