A new measurement system for chemical optrodes based on the use of multi-LEDs and double-photodiodes is presented. A sensing area is illuminated through one branch of a bifurcated fibre optics with blue, yellow, red and IR LEDs of high intensities. The second arm of the fibre optics guides reflected light from the sensing area to the detector. The sensing system is simple as no monochromators or optical filters are employed and also no need for separate optical modulation as LED is amenable to electronic modulation. The light source and detector consisted of only solid-state electronic components. Therefore, the sensor system is simpler, inexpensive, compact and highly suitable for field applications. The optrode was tested for Hg(II) ions using 4-(2-thiazolylazo) resorcinol (TAR) immobilised onto XAD-4 with yellow LED (590 nm) as light source. The sensor showed good sensitivity in the range 0.1–20 ppm of Hg(II) ion and also gave similar responses to many other toxic metal ions. The sensor can be regenerated rapidly using 0.1 M HCl.
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