All-solid-state instrument for fluorescence-based fibre-optic chemical sensors
A detector system for fibre-optic sensors relying on a light-emitting diode and a photodiode is described. A sensing membrane is irradiated through one branch of a bifurcated optical cable with a blue light-emitting diode of high intensity. The second arm of the cable guides fluorescent light from the membrane back to the detector. Wavelength separation was found to be unnecessary and lock-in amplification alone provided excellent discrimination against fluctuations in ambient light intensity. The entire system is therefore significantly simpler than most fluorescence-based sensors as no monochromators or optical filters are employed and both the light source and detector consist only of solid-state electronic components. The light-emitting diode can be square-wave modulated electronically rather than with a mechanical chopper as necessary for other types of light sources. It was possible to carry out the lock-in detection with a circuitry based on a demodulator contained in one integrated circuit package. The entire detector system is therefore very compact, simple and inexpensive. The performance was tested with two types of sensing membranes. An application is demonstrated with the determination of oxygen employing the quenching of the fluorescence of tris(2,2′-bipyridyl)ruthenium(II) chloride contained in a silicone membrane. A further application is shown with a nitrate-selective membrane based on poly(vinyl chloride), which contains a tridodecylmethylammonium salt as ionophore and a lipophilic derivative of fluorescein. Nitrate concentrations over four orders of magnitude could be determined with this system.