Principles of Fluorescence Spectroscopy

628 FLUORESCENCE SENSING
Figure 19.9. Stern-Volmer plots for oxygen quenching of
[Ru(phen) 3 ] 2+ and [Ru(Ph 2 phen) 3 ] 2+ in GE RTV 118 silicon.
Phen is 1,10-phenanthroline; ph 2 phen is 4,7-diphenyl-1,10phenanthroline.
Revised from [24].
Figure 19.10. Luminescence intensity of an oxygen sensor with
[Ru(Ph 2 phen) 3 ] 2+ as the probe, when exposed to breathing. Revised
from [24].
than [Ru(phen) 3 ] 2+ . The difference in sensitivity is due to
the longer unquenched lifetime (τ 0 ) of the diphenyl derivative,
and thus the larger Stern-Volmer quenching constant
(eq. 19.1). These long-lifetime probes have been used in
real-time oxygen sensors. For example, Figure 19.10 shows
the intensity of [Ru(Ph 2 phen) 3 ] 2+ in silicone while exposed
to exhaled air. The intensity increases with each exhale
because of the lower O 2 and higher CO 2 content of the
exhaled air. The higher intensity on the first exhale after the
breath was held is due to the lower O 2 content in the air that
was retained longer in the lungs. The oxygen sensitivity of
the sensor can be adjusted by selecting probes with different
lifetimes or by modifying the chemical composition of
the supporting media. The sensitivity to oxygen can be
increased by using MLCs with longer lifetimes, some of
which are as long as 50 µs. 32
19.4.2. Lifetime-Based Sensing of Oxygen
For practical sensing applications the device must be simple
and inexpensive, which is possible using the long-lifetime
MLCs. The oxygen-sensitive MLCs in Figure 19.9
absorb near 450 nm, and are thus easily excited with blue
light-emitting diodes (LEDs). One simple oxygen sensor
device is shown in Figure 19.11. Because of the long decay
times and simple instrumentation, oxygen sensors were
used to demonstrate the stability of phase-angle sensing in
the presence of large-amplitude intensity fluctuations. 33 The
intensity was varied by waving fingers in the light path,
resulting in fivefold changes in intensity (Figure 19.11). In
contrast to the measured intensities measurements, the
phase angles remained constant.
Figure 19.11. Phase-angle stability with intensity fluctuations measured with an oxygen-sensing device. The amplitude of the incident light was varied
by waving fingers between the LED and the sensor. Revised from [33].