Page 206 - Organic Electronics in Sensors and Biotechnology

Progress and Challenges in OLED-Based Chemical and Biological Sensors 183

The foregoing review also highlighted the advantages of operat-
ing PL-based sensors in the τ mode, as this mode eliminates the need
for frequent sensor calibration, a reference sensor, and optical filters
that block the EL from the PD. As the EL decay time of fluorescent
OLEDs is typically < 100 ns, they are particularly suitable for pulsed
excitation of oxygen-sensitive phosphorescent dyes, whose radiative
decay time τ is typically > 1 μs. Consequently, the PtOEP and
rad
PdOEP dyes, with a large PL quantum yield and τ of ~100 and
rad
~1000 μs, respectively, were particularly rewarding.
With absorption bands around ~380 and ~540 nm (the latter
closely overlapping the EL of Alq -based OLEDs) and PL peaking at
3
~640 nm, the PtOEP and PdOEP dyes also have a large Stokes shift.
This enables their use not only in the τ mode, but also in the I mode,
as the EL tail at the PL band is minimal.
Another example of a successful OLED-based sensor is that of
hydrazine, which is highly toxic and volatile, but a powerful mono-
15
propellant used in space shuttles. It is also a common precursor for
some polymer synthesis, plasticizers, and pesticides. Due to its
extreme toxicity, the American Conference of Governmental Indus-
trial Hygienists recommended that the threshold limit value (TLV)
for hydrazine exposure, i.e., the time-weighted average concentra-
tion of permissible exposure within a normal 8 h workday, not exceed
73
10 ppb in air. The OSHA recommended skin exposure limit is
0.1 ppm (0.1 mg/m ), and the immediate threat to life is less than
3
60 ppm. 74
The hydrazine sensor is based on the reaction between N H and
2 4
anthracene 2,3-dicarboxaldehyde. 15, 75 The reactants are not emissive,
but the reaction product fluoresces around 550 nm when excited
around 475 nm by, e.g., a blue DPVBi-based OLED; the signal is pro-
portional to the N H level.
2 4
The OLEDs used for the N H sensor were operated in a dc mode
2 4
at 9 to 20 V, or in a pulsed mode at up to 35 V. The PD was a PMT. At
~60 ppb, the PL was detected after ~1 min. Therefore, at ~1 ppb, the
PL would be detected in ~1 h. Thus, this capability exceeds the OSHA
requirements by a factor of ~80.
A third example of an OLED-based sensor on which preliminary
evaluations have been reported is that of Bacillus anthracis, via the
lethal factor (LF) enzyme it secretes. The need for a compact, low-
15
cost, field-deployable sensor for rapid, on-site detection of this bacte-
rium, which would eliminate the need to send samples for diagnosis
to a remote site, is obvious. The detection of LF, which is one of the
three proteins secreted by the live anthrax bacterium, is based on its
property of cleaving certain peptides at specific sites. 76–78 As the LF
cleaves the peptide, which is labeled with a fluorescence resonance
energy transfer (FRET) donor-acceptor pair, with the donor and
acceptor on opposite sides of the cleaving site and the two cleaved