Thrombin binding stabilizes the alternative G-quadruplex conformation of the aptamer, liberating the methylene blue (MB)-tagged oligonucleotide to produce a flexible, single-stranded DNA element. This allows the MB tag to collide with the gold electrode surface, producing a readily detectable Faradaic current at thrombin concentrations as low as approximately 3 nM.

We present a microfluidic technique for sensitive, real-time, optimized detection of airborne water-soluble molecules by surfaceenhanced Raman spectroscopy (SERS). The method is based on a free-surface fluidic device in which a pressure-driven liquid microchannel flow is constrained by surface tension. A colloidal suspension of silver nanoparticles flowing through the microchannel that is open to the atmosphere absorbs gas-phase 4-aminobenzenethiol (4-ABT) from the surrounding environment. As surface ions adsorbed on the colloid nanoparticles are substituted by 4-ABT, the colloid aggregates, forming SERS ''hot spots'' whose concentrations vary predictably along the microchannel flow. 4-ABT confined in these hot spots produces SERS spectra of very great intensity. An aggregation model is used to account quantitatively for the extent of colloid aggregation as determined from the variation of the SERS intensity measured as a function of the streamwise position along the microchannel, which also corresponds to nanoparticle exposure time. This allows us to monitor simultaneously the nanoparticle aggregation process and to determine the location at which the SERS signal is optimized. chemical detection ͉ nanoparticle aggregationA n important trend in modern chemical reaction engineering is the miniaturization of reactors and other devices to the micro and nano scale (1). Such lab-on-a-chip systems, first postulated in 1990, and based on small-scale physics, now complement traditional reaction design approaches (2). In this work, we describe a microfluidic stream in which the spanwise dimension is miniaturized to a sufficiently small size to cause the flowing liquid to be confined solely by surface tension. This architecture allows a small-scale chemical processing device to be fabricated that allows gas-phase or airborne species to enter the flowing microfluidic circuit directly from the ambient environment. Once absorbed, these chemical species can potentially be subjected to further chemical reactions and analysis. Here, we demonstrate the absorption of a gas-phase species at a low partial pressure into a silver colloid flow confined to the microchannel and the subsequent detection of the analyte by surface-enhanced Raman spectroscopy (SERS). In this device, reaction time scales proportionally with distance along the microchannel. This architecture allows the aggregation dynamics to be modeled and understood, making it possible to optimize the SERS signal intensity.SERS spectra contain vibrational information that allows molecular species to be detected and identified (3, 4). First observed by Fleischmann and coworkers (5), and shown to be a highly enhanced form of Raman spectroscopy by Van Duyne (6), the SERS enhancement can be so great that analytes with femtomolar or lower concentrations can be detected (7), potentially making SERS a sensitive approach to detecting gaseous molecules emanating from explosive devices, toxic airborne compounds, or other atmospheric environmental contaminants (8).The theory of SERS i...

The dominant physical transport processes are analyzed in a free-surface microfluidic and surface-enhanced Raman spectroscopy (SERS) chemical detection system. The analysis describes the characteristic fluid dynamics and mass transport effects occurring in a microfluidic detection system whose analyte absorption and concentration capability is designed to operate on principles inspired by canine olfaction. The detection system provides continuous, real-time monitoring of particular vapor-phase analytes at concentrations of 1 ppb. The system is designed with a large free-surface-to-volume ratio microfluidic channel which allows for polar or hydrophilic airborne analytes to readily be partitioned from the surrounding gas phase into the aqueous phase for detection. The microfluidic stream can concentrate certain molecules by up to 6 orders of magnitude, and SERS can enhance the Raman signal by 9-10 orders of magnitude for molecules residing in the so-called SERS "hot spots", providing extremely high detection sensitivity. The resulting vibrational spectra are sufficiently specific to identify the detected analyte unambiguously. Detection performance was demonstrated using a nominal 1 ppb, 2,4-dinitrotoluene (2,4-DNT) vapor stream entrained within N(2) gas. Applications to homeland security arise from the system's high sensitivity and its ability to provide highly reproducible, continuous chemical detection monitoring with minimal sampling requirements.