Lifetime-based sensors

TABLE 11.2 Calculated Sensor Lifetimes Based on the Fluorophore Photobleaching Rate Constants from the Literature... 

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

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 ... 

Neurauter G, Klimant I, Wolfbeis OS (1999) Microsecond lifetime-based optical carbon dioxide sensor using luminescence resonance energy transfer. Anal Chim Acta 382 67-75... 

Cajlakovic M, Bizzarri A, Ribitsch V (2006) Luminescence lifetime-based carbon dioxide optical sensor for clinical applications. Anal Chim Acta 573-574 57-64... 

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. 

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.)...

Figure 9.6 shows the stability of the lifetime-based system here described over a period of 100 h of continuous operation. The sensor was immersed in saline solution in equilibrium with air at 760 torr. The intensity of the sensor decays with time due to the photodestruction of the porphyrin molecules (photobleaching). However, the lifetime value measured by the instrument remains relatively invariant over this period with a system drift of less than 1%. The instrument adjusts the gain of the programmable amplifier to compensate for the reduction in signal. 

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. 

Of course, if not accompanied by a full understanding of the working principles of these devices, the status of their development, and their potential, the results of this test would be misleading when deciding on the choice of the right type of sensor for further development. It has been shown by later development, as discussed, that the lifetime-based scheme is much to be favored in terms of performance and cost. This was the reason that a major manufacturer, Luxtron, substituted the lifetime-based thermometer for the early, intensity-based one in its commercial production/5 ... 

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... 

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). 

S. B. Bambot, J, Sipior, J. R. Lakowicz, and G. Rao, Lifetime-based optical sensing of pH using resonance energy transfer in sol-gel sensors, Sensors Actuators B, in press. 

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. 

Luminescence Lifetime-Based Imaging of Sensor Arrays for High-Throughput Screening Applications... 

Flow-Based Systems Needle-type sensors with a fluid flowing over the sensor tip seem to resist biofouling and extend sensor lifetime.31 There are numerous methods that have been investigated for flow-based sensors, such as microperfusion systems,75 microdialysis,76 77 and ultrafiltration.78 Reduced fouling was found with an open microflow system where slow flow of protein-free fluid over the sensor surface at the implant site is effected.73 Different from the other flow-based sensors, the open microflow is controlled by the subcutaneous tissue hydrostatic pressure and does not require a pump. 

Tolosa L, Gryczynski I, Eichhom LR, Dattelbaum JD, Castellano FN, Rao G, Lakowicz JR. Glucose sensor for low-cost lifetime-based sensing using a genetically engineered protein. Analytical Biochemistry 1999, 267, 114—120. 

Problems such as peeling of the polymer films and leaching of the membranes need to be overcome to increase sensor lifetime. New membranes such as those based on siloxanes show possible increases in performance and stability. 

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