Optical oxygen sensor instrumentation based on the detection of luminescence lifetime.

Optical oxygen sensors are mainly based on the principle of luminescence quenching. In contrast to arready existing intensity-based systems, the measurement of the luminescence lifetime provides certain advantages, such as insensitivity to photobleaching or leaching of the dye, or changes in the intensity of excitation light. This facilitates the use of simple optical systems or optical fibres. A new family of oxygen-sensitive dyes, the porphyrin-ketones, has been introduced, which exhibits favorable spectral properties and decay times in the order of tens and hundreds of microseconds. This allows the use of simple optoelectronic circuitry and low-cost processing electronics. An optical oxygen sensor module has been developed with the dimensions of only 120 x 60 x 30 mm. The prototype is based on the measurement of the decay time of the luminophore by measuring the phase shift between the square-wave excitation and the detected square-wave of the emission coming from the sensor. The instrument is based on semiconductor devices (light-emitting diodes, photodiodes) and may be used for the detection of oxygen in gaseous or liquid samples. The measurement range of the device is from 0 to 200 hPa oxygen partial pressure with a resolution of < 1 hPa over the whole measurement range. The overall measurement accuracy of < +/- 1 hPa has been obtained for periods of 24 h of continuous measurement in a thermostatted environment. The sensor response times t90 are typically < 1 s for gases and 0.5 to 5 min for liquid samples.

New instrumentation for optical measuring of oxygen in gas or dissolved in liquids.

The optical oxygen sensor is a novel device for the determination of oxygen in gases or dissolved in liquids. It is based on the measurement principle of fluorescence quenching, which is completely different from that of polarographic oxygen sensors (today the most widespread devices of oxygen detection). The new instrument offers features and advantages, which render it not only a realistic alternative, but, for specific applications, make it superior to existing electrochemical methods. The system is based on low-cost semiconductor devices (light-emitting diodes, photodiodes, low-cost analogue and digital components) and new LED-compatible oxygen-sensitive membranes. The flow cell of the instrument may be thermostatted and the sensor can be calibrated by a simple two-point calibration procedure. The optical oxygen sensor is particularly suitable for measuring dissolved oxygen in respirometry, since no oxygen is consumed by the device and the signal is independent of sample flowrate or stirring speed. Typical fields of application are monitoring of oxygen in ground and drinking water, in process control in bioreactors and in breath gas and blood gas analysis.

Effects of polymer matrices on calibration functions of luminescent oxygen sensors based on porphyrin ketone complexes.

The design of luminescent oxygen sensors is guided by optimizing sensitivity and/or the form of the calibration function. Both qualities are governed by the molecular processes of luminescence quenching. To evaluate the influence of matrix effects, we prepared membranes based on oxygen-sensitive phosphorescent complexes of porphyrin ketones dissolved in plasticizer-free poly(vinyl chloride) (PVC) and polystyrene (PS). In a PVC matrix, both platinum(II) and palladium(II) octaethylporphyrin ketones exhibited perfectly linear Stern-Volmer intensity plots and almost single-exponential excited state decays. In a PS matrix, the sensitivity of palladium(II) octaethylporphyrin ketone was among the highest reported to date. Yet, slightly nonlinear Stern-Volmer plots and nonexponential decays illustrate the significance of matrix effects of PS. Addition of plasticizers to PVC-based sensors allowed tuning of the oxygen sensitivity in a wide range, while the Stern-Volmer plots became pronouncedly nonlinear. For the plasticizer bis(2-ethylhexyl) adipate, the decay profile was single-exponential in the absence but nonexponential in the presence of oxygen, which is attributed to a distribution of quenching rate constants.

Optical triple sensor for measuring pH, oxygen and carbon dioxide.

A triple sensor unit consisting of opto-chemical sensors for measurement of pH, oxygen and carbon dioxide in bioreactors is presented. The pH and the CO2 sensor are based on the color change of a pH-sensitive dye immobilized on a polymeric support. The resulting changes in absorption are monitored through optical fibers. The oxygen sensor is based on the quenching of the fluorescence of a metal-organic dye. All three sensors are fully LED compatible. The sensitive membranes consist of plastic films and can be stored and replaced conveniently. The sensors are sterilizable with hydrogen peroxide and ethanol. In addition, the pH sensor is steam sterilizable. Accuracy, resolution and reproducibility fulfill the requirements for use in biotechnological applications. Calibration procedures for each sensor are presented. The working principle and the performance of all three sensors are described, with particular emphasis given to their application in bioreactors.

A fiberoptic cholesterol biosensor with an oxygen optrode as the transducer.

A biosensor for the continuous optical determination of cholesterol is presented. Cholesterol oxidase is immobilized covalently on a nylon membrane and the consumption of oxygen is measured by following, via fiberoptic bundles, the changes in fluorescence of an oxygen-sensitive dye whose fluorescence is dynamically quenched by molecular oxygen. The dye is dissolved in a very thin silicone membrane placed beneath the enzyme layer. During interaction of the enzyme with cholesterol, oxygen is consumed, which is indicated by the fluorescent dye. At pH 7.25, the analytical range of the sensor is 0.2 to 3 mM and the time to reach a full steady state in a flowing solution ranges from 7 to 12 min.