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.     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized into poly (lactic-co-glycolic acid)/room temperature ionic liquid composite film

A promising material of poly(lactic-co-glycolic acid) (PLGA) and, room temperature ionic liquid (ILs) (1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄) was firstly used as an immobilization matrix to entrap proteins and its bioelectrochemical properties were studied. Direct electrochemistry and electrocatalytic behaviors of hemoglobin (Hb) entrapped in the PLGA/ILs composite film on the surface of glass carbon electrode were investigated. UV-vis spectroscopy, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the composite film. The obtained results demonstrated that the Hb molecule in the film kept its native structure and showed its good electrochemical behavior. A pair of well-defined redox peaks of Hb was obtained at the Hb/PLGA/ILs composite film-modified GC electrode through direct electron transfer between the protein and the underlying electrode. The proposed biosensor showed good reproducibility and high sensitivity to H₂O₂ with the detection limit of 2.37 × 10⁻⁷ M (S/N = 3). In the range of 5.0 × 10⁻⁶ to 8.05 × 10⁻³ M, the catalytic reduction current of H₂O₂ was proportional to its concentration. The apparent Michaelis-Menten constant of Hb in the PLGA/ILs composite film was estimated to be 0.069 mM, showing its high affinity.     

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.     

H2 sensing properties of diode-type gas sensors fabricated with Ti- and/or Nb-based materials

A thermally oxidized TiO₂ or Nb₂O₅ film equipped with a top Pd film electrode and a bottom Ti or Nb plate electrode (Pd/MO(n)/M, MO: oxide film, M: metal plate, n: annealing temperature (°C)) has been investigated as a diode-type H₂ sensor under air or N₂ atmosphere. Pd/TiO₂(n)/Ti sensors showed relatively poor H₂ sensing properties in air, in comparison with Pd/anodic-TiO₂(n)/Ti sensors constructed with an anodized TiO₂ film equipped with a top Pd film electrode and a bottom Ti plate electrode, which were reported in our previous studies. On the other hand, Pd/Nb₂O₅(n)/Nb sensors showed relatively larger H₂ response with fast response and recovery speeds than Pd/TiO₂(n)/Ti sensors in air under high forward bias conditions. A Pd/Nb₂O₅(450)/Ti sensor, which was fabricated by radio-frequency magnetron sputtering of Nb metal on a Ti substrate followed by thermal oxidation at 450 °C, showed the largest H₂ response and relatively fast response and recovery speeds in air, among the sensors tested. In addition, H₂ response of the Pd/Nb₂O₅(450)/Ti sensor in air was much lower than that in N₂, but the logarithm of H₂ response was almost proportional to the logarithm of H₂ concentration in a wide range of H₂ concentration (10–8000 ppm) in air, and the H₂ sensitivity in air was much higher than that in N₂.     

Direct electrochemistry and electrocatalysis with horseradish peroxidase immobilized in polyquaternium-manganese oxide nanosheet nanocomposite films

A novel biocompatible polyquaternium (QY)-manganese oxide nanosheet (MNS) nanocomposite has been prepared and shown to be a promising matrix for horseradish peroxidase (HRP) immobilization. The resulting HRP-QY-MNS film was characterized by Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopy, which indicated that HRP retained its native structure in the nanocomposite film. An HRP-QY-MNS film-modified glassy carbon electrode exhibited a pair of well-defined and quasi-reversible cyclic voltammetric peaks centered at −0.272 V (vs. Ag/AgCl) in pH 7.0 phosphate buffer solution. The direct electrochemical behavior of HRP was greatly enhanced in the QY-MNS nanocomposite film compared with that in single-component QY or MNS films. The immobilized HRP showed excellent electrocatalysis in the reduction of hydrogen peroxide (H₂O₂), which was exploited in the construction of an H₂O₂ biosensor. The linear range of the biosensor for H₂O₂ was found to be from 1.0 × 10⁻⁷ to 3.2 × 10⁻⁵ M with a correlation coefficient of 0.998. The detection limit was 7.8 × 10⁻⁸ M at a signal-to-noise ratio of 3. The biosensor exhibited rapid response and good long-term stability.     

pH sensor based on the crown heteropolyanion K28Li5H7P8W48O184·92H2O immobilized using a layer by layer assembly process

A new sensitive pH sensor based on immobilization of the crown heteropolyanion K₂₈Li₅H₇P₈W₄₈O₁₈₄·92H₂O (P₈W₄₈) on a electrode surface through a layer by layer assembly process is described. The immobilization is based on the electrostatic adsorption of the complex in layers of charged polyelectrolyte poly(allylamine hydrochloride) (PAH). The deposited P₈W₄₈/LBL film was investigated by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. Compared to the electrochemical behavior of dissolved P₈W₄₈, a slight shift in the redox peak towards negative potentials is observed, which have been attributed to a slight decrease in the acidity of the interior of the P₈W₄₈/LBL film compared to the testing buffer solution. The relationship between the peak currents of the deposited P₈W₄₈/LBL film and the number of layers is shown to be linear, which demonstrates that equal amounts of P₈W₄₈ are adsorbed in each deposition layer. The P₈W₄₈/LBL modified electrode showed high sensitivities toward pH. Therefore, such electrodes were tested as pH sensors using the titration method. The resulting pH sensor has a detection range of pH 1–13, a sensitivity of 69 ± 2 mV/pH, high repeatability (<3 mV), fast response time (<7 s), low sensitivity toward change in ionic strength and nature of the supporting electrolyte, low internal resistance and a working life time of at least 3 months. Moreover, the sensor is easy to manufacture and can be easily miniaturized for measurements in micro- and nano-systems.     

Ionic liquid and nanogold-modified immunosensing interface for electrochemical immunoassay of hepatitis B surface antigen in human serum

A mediator-free electrochemical immunoassay protocol based on a disposable immunosensor for the detection of hepatitis B surface antigen (HBsAg) in human serum was developed. To fabricate such an immunosensor, a layer of sol–gel composite film containing room temperature ionic liquid and chitosan was initially formed on a glassy carbon electrode. Nanogold particles were then adsorbed onto the membrane via the amine groups of chitosan molecules, and then horseradish peroxidase (HRP)-labeled hepatitis B surface antibodies (HRP-anti-HBs) were immobilized onto the nanogold surface. With a non-competitive immunoassay format, the antibody–antigen complex could be formed by a simple one-step immunoreaction between the immobilized HRP-anti-HBs and HBsAg in sample solution. The formed immunocomplex inhibited partly the active center of the HRP, which decreased the immobilized HRP toward the reduction of H₂O₂. The performance and factors influencing the performance of the immunosensor were evaluated. Under optimal conditions, the current change obtained from the carried HRP relative to H₂O₂ system was proportional to HBsAg concentration in the range of 1.5–400 ng/mL with a detection limit of 0.5 ng/mL (at 3δ). The reproducibility, selectivity, and stability of the proposed immunosensor were acceptable. Moreover, the proposed immunosensors were used to analyze HBsAg in human serum specimens. Analytical results of clinical samples suggested that the developed immunosensor has a promising alternative approach for detecting HBsAg in the clinical diagnosis.     

The electrochemical method for detecting 26S proteasome

Detection of 26S proteasome, a multiproteolytic complex that degrades intracellular proteins in eukaryotic cells, by electrochemical methods is of interest for improved understanding of living cells and detection of cancer. This study develops an electrochemical system to detect 26S using a gold electrode modified by a self-assembled monolayer of 1,6-hexanedithiol (HDT) for capture of 26S proteasomes. When 26S is fixed on a HDT-gold electrode, it is found that electrolyte anions can enhance detection but also cause damage to the HDT layer. Cyclic voltammetry and electrochemical impedance spectroscopy demonstrate that HDT stability is better in LiClO₄ solution than in sodium sulfate (Na₂SO₄) solution. Chymotrypsin-like activity of 26S as measured by fluorescence with Suc-LLVY-AMC substrate declines 10% with LiClO₄ and 25% with Na₂SO₄. LiClO₄ is a better electrolyte salt in a 26S-HDT-gold electrode application in Tris buffer. Increased electron transfer resistance is observed after binding 26S on the HDT-gold electrode. Stable 26S concentration is from 2 to 100 nM. As 26S concentration increases from 2 to 100 nM, the electron-transfer impedance of Fe(CN)₆^4−/3− redox rises logarithmically. The range of electrochemical detection of 26S proteasome is nanomolar.     

Electrochemical sensor for hydroperoxides determination based on Prussian blue film modified electrode

An electrochemical sensor for hydroperoxides determination was investigated. The sensor was based on the electrocatalytic reduction of hydroperoxides on Prussian blue (PB)-modified glassy carbon electrode. The modified electrode possesses a high electrocatalytic effect towards all studied peroxides with the highest effect obtained with H₂O₂ followed by tert-butyl hydroperoxide (TBH), cumene hydroperoxide (CH) and linoleic acid hydroperoxide (LAH). In addition, the modified electrode showed a good stability and a fast response time (<20 s). The lower detection limits of H₂O₂, TBH, CH and LAH were found to be 10⁻⁷ mol L⁻¹, 2 × 10⁻⁷ mol L⁻¹, 3.5 × 10⁻⁷ mol L⁻¹ and 4 × 10⁻⁷ mol L⁻¹, respectively. The electrochemical sensor was then applied for amperometric determination of peroxide value (PV) in edible oil at an applied potential of 50 mV (vs. Ag/AgCl (1 M KCl)). A good linearity has been found in the range 0.02–1.0 mequiv. O₂/kg, with a detection limit (S/N = 3) of 0.001 mequiv. O₂/kg. The precision of the method (R.S.D., n = 9) for within and between-days is better than 1.9% and 2.7%, respectively at 0.1 mequiv. O₂/kg. The method was successfully applied to the determination of PV in real edible oil samples with an excellent agreement with results obtained with the official standard procedure. The proposed method is accurate, simple, cheap and could be used to control edible oil rancidity with a high sample throughputs (more than 120 samples/h).     

Amperometric glucose biosensor based on integration of glucose oxidase with platinum nanoparticles/ordered mesoporous carbon nanocomposite

This article reports a new amperometric glucose biosensor based on ordered mesoporous carbon (OMC) supported platinum nanoparticles (Pt/OMC) modified electrode. The Pt/OMC nanocomposite modified electrode exhibited excellent electrocatalytic activities towards the reduction and oxidation of H₂O₂ as well. This feature allowed us to use it as bioplatform on which glucose oxidase (GOD) was immobilized by entrapment in electropolymerized pyrrole film for the construction of the glucose biosensor. The biosensor showed good analytical performances in terms of low detection (0.05 mM), high sensitivity (0.38 μA/mM) and wide linear range (0.05-3.70 mM). In addition, the effects of pH value, applied potential, electroactive interference and the stability of the biosensor were discussed. The applicability to blood analysis was also evaluated.     

Amperometric glucose microbiosensor based on a Prussian Blue modified carbon fiber electrode for physiological applications

Results of experiments in vitro and in vivo, using an amperometric glucose microbiosensor based on a Prussian Blue (PB) modified carbon fiber electrode with very low dimensions (∼10 μm diameter), are presented. The electrocatalytic properties of the PB film enable detection of an enzymatic by-product (H₂O₂) at a very low applied potential: 0.0 V against SCE. The main steps during glucose microbiosensor construction were examined by cyclic voltammetry and electrochemical impedance spectroscopy. Excellent selectivity of the glucose microbiosensors against a large number of physiological interference compounds is demonstrated. Finally, microbiosensor responses during intraperitoneal injection, local infusion and local electrical stimulation showed sufficient sensitivity and stability to monitor multi-phasic and reversible changes in brain ECF glucose levels during physiological experiments, illustrating the excellent properties and utility of this biosensor design in the neurosciences.     

Diesel engine dynamometer testing of impedancemetric NOx sensors

Prototype solid-state electrochemical sensors using a dense gold sensing electrode, porous yttria-stabilized zirconia (YSZ) electrolyte, and a platinum counter electrode (Au/YSZ/Pt) were evaluated for measuring NO_x (NO and NO₂) in diesel exhaust. Both electrodes were exposed to the test gas (i.e., there was no reference gas for the counter electrode). An impedancemetric method was used for NO_x measurements, where the phase angle was used as the response signal. A portion of the tailpipe exhaust from the dynamometer test stand was extracted and fed into a furnace containing the experimental sensor. The prototype sensor was tested along with a commercially available NO_x sensor. Simultaneous measurements for NO_x, O₂, CO₂, H₂O, CO, and CH₄ in a separate feed stream were made using Fourier transform infrared (FTIR) spectroscopy and an oxygen paramagnetic analyzer. The experimental sensor showed very good measurement capability for NO in the range of 25-250 ppm, with a response paralleling that of the FTIR and commercial sensor. The prototype sensor showed better sensitivity to NO_x at the lower concentration ranges. O₂ is an interferent for the experimental sensor, resulting in decreased sensitivity for measurement of NO_x. Methods to overcome this interference are discussed.     

Electrochemical determination of ascorbic acid in fruits on a vanadium oxide polypropylene carbonate modified electrode

A novel vanadium oxide polypropylene carbonate modified glassy carbon electrode was developed and used for the measurement of ascorbic acid (AA). The electrode was prepared by casting a mixture of vanadium tri(isopropoxide) oxide (VO(OC₃H₇)₃) and poly(propylene carbonate) (PPC) onto the surface of a glassy carbon electrode. The electrochemical behavior of the VO(OC₃H₇)₃–PPC film modified glassy carbon electrode was investigated by cyclic voltammetry and amperometry. This modified electrode exhibited electrocatalytic response to the oxidation of ascorbic acid. Compared with a bare glassy carbon electrode, the modified electrode exhibits a 220 mV shift of the oxidation potential of ascorbic acid in the cathodic direction and a marked enhancement of the current response. The response current revealed a good linear relationship with the concentration of ascorbic acid in the range of 4 × 10⁻⁸ and 1 × 10⁻⁴ mol L⁻¹ and the detection limit of 1.5 × 10⁻⁸ mol L⁻¹ (S/N = 3) in the pH 8.06 Britton–Robinson solution. Quantitative recovery of the ascorbic acid in synthetic samples has been obtained and the interferences from different species have been studied. The method has been successfully applied to the determination of ascorbic acid in fruits. The concentrations of ascorbic acid measured by this method are in good agreement with the literature value. It is much promising for the modified films to be used as an electrochemical sensor for the detection of ascorbic acid.     

A novel bienzyme glucose biosensor based on three-layer Au-Fe3O4@SiO2 magnetic nanocomposite

The magnetic core-shell Au-Fe₃O₄@SiO₂ nanocomposite was prepared by layer-by-layer assembly technique and was used to fabricate a novel bienzyme glucose biosensor. Glucose oxidase (GOD) and horseradish peroxidase (HRP) were simply mixed with Au-Fe₃O₄@SiO₂ nanocomposite and cross-linked on the ITO magnetism-electrode with nafion (Nf) and glutaraldehyde (GA). The modified electrode was designated as Nf-GOD-HRP/Au-Fe₃O₄@SiO₂/ITO. The effects of some experimental variables such as the pH of supporting electrolyte, enzyme loading, the concentration of the mediator methylene blue (MB) and the applied potential were investigated. The electrochemical behavior of the biosensor was studied using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronoamperometry. Under the optimized conditions, the biosensor showed a wide dynamic range for the detection of glucose with linear ranges of 0.05-1.0 mM and 1.0-8.0 mM, and the detection limit was estimated as 0.01 mM at a signal-to-noise ratio of 3. The biosensor exhibited a rapid response, good stability and anti-interference ability. Furthermore, the biosensor was successfully applied to detect glucose in human serum samples, showing acceptable accuracy with the clinical method.     

A sensitive nonenzymatic hydrogen peroxide sensor based on DNA–Cu complex electrodeposition onto glassy carbon electrode

In this paper, DNA–Cu²⁺ complex was electrodeposited onto the surface of glassy carbon (GC) electrode, which fabricated a DNA–Cu²⁺/GC electrode sensor to detect H₂O₂ with nonenzyme. Cyclic voltammogram of DNA–Cu²⁺/GC electrode showed a pair of well-defined redox peaks for Cu²⁺/Cu⁺. Moreover, the electrodeposited DNA–Cu²⁺ complex exhibited excellent electrocatalytic behavior and good stability for the detection of H₂O₂. The effects of Cu²⁺ concentration, electrodeposition time and determination conditions such as pH value, applied potential on the current response of the DNA–Cu²⁺/GC electrode toward H₂O₂ were optimized to obtain the maximal sensitivity. The linear range for the detection of H₂O₂ is 8.0 × 10⁻⁷ M to 4.5 × 10⁻³ M with a high sensitivity of −40.25 μA mM⁻¹, a low detection limit of 2.5 × 10⁻⁷ M and a fast response time of within 4 s. In addition, the sensor has good reproducibility and long-term stability and is interference free.     

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.     

Temperature dependent H2S and Cl2 sensing selectivity of Cr2O3 thin films

The conductometric gas sensing characteristics of Cr₂O₃ thin films - prepared by electron-beam deposition of Cr films on quartz substrate followed by oxygen annealing - have been investigated for a host of gases (CH₄, CO, NO₂, Cl₂, NH₃ and H₂S) as a function of operating temperature (between 30 and 300 °C) and gas concentration (1-30 ppm). We demonstrate that these films are highly selective to H₂S at an operating temperature of 100 °C, while at 220 °C the films become selective to Cl₂. This result has been explained on the basis of depletion of chemisorbed oxygen from the surface of films due to temperature and/or interaction with Cl₂/H₂S, which is supported experimentally by carrying out the work function measurements using Kelvin probe method. The temperature dependent selectivity of Cr₂O₃ thin films provides a flexibility to use same film for the sensing of Cl₂ as well as H₂S.     

Gas sensors based on anodic tungsten oxide

Nanostructured porous tungsten oxide materials were synthesized by the means of electrochemical etching (anodization) of tungsten foils in aqueous NaF electrolyte. Formation of the sub-micrometer size mesoporous particles has been achieved by infiltrating the pores with water. The obtained colloidal anodic tungsten oxide dispersions have been used to fabricate resistive WO₃ gas sensors by drop casting the sub-micrometer size mesoporous particles between Pt electrodes on Si/SiO₂ substrate followed by calcination at 400 °C in air for 2 h. The synthesized WO₃ films show slightly nonlinear current-voltage characteristics with strong thermally activated carrier transport behavior measured at temperatures between −20 °C and 280 °C. Gas response measurements carried out in CO, H₂, NO and O₂ analytes (concentration from 1 to 640 ppm) in air as well as in Ar buffers (O₂ only in Ar) exhibited a rapid change of sensor conductance for each gas and showed pronounced response towards H₂ and NO in Ar and air, respectively. The response of the sensors was dependent on temperature and yielded highest values between 170 °C and 220 °C.     

Nanolayer treatment to realize suitable configuration for electrochemical allopurinol sensor based on molecular imprinting recognition sites on multiwall carbon nanotube surface

In this work, nanolayer MIP (molecularly imprinted polymer) is used to achieve suitable conformation for catalyzing AP (allopurinol) redox reaction. Also, a sensitive electrochemical sensor was fabricated for AP based on MIP immobilized on multiwall carbon nanotube (MWCNT) surface. Thin film of MIP immobilized on MWCNT surface (MIPCNT) with specific recognition sites for AP was cast on glassy carbon electrode (GCE). The morphology and features of the film were characterized by field emission scanning electron microscopy (FE-SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometric measurements (I–t) in detail. The near equilibrium time to adsorb AP on the surface of electrode is about 9 min. The modified electrode was used to detect the concentration of AP with a linear range and detection limit (S/N = 3) of 0.01–1.0 μM and 6.88 nM, respectively. The MIPCNT film displayed an excellent selectivity toward AP. Finally, the modified electrode was successfully applied to determine AP in the human serum sample and two brand tablets.