Cholesterol and Albumin as Key Modulators of ICG Photostability in Aqueous Solution

Indocyanine green (ICG) is a clinically approved near-infrared fluorescent dye used in medical imaging and diagnostics but has limitations owing to its poor photostability in aqueous environments. This paper has explored the role of human serum albumin (HSA) and cholesterol in protecting ICG photostability. Pure ICG, an HSA-ICG complex, and an ICG-cholesterol colloidal assembly solution were stirred in the dark (control) and under continuous broadband irradiation (400-1600 nm, approximately 1.4 W), and absorption spectra (550-950 nm) were taken every 1 min, over 15 min. All the formulations were stable in the dark, with minimal total variance in absorbance. Pure ICG significantly photodegraded under irradiation (55 ± 2.75-65 ± 3.25% loss of maximum absorbance). The introduction of HSA and cholesterol limited the photodegradation, resulting in 15 ± 0.75-30 ± 1.5% and 25 ± 1.25% losses in maximum absorbance, respectively, upon irradiation. The modulators produced a significant increment in initial NIR absorbance (p < 0.001) and retained significantly high stability during irradiation (p < 0.01). Moreover, both modulators reduced photooxidative damage, as shown by the lower level of singlet oxygen (¹O₂) generation in the presence of HSA-ICG (35 ± 1.75%) and ICG-cholesterol (19 ± 0.95%) compared to pure ICG (57 ± 2.85% after 15 min). These results reveal that cholesterol is the best stabilizer of ICG photostability. By safely dissipating excitation energy via non-radiative decay, cholesterol demonstrates strong potential for enhancing ICG performance in photothermal therapy (PTT), whereas HSA remains the optimal modulator for near-infrared fluorescence imaging and photodynamic therapy.