A flow system for calibration of dissolved oxygen sensors

Well-defined oxygen standard solutions were obtained by the electrolysis of water in a coulometric oxygen generator. The generator was integrated into a flow system that includes the degassing of the carrier electrolyte, the generation of dissolved oxygen and the temperature control of the carrier electrolyte. The current efficiency of oxygen generation was found to be 100% by the Winkler titration method. Calibrations of a home made laboratory sensor and a WTW CellOx dissolved oxygen sensor have been made in a concentration range of 0.02 to 8 mg/L at temperatures from 5°C to 30°C. The calibration of the WTW sensor on water vapour saturated air was compared with the electrochemical calibration method. Both methods gave reliable results provided that the temperature equilibration between the sensor and the ambient air was successful. Received: 24 March 1997 / Revised: 15 May 1997 / Accepted: 15 May 1997     

Electroluminescent materials for white organic light emitting diodes

White organic light emitting diodes (WOLEDs) are promising devices for application in low energy consumption lighting since they combine the potentialities of high efficiency and inexpensive production with the appealing features of large surfaces emitting good quality white light. However, lifetime, performances and costs still have to be optimized to make WOLEDs commercially competitive as alternative lighting sources. Development of efficient and stable emitters plays a key role in the progress of WOLED technology. This tutorial review discusses the main approaches to obtain white electroluminescence with organic and organometallic emitters. Representative examples of each method are reported highlighting the most significant achievements together with open issues and challenges to be faced by future research.     

Microparticle ratiometric oxygen sensors utilizing near-infrared emitting quantum dots

Luminescent sensors incorporating two luminophores, an indicator and a reference, offer many advantages over intensity measurements from sensors made with one indicator dye. Quantum dots have yet to be widely employed as insensitive reference luminophores in such systems. This work describes the use of near-infrared emitting quantum dots in conjunction with a long-lifetime platinum(II) porphyrin phosphor in a microsphere-based, ratiometric oxygen sensor. The process for self-assembly of the nanocomposite system was developed, and the response and photostability of the prototypes were investigated. Results indicate the sensors possess excellent sensitivity (K(SV) = 0.00826 μM(-1)) at oxygen concentrations below 300 μM and were resistant to photobleaching. The sensor luminophores displayed minimal spectral overlap and little interference from excitation light, preventing the need for optical filters. A reversible photoenhancement of the quantum dot signal was also observed when exposed for extended periods of time. This work demonstrates the advantages of incorporating long-wavelength quantum dots into ratiometric intensity sensing schemes and highlights some key limitations that must be considered in their use.     

An integrated light emitting diode-induced fluorescence detector for capillary electrophoresis

An integrated light emitting diode (LED)-induced fluorescence detector was described and evaluated. The LED and its related components including lens and interference filter, the optical fiber used to collect fluorescence, and the capillary column are integrated into a substrate block, which eliminates the need of align procedure of the fiber and the capillary. Forty-fold enhancement of sensitivity was obtained compared with our previous work and the detection limit for fluorescein was 5 nM. Application of the detector for the analysis of FITC-labeled Ephedrine extract was demonstrated.     

A microfluidic device using a green organic light emitting diode as an integrated excitation source

A simply fabricated microfluidic device using a green organic light emitting diode (OLED) and thin film interference filter as integrated excitation source is presented and applied to fluorescence detection of proteins. A layer-by-layer compact system consisting of glass/PDMS microchip, pinhole, excitation filter and OLED is designed and equipped with a coaxial optical fiber and for fluorescence detection a 300 microm thick excitation filter is employed for eliminating nearly 80% of the unwanted light emitted by OLEDs which has overlaped with the fluorescence spectrum of the dyes. The distance between OLED illuminant and microchannels is limited to approximately 1 mm for sensitive detection. The achieved fluorescence signal of 300 microM Rhodamine 6G is about 13 times as high as that without the excitation filter and 3.5 times the result of a perpendicular detection structure. This system has been used for fluorescence detection of Rhodamine 6G, Alexa 532 and BSA conjugates in 4% linear polyacrymide (LPA) buffer (in 1 x TBE, pH 8.3) and 1.4 fmol and 35 fmol mass detection limits at 0.7 nl injection volume for Alexa and Rhodamine dye have been obtained, respectively.     

New low-molecular-weight electroluminescent materials for green organic light emitting diodes

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Zeolite-fiber integrated optical chemical sensors for detection of dissolved organics in water

MFI zeolite coated optical fiber sensors have been developed for in situ detection of dissolved organics in water. The sensors operate by monitoring the optical reflectivity changes caused by the selective adsorption of organic molecules, i.e., 2-propanol or pentanoic acid in this study, from aqueous solutions in the zeolitic pores. Reversible and monotonic sensor signals were observed in response to the variation of 2-propanol concentration in water with fast response. However, the sensor exhibited a much slower response to pentanoic acid than to 2-propanol. It was also found that substitution of Si by Al in the MFI framework increased the adsorption of pentanoic acid that resulted in enhanced sensor responses.     

Recent Progress toward white organic light emitting diodes

An efficient and stable white organic light emitting diode (WOLED) is highly desirable in potential applications such as lighting, background light source, and full color display.A series of highly fluorescent dyes based on a dipyrazolopyridine skeleton,1,7-diphenyl-l,7-dihydrodipyrazolo[3,4-b,4′,3′-e]pyridine, were synthesized and evaluated as emitting as well as charge-transporting material in the fabrication of electroluminescent devices.Several of the blue derivatives are found to be useful as the source of blue emission in fabricating bright white-emitting devices. The choice of dopants, cathode materials, electron-transporting materials as well as the device configurations greatly affect the emission profile, efficiencies, as well as the device lifetime. The latest progress in achieving a more efficient, color stable, durable white light device will be discussed.     

Feasibility studies for the detection of volatile organic compounds in the gas phase using microelectrode sensors

The use of microelectrode sensors to detect volatile organic compounds (VOCs) in air is demonstrated. In general, VOCs that oxidize easily to form protons gave a larger electrochemical response. The use of voltammetry for speciation and the effect of electrode size on the electrochemical response are discussed. We demonstrate that surface enhanced Raman spectroscopy (SERS) can be used to monitor the electrochemical reactions in situ and discuss its applicability in identifying the electroactive species.     

Carbazole-oxadiazole containing polyurethanes as phosphorescent host for organic light emitting diodes

New types of polyurethanes (PUs) were prepared from condensation polymerization of isophorone diisocyanate (IPDI) with various combination of 9-butyl-3,6-bis(4-hydroxyphenyl)carbazole (Cz) and 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole (OXD), and end-capped with 4-tert-butyl phenol. The Cz-OXD PUs can also be used as host for phosphorescent dye. Red EL emission was obtained when Ir(btp)₂(acac) or Ir(2-phq)₂(acac) was used as the phosphorescent dyes in Cz-OXD (3:1) PU. Maximum brightness of 394 cd/m² and EL efficiency of 1 cd/A were achieved for the Ir(2-phq)₂(acac) base device. In addition, white light PLED was demonstrated when co-dopant of Ir(btp)₂(acac) and Firpic were used.     

A novel power-controlling approach for integrated,conductometric gas sensors

An original power controlling driving/reading circuit for Porous Silicon JFET (PSJFET) gas sensors is presented. The PSJFET is an integrated p-channel JFET with two independent gates: a meso-structured PS layer, acting as a sensing, floating gate, which modulates the JFET current upon adsorption/desorption of specific analytes, and a high-impedance electric gate, which allows the JFET current tuning independently from analytes in the environment. The circuit exploits the independence of the sensing and electrical gate terminals to set/control the sensor power-dissipation, which is kept almost constant independently from adsorption/desorption-induced effects, while simultaneously carrying out a current-voltage conversion. For such a purpose, a negative feedback loop is used to modulate the PSJFET electric gate voltage, which becomes the output signal, while keeping constant the source-drain sensor current and, hence, the power dissipation. The proposed approach is validated by performing time-resolved measurements on PSJFET sensors under different NO₂ concentrations (100ppb, 300ppb, 500ppb), at room temperature.     

Thin-film polymer light emitting diodes as integrated excitation sources for microscale capillary electrophoresis

We report the use of a thin-film polymer light emitting diode as an integrated excitation source for microfabricated capillary electrophoresis. The polyfluorene-based diode has a peak emission wavelength of 488 nm, an active area of 40 microm x 1000 microm and a thickness of similar 2 mm. The simple layer-by-layer deposition procedures used to fabricate the polymer component allow facile integration with planar chip-based systems. To demonstrate the efficacy of the approach, the polyfluorene diode is used as an excitation source for the detection of fluorescent dyes separated on-chip by electrophoresis. Using a conventional confocal detection system the integrated pLED is successfully used to detect fluorescein and 5-carboxyfluorescein at concentrations as low as 10(-6) M with a mass detection limit of 50 femtomoles. The drive voltages required to generate sufficient emission from the polymer diode device are as low as 3.7 V.     

Electronic properties of anthracene derivatives for blue light emitting electroluminescent layers in organic light emitting diodes: a density functional theory study

Molecular level parameters are investigated computationally to understand the factors that are responsible for the higher efficiency in derivatives of 9,10-bis(1-naphthyl)anthracene (alpha-ADN), 9,10-bis(2-naphthyl)anthracene (beta-ADN), their tetramethyl derivatives (alpha,beta-TMADN) and the t-Bu derivative (beta-TBADN) as blue light emitting electroluminescent (EL) layers in organic light emitting diodes (OLEDs). DFT studies at the B3LYP/6-31G(d,p) level have been carried out on the substituted anthracenes. The absorption spectra are simulated using time dependent DFT methods (TD-DFT) whereas the emission spectra are approximated by optimizing the excited state by HF/CI-Singles and then carrying out the vertical CI calculations by the TD-DFT method. The reorganization energy for estimating the hole and electron transport is calculated. The transfer integrals between parallely stacked molecules in the bulk state are estimated by calculating the electronic splitting. The substituted anthracenes are compared with unsubstituted anthracene and yet untested 9,10-dianthrylanthracene (TANTH). A larger and slower buildup of the electrons and holes in the EL layer, due to the higher reorganization energy and smaller electronic coupling between the adjacent molecules could lead to an increase in hole-electron recombination in the layer and thus increase the efficiency.     

Neutral cuprous complexes as ratiometric oxygen gas sensors

Four neutral mononuclear Cu(I) complexes, [Cu(pyin)(PPh(3))(2)] (1a), [Cu(pyin)(DPEphos)] (1b), [Cu(quin)(PPh(3))(2)] (2a) and [Cu(quin)(DPEphos)] (2b) (Hpyin = 2-(2-pyridyl)indole, Hquin = 2-(2-quinolyl)indole and DPEphos = bis(2-(diphenylphosphino)phenyl)ether) have been synthesized. X-Ray crystal structure analysis revealed that the central Cu(I) ion in all complexes is in a distorted tetrahedral coordination environment. All four complexes display the typical metal-to-ligand charge transfer (MLCT) absorption band at 371, 363, 413 and 402 nm, respectively. No emission was observed from any complexes in the solid state due to triplet-triplet annihilation. However, the complexes show unusual dual-emission originating from intraligand charge-transfer (ILCT) and MLCT transitions, when dispersed in a rigid matrix (e.g. PMMA) or in frozen CH(2)Cl(2). The oxidation potential of Cu(I)/Cu(II) in these neutral complexes, ～0.5 V (vs. Ag/AgCl), is lower than those of cationic Cu(I) complexes. Films containing 10 wt% of these complexes in PMMA shows ratiometric fluorescent oxygen gas sensing property with a response ratio of 0.3-3.2 and response time of 3-4 s. Complex 2b acts as a ratiometric oxygen gas sensor with good reversibility through energy and electron transfer mechanisms under the loss of a counteranion.     

Carbazole-linked porphyrin dimers for organic light emitting diodes: synthesis and initial photophysical studies

Carbazole linked porphyrin dimers were synthesized in good yields via stepwise Suzuki coupling reactions using bromoporphyrins and borylated carbazoles as the precursors, the latter of which were synthesized via known procedures from biphenyl derivatives. For comparative purposes porphyrin-carbazole monomers were synthesized. Single layer organic light emitting diodes (OLEDs) were created to demonstrate the optical properties of these materials. Light emission from these carbazole substituted porphyrins showed better results compared to previously examined bromo substituted porphyrins with better electroluminescence and lower turn-on voltages. Dimers exhibited turn-on voltages of 3 V compared to 6 V for monomeric porphyrin-carbazoles.     

Towards microalbuminuria determination on a disposable diagnostic microchip with integrated fluorescence detection based on thin-film organic light emitting diodes

As a first step towards a fully disposable stand-alone diagnostic microchip for determination of urinary human serum albumin (HSA), we report the use of a thin-film organic light emitting diode (OLED) as an excitation source for microscale fluorescence detection. The OLED has a peak emission wavelength of 540 nm, is simple to fabricate on flexible or rigid substrates, and operates at drive voltages below 10 V. In a fluorescence assay, HSA is reacted with Albumin Blue 580, generating a strong emission at 620 nm when excited with the OLED. Filter-less discrimination between excitation light and generated fluorescence is achieved through an orthogonal detection geometry. When the assay is performed in 800 microm deep and 800 microm wide microchannels on a poly(dimethylsiloxane)(PDMS) microchip at flow rates of 20 microL min(-1), HSA concentrations down to 10 mg L(-1) can be detected with a linear range from 10 to 100 mg L(-1). This sensitivity is sufficient for the determination of microalbuminuria (MAU), an increased urinary albumin excretion indicative of renal disease (clinical cut-off levels: 15-40 mg L(-1)).     

Optical sensors for dissolved sulfur dioxide

Colorimetric sensing membranes for the determination of sulfur dioxide were developed and characterized. These films can be used for sensing trace amounts of sulfur dioxide both in the gas phase and in aqueous solutions. Lipophilic pH indicator ion pairs were immobilized in hydrophobic gas-permeable silicone and phenyl substituted ormosil. On exposure to SO₂ the films undergo a visually detectable color change from blue to yellow. No cross-sensitivity to pH and CO₂ was observed. Response times depend on the thickness of the sensing membranes, the indicator concentration in the film as well as on the respective SO₂ concentration. Membranes with response times of < 1 min (t₉₀) were developed. The sensitivity to sulfur dioxide depends on the pK_a of the indicator. An increase in the pK_a results in a lower detection limit. The new optical SO₂ sensors are chemically and mechanically stable and are easy to manufacture. The storage stability of the membranes is at least 7 months if stored in the dark. Received: 17 December 1997 / Revised: 12 June 1998 / Accepted: 15 June 1998     

Spiro linked compounds for use as active materials in organic light emitting diodes

Spiro-linkage of low molecular weight entities as a new structural concept for the design of new active materials for electroluminescent applications is presented. These spiro linked compounds result in nonpolymeric organic glasses with high thermal stability as can be derived from their high glass transition temperatures (T_g), and characterized by differential scanning calorimetry. Blue emitters based on spiro linked oligophenyles are presented. These compounds are soluble in common organic solvents and show high photoluminescence quantum efficiency in the solid state and high morphologic stability with glass transition temperatures up to 250°C. Charge transport materials based on spiro linked versions of 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) for electron transport, and spiro linked versions of triphenyldiamin derivatives (TPD) for hole transport show improved morphologic properties with nearly unchanged electronic properties compared to the parent compounds. High quality amorphous films can be prepared with the spiro compounds by vapor deposition as well as by simple spin coating.     

Polyfluorene containing diphenylquinoline pendants and their applications in organic light emitting diodes

We present a short, efficient synthetic route for the preparation of a novel polyfluorene copolymer (PF‐Q) containing two electron‐deficient, 2,4‐diphenylquinoline groups functionalized at the C‐9 positions of alternate fluorene units that form a three‐dimensional cardostructure. The presence of the rigid bulky pendent groups leads to a polyfluorene possessing a high glass‐transition temperature (207 °C) and very good thermal stability (5% weight loss observed at 460 °C). A photoluminescence study revealed that the Förster energy transfer from the excited quinoline groups to the polyfluorene backbone is very efficient; it also demonstrated that the commonly observed aggregate/excimer formation in polyfluorenes is suppressed very effectively in this polymer, even after it has been annealed at 150 °C for 20 h. A light emitting diode (LED) device prepared with PF‐Q as the emitting layer exhibits a stable blue emission with a maximum brightness of 1121 cd/m² at 12 V and a maximum external quantum efficiency of 0.80% at 250 cd/m². We also used PF‐Q, which contains diphenylquinoline units that behave as electron‐transporting side chains, as a host material and doped it with 2.4 wt % of a red‐emitting phosphorescent dye, Os(fppz), to realize a red electroluminescence with CIE color coordinates of (0.66, 0.34). The doped device exhibits a maximum external quantum efficiency of 6.63% (corresponding a luminance efficiency of 8.71 cd/A) at a current density of 47.8 mA/cm², together with a maximum brightness of 10457 cd/m². © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 859–869, 2005     

Optical sensors for dissolved ionic chemical species

The sensing of ions in various media is a continuing problem that often requires new techniques as existing sensors may be inadequate for a particular example. Chemical sensors based on fibre optics have recently been the subject of considerable interest, as they have a number of advantages over conventional systems. This paper reviews those optical fibre chemical sensors that have been developed for the sensing of ionic chemical species in solution.