<title>Waveguide immunofluorescence sensor for water pollution analysis</title>

Abstract

A regenerable channel waveguide fluorescence sensor for environmental monitoring is reported. The sensor has been characterised as a detector of the pesticide 2,4 dichlorophenoxyacetic acid. A binding inhibition assay, using fluorescent Cy5.5 dye-labelled antibodies, was monitored at the modified surface of the glass waveguide to detect the target analyte. Three calibration curves were determined and averaged. The averaged calibration curve has a mid-point of 0.68 ppb and a calculated detection limit of 0.28 ppb. Incorporation of a 20 nm thick tantalum pentoxide film at the waveguide surface enhanced the peak fluorescence signal by a factor of approximately 6 compared with an uncoated sensor. Due to the high optical field strengths at the surface of the waveguide, which is approximately 10µm wide, significant photobleaching of the dye molecules occurs. The rate of photobleaching will be reduced if the power density of the excitation radiation at the surface of the waveguide is reduced, offering the potential for enhanced device sensitivity. This may be achieved, without reducing the total power, by broadening the 10µm wide optical waveguide through a tapered region to a final width in excess of 50µm. A distinct advantage of this broadening is to improve the signal to noise ratio of the sensor as the number of bound fluorophores at the waveguide surface increases linearly with the waveguide width. Theoretical modelling of tapered waveguides, using a beam propagation method package, has indicated that the peak field intensity of radiation in the 10µm guide may be reduced by 85% if the guide is broadened through a taper to a final width of 50µm.