Mercury-free simultaneous determination of cadmium and lead at a glassy carbon electrode modified with multi-wall carbon nanotubes

A multi-wall carbon nanotube (MWNT) modified glassy carbon electrode (GCE) was described for the simultaneous determination of trace levels of cadmium and lead by anodic stripping voltammetry (ASV). In pH 4.5 NaAc-HAc buffer containing 0.02 mol/l KI, Cd²⁺ and Pb²⁺ first adsorb onto the surface of a MWNT film coated GCE and then reduce at −1.20 V. During the positive potential sweep, reduced cadmium and lead were oxidized, and two well-defined stripping peaks appeared at −0.88 and −0.62 V. Compared with a bare GCE, a MWNT film coated GCE greatly improves the sensitivity of determining cadmium and lead. Low concentration of I⁻ significantly enhances the stripping peak currents since it induces Cd²⁺ and Pb²⁺ to adsorb at the electrode surface. The striping peak currents change linearly with the concentration of Cd²⁺ from 2.5×10⁻⁸ to 1×10⁻⁵ mol/l and with that of Pb²⁺ from 2×10⁻⁸ to 1×10⁻⁵ mol/l. The lowest detectable concentrations of Cd²⁺ and Pb²⁺ are estimated to be 6×10⁻⁹ and 4×10⁻⁹ mol/l, respectively. The high sensitivity, selectivity, and stability of this MWNT film coated electrode demonstrated its practical application for a simple, rapid and economical determination of trace levels of Cd²⁺ and Pb²⁺ in water samples.     

Anodic stripping voltammetric determination of mercury using multi-walled carbon nanotubes film coated glassy carbon electrode

An electrochemical method for the determination of trace levels of mercury based on a multi-walled carbon nanotubes (MWNT) film coated glassy carbon electrode (GCE) is described. In 0.1 mol L^–1 HCl solution containing 0.02 mol L^–1 KI, Hg²⁺ was firstly preconcentrated at the MWNT film and then reduced at –0.60 V. During the anodic potential sweep, reduced mercury was oxidized, and then a sensitive and well-defined stripping peak at about –0.20 V appeared. Under identical conditions, a MWNT film coated GCE greatly enhances the stripping peak current of mercury in contrast to a bare GCE. Low concentrations of I^– remarkably improve the determining sensitivity, since this increases the accumulation efficiency of Hg²⁺ at the MWNT film coated GCE. The stripping peak current is proportional to the concentration of Hg²⁺ over the range 8×10^–10–5×10^–7 mol L^–1. The lowest detectable concentration of Hg²⁺ is 2×10^–10 mol L^–1 at 5 min accumulation. The relative standard deviation (RSD) at 1×10^–8 mol L^–1 Hg²⁺ was about 6% (n=10). By using this proposed method, Hg²⁺ in some water samples was determined, and the results were compared with those obtained by atomic absorption spectrometry (AAS). The two results are similar, suggesting that the MWNT-film coated GCE has great potential in practical analysis.     

Adsorptive stripping voltammetric determination of 4-aminophenol at a single-wall carbon nanotubes film coated electrode

A single-wall carbon nanotubes (SWNT)-Nafion film coated glassy carbon electrode (GCE) was described for the determination of 4-aminophenol. In pH 3.0 sodium citrate-HCl buffer, the oxidation peak current of 4-aminophenol increases greatly at the SWNT-Nafion film coated GCE in contrast to that at both bare GCE and Nafion-film coated GCE. Moreover, the oxidation peak potential shifts to more negative potential. All the experimental parameters were optimized for the determination of 4-aminophenol. The oxidation peak current is proportional to the concentration of 4-aminophenol over the range from 5×10⁻⁹ to 2×10⁻⁶ mol l⁻¹. The detection limit is 8×10⁻¹⁰ mol l⁻¹ at 4 min of accumulation. Using the proposed method, 4-aminophenol in water samples was determined.     

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.     

Effect of surface modification of indium tin oxide by nanoparticles on the electrochemical determination of tryptophan

The effect of surface modification of indium tin oxide (ITO) by multi wall carbon nanotube (MWNT) and gold nanoparticles attached multi wall carbon nanotube (AuNP-MWNT) has been studied to determine tryptophan, an important and essential amino acid for humans and herbivores. A detailed comparison has been made among the voltammetric response of bare ITO, MWNT/ITO and AuNP-MWNT/ITO in respects of several essential analytical parameters viz. sensitivity, detection limit, peak current and peak potential of tryptophan. The AuNP-MWNT/ITO exhibited a well defined anodic peak at pH 7.2 at a potential of ∼669 mV for the oxidation of tryptophan as compared to 760 mV at MWNT/ITO electrode. Under optimum conditions linear calibration curve was obtained over tryptophan concentration range 0.5-90.0 μM in phosphate buffer solution of pH 7.2 with detection limit and sensitivity of 0.025 μM and 0.12 μA μM⁻¹, respectively. The oxidation of tryptophan occurred in a pH dependent, 2e⁻ and 2H⁺ process and the electrode reaction followed adsorption controlled pathway. The method has been found selective and successfully implemented for the determination of tryptophan in human urine and plasma samples using standard addition method. The electrode exhibited an efficient catalytic response with good reproducibility and stability.     

Voltammetric detection of the α-dicarbonyl compound: Methylglyoxal as a flavoring agent in wine and beer

A simple, rapid and highly selective method for the determination of the most abundant α-dicarbonyl compound in wine and beer has been developed for the first time by employing square wave voltammetry. A novel electrochemical sensor, based on the electrodeposition of platinum nanoparticles onto single wall carbon nanotubes that were cast on a glassy carbon electrode (GCE) substrate, is presented in this paper. This modified electrode exhibited excellent catalytic activity in the electroreduction of methylglyoxal, showing much higher peak currents than those measured on an unmodified GCE. The effects of different experimental and instrumental parameters, such as solution pH and square wave frequency, were examined. The reduction peak current showed a linear range of from 0.1 × 10⁻⁶ to 100 × 10⁻⁶ M with a 0.9979 correlation coefficient; and a low detection limit of 2.8 × 10⁻⁹ M was also obtained. The proposed methodology was successfully applied to the quantitative analysis of methylglyoxal in wine and beer samples. The developed sensor possesses advantageous properties such as a high active surface area, stability, and rapid electron transfer rate, which cumulatively demonstrate high performance toward the electrocatalytic reduction and detection of methylglyoxal.     

Electrocatalytic Properties and Voltammetric Determination of Melatonin at a Nanostructured Film Electrode

A multi-wall carbon nanotube (MWNT) film coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behavior of melatonin on the MWNT film coated GCE was investigated. The oxidation peak current of melatonin increases significantly and the oxidation peak position shifts positively at the MWNT film modified GCE compared to that at a bare GCE. This indicates that MWNTs feature highly effective catalysis to the electrochemical oxidation of melatonin. A simple and sensitive electroanalytical method was developed for the determination of melatonin. The oxidation peak current is proportional to the concentration of melatonin from 8×10^–8 to 1×10^–5molL^–1. The detection limit is about 2×10^–8molL^–1 for 3min accumulation. The proposed method was demonstrated to work satisfactorily with commercial capsules.     

Voltammetry and determination of metronidazole at a carbon fiber microdisk electrode

The electrochemistry of metronidazole, 1-(hydroxyethyl)-2-methyl-5-nitroimidazole, was investigated at a carbon fiber microdisk electrode in pH 9 Britton Robinson buffer. Under these conditions, the reduction of metronidazole is controlled by both mass transport to the microdisk and adsorption with an equilibrium constant of 4 × 10³ mol⁻¹ dm³ and a saturation coverage of 0.88 × 10⁻⁸ mol cm⁻². The adsorption and accumulation of metronidazole on the surface of the carbon fiber allows its determination at low concentrations by square wave adsorptive stripping voltammetry. A detection limit for metronidazole of 5 × 10⁻⁷ mol dm⁻³ and a R.S.D. of 3.7% at 1 × 10⁻⁶ mol dm⁻³ (n = 4) were obtained with a two electrode system with no stirring during the accumulation step. Based on this method, a simple procedure for the determination of metronidazole in urine is described which requires no pre-treatment of the sample before analysis.     

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.     

Electrocatalytic reduction of nitrite at polypyrrole nanowire-platinum nanocluster modified glassy carbon electrode

Pt nanoclusters were deposited in polypyrrole (PPy) nanowires by cyclic voltammetry method, fabricating a PPy-Pt nanocomposite on glassy carbon electrode (PPy-Pt/GCE). The electrocatalytic reduction of nitrite at PPy-Pt/GCE has been investigated using 0.5 M H₂SO₄ solution. The sensor exhibited excellent electrocatalytic activity toward nitrite reduction. In acidic medium, the cyclic voltammetry at 20 mV s^− 1 gave a nitrite reduction peak at − 0.124 V with 0.566 μA μM^− 1 current sensitivity in the range of 5.0 × 10^− 7-1.0 × 10^− 3 M. The detection limit was 1.5 × 10^− 7 M (s/n = 3). The proposed method was successfully applied in the detection of nitrite in real water samples and obtained satisfactory results. The PPy-Pt composite modified electrode had good storage stability, reproducibility and anti-interference ability.     

Simultaneous determination of epinephrine and norepinephrine in human blood plasma and urine samples using nanotubes modified edge plane pyrolytic graphite electrode

The simultaneous determination of catecholamines - epinephrine and norepinephrine by square wave voltammetry (SWV) at physiological pH 7.2 is reported using multi-walled carbon nanotubes modified edge plane pyrolytic graphite electrode (MWNT/EPPGE). A broad bump at ∼250 mV is appeared for the oxidation of epinephrine (EP) and norepinephrine (NE) at bare EPPGE whereas at MWNT/EPPGE two well-separated peaks at ∼150 and ∼215 mV are appeared for the oxidation of EP and NE, respectively. The oxidation peak current of both the neurotransmitters also increased significantly along with the negative shift of peak potentials using MWNT/EPPGE. The oxidation of both compounds occurred in a pH dependent, 2e and 2H⁺ process and the electrode reaction followed diffusion controlled pathway. Linear calibration curves were obtained for epinephrine and norepinephrine in the range 0.5-100 nM with limits of detection 0.15 × 10⁻⁹ and 0.90 × 10⁻¹⁰ M, respectively. The developed protocol is implemented for the simultaneous determination of epinephrine and norepinephrine in blood plasma and urine samples of smokers as well as in athletes.     

Electrochemical determination of some antidiabetic drugs for type 2 diabetic patients

Quantitative determination of rosiglitazone, pioglitazone, glimepiride and glyburide as antidiabetic drugs for type 2 diabetic patients was performed conveniently and economically using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Carbon paste (CPE) and glassy carbon (GCE) electrodes were successfully used as sensors for these drugs in Briton-Robinson (B-R) as buffer solution. The preparation of CPE and the GCE as ion selective electrodes is based on the construction of 10% standard drug ion pair with reineckate or tungstophosphate imbedded as electroactive material. Working standards were freshly prepared just before the assay by dilution from a 10⁻² mol L⁻¹ drug stock solution. At a scan rate of 100 mV s⁻¹ the cyclic voltammograms showed a well defined anodic peak with high selectivity. The DVP gave a reproducible well defined diffusion controlled peak for each drug at a scan rate of 10 mV s⁻¹. The oxidation peaks were used to determine the tested drug concentrations. The quantitative determination of the four drugs in their pharmaceutical preparations by the proposed electrochemical technique was found to be identical with the values obtained by the standard HPLC method. A mean % recovery of 100 ± 1 was obtained and the % relative standard deviation was 1.62 indicating the high precision of the method and the confidence in its repeatability. The proposed electroanalytical technique using either the CPE or the GCE is economic, selective and can be applied for both the qualitative and quantitative determination of the drugs in their pharmaceutical preparations, without special drug separation.     

Voltammetric determination of Cd2+ based on the bifunctionality of single-walled carbon nanotubes-Nafion film

In the presence of Nafion, single-walled carbon nanotubes (SWNTs) were easily dispersed into ethanol, resulting in a homogeneous SWNTs/Nafion suspension. After evaporating ethanol, a SWNTs/Nafion film with bifunctionality was constructed onto glassy carbon electrode (GCE) surface. Attributing to the strong cation-exchange ability of Nafion and excellent properties of SWNTs, the SWNTs/Nafion film-coated GCE remarkably enhances the sensitivity of determination of Cd²⁺. Based on this, an electrochemical method was developed for the determination of trace levels of Cd²⁺ by anodic stripping voltammetry (ASV). In pH 5.0 NaAc-HAc buffer, Cd²⁺ was firstly exchanged and adsorbed onto SWNTs/Nafion film surface, and then reduce at −1.10 V. During the positive potential sweep, reduced cadmium was oxidized, and a well-defined stripping peak appeared at −0.84 V, which can be used as analytical signal for Cd²⁺. The linear range is found to be from 4.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol L⁻¹, and the lowest detectable concentration is estimated to be 4.0 × 10⁻⁹ mol L⁻¹. Finally, this method was successfully employed to detect Cd²⁺ in water samples.     

Preconcentration of palladium in a flow-through electrochemical cell for determination by graphite furnace atomic absorption spectrometry

An electrochemical preconcentration at a controlled potential on the electrode in a flow-through mode followed by graphite furnace atomic absorption spectrometric (GFAAS) detection is proposed for determination of trace amounts of palladium. After electrolysis the polarization of the electrodes was changed and deposited metal was dissolved electrochemically in the presence of an appropriate stripping reagent. Conditions for the electrodeposition, such as pH of the solutions, a deposition potential, dissolution potential and a composition of stripping solution were optimised. The graphite electrode (GE) and glassy carbon electrode (GCE) were tested for the palladium reduction process. The detection limit of 0.05 ng ml⁻¹ Pd (1 pg) was obtained after palladium preconcentration on the GCE and dissolution with 0.2 mol l⁻¹ thiourea in 0.1 mol l⁻¹ HCl followed by GFAAS detection. The method was applied for the determination of palladium in spiked tap water and road dust samples.     

Direct electrocatalytic oxidation of nitric oxide and reduction of hydrogen peroxide based on α-Fe2O3 nanoparticles-chitosan composite

α-Fe₂O₃ nanoparticles prepared using a simple solution-combusting method have been dispersed in chitosan (CH) solution to fabricate nanocomposite film on glass carbon electrode (GCE). The as-prepared α-Fe₂O₃ nanoparticles were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM). The nanocomposite film exhibits high electrocatalytic oxidation for nitric oxide (NO) and reduction for hydrogen peroxide (H₂O₂). The electrocatalytic oxidation peak is observed at +0.82 V (vs. Ag/AgCl) and controlled by diffusion process. The electrocatalytic reduction peak is observed at −0.45 V (vs. Ag/AgCl) and controlled by diffusion process. This α-Fe₂O₃-CH/GCE nanocomposite bioelectrode has response time of 5 s, linearity as 5.0 × 10⁻⁷ to 15.0 × 10⁻⁶ M of NO with a detection limit of 8.0 × 10⁻⁸ M and a sensitivity of −283.6 μA/mM. This α-Fe₂O₃-CH/GCE nanocomposite bioelectrode was further utilized in detection of H₂O₂ with a detection limit of 4.0 × 10⁻⁷ M, linearity as 1.0 × 10⁻⁶ to 44.0 × 10⁻⁶ M and with a sensitivity of 21.62 μA/mM. The shelf life of this bioelectrode is about 6 weeks under room temperature conditions.     

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.     

Simultaneous determination of guanine and adenine in DNA using an electrochemically pretreated glassy carbon electrode

A simple and reliable method based on adsorptive stripping at an electrochemically pretreated glassy carbon electrode (GCE) was proposed for simultaneous or individual determination of guanine and adenine in DNA. The detection sensitivity of guanine and adenine was improved greatly by activating the GCE electrochemically. After accumulation on pretreated GCE at open circuit for 5 min or at the potential of +0.3 V for 120 s, guanine and adenine produced well-defined oxidation peaks at about +0.8 and +1.1 V, respectively in pH 5 phosphate buffer. The detection limit for individual measurement of guanine and adenine was 4.5 ng ml⁻¹ (3×10⁻⁸ mol l⁻¹) and 4 ng ml⁻¹ (3×10⁻⁸ mol l⁻¹), respectively. Acid-denatured DNA showed two oxidation peaks corresponding to guanine and adenine residues in the same buffer. The proposed method can be used to estimate the guanine and adenine contents in DNA with good selectivity in a linear range of 0.25-5 μg ml⁻¹.     

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.     

A novel high selective and sensitive metronidazole voltammetric sensor based on a molecularly imprinted polymer-carbon paste electrode

The design and construction of a highly selective voltammetric sensor for metronidazole by using a molecularly imprinted polymer (MIP) as recognition element were introduced. A metronidazole selective MIP and a nonimprinted polymer (NIP) were synthesized and then incorporated in the carbon paste electrodes (CPEs). The sensor was applied for metronidazole determination using cathodic stripping voltammetric method. The MIP-CP electrode showed very high recognition ability in comparison to NIP-CPE. Some parameters affecting the sensor response were optimized and then the calibration curve was plotted. Two dynamic linear ranges of 5.64 × 10⁻⁵ to 2.63 × 10⁻³ mg L⁻¹ and 2.63 × 10⁻³ to 7.69 × 10⁻² mg L⁻¹ were obtained. The detection limit of the sensor was calculated as 3.59 × 10⁻⁵ mg L⁻¹. This sensor was used successfully for metronidazole determination in biological fluids.     

Electrochemical Determination of Tannins Using Multiwall Carbon Nanotubes Modified Glassy Carbon Electrode

A novel chemically modified electrode is prepared on the basis of the attachment of multiwall carbon nanotubes (MWNTs) to the surface of a glassy carbon electrode (GCE) in the presence of a hydrophobic surfactant. The electrochemical behavior of tannins at the MWNTs-modified GCE is investigated. Tannins yield a well-defined oxidation at about 0.30 V (SCE) at the MWNTs-modified GCE. MWNT-film shows remarkable enhancement effect on the oxidation peak current of tannins. The experimental parameters are optimized, and a direct electrochemical method to detect tannins is proposed. The oxidation peak current is proportional to the concentration of tannins over the range from 4 × 10^–7 to 2 × 10^–4 M, and the detection limit is 1 × 10^–7 mol/l after 5 min of accumulation. The relative standard deviation of 6% for determination of 2 × 10^–6 mol/l tannins indicates excellent reproducibility. The analysis method is demonstrated by using tea and Chinese gall samples.