Rapid chemical vapor sensing and micro gas chromatography detection using optofluidic ring resonators
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Rapid chemical vapor sensing and micro gas chromatography detection using optofluidic ring resonators Yuze Sun 1, Siyka I. Shopova 1, Ian M. White 1, Hongying Zhu 1, Greg Frye-Mason 2, Shiou-jyh Ja 3, Aaron Thompson 3, and Xudong Fan 1 1 Department of Biological Engineering, University of Missouri 2 ICx Technologies, 1001 Menaul Blvd NE, Suite A, Albuquerque, New Mexico 87107 3 ICx Nomadics, 1024 South Innovation Way, Stillwater, Oklahoma 74074 ABSTRACT We develop rapid chemical vapor sensors and micro gas chromatography (μGC) analyzers based on the optofluidic ring resonator (OFRR). An OFRR is a micro-sized thin-walled glass capillary; the circular cross-section of the capillary acts as an optical ring resonator while the whispering gallery modes or circulating waveguide modes (WGMs) supported by the ring resonator interact with the vapor samples passing through the capillary. The OFRR interior surface is coated with a vapor-sensitive polymer. The analyte and polymer interaction causes the polymer refractive index (RI) and the thickness to change, which is detected as a WGM spectral shift. Owing to the excellent fluidics, the OFRR vapor sensor exhibits sub-second detection and recovery time with a flow rate of 1 mL/min. On-column separation and detection in the OFRR based μGC system is also demonstrated, showing efficient separation of vapor mixtures and presenting highly reproducible retention time for the individual analyte. Compared to the conventional GC system, the OFRR μGC has the advantage of small size, rapid response, and high selectivity over a short length of column. INTRODUCTION Optical vapor sensors have recently generated much interest in environmental monitoring, homeland security, and health care. Various optical techniques have been developed, which include optical absorption measurement [1], photoluminescence measurement [2], spectroscopy [3], and refractive index (RI) measurement. Compared to other optical sensing techniques, the RI measurement demonstrates great potential for the development of rapid, miniaturized, and simple vapor sensors with high sensitivity and reasonable selectivity. The optical structures that have been investigated include waveguides [4], fiber Bragg gratings [5], surface Plasmon resonance (SPR) [6], ring resonators [7-12], and interferometers [13]. Optical ring resonator is a promising platform to develop high sensitivity label-free biosensors and chemical vapor sensors owing to the high-Q factors of the whispering gallery modes (WGMs) that they support. Although a detection limit (DL) of 10-6 – 10-7 refractive index units (RIU) has been achieved in detecting biomolecules [14], the potential of ring resonator in vapor sensing has not been fully exploited. Only a handful of studies have in the past few years been carried out on planar ring resonators [7-9], showing a DL of tens of parts per million (ppm).
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