(237a) A Hybrid Nanoscale Biosensing Platform Based On Dielectrophoresis and Surface Plasmonics

We report a new platform that combines the advantages of dielectrophoresis (DEP) and Surface Plasmon Resonance (SPR) for pH and molecular sensing. Optical sensing offers numerous advantages over Electrical Impedance Spectroscopy (EIS), as its optical frequency does not alter the conformation of the hybridized target molecules, which presents a major robustness issue with EIS detection. SPR relies on the evanescent optical wave extending just few hundred nanometers above metal surface to sense the presence of target molecules, eliminating the sensing of bulk debris. The platform involves a nanostructure array which can sustain both high electric field and a resonant grating for plasmonics.  The high electric field is generated at the nanostructure array with a planar electrode pair and is used to trap target molecules from a flowing solution to the nanostructure. The hydrodynamic shear offered by the large flow (microliter/minute) also allows mismatch discrimination in DNA sensing and minimize non-specific binding in general (Cheng et al, Lab on a Chip, 9, 3193(2009)).  Instead of an expensive laser light source at a specific incident angle, efficient coupling by the resonant grating allows the light source to be a broadband Halogen lamp or an embedded LED source. Detection is also done with a miniature spectrometer such that the entire detector footprint is smaller than a brief case. The reflection spectrum of the nano-array exhibits several high-Q distinct peaks at frequencies that are sensitive to the presence of hybridized target molecules within the holes of the array.  For pH sensing, we functionalize a single monolayer of poly(2-vinyl-pyridine) on the nanostructure array and exploit how their pH-sensitive persistent length (monolayer height) and refractive index produces a large shift in the resonant frequency. Charging and discharging of counter-ions by the AC field is found to induce a coil-stretch transition and a shift in the Plasmonics resonant peaks. We have developed an ion-condensation strategy to either eliminate this ionic strength sensitivity or strengthen it. Sensitivity and selectivity for a range of sample tests will be reported.