Fiber-optic fluorometer signal enhancement and application to biosensor design

Fiber-optic fluorescence spectroscopy is currently the focus of active research because of the high sensitivity of fluorimetry and capacity for remote analysis with optical fibers. It is shown here that a further increase in sensitivity can be achieved by placing a mirror within a few fiber diameters of the distal end of the optical fiber in a solution of flourophore. A potentially four-fold signal enhancement results which is due to an increase in the excitation intensity as well as the reflection of the collected emitted light. The enhancement is shown to be dependent on the pathlength and concentration of the fluorophore, as well as the numerical aperture and core diameter of the optical fiber. The limit of detection of the fluorometer, which incorporates a mirror, is nanomolar dye concentrations, with the maximum response in path lengths of dye that are in the order of 6 fiber diameters. This principle can be applied to the design of optical sensors for continuously monitoring the concentration of specific biochemicals, and a conceptual design for such a sensor is described. A mathematical model is presented which describes the fluorescence output signal of a fiber-optic flourometer which incorporates a mirror.