Kee Scholten

Microfabricated optofluidic ring resonator structures.

We describe the fabrication and preliminary optical characterization of rugged, Si-micromachined optofluidic ring resonator (μOFRR) structures consisting of thin-walled SiO(x) cylinders with expanded midsections designed to enhance the three-dimensional confinement of whispering gallery modes (WGMs). These μOFRR structures were grown thermally at wafer scale on the interior of Si molds defined by deep-reactive-ion etching and pre-treated to reduce surface roughness. Devices 85-μm tall with 2-μm thick walls and inner diameters ranging from 50 to 200 μm supported pure-mode WGMs with Q-factors >10(4) near 985 nm. Advantages for eventual vapor detection in gas chromatographic microsystems are highlighted.

Vapor discrimination by dual-laser reflectance sensing of a single functionalized nanoparticle film

Sorption-induced changes in the localized surface-plasmon resonance (LSPR) of an n-octanethiolate-monolayer-protected gold nanoparticle film on a Si chip are exploited to differentiate two volatile organic compounds (VOC) with a single sensor. Probing the film with 488 nm and 785 nm lasers gave reflectance sensitivity ratios at the two wavelengths of 0.68 and 0.80 for toluene and n-heptane, respectively, permitting their discrimination. Swelling-induced increases in inter-particle distance appear to predominate over changes in the refractive index of the inter-particle matrix in the sensor responses. The corresponding ratios of sensitivities with a reference film of polydimethylsiloxane did not differ for the two vapors. Approaches for extending the capability for VOC discrimination by use of arrays of such LSPR sensors are discussed, along with the advantages of employing this simple platform in compact, field-deployable environmental VOC monitoring systems.

Polymer-coated micro-optofluidic ring resonator detector for a comprehensive two-dimensional gas chromatographic microsystem: μGC × μGC–μOFRR

We describe first results from a micro-analytical subsystem that integrates a detector comprising a polymer-coated micro-optofluidic ring resonator (μOFRR) chip with a microfabricated separation module capable of performing thermally modulated comprehensive two-dimensional gas chromatographic separations (μGC ×μGC) of volatile organic compound (VOC) mixtures. The 2 × 2 cm μOFRR chip consists of a hollow, contoured SiO(x) cylinder (250 μm i.d.; 1.2 μm wall thickness) grown from a Si substrate, and integrated optical and fluidic interconnection features. By coupling to a 1550 nm tunable laser and photodetector via an optical fiber taper, whispering gallery mode (WGM) resonances were generated within the μOFRR wall, and shifts in the WGM wavelength caused by transient sorption of eluting vapors into the PDMS film lining the μOFRR cylinder were monitored. Isothermal separations of a simple alkane mixture using a PDMS coated 1st-dimension ((1)D) μcolumn and an OV-215-coated 2nd-dimension ((2)D) μcolumn confirmed that efficient μGC ×μGC-μOFRR analyses could be performed and that responses were dominated by film-swelling. Subsequent tests with more diverse VOC mixtures demonstrated that the modulated peak width and the VOC sensitivity were inversely proportional to the vapor pressure of the analyte. Modulated peaks as narrow as 120 ms and limits of detection in the low-ng range were achieved. Structured contour plots generated with the μOFRR and a reference FID were comparable.

μGC × μGC microsystem with resistive and optical detection

The integration of microfabricated chemiresistor (CR) and micro-optofluidic ring resonator (μOFRR) sensors with other microfabricated components to yield a microsystem capable of performing comprehensive two dimensional gas chromatography (μGC × μGC) is described. Two CRs with different monolayer protected Au nanoparticle (MPN) interface layers were tested along with one μOFRR with a PDMS interface layer. All three provided modulation numbers similar to those of a reference flame ionization detector (FID). Modulated peak widths (full width at half maximum, fwhm) for the CR detectors ranged from 0.31 to 1.9 sec, and those for the μOFRR ranged from 0.24 to 1.2 sec. Although these fwhm values were greater than those from the FID and they increased with the vapor pressure of the analyte, most were <; 1 sec and all peaks were narrow enough to permit efficient μGC × μGC separations. Analyses of mixtures of 6-18 volatile organic compounds are demonstrated.

Multi-variable micro opto-fluidic ring resonator sensing with plasmonic nanoparticle films

Localized surface plasmon resonance (LSPR) in films of thiolate- and dithianate-monolayer-protected gold nanoparticles (MPN) having different monolayers and Au core sizes (4, 5, and 40 nm), and probed at three visible wavelengths near the LSPR maximum, has been exploited to discriminate among 6 vapors with a single sensing film. Arrays of just two MPN films (4- or 5-nm core diameters) were also remarkably selective (> 95% discrimination) even probing each film with just a single wavelength. Initial results obtained from the first MPN-coated microfabricated optofluidic ring resonator sensor (μOFRR) used as a μGC detector are also presented and appear promising.