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It is known that singlet oxygen (1O2is the main factor mediating cytotoxicity in photodynamic therapy (PDT). The effectiveness of a PDT treatment is directly linked to the 1O2 produced in the target. Although the luminescence from 1O2 is suggested as an indicator for evaluating photodynamic therapy, the inherent disadvantages limit its potential for in vivo applications. We have previously reported that chemiluminescence can be used to detect1O2 production in PDT and have linked the signal to the cytotoxicity. We further our investigation for monitoring 1O2 production during PDT. The lifetime of 3,7-dihydro-6-{4-[2-(N′-(5-fluoresceinyl)thioureido)ethoxy]phenyl}-2-methylimidazo {1,2-a} pyrazin-3-one-chemiluminescence (FCLA-CL) is evaluated, and the results show that it is much longer than that of direct luminescence of 1O2. A gated measurement algorithm is developed to fully utilize the longer lifetime for a clean measurement of the CL without the interference from the irradiation light. The results show that it is practically feasible to use the technique to monitor the 1O2. Compared to the direct 1O2 luminescence measurement, our new technique is sensitive and can be realized with a conventional optical detector with excellent signal-to-noise ratio. It thus provides a means for real-time in vivo monitoring of 1O2 production during PDT.
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