Recent advances toward commercialization of a new generation of
low-cost LED- and OLED-based monitors for
dissolved oxygen (DO), and multiple (bio)analytes such as glucose, lactate, alcohol, and cholesterol are described.
The design of the DO monitors, which contain no optical fibers, filters, mirrors, or lens, is significantly simpler and
consequently lower-cost than that of commercial LED-based DO monitors. The multiple (bio)analyte monitors are
based on a DO monitor and the oxidase enzyme specific to each analyte. The potential advantages and
disadvantages of the OLED- vs LED-based monitors is also discussed.
Typical guest-host small molecular OLEDs (SMOLEDs) exhibit an emission spike at 100 - 200 ns and a tail that
extends over several μs following a bias pulse. The spike and tail are attributed to recombination of correlated
charge pairs and detrapped charges (mostly from the host shallow states), respectively. They may also be associated
with other OLED layers and other phenomena, e.g., triplet-triplet annihilation. The implications of the spike and tail
for OLED-based, photoluminescent oxygen sensors operated in the time domain are evaluated and compared to the
behavior observed when using undoped OLEDs or inorganic LEDs as the excitation sources.
A photoluminescence (PL)-based O2 sensor utilizing inorganic light emitting diode (LED) as the light source and a
polymer-based photodetector (PD) is demonstrated. The device structure is compact and the sensor integrates the sensing
element, light source, and organic PD as thin films that are attached such that the sensing element is sandwiched between
the LED and the PD. The sensing elements are based on the oxygen-sensitive dyes Pt-octaethylporphyrin embedded in a
polystyrene matrix. A green inorganic LED (peak emission ~525 nm) light source was used to excite the porphyrin dye,
which emits at ~640 nm. This emission can be measured using P3HT:PCBM bulk heterojunction photodiodes, which
have been shown earlier to have efficient photodetection at this wavelength if the active layer is sufficiently thick. The
time constant associated with sweeping out the photogenerated carriers is found to be ~ 10μs. Such a fast decay of
photocurrent is useful for oxygen monitoring, determined by measuring the Pl decay time rather than the PL intensity, of
the sensing film. This approach can eliminate the need for frequent sensor calibration and optical filters (as pulsed LED
excitation is employed in this mode) which lead to bulkier design.
Organic light-emitting diode (OLED)-based sensing platforms are attractive for photoluminescence (PL)-based
monitoring of a variety of analytes. Among the promising OLED attributes for sensing applications is the thin and
flexible size and design of the OLED pixel array that is used for PL excitation. To generate a compact, fielddeployable
sensor, other major sensor components, such as the sensing probe and the photodetector, in addition to
the thin excitation source, should be compact. To this end, the
OLED-based sensing platform was tested with
composite thin biosensing films, where oxidase enzymes were immobilized on ZnO nanoparticles, rather than
dissolved in solution, to generate a more compact device. The analytes tested, glucose, cholesterol, and lactate, were
monitored by following their oxidation reactions in the presence of oxygen and their respective oxidase enzymes.
During such reactions, oxygen is consumed and its residual concentration, which is determined by the initial
concentration of the above-mentioned analytes, is monitored. The sensors utilized the oxygen-sensitive dye Pt
octaethylporphyrin, embedded in polystyrene. The enzymes were sandwiched between two thin ZnO layers, an
approach that was found to improve the stability of the sensing probes.
With the goal of developing microfluidic platforms for sensing applications, flash-free micro
patterns were embossed in polypropylene surfaces with ultrasonic heating for a biosensing
lab-on-CD application. The embossed features were designed to act as reservoirs, valves, and reaction
chambers to allow, in combination with a compact sensing platform, the monitoring of analyte levels
using a standard PC-CD player. To generate the compact sensor, as an example, we chose the
photoluminescence (PL)-based detection of lactate and glucose using an OLED-based sensing
platform. Once embossed, the surface energy of the plastic substrate was chemically modified to
make it hydrophilic. Reagents, placed in separate reservoirs, were directed through burst valves
towards a reaction chamber via CD rotation. Lactate or glucose were monitored by measuring the
effect of the related dissolved oxygen level on the PL decay time of an oxygen-sensitive dye,
following analyte oxidation catalyzed by a suitable specific oxidase enzyme. The results demonstrate
the potential of integrating OLEDs as excitation sources in PL-based sensors with microfluidic CD-based
platforms, including for simultaneous multiple analyses.
Recent developments in the structurally integrated OLED-based platform of luminescent chemical and biological sensors
are reviewed. In this platform, an array of OLED pixels, which is structurally integrated with the sensing elements, is
used as the photoluminescence (PL) excitation source. The structural integration is achieved by fabricating the OLED
array and the sensing element on opposite sides of a common glass substrate or on two glass substrates that are attached
back-to-back. As it does not require optical fibers, lens, or mirrors, it results in a uniquely simple, low-cost, and
potentially rugged geometry. The recent developments on this platform include the following:
(1) Enhancing the performance of gas-phase and dissolved oxygen sensors. This is achieved by (a) incorporating high-dielectric
TiO2 nanoparticles in the oxygen-sensitive Pt and Pd octaethylporphyrin (PtOEP and PdOEP, respectively)-
doped polystyrene (PS) sensor films, and (b) embedding the oxygen-sensitive dyes in a matrix of polymer blends such as
PS:polydimethylsiloxane (PDMS).
(2) Developing sensor arrays for simultaneous detection of multiple serum analytes, including oxygen, glucose, lactate,
and alcohol. The sensing element for each analyte consists of a PtOEP-doped PS oxygen sensor, and a solution
containing the oxidase enzyme specific to the analyte. Each sensing element is coupled to two individually addressable
OLED pixels and a Si photodiode photodetector (PD).
(3) Enhancing the integration of the platform, whereby a PD array is also structurally integrated with the OLED array
and sensing elements. This enhanced integration is achieved by fabricating an array of amorphous or nanocrystalline Si-based
PDs, followed by fabrication of the OLED pixels in the gaps between these Si PDs.
Steps towards the improvement of a compact photoluminescence (PL)-based sensor array that is fully structurally
integrated are described. The approach is demonstrated for oxygen sensing, which can be monitored via its effect on the
PL intensity I or decay time τ of oxygen-sensitive dyes such as Pt octaethylporphryn (PtOEP) and its Pd analog
(PdOEP). The integrated components include (1) an organic light emitting device (OLED) excitation source, which is an
array of coumarin-doped tris(quinolinolate) Al (Alq3) pixels, (2) the sensor film, i.e., PdOEP embedded in polystyrene,
and (3) the photodetector (PD), which is a plasma-enhanced CVD-grown p-i-n or n-i-p structure, based on amorphous or
nanocrystalline (Si,Ge):H. These components are fabricated on common or separate substrates that are attached back-to-back,
resulting in sensors with a thickness largely determined by that of the substrates. The fully integrated oxygen
sensor is demonstrated first by fabricating each of the three components on a separate substrate. The PD was placed in
front of a flow cell containing the sensor film, while the OLED array was "behind" the sensor film. This design showed
the expected trend in monitoring different concentration of O2 via their effect on I, with improved detection sensitivity
achieved by shielding the electromagnetic noise synchronous with the pulsed OLED. The detection sensitivity using the
I monitoring mode is expected to further increase by reducing the OLED tail emission. The issue of the OLED
background can be eliminated by monitoring the oxygen concentration via its effect on τ, where the OLED is pulsed and
τ is measured while the OLED is off. Steps therefore focused also on shortening the response time of the PDs, and
understanding the factors affecting their speed. Development of a sensor array, where the PD pixels are fabricated
between the OLED pixels on the same side of a common substrate, is also discussed.
A compact, photoluminescence (PL)-based sensor array, utilizing tris(quinolinolate) Al OLED pixels as the excitation
sources, for sequential or simultaneous monitoring of dissolved oxygen (DO), glucose, lactate, and alcohol, is described.
The DO is monitored through its effect on the PL lifetime of the oxygen-sensitive dye Pt octaethylporphyrin (PtOEP)
embedded in a polystyrene film. The other analytes are monitored through their oxidation, catalyzed by an appropriate
oxidase, which reduces the amount of DO in their vicinity. The OLED pixels are fabricated on a glass substrate; each
pixel is typically 2×2 mm2, with a 2 mm gap between the pixels. Two OLED pixels are associated with the detection of
each of the analytes. The pixels are individually addressable, enabling consecutive detection of the different analytes
within a few minutes utilizing a single photodetector (PD). Simultaneous detection is achieved by using an array of PDs.
The OLED-based sensing array is compact and uniquely simple in its ease of fabrication and integration. Its performance
attributes are comparable to those obtained for a single analyte using any excitation source. The potential of small-size,
multi-color OLED pixel arrays for multianalyte detection is also discussed.
The development of a compact structurally integrated platform for detection of multianalytes that consume oxygen in the presence of specific oxidase enzymes is described. The detection is based on monitoring the photoluminescence (PL) intensity or lifetime of a sensing element based on the oxygen sensitive dye Pt octaethyl porphyrin (PtOEP). The excitation source for the PL is an array of individually addressable green OLED pixels. The analytes are gas- phase and dissolved oxygen, glucose, lactate, and alcohol. The sensing element for each analyte includes a layer of PtOEP-doped polystyrene, whose PL lifetime decreases with increasing O2 level, and a film or solution containing the oxidase enzyme specific to the analyte. Each sensing element is associated with two addressable ~2x2 mm2 OLED pixels. The operation and performance metrics of the sensor under various conditions are described and discussed.
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