Biological response, time scale

Structure and function need to be jointly considered in the assessment of effects of stressors on river systems. It has been shown that the two sets of parameters offer complementary information since they cover different time scales and responses. This being shown in the case of biofilms is not a unique characteristic of them, but it might be applied to all other biological communities (e.g. macroinvertebrates, fish). These differ from the biofilm in its higher size and life span, and therefore in their integrative capacity to reflect effects in one part of the ecosystem. Higher traffic levels in addition to biofilms should be considered to study the whole ecosystem. In all of these biological compartments, the combined use of descriptors may amplify our ability to predict the effect of stressors on river basins. 

It would obviously be informative if the release of insulin could be monitored spatially and on a useful time scale. In many microelectrode studies in biology, the substances concerned are easy to reduce or oxidize electrochemically. This is not so with sulfur-containing insulin, and it was not until a suitable (if complex) electrocatalyst was discovered that fast scan cychc voltammetry with microelectrodes could be used to monitor it. The substance that promotes reaction with the sulfide atoms in insulin is a mixed-valent RuOz-cyanoruthenate mixture. The catalyst is prepared in situ and deposited onto the carbon fiber of the microelectrode. The response time of electrodes thus prepared (Kennedy and Huang, 1995) is < 100 ms, and detection limits are as low as 5 pM. 

The response time of a chemical sensor should be appropriate for the application for which it is intended. For example, if the sensor is used to monitor acutely toxic (lethal) substances in the workplace, the response time should be faster than the biological/ toxicological re.sponse — perhaps only a few seconds. On the other hand, some applications, such as monitoring the spread of a chemical waste plume underground, have characteristic time scales of days to years, permitting utilization of sensors that respond more slowly. 

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