Nada F. Atta

Simultaneous determination of catecholamines, uric acid and ascorbic acid at physiological levels using poly(N-methylpyrrole)/Pd-nanoclusters sensor

An interesting electrochemical sensor has been constructed by the electrodeposition of palladium nanoclusters (Pd(nano)) on poly(N-methylpyrrole) (PMPy) film-coated platinum (Pt) electrode. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy were used to characterize the properties of the modified electrode. It was demonstrated that the electroactivity of the modified electrode depends strongly on the electrosynthesis conditions of the PMPy film and Pd(nano). Moreover, the modified electrode exhibits strong electrocatalytic activity toward the oxidation of a mixture of dopamine (DA), ascorbic acid (AA), and uric acid (UA) with obvious reduction of overpotentials. The simultaneous analysis of this mixture at conventional (Pt, gold [Au], and glassy carbon) electrodes usually struggles. However, three well-resolved oxidation peaks for AA, DA, and UA with large peak separations allow this modified electrode to individually or simultaneously analyze AA, DA, and UA by using differential pulse voltammetry (DPV) with good stability, sensitivity, and selectivity. This sensor is also ideal for the simultaneous analysis of AA, UA and either of epinephrine (E), norepinephrine (NE) or l-DOPA. Additionally, the sensor shows strong electrocatalytic activity towards acetaminophen (ACOP) and other organic compounds. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.05 to 1mM, 0.1 to 10 microM, and 0.5 to 20 microM, respectively. The detection limits (signal/noise [S/N]=3) were 7 microM, 12 nM, and 27 nM for AA, DA, and UA, respectively. The practical application of the modified electrode was demonstrated by measuring the concentrations of AA, DA, and UA in injection sample, human serum, and human urine samples, respectively, with satisfactory results. The reliability and stability of the modified electrode gave a good possibility for applying the technique to routine analysis of AA, DA, and UA in clinical tests.

Poly(3,4-ethylene-dioxythiophene) electrode for the selective determination of dopamine in presence of sodium dodecyl sulfate

A novel biosensor using poly(3,4-ethylene dioxythiophene) (PEDOT) modified Pt electrode was developed for selective determination of dopamine (DA) in presence of high concentrations of ascorbic acid (AA) and uric acid (UA) with a maximum molar ratio of 1/1000, and 1/100 in the presence of sodium dodecyl sulfate (SDS). SDS forms a monolayer on PEDOT surface with a high density of negatively charged end directed outside the electrode. The electrochemical response of dopamine was improved by SDS due to the enhanced accumulation of protonated dopamine via electrostatic interactions. The common overlapped oxidation peaks of AA, UA and DA can be resolved by using SDS as the DA current signal increases while the corresponding signals for AA and UA are quenched. The use of SDS in the electrochemical determination of dopamine using linear sweep voltammetry at modified electrode PEDOT/Pt resulted in detecting dopamine at relatively lower concentrations. The DA concentration could be measured in the linear range of 0.5 to 25μmol L(-1) and 30μmol L(-1) to 0.1mmol L(-1) with correlation coefficients of 0.998 and 0.993 and detection limits 61nmol L(-1) and 86nmol L(-1), respectively. The validity of using this method in the determination of dopamine in human urine was also demonstrated.

Poly(3-methylthiophene)/palladium sub-micro-modified sensor electrode. Part II: Voltammetric and EIS studies, and analysis of catecholamine neurotransmitters, ascorbic acid and acetaminophen

Promising voltammetric sensors based on the modification of Pt and poly(3-methylthiophene) (PMT) electrodes with Pd nanoparticles were achieved for the determination of catecholamine neurotransmitters, ascorbic acid and acetaminophen. Electrochemistry of the indicated compounds was studied at these electrodes and interesting electrocatalytic effects were found. Furthermore, simple, easily prepared one electrochemical step Pd-modified Pt electrode (Pt/Pd) is reported for the first time. Cyclic voltammetry (CV) and chronocoulometry (CC) were used for the determination of the apparent diffusion coefficients in different electrolytes at these electrodes and the values are in the range from 10(-4) to 10(-5)cm(2)s(-1). Furthermore, it was found that the method of polymer formation had a substantial effect on the synergism between the polymer film and the loaded metal particles towards the oxidation of dopamine (DA) in different supporting electrolytes. This was confirmed by the CV, CC and EIS (electrochemical impedance spectroscopy) as well as SEM (Scanning Electron Microscopy) results. Pt and PMT electrodes modified with Pd nanoparticles showed excellent results for the simultaneous determination of tertiary and quaternary mixtures of the studied compounds.

Effect of surfactants on the voltammetric response and determination of an antihypertensive drug

The effect of adding surface-active agents to electrolytes containing terazosin, an antihypertensive drug, on the voltammetric response of glassy carbon electrode was studied. The current signal due to the oxidation process was a function of the amount of terazosin, pH of the medium, type of surfactant, and accumulation time at the electrode surface. Two surfactants were used, an anionic type, sodium dodecyl sulfate (SDS) and a cationic type, cetyl trimethyl ammonium bromide (CTAB). Addition of SDS to the terazosin-containing electrolyte was found to enhance the oxidation current signal while CTAB showed an opposite effect. Beside the interfacial interaction of the surfactant with the electrode surface in reference to the bias applied potential and the charge of surfactant, terazosin-surfactant interaction in the electrolytic solution was found to be critical to the magnitude of current signal. Addition of SDS to terazosin-containing buffer solution resulted in a decrease in the drug absorption spectrum both in the ultra-violet and visible (UV-vis) regions. Moreover, NMR measurements showed considerable chemical shifts for the aromatic protons of the quinazolinyl moiety of the terazosin in presence of SDS. The affected aromatic protons are positioned next to the interacting protonated amino-group of the terazosin with the charged sulfonate-group of SDS. On the other hand, addition of CTAB did not cause noticeable changes both to the UV-vis and NMR spectra of the drug. The use of SDS in the electrochemical determination of terazosin using linear sweep voltammetry and differential pulse voltammetry at solid glassy carbon electrode enhanced the detection limit from 6.00x10(-7)molL(-1) in absence of surfactant to 4.58x10(-9)molL(-1) when present. The validity of using this method in the determination of drug active ingredient in urine samples and tablet formulations was also demonstrated.

Carbon Paste Gold Nanoparticles Sensor for the Selective Determination of Dopamine in Buffered Solutions

�1 universal buffer solution pH 7.4 is introduced. The sensor is based on a carbon paste CP electrode modified with gold nanoparticles. In a mixture of DA, AA, and UA, the sensor shows high selective response toward DA and no response for AA or UA. The effect of various experimental parameters including time of deposition of gold nanoparticles on the CP electrode, pH, and scan rate on the voltammetric response of DA was investigated. At the optimum conditions, the concentration of DA was determined using differential pulse voltammetry in a linear range of 1.0 10 �7 to 5.0 10 �6 mol L �1 and 5.0 10 �6 to 1.3 10 �4 mol L �1 with correlation coefficients of 0.9995 and 0.9988 and a detection limit of 5.9 10 �9 and 8.2 10 �8 mol L �1 , respectively. The modified electrode can be used for the determination of DA spiked into human serum samples, and excellent recovery results were obtained over a wide concentration range of DA. Moreover, validation parameters, such as reproducibility, sensitivity, and recovery were evaluated successfully in the determination of DA in diluted human urine.

Probing cysteine self-assembled monolayers over gold nanoparticles – Towards selective electrochemical sensors

Cysteine forms self-assembled monolayers over gold nanoparticles. Based on this knowledge, a novel electrochemical sensor (Au-Au(nano)-Cys-SDS) has been constructed by the formation of self-assembly monolayer (SAM) of cysteine on gold-nanoparticles modified gold electrode (Au-Au(nano)-Cys) to be utilized for determination of dopamine in the presence of sodium dodecyl sulfate (SDS). Electrochemical investigation and characterization of the modified electrode sensor was achieved using cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron, and atomic force microscopies. Au-Au(nano)-Cys electrode in the presence of SDS gave comparable high current response to that of the gold nanoparticles modified gold electrode (Au-Au(nano)). The Au-Au(nano)-Cys-SDS electrode current signal was remarkably stable via repeated cycles and long term stability due to the strong AuS bond. Very small peak separation, almost zero or 15 mV peak separation was also obtained by repeated cycles indicating unusual high reversibility. The oxidation peak current was determined to be linearly dependent on the dopamine concentration. A resulting calibration curve using square wave voltammetry (SWV) was obtained over concentration range of 30-100 μmol L(-1) and 120-320 μmol L(-1) with correlation coefficients of 0.996 and 0.994 and a limit of detection of 16 and 57 nmol L(-1), respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of tertiary mixture of ascorbic acid AA, dopamine, and acetaminophen APAP was explored at this modified electrode. It has been demonstrated that Au-Au(nano)-Cys-SDS electrode can be used as a sensor with excellent reproducibility, sensitivity, and long term stability.

Voltammetric studies of the oxidation of reduced nicotinamide adenine dinucleotide at a conducting polymer electrode

Voltammetric studies of the redox behaviour of NADH at a poly(3-methylthiophene) conducting polymer electrode showed a large electrocatalytic effect without the use of electron-transfer mediators.

Simultaneous Determination of Catecholamines and Serotonin on Poly(3,4-ethylene dioxythiophene) Modified Pt Electrode in Presence of Sodium Dodecyl Sulfate

A promising electrochemical sensor was developed using poly(3,4-ethylene dioxythiophene) modified platinum electrode in the presence of sodium dodecyl sulphate (SDS). This sensor is sensitive for the determination of catecholamine compounds, namely dopamine, epinephrine, L-norepinephrine, and L-DOPA, as well as serotonin (ST) in the presence of interference molecules such as uric acid, ascorbic acid (AA), and glucose. The presence of SDS in the medium plays a key role in the electrostatic attraction of these compounds toward the polymeric surface and causes repulsion toward the interfering ones. Cyclic voltammetry, linear sweep voltammetry, ultraviolet-visible (UV‐vis), nuclear magnetic resonance and electrochemical impedance spectroscopy were used to verify the behavior of the studied compounds in micellar media. In the presence of an anionic surfactant, the presence of large excess of AA and glucose did not interfere with the voltammetric responses of catecholamine and ST. The linear response was obtained for serotonin in the range of 0.05‐10 lmol l � 1 and 20‐100 lmol l � 1 with correlation coefficients of 0.997 and 0.998 and detection limits 48 and 71 nmol l � 1 , respectively. The utility of this modified electrode was demonstrated for the determination of

Conducting polymer ion sensor electrodes–III. Potentiometric sulfide ion selective electrode

A new potentiometric sensor electrode for sulfide based on conducting polymer films is introduced. The electrode is formed by electrochemically depositing a film of poly(3-methylthiophene) and poly(dibenzo-18-crown-6) onto an alloy substrate. Different methods were used for the electrode preparations. The alloy used has a low melting point, which allowed its use for manufacturing a microsize version of this electrode. The electrode response is stable for 3 days. The working temperature range for this electrode is between 10 and 40 degrees C. The linear dynamic range is 1.0x10(-7)-1.0x10(-2) M and measures total sulfide concentration over a range of pH from 1 to 13. The polymer electrode showed high selectivity for sulfide in the presence of many common interfering anions. The electrode is useful for the measurement of total sulfide in biological environments and can be manufactured in the micron scale. Therefore, it will be useful for the measurement within biofilms.

Computational investigation and synthesis of a sol–gel imprinted material for sensing application of some biologically active molecules

A hybrid sol-gel material was molecularly imprinted with a group of neurotransmitters. Imprinted material is a sol-gel thin film that is spin coated on the surface of a glassy carbon electrode. Imprinted films were characterized electrochemically using cyclic voltammetry (CV) and the encapsulated molecules were extracted from the films and complementary molecular cavities are formed that enable their rebind. The films were tested in their corresponding template solutions for rebinding using square wave voltammetry (SWV). Computational approach for exploring the primary intermolecular forces between templates and hydrolyzed form of the precursor monomer, tetraethylorthosilicate (TEOS), were carried out using Hartree-Fock method (HF). Interaction energy values were computed for each adduct formed between a monomer and a template. Analysis of the optimized conformations of various adducts could explain the mode of interaction between the templates and the monomer units. We found that interaction via the amino group is the common mode among the studied compounds and the results are in good agreement with the electrochemical measurements.