Free Bubble-Assisted Interfacial Assembly of Amphiphilic NIR-II Small Molecules with Enhanced Fluorescence Brightness

The second near-infrared window (NIR-II, 1000-1700 nm) holds promise for in vivo bioimaging owing to its inherent advantages such as minimized background autofluorescence, reduced light scattering, and deep tissue penetration. Donor-acceptor-donor (D-A-D)-type small molecules have emerged as promising NIR-II fluorescent probes. However, their practical application is severely hindered by low molar extinction coefficients, aggregation-caused quenching, and twisted intramolecular charge-transfer (TICT) effects. Herein, a novel free-bubble-assisted assembly strategy to enhance the fluorescence brightness of NIR-II small molecules is proposed. Three NIR-II fluorescent molecules (TF, TF-N+, and TF-OEGMA) with tunable hydrophilicity-hydrophobicity were rationally designed and synthesized. Through simple co-incubation with free nanobubbles (NBs), these molecules were successfully assembled into stable fluorescent nanobubble complexes (TF NBs, TF-N+ NBs, and TF-OEGMA NBs). Systematic characterizations revealed that hydrophilic TF-N+ and amphiphilic TF-OEGMA exhibit superior binding capability and stability at the bubble interface compared with hydrophobic TF. Notably, TF-OEGMA NBs display significantly enhanced NIR-II fluorescence intensity and imaging contrast relative to free TF-OEGMA, which may be attributed to the unique bubble microenvironment that effectively suppressed molecular aggregation and TICT. This work provides a new paradigm for constructing high-performance NIR-II fluorescent probes through bubble-assisted interfacial assembly, offering great potential for clinical applications in precise disease diagnosis and therapeutic monitoring.