Facilitation of high-rate NADH electrocatalysis using electrochemically activated carbon materials

Review on Microreactors for Photo-Electrocatalysis Artificial Photosynthesis Regeneration of Coenzymes

In recent years, with the outbreak of the global energy crisis, renewable solar energy has become a focal point of research. However, the utilization efficiency of natural photosynthesis (NPS) is only about 1%. Inspired by NPS, artificial photosynthesis (APS) was developed and utilized in applications such as the regeneration of coenzymes. APS for coenzyme regeneration can overcome the problem of high energy consumption in comparison to electrocatalytic methods. Microreactors represent a promising technology. Compared with the conventional system, it has the advantages of a large specific surface area, the fast diffusion of small molecules, and high efficiency. Introducing microreactors can lead to more efficient, economical, and environmentally friendly coenzyme regeneration in artificial photosynthesis. This review begins with a brief introduction of APS and microreactors, and then summarizes research on traditional electrocatalytic coenzyme regeneration, as well as photocatalytic and photo-electrocatalysis coenzyme regeneration by APS, all based on microreactors, and compares them with the corresponding conventional system. Finally, it looks forward to the promising prospects of this technology.

Polydopamine nanofilms for high-performance paper-based electrochemical devices

Since the discovery of polydopamine (PDA), there has been a lot of progress on using this substance to functionalize many different surfaces. However, little attention has been given to prepare functionalized surfaces for the preparation of flexible electrochemical paper-based devices. After fabricating the electrodes on paper substrates, we formed PDA on the surface of the working electrode using a chemical polymerization route. PDA nanofilms on carbon were characterized by contact angle (CA) experiments, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy (topography and electrical measurements) and electrochemical techniques. We observed that PDA introduces chemical functionalities (RNH₂ and RC═O) that decrease the CA of the electrode. Moreover, PDA nanofilms did not block the heterogeneous electron transfer. In fact, we observed one of the highest standard heterogeneous rate constants (k_s ) for electrochemical paper-based electrodes (2.5 ± 0.1) × 10^-3  cm s^-1 , which is an essential parameter to obtain larger currents. In addition, our results suggest that carbonyl functionalities are ascribed for the redox activity of the nanofilms. As a proof-of-concept, the electrooxidation of nicotinamide adenine dinucleotide showed remarkable features, such as, lower oxidation potential, electrocatalytic peak currents more than 30 times higher when compared to unmodified paper-based electrodes and electrocatalytic rate constant (k_obs ) of (8.2 ± 0.6) × 10²  L mol^-1  s^-1 .

Mediated electrochemical oxidation of glucose via poly(methylene green) grafted on the carbon surface catalyzed by flavin adenine dinucleotide-dependent glucose dehydrogenase

Electrochemically polymerized phenothiazines (thionine, methylene green, methylene blue, and toluidine blue) on carbon electrodes were investigated as electron transfer mediators of glucose oxidation by flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) for biosensor and biofuel cell applications. Among the tested polyphenothiazines grafted on a glassy carbon electrode, clear redox-mediating activity was observed for poly(methylene green), and the catalytic oxidation current depended on the concentrations of glucose and enzymes and the amount of polymer deposited on the electrode surface. The poly(methylene green)-grafted porous carbon electrodes showed 3 mA cm-2 of glucose oxidation current catalyzed by FAD-GDH.

A Self-Powered Biosensor with a Flake Electrochromic Display for Electrochemical and Colorimetric Formaldehyde Detection

The formaldehyde biosensors with the features of cost effectiveness, high specificity, easy operation, and simplicity are urgently desired in routing and field detection of formaldehyde. Here, we report a new design of an enzymatic self-powered biosensor (ESPB) toward formaldehyde detection. The ESPB involves a formaldehyde dehydrogenase/poly-methylene green/buckypaper bioanode as the sensing electrode and a Prussian blue/Au nanoparticles/carbon fiber paper cathode as the electrochromic display. Formaldehyde acts as the fuel to drive the ESPB, relying on that the concentration of formaldehyde can be determined with the ESPB by both directly measuring the variance in short circuit current and observing the color change of the cathode. By measuring the variance in short circuit current, a linear detection range from 0.01 to 0.35 mM and a calculated detection limit of 0.006 mM are obtained, comparable to or better than those reported before. The color change of the cathode can be distinguished easily and exactly via the naked eye after immersing the ESPB in formaldehyde solution for 90 s with the concentration up to 0.35 mM, covering the permissive level of formaldehyde in some standards associated with environmental quality control. Specially, the formaldehyde concentration can be precisely quantified by analyzing the color change of the cathode digitally using the equation of B/(R + G + B). In the following test of real spiked samples of tap water and lake water, the recovery ratios of formaldehyde with the concentrations from 0.010 to 0.045 mM are tested to be between 95 and 100% by both measuring the variance in short circuit current and analyzing the color change of the cathode digitally. In addition, the ESPB exhibits negligible interference from acetaldehyde and ethanol and can be stored at 4 °C for 21 days with a loss of less than 8% in its initial value of short circuit current. Therefore, the ESPB with the capability of working like disposable test paper can be expected as a sensitive, simple, rapid, cost-effective colorimetric method with high selectivity in routing and field formaldehyde detection.

A novel non-enzymatic zinc oxide thin film based electrochemical recyclable strip with device interface for quantitative detection of catechol in water

Catechol, one of the major effluents released by various chemical and metal processing industries, causes severe pollution of groundwater. Monitoring of catechol in water using cost-effective, handheld sensor is demanding for the safety of the environment. In this work, non-enzymatic zinc oxide thin film based electrochemical strip sensor is developed on conducting glass substrate for detection of catechol. The preparation of strip without employing standard Pt or Ag/AgCl electrodes and simply depositing ZnO through wet chemical process represents a cost-effective innovative technique. The ZnO thin film is characterized using field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM) and grazing incidence X-ray diffractometer (GIXRD). Catechol is electrochemically detected by means of cyclic voltammetry and amperometry. A prominent redox peak of the developed strip attributed to the detection of catechol is observed at -0.26 V in cyclic voltammetry. The strip is integrated with readout meter and an algorithm is built based on the experimentally observed linear variation of amperometric current with catechol concentration. The quantitative detection performance is demonstrated by testing 0.1-12 ppm catechol solutions.

Direct Drawing Method of Graphite onto Paper for High-Performance Flexible Electrochemical Sensors

A simple and fast fabrication method to create high-performance pencil-drawn electrochemical sensors is reported for the first time. The sluggish electron transfer observed on bare pencil-drawn surfaces was enhanced using two electrochemical steps: first oxidizing the surface and then reducing it in a subsequent step. The heterogeneous rate constant was found to be 5.1 × 10^-3 cm s^-1, which is the highest value reported so far for pencil-drawn surfaces. We mapped the origin of such performance by atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Our results suggest that the oxidation process leads to chemical and structural transformations on the electrode surface. As a proof-of-concept, we modified the pencil-drawn surface with Meldola's blue to electrocatalytically detect nicotinamide adenine dinucleotide (NADH). The electrochemical device exhibited the highest catalytic constant (1.7 × 10⁵ L mol^-1 s^-1) and the lowest detection potential for NADH reported so far in paper-based electrodes.

Fluorinated graphenes as advanced biosensors - effect of fluorine coverage on electron transfer properties and adsorption of biomolecules

Graphene derivatives are promising materials for the electrochemical sensing of diverse biomolecules and development of new biosensors owing to their improved electron transfer kinetics compared to pristine graphene. Here, we report complex electrochemical behavior and electrocatalytic performance of variously fluorinated graphene derivatives prepared by reaction of graphene with a nitrogen-fluorine mixture at 2 bars pressure. The fluorine content was simply controlled by varying the reaction time and temperature. The studies revealed that electron transfer kinetics and electrocatalytic activity of CFx strongly depend on the degree of fluorination. The versatility of fluorinated graphene as a biosensor platform was demonstrated by cyclic voltammetry for different biomolecules essential in physiological processes, i.e. NADH, ascorbic acid and dopamine. Importantly, the highest electrochemical performance, even higher than pristine graphene, was obtained for fluorinated graphene with the lowest fluorine content (CF0.084) due to its high conductivity and enhanced adsorption properties combining π-π stacking interaction with graphene regions with hydrogen-bonding interaction with fluorine atoms.

Synergistic effect of pyrroloquinoline quinone and graphene nano-interface for facile fabrication of sensitive NADH biosensor

A self-assembly composite of graphene-pyrroloquinoline quinone (PQQ) was fabricated and modified on glassy carbon electrode (GCE) for sensitive detection of nicotinamide adenine dinucleotide (NADH). Chitosan (CTS) was applied to disperse graphene to form a stable robust film on GCE. A synergistic effect between PQQ and graphene was observed during the electrocatalytic oxidation of NADH, with about 260mV reduction in the oxidation potential and 2.5-fold increase in the oxidation current compared with those on the bare GCE. The electrochemical sensors based on the modified electrodes allowed the detection of NADH with a good linear dependence from 0.32 to 220µM with a high sensitivity of 0.421µAµM^-1cm^-2 and a low detection limit of 0.16µM (S/N=3). It could also eliminate the interference of electroactive substances like ascorbic acid (AA), uric acid, and dopamine and its derivatives. The outstanding performances of graphene-PQQ/CTS composite capable of improving the electrical conductivity and accelerating the electron transport suggested its promising applications for design of different graphene based composites used in electrochemical sensing and energy fields.

Au nanoparticle/graphene nanocomposite as a platform for the sensitive detection of NADH in human urine

Here we report on a facile, rapid, sensitive, selective and highly stable electrochemical sensing platform for β-nicotinamide adenine dinucleotide (NADH) based on uncapped Au nanoparticle/reduced graphene oxide (rGO) nanocomposites without the aid of any redox mediators and enzymes. The Au nanoparticle/rGO composite sensing platform was directly formed on a glassy carbon electrode through an in situ electrochemical reduction of GO and Au(3+) with a 100% usage of the precursors. The as-prepared Au nanoparticle/rGO composites demonstrated excellent direct electrocatalytic oxidation toward NADH, providing a large electrochemical active surface area as well as a favorable environment for electron transfer from NADH to the electrode via the enhanced mobility of charge carriers. The Au nanoparticle/rGO composites offered a ~2.3 times higher electrocatalytic current density with a negative shift of 112mV, in comparison to Au nanoparticles. The sensor developed in this study displayed a high sensitivity of 0.916µA/µMcm(2) and a wide linear range of from 50nM to 500µM with a limit of detection of 1.13nM (S/N=3). The interferences from the common interferents such as glutathione, glucose, ascorbic acid and quanine were negligible. The prepared sensor was further tested for the determination of NADH in human urine samples, showing the Au nanoparticle/rGO nanocomposites simultaneously formed by one-step electrochemical reduction have promising biomedical applications.