Electrochemical biosensors have been of increasing interest, especially those developed to
be directly applied in diagnostic areas, such as neuroscience. We have been interested in
developing a range of biosensors for monitoring glucose, lactate, pyruvate, and glutamate
in order to study on-line both brain function in the laboratory and to monitor brain health in neurointensive care. For a biosensor to function effectively in these situations,
it has to combine the following characteristics: quick response and high sensitivity, good reproducibility and adequate stability. In this study we compared the performance of a number of different amperometric
biosensors strategies. These included ferrocene mediation of immobilised enzymes (system
A), a redox hydrogel based system (system B), and a conducting polymer approach using
polyaniline (system C). All assays were operated as flow-injection systems with upstream
immobilised enzyme beds if necessary. When calibrated for H2O2 systems A and B reacted
quickly enough to give quantitative conversion up to 0.2 mM. Above this concentration
the response was limited by horseradish peroxidase enzyme kinetics and eventually
enzyme loading. System C showed a restricted H2O2 response. When calibrated for
glucose (by use of immobilised glucose oxidase) system B exhibited the highest sensitivity
but its analytical range was restricted because the system became limited by H2O2
response. System A had low sensitivity for analyte compared to H2O2 and system B, but a
greater useful range. Problems of mediator cycling between the immobilised enzymes are
discussed. System C gave an excellent linear range but sensitivity was limited by
background noise. Stability and reproducibility of the systems are also described. In
conclusion, from this study the ferrocene system proved to be overall most useful and has
now been used in the first dual on-line monitoring of glucose and lactate in patients in
neurointensive care.
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