Pretreated Glassy Carbon

The influence of different pretreatment strategies on the adsorption of DNA on the GC surface has been extensively discussed. The sensitivity for ssDNA detection at the GC surface was improved greatly (tenfold) by modifying the electrode surface with an electrochemical oxidation treatment at -i-1.75 V (vs SCE) for 300 s in PBS, pH 5.0. The same results were reported when GC(ox) was obtained at (1) 1.60 V (vs SCE) for 15 s in 10% HNO3 solution with 2.5% K2Cr207, [35] and, (2) 1.20 V (vs Ag/AgCl) in 0.5 M NaOH for 10 min [45]. 

This improvement was due to an easy adsorption of ss- and dsDNA on the GC(ox) surface [46,47]. Regarding the nonconductive nature of graphite oxide film formed on the surface during anodization [15], the activation of GC woifld affect primarily the adsorption process but not the charge transfer of the G and A residues. The ssDNA was preconcentrated on GC(ox) surface 

Not only was an improvement in the detection for ssDNA at GC(ox) observed, but also for G and A bases [47]. These results suggest that the increased adsorption of DNA on the GC(ox) depends more on the DNA bases than on the phosphate-sugar DNA backbone. 

This conclusion is also supported by the fact that, in contrast to ssDNA, the oxidation signal coming from dsDNA is poorly developed at both GC and GC(ox). This is probably attributable to the electroactive A and G residues in dsDNA being inaccessible to the surface, while most bases in denatured DNA can freely interact with the GC(ox) surface. On the other hand, the hydrogen-bonded bases in native DNA are hidden within the double heUx, a serious steric barrier to electron transfer between the purine and the GC(ox). 

However, when the potential of the pretreatment of the GC exceeded -I- 1.75 V (vs SCE) or it was driven longer than 300 s in PBS (pH 5.0), the adsorption of ssDNA at the electrode was found to decrease [46], showing that different conditions for obtained GC(ox) were detrimental for the DNA adsorption and oxidation. A similar negative effect was observed when the adsorption of the DNA was performed on polished GC previously exposed to air for a given time [44]. 

---

Dai HP, Shin KK (1988) Voltammetric behavior of ahzarin S adsorbed on electrochemically pretreated glassy carbon electrodes. Electrochim Acta 43 2709-2715. 

Ravichandran K, Baldwin RP. 1983. Liquid chromatographic determination of hydrazines with electrochemically pretreated glassy carbon electrodes. Anal Chem 55 1782-1786. 

A. G. Fogg, M. A. Femdndez-Arciniega, and R. M. Alonso, Oxidative Amperometric Flow Injection Determination of Sulphite at an Electro-chemically Pretreated Glassy Carbon Electrode. Analyst, 110 (1985) 851. 

Cenas, N. K., Rozgaite, J., Pocius, A., Kulys, J. J., Elearocatalytic Oxidation of NADH and Ascorbic Acid on Electrochemically Pretreated Glassy Carbon Electrodes , J. Electroanal. Chem. 154 (1983) 121-128. 

Engstrom, R. C., Strasser, V. A., Characterization of Electrochemically Pretreated Glassy Carbon Electrodes , Anal. ChettL 56 (1984) 136-141. 

The polymer film of N,N-dimethylaniline is deposited on the electrochemically pretreated glassy carbon electrode by continuous electrooxidation of the monomer. This film-coated electrode is be used as an amperometric sensor of AA Based on a l-AA biosensor based on ascorbate oxidase. The enzyme is extracted from the mesocarp of cucumber by using 0.05 mol L" phosphate buffei pFl 5.8 containing 0.5 mol L NaCl Methylene blue (MB) is incorporated into titanium phosphate (TiP) after pretreatment of TiP with the gas butylamine. The dye is strongly retained and not easily leached from the layered host matrix. The adsorbed MB on TiP is used to prepare modified carbon paste electrodes (MCPE)... 

Engstrom, R.C. and V.A. Strasser, Characterization of electrochemically pretreated glassy carbon electrodes. Anal. Chem., 1984. 56 p. 136-41. 

Utilization of resonance effects can facilitate unenhanced Raman measurement of surfaces and make the technique more versatile. For instance, a fluorescein derivative and another dye were used as resonantly Raman scattering labels for hydroxyl and carbonyl groups on glassy carbon surfaces. The labels were covalently bonded to the surface, their fluorescence was quenched by the carbon surface, and their resonance Raman spectra could be observed at surface coverages of approximately 1%. These labels enabled assess to changes in surface coverage by C-OH and C=0 with acidic or alkaline pretreatment [4.293]. 

The actual value varies slightly depending on the way the glassy carbon electrode was pretreated. 

Several types of carbon are in common use as electrodes. The most used of these is glassy carbon (GC) [100] it is the most reproducible but very difficult to machine as it is hard and brittle. One thus tends to be confined to the dimensions and shapes which come from the manufacturer. Each manufacturer has his own fabrication method (or more than one). A common problem is that small holes can appear in the middle of the piece of GC if this occurs, there is no option but to machine more away. Additionally, the GC is not always homogeneous. In the authors opinion, the best glassy carbon is Tokai. While it is not always possible to use this because of geometrical considerations, better reproducibility will be obtained with it, especially after electrochemical pretreatment [101]. 

Platinum, gold and carbon are the most frequently employed materials for solid electrodes. Platinum has a high background current, but it also exhibits the highest degree of reproducibility of the obtained voltammet-ric curves. Carbon must not be porous and for this reason the pores must be filled, e.g., with paraffin. The so-called Glassy carbon [9] with a very low porosity is recommended. The electrode surface must in all cases be respected, this makes a mechanical or an electrochemical pretreatment necessary before each recording of a voltammetric curve. 

They arrived at this conclusion by comparing the effect of electron donor or acceptor substituents in nitrobenzene on the magnitude of the potential shift at a freshly polished glassy carbon electrode vs. one that was electrochemically pretreated. In the latter case, the meta derivatives, for which adsorption increases with increasing electron-withdrawing power of the substituent, the potential shift results were interpreted as evidence that adsorption weakened the electronic effect exerted by electron-withdrawing substituents on the electroreactivity of their nitro group. (For a more detailed discussion of the effects of adsorption on electron transfer at carbon electrodes, see Refs. 673 and 751.)... 

On electrooxidative pretreatment in aqueous media of glassy carbon the surface is modified the anodization results in a higher background current, increased reversibility, and decreased oxidation potential of some compounds. The effect depends on pH and the supporting electrolyte [150]. 

Fig. 6.12 Sample pretreatment and conversion of organic compounds into gases for isotope ratio mass spectrometry (IRMS). The reductive pyrolysis is normally performed on glassy carbon (for a summary see e.g. [ 1291). In case of coupled HPLC-IRMS for determination wet oxidation is used [ 122f In the routine isotope ratio analysis of water often isotope equilibration with gases are used for 0-analysis CO2 [ 130], for S H-analysis H2 gas in the presence of Pt [131]. Recently an on-line method for S O and in water has been described showing dual-inlet like performance [ 132]. Low-temperature pyrolysis in connection with GC is used for compound fragmentation with the aim of partial isotope pattern analysis [133]. TC-EA = thermo conversion elemental analyser...

One typical example of this behavior is the voltammogram of the ferro/ferricyanide couple (test reaction) that at carbon electrodes is less reversible than at noble metal electrodes. The kinetics of the test reaction in 1 M aqueous KCl was used as the reference to compare its electrochemical behavior on different carbon electrodes [20]. This electrochemical reaction occurs via an outer sphere mechanism and its rate depends on the electrolyte composition and can be increased by appropriate treatment of carbon electrodes, for instance, by application of a high current potential routine to electrodes of carbon fibers. Similar results have been obtained with glassy carbon surfaces that had been pretreated at 500°C under reduced pressure. An alternative activation method is based on careful electrode surface polishing [6]. 

Carbons exhibit a low electrocatalytic activity for the hydrogen electrode reaction (HER). Structural characteristics have significant electrocatalytic effects on the HER as changes from 2 X 10 to 2.5 x 10 A/cm on going from the basal plane to the side face of pyrolytic graphite. On glassy carbon, the HER overpotential decreases as the pretreatment temperature is increased. This thermal treatment leads to stmctural and chemical transformations from carbonization, precrystaUization, and to graphitization. 

The rate of the oxygen evolution reaction (OER) on pyrolytic graphite is higher than that for glassy carbon. For both the carbon electrodes, the temperature pretreatment has no influence on the current measured at constant potential. 

For anodic NADH oxidation a potential of more than +0.4 V is necessary. Since this high overpotential favors electrochemical interferences, various investigations have been conducted to decrease the oxidation potential by using mediators or pretreating the electrode. Cenas et al. (1984) found that after electrochemical pretreatment of glassy carbon electrodes in the range of -0.8-1.8 V, NADH oxidation occurs at 6-0.2 V vs Ag/AgCl. LDH was entrapped on top of the electrode by means of a dialysis membrane. The oxidation current was proportional to lactate concentration up to 10 mmol/1. Presumably because of adsorption of NADH oxidation products the half life of the sensor was less than 3 days. 

---