Nanoelectronic Chemical Sensors for Chemical Agent and Explosives Detection

A new class of nanometer-scale, low power, solidstate devices is being investigated for the detection of CW agents and other hazardous vapors. These nanoelectronic chemical vapor sensors, or “chemiresistors” are comprised of nanometer-sized gold particles (1.2-2.4nm) encapsulated by monomolecular layers of functionalized alkanethiols (R-SH) deposited as thin films on interdigitated microelectrodes (Fig 1). Fig. 1. Nanoelectronic Chemical Sensor Concept. When chemical (agent, explosive) vapors reversibly absorb into these thin films, a large modulation of the electrical conductivity of the film is observed. The measured current between gold clusters is extremely sensitive to very small amounts of monolayer swelling or dielectric alteration caused by absorption of vapor molecules. For chemical agent simulants, a large dynamic range (5-logs) of sensitivities is observed and extends down to ppb (parts-per-billion) vapor concentrations. For explosive vapors of TNT/DNT detection limits in the femtogram range have been observed. Complete reversibility has been observed for all analyte vapors and the devices exhibit relatively low sensitivity to water vapor (a major interferent). Tailored selectivities of the sensors are accomplished by incorporation of chemical functionalities at the terminal structure of the alkanethiol or substitution of the entire alkane structure. 1. BACKGROUND Over the past ten years, there has been considerable interest in nanometer-sized materials largely due to the wide range of applications of these materials in several fields including advanced electronics, nonlinear optics, catalysis and hydrogen adsorption. Recent interest in this “intermediate state of matter” stems largely from a seminal 1994 study by Brust et al. where a new method was developed for preparing and stabilizing nanometer-sized gold colloids that are easily dispersed in organic solvents and isolated as pure powders. Colloids have been studied since Faraday’s examination of them but they were only stable in solution. Depending on the preparative conditions, the particles had a tendency to agglomerate slowly, eventually lose their disperse character and flocculate. The removal of solvent generally led to the complete loss of the ability to reform a colloidal solution. Brust and coworkers solved this problem by “protecting” the gold colloids or clusters with the self-assembled surfactant, dodecanethiol (C12H25SH), which was then well known to form self-assembled monolayers on planar gold surfaces. Leff and coworkers further demonstrated that control of the gold particle size in this system could be achieved by varying the gold-tothiol reactant ratio and applied a model in which the role of the thiol is analogous to that of the surfactant in water-in-oil microemulsions. In many respects these new cluster compounds behave like simple chemical compounds; they can be precipitated, redissolved and chromatographed without any apparent change in properties. This preparative methodology allowed a reasonable degree of control of the size of the nanoparticles, and most research groups have targeted the technologically-attractive 1-5 nm size range for these monolayer-protected clusters (MPCs). In 1998, Wohltjen and Snow demonstrated that gold nanoclusters with functionalized protecting monolayers form the basis of sensitive vapor “chemiresistors” by depositing these MPCs to produce metal-insulatormetal-ensemble (MIME) films on interdigitated Report Documentation Page Form Approved