Photometric detection in flow analysis systems using integrated PEDDs

Abstract

A novel inexpensive optical sensing technique has been developed for colorimetric flow analysis. This sensing system employs two LEDs whereby one is used as the light source and the other as a light detector. The LED used as light detector is reverse biased with a 5V supply so that the photocurrent generated by the incident light discharges the capacitance. Direct digital output is provided by a simple timer circuit that measures the time taken for this discharge process from 5V (logic 1) to 1.7V (logic 0). This sensing concept has been applied in flow analysis by constructing an optical flow cell with a pair of LEDs. Calibration of the integrated optical flow cell using a dye resulted in a linear response that obeys the Beer-Lambert Law. The flow rate, dynamic range, sensitivity and limits of detection were investigated. The system was also used for pH determination in the range of pH 2.5 -6.8 using bromocresol green (BCG). The pK a of BCG was successfully determined by this technique.

Determination of phosphate using a highly sensitive paired emitter–detector diode photometric flow detector

“…The integrated PEDD cell was fabricated as previously described [25] using two 5 mm LEDs (Kingbright, Ireland) as shown in Fig. 1.…”

“…The use of LEDs offer advantages such as low cost, compact form, availability across a broad spectral range from UV to near-IR, robust and long lifetimes [21][22][23]. In most microanalytical systems the light source and the photodetector are normally separate units integrated with the microfluidic manifold [24], however in this case the PEDD is a single unit containing light source, fluid channel and detector for flow analysis [25][26][27].…”

Inkjet printed LED based pH chemical sensor for gas sensing

“…The sensing technique employed with the PEDD sensor was as previously reported [21,22]. The detector LED in output mode was charged up to 5 V for 100 µs and then switched to high impedance input mode.…”

“…The detector LED in output mode was charged up to 5 V for 100 µs and then switched to high impedance input mode. Photons from the emitter LED generate a photocurrent in the reverse biased detector LED, which discharges a capacitance based voltage from an initial value of 5 V (logic 1) to a preset value of 1.7 V (logic 0) [21,22].…”

“…A PEDD consists of two LEDs, whereby one serves as the light source and the other in reverse bias mode as the light detector. Instead of measuring the photocurrent directly, a simple timer circuit is used to measure the time taken for the photocurrent generated by the emitter LED to discharge the detector LED from 5 V (logic 1) to 1.7 V (logic 0) to give digital output directly without using an A/D converter or operation amplifier [21,22]. This method achieves excellent sensitivity and signal-tonoise ratio in comparison to the more commonly employed method of coupling an LED to a photodiode [23].…”