A compact instrument includes a fiber-optic probe that utilizes the evanescent-wave interaction to measure the accumulation of a biofilm. This instrument is a prototype of instruments that could be used to effect continuous monitoring and provide early warning of biofilms associated with bacterial contamination in diverse water systems, including potable-water supply systems, industrial heat-exchanger systems, and heating and cooling systems for buildings. The instrument makes it possible to detect biofouling of such systems sooner than the ends of the 24-to-48 hour incubation periods needed for conventional detection of bacteria by culture-plate counting.
The instrument has overall dimensions of 4 by 17 by 1.5 in. (about 10 by 43 by 4 cm) and is made from inexpensive optoelectronic components. One of the components is a light-emitting diode (LED). Modulated light from the LED is launched into the fiber-optic probe, which includes an unclad length of optical fiber that serves as a sensory element. Light travels along the fiber, past the sensory element, to a photodetector. The output of the photodetector is processed by a digital-interface circuit board connected to the parallel input port of a computer. In essence, what one seeks to compute is the proportion of light reaching the photodetector as an indication of the amount (if any) of biofouling on the sensory element.
Any biofilm attached to the sensory length of fiber affects the evanescent wave of the light propagating in the fiber. The effect is primarily a result of (1) a change in the index of refraction to which the evanescent wave is subject and (2) increased scattering of light. The evanescent wave is shallow enough that the instrument exhibits a significant response to as little as a monolayer of bacteria.
The design of the instrument was guided by a mathematical model that assisted in the optimization of the instrument performance and the prediction of the response of the instrument to specified changes in the index of refraction of the medium surrounding the sensory element. The overall response of the instrument to a change in the index of refraction is characterized by, among other things, a time of less than 1 minute. In tests in which bacterial cultures were added to, variously, distilled water or plant-nutrient solutions, peak first-stage responses occurred within 1 to 6 hours and peak second-stage responses took place after 3 to 10 hours.