Langmuir 2004, 20, 299-305
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Detection of Organic Mercaptan Vapors Using Thin Films of Alkylamine-Passivated Gold Nanocrystals Shawn M. Briglin, Ting Gao, and Nathan S. Lewis* Noyes Laboratory, 127-72, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received July 1, 2003. In Final Form: September 17, 2003 Alkylamine-capped gold nanocrystals have been used as chemically sensitive resistors for the detection of volatile organic mercaptan vapors. Thin (600 s for low analyte concentrations. E. Analysis of Resistance Response Data. The magnitude of the electrical resistance of each detector was initially very high (10 kΩ to 100 MΩ). Furthermore, similarly prepared films exhibited a wide range of initial resistance values. Consequently, a suitable descriptor was required for interpretation of the effects of analyte exposure on such films. Additionally, all of the detectors exhibited some drift in their resistance versus time even in the absence of thiol vapors. A small amount of sulfides are present in the atmosphere, and these vapors could in principle produce the observed resistance drift. However, similar values of the resistance drift were observed even when the detector chamber was maintained under a flow of compressed N2(g). A suitable descriptor for CH3SH exposure was found by taking the natural logarithm of the resistance of each detector and then fitting a straight line to the data during an analyte exposure, typically when the detector resistance was between 95% and 70% of its initial resistance value. The detectors exhibited variable responses to H2S. Only three of the four detectors studied exhibited the characteristic response. Two showed slight initial increases in resistance followed by decreasing resistance, and one did not respond significantly, but this detector was very noisy prior to initiating the exposure. Because the IDE substrates were made from Au, a control experiment was performed to ensure that the Au electrodes themselves did not produce a detectable change in resistance during exposures to S-containing gases. For this purpose, a highly resistive (R > 100 kΩ) poly(ethylene-co-vinyl acetate) carbon black-polymer composite film was deposited onto an IDE substrate and then exposed to CH3SH for several minutes. No change in the resistance of the carbon black-polymer detector/ IDE structure was observed. Exposure to H2S(g) did in some cases result in a slight drift in the direction of increasing resistance, consistent with corrosion of the electrical contacts or
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Figure 1. Resistance vs time of a dodecylamine-capped Au nanocrystal thin film vapor detector exposed to 120 s pulses of 12 ppth of acetone at 720 s and to 1.5 ppth of toluene at 1080 s, respectively, at a flow velocity of 4.0 L min-1 in a background of laboratory air. the electrodes, but these control tests never produced the characteristic response of decreasing resistance that was observed for the Au nanocrystal films upon exposure to the thiol-containing vapors. F. Transmission Electron Microscopy. Transmission electron microscopy (TEM) was performed using a Philips EM430 (300 kV) with 2.3 Å point-to-point resolution. The dodecylamineterminated Au nanocrystal samples were prepared by evaporating one drop of a dilute solution of the nanoparticles in hexane onto an amorphous-carbon-coated Cu TEM grid. One of the grids was exposed to 160 ppth propanethiol vapor for
