Improvement of a mediator-type biochemical oxygen demand sensor for on-site measurement.

We characterized a mediator-type biochemical oxygen demand (BOD) sensor with a three-electrode system using potassium ferricyanide (FC) and Pseudomonas fluorescens in our previous study. In the present study, we have utilized the advantages of a mediator-type biosensor, which does not require air-supply equipment for on-site measurements, and made a fully disposable sensor tip for a portable device. The tip consists of a two-electrode system with P. fluorescens immobilized on a cellulose acetate membrane and is packaged in polyester film to prevent it from drying out. By aeration with a 0.1 M NaCl solution of P. fluorescens (after growth), the sensor responses as well as their reproducibility and stability have been successfully improved. The responses increased more than seven times, and the calibration curve from 15 to 260 mg l(-1) also remained linear although the response decreased approximately half the original after at least 35 days in storage. The reproducibility of the sensor responses improved to 12.7% (average of relative standard deviations (RSDs)) in the calibration curve obtained by using the Organization for Economic Cooperation and Development synthetic sewage. Examination of real samples from three different sources showed that the BOD as determined by the sensor correlates well with the conventional 5-day BOD method (r(2)=0.982, 0.823, and 0.809). Consequently, the aeration process makes it possible to realize rapid, and in situ measurements without the long conditioning process that is generally required to activate the microorganisms immobilized on bio-films before use. Finally, we have designed a portable device that utilizes our disposable sensor tip.