Using Chemiresistor Sensor Arrays to Test Petrol Station Groundwater Samples for Hydrocarbon Pollutants
Authors: Cooper James Scott , Chow Edith , Hubble Lee John , Chai Roger , Sosa-Pintos Andrea 
Published: May 2020

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