No abstract

Three biosensor configurations that included a dissolved oxygen electrode and yeast cells (Saccharomyces cerevisiae) were evaluated as a means to assess the environmental effects of toxic chemicals. The configurations were: closed suspension—assay medium closed to air and cells suspended in the medium; open suspension—assay medium open to air and cells suspended in medium; and biofilm electrode (BFE)—assay medium open to air but cells immobilized on the surface of the electrode. The BFE was the most advantageous configuration based on assay times and detection limits.

A biosensor electrode system with unique configuration and a thin layer of immobilized yeast cells, set on the surface of an amperometric oxygen membrane electrode, was developed for rapid screening of toxic chemicals in a variety of pollution and process control applications. Measurement is based on the instantaneous detection of changes in oxygen respiratory activity of biofilm of yeast cells upon exposure to toxic chemicals.The design of this electrode system, referred to as biofilm electrodes, was based on a mathematical model of oxygen transport in the biofilm and the electrochemical current response. The biofilm, which consists of three sublayers-boundary layer, filter pad, and yeast cell layer-was modeled as a one composite diffusion layer, or three separate layers in series. While the three layer model is more theoretically complete, the one layer model was more reliable and simpler to use. IntroductionBiological oxygen uptake rates of suspensions of microorganisms have been conveniently determined by using closed-cell (reactor) respirometers, as shown in Figure la. This is based on in-situ monitoring of dissolved oxygen by amperometric membrane electrodes. Since this is a closed system from the atmosphere, the test period is limited by the amount of oxygen in test medium. Once the dissolved oxygen is consumed, respiration rate monitoring will terminate. In certain applications, it is desirable to monitor respiration rates over extended periods of times. This can be achieved by using a reactor open to the atmosphere, as shown in Figure 1 b. Under these conditions, respiration rate monitoring can be extended by replenishing oxygen from the atmosphere (Goldblum, 1988). Further optimization of this system led to the development of the biofilm electrode, shown in Figure Ic, where the microorganisms form a thin layer separating the oxygen electrode from the test solution. The oxygen concentration in the test solution is maintained at equilibrium with the oxygen atmospheric partial pressure. With this configuration, the electrode response indicates the oxygen flux across the biofilm, which under controlled experimental conditions, is solely dependent on the biological oxygen uptake rate.Electrode systems of this type are referred to in this article as biofilm electrodes (BFE). The biofilm can be made of enzymes (Davis, 1986;Lowe, 1985;Mancy, 1984), organelles such as mitochondria (Haubenstricker, 1984), bacteria (Holodnick, 1988), yeast, and mammalian cells (Goldblum and Holodnick, 1988). The BFE system under investigation consists of an amperometric oxygen membrane electrode, also known as a dissolved oxygen electrode (DOE), and a biofilm of immobilized yeast cells. This electrode system shows great commercial potential in both industrial and regulatory applications, largely based on its ability for rapid screening of toxic substances at a great saving in time and expenses. Details of the BFE are illustrated in Figure 2. Oxygen transport in such BFE systems is the physical phenomenon that renders it advantage...