Plasmonic sensors for aromatic hydrocarbon detection

Abstract

The development of innovative materials for sensitive and selective gas sensing is a very relevant field for the current nanotechnology research. A strong effort is dedicated to the fabrication of low-cost and efficient nanoscale devices capable of a fast detection. Resistive electrical devices are the most adopted solutions for in-situ and real-time detection, but their main drawbacks are the low selectivity, response drift, electromagnetic noise dependence and need of contact measurements. Optical gas sensors allow to overcome such limits, and could moreover exhibit thermal and mechanical stability, operate at room temperature, and be integrated on-chip. Within this framework, plasmon-based optical devices are knowing an increasing development and diffusion. Herein plasmonic sensors for aromatic hydrocarbon detection are presented. These systems are based on arylbridged polysilsesquioxanes (aryl-PSQs), obtained either ± coupling such hybrid films with Au nanoparticles (NPs), aiming to the excitation of localized surface plasmon resonances (LSPRs), or ±A depositing them onto metallic waveguiding layers, to form gratings supporting the propagation of surface plasmon polaritons (SPPs). Aryl-PSQs are sol-gel materials characterized by a native controlled porosity and other functionalities (Loy and Shea, Chem Rev 95:1431.1442, 1995; Dabrowski et al., Appl Surf Sci 253:5747.5751, 2007; Brigo et al., Nanotechnology 23:325302, 2012). Temperature programmed desorption investigations of xylene on phenyl-bridged (ph-PSQ) and diphenyl-bridged (diph-PSQ) PSQ films indicate a specific π-π interaction between the organic component of the films and xylene molecules: the interaction energy is quantified in 38 ± 14 kJ/mol and 115 ± 13 kJ/mol, respectively (Brigo et al., J Mater Chem C 1:4252, 2013). For type 2 sensors, a thin film of aryl-PSQ was deposited on a submonolayer of Au NPs coating a fused silica substrate. These sensors were tested monitoring the variation of the LSPRs under cycles of exposure to N2 and to 30 ppm xylene in N2. Figure 38.1a shows that in the presence of xylene molecules, the resonance undergoes an intensity increase and red-shift. An increase in refractive index of the dielectric, as a consequence of the benzene-xylene coupling, might determine the resonance shift to lower frequencies. The plot of Fig. 38.1b shows the dynamic response of a 150 nm thick sensor: response intensity of 0.068, response time of 10 and recovery time of 200. For type ±A, sinusoidal surface plasmon gratings were embedded in ph-PSQs (Fig. 38.2). The sensor exhibits two SPP modes: the Long Range (LR) and the Short Range (SR) SPP. Figure 38.3 shows the reflectance upon exposure to 30 ppm xylene, and after regeneration. A 2:9 ± 0:9 nm red-shift of the LR and a 2:5±1:3 nm red-shift of the SR were correlated to the interaction with the analyte. Collected data are compatible with theoretical predictions assuming a film ⃤n of 0.011±0.005 (Brigo et al., Nanotechnology 24:155502, 2013). The reported sensors demonstrate a major performance in terms of sensitivity, ease in fabrication procedure, and promising dynamics.