Lifetime oxygen sensors

In phase-fluorimetric oxygen sensors, active elements are excited with periodically modulated light, and changes in fluorescence phase characteristics are measured. The delay or emission (phase shift, ( ), measured in degrees angle) relates to the lifetime of the dye (x) and oxygen concentration as follows ... 

Figure 9.4. Stem-Volmer plot of a fiberoptic oxygen sensor at different temperatures. , 43°C a, 37°C , 31°C , 25°C. The sensing capabilities of the fiberoptic sensor are limited by diffusion processes as suggested by the decreasing value of the lifetimes with increasing temperature. (From Ref. 21 with permission.)...

Significant curvature may be observed in the case of lifetime- (and intensity-) based sensors, mainly when the relation knri [Parameter]) is not linear. Figure 9.4 shows this type of nonlinear behavior for a fiberoptic oxygen sensor. The figure shows Stern-Volmer-type plots (r l versus [02]) at four different temperatures. The curvature is caused by the inability of the carrier to transport oxygen proportionally to the equilibrium partial pressure of oxygen. 

Figure 9.6. Stability of a lifetime-based fiberoptic oxygen sensor over a period of 100 h of continuous operation. Lifetime techniques are insensitive to the process of photobleaching only in the absence of excited state reactions. Excited state reactions of the sensor-carrier system cause drifts in the observed lifetime with photobleaching. They are avoided by limiting the concentration of the sensor in the carrier. (From Ref. 21 with permission.)...

Platinum and palladium porphyrins in silicon rubber resins are typical oxygen sensors and carriers, respectively. An analysis of the characteristics of these types of polymer films to sense oxygen is given in Ref. 34. For the sake of simplicity the luminescence decay of most phosphorescence sensors may be fitted to a double exponential function. The first component gives the excited state lifetime of the sensor phosphorescence while the second component, with a zero lifetime, yields the excitation backscatter seen by the detector. The excitation backscatter is usually about three orders of magnitude more intense in small optical fibers (100 than the sensor luminescence. The use of interference filters reduce the excitation substantially but does not eliminate it. The sine and cosine Fourier transforms of/(f) yield the following results ... 

Table 10.3. Mean Lifetimes (r) in Solvents Purged by Nitrogen (N2), Air, and Oxygen (O2) and Sensing Parameters (Changes in Phase Angle, AO, and in Modulation, Am, "Air - N2 ) of Potential Oxygen Sensors 33 ...

Figure 10.10. Frequency responses of intensity decays of possible oxygen sensors with various lifetimes. The arrows indicate the magnitude of changes in phase and modulation in the range from 0% oxygen (N2, deaerated solution) to about 20% oxygen (Air, equilibrium with air).

The overall oxygen sensitivity exhibited by an optical sensor is basically predefined by the Stern-Volmer constant Ksv. The sensitivity of the final optical oxygen sensor increases with Ksv [65]. Generally, high Ksv values are provided by the Pd- and Pt-porphyrin complexes, by Ru(dpp)3, and by pyrene. Fluorescence quenching by oxygen not only affects the fluorescence intensity of the dye, but also has an influence on its lifetime r (Fig. 6) ... 

Demas et al. described optical oxygen sensors using analogous osmium(II) complexes that have intense red absorptions and that can be excited with low-cost, high-intensity red diode lasers [25]. The osmium(II) complexes are probably more photochemically robust than ruthenium(II) complexes because of the larger energy gap between emitting state and the photochemically destructive upper d-d state. In Table 2, the photochemical and photophysical properties of osmium(II) tris(l,4-diphenyl-l,10-phenanthroline) (Os(dpp)3+) and osmium(II) tris(l,10-phenanthroline) (Os(phen)3+) are indicated as examples of osmium(II) complexes. The luminescence lifetimes of Os(dpp)3+ and Os(phen)3+ are 4.6 and 6.0 ns in dichloromethane solution,... 

The intense and long-lived luminescence of these complexes has been utilized in the development of the oxygen sensor. Four mononuclear and dinuclear cyclometalated iridium(III) diimine complexes were immobilized in polymerized poly(ethylene glycol) ethyl ether methacrylate matrices, and the oxygen quenching on the luminescence of the films was studied. Tinear Stem-Vohner plots were obtained. Despite the difference in the luminescence energy and lifetimes of the complexes, similar Stem-Volmer slopes were observed. The size and charge of the complexes played an important role in the sensitivity of the systems. 

A fiber-optic oxygen sensor with the fluorescence decay time (rather than its intensity) as the information carrier has been described by two groups [119, 120]. In the former work, a ruthenium complex is immobilized in silicone-rubber, and quenching by oxygen is measured by either lifetime or intensity measurements. The 337-nm line of a nitrogen laser served as the excitation line, and the dye was dissolved in a silicone-rubber membrane placed in the fluorimeter. This sensing membrane is reported to be highly specific, and chlorine and sulfur dioxide were the only interferents. 

Figure 17-lStem-Volmer plots of the quenching of a silicone-entrapped ruthenium complex by oxygen. Upper curve based on intensity Iq/I easuiements, lower curve based on lifetimes Zg/t [120]. Note the distinctly better linearity of the lifetime-based sensor at oxygen levels up to 30%. 

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