Harry B. Mark

Hydrogen peroxide sensitive amperometric biosensor based on horseradish peroxidase entrapped in a polypyrrole electrode

The enzyme horseradish peroxidase (HRP) has been entrapped in situ by electropolymerization of pyrrole onto a platinum electrode. The latter was previously coated by a polypyrrole layer for better adhesion of the biocatalyst film and in order to avoid the enzyme folding onto the Pt electrode. The biosensor allowed the determination of hydrogen peroxide in the concentration range comprised between 4.9 x 10(-7) and 6.3 x 10(-4) M. The biosensor retained more than 90% of its original activity after 35 days of use.

Electrochemistry and detection of some organic and biological molecules at conducting poly(3-methylthiophene) electrodes

Electrodes modified by the electrodeposition of poly(3-methylthiophene) were used as chemical sensors for some organic and biological molecules of industrial and medicinal interest. The electrochemical behaviors of ferri/ferrocyanide, catechol, ascorbic acid, hydroquinone, dopamine epinephrine, acetaminophen, p-aminophenol and NADH were examined by cyclic voltammetry. The results showed that the proposed modified surface catalyzes the oxidation of these compounds. Differential pulse and square wave techniques were used for the analysis of binary mixture of ascorbic acid with catechol, NADH, dopamine and p-aminophenol. Voltammetric peak resolution was also demonstrated for a ternary mixture of ascorbic acid, catechol and p-aminophenol. Polymer coated electrode was also used in an amperometric detector for flow injection analysis of most of the aforementioned compounds. The responses of the polymer electrode were 4-10 times larger as compared to those of platinum. The modified electrode displayed excellent response stability for successive injections and detection limits were 10 ppb for catechol, dopamine, epinephrine, NADH and p-aminophenol, 1 ppb for acetaminophen and 100 ppb for ascorbic acid. Voltammetric peak positions were affected by the nature of the electrolyte and its pH. Also, film thicknesses were shown to be a factor affecting both the current magnitudes and oxidation peak potential of NADH.

Electrochemical control of solid phase micro-extraction: conducting polymer coated film material applicable for preconcentration/analysis of neutral species

Exploitation of the physical, chemical and electrically conductive properties of poly(3-dodecylthiophene) (P3DDT) for the preconcentration and release in solid phase microextraction (SPME) of organometallic arsenobetaine (AsB) from aqueous media was investigated. Hydrophobic interactions between this neutral arsenic species and an undoped polythiophene (no applied potential) with n-substituted alkyl groups (n=12) in the three position were used for the diffusion-controlled preconcentration. After absorption into the polymer matrix, the chemical properties of this conductive polymer were changed by applying an external potential. This potential provides a sufficient driving force for desorption of the analyte from the extraction phase into an aqueous solution for subsequent analysis. The applied positive potential oxidizes the polymer to its charged hydrophilic state, which releases the neutral analyte. The concentration and speciation of the analyte from the sample matrix was analyzed by HPLC coupled to an ICP-MS. The diffusion-controlled uptake was fast (equilibrium attained within minutes) and did not require pretreatment of the analyte. The electrochemically-controlled release of the analyte is also very rapid (within minutes). This conducting polymer film system, therefore, can offer analytical applications for the convenient preconcentration and subsequent analysis of neutral environmental species.

Voltammetric resolution of ascorbic acid and dopamine at conducting polymer electrodes

Abstract Electrochemical behavior of dopamine and ascorbic acid binary mixture was examined by cyclic voltammetry and the analysis of a each component was performed by differential pulse voltammetry. The electrochemical behavior of these molecules on poly-3-methylthiophene, polypyrrole, polyaniline electrodes were compared. The effects of electrolyte type and its pH, and the film thickness on the voltammetric resolution of these molecules were systematically examined. At all the modified electrodes both dopamine and ascorbic acid peaks shifted their positions to more negative potentials with increasing pH, difference between the positions remaining almost constant. Type of electrolyte seems to have an effect on ascorbic acid peak only at the poly-3-methylthiophene electrode. The effect of increasing film thickness was to observe increased peak heights.

An amperometric aqueous ethanol sensor based on the electrocatalytic oxidation at a cobalt-nickel oxide electrode

Abstract An electrodeposited mixed oxide film of Coue5f8Ni has been applied as a sensor for the amperometric determination of aqueous ethanol samples. The electrode has a linear response over a 1.5–7.9 mM ethanol concentration range with a detection limit of 32 μM ethanol. Deposition conditions for maximum catalytic efficiency, electrode lifetime and optimum analytical analysis potential are discussed. SEM, EDAX and electrochemical kinetic studies indicate that the active catalytic species in the electrode film is a 2:1 Coue5f8Ni oxide species and suggest a partial mechanism of the ethanol oxidation reaction. The feasibility of this modified electrode for the determination of atmospheric ethanol is also examined. All other organic alcohols tested were interferants. Conditions affecting the stability of the sensor are discussed.

Electrochemistry and detection of some organic and biological molecules at conducting polymer electrodes. Part 3. Evidence of the electrocatalytic effect of the heteroatom of the poly(hetetroarylene) at the electrode/electrolyte interface

Abstract The electrocatalytic effect of the conducting poly(3-methylthiophene) film (3-PMT) electrode on the redox behavior of catechol compounds has been examined. The nature of the electrode material substrate had no effect on the peak potentials in the cyclic voltammograms (CVs) of different redox systems of biological interest: p -aminophenol, dopamine, catechol and ascorbic acid. The nature of the heteroatom of 3-PMT, polypyrrole, polyfuran and polyanaline had a very large effect on their electrokinetic properties. The adsorption of molybdenum ion results in the blocking of ‘the active sites’ at the interface, as indicated by the disappearance of the electrochemical activity of the 3-PMT film electrode. The surface examination by X-ray photoelectron spectroscopy (XPS) of the molybdenum exposed polymer suggests the possibility of a chemical bonding between the metal ions and the sulfur heteroatom of the polymer surface. The ‘apparent’ diffusion coefficients D app were determined from the effect of the scan rate dependency of CVs of the catechol, etc. The results showed that the electron transfer is predominantly diffusion controlled at the polymer/vbsolution interface and probably involves the sulfur heteroatom specifically.

Optimization of preparation of poly(3-methylthiophene)-modified Pt microelectrodes for detection of catecholamines

Abstract Prior studies have shown that poly(3-methylthiophene)-modified platinum electrodes (P3MT/Pt) have improved electrochemical reversibility, selectivity, and sensitivity for the detection of catecholamines when compared with conventional electrodes. Difficulties have, however, been encountered, as a P3MT film grows beyond the edges of the Pt electrode substrate during electropolymerization, and, consequently, increases the surface area of the electrode. The width of the film growth beyond the edges of the substrate depends on electropolymerization conditions and the size of the electrode substrate. This makes the catecholamine analysis irreproducible from electrode to electrode, and especially limits the application of the P3MT/Pt system as micro- and ultramicro-electrodes. In order to control the edge effect, optimization of the preparation conditions of P3MT/Pt electrodes, including temperature, monomer concentration, deposition time, voltage, and electrolyte concentration, was studied by fractional factorial design (FFD). Improved P3MT/Pt microelectrodes were achieved by applying the optimal conditions, which simultaneously reduced the edge effect. Also, P3MT/Pt film electrodes polymerized at −20°C exhibited the same electrocatalytic properties, which are essential for detection of catecholamines, as those electrodes made at room temperature.

STUDIES OF SOME HINDERED 2,2′-BITHIENYLS AND 3,3′-BRIDGED 2,2′-BITHIENYLS WITH SPECIAL REFERENCE TO THEIR UV SPECTRA AND OXIDATION POTENTIALS

Abstract Some hindered bithienyl derivatives have been synthesized by cross-coupling reaction of Grignard reagents using NiCl2(dppp)(dppp = Ph2PCH2CH2CH2PPh2) or by reductive dimerization in the presence of Zn/NiCl2(PPh3)2. Their UV spectra and oxidation potentials are discussed in terms of their coplanarity and of the inductive effect of the substituent.

SYNTHESIS OF MIXED OLIGOMERIC HETEROARYLENES CONTAINING FURAN, THIOPHENE, AND SELENOPHENE RINGS; THEIR UV SPECTRA AND OXIDATION POTENTIALS

Abstract Mixed oligomeric five-membered heteroarylenes 3a-f and 4a-e have been prepared by cross-coupling reaction of the Grignard reagents 2 derived of the appropriately substituted thiophenes 1 with either the 2-bromoheteroarylenes or the 2,5-dibromoheteroarylenes derived of furan, thiophene, and/or selenophene respectively. The UV spectra and oxidation potential of compounds 3a-f and 4a-e are discussed on the basis of co-planarity.

A novel sorbent tube for ambient hydrogen sulfide determination

A novel tubular device has been developed for hydrogen sulfide determination in air. Several substrates such as commercial silica gel and alumina TLC plates, silica gel powder, alumina, CaSO(4), CaCO(3), BaSO(4), MgO, chalk, alpha-cellulose and ethyl cellulose were tested as solid substrates. 30-70-mesh silica gel was finally employed in glass tubes of 4.0 mm internal diameter. Silica gel is treated with 0.5 M aqueous CdCl(2) solution, dried, filled into the glass tube and sample gas is passed through the device using nitrogen as the carrier gas where 70% relative humidity is employed. The analyte reacts with the solid substrate to form a luminescent spot whose length in the tube is measured and correlated to concentration. The flow rate was 68 ml min(-1). The analytical system is linear in the range of 0.2-1.3 ppm H(2)S for the specified conditions. The prepared devices are stable at least for 3 months prior to sampling; and after sampling, the luminescing spot is stable also at least for 3 months.