MHz LED source for nanosecond fluorescence sensing

An inexpensive and portable pulsed light emitting diode (LED) source with variable repetition rate up to 10 MHz suitable for time-resolved fluorescence sensing in medical, environmental and industrial applications is described. The pocket-sized battery-driven source is pulsed by signals from a simple logic circuit, giving measured optical pulse widths of ~1.9 ns at 525 nm as recorded using time-correlated single-photon counting. Pulses are presented for three LEDs with peak emissions at 525, 560 and 590 nm. The measurement performance is illustrated by the fluorescence lifetime and anisotropy decay of rhodamine 6G in various solvents and a fluorescence resonance energy transfer (FRET) sensor for detecting metal ions in water which uses a new measurand, namely the site distribution function ρ( r ) of metal ion acceptors accumulating in an anionic porous polymer with respect to the donor fluorophores. The ρ( r ) dependence reveals a minimum fluorophore-metal ion separation of ~7 Å which explains the selectivity of such FRET sensors in discriminating against other quenching mechanisms.