Nucleic acid biosensor for DNA hybridization detection using rutin-Cu as an electrochemical indicator

The complex of rutin-Cu (C₈₁H₈₆Cu₂O₄₈, abbreviated by Cu₂R₃, R = rutin) was synthesized and characterized by elemental analysis and IR spectra. Cyclic voltammetry (CV) and fluorescence spectroscopy were used to investigate the interaction of Cu₂R₃ with salmon sperm DNA. It was revealed that Cu₂R₃ could interact with double-stranded DNA (dsDNA) by a major intercalation role. Using Cu₂R₃ as a novel electroactive indicator, an electrochemical DNA biosensor for the detection of specific DNA fragment was developed and its selectivity for the recognition with different target DNA was assessed by differential pulse voltammetry (DPV). The target DNA related to coliform virus gene could be quantified ranged from 1.62 × 10⁻⁸ mol L⁻¹ to 8.10 × 10⁻⁷ mol L⁻¹ with a good linearity (r = 0.9989) and a detection limit of 2.3 × 10⁻⁹ mol L⁻¹ (3σ, n = 7) was achieved by the constructed electrochemical DNA biosensor.     

A sensitive DNA biosensor fabricated from gold nanoparticles, carbon nanotubes, and zinc oxide nanowires on a glassy carbon electrode

We outline here the fabrication of a sensitive electrochemical DNA biosensor for the detection of sequence-specific target DNA. Zinc oxide nanowires (ZnONWs) were first immobilized on the surface of a glassy carbon electrode. Multi-walled carbon nanotubes (MWCNTs) with carboxyl groups were then dropped onto the surface of the ZnONWs. Gold nanoparticles (AuNPs) were subsequently introduced to the surface of the MWNTs/ZnONWs by electrochemical deposition. A single-stranded DNA probe with a thiol group at the end (HS-ssDNA) was covalently immobilized on the surface of the AuNPs by forming an Au-S bond. Scanning electron microscopy (SEM) and cyclic voltammetry (CV) were used to investigate the film assembly process. Differential pulse voltammetry (DPV) was used to monitor DNA hybridization by measuring the electrochemical signals of [Ru(NH₃)₆]³⁺ bounding to double-stranded DNA (dsDNA). The incorporation of ZnONWs and MWCNTs in this sensor design significantly enhances the sensitivity and the selectivity. This DNA biosensor can detect the target DNA quantitatively in the range of 1.0 × 10⁻¹³ to 1.0 × 10⁻⁷ M, with a detection limit of 3.5 × 10⁻¹⁴ M (S/N = 3). In addition, the DNA biosensor exhibits excellent selectivity, even for single-mismatched DNA detection.     

Hybridization biosensor using sodium tanshinone IIA sulfonate as electrochemical indicator for detection of short DNA species of chronic myelogenous leukemia

Cyclic voltammetry (CV) was used to investigate electrochemical behavior of sodium tanshinone IIA sulfonate (STS) and the interaction between STS and salmon sperm DNA. STS had excellent electrochemical activity on the glassy carbon electrode (GCE) with a couple reversible redox peaks. In pH 4.0 phosphate buffer solution (PBS), the binding ratio between STS and salmon sperm DNA was calculated to be 1:1 and the binding constant was 1.67 × 10⁴ L/mol. A chronic myelogenous leukemia (CML, Type b3a2) DNA biosensor was developed by immobilizing covalently single-stranded CML DNA fragment to a modified GCE. The surface hybridization of the immobilized single-stranded CML DNA fragment with its complementary DNA fragment was evidenced by electrochemical methods using STS as a novel electrochemical indicator, with a detection limit of 6.7 × 10⁻⁹ M and a linear range from 2.0 × 10⁻⁸ M to 2.0 × 10⁻⁷ M. Selective determination of complementary ssDNA was achieved using differential pulse voltammetry (DPV).     

Amperometric flow injection analysis as a new approach for studying disperse systems

Fast and simple quantitative determination in dispersed systems (layered double hydroxides - LDHs - suspensions in aqueous solutions) was performed by a procedure that couples flow injection and amperometric detection (FI-AM). LDH dispersions are injected in a continuous flow (1 mL min⁻¹) of 0.05 mol L⁻¹ KNO₃ solution and [Cu(H₂O)₆]²⁺, used as a probe, is detected at a glassy carbon electrode housed in a flat electrochemical cell. The current intensity, recorded at the selected working potential (−0.25 V vs Ag/AgCl/NaCl (3 mol L⁻¹)), presents a linear relationship with [Cu(H₂O)₆]²⁺ concentration and the procedure offers high sensitivity (slope = 0.036 μA/(μmol L⁻¹)), a low detection limit (=0.7 μmol L⁻¹) and a wide quantification range (4-200 μmol L⁻¹).The method was applied to [Cu(H₂O)₆]²⁺ determination in two particular LDH-aqueous solution dispersed systems: (1) [Cu(H₂O)₆]²⁺ scavenging by etilendiammintetraacetic acid (EDTA) modified Zn-Al-LDHs, and (2) [Cu(H₂O)₆]²⁺ release from a copper doped Mg-Al-LDHs. The results obtained are comparable to those reported in previous works using different quantification techniques. FI-AM determination is applied without sample pretreatment (solid-supernatant separation) providing a high sampling rate (above 120 samples h⁻¹) that allows a better comprehension of the processes, particularly at the initial stages.     

Electrochemical and spectroscopic properties of dendritic cobalto-salicylaldiimine DNA biosensor

A metallodendrimer-based electrochemical DNA biosensor was constructed by a layer-by-layer assembly of cobalt(II) salicylaldiimine metallodendrimer (SDD-Co(II)) and a 21 bases oligonucleotide NH₂-5′-GAGGAGTTGGGGGAGCACATT-3′ (pDNA) on a gold electrode. The complementary oligonucleotide was 5′-AATGTGCTCCCCCAACTCCTC-3′ (tDNA). UV-visible spectra of SDD-Co(II) in 1:1 (v/v) acetone-ethanol solution showed absorption bands at 325 nm and 365-420 nm related to π-π* intra-dendrimer transitions and d-π* metal-dendrimer charge transfer transitions, respectively. Square-wave voltammetry (SWV) characterisation of the Au|SDD-Co(II)|pDNA biosensor system in phosphate buffer saline solution of pH 7.4, indicated a reversible one-electron electrochemical process with a formal potential, E°′, value of +210 mV. Electrochemical impedance spectroscopy (EIS) data confirmed that the hybridisation of the biosensor's pDNA with the tDNA to form double-stranded DNA (dsDNA) resulted in an increase of the impedimetric charge transfer resistance, R_ct, value from 6.52 to 12.85 kΩ. The limit of detection (LOD), calculated as 3σ of the background noise, and sensitivity of the sensor were 1.29 kΩ/nmol L⁻¹ and 0.34 pmol L⁻¹, respectively.     

Variations in the crystalline deposits formed upon electrochemical oxidation of the anions, [Ir(CO)2X2] (X = Cl, Br, and I)

The electrochemical properties of the ions [Ir(CO)₂X₂]⁻ (X = Cl, Br, and I) have been studied in dichloromethane solutions using cyclic voltammetry, chronoamperometry, electrochemical quartz crystal microbalance (EQCM), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy. For the chloro and bromo salts, the anion [Ir(CO)₂X₂]⁻ is oxidized initially to form [Ir(CO)₂X₂]⁺. The standard rate constants for the two-electron oxidations of [Ir(CO)₂X₂]⁻ are 7.8(±0.6) × 10⁻³ and 9.2(±0.9) × 10⁻⁴ cm s⁻¹ for the redox couples, [Ir(CO)₂Cl₂]^−/+ and [Ir(CO)₂Br₂]^−/+, respectively. The processes following electrolysis lead to the formation of two types of crystalline deposits on the electrode surface: needles and plates. The relative amounts of these solid phases that form depend mainly on the concentration of iridium complex in solution and on the time window of experiment. The strong intermetallic Ir-Ir interaction is responsible for the formation of the one-dimensional iridium complex chain. The crystal structures of the needle phases formed from [Ir(CO)₂Cl₂]⁻ and [Ir(CO)₂Br₂]⁻ are the same and belong to the space group Cmcm (no. 63). The stoichiometry of the one-dimensional crystals depends on the constitution of the supporting electrolyte: (TAA)_0.6[Ir(CO)₂Cl₂] and (TAA)_0.7[Ir(CO)₂Br₂] (TAA is tetra(alkyl)ammonium cation) salts are formed on the electrode surface. The formation of large three-dimensional crystal is responsible for the accumulation of electroactive materials on the electrode surface. The irreversible oxidation of [Ir(CO)₂I₂]⁻ leads only to the formation of large, plate-like crystals on the electrode surface, no needles are formed.     

An electrochemical DNA biosensor developed on novel multinuclear nickel(II) salicylaldimine metallodendrimer platform

An electrochemical DNA biosensor (EDB) was prepared using an oligonucleotide of 21 bases with sequence NH₂-5′-GAGGAGTTGGGGGAGCACATT-3′ (probe DNA) immobilized on a novel multinuclear nickel(II) salicylaldimine metallodendrimer on glassy carbon electrode (GCE). The metallodendrimer was synthesized from amino functionalized polypropylene imine dendrimer, DAB-(NH₂)₈. The EDB was prepared by depositing probe DNA on a dendrimer-modified GCE surface and left to immobilize for 1 h. Voltammetric and electrochemical impedance spectroscopic (EIS) studies were carried out to characterize the novel metallodendrimer, the EDB and its hybridization response in PBS using [Fe(CN)₆]^3−/4− as a redox probe at pH 7.2. The metallodendrimer was electroactive in PBS with two reversible redox couples at E°′ = +200 mV and E°′ = +434 mV; catalytic by reducing the E_pa of [Fe(CN)₆]^3−/4− by 22 mV; conducting and has diffusion coefficient of 8.597 × 10⁻⁸ cm² s⁻¹. From the EIS circuit fitting results, the EDB responded to 5 nM target DNA by exhibiting a decrease in charge transfer resistance (R_ct) in PBS and increase in R_ct in [Fe(CN)₆]^3−/4− redox probe; while in voltammetry, increase in peak anodic current was observed in PBS after hybridization, thus giving the EDB a dual probe advantage.     

Electrochemical study on cobalt film modified glassy carbon electrode and its application

A novel electroactive material for ascorbic acid (AA) determination was successfully prepared by plating/potential cycling method. The cobalt film was first deposited on the surface of glassy carbon electrode (GCE) in CoSO₄ solution by potential cycling, and then a cobalt film on the surface of GCE was activated by potential cycling in 0.1 mol L⁻¹ NaOH. The electrochemical performance of the resulted film (Co/GCE) and factors affecting its electrochemical activity were investigated by cyclic voltammetry and amperometry. This film electrode exhibited good electrocatalytic activity to the oxidation of AA. This biosensor had a fast response of AA less than 3 s and excellent linear relationships were obtained in the concentration range of 3 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 2 × 10⁻⁷ mol L⁻¹ (S/N = 3) under the optimum conditions. Moreover, the selectivity, stability and reproducibility of this biosensor were evaluated with satisfactory results.     

Electrocatalytic reduction of bromate ion using a polyaniline-modified electrode: An efficient and green technology for the removal of BrO3 in aqueous solutions

The electrocatalytic reduction of bromate ion (BrO₃⁻) was investigated in a three-electrode system using polyaniline (PANI) as the electrode material. Bromate ion reduction and Br⁻ removal were observed during electrochemical treatment because of the catalytic and doping capabilities of the PANI film. BrO₃⁻ removal efficiency in the 0.10 mol L⁻¹ Na₂SO₄ supporting electrolyte achieved 99% at pH 7 in 25 min, with no bromide ion detected in the solution. Optimal removal was found in pH range 6-7, and the pH of the solution had a significant impact on bromate reduction. A reduction mechanism was also discussed by analyzing the cyclic voltammograms of the reduction process and X-ray photoelectron spectra of the main elements (N 1s and Br 3d) on the PANI surface. We propose that during the electrocatalytic reduction process, bromate is reduced to bromide because of the loss of electrons from the nitrogen atoms on the PANI chains. The doping of the resultant Br⁻ ions in the PANI film has an important role in avoiding further oxidation of Br⁻ to BrO₃⁻. The used PANI film can be regenerated by de-doping the Br⁻ ions with a 0.5 mol L⁻¹ H₂SO₄ solution. Thus the process can be considered efficient and green.     

A DNA electrochemical sensor with poly-l-lysine/single-walled carbon nanotubes films and its application for the highly sensitive EIS detection of PAT gene fragment and PCR amplification of NOS gene

A sensitive and novel DNA electrochemical biosensor for the detection of the transgenic plants gene fragment by electrochemical impedance spectroscopy (EIS) was presented. The well-dispersed carboxylic group-functionalized single-walled carbon nanotubes (SWNTs) were dripped onto the carbon paste electrode (CPE) surface firstly, and poly-l-lysine films (pLys) were subsequently electropolymerized by cyclic voltammetry (CV) to prepare pLys/SWNTs/CPE. The morphology of pLys/SWNTs films was examined using a field emission scanning electron microscope (SEM). The pLys/SWNTs films modified electrode exhibited very good conductivity. DNA probes were easily immobilized on the poly-l-lysine films via electrostatic adsorption. The hybridization events were monitored with electrochemical impedance spectroscopy using [Fe(CN)₆]^3−/4− as indicator. The PAT gene fragment from phosphinothricin acetyltransferase gene was detected by this DNA electrochemical sensor. The dynamic detection range of this sensor to the PAT gene fragment was from 1.0 × 10⁻¹² to 1.0 × 10⁻⁷ mol/L. A detection limit of 3.1 × 10⁻¹³ mol/L could be estimated. The PCR amplification of NOS gene from the sample of a kind of transgenic modified bean was also detected satisfactorily by EIS.     

Interaction between promethazine hydrochloride and DNA and its application in electrochemical detection of DNA hybridization

The interactions of promethazine hydrochloride (PZH) with thiolated single-stranded DNA (HS-ssDNA) and double-stranded DNA (HS-dsDNA) self-assembled on gold electrodes have been studied electrochemically. The binding of PZH with ssDNA shows a mechanism containing an electrostatic interaction, while the mode of PZH interaction with dsDNA contains both electrostatic and intercalative bindings. The redox system belongs to the category of diffusion control approved by cyclic voltammetry (CV). The diffusion coefficients of PZH at the bare, HS-dsDNA and HS-ssDNA modified gold electrodes decrease regularly as 1.34 × 10⁻³ cm² s⁻¹, 1.04 × 10⁻³ cm² s⁻¹, 7.47 × 10⁻⁴ cm² s⁻¹, respectively. The electron transfer standard rate constant k_s of PZH at bare gold, HS-ssDNA and HS-dsDNA modified electrodes are 0.419 s⁻¹, 0.131 s⁻¹, and 0.154 s⁻¹, respectively. The presence of adsorbed dsDNA results in a great increase in the peak currents of PZH in comparison with those obtained at a bare or ssDNA adsorbed gold electrode. The difference between interactions of PZH with HS-ssDNA and HS-dsDNA has been used for hybridization recognition of 14-mer DNA oligonucleotide. The peak current (i_pa) of PZH is linearly proportional to the logarithmic concentration of complementary target DNA in the range from 2.0 × 10⁻⁹ mol L⁻¹ to 5.0 × 10⁻⁷ mol L⁻¹ with the detection limit of 3.8 × 10⁻¹⁰ mol L⁻¹.     

Electrochemical determination of bromide at a multiwall carbon nanotubes-chitosan modified electrode

A multiwall carbon nanotubes (MWNTs)-chitosan modified glassy carbon electrode (GCE) exhibits attractive ability for highly sensitive cathodic stripping voltammetric measurements of bromide (Br⁻). In pH 1.8 H₂SO₄ solution, a substantial increase in the stripping peak current of Br⁻ (compared to bare GCE and chitosan modified GCE) is observed using MWNTs-chitosan modified electrode. Operational parameters were optimized and the electrochemical behaviors of Br⁻ were studied by different electrochemical methods. The kinetics parameters were measured, the number of electron transfer (n) was 1 and the transfer coefficient (α) is 0.17. A wide linear calibration range (3.6 × 10⁻⁷-1.4 × 10⁻⁵ g mL⁻¹) was achieved, with a detection limit of 9.6 × 10⁻⁸ g mL⁻¹. The mechanism of electrode reaction was fully discussed.     

Sensitive DNA impedance biosensor for detection of cancer, chronic lymphocytic leukemia, based on gold nanoparticles/gold modified electrode

A simple and sensitive DNA impedance sensor was prepared for the detection of chronic lymphocytic leukemia. The DNA electrochemical biosensor is worked based on the electrochemical impedance spectroscopic (EIS) detection of the sequence-specific DNA related to chronic lymphocytic leukemia. The ssDNA probe was immobilized on the surface of the gold nanoparticles. Compared to the bare gold electrode, the gold nanoparticles-modified electrode could improve the density of the probe DNA attachment and hence the sensitivity of the DNA sensor greatly. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy were performed in a solution containing 1.0 mmol L⁻¹ K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 50 mmol L⁻¹ phosphate buffer saline pH 6.87 plus 50 mmol L⁻¹ KCl. In the CV studied, the potential was cycled from 0.0 to +0.65 V with a scan rate of 50 mV s⁻¹. Using EIS, the difference of the electron transfer resistance (ΔR_et) was linear with the logarithm of the complementary oligonucleotides sequence concentrations in the range of 7.0 × 10⁻¹²–2.0 × 10⁻⁷ mol L⁻¹, with a detection limit of 1.0 × 10⁻¹² mol L⁻¹. In addition, the DNA sensor showed a good reproducibility and stability during repeated regeneration and hybridization cycles.     

Spectroscopic study of the electrochemical behaviour of tantalum(V) chloride and oxochloride species in 1-butyl-1-methylpyrrolidinium chloride

FTIR spectroscopy was used to identify the oxochloride species of tantalum(V) in ionic liquids and to confirm the correlations between their presence in electrolytes and the changes in the route of electrochemical reduction of tantalum(V). Electrochemical behaviour of the mixtures (x)1-butyl-1-methyl-pyrrolidinium chloride-(1 − x)TaCl₅ at x = 0.80, 0.65, and 0.40 was investigated over the temperature range 90-160 °C with respect to the electrochemical deposition of tantalum and was discussed in terms of spectroscopic data. The mechanism of electrochemical reduction of tantalum(V) in the basic and acidic electrolytes depends strongly on the structure and composition of the electro active species of tantalum(V) defined by the molar composition of ionic liquids and on the competition between tantalum(V) chloride and oxochloride species. In the basic mixture at x = 0.80, with octahedral [TaCl₆]⁻ ions as the electrochemically active species only the first reduction step Ta⁵⁺ → Ta⁴⁺ at −0.31 V was observed. The competitive reduction of tantalum(V) oxochloride species occurs at more anodic potential (−0.01 V) than the reduction of the chloride complexes and can restrict the further reduction of tantalum(IV). In the basic ionic liquid at x = 0.65, the cyclic voltammograms exhibit reduction peaks at −0.31 V and −0.51 V attributed to the diffusion controlled process as [TaCl₆]⁻ + e⁻ → [TaCl₆]²⁻ and [TaCl₆]²⁻ + e⁻ → [TaCl₆]³⁻. The further irreversible reduction of tantalum(III) to metallic state may occur at −2.1 V. In the acidic ionic liquids, at x = 0.40 the electrochemical reduction of two species occurs, TaCl₆⁻ and Ta₂Cl₁₁⁻ and it is limited by two electron transfer for both of them at −0.3 V and −1.5 V, respectively.     

Effects of NO2 ions on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 mol L⁻¹ NaHCO₃ + 0.02 mol L⁻¹ NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with NO₂⁻ concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [NO₂⁻] = 0 up to 400 ± 50 mV/SCE for [NO₂⁻] = 0.1 mol L⁻¹. During anodic polarisation at potentials comprised between E_b([NO₂⁻] = 0) and E_b([NO₂⁻] ≠ 0), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via scanning vibrating electrode technique (SVET). Addition of a NaNO₂ solution after the beginning of the polarisation led to a rapid repassivation of pre-existing well-grown pits. In situ micro-Raman spectroscopy was then used to identify the corrosion products forming inside the pits. The first species to be detected in the presence of NO₂⁻ were mainly dissolved Fe(III) species, more likely [Fe^III(H₂O)₆]³⁺ complexes. Iron(II) carbonate FeCO₃, siderite, and carbonated green rust GR(CO₃²⁻) were also detected in the active pits, as in the absence of nitrite. But they were accompanied by maghemite γ-Fe₂O₃, a phase structurally similar to the passive film, that forms from the Fe(III) complexes. The Raman analyses then correlate with the SVET observations and confirm that the main effect of nitrite ions is to oxidize iron(II) into iron(III). The passive film would then form from the Fe(III) species still bound to the steel surface.     

Amperometric biosensor for ethanol based on co-immobilization of alcohol dehydrogenase and Meldola's Blue on multi-wall carbon nanotube

In this work, multi-wall carbon nanotube (MWCT) is evaluated as transducer, stabilizer and immobilization matrix for the construction of amperometric biosensor based on alcohol dehydrogenase (ADH) and Meldola's Blue (MB). The amperometric response was based on the electro catalytic properties of MB to oxidize NADH, which was generated in the enzymatic reaction of ethanol with NAD⁺ under catalysis of ADH. It is shown that the employed materials are promising as electrochemical mediators and enzyme stabilizers. The enzyme was immobilized onto the MWCT adsorbed with MB by cross-linking with glutaraldehyde. The dependence on the biosensor response for ethanol was investigated in terms of pH, supporting electrolyte, ADH and NAD⁺ amounts and working potential. The amperometric response for alcohol using this biosensor showed excellent sensitivity (4.75 μA cm⁻² mmol L⁻¹), operational stability (around 95% of the activity was maintained after 300 determinations) and wide linear response range (0.05-10 mmol L⁻¹). These favorable characteristics allowed its application for measurements of ethanol in a great variety of alcoholic beverages with a simple dilution. The precision and recovery data showed by the proposed biosensor may give reliable results for real complex matrices.     

DNA Langmuir-Blodgett modified glassy carbon electrode as voltammetric sensor for determinate of methotrexate

A highly sensitive electrochemical biosensor for the detection of trace amounts of methotrexate has been designed. Double stranded (ds)DNA molecules are immobilized onto a pretreated glassy carbon electrode (GCE(ox)) surface with Langmuir-Blodgett (LB) technique. The adsorptive voltammetric behaviors of methotrexate on DNA-modified electrode were explored by means of cyclic voltammetry (CV) and square wave voltammetry (SWV). The oxidation mechanism was proposed and discussed in this work. In addition, the optimum experimental conditions for the detection of methotrexate were explored, and the currents measured by SWV presented a good linear property as a function of the concentrations of methotrexate in the range of 2.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol L⁻¹, with an LOD of 5.0 × 10⁻⁹ mol L⁻¹. The method proposed was applied for the determination of methotrexate in pharmaceutical dosage and diluted human urine with wonderful satisfactory successfully.     

Preparation, structure and electrochemistry of a polypyrrole hybrid film with [Pd(dmit)2], bis(1,3-dithiole-2-thione-4,5-dithiolate)palladate(II)

The synthesis of a hybrid material obtained by electropolymerization of a solution of pyrrole and [NEt₄]₂[Pd(dmit)₂] (1,3-dithiole-2-thione-4,5-dithiolate, [dmit]²⁻, [C₃S₅]²⁻) in acetonitrile solution is reported. FTIR and UV-vis spectroscopy showed that the [Pd(dmit)₂]²⁻ anion had been inserted in the polypyrrole framework without modification during the electropolymerization process. Cyclic voltammetry showed that the material has electroactivity undergoing redox processes related to the conducting polymer and the counteranion. The electrochemical results also suggest that the counteranion is not trapped in the PPy matrix undergoing anion exchange during the redox cycle of PPy. The PPy/[Pd(dmit)₂]²⁻ exhibits good thermal stability and has a higher intrinsic conductivity value (4.27 × 10⁻³ S cm⁻¹) than do other PPy/dmit films previously studied.     

Direct electrochemistry and electrocatalysis of cytochrome c based on chitosan-room temperature ionic liquid-carbon nanotubes composite

A robust and effective composite film combined the benefits of room temperature ionic liquid (RTIL), chitosan (Chi) and multi-wall carbon nanotubes (MWNTs) was prepared. Cytochrome c (Cyt c) was successfully immobilized on glassy carbon electrode (GCE) surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Cyt c were investigated in detail. A pair of well-defined and quasi-reversible redox peaks of Cyt c was obtained in 0.1 mol L⁻¹ pH 7.0 phosphate buffer solution (PBS), indicating the Chi-RTIL-MWNTs film showed an obvious promotion for the direct electron transfer between Cyt c and the underlying electrode. The immobilized Cyt c exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0 × 10⁻⁶ to 2.6 × 10⁻⁴ mol L⁻¹, with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹ (S/N = 3). The apparent Michaelis-Menten constant (K_m) was calculated to be 0.45 ± 0.02 mmol L⁻¹. Moreover, the modified electrode displayed a rapid response (5 s) to H₂O₂, and possessed good stability and reproducibility. Based on the composite film, a third-generation reagentless biosensor could be constructed for the determination of H₂O₂.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.