Fluorescence of Indocyanine Green in Blood: Intensity Dependence on Concentration and Stabilization with Sodium Polyaspartate

Indocyanine green (ICG) has been widely used in cardiovascular, hepatic, and ophthalmologic studies. Application of this fluorescent dye has been handicapped by its poor stability in solution and by the complex dependence of its fluorescence intensity on concentration. Noncovalent interactions between ICG and sodium polyaspartate (PASP) stabilize ICG fluorescence in aqueous solution, but the effect of PASP on ICG fluorescence in blood has not been described. The current study had two main goals: first, to characterize in vitro in blood the relationship between fluorescence intensity and concentration of ICG-PASP (ICG) and the stability of this relationship over time; second, to test a new phenomenological model describing the dependence of ICG fluorescence on concentration. Freshly-prepared ICG and ICG-PASP solutions produced the same fluorescence intensity over a wide range of concentrations (0.0005-0.1271 mg/ml). The peak fluorescence of ICG was reduced by 11% after 10 h and by 72% at 7 days. In contrast, the peak fluorescence intensity of ICG-PASP solutions was nearly unchanged for up to 14 days. The dependence of the fluorescence intensity on concentration was accurately represented by our model that accounted for the generation of fluorescence following light absorption, and for the reabsorption of the emitted fluorescence by ICG.