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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 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 (<1.5 kg), low birth weight (1.5 to 2.5 kg) or healthy (>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.

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.

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. (Graph Presented).

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 Royal Society of Chemistry 2013.

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 withchemicals/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.