Measurement of Fluorescence Decay Curves
Here, the black trace shows the optical pulse profile produced by a 435 nm blue LED using a nanosecond pulse generator built from 74F series logic IC’s. The LED pulse in this experiment has a FWHM of just under 20 ns. This experiment was performed using an analog PMT and an oscilloscope.
The blue trace is the fluorescence decay profile of a ruthenium dye that has been excited by this LED. The LED pulse can be seen in the early part of the trace – followed by the exponential decay of the Ru fluorescence. Analysis of the data yields a fluorescence lifetime of 285 nsec.
Ruthenium fluorescence is strongly quenched by dissolved oxygen – here, if the solution is degassed with N2 the measured lifetime increases by a factor of approximately 1.6.
Quinine Sulphate (QS)
QS is a well known standard for fluorescence measurements, with a reported fluorescence lifetime of 19 ns. Exciting the sample with ~2ns FWHM pulses at 370nm using the ultra-short pulse generator, and recording the emission at 450 nm, resulted in this decay curve.
The fluorescence lifetime of QS from this data is approximately 20 ns, in good agreement with the literature value.
Ruthenium Dye Fluorescence DecayThe decay curve shown at left was obtained using a 435 nm blue LED, driven by the ultra-short pulse generator. Emission from the sample cuvette was coupled into a small monochromator.
The orange-red Ru dye fluorescence was monitored at 620 nm by a Hamamatsu photon counting head mounted at the exit port of the monochromator with photon counting via the XMOS Startkit TDC-GP2 shield.
There is a sizeable offset in the trace at left, indicating that there is a significant stray light contribution in this run. Photon counting experiments require great care to exclude unwanted stray light and work is currently in progress to improve the system in this regard.
Temporal Profile of a Xe Flashlamp
In flash photolysis experiments a photophysical process is initiated by a high intensity, short burst of light from a flashlamp. The present system fires a camera flashlamp by connecting a hot shoe to mini-DIN connector J4 as described earlier.
An externally connected TSL235 light-to-frequency converter monitors the intensity temporal profile of the photoflash. Here, the tail of the emission is exponential and the emission has decreased to 50% of its peak value after 2.4 msec.
Pulse OximetryAn important item of medical monitoring equipment in hospitals is the oximeter, which non-invasively monitors a patients’ heart rate and oxygen saturation. In the former measurement the pumping action of blood through the body can be monitored by a simple “clip-on” finger probe that contains an LED and an optical sensor.
Data from a typical experiment using our instrument is shown here. A red LED is used as the light source and a TSL235 detector connected at J4 is used as the detector.
The “pleth” trace shown here was recorded over a period of ~8 seconds. Peaks in the waveform are indicated by green markers, and valleys by yellow markers. The heart rate can easily be computed from the measured time interval between any two peaks.