Alcohol dehydrogenase amperometric biosensor based on a colloidal gold-carbon nanotubes composite electrode

A novel ethanol biosensor based on the bulk incorporation of alcohol dehydrogenase (ADH) into a colloidal gold (Au_coll)-multiwalled carbon nanotubes (MWCNTs) composite electrode using Teflon as binding material is reported. The composite Au_coll-MWCNTs-Teflon electrode exhibited significantly improved electrooxidation of NADH when compared with other carbon composite electrodes, including those based on carbon nanotubes. Amperometric measurements for NADH at +0.3 V showed significant differences in sensitivity between Au_coll-MWCNTs-Teflon and MWCNTs-Teflon composite electrodes. Incorporation of ADH into the bulk electrode material allowed the construction of a mediatorless ethanol biosensor. Both the enzyme loading and the NAD⁺ concentration in solution were optimized. The ADH-Au_coll-MWCNTs-Teflon biosensor allowed a limit of detection for ethanol of 4.7 μmol l⁻¹, which is remarkably better than those reported for other CNTs-based ADH biosensors. The apparent Michaelis-Menten constant was 4.95 mmol l⁻¹, which is much lower than that reported by immobilization of ADH onto a gold electrode. Both repeatability of the ethanol amperometric measurements, reproducibility with different biosensors, lifetime and storage ability can be, in general, advantageously compared with other ADH-CNTs biosensors. The biosensor was applied for the rapid determination of ethanol in commercial and certified beer samples.     

Amperometric bienzyme glucose biosensor based on carbon nanotube modified electrode with electropolymerized poly(toluidine blue O) film

The amperometric bienzyme glucose biosensor utilizing horseradish peroxidase (HRP) and glucose oxidase (GOx) immobilized in poly(toluidine blue O) (PTBO) film was constructed on multi-walled carbon nanotube (MWNT) modified glassy carbon electrode. The HRP layer could be used to analyze hydrogen peroxide with toluidine blue O (TBO) mediators, while the bienzyme system (HRP + GOx) could be utilized for glucose determination. Glucose underwent biocatalytic oxidation by GOx in the presence of oxygen to yield H₂O₂ which was further reduced by HRP at the MWNT-modified electrode with TBO mediators. In the absence of oxygen, glucose oxidation proceeded with electron transfer between GOx and the electrode mediated by TBO moieties without H₂O₂ production. The bienzyme electrode offered high sensitivity for amperometric determination of glucose at low potential, displaying Michaelis-Menten kinetics. The bienzyme glucose biosensor displayed linear response from 0.1 to 1.2 mM with a sensitivity of 113 mA M⁻¹ cm⁻² at an applied potential of −0.10 V in air-saturated electrolytes.     

Electrochemical study of rat brain acetylcholinesterase inhibition by chlorofos: Kinetic aspects and analytical applications

An electrochemical sensor was applied for investigating the immobilized rat brain acetylcholinesterase inhibition by chlorofos. Two alternative routes were explored as response-generating reactions: (i) direct electrochemical oxidation of thiocholine produced upon acetylthiocholine enzymatic hydrolysis and (ii) reduction of the produced thiocholine with hexacyanoferrate (III), followed by hexacyanoferrate (II) electrochemical detection. The advantages of the direct way are simplicity and higher sensitivity compared to the indirect one, which however avoids the interferences because of the lower potential applied.Enzyme inhibition was identified as competitive, the increasing from 1.31 to 1.43 mmol L⁻¹ with chlorofos concentration in the range 0.2-1.0 mmol L⁻¹ and the maximal rate of the enzyme reaction remaining constant (I_max = 579.30 ± 5.71 μA) in the presence of chlorofos. The inhibition constant was calculated using the Dixon method (K_I = 10.07 mmol L⁻¹).The suppression of the acetylcholinesterase activity by the inhibitor, expressed as current decrease at a constant substrate concentration, was exploited for chlorofos quantification optimized by the design of experiments methodology. Optimal response was obtained for an acetylthiocholine concentration of 0.2 mmol L⁻¹, at 26 °C and pH 7.     

Low potential detection of glucose at carbon nanotube modified glassy carbon electrode with electropolymerized poly(toluidine blue O) film

A mediator glucose biosensor has been constructed by immobilizing glucose oxidase at electropolymerized poly(toluidine blue O) film on carbon nanotube modified glass carbon electrode. The toluidine blue O moieties served as redox mediators for enzymatic glucose oxidation and as polymeric network to maintain the biosensor activity. Great enhancement in current response was observed for the glucose biosensor. The detection potential could be decreased to −0.1 V (versus Ag|AgCl), where common interferences such as ascorbic acid, uric acid and acetamidophenol were not oxidized to cause interferences. The amperometric glucose biosensor offered a sensitivity of 14.5 mA M⁻¹ cm⁻² for the linear range of 1-7 mM.     

Direct electrochemistry and electrocatalysis of cytochrome c based on chitosan-room temperature ionic liquid-carbon nanotubes composite

A robust and effective composite film combined the benefits of room temperature ionic liquid (RTIL), chitosan (Chi) and multi-wall carbon nanotubes (MWNTs) was prepared. Cytochrome c (Cyt c) was successfully immobilized on glassy carbon electrode (GCE) surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Cyt c were investigated in detail. A pair of well-defined and quasi-reversible redox peaks of Cyt c was obtained in 0.1 mol L⁻¹ pH 7.0 phosphate buffer solution (PBS), indicating the Chi-RTIL-MWNTs film showed an obvious promotion for the direct electron transfer between Cyt c and the underlying electrode. The immobilized Cyt c exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0 × 10⁻⁶ to 2.6 × 10⁻⁴ mol L⁻¹, with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹ (S/N = 3). The apparent Michaelis-Menten constant (K_m) was calculated to be 0.45 ± 0.02 mmol L⁻¹. Moreover, the modified electrode displayed a rapid response (5 s) to H₂O₂, and possessed good stability and reproducibility. Based on the composite film, a third-generation reagentless biosensor could be constructed for the determination of H₂O₂.     

HRP biosensor based on sugar-lectin biospecific interactions for the determination of phenolic compounds

A layer-by-layer self-assembly of concanavalin A (Con A) and glycoprotein horseradish peroxidase (HRP) afforded multilayer thin films on the surface of a thiol-modifed gold electrode, through biospecific complexation of Con A and sugar residues in the glycoenzymes. The performance of the HRP biosensor is reported for the amperometric detection of phenolic compounds. The concentration of hydrogen peroxide and assembly conditions of the precursor film, such as pH, the ionic strength of the polyelectrolyte solutions and the number of assembled bilayers were investigated using catechol. With optimized conditions, the biosensor presented a linear response for catechol from 6.0 to 48.0 μmol l⁻¹, with a high sensitivity of 160 μmol⁻¹ l nA and a detection limit of 0.6 μmol l⁻¹. The response time of the biosensor for phenolic compounds was very short, reaching 95% of its maximum response in about 2 s. The differences in sensitivity observed for a series of phenolic substrates were discussed in terms of the stability of the oxidized phenolic compounds and the properties of substituents.     

Electrochemical study on cobalt film modified glassy carbon electrode and its application

A novel electroactive material for ascorbic acid (AA) determination was successfully prepared by plating/potential cycling method. The cobalt film was first deposited on the surface of glassy carbon electrode (GCE) in CoSO₄ solution by potential cycling, and then a cobalt film on the surface of GCE was activated by potential cycling in 0.1 mol L⁻¹ NaOH. The electrochemical performance of the resulted film (Co/GCE) and factors affecting its electrochemical activity were investigated by cyclic voltammetry and amperometry. This film electrode exhibited good electrocatalytic activity to the oxidation of AA. This biosensor had a fast response of AA less than 3 s and excellent linear relationships were obtained in the concentration range of 3 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 2 × 10⁻⁷ mol L⁻¹ (S/N = 3) under the optimum conditions. Moreover, the selectivity, stability and reproducibility of this biosensor were evaluated with satisfactory results.     

Glucose biosensor based on multiwalled carbon nanotubes grown directly on Si

An amperometric biosensor based on covalent immobilization of glucose oxidase (GOx) on multiwalled carbon nanotubes (MWCNTs) with potassium ferricyanide as the redox mediator was developed. The MWCNTs were grown directly on a layered structure of Co/Ti/Cr on a SiO₂/Si substrate by microwave-heated chemical vapor deposition. The mediator helps to shuttle the electrons between the immobilized GOx and the MWCNT electrode, therefore operating at a potential of 0.25 V vs. the saturated calomel electrode. This potential precludes the interfering compounds from oxidization. The sensitivity of biosensors to glucose was found to depend on the acid pretreatment and GOx reaction times. The steady-state response of the optimized biosensor exhibits a sensitivity of 20.6 μA mM⁻¹ cm⁻², a linear range of up to 8 mM, and a response time of <5 s.     

Organic phase PPO biosensor based on hydrophilic films of electropolymerized polypyrrole

We described herein, the construction of an organic phase enzyme electrode (OPEE) via polyphenol oxidase (PPO) entrapment within a hydrophilic polypyrrole film electrogenerated from on a new bispyrrolic derivative (1) containing a long hydrophilic spacer. The so-called “adsorption step procedure” was adopted for the preparation of the organic phase PPO biosensor. The amperometric detection of catechol was carried out in anhydrous chloroform at −0.2 V versus Ag/AgCl. The electroanalytical parameters of the biosensor strongly depend on its configuration and on the hydration state of the enzyme matrix. The best sensitivity obtained for catechol in chloroform was 15.6 mA M⁻¹ cm⁻².     

Investigation on the electro-oxidation of iodide in the room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate at platinum electrode

The oxidation behavior of iodide has been investigated by linear sweep voltammetry and cyclic voltammetry at a platinum electrode in the room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]). The experimental results showed that iodide yielded two oxidation peaks P_a1 and P_a2 in [C₄mim][BF₄], and both of P_a1 and P_a2 are diffusion-controlled. P_a1 yielded from iodide to triiodide at +0.58 V is a two-electron oxidation wave, and P_a2 yielded from triiodide to iodine at +1.00 V is one-electron oxidation wave. Linear relationships between I_pa1 and the concentration of iodide can be established from 0.45 to 7.2 mmol L⁻¹ with a detect limit of 0.3 mmol L⁻¹ by linear sweep voltammetry, and from 0.30 to 7.8 mmol L⁻¹ with a detect limit of 0.2 mmol L⁻¹ by differential pulse voltammetry. These methods can be used for simple, rapid determination of iodide in the crude [C₄mim][BF₄].     

A polyblend electrolyte (PVP/PEG+KI+I2) for dye-sensitized nanocrystalline TiO2 solar cells

A novel polyblend electrolyte consisting of KI and I₂ dissolved in a blending polymer of polyvinyl pyrrolidone (PVP) and polyethylene glycol (PEG) was prepared. The formation of ⁻I₃ in the polymer electrolyte was confirmed by X-ray photoelectron spectroscopy (XPS) characterization. Due to the coordinating and plasticizing effect by PVP, the ionic conductivity of the polyblend electrolyte is enhanced. The highest ionic conductivity of 1.85 mS cm⁻¹ for the polyblend electrolyte was achieved by optimizing the compositions as 40 wt.% PVP + 60 wt.% PEG + 0.05 mmol g⁻¹ I₂ + 0.10 mmol g⁻¹ KI. Based on the polyblend electrolyte, a DSSC with fill factor of 0.59, short-circuit density of 9.77 mA cm⁻², open-circuit voltage of 698 mV and light-to-electricity conversion efficiency of 4.01% was obtained under AM 1.5 irradiation (100 mW cm⁻²).     

Supercritical fluid extraction of lycopene from tomato juice and characterization of its antioxidation activity

A new method to extract lycopene from tomato juice using supercritical CO₂ as solvent and without the need to dry the raw material is presented. To conduct the extraction, the tomato juice was subjected to cycles of centrifugation followed by rinsing with absolute ethanol to partially remove the water present in the solid part of the juice. The influence of the temperature and pressure on the extraction efficiency and on the extract antioxidant activity was studied using a factorial experimental design. The extraction efficiency varied from 7.7% to 76.7% and only extraction temperature had a statically significant effect on the process. The reversed phase HPLC analysis showed that lycopene is the major compound of the extract. The extract that presented higher antioxidant activity was obtained at 40 °C and 350 bar with 12.7 mmol L⁻¹ Trolox/g of extract using the DPPH radical scavenging method and 61.3 mmol L⁻¹ Trolox/g of extract using the rubrene singlet oxygen quenching method.     

The use of silver solid amalgam electrode for voltammetric and amperometric determination of nitroquinolines

Solid amalgam electrodes represent a suitable alternative to mercury electrodes due to their similar electrochemical properties and non-toxicity of the amalgam material. Nitro derivatives of quinoline have been proven to be genotoxic, thus their presence in environmental samples is a legitimate cause for concern.In this contribution, meniscus modified silver solid amalgam electrode (m-AgSAE) was employed for the batch voltammetric determination and amperometric determination in connection with flow injection analysis of 5-nitroquinoline and 6-nitroquinoline (5-NQ, 6-NQ). Their electrochemical behavior was characterized by cyclic voltammetry, for their determination direct current voltammetry and differential pulse voltammetry were used. Linear calibration curves in the concentration range of 2 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ were obtained. These results are comparable with results obtained for polarographic determination of the same substances using mercury electrodes. Further, the meniscus modified silver solid amalgam electrode was employed in amperometric detection cell in “wall jet” arrangement for determination of 5-NQ in flow injection analysis. Under optimized conditions (run buffer 0.05 mol L⁻¹ borate buffer, pH 9.0; flow rate 4 mL min⁻¹; detection potential −1.6 V; injection volume 0.1 mL), the limit of quantitation of ∼4 × 10⁻⁶ mol L⁻¹ was achieved. The repeatability of the detector response is satisfactory (relative standard deviation ∼2.15% for c(5-NQ) = 1 × 10⁻⁴ mol L⁻¹). Practical applicability of the method was verified for the determination of micromolar concentrations of 5-NQ in drinking and river water model samples.     

Immobilization of Nafion-ordered mesoporous carbon on a glassy carbon electrode: Application to the detection of epinephrine

A stable suspension of ordered mesoporous carbon (OMC) was obtained by dispersing OMC in a solution of Nafion. By coating the suspension onto glassy carbon (GC) electrode, cyclic voltammetry was used to evaluate the electrochemical behaviors of Nafion-OMC-modified GC (Nafion-OMC/GC) electrode in 0.1 mmol L⁻¹ hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃)/0.1 mol L⁻¹ KCl solution, where Nafion-OMC/GC electrode shows a faster electron transfer rate as compared with OMC/GC, Nafion/GC and GC electrodes. Due to the unique properties of Nafion-OMC, an obvious decrease in the overvoltage of the epinephrine (EP) oxidation (ca. 100 mV at pH 4.1 and 115 mV at pH 7.0) as well as a dramatic increase in the peak current (12 times at pH 4.1 and 6 times at pH 7.0) was observed at Nafion-OMC/GC electrode compared to that seen at GC electrode. By combining the advantages of OMC with those of Nafion, the anodic peak of EP and that of ascorbic acid (AA) were separated successfully (by ca. 144-270 mV) in the pH range of 2.0-10.0, which may make Nafion-OMC/GC electrode potential for selective determination of EP in the presence of AA at a broad pH range. As an EP sensor, the EP amperometric response at Nafion-OMC/GC electrode in pH 7.0 PBS is extremely stable, with 99% of the initial activity remaining (compared to 32% at GC surface) after 120 min stirring of 0.20 mmol L⁻¹ EP. And Nafion-OMC/GC electrode can be used to readily detect the physiological concentration of EP at pH 7.0. These make Nafion-OMC/GC electrode potential candidates for stable and efficient electrochemical sensor for the detection of EP. The solubilization of OMC by Nafion may provide a route to more precise manipulation, and functionalization for the construction of OMC-based sensors, as well as allowing OMC to be introduced to biologically relevant systems.     

Capillary electrophoretic separation and post-column electrochemical detection of mercury and methyl mercury and applications to coal samples

A study of the electrochemistry of mercury and organo mercury complexes with cysteine indicated the formation of stable complexes which can be utilized for the determination of the species in environmental matrices. A technique based on capillary electrophoresis and amperometric detection (CE-AD) has been developed for the speciation of mercury with a newly developed cross hair design electrochemical detector. This technique has the capability to detect mercury species that are electrochemically active. For CE-AD the detection limits were 0.005 ± 0.002 μg L⁻¹ for Hg²⁺ and 0.4 ± 0.05 μg L⁻¹ for MeHg⁺. These detection sensitivities are very attractive for environmental monitoring and the method was utilized for the determination of mercury in coal.     

A new amperometric glucose biosensor based on platinum nanoparticles/polymerized ionic liquid-carbon nanotubes nanocomposites

A new amperometric glucose biosensor has been developed based on platinum (Pt) nanoparticles/polymerized ionic liquid-carbon nanotubes (CNTs) nanocomposites (PtNPs/PIL-CNTs). The CNTs was functionalized with polymerized ionic liquid (PIL) through directly polymerization of the ionic liquid, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM]BF₄), on carbon nanotubes and then used as the support for the highly dispersed Pt nanoparticles. The electrochemical performance of the PtNPs/PIL-CNTs modified glassy carbon (PtNPs/PIL-CNTs/GC) electrode has been investigated by typical electrochemical methods. The PtNPs/PIL-CNTs/GC electrode shows high electrocatalytic activity towards the oxidation of hydrogen peroxide. Taking glucose oxidase (GOD) as the model, the resulting amperometric glucose biosensor shows good analytical characteristics, such as a high sensitivity (28.28 μA mM⁻¹ cm⁻²), wide linear range (up to 12 mM) and low detection limit (10 μM).     

A-type zeolite containing Ag/Zn as inorganic antifungal for waterborne coating formulations

The biocide cations Ag⁺ and Zn²⁺ were hosted in the cavities of an ordered aluminosiliceous framework. Starting from sodium A-type zeolite (NaA), LTA containing Ag⁺ (AgA), Zn²⁺ (ZnA) and Ag⁺/Zn (AgZnA) at different cation exchanged levels was obtained and its antifungal properties were evaluated. To determine the minimum inhibitory concentration (MIC) of the exchanged zeolites against Aspergillus niger, [Ag⁺] and [Zn²⁺] values ranging from 50 < [Ag⁺] < 1000 mg L⁻¹ to 650 < [Zn²⁺] < 2000 mg L⁻¹, respectively, were used for NaA, and for AgZnA: 30 < Ag⁺ < 250 mg L⁻¹. The zeolite sample having [Ag⁺] = 100 mg L⁻¹, [Zn²⁺] = 90 mg L⁻¹ produces a growth inhibition comparable to that achieved with 230 mg L⁻¹ of Ag⁺¹ (MIC value obtained for the single cation). The antifungal activity of these products after incorporation in waterborne coating formulations was also determined. Results indicate that Ag⁺ and Zn²⁺ supported on A-type zeolite could be a beneficial tool for the development of waterborne coatings with a longer protection against microbiological attack when compared to traditional organic biocides.     

Application of bismuth-film electrode for cathodic electroanalytical determination of sulfadiazine

A bismuth-film electrode for use in cathodic electrochemical detection was employed in order to quantify sulfadiazine in pharmaceutical formulations. The bismuth film was deposited ex situ onto a glassy carbon substrate. Analysis of two sulfa drugs was carried out by differential-pulse voltammetry in 0.05 mol L⁻¹ Britton-Robinson pH 4.5 solution. Sulfadiazine reduction was observed at −0.74 V vs. Ag/AgCl in one well-resolved irreversible reduction peak. The analytical curve with two slopes was obtained in the concentration range of 3.2-97.0 μmol L⁻¹. The detection limit was 2.1 μmol L⁻¹ for concentrations of 3.2-20.0 μmol L⁻¹ (r = 0.9949) and 12.2 μmol L⁻¹ for concentrations between 20.0 and 97.0 μmol L⁻¹ (r = 0.9951). Recovery studies carried out with both sulfadiazine samples gave values from 93.6 to 109.3%. The accuracy of the results supplied by the bismuth-film electrode was compared to those obtained by the standard amperometric titration method. The relative error between them was lower than 2.0%.     

Electrochemical sensing of NADH on NiO nanoparticles-modified carbon paste electrode and fabrication of ethanol dehydrogenase-based biosensor

In this work, an electrochemical β-nicotinamide adenine dinucleotide (NADH) sensor based on a carbon paste electrode modified with nickel oxide nanoparticles (NiONPs) was developed. The key highlights of this work are ease of preparation of the NiONPs-modified carbon paste electrode (NiONPs/MCPE), and its high sensitivity to NADH. The electrochemical characterization of NiONPs/MCPEs was performed via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical oxidation response of NADH was investigated by differential pulse voltammetry and chronoamperometry. The results indicated that the electrocatalytic effects of NiONPs on the response current of NADH significantly facilitated the electron transfer and improved the performance of the biosensor. Compared to bare carbon paste electrode (BCPE), the oxidation potential was shifted toward more negative potentials and the oxidation current was increased remarkably. Under optimum conditions, NADH could be detected in the range from 1.0 × 10^?4 to 1.0 mmol L^?1 with lower detection limit (0.05 μmol L^?1). The proposed NADH sensor demonstrated fast and reproducible response. Furthermore, an ethanol biosensor was prepared using NiONPs and NAD⁺-dependent alcohol dehydrogenase enzyme giving linear responses over the concentration range of 1.6 and 38 mmol L^?1 of ethanol.     

Amperometric urea biosensors based on the entrapment of urease in polypyrrole films

Urease (Urs) was immobilized in electrochemically prepared polypyrrole (PPy) and the resulting films were characterized by cyclic voltammetry, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and ultraviolet visible spectroscopy (UV-VIS). The enzymatic activity of Urs entrapped in the PPy matrix was confirmed by the catalytic conversion of urea into carbon dioxide and ammonia, when urea was detected amperometrically at different concentrations in standard samples and commercial fertilizers. The PPy/Urs biosensors exhibited selectivity, a relatively high efficiency at urea concentrations below 3.0 mmol L⁻¹, and a sensitivity to urea of 2.41 μA cm⁻² mmol⁻¹ L.