The development of efficient and selective luminescent probes for reactive oxygen species, particularly for singlet molecular oxygen, is currently of great importance. In this study, the photochemical behavior of Singlet Oxygen Sensor Green(®) (SOSG), a commercially available fluorescent probe for singlet oxygen, was examined. Despite published claims to the contrary, the data presented herein indicate that SOSG can, in fact, be incorporated into a living mammalian cell. However, for a number of reasons, caution must be exercised when using SOSG. First, it is shown that the immediate product of the reaction between SOSG and singlet oxygen is, itself, an efficient singlet oxygen photosensitizer. Second, SOSG appears to efficiently bind to proteins which, in turn, can influence uptake by a cell as well as behavior in the cell. As such, incorrect use of SOSG can yield misleading data on yields of photosensitized singlet oxygen production, and can also lead to photooxygenation-dependent adverse effects in the system being investigated.

Singlet oxygen, O(2)(a(1)Δ(g)), was produced upon pulsed-laser irradiation of an intracellular photosensitizer and detected by its 1275 nm O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence in time-resolved experiments using (1) individual mammalian cells on the stage of a microscope and (2) suspensions of mammalian cells in a 1 cm cuvette. Data were recorded using hydrophilic and, independently, hydrophobic sensitizers. The microscope-based single cell results are consistent with a model in which the behavior of singlet oxygen reflects the environment in which it is produced; nevertheless, the data also indicate that a significant fraction of a given singlet oxygen population readily crosses barriers between phase-separated intracellular domains. The singlet oxygen phosphorescence signals reflect the effects of singlet-oxygen-mediated damage on cell components which, at the limit, mean that data were collected from dead cells and, in some cases, reflect contributions from both intracellular and extracellular populations of singlet oxygen. Despite the irradiation-induced changes in the environment to which singlet oxygen is exposed, the "inherent" intracellular lifetime of singlet oxygen does not appear to change appreciably as the cell progresses toward death. The results obtained from cell suspensions reflect key features that differentiate cell ensemble from single cell experiments (e.g., the ensemble experiment is more susceptible to the effects of sensitizer that has leaked out of the cell). Overall, the data clearly indicate that measuring the intracellular lifetime of singlet oxygen in a O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence experiment is a challenging endeavor that involves working with a dynamic system that is perturbed during the measurement. The most important aspect of this study is that it establishes a useful framework through which future singlet oxygen data from cells can be interpreted.

Singlet molecular oxygen, O(2)(a(1)Delta(g)), can be created in photosensitized experiments with sub-cellular spatial resolution in a single cell. This cytotoxic species can subsequently be detected by its 1270 nm phosphorescence (a(1)Delta(g)--> X(3)Sigma). Cellular responses to the creation of singlet oxygen can be monitored using viability assays. Time- and spatially-resolved optical measurements of both singlet oxygen and its precursor, the excited state sensitizer, reflect the complex and dynamic morphology of the cell. These experiments help elucidate photoinduced, oxygen-dependent events that compromise cell function and ultimately lead to cell death. In this perspective, recent work on the photosensitized production and detection of singlet oxygen in single cells is summarized, highlighting the advantages and current limitations of this unique experimental approach to study an old problem.

The response of a given cell to spatially-resolved sub-cellular irradiation of a singlet oxygen photosensitizer (protoporphyrin IX, PpIX) using a focused laser was assessed. In these experiments, incident light was scattered over a volume greater than that defined by the dimensions of the laser beam as a consequence of the inherent inhomogeneity of the cell. Upon irradiation at a wavelength readily absorbed by PpIX in a one-photon transition, this scattering of light eliminated any advantage accrued to the use of focused irradiation. However, upon irradiation at a longer wavelength where PpIX can only absorb light under non-linear two-photon conditions, meaningful intracellular resolution was achieved in the small spatial domain where the light intensity was high enough for absorption to occur.

The unique optical properties of squaraines render these molecules useful for applications that range from xerography to photodynamic therapy. In this regard, squaraines derived from the condensation of nitrogen-based heterocycles and squaric acid have many promising attributes. Key solution-phase photophysical properties of six such squaraines have been characterized in this study. One feature of these molecules is a pronounced absorption band in the region approximately 600-720 nm that has significant spectral overlap with the fluorescence band (i.e., the Stokes shift is small). As such, effects of emission/reabsorption yield unique excitation wavelength dependent phenomena that are manifested in quantum yields of both fluorescence and sensitized singlet oxygen production. Comparatively small squaraine-sensitized yields of singlet oxygen production and, independently, large rate constants for squaraine-induced deactivation of singlet oxygen are consistent with a model in which there is appreciable intra- and intermolecular charge-transfer in the squaraine and squaraine-oxygen encounter complex, respectively. The results reported herein should be useful in the further development of these compounds for a range of oxygen-dependent applications.