Microfluidic/SERS Detection of Trace Explosives

Abstract : We have developed a chemical detector combining Free-Surface microfluidics (FSF) with surface-enhanced Raman spectroscopy (SERS) to measure air-borne molecules with extremely high sensitivity (parts per trillion) and molecular specificity. The free-surface of a microfluidic material stream allows polar airborne molecules to absorb directly into a microchannel flow, and provides a barrier to non-polar, water insoluble airborne molecules. Explosives molecules are polar and readily absorb into the microchannel flow, whereas many interferent molecules do not absorb. Once absorbed into the flowing liquid within the microchannel, the explosives molecules combine with SERS-active nanoparticles, thereby enhancing the Raman signal by approximately 8-10 orders of magnitude. The molecules adsorbed into the microchannel flow are detected with surface enhanced Raman spectroscopy (SERS), an effect first observed in 1978, which is a highly amplified form of Raman spectroscopy that occurs when molecules reside near appropriately nanostructured surfaces of certain metals. Aside from the alkali metals, silver, gold or copper are ideal SERS-enabling metals, less ideally indium, aluminum and platinum and even less so other metals. Currently, the best reported results are obtained when the SERS-active substrate is composed of closely arrayed, interacting nanoparticles or nanostructures so constructed as to allow the analyte or a portion thereof to occupy small crevices between particles with dimensions in the range 0.5 to 40 nm. The enhancement in signal and therefore in analysis sensitivity brought about by placing the analyte in close proximity to the SERS-active materials is large (up to ~7-10 orders in magnitude), so large that Raman spectroscopic signals arising from single analyte molecules or a small number of analyte molecules may be recorded under favorable circumstances.