Gas-phase and dissolved oxygen

Despite the established sensing approach, in particular for gas phase measurements, extensive studies of optical O2 sensors are still continuing in an effort to enhance sensor performance, reduce sensor cost and size, simplify fabrication, and develop an O2 sensor that is compatible with in vivo biomedical monitoring [56]. Development of field deployable, compact sensors such as those envisioned for the structurally integrated OLED-based platform is therefore expected to be beneficial for the varied needs of gas 

The similarity of the responses and detection sensitivities for water and ethanol, despite the larger solubility of oxygen in ethanol ( 8 times higher at 25°C) [67], may indicate that the oxygen concentration in the PS host, which is in equilibrium with the DO, is comparable for both liquids. We note that the detection sensitivity of DO in water measured with the OLED-based sensors is among the highest reported for PtOEPiPS. 

The OLED-based sensors were tested in the 23-60°C temperature range. The values of t are expected to generally decrease with increasing temperature, as the PL quenching is enhanced at elevated temperatures [47]. However, in the 23-60°C studied range, the phosphorescence of porphyrins is only slightly dependent on the temperature [68]. Indeed, the temperature effect on the SV plots was minimal small reductions in tq and t (100% O2) were observed as the temperature increased, e.g., for one film, tq decreased from 91 to 84 js with Sg varying from 36.5 to 37.5. 

OLED was switched to a pulsed mode, and t (corresponding to the 21% O2 in air) was determined. In this pulsed mode, the pulse amplitude, width, and repetition rate were 20 V, 100 ps, and 50 Hz, respectively. The displayed value of T was determined by averaging the decay curves over 1,000 sweeps. 

As Fig. 3.6 shows, t slowly decreased from 20.2 0.1 to 19.7 0.05 ps during this 30 day test. In other words, the relative error actually decreased with time, from 0.5% to 0.25%. These and later results demonstrated that the lifetime of this sensor module is well beyond the 30 day test. 

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Examples of Analyte Monitoring 16.3.1 Gas-Phase and Dissolved Oxygen... 

Recall that the mass balance equations of Eqs. (1.1a) and (1.1b) incorporate not only terms for internal chemical reactions but also terms for physical mass transport across the boundaries of the control volume. Often, useful control volume boundaries coincide with boundaries between phases, such as between air and water or between water and solid bottom sediment, as discussed for the lake control volume in Section 1.3.1. Note, however, that the terms "environmental media" and "phases" are not interchangeable. For example, chemicals in the gas phase can refer to chemicals present in gaseous form in the atmosphere or in air bubbles in surface waters or in air-filled spaces in the subsurface environment. Chemicals in the aqueous phase are chemicals dissolved in water. Chemicals in the solid phase include chemicals sorbed to solid particles suspended in air or water, chemicals sorbed to soil grains, and solid chemicals themselves. In addition, an immiscible liquid (i.e., a liquid such as oil or gasoline that does not mix freely with water) can occur as its own nonaqueous phase liquid (NAPL, pronounced "napple"). Some examples of mass transport between phases are the dissolution of oxygen from the air into a river (gas phase to aqueous phase), evaporation of solvent from an open can of paint (nonaqueous liquid phase to gas phase), and the release of gases from new synthetic carpet (solid phase to gas phase). Mass transport between phases is affected both by physics and by the properties of the chemical involved. Thus, it is important to imderstand both the types of chemical reactions that are common in the environment, and the relative affinities that various chemicals have for gas, liquid, and solid phases. 

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