Page 210 - Organic Electronics in Sensors and Biotechnology

Progress and Challenges in OLED-Based Chemical and Biological Sensors 187

5.6 Concluding Remarks
Recent advances in the development of the OLED-based luminescent
chemical and biological sensor platform were reviewed. The advan-
tages of this platform include its ease of fabrication, uniquely simple
integration with the sensing elements, and consequent promise as
a compact field-deployable low-cost monitor for various analytes.
In addition, the OLEDs could be efficient light sources in sensor
(micro)arrays for detection of multiple analytes in microfluidic
architectures.
The viability of the OLED-based platform was demonstrated for
gas-phase and dissolved oxygen, hydrazine, and simultaneous moni-
toring of multiple analytes, such as mixtures of dissolved O , glucose,
2
lactate, and ethanol. Initial results on the structural integration of the
OLED excitation source, the thin-film sensing element, and a Si-based
thin-film PD were also reviewed. This effort is motivated by its poten-
tial to lead to badge-size monitors.
For the sensors based on monitoring oxygen, the fluorescent
OLEDs are operated in a pulsed mode. This enables their use not only
in the PL intensity I mode, but also in the PL decay time τ mode, as τ
is determined by the analyte concentration. Since τ > 1 μs, it is much
longer than the ~100 ns EL decay time τ of typical fluorescent
EL
OLEDs. Indeed, even some phosphorescent OLEDs, where τ ~ 1 μs,
EL
can be used to excite the sensor films and monitor the O in the τ mode.
2
This mode is strongly preferable over the I mode: (1) It removes the
need for a reference sensor and frequent sensor calibration, since τ is
independent of moderate changes in the sensing film, light source
intensity, and background light. (2) It removes the need for optical
filters that block the EL from the PD, as the PL decay curve of the O
2
sensing dye is monitored after the EL is turned off.
Some current and future efforts to enhance the OLED-based sen-
sor platform include these: (1) Applications of microcavity, stacked
(tandem), and/or mixed layer phosphorescent OLEDs. 82, 83 These
would be brighter, more efficient, and longer-lived than the conven-
tional fluorescent OLEDs; the microcavity OLEDs’ narrow EL spectra
could be tailored to the absorption spectrum of the sensing element,
resulting in drastic improvement of the sensor platform. However,
preliminary studies indicated, as expected, that the relatively thicker
microcavity OLEDs require longer pulses to obtain the maximal EL
and therefore PL (see Fig. 5.18). Increased PL intensity can improve
the limit of detection; however, the use of longer pulses may adversely
affect the OLED lifetime. (2) Development of the platform for various
biological analytes, notably foodborne pathogens, which are a major
ongoing issue in the global food supply chain. (3) Improvement of
nc-Si-based PDs to shorten their response time and evaluation of
organic PDs (OPDs) for integration with the OLED/sensor film mod-
ules, as Peumans et al. have demonstrated fast OPDs. 84