Thermodynamic properties and photodegradation kinetics of indocyanine green in aqueous solution

Aqueous solutions of indocyanine green (ICG) are spectrophotometrically and fluorimetrically studied as a function of the dye concentration. The integral absorbance concept is introduced to minimize uncertainties due to the use of only a point. This new method, provides a value of the equilibrium constant consistent with those obtained, by a different method, for safranine and thionine. The influence of ICG concentration on its fluorescence spectrum is determined. When ICG is exposed to white light degrades through the formation of free radicals. The overall kinetics of degradation is monitored spectrophotometrically and studied in an unconventional way. The results exhibit a power law, regardless of the initial concentration of dye. An apparent order of 1.5 suggests a complex mechanism where the collision between radicals is very important whilst a lifetime of 4 h it is indicative of a rapid degradation kinetics.