Decay time sensing:

To illustrate the use of time-resolved data for sensing, assume that each form (free and bound) has a unique decay time, rF and rb- The intensity decay of a mixture is a double-exponential... 

Photodiode detectors have already been cited in this chapter in relation to near-IR fluorescence measurements on singlet oxygen,(8 16 18) in decay-time temperature sensing,(50) in liquid chromatography,(62) the study of proteins labelled with Nile Red,(64) and diode laser spectrometry,(67) Photodiodes are also conveniently packaged for many applications in an array form enabling rapid data acquisition e.g., in spectrophotometry, (35)... 

Isotopes can be divided into two fundamental kinds, stable and unstable (radioactive) species. The number of stable isotopes is about 300 whilst over 1,200 unstable ones have been discovered so far. The term stable is relative, depending on the detection limits of radioactive decay times, hi the range of atomic numbers from 1 (H) to 83 (Bi), stable nuclides of all masses except 5 and 8 are known. Only 21 elements are pure elements, in the sense that they have only one stable isotope. AU other elements are mixtures of at least two isotopes. The relative abundance of different isotopes of an element may vary substantially. In copper, for example, Cu accounts for 69% and Cu for 31% of all copper nuclei. For the light elements, however, one isotope is predominant, the others being present only in trace amounts. 

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. 

Fig. 3.22. Structurally integrated OLED-based multianalyte sensor for oxygen, glucose, alcohol, and lactate. All of the sensing elements were based on a PSiPtOEP film, and the analytes were monitored via the PL decay time r of the PtOEP. The figure shows the intensity as a function of time (black lines) and the exponential fit (white lines). Measurements were conducted in air at 23°C...

One common approach to fluorescence sensing is to rely on fluorophores which are collisionally quenched by the analyte. There are many known collisional quenchers (andytes) which alter the fluorescence intensity and decay time. These include O2 (27-31), chloride (32-33), chlorinated hydrocarbons (34), iodide (35), bromate (36), xenon (37), acrylamide (38), succinimide (39), sulfur dioxide (40), and halothane (41), to name a few. The quenching process obeys the Stem-Volmer equation ... 

It seems valuable to speculate on the potential future applications of lifetime-based sensing. In our opinion, the dominant advantages of lifetime-based sensing are that the measurements can be independent of the local probe concentration, and the decay time measurements are mostly self-calibrating. The insensitivity of the lifetime to the local probe concentration enables the use of lifetime-based sensing in flow cytometry (iiO), fluorescence microscopy 25,88,111-112, Lakowicz, J.R. et al. Cell Calcium, in press), and clinicsd chemistry 25,65-66,113, Lakowicz, J.R. et al. SPIE Proceedings 1993, Vol.1895, in press)... 

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