Electrochemical property and electroanalytical application of large mesoporous carbons

This article reports on the electrochemical property of large mesoporous carbons (LMC) synthesized using nano-CaCO₃ as a template and sucrose as a carbon precursor. Though a simple preparation method for LMC, the LMC modified glassy carbon electrode (LMC/GCE) exhibited good electrochemical activity for some common electroactive compounds, such as ascorbic acid (AA), uric acid (UA), epinephrine (EP) and tryptophan (Trp). The unique properties of LMC were also compared with those of carbon nanotubes (CNT), and the results showed that the LMC possessed a much better property than CNT. In addition, the LMC/GCE was also used to analyze the β-nicotinamide adenine dinucleotide (NADH) and hydrogen peroxide (H₂O₂) and showed a high sensitivity and low detection limit. Based on a simple preparation method and good electrochemical property, the LMC represents a new class of carbon electrode for electrochemical sensor applications.     

A Biofuel Cell Based on Biocatalytic Reactions of Glucose on Both Anode and Cathode Electrodes

Biofuel cells based on electrocatalytic oxidation of NADH and reduction of H₂O₂ have been prepared using carbon fiber electrodes functionalized with graphene nano‐flakes. The electrochemical oxidation of NADH was catalyzed by Meldola's blue (MB), while the reduction of H₂O₂ was catalyzed by hemin, both catalysts were adsorbed on the graphene flakes due to their π‐π staking. In the next set of experiments, the MB‐ and hemin‐electrodes were additionally modified with glucose dehydrogenase (GDH) and glucose oxidase (GOx), respectively. The enzyme catalyzed reactions in the presence of glucose, NAD⁺ and O₂ resulted in the production of NADH and H₂O₂ in situ. The produced NADH and H₂O₂ were oxidized and reduced, respectively, at the bioelectrocatalytic electrodes, thus producing voltage and current generated by the biofuel cell. The enzyme‐based biofuel cells operated in a human serum solution modelling an implantable device powered from the natural biofluid. Finally, two enzyme‐based biofuel cell connected in series and operating in the serum solution produced electrical power sufficient for activation of an electronic watch used as an example device.     

Direct Electrochemistry of Hemoglobin and its Electrocatalysis Based on a Carbon Nanotube Paste Electrode

A new hemoglobin (Hb) and carbon nanotube (CNT) modified carbon paste electrode was fabricated by simply mixing the Hb, CNT with carbon powder and liquid paraffin homogeneously. To prevent the leakage of Hb from the electrode surface, a Nafion film was further applied on the surface of the Hb‐CNT composite paste electrode. The modified electrode was characterized by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Direct electrochemistry of hemoglobin in this paste electrode was easily achieved and a pair of well‐defined quasi‐reversible redox peaks of a heme Fe(III)/Fe(II) couple appeared with a formal potential (E^0′) of ?0.441 V (vs. SCE) in pH 7.0 phosphate buffer solution (PBS). The electrochemical behaviors of Hb in the composite electrode were carefully studied. The fabricated modified bioelectrode showed good electrocatalytic ability for reduction of H₂O₂ and trichloroacetic acid (TCA), which shows potential applications in third generation biosensors.     

A Biofuel Cell Based on Biocatalytic Reactions of Lactate on Both Anode and Cathode Electrodes – Extracting Electrical Power from Human Sweat

Electrodes composed of carbon fibers were modified with graphene nano‐sheets in order to increase their surface area and facilitate electrochemical reactions. Electrocatalytic species, such as Meldola's blue (MB) and hemin were immobilized on the graphene surface due to their π‐π stacking and then used for electrocatalytic oxidation of NADH and reduction of H₂O₂, respectively. Further modification of these electrodes with enzymes producing NADH and H₂O₂ in situ (lactate dehydrogenase, LDH, and lactate oxidase, LOx, respectively), allowed assembling of a biofuel cell operating in the presence of lactate, oxygen and NAD⁺. The cathode of the biofuel cell required lactate and O₂ for its operation, while the anode operated in the presence of lactate and NAD⁺. Notably, both bioelectrocatalytic electrodes operated in the presence of lactate, one producing H₂O₂ in the reaction catalyzed by LOx in the presence of O₂, second producing NADH in the reaction catalyzed by LDH in the presence of NAD⁺. Both reactions were performed in the biofuel cell without separation of the cathodic and anodic solutions and with no need of a membrane. The biofuel cell was tested in solutions mimicking human sweat and then in real human sweat samples, demonstrating substantial power release being able to activate electronic devices.     

Electrochemical Fabrication of Prussian Blue Nanocube‐decorated Electroreduced Graphene Oxide for Amperometric Sensing of NADH

In this study, Prussian blue (PB) film on the electroreduced graphene oxide (ERGO)‐modified Au electrode surface (ERGO/PB) is easily prepared by means of cyclic voltammetric technique in the mixture of K₃Fe(CN)₆ and FeCl₃. Its electrochemical behaviors for NADH biosensor are studied. The structural and morphological characters of modified electrode material are analyzed with using of XPS, XRD, Raman, EDS, and SEM techniques. ERGO/PB hybrid nanocomposite for NADH biosensor is exhibited to the higher catalytic effect (linear range from 1.0 to 100 μM, detection limit of 0.23 μM at S/N=3) compared to naked Au, ERGO‐modified Au, and PB‐modified Au electrodes. In addition to, ERGO/PB electrode was used to voltammetric and amperometric detection of H₂O₂. ERGO/PB electrodes also showed the same behavior as the NADH sensor. This ERGO/PB‐modified electrode supplied a simple, new, and low‐cost route for amperometric sensing of both NADH and H₂O₂.     

Effects of CNT‐film Pretreatment on the Characteristics of NiCo2O4/CNT Core‐shell Hybrids as Electrode Material for Electrochemistry Capacitor

NiCo₂O₄/CNT nanocomposite films were fabricated by in‐situ growing ultrafine NiCo₂O₄ nanoparticles on acid‐modified carbon nanotube (CNT) films. The effects of CNT‐film pretreatment were investigated thoroughly by various characterization outfits including Fourier Transform Infrared spectroscopy (FT‐IR), X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, RTS‐9 four‐point probes resistivity measurement system, X‐ray powder diffraction (XRD), scanning electron microscopy (SEM) and CHI660D electrochemical workstation. These results suggested that carbon nanotubes were uniformly wrapped by NiCo₂O₄ nanoparticles forming a hierarchical core‐shell structure. And the crystallinity, conductivity of the CNTs and detail structure (both morphology and size) of the NiCo₂O₄ nanoparticles varied with prolonged acid treatment time which resulted in increased functional groups and defects on CNT films and further affected the electrochemical properties. The composite film composed of the CNT film pretreated by mixed acid for 12 h exhibited excellent electrochemical properties: 828 F/g at 1 A/g and 656 F/g at 20 A/g, and maintained over 99 % of its capacitance after 3000 cycles of charge/discharge at 5 A/g. Acid treatment for either too long or too short is detrimental to the electrochemical properties of the composite films. Such work should be of fundamental importance for tailoring electrochemical properties by elaborate design of acid treatment on CNTs.     

Magnetic loading of carbon nanotube/nano-Fe3O4 composite for electrochemical sensing

An electrochemical sensing platform was developed based on the magnetic loading of carbon nanotube (CNT)/nano-Fe₃O₄ composite on electrodes. To demonstrate the concept, nano-Fe₃O₄ was deposited by the chemical coprecipitation of Fe²⁺ and Fe³⁺ in the presence of CNTs in an alkaline solution. The resulting magnetic nanocomposite brings new capabilities for electrochemical devices by combining the advantages of CNT and nano-Fe₃O₄ and provides an alternative way for loading CNT on electrodes. The fabrication and the performances of the magnetic nanocomposite modified electrodes have been described. Cyclic voltammetry (CV) and constant potential measurement indicated that the incorporated CNT exhibited higher electrocatalytic activity toward the redox processes of hydrogen peroxide. In addition, chitosan (CTS) has also been introduced into the bulk of the CNT/nano-Fe₃O₄ composite by coprecipitation to immobilize glucose oxidase (GOx) for sensing glucose. The marked electrocatalytic activity toward hydrogen peroxide permits effective low-potential amperometric biosensing of glucose, in connection with the incorporation of GOx into CNT/Fe₃O₄/CTS composite. The accelerated electron transfer is coupled with surface renewability. TEM images and XRDs offer insights into the nature of the magnetic composites. The concept of the magnetic loading of CNT nanocomposites indicates great promise for creating CNT-based biosensing devices and expands the scope of CNT-based electrochemical devices.     

An Electrochemical Sensor Fabricated by Sonochemical Approach for Determination of the Antipsychotic Drug Haloperidol

A nanocomposite (Ho₂O₃NPs/BNT) was synthesized by decorating holmium(III)oxide nanoparticles (H₂O₃NPs) on bentonite (BNT) through a realizable sonochemical approach for the electrochemical detection of haloperidol (Hlp). A glassy carbon electrode was modified with this nanocomposite. The Ho₂O₃NPs/BNT modified electrode outperformed bare and other modified electrodes in terms of electrochemical performance for Hlp detection in a pH 8.0 phosphate buffer. The proposed electrochemical platform showed a wide linear range (0.01 μM–24 μM), low detection limit (2.4 nM), and high sensitivity by square wave voltammetry. In addition, the proposed electrochemical sensor met the clinical criteria in terms of stability, selectivity, and repeatability.     

Electroanalytical Study of Prussian Blue Modified Glassy Carbon Paste Electrodes

Two types of glassy carbon (GC) powder (i.e., Sigradur K and Sigradur G) have been mixed with mineral oil to obtain glassy carbon paste electrodes (GCPE's). The electrochemical behavior of such electrodes at different percentages of glassy carbon has been evaluated with respect to the electrochemistry of ferricyanide as revealed with cyclic voltammetry and the best paste composition was chosen. GC was then modified with Prussian Blue (PB), mixed at different percentages with unmodified GC and with a fixed amount of mineral oil in order to obtain PB modified glassy carbon paste electrodes (PB‐GCPE's). PB‐GCPE's with different percentages of GC modified with PB (PB‐GC) were compared and the dependence on the amount of PB on their performances was evaluated by studying the parameters of cyclic voltammetry (i.e., current peak, ΔE_p, anodic and cathodic current ratio, charge density) and the amperometric response to H₂O₂. Data interpretation based on the GC surface area is presented. GCPE's with a selected amount of PB‐GC were then tested as H₂O₂ probes and all the analytical parameters together with the dependence on pH were evaluated. Some preliminary experiments with these electrodes assembled as glucose, lysine and lactate biosensors are also reported.     

Direct Electron Transfer and Bioelectrocatalysis by a Hexameric,Heme Protein at Nanostructured Electrodes

A nanohybrid consisting of poly(3‐aminobenzenesulfonic acid‐co‐aniline) and multiwalled carbon nanotubes [MWCNT‐P(ABS‐A)]) on a gold electrode was used to immobilize the hexameric tyrosine‐coordinated heme protein (HTHP). The enzyme showed direct electron transfer between the heme group of the protein and the nanostructured surface. Desorption of the noncovalently bound heme from the protein could be excluded by control measurements with adsorbed hemin on aminohexanthiol‐modified electrodes. The nanostructuring and the optimised charge characteristics resulted in a higher protein coverage as compared with MUA/MU modified electrodes. The adsorbed enzyme shows catalytic activity for the cathodic H₂O₂ reduction and oxidation of NADH.     

Modified electrodes with Keggin-type silicotungstates and poly(brilliant cresyl blue)

Electrosynthesis of poly(brilliant cresyl blue) in aqueous solution in the presence of Keggin-type polyoxotungstates, [SiW₁₁Fe(H₂O)O₃₉]^5? or [SiW₁₁Co(H₂O)O₃₉]^6?, was used to prepare modified glassy carbon electrodes. The deposited hybrid organic/inorganic films were studied and characterised by cyclic voltammetry and electrochemical impedance spectroscopy. Cyclic voltammetry showed that the electrochemical features of the polyoxoanions were maintained after immobilisation, with the first tungsten reduction peak involving the uptake of protons from the solution. The chemically modified electrodes were stable, and their preparation was easy to perform. The results provide valuable information for exploring future applications of these films in electrochemical sensors or electrocatalysis.     

The effects of carbon nanotubes on the electrocatalysis of hydrogen peroxide by metallo-phthalocyanines

The pre-grafted screen-printed gold electrode modified with phenyl-amino monolayer was investigated for covalent immobilization of phenyl-amine functionalized single-walled carbon nanotubes (PA-SWCNT) and metal tetra-amino phthalocyanine (MTAPc) using Schiff-base reactions with benzene-1,4-dicarbaldehyde (BDCA) as cross-linker. The PA-SWCNT and MTAPc modified electrodes were applied as hybrids for electrochemical sensing of H₂O₂. The step-by-step fabrication of the electrode was followed using electrochemistry, impedance spectroscopy, scanning electron microscopy and Raman spectroscopy and all these techniques confirmed the fabrication and the immobilization of PA-SWCNT, MnTAPc and CoTAPc onto gold surfaces. The apparent electron transfer constant (k_app) showed that the carbon nanotubes and metallo-phthalocyanines hybrids possess good electron transfer properties compared to the bare, pre-grafted and the MTAPc modified gold electrode surfaces without PA-SWCNT. The electrochemical sensing of hydrogen peroxide was successful with PA-SWCNT-MTAPc hybrid systems showing higher electrocatalytic currents compared to the other electrodes. The analytical parameters obtained using chronoamperometry gave good linearity at H₂O₂ concentrations ranging from 1.0 to 30.0 μmol L⁻¹. The values for the limit of detection (LoD) were found to be of the orders of 10⁻⁷ M using the 3δ for all the electrodes. The PA-SWCNT-MTAPc modified SPAuEs were much more sensitive compared to PA-MTAPc modified SPAuEs.     

Analytical sensing of hydrogen peroxide on Ag nanoparticles–multiwalled carbon nanotube-modified glassy carbon electrode

A sensor based on silver nanoparticles (NPs)/multiwalled carbon nanotube (CNT)-modified glassy carbon electrode (GCE) was prepared and employed for accurate and rapid determination of hydrogen peroxide (H₂O₂). In summary, by using a mechanochemical method, multiwalled CNTs dispersed in ethanol and used for modification of GCE. After that, by using a double-pulse technique, silver NPs are electrodeposited on surface of multiwalled CNTs/ GCE. Parameters that are affected by electrocatalytic reduction of H₂O₂ on the modified electrode, such as multiwalled CNT concentration and double-pulse parameters, were optimized using Minitab software. The optimal modified electrode was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and cyclic voltammetry. The proposed H₂O₂ sensor exhibited excellent characteristics for the sensing of H₂O₂, such as wide linear range from 0.1 to 10 mM, a low detection limit of 2 μM, high repeatability, and no interference by a number of substances.     

An Electrochemical Nanosensor for Monitoring the Dynamics of Intracellular H2O2 Upon NADH Treatment

Reactive oxygen species (ROS)-based therapeutic strategies play an important role in cancer treatment. However, in situ, real-time and quantitative analysis of intracellular ROS in cancer treatment for drug screening is still a challenge. Herein we report one selective hydrogen peroxide (H₂O₂) electrochemical nanosensor, which is prepared by electrodeposition of Prussian blue (PB) and polyethylenedioxythiophene (PEDOT) onto carbon fiber nanoelectrode. With the nanosensor, we find that the level of intracellular H₂O₂ increases with NADH treatment and that increase is dose-dependent to the concentration of NADH. High-dose of NADH (above 10 mM) can induce cell death and intratumoral injection of NADH is validated for inhibiting tumor growth in mice. This study highlights the potential of electrochemical nanosensor for tracking and understanding the role of H₂O₂ in screening new anticancer drug.     

Electrochemical Biosensing for Cancer Cells Based on TiO2/CNT Nanocomposites Modified Electrodes

Both TiO₂ nanoparticles and carbon nanotubes have been usually utilized to modify the electrodes to enhance the detection sensitivity of biomolecular recognition. In this research, novel TiO₂/CNT nanocomposites have been prepared and doped on the carbon paper as the modified electrodes. Subsequently, the redox behavior of the ferricyanide probe and the surface properties of the cancer cells coated on the modified electrodes have been investigated by using electrochemical and contact angle measurements. Compared with electrochemical signals on bare carbon paper and nanocomposite modified substrates, the significantly enhanced electrochemical signals on the modified electrodes covered with cancer cells have been observed. Meanwhile, different leukemia cells (i.e., K562/ADM cells and K562/B.W. cells) could be also recognized because of their different electrochemical behavior and hydrophilic/hydrophobic features on the modified electrodes due to the specific components on the plasma membranes of the target cells. This new strategy may have potential application in the development of the biocompatible and multi‐signal responsive biosensors for the early diagnosis of cancers.     

The Interaction of Iodide Film with Platinum Microparticles on Different Electrode Materials for Various Electrocatalytic Reactions

The electrochemical depositions of Pt microparticles and KI film were successfully carried out on glassy carbon electrodes (GCE), gold electrodes (GE), and indium tin oxide electrodes (ITO). The electrochemical studies of Pt micro/KI film on GCE show that the film was stable, active at pH 1.5 electrolyte solutions. The Pt microparticle/KI film modified ITO electrodes were examined by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The SEM and AFM results show that the Pt particle sizes were in the range of 120 nm–1.4 μm, respectively. The proposed film on GCE shows efficient electrocatalysis for oxygen, Cr₂O reduction by using cyclic voltammetry. Further the electrochemical oxidation of sodium meta‐arsenite (As(III)), H₂O₂ were successfully carried out and the detection of H₂O₂ in real samples has been validated.     

Electrodes modified with iron porphyrin and carbon nanotubes: application to CO2 reduction and mechanism of synergistic electrocatalysis

Electrodes modified with iron porphyrin and carbon nanotubes (FeP–CNTs) were prepared and used for CO₂ electroreduction. The adsorption of iron porphyrin onto the multiwalled carbon nanotubes was characterized by scanning electron microscopy and ultraviolet and visible spectroscopy. The electrochemical properties of the modified electrodes for CO₂ reduction were investigated by cyclic voltammetry and CO₂ electrolysis. The FeP–CNT electrodes exhibited less negative cathode potential and higher reaction rate than the electrodes modified only with iron porphyrin or carbon nanotubes. A mechanism of the synergistic catalysis was proposed and studied by electrochemical impedance spectroscopy and electron paramagnetic resonance. The direct electron transfer between iron porphyrin and carbon nanotubes was examined. The current study shed light on the mechanism of synergistic catalysis between CNTs and metalloporphyrin, and the iron porphyrin–CNT-modified electrodes showed great potential in the efficient CO₂ electroreduction.     

NADH Oxidation Catalyzed by Electropolymerized Azines on Carbon Nanotube Modified Electrodes

Electropolymerizing azines on a carbon nanotube (CNT) modified electrode yields a high‐surface area interface with excellent electrocatalytic activity towards NADH oxidation. Electrodeposition of poly(methylene green) (PMG) and poly(toluidine blue) (PTBO) on the carboxylated CNT‐modified electrodes was achieved by cyclic voltammetry. The PMG‐CNT interface demonstrates 5.0 mA cm^?2 current density for NADH oxidation at 50 mV vs. Ag|AgCl in 20 mM NADH solution. The kinetics of NADH electrocatalysis were analyzed using a quantitative mass‐transport‐corrected model with NADH bulk concentration and applied potential as independent variables. This high‐rate poly(azine)‐CNT interface is potentially applicable to high‐performance bioconversion, bioenergy and biosensors involving NADH‐dependent dehydrogenases.     

Synergistic effect of mediator–carbon nanotube composites for dehydrogenases and peroxidases based biosensors

Very sensitive, low cost and reliable NADH and H₂O₂ sensors were realised and used for development of enzyme based biosensors. The active surface of the electrodes was modified with a nanocomposite obtained by modification of SWNT with a proper mediator: Meldola Blue (for NADH) and Prussian Blue (for H₂O₂). Low applied potential of − 50 mV vs. Ag/AgCl reference electrode proved the synergistic effect of nanocomposite materials towards NADH and H₂O₂ detection. Biosensors for malic acid and alkylphenols have been developed, using mediator-functionalised-SWNT-based electrodes and two different classes of enzymes: NAD⁺-dependent dehydrogenases and peroxidases. Immobilization of the enzymes was realised using a series of different procedures — adsorption, Nafion membrane, sol–gel and glutaraldehyde, in order to find the best configuration for a good operational stability. A higher sensitivity comparing with other reported biosensors of about 12.41 mA/M·cm² was obtained for l-malic acid biosensor with enzyme immobilised in Nafion membrane. Phenol, 4-t-octylphenol and 4-n-nonylphenol were used as standard compounds for HRP based biosensor. Fast biosensor response and comparable detection limit with HPLC methods were achieved.     

Direct electrochemistry and electrocatalysis of heme proteins on SWCNTs-CTAB modified electrodes

Direct electrochemistry and electrocatalysis of heme proteins including hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP) were studied with the protein incorporated single walled carbon nanotubes (SWCNTs)-cetylramethylammonium bromide (CTAB) nanocomposite film modified glassy carbon electrodes (GCEs). The incorporated heme proteins were characterized with Fourier transform infrared spectroscopy (FTIR), ultraviolet visible (UV) spectroscopy, atomic force microscopy (AFM) and electrochemistry, indicating the heme proteins in SWCNTs-CTAB nanocomposite films keep their secondary structure similar to their native states. The direct electron transfer between the heme proteins in SWCNTs-CTAB films and GCE was investigated. The electrochemical parameters such as formal potentials and apparent heterogeneous electrontransfer rate constants (k_s) were estimated by square wave voltammetry with nonlinear regression analysis. The heme protein-SWCNT-CTAB electrodes show excellent electrocatalytic activities for the reduction of H₂O₂ and NO₂⁻, which have been utilized to determine the concentrations of H₂O₂ and NO₂⁻.