Determination of Antioxidant Capacity by Using Xanthine Oxidase Bioreactor Coupled with Flow‐through H2O2 Amperometric Biosensor

A new flow system for antioxidant capacity (AOC) estimation, consisting of a bioreactor, containing immobilized xanthine oxidase (XOD), coupled with a H₂O₂ amperometric biosensor, based on Os‐wired horseradish peroxidase, was developed. The H₂O₂, resulting from the enzymatic reaction between xanthine (XA) and XOD, was amperometrically monitored at ?0.1 V vs. Ag/AgCl/KCl_sat, in order to avoid the electrochemical interferences. Two protocols were used to perform the AOC evaluation: “steady‐state”, when the antioxidant (AOX) was injected in the XA flow, and “transient state”, when XA and AOX were simultaneously injected in the carrier flow. The AOC of some commercial beverages were evaluated and compared with those obtained with 2,2‐diphenyl‐1‐picrylhydrazyl radical and Folin–Ciocalteu methods.     

Amperometric Biosensor for Choline Based on Gold Screen‐Printed Electrode Modified with Electrochemically‐Deposited Silica Biocomposite

A mediator‐free choline biosensor was developed using the electrochemically assisted sol‐gel deposition on gold screen‐printed electrodes. The addition of 12 mM of cationic surfactant CTAB in silica sol allowed enhancing the stability of the sensor. The modified electrode demonstrated catalytic activity and stable amperometric response to choline for over 3 weeks of exploitation with the sensitivity of 6 µA mM^?1 and LOD of 6 µM. The interference of ascorbic acid was reduced by pretreating the analyzed solution with MnO₂ powder. The application of the sensor with the purpose of identifying choline in the baby milk demonstrated satisfactory metrological characteristics.     

An Electrochemical Method for Sensitive Determination of Antioxidant Capacity

A simple and sensitive electrochemical method for the determination of antioxidant capacity has been studied. 4‐hydroxybenzoic acid (4‐HBA) was used as a trapping agent for photogenerated ^.OH radicals, leading to 3,4‐dihydroxybenzoic acid (3,4‐DHBA). Square‐wave voltammetry was used to quantify the amount of 3,4‐DHBA formed from the reaction between 4‐HBA and ^.OH. Addition of antioxidants induced the competition between 4‐HBA and antioxidants toward ^.OH elimination, resulting in a decrease of the measured current. The IC₅₀ for different standard antioxidants was calculated and expressed in Trolox equivalent antioxidant capacity (TEAC). The proposed method was successfully compared to a fluorimetric one.     

Preparation of GOD/Sol‐Gel Silica Film on Prussian Blue Modified Electrode for Glucose Biosensor Application

A novel amperometric glucose biosensor was fabricated by in situ incorporating glucose oxidase (GOD) within the sol‐gel silica film on a Prussian blue (PB) modified electrode. The method is simple and controllable, which combined the merits of in situ immobilizing biomolecules in sol‐gel silica film by electrochemical method and the synergic catalysis effects of PB and GOD molecules. Scanning electron microscopy (SEM) showed that the GOD/sol‐gel silica film was homogeneous with a large number of three‐dimensional nanopores, which not only enhanced mass transport, but also maintained the active configuration of the enzyme molecule and prevented the leakage of enzyme, therefore improved the stability and sensitivity of the biosensor. The fabricated biosensor showed fast response time (10 s), high sensitivity (26.6 mA cm^?2 M^?1), long‐term stability, good suppression of interference, and linear range of 0.01 mM–5.8 mM with a low detection limit of 0.94 μM for the detection of glucose. In addition, the biosensor was successfully applied to determine glucose in human serum samples.     

Voltammetric Determination of Purine Bases Using a Carbon Electrode Modified With Hybrid Silica Film

Hybrid silica films containing cation‐exchange polyelectrolytes were designed and used to modify a glassy carbon electrode (GCE) for voltammetric determination of purine bases. Hybrid silica‐polyelectrolyte films synthesised in the presence of adenine as structure directed component have demonstrated enhanced sorption capacity to purine base. The anodic peaks of adenine and guanine at a hybrid film‐modified GCE were observed at +1.55 and +1.1 V, respectively, in phosphate buffer solution at pH 3.5. Oxidation currents of adenine and guanine were proportional to their concentration in the range of 0.02–0.50 mM, with a detection limit of 0.015 mM. The developed method was used to determine adenine in adenosine triphosphate and purine bases in hydrolyzed solutions of DNA and demonstrated good metrological characteristics.     

A Cholesterol Biosensor Based on Entrapment of Cholesterol Oxidase in a Silicic Sol‐Gel Matrix at a Prussian Blue Modified Electrode

A cholesterol biosensors fabricated by immobilization of cholesterol oxidase (ChOx) in a layer of silicic sol‐gel matrix on the top of a Prussian Blue‐modified glassy carbon electrode was prepared. It is based on the detection of hydrogen peroxide produced by ChOx at ?0.05 V. The half‐lifetime of the biosensor is about 35 days. Cholesterol can be determined in the concentration range of 1×10^?6?8×10^?5 mol/L with a detection limit of 1.2×10^?7 mol/L. Normal interfering compounds, such as ascorbic acid and uric acid do not affect the determination. The high sensitivity and outstanding selectivity are attributed to the Prussian Blue film modified on the sensor.     

Preparation of a Xanthine Sensor Based on the Immobilization of Xanthine Oxidase on a Chitosan Modified Electrode by Cross‐linking

Here in this paper, xanthine oxidase (XOD) was immobilized onto the chitosan (CHT) modified electrode by a simple way of cross‐linking with glutaraldehyde (GTD) and 3‐aminopropyltriethoxysilane (KH). The electrode displayed a sharp peak to the oxidation of xanthine at a potential about 0.67 V and the optimum of pH for determination was investigated. Under the optimum conditions, the biosensor fabricated on the KH/GTD/XOD/CHT modified electrode showed excellent response to the oxidation of xanthine within the range of 0.5 to 18 μmol/L with a low detection limit of 0.0215 µmol/L, a good stability and a high selectivity. The sensor can also be used for the determination of hypoxanthine. The electrochemical results indicated that the immobilized enzyme still retained its biological activity and this provided a new way for the construction of biosensors and determination of xanthine.     

Renewable Surface Sol‐gel Derived Carbon Ceramic Electrode Modified with Copper Complex and Its Application as an Amperometric Sensor for Bromate Detection

A sol‐gel technique was used for the preparation of a three dimensional carbon composite electrode modified with [Cu(bpy)₂]Br₂ complex. A reversible redox couple of Cu(II)/Cu(I) is observed at the electrode surface. The electrochemical behavior and stability of the modified electrode was characterized by cyclic voltammetry. The charge transfer coefficient (α) and charge transfer rate constant (K_s) for the modified electrode were determined by cyclic voltammetry, which were found to be 0.46 and 14.2 s^?1, respectively. The modified electrode showed excellent catalytic activity toward bromate reduction at significantly reduced overpotentials and can be used successfully for amperometric detection of bromate. Under the optimized conditions, the calibration plots are linear in the concentration range 0.5 μM ?200μM. Detection limit (signal to noise is 3) and sensitivity were found to be 0.1 μM and 20 nA / μM, respectively. These analytical parameters compare favorably with those obtained with modern analytical techniques. The modified carbon ceramic electrode doped with Cu‐Complex shows a good reproducibility, a short response time (t<2 s), remarkable long term stability (>4 months) and especially good surface renewability by simple mechanical polishing (RSD for 6 successive polishing is 1.5%).     

Preparation and Characterization of a Tin Pentacyanonitrosylferrate‐Modified Carbon Ceramic Electrode: Application to Electrocatalytic Oxidation and Amperometric Detection of L‐Cysteine

The sol‐gel technique was used to construct tin pentacyanonitrosylferrate (SnPCNF) modified composite carbon ceramic electrode (CCE). This involves two steps: construction of CCE containing metallic Sn powder and then electrochemical creating of SnPCNF on the surface of CCE. The SnPCNF modified CCE (SnPCNFlCCE) was characterized by energy‐dispersive X‐ray (EDX), FTIR and cyclic voltammetry (CV) techniques. The SnPCNF film showed electrocatalytic activity toward the oxidation of L ‐cysteine. A linear calibration plot was obtained over the L ‐cysteine concentration range 1–51 μM using chronoamperometry. L ‐cysteine was determined amperometrically at the surface of this modified electrode. The detection limit (for a signal to noise of 3) and sensitivity were found to be 0.62 μM and 126 μA/mM, respectively.     

Development of a cytochrome c-based screen-printed biosensor for the determination of the antioxidant capacity of orange juices

This paper describes the development of an amperometric cytochrome c (cyt c)-based biosensor and its later application to the quantification of the scavenging capacity of antioxidants. The enzymatic biosensor was constructed by covalently co-immobilizing both cyt c and XOD on a mercaptoundecanol/mercaptoundecanoic acid (MU/MUA) mixed self-assembled monolayer (SAM)-modified screen-printed gold electrode. The applicability of this method was shown by analyzing the antioxidant capacity of pure substances, such as ascorbic acid and Trolox, and natural sources of antioxidants, particularly 5 orange juices.     

Antioxidant capacity of the algae using a biosensor method

Three different methods, i.e. a biosensor method, a voltammetric method and a spectrophotometric method, have been used to evaluate the total antioxidant capacity of certain types of algae. In the final evaluation of the data also the variation in time of the antioxidant capacity of cultivated algae was considered and some experimental factors, such as the use of different solvent mixtures to extract the antioxidant substances contained in the algae, were discussed.     

Characterization of ABTS at a Polymer‐Modified Electrode

ABTS, 2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonate), a colorless dianion that forms a colored radical upon oxidation, was characterized with electrochemistry and spectroscopy and demonstrated to be a detectable analyte in a polymer‐modified spectroelectrochemical sensor. Three positively charged polymers were incorporated into a thin film on an indium tin oxide (ITO) optically transparent electrode and used to concentrate ABTS at the electrode surface. Of the three films, poly(vinylbenzyl trimethyl ammonium chloride)‐poly(vinyl alcohol) (PVTAC‐PVA), poly(diallyldimethylammonium chloride)‐silica (PDMDAAC‐SiO₂), and quaternized poly(4‐vinyl‐N‐methylpyridinium nitrate)‐silica (QPVP‐SiO₂), PVTAC‐PVA demonstrated the best ability to absorb ABTS. Within 20 min, a change of 0.2 absorbance units at 417 nm and 13.6 μA/cm² in anodic peak current density in cyclic voltammetry at a scan rate of 0.025 V/s were observed.     

Adsorption and Reactivity of Chlorogenic Acid at a Hydrophobic Carbon Ceramic Composite Electrode: Application for the Amperometric Detection of Hydrazine

Sol-gel technique was used for construction of a carbon composite electrode. The prepared carbon ceramic electrode was modified with electroless deposition of chlorogenic acid for less than 1 min. The adsorbed thin films of chlorogenic acid on carbon composite electrode show one pair of peaks with a surface confined characteristic, which strongly depends on the solution pH, as anticipated for quinone /hydroquinone functionalities. The modified electrode shows highly catalytic activity toward hydrazine electrooxidation at wide pH range (5–11). Also the rotating modified electrode shows excellent analytical performance for amperometric determination of hydrazine. The detection limit, sensitivity, response time and linear dynamic range are 20 nM, 220 nA / μM, 1 second and 0.1 μM-1 mM, respectively. The catalytic rate constant for hydrazine oxidation at the surface of modified electrode was evaluated by cyclic voltammetry and was found to be around 1.5×10³ M⁻¹s⁻¹in phosphate buffer solution (pH 8). The precision of chronoamperometric measurements was 1–3% for 5 replicate determinations in the concentration range of the linear calibration. The reproducibility of modified CCE was evaluated with 8 successive polishing and modifications and then the anodic peak current was measured (RSD 2%). The advantages of this sensor are excellent catalytic activity, high sensitivity, good reproducibility and simplicity of preparation at short time periods.     

An L‐Lactate Amperometric Enzyme Electrode Based on L‐Lactate Oxidase Immobilized in a Laponite Gel on a Glassy Carbon Electrode. Application to Dairy Products and Red Wine

A biosensor based on the immobilization of Lactate oxidase in laponite–organosilasesquioxane films on glassy carbon electrode for the quantification of L ‐lactate in wine and dairy products is presented. The bioelectrode showed a very high sensitivity (0.33±0.01) A cm^?2 M^?1 and a short time response (10 s) for less than 1 U of enzyme. No significant interferences, including ascorbic acid, were detected. For red wine, matrix effects assigned to polyphenols and anthocyanins were observed, which ware easily overcome by sample dilution. Our L ‐lactate determinations were in good agreement with those of two standard methods.     

铂黑修饰酶电极测定胆碱

利用电化学方法在玻碳电极上均匀沉积亚微米粒度铂黑并进而修饰一层胆碱氧化酶膜,制成了胆碱酶电极。以计时电流法测定氯化胆碱,检测下限为 ７．４ μｍｏｌ／Ｌ,线性范围２．４×１０－２～ １．０ ｍｍｏｌ／Ｌ,重现性好,使用寿命较长。     

A Mediator‐Free Bienzyme Amperometric Biosensor Based on Horseradish Peroxidase and Glucose Oxidase Immobilized on Carbon Nanotube Modified Electrode

Multiwalled carbon nanotube (CNT) modified glassy carbon electrode immobilized with horseradish peroxidase (HRP) in Nafion coating showed direct electron transfer between HRP enzyme and the CNT‐modified electrode. A mediator‐free bienzyme glucose biosensor based on horseradish peroxidase and glucose oxidase was constructed. The bienzyme biosensor exhibited a high sensitivity for glucose detection at zero applied potential.     

Electrochemical Determination of the Antioxidant Capacity: The Ceric Reducing/Antioxidant Capacity (CRAC) Assay

The CRAC assay is a direct electron transfer test of antioxidant capacity for several organic compounds. The ability of eight different compounds in reducing Ce⁴⁺ was studied by chronoamperometric measurements of the remaining Ce³⁺ species. The following antioxidant classification was observed: tannic acid quercetin>rutin>gallic acid≈catechin>ascorbic acid>BHA>Trolox. These results agree with others already published and a good correlation (R²=0.937) was found with the classical spectrophotometric FRAP assay. The CRAC assay is simple, fast, free from sample pretreatment and applicable to nontransparent samples.     

An Electrochemical Biosensor with Cholesterol Oxidase/ Sol‐Gel Film on a Nanoplatinum/Carbon Nanotube Electrode

The carbon nanotubes decorated nanoplatinum (CNT‐Pt) were prepared using a chemical reduction method and a novel base electrode was constructed by intercalating CNT‐Pt on the surface of a waxed graphite electrode. The results showed that the nano‐particles of platinum at a waxed graphite electrode exhibits high catalytic activity for the reduction of hydrogen peroxide. The cholesterol oxidase (ChOx), chosen as a model enzyme, was immobilized with sol‐gel on the CNT‐Pt base electrode to construct a biosensor. The current response of the biosensor for cholesterol was very rapid (<20 s). The linear range for cholesterol measurement was 4.0×10^?6 mol/L ?1.0×10^?4 mol/L with a detection limit of 1.4×10^?6 mol/L. The experiments also showed that the ChOx/sol‐gel/CNT‐Pt biosensor was sensitive and stable in detecting cholesterol in serum samples.     

Flow Injection Analysis of Ascorbic Acid in Real Samples Using a Highly Stable Chemically Modified Screen‐Printed Electrode

A silica sol‐gel glass‐coated ferricyanide‐doped Tosflex‐modified screen‐printed electrode (designated as SG/FeCN‐Ts/SPE) was used for the mediated oxidation of ascorbic acid in neutral pH. The sol‐gel overlayer was found to stabilize the Fe(CN) ‐Tosflex system without any foul‐off and deterioration behavior through chemical interaction between the hydrophilic sites of Tosflex and oxy‐ and/or ‐hydroxy surface functional groups of silicate. The electrochemical mediation of ascorbic acid was found to follow the Michaelis‐Menten kinetic pathway. The SG/FeCN‐Ts/SPE was very stable for repetitive measurements by flow injection analysis at 0.3 V (vs. Ag/AgCl). A linear calibration plot was obtained for ascorbic acid determination up to 300 μM with sensitivity and detection limit (S/N=3) of 2.85 nA/μM and 46 nM, respectively. The system also showed tolerable interference to uric acid, urea and carbohydrates and was used for the determination of ascorbic acid in human urine, clinical tablet and apple juice.     

Modified Polyaniline-Based Electrode as a Sensor for the Antioxidant Determination

An electrochemical sensor for the antioxidant concentration determination in solutions and mixtures is proposed. An organic or inorganic oxidant is introduced into polyaniline synthesized electrochemically; at a preset potential, the oxidant reacts with antioxidants in solution. The antioxidant concentration is judged upon from the current density.