Chemiresistive vapor sensing with microscale films of gold monolayer protected clusters

Here we report the stability, conductivity, and vapor-sensing properties of microcontact-printed films of 1.6 nm average diameter hexanethiolate-coated gold monolayer protected clusters (C6 An MPCs). The C6 Au MPCs were stamped into parallel lines (similar to 1.2,mu m wide and 400 nm thick) across two Au electrodes separated by a 1-mu m gap. The chemiresistive vapor-sensing properties were measured for saturated toluene and 2-propanol vapors. As-prepared patterned Au MPC films were unstable in the presence of saturated toluene vapor, and their current response was irreversible. Chemically linking the films with vapor-phase hexanedithiol greatly improves their stability and leads to reversible responses. The extent of Au MPC cross-linking and vapor response to organic vapors varies with different exposure times to dithiol vapor. The response to toluene changed from 61 to 8% for exposures of 1 and 60 min, respectively, which is likely due to greater film flexibility with less dithiol exposure. The current measured through the films varies from 10(-11) to 10(-3) A as a function of the temperature between 250 and 320 degrees C, which correlates with the loss of organic material as measured by FT-IR spectroscopy and the change in thickness and width of the film as measured by atomic force microscopy. The vapor-sensing properties vary with temperature, current, and organic content in the film, which are all interrelated. Response to toluene decreased with increasing temperature and conductivity, while the response to 2-propanol was less predictable. Reducing the size of vapor-sensing devices based on An MPCs is important for creating highly portable devices that can simultaneously detect multiple analytes. This work demonstrates a simple method for reducing the size of such devices down to the microscale and describes methods for maximizing response, stability, and reversibility.