Abstract
Bright anti-Stokes fluorescence (ASF) in the first near-infrared spectral region (NIR-I, 800 nm–900 nm) under the excitation of a 915 nm continuous wave (CW) laser, is observed in Indocyanine Green (ICG), a dye approved by the Food and Drug Administration for clinical use. The dependence of fluorescence intensity on excitation light power and temperature, together with fluorescence lifetime measurement, establish this ASF to be originated from absorption from a thermally excited vibrational level (hot-band absorption), as shown in our experiments, which is stronger than the upconversion fluorescence from widely-used rare-earth ion doped nanoparticles. To test the utility of this ASF NIR-I probe for advanced bioimaging, we successively apply it for biothermal sensing, cerebral blood vessel tomography and blood stream velocimetry. Moreover, in combination with L1057 nanoparticles, which absorb the ASF of ICG and emit beyond 1100 nm, these two probes generate multi-mode images in two fluorescent channels under the excitation of a single 915 nm CW laser. One channel is used to monitor two overlapping organs, urinary system & blood vessel of a live mouse, while the other shows urinary system only. Using in intraoperative real-time monitoring, such multi-mode imaging method can be beneficial for visual guiding in anatomy of the urinary system to avoid any accidental injury to the surrounding blood vessels during surgery.
Highlights
Introduction AntiStokes luminescence is an optical process, wherein the absorption of long-wavelength photons produces short-wavelength emission light
There are four processes that can produce anti-Stokes fluorescence (ASF) - (i) direct multiphoton absorption (MPA) process, (ii) upconversion process based on multistep absorption through intermediate energy levels, (iii) thermally activated delayed fluorescence (TADF) process, and (iv) hotband absorption (HBA) process[1,2]
Excitation power dependence, temperature dependence, and lifetime measurement were carried out to establish that this ASF is originated from HBA, and biothermal sensing imaging was achieved. We show that this ASF is much stronger than typical upconversion fluorescence in rare-earth ion doped nanoparticles (UCNPs) excited at 980 nm, with negligible thermal damage to biological tissues
Summary
No fluorescence signal could be detected in blood vessels and the biliary tract of the rat intravenously injected with NaYF4: Yb3+, Tm3+ (Fig. 3e). In channel 2, only ureters are visualized without crosstalk of fluorescence signals in blood vessels Another impressive example of multi-mode imaging application is shown, right two columns, when ICG intravenously and L1057 NPs retrogradely to bladder were injected. Since ICG can produce fluorescence signals above 1100 nm (Fig. S11), when ICG is excited by the 793 nm laser (strongly absorbed by ICG), the SF of ICG in channel 2 is very strong and has serious crosstalk with that of L1057 NPs (which can be effectively excited by 793 nm laser), rendering the blood vessels and ureters all bright and they could not be distinguished at all (Fig. S12).
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