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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.

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 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 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.

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.

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.

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)

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.

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.

Biosensors combine a biological recognition mechanisms with a physical transduction technique. In nature, the transduction mechanism for high sensitivity molecular detection is modulation of cell membrane ionic conductivity, through specific ligand - receptor binding induced switching of ion channels. This effects an inherent signal amplification of 6-8 orders of magnitude, corresponding to the total ion flow arising from the single channel gating event. Here we describe the first reduction of this principle to a practical sensing device, which is a planar impedance element composed of a macroscopically supported synthetic bilayer membrane incorporating ion channels. The membrane and ionic reservoir are covalently attached to an evaporated gold surface. The channels have specific receptor groups attached which permit switching of the channels by analyte binding to the receptors. The device may then be made specific for the detection of a very wide range of analytes, including proteins, drugs, hormones, antibodies, DNA, etc., currently in the 10-7-10-12 M range. It also lends itself readily to microelectronic fabrication, the optimum sensitivity range of the device may be tuned over several orders of magnitude.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

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

Tactile feedback in robotic and prosthetic applications requires high resolution sensing with short response times, high sensitivity and conformability. Nanoparticle films offer the opportunity to realise high resolution sensors on flexible substrates through ink-jet printing and the ability to tailor devices 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 response time to a ∼100 mN load was less than 20 ms. The sensor had a sensitivity factor of 0.0039 mN -1 and a detection limit of 10 mN. The model fit to the experimental data has a correlation coefficient R 2 of 0.987.