Alan L. Hart

Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Part 1. An adsorption-controlled mechanism

The electrochemical oxidation of H2O2 at a platinum rotating disk electrode was studied at pH 7.26 for the [H2O2] range 0–80 mM and for rotation rates 630–10,000 r.p.m. A mechanism is proposed to account for the steady-state current response as a function of both rotation rate and [H2O2]. The mechanism incorporates reversible binding of hydrogen peroxide to electrochemically generated Pt(II) surface sites with inhibiting competitive adsorption of dioxygen at these sites. A further inhibiting side reaction is also identified involving protonation of the surface adsorbed H2O2 complex.

Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Part II: effect of potential

The electrochemical oxidation of H2O2 at a platinum rotating disc electrode was studied in 0.1 M phosphate buffer at pH 7.26 for the [H2O2] range 0–40 mM, rotation rates 630–10000 rpm and anodic potential +264 to +712 mV vs. Ag/AgCl using staircase potentiometry over the temperature range 5–35°C. The results were interpreted in terms of a surface binding site model involving two modes of product inhibition. Potential- and temperature-invariant values of the constants for H2O2, H+ and O2 binding were determined and the rate constants for reduction and the re-oxidation of the binding site as a function of potential and temperature were evaluated together with the activation energy for the reduction of the binding site.

Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Part V: inhibition by chloride

The electrochemical oxidation of H2O2 at platinum rotating disc electrodes and microelectrodes was studied in the presence of chloride in the range 0‐120 mM at pH 7.3. Oxidation was inhibited by chloride with greater inhibition at higher electrode rotation rates. The mode of inhibition was deduced to be non-competitive with the chloride interacting with precursor binding sites at the electrode surface, since increased [H2O2] did not decrease the degree of inhibition. The rotation rate dependence indicates that it is likely a soluble platinum complex involving chloride forms to remove binding sites from the electrode surface.

The response of screen-printed enzyme electrodes containing cholinesterases to organo-phosphates in solution and from commercial formulations

Abstract Electrodes containing a layer of cobalt phthalocyanine, mixed with graphite ink, and a layer of acetyl- or butyryl cholinesterase were constructed by screen-printing, except for a thin layer of polyurethane applied by an airbrush. When the electrodes were made the anodes of electrochemical cells, the generation of current by the addition of enzyme substrate was inhibited by standard solutions of organo-phosphate pesticides. Cholinesterase activity was inhibited by paraoxon and dichlorvos concentrations as low as 10−8 M. The relationship between the logarithm of concentration and the inhibition quotient (ratio between initial current and the rate of change of the inhibited current) was curvilinear. Malathion only caused an inhibition of current generated by electrodes containing acetyl cholinesterase. Inhibition by malathion required a period of pre-incubation in the pesticide before addition of enzyme substrate, and was evident down to, and including, a concentration of 10−7 M. The addition of diluted commercial formulations of some organo-phosphate pesticides caused rapid inhibition of electrode function. Nearly linear relationships were observed between the logarithm of the degree of dilution of commercial formulations containing PO groups and the natural logarithms of the inhibition quotients. Inhibition by formulations containing malathion did not require pre-incubation of the electrodes in the mixture (only electrodes containing acetyl cholinesterase were tested). The relationship between the logarithm of the degree of dilution and the natural logarithm of the inhibition quotient was curvilinear. The responses of the screen-printed electrodes to commercial formulations of pesticides indicate that they may be a suitable platform for further development of sensors capable of detecting the presence of these pesticides in the environment.

Electrochemical oxidation of hydrogen peroxide at platinum electrodes. Part IV: phosphate buffer dependence

Abstract The electrochemical oxidation of H 2 O 2 at platinum rotating disc electrodes and microelectrodes was studied as a function of phosphate buffer concentration in the range 0–100 mM and pH from pH 4 to pH 10. The results were interpreted in terms of development of a surface binding site for H 2 O 2 from a precursor site through interaction with H 2 PO 4 − from the electrolyte. In the absence of phosphate an alternative binding site mechanism was evident. The precursor site was shown to be inhibited by protons at low pH producing an inactive site.

Milk L -lactate concentration is increased during mastitis

A study was undertaken in cattle to evaluate changes in milk L-lactate in relation to mastitis. A healthy, rear quarter of the udder of each of ten cows in mid-lactation was infused with 1000 colony-forming units (cfu) of Streptococcus uberis following an afternoon milking. Foremilk samples were taken at each milking from control and treated quarters and antibiotic treatment was applied following the onset of clinical mastitis or after 72 h. One cow did not become infected. Six quarters showed clinical symptoms of mastitis within 24-40 h and this was associated with a more than 30-fold increase in milk L-lactate (to 3.3 mM) and an increase in somatic cell count (SCC) from 4.5 x 10(3) to 1 x 10(7) cells/ml. Three cows were subclinical, with cell counts ranging from 1.5 x 10(6) to 1 x 10(7) cells/ml. In these animals, milk lactate ranged from 0.7 to 1.5 mM in the infected quarters up to 40 h post-infection, compared with less than 0.1 mM in control quarters. Milk was examined from 137 cows in mid-lactation which were known to have mastitis. Foremilk samples were taken aseptically from control and infected quarters of cows on commercial farms. Mean milk L-lactate concentrations and SCC were 0.14 +/- 0.02 mM and 1.85 +/- 0.3 x 10(5) cells/ml, respectively, in control (bacteriologically negative) samples. However, L-lactate concentrations exceeded 2.5 mM in the presence of some types of infection, the level of the lactate response being closely related to the impact of the infection on SCC. L-Lactate concentrations were relatively elevated in milk samples taken post partum, declining from 0.8 to 0.14 mM oyer the first few days of lactation. In conclusion, milk L-lactate has potential as an indicator of clinical and subclinical mastitis in dairy cows.

Estimation of soluble L-lactate in dairy products using screen-printed sensors in a flow injection analyser

Screen-printed lactate sensors, with an outer membrane applied by an air-brush, were mounted in a flow injection analyser. Diluted extracts of buttermilk and yoghurt were passed over the sensors. The lactate concentration in the extracts was determined with a high degree of accuracy and precision (based on variation among sensors). Fresh sensors were used for each new buttermilk or yoghurt sample, but individual sensors were capable of giving repeatable results, with standard lactate solutions, over a large number of injections.

Measurement of soluble L-lactate in dairy products using screen-printed sensors in batch mode

Amperometric sensors based on lactate oxidase and platinized carbon were constructed entirely by screen-printing. They were used to estimate the lactate concentration in diluted samples of yoghurt and buttermilk. Estimates made in untreated (apart from dilution) solutions were subject to bias, relative to values obtained from spectrophotometric assays. Removal of cations improved the accuracy of the estimates. Calcium may have made a small contribution to the bias but the nature of the inhibition is otherwise unknown. With untreated samples, determinations of any accuracy require sufficient dilution to avoid such bias, although the smaller signals resulting from dilution place more demands on calibration and manufacturing standards. The relative standard deviations of estimates, based on variation among sensors, of the most dilute samples were generally too high for practical use. The errors of estimates at lower dilutions approached those of screen-printed sensors made under industrial conditions.

Mathematical modelling of oxygen transport-limited follicle growth

Mathematical modelling was used to investigate oxygen transport in the developing ovarian follicle. In contrast to previous findings, the results show that oxygen can reach the oocyte in large preantral follicles. This is largely due to the inclusion of fluid voidage in the model and improved estimates of oxygen diffusion coefficients through the granulosa. The results also demonstrate that preantral follicles will eventually reach a size beyond which further growth will result in the follicle becoming increasingly anoxic. The predicted size range at which this occurs is consistent with the size range at which antrum formation is observed in many mammals. This suggests that the antrum formation stage of follicular growth may be pivotal to the further development and ultimate fate of the follicle, and that antrum formation itself may represent a mechanism by which the follicle can overcome oxygen limitations. This was supported through extension of the model to the antral follicle, which showed that antrum formation can provide a way in which the follicle can continue to grow and yet avoid becoming hypoxic. The results of the model were consistent with observed follicle development.

Theoretical investigation into the dissolved oxygen levels in follicular fluid of the developing human follicle using mathematical modelling

Oxygen levels in the follicle are likely to be critical to follicle development. However, a quantitative description of oxygen levels in the follicle is lacking. Mathematical modelling was used to predict the dissolved oxygen levels in the follicular fluid of the developing human follicle. The model predictions showed that follicular fluid dissolved oxygen levels are highly variable among follicles, due to the unique geometry of individual follicles. More generally, predictions showed that oxygen levels in follicular fluid increase rapidly during the initial early antral stages of follicle growth before peaking in the later early antral phase. Follicular fluid dissolved oxygen levels then decline through to the beginning of the pre-ovulatory phase, from which they increase through to ovulation. Based on the best available parameter estimates, the model predictions suggest that the mean dissolved oxygen levels in human follicular fluid during the late antral and pre-ovulatory phases range between 11 and 51 mmHg (approximately 1.5-6.7 vol%). These predictions suggest that the human ovarian follicle is a low-oxygen environment that is often challenged by hypoxia, and are in agreement with only some published data on follicular fluid oxygen levels. Predictions are discussed in relation to follicle health and oocyte culture.