Xanthine biosensor based on the direct oxidation of xanthine at an electrogenerated oligomer film.

Poly(o-aminophenol-co-pyrogallol) (PAP) was first synthesized via the electrochemical copolymerization of o-aminophenol and pyrogallol in the acidic solution, using a reduced graphene oxide/glassy carbon (RGO/GC) electrode as a working electrode. Reduced graphene oxide played an important role in increasing PAP amount deposited on the RGO/GC electrode compared to that on the bare GC electrode, which is due to that RGO has the large specific surface area. The results from the spectra of IR, (1)H NMR and ESR and the measurement of molecular weight demonstrated that PAP is an oligomer with the free radicals and exhibited good redox activity in a wide pH range from pH<1-9.0 and can effectively catalyze xanthine oxidation due to the presence of the free radicals and the reversible redox groups in the copolymer chain. On the basis of the direct oxidation of xanthine on PAP, the PAP/RGO/GC electrode was used as a xanthine biosensor. The biosensor showed a linear range from 1.0 to 120µM xanthine at pH 6.0 with a correction coefficient of 0.9965 and fast response to xanthine oxidation. The peak potential of xanthine oxidation shifted from 0.814 to 0.668V as pH increased from 5.0 to 7.5.

Electrochemical oxidation of pyrogallol: formation and characterization of long-lived oxygen radicals and application to assess the radical scavenging abilities of antioxidants.

Electrochemical oxidation of pyrogallol in a pH 5.0 phosphate buffer was carried out on a reduced graphene oxide/glassy carbon (RGO/GC) electrode. Reduced graphene oxide plays an important role in catalyzing the electrochemical oxidation of pyrogallol. A deep yellow film deposited on the electrode exhibits electroactivity in a wide pH range. On the basis of the experimental results from measurements of (1)H NMR, (13)C NMR, and IR spectra, there are hydroxyl, carbonyl, and aldehyde groups in the product. No visible absorption peak occurs in the UV-vis spectrum of the product, and its molecular weight is lower than that of the dipolymer but higher than that of the monomer. Therefore, the film is neither a polymer nor a dipolymer and is only a product of pyrogallol oxidation with oxygen radicals. No tendency toward the decay of the ESR signal intensity of the electrogenerated film deposited on the RGO/graphite electrode was observed after 210 days. Electrogenerated film was used as a radical source to test the radical scavenging abilities of ascorbic acid, catechin, and catechol in aqueous solutions based on the ESR signal intensity. The result indicates that ascorbic acid and catechin can scavenge the free radicals, but catechol can hardly scavenge the free radicals.

Influence of potential and temperature on the ESR spectra of polyaniline synthesized using the interface polymerization.

In situ ESR-electrochemical measurements indicate that the distinct redox properties of polyaniline synthesized using the interface polymerization method (labeled IP-polyaniline) are strongly related to its unpaired spin density. IP-polyaniline in a 1.0 M NaCl solution of pH 5.5 still holds stronger ESR signals at a wide potential range, which results in its high redox activity in this solution. The influence of pH on the potential range for the formation of polaron is detected. Also, some unusual phenomena are observed in the measurements of ESR signal intensity as a function of applied potential, for example, the ESR signal intensity of IP-polyaniline in 0.20 M HCl solution decreases with increasing potential from 0.30 to 0.80 V accompanied with the peak-to-peak line width ΔH(pp) of the ESR signal increasing from 0.30 to 0.60 V, and then, however, ΔH(pp) decreases pronouncedly as the potential increases further. The results from measurements for the ESR susceptibility of IP-polyaniline as a function of temperature demonstrate the presence of the conversion of the temperature-dependent Curie susceptibility to the temperature-independent Pauli susceptibility at the temperature range 135-335 K; however, the ESR susceptibility of IP-polyaniline increases again from 335 to 375 K. The ΔH(pp) value increases very obviously from 135 to 195 K and then decreases with increasing temperature up to 375 K.

Spectral characteristics of polyaniline nanostructures synthesized by using cyclic voltammetry at different scan rates.

The polyaniline nanofibers with different sizes were synthesized by using cyclic voltammetry at different potential scan rates, in the presence of ferrocenesulfonic acid. The potential scan rate controlled the formation and growth of polyaniline nuclei, which plays a key role in controlling nanofiber sizes. The average diameters of nanofibers decreased from about 130 nm to about 80 nm as the potential scan rate increased from 6 to 60 mV s (-1). We first observed an ordered change in the following spectra with the nanofiber sizes of polyaniline. The spectra of the X-ray diffraction indicated that the partially crystalline form existed in the polyaniline nanofibers and that the crystallinity of polyaniline increased with decreasing diameter of polyaniline nanofibers. The ESR spectra revealed the fact that the decrease in the intensity of the ESR signal was accompanied by the increase in the value of the peak-to-peak line width Delta H pp as the diameter of polyaniline nanofibers decreased. The 1H NMR spectra showed that a peak in a triplet caused by the +/- NH free radical was split into two peaks with different intensities and that their relative intensity also changed with the diameter of the polyaniline nanofibers.

Synthesis and electronic properties of poly(aniline-co-2-amino-4-hydroxybenzenesulfonic acid).

Poly(aniline- co-2-amino-4-hydroxybenzenesulfonic acid) (PAAHB) was first synthesized via the electrochemical copolymerization of aniline and 2-amino-4-hydroxybenzenesulfonic acid (AHB) in the presence of an ionic liquid. The conductivity of PAAHB in the oxidized state is 0.45 S cm(-1). The conductivity and ESR spectra of PAAHB are slightly affected by water. The cyclic voltammograms of PAAHB reveal excellent redox activity from pH<1 to pH 11.0. This is attributed to the synergistic effect of the -SO3- and -OH functional groups in the copolymer chain and the ionic liquid incorporated into the PAAHB film. It is evident that the pH dependence of the redox activity and conductivity of PAAHB is much better than that of polyaniline. The proton NMR spectra of PAAHB and AHB demonstrate that the -SO3- group exists in the copolymer chain instead of the -SO3H group. Therefore, PAAHB can be used for the determination of dopamine in the presence of ascorbic acid as a result of the -SO3- group, which plays an important role in the selectivity.

ESR studies of poly(aniline-co-m-aminophenol) in the solid state and nonaqueous solution.

The copolymers, poly(aniline-co-m-aminophenol)s, used for the ESR studies were synthesized chemically in the solutions consisting of different concentration ratios of m-aminophenol to aniline. On the basis of the ESR measurements, the unpaired spin (polaron) densities of the copolymers were calculated to be 1.14 x 10(19) spins per gram for copolymer-A with the conductivity of 7.02 x 10(-6) S cm-1 and 2.03 x 10(20) spins per gram for copolymer-C with the conductivity of 2.34 S cm-1. The ESR measurements of the copolymers in the solid states show that the peak-to-peak line width DeltaHpp decreases with a decreasing concentration ratio of m-aminophenol to aniline, but the g-value hardly changes. A conversion of Curie spins to Pauli spins for the copolymers is observed as the temperature changes in going from low temperature to high temperature between 136 and 356 K. The ESR studies of the copolymers in a nonaqueous solution first reveal that there are two free radicals in the copolymer, and the unpaired spins in the copolymers arise from nitrogen nuclei.

Improvement in selectivity and storage stability of a choline biosensor fabricated from poly(aniline-co-o-aminophenol).

A choline biosensor was fabricated by using electrochemical doping to immobilize choline oxidase in poly(aniline-co-o-aminophenol) film that exhibits a good electric activity over a wide pH range. Using cyclic voltammetry, impedance measurement and scanning electron microscopy characterized the poly(aniline-co-o-aminophenol) film doped with choline oxidase. The amperometric detection of choline is based on the oxidation of the H2O2 enzymatically produced on the choline biosensor. The choline biosensor has a lower potential dependence. Thus, its operational potential was controlled at a low potential of 0.40 V (vs.SCE). The response current of the choline biosensor increases with increasing temperature from 277.1 to 308.1 K. An apparent activation energy of 30.8 kJ mol(-1) was obtained. The choline biosensor has a wide linear response range from 1x10(-7) to 1x10(-4) M choline with a correlation coefficient of 0.9999 and has a high sensitivity of 127 microA cm(-2), at 0.40 V and pH 8.0. The response time of the biosensor is 15-25 s, depending on the applied potentials. An apparent Michaelis constant and an optimum pH for the immobilized enzyme are 1.8 mM choline and 8.4, respectively, which are very close to those of choline oxidase in solution. The effect of selected organic compounds on the response of the choline biosensor was studied. Together, these findings show that the choline biosensor exhibits a better selectivity to interfering species and a better storage stability.

Catechol sensor using poly(aniline-co-o-aminophenol) as an electron transfer mediator.

In this work, poly(aniline-co-o-aminophenol) (copolymer) was used as an electron transfer mediator in the electrochemical oxidation of catechol due to its reversible redox over a wide range of pH. The experimental results indicate that the anodic peak potential of catechol at the copolymer electrode is lower than that at the platinum electrode in a solution consisting of catechol and sodium sulfate with pH 5.0, and the activation energy for the electrochemical oxidation of catechol at the copolymer electrode is low (23.6 kJ mol(-1)). These are strong evidence for the electrocatalytic oxidation of catechol at the copolymer electrode. The -OH group on the copolymer chain plays an important role in the electron transfer between the copolymer electrode and catechol in the solution. Based on the catalytic oxidation, the copolymer is used as a sensor to determine the concentration of catechol. The response current of the sensor depends on the concentration of catechol, pH, applied potential and temperature. At 0.55 V (versus saturated calomel reference electrode (SCE)) and pH 5.0, the sensor has a fast response (about 10s) to catechol and good operational stability. The sensor shows a linear response range between 5 and 80 microM catechol with a correlation coefficient of 0.997. It was found that phenol and resorcinol cannot be oxidized at the copolymer electrode at potentials < or =0.55 V, so controlling the sensor potential affords a good way of avoiding the effect of phenol and resorcinol on the determination of catechol.

Determination of hydrogen peroxide using amperometric sensor of polyaniline doped with ferrocenesulfonic acid.

The result of cyclic voltammetry shows that polyaniline doped with ferrocenesulfonic acid (PAnFc) can effectively catalyze the oxidation of hydrogen peroxide, so PAnFc is used as a sensor to determine the concentration of H2O2. Iron in the ferrocenesulfonic acid exists in two oxidation states: Fe2+ and Fe3+. They can reversibly oxidize and reduce between the Fe2+ and Fe3+ oxidation states, which play an important role in the catalytic oxidation of H2O2. The response current of the sensor depends on the pH, applied potential and temperature at a given concentration of H2O2. At optimum conditions, the sensor has a fast response to H2O2, good operational stability, a good linear response to H2O2 in the range from 4 to 64 microM, and a small temperature dependence of the response current.