Surface Of Modified Glassy Carbon Electrode Research Articles

Detection of Axitinib Using Multiwalled Carbon Nanotube-Fe2O3/Chitosan Nanocomposite-Based Electrochemical Sensor and Modeling with Density Functional Theory.

In this study, axitinib (AXI), a potent and selective inhibitor of vascular endothelial growth factor receptor (VEGFR) tyrosine kinase and used as a second-generation targeted drug, was investigated electrochemically under optimized conditions using multiwalled carbon nanotubes/iron(III) oxide nanoparticle–chitosan nanocomposite (MWCNT/Fe2O3@chitosan NC) modified on the glassy carbon electrode (GCE) surface. Characterization of the modified electrode was performed using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The adsorptive stripping differential pulse voltammetric (AdSDPV) technique was used for the sensitive, rapid, and precise detection of AXI. The current peak obtained with the MWCNT/Fe2O3@chitosan NC modified electrode was 23 times higher compared to the bare electrode. The developed modified electrode showed excellent electrocatalytic activity in AXI oxidation. Under optimized conditions, the effect of supporting electrolyte and pH was investigated, and 0.1 M H2SO4 was chosen as the electrolyte with the highest peak current for the target analyte. In the concentration range of MWCNT/Fe2O3@chitosan NC/GCE, 6 × 10–9 and 1 × 10–6 M, the limit of detection (LOD) and limit of quantification (LOQ) values were calculated to be 0.904 and 0.0301 pM, respectively. Tablet and serum samples were used for the applicability of the developed sensor, relative standard deviation (RSD) values for all samples were below 2%, and the recovery results were 99.23 and 101.84%, respectively. The MWCNT/Fe2O3@chitosan NC/GCE designed to determine AXI demonstrated the applicability, selectivity, precision, and accuracy of the sensor. The mechanism of electron transfer from the modified GCE surface to the analyte solution is studied via modeling the modified GCE surface by the density functional theory (DFT) method at B3LYP/6-311+g(d,p) and M062X/6-31g(d,p) levels. We observed that the iron oxide nanoparticles play an important role in channeling electron flow from the analyte solution to the MWCNT-coated GCE electrode surface. Adsorption of the nanocomposite material onto the GCE surface occurs via strong electrostatic interactions, including ionic and hydrogen bond formations. During the adsorption-controlled oxidation process of the axitinib, the electrons are transferred via the highest occupied molecular orbital (HOMO) localized on the iron oxide moiety to the lowest unoccupied molecular orbital (LUMO) of the MWCNT/GCE surface.

Pillar[5]arene Based Non-Enzymatic and Enzymatic Tyramine Sensor

In this study, non-enzymatic and enzymatic tyramine sensors were prepared by using P[5]A as a surface modification material. P[5]A, which belongs to the group of ionic liquids, enables the development of a biocompatible and stable electroactive electrode surface. In the modification process, P[5]A dispersed in gelatin solution was dropped on a glassy carbon electrode (GCE). In the preparation of enzymatic sensor, tyrosinase (TYR) was immobilized by used glutaraldehyde as a cross-linker on P[5]A modified GCE surface. Surface morphology and electrochemical behavior of P[5]A modified GCE was investigated by scanning electron microscope (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Experimental conditions such as pH, working potential and P[5]A amount were optimized as GCE/Gel/P[5]A and GCE/Gel/P[5]A/TYR. Under optimum experimental conditions, linear working ranges of GCE/Gel/P[5]A and GCE/Gel/P[5]A/TYR was $8.0\times 10^{-8} - 1.5\times 10^{-6}$ M with LOD of $4.0\times 10^{-8}$ M and $1.6\times 10^{-6} - 4.2\times 10^{-5}\text{M}$ with LOD of $5.0\times 10^{-7}$ M, respectively. These sensors showed a lower detection limit, high selectivity and good repeatability for tyramine. Analytical applicability of non-enzymatic and enzymatic sensor was examined by tyramine analysis and recovery studies in the sauerkraut sample.

Application of carbon nanoonion-NiMoO4-MnWO4 nanocomposite for modification of glassy carbon electrode: Electrochemical determination of ascorbic acid

In this paper, sonochemical synthesized nanocomposite was used in the construction of an electrochemical sensor. Decorated carbon nanoonions (CNOs) with nickel molybdate (NiMoO4) and manganese tungstate (MnWO4) nanocomposite were used to modify the glassy carbon electrode (GCE). The high conductivity and large surface area of the CNOs and the electrocatalytic effect of NiMoO4 and MnWO4 nanocomposite provide a synergistic effect on the electrooxidation processes. The modified GCE surface with a synthesized nanocomposite of CNO-NiMoO4-MnWO4 was used to determine ascorbic acid using differential pulse voltammetry (DPV) method. The developed ascorbic acid sensor displayed a wide dynamic calibration range (1–100 μM) with trace level LOD (0.33 μM) and appropriate sensitivity (0.1768 μA μM−1cm−2). The determination of ascorbic acid in the serum and urine as the real samples exhibited a reasonable relative recovery in the range of 100–104%.

Development of a Molecularly Imprinted Poly(Acrylic Acid)‐MWCNT Nanocomposite Electrochemical Sensor for Tramadol Determination in Pharmaceutical Samples

AbstractA new voltammetric sensor based on molecularly imprinted poly(acrylic acid)‐MWCNT nanocomposite (MIP‐MWCNT) drop‐coated onto glassy carbon electrode (GCE) was developed and applied to tramadol (TR) determination in pharmaceutical samples. The voltammetric sensor prepared by suspension of MIP‐MWCNT at 1 : 1 (w/w) ratio show an improved performance compared to unmodified GCE. The electrochemical method is based on preconcentration of tramadol onto MIP‐MWCNT modified GCE surface at −1.5 V vs Ag/AgCl for 180 s in 0.1 Britton‐Robinson buffer (pH 8.0) at stirred solution. Upon preconcentration, the differential anodic voltammogram was recorded under the optimized condition giving rise to an analytical curve varying from 9.0 up to 30.0 μmol L−1 (R2=0.997) and limits of detection and quantification of 1.4 and 4.8 μmol L−1, respectively. The method precision was assessed in terms of intraday (n=6) and interday (two consecutive days) precision, giving relative standard deviations (RSD%) values between 2.8 to 7.4 %. Excipients usually found in pharmaceutical pills (magnesium stearate, microcrystalline cellulose, starch, and silica) and paracetamol were evaluated as potential interferents, however no interference was evidenced in TR determination. The method applicability was evaluated by TR analysis in pharmaceutical samples. Moreover, the method accuracy was attested by comparison of addition and recovery assays with a reference technique (high‐performance liquid chromatography).

Real Time Detection of Hazardous Hydroxyl Radical Using an Electrochemical Approach

AbstractThe proposed work demonstrates an electrochemical strategy for hydroxyl radical (•OH) detection based on aptamer immobilized glassy carbon electrode (GCE). To enhance the sensitivity of the detection method, nitrogen doped porous carbon nanostructures (N−C) and gold nanoparticles (AuNPs) were used as signal amplifiers. During biosensor fabrication, thiolated DNA and 6‐mercaptohexanol (MCH) were self assembled on N−C/AuNPs modified GCE surface. The fabricated biosensor was characterized electrochemically by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) which justified its high electronic conductivity. The aptamer immobilized electrode (N−C/AuNPs/DNA/MCH/GCE) was immersed in Fenton's reagent where oxidative damage of DNA occurred due to generated •OH. The DNA damage was monitored by hexaammineruthenium(III) (Ru(NH3)63+) which acted as electrochemical probe for sensitive detection of •OH. Square wave voltammetry (SWV) was used for •OH biosensing using simple, sensitive and rapid strategy at the fabricated electrode.

An electrochemical tyrosinamide aptasensor using a glassy carbon electrode modified by N-acetyl-l-cysteine-capped Ag-In-S QDs

This paper reports an aptamer-based green approach for the electrochemical evaluation of tyrosinamide (Tyr-NH2). In this regard, at the first step, an aqueous synthetic strategy for preparing N-acetyl-l-cysteine (NAC)-capped Ag-In-S (AIS) quantum dots (QDs) with bright yellow/orange emission was developed. The conjugation of AIS QDs to NAC-biomolecules provides opportunities for using them as luminescent contrast agents for living cell tracking and labeling or sensing studies. In the next step, the design stage of the aptasensor, the glassy carbon electrode (GCE) was modified with the AIS QDs and then the Tyr-NH2 special aptamer, which has an amine group at its end, interacts with silver and indium ions at the surface of the AIS QDs and through the formation of covalent bonding of AgN and InN, attaches to the GCE surface modified with the AIS QDs. In this approach, for the first time, NAC-capped AIS QDs have been used to modify the electrode surface in the aptamer-based electrochemical sensor. The response changes of the [Fe(CN)6]4−/3− as redox probe, during the modification of GCE surface, the fabrication and assessment of proposed aptasensing, using the cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy were recorded. The designed aptasensor for the Tyr-NH2 evaluation showed good linearity from 0.01 to 2.81 nM and 2.81–10.81 nM, and low detection limit of 3.34 pM. The obtained results of the stability, reproducibility and selectivity investigations implying that the reported aptasensor as the first aptamer-based electrochemical assay for Tyr-NH2, can be reliable for the determination of Tyr-NH2 in serum samples.

A novel electrochemical biosensor for antioxidant evaluation of phloretin based on cell-alginate/ʟ-cysteine/gold nanoparticle-modified glassy carbon electrode

Antioxidant evaluation of bioactive compounds is limited, since many methods lack a real physiological environment that can be used conveniently and intuitively. In this study, a simple, label-free and effective electrochemical biosensor method has been developed to evaluate the antioxidant effect of phloretin (Ph) by 3D cell modification on a glassy carbon electrode (GCE). In response to this, A549 cells were immobilized onto a self-assembled ʟ-cysteine/gold nanoparticle (AuNPs/ʟ-Cys)-modified GCE surface by a simple drop casting after encapsulated in alginate. The electrochemical impedance spectroscopy (EIS) results showed that the impedance value (Ret) increased with the concentration of H2O2 in the range of 0–60 μmol/L with the correlation of 0.990 which acted as an oxidative stress model inducer. However, the EIS value decreased with the co-incubation of Ph ranging from 10 to 100 μmol/L, showing a dose-dependent manner and time effect, indicating that the variation of Ret was responded to the antioxidant effect. The response impedance of the biosensor is linear to Ph concentrations from 20 μmol/L to 100 μmol/L with the detection limit (LOD) as 1.96 μmol/L. A significant correlation was observed between reactive oxygen species (ROS) values and Ret values following the concentrations of Ph, thus demonstrating the good biological relevance of cell-based electrochemical method. The strategy has been used to evaluate Ph antioxidant capacity in real cells with satisfactory results, indicating the feasibility of biosensor analysis for antioxidant evaluation.

Conformation switching of an aptamer based on cocaine enhancement on a surface of modified GCE

An ultrasensitive aptasensor was fabricated as an electrochemical nanotool based on the conformation switching of an aptamer (Apt). The Apt which was covalently attached on the surface of a glassy carbon electrode (GCE) covered with cadmium telluride (CdTe) quantum dots (QDs) works as a unique modifier for assaying cocaine. The Apt was combined with cocaine to form a three-way junction complex; this complex increased the steric hindrance of the modified GCE surface and resulted in a variation of the corresponding current of a redox probe. In the present study, DPV technique for cocaine detection was applied and resulted in an unprecedented detection limit (LOD) of 5.0±0.1pmolL−1, which is more sensitive than previously reported methods. One of the greatest advantages of this aptasensor is the elimination of enzymes or antibodies. It is also relatively a highly sensitive, simple, reproducible, and controllable nanotool. Likewise, it can be easily miniaturized, which is a necessary condition for the high-throughput system and on-site applications. The offered nanotool has a great promise for the routine analysis of the ultra-trace amounts of cocaine, which is important for law enforcement and clinical medicine. It is notable to say that further attempts are under way in our laboratory for the construction of other aptasensors with higher performance for specific targets such as the detection of methadone (MTD) and ibuprofen (IBP).

Determination of metformin at metal-organic framework (Cu-BTC) nanocrystals/multi-walled carbon nanotubes modified glassy carbon electrode

Glassy carbon (GC) electrode modified with Cu-based metal-organic frameworks (Cu-BTC, BTC=1,3,5-benzenetricarboxylate) and multi-walled carbon nanotubes (CNTs) composites was developed. The Cu-BTCs were synthesized by two methods, traditional hydrothermal and ultrasonic irradiation methods. It was found that the Cu-BTC that was synthesized by the latter method, with mean crystallite sizes in the nanometers range, showed a higher impact on enhancing the electrocatalytic activity of the modified electrode toward the electro-oxidation of metformin compared to that synthesized with the former method with a larger crystallite size of micrometer-scale. The morphological characteristics of Cu-BTCs and Cu-BTC nanocrystals/CNTs modified GC electrode surface were characterized by scanning electron microscopy. The modified electrode showed excellent response of oxidation current for the determination of metformin. Under optimum conditions, the peak current showed good linearity with concentrations in the range of 0.5 μM to 25 μM and a detection limit of 0.12 μM, as well as good repeatability. The Cu-BTC nanocrystals/CNTs modified GC electrode was applied successfully to determine metformin in pharmaceutical samples with good accuracy.

Morphology-dependent NiO modified glassy carbon electrode surface for lead(II) and cadmium(II) detection

Glassy carbon electrode (GCE) surfaces have been modified with different NiO morphologies consisting of rods NiO, flakes NiO and balls NiO prepared via the hydrothermal synthesis method for Pb(II) and Cd(II) detection by using the square wave anodic stripping voltammetry (SWASV). Meanwhile, the typical cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), BET surface area and adsorption property of the modified electrode surfaces have been investigated to evaluate their electrochemical detection effect. Results show that balls NiO modified GCE can get the optimal detection ability for its highest detection sensitivity to Pb(II) (13.46AM−1) and Cd(II) (5.10AM−1), the lowest detection limit (DL) to Pb(II) (0.08μM) and Cd(II) (0.07μM) as well as the superior linear relativity. In addition, an enhanced current at redox peaks, lower electron transfer resistance, larger BET surface area and stronger adsorption capacity have been confirmed for the balls NiO modified GCE surface. Finally, excellent stability and reproducibility of balls NiO modified electrodes for Pb(II) and Cd(II) detection have also been proved via the SWASV responses.

Electrochemical determination of Cu(II) ions using glassy carbon electrode modified by some nanomaterials and 3-nitroaniline

Abstract The aim of this research was to investigate the effect of the several nanomaterials in electrochemical determination of Cu(II) ions. For this aim, firstly the deposition of graphene oxide (GO), graphene, magnetite (Fe3O4), gold-chitosan (AuChts) or multilayer carbon nanotubes (MWCNTs) on the glassy carbon (GC) electrode surface was performed. Then the electrochemical modification of electrode by poly-3-nitroaniline (poly-3NA) was performed by 100 potential cycles in the range between +0.9 V and +1.4 V vs. Ag/AgNO3 at the sweep rate of 100 mV/s. For electrochemical reduction of nitro groups present on modified GC electrode surface, potential cycling was performed in 100 mM HCl between −0.1 V and −0.8 V vs. Ag/AgCl/(KClsat.) at the sweep rate of 100 mV/s. Nanomaterial and poly-3NA modified electrodes were applied in the determination of Cu(II) ions by differential pulse voltammetry. It was determined that GC electrodes consecutively modified with MWCNTs, poly-3NA and then by electrochemical reduction of nitro groups were the most sensitive towards Cu(II) ions with detection limit of 0.5 × 10−9 M.

Electrochemical determination of copper(II) ions at naringenin-modified glassy carbon electrode: application in lake water sample

Abstract Electrochemical modification of glassy carbon (GC) electrode by naringenin (NG), a fl avonoid derivative, and applicability of electrode modified in this way for determination of copper(II) (Cu(II)) ions is reported in this study. Surface modification experiments were performed in phosphate buffer solution (PBS), pH 7, in the 0.0 mV and +1400 mV potential ranges with a sweep rate of 100 mV s–1 and 30 cycles. The surface modification of GC electrode was performed with NG using cyclic voltammetry (CV), whereas the characterization of this sensor electrode was performed using CV and electrochemical impedance spectroscopy (EIS). The sensitivity of GC electrode modified in described way towards Cu(II) ions was investigated in Britton–Robinson (BR) buffer solution, pH 5, by differential pulse voltammetry (DPV). For the calibration curve, a series of standard Cu(II) solution from 1.0 × 10−12 M to 1.0 × 10−6 M was prepared. The NG modified GC (NG/GC) sensor electrode, has a good repeatability, was applie...

Highly‐Ordered Covalent Anchoring of Carbon Nanotubes on Electrode Surfaces by Diazonium Salt Reactions

Since their discovery in 1991, carbon nanotubes (CNTs) have attracted growing interest in view of their unique mechanical and physicochemical properties, and have successfully been applied in microelectronics such as memory devices, switches, supercapacitors, and in the biomedical area as drug-delivery devices and biosensors. The oriented assembly of CNTs on an electrode surface offers further possibilities with respect to both electrochemical biosensors because of the high electron-transfer rate along the tube, and the attachment of redoxactive molecules (or redox enzymes) to the ends of the CNTs. Electrodes modified with vertically aligned CNTs have been accomplished through direct growth or self-assembly of CNTs. Although different procedures can be followed to achieve a covalent attachment of molecules, the most applied method to anchor CNTs is perhaps that based on the formation of amide bonds from the reaction between the amines located on the modified electrode and the carboxylic groups at the ends and side-wall defects of the nanotubes. Since 1992, when an aryl diazonium salt was used for the covalent functionalization of a carbon electrode, this approach has been applied to a variety of surfaces such as carbon-based materials, metals, and semiconductors. The method has also been used for the derivatization of CNTs, that is, multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs). To the best of our knowledge, no report of CNTanchoring on a surface by the use of diazonium salt reactions has been published to date. The aim of the present work is to develop a new approach based on diazonium salt reactions that provides a simple, stable, and well-organized assembly of CNTs for a wide range of substrates. To chemically modify an electrode surface, a diazonium salt is reduced to the corresponding aryl radical, which binds to the surface rapidly. The process is promoted by a variety of experimental conditions, and can even be used in combination with click chemistry. Herein, the procedures adopted for the derivatization of a glassy carbon electrode (GCE) are shown in Figure 1. For the immobilization of unfunctionalized SWCNTs on the chemically modified GCE surface, the p-nitro diazonium ions, which are formed in situ from p-nitroaniline (see Experimental Section for details), bind to the CGE surface (Figure 1a). The amine groups obtained by electrochemical reduction of nitro groups are converted into the corresponding diazonium functionalities, which covalently bind SWCNTs. The incorporation of phenyl groups to form a mixed monolayer, which is anticipated to prevent the diazo coupling, a reaction that occurs in a densely packed p-aniline monolayer. Indeed, the electrophilic attack of the diazonium functionality at the ortho position of a vicinal p-aniline (diazo coupling) or to the vicinal amine group, avoids CNTs immobilization. Alternatively, SWCNTs are first functionalized with p-nitrobenzene groups, then the amines formed by reduction of the nitro groups are converted into diazonium ions, which covalently bind SWCNTs on a clean GCE surface (Figure 1b). The orientation of SWCNTs attached to a GCE surface was investigated by scanning electron microscopy (SEM). A typical SEM image, taken from a 458 tilted view, is shown in Figure 1c; the image clearly shows that SWCNTs are arranged vertically on the surface. The obtained arrangement is independent of the reaction route. TEM images clearly show that SWCNTs are perpendicularly oriented with only one end anchored on the CGE surface (Figure 1d,e). The SWCNTs attached to the GCE were 80–120 nm in length, with a diameter of 2–8 nm. Cyclic voltammograms (CVs) were recorded in a solution of K3[Fe(CN)6] in water (2 10 3 molL ) using bare GCEs or GCEs modified with SWCNTs according to the procedure described above (Figure 2). The assembly of CNTs on the electrode leads to a 1.5-fold increase of the current intensity, which results from the increased active electrode surface; the double-layer capacitance increases accordingly. Sonication of the modified electrode slightly decreases the current intensity, thus suggesting that some SWCNTs may be adsorbed on the surface. Also, the electrode modification slightly improves the reversibility, as indicated by the peak separation values. The electron-transfer rate constants (kET) determined for both bare and SWCNT-modified GCEs, were calculated from the peak separation assuming the [Fe(CN)6] 3 /4 diffusion coef[*] Prof. T. Ferri, V. Paolini Department of Chemistry, “Sapienza” University of Rome Piazzale Aldo Moro 5, 00185 Rome (Italy) E-mail: tommaso.ferri@uniroma1.it

Simultaneous voltammetric determination of tramadol and acetaminophen using carbon nanoparticles modified glassy carbon electrode

A sensitive and selective electrochemical sensor was fabricated via the drop-casting of carbon nanoparticles (CNPs) suspension onto a glassy carbon electrode (GCE). The application of this sensor was investigated in simultaneous determination of acetaminophen (ACE) and tramadol (TRA) drugs in pharmaceutical dosage form and ACE determination in human plasma. In order to study the electrochemical behaviors of the drugs, cyclic and differential pulse voltammetric studies of ACE and TRA were carried out at the surfaces of the modified GCE (MGCE) and the bare GCE. The dependence of peak currents and potentials on pH, concentration and the potential scan rate were investigated for these compounds at the surface of MGCE. Atomic force microscopy (AFM) was used for the characterization of the film modifier and its morphology on the surface of GCE. The results of the electrochemical investigations showed that CNPs, via a thin layer model based on the diffusion within a porous layer, enhanced the electroactive surface area and caused a remarkable increase in the peak currents. The thin layer of the modifier showed a catalytic effect and accelerated the rate of the electron transfer process. Application of the MGCE resulted in a sensitivity enhancement and a considerable decrease in the anodic overpotential, leading to negative shifts in peak potentials. An optimum electrochemical response was obtained for the sensor in the buffered solution of pH 7.0 and using 2 μL CNPs suspension cast on the surface of GCE. Using differential pulse voltammetry, the prepared sensor showed good sensitivity and selectivity for the determination of ACE and TRA in wide linear ranges of 0.1–100 and 10–1000 μM, respectively. The resulted detection limits for ACE and TRA was 0.05 and 1 μM, respectively. The CNPs modified GCE was successfully applied for ACE and TRA determinations in pharmaceutical dosage forms and also for the determination of ACE in human plasma.

Electrochemical study of bergenin on a poly(4-(2-pyridylazo)-resorcinol) modified glassy carbon electrode and its determination in tablets and urine

A very sensitive electroanalytical method was employed to determine bergenin in phosphate buffer with a pH of 6.0, bergenin was accumulated at a 4-(2-pyridylazo)-resorcinol (PAR) polymer film modified glassy carbon electrode (GCE) surface under the condition of open-circuit. In the following anodic sweep from −0.4 to 0.8 V, bergenin adsorbed at the PAR polymer film modified electrode surface, was oxidized and yielded a sensitive oxidation peak at 0.595 V. Due to its unique structure and extraordinary properties, the PAR polymer film shows higher accumulation efficiency toward bergenin compared with a bare GCE. Hence, the amount of bergenin at the PAR polymer film modified GCE surface increases significantly, and finally the oxidation peak current improves greatly. The experimental conditions, such as supporting electrolyte, pH value, accumulation time and scan rate, were optimized for the measurement of bergenin, and a sensitive electroanalytical method was proposed for bergenin determination. The oxidation peak current varies linearly with the concentration of bergenin over the range of 2.0 × 10 −7 to 1.2 × 10 −5 mol/L, and the detection limit is 2.0 × 10 −8 mol/L after 3 min open-circuit accumulation. The relative standard deviation of the same electrode in 10 successive scans is 1.8% for 1.0 × 10 −6 mol/L bergenin and 2.1% for interelectrodes, indicating excellent reproducibility. This new method was successfully demonstrated with bergenin tablets and diluted urine.

A new scheme of hybridization based on the Au nano–DNA modified glassy carbon electrode

DNA hybridization on the Au nano–DNA modified glassy carbon electrode (GCE) was investigated. The thiol modified probe oligonucleotides (SH–ssDNA) at the 5′ phosphate end were assembled on the Au nano–DNA modified GCE surface. The electrochemical response of the probe immobilization and hybridization with target DNA was measured by differential pulse voltammetry (DPV) using methylene blue (MB) as the electroactive indicator. Gold nanoparticles can be dispersed effectively on the GCE surface in the presence of calf thymus DNA. Au nano–DNA modified GCE could greatly increase the active sites and enhance the response signal during immobilization and hybridization. The hybridization amount of target DNA could be greatly increased. The linear detection range of Au nano–DNA electrode for the complementary 21-mer oligonucleotide (cDNA) was achieved from 1.52 × 10 −10 to 4.05 × 10 −8 mol L −1. The detection limit could reach the concentration of 10 −10 mol/L.

Hematoxylin multi-wall carbon nanotubes modified glassy carbon electrode for electrocatalytic oxidation of hydrazine

A new hydrazine sensor has been fabricated by immobilizing hematoxylin at the surface of a glassy carbon electrode (GCE) modified with multi-wall carbon nanotube (MWCNT). The adsorbed thin films of hematoxylin on the MWCNT modified GCE show one pair of peaks with surface confined characteristics. The hematoxylin MWCNT (HMWCNT) modified GCE shows highly catalytic activity toward hydrazine electro-oxidation. The results show that the peak potential of hydrazine at HMWCNT modified GCE surface shifted by about 167 and 255 mV toward negative values compared with that at an MWCNT and activated modified GCE surface, respectively. In addition, at HMWCNT modified electrode surface remarkably improvement the sensitivity of determination of hydrazine. The kinetic parameters, such as the electron transfer coefficient, α, and the standard heterogeneous rate constant, k 0, for oxidation of hydrazine at the HMWCNT modified GCE were determined and also is shown that the heterogeneous rate constant, k′, is strongly potential dependent. The overall number of electron involved in the catalytic oxidation of hydrazine and the number of electrons involved in the rate-determining steps are 2 and 1, respectively. The amperometric detection of hydrazine is carried out at 220 mV in 0.1 M phosphate buffer solution (pH 7) with linear response range 2.0–122.8 μM hydrazine, detection limit of 0.68 μM and sensitivity of 0.0208 μA μM −1. Finally the amperometric response for hydrazine determination is reproducible, fast and extremely stable, with no loss in sensitivity over a continual 400 s operation.

Layer-by-layer assembly of multilayer films consisting of silicotungstate and a cationic redox polymer on 4-aminobenzoic acid modified glassy carbon electrode and their electrocatalytic effects

A novel 4-aminobenzoic acid (4-ABA) monolayer film is formed on glassy carbon electrode (GCE) by amino cation radical method. Silicotungstic heteropolyanion (SiW 12O 40 4−, denoted as SiW 12)-containing multilayer films have been fabricated on the 4-ABA modified GCE surface by alternate deposition with a quaternized poly(4-vinylpyridine) partially complexed with [Os(bpy) 2Cl] 2+/+ (denoted as QPVP-Os). Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XR) have been used to characterise the as-prepared multilayer films. It is proved that the multilayer films are uniform and stable. The average thickness for a bilayer of QPVP-Os/SiW 12 in the multilayer film is 30.2 Å. The electrocatalytic activities of the multilayer films have been investigated on the reduction of three substrates of important analytical interests, HNO 2, BrO 3 − and H 2O 2. Especially, the influence of layer number of the multilayer films on the electrocatalytic reduction of HNO 2 has been investigated in detail.