Silk provides a new avenue for third generation biosensors: Sensitive, selective and stable electrochemical detection of nitric oxide.

Using heme entrapped in recombinant silk films, we have produced 3rd generation biosensors, which allow direct electron transfer from the heme center to an electrode avoiding the need for electron mediators. Here, we demonstrate the use of these heme-silk films for the detection of nitric oxide (NO) at nanomolar levels in the presence and absence of oxygen. The sensor was prepared by drop-casting a silk solution on a glassy carbon electrode modified with multiwalled carbon nanotubes (MWCNT) followed by infusion with heme. The sensor was characterized by cyclic voltammetry and showed well defined and reversible Fe+/ Fe3+ redox couple activity, with NO detection by oxidation at potentials above +0.45V or reduction at potentials below - 0.7V. Evaluation of the effect of pH on the sensor response to NO reduction indicated a maximum response at pH 3. The sensor showed good linearity in the concentration range from 19 to 190nM (R2 = 0.99) with a detection limit of 2nM. The sensor had excellent selectivity towards NO with no or negligible interference from oxygen, nitrite, nitrate, dopamine and ascorbic acid and retained 86% of response after 2 months of operation and storage at room temperature.

Detection of vapor-phase organophosphate threats using wearable conformable integrated epidermal and textile wireless biosensor systems.

Flexible epidermal tattoo and textile-based electrochemical biosensors have been developed for vapor-phase detection of organophosphorus (OP) nerve agents. These new wearable sensors, based on stretchable organophosphorus hydrolase (OPH) enzyme electrodes, are coupled with a fully integrated conformal flexible electronic interface that offers rapid and selective square-wave voltammetric detection of OP vapor threats and wireless data transmission to a mobile device. The epidermal tattoo and textile sensors display a good reproducibility (with RSD of 2.5% and 4.2%, respectively), along with good discrimination against potential interferences and linearity over the 90-300mg/L range, with a sensitivity of 10.7µA∙cm3∙mg-1 (R2 = 0.983) and detection limit of 12mg/L in terms of OP air density. Stress-enduring inks, used for printing the electrode transducers, ensure resilience against mechanical deformations associated with textile and skin-based on-body sensing operations. Theoretical simulations are used to estimate the OP air density over the sensor surface. These fully integrated wearable wireless tattoo and textile-based nerve-agent vapor biosensor systems offer considerable promise for rapid warning regarding personal exposure to OP nerve-agent vapors in variety of decentralized security applications.

Wearable Flexible and Stretchable Glove Biosensor for On-Site Detection of Organophosphorus Chemical Threats.

A flexible glove-based electrochemical biosensor with highly stretchable printed electrode system has been developed as a wearable point-of-use screening tool for defense and food security applications. This disposable-mechanically robust "lab-on-a-glove" integrates a stretchable printable enzyme-based biosensing system and active surface for swipe sampling on different fingers, and is coupled with a compact electronic interface for electrochemical detection and real-time wireless data transmission to a smartphone device. Stress-enduring inks are used to print the electrode system and the long serpentine connections to the wireless electronic interface. Dynamic mechanical deformation, bending, and stretching studies illustrate the resilience and compliance of the printed traces against extreme mechanical deformations expected for such on-glove sampling/sensing operation. An organophosphorus hydrolase (OPH)-based biosensor system on the index finger enables rapid on-site detection of organophosphate (OP) nerve-agent compounds on suspicious surfaces and agricultural products following their swipe collection on the thumb finger. The new wireless glove-based biosensor system offers considerable promise for field screening of OP nerve-agents and pesticides in defense and food-safety applications, with significant speed and cost advantages. Such "lab-on-a-glove" demonstration opens the area of flexible wearable sensors to future on-the-hand multiplexed chemical detection in diverse fields.

Carbon nanotube-ionic liquid composite sensors and biosensors.

A new composite electrode has been fabricated using multiwall carbon nanotubes (MWCNT) and the ionic liquid n-octylpyridinum hexafluorophosphate (OPFP). This electrode shows very attractive electrochemical performances compared to other conventional electrodes using graphite and mineral oil, notably improved sensitivity and stability. One major advantage of this electrode compared to other electrodes using carbon nanotubes and other ionic liquids is its extremely low capacitance and background currents. A 10% (w/w) loading of MWCNT was selected as the optimal composition based on voltammetric results, as well as the stability of the background response in solution. The new composite electrode showed good activity toward hydrogen peroxide and NADH, with the possibility of fabricating a sensitive biosensor for glucose and alcohol using glucose oxidase and alcohol dehydrogenase, respectively, by simply incorporating the specific enzyme within the composite matrix. The marked electrode stability and antifouling features toward NADH oxidation was much higher for this composite compared to a bare glassy carbon electrode. While a loading of 2% MWCNT showed very poor electrochemical behavior, a large enhancement was observed upon gentle heating to 70 degrees C, which gave a response similar to the optimum composition of 10%. The ease of preparation, low background current, high sensitivity, stability, and small loading of nanotubes using this composite can create new novel avenues and applications for fabricating robust sensors and biosensors for many important species.

Ionic liquid-carbon composite glucose biosensor.

The use of ionic liquids that are solid at room temperature such as n-octyl-pyridinium hexafluorophosphate (nOPPF(6)) is shown to be advantageous in the fabrication of new form of biocomposite materials with attractive performance over other types of composites and pastes involving non-conductive binders. The resulting IL/graphite material brings new capabilities for electrochemical devices by combining the advantages of ILs and "bulk" composite electrodes. The electrocatalytic properties of the ILs are not impaired by their association with the graphite powder. The marked electrocatalytic activity towards hydrogen peroxide permits effective amperometric biosensing of glucose in connection with the incorporation of glucose oxidase within the three-dimensional IL/graphite matrix. The accelerated electron transfer is coupled with low background current and improved linearity. The advantages of these IL-based biocomposite devices are illustrated from comparison to conventional mineral oil/graphite biocomposite. The influence of the IL and glucose oxidase (GOx) loading upon the amperometric and voltammetric data, as well as the electrode capacitance and resistance, is examined. The preparation of IL/graphite composites overcomes a major obstacle for creating IL-based biosensing devices and expands the scope of IL-based electrochemical devices.

Enhanced stability and sensitivity of ionic liquid-carbon paste electrodes at elevated temperatures.

We describe the operation of ionic liquid-carbon paste electrodes at elevated temperatures and the effect of heating on the electrode performance and response. Using cyclic and square wave voltammetry and amperometry, it is shown that signals can be enhanced and stabilized by increasing the temperature of the operating solution. At low temperature, the electrode was susceptible to electrode fouling and showed poor stability, sensitivity, and linearity. An order of magnitude improvement of response for ascorbic acid was possible by operating the electrode at 60 degrees C compared to 0 degrees C. This study represents the first report showing that the analytical response of ionic liquid-carbon paste electrodes can be improved by operating them at elevated temperatures for a number of applications.

Oxygen-independent poly(dimethylsiloxane)-based carbon-paste glucose biosensors.

Several silicone oils have been assessed and compared as an internal source of oxygen in connection to their use as binders for carbon-paste glucose biosensors. All four poly(dimethylsiloxane) (PDMS) oils tested a dramatic increase in the oxygen capacity of carbon-paste enzyme electrodes to allow convenient biosensing under severe oxygen-deficit conditions. The resulting oxygen independence is better than that exerted by perfluorocarbon binders or that displayed by mediator-based bioelectrodes. The resistance to oxygen effects is indicated from the identical response (observed in the presence and absence of oxygen) up to 2 x 10(-2) M glucose and the slight (12%) sensitivity loss at 4 x 10(-2) M. The influence of the viscosity of the PDMS binder upon the internal oxygen supply is examined. The PDMS carbon-paste enzyme electrode displays a stable glucose response over prolonged (15 h) operation in an oxygen-free solution. On-line continuous testing indicates favorable dynamic properties with no carry-over effects over the physiological and pathophysiological range (3-12 mM glucose).