BLUE-LIGHT-EMITTING DIODE DEMONSTRATED AS AN ULTRAVIOLET EXCITATION SOURCE FOR NANOSECOND PHASE-MODULATION FLUORESCENCE LIFETIME MEASUREMENTS

We have produced amplitude-modulated near-ultraviolet light, centered at 390 nm, using an inexpensive, commercially available blue light-emi tting diode (LED). The LED was amplitude modulated with the +13 dBm ac output from a frequency generator while biased through a bias tee wit h 60 mA of de current, The LED produced 45 to 54 mu W of UV light over the modulation bandwidth of 0.01 to 200 MHz, when measured after opti cal filters to remove the residual blue output. Since the filter atten uated the UV output about 3 dB, more than 100 mu W of UV light was ini tially produced. Modulated UV light was available to approximately 200 MHz, with a -3 dB point of 31 MHz, allowing the measurement of ns flu orescence lifetimes. The fluorescence lifetimes of standard fluorophor es (9-cyanoanthracene and green fluorescent protein) were measured in the frequency domain using the phase-modulation technique, producing l ifetimes that closely agree with those reported in the literature, con firming that the UV-emitting blue LED is practical for spectroscopic a nd sensor applications. When compared to a laser modulated with a Pock els cell, the LED was smaller, less expensive, required less power, ge nerated less heat, and required less alignment. The ability to modulat e the LED at high frequencies, along with the UV output, allows its us e as an inexpensive UV light source Ln fluorescence lifetime optical s ensors and even frequency-domain fluorometers. (C) 1997 American Insti tute of Physics.