title: A microfabricated optofluidic ring resonator for sensitive, high-speed detection of volatile organic compounds.
abstract: Advances in microanalytical systems for multi-vapor determinations to date have been impeded by limitations associated with the microsensor technologies employed. Here we introduce a microfabricated optofluidic ring resonator (μOFRR) sensor that addresses many of these limitations. The μOFRR combines vapor sensing and fluidic transport functions in a monolithic microstructure comprising a hollow, vertical SiOx cylinder (250 μm i.d., 1.2 μm wall thickness; 85 μm height) with a central quasi-toroidal mode-confinement section, grown and partially released from a Si substrate. The device also integrates on-chip fluidic-interconnection and fiber-optic probe alignment features. High-Q whispering gallery modes generated with a tunable 1550 nm laser exhibit rapid, reversible shifts in resonant wavelength arising from polymer swelling and refractive index changes as vapors partition into the ~300 nm PDMS film lining the cylinder. Steady-state sensor responses varied in proportion to concentration over a 50-fold range for the five organic vapors tested, providing calculated detection limits as low as 0.5 ppm (v/v) (for m-xylene and ethylbenzene). In dynamic exposure tests, responses to 5 μL injected m-xylene vapor pulses were 710 ms wide and were only 18% broader than those from a reference flame-ionization detector and also varied linearly with injected mass; 180 pg was measured and the calculated detection limit was 49 pg without use of preconcentration or split injection, at a flow rate compatible with efficient chromatographic separations. Coupling of this μOFRR with a micromachined gas chromatographic separation column is demonstrated.
authors.affiliation: Applied Physics Program, University of Michigan, Ann Arbor, MI 48109-1040, USA.
authors.forename: Kee
authors.initials: K
authors.lastname: Scholten
authors.forename: Xudong
authors.initials: X
authors.lastname: Fan
authors.forename: Edward T
authors.initials: ET
authors.lastname: Zellers
mesh_terms: D057230:Limit of Detection; D016014:Linear Models; D046210:Microfluidic Analytical Techniques / Q000295:instrumentation*; D055100:Optical Fibers; D055095:Optics and Photonics / Q000295:instrumentation*; D015203:Reproducibility of Results; D055549:Volatile Organic Compounds / Q000032:analysis* / Q000302:isolation & purification
chemical_list: D055549:Volatile Organic Compounds