A compact fiber-optic immunosensor for Salmonella based on evanescent wave excitation

A compact fiber-optic evanescent-wave sensing system that features all-fiber optical design and red semiconductor-laser excitation has been developed and tested. A 2 x 2 fiber coupler directs the input light to the SMA-connected sensing fiber tip and the fluorescent signal back to a CCD fiber spectrophotometer. In this system,the fluorescent signal is confined in the fiber system so the signal-to-noise ratio is greatly improved and the system can be operated in ambient light conditions. A diode laser as the source has the advantages of small volume, ruggedness, low cost and stability; more importantly, since biological matrices demonstrate minimal fluorescent background at the laser wavelength of 650 nm, this system can reduce the background signal of non-essential biomolecules. To illustrate the biosensor's diagnostic capabilities, a sandwich immunoassay to detect Salmonella was developed. Tapered fiber tips with different shapes and treatments were studied and optimized, The system could detect Salmonella with a concentration as low as 10(4) colony-forming units per milliliter (CFU ml(-1)). (C) 1997 Elsevier Science S.A.