Principles of Fluorescence Spectroscopy

122 TIME-DOMAIN LIFETIME MEASUREMENTS
Figure 4.29. Electronic schematic for TCSPC with multiple detectors. Revised from [134].
this is a minor problem compared to the advantages of multidimensional
measurements.
Advances in electronics for TCSPC have made multidimensional
measurements rather simple, reliable, and relatively
inexpensive. Multiple intensity decays can be collected
simultaneously at different emission wavelengths or
at different locations in a sample. There are numerous
potential applications for such measurements in analytical
chemistry and cellular imaging. In the future we can expect
many if not most TCSPC experiments to be performed with
multiple detectors.
Figure 4.30. Multidimensional TCSPC of a mixture of fluorescein
and rhodamine 6G. Excitation was a 405-ns pulse with a ps diode
laser, 20 MHz repetition rate. The detector was an R5900-L16, which
has 16 separate anodes in a linear geometry. Revised from [134].
Multidimensional capabilities are now easily accessible
and are standard in some of the TCSPC electronics. A
typical circuit is shown in Figure 4.29. The pulses from 4 to
16 separate detectors are sent to a rotating module that
keeps track of their origins and sends them to the TAC and
ADC. A single TAC and ADC are used because these are
the most expensive, complex, and power-consuming parts
of the electronics. After conversion the data are sent to separate
blocks of memory as directed by the routing module.
If two photons are detected in different channels at the same
time, the routing module discards their pulses.
The power and simplicity of multidimensional TCSPC
is shown in Figure 4.30 for a mixture of fluorescein and
rhodamine 6G. The light source was a ps diode laser. The
emission wavelengths were separated and focused onto a
multi-anode PMT (Figure 4.31). Such PMTs have a single
photocathode and dynodes that maintain the position of the
electrons so they arrive at the appropriate anode. The transit
time spreads of these PMTs are 0.3 to 0.6 ns, so that
nanosecond decay times can be measured. The output from
Figure 4.31. Multi-anode PMT for Hamamatsu, R5900. Reprinted
with permission from [11,136].