Fluorescence lifetime based sensors

Even though sensors based on the measurement of fluorescence intensity are the most numerous, the high-intensity light sources employed accelerate the possible photodecomposition of the active molecules. Fluorescence-lifetime-based sensors avoid this problem by the excitation of molecules with light pulses. 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

Lifetime [3,9-11] based sensors rely on the determination of decay time of the fluorescence or phosphorescence. Typically, the fluorescence lifetime is 2-20 ps and phosphorescence lifetime is 1 ps to 10 s. Lifetime-based sensors utilize the fact that analytes influence the lifetime of the fluorophore. Thus all dynamic quenchers of luminescence or suitable quenchers can be assayed this way. The relationship between lifetimes in the absence (t0) and presence (t) of a quencher is given by Stern and Volmer ... 

J. R. Lakowicz, H. Szmacinski and K. W. Berndt, Fluorescence lifetime-based sensing of blood gases and cations, in Fiber Optic Medical and Fluorescent Sensors and Applications (J. R. Lakowicz, ed.), Proc. SPIE. 1648, 150-163 (1992). 

Szmacinski, H. and J.R. Lakowicz, Fluorescence lifetime-based sensing and imaging. Sensors Actuators B-Chemical, 1995(1-3) p. 16-24. 

What mechanisms can be used to create a lifetime-based glucose sensor In our opinion, the mechanism should be fluorescence resonance energy transfer (FRET). The phenomenon of FRET results in transfer of the excitation from a donor fluorophore to an acceptor chromophore, which need not itself be fluorescent. FRET is a through-space interactor which occurs over distances of 20-60 A. 

R. B. Thompson and J. R. Lakowicz, Fiber optic pH sensor based on phase fluorescence lifetimes, Anal. Chem. 65, 853-856 (1993). 

Z. Zhang, K. T. V. Grattan, and A.W. Palmer, Fiber-optic high-temperature sensor based on the fluorescence lifetime of alexandrite, Rev. Sci. Instrum. 63, 3869-3873 (1992). 

Finally, we note that future instrument for lifetime-based sensing and imaging can be based on laser diode light sources. At present it is desirable to develop specific probes which can be excited from 630 to 780 nm, the usual range of laser diodes. The use of such probes will allow us to avoid the use of complex laser sources, which should result in the expanded use of fluorescence detection in the chemical and biomedical sensors. 

Sensors based on the fluorescence quenching ofrhodamine 6G in resins by iodide ions(43) and in Nafion polymer by metal ions in solution 44,45) have been demonstrated. However, complex fluorescence decay mechanisms often hinder interpretation in lifetime-based sensing and much progress is still to be made in this area before the true potential of lifetime-based sensing becomes a reality. For example, rhodamine 6G in... 

Emission lifetime provides a powerful selectivity dimension as each fluorophore has a unique decay rate. While laboratory fluorescent lifetime measurement systems are widely available and the concept of lifetime LIF sensing has been demonstrated,commercial LIF lifetime sensors are not yet available on the market for PAT and field applications to monitor organic analytes or bioagents. Once available, lifetime LIF sensors could revolutionize fluorescent based monitoring by affording superior detection merits along with sufficient recognition power. 

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