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We present the flow synthesis of 4-(dimethylamino)pyridine (DMAP)-stabilised gold nanoparticles in a biphasic flow reactor system. In the first stage, the reduction of gold chloride using sodium borohydride took place via gas permeable tubing to minimise pressure build-up in the lines due to hydrogen gas liberation. The use of liquid–liquid membrane separators enabled automatic washing of the intermediate tetraoctylammonium bromide (TOAB)-stabilised gold nanoparticles and for subsequent phase transfer of the organic nanoparticles to the aqueous phase using DMAP. Through a systematic investigation of DMAP concentration and ratio, the amount of DMAP necessary for full transfer of the gold nanoparticles was elucidated. This semi-automated, scalable flow process enabled the production of gold nanoparticles in high yield, with good batch-to-batch reproducibility and improved monodispersity compared to conventional batch methods.

The analyte sensitivity of a chemical sensor is commonly tuned by the type of chemical recognition element. Here, it is shown that the solvent also plays a critical role in the sensing performance of thiol-capped gold nanoparticle film chemiresistors. Both the resistance change and speed of response of the chemiresistor can be optimised by adjusting the composition of methanol and water used for detection of the pesticide permethrin. It is shown that the resistance change is governed by the partitioning of the analyte between the thiol coating of the gold nanoparticles and the solvent. The partition coefficient, log P, has a strong influence on the response magnitude and is directly correlated to the response time. By varying the solvent, the log P of extremely high partitioning analytes such as permethrin in water can be lowered, and a much larger resistance change can be obtained in a shorter time period (5 min). A theoretical model is developed that correlates the experimental findings to changes in analyte partition coefficient with solvent type and composition. The results have significant implications for the design of chemical sensors and solvent conditions that can offer an enhanced signal within a practical timeframe.

Plasma printing and plasma-assisted printing of functional nanomaterials are being developed as important alternative fabrication techniques. Nano-gold is used in many applications including organic photovoltaics, flexible electronics, nanomedicine, catalysis and sensing, taking advantage of its unique optical, electrical and physical properties, which depend on particle shape, size and distribution. A direct one-step nano-gold printing process using an HAuCl4 solution precursor injected into an atmospheric-pressure plasma jet is demonstrated. Atomized droplets of the solution are reduced to gold nanoparticles in the plasma and deposited on the substrate. The gold film has minimal Cl content, and its structure can be controlled by the deposition time, from nanometer-size particles to a dense film that fully covers the substrate. Printing is demonstrated on substrates including silicon, alumina filter membrane, vertical graphene, and paper. The applicability of the nano-gold film as a SERS (surface-enhanced Raman scattering) platform is demonstrated by sensing of a 0.25 and 0.7 amol μm−2 of Rhodamine B on an Si and paper substrate respectively, a level undetectable in the absence of nano-gold.

Surface-enhanced Raman spectroscopy (SERS) technology is an attractive method for the prompt and accurate on-site screening of illicit drugs. As portable Raman systems are available for on-site screening, the readiness of SERS technology for sensing applications is predominantly dependent on the accuracy, stability and cost-effectiveness of the SERS strip. An atmospheric-pressure plasma-assisted chemical deposition process that can deposit an even distribution of nanogold particles in a one-step process has been developed. The process was used to print a nanogold film on a paper-based substrate using a HAuCl4 solution precursor. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the gold has been fully reduced and that subsequent plasma post-treatment decreases the carbon content of the film. Results for cocaine detection using this substrate were compared with two commercial SERS substrates, one based on nanogold on paper and the currently available best commercial SERS substrate based on an Ag pillar structure. A larger number of bands associated with cocaine was detected using the plasma-printed substrate than the commercial substrates across a range of cocaine concentrations from 1 to 5000 ng/mL. A detection limit as low as 1 ng/mL cocaine with high spatial uniformity was demonstrated with the plasma-printed substrate. It is shown that the plasma-printed substrate can be produced at a much lower cost than the price of the commercial substrate.

Nanozymes are advanced nanomaterials which mimic natural enzymes by exhibiting enzyme-like properties. As nanozymes offer better structural stability over their respective natural enzymes, they are ideal candidates for real-time and/or remote environmental pollutant monitoring and remediation. In this review, we classify nanozymes into four types depending on their enzyme-mimicking behaviour (active metal centre mimic, functional mimic, nanocomposite or 3D structural mimic) and offer mechanistic insights into the nature of their catalytic activity. Following this, we discuss the current environmental translation of nanozymes into a powerful sensing or remediation tool through inventive nano-architectural design of nanozymes and their transduction methodologies. Here, we focus on recent developments in nanozymes for the detection of heavy metal ions, pesticides and other organic pollutants, emphasising optical methods and a few electrochemical techniques. Strategies to remediate persistent organic pollutants such as pesticides, phenols, antibiotics and textile dyes are included. We conclude with a discussion on the practical deployment of these nanozymes in terms of their effectiveness, reusability, real-time in-field application, commercial production and regulatory considerations.

Exposing a thiol-functionalised gold nanoparticle film chemiresistor to methanol in aqueous solution results in only a small electric current response as the thiol ligand/water partition coefficient of methanol is small, leading to only minor swelling of the chemiresistor film. Nevertheless, the current response to methanol can be enhanced if the chemiresistor becomes pre-exposed to a molecule with a large ligand/water partition coefficient P (e.g. octane with Po = 104.3). The large response enhancement is achieved because methanol, when added to an aqueous solution of octane, lowers the large initial partition coefficient of octane. Octane exiting the thiol ligands then leads to strong film shrinkage resulting in a relative current change much greater than the one otherwise induced by methanol alone. This was theoretically modelled for octane and heptane (Ph = 103.6). A strong response enhancement to methanol (>20 times) was observed experimentally by exposure to 2 ppm octane compared to direct testing of methanol in aqueous solution. Besides octane and heptane, molecules with P > 107 (e.g. permethrin) can theoretically be used to provide enhancement factors of several orders of magnitude. For practical reasons, heptane and octane saturate more quickly, thus enabling more rapid detection of methanol than higher P organic molecules.

The birth weight of babies is generally used as a health state indicator but is not often recorded in resource-poor settings due to limited access to medical clinics. In this work we develop a low-cost and lightweight “balance-in-a-box” that could allow remote communities to have access to a weighing balance. The “balance-in-a-box” is a cardboard mailing box which is reconstructed to form the balance tray, with a novel piezoresistive pressure sensor underneath, a paper-based readout system and battery integrated with copper tape to form an electrical circuit. The readout system comprises multiple segments of electrochromic Prussian blue/polyaniline on conductive gold nanoparticle films connected by graphite resistive separators. The color of each segment is voltage dependent and is directly correlated to the weight applied to the pressure sensor. It is shown that by varying the properties of the readout system (total resistance, individual resistance values of the graphite separators and number of gold nanoparticle segments) that it is possible to tune the critical weight values in which the gold nanoparticle segments change color. Overall, this work demonstrates that babies can be classified as very low birth weight ( 2.5 kg) using a simple, integrated paper-based system that can be deployed to resource-poor settings where babies are at greatest risk of being underweight and requiring health care treatment.

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

Paper-based sensors are gaining increasing attention for their potential applications in resource-limited settings and for point-of-care analysis. However, chemical analysis of paper-based electronic sensors is frequently interpreted using complex software and electronic displays which compromise the advantages of using paper. In this work, we present two semiquantitative paper-based readout systems that can visually measure a change in resistance of a resistive-based sensor. The readout systems use electrochromic Prussian blue/polyaniline as an electrochromic indicator on a resistive gold nanoparticle film that is fabricated on paper. When the readout system is integrated with a resistive sensor in an electrical circuit, and a voltage is applied, the voltage drop along the readout system varies depending on the sensor's resistance. Due to the voltage gradient formed along the gold nanoparticle film, the overlaying Prussian blue/polyaniline will change color at voltages greater than its reduction voltage (green/blue for oxidized state and transparent for reduced state). Thus, the changes in resistances of a sensor can be semiquantified through color visualization by either measuring the length of the transparent film (analog readout system) or by counting the number of transparent segments (digital readout system). The work presented herein can potentially serve as an alternative paper-based display system for resistive sensors in instances where cost and weight is a premium.

Partially selective gold nanoparticle sensors have the sensitivity and selectivity to discriminate and quantify benzene, toluene, ethylbenzene, p-xylene and naphthalene (BTEXN) at concentrations relevant to the US Environmental Protection Agency. In this paper we demonstrate that gold nanoparticle chemiresistors can do so in the presence of 16 other hydrocarbons and that they did not reduce the discriminating power of the array. A two-level full factorial designed experiment was performed on unary, binary, ternary, quaternary, quinary combinations of BTEXN analytes with and without the possibly interfering hydrocarbons. The nominal component concentration of the mixtures was 100 μg L(-1), equivalent to approximately 100 parts per billion (ppb). Concentrations predicted with the random forests method had an average root mean square error of 10-20% of the component concentrations. This level of accuracy was achieved regardless of whether or not the 16 possibly interfering hydrocarbons were present. This work shows that the sensitivity and selectivity of gold nanoparticles chemiresistor sensors towards BTEXN analytes are not unduly affected by the other hydrocarbons that are expected to be present at a petroleum remediation site.

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.

Paper-based sensors are a new alternative technology for fabricating simple, low-cost, portable and disposable analytical devices for many application areas including clinical diagnosis, food quality control and environmental monitoring. The unique properties of paper which allow passive liquid transport and compatibility with chemicals/biochemicals are the main advantages of using paper as a sensing platform. Depending on the main goal to be achieved in paper-based sensors, the fabrication methods and the analysis techniques can be tuned to fulfill the needs of the end-user. Current paper-based sensors are focused on microfluidic delivery of solution to the detection site whereas more advanced designs involve complex 3-D geometries based on the same microfluidic principles. Although paper-based sensors are very promising, they still suffer from certain limitations such as accuracy and sensitivity. However, it is anticipated that in the future, with advances in fabrication and analytical techniques, that there will be more new and innovative developments in paper-based sensors. These sensors could better meet the current objectives of a viable low-cost and portable device in addition to offering high sensitivity and selectivity, and multiple analyte discrimination. This paper is a review of recent advances in paper-based sensors and covers the following topics: existing fabrication techniques, analytical methods and application areas. Finally, the present challenges and future outlooks are discussed.

The response time of chemiresistors consisting of gold nanoparticles coated with self-assembled monolayers (SAM) of 1-hexanethiol, exposed to different hydrocarbons dissolved in water, is investigated with respect to the analyte SAM/water partition coefficient, the analyte−water flow velocity, and the nanoparticle film thickness. We show experimentally that the response time is proportional to the analyte SAM/water partition coefficient and that for sufficiently large velocities the response time becomes flow velocity independent. The dependence of the response time on film thickness is found to be different in the two different flow velocity regimes examined. We outline in detail a theoretical model that explains our experimental data.

Gold nanoparticle films (Au(NPF)) functionalized with a range of hydrophobic and hydrophilic thiols were assembled in chemiresistor sensor arrays that were used to differentiate between complex mixtures of analytes in the aqueous phase. A chemiresistor array sampled a simple system of linear alcohols (methanol, ethanol, propan-1-ol, and butan-1-ol) dissolved in water over a range of concentrations. Discriminant analysis confirmed that the response patterns of the array could be used to successfully distinguish between the different alcohol solutions at concentrations above 20 mM for all of the alcohols except methanol, which was distinguished at concentrations above 200 mM. Alcohol solutions more dilute than these concentrations had response patterns that were not consistently recognizable and failed cross validation testing. This defined the approximate limit of discrimination for the system, which was close to the limits of detection for the majority of the individual sensors. Another Au(NPF) chemiresistor array was exposed to, and successfully identified crude oil, diesel, and three varieties of gasoline dissolved in artificial seawater at a fixed concentration. This work is a demonstration that the pattern of responses from an array of differently functionalized Au(NPF) sensors can be used to distinguish analytes in the aqueous phase.

Gold nanoparticles functionalized with mixtures of 1-hexanethiol and 4-mercaptophenol were investigated as chemiresistor sensors for use in aqueous solutions. X-ray photoelectron spectroscopy (XPS) studies confirmed that it was possible to form mixed ligand compositions on the surface of the nanoparticles. Nanoparticle films with a higher proportion of 1-hexanethiol as the ligand resulted in higher impedances than those with lower proportions. The electrical response of the chemiresistor to toluene, dichloromethane, hexane and ethanol dissolved in water was found to depend strongly on the composition of the organic ligand used to cap the gold nanoparticles. 1-Hexanethiol-coated gold nanoparticles were sensitive to non-polar analytes whereas the sensitivity was reduced upon increasing the composition of 4-mercaptophenol on the surface. It was also shown that the sensitivity to ethanol could be enhanced 3-fold by using a sensor that was functionalized with a mixture of 60% 1-hexanethiol and 40% 4-mercaptophenol compared to a sensor functionalized with pure thiols only.

Chemiresistor sensors based on thin films of gold nanoparticles capped with hydrophobic hexanethiol or hydrophilic 6-hydroxyhexanethiol self-assembled monolayers are shown to respond to organic analytes in aqueous solution. Although considerable swelling of the 6-hydroxyhexanethiol-capped nanoparticle film by water occurred, the 6-hydroxyhexanethiol-capped nanoparticle film still responded toward the presence of organic analytes in aqueous solution. The response toward nonpolar analytes (toluene, hexane, dichloromethane) was reduced for the 6-hydroxyhexanethiol-capped chemiresistor compared to the hexanethiol-capped chemiresistor. However, for polar analytes such as ethanol the response sensitivity was reversed. A simple theoretical model describing the chemiresistor response in water is presented and was used to determine the partition coefficients between the nanoparticle film and water for ethanol and toluene.

A novel chemiresistor sensor for detection of organic analytes in high-conductivity aqueous electrolyte solution is reported. The chemiresistor sensor is based on thin films of gold nanoparticles capped with a 1-hexanethiol monolayer that is inkjet printed onto a microelectrode. In order for a change in nanoparticle film resistance to be measured, the electronic conduction must preferentially occur through the nanoparticle film rather than through the high-conductivity electrolyte solution. This was achieved by miniaturizing the chemiresistor device such that the double layer capacitance of the electrodes in contact with the electrolyte solution gives rise to a significantly larger impedance compared to the nanoparticle film resistance. This system was shown to be sensitive to simple organics dissolved in an aqueous electrolyte solution. The organic analytes, dissolved in the aqueous solution, partition into the hydrophobic nanoparticle film causing the nanoparticle film to swell, resulting in an increase in the low-frequency impedance of the sensor. An increase in the impedance, at 1 Hz, of the gold nanoparticle chemiresistor on exposure to toluene, dichloromethane, and ethanol dissolved in 1 M KCl solution was demonstrated with detection limits of 0.1, 10, and 3000 ppm, respectively. Titration curves over 3 orders of magnitude could be obtained for analytes such as toluene.

An electrochemical assay based on underpotential deposition (UPD) of metal has been utilised for the detection of Cd2+ using surface immobilized single-stranded DNA (ss-DNA). Since Cd2+ is able to undergo UPD on gold surface, this allows the opportunity to detect the amount of Cd2+ accumulated by immobilized ss-DNA directly via voltammetry. This is evidenced by the appearance of Cd2+/Cd0 electrochemistry at E0′ of 92 mV (rather than E0′ of −795 mV in bulk solution) at the ss-DNA modified gold electrode only after an exposure to Cd2+ solution. An association constant of 8.33 × 105 M−1 was determined from a Cd2+ calibration curve assuming a Langmuir-binding model for Cd2+ with surface-immobilized DNA. The high association constant is reflected in a low detectable concentration of 10 pM. The sensing layer can also be regenerated to metal-free status and can be reused up to 18 times without significant signal degradation. Keywords: Underpotential deposition, Cadmium detection, DNA modified surface, Electrochemical assay, Heavy metals

Multiple electrodes, combined with a chemometric strategy to calibrate the measurement response, have been used for the determination of an analyte across a broader dynamic range than is possible with a single electrode. The model system used for the detection of copper comprised electrodes modified with a self-assembled monolayer. The electrodes were modified with the copper-complexing species (3-mercaptopropionic acid, thioctic acid, and the peptides cysteine and Gly-Gly-His) and copper was determined over concentrations ranging from nanomolar to millimolar using voltammetric analysis. We have demonstrated that by combining the calibration functions from the four electrodes a better estimate (i.e. with smaller variance) of the concentration of the analyte is obtained. Measurement uncertainty is expressed for independently prepared electrodes, which allows the possibility of commercial production and factory calibration. The principles of using multiple electrodes modified with recognition elements with different affinities for the target analyte to extend the dynamic range of sensors is a general one that could be applied to other analytes.

Chemiresistor sensor arrays are a promising technology to replace current laboratory-based analysis instrumentation, with the advantage of facile integration into portable, low-cost devices for in-field use. To increase the performance of chemiresistor sensor arrays a high-throughput fabrication and screening methodology was developed to assess different organothiol-functionalized gold nanoparticle chemiresistors. This high-throughput fabrication and testing methodology was implemented to screen a library consisting of 132 different organothiol compounds as capping agents for functionalized gold nanoparticle chemiresistor sensors. The methodology utilized an automated liquid handling workstation for the in situ functionalization of gold nanoparticle films and subsequent automated analyte testing of sensor arrays using a flow-injection analysis system. To test the methodology we focused on the discrimination and quantitation of benzene, toluene, ethylbenzene, p-xylene, and naphthalene (BTEXN) mixtures in water at low microgram per liter concentration levels. The high-throughput methodology identified a sensor array configuration consisting of a subset of organothiol-functionalized chemiresistors which in combination with random forests analysis was able to predict individual analyte concentrations with overall root-mean-square errors ranging between 8-17 μg/L for mixtures of BTEXN in water at the 100 μg/L concentration. The ability to use a simple sensor array system to quantitate BTEXN mixtures in water at the low μg/L concentration range has direct and significant implications to future environmental monitoring and reporting strategies. In addition, these results demonstrate the advantages of high-throughput screening to improve the performance of gold nanoparticle based chemiresistors for both new and existing applications.

Sensors for the detection of metal ions are of considerable importance for enabling the monitoring of environmental samples for metal ion contamination directly in the field. This review outlines the use of peptides and amino acids as the recognition element of electrochemical sensors for metal ion detection. Initially the complexation of metals by peptides is discussed followed by the immobilization of peptides on electrode surfaces. Subsequently, the application of peptide modified electrodes for detecting metals is reviewed and finally challenges and future prospects are outlined.

This paper reports a study of the factors affecting the analytical performance of gold and glassy carbon electrodes modified with the tripeptide Gly-Gly-His for the detection of copper ions. Gly-Gly-His is attached to a glassy carbon (GC) surface modified with 4-carboxyphenyl moieties or a gold surface modified with 3-mercaptopropionic acid by the reaction of the N-terminal amine group of the peptide with the carboxylic acid groups of the monolayer via carbodiimide activation. X-ray photoelectron spectroscopy was used to characterize the steps in the biosensor fabrication. It was found that the analytical performance of a sensor prepared with Gly-Gly-His on a GC electrode was similar to that on a gold electrode under the same conditions. The performance was greatly enhanced at higher temperature, no added salt during copper accumulation and longer accumulation time within a pH range of 7 – 9. Interference studies and investigations of stability of the Gly-Gly-His sensor are reported. Analysis of natural water samples show that the sensors measure only copper ions that can complex at the sensor surface. Strongly complexed copper in natural water is not measured. Despite greater stability of diazonium salt derived monolayers on carbon surfaces compared with alkanethiols self-assembled monolayers on gold, the stability of the sensors was essentially the same regardless of the modification procedure.

This paper studies the analytical performance and characterization of a peptide modified biosensor towards detection of Cu2+ ions. The peptide modified biosensor is developed by carbodiimide attachment of the Gly-Gly-His tripeptide onto a 3-mercaptopropionic acid (MPA) modified gold surface. X-ray photoelectron spectroscopy (XPS) was used to characterize the steps in the biosensor fabrication. Detection of Cu2+ ions are determined by cyclic and Osteryoung square wave voltammetry. The analytical performance of this biosensor is found to be affected by the pH, salt concentration, temperature and accumulation time of the test solution. Interference studies and investigations of the reusability of this biosensor have also been performed.

An electrochemical sensor for the detection of lead ions is described which is made by modifying a gold electrode substrate with self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) or thioctic acid (TA) followed by covalent attachment of a lead binding peptide, human angiotensin I. Cyclic voltammetry of MPA–angiotensin modified gold electrodes complexed with lead displayed voltammograms with prominent lead peaks at E 0′ , −0.29 V. A detection limit of 1 nM was achieved using Osteryoung square wave voltammetry. However, the electrodes were not stable over repeated electrochemical cycles due to partial electrochemical desorption of the SAM. The TA–angiotensin modified gold electrode showed greater stability and were able to be regenerated several times. Using Osteryoung square wave voltammetry for TA–angiotensin modified electrodes, lead concentrations down to 1.9 nM were detected. Although the detection limit of the TA–angiotensin modified electrode is higher than achieved with MPA–angiotensin, it is still well below Australian drinking water guidelines. Studies of interference effects on the Pb 2+ current showed Hg 2+ as a significant interferent but only at levels significantly greater than those found in natural waters.

The effect of the surface chemistry of DNA recognition interfaces on DNA hybridization at a gold surface was investigated using both electrochemistry and the quartz crystal microbalance (QCM) technique. Different DNA recognition interfaces were prepared using a two-component self-assembled monolayer consisting of thiolated 20-mer probe single-stranded DNA (ss-DNA) containing either a 3'-mercaptopropyl or a 3'-mercaptohexyl linker group and an alcohol-terminated diluent layer with 2-, 6-, or 11-carbon length. The influence of the interfacial design on the hybridization efficiency, the affinity constant (Ka) describing hybridization, and the kinetics of hybridization was assessed. It was found that the further the DNA was above the surface defined by the diluent layer the higher the hybridization efficiency and Ka. The kinetics of DNA hybridization was assessed using both a QCM and an electrochemical approach to ascertain the influence of the interface on both the initial binding of target DNA to the surface and the formation of a complete duplex. These measurements showed that the length of the diluent layer has a large impact on the time taken to form a perfect duplex but no impact on the initial recognition of the target DNA by the immobilized probe DNA.

An electrochemical sensor for cadmium ions is described which is made by modifying a gold electrode with 3-mercaptopropionic acid followed by covalently coupling the cadmium-selective hexapeptide His–Ser–Gln–Lys–Val–Phe. The resultant electrode had significantly superior analytical performance compared with a glutathione (GSH) modified electrode for the detection of Cd2+, having a greater sensitivity, broader detectable concentration range and a five times lower detection limit of 0.9 nM. Interference from Pb2+ was observed which is an improvement from the GSH modified electrode which had Pb2+ and Cu2+ as interfering ions. The effects of Cu2+, Ni2+, Zn2+, Cr3+ and Ba2+were insignificant. Keywords: Heavy metals, Biosensors, Electrochemistry, Cadmium, His–Ser–Gln–Lys–Val–Phe, Glutathione

Paper-based potentiometric pH sensors allow multiple measurements to be recorded in a cost-effective manner but usually in combination with an external display unit. In this work, a potentiometric pH sensor is integrated with an electrochromic readout system all on paper. The potentiometric pH sensor is based on electropolymerised aniline on a conductive gold nanoparticle film working electrode. The voltage output of the sensor is amplified using an operational amplifier and generated across an electrochromic readout system. The readout system comprises four segments of electrochromic Prussian blue/polyaniline on conductive gold nanoparticle films connected by graphite resistive separators. The colour of each segment is dependent on the voltage output from the potentiometric sensor and can be used to determine the pH range of a sample or whether the sample pH falls outside a critical value. This type of integrated paper device can be used for multiple measurements and also be applied to the development of other types of potentiometric sensors.

The performance of chemiresistor sensors made from thin film assemblies of single-wall carbon nanotubes, multiwall carbon nanotubes, reduced graphene oxide nanosheets (RGON), and gold nanoparticles (AuNP) was assessed with an immersible microelectrode array. Carbon nanotube and RGON chemiresistors were functionalized with octadecyl-1-amine and the AuNP chemiresistors were functionalized with 1-hexanethiol. The analytes examined were aqueous solutions of petroleum hydrocarbons: cyclohexane, naphthalene, benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX analytes). Titrations were performed to determine the detection limits of the different chemiresistors. The AuNP chemiresistor was the most sensitive to all the analytes with limits of detection between 0.2 and 0.6 ppm in water. In contrast, the multiwall carbon nanotube chemiresistor was the least sensitive to the analytes with limits of detection between 20 and 200 ppm. These sensitivities show that these nanomaterials have the potential, with further optimization, of being incorporated into devices that would respond to hydrocarbons in water at concentrations relevant to the regulations of the US Environmental Protection Agency. The stability of the sensors over 26 days was also assessed. Remarkably, there was a negligible change in the electrical resistance of the RGON sensors over this time. The nanotube sensors increased in resistance and the AuNP decreased in resistance over the same period. The drifting resistances did not affect the sensitivity of the nanomaterials, which remained constant with time.

The porosity of 1-hexanethiol-functionalised gold nanoparticle films was assessed and utilised as chemiresistor sensors. Electrochemical capacitance measurements showed that the accessibility of electrolytes of different ionic strengths into the pores depended on the thickness of the electric double layer formed. A large variation in capacitance was measured in 0.01–1000 mM NaClO4, implying a wide pore size distribution. The change in morphology of the nanoparticle films upon storage in air, water and ethanol for two weeks was investigated. There was a significant decrease in the electrochemical capacitance at high electrolyte concentrations for the ethanol-stored films compared to the freshly-prepared films suggesting a decrease in the number of small pores of radii in the range of 0.3–3 nm. This was further supported by optical topographical measurements where a decrease in the thickness of ethanol-stored films was observed relative to the freshly-prepared films. The porous nature of the nanoparticle films was found to have an effect on the chemical sensing behaviour. When used as chemiresistor sensors, for the detection of heptane in water, the ethanol-stored films provided larger resistance changes and longer response times. This suggests that the more densely packed ethanol-stored films provided more sites that enabled film swelling, and that diffusion of the analyte occurred through the narrower water-filled pores. This demonstrates the effect of different storage conditions on film morphology and subsequently sensor response.

The treatment of venous leg ulcers is commonly performed through the use of compression bandaging. However, applying an effective pressure can be a challenge, even for an experienced nurse. In this work, a simple and portable integrated platform consisting of a piezoresistive pressure sensor, paper-based electrochromic readout system and battery is developed to directly monitor the applied pressure. The pressure sensor is fabricated using a mixture of copper particles and polydimethylsiloxane and tuned to the desired pressure sensitivity via the addition of carbon paste. Since the sensor is to be incorporated within the bandage, an external electronic display is not practical and thus a lightweight, paper-based readout system comprising an electrochromic coating of Prussian blue/polyaniline on top of segments of gold nanoparticle films connected via graphite resistive films is developed. The readout system functions by measuring changes in the voltage drop across the readout system due to changes in the resistance of the pressure sensor. This allows the applied pressure to be visualized by the number of gold nanoparticle segments that have changed color in the readout system. This work opens the door to a lightweight and low-cost sensing approach for compression bandaging at home or in community locations.

Functionalised gold nanoparticle (Au(NP)) chemiresistors are investigated for direct sensing of small organic molecules in biological fluids. The principle reason that Au(NP) chemiresistors, and many other sensing devices, have limited operation in biological fluids is due to protein and lipid fouling deactivating the sensing mechanism. In order to extend the capability of such chemiresistor sensors to operate directly in biofluids, it is essential to minimise undesirable matrix effects due to protein and lipidic components. Ultrafiltration membranes were investigated as semi-permeable size-selective barriers to prevent large biomolecule interactions with Au(NP) chemiresistors operating in protein-loaded biofluids. All of the ultrafiltration membranes protected the Au(NP) chemiresistors from fouling by the globular biomolecules, with the 10 kDa molecular weight cut-off size being optimum for operation in biofluids. Titrations of toluene in different protein-loaded fluids indicated that small molecule detection was possible. A sensor array consisting of six different thiolate-functionalised Au(NP) chemiresistors protected with a size-selective ultrafiltration membrane successfully identified, and discriminated the spoilage of pasteurised bovine milk. This proof-of-principle study demonstrates the on-chip protein separation and small metabolite detection capability, illustrating the potential for this technology in the field of microbial metabolomics. Overall, these results demonstrate that a sensor array can be protected from protein fouling with the use of a membrane, significantly increasing the possible application areas of Au(NP) chemiresistors ranging from the food industry to health services.

The diagnosis of diseases in developing countries remains a significant challenge. Here, we report on the design and development of a novel diagnostic device for use in the developing world. Patient diagnosis will be based on urine samples analysed by low cost chemiresistor sensor arrays to generate a metabolomic profile with mobile phone technology being utilised as the user interface for the healthcare worker. Gold nanoparticle chemiresistor sensor technology is showing great promise for disease detection capabilities within this device framework. Preliminary results indicate that an array of different chemiresistors is able to discriminate between synthetic models representing the difference between healthy and TB infected urine. (4 pages)

We investigate the response dynamics of 1-hexanethiol-functionalized gold nanoparticle chemiresistors exposed to the analyte octane in aqueous solution. The dynamic response is studied as a function of the analyte-water flow velocity, the thickness of the gold nanoparticle film and the analyte concentration. A theoretical model for analyte limited mass-transport is used to model the analyte diffusion into the film, the partitioning of the analyte into the 1-hexanethiol capping layers and the subsequent swelling of the film. The degree of swelling is then used to calculate the increase of the electron tunnel resistance between adjacent nanoparticles which determines the resistance change of the film. In particular, the effect of the nonlinear relationship between resistance and swelling on the dynamic response is investigated at high analyte concentration. Good agreement between experiment and the theoretical model is achieved.

A sensor array of gold nanoparticle chemiresistors, functionalized with a variety of thiolate ligands, was used to detect different hydrocarbons dissolved in water. Discriminant analysis of the sensor responses showed that the diluted saturated solutions of kerosene, turpentine, two types of oil and three types of petrol could be clearly identified across a range of concentrations.

This study was conducted to assess and characterize the intrinsic electrical noise associated with thiolate functionalized gold nanoparticle films utilized as chemiresistor sensors. Recently, this sensor type has been demonstrated to have liquid-based sensing capabilities. Differences in the thermal noise component of the different thiolate functionalized nanoparticle films in ambient air and in deionized water are detailed. Comparable values are found between real impedance values and resistance values derived from the Nyquist equation, in the frequency domain, for differing chemical measurement environments. Voltage-biased studies in aqueous media demonstrate the device's retention of the intrinsic 1/f noise, known to exist in gold nanoparticle chemiresistors operating in ambient air. These results have implications on the ultimate sensitivity of devices utilized for liquid-based analyte detection.

The electrical impedance change and response time of chemiresistor sensors based on gold nanoparticles capped with 1-hexanethiolate ligands were investigated for several organic analytes dissolved in aqueous solution. Analytes that partitioned strongly into the nanoparticle film resulted in greater impedance changes and longer response times than analytes which had a smaller partition coefficient.

A peptide-modified electrode array with a different peptide on each electrode is compared with a single electrode modified with many peptides for the voltammetric measurement of concentrations of Cu(2+), Cd(2+) and Pb(2+) in solution. The single gold electrode was modified simultaneously with peptides Gly-Gly-His, glutathione and angiotensin I each coupled to thioctic acid, and thioctic acid itself, and the calibration of mixtures of ions was compared with previously published data from an array of four sensors each with an individual modification. Calibration at the multi-peptide single-electrode sensor was by two-way partial least squares (voltammetric current measured with variables 'sample' x 'potential') and for the electrode array by three-way NPLS1 ('sample' x 'potential' x 'electrode'). The advantage of designing experiments to yield multi-way data is demonstrated and discussed.

The influence of film morphology on the performance of inkjet-printed gold nanoparticle chemiresistors has been investigated. Nanoparticles deposited from a single-solvent system resulted in a "coffee ring"-like structure with most of the materials deposited at the edge. It was shown that the uniformity of the film could be improved if the nanoparticles were deposited from a mixture of solvents comprising N-methyl-2-pyrrolidone and water. Electrical conductivity measurements showed that both "coffee ring" and "flat" films were qualitatively similar suggesting that the films have similar nanoscale structures. To form the functional chemiresistor device, the 4-(dimethylamino)pyridine coating on the nanoparticle was exchanged with 1-hexanethiol to provide a hydrophobic sensing layer. The performance of 1-hexanethiol coated gold nanoparticle chemiresistors to small organic molecules, toluene, dichloromethane and ethanol dissolved in 1 M KCl in regard to changes in impedance and response times was unaffected by the film morphology. For larger hydrocarbons such as octane, the rate of uptake of the analyte into the film was significantly faster when the flatter nanoparticle film was used as opposed to the "coffee ring" film which has a thicker edge. Furthermore, the presence of potassium and chloride ions in the solution media does not significantly affect the impedance of the nanoparticle film at 1 Hz (

Publisher Summary This chapter presents a procedure to construct a peptide-modified electrode for analyzing copper in water samples and to calibrate the peptide-modified electrode using standard solutions of metal ions. All electrochemical measurements were performed with an Autolab PGSTAT 12 potentiostat. Measurements were carried out with a conventional three-electrode system. When the concentrations of Cu2+ in tap water and lake water determined by the MPA–Gly–Gly–His modified electrode, it was found to be 0.55±0.08 μM. This value is much lower than the copper concentration measured by ICP-MS of 1.70±0.15 μM (95% confidence interval). The peptide-modified electrode measures free Cu2+ or weakly bound copper complexes rather than the total metal-ion concentration measured by ICP-MS. Hence the use of peptides has the ability to measure metal-ion bioavailability for specific species. The analytical application of MPA–Gly–Gly–His modified electrodes was also demonstrated in the determination of Cu2+ in lake water.

A simple and economical process for fabricating gold electrodes on paper is presented. Gold nanoparticles stabilised with 4-(dimethylamino)pyridine were applied to nail-polish coated filter paper and made conductive using a camera flash sintering step. To test the ability of the sintered gold nanoparticle film to function as a sensing platform, cysteine was self-assembled on gold and used for the electrochemical determination of copper ions. The cysteine-sintered gold nanoparticle film was able to successfully complex copper ions, with only minor differences in performance compared with a standard cysteine-modified solid-state gold disk electrode. Investigations by Raman spectroscopy revealed the successful removal of the 4-(dimethylamino)pyridine coating during sintering, whereas electrochemical impedance spectroscopy and scanning electron microscopy suggested that differences in the sensing performance could be attributed to the rougher morphology of the sintered gold nanoparticle electrode.

The simultaneous determination of Cu(2+), Cd(2+) and Pb(2+) is demonstrated at four modified gold electrodes using N-PLS calibration. Three of the electrodes were modified with the peptides Gly-Gly-His, gamma-Glu-Cys Gly and human angiotensin I which were covalently attached to thioctic acid self-assembled monolayers and the fourth electrode was modified with thioctic acid only. Voltammetry at the modified electrodes in the presence of the three metal ions revealed one peak due to the reduction of copper and another due to the overlapping peaks of cadmium and lead which made quantification using conventional methods difficult. N-PLS was used to calibrate and predict trace concentrations (100 nM to 10 microM) of mixtures of Cu(2+), Cd(2+) and Pb(2+).

Using biological components to provide selectivity for monitoring heavy metal ions in sensory devices are attractive due to the natural occurrence of metal-binding peptides and proteins. They have advantages over conventional analytical techniques such as atomic absorption spectroscopy, inductively coupled plasma mass spectrometry and anodic stripping voltammetry, because they have the potential to give an estimate of the bioavailability of heavy metals as opposed to measuring total metal concentrations. The metal-binding properties of peptides have been studied extensively [ 1 ] but research into using amino acids and peptides as biosensors for the detection of heavy metal ions is relatively new. Apart from the selectivity imparted from using different peptide ligands, an additional level of selectivity can be achieved by exploiting the different redox potentials of different metals. As a consequence, the following work describes the use of peptide-modified electrodes as highly selective and sensitive metal ion biosensors. [ 2 ]

An electrochemical sensor for the detection of cadmium ions is described using immobilized glutathione as a selective ligand. First, a self-assembled monolayer of 3-mercaptopropionic acid (MPA) was formed on a gold electrode. The carboxyl terminus then allowed attachment of glutathione (GSH)via carbodiimide coupling to give the MPA-GSH modified electrode. A cadmium ion forms a complex with glutathione via the free sulfhydryl group and also to the carboxyl groups. The complexed ion is reduced by linear and Osteryoung square wave voltammetry with a detection limit of 5 nM. The effect of the kinetics of accumulation of cadmium on the measured current was investigated and modeled. Increasing the temperature of accumulation and electrochemical analysis caused an increase in the voltammetric peak of approximately 4% per degrees C around room temperature. The modified electrode could be regenerated, being stable for more than 16 repeated uses and more than two weeks if used once a day. Some interference from Pb(2+) and Cu(2+) was observed but the effects of Zn(2+), Ni(2+), Cr(3+) and Ba(2+) were insignificant.

The toxicity of metal ions to flora and fauna makes the monitoring of metals in the environment vital. Current methods of metal ion monitoring involve using classical elemental analysis techniques such as atomic absorption spectroscopy (AAS), inductively coupled plasma mass spectroscopy (ICPMS), and anodic stripping voltammetry (ASV). These classical analytical techniques are reliable and fast but suffer from two key problems. Firstly, they require the sample to be transported from the site of collection to a laboratory; secondly, they monitor either the total metal ion concentration or a labile concentration. The true toxicity of metal ions in the environment, however, is related to the amount of bioavailable metal rather than total metal.[1] Bioavailability is a somewhat loosely defined term as what is regarded as a bioavailable metal may vary from one species to the next.[2] As the water safety guidelines can only reflect the reliable measurement techniques available, the current guidelines attempt to assess bioavailability using a combination of techniques including filtration, ion exchange, ASV, chemical modeling of toxicity; and, if necessary, the effect on the target organism.[3] Such analyses are complicated and certainly not particularly compatible with monitoring in the field. So the challenges to the analytical community are to develop methods for on-site measurement of metal ion levels which may give an indication of the bioavailability of at least some metals. For any new analytical method to be competitive with AAS, ICP-MS, and ASV it is essential that it is reliable, robust, easy to use, and able to measure a suite of metals (all criteria met by these classical methods). The ability to undertake analyses in the field, cheaply and easily, is also desirable. These essential and desired criteria for a new analytical method are compatible with what biosensors are claimed to be able to achieve.[4] Biosensors have most frequently been applied to the detection of organic and biological molecules.[4–8] Thus far there has been little research into the detection of metals using biosensors. In the quest to have analytical methods which measure metals in the field, biosensors appear to be ideal as the recognition molecule is a biological molecule and hence could provide an indication of how the metal ions interact with a particular organism. The possibilities of detecting metal ions using biosensors and biological molecules have begun to attract more interest recently. The purpose of this article is to outline what has been achieved thus far in the detection of metal ions using biosensors. Initially we will discuss more mature biosensor technologies for monitoring metals which use enzymes and bacteria whereupon attention will turn to some new approaches which have recently been described. It is important to emphasize that selectivity in metal ion sensing is often more important than sensitivity because trace levels of heavy metals are present in a sample containing other ionic species which are often a million-fold more concentrated.[9]

Mixtures of benzene, toluene, ethylbenzene, p-xylene and naphthalene dissolved in water were probed with an array of partially selective gold nanoparticle chemiresistor sensors. A full factorial experimental design was followed to generate every possible combination (unary, binary, ternary, quaternary and quinary). The nominal concentrations of the individual components in the mixtures were 0, 0.5, 1, 5 or 10 mg/L and the combined concentrations were between 0 and 45 mg/L, which are relevant to EPA defined maximum contaminant levels in drinking water. Several different statistical techniques were used to predict the component concentrations in the mixtures based on the sensor array responses. The most accurate technique was a non-linear ensemble method called random forests. The overall root mean square error between the predicted and measured concentrations (residuals) was 0.2–1.5 mg/L for the mixtures with a nominal component concentration of 10 mg/L. The accuracy of the random forests predictions was not unduly affected by increasing mixture complexity. Random forests analysis is a statistical technique suitable for quantifying the relationship between responses of partially selective sensors to the concentration of different hydrocarbons in water.

We report on the development of a sensor system for the detection of hydrocarbons in water. The sensor system is based on functionalized gold nanoparticle chemiresistors that change their electrical resistance when exposed to chemical compounds. We fabricated arrays with sixteen chemiresistors, where each chemiresistor can contain gold nanoparticles functionalized with a different linker. The array of semi-selective sensors was exposed to solutions containing benzene, toluene, ethylbenzene, and xylenes (BTEX) dissolved in water. Using linear discriminant analysis we demonstrated that each one of the BTEX compounds could be distinguished.