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.     

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 oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode

A new gold nanoparticles-modified electrode (GNP/LC/GCE) was fabricated by self-assembling gold nanoparticles to the surface of the l-cysteine-modified glassy carbon electrode. The modified electrode showed an excellent character for electrocatalytic oxidization of uric acid (UA) and ascorbic acid (AA) with a 0.306 V separation of both peaks, while the bare GC electrode only gave an overlapped and broad oxidation peak. The anodic currents of UA and AA on the modified electrode were 6- and 2.5-fold to that of the bare GCE, respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of UA and AA has been explored at the modified electrode. DPV peak currents of UA and AA increased linearly with their concentration at the range of 6.0 × 10⁻⁷ to 8.5 × 10⁻⁴ mol L⁻¹ and 8.0 × 10⁻⁶ to 5.5 × 10⁻³ mol L⁻¹, respectively. The proposed method was applied for the detection of UA and AA in human urine with satisfactory result.     

The use of silver solid amalgam electrode for voltammetric and amperometric determination of nitroquinolines

Solid amalgam electrodes represent a suitable alternative to mercury electrodes due to their similar electrochemical properties and non-toxicity of the amalgam material. Nitro derivatives of quinoline have been proven to be genotoxic, thus their presence in environmental samples is a legitimate cause for concern.In this contribution, meniscus modified silver solid amalgam electrode (m-AgSAE) was employed for the batch voltammetric determination and amperometric determination in connection with flow injection analysis of 5-nitroquinoline and 6-nitroquinoline (5-NQ, 6-NQ). Their electrochemical behavior was characterized by cyclic voltammetry, for their determination direct current voltammetry and differential pulse voltammetry were used. Linear calibration curves in the concentration range of 2 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ were obtained. These results are comparable with results obtained for polarographic determination of the same substances using mercury electrodes. Further, the meniscus modified silver solid amalgam electrode was employed in amperometric detection cell in “wall jet” arrangement for determination of 5-NQ in flow injection analysis. Under optimized conditions (run buffer 0.05 mol L⁻¹ borate buffer, pH 9.0; flow rate 4 mL min⁻¹; detection potential −1.6 V; injection volume 0.1 mL), the limit of quantitation of ∼4 × 10⁻⁶ mol L⁻¹ was achieved. The repeatability of the detector response is satisfactory (relative standard deviation ∼2.15% for c(5-NQ) = 1 × 10⁻⁴ mol L⁻¹). Practical applicability of the method was verified for the determination of micromolar concentrations of 5-NQ in drinking and river water model samples.     

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.     

Electrochemical behavior of p-tert-butyl calix[8]arene in CH2Cl2

The electrochemical behavior of p-tert-butyl calix[8]arene has been investigated by cyclic voltammetry. The result shows that there is an irreversible electrochemical oxidative wave when the potential ranges from −0.3 to 1.6 V versus Ag/0.1 M AgNO₃ in acetonitrile (Ag/Ag⁺). At 25 °C, the peak potential is ca. 1.43 V (versus Ag/Ag⁺) at scan rate of 0.05 V s⁻¹. The number of the electrons transferred in the electrochemical reaction is four. The diffusion coefficient of p-tert-butyl calix[8]arene is 2.8 × 10⁻⁵ cm² s⁻¹. The diffusion activation energy is 12.3 kJ mol⁻¹.     

Electrochemical biosensor based on the interaction between copper(II) complex with 4,5-diazafluorene-9-one and bromine ligands and deoxyribonucleic acid

The copper complex of 4,5-diazafluorene-9-one (dafone) and bromine ligands ([Cu(dafone)₂]Br₂) was prepared and its interaction with double-stranded salmon sperm DNA (dsDNA) in pH 8.0 Britton-Robinson (B-R) buffer solution was studied by electrochemical experiments at the glassy carbon electrode (GCE). It was revealed that Cu(dafone)₂Br₂ could bind with salmon sperm DNA strands mainly by intercalation mode. The binding number of [Cu(dafone)₂]Br₂ for each salmon sperm dsDNA chain and equilibrium constant of the binding reaction were calculated to be 3 and 2.8 × 10¹² L³ mol⁻³, respectively. The Cu(dafone)₂Br₂ was further utilized as a new electrochemical DNA indicator for the detection of human hepatitis B virus (HBV) DNA fragment by differential pulse voltammetry (DPV). The difference of its electrochemical responses occurred between hybridized dsDNA duplex and probe DNA was explored to assess the selectivity of the developed electrochemical DNA biosensor. The constructed electrochemical DNA biosensor achieved a detection limit of 3.18 × 10⁻⁹ mol L⁻¹ for complementary target DNA and also realized a robust stability and good reusability.     

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.     

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).     

Determination of amiloride using a ds-DNA-modified pencil graphite electrode based on guanine and adenine signals

A sensitive electrochemical procedure based on ds-DNA interaction with amiloride at a ds-DNA-modified pencil graphite electrode (PGE) was introduced as a promising tool for determination of amiloride. An adsorptive stripping voltammetry was applied for the immobilization of ds-DNA on PGE in acetate buffer (pH 4.8). Differential pulse voltammetry (DPV) was carried out to obtain the change in the oxidation signal intensity of guanine and adenine before and after interaction with amiloride. The decrease in intensity of the guanine and adenine oxidation signals was used as an indicator for the sensitive determination of amiloride. Under the optimum conditions, a linear dependence of the guanine and adenine oxidation signals was observed to the amiloride concentration in the range of 0.75-240 μmol L⁻¹ with a detection limit of 0.5 μmol L⁻¹. The relative standard deviations of 10 replicate measurements of 1.0 and 10.0 μmol L⁻¹ amiloride concentrations were 4.7% and 5.3%, respectively. UV-vis measurements combined with DPV were also carried out to propose the most plausible mechanism for the interaction of amiloride and ds-DNA. The influence of potential interfering substances on the amiloride determination was studied. Finally, the ds-DNA-modified PGE biosensor was applied for the determination of amiloride in tablets and urine samples with satisfactory results.     

A novel graphite-polyurethane composite electrode modified with thiol-organofunctionalized silica for the determination of copper ions in ethanol fuel

A graphite-polyurethane composite modified with 2-benzothiazolethiol organofunctionalized silica was evaluated as an alternative electrode in the determination of Cu²⁺ ions in ethanol fuel samples, on the basis of a differential pulse anodic stripping voltammetry procedure. This metal can be quantified by mixing ethanol fuel with 0.10 mol L⁻¹ KNO₃ aqueous solution and subsequent voltammetric measurement after the accumulation step. A maximum limit of 70% (v/v) ethanol in potassium nitrate aqueous solution was obtained for voltammetric measurements without loss of sensitivity for metal species. Factors affecting the pre-concentration and stripping steps were investigated and optimum conditions were employed to develop the analytical procedure. Using 20 min of accumulation time, the linear range of 0.1-1.2 μmol L⁻¹ was obtained with the limit of detection of 3.9 × 10⁻⁸ mol L⁻¹. The developed electrode was successfully applied to determine Cu²⁺ in commercial ethanol fuel samples. The proposed method was compared with a traditional analytical technique, the flame atomic absorption spectrometry, and no significant differences between the results obtained by both methods were observed according to statistical evaluation.     

Electrochemical sensor for sulfite determination based on a nanostructured copper-salen film modified electrode

The electrochemical preparation described herein involved the electrocatalytic oxidation of sulfite on a platinum electrode modified with nanostructured copper salen (salen = N,N′-ethylenebis(salicylideneiminato)) polymer films. The complex was prepared and electropolymerized at a platinum electrode in a 0.1 mol L⁻¹ solution of tetrabutylammonium perchlorate in acetonitrile by cyclic voltammetry between 0 and 1.4 V vs. SCE. After cycling the modified electrode in a 0.50 mol L⁻¹ KCl solution, the estimated surface concentration was found to be equal to 2.2 × 10⁻⁹ mol cm⁻². This is a typical behavior of an electrode surface immobilized with a redox couple that can usually be considered as a reversible single-electron reduction/oxidation of the copper(II)/copper(III) couple. The potential peaks of the modified electrode in the electrolyte solution (aqueous) containing the different anions increase with the decrease of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the sensor. The potential peak was found to be linearly dependent upon the ratio [ionic charge]/[ionic radius]. The oxidation of the sulfite anion was performed at the platinum electrode at +0.9 V vs. SCE. However, a significant decrease in the overpotential (+0.45 V) was obtained while using the sensor, which minimized the effect of oxidizable interferences. A plot of the anodic current vs. the sulfite concentration for chronoamperometry (potential fixed = +0.45 V) at the sensor was linear in the 4.0 × 10⁻⁶ to 6.9 × 10⁻⁵ mol L⁻¹ concentration range and the concentration limit was 1.2 × 10⁻⁶ mol L⁻¹. The reaction order with respect to sulfite was determined by the slope of the logarithm of the current vs. the logarithm of the sulfite concentration.     

The ultrasonic electropolymerization of an 5-[o-(4-bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) film electrode and its electrocatalytic properties to dopamine oxidation in aqueous solution

5-[o-(4-Bromine amyloxy)phenyl]-10,15,20-triphenylporphrin (o-BrPETPP) was electropolymerized on a glassy carbon electrode (GCE), and the electrocatalytic properties of the prepared film electrode response to dopamine (DA) oxidation were investigated. A stable o-BrPETPP film was formed on the GCE under ultrasonic irradiation through a potentiodynamic process in 0.1 M H₂SO₄ between −1.1 V and 2.2 V versus a saturated calomel electrode (SCE) at a scan rate of 0.1 V s⁻¹. The film electrode showed high selectivity for DA in the presence of ascorbic acid (AA) and uric acid (UA), and a 6-fold greater sensitivity to DA than that of the bare GCE. In the 0.05 mol L⁻¹ phosphate buffer (pH 6.0), there was a linear relationship between the oxidation current and the concentration of DA solution in the range of 5 × 10⁻⁷ mol L⁻¹ to 3 × 10⁻⁵ mol L⁻¹. The electrode had a detection limit of 6.0 × 10⁻⁸ mol L⁻¹(S/N = 3) when the differential pulse voltammetric (DPV) method was used. In addition, the charge transfer rate constant k = 0.0703 cm s⁻¹, the transfer coefficient α = 0.709, the electron number involved in the rate determining step n_α = 0.952, and the diffusion coefficient D_o = 3.54  10⁻⁵ cm² s⁻¹ were determined. The o-BrPETPP film electrode provides high stability, sensitivity, and selectivity for DA oxidation.     

Development of a voltammetric sensor based on a molecularly imprinted polymer (MIP) for caffeine measurement

A new voltammetric sensor for caffeine measurement is introduced. A caffeine-selective molecularly imprinted polymer (MIP) and a non-imprinted polymer (NIP) were synthesized and then used for carbon paste (CP) electrode preparation. The MIP, embedded in the carbon paste electrode, functioned as a selective recognition element and a pre-concentrator agent for caffeine determination. The prepared electrode was used for caffeine measurement via a three-step procedure including analyte extraction in the electrode, electrode washing and electrochemical measurement of caffeine. The MIP-CP electrode showed very high recognition ability in comparison to NIP-CP. It was shown that electrode washing after caffeine extraction led to enhanced selectivity. Differential pulse voltammetry for caffeine determination was more effective than square wave voltammetry. Some parameters affecting sensor response were optimized, and a calibration curve was then plotted. A linear range of 6 × 10⁻⁸ to 2.5 × 10⁻⁵ mol L⁻¹ was obtained. The detection limit of the sensor was calculated to be equal to 1.5 × 10⁻⁸ mol L⁻¹. This sensor was used successfully for caffeine determination in spiked beverage and tea samples.     

Promethazine determination in plasma samples by using carbon paste electrode modified with molecularly imprinted polymer (MIP): Coupling of extraction, preconcentration and electrochemical determination

A promethazine (PMZ) molecularly imprinted polymer (MIP) and a non-imprinted polymer (NIP) were synthesized by two different formulations of methacrylic acid-ethylene glycol dimethacrylate (MAA-EGDMA) and vinyl benzene-divinyl benzene (VB-DVB). Then, the MIPs were used to modify the carbon paste electrode (CP). The response difference between MIP-CP and NIP-CP electrodes, containing VB-DVB polymer, was higher than that for MIP-CP and NIP-CP modified with polymer of MAA-EGDMA, indicating the lower nonselective surface adsorption property of the VB-DVB based MIP. The MIP, incorporated in the carbon paste electrode, functioned as selectively recognition element and pre-concentrator agent for PMZ determination. The prepared electrode was used for PMZ measurement by the three steps procedure including analyte extraction in the electrode, electrode washing and electrochemical measurement of PMZ. It was shown that the electrode washing, after PMZ extraction, led to enhanced selectivity. Square wave voltammetry (SWV) for PMZ determination by proposed electrode was proved to be better than that of differential pulse voltammetry. Some parameters, effective on the electrode response, were optimized and then a calibration curve was plotted. Two dynamic linear range of 7 × 10⁻⁹ to 4 × 10⁻⁷ and 4 × 10⁻⁷ to 7 × 10⁻⁶ mol L⁻¹ were obtained. The detection limit of the method was calculated equal to 3.2 × 10⁻⁹ mol L⁻¹. This method was used successful for PMZ determination in blood serum sample.     

Catalytic adsorptive stripping voltammetric determination of Cr(VI) in EDTA extracts from solid samples

A catalytic adsorptive stripping voltammetric procedure which allows for Cr(VI) determination in EDTA extracts is presented. EDTA used for extraction can be exploited as a masking agent for Cr(III). The calibration graph for Cr(VI) for an accumulation time of 60 s was linear in the range from 2 × 10⁻¹⁰ to 3 × 10⁻⁸ mol L⁻¹. The relative standard deviation for a Cr(VI) concentration of 2 × 10⁻⁹ mol L⁻¹ was 5.1% (n = 5). The detection limit estimated from three times the standard deviation of low Cr(VI) concentration and accumulation time of 60 s was about 7 × 10⁻¹¹ mol L⁻¹. The influence of foreign ions commonly present in extracts from solid samples is presented. The proposed voltammetric procedure was applied to determine Cr(VI) in EDTA extractable chromium from soil certified reference materials CRM 483 and CRM 041. The performance of the method was also verified by studying the recovery of Cr(VI) from spiked river water.     

Electroanalytical determination of TBHQ, a synthetic antioxidant, in soybean biodiesel samples

This paper reports an electroanalytical method developed for tert-butylhydroquinone (TBHQ) determination in biodiesel in the presence of surfactant Triton X-100 (T-100). T-100 was shown to improve the electroanalytical signal and its use was decisive for direct analysis of TBHQ in biodiesel, only requiring previous dilution of biodiesel samples in methanol. Several parameters were studied and optimized for the development of this methodology. Under optimal conditions, oxidation peak current was proportional to TBHQ concentration in the range of 1.05-10.10 × 10⁻⁶ mol L⁻¹, with limits of detection and quantification of 3.43 × 10⁻⁸ mol L⁻¹ and 1.14 × 10⁻⁷ mol L⁻¹, respectively, by square wave voltammetry (SWV). The results achieved with the proposed method were satisfactory, relative to those obtained using high-performance liquid chromatography (HPLC).     

A glassy carbon supported bilayer lipid-like membrane of 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide for electrochemical sensing of epinephrine

A self-assembled bilayer lipid-like membrane (BLM) supported on glassy carbon electrode (GCE) was fabricated using 5,5-ditetradecyl-2-(2-trimethyl-ammonioethyl)-1,3-dioxane bromide (DTDB) for epinephrine (EP) determination in the presence of ascorbic acid (AA). This modified electrode (DTDB/GCE) has strong membrane adsorption accumulation and electrocatalytic ability toward EP and AA. The oxidation of EP was controlled by double step adsorption accumulation process of the DTDB-BLM. The parameters of fitted Langmuir isotherm Γ_max, B_ADS, and ΔG_ADS values were determined as 1.0×10⁻¹¹ mol cm⁻², 2.04×10⁶ dm³ mol⁻¹, and −45.17 kJ mol⁻¹ for the fist step for EP concentration less than 1 mM, and 4.92×10⁻¹¹ mol cm⁻², 7.35×10⁴ dm³ mol⁻¹, and −37.1 kJ mol⁻¹ for the second step for EP concentration higher than 1 μM. The DPV peaks for EP and AA oxidations were appeared at 0.220 and 0.085 V versus SCE, respectively, allowing the determination of EP in the presence of high concentration of AA. The advantage of DTDB-BLM was demonstrated experimentally in comparison with other three BLMs, and attributed to the dioxane group as well as the suitable length of the carbon chain of DTDB molecule. The current response of the DTDB/GCE was fast and reproducible, suitable for the electrochemical sensing in flow-injection systems. A linear range of 1×10⁻⁸ to 1×10⁻⁴ M EP was preliminary obtained using a simple setup.     

Application of bismuth-film electrode for cathodic electroanalytical determination of sulfadiazine

A bismuth-film electrode for use in cathodic electrochemical detection was employed in order to quantify sulfadiazine in pharmaceutical formulations. The bismuth film was deposited ex situ onto a glassy carbon substrate. Analysis of two sulfa drugs was carried out by differential-pulse voltammetry in 0.05 mol L⁻¹ Britton-Robinson pH 4.5 solution. Sulfadiazine reduction was observed at −0.74 V vs. Ag/AgCl in one well-resolved irreversible reduction peak. The analytical curve with two slopes was obtained in the concentration range of 3.2-97.0 μmol L⁻¹. The detection limit was 2.1 μmol L⁻¹ for concentrations of 3.2-20.0 μmol L⁻¹ (r = 0.9949) and 12.2 μmol L⁻¹ for concentrations between 20.0 and 97.0 μmol L⁻¹ (r = 0.9951). Recovery studies carried out with both sulfadiazine samples gave values from 93.6 to 109.3%. The accuracy of the results supplied by the bismuth-film electrode was compared to those obtained by the standard amperometric titration method. The relative error between them was lower than 2.0%.     

Study of the potentiometric response of the doped spinel Li1.05Al0.02Mn1.98O4 for the optimization of a selective lithium ion sensor

In this paper, we studied the development of a selective lithium ion sensor constituted of a carbon paste electrode modified (CPEM) with an aluminum-doped spinel-type manganese oxide (Li_1.05Al_0.02Mn_1.98O₄) for investigating the influence of a doping ion in the sensor response. Experimental parameters, such as influence of the lithium concentration in the activation of the sensor by cyclic voltammetry, pH of the carrier solution and selectivity for Li⁺ against other alkali and alkaline-earth ions were investigated. The sensor response to lithium ions was linear in the concentration range 5.62 × 10⁻⁵ to 1.62 × 10⁻³ mol L⁻¹ with a slope 100.1 mV/decade over a wide pH 10 (Tris buffer) and detection limit of 2.75 × 10⁻⁵ mol L⁻¹, without interference of other alkali and alkaline-earth metals, demonstrating that the Al³⁺ doping increases the structure stability and improves the potentiometric response and sensitivity of the sensor. The super-Nernstian response of the sensor in pH 10 can be explained by mixed potential arising from two equilibria (redox and ion-exchange) in the spinel-type manganese oxide.