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Fiber-optic sensors allow remote analyses of chemical substances and they now find many applications in chemistry and biology [1,2]. The purpose of this short report is to give our first results in the development of optical-fiber chemical sensors. Among the numerous known spectrometric methods, we chose the fluorometric one, generally described as a suitable method for determining substances at the parts per million or parts per billion level, with the objective of analyzing nitrate and nitrite anions, using modifications of the fluorescence emission of suitable dyes. The detection of nitrates is based on the irreversible nitration of fluorescein, which leads to a subsequent inhibition of fluorescence emission [3]; determination of nitrites corresponds to their addition on 2,3-diaminonaphthalene, which on the contrary, improves the fluorescence emission [4]. To set up simple instrumentation, we are developing fiber-optic sensors. This consists of (i) realizing an extrinsic active optical fiber by chemical linkage of suitable fluorescent dyes on silica fiber involving silanization reaction (APTES) and chemical methods and (ii) designing an optical device which is appropriate for measurements with optical fibers. The threshold of detection, coating efficiency, and stability with time are presented.

Photophysical techniques have potential for the development of optical sensors in monitoring and controlling fermentors. In the particular case of acetone-butanol-ethanol (ABE) fermentation, carried out by bacteria of the speciesClostridium acetobutylicum, we have developed two studies based on fluorescence spectroscopy. First, we measured the intrinsic fluorescence of NADH related to bacteria metabolism, leading to a linear relationship between the NADH specific fluorescence and the specific rate of butyric acid production. At the same time, we have correlated enzymatic activities (acetate kinase, butyrate kinase, acetoacetate decarboxylase) with NADH specific fluorescence. Second, we studied the fluorescence polarization of extrinsic DPH (1,6-diphenyl-1,3,5-hexatriene) related to membrane fluidity. A simultaneous increase in both DPH anisotropy (order parameter increase) and butanol production is observed. Even though these results seem contradictory, because of the well-known fluidizing effect of butanol on lipids, they can be explained by a homeoviscous response ofC. acetobutylicum to the presence of butanol during fermentation. Thus the apparent changes in fluidity could be the result of the adaptative membrane alteration.