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Correlation of properties of common carbon fiber precursor materials, polyacrylonitrile (PAN) and pitch, with kinetics and sensitivity of microdisk electrodes, PAN T650, PAN HCB and Pitch P25, nanostructured by an electrochemical etching method was tested. Sensitivity of PAN electrodes is higher but kinetics are slower than at pitch electrodes due to more defects in PAN. Because of more surface oxides at PAN T650 adsorption of dopamine is greater than at PAN HCB electrodes but for uric acid adsorption is greater at PAN HCB. Uric acid sensitivity is related to high conductivity of PAN fibers, with an inverse relationship observed for dopamine.

High signal intensities of glutathione (GSH), [GSH+H]+(m/z308), cysteine (CySH), [CySH+H]+(m/z122), and homocysteine (hCySH), [hCySH+H]+(m/z136), are observed in ESI MS with on‐line electrochemistry (EC). Dimers formed by H‐bonding, which are not electrochemical products, are detected as [2GSH+H]+(m/z615), [2CySH+H]+(m/z243) and [2hCySH+H]+(m/z271) together with disulfide dimers GSSG, CySSCy and hCySSCyh, [GSSG+H]+(m/z613), [CySSCy+H]+(m/z241) and [hCySSCyh+H]+(m/z269). When dopamine is present a thiol/dopamine quinone (DAQ) adduct is observed. Formation of this adduct is proposed to occur by an electrochemical mechanism during ESI. Catalysis of thiol oxidation and analysis of thiol mixtures is addressed.

The sensitivity of detection of uric acid (H2U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pKa1=5.4 of H2U, protonated unoxidized uric acid [H2U+H]+(m/z169) was detected together with the protonated uric acid dimer [2H2U+H]+(m/z337). The dimer likely forms by 1e−oxidation of urate (HU−) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e−, 2H+oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pKa2=9.8 of H2U, allantoin (Allnt) (MW 158.04), the final 2e−, 2H+oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]+(m/z235) and the [2H2U+H]+dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]+(pKspNaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH4Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.

Dopamine [DA]+(m/z154), DA dimer [2DA‐H]+(m/z307) and DA quinone [DAQ]+(m/z152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e−, 1H+oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e−, 2H+oxidation reaction. On‐line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone‐cysteine adduct [DAQ+Cys]+was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone‐cysteine adduct by 1e−pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.

A new fast scan voltammetry (FSV) method was tested in the determinations of adenosine in physiological buffers at pH 7.4. The buffers can be used in the determinations of adenosine in vivo and include 7 × 10−2 M phosphate, Krebs–Henseleit (K-H) and Hanks' Balanced Salts (HBSS). A new method of fabrication of carbon fiber electrodes (CFEs) by polishing, followed by electrochemical pretreatment (ECP) was developed for the determination. After the ECP of CFE, CFE background current was stable in FSV determinations even though an increase in the background current was observed after the ECP in the buffers at pH 7.4. The sensitivity in FSV determinations of adenosine at the pretreated electrodes was tested in the physiological buffers at the potential scan rate of 500 V s−1 at pH 7.4. Buffer composition and pH was the same during the ECP of CFE and in the FSV determinations. The sensitivity in the FSV determinations of adenosine at the new CFEs was high, compared to that previously reported at CFEs prepared by other methods, and showed a limited dependence on buffer composition. However, a small increase in buffer pH above 7.4 resulted in a decrease in sensitivity in the determinations of adenosine. The decrease in sensitivity was associated with an additional increase in CFE background current at pH > 7.4. At pH 7.4 best sensitivity and limit of detection was obtained in 7 × 10−2 M phosphate buffer, where the background current was lowest. The sensitivity was half that in K-H and HBSS. Standard deviation of measurements was ca. 1%. The results demonstrate the feasibility of sensitive FSV determinations of adenosine in physiological buffers at pH 7.4 at CFEs.

A new fast scan voltammetry FSV method was tested in the determinations of adenosine in physiological buffers at pH 7.4. The buffers can be used in the determinations of adenosine in vivoand include 7 × 10−2M phosphate, Krebs–Henseleit K-H and Hanks' Balanced Salts HBSS. A new method of fabrication of carbon fiber electrodes CFEs by polishing, followed by electrochemical pretreatment ECP was developed for the determination. After the ECP of CFE, CFE background current was stable in FSV determinations even though an increase in the background current was observed after the ECP in the buffers at pH 7.4. The sensitivity in FSV determinations of adenosine at the pretreated electrodes was tested in the physiological buffers at the potential scan rate of 500 V s−1at pH 7.4. Buffer composition and pH was the same during the ECP of CFE and in the FSV determinations. The sensitivity in the FSV determinations of adenosine at the new CFEs was high, compared to that previously reported at CFEs prepared by other methods, and showed a limited dependence on buffer composition. However, a small increase in buffer pH above 7.4 resulted in a decrease in sensitivity in the determinations of adenosine. The decrease in sensitivity was associated with an additional increase in CFE background current at pH > 7.4. At pH 7.4 best sensitivity and limit of detection was obtained in 7 × 10−2M phosphate buffer, where the background current was lowest. The sensitivity was half that in K-H and HBSS. Standard deviation of measurements was ca.1. The results demonstrate the feasibility of sensitive FSV determinations of adenosine in physiological buffers at pH 7.4 at CFEs.

A direct and fast voltammetric method of determination of adenosine, with a novel carbon fiber ultramicroelectrode (CFE), is demonstrated here. To achieve high sensitivity required in biological determinations, before the determination of adenosine, carbon fiber ultramicro electrodes were activated by electrochemical pretreatment. The electrochemical activation process generates a nanostructured carbon fiber surface which is stable. The high stability of the active surface, and the high reproducibility of the activation process, leads to reproducible and sensitive fast scan voltammetric (FSV) determinations of adenosine. Adenosine sensitivity was determined in 7×10–2M phosphate buffer, pH 7.4, at a scan rate of 500 V s–1. The potential window in the determinations was from –1.0 to 1.5 V (vs. SCE). A high sensitivity of the new method of adenosine determinations was achieved under physiological conditions.

X-Ray photoelectron spectroscopy (XPS) analysis was used to verify the structure of nanosized overoxidized polypyrrole (OPPy) films that were grown on glassy carbon. The films were electrochemically grown to ca. 1.2–1.6 nm thickness from acetonitrile with tetrabutylammonium perchlorate (TBAP) and from water with adenosine triphosphate (ATP) as dopant. The films have been previously characterized by electrochemical methods, but this represents the first study of the ultrathin films by XPS. XPS analysis verifies that the key structural features of nanosized OPPy films are the same as those of much thicker films previously studied by XPS on metal substrates, where differential charging effects are much less severe. Some differences between the microstructures of OPPy and OPPy/ATP films are highlighted by the results. In addition, XPS results confirm a two-domain model of the polymer films that has been proposed from previous studies of PPy and OPPy. The results show that XPS can be used to characterize nanosized films on graphite, after correction for differential charging. Graphite represents a substrate of choice for the fabrication of permselective ultrathin membranes for biosensors from materials such as OPPy.

Uric acid (UA) has been under intensive investigation by electrochemists owing to its important role as a metabolite in biological fluids. One of the major problems in biological determinations of uric acid comes from electrochemical interferences such as ascorbic acid (AA), which has a similar oxidation potential, E1/2 ≈ 200 mV versus SCE, at graphite electrodes, and is present at high concentrations in biological systems. UA undergoes a 2 H+, 2 e– oxidation in aqueous buffers. The oxidation product, a diimine, is an unstable intermediate with a half-life of less than 22 ms. A follow-up hydration reaction converts the diimine to an imine alcohol. Results of previous work show that UA weakly adsorbs and undergoes a fast electron transfer reaction, ks = 54 s–1, at carbon fiber electrodes. These characteristics make UA an excellent candidate for fast scan voltammetric (FSV) determinations. This paper presents the results of FSV at bare carbon fiber electrodes. The results show good selectivity and sensitivity in the determination of low concentrations of UA in the presence of high concentrations of AA. By increasing the scan rate above 500 V s–1, voltammograms of UA in the presence of AA can be resolved because of the kinetic differences in the response of the two anions, without the need for a permselective film on the electrode. Results are also presented that demonstrate an effective way to reach a stable background current at bare carbon fiber electrodes, which is required in FSV because the signal from the analyte is smaller than the electrochemical signal from the background current. Signal-to-noise ratios at bare carbon fiber electrodes in FSV are improved, because the high temporal resolution in fast scan methods allows the acquisition of a large number of scans that can be signal averaged in a short period of time. In addition, large signals can be measured because the voltammetric peak current increases with increase in scan rate.

Copper deposition patterns on graphite electrodes were analyzed by scanning electron microscopy. The deposition patterns correlate very well with the electrochemical activity of different graphites. The results indicate that treatment of graphite by electroactivation, laser irradiation of polishing on 600-grit silicon carbide paper produces active sites on the surface. The density of the sites directly reflects the electrochemical reactivity of the surface.

In this paper a simple current transducer is described which can measure pA currents at low scan rates in conjunction with a commercial potentiostat such as the one supplied by Bioanalytical Systems (BAS), and which will allow fast-scan voltammetry measurements up to 4000 V/s in conjunction with a waveform generator and a data acquisition system.