Light scattering to determine the relative phase of two Bose-Einstein Condensates

chemical nerve agents. HC is an acidic, strong– hydrogen-bonding polycarbosilane (25). We coated a sensor with a thin layer (È100 nm) of HC and tested the response to several analytes. The response of this polymer-coated sensor to repeated 10-s doses of acetone ranging from 60 to 540 ppm is shown in Fig. 4. The acetone produces a large, rapid response that is È100 times larger than the response measured in the same sensor before the HC deposition. The HC concentrates the acetone vapor in the vicinity of the SWNTs, which increases the response while maintaining a rapid response time. The response to a single 200-s dose of DMMP delivered at 320 ppb shows that the measured gain for DMMP relative to the uncoated sensor is about 500 (Fig. 4). Note that the low diffusion rate of DMMP in the HC causes a slower recovery rate, t 90 0 370 s. For water and chloroform, the polymer coating produces much lower response gains of 1 and 10, respectively. Thus, the HC provides a large chemically selective gain, demonstrating the feasibility of SWNT sorption-based chemical sensing. These sensor characteristics compare favorably with those of commercial chemicapacitors. Using a signal-to-noise ratio of 3:1 as a detection criterion, we estimate that MDL 0 0.5 ppm and t 90 G 4 s for acetone and MDL 0 0.5 ppb and t 90 0 370 s for DMMP. For these same analytes, the commercial sensor achieves a MDL 0 2 ppm and t 90 0 228 s for acetone and MDL 0 2 ppb and t 90 0 3084 s for DMMP (4). We attribute our faster response and recovery times to the use of a much thinner layer of chemoselective material. For HC, the minimum layer thickness was limited by the tendency of the HC to form a discontinuous film below È100 nm. Our initial polymer-coated SWNT sensors achieve both higher sensitivity and faster response times than do current chemicapacitors. However, both of these properties can be substantially improved with a few design modifications. The sensitivity is currently limited by the small series capacitance of the thick SiO 2 layer. By thinning the SiO 2 layer or replacing it with a high–dielectric constant insulator (28), we estimate that we can increase the series capacitance by about a factor of 10, which should produce a comparable increase in response. The response time for analytes such as DMMP is limited by diffusion through the layer of HC. Because the SWNT capacitor is based on a surface effect, we can improve the response time and still achieve chemical gain by using extremely thin layers of chemoselective material down to, and including, a single molecular monolayer. To explore this limit of a chemoselective monolayer, we coated the SiO 2 surface with a self-assembled monolayer (SAM) of allyltrichlorosilane. We then reacted the terminal alkenes with hexafluoroacetone to produce a monolayer of hexafluoroisopropanol that partially covers the SWNTs with fluoroalchohol groups. The response of this SAMcoated sensor to repeated 10-s doses of DMMP ranging from 320 ppb to 2.9 ppm is shown in Fig. 4. For this sensor, the response tracks our vapor-delivery system, indicating that t 90 G 4 s, and we measured a MDL 0 50 ppb. Notably, with the SAM coating, the capacitance response of DMMP relative to that of water is increased by a factor of 40, indicating that we achieved substantial chemically selective gain. These initial promising results indicate that optimization of the chemoselective monolayers to better cover the SWNTs, combined with improved sensor design, can result in a new class of sorption-based sensors that combine the features of low power, high sensitivity, and fast response time.