Electrocatalytic determination of hydrazine by a glassy carbon electrode modified with PEDOP/MWCNTs-Pd nanoparticles

The electrocatalysis of hydrazine oxidation by poly-ethylenedioxy pyrrole (PEDOP)-coated MWCNTs-palladium nanoparticles [PEDOP/MWCNTs-Pd] was investigated as an electrochemical sensor on the surface of glassy carbon electrode (GCE) in aqueous medium. Electrochemical oxidation of hydrazine in phosphate buffer (pH 7.4) was performed using cyclic voltammetry (CV) and chronoamperometry (CA) methods. Using the proposed electrode, the catalytic oxidation peak current of hydrazine was high and the overpotential of its oxidation decreased. Based on the obtained results, a mechanism for electrooxidation of hydrazine at [PEDOP/MWCNTs-Pd/GCE] demonstrated an irreversible diffusion-controlled electrode process and a four-electron transfer involved in the overall reaction. The experimental results showed that the mediated oxidation peak currents of the hydrazine were linearly dependent on the concentration of hydrazine in the range of 1.0 × 10⁻⁷ to 5.0 × 10⁻³ M. The detection limit (S/N = 3) was found to be 4 × 10⁻⁸ M with a fast response time of 10 s.     

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.     

A sensitive hydrazine electrochemical sensor based on electrodeposition of gold nanoparticles on choline film modified glassy carbon electrode

A highly sensitive hydrazine sensor was developed based on the electrodeposition of gold nanoparticles onto the choline film modified glassy carbon electrode (GNPs/Ch/GCE). The electrochemical experiments showed that the GNPs/Ch film exhibited a distinctly higher activity for the electro-oxidation of hydrazine than GNPs with 3.4-fold enhancement of peak current. The kinetic parameters such as the electron transfer coefficient (α) and the rate of electron exchange (k) for the oxidation of hydrazine were determined. The diffusion coefficient (D) of hydrazine in solution was also calculated by chronoamperometry. The sensor exhibited two wide linear ranges of 5.0 × 10⁻⁷-5.0 × 10⁻⁴ and 5.0 × 10⁻⁴-9.3 × 10⁻³ M with the detection limit of 1.0 × 10⁻⁷ M (s/n = 3). The proposed electrode presented excellent operational and storage stability for the determination of hydrazine. Moreover, the sensor showed outstanding sensitivity, selectivity and reproducibility properties. All the results indicated a good potential application of this sensor in the detection of hydrazine.     

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.     

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.     

Poly(brilliant cresyl blue) electrogenerated on single-walled carbon nanotubes modified electrode and its application in mediated biosensing system

Single-walled carbon nanotubes (SWCNTs) functionalized with carboxylic acid groups were cast to glassy carbon electrode (GCE) to construct a three-dimensional nano-micro structured scaffold. Brilliant cresyl blue (BCB) was electropolymerized on the above-mentioned SWCNTs/GCE using continuous cycling between −0.7 and 0.9 V vs. SCE. PolyBCB yielded on SWCNTs/GCE exhibited the enhanced electrochemical redox behavior compared with that electrogenerated on bare GCE. The apparent surface coverage of PolyBCB obtained by SWCNTs/GCE was at least 10 times higher than that obtained by bare GCE, namely 4.8 × 10⁻⁹ and 3.6 × 10⁻¹⁰ mol cm⁻². The cyclic voltammograms recorded by PolyBCB/SWCNTs/GCE exhibited well-defined two peaks located at −0.25 V and −0.06 V, respectively, with a surface-controlled mechanism. In addition, morphologies of PolyBCB electrogenerated on GCE and SWCNTS/GCE were characterized by atomic force microscopy. Finally, this proposed PolyBCB/SWCNTs/GCE was used in the construction of the second-generation biosensors to hydrogen peroxide and glucose, with the enhanced analytical performance.     

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.     

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.     

DC humidity sensing properties of BaTiO3 nanofiber sensors with different electrode materials

Barium titanate (BaTiO₃) nanofibers were synthesized by electrospinning and calcination techniques. Two direct current (DC) humidity sensors with different electrodes (Al and Ag) were fabricated by loading BaTiO₃ nanofibers as the sensing material. Compared with the Al electrode sensor, the Ag electrode sensor exhibits larger sensitivity and quicker response/recovery. The current of Al electrode sensor increases from 4.08 × 10⁻⁹ to 1.68 × 10⁻⁷ A when the sensor is switched from 11% to 95% relative humidity (RH), while the values are 2.19 × 10⁻⁹ and 3.29 × 10⁻⁷ A for the Ag electrode sensor, respectively. The corresponding response and recovery times are 30 and 9 s for Al electrode sensor, and 20 and 3 s for Ag electrode sensor, respectively. These results make BaTiO₃ nanofiber-based DC humidity sensors good candidates for practical application. Simultaneously, the comparison of sensors with different electrode materials may offer an effective route for designing and optimizing humidity sensors.     

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.     

Screen-printed carbon electrode for choline based on MnO2 nanoparticles and choline oxidase/polyelectrolyte layers

This paper presents the amperometric biosensor that determines choline and cholinesterase activity using a screen printed graphite electrode. In order to detect H₂O₂ we have blanket modified the electrode material with manganese dioxide nanoparticles layer. Using layer-by-layer technique on the developed hydrogen peroxide sensitive electrode surface choline oxidase was incorporated into the interpolyelectrolyte nanofilm. Its ability to serve as a detector of choline in bulk analysis and cholinesterase assay was investigated. We examined the interferences from red-ox species and heavy metals in the blood and in the environmental sample matrixes. The sensor exhibited a linear increase of the amperometric signal at the concentration of choline ranging from 1.3 × 10⁻⁷ to 1.0 × 10⁻⁴ M, with a detection limit (evaluated as 3σ) of 130 nM and a sensitivity of 103 mA M⁻¹ cm⁻² under optimized potential applied (480 mV vs. Ag/AgCl). The biosensor retained its activity for more than 10 consecutive measurements and kept 75% of initial activity for three weeks of storage at 4 °C. The R.S.D. was determined as 1.9% for a choline concentration of 10⁻⁴ M (n = 10) with a typical response time of about 10 s. The developed choline biosensor was applied for butyrylcholinesterase assay showing a detection limit of 5 pM (3σ). We used the biosensor to develop the cholinesterase inhibitor assay. Detection limit for chlorpyrifos was estimated as 50 pM.     

A glucose oxidase immobilization platform for glucose biosensor using ZnO hollow nanospheres

A good route (template-directed synthetic route) for the fabrication of ZnO hollow nanospheres (ZnO-HNSPs) was proposed. ZnO hollow nanosphere is a wonderful platform to immobilize glucose oxidase for glucose biosensor owing to the high specific surface area and high isoelectric point (IEP). Along with nafion and glucose oxidase (GOD), a glucose sensor was designed. Nafion/ZnO-HNSPs/GOD/GCE displays higher catalytic activity toward the glucose oxidation than Nafion/ZnO nano-Flowers/GOD/GCE. Linear response was obtained over a concentration range from 5.0 × 10⁻³ mM to 13.15 mM with a detection limit of 1.0 μM (S/N = 3), and the sensitivity was 65.82 μA/(mM cm²). Satisfyingly, the Nafion/ZnO-HNSPs/GOD/GCE could effectively avoid the interferences from the common interfering species such as uric acid (UA), ascorbic acid (AA), dopamine (DA) and fructose. The Nafion/ZnO-HNSPs/GOD modified electrode allows high sensitivity, excellently selective, stable, and fast amperometric sensing of glucose and thus is promising for the future development of glucose sensors.     

Development of an optical sensor for determination of zinc by application of PC-ANN

A very sensitive and reversible optical chemical sensor based on dithizone as chromoionophore immobilized within a plasticized carboxylated PVC film for Zn²⁺ determination is described. At optimum conditions (i.e. pH 5.0), the proposed sensor displays a linear response to Zn²⁺ over 5.0 × 10⁻⁸-5.0 × 10⁻⁶ mol L⁻¹ range. This range was improved to 2.5 × 10⁻⁸-5.8 × 10⁻⁵ mol L⁻¹ range by applying principle component-feed forward artificial neural network with back-propagation training algorithm (PC-ANNB). Detection limit of 8.0 × 10⁻⁹ mol L⁻¹ was obtained. The sensor is fully reversible within the dynamic range and the response time (t_95%) is approximately 4 min under batch conditions. In addition to its high stability and reproducibility, the sensor shows good selectivity towards Zn²⁺ ion with respect to common metal cations. The sensor was successfully applied for determination of Zn²⁺ ion in hair sample.     

An All-solid-state Cd-selective electrode with a low detection limit

A new all-solid-state Cd²⁺-selective electrode with a low detection limit was prepared by using conjugated thiophene oligomer α-sexithiophene (α-6T) as solid contact deposited between an ionophore-doped poly(vinyl chloride) membrane and a gold disc substrate. The electrode exhibited a Nernstian response for Cd²⁺ ions over a wide concentration range of 10⁻³-10⁻⁷ M with a detection limit as low as 1.3 × 10⁻⁸ M. Results showed that the fabricated potentiometric sensor was suitable for use within the pH range of 2.0-9.0 and exhibited good reproducibility for long-term measurements.     

Hybridization biosensor based on the covalent immobilization of probe DNA on chitosan-mutiwalled carbon nanotubes nanocomposite by using glutaraldehyde as an arm linker

A label-free DNA biosensor for hybridization detection of short DNA species related to the transgenic plants gene fragment of cauliflower mosaic virus (CaMV) 35S promoter was developed in this paper. The nanocomposite containing chitosan (CS) and mutiwalled carbon nanotubes (MWNTs) was first coated on a glassy carbon electrode. Then a highly reactive dialdehyde reagent of glutaraldehyde (GTD) was applied as an arm linker to covalently graft the 5′-amino modified probe DNA to the CS-MWNTs surface via the facile aldehyde-ammonia condensation reaction. The hybridization capacity of the developed biosensor was monitored with electrochemical impedance spectroscopy (EIS) using [Fe(CN)₆]^3−/4− as an indicating probe, and the experimental results showed that the biosensor had fast hybridization rate and low background interference. A wide dynamic detection range (1.0 × 10⁻¹³-5 × 10⁻¹⁰ M) and a low detection limit (8.5 × 10⁻¹⁴ M) were achieved for the complementary sequence. In addition, the hybridization specificity experiments showed that the sensing system can accurately discriminate complementary sequence from mismatch and noncomplementary sequences.     

Lead (II) carbon paste electrode based on derivatized multi-walled carbon nanotubes: Application to lead content determination in environmental samples

For the first time a novel derivatized multi-walled carbon nanotubes-based Pb²⁺ carbon paste electrode is reported. The electrode with optimum composition, exhibits an excellent Nernstian response to Pb²⁺ ion ranging from 5.9 × 10⁻¹⁰ to 1.0 × 10⁻² M with a detection limit of 3.2 × 10⁻¹⁰ M and a slope of 29.5 ± 0.3 mV dec⁻¹ over a wide pH range (2.5-6.5) with a fast response time (25 s) at 25 °C. Moreover, it also shows a high selectivity and a long life time (more than 3 months). Importantly, the response mechanism of the proposed electrode was investigated using AC impedance technique. Finally, the electrode was successfully applied for the determination of Pb²⁺ ion concentration in environmental samples, e.g. soils, waste waters, lead accumulator waste and black tea, and for potentiometric titration of sulfate anion.     

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.     

Aptamer-based colorimetric biosensing of dopamine using unmodified gold nanoparticles

A simple, sensitive and selective colorimetric biosensor for the detection of dopamine (DA) was demonstrated with a 58-mer dopamine-binding aptamer (DBA) as recognition element and unmodified gold nanoparticles (AuNPs) as probes. Upon the addition of DA, the conformation of DBA would change from a random coil structure to a rigid tertiary structure like a pocket and this change has been demonstrated by circular dichroism spectroscopic experiments. Besides, the conformational change of DBA could facilitate salt-induced AuNP aggregation and lead to the color change of AuNPs from red to blue. The calibration modeling showed that the analytical linear range covered from 5.4 × 10⁻⁷ M to 5.4 × 10⁻⁶ M and the corresponding limit of detection (LOD) was 3.6 × 10⁻⁷ M. Some common interferents such as 3,4-dihydroxyphenylalanine (DOPA), catechol, epinephrine (EP), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and ascorbic acid (AA) showed no or just a little interference in the determination of DA.     

Sensitive amperometric determination of chemical oxygen demand using Ti/Sb-SnO2/PbO2 composite electrode

A novel Ti/Sb-SnO₂/PbO₂ composite electrode was fabricated for COD determination. The new electrode configuration improved the sensitivity of the amperometric method apparently. Effects of common experimental parameters, such as applied potential, pH and concentration of the electrolyte on its analytical performance were investigated. A linear range of 0.5-200 mg L⁻¹ COD and a detection limit (a signal-to-noise ratio of 3) of 0.3 mg L⁻¹ were achieved under optimized conditions. The experiments for detecting COD in model samples and real samples were carried out to evaluate the electrode's performance. The obtained results were in good agreement with those determined by the standard dichromate method, with a relative error less than 12%.