Changes in absorption spectra of indocyanine green after visible light exposure and cold dark storage

Indocyanine green (ICG) is commonly used as a fluorescent cardiovascular imaging agent, but its stability under different conditions remains insufficiently studied. This research examines the photochemical stability and aggregation behaviour of ICG in aqueous solutions subjected to visible light irradiation and during cold, dark storage. ICG solutions at different concentrations were exposed to visible light, and changes in absorption spectra were monitored over time. Exponential decreases in the absorption maxima were observed for both monomeric and dimeric ICG during irradiation. These changes were modelled, allowing for the description of decay processes caused by irradiation for monomers and dimmers, respectively. Furthermore, after a month of dark storage at 4 °C, a decrease in absorption was observed, accompanied by the formation of J-aggregates. These results indicate that ICG undergoes significant structural changes that may affect its fluorescence properties, with potential applications for use in photodynamic therapy. More research is needed to explore the impact of these changes on the performance of ICG in vivo.