Page 193 - Organic Electronics in Sensors and Biotechnology

170 Chapter Five

background light stemming from the tail of the broad EL band is
practically eliminated.

5.3.1 Advances in Monitoring Gas-Phase
and Dissolved Oxygen
The first effort to develop the OLED-based sensor platform focused
37
on O sensors, which command a large commercial market. The
2
well-known method for monitoring gas-phase and dissolved O (DO)
2
is based on the collisional quenching of the phosphorescence of
oxygen-sensitive dyes such as Ru, Pt, or Pd chelates by gas-phase O
2
and DO. In a homogeneous matrix, the O concentration can be deter-
2
mined ideally from changes in I under steady-state conditions or
from τ using the Stern-Volmer (SV) equation
I /I =τ /τ= 1 + K [O ] (5.1)
0 0 SV 2
where I and τ are the unquenched values and K is a temperature-
0 0 SV
and film-dependent constant.
Although this PL-based sensing is well established, extensive
studies of optical O sensors are still continuing in an effort to
2
enhance the sensors’ accuracy, reliability, limit of detection, and
operational lifetime; reduce their cost and size, and develop an O
2
38
sensor for in vivo applications. The large market for O sensors is
2
also an incentive for continued research in the field. The low-cost
field-deployable, compact oxygen sensors will therefore be very
attractive for the various biological, medical, environmental, and
industrial applications. 37–40
Among the structurally integrated OLED/dye-doped O sensor
2
film modules evaluated to date, those based on Pt and Pd octaethyl-
porphyrine (PtOEP and PdOEP, respectively), embedded in poly-
styrene (PS) and excited by Alq -based OLEDs, exhibited the highest
3
sensitivities. Their dynamic range and sensitivity, defined as S ≡
17
g
τ /τ(100% O ) in the gas phase and as the ratio S of τ measured in
0 2 DO
a deoxygenated solution to that of an oxygen–saturated solution,
from 0 to 60°C, are comparable to the best O sensors reported to date.
2
This demonstrates the viability of the platform for high-sensitivity
monitoring.
The linear SV relation was confirmed for DO monitored by dyes
41
in solution. It was observed also for the solid [DPVBi OLED]/[Ru
17
dye sensor film] matrix, and as seen in Figs. 5.2 and 5.3 for DO
measurements with the oxygen-sensitive dyes embedded in a PS
film. The pulse width used for obtaining the data shown in Figs. 5.2
and 5.3 was 100 μs. We note that S ~14 of DO in water measured
DO
with the OLED-based sensors is among the highest reported for
PtOEP:PS.