Sensing Lorazepam with a glassy carbon electrode coated with an electropolymerized-imprinted polymer modified with multiwalled carbon nanotubes and gold nanoparticles

We report on an electrode for the amperometric determination of lorazepam. A glassy carbon electrode was coated with a molecular imprint made by electropolymerization of ortho-phenylenediamine and filled with multiwalled carbon nanotubes and gold nanoparticles, which enhances the transmission of electrons. The sensor was studied with respect to its response to hexacyanoferrate (III) as a probe and by electrochemical impedance spectroscopy, cyclic voltammetry and square wave voltammetry. The linear response range to Lorazepam is from 0.5 nM to 1.0 nM and from 1.0 nM to 10.0 nM, with a detection limit of 0.2 nM (at an S/N of 3). The electrode was successfully applied to determine Lorazepam in spiked human serum.

Determination of trace thiocyanate with nano-silver coated multi-walled carbon nanotubes modified glassy carbon electrode

Novel nano-silver coated multi-walled carbon nanotube composites were prepared and used to fabricate a modified electrode. The application of the nano-silver coated multi-walled carbon nanotube composites modified electrode for determination of trace thiocyanate is demonstrated for the first time. The influence of substrate, pH and interference of coexisting substances was investigated for response properties of the electrode. There was a linear relationship at the range 2.5 × 10⁻⁹ to 5 × 10⁻⁸ mol L⁻¹ and 5 × 10⁻⁸ to 1 × 10⁻⁶ mol L⁻¹ of thiocyanate with the decrement of anodic DPV peak currents. The limit of detection was 1 × 10⁻⁹ mol L⁻¹(S/N = 3). The constructed electrode showed excellent reproducibility and stability. Actual urine and saliva samples of smoker and non-smoker were analyzed and satisfactory results were obtained. This method provides a new way to construct any electrode for biological and environmental analysis.     

Adsorptive potentiometric stripping analysis of trace tamoxifen at a glassy carbon electrode

A highly sensitive adsorptive stripping procedure for trace measurement of the anticancer drug tamoxifen is described. The method is based on controlled adsorptive accumulation of the drug at an electrochemically treated glassy carbon electrode, followed by chronopotentiometric measurement of the surface species. The chronopotentiometric operation effectively addresses the large background contribution inherent to the glassy carbon electrode to yield a detection limit of 4 x 10(-10) M after 4 min preconcentration. The adsorptive stripping response is evaluated with respect to electrode type and conditioning, accumulation potential and time, stripping current, pH, drug concentration, potential interferences, and other variables. Applicability to urine samples is illustrated.     

Theoretical predictions on the response properties of potentiometric gas sensors based on internal polymer membrane electrodes

The appropriate equilibrium expressions and known thermodynamic equilibrium constants are used in calculations on the expected response properties of polymer membrane electrode-based ammonia and carbon dioxide gas sensors. Slopes, detection limits, Nernstian response ranges and selectivities of such devices are shown to be a function of the initial pH, ionic strength and equilibrium constant of the internal electrolyte buffer used within these probes. Previously reported data for an ammonia sensor of this type correlate well with the theory. The poor response characteristics of carbon dioxide sensors based on internal carbonate-responsive membranes is also explained via the model presented. Future prospects and considerations for the development of other gas sensors of this type are discussed.     

Determination of catechol derivatives on pretreatment and copolymer coated glassy carbon electrode

A glassy carbon electrode was pretreated electrochemically and was coated with a copolymer of maleic acid anhydride attached with Eastman-AQ55D (MA/AQ). The voltammetric behavior of a series of biologically important compounds, such as dopamine, L-DOPA, DOPAC, ascorbic acid and uric acid were examined at both pretreated and coated electrodes. Electrochemical pretreatment increased peak current of dopamine and L-DOPA while decreased that of ascorbic acid, uric acid and DOPAC. The copolymer coating caused a decrease of peak currents, but effectively hindered the anionic species (ascorbic acid, uric acid and DOPAC) access to the electrode surface. In flow injection and liquid chromatographic analysis. The dopamine and L-DOPA yielded the better selectivity response at MA/AQ electrode than at bare and AQ electrodes.     

Derivative potentiometric stripping analysis with a thin film of mercury on a glassy carbon electrode

Derivative potentiometric stripping analysis is based on the pre-concentration of metal analytes in a thin film of mercury on a glassy carbon electrode and subsequent measurement of the electrode potential subject to controlled transport of oxidant to the electrode surface. The mechanism for the stripping process with respect to oxidant concentration, electrode rotation rate and instrument parameters is discussed, together with analytical results and applications.     

Electroanalysis of some common pesticides using conducting polymer/multiwalled carbon nanotubes modified glassy carbon electrode

The cyclic voltammetric behaviour of three common pesticides such as isoproturon (ISO), voltage (VOL) and dicofol (DCF) was investigated at glassy carbon electrode (GCE), multiwalled carbon nanotubes modified GCE (MWCNTs/GCE), polyaniline (PANI) and polypyrrole (PPY) deposited MWCNT/GCE. The modified electrode film was characterized by scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). The electroactive behaviour of the pesticides was realized from the cyclic voltammetric studies. The differential pulse voltammetric principle was used to analyze the above-mentioned pesticides using MWCNT/GCE, PANI/MWCNT/GCE and PPY/MWCNT/GCE. Effects of accumulation potential, accumulation time, Initial scan potential, amplitude and pulse width were examined for the optimization of stripping conditions. The PANI/MWCNT/GCE performed well among the three electrode systems and the determination range obtained was 0.01-100 mgL(-1) for ISO, VOL and DCF respectively. The limit of detection (LOD) was 0.1 microgL(-1) for ISO, 0.01 microgL(-1) for VOL and 0.05 microgL(-1) for DCF on PANI/MWCNT/GCE modified system. It is significant to note that the PANI/MWCNT/GCE modified system results in the lowest LOD in comparison with the earlier reports. Suitability of this method for the trace determination of pesticide in spiked samples was also realized.     

Surfactant effects on methanol oxidation at Pt–Ru/C coated glassy carbon electrode

The role of surfactants, cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and Triton X-100 (in the catalyst), on methanol oxidation at commercial 50:50 Pt–Ru/C catalyst-coated glassy carbon has been studied using cyclic voltammetry, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Surfactant containing catalysts showed a considerable reduction in the methanol oxidation potential. In terms of oxidation potential, better results (lower methanol oxidation potential) were observed in the order SDS > Triton X-100 > CTAB > no surfactant. SEM studies on the catalyst ink showed better homogeneity in the sample prepared using surfactant. This indicates better Pt Pt contact, which is likely to favour methanol adsorption and its oxidation. Hence, lowering of oxidation potentials for methanol oxidation could be seen with use of surfactants. Results of FT-IR on the catalyst ink showed definite changes in the frequencies in the case of Pt–Ru/C containing surfactants indicating definite interaction between catalyst and surfactant. Catalysts, with and without surfactants, yielded linear plots of concentration vs peak currents for methanol oxidation (0–2 M). With surfactant containing catalysts, reduction in methanol oxidation current was observed, and the order followed was the reverse of the above.     

Voltammetric determination of 2-chlorophenol using a glassy carbon electrode coated with multi-wall carbon nanotube-dicetyl phosphate film

Multi-wall carbon nanotubes (MWNT) were dispersed into water in the presence of dicetyl phosphate (DCP), and MWNT-DCP composite film coated glassy carbon electrodes (GCE) were constructed. The electrochemical properties of 2-chlorophenol at a bare GCE and MWNT-DCP modified GCE were compared. It was found that MWNT-DCP modified GCEs significantly enhance the oxidation peak current of 2-chlorophenol and lowers its oxidation overpotential, suggesting great potential in the sensitive determination of 2-chlorophenol. Finally, a sensitive and simple voltammetric method was developed for the determination of 2-chlorophenol. The oxidation peak current increases linearly with the concentration in the range of 1.0 × 10⁻⁷–2.0 × 10⁻⁵ mol L⁻¹, and the detection limit is 4.0 × 10⁻⁸ mol L⁻¹ for 2 min accumulation. The method was successfully used to determine 2-chlorophenol in waste water samples.     

Electrochemical determination of acetaminophen using a glassy carbon electrode coated with a single-wall carbon nanotube-dicetyl phosphate film

Single-wall carbon nanotubes (SWNT) were dispersed into water in the presence of dicetyl phosphate (DCP), and then a SWNT-DCP film-coated glassy carbon electrode (GCE) was constructed. The electrochemical behavior of acetaminophen at bare GCE and SWNT-DCP modified GCE were compared, suggesting that the SWNT-DCP-modified GCE significantly enhances the oxidation peak current of acetaminophen. A sensitive and simple electrochemical method with a good linear relationship in the range of 1.0 × 10⁻⁷–2.0 × 10⁻⁵ mol L⁻¹, was developed for the determination of acetaminophen. The detection limit is 4.0 × 10⁻⁸ mol L⁻¹ for 3-min accumulation. This method was successfully demonstrated with tablets.     

Stripping voltammetric determination of traces of technetium with a glassy carbon electrode coated with tri-n-octylphosphine oxide

Technetium(VII) and Tc(IV) are concentrated from 3 M hydrochloric acid media by complexing with tri-n-octylphosphine oxide applied as a thin layer to a glassy carbon electrode. Differential-pulse cathodic stripping voltammetry from 0 V provides a stripping peak for Tc(VII) at ?350 mV (vs. Ag/AgCl). The detection limit after an enrichment time of 10 min is about 1.8×10^?8 M Tc(VII). Technetium(IV) produces a stripping peak near the Tc(VII) peak which can be used for rough estimates of the Tc(VII)/Tc(IV) ratio within limited ranges. Uranium(VI) in equimolar concentrations interferes.     

Amperometric detection of carbohydrates and thiols by using a glassy carbon electrode coated with Co oxide/multi-wall carbon nanotubes catalytic system

A glassy carbon electrode coated with cobalt oxide/multi-wall carbon nanotubes (MWCNT) system was used for the detection of carbohydrates and thiols. The modification of the glassy carbon electrode increased the anodic current response of these organic compounds and decreased their overvoltage. The amperometric responses were extremely stable with no loss of sensitivity over many days of storage. Such attractive performance characteristics indicate great promise for using this new catalytic system for monitoring in fast and simple way compounds of great interest for food industry, biotechnology and clinical diagnostics.     

Voltammetric determination of salbutamol on a glassy carbon electrode coated with a nanomaterial thin film

A rapid and convenient electrochemical method is described for the determination of salbutamol based on multi-carbon nanotubes (MWNT) film coated glassy carbon electrode (GCE). The electrochemical behavior of salbutamol at this modified electrode was studied by square wave voltammetry, which indicated that the oxidation peak potential of salbutamol shifted on 40 mV to less positive potential and the peak current increased 4.5 fold, in contrast to that at a bare electrode. Various experimental parameters such as pH value of supporting electrolyte, the amount of modifier, and accumulation time were optimized. Under optimal measurement conditions, there is a good linear relationship between the peak current (I _pa) and salbutamol concentration in the range from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M, and the detection limit is 2.0 × 10⁻⁷ M (S/N = 3) at 2 min accumulation. The method has been successfully employed to detect salbutamol in pharmaceutical formulations.     

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.     

Electrocatalysis and amperometric detection of aliphatic aldehydes at platinum-palladium alloy coated glassy carbon electrode

Aliphatic aldehydes undergo an electrocatalytic oxidation at a platinum-palladium (Pt-Pd) alloy coated glassy carbon electrode. The alloy modification offers a highly sensitive and stable, constant (+0.35 V) potential detection of simple aldehydes. The drastically enhanced catalytic response of the alloy deposit, compared to the single component metals, is attributed to synergistic effect associated with changes in the adsorption features of the surface. The influence of the alloy deposition conditions upon the amperometric response is assessed. Scanning electron microscopy, energy dispersive X-ray analysis and elemental distribution mapping offer useful insights into the microstructure and composition of the alloy deposit. Formaldehyde, acetaldehyde and propionaldehyde were detected in flow-injection analysis at levels as low as 0.9, 6.4 and 6.5 ng (30, 160 and 120 pmol), respectively. Such operation offers lower operating potentials and detection limits compared to the recently developed mixed-valent ruthenium coated detector for aldehydes [2].     

A study of differential potentiometric stripping analysis for lead and cadmium with Nafion coated carbon fibre electrode

The differential potentiometric stripping analysis (DPSA) with Nafion coated carbon fibre electrode has been studied, and a method to determine lead and cadmium in urine directly has been attempted. The effect of various experimental parameters on the DPSA response is discussed. The experimental conditions include 0.2M sodium perchlorate, deposition potential of ?1.0 or ?1.1 V and using 20 ppm mercuric ion as oxidizing agent. The response of the signal is in linear relation with the concentrations of lead and cadmium respectively up to 0.5 ppm. The electrode coated with Nafion film alleviates the interference from organics in urine samples.     

Direct electron transfer and voltammetric determination of roxithromycin at a single-wall carbon nanotube coated glassy carbon electrode

The electrochemical behavior of roxithromycin (RM) at a single-wall carbon nanotube (SWNT) coated glassy carbon (GC) electrode was studied. It was found that RM could produce an irreversible anodic peak at the electrode. When the pH of supporting electrolyte (i.e. phosphate buffer solution) was 7 the peak potential was 0.86V (vs. SCE). The electrochemical reaction contained electron and proton transfer, and the electron-transfer coefficient (α) was ca. 0.87. The anodic peak depended on the adsorption of RM, the maximum adsorption amount was about 3.99×10(-10)molcm(-2). The adsorbed RM could be removed by cycling between 0.1 and 1.1V in a blank solution for about two minutes, and the electrode thus could be regenerated. Under the optional conditions, the anodic peak current was linear to RM concentration over the range of 5.0×10(-6) to 1.0×10(-4)M. The limit of detection was 5.0×10(-7)M (S/N=3) for 180s accumulation at -0.8V. The modified electrode had good stability and repeatability, and it was successfully applied to the determination of RM in medicine samples.     

唑类有机化合物修饰电极同时测定尿液中的多巴胺和黄嘌呤

建立以唑类有机化合物(4-氨基-4H-1,2,4,-三唑)修饰玻璃电极,得到聚唑类有机合物膜修饰电极,并以该电极对多巴胺(DA)和黄嘌呤(XN)进行同时分析测定。结果表明,在0.2 mol·L-1的磷酸盐缓冲液(p H=5.0)中,DA和XN在聚唑类有机化合物修饰的电极上具有比较好的电化学行为。多巴胺和黄嘌呤分别5×10-5~5×10-9mol·L-1和1×10-4~1×10-7mol·L-1浓度范围内有较好的电化学响应,DA和XN检出限分别为5×10-10mol·L-1和1×10-8mol·L-1,该修饰电极具有较高的灵敏度、重现性和稳定性。该方法可用于人体的尿液中两种化合物的同时测定分析,回收率分别为94.7%~99.3%与93.6%~95.3%.     

Modification of glassy carbon electrode with a polymer/mediator composite and its application for the electrochemical detection of iodate

A modified glassy carbon electrode was prepared by depositing a composite of polymer and mediator on a glassy carbon electrode (GCE). The mediator, flavin adenine dinucleotide (FAD) and the polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) were electrochemically deposited as a composite on the GCE by applying cyclic voltammetry (CV). This modified electrode is hereafter designated as GCE/PEDOT/FAD. FAD was found to significantly enhance the growth of PEDOT. Electrochemical quartz crystal microbalance (EQCM) analysis was performed to study the mass changes in the electrode during the electrodeposition of PEDOT, with and without the addition of FAD. The optimal cycle number for preparing the modified electrode was determined to be 9, and the corresponding surface coverage of FAD (Γ_FAD) was ca. 5.11 × 10⁻¹⁰ mol cm⁻². The amperometric detection of iodate was performed in a 100 mM buffer solution (pH 1.5). The GCE/PEDOT/FAD showed a sensitivity of 0.78 μA μM⁻¹ cm⁻², a linear range of 4–140 μM, and a limit of detection of 0.16 μM for iodate. The interference effects of 250-fold Na⁺, Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Cl⁻, NO₃⁻, I⁻, SO₄²⁻ and SO₃²⁻, with reference to the concentration of iodate were negligible. The long-term stability of GCE/PEDOT/FAD was also investigated. The GCE/PEDOT/FAD electrode retained 82% of its initial amperometric response to iodate after 7 days. The GCE/PEDOT/FAD was also applied to determine iodate in a commercial salt.