Page 192 - Organic Electronics in Sensors and Biotechnology

Progress and Challenges in OLED-Based Chemical and Biological Sensors 169

Recent developments demonstrated the viability of the structurally
integrated OLED array/sensing film, where these components are fab-
ricated on two separate glass slides that are attached back-to-back. This
geometry is unique in its ease of fabrication, and it eliminates the need
for components such as optical fibers, lens, and mirrors, resulting in a
compact and potentially very low cost OLED/sensor film module,
whose ~2 mm thickness is determined by that of the substrates. 10–18
Additionally, the nearly ideal coupling between the excitation source
and the sensor film enables operation at relatively low power, which
minimizes heating that is a critical issue for sensor materials and ana-
lytes involving heat-sensitive bio(chemical) compounds.
The PD, usually a photomultiplier tube (PMT) or a Si photodiode,
is typically positioned either in front of the sensing film (“front detec-
tion”) or behind the OLED array (”back detection”). In the latter con-
figuration, the PD collects the PL that passes through the gaps
between the OLED pixels. As the PD is not structurally integrated
with the other components, in this configuration the OLED array is
sandwiched between the PD and the sensing component, unlike the
envisioned structure of Fig. 5.1a, where the OLED and PD pixels are
fabricated on the same side of a common substrate. The resulting sen-
sor probe, which includes the OLED array, the sensor film, and the
PD, is very compact when using, e.g., a surface-mount Si photodiode,
which is < 2 mm thick.
The OLED pixel array enables fabrication of a new platform of
compact multianalyte sensor arrays. The pixels are individually
addressable and can be associated with different sensor films, and their
emission can vary from single color (at any peak wavelength from
380 to 650 nm) to multicolor combinatorial arrays. 31, 35, 36 As the volume
of manufactured OLEDs increases, they will eventually become dis-
posable. Thus, the development of OLED-based field-deployable,
compact, low-cost, user-friendly, and autonomous chemical and
biological sensor arrays is promising.

5.3 Sensors Based on Oxygen Monitoring
In the examples provided below, typically 2 to 4 OLED pixels, 2 × 2
2
mm each, were utilized as the excitation source; 0.3 × 0.3 mm pixels
2
were also successfully employed. Importantly, the OLEDs were often
operated in a pulsed mode, enabling monitoring of the effect of the
analytes not only on the PL intensity I, but also on the PL decay time τ.
The τ-based detection mode is advantageous since moderate changes
in the intensity of the excitation source, dye leaching, or stray light
have essentially no effect on the measured value of τ. Thus, the need
for a reference sensor or frequent sensor calibration is avoided. Addi-
tionally, as τ is measured during the off period of the OLED, no filters
are needed to block any OLED light from the PD, and the issue of