Microfabricated Optofluidic Ring Resonators for Sensitive, High-Speed Detection of Volatile Organic Compounds

Abstract

The development of microfabricated sensors and sensor arrays for volatile organic compounds (VOC) and their evaluation as detectors in micro-scale gas chromatographic (μGC) instrumentation are described. Initial efforts explored the discrimination of VOCs with arrays of chemiresistors (CR) employing interface layers of thiolate-monolayer-protected gold nanoparticles (MPNs) or tin-oxide nanowires (NWs). The response diversity of several possible MPN-CR arrays was found to exceed that of the NW-CR array, and was not enhanced by combining the former with the latter. The next study demonstrated that the response diversity of MPN-CR arrays could be enhanced moderately by combining them with arrays of mass-sensitive MPN-coated thickness-shear-mode resonators. However, the analysis of binary VOC mixtures was not satisfactory even with the best of these multi-transducer arrays. A new type of optical vapor sensor was then created: the microfabricated optofluidic ring resonator (μOFRR). This sensor combines vapor sensing and fluidic transport functions in a monolithic microstructure comprising a hollow, vertical SiOx cylinder (250 μm i.d.) with a central quasi-toroidal mode-confinement section, grown and partially released from a Si substrate. It also integrates fluidic-interconnection and fiber-optic probe alignment features. High-Q whispering gallery modes (WGM) generated with a tunable near-IR laser exhibited shifts in resonant wavelength, λWGM, from polymer swelling and refractive index changes as vapors reversibly partitioned into the thin sorptive-polymer film lining the cylinder. Remarkably high sensitivity and rapid responses were obtained with this μOFRR sensor installed downstream from a single μGC separation column and a two-dimensional μGC subsystem. Since MPN films exhibit localized surface plasmon resonance (LSPR) they also have the potential to serve as interface layers in optical sensor arrays. Indeed, it was shown that VOC discrimination was possible by probing an MPN film at just two wavelengths flanking its LSPR absorbance maximum in a custom-built reflectance measurement system. In a first attempt to adapt multi-wavelength plasmonic sensing to the μOFRR platform, measured shifts in λWGM from an MPN coated μOFRR sensor were shown to be proportional to concentration for several VOCs. Results suggest that arrays of MPN-coated μOFRR sensors show great promise as detectors in single- and multi-dimensional μGC systems.