Anal. Chem. 2006, 78, 2335-2341
Nanofluidic Injection and Heterogeneous Kinetics of Organomercaptan Surface Displacement Reactions on Colloidal Gold in a Microfluidic Stream John S. Kirk, Jonathan V. Sweedler, and Paul W. Bohn*
Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at UrbanasChampaign, 600 South Mathews Avenue, Urbana, Illinois 61801
Colloidal gold is developed as a molecular capture reagent in hybrid nanofluidic-microfluidic devices for masslimited sample analysis. Two fluorescent organomercaptans are injected through a nanocapillary array membrane and subsequently captured at the surface of 19-nmdiameter colloidal Au nanoparticles. The surface displacement kinetics are monitored via quenching of the organomercaptan fluorescence by the metallic particles coupled to a distance-time conversion based on fluid velocity in the microfluidic channel using the point of mixing as the zero of time. The adsorbate concentration, colloid concentration, and fluid velocity are varied to determine the surface displacement rate constants for these heterogeneous reactions in the microfluidic device. Surface displacement rate constants are ∼104 M-1 s-1 for a small organic molecule and for an octapeptide. These values are similar to values determined in macroscale measurements made with a traditional fluorometer and are 1 order of magnitude larger than values reported for adsorption of organomercaptans on planar Au, indicating faster kinetics in the colloid-adsorbate system. These results highlight the utility of colloidal Au nanoparticles as molecular carriers for the sequestration of analytes, allowing the manipulation of mass-limited samples and ultimately the capture and delivery of selected analytes from a microfabricated device to an off-line detector. The development of better sample handling methods is required to fully exploit recent technological advances in analytical methodology applied to mass-limited samples. The range of applications that requires analysis of mass-limited samples typically encompasses two categories: those in which the sample is only available in small amounts, e.g., characterization of the chemical composition of subcellular domains, and those in which the sample has some property, e.g., high cost or toxicity, that requires analysis in small amounts. Especially when samples composed of
