Electrical Detection of Fast Reaction Kinetics in Nanochannels with an Induced Flow

Abstract

Nanofluidic channels can be used to enhance surface binding reactions, since the target molecules are closely confined to the surfaces that are coated with specific binding partners. Moreover, diffusion-limited binding can be significantly enhanced if the molecules are steered into the nanochannels via either pressure-driven or electrokinetic flow. By monitoring the nanochannel impedance, which is sensitive to surface binding, low analyte concentrations have been detected electrically in nanofluidic channels within response times of 1-2 hours. This represents a ~54 fold reduction in the response time using convective flow compared to diffusion-limited binding. At high flow velocities the presented method of reaction kinetics enhancement is potentially limited by forceinduced dissociations of the receptor-ligand bonds. Optimization of this scheme could be useful for label-free, electrical detection of biomolecule binding reactions within nanochannels on a chip. Keywords NANOFLUIDICS; BINDING KINETICS; ENHANCED MASS TRANSPORT; CONVECTION; ELECTRICAL BIOSENSOR; IMPEDANCE SPECTROSCOPY; FUNCTIONALIZATION; STREPTAVIDIN-BIOTINOne limiting factor for low-abundance analyte detection by immunoassay is the existence of surface diffusion layers, which limits the binding kinetics. In typical ELISA (Enzyme Linked ImmunoSorbent Assays) or bead-based immunoassays, target molecules need to be transported (primarily by diffusion) to the surface-bound antibodies for a binding reaction to occur. The distance for this diffusive transport is roughly corresponding to the average distance between the two target molecules in the sample solution, which can be as large as ~10 µm at lower concentrations (~pM). Diffusive transport at that length scale is relatively slow and inefficient, therefore leading to analyte depletion near the binding surface. This can significantly limit the speed of assays, requiring long incubation times to reach binding equilibrium. 1One way to deal with this problem would be shortening this distance, by confining both target molecules and the antibodies within a nanofluidic channel. Karnik et al. 2 have studied streptavidin-biotin binding in nanochannels, demonstrating the dominance of size and charge effects at high and low ionic strengths, respectively. Largest conductance changes after streptavidin immobilization to biotinylated surfaces were observed at low ionic strength, and *To whom correspondence should be addressed. E-mail: jyhan@mit.edu, Tel: +1-617-253-2290, Fax: +1-617-258-5846 We demonstrate that diffusion-limited reactions can be overcome by performing the detection of analytes in nanochannels with an applied convective flow through them to enhance mass transport. This results in fast reaction kinetics in nanofluidic channels and thus a reduction in the response time to detect a specific target molecule even at low analyte concentrations. Compared to the diffusional transport limit, the response time is decreased by a factor of ~54 by using pressure-driven flow through nanochannels. Surface...

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