Low cost phase-modulation measurements of nanosecond fluorescence lifetimes using a lock-in amplifier

The use of a 200 MHz lock-in amplifier was demonstrated as a low cost instr ument for frequency domain measurements of nanosecond fluorescence lifetime s. The lock-in directly provided both the dc bias and the ac signal used to modulate the intensity of a blue light emitting diode excitation source. T he emission was measured by a photomultiplier tube and the resulting signal was sent back through a dc block to the lock-in with no external signal pr ocessing or heterodyning required. The system was highly accurate at measur ing phase and modulation up to 80 MHz and moderately so up to 100 MHz. The fluorescence lifetimes of several standard fluorophores (Fluorescein,Rhodam ine B, and [Ru(bpy)(3)](2+)) were measured by the lock-in, and the results agreed closely with those made on a research grade fluorometer. The entire lock-in based system costs less than US $10,000 to build and can be control led by any standard computer through a GPIB or serial connection. The syste m is also portable, consumes little power, and can easily be configured for use with fiber optics, making it ideal for use with fluorescence lifetime based sensors. (C) 1999 American Institute of Physics. [S0034-6748(99)05402 -7].