Bulusu V. Sarada

Anodic Voltammetry of Xanthine, Theophylline, Theobromine and Caffeine at Conductive Diamond Electrodes and Its Analytical Application

Boron-doped diamond (BDD) electrodes were used to examine the electrochemical oxidation of xanthine and its naturally occurring N-methyl derivatives, theophylline, theobromine and caffeine. Voltammetric studies showed that the mechanism of the overall reaction is similar to that of the oxidation of purine derivatives at the pyrolytic graphite electrode. The effects of pH, concentration and potential sweep rate on the voltammetric response were thoroughly investigated, and it was found that BDD exhibits excellent behavior, in terms of very well-defined, reproducible oxidation peaks, for xanthine, theophylline, theobromine and caffeine determination. The results enabled the measurement of the oxidation peak current to be used as the basis for a simple, accurate and rapid method for determining the investigated compounds, within a concentration range of 1 to 400 μM for theophylline, theobromine and caffeine, and of 1 to 100 μM for xanthine. Promising results were obtained for caffeine determination in real samples of commercially available products, without separation from the matrix.

Electroanalysis of dopamine and NADH at conductive diamond electrodes

Abstract Highly boron-doped diamond thin-film electrodes were examined for various possible applications in electroanalysis. Electrochemical oxidation of dopamine and NADH was investigated using cyclic voltammetry and chronoamperometry. Comparison experiments were performed using glassy carbon electrodes. Anodically treated diamond electrodes made it possible to determine dopamine selectively with high sensitivity in the presence of a large excess of ascorbic acid in acidic media. A detection limit of 50 nM was obtained using chronoamperometry. The treated electrodes were found to be stable for several months. Electrochemical oxidation of NADH was carried out at as-deposited diamond electrodes, with which very stable and reproducible cyclic voltammograms for NADH oxidation were obtained, unlike glassy carbon, at which a significant positive shift (∼200 mV) in the peak potential was observed within 1 h. The amperometric detection limit was found to be ∼10 nM. Interference of ascorbic acid was minimal using untreated electrodes when the concentration of ascorbic acid was comparable to the NADH concentration. Diamond microelectrodes small enough to consist of only one or two high quality microcrystals were fabricated in order to compare the electrochemical behavior with that of polycrystalline thin film electrodes, which contain large numbers of grain boundaries, at which non-diamond (sp 2 ) carbon can exist. This work demonstrates the potential of diamond electrodes for electroanalytical applications.

Electroanalytical study of sulfa drugs at diamond electrodes and their determination by HPLC with amperometric detection

Abstract Conductive boron-doped diamond thin film electrodes were examined for the electroanalysis of three sulfa drugs, sulfadiazine, sulfamerazine and sulfamethazine. Cyclic voltammetry, flow injection analysis and liquid chromatography with electrochemical detection were used to study the oxidation reactions. At diamond electrodes, highly reproducible and well-defined cyclic voltammograms were obtained for all three drugs with a signal to background (S/B) ratio of a factor of ten greater than that obtained at two types of freshly polished glassy carbon (GC) electrodes. Diamond exhibited a highly reproducible amperometric response, with a peak variation of ∼5%, even at a concentration of 100 nM. A detection limit of 50 nM and a linear dynamic range of three orders of magnitude were obtained. No fouling of the electrode was observed within the experimental period of several hours. A rapid stabilization of the background current is achieved, within 10 min after the application of the operating potential under flow conditions, unlike the case of GC electrodes, which requires more than 30 min for reasonable stabilization. We have demonstrated the application of the diamond electrode for the simple and sensitive amperometric detection of the sulfa drugs in a standard mixture after their separation with reverse-phase HPLC to produce chromatograms with a flat baseline and high reproducibility, even at concentrations as low as 100 nM.

Electrodeposition of hydrous iridium oxide on conductive diamond electrodes for catalytic sensor applications

Abstract Boron-doped diamond (BDD) electrodes have been modified by hydrous iridium oxide (IrO x ) electrodeposition, and depending on the deposition conditions, either highly dispersed iridium oxide nanoparticles or continuous oxide films were obtained. The electrochemical characteristics of IrO x on BDD compare well with those obtained on other substrate materials. The proposed method allows excellent control of the deposited amount, and high reproducibility of the electrochemical performance. Electrodes obtained by the deposition of a very low amount of IrO x (ca. 2 nmol cm −2 ) on the diamond surface exhibited an excellent analytical performance for hydrogen peroxide amperometric detection, that compares favorably with those obtained by using other substrate materials. Furthermore, these electrodes enabled H 2 O 2 detection in neutral media, which is an advantage compared to sensors obtained by iridium oxide deposition on other substrates. The detection limit for H 2 O 2 was ca. 10 times lower than that of commercially available platinum bulk electrodes. By proper selection of the diamond substrate structure, continuous IrO x layers of different thickness were formed. The IrO x /BDD electrodes have also exhibited excellent pH-sensing properties in a wide pH range, and high stability of the potentiometric response. These studies indicated that, by proper selection of the diamond substrate structure, robust IrO x layers could be prepared for use as H 2 O 2 detection and pH sensors.

Electrochemical oxidation of underivatized-nucleic acids at highly boron-doped diamond electrodes

Boron-doped diamond (BDD) electrodes have been examined for the electrochemical oxidation of underivatized-nucleic acids in terms of single stranded and double stranded DNA. Cyclic voltammetry and square wave voltammetry have been used to study the oxidation reactions and to detect DNA without derivatization or hydrolysis steps. At the diamond electrode, at least two well-defined voltammetric peaks were observed for both single stranded and double stranded DNA. Diamond electrode is the first material to show a well-defined voltammetric peaks for adenine group oxidation directly in the helix structure of nucleic acid due to its wide potential window. For single stranded DNA, a third peak, related to the pyrimidine group oxidation was also observed. As-deposited diamond film with predominantly hydrogen-terminated surface exhibited superior performance over oxygen-terminated diamond in terms of sensitivity. However, by optimizing the ionic strength, sensitivity of O-terminated films could be improved. Linear calibration results have shown linearity of current with concentration in the range 0.1-8 microg mL(-1) for both guanine and adenine residues at as-deposited BDD. Detection limits (S/N = 3) of 3.7 and 10 ng mL(-1) for adenine and guanine residue in single stranded DNA, respectively, and 5.2 and 10 ng mL(-1) for adenine and guanine residue in double stranded DNA, respectively, were observed. This work shows the promising use of diamond as an electrochemical detector for direct detection of nucleic acids. The results also show the possibility of using the oxidation peak current of adenine group that is more sensitive for the direct detection of nucleicacids.

The electrooxidation of organic acids at boron-doped diamond electrodes

Abstract The electrooxidation of citric acid, malic acid, alanine and cysteine at boron-doped diamond (BDD) electrodes and glassy carbon (GC) electrodes was investigated by use of cyclic voltammetry. Well-defined, irreversible peaks were obtained for the oxidation of citric acid and cysteine. Malic acid and alanine exhibit discernible responses. This preliminary study has shown that BDD has better sensitivity than GC for these compounds. Except for cysteine, none of the studied compounds exhibits a recognizable oxidation peak at GC electrodes at millimolar concentration levels.

Homoepitaxial Single-Crystal Boron-Doped Diamond Electrodes for Electroanalysis

In order to study the properties of single-crystal boron-doped diamond electrodes for electroanalysis, (100) and (111) boron-doped homoepitaxial single-crystal and polycrystalline diamond thin films were deposited by means of microwave plasma-assisted chemical vapor deposition. Features in the cyclic voltammograms (CVs) for aqueous H 2 SO 4 supporting electrolyte and Fe(CN) 3-/4- 6 in aqueous Na 2 SO 4 for polycrystalline electrodes were dominated by behavior typical of (111) single-crystal facets, rather than (100) facets. Apparent heterogeneous electron-transfer rate constants (k 0 ) for various redox systems were estimated with CV simulation at polycrystalline, (111) and (100, off-axis. 4°). Based on comparison with the Marcus model, the behavior of the k° values, except for that of Fe(CN) 3-/4- 6 , indicates apparent outer-sphere electron-transfer, but with a lower density of states compared to glassy carbon or typical metal electrodes. The (111) homoepitaxial film performed better than a polycrystalline film in the cyclic voltammetric detection of serotonin, with signal-to-background ratios of 5 and 2, respectively. Single-crystal diamond may thus be an even more optimal electrode material for electroanalysis than polycrystalline diamond, which has been shown to have superior characteristics as an electrode material.

Selective amperometric detection of dopamine using OPPy-modified diamond microsensor system

Highly boron-doped diamond microfiber electrodes (BDDMF) were fabricated and characterized by the use of Scanning Electron Microscopy (SEM), Raman spectroscopy, and cyclic voltammetry. Amperometric detection of dopamine (DA), a neurotransmitter was achieved at pH 7.0, using BDDMF electrodes. The interferences from ascorbic acid (AA) and DOPAC were efficiently eliminated by using overoxidized polypyrrole-modified BDDMF electrodes, which also increased the sensitivity for the detection of dopamine. The limit of detection (S/N = 3) for dopamine was 0.1 nM, which is one order lower than that observed for carbon microfiber electrodes (CMFE), and the linear dynamic range was obtained from 0.5 nM to 100 microM (r2 = 0.997). The amperometric response for 0.5 nM dopamine has shown high stability with an RSD of 5.4% (n = 5). Highly reproducible results were obtained with an RSD of 6.2% for 10 measurements of 1 nM DA taken during 10 h and also remained the same, during measurements for 7 days, with no variation in efficiency for rejection of AA and DOPAC.

Gradient liquid chromatography of leucine-enkephalin peptide and its metabolites with electrochemical detection using highly boron-doped diamond electrode

Boron-doped diamond thin film (BDD) electrodes have been used to study the oxidation reactions and to detect leucine-enkephalinamide (LEA) and its metabolites, tyrosine (T), tyrosyl-alanine (TA), tyrosyl-alanine-glycine (TAG) and leucine-enkephalin (LE) using cyclic voltammetry (CV), flow-injection analysis (FIA), and gradient liquid chromatography (LC) with amperometric detection. At diamond electrodes, well-defined and highly reproducible cyclic voltammograms were obtained with signal-to-background (S/B) ratios 5-10 times higher than those observed for glassy carbon (GC) electrodes. The analytical peaks of LC for LEA and its metabolites were well resolved. No deactivation of BDD electrodes was found after several experiments with standard as well as plasma samples, indicating high stability of the electrode. Calibration curves were linear over a wide range from 0.06 to 30 microM with regression coefficients of 0.999 for all compounds. The limits of detection obtained based on a signal-to-noise ratio of 3:1 were 3, 2.2, 2.7, 20 and 11 nM for T, TA, TAG, LE and LEA, respectively. These values were at least one order lower than those obtained at GC electrodes, which has given limits of detection of 22.88, 20.64, 89.57, 116.04 and 75.67 for T, TA, TAG, LE and LEA, respectively. Application of this method to real samples was demonstrated and validated using rabbit serum samples. This work shows the promising use of conducting diamond as an amperometric detector in gradient LC, especially for the analysis of enkephalinamide and its metabolites.

The electrochemical oxidation of homocysteine at boron-doped diamond electrodes with application to HPLC amperometric detection

The electrochemical oxidation of homocysteine was studied at as-deposited and anodized (oxidized) boron-doped diamond (BDD) thin film electrodes with cyclic voltammetry, flow injection analysis and high-pressure liquid chromatography with amperometric detection. At anodized boron-doped diamond electrodes, highly reproducible, well-defined cyclic voltammograms for homocysteine oxidation were obtained in acidic media, while as-deposited diamond did not provide a detectable signal. In alkaline media, however, the oxidation response was obtained both at as-deposited and anodized diamond electrodes. The potential sweep rate dependence of homocysteine oxidation (peak currents for 1 mM homocysteine linearly proportional to v(1/2), within the range of 0.01 to 0.3 V s(-1)) indicates that the oxidation involves a diffusing species, with negligible adsorption on the BDD surface at this concentration. In the flow system, BDD exhibited a highly reproducible amperometric response, with a peak variation less than 2%. An extremely low detection limit (1 nM) was obtained at 1.6 V vs. Ag/AgCl. In addition, the determination of homocysteine in a standard mixture with aminothiols and disulfide compounds by means of isocratic reverse-phase HPLC with amperometric detection at diamond electrodes has been investigated. The results showed excellent separation, with a detection limit of 1 pmol and a linear range of three orders of magnitude.