A reagentless amperometric electrode based on carbon paste, chemically modified with D ‐lactate dehydrogenase, NAD + , and mediator containing polymer for D ‐lactic acid analysis. I. Construction, composition, and characterization

A reagentless carbon paste electrode was designed for D ‐lactic acid analysis in a flow injection system for the monitoring of the production of D ‐lactate in a batch fermentation. D ‐Lactate dehydrogenase, nicotinamide adenine dinucleotide (NAD + ), a synthetic redox polymer containing covalently attached toluidine blue O as mediator, graphite powder, and paraffin oil were used for the construction of the modified carbon paste electrode. D ‐Lactate selectivity was indicated by insignificant responses from a variety of possible interfernces including L ‐lactate. The electrodes gave a linear response in the range between 0.05 and 5 m M D ‐lactate, with a detecting limit of 30 μ M , allowing a sample throughput of 20 h −1 . Preliminary investigations were made by covering the electrode surface with electropolymerized membranes. Satisfactory stability was observed, indicated by a reproducibility of 3.3% relative standard deviation (RSD, n = 31), with a non‐membrane‐covered electrode for the analysis of D ‐lactate in fermentation broth. A long‐term stability (230 broth samples) was proven, suggesting the electrodes to have a good potential for use in on‐line monitoring of fermentation processes. © 1995 John Wiley & Sons, Inc.