Carbon felt-based bioelectrocatalytic flow-through detectors: Highly sensitive amperometric determination of H2O2 based on a direct electrochemistry of covalently modified horseradish peroxidase using cyanuric chloride as a linking agent

Horseradish peroxidase (HRP) was chemically modified using cyanuric chloride (CC) as a linking agent onto a carbon felt (CF), which is a microelectrode ensemble of micro carbon fiber (>7 μm, diameter) with a random three-dimensional structure. The resulting HRP-modified CF (HRP-ccCF) exhibited well-defined redox waves based on the HRP heme Fe^III/Fe^II redox couple at −0.23 V vs. Ag/AgCl (at pH 7.0), while the HRP-adsorbed CF (HRP-CF) showed no apparent redox couple in the same potential range, indicating that the chemical modification of HRP via CC facilitated the direct electron transfer (DET) between HRP and CF. The apparent heterogeneous electron transfer rate constant k_s was estimated to be 35 s⁻¹. Cyclic voltammetry and electrochemical impedance spectroscopy revealed that the interfacial properties (i.e., structure, morphology of enzyme-layer) of covalently modified HRP (HRP-ccCF) and physically adsorbed HRP (HRP-CF) are different, resulting in the difference in the electron transfer properties. The HRP-ccCF was successfully used as a working electrode unit in bioelectrocatalytic flow-through detector for highly sensitive amperometric determination of H₂O₂. Under the optimized conditions (i.e., applied potential, 0 V vs. Ag/AgCl; carrier flow rate, 3.25 ml/min; and carrier pH 7.0), the cathodic peak current of H₂O₂ linearly increased up to 3 μM (sensitivity, 1.94 μA/μM; the detection limit, 0.08 μM [S/N = 3]) with sample through-put of ca. 90 samples/h.     

Electrochemical genosensor based on modified octadecanethiol self-assembled monolayer for Escherichia coli detection

An electrochemical genosensor based on 1-fluoro-2-nitro-4-azidobenzene (FNAB) modified octadecanethiol (ODT) self-assembled monolayer (SAM) has been fabricated for Escherichia coli detection. The results of electrochemical response measurements investigated using methylene blue (MB) as a redox indicator reveal that this nucleic acid sensor has 60 s of response time, high sensitivity (0.5 × 10⁻¹⁸ M) and linearity as 0.5 × 10⁻¹⁸-1 × 10⁻⁶ M. The sensor has been found to be stable for about four months and can be used about ten times. It is shown that water borne pathogens like Klebsiella pneumonia, Salmonella typhimurium and other gram-negative bacterial samples has no significant effects in the response of this sensor.     

Electrochemical behavior of graphene doped carbon paste electrode and its application for sensitive determination of ascorbic acid

In present paper, the graphene doped carbon paste electrode (CPE) was firstly prepared with the addition of graphene into the carbon paste mixture. Compared with conventional CPE, an improved electrochemical response of graphene doped CPE toward the redox couple of Fe(CN)₆^3−/4− was demonstrated owing to the excellent electrical conductivity of graphene. The graphene doped CPE was further used for the successful determination of ascorbic acid (AA), and it showed an excellent electrocatalytic oxidation activity toward AA with a lower overvoltage, pronounced current response, and good sensitivity. Under the optimized experimental conditions, the proposed electrochemical AA sensor exhibited a rapid response to AA within 5 s and a linear calibration plot ranged from 1.0 × 10⁻⁷ to 1.06 × 10⁻⁴ M was obtained with a detection limit of 7.0 × 10⁻⁸ M.     

Facile synthesis of NiO nanoflowers and their electrocatalytic performance

In this paper, hierarchically structured NiO nanoflowers were facile synthesized by incorporating a convenient solution process with a subsequent thermal treating process. Their catalytic activity was then electrochemically investigated in detail. The NiO nanoflower modified biosensor exhibits excellent sensing performance for the determination of l-ascorbic acid with a response time less than 8 s, linear range between 0.005 and 3.5 mM, and sensitivity as 220.4 μA mM⁻¹ cm⁻². Besides, a high selectivity towards the oxidation of AA in the presence of dopamine (DA) and nitrite was also observed at their maximum physiological concentrations. The good analytical performance, long-term stability, low cost and straightforward fabrication process made the NiO nanomaterials promising for the development of effective electrochemical sensors for a wide range of potential applications in medicine, biotechnology and environmental chemistry.     

Graphene-polyaniline composite film modified electrode for voltammetric determination of 4-aminophenol

An electrochemical sensor based on graphene-polyaniline (GR-PANI) nanocomposite for voltammetric determination of 4-aminophenol (4-AP) is presented. The electrochemical behavior of 4-AP at the GR-PANI composite film modified glassy carbon electrode (GCE) was investigated by cyclic voltammetry. 4-AP exhibits enhanced voltammetric response at GR-PANI modified GCE. This electrochemical sensor shows a favorable analytical performance for 4-AP detection with a detection limit of 6.5 × 10⁻⁸ M and high sensitivity of 604.2 μA mM⁻¹. Moreover, 4-AP and paracetamol can be detected simultaneously without interference of each other in a large dynamic range.     

Voltammetric studies of the interaction of rutin with DNA and its analytical applications on the MWNTs-COOH/Fe3O4 modified electrode

Multi-walled carbon nanotubes functionalized with a carboxylic acid group (MWNTs-COOH)/iron oxide (Fe₃O₄) modified glassy carbon electrode (MWNTs-COOH/Fe₃O₄/GCE) and DNA/MWNTs-COOH/Fe₃O₄/GCE were prepared. The electrochemical behaviors of rutin (RU) were investigated on MWNTs-COOH/Fe₃O₄/GCE by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in britton-robinson buffer solution (B-R). The interaction of RU with DNA was also explored. Dramatic decrease of peak current without obvious peak potential shift were observed in both cases of DNA in the solution and immobilized on the electrode surface. In addition, the electron transfer coefficient (α) and the rate constant (k_s) kept unchanged in the absence and presence of DNA. So interaction of DNA with RU formed a non-electroactive complex. The binding constant and binding ratio was obtained in the process. The interaction was also confirmed by UV-visible spectroscopy. The reduction peak current was linear with the concentration of RU in the range of 2.50 × 10⁻⁸ to 1.37 × 10⁻⁶ M, with a detection limit of 7.5 nM. The MWNTs-COOH/Fe₃O₄/GCE showed comparatively low detection limit, rapid response, simplicity for the determination of RU.     

Electrochemical behaviors and determination of isoniazid at ordered mesoporous carbon modified electrode

In this work, an electrochemical sensor based on ordered mesoporous carbon (OMC) for the amperometric detection of isoniazid was developed. OMC was dispersed in a solution of Nafion, and the suspension was modified onto the surface of glassy carbon (GC) electrode. Cyclic voltammetry and amperometry were used to investigate the electrochemical behaviors of isoniazid on Nafion-OMC modified electrode (Nafion-OMC/GC). The results indicate that OMC can facilitate the electrochemical oxidation of isoniazid with a great decrease of overpotential in pH 7.0 phosphate buffer solution. The proposed biosensor provides excellent performance towards the determination of isoniazid with a high sensitivity of 0.031 μA/μM, a low detection limit of 8.4 × 10⁻⁸ M and wide linear range from 1.0 × 10⁻⁷ M to 3.7 × 10⁻⁴ M at +0.20 V vs. Ag/AgCl. The method was successfully applied to the determination of isoniazid tablets with satisfying results. All the results suggest that Nafion-OMC/GC electrode is a potential candidate for a stable and efficient electrochemical sensor to detect isoniazid.     

Sensitive electrochemical determination of acetaminophen in pharmaceutical formulations at multiwalled carbon nanotube-alumina-coated silica nanocomposite modified electrode

A highly sensitive electrochemical sensor for the determination of acetaminophen at the multiwalled carbon nanotube-alumina-coated silica (MWCNT-ACS) nanocomposite modified glassy carbon electrode is reported. The morphology of the MWCNT-ACS nanocomposite was characterized by field emission scanning electron microscopy. The electrocatalytic properties of the MWCNT-ACS nanocomposite modified glassy carbon electrode were characterized by cyclic voltammetry and square-wave voltammetry in the presence of acetaminophen. The MWCNT-ACS nanocomposite modified glassy carbon electrode exhibited the abilities to raise the current response and to decrease the electrooxidation potential. In cyclic voltammetric responses, the oxidation peak current of acetaminophen obtained at the MWCNT-ACS modified glassy carbon electrode was 100 times greater than that of bare glassy carbon electrode. The MWCNT-ACS nanocomposite modified glassy carbon electrode for the determination of acetaminophen displayed a sensitivity of 376.5 A M⁻¹ cm⁻² and a detection limit of 0.05 μM using square-wave voltammetry. The analytical applicability of the developed method was achieved by analyzing the content of acetaminophen in five commercial drugs without pretreatment.     

Correlation between cell growth rate and glucose consumption determined by electrochemical monitoring

The electrochemical monitoring of glucose consumption is relevant for cell biology studies because of its wide detection range, high sensitivity and easy implementation. Whereas the glucose consumption and cell growth rate can be tightly correlated, they should also be cell population density dependent. In this work, we fabricated high sensitive enzyme electrodes for accurate monitoring of glucose consumption of cells in different growth stages. The performance of the fabricated device was firstly evaluated by cyclic voltammetry (CV) with p-benzoquinone (PBQ) as redox mediator, showing a linear response over a wide detection range (0.3-60 mM), a high sensitivity (1.61 ± 0.10 μA mM⁻¹ mm⁻² (n = 5)) and a low detection limit (80 μM). Then, daily glucose consumptions of NIH 3T3 cells in 24-well plates were determined for a period of 7 days. The results could be compared to the cell population growth curve, showing a close correlation but different behavior. We found that the increase of the glucose consumption took place prior the cell number increase but the glucose consumption per cell decreases linearly in the exponential growth stage of cells.     

Photoelectrocatalytic regeneration of NADH at poly(4,4′-diaminodiphenyl sulfone)/nano TiO2 composite film modified indium tin oxide electrode

Herein we report the photoelectrocatalytic regeneration of NADH at poly(4,4′-diaminodiphenyl sulfone)/nano TiO₂ (PDDS/TiO₂) composite modified indium tin oxide (ITO) electrode. The PDDS film growth was confirmed through in situ electrochemical quartz crystal microbalance (EQCM) studies. The prepared PDDS/TiO₂ composite was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) studies. SEM and AFM results confirmed that TiO₂ nanoparticles size is between 130 and 180 nm. XRD results showed that TiO₂ nanoparticles are crystalline and belong to anatase phase. Electrochemical impedance spectroscopy (EIS) and light induced EIS results substantiate a rapid electron transfer process at PDDS/TiO₂ composite surface. Cyclic voltammetry (CV) results demonstrated that composite film showed excellent response to the photoelectrocatalytic regeneration of NADH. The photoelectrocatalytic oxidation of NADH at composite film surface irradiated for 5 min (optimized irradiation time) produced a notable enhancement in anodic peak current and it was 18-fold higher than that of PDDS film and several folds higher than that of TiO₂ and bare ITO electrodes. Further, composite film showed higher sensitivity of 124.1 μA μM⁻¹ for NADH. From Square wave voltammetry (SWV) results, sensitivity of the irradiated composite film was obtained as 0.252 μA nM⁻¹ of NADH. The linear concentration range was between 23 and 39 nM NADH respectively. Further, the composite film exhibits good selectivity towards NADH and no significant interference effect was observed even when 200-fold excess of ascorbic acid (AA), dopamine (DA) and uric acid (UA) coexist in the same supporting electrolyte solution.     

Electrochemical studies of bovine serum albumin immobilization onto the poly-o-phenylenediamine and carbon-coated nickel composite film and its interaction with papaverine

A biosensor based on bovine serum albumin (BSA) and poly-o-phenylenediamine (PoPD)/carbon-coated nickel (C-Ni) nanobiocomposite film modified electrode has been developed to study the interaction of BSA with papaverine (PAP). The well-dispersed C-Ni nanoparticles were dripped onto the glassy carbon electrode (GCE) surface firstly, and PoPD films were subsequently electropolymerized by cyclic voltammetry (CV) to prepare PoPD/C-Ni/GCE. Finally, the BSA was easily immobilized on the PoPD films via electrostatic adsorption. The morphology and the electrochemical properties of the fabricated composite electrodes were examined by scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS), respectively. The interaction of PAP with BSA was monitored by differential pulse voltammetry (DPV), using PoPD as the electrochemical indicator. The binding constant (K), obtained by DPV, was 1.7 × 10⁴ L/mol, which was consistent with the fluorescence analysis. This constructed biosensor also exhibited a fine linear correlation with PAP concentration range of 2.5 × 10⁻⁹-4.5 × 10⁻⁵ mol/L and a detection limit of 8.3 × 10⁻¹⁰ mol/L was achieved by DPV.     

An amperometric glucose biosensor based on layer-by-layer GOx-SWCNT conjugate/redox polymer multilayer on a screen-printed carbon electrode

An amperometric glucose biosensor based on a multilayer made by layer-by-layer assembly of single-walled carbon nanotubes modified with glucose oxidase (GOx-SWCNT conjugates) and redox polymer (PVI-Os) on a screen-printed carbon electrode (SPCE) surface was developed. The SPCE surface was functionalized with a cationic polymer by electrodeposition of the PVI-Os, followed by alternating immersions in anionic GOx-SWCNT conjugate solutions and cationic PVI-O solutions. The purpose is to build a multilayer structure which is further stabilized through the electrodeposition of PVI-Os on the multilayer film. The electrochemistry of the layer-by-layer assembly of the GOx-SWCNT conjugate/PVI-Os bilayer was followed by cyclic voltammetry. The resultant glucose biosensor provided stable and reproducible electrocatalytic responses to glucose, and the electrocatalytic current for glucose oxidation was enhanced with an increase in the number of bilayers. The glucose biosensor displayed a wide linear range from 0.5 to 8.0 mM, a high sensitivity of 32 μA mM⁻¹ cm⁻², and a response time of less than 5 s. The glucose biosensor proved to be promising amperometric detectors for the flow injection analysis of glucose.     

Application of modified pyrolytic graphite electrode as a sensor in the simultaneous assay of adenine and adenosine monophosphate

A simple and sensitive method based on square wave voltammetry (SWV) at single-walled carbon nanotube (SWNT) modified edge plane pyrolytic graphite electrode (EPPGE) is proposed for the simultaneous determination of adenine and adenosine-5′-monophosphate (5′-AMP). The modified electrode exhibits remarkable electrocatalytic properties towards adenine and 5′-AMP oxidation with a peak potential of ∼850 and 1165 mV respectively. Linear calibration curves are obtained over the concentration range of 5-100 nM for adenine and 10-100 nM for 5′-AMP with sensitivity of 677 and 476 nA nM⁻¹ for adenine and 5′-AMP respectively. The limit of detection for adenine and 5′-AMP was found to be 37 × 10⁻¹⁰ M and 76 × 10⁻¹⁰ M, respectively. The effect of pH revealed that the oxidation of adenine and 5′-AMP at SWNT modified EPPGE involved equal number of electrons and protons. The modified electrode exhibited high stability and reproducibility.     

Electrocatalytic reduction of hydrogen peroxide and its determination in antiseptic and soft-glass cleaning solutions at phosphotungstate-doped-glutaraldehyde-cross-linked poly-l-lysine film electrodes

The present work describes the electrocatalytic behavior of phosphotungstate-doped glutaraldehyde-cross-linked poly-l-lysine (PLL-GA-PW) film electrode towards reduction of hydrogen peroxide (H₂O₂) in acidic medium. The modified electrode was prepared by means of electrostatically trapping the phosphotungstate anion into the cationic PLL-GA coating on glassy carbon electrode. The PLL-GA-PW film electrode showed excellent electrocatalytic activity towards H₂O₂ reduction in 0.1 M H₂SO₄. Under the optimized conditions, the electrochemical sensor exhibited a linear response for H₂O₂ concentration over the range 2.5 × 10⁻⁶ to 6.85 × 10⁻³ M with a sensitivity of 1.69 μA mM⁻¹. The curvature in the calibration curve at high concentration is explained in terms of Michaelis-Menten (MM) saturation kinetics, and the kinetics parameters calculated by three different methods were compared. The PLL-GA-PW film electrode did not respond to potential interferents such as dopamine, ascorbic acid and uric acid. This unique feature of PLL-GA-PW film electrode allowed selective determination of H₂O₂. Finally, the proposed electrochemical sensor was successfully applied to determine H₂O₂ in commercially available antiseptic solution and soft-contact lenses cleaning solution and the method has been validated using independent estimation by classical potassium permanganate titration method. Major advantages of the method are simple electrode fabrication, stability and high selectivity towards hydrogen peroxide.     

Fe2O3 modified thick films of nanostructured SnO2 powder consisting of hollow microspheres synthesized from pyrolysis of ultrasonically atomized aerosol for LPG sensing

Nanostructured hollow spheres of SnO₂ with fine nanoparticles were synthesized by ultrasonic atomization. Thick film gas sensors were fabricated by screen printing technique. Different surface modified films (Fe₂O₃ modified SnO₂) were obtained by dipping them into an aqueous solution (0.01 M) of ferric chloride for different intervals of time followed by firing at 500 °C. The structural and microstructural studies of the samples were carried out using XRD, SEM, and TEM. The sensing performance of pure and modified films was studied by exposing various gases at different operating temperatures. One of the modified sample exhibited high response (1990) to 1000 ppm of LPG at 350 °C. Optimum amount of Fe₂O₃ dispersed evenly on the surface_, adsorption and spillover of LPG on Fe₂O₃ misfits and high capacity of adsorption of oxygen on nanostructured hollow spheres may be the reasons of high response.     

High sensitive simultaneous determination of hydroquinone and catechol based on graphene/BMIMPF6 nanocomposite modified electrode

A novel nanocomposite, comprising of graphene sheet (GS) and ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆), was developed on the glassy carbon electrode (GCE) for the simultaneous determination of hydroquinone and catechol in 0.10 M acetate buffer solution (pH 5.0). At the GS/BMIMPF₆/GCE, both hydroquinone and catechol can cause a pair of quasi-reversible and well-defined redox peaks. In comparison with bare GCE and GS modified electrode, GS/BMIMPF₆/GCE showed larger peak currents, which was related to the higher specific surface area of graphene and high ionic conductivity of BMIMPF₆. Under the optimized condition, the cathodic peak current were linear over ranges from 5.0 × 10⁻⁷ M to 5.0 × 10⁻⁵ M for hydroquinone and from 5.0 × 10⁻⁷ M to 5.0 × 10⁻⁵ M for catechol, with the detection limits of 1.0 × 10⁻⁸ M and 2.0 × 10⁻⁸ M, respectively. The proposed method was successfully applied to the simultaneous determination of hydroquinone and catechol in artificial sample, and the results are satisfactory.     

Multifunctional mesoporous silica nanoparticles as sensitive labels for immunoassay of human chorionic gonadotropin

A novel method to construct amperometric immunosensor for human serum chorionic gonadotrophin (hCG) has been described. In this study, horseradish peroxidase (HRP), Pt nanoparticles and secondary antibody (Ab₂) modified MSN (Pt@MSN/HRP/Ab₂) was synthesized and the multifunctional MSN was used as label for the preparation of immunosensor. With the hCG primary antibody immobilized onto thionine/graphene modified glassy carbon electrode (GCE) via crosslinking with glutaraldehyde, the electrochemical immunosensor was able to realize a reliable determination of hCG in the range of 0.01-12 ng mL⁻¹ with a detection limit of 7.50 pg mL⁻¹. This immunoassay system has many desirable merits including high sensitivity, accuracy, and little instrumentation requirement. Significantly, the new method may be quite promising, with potentially broad applications for clinical immunoassays.     

An electrochemical biosensor for determination of hydrogen peroxide using nanocomposite of poly(methylene blue) and FAD hybrid film

An electrochemical biosensor for determination of hydrogen peroxide (H₂O₂) has been developed by the hybrid film of poly(methylene blue) and FAD (PMB/FAD). The PMB/FAD hybrid film was performed in PBS (pH 7) containing methylene blue and FAD by cyclic voltammetry. Repeatedly scanning potential range of −0.6-1.1 V, FAD was immobilized on the electrode surface by electrostatic interaction while methylene blue was electropolymerized on electrode surface. This modified electrode was found surface confined and pH dependence. It showed good electrocatalytic reduction for H₂O₂, KBrO₃, KIO₃, and NaClO as well as electrocatalytic oxidation for NADH. At an applied potential of −0.45 V vs. Ag/AgCl, the sensor showed a rapid and linear response to H₂O₂ over the range from 0.1 μM to 960 μM, with a detection limit of 0.1 μM and a significant sensitivity of 1109 μA mM⁻¹ cm⁻² (S/N = 3). It presented excellent stability at room temperature, with a variation of response current less than 5% over 30 days.     

Locked nucleic acids biosensor for detection of BCR/ABL fusion gene using benzoate binuclear copper (II) complex as hybridization indicator

Benzoate binuclear copper (II) complex, [Cu₂(C₇H₅O₂)₄(C₂H₆O)₂] (abbreviated as CuR₂) was prepared and its interaction with double-stranded salmon sperm DNA (dsDNA) in pH 7.4 phosphate buffer solution was studied by electrochemical experiments at the Au electrode (AuE). It was revealed that CuR₂ presented an excellent electrochemical activity on AuE and could bind with dsDNA by intercalation mode. The CuR₂ was further utilized as a new indicator in the fabrication of an electrochemical DNA biosensor for detection of BCR/ABL fusion gene. The biosensor based on nanogold (NG) modified AuE was developed by using thiolated-hairpin locked nucleic acids (LNA) as the capture probe for hybridization with BCR/ABL fusion gene. The results indicated this new method has excellent specificity for single-base mismatch and complementary after hybridization. The constructed electrochemical DNA biosensor achieved a detection limit of 1.0 × 10⁻¹⁰ M for complementary target DNA with a good stability.     

Impedimetric immunosensor for electronegative low density lipoprotein (LDL) based on monoclonal antibody adsorbed on (polyvinyl formal)-gold nanoparticles matrix

Monoclonal antibodies (MAb) have been commonly applied to measure LDL in vivo and to characterize modifications of the lipids and apoprotein of the LDL particles. The electronegative low density lipoprotein (LDL⁻) has an apolipoprotein B-100 modified at oxidized events in vivo. In this work, a novel LDL⁻ electrochemical biosensor was developed by adsorption of anti-LDL⁻ MAb on an (polyvinyl formal)-gold nanoparticles (PVF-AuNPs)-modified gold electrode. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to characterize the recognition of LDL⁻. The interaction between MAb-LDL⁻ leads to a blockage in the electron transfer of the [Fe(CN)₆]⁴⁻/K₄[Fe(CN)₆]³⁻ redox couple, which may could result in high change in the electron transfer resistance (R_CT) and decrease in the amperometric responses in CV analysis. The compact antibody-antigen complex introduces the insulating layer on the assembled surface, which increases the diameter of the semicircle, resulting in a high R_CT, and the charge transferring rate constant κ⁰ decreases from 18.2 × 10⁻⁶ m/s to 4.6 × 10⁻⁶ m/s. Our results suggest that the interaction between MAb and lipoprotein can be quantitatively assessed by the modified electrode. The PVF-AuNPs-MAb system exhibited a sensitive response to LDL⁻, which could be used as a biosensor to quantify plasmatic levels of LDL⁻.