Catalytic Currents buffer concentration

Figure 12.5 (a) Layer-by-layer deposition of glucose oxidase and the polyallylamine Os3 +n + -polypyridine polyelectrolyte on the electrode, (b) Typical catalytic current responses for different glucose concentrations obtained by self-assembled nanostructured thin films based on different architectures (i) PAH/Os/GOx, (ii) cysteamine/GOx/PAH-Os, (iii) PAH/GOx/ -Os, and (iv) (PAH-Os)2/(GOx)i. All measurements were performed in 0.1 M tris buffer at pH 7.5. Part (b) Reproduced with permission from Ref. 34a. Copyright Wiley-VCH Verlag GmbH Co. KGaA. 

Figure 13. (A) Chronoamperometric response of Mb/Au/ITO electrode at -0.4 V in pH 7.0 phosphate buffer while successively injecting 10 pM H202. (B) Plot of catalytic current vs. H202 concentration. Inset Linear calibration curve of 1/i vs. 1/Ch2o2. Reproduced from [44], copyright 2005, with permission from Elsevier.

Criteria used to distinguish among diffusion, kinetic, adsorption, and catalytic currents include changes in the wave height (limiting current) with (1) concentration of the electroactive species, (2) mercury pressure, (3) pH, (4) buffer concentration, and... 

Whereas diffusion and adsorption currents are usually pH-independent, most kinetic and catalytic currents (in particular catalytic currents due to hydrogen evolution) change markedly with pH. Catalytic currents are frequently characterized by strong dependence on buffer concentration and kinetic currents sometimes show great changes with increasing temperature (Table 3.1). 

Figure 2. Dependence of the catalytic current on glucose concentration gold electrode, 60 m diameter using 0.2mM DMHAE-ferrocene and 20JXM GOD in phosphate buffer, pH 7.0, at 5mV s . ...

Whereas diffusion and adsorption currents are usually independent of the buffer concentration, kinetic currents (in some instances) and catalytic currents are very often a function of the concentration and composition of the buffer. The ionic strength was also observed in some instances to affect catalytic currents. 

Conversely the presence of such a catalytic wave was claimed for some substances containing no sulphur. From the study of the influence of the composition of the buffer solution, of the ionic strength and of the concentration of cobalt as well as of the concentration of the catalytically active substance, it has recently been possible to distinguish at least three different types of catalytic currents. Thus it is possible that the catalytic effects e.g. for uric acid, ) glycerinaldehyde, or dihydroxyacetone, ) have characteristics other than the catalytic waves due to thio-compounds. 

FIGURE 8.37 Collisions of Pt NPs at SAM-covered Au electrodes, (a) Schematic illustration of particle collisions at the SAMs catalytic hydrazine oxidation and suppression of electron transport through long molecular spacers, (b) Plot of representative catalytic current versus the number of carbon atoms in SAMs, (c) Current transients recorded at different SAMs. Au electrode 10 pm in diameter electrode potential, 0.1 V particle size, 3.6 nm particle concentration, 50 pM electrolyte, 12 mM hydrazine+50 mM PBS buffer pH 7.5. 

Adiponitrile is readily hydrogenated catalytically to hexamethylenediamine, which is an important starting material for the prodnction of nylons and other plastics. The electrochemical production of adiponitrile was started in the United States in 1965 at present its volume is about 200 kilotons per year. The reaction occurs at lead or cadmium cathodes with current densities of np to 200 mA/cm in phosphate buffer solutions of pH 8.5 to 9. Salts of tetrabntylammonium [N(C4H9)4] are added to the solution this cation is specihcally adsorbed on the cathode and displaces water molecules from the first solution layer at the snrface. Therefore, the concentration of proton donors is drastically rednced in the reaction zone, and the reaction follows the scheme of (15.36) rather than that of (15.35), which wonld yield propi-onitrile. 

A biosensor based on mediator-free CYP2B4 catalysis by immobilizing monomer-ized CYP2B4 in montmorillonite was studied by Shumyantseva [222], When substrates were added to air saturated buffer solution, there was an increase in the reduction current. A typical concentration dependence measured in chronamperometry is shown for aminopyrine and benzphetamine (Fig. 17.4). The reaction was inhibited by metyrap-one. This indicates the catalytic activity of CYP2B4 in the presence of substrate. 

Mb-CMC fihn was made by casting solution of myoglobin and carboxymethyl cellulose (CMC) on pyrolytic graphite electrode. Trichloroacetic acid (TCA), nitrite, oxygen, and hydrogen peroxide can be catalytically reduced at the Mb-CMC film electrode. When TCA was added to a pH 3.0 buffer, an increase in the MbFe reduction peak at about -0.2 V was observed, accompanied by a decrease of the MbFe oxidation peak. The reduction peak current increased as the TCA concentration increased. The catalytic reduction peak current showed a linear relationship with TCA concentration in the range of 7.5 X 10 - 2 X 10 M. Compared with the direct reduction... 

V. The peak current increased with the concentration of NO2. Electrochemical catalytic response of hydrogen peroxide was also observed by HRP-AQ films. When H2O2 was added to a pH 7.0 buffer, an increase in reduction peak at about 0.36V was seen with the disappearance of oxidation peak for HRP-Fe(ll). The reduction peak current increased with the concentration of H2O2 in solution. However, there was no reduction current observed at AQ film electrodes in the presence of H2Q2. 

FIG. 21 Holo-GDH formation in E. coli cells immobilized on an electrode was monitored by measurement of the current for the catalytic reaction of the immobilized cells. Panel (A) the E. coft-modified electrode was incubated in buffer containing 5 mM for 15 min, then 0.3 mM Qo and 10 mM o-glucose was added to the buffer solution and the current was recorded at 0.5 V to measure the increase in current on the addition of PQQ at a final concentration of (a) 100, (b) 60, and (c) 30 nM. Panel (B) the data used in these plots were taken from the current-time curves in panel (A). (From Ref. 54.)... 

---