Abstract: A biconically tapered optical fiber serves as a biosensor at all optical wavelengths of interest ranging from UV to far IR at subfemtogram per mL sensitivity. The biconically tapered sensor detects biomaterials such as pathogenic species, proteins and DNA and others biological analytes. Although it uses the principles of evanescent fields, absorption, adsorption, fluorescence capture and retransmission through the fiber, the detection at the sub-nanogram per mL level is achieved primarily by adsorption or a surface activity due to a refractive index change. The geometry of the biconically tapered optical sensor affects its performance. The sensing modality is achieved in situ with a source connected at one end of a tapered fiber and a suitable detector at the other end. The tapered region is optionally immobilized with a recognition molecule such as an antibody to the target antigen or a complementary DNA strand. The sample is brought in contact with the tapered region either in batch mode or in a flow mode.