We demonstrate folded waveguide ring resonators for biomolecular sensing. We show that extending the ring cavity length increases the resonator quality factor, and thereby enhances the sensor resolution and minimum level of detection, while at the same time relaxing the tolerance on the coupling conditions to provide stable and large resonance contrast. The folded spiral path geometry allows a 1.2 mm long ring waveguide to be enclosed in a 150 microm diameter sensor area. The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of approximately 3 pg/mm(2), or a total mass of approximately 5 fg. The real time measurements are used to analyze the kinetics of biotin-streptavidin binding.
We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 microm diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of approximately 10 fg of streptavidin protein is demonstrated with near temperature-independent response.
Reagentless micropatterning of hydrogen-terminated Si(111) via UV irradiation through a photomask has proven
to be a convenient strategy for the preparation of ordered bicomponent monolayers. The success of this technique
relies upon the differential rate of reaction of an alkene with the hydrogen-terminated and photooxidized regions of
the surface. Monolayer formation can be accomplished under either thermal or photochemical conditions. It was
observed that, after 3 h, reaction in neat alkene solution irradiation (Rayonet, 300 nm) afforded the expected patterned
surface, while thermal conditions (150 °C) resulted in a partial loss of pattern fidelity. Monolayer properties and
formation were studied on oxidized and hydrogen-terminated silicon under thermal and photochemical initiation, by
contact angle, ellipsometry, Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy,
and X-ray photoelectron spectroscopy. Results show that alkenes add to silanol groups on the silica surface in a manner
consistent with acid catalysis: once attached to the surface, the silica oxidized the hydrocarbon.
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