Yuankun Cai

Polythiophene‐Fullerene Based Photodetectors: Tuning of Spectral Response and Application in Photoluminescence Based (Bio)Chemical Sensors

A photoluminescence (PL)-based oxygen and glucose sensor utilizing inorganic or organic light emitting diode as the light source, and polythiophene: fullerene type bulk-heterojunction devices as photodetectors, for both intensity and decay-time based monitoring of the sensing elements PL. The sensing element is based on the oxygen-sensitive dye Pt-octaethylporphyrin embedded in a polystyrene matrix.

Structurally integrated organic light-emitting device (OLED)-based multianalyte sensing through analyte-oxidase interactions

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.

Transient emission in OLEDs: implications for sensing applications

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.

Advances toward commercialization of a new generation of low cost (O)LED-based dissolved oxygen and bioanalyte monitors

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.

OLED-Based Biosensing Platform with ZnO Nanoparticles for Enzyme Immobilization

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.

Polymer-based photodetectors for structurally integrated photoluminescence based oxygen sensors

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.

OLED-polypropylene bio-CD sensor

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 OLED-based chemical and biological sensors

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.

OLED-based sensor array for simultaneous monitoring of multiple analytes

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.