Single-mode optical fibers (core diameter 8 mm, cladding diameter 125 mm) were tapered to enhance evanescent field for detection. Recently (Rijal et al., Biosensors and Bioelectronics, (2005), in press; published on line) we showed that such a taper can be immobilized with an antibody to the pathogen, E coli 0157:H7 for detection of the pathogen using 470 nm light. The selectivity of the antibody-immobilized taper sensor was determined from response to a mixture of the pathogen and a non-pathogenic variant (JM101) at 0%, 50% and 70% by concentration. We showed that 70 cells/mL detection is possible.

In the current investigation we have explored the relationship between wavelength used (400 to 700 nm) and the geometry of taper (convergent length: 400 to 4000 microns; waist length: 50 to 2500 microns; divergent length: 400 to 3000 microns; waist diameter: 3 to 20 microns) on sensitivity to low concentration of E coli and yeast ( 10 to 10^6 cells/mL). We have attempted to relate sensitivity to cells with sensitivity of tapers to refractive index change by measuring air and water transmission ratios. We also show that use of longer wavelength light (1330 nm and 1550 nm) provides a platform for enhanced sensitivity. Using both E. coli and a common yeast strain we show that cell concentration less than 10 cells/mL can be determined. At the longer wavelength the two cell types appear to exhibit higher than normal absorption and scatter than at visible wavelength. Current effort is on determining sensor sensitivity using immobilized antibody on the sensor to both E. coli 0157:H7 and Bacillus anthracis (strain 7702). Acknowledgement: This work was supported in part by National Science Foundation (NSF) under Grant No. BES 0329793 and Ben Franklin Partenrship's Nanotechnology Institute.