Ultrasensitive NIR-II Surface-Enhanced Resonance Raman Scattering Nanoprobes with Nonlinear Photothermal Effect for Optimized Phototheranostics.

Surface-enhanced resonance Raman scattering (SERRS) in the second near-infrared (NIR-II) window has great potential for improved phototheranostics, but lacks nonfluorescent, resonant and high-affinity Raman dyes. Herein, it is designed and synthesize a multi-sulfur Raman reporter, NF1064, whose maximum absorption of 1064 nm rigidly resonates with NIR-II excitation laser while possessing absolutely nonfluorescent backgrounds. Ultrafast spectroscopy suggests that the fluorescence quenching mechanism of NF1064 originates from twisted intramolecular charge transfer (TICT) in the excited state. Gold nanorods (AuNRs) decorated with such nonfluorescent NF1064 (AuNR@NF1064) show remarkable SERRS performances, including zero-fluorescence background, femtomolar-level sensitivity as well as superb photostability without fluorescence photobleaching. More importantly, AuNR@NF1064 exhibits a nonlinear photothermal effect upon plasmonic fields of AuNRs by amplifying the non-radiative decay of nonfluorescent NF1064, thus achieving a high photothermal conversion of 68.5% in NIR-II window with potential for further augmentation. With remarkable SERRS and photothermal properties, the NIR-II nanoprobes allow for high-precision intraoperative guided tumor resection within 8 min, and high-efficient hyperthermia combating of drug-resistant bacterial infection within living mouse body. This work not only unlocks the potential of nonfluorescent resonant dyes for NIR-II Raman imaging, but also opens up a new method for boosting photothermal conversion efficiency of nanomaterials.