Visible light-emitting diodes (LEDs) were developed in 1962, containing p-n junctions emitting a narrow band of light wavelengths. LEDs have found a wide application for the miniaturization of optical sensors due to their low cost, small size, robustness, and high difference is generated across the p-n junction [1]. Therefore, LEDs can be used as both the light source and the detector in paired emitter-detector diodes (PEDD) optical sensors [2]. PEDD-based photometers provide low fabrication cost, low power consumption, ease of miniaturization, and a high signal-noise ratio response in a large wavelength range [3]. Furthermore, their output is a direct pulse-duration-modulated signal, eliminating the need for a costly analog-to-digital converter. These advantages have led to the utilization of PEDD- -through optical This paper reports on developing a cheap but efficient PEDD-based photometer. The photometer consists of a white light-emitting diode (LED) as the emitter diode, an RGB LED as the detector diode and a multimeter for recoding the signal. The developed PEDD-based photometer was used for the determination of 2,4,6- trinitrotoluene (TNT) in soil samples. A Meisenheimer complex in acetonitrile was used as a probe to monitor the presence of residual TNT in soil samples. The calibration curve was linear in the concentration range of 1.5 to 97.0 M, with a detection limit of 0.32 M and quantification limit of 1.05 M. Analysis of the soil samples collected from an Iranian cinema town using the developed method showed considerable residual of TNT.
