Nanostructured nickel oxide electrodes for non-enzymatic electrochemical glucose sensing.

Nanostructured nickel (Ni) and nickel oxide (NiO) electrodes were fabricated on Ni foils using the glancing angle deposition (GLAD) technique. Cyclic voltammetry and amperometry showed the electrodes enable non-enzymatic electrochemical determination of glucose in strongly alkaline media. Under optimized conditions of NaOH concentration and working potential (~ 0.50 V vs. Ag/AgCl), the GLAD electrodes performed far better than bare Ni foil electrodes, with the GLAD NiO electrode showing an outstanding sensitivity (4400 µA mM-1 cm-2), superior detection limit (7 nM), and wide dynamic range (0.5 µM-9 mM), with desirable selectivity and reproducibility. Based on their performance at a low concentration, the GLAD NiO electrodes were also used to quantify glucose in artificial urine and sweat samples which have significantly lower glucose levels than blood. The GLAD NiO electrodes showed negligible response to the common interferents in glucose measurement (uric acid, dopamine, serotonin, and ascorbic acid), and they were not poisoned by high amounts of sodium chloride. Graphical abstract The figures depict (A) SEM image of vertical post-GLAD NiO electrodes used for non-enzymatic electrochemical glucose monitoring, and (B) calibration plots of the three different electrodes.

An impedimetric biosensor for E. coli O157:H7 based on the use of self-assembled gold nanoparticles and protein G.

Two kinds of electrochemical impedimetric biosensors for the detection of E. coli O157:H7 are described and compared. They were fabricated using self-assembled layers of thiolated protein G (PrG-thiol) on (i) planar gold electrodes and (ii) gold nanoparticles (Au NPs) modified gold electrodes. The fabrications of the biosensors were characterized using cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy and atomic force microscopy techniques. The modification of the planar gold electrode by Au NPs via self-assembled monolayer of 1,6-hexadithiol as a linker molecule increased the electrochemically active surface area by about 2.2 times. The concentration of PrG-thiol and its incubation time, as well as the concentration of IgG were optimized. The Au NP-based biosensor exhibited a limit of detection of 48 colony forming unit (cfu mL-1) which is 3 times lower than that of the planar gold electrode biosensor (140 cfu mL-1). It also showed a wider dynamic range (up to 107 cfu mL-1) and sensitivity. The improved analytical performance of the Au NP-modified biosensor is ascribed to the synergistic effect between the Au NPs and the PrG-thiol scaffold. The biosensor exhibited high selectivity for E. coli O157:H7 over other bacteria such as Staphylococcus aureus and Salmonella typhimurium. Graphical abstract Schematic representations of sensor fabrication using Au NP-modified electrode (HKEC = heat- killed E. coli O157:H7).

Electrochemical Determination of Fentanyl Using Carbon Nanofiber-Modified Electrodes.

In this work, we report the direct electrochemical oxidation of fentanyl using commercial screen-printed carbon electrodes (SPCEs) modified with carboxyl-functionalized carbon nanofibers (fCNFs). CNFs have surface chemistry and reactivity similar to carbon nanotubes (CNTs), yet they are easier to produce and are of a lower cost than CNTs. By monitoring the current produced during the electrochemical oxidation of fentanyl, variables such as fCNF loading, fentanyl accumulation time, electrolyte pH, and differential pulse voltammetry parameters were optimized. Under an optimized set of conditions, the fCNF/SPCEs responded linearly to fentanyl in the concentration range of 0.125-10 µM, with a limit of detection of 75 nM. The fCNF/SPCEs also demonstrated excellent selectivity against common cutting agents found in illicit drugs (e.g., glucose, sucrose, caffeine, acetaminophen, and theophylline) and interferents found in biological samples (e.g., ascorbic acid, NaCl, urea, creatinine, and uric acid). The performance of the sensor was also successfully tested using fentanyl spiked into an artificial urine sample. The straightforward electrode assembly process, low cost, ease of use, and rapid response make the fCNF/SPCEs prime candidates for the detection of fentanyl in both physiological samples and street drugs.

Spatially resolved Raman spectroelectrochemistry of solid-state polythiophene/viologen memory devices.

A three terminal molecular memory device was monitored with in situ Raman spectroscopy during bias-induced switching between two metastable states having different conductivity. The device structure is similar to that of a polythiophene field effect transistor, but ethylviologen perchlorate was added to provide a redox counter-reaction to accompany polythiophene redox reactions. The conductivity of the polythiophene layer was reversibly switched between high and low conductance states with a "write/erase" (W/E) bias, while a separate readout circuit monitored the polymer conductance. Raman spectroscopy revealed reversible polythiophene oxidation to its polaron form accompanied by a one-electron viologen reduction. "Write", "read", and "erase" operations were repeatable, with only minor degradation of response after 200 W/E cycles. The devices exhibited switching immediately after fabrication and did not require an "electroforming" step required in many types of memory devices. Spatially resolved Raman spectroscopy revealed polaron formation throughout the polymer layer, even away from the electrodes in the channel and drain regions, indicating that thiophene oxidation "propagates" by growth of the conducting polaron form away from the source electrode. The results definitively demonstrate concurrent redox reactions of both polythiophene and viologen in solid-state devices and correlate such reactions with device conductivity. The mechanism deduced from spectroscopic and electronic monitoring should guide significant improvements in memory performance.

Self-assembly of alkylthiosulfates on gold: role of electrolyte and trace water in the solvent.

Spontaneous self-assembly of alkylthiosulfates on gold produce monolayers similar to the corresponding alkanethiols. Alkylthiosulfate self-assembly from THF solutions is inhibited in the presence of tetrabutylammonium tetrafluoroborate electrolyte. The mechanism of alkylthiosulfate self-assembly and the role of electrolyte and trace water in the solvent are investigated using open-circuit potential measurements, contact angle goniometry and redox electron transfer blocking experiments to explore the hypothesis that trace water present in the solvent facilitates monolayer formation on gold. Furthermore, the unique behavior of tetrabutylammonium tetrafluoroborate, compared to other tetrabutylammonium electrolytes, on the inhibition of alkylthiosulfate self-assembly has been explained.

Electrochemically assisted self-assembly of alkylthiosulfates and alkanethiols on gold: the role of gold oxide formation and corrosion.

Electrochemically directed self-assembly of alkylthiosulfates enables the selective formation of monolayers on gold surfaces. These monolayers are identical to those formed from the corresponding alkanethiols. However, the mechanistic details of monolayer formation under electrochemical conditions as well as the role of other variables and residual water in the solvent have not been extensively studied. A systematic investigation shows that self-assembly is not a result of an outer-sphere one-electron oxidation of alkylthiosulfate. Voltammetry and electrochemical quartz crystal microbalance techniques reveal that self-assembly involving alkylthiosulfates as well as alkanethiols under oxidative conditions proceed through direct reaction with gold oxide and in some cases is accompanied by corrosion. X-ray photoelectron spectroscopy indicates that monolayers can undergo rapid exchange with molecules in solution under electrochemically directed self-assembly conditions.

Field enhanced charge carrier reconfiguration in electronic and ionic coupled dynamic polymer resistive memory.

Dynamic resistive memory devices based on a conjugated polymer composite (PPy(0)DBS(-)Li(+) (PPy: polypyrrole; DBS(-): dodecylbenzenesulfonate)), with field-driven ion migration, have been demonstrated. In this work the dynamics of these systems has been investigated and it has been concluded that increasing the applied field can dramatically increase the rate at which information can be 'written' into these devices. A conductance model using space charge limited current coupled with an electric field induced ion reconfiguration has been successfully utilized to interpret the experimentally observed transient conducting behaviors. The memory devices use the rising and falling transient current states for the storage of digital states. The magnitude of these transient currents is controlled by the magnitude and width of the write/read pulse. For the 500 nm length devices used in this work an increase in 'write' potential from 2.5 to 5.5 V decreased the time required to create a transient conductance state that can be converted into the digital signal by 50 times. This work suggests that the scaling of these devices will be favorable and that 'write' times for the conjugated polymer composite memory devices will decrease rapidly as ion driving fields increase with decreasing device size.

Redox-gated three-terminal organic memory devices: effect of composition and environment on performance.

The performance of redox-gated organic nonvolatile memory (NVM) based on conducting polymers was investigated by altering the polymer structure, composition, and local environment of three-terminal devices with a field-effect transistor (FET) geometry. The memory function was dependent on the presence of a redox active polymer with high conducting and low conducting states, the presence of a redox counter-reaction, and the ability to transport ions between the polymer and electrolyte phases. Simultaneous monitoring of both the "write" current and "readout" current revealed the switching dynamics of the devices and their dependence on the local atmosphere. Much faster and more durable response was observed in acetonitrile vapor than in a vacuum, indicating the importance of polar molecules for both ion motion and promotion of electrochemical reactions. The major factor determining "write" and "erase" speeds of redox-gated polymer memory devices was determined to be the rate of ion transport through the electrolyte layer to provide charge compensation for the conducting polarons in the active polymer layer. The results both confirm the mechanism of redox-gated memory action and identify the requirements of the conducting polymer, redox counter reaction, and electrolyte for practical applications as alternative solid-state nonvolatile memory devices.