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Lithographically Patterned Nanowires in Sensors and Transducers


Lithographically patterned metal nanowires were utilized in two studies on sensing and transduction. First, ultra-long (mm scale) polycrystalline gold nanowires were investigated for their ability to perform as thermophones, or thermoacoustic sound emitters. Arrays of ~4000 linear nanowires were fabricated at 5 um pitch on glass surfaces. Sound generation by the nanowires was evaluated as a function of acoustic frequency (from 5 - 120 kHz), angle from the plane of the nanowires, input power (from 0.30 - 2.5 W) and the width of the nanowires in the array (from 270 to 500 nm.) Classical theory based upon metal films accurately predicts the measured properties of these gold nanowire arrays. Angular "nodes" for the off-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, were faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (~10^{-10} at 25 kHz), the power dependence, and the frequency dependence were independent of the lateral dimensions of these wires over the range from 270 to 500 nm. Second, a PEDOT-deferoxamine nanojunction chemiresistor was developed for the rapid detection of Fe(III) at sub-nanomolar concentrations. The backbone of the sensor is a single lithographically patterned metal nanowire in which a nanogap is formed by focused ion beam (FIB). The nanowire is then electrochemically reconnected by the ionophore-doped polymer PEDOT-deferoxamine, creating a chemically responsive junction selective for Fe(III). Fabrication challenges, centered on the adhesion between the metal nanowire core and the PEDOT-DFA transduction layer, led to three design iterations of the sensor. Two of these nanojunctions were able to detect 10^{-11}-10^{-4} M Fe(III), demonstrating a dynamic range that is on par with ion selective electrodes and a limit of detection that is three order of magnitude better. However, these junctions fail to decrease the detection time and show a significant response to the control ion Zn(II).

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