Light emitting diode-based detectors: Absorbance, fluorescence and spectroelectrochemical measurements in a planar flow-through cell

Abstract

Light emitting diodes (LEDs) were first used for chemical analysis three decades ago. They are finally making their appearance in commercial analytical systems and dedicated detectors. LEDs are the most energy-efficient means of producing monochromatic light, and provide a concentrated small cool emitter ideal for miniature analytical devices. Although they rank behind fluorescent and halogen discharge lamps in total conversion efficiency (lm/W), new efficiency records are being set every year such that by next decade broadband (white) LED sources are not only likely in analytical instrumentation, but for general illumination. This paper begins with a review of analytical use of LEDs that has been advanced in the last decade. LED-based absorbance measurement and its use in pedagogy, titrations, in providing immunity to refractive index and turbidity effects, in field and process analysis, in capillary electrophoresis (CE), in liquid–liquid extraction systems, in film and drop-based analytical systems and with liquid core waveguides (LCWs) are discussed. LED-based fluorescence and spectroelectrochemical detection follows next. Multipurpose LED-based analytical instrumentation and special analytical applications and general applications are discussed. A listing of (mostly web-based) resources for fabricating LED-based detectors is then provided. Detector circuits and available components are considered and different modes of driving LEDs are compared. The temperature dependence of LED characteristics and strategies to ameliorate this problem are discussed. The review and general resource material is followed with the construction details, operation and performance observed for a simple-to-fabricate multipurpose cell that allows simultaneous multiwavelength absorbance, fluorescence and spectroelectrochemical detection.