Nitrogen-doped Carbon Nanotube Electrodes Research Articles

Investigating the role of metals loaded on nitrogen-doped carbon-nanotube electrodes in electroenzymatic alcohol dehydrogenation

A new enzymatic biofuel cell (EBFC) is developed using conductive metal alloy nanoparticles with carbon cloth (CC) as an immobilization support for ethanol dehydrogenase (EtDH) and formolase (FLS). Ethanol (EtOH) dehydrogenation to acetaldehyde via direct electron transfer (DET) is pursued as the first step, followed by the condensation of acetaldehyde to acetoin. Metals are deposited onto novel three-dimensional jellyfish (JF)-shaped nanoparticles (SiO2–NCNT–CoFe2), where NCNT denotes “N-doped carbon nanotube”. The fabricated JF–metal–CC–EtDH bioelectrodes exhibit a variation in power generation with varying metals, with a value 37.6-fold higher than that of previously reported EBFC operations with DET for EtOH oxidation. The highest acetoin content is also found in JF-Os–CC–EtDH–FLS, attributable to faster electron uptake by the bioelectrode. First-principles calculations suggest that the d-state delocalization of metal-loaded JF particles is the cause of the enhanced catalytic activity, and it can be utilized in designing electrocatalysts.

Amperometric Detection of Aqueous Silver Ions by Inhibition of Glucose Oxidase Immobilized on Nitrogen-Doped Carbon Nanotube Electrodes.

An amperometric glucose biosensor based on immobilization of glucose oxidase on nitrogen-doped carbon nanotubes (N-CNTs) was successfully developed for the determination of silver ions. Upon exposure to glucose, a steady-state enzymatic turnover rate was detected through amperometric oxidation of the H2O2 byproduct, directly related to the concentration of glucose in solution. Inhibition of the steady-state enzymatic glucose oxidase reaction by heavy metals ions such as Ag(+), produced a quantitative decrease in the steady-state rate, subsequently creating an ultrasensitive metal ion biosensor through enzymatic inhibition. The Ag(+) biosensor displayed a sensitivity of 2.00 × 10(8) ± 0.06 M(-1), a limit of detection (σ = 3) of 0.19 ± 0.04 ppb, a linear range of 20-200 nM, and sample recovery at 101 ± 2%, all acquired at a low-operating potential of 0.05 V (vs Hg/Hg2SO4). Interestingly, the biosensor does not display a loss in sensitivity with continued use due to the % inhibition based detection scheme: loss of enzyme (from continued use) does not influence the % inhibition, only the overall current associated with the activity loss. The heavy metals Cu(2+) and Co(2+) were also detected using the enzyme biosensor but found to be much less inhibitory, with sensitivities of 1.45 × 10(6) ± 0.05 M(-1) and 2.69 × 10(3) ± 0.07 M(-1), respectively. The mode of GOx inhibition was examined for both Ag(+) and Cu(2+) using Dixon and Cornish-Bowden plots, where a strong correlation was observed between the inhibition constants and the biosensor sensitivity.

Investigating the Electrocatalytic Oxidation of Dihydronicotinamide Adenine Dinucleotide at Nitrogen-Doped Carbon Nanotube Electrodes: Implications to Electrochemically Measuring Dehydrogenase Enzyme Kinetics

Nitrogen-doped carbon nanotubes (N-CNTs) have been shown to be electrocatalytic toward the oxidation of dihydronicotinamide adenine dinucleotide (NADH), the reduced form of the coenzyme necessary for enzymatic turnover in NAD+-dependent dehydrogenases. The observed oxidation potential of the electrocatalyst, however, still shows a significant overpotential, suggesting that even for effective electrocatalysts, electrooxidation may be kinetically controlled. We demonstrate using the Koutecky–Levich rotating disk electrode technique that the observed electron transfer rate constant (kobs) is a function of potential over a wide potential window; however, kobs could only be accurately measured for a portion of that window for the electrocatalytic N-CNTs. More importantly, electrochemically measured enzyme kinetics, acquired after adsorption of glucose dehydrogenase onto the N-CNTs, are never independent of potential, even when the electron transfer rate constant is too fast to measure by the rotating disk tech...

Electrochemical Behavior of Flavin Adenine Dinucleotide Adsorbed onto Carbon Nanotube and Nitrogen-Doped Carbon Nanotube Electrodes

Flavin adenine dinucleotide (FAD) is a cofactor for many enzymes, but also an informative redox active surface probe for electrode materials such as carbon nanotubes (CNTs) and nitrogen-doped CNTs (N-CNTs). FAD spontaneously adsorbs onto the surface of CNTs and N-CNTs, displaying Langmuir adsorption characteristics. The Langmuir adsorption model provides a means of calculating the electroactive surface area (ESA), the equilibrium constant for the adsorption and desorption processes (K), and the Gibbs free energy of adsorption (ΔG°). Traditional ESA measurements based on the diffusional flux of a redox active molecule to the electrode surface underestimate the ESA of porous materials because pores are not penetrated. Techniques such as gas adsortion (BET) overestimate the ESA because it includes both electroactive and inactive areas. The ESA determined by extrapolation of the Langmuir adsorption model with the electroactive surface probe FAD will penetrate pores and only include electroactive areas. The redox activity of adsorbed FAD also displays a strong dependency on pH, which provides a means of determining the pKa of the surface confined species. The pKa of FAD decreases as the nitrogen content in the CNTs increases, suggesting a decreased hydrophobicity of the N-CNT surface. FAD desorption at N-CNTs slowly transforms the main FAD surface redox reaction with E1/2 at -0.84 V into two new, reversible, surface confined redox reactions with E1/2 at -0.65 and -0.76 V (vs Hg/Hg2SO4), respectively (1.0 M sodium phosphate buffer pH = 6.75). This is the first time these redox reactions have been observed. The new surface confined redox reactions were not observed during FAD desorption from nondoped CNTs.

Electrochemical Oxidation of Dihydronicotinamide Adenine Dinucleotide at Nitrogen-Doped Carbon Nanotube Electrodes

Nitrogen-doped carbon nanotubes (N-CNTs) substantially lower the overpotential necessary for dihydronicotinamide adenine dinucleotide (NADH) oxidation compared to nondoped CNTs or traditional carbon electrodes such as glassy carbon (GC). We observe a 370 mV shift in the peak potential (Ep) from GC to CNTs and another 170 mV shift from CNTs to 7.4 atom % N-CNTs in a sodium phosphate buffer solution (pH 7.0) with 2.0 mM NADH (scan rate 10 mV/s). The sensitivity of 7.4 atom % N-CNTs to NADH was measured at 0.30 ± 0.04 A M(-1) cm(-2), with a limit of detection at 1.1 ± 0.3 μM and a linear range of 70 ± 10 μM poised at a low potential of -0.32 V (vs Hg/Hg2SO4). NADH fouling, known to occur to the electrode surface during NADH oxidation, was investigated by measuring both the change in Ep and the resulting loss of electrode sensitivity. NADH degradation, known to occur in phosphate buffer, was characterized by absorbance at 340 nm and correlated with the loss of NADH electroactivity. N-CNTs are further demonstrated to be an effective platform for dehydrogenase-based biosensing by allowing glucose dehydrogenase to spontaneously adsorb onto the N-CNT surface and measuring the resulting electrode's sensitivity to glucose. The glucose biosensor had a sensitivity of 0.032 ± 0.003 A M(-1) cm(-2), a limit of detection at 6 ± 1 μM, and a linear range of 440 ± 50 μM.

Nitrogen-doped Carbon Nanotube Electrodes for Enzyme Based Electrochemical Biosensing

not Available.

Influence of Surface Adsorption on the Interfacial Electron Transfer of Flavin Adenine Dinucleotide and Glucose Oxidase at Carbon Nanotube and Nitrogen-Doped Carbon Nanotube Electrodes

The adsorption of flavin adenine dinucleotide (FAD) and glucose oxidase (GOx) onto carbon nanotube (CNT) and nitrogen-doped CNT (N-CNT) electrodes was investigated and found to obey Langmuir adsorption isotherm characteristics. The amount adsorbed and adsorption maximum are dependent on exposure time, the concentration of adsorbate, and the ionic strength of the solution. The formal potentials measured for FAD and GOx are identical, indicating that the observed electroactivity is from FAD, the redox reaction center of GOx. When glucose is added to GOx adsorbed onto CNT/N-CNT electrodes, direct electron transfer (DET) from enzyme-active FAD is not observed. However, efficient mediated electron transfer (MET) occurs if an appropriate electron mediator is placed in solution, or the natural electron mediator oxygen is used, indicating that GOx is adsorbed and active on CNT/N-CNT electrodes. The observed surface-confined redox reaction at both CNT and N-CNT electrodes is from FAD that either specifically adsorbs from solution or adsorbs from the holoprotein subsequently inactivating the enzyme. The splitting of cathodic and anodic peak potentials as a function of scan rate provides a way to measure the heterogeneous electron-transfer rate constant (k(s)) using Laviron's method. However, the measured k(s) was found to be under ohmic control, not under the kinetic control of an electron-transfer reaction, suggesting that k(s) for FAD on CNTs is faster than the measured value of 7.6 s(-1).

Activity, Selectivity, and Anion-Exchange Membrane Fuel Cell Performance of Virtually Metal-Free Nitrogen-Doped Carbon Nanotube Electrodes for Oxygen Reduction Reaction

Virtually metal-free, aligned nitrogen-doped carbon nanotubes (N-CNTs) are prepared by an alumina template technique and characterized using XRD, Raman spectroscopy, SEM, and XPS. Highly uniform and aligned nanotubes are identified from SEM. The surface nitrogen concentration in the N-CNTs is estimated to be ∼8.0 at.%. Electrocatalytic oxygen reduction reaction (ORR) activity in alkali media is investigated by rotating disk electrode (RDE) voltammetry and compared with commercial Pt/C (E-TEK). RDE measurements indicated the comparable ORR activity of nitrogen-doped carbon nanotubes with Pt/C. Kinetic analysis reveals that the ORR on nitrogen-doped carbon nanotubes follows the four-electron pathway leading to water. Moreover, during the ORR, nitrogen-doped carbon nanotubes exhibited substantially higher ethanol tolerance than Pt/C. A maximum power density of 37 and 62 mW/cm2 is observed with N-CNT and Pt/C cathodes, respectively, under anion-exchange membrane fuel cell conditions. Durability measurements performed under fuel cell operating conditions for 30 h indicate the good stability of the catalysts.

Indirect Electrocatalytic Degradation of Cyanide at Nitrogen-Doped Carbon Nanotube Electrodes

Nitrogen-doped carbon nanotube (N-CNT) mat electrodes exhibit high catalytic activity toward O(2) reduction, which can be exploited for the remediation of free cyanide (CN(-)). During the electrochemical O(2) reduction process, the hydroperoxide anion (HO(2)(-)) is formed and then reacts to chemically oxidize cyanide (CN(-)) to form cyanate (OCN(-)). The proposed electrochemical-chemical (EC) mechanism for CN(-) remediation at N-CNTs is supported by cyclic voltammetry and bulk electrolysis, and the formation of OCN(-) is confirmed via spectroscopic methods and electrochemical simulations. Our results indicate that by exploiting their catalytic behavior for O(2) reduction, N-CNTs can efficiently convert toxic CN(-) to the nontoxic OCN(-).

In Search of the Active Site in Nitrogen-Doped Carbon Nanotube Electrodes for the Oxygen Reduction Reaction

Nitrogen-doped carbon nanomaterials are known to exhibit good electrocatalytic activity for the oxygen reduction reaction (ORR). However, the structure of the ORR active site and optimum content of nitrogen in the carbon lattice for ORR activity remains unknown. In this study, a series of vertically aligned carbon nanotubes (VA-CNTs) with a surface nitrogen concentration of 0, 4.3, 5.6, 8.4, and 10.7 atom % is prepared by the alumina template technique and characterized with XRD, Raman spectroscopy, SEM, and XPS. Electrocatalytic ORR activity is investigated by rotating disk electrode (RDE) voltammetry. Among them, VA-CNTs with a nitrogen concentration of 8.4 atom % exhibited the best ORR performance. This is ascribed to a greater number of pyridinic-type nitrogen sites. The good performance of less expensive nitrogen-doped CNTs makes the ORR electrodes a viable alternative to platinum for energy conversion device applications.

Synergistic Assembly of Dendrimer-Templated Platinum Catalysts on Nitrogen-Doped Carbon Nanotube Electrodes for Oxygen Reduction

A study of the synergistic tuning of nitrogen-doped carbon nanotubes (NCNTs) as support- and size-monodisperse platinum nanoparticles templated from G4-NH2 dendrimers (Pt-DEN's) as catalysts targeted toward oxygen reduction is reported. UV-vis spectroscopy, adsorption isotherms, TGA, TEM, and voltammetry were used to characterize the loading and activity of Pt-DENs immobilized on CNT and NCNT supports. The facile uptake of Pt-DENs was found to be influenced by the number of edge plane sites on the NCNT support with higher adsorption rates observed for NCNTs with increased nitrogen content. Pt-DEN/NCNT composites exhibit high activity with a mass-transport-limited current density and mass activity of 2.3 mA cm(-2) and 0.05 mA g(-1), respectively, for the oxygen reduction reaction (ORR).