Electrocatalytic Oxidation Of NADH Research Articles

Electrocatalytic oxidation of NADH at gold nanoparticles loaded poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid) film modified electrode and integration of alcohol dehydrogenase for alcohol sensing

A new modified electrode is fabricated by dispersing gold nanoparticles onto the matrix of poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonic acid), PEDOT–PSS. The electrocatalytic activity of the PEDOT–PSS-Au nano electrode towards the oxidation of β-nicotinamide adenine dinucleotide (NADH) is investigated. A substantial decrease in the overpotential (>0.7 V) has been observed for the oxidation of NADH at the PEDOT–PSS-Au nano electrode in comparison to the potential at PEDOT–PSS electrode. The Au nanoparticles dispersed in the PEDOT–PSS matrix prevents the fouling of electrode surface by the oxidation products of NADH and augments the oxidation of NADH at a less positive potential (+0.04 V vs. SCE). The electrode shows high sensitivity to the electrocatalytic oxidation of NADH. Further, the presence of ascorbic acid and uric acid does not interfere during the detection of NADH. Important practical advantages such as stability of the electrode (retains ∼95% of its original activity after 20 days), reproducibility of the measurements (R.S.D.: 2.8%; n = 5), selectivity and wide linear dynamic range (1–80 μM; R 2 = 0.996) are achieved at PEDOT–PSS-Au nano electrode. The ability of PEDOT–PSS-Au nano electrode to promote the electron transfer between NADH and the electrode makes us to fabricate a biocompatible dehydrogenase-based biosensor for the measurement of ethanol. The biosensor showed high sensitivity to ethanol with rapid detection, good reproducibility and excellent stability.

Meldola blue immobilized on a new SiO 2/TiO 2/graphite composite for electrocatalytic oxidation of NADH

Meldola blue immobilized on a new SiO 2/TiO 2/graphite composite was applied in the electrocatalytic oxidation of NADH. The materials were prepared by the sol–gel processing method and characterized by several techniques including scanning electronic microscopy coupled to energy dispersive spectroscopy (SEM–EDS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electronic microscopy (HRTEM). Si and Ti mapping profiles on the surface showed a homogeneous distribution of the components. Ti2p binding energy peaks indicate that the formation of Si–O–Ti linkage is presumably the responsible for the high rigidity of the matrices. The good electrical conductivity presented by the composites (5 and 11 S cm −1) can be related to a homogeneous distribution of graphite particles observed by TEM. After the materials characterization, a SiO 2/TiO 2/graphite electrode was prepared and some chemical modifications were performed on its surface to promote the adsorption of meldola blue. The resulting system presented electrocatalytic properties toward the oxidation of NADH, decreasing the oxidation potential to −120 mV. The proposed sensor showed a wide linear response range from 0.018 to 7.29 mmol l −1 and limit of detection of 0.008 mmol l −1. SiO 2/TiO 2/graphite has shown to be a promising material to be used as a suitable support in the construction of new electrochemical sensors.

Electrocatalytic Oxidation of NADH on Electrochemically Oxidized Carbon Fiber Paper Electrodes for a Simple Biofuel Cell Anode

An electrochemically oxidized carbon fiber paper (CFP) electrode was fabricated and the direct electrochemical oxidation of NADH was examined for a biofuel cell application. The Raman spectra of the oxidized CFP electrode indicated that edge plane sites were generated by electrochemical activation. On a cyclic voltammogram of the electrode at pH 7.0, an oxidation-reduction peak with a redox potential of about 0 mV (vs. Ag/AgCl) appeared. The electrocatalytic oxidation of NADH with an anodic peak at low potential (+50 mV) occurred and the current density reached as large as a few mA/cm2. The quinoidal moieties which are generated at the edge plane site act as an electron transfer mediator. With NAD+-dependent alcohol dehydrogenase (ADH), the ADH catalyzed bioanode could be constructed by entrapment with poly(diallyldimethylammonium chloride) on the oxidized CFP electrode.

Mediated electrocatalytic oxidation of bioanalytes and biosensing of glutamate using functionalized multiwall carbon nanotubes-biopolymer nanocomposite

Abstract The electrochemical and electrocatalytic properties of functionalized multiwall carbon nanotubes (FCNTs)-biopolymer nanocomposite is described. The multiwall carbon nanotubes (MWCNTs) were functionalized by sonochemical treatment in acidic solution. Sonochemical treatment shortens the length of MWCNTs and generates quinone-like functional group on the tube ends. The FCNTs were characterized by FTIR, FESEM and electrochemical measurements. The nanocomposite was prepared by mixing FCNTs with the biopolymer chitosan (CHIT). The nanocomposite-modified electrode shows a well-defined redox peak at −0.05 V. The voltammetric peak shifts by −59 ± 1 mV while changing the solution pH by one unit, suggesting that protons and electrons take part in the redox reaction in the ratio of 1:1. The redox response of the nanocomposite-modified electrode is ascribed to the quinone-like functional groups of FCNT. The surface coverage of the redox species was 1.3 × 10−10 mol/cm2. The existence of quinone-like functional groups on the FCNTs was confirmed by FTIR spectral measurement. The mediated electrocatalytic oxidation of NADH and ascorbate (AA) by the FCNT nanocomposite-modified electrode was observed for the first time at less positive potential (−0.05 V) in the absence of any additional redox mediators. The oxidation potential for NADH and AA on the FCNT nanocomposite- modified electrode is much less positive than that on the as-purchased MWCNT modified-electrode. Compared with the as-purchased MWCNTs, the FCNTs exhibit very high catalytic activity toward oxidation of the bioanalytes. The high catalytic activity is attributed to the presence of mediating quinone-like functionalities. The FCNT nanocomposite electrode was successfully used for the development of dehydrogenase based glutamate biosensor.

Electrochemical Preparation of Poly(acriflavine) Film‐Modified Electrode and Its Electrolcatalytic Properties Towards NADH, Nitrite and Sulfur Oxoanions

AbstractElectrochemical polymerization of acriflavine (AF) was carried out onto glassy carbon electrodes (GCE) from the aqueous buffer solution containing 1.5×10−3 M AF monomer (pH 3.5) which produced a thin electrochemically active film. This is noted as poly(AF) film modified electrodes (PAF/GCE). This modified electrode was shown a stable reversible redox couple centered at +0.22 V in pH 3.5 buffer solutions. PAF/GCE was found to be more stable in acidic solutions and its formal potential was found to be pH dependent with a slope close to −60 mV/pH. The electrochemical deposition kinetics of poly(AF) onto gold coated quartz crystal was studied by using electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry (CV). PAF/GCE was found be good mediator for electrochemical oxidation of reduced nicotinamide adenine dinucleotide (NADH) in pH 5 buffer solutions. The electrocatalytic oxidation of SO$\rm{ _3^{2 - } }$ and electrocatalytic reduction of NO$\rm{ _2^ - }$, SO$\rm{ _5^{2 - } }$ and S2O$\rm{ _8^{2 - } }$ were carried out at PAF/GCE electrode in acidic aqueous solutions. The electrocatalytic oxidation of NADH was also investigated by using amperometric method.

금 나노입자를 회합시킨 수식된 흑연전극으로 NADH의 전기촉매 산화반응

Mercaptopropionic acid(mpa) has been used to make self-assembled monolayer(SAMs) on the surface of graphite electrode incorporating gold nano particles, which are subsequently modified with dopamine(dopa). Such modified electrodes haying types of Gr(Au)/mpa-dopa were employed in the electrocatalytic oxidation of NADH. The responses of such modified electrodes were studied in terms of electron transfer kinetics and reaction procedure in the reaction. The reaction of the surface immobilized dopa with NADH was studied using the rotating disk electrode technique and a value of was obtained for the second-order rate constant in 0.1 M phosphate buffer(pH

The Flavoprotein Subcomplex of Complex I (NADH:Ubiquinone Oxidoreductase) from Bovine Heart Mitochondria: Insights into the Mechanisms of NADH Oxidation and NAD+ Reduction from Protein Film Voltammetry

Complex I (NADH:ubiquinone oxidoreductase) from bovine heart mitochondria contains 45 different subunits and nine redox cofactors. NADH is oxidized by a noncovalently bound flavin mononucleotide (FMN), then seven iron-sulfur clusters transfer the two electrons to quinone, and four protons are pumped across the inner mitochondrial membrane. Here, we use protein film voltammetry to investigate the mechanisms of NADH oxidation and NAD+ reduction in the simplest catalytically active subcomplex of complex I, the flavoprotein (Fp) subcomplex. The Fp subcomplex was prepared using chromatography and contained the 51 and 24 kDa subunits, the FMN, one [4Fe-4S] cluster, and one [2Fe-2S] cluster. The reduction potential of the FMN in the enzyme's active site is lower than that of free FMN (thus, the oxidized state of the FMN is most strongly bound) and close to the reduction potential of NAD+. Consequently, the catalytic transformation is reversible. Electrocatalytic NADH oxidation by subcomplex Fp can be explained by a model comprising substrate mass transport, the Michaelis-Menten equation, and interfacial electron transfer kinetics. The difference between the "catalytic" potential and the FMN potential suggests that the flavin is reoxidized before NAD+ is released or that intramolecular electron transfer from the flavin to the [4Fe-4S] cluster influences the catalytic rate. NAD+ reduction displays a marked activity maximum, below which the catalytic rate decreases sharply as the driving force increases. Two possible models reproduce the observed catalytic waveshapes: one describing an effect from reducing the proximal [2Fe-2S] cluster and the other the enhanced catalytic ability of the semiflavin state.

Electrocatalytic oxidation of NADH with Meldola's blue functionalized carbon nanotubes electrodes

Meldola's blue (MB) functionalized carbon nanotubes (CNT) nanocomposite film (MB/CNT) electrode was prepared by non-covalent adsorbing MB on the surface of a carbon nanotubes modified glassy carbon electrode (CNT/GCE). Electrochemical behaviors of the resulting electrode were investigated thoroughly with cyclic voltammetry in the potential range of −0.6 to 0.2 V, and two well-defined redox couples were clearly visualized. We also studied the electron transfer kinetics of MB loaded on CNT (MB/CNT) in comparison with that of MB on conventional graphite powder (MB/GP). The heterogeneous electron transfer rate constant ( k s) of MB/CNT was calculated to be about three times larger than that of MB/GP. The accelerated electron transfer kinetics was attributed to the unique electrical and nanostructural properties of CNT supports as well as the interaction between MB and CNT. In connection with the oxidation of nicotinamide adenine dinucleotide (NADH), excellent electrocatalytic activities were observed at MB/CNT/GCE compared with MB/GP modified glassy carbon electrode (MB/GP/GCE). Based on the results, a new NADH sensor was successfully established using the MB/CNT/GCE. Under a lower operation potential of −0.1 V, NADH could be detected linearly up to a concentration of 500 μM with an extremely lower detection limit of 0.048 ± 0.02 μM estimated at a signal-to-noise ratio of 3. Sensitivity, selectivity, reproducibility and stability of the NADH sensor were also investigated and the main analytical data were also compared with those obtained with the MB/GP/GCE.

Electrocatalytic sensing of NADH on a glassy carbon electrode modified with electrografted o-aminophenol film

A simple and sensitive method for the electrocatalytic detection of NADH on a glassy carbon electrode modified with electrografted o-aminophenol film ( o-AP) is presented. The modification of a glassy carbon electrode surface with an o-AP film was achieved by electrochemical reduction of the corresponding, in situ generated nitrophenyl diazonium cation. The functionalized electrode shows an efficient electrocatalytic activity towards the oxidation of NADH with activation overpotential, which is ca. 350 mV lower than that at the bare electrode. The stability and the electrocatalytic activity of the modified electrode have been critically addressed. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed o-AP. The second-order rate constant for electrocatalytic oxidation of NADH, k obs, and the apparent Michaelis–Menten constant K M, at pH 7.0 were evaluated with rotating disk electrode (RDE) experiments, using the Koutecky–Levich approach. Using the o-AP-GC electrode, at an applied potential of +150 mV (vs. Ag–AgCl) with amperometric detection of NADH, a calibration range from 7.5 × 10 −7 to 2.5 × 10 −6 with a detection limit of 1.5 × 10 −7 M was obtained, and excellent reproducibility was demonstrated with an RSD% = 2.1, n = 10 using a concentration of 1.0 × 10 −6 M NADH.

Synergistic effect of a catechin-immobilized poly(3,4-ethylenedioxythiophene)-modified electrode on electrocatalysis of NADH in the presence of ascorbic acid and uric acid

Catechin is a polyphenolic flavonoid that can be isolated from a variety of natural sources, including tea leaves, grape seeds, and the wood and bark of trees such as acacia and mahogany. In our experiments, catechin was immobilized on PEDOT/GC (poly(3,4-ethylenedioxythiophene)/glassy carbon)-modified electrodes and used as a mediator for NADH (nicotinamide adenine dinucleotide) oxidation. The effect of the PEDOT thickness on the surface coverage of the catechin molecules was studied using cyclic voltammetry. The electrochemical properties and the effect of pH on the redox properties of the immobilized catechin molecules were studied by cyclic voltammetry in phosphate solution. The electrocatalytic oxidation of NADH at different electrode surfaces such as the bare GC-, the PEDOT/GC-, the catechin/GC- and the catechin/PEDOT/GC-modified electrodes was explored in phosphate solution at pH 7. In the catechin/PEDOT/GC-modified electrode, the PEDOT film plays an important role in resolving the oxidation potentials of ascorbic acid and NADH into two peaks that occur at the same potential for the catechin/GC-modified electrode surface. The heterogeneous electron transfer rate constant for NADH oxidation at the catechin/PEDOT/GC-modified electrode was determined using the rotating disk electrode technique and found to be 9.88 × 10 3 M −1 s −1. The amperometric determination of NADH at the catechin/PEDOT/GC electrode was tested. The sensitivity of the electrode was 19 nA/μM.

Layer-by-Layer Assembled Film Based on Chitosan/Carbon Nanotubes, and its Application to Electrocatalytic Oxidation of NADH

Multilayer films of multiwalled carbon nanotubes (MWNTs) were homogeneously and stably assembled on a glassy carbon (GC) electrode using the layer-by-layer (LBL) method based on electrostatic interaction between MWNTs (negatively charged) and a biopolymer chitosan (CHIT) (positively charged). Scanning electron microscopy (SEM) image of the resulting {CHIT/MWNTs}9 film indicated that the substrate was mostly covered with MWNTs in the form of small bundles or single nanotubes. The multilayer film was used to study the electrocatalytic oxidation of NADH. The assembled {CHIT/MWNTs}9/GC electrode could decrease the oxidation overpotential of NADH by more than 350 mV. The {CHIT/MWNTs}9/GC electrode exhibited a wide linear response range of 8 × 10−7 to 1.6 × 10−3 mol · L−1 with a correlation coefficient of 0.997 for the detection of NADH. The response time and detection limit (S/N = 3) were determined to be 3 s and 0.3 × 10−6 mol · L−1, respectively. Another attractive characteristic was that the method was simple and the assembled {CHIT/MWNTs}9/GC electrode was highly stable.

Carbon nanotubes–polymer–redox mediator hybrid thin film for electrocatalytic sensing

Electrocatalytic sensing of NADH using a hybrid thin film derived from multi-wall carbon nanotubes (CNTs), Nafion (Nf) polymer and electrogenerated redox mediator is described. The redox mediator was electrochemically generated by the oxidation of serotonin on the hybrid thin film modified glassy carbon electrode (GC/Nf-CNT). Controlled potential electrolysis of serotonin at 0.1 V in neutral solution results in the generation of the redox mediator 5,5'-dihydroxy-4,4'-bitryptamine (DHB) on the hybrid thin film. The electrogenerated DHB has redox active quinone-imine structure and was electrochemically characterized by studying the pH dependent redox response. DHB on the hybrid thin film exhibits reversible redox peak at -0.05 V and the formal potential shifts by -55 mV while increasing the solution pH by 1 unit. The quinone-imine structure of DHB efficiently catalyzes the oxidation of NADH with a decrease in the overpotential of about 500 mV compared to the unmodified electrode. The CNTs of the hybrid thin film facilitates the mediated electrocatalytic oxidation of NADH. The hybrid thin film modified electrode exhibits stable amperometric response and it linearly responds to NADH (0.5-400 microM). This hybrid thin film modified electrode could detect NADH as low as 0.1 microM at -0.05 V with a sensitivity of 11.1 nA/microM in physiological pH.

Reversible cyclic voltammetry of the NADH/NAD+ redox system on hybrid poly(luminol)/FAD film modified electrodes

Hybrid films composed of electropolymerized luminol–flavin adenine dinucleotide (FAD) adsorbed film modified electrodes have been prepared in aqueous acidic solutions. These films are stable and electrochemically active, and can be produced on glassy carbon, platinum, gold, and transparent semiconductor tin oxide electrodes. The hybrid poly(luminol)/FAD film showed two redox couples. Electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of hybrid poly(luminol)/FAD and poly(luminol) films. The modified electrode was transferred into various aqueous acidic solutions, the two redox couples and the formal potentials of the hybrid films were observed to be pH-dependent. The electrocatalytic oxidation and reduction of NADH and NAD+ by a polyluminol/FAD hybrid film in aqueous solutions with various pH were carried out. The electrocatalytic oxidation of NADH and the reversible electrocatalytic reactions of NADH/NAD+ using a poly(luminol)/FAD hybrid film were found to be active.

Electrocatalytic properties of NDGA and NDGA/FAD hybrid film modified electrodes for NADH/NAD + redox reaction

The fabrication of monolayers composed of nordihydroguaiaretic acid (NDGA), and hybrid films composed of NDGA-flavin adenine dinucleotide (FAD) adsorbed films was performed in neutral aqueous solutions to produce electrochemically active thin films exhibiting one and two redox couples, respectively. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of the NDGA and hybrid NDGA/FAD film monolayers. The NDGA modified film electrocatalytically oxidized NADH, ascorbic acid, dopamine, and N 2H 4 in neutral aqueous solutions. Well-separated voltammetric peaks were observed for dopamine and uric acid mixtures, and also for ascorbic acid and uric acid mixtures using the NDGA/GC modified electrode. When transferred to various aqueous buffered solutions, the two redox couples of the NDGA/FAD hybrid film and their formal potentials were observed to be pH-dependent. The electrocatalytic oxidation and reduction of NADH and NAD + by a NDGA/FAD hybrid film in neutral aqueous solutions was carried out, and the electrocatalytic oxidation of NADH was performed using a NDGA/FAD hybrid film.

Electrocatalytic Oxidation of NADH Using a Novel Modified Electrode with a Ruthenium Complex Containing Phenanthroline Quinone

Abstract A ruthenium complex containing phenanthroline quinone, [Ru(pdon)2(aphen)](ClO4)2 (pdon = 1,10-phenanthroline-5,6-dione, aphen = 5-amino-1,10-phenanthroline) (1), was prepared as an electrocatalytic adsorbate on a gold electrode. Immobilization of 1 on the gold electrode was confirmed by surface enhanced resonance Raman spectroscopy. The rate constant ks was calculated as 10 s−1 at pH 7.0. The electrocatalytic oxidation of NADH using this modified electrode was investigated.

Enzyme-based glucose fuel cell using Vitamin K 3-immobilized polymer as an electron mediator

To create an enzyme-based biological fuel cell generating electricity from glucose and O 2, we modified a glassy carbon electrode with a bi-layer polymer membrane, the inner layer containing diaphorase (Dp) and the outer, glucose dehydrogenase (GDH, an NAD +-dependent enzyme). The Dp membrane was formed from a newly synthesized 2-methyl-1,4-naphthoquinone (Vitamin K 3; VK 3)-based polymer. This polymer showed reversible redox activity at a potential close to that of free VK 3 (−0.25 V vs. Ag/AgCl sat. KCl), and served as an electron mediator of Dp for the electrocatalytic oxidation of NADH to NAD +. The addition of Ketjenblack into the Dp/VK 3 film enhanced the generation of NAD +. The outer GDH membrane oxidized glucose continuously using NAD + generated at the inner Dp film. To construct the glucose/O 2 biological fuel cell, we coupled the enzyme-modified anode with a polydimethylsiloxane-coated Pt cathode. The cell’s open circuit voltage was 0.62 V and its maximum power density was 14.5 μW/cm 2 at 0.36 V in an air-saturated phosphate buffered saline solution (pH 7.0) at 37 °C containing 0.5 mM NADH and 10 mM glucose. Although its performance deteriorated to ca. 4 μW/cm 2 over 4 days, the cell subsequently maintained this power density for more than 2 weeks.

Electrocatalytic oxidation of NADH at carbon paste electrodes modified with meldola blue adsorbed on zirconium phosphate: effect of Ca2+ and polyethyleneimine

The basic electrochemistry of carbon paste electrodes modified with Meldola Blue adsorbed on zirconium phosphate (MB-ZP-CPEs) and their ability to oxidize NADH have been investigated. Three types of carbon powder (graphite and glassy carbon-type Sigradur K and G) were used to obtain MB-ZP-CPEs. On comparing cyclic voltammograms recorded at MB-ZP-CPEs, similarly prepared from the three different carbon powders, those made with Sigradur K exhibited the lowest background current, and the best MB electrochemistry, seen as the highest peak intensities and smallest peak separation. Using MB-ZP-CPEs based on Sigradur K a study on NADH oxidation was done focusing on the effect of the Ca2+ concentration in the contacting solution and on the addition of polyethyleneimine (PEI) into the paste. It can be stated that MB-ZP-CPEs based on Sigradur K and containing 1.23% (w/w) PEI exhibited the best behavior for NADH oxidation, measured by the highest electrocatalytic rate constant (8.2×103 M−1 s−1).

Electrocatalytic oxidation of NADH at single-wall carbon-nanotube-paste electrodes: kinetic considerations for use of a redox mediator in solution and dissolved in the paste

Cyclic voltammetry has been successfully used to study the oxidation of nicotinamide adenine dinucleotide (NADH) at single-wall carbon-nanotube-paste (CNTP) electrodes modified with p-methylaminophenolsulfate (p-MAP) and 3,4-dihydroxybenzaldehyde (3,4-DHB). Diffusion-like behaviour was observed for p-MAP-modified electrodes, and a diffusion coefficient of 2.4x10(-6) cm2 s(-1) was calculated for p-MAP in the paste. The behaviour of 3,4-DHB-modified CNTP electrodes was typical of that of surface-confined mediators. p-MAP electrocatalytic activity was first checked in solution, and a rate constant of 9.2 mol(-1) L s(-1) was obtained for the reaction between NADH and the mediator. The p-MAP-modified electrode did not have significant electrocatalytic activity for electro-oxidation of NADH, probably because of the formation of a complex between NADH and the confined mediator. In contrast, the 3,4-DHB-modified electrode had very good NADH electrocatalytic activity, with a heterogeneous rate constant of approximately 20x10(2) mol(-1) L s(-1).

Efficient electrocatalytic oxidation of NADH at gold nanoparticles self-assembled on three-dimensional sol-gel network

It is demonstrated that gold (Au) nanoparticles self-assembled on a sol-gel derived three-dimensional silicate network efficiently catalyze the oxidation of NADH in the absence of any electron transfer mediators with a decrease in overpotential of 780 mV in neutral solution.

チオニン修飾金ナノ粒子を用いた電極表面の機能化と電気化学的グルコースセンサへの応用

The electrochemically effective oxidation of NADH has drawn striking attention because it is widely applicable for fabricating biosensors. The mediators are normally used as electron shuttle molecules between the electrode and enzymes. Phenothiazine compounds deserve as suitable mediators due to theirs negative formal potential and large rate constant. For the practical use, both the enzyme and mediator are required to be co-immobilized on the electrode surface. In this study, gold nano-particles modified with thionine (Th+) were immobilized on the surface of the gold electrode by physical adsorption. Th+ was covalently bound to the surface of the gold nano-particles using 3,3'-dithiobis (succinimidyl propionate) (DSP) as a cross linker. The electrochemistry of Th+ modified on the gold nano-particles was characterized by voltammetry. Th+ molecules on the gold nano-particle modified electrodes showed two redox couples at −30mV and 240mV vs Ag/AgCl at pH7. A well-defined peak for the electrocatalytic oxidation of NADH was observed at 240mV. The nano-particle electrodes modified with glucose dehydrogenase (GDH) and Th+ mediators were fabricated and characterized aiming at fabricating a reagentless glucose biosensor. Upon the addition of glucose to the electrochemical cell, the oxidation current dramatically increased due to an electrocatalytic reaction.