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The synthesis and employment of volatile toxic compounds as chemical weapons with a large-scale destructive power has introduced a new insidious threat over the last century. In this framework, the development of wearable sensing tools represents a critical point within the security field, in order to provide early alarm systems. Herein, a novel wearable electrochemical biosensor was developed for the rapid and on-site detection of mustard agents. Since a chemical attack is typically carried out by spraying these volatile agents into air, the sensor was designed in order to be able to measure mustard agents directly in the aerosol phase, further than in the liquid phase. The electrodes were screen-printed onto a filter paper support, which allowed to harness the porosity of paper to pre-load all the needed reagents into the cellulose network, and hence to realise an origami-like and reagent-free device. Mustard agent detection was carried out by monitoring their inhibitory effects toward the choline oxidase enzyme, through the amperometric measurement of the enzymatic by-product hydrogen peroxide. A carbon black/Prussian blue nanocomposite was used as a bulk-modifier of the conductive graphite ink constituting the working electrode, allowing for the electrocatalysis of the hydrogen peroxide reduction. After having verified the detecting capability toward a mustard agent simulant, the applicability of the resulting origami-like biosensor was demonstrated for the rapid and real-time detection of real sulfur mustard, obtaining limits of detection equal to 1 mM and 0.019 g·min/m3 for liquid and aerosol phase, respectively.

A novel recognition element of molecularly imprinted films (MIFs) was synthesized by wet phase inversion (WPI) on the surface of Ti/TiO2 electrode for highly selective and sensitive electrochemical detection of bisphenol A (BPA). The Ti/TiO2/MIFs sensor was constructed by casting the precursor poly(acrylonitrile-co-acrylic acid) (p(AN-co-AA)) in dimethyl sulfoxide containing template molecule BPA onto the electrode and then immersing into water, resulting in simultaneous p(AN-co-AA) precipitation and BPA imprinting via the facile WPI. The imprinted sites could selectively rebind BPA through hydrogen bonding and hence lead to the equalizing current increase in amperometric detection, by which the BPA could be sensed electrochemically. Accordingly, the Ti/TiO2/MIFs sensor offered a favorable linearity within the wide range over five orders of magnitude (4.4nM-0.13mM), and a low detection limit down to 1.3nM. Excellent recognition selectivity for BPA was also attained over its analogues. Furthermore, this sensor was successfully applied to detect BPA in seawater and paper cup samples, and high recoveries were 86-110% with low relative standard deviations of 1.3-3.2%. By using BPA as a model, the MIFs-based method may provide a facile, rapid, and cost-effective way for ultrasensitive electrochemical measurements of various targeted compounds with good applicability to WPI.

Exhaled breath contains a large amount of biochemical and physiological information concerning one’s health and provides an alternative route to noninvasive medical diagnosis of diseases. In the case of lung diseases, hydrogen peroxide (H2O2) is an important biomarker associated with asthma, chronic obstructive pulmonary disease, and lung cancer and can be detected in exhaled breath. The current method of breath analysis involves condensation of exhaled breath, is not continuous or real time, and requires two separate and bulky devices, complicating the periodic or long-term monitoring of a patient. We report the first disposable paper-based electrochemical wearable sensor that can monitor exhaled H2O2 in artificial breath calibration-free and continuously, in real time, and can be integrated into a commercial respiratory mask for on-site testing of exhaled breath. To improve precision for sensing H2O2, we perform differential electrochemical measurement by amperometry in which screen-printed Prussian Blue-mediated and nonmediated carbon electrodes are used for differential analysis. We were able to measure H2O2 in simulated breath in a concentration-dependent manner in real time, confirming its functionality. This proposed system is versatile, and by modifying the chemistry of the sensing electrodes, our method of differential sensing can be extended to continuous monitoring of other analytes in exhaled breath.

A paper-based analytical device (PAD) with an integrated composite electrode for non-enzymatic creatinine sensing was developed. The electrode was produced and optimization was efficiently accomplished using a rapid digital dispensing approach. The electrochemical sensor was fabricated using an HP D300 digital dispenser to deliver a copper oxide and ionic liquid composite onto an electrochemically reduced graphene modified screen-printed carbon electrode (CuO/IL/ERGO/SPCE) on a PAD. The modified electrode was characterized using electrochemical and microscopic techniques. Electrochemical detection of creatinine was performed on the SPCE using amperometry at a constant potential. Under optimized conditions, the paper-based sensor exhibited a linear range of 0.01–2.0 mM (R2 = 0.99) and the limit of detection was 0.22 μM (S/N = 3, IUPAC definition) for creatinine. The simple fabrication process, low cost, and clinically appropriate creatinine sensitivity make this device applicable for point-of-care use.

We have explored AuNPs (13 nm) both as a catalyst and as a core for synthesizing water-dispersible and highly stable core-shell structural gold@Prussian blue (Au@PB) nanoparticles (NPs). Systematic characterization by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) disclosed AuNPs coated uniformly by a 5 nm thick PB layer. Au@PB NPs were attached to single-layer graphene oxide (GO) to form Au@PB decorated GO sheets. The resulting hybrid material was filtered layer-by-layer into flexible and free-standing GO paper, which was further converted into conductive reduced GO (RGO)/Au@PB paper via hydrazine vapour reduction. High-resolution TEM images suggested that RGO papers are multiply sandwich-like structures functionalized with core-shell NPs. Resulting sandwich functionalized graphene papers have high conductivity, sufficient flexibility, and robust mechanical strength, which can be cut into free-standing electrodes. Such electrodes, used as non-enzymatic electrochemical sensors, were tested systematically for electrocatalytic sensing of hydrogen peroxide. The high performance was indicated by some of the key parameters, for example the linear H2O2 concentration response range (1-30 μM), the detection limit (100 nM), and the high amperometric sensitivity (5 A cm(-2) M(-1)). With the advantages of low cost and scalable production capacity, such graphene supported functional papers are of particular interest in the use as flexible disposable sensors.

Introduction: The PKU is an inborn error of amino acid metabolism, in which phenylalanine (Phe) accumulated in the blood causing alterations at the central nervous system. We report a novel paper-based enzymatic platform coupled to screen printed graphene-modified electrode for the neonatal screening of phenylketonuria (PKU0. Methods: The paper-based analytical device coupled to electrochemical detection (EPAD) is based on the use of paper microzones modified with phenylalanine dehydrogenase enzyme (PheDH). The modified PADs were placed on the surface of an electrode modified with electrochemically reduced graphene (ERGO). PheDH in the presence of NAD + catalyzes the reversible deamination of Phe to form phenylpyruvate, ammonia, and NADH. The electrochemical oxidation of NADH was monitored by differential pulse amperometry (DPA) at 0.6 V. The method was linear in the concentration range from 1 to 600 μmol/L of Phe with a LOQ of 1 μmol/L and LOD of 0.2 μmol/L. Within day precision was 5.7% across 3 levels of control samples. Between-day precision was 8.3%. The comparison with the standard Phe enzyme assay kit showed good agreement. The time required for the overall assay was

Paper-based electrochemical biosensors are a promising technology that enables rapid, quantitative measurements on an inexpensive platform. However, the control of liquids in paper networks is generally limited to a single sample delivery step. Here, we propose a simple method to automate the loading and delivery of liquid samples to sensing electrodes on paper networks by stacking multiple layers of paper. Using these stacked paper devices (SPDs), we demonstrate a unique strategy to fully immerse planar electrodes by aqueous liquids via capillary flow. Amperometric measurements of xanthine oxidase revealed that electrochemical sensors on four-layer SPDs generated detection signals up to 75% higher compared with those on single-layer paper devices. Furthermore, measurements could be performed with minimal user involvement and completed within 30 min. Due to its simplicity, enhanced automation, and capability for quantitative measurements, stacked paper electrochemical biosensors can be useful tools for point-of-care testing in resource-limited settings.

Recent progress in flexible and lightweight electrochemical sensor systems requires the development of paper-like electrode materials. Here, we report a facile and green synthesis of a new type of MnO 2 nanowires–graphene nanohybrid paper by one-step electrochemical method. This strategy demonstrates a collection of unique features including the effective electrochemical reduction of graphene oxide (GO) paper and the high loading of MnO 2 nanowires on electrochemical reduced GO (ERGO) paper. When used as flexible electrode for nonenzymatic detection of hydrogen peroxide (H 2 O 2 ), MnO 2 –ERGO paper exhibits high electrocatalytic activity toward the redox of H 2 O 2 as well as excellent stability, selectivity and reproducibility. The amperometric responses are linearly proportional to H 2 O 2 concentration in the range 0.1–45.4 mM, with a detection limit of 10 μM (S/N = 3) and detection sensitivity of 59.0 μA cm −2 mM −1 . These outstanding sensing performances enable the practical application of MnO 2 –ERGO paper electrode for the real-time tracking H 2 O 2 secretion by live cells macrophages. Therefore, the proposed graphene-based nanohybrid paper electrode with intrinsic flexibility, tailorable shapes and adjustable properties can contribute to the full realization of high-performance flexible electrode material used in point-of-care testing devices and portable instruments for in-vivo clinical diagnostics and on-site environmental monitoring.

In this work we have developed an amperometric enzymatic biosensor in a paper-based platform with a mixed electrode configuration: carbon ink for the working electrode (WE) and metal wires (from a low-cost standard electronic connection) for reference (RE) and auxiliary electrodes (AE). A hydrophobic wax-defined paper area was impregnated with diluted carbon ink. Three gold-plated pins of the standard connection are employed, one for connecting the WE and the other two acting as RE and AE. The standard connection works as a clip in order to support the paper in between. As a proof-of-concept, glucose sensing was evaluated. The enzyme cocktail (glucose oxidase, horseradish peroxidase and potassium ferrocyanide as mediator of the electron transfer) was adsorbed on the surface. After drying, glucose solution was added to the paper, on the opposite side of the carbon ink. It wets RE and AE, and flows by capillarity through the paper contacting the carbon WE surface. The reduction current of ferricyanide, product of the enzymatic reaction, is measured chronoamperometrically and correlates to the concentration of glucose. Different parameters related to the bioassay were optimized, adhering the piece of paper onto a conventional screen-printed carbon electrode (SPCE). In this way, the RE and the AE of the commercial card were employed for optimizing the paper-WE. After evaluating the assay system in the hybrid paper-SPCE cell, the three-electrode system consisting of paper-WE, wire-RE and wire-AE, was employed for glucose determination, achieving a linear range between 0.3 and 15mM with good analytical features and being able of quantifying glucose in real food samples.

A novel nanocomposite of graphene (G), polyvinylpyrrolidone (PVP) and polyaniline (PANI) has been successfully prepared and used for the modification of paper-based biosensors via electrospraying. The droplet-like nanostructures of G/PVP/PANI-modified electrodes are obtained with an average size of 160±1.02 nm. Interestingly, the presence of small amount of PVP (2 mg mL −1 ) in the nanocomposites can substantially improve the dispersibility of G and increase the electrochemical conductivity of electrodes, leading to enhanced sensitivity of the biosensor. The well-defined cyclic voltammogram of standard ferri/ferrocyanide is achieved on a G/PVP/PANI-modified electrode with a 3-fold increase in the current signal compared to an unmodified electrode. This modified electrode also exhibits excellent electrocatalytic activity towards the oxidation of hydrogen peroxide (H 2 O 2 ). Furthermore, cholesterol oxidase (ChOx) is attached to G/PVP/PANI-modified electrode for the amperometric determination of cholesterol. Under optimum conditions, a linear range of 50 μM to 10 mM is achieved and the limit of detection is found to be 1 μM for cholesterol. Finally, the proposed system can be applied for the determination of cholesterol in a complex biological fluid ( i.e. human serum).

The growing demand for compact point-of-care medical devices and portable instruments for on-site environmental sampling has stimulated intense research on flexible sensors that can be miniaturized and function under considerable physical deformation. We report a new type of flexible electrochemical biosensors based on free-standing graphene paper carrying binary nanocomposites of PtAu alloy and MnO 2 . The coral-like PtAu–MnO 2 nanocomposites are grown on the substrate through one-step template-free electrodeposition, leading to an intimate contact between the PtAu alloy and MnO 2 matrix. The flexible electrode exhibits a unique set of structural and electrochemical properties such as better uniformity, larger active surface areas, and faster electron transfer in comparison with the control electrode prepared by tandem growth of MnO 2 network and PtAu alloy in two steps. In nonenzymatic amperometric glucose detection, the PtAu–MnO 2 binary nanostructure-decorated graphene paper has shown greatly enhanced sensing performance such as wide liner range (0.1 mM to 30.0 mM), high sensitivity (58.54 μA cm −2 mM −1 ), low detection limit (0.02 mM, S / N =3), satisfactory selectivity, excellent reproducibility and stability, and tolerability to mechanical stress. The strategy of co-growth of metal and metal oxides on freestanding carbon substrates opens new possibility to develop high-performance flexible electrochemical sensors.

Segmented flow analysis (SFA) and ion chromatography with pulsed amperometric detection (IC-PAD) are widely used analytical techniques for the analysis of glucose, fructose, and sucrose in tobacco. In the work presented here, 27 cured tobacco leaves and 21 tobacco products were analyzed for sugars using SFA and IC. The results of these analyses demonstrated that both techniques identified the same trends in sugar content across tobacco leaf and tobacco product types. However, comparison of results between techniques was limited by the selectivity of the SFA method, which relies on the specificity of the reaction of p-hydroxybenzoic acid hydrazide (PAHBAH) with glucose and fructose to generate a detectable derivative. Sugar amines and chlorogenic acid, which are found in tobacco, are also known to react with PAHBAH to form a reaction product that interferes with the analysis of fructose and glucose. To mitigate this problem, solid phase extraction (SPE) was used to remove interferences such as sugar amines and chlorogenic acid from sample matrices prior to SFA. A combination of C18 and cation exchange solid phase extraction cartridges was used, and the results from SFA and IC analyses showed significant convergence in the results of both analytical methods. For example, the average difference between the results from the SFA and IC analyses for flue-cured tobacco samples dropped by 73% when the two-step C18/cation exchange resin sample cleanup was used.

A sensitive and fast-responding membrane-free amperometric gas sensor is described, consisting of a small filter paper foil soaked with a room temperature ionic liquid (RTIL), upon which three electrodes are screen printed with carbon ink, using a suitable mask. It takes advantage of the high electrical conductivity and negligible vapour pressure of RTILs as well as their easy immobilization into a porous and inexpensive supporting material such as paper. Moreover, thanks to a careful control of the preparation procedure, a very close contact between the RTIL and electrode material can be achieved so as to allow gaseous analytes to undergo charge transfer just as soon as they reach the three-phase sites where the electrode material, paper supported RTIL and gas phase meet. Thus, the adverse effect on recorded currents of slow steps such as analyte diffusion and dissolution in a solvent is avoided. To evaluate the performance of this device, it was used as a wall-jet amperometric detector for flow injection analysis of 1-butanethiol vapours, adopted as the model gaseous analyte, present in headspace samples in equilibrium with aqueous solutions at controlled concentrations. With this purpose, the RTIL soaked paper electrochemical detector (RTIL-PED) was assembled by using 1-butyl-3- methylimidazolium bis(trifluoromethanesulfonyl)imide as the wicking RTIL and printing the working electrode with carbon ink doped with cobalt(ii) phthalocyanine, to profit from its ability to electrocatalyze thiol oxidation. The results obtained were quite satisfactory (detection limit: 0.5 ?M; dynamic range: 2-200 ?M, both referring to solution concentrations; correlation coefficient: 0.998; repeatability: ±7% RSD; long-term stability: 9%), thus suggesting the possible use of this device for manifold applications. © 2012 The Royal Society of Chemistry.

A novel paper-based analytical device (PAD) coupled with a silver nanoparticle-modified boron-doped diamond (AgNP/BDD) electrode was first developed as a cholesterol sensor. The AgNP/BDD electrode was used as working electrode after modification by AgNPs using an electrodeposition method. Wax printing was used to define the hydrophilic and hydrophobic areas on filter paper, and then counter and reference electrodes were fabricated on the hydrophilic area by screen-printing in house. For the amperometric detection, cholesterol and cholesterol oxidase (ChOx) were directly drop-cast onto the hydrophilic area, and H2O2 produced from the enzymatic reaction was monitored. The fabricated device demonstrated a good linearity (0.39 mg dL(-1) to 270.69 mg dL(-1)), low detection limit (0.25 mg dL(-1)), and high sensitivity (49.61 μA mM(-1) cm(-2)). The precision value for ten replicates was 3.76% RSD for 1 mM H2O2. In addition, this biosensor exhibited very high selectivity for cholesterol detection and excellent recoveries for bovine serum analysis (in the range of 99.6-100.8%). The results showed that this new sensing platform will be an alternative tool for cholesterol detection in routine diagnosis and offers the advantages of low sample/reagent consumption, low cost, portability, and short analysis time.

Microfluidic biosensing systems with enzyme-based detection have been extensively studied in the last years owing to features such as high specificity, a broad range of analytes and a high degree of automation. This review gives an overview of the most important factors associated with these systems. In the first part, frequently used immobilization protocols such as physisorption and covalent bonding and detection techniques such as amperometry and fluorescence measurements are discussed with respect to effort, lifetime and measurement range. The Michaelis–Menten model describing the kinetics of enzymatic reactions, the role of redox mediators and the limitations of the linear measurement range of enzymatic sensors are introduced. Several possibilities of extending the linear measurement range in microfluidic systems such as diffusion-limiting membranes and the flow injection setup are presented. Regarding the integration of enzymes into microfluidic systems during the fabrication process, the constraints imposed by the biomolecules due to the limited usage of high temperatures and solvents are addressed. In the second part, the most common forms of enzyme integration into microfluidic systems, i.e. in channels and on electrodes, on microparticles, on paper and thread and as injected enzyme solutions, are reviewed, focusing on fabrication, applications and performance.

Recent advances in electrochemical analysis on filter paper exemplify the versatility of this substrate for high performance testing. Its low-cost, light-weight, and environmentally friendly properties make it particularly attractive for applications in addressing health and environmental safety needs in low-resource settings and developing countries. However, the main drawback to sensitive electrochemical testing is the use of a potentiostat, a bench-top instrument that is extremely expensive, thereby negating the some of the benefits of paper-based devices. Hence there is a need to develop paper-devices for use with handheld, portable device readers that can extract quantitative readouts. In this study, we developed a method to use micro-paper electrochemical devices, or µPEDs, with a glucose meter, which are used for personal monitoring of blood glucose levels. Ethanol was chosen as a model target analyte due to its importance in the global issue of road safety. µPEDs were simple in design and could be tested with a potentiostat. We observed that inclusion of the stabilizer trehalose was critical to preparing µPEDs for later analysis. In addition, an NAD+-dependent enzyme was used to impart selectivity to the biosensor, which also represents a class of enzymes with targets relevant to the health and food industry.