Counterion‐Paired Bright Heptamethine Fluorophores with NIR‐II Excitation and Emission Enable Multiplexed Biomedical Imaging

Abstract

Photon excitation and emission at the NIR‐II spectral window enable high‐contrast deep‐tissue bioimaging. However, multiplexed imaging with NIR‐II excitation and emission has been hampered by the limited chemical strategies to develop bright fluorophores with tunable absorption in this spectral regime. Herein, we developed a series of heptamethine cyanines (HCs) with varied absorption/emission maxima spanning from 1100 to 1600 nm through a physical organic approach. A bulky counterion paired to HCs was found to elicit substantial improvements in absorptivity (7‐fold), brightness (14‐fold), and spectral profiles in water, addressing a notorious quenching problem of NIR‐II cyanines due to aggregation and polarization. We demonstrated the utilities of HC1222 and HC1342 for high‐contrast dual‐color imaging of circulatory system, lymphatic structures, tumor, and organ function in living mice under 1120 nm and 1319 nm excitation, showing HCs as a promising platform for non‐invasive bioimaging.

Photophysics of fluorescent nanoparticles based on organic dyes – challenges and design principles

“…[132][133][134] NPs based on NIR dyes are however still challenging and oen show low quantum yields. 135,136 Typical NIR dyes like cyanines present challenges stemming from their large p-surfaces and tendency for aggregation, large oscillator strength that also favours strong coupling, their sensitivity to quenching by water and generally very fast non-radiative deactivation rates as dictated by the energy gap law. 48,137 Strategies that will address these challenges and provide bright tuneable NIR-I and NIR-II NPs will be highly valuable.…”