Controllable Self-Assembly of Amphiphilic Heptamethine Cyanines for Turn-On Fluorescence-Guided Tumor Phototherapy

Heptamethine cyanines are attractive NIR fluorophores for image-guided tumor phototherapy. However, most cyanines exhibit unignorably high background fluorescence, which would lead to low-contrast images in the tumor. Inspired by findings that proper modulations of cyanines can self-quench fluorescence in the aggregated state but retrieve fluorescence when they are switched to the monomer, we herein engineer four amphiphilic heptamethine cyanines (IR780-NC_<i>n</i>, <i>n</i> = 3/6/12/18) with extended hydrophobic alkyl chains. By studying the optimal properties and morphology of the dyes, we finally optimize IR780-NC₁₂ and IR780-NC₁₈ which can form nonfluorescent nanoaggregates in PBS but disassemble and recover intense NIR emission in hydrophobic media. Upon cellular uptake, the amphiphilic cyanines show fluorescence in membranous organelles such as mitochondria (IR780-NC₃ and IR780-NC₆) or the cell membrane (IR780-NC₁₂ and IR780-NC₁₈). Also, hydrophobic shielding enhances ROS generation under laser irradiation, as observed in IR780-NC₁₂ and IR780-NC₁₈. <i>In vivo</i>, IR780-NC₁₈ administration (i.v.) exhibits the brightest and most prolonged tumor fluorescence up to 96 h. Guided by the NIR images in the tumor, an efficient PTT/PDT antitumor effect is confirmed for IR780-NC₁₂ and IR780-NC₁₈ phototherapy. We believe that self-assembly tuning design is a simple and powerful strategy to develop fluorescent "turn-on" cyanines as high-contrast NIR theranostic agents to guide tumor phototherapy.