Unravelling the Tunable Near-Infrared-II Fluorescence from FDA-Approved Indocyanine Green via Supramolecular Complexation with Albumin

Indocyanine green (ICG) is a clinically approved near-infrared (NIR) dye widely used in angiography and tumor imaging, yet its emission is primarily restricted to the NIR-I window. Although protein binding has been reported to enhance its NIR-II fluorescence, the supramolecular basis of this behavior remains insufficiently understood. Here, we identify two reversible binding modes with serum albumin: an emissive monomer-associated type I complex and a quenched multivalent type II complex, governed by the ICG-to-albumin stoichiometric ratio. Spectroscopic analyses and gel electrophoresis validate these interconvertible states and demonstrate their tunability under physiologically relevant conditions. This stoichiometry-controlled switching framework clarifies the concentration-dependent optical behavior of ICG and provides practical guidance for optimizing NIR-II imaging performance in clinical applications.