Direct electrochemistry and electrocatalysis of hemoglobin in gelatine film modified glassy carbon electrode

Direct electrochemical and electrocatalytic behavior of hemoglobin (Hb) immobilized on glass carbon electrode (GCE) containing gelatine (Gel) films was investigated. The characteristics of Hb/Gel film modified GC electrode were performed by using SEM microscopy, UV-vis spectroscopy and electrochemical methods. The immobilized Hb showed a couple of quasi-reversible redox peak with a formal potential of −0.38 V (versus SCE) in 0.1 M pH 7.0 PBS. The formal potential changed linearly from pH 4.03 to 8.41 with a slope value of −52.0 mV pH⁻¹, which suggested that a proton transfer was accompanied with each electron transfer (ET) in the electrochemical reaction. The Hb/gelatine/GCE displayed a rapid amperometric response to the reduction of H₂O₂ and nitrite.     

Electrocatalytic oxidation behavior of guanosine at graphene, chitosan and Fe3O4 nanoparticles modified glassy carbon electrode and its determination

A graphene, chitosan and Fe₃O₄ nanoparticles (nano-Fe₃O₄) modified glassy carbon electrode (graphene-chitosan/nano-Fe₃O₄/GCE) was fabricated. The modified electrode was characterized by scanning electron microscope and electrochemical impedance spectroscopy. The electrochemical oxidation behavior of guanosine was investigated in pH 7.0 phosphate buffer solution by cyclic voltammetry and differential pulse voltammetry. The experimental results indicated that the modified electrode exhibited an electrocatalytic and adsorptive activities towards the oxidation of guanosine. The transfer electron number (n), transfer proton number (m) and electrochemically effective surface area (A) were calculated. Under the optimized conditions, the oxidation peak current was proportional to guanosine concentration in the range of 2.0 × 10⁻⁶ to 3.5 × 10⁻⁴ mol L⁻¹ with the correlation coefficient of 0.9939 and the detection limit of 7.5 × 10⁻⁷ mol L⁻¹ (S/N = 3). Moreover, the modified electrode showed good ability to discriminate the electrochemical oxidation response of guanosine, guanine and adenosine. The proposed method was further applied to determine guanosine in spiked urine samples and traditional Chinese medicines with satisfactory results.     

Study of the electrochemical behavior of isorhamnetin on a glassy carbon electrode and its application

The electrochemical behavior of isorhamnetin (ISO) at a glassy carbon electrode was studied in a phosphate buffer solution (PBS) of pH 4.0 by cyclic voltammetry (CV) and differential pulse voltammetric method (DPV). A well-defined redox wave of ISO involving one electrons and one proton appeared. The electrode reaction is a reactant weak adsorption-controlled process with a charge transfer coefficient (α) of 0.586. Based on the understanding of the electrochemical process of ISO at the glassy carbon electrode, analysis of ISO can be realized. Under optimal conditions, the oxidation peak current showed linear dependence on the concentration of ISO in the range of 1.0 × 10⁻⁸ to 4.0 × 10⁻⁷ M and 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ M. The detection limit is 5.0 × 10⁻⁹ M. This method has been successfully applied to the detection of ISO in tablets.     

Myoglobin/arylhydroxylamine film modified electrode: Direct electrochemistry and electrochemical catalysis

The adsorption processes and electrochemical behavior of 4-nitroaniline (4-NA) adsorbed onto glassy carbon electrodes (GCE) have been investigated in aqueous 0.1 M nitric acid (HNO₃) electrolyte solutions using cyclic voltammetry (CV). 4-NA adsorbs onto GCE surfaces, and upon potential cycling past −0.2 V, is transformed into the arylhydroxylamine (ArHA) derivative which exhibits a well-behaved pH dependent redox couple centered at 0.32 V at pH 1.5. It is noted as arylhydroxylamine modified glassy carbon electrodes (HAGCE). This modified electrode can be readily used as an immobilization matrix to entrap proteins and enzymes. In our studies, myoglobin (Mb) was used as a model protein for investigation. A pair of well-defined reversible redox peaks of Mb (Fe(III)-Fe(II)) was obtained at the Mb/arylhydroxylamine modified glassy carbon electrode (Mb/HAGC) by direct electron transfer between the protein and the GCE. The formal potential (E^0′), the apparent coverage (Γ^*) and the electron-transfer rate constant (k_s) were calculated as −0.317 V, 8.26 × 10⁻¹² mol/cm² and 51 ± 5 s⁻¹, respectively. Dramatically enhanced biocatalytic activity was exemplified at the Mb/HAGC electrode by the reduction of hydrogen peroxide (H₂O₂), trichloroacetic acid (TCA) and oxygen (O₂). The Mb/arylhydroxylamine film was also characterized by UV-visible spectroscopy (UV-vis), scanning electron microscope (SEM) indicating excellent stability and good biocompatibility of the protein in the arylhydroxylamine modified electrode. This new Mb/HAGC electrode exhibited rapid electrochemical response (2 s) for H₂O₂ and had good stability in physiological condition, showing the potential applicability of the films in the preparation of third generation biosensors or bioreactors based on direct electrochemistry of the proteins.     

Electrochemical deoxyribonucleic acid biosensor based on carboxyl functionalized graphene oxide and poly-l-lysine modified electrode for the detection of tlh gene sequence related to vibrio parahaemolyticus

A carboxyl functionalized graphene oxide (GO-COOH) and electropolymerized ploy-l-lysine (PLLy) modified glassy carbon electrode (GCE) was fabricated and used for the construction of an electrochemical deoxyribonucleic acid (DNA) biosensor. The NH₂ modified probe ssDNA sequences were immobilized on the surface of GO-COOH/PLLy/GCE by covalent linking with the formation of amide bonds, which was stable and furthur hybridized with the target ssDNA sequence. Differential pulse voltammetry (DPV) was used to monitor the hybridization events with methylene blue as electrochemical indicator, which gave a sensitive reduction peak at −0.287 V (vs. SCE). Under the optimal conditions the reduction peak current was proportional to the concentration of tlh gene sequence in the range from 1.0 × 10⁻¹² to 1.0 × 10⁻⁶ mol L⁻¹ with a detection limit as 1.69 × 10⁻¹³ mol L⁻¹ (3σ). The polymerase chain reaction products of tlh gene from oyster samples were detected with satisfactory results, indicating the potential application of this electrochemical DNA sensor.     

Electrical detection of deoxyribonucleic acid hybridization based on carbon-nanotubes/nano zirconium dioxide/chitosan-modified electrodes

A novel and sensitive electrochemical DNA biosensor based on nanoparticles ZrO₂ and multi-walled carbon nanotubes (MWNTs) for DNA immobilization and enhanced hybridization detection is described. The MWNTs/nano ZrO₂/chitosan-modified glassy carbon electrode (GCE) was fabricated and oligonucleotides were immobilized to the GCE. The hybridization reaction on the electrode was monitored by differential pulse voltammetry (DPV) analysis using electroactive daunomycin as an indicator. Compared with previous DNA sensors with oligonucleotides directly incorporated on carbon electrodes, this carbon nanotube-based assay with its large surface area and good charge-transport characteristics increased DNA attachment quantity and complementary DNA detection sensitivity. The response signal increases linearly with the increase of the logarithm of the target DNA concentration in the range of 1.49 × 10⁻¹⁰ to 9.32 × 10⁻⁸ mol L⁻¹ with the detection limit of 7.5 × 10⁻¹¹ mol L⁻¹ (S/N = 3). The linear regression equation is I = 32.62 + 3.037 log C_DNA (mol L⁻¹) with a correlation coefficient value of 0.9842. This is the first application of carbon nanotubes combined with nano ZrO₂ to the fabrication of an electrochemical DNA biosensor with a favorable performance for the rapid detection of specific hybridization.     

Studies on electrochemical behaviors of acyclovir and its voltammetric determination with nano-structured film electrode

A multi-wall carbon nanotubes (MWNTs)-dihexadecyl hydrogen phosphate (DHP) film-coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behaviors of acyclovir on the MWNTs-DHP film-coated GCE were investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronocoulometry (CC). The oxidation peak current of acyclovir increased significantly and the peak potential shifted negatively at the MWNTs-DHP film-modified GCE, compared with that at a bare GCE. The results showed that this nano-structured film electrode exhibited excellent enhancement effects on the electrochemical oxidation of acyclovir. Consequently, a simple and sensitive electroanalytical method was developed for the determination of acyclovir. The oxidation peak current was proportional to the concentration of acyclovir from 8.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol/L. The detection limit was about 3.0 × 10⁻⁸ mol/L for 60 s accumulation at 0.00 V. The proposed method was demonstrated by using acyclovir tablets and the result was satisfying.     

Amperometric sulfite sensor based on multiwalled carbon nanotubes/ferrocene-branched chitosan composites

A novel amperometric sensor for the determination of sulfite was fabricated based on multiwalled carbon nanotubes (MWCNTs)/ferrocene-branched chitosan (CHIT-Fc) composites-covered glassy carbon electrode (GCE). The electrochemical behavior of the sensor was investigated in detail by cyclic voltammetry. The apparent surface electron transfer rate constant (K_s) and charge transfer coefficient (α) of the CHIT-Fc/MWCNTs/GCE were also determined by cyclic voltammetry, which were about 1.93 cm s⁻¹ and 0.42, respectively. The sensor displayed good electrocatalytic activity towards the oxidation of sulfite. The peak potential for the oxidation of sulfite was lowered by at least 330 mV compared with that obtained at CHIT/MWCNTs/GCE. In optimal conditions, linear range spans the concentration of sulfite from 5 μM to 1.5 mM and the detection limit was 2.8 μM at a signal-to-noise ratio of 3. The proposed method was used for the determination of sulfite in boiler water. In addition, the sensor has good stability and reproducibility.     

Electrochemical behaviors of guanosine on carbon ionic liquid electrode and its determination

The electrochemical behaviors of guanosine on the ionic liquid of N-butylpyridinium hexafluorophosphate (BPPF₆) modified carbon paste electrode (CPE) was studied in this paper and further used for guanosine detection. Guanosine showed an adsorption irreversible oxidation process on the carbon ionic liquid electrode (CILE) with the oxidation peak potential located at 1.12 V (vs. SCE) in a pH 4.5 Britton-Robinson (B-R) buffer solution. Compared with that on the traditional carbon paste electrode, small shift of the oxidation peak potentials appeared but with a great increment of the oxidation peak current on the CILE, which was due to the presence of ionic liquid in the modified electrode adsorbed the guanosine on the surface and promoted the electrochemical response. The electrochemical parameters such as the electron transfer coefficient (α), the electron transfer number (n), and the electrode reaction standard rate constant (k_s) were calculated as 0.74, 1.9 and 1.26 × 10⁻⁴ s⁻¹, respectively. Under the optimal conditions the oxidation peak current showed a good linear relationship with the guanosine concentration in the range from 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol/L by cyclic voltammetry with the detection limit of 2.61 × 10⁻⁷ mol/L (3σ). The common coexisting substances showed no interferences to the guanosine oxidation. The CILE showed good ability to distinguish the electrochemical response of guanosine and guanine in the mixture solution. The urine samples were further detected by the proposed method with satisfactory results.     

Simultaneous determination of uric acid,xanthine, hypoxanthine and caffeine in human blood serum and urine samples using electrochemically reduced graphene oxide modified electrode

This paper describes the fabrication of graphene on glassy carbon electrode (GCE) by electrochemical reduction of graphene oxide (GO) attached through 1,6-hexadiamine on GCE and the simultaneous determination of structurally similar four purine derivatives using the resultant electrochemically reduced GO (ERGO) modified electrode. The electrocatalytic activity of ERGO was investigated toward the oxidation of four important purine derivatives, uric acid (UA), xanthine (XN), hypoxanthine (HXN) and caffeine (CAF) at physiological pH. The modified electrode not only enhanced the oxidation currents of the four purine derivatives but also shifted their oxidation potentials toward less positive potentials in contrast to bare GCE. Further, it successfully separates the voltammetric signals of the four purine derivatives in a mixture and hence used for the simultaneous determination of them. Selective determination of one purine derivative in the presence of low concentrations other three purine derivatives was also realized at the present modified electrode. Using differential pulse voltammetry, detection limits of 8.8 × 10⁻⁸ M, 1.1 × 10⁻⁷ M, 3.2 × 10⁻⁷ M and 4.3 × 10⁻⁷ M were obtained for UA, XN, HXN and CAF, respectively. The practical application of the modified electrode was demonstrated by simultaneously determining the concentrations of UA, XN, HXN and CAF in human blood plasma and urine samples.     

Selective detection of dopamine in the presence of uric acid using a gold nanoparticles-poly(luminol) hybrid film and multi-walled carbon nanotubes with incorporated β-cyclodextrin modified glassy carbon electrode

Gold nanoparticles-poly(luminol) (Plu-AuNPs) hybrid film and multi-walled carbon nanotubes with incorporated β-cyclodextrin modified glassy carbon electrode (β-CD-MWCNTs/Plu-AuNPs/GCE) was successfully prepared for simultaneous determination of dopamine (DA) and uric acid (UA). The surface of the modified electrode has been characterized by X-ray photo-electron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), field-emission scanning electron microscope (SEM) and transmission electron microscope (TEM). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) have been used to investigate the β-CD-MWCNTs/Plu-AuNPs composite film. Gold nanoparticles anchored into poly(luminol) film exhibited catalytic activity for DA. MWCNTs with incorporated β-CD can greatly promote the direct electron transfer. In 0.10 M phosphate buffer solution (PBS, pH 7.0), the DPV response of the β-CD-MWCNTs/Plu-AuNPs/GCE sensor to DA is about 8-fold as compared with the Plu-AuNPs/GCE sensor, and the detection limit for DA is about one order of magnitude lower than the Plu-AuNPs/GCE sensor. The steady-state current response increases linearly with DA concentration from 1.0 × 10⁻⁶ to 5.6 × 10⁻⁵ M with a low detection limit (S/N = 3) of 1.9 × 10⁻⁷ M. Moreover, the interferences of ascorbic acid (AA) and uric acid (UA) are effectively diminished. The applicability of the prepared electrode has been demonstrated by measuring DA contents in dopamine hydrochloride injection.     

Simultaneous detection of ascorbate and uric acid using a selectively catalytic surface

The direct and selective detection of ascorbate at conventional carbon or metal electrodes is difficult due to its large overpotential and fouling by oxidation products. Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A monolayer of o-aminophenol (o-AP) was grafted on a glassy carbon electrode (GCE) via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution. The o-aminophenol confined surface was characterized by cyclic voltammetry. The grafted film demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbate in phosphate buffer of pH 7.0 shifting the overpotential from +462 to +263 mV versus Ag/AgCl. Cyclic voltammetry and d.c. amperometric measurements were carried out for the quantitative determination of ascorbate and uric acid. The catalytic oxidation peak current was linearly dependent on the ascorbate concentration and a linear calibration curve was obtained using d.c. amperometry in the range of 2-20 μM of ascorbate with a correlation coefficient 0.9998, and limit of detection 0.3 μM. The effect of H₂O₂ on the electrocatalytic oxidation of ascorbate at o-aminophenol modified GC electrode has been studied, the half-life time and rate constant was estimated as 270 s, and 2.57 × 10⁻³ s⁻¹, respectively. The catalytically selective electrode was applied to the simultaneous detection of ascorbate and uric acid, and used for their determination in real urine samples. This o-AP/GCE showed high stability with time, and was used as a simple and precise amperometric sensor for the selective determination of ascorbate.     

Poly(methylene blue) functionalized graphene modified carbon ionic liquid electrode for the electrochemical detection of dopamine

An ionic liquid 1-butylpyridinium hexafluorophosphate based carbon ionic liquid electrode (CILE) was used as the substrate electrode and a poly(methylene blue) (PMB) functionalized graphene (GR) composite film was co-electrodeposited on CILE surface by cyclic voltammetry. The PMB–GR/CILE exhibited better electrochemical performances with higher conductivity and lower electron transfer resistance. Electrochemical behavior of dopamine (DA) was further investigated by cyclic voltammetry and a pair of well-defined redox peaks appeared with the peak-to-peak separation (ΔE_p) as 0.058 V in 0.1 mol L⁻¹ pH 6.0 phosphate buffer solution, which proved a fast quasi-reversible electron transfer process on the modified electrode. Electrochemical parameters of DA on PMB–GR/CILE were calculated with the electron transfer number as 1.83, the charge transfer coefficients as 0.70, the apparent heterogeneous electron transfer rate constant as 1.72 s⁻¹ and the diffusional coefficient (D) as 3.45 × 10⁻⁴ cm² s⁻¹, respectively. Under the optimal conditions with differential pulse voltammetric measurement, the linear relationship between the oxidation peak current of DA and its concentration was obtained in the range from 0.02 to 800.0 μmol L⁻¹ with the detection limit as 5.6 nmol L⁻¹ (3σ). The coexisting substances exhibited no interference and PMB–GR/CILE was applied to the detection of DA injection samples and human urine samples with satisfactory results.     

Glassy carbon electrodes sequentially modified by cysteamine-capped gold nanoparticles and poly(amidoamine) dendrimers generation 4.5 for detecting uric acid in human serum without ascorbic acid interference

Glassy carbon electrodes (GCE) were sequentially modified by cysteamine-capped gold nanoparticles (AuNp@cysteamine) and PAMAM dendrimers generation 4.5 bearing 128-COOH peripheral groups (GCE/AuNp@cysteamine/PAMAM), in order to explore their capabilities as electrochemical detectors of uric acid (UA) in human serum samples at pH 2. The results showed that concentrations of UA detected by cyclic voltammetry with GCE/AuNp@cysteamine/PAMAM were comparable (deviation <±10%; limits of detection (LOD) and quantification (LOQ) were 1.7 × 10⁻⁴ and 5.8 × 10⁻⁴ mg dL⁻¹, respectively) to those concentrations obtained using the uricase-based enzymatic-colorimetric method. It was also observed that the presence of dendrimers in the GCE/AuNp@cysteamine/PAMAM system minimizes ascorbic acid (AA) interference during UA oxidation, thus improving the electrocatalytic activity of the gold nanoparticles.     

A hydrogen peroxide biosensor based on Langmuir-Blodgett technique: Direct electron transfer of hemoglobin in octadecylamine layer

The present paper describes the modification of hemoglobin (Hb)-octadecylamine (ODA) Langmuir-Blodgett (LB) film on a gold electrode surface to develop a novel electrochemical biosensor for the detection of hydrogen peroxide. Atomic force microscopy (AFM) image of Hb-ODA LB film indicated Hb molecules existed in ODA layer in a well-ordered and compact form. The immobilized Hb displayed a couple of stable and well-defined redox peaks with an electron transfer rate constant of 4.58 ± 0.95 s⁻¹ and a formal potential of −185 mV (versus Ag/AgCl) in phosphate buffer (1.0 mM, pH 5.0) contain 0.1 M KCl at a scan rate of 200 mV s⁻¹, characteristic of Hb heme Fe(III)/Fe(II) redox couple. The formal potential of Hb heme Fe(III)/Fe(II) redox couple in ODA film shifted linearly between pH 5 and 8 with a slope of −23.8 mV pH⁻¹, suggesting that proton took part in electrochemical reaction. The ODA could accelerate the electron transfer between Hb and the electrode. This modified electrode showed an electrochemical activity to the reduction of hydrogen peroxide (H₂O₂) without the aid of any electron mediator.     

Sensitive and selective electrochemical determination of quinoxaline-2-carboxylic acid based on bilayer of novel poly(pyrrole) functional composite using one-step electro-polymerization and molecularly imprinted poly(o-phenylenediamine)

A facile and efficient molecularly imprinted polymer (MIP) recognition element of electrochemical sensor was fabricated by directly electro-polymerizing monomer o-phenylenediamine (oPD) in the presence of template quinoxaline-2-carboxylic acid (QCA), based on one-step controllable electrochemical modification of poly(pyrrole)-graphene oxide-binuclear phthalocyanine cobalt (II) sulphonate (PPY-GO-BiCoPc) functional composite on glassy carbon electrode (GCE). The MIP film coated on PPY-GO-BiCoPc functional composite decorated GCE (MIP/PPY-GO-BiCoPc/GCE) was presented for the first time. The synergistic effect and electro-catalytic activity toward QCA redox of PPY-GO-BiCoPc functional composite were discussed using various contrast tests. Also, the effect of experimental variables on the current response such as, electro-polymerization cycles, template/monomer ratio, elution condition for template removal, pH of the supporting electrolyte and accumulation time, were investigated in detail. Under the optimized conditions, the proposed MIP sensor possessed a fast rebinding dynamics and an excellent recognition capacity to QCA, while the anodic current response of square wave voltammetry (SWV) was well-proportional to the concentration of QCA in the range of 1.0 × 10⁻⁸–1.0 × 10⁻⁴ and 1.0 × 10⁻⁴–5.0 × 10⁻⁴ mol L⁻¹ with a low detection limit of 2.1 nmol L⁻¹. The established sensor was applied successfully to determine QCA in commercial pork and chicken muscle samples with acceptable recoveries (91.6–98.2%) and satisfactory precision (1.9–3.5% of SD), demonstrating a promising feature for applying the MIP sensor to the measurement of QCA in real samples.     

Differential pulse voltammetric simultaneous determination of noradrenalin and acetaminophen using a hematoxylin biosensor

A highly efficient noradrenalin (NA) biosensor was fabricated on the basis of hematoxylin electrodeposited on a glassy carbon electrode, GCE. The cyclic voltammetric responses of the hematoxylin biosensor at various scan rates, which were obtained in a 0.25 mmol L⁻¹ NA solution, showed the characteristic shape typical of an EC_cat process. The kinetic parameters such as electron transfer coefficient, α, the catalytic electron transfer rate constant, k′, and the standard catalytic electron transfer rate constant, k⁰, for oxidation of NA at the hematoxylin biosensor surface were estimated using cyclic and RDE voltammetry. The peaks of differential pulse voltammetric (DPV) for NA and acetaminophen (AC) oxidation at the hematoxylin biosensor surface were clearly separated from each other when they co-exited in the physiological pH (pH 7.0). It was, therefore, possible to simultaneously determine NA and AC in the samples at a hematoxylin biosensor. Linear calibration curves were obtained for 5.0 × 10⁻¹ to 65.40 μmol L⁻¹ and 65.40-274.20 μmol L⁻¹ of NA, and for 12.00-59.10 μmol L⁻¹ and 59.10-261.70 μmol L⁻¹ of AC. The sensitivities of the biosensor to NA in the absence and presence of AC were found virtually the same, which indicates the fact that the electrocatalytic oxidation processes of NA are independent of AC and, therefore, simultaneous or independent measurements of the two analytes (NA and AC) are possible without any interference. The results of 16 successive measurements show an average voltammetric peak current of 1.13 ± 0.03 μA for an electrolyte solution containing 5.00 μmol L⁻¹ NA. The hematoxylin biosensor has been satisfactorily used for the determination of NA and AC in pharmaceutical formulations. The results obtained, using the biosensor, are in very good agreement with those declared in the label of pharmaceutical inhalation products.     

Glassy carbon electrodes modified with gold nanoparticles for the simultaneous determination of three food antioxidants

Electrochemical behavior of three antioxidants: butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and butylated hydroquinone (TBHQ), was investigated at a glassy carbon electrode modified with gold nanoparticles (AuNPs/GCE). This electrode was characterized by scanning electron microscopy (SEM). The experimental results indicated that the modified electrode was strongly electroactive during the redox reactions of BHA, BHT and TBHQ, and this was confirmed by the observed increased redox peak currents and shifted potentials; in addition, the oxidation products of BHA and TBHQ were found to be the same. The experimental conditions were optimized and the oxidation peaks of BHA and BHT were clearly separated. Based on this, an electrochemical method was researched and developed for the simultaneous determination of BHA, BHT and TBHQ in mixtures with the use of first derivative voltammetry; the linear concentration ranges were 0.10–1.50 μg mL⁻¹, 0.20–2.20 μg mL⁻¹ and 0.20–2.80 μg mL⁻¹, and detection limits were 0.039, 0.080 and 0.079 μg mL⁻¹, for BHA, BHT and TBHQ, respectively. The proposed method was successfully applied for the analysis of the three analytes in edible oil samples.     

In situ copper oxide modified molecularly imprinted polypyrrole film based voltammetric sensor for selective recognition of tyrosine

Organic-inorganic hybrids are promising functional materials as they combine the special characteristics of both organic (polymer) and inorganic phases. Among different existing approaches for the preparation of such polymer-inorganic hybrid coatings, in situ electrochemical methods are very advantageous because of their high sensitivity and simplicity. In the present study, voltammetric sensors for tyrosine are designed and developed via various modifications on glassy carbon electrode such as polypyrrole coated GCE, molecularly imprinted polypyrrole coated GCE (MIPPy) and in situ copper oxide modified MIPPy coated GCE. Of these, in situ copper oxide modified MIPPy coated GCE sensor responds to tyrosine concentrations in the range 1 × 10⁻⁸ to 1 × 10⁻⁶ and 2 × 10⁻⁶ to 8 × 10⁻⁶ M with a very low detection limit of 4.0 × 10⁻⁹ M and by far the most sensitive one. Detailed linear sweep voltammetric and chronoamperometric experiments were undertaken to investigate the electrocatalytic behavior of tyrosine. The electron transfer coefficient, diffusion coefficient and charge transfer rate constants involved in the sensing process using in situ copper oxide modified MIPPy film coated GCE are 0.47, 1.88 × 10⁻⁶ cm² s⁻¹, 4.7 × 10⁶ L mol⁻¹ s⁻¹, respectively. Furthermore, the designed sensor is highly selective and has been applied successfully for the analysis of synthetic and real samples of human urine.     

Amplified impedimetric aptasensor based on gold nanoparticles covalently bound graphene sheet for the picomolar detection of ochratoxin A

An amplified electrochemical impedimetric aptasensor for ochratoxin A (OTA) was developed with picomolar sensitivity. A facile route to fabricate gold nanoparticles covalently bound reduced graphene oxide (AuNPs–rGO) resulted in a large number of well-dispersed AuNPs on graphene sheets with tremendous binding sites for DNA, since the single rGO sheet and each AuNP can be loaded with hundreds of DNA strands. An aptasensor with sandwich model was fabricated which involved thiolated capture DNA immobilized on a gold electrode to capture the aptamer, then the sensing interface was incubated with OTA at a desired concentration, followed by AuNPs–rGO functionalized reporter DNA hybridized with the residual aptamers. By exploiting the AuNPs–rGO as an excellent signal amplified platform, a single hybridization event between aptamer and reporter DNA was translated into more than 10⁷ redox events, leading to a substantial increase in charge-transfer resistance (R_ct) by 7∼ orders of magnitude compared with that of the free aptamer modified electrode. Such designed aptasensor showed a decreased response of R_ct to the increase of OTA concentrations over a wide range of 1 pg mL⁻¹–50 ng mL⁻¹ and could detect extremely low OTA concentration, namely, 0.3 pg mL⁻¹ or 0.74 pM, which was much lower than that of most other existed impedimetric aptasensors. The signal amplification platform presented here would provide a promising model for the aptamer-based detection with a direct impedimetric method.