We demonstrate that thin films consisting of cross-linked nanoparticle aggregates function as highly sensitive strain gauges. The sensors exploit the exponential dependence of the interparticle tunnel resistance on the particle separation. Their sensitivity (gauge factor) is two orders of magnitude higher than that of conventional metal foil gauges and rivals that of state-of-the-art semiconductor gauges. We describe the strain gauge behavior in a tunneling model that predicts the dependence of the gauge factor on several parameters, in particular, the nanoparticle size, the interparticle separation gap, and the conductance of the linker molecules.

4-Dimethylaminopyridine-stabilised gold nanoparticles are synthesised in a biphasic flow reactor system using organic/aqueous membrane separators and gas-permeable tubing.

Environmental contextRegular insecticide treatments on the interior of aircraft impedes the spread of mosquitos and other pests internationally, but border protection agencies lack effective tools to ensure airlines have complied. We report the first use of chemiresistor sensors to detect and identify insecticide residue on an interior aircraft surface. The method could be developed into a tool that helps lower the risk of vector-borne diseases like malaria entering international ports. AbstractAustralia and other island nations are protected from stowaway pest vectors, like mosquitos, by aircraft disinsection – spraying the airplane interior with an insecticide. It is a simple biosecurity measure that can reduce the spread of malaria, Zika and other mosquito-borne diseases. However, checking airline compliance and the efficacy of the insecticide residue is a difficult task for border protection officials, which requires either a live fly bioassay or off-site laboratory testing. Neither of these methods are ideal for the hectic schedules of airlines. As such, we propose using gold nanoparticle chemiresistor sensor arrays, to detect and identify insecticide residue on the interior surface of aircraft. We have shown that hexanethiol functionalised sensors have a limit of detection of 3 parts per billion (ppb) for permethrin in solution and have a broad dynamic range responding to concentrations up to 1000 ppb. The chemical residues of three different insecticide products were lifted off an interior aircraft surface and identified with an array of seven uniquely functionalised sensors. This is the first ever demonstration of gold nanoparticle chemiresistor sensors being used for the analysis of chemical residues. These sensors have the potential to rapidly check the efficacy of insecticide residues on aircraft surfaces.

Groundwater monitoring is a cumbersome and expensive process. Typically, once every three to six months a technician visits a site, collects water samples, and transports them to an analytical laboratory for testing. Not only is the testing expensive, but potentially catastrophic events could remain undetected for several months. A plausible alternative could be gold nanoparticle chemiresistors sensor arrays [1]. We have previously demonstrated that gold nanoparticle chemiresistors can operate in water irrespective of the salinity of the aqueous solution [2], and an array of these chemiresistors could discriminate between different complex mixtures of hydrocarbons such as gasoline, diesel, kerosene or crude oil dissolved in artificial seawater [3]. In addition, we have demonstrated that an array can identify BTEXN in the presence of 15 other structurally relevant hydrocarbons in laboratory-grade water. In the current study we investigate how feasible is it for these chemiresistor sensors to function in real groundwater samples. Using standard photolithography techniques, we fabricate our own interdigitated electrodes. (a) in the figure depicts the glass substrate and electrodes on which an array of 16 gold nanoparticle chemiresistor sensors are deposited on. Gold nanoparticle sensors can be very small, they consist of interdigitated microelectrodes just 0.3 mm wide as shown in (b) of the figure. The sensors can be given different affinities for different analytes by changing the chemistry of the molecules that coat the gold nanoparticles, (c) in the figure demonstrates a gold nanoparticle that is functionalised with 1-hexanethiol. This work focused on eight different sensor chemistries that impart chemical sensitivity and selectivity. Depending on the chemicals present in a water sample, each sensor in the array will change their electrical resistance to a different extent, providing a pattern of response or fingerprint. Groundwater samples were collected from 14 sites across western, central, and northern Sydney, Australia. From these sites, 48 samples were tested. Supplementary laboratory testing showed there was a variety of hydrocarbon contamination in the different samples. Though the testing order was randomised, generally the samples were tested in increasing order of known hydrocarbon contamination. Experiments were performed to determine the limit of detection in one groundwater sample, and the effect of the 48 different groundwater samples on the sensitivity of chemiresistor sensors. The limit of detection for benzene in a real groundwater sample was 70 µg/L. The sensor’s performance was determined to be even better for the other analytes: toluene was 30 µg/L, ethylbenzene was 11 µg/L, p-xylene was 13 µg/L, and naphthalene was 6 µg/L. BTEXN Limits of detection in groundwater are equivalent to the limits of detection previously determined when operating in laboratory-grade water. One type of chemiresistor sensor functionalised with 1-heptanethiol was especially resilient to 90% of the groundwater samples provided from across Sydney. On average, this sensor type experienced a negligible loss in sensitivity; it demonstrated an average normalised sensitivity decrease to the internal standard of only 6% after exposure to 48 different groundwater samples. For the remaining six other chemiresistor sensor types there was a range of normalised sensitivity losses between 15% – 58%. Gold nanoparticle chemiresistor sensors have been demonstrated to maintain their limits of detection to BTEXN analytes in real groundwater samples. Exposure to a variety of different groundwater samples from numerous sites across Sydney afforded valuable information on the performance of different chemiresistor sensor types. This feasibility study has laid a strong foundation for the next stage of work; developing a remotely deployed device that monitors the hydrocarbon ‘health’ of groundwater in a well, remotely and in real-time. 1. Ho, C.K., Robinson, A., Miller, D.R., Davis, M.J., Overview of sensors and needs for environmental monitoring, Sensors, 5 (2005) 4-37. 2. Raguse, B., Chow, E., Barton, C.S., Wieczorek, L., Gold nanoparticle chemiresistor sensors: Direct sensing of organics in aqueous electrolyte solution, Analytical Chemistry, 79 (2007) 7333-7339. 3. Cooper, J.S., Raguse, B., Chow, E., Hubble, L., Müller, K.H., Wieczorek, L., Gold nanoparticle chemiresistor sensor array that differentiates between hydrocarbon fuels dissolved in artificial seawater, Analytical Chemistry, 82 (2010) 3788-3795. Figure 1