Optical excitation of chemicals provides reactive excited states, which can initiate chemical reactions and can alsorelax via radiative photophysical processes, providing the basis for fluorescence diagnostics. The best-knownexample of the former is Photodynamic Therapy (PDT), which is now approved for the treatment of a number ofneoplastic and non-neoplastic pathologies. Although the concept of the use of photodynamic agents in diagnostics isas old as their use in therapy, the focused development of this aspect has been relatively recent. Typically,photodynamic agents have high triplet yields and relatively long triplet lifetimes (microsecond range), which allowsthem to interact and destroy molecular targets near them either directly or indirectly by producing other toxicmolecular species. Associated with a high triplet yield is the fortunate attribute of most PDT agents in having lowbut finite fluorescence quantum yields. Fluorescence from these molecules may be used not only for diagnostics ofdisease de novo but also for guided surgery, PDT dosimetry and therapeutic monitoring. Other uses of fluorescencein PDT (not necessarily from the PDT agents) include the development of technologies that allow tracking of cellsduring treatment in vivo, studies of sub-cellular localization of molecules for mechanistic studies and photosensitizertracking for specific targeting. An overview of studies on these aspects from different laboratories will be presented.
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