Fibre-fluorescence immunosensor based on evanescent wave detection

A fibre-optic sensor capable of detecting an immune reaction between fluorescently labelled antigens and antibodies bound at the waveguide surface is described. In a configuration where skew rays were detected, a signal level was found that made detection of a concentration of 10e9 M antigen possible. Keywork Fibre-optic sensor; Immune reactions The evanescent wave is a well known physical phenomenon used in several types of chemo-optical sensors such as grating couplers, surface plasmon resonance sensors and fibre-fluorescence sensors [l-4]. It occurs on total internal reflection [5] of plane waves at the interface of two dielectrics and the relatively short penetration depth of the evanescent wave makes it very suitable for measuring optical changes at this interface. Additional specific advantages of a fibre-fluorescence sensor are the long interaction length and the high efficiency for collection of fluorescence emitted near the core of the fibre [6,7]. For these reasons it is attractive to consider the use of the fibre-fluorescence sensor as an immunosensor. In an evanescent fluorescence immunosensor, an antibody (Ab), immobilized onto the waveguide surface, is used as a selector molecule to detect quantitatively and specifically the presence of an antigen in the sample to be analysed. This is accomplished by adding a known amount of fluorescently labelled antigen, which will compete for Ab binding with the antigen present in the analyte solution. On evanescent excitation of the label molecule the resulting fluorescence intensity is a measure of the concentration of non-labelled antigens. In recent years, several fibre-fluorescence immunosensors with relatively low sensitivity have been described [2,8-lo]. In this paper it is shown how the sensitivity can be improved considerably by taking into account more properly the propagation of rays inside the fibre with emphasis on skew rays.