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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.

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.

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.

Tactile feedback in biomedical applications, e.g. prosthetic artificial skin and minimally invasive surgery, requires multi-axial detection capabilities, with high sensitivity and reliability. This paper presents the fabrication, modeling and experimental characterization of a novel nanoparticle resistive-based multi-axial sensor over the range of 0–300 mN for use in tactile feedback. The fabrication uses inkjet printing, and micro-molding techniques scalable to sub millimeter dimensions. Sensitivities of 0.037%/mN in the normal, 0.036%/mN in the shear x and 0.048%/mN shear y directions respectively were achieved. Dynamic characteristics indicated minimal change in hysteresis (standard deviation – 1.2%) and response (standard deviation – 0.4%) compared with the applied load for increasing strain-rates.

Tactile feedback in robotic and prosthetic applications requires high resolution sensing with rapid response times, high sensitivity and conformability. Nanoparticle films offer the opportunity to realise high spatial resolution sensors on various substrates through ink-jet printing and the ability to tailor device characteristics to achieve sensitive and rapid transduction. This paper presents modelled and experimental characteristics of a novel nanoparticle resistive-based sensor over the range of 0–250 mN for use as a tactile sensor. The frequency response on a flexible substrate showed a cut-off frequency of 80 Hz to 100 μm amplitude displacements. The sensor had a detection limit of ∼10 mN with a sensitivity factor of 0.3% mN −1 and 0.001% mN −1 on a flexible and rigid substrate respectively.

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.

The electrical resistances of films of gold nanoparticles linked by hexanedithiol and octanedithiol molecules were measured as a function of temperature from 4 up to 375 K. The electrical resistance of these films goes through a minimum at a temperature T*. The change of resistance with temperature is semiconductor-like below T* and metal-like above T*. We argue that this behavior of the electrical resistance might not be the signature of a Mott−Hubbard metal-to-insulator transition but instead the signature of thermally activated electron conduction with a temperature-dependent electron tunneling where the tunneling gap between nanoparticles increases when the films expand thermally. Measuring the electrical resistances of different films revealed how T* relates to the size of the nanoparticles and to the expansion coefficients of the substrates on which the films were deposited. Simple model calculations agree well with the experimental data. Water/ice trapped within nanopores in the films leads to a hy...

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.

The interaction between cucurbit[8]uril (Q[8]) and a series of symmetric viologen derivatives having aliphatic substituents of variable length [N,N′-dialkyl-4,4′-bipyridinium dianions; alkyl = CH3(CH2) n –, n = 0 (MV2+), 1 (EV2+), 2 (PV2+), 3 (BV2+), 4 (FV2+), 5 (HV2+) or 6 (SV2+); BPY2+ = diprotonated 4,4-bipyridine], determined by 1H NMR and electronic absorption spectroscopy methods, is described. Some different binding models were observed in this work when compared to the interactions between cucurbit[7]uril (Q[7]) and these guests. The experimental results revealed that the binding site of the guests by Q[8] depended strongly on the length of the aliphatic substituents on the 4,4′-bipyridinium nucleus. While a 1:2 complex was observed for Q[8]-BPY2+ under acidic conditions, a 1:1 complex was formed for Q[8]-viologen derivatives with chains shorter than four carbon atoms. However, multiple Q[8] molecules could be threaded on the longer-chain FV2+, HV2+ or SV2+ molecules to form 2:1 and even possibly 3:1 complexes.

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.

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.

We have investigated the performance of a porous Au film electrode for future applications in energy storage by the reduction of CO 2 to CO. The electrode consists of a porous hydrophilic polymer membrane with a 260 nm Au film, applied by vapour deposition. The hydrophilic property of the polymer substrate was necessary for obtaining current densities comparable to those at an ordinary Au electrode under similar conditions: aqueous KHCO 3 , ambient temperature and approximately one atmosphere pressure of CO 2 . At potentials more negative than the activation potential for CO 2 reduction, −1.2 V versus SCE, the current density was of fractional order with respect to the CO 2 pressure, indicating the involvement of an adsorbed reaction intermediate, possibly a radical anion complex (CO 2 ) 2 − . Analysis by GC and FTIR of the gas reaction product showed that the energy storage efficiency was maximal near the activation potential. Close to this potential, CO was formed with a Faradaic efficiency of 75% and an enthalpic efficiency of 35%. Under these conditions, the power storage capacity of the electrode was 50 W m −2 .

A new hybrid material consisting of a nanoparticle film on a flexible, porous substrate is formed. The hybrid nanoparticle films are non-redispersable in solvents, yet remain porous and flexible. Visually, the hybrid films are highly reflective and metallic gold in appearance. However, the electronic properties of the films are characteristic for materials made from separate, non-sintered nanoparticles. Films of large area (several tens of cm2) and several microns in thickness can be formed. The method of formation is based on cross-linking gold nanoparticles using alkane-dithiols followed by filtration onto nanoporous supports. The films were characterized by transmission electron microscopy, atomic force microscopy and resistance measurements. The effect of the ratio of alkane-dithiol cross-linker to gold nanoparticles on the resistance of the nanoparticle films was also studied.

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.

An efficient and convenient route to two novel quasimacrocyclic archaebacterial lipid analogues is presented. The target compounds 2 and 3 are prepared in seven and four steps, respectively, from known starting materials, and are useful for the study of synthetic tethered bilayer membranes.

Using novel synthetic lipids, a tethered bilayer membrane (tBLM) was formed onto a gold electrode such that a well-defined ionic reservoir exists between the gold surface and the bilayer membrane. Self-assembled monolayers of reservoir-forming lipids were first adsorbed onto the gold surface using gold−sulfur interactions, followed by the formation of the tBLM using the self-assembly properties of phosphatidylcholine-based lipids in aqueous solution. The properties of the tBLM were investigated by impedance spectroscopy. The capacitance of the tBLM indicated the formation of bilayer membranes of comparable thickness to solvent-free black (or bilayer) lipid membranes (BLM). The ionic sealing ability was comparable to those of classical BLMs. The function of the ionic reservoir was investigated using the potassium-specific ionophore valinomycin. Increasing the size of the reservoir by increasing the length of the hydrophilic region of the reservoir lipid or laterally spacing the reservoir lipid results in a...

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.

A large area nanogap electrode fabrication method combinig conventional lithography patterning with the of focused ion beam (FIB) is presented. Lithography and a lift-off process were used to pattern 50 nm thick platinum pads having an area of 300μm × 300μm. A range of 30-300 nm wide nanogaps (length from 300μm to 10 mm ) were then etched using an FIB of Ga+at an acceleration voltage of 30 kV at various beam currents. An investigation of Ga+beam current ranging between 1-50 pA was undertaken to optimise the process for the current fabrication method. In this study, we used Monte Carlo simulation to calculate the damage depth in various materials by the Ga+. Calculation of the recoil cascades of the substrate atoms are also presented. The nanogap electrodes fabricated in this study were found to have empty gap resistances exceeding several hundred MΩ. A comparison of the gap length versus electrical resistance on glass substrates is presented. The results thus outline some important issues in low-conductance measurements. The proposed nanogap fabrication method can be extended to various sensor applications, such as chemical sensing, that employ the nanogap platform. This method may be used as a prototype technique for large-scale fabrication due to its simple, fast and reliable features.

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.

The reaction of CH3TiCl3 with 2-benzyloxy propanal 10 at low temperatures in CD2Cl2 leads to the formation of two diastereomeric Cram-type chelates 13a/13b which can be observed directly by 13C-NMR spectroscopy. The octahedral chelates undergo CC bond formation with first order kinetics, suggesting intramolecular 1,3 methyl transfer. Crossover experiments are unsuitable for definitive proof of intramolecularity due to ligand exchange reactions prior to Grignard-type addition.

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.

The diffuse reflectance of films formed by the deposition of gold nanoparticles on a porous polymer substrate is investigated. Measured reflectances are compared with those calculated using a two-flux model of radiative transfer, modified to take into account the optically-rough surface. The required scattering and absorption coefficients of the nanoparticles are calculated using a standard Mie scattering method for optically-isolated particles, and the generalized-multiparticle-Mie method for clusters of particles. The calculated reflectances show good agreement with measurements for the films containing optically-isolated particles. Progress is described for the films containing clusters of particles.

The electrical resistance of nanoparticle assemblies in the form of thin films shows semiconductor-like behavior at low temperatures and metal-like behaviour at high temperatures. We show that this resistance behavior is due to the interplay between Coulomb blockade reduced tunneling of electrons between nanoparticles and the thermal expansion of the nanoparticle assembly. We develop a detailed theoretical model that describes the electron transport process and show that the model agrees well with experimental data.

The concept of chelation controlled addition to chiral alkoxy aldehydes using Lewis acids and mild C-nucleophiles has been extended to include cyclopropylation. The mechanism of the TiCl4-mediated chelation controlled addition of enolsilanes and allylsilanes to chiral α-alkoxy aldehydes has been studied by rapid injection NMR techniques. Accordingly, an acyclic transition state is involved in which the silyl group does not migrate to the carbonyl oxygen atom. In contrast, LiClO4 is an effective Lewis acid (excess or catalytic amounts) which induces chelation controlled group transfer type of aldol additions.

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 (

The ability to express and purify modified recombinant signalling proteins such that they retain their biological function in a cell-free context has provided a basis for production of molecular biosensors. Here the authors utilise G-protein coupled receptors (GPCRs) and their G-proteins to detect various binding partners in a cell-free environment. Molecular biology approaches were employed to express these proteins using baculovirus and bacteria, and to alter their characteristics to improve surface-attachment and fluorescent labelling capabilities. Ligand-mediated signalling of a GPCR could be measured (using [35S]GTPgammaS-binding assays) in a reconstituted system with recombinant proteins either free in solution or attached to Ni2+-coated beads. Affinity of histidine-tagged proteins for a Ni2+-coated surface was significantly enhanced by addition of extra histidine residues to the tag, as determined by surface plasmon resonance. This was due to the longer tag occupying, on average, a greater number of available histidine-binding sites. Further, a novel homogeneous fluorescence resonance energy transfer (FRET)-based assay has been developed to detect rearrangements in the G-protein heterotrimer. Investigation of small peptides that can be fused to G-protein subunits, allowing for site-specific fluorescent labelling, was undertaken in order to improve the resolution of the "first generation" FRET assay. By utilizing this improved G-protein heterotrimer "molecular switch", we are developing a generic technology such that a range of GPCRs could be assayed for ligand-mediated activation while attached to surfaces (e.g. on beads or as microarrays) or in solution (e.g. multi-well plates), with increased throughput.

Functionalised gold nanoparticles, capable of specifically binding histidine-tagged proteins, are of interest as their unique optical properties can facilitate the study of protein-protein interactions. Citrate-stabilized gold nanoparticles were functionalised with surface capping agents to specifically bind histidine-tagged proteins, and to test the stability of the nanoparticle solutions under various conditions.

Films consisting of self-assembled gold nanoparticles cross-linked with alkane-dithiols were prepared by a filtration method and studied with scanning electron microscopy to determine the structure of the films and spectrophotometry and ellipsometry to ascertain their optical properties. The structural characterization showed the existence of nanometer sized voids within the films. This previously unmentioned feature is responsible for the previous difficulties in modelling the optical properties with effective medium models.This can be remedied, using a two-tiered hierarchical effective medium model, which takes into account the existence of the voids. Using this model we were able to fit the experimental data,with only the void volume fraction to be determined by the overall fit, while the gold volume fraction in the linker medium is fixed by the wavelength of the resonance peak. Our model should be applicable to all such films, when the deposition method, which determines the microstructure, is properly taken into account.

We study experimentally and theoretically the electrical conductivity of films made of gold nanoparticles linked by alkanedithiol molecules. The dependence of the conductivity on the length of the alkanedithiol molecule and on the thickness of the nanoparticle films at room temperature is investigated. We describe theoretically conductance between adjacent metal nanoparticles in terms of single electron tunneling along the linker molecules. Due to variations in the separation gaps between neighboring nanoparticles a film can be approximated by a network of widely varying tunnel conductances and the film conductivity can be described in terms of percolation theory. We demonstrate that the expected exponential decrease of the conductivity with increasing length of linker molecules is weakened by the presence of high conductance percolation pathways and we show that due to three-dimensional current percolation the conductivity of the nanoparticle films becomes film thickness dependent.

We have investigated theoretically and experimentally the temperature dependence of the conductance of films of Au nanoparticles linked by alkane dithiol molecules in the temperature range between 5 and 300 K. Conduction in these films is due to tunneling of single electrons between neighboring metal nanoparticles via the linker molecules. During tunneling an electron has to overcome the Coulomb charging energy. We find that the observed temperature dependence of the conductance is non-Arrhenius-like and can be described in terms of a percolation theory which takes account of disorder in the system. Disorder in our nanoparticle films is caused by variations in the nanoparticle size, fluctuations in the separation gaps between adjacent nanoparticles and by offset charges. To explain in detail our experimental data, a wide distribution of separation gaps and charging energies has to be assumed. We find that a wide Coulomb charging energy distribution can arise from random offset charges even if the nanoparticle size distribution is narrow.