Hot-band absorption of indocyanine green for advanced anti-stokes fluorescence bioimaging

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.

Hot-Band-Absorption-Induced Anti-Stokes Fluorescence of Aggregation-Induced Emission Dots and the Influence on the Nonlinear Optical Effect

“…The phenomena of anti-Stokes luminescence can be divided into two broad categories according to different sources of additional energy [ 13 , 14 ]. One source of additional energy is another excitation photon, as in the multi-photon absorption process and upconversion processes based on lanthanide or triplet–triplet annihilation [ 5 , 9 , 15 , 16 , 17 ].…”

“…One source of additional energy is another excitation photon, as in the multi-photon absorption process and upconversion processes based on lanthanide or triplet–triplet annihilation [ 5 , 9 , 15 , 16 , 17 ]. Thermal photons are another source, as in the hot-band absorption (HBA) process and the thermally activated delayed fluorescence (TADF) process [ 14 , 18 , 19 ]. All these processes have been successfully utilized in in vivo bioimaging, while they all have certain different limitations.…”

“…The bio-applications of the upconversion process are restricted by the small absorption cross-section and heavy metal ions for lanthanide-based upconversion, and the low photostability for triplet–triplet-annihilation-based upconversion [ 13 , 20 , 21 , 22 ]. The processes of thermally activated delayed fluorescence and HBA have drawn interest in recent years because they can be observed in organic dyes with high absorption [ 6 , 14 , 23 ]. In general, the process of anti-Stokes thermally activated delayed fluorescence is similar to the process of HBA-induced anti-Stokes fluorescence.…”

“…In general, the process of anti-Stokes thermally activated delayed fluorescence is similar to the process of HBA-induced anti-Stokes fluorescence. However, the former has a long lifetime on the order of milliseconds to microseconds due to the involvement of the triplet states, in comparison with the short lifetime of the latter [ 14 , 24 , 25 , 26 , 27 , 28 ]. Because of its short fluorescence lifetime, the HBA process is suitable for fast-scanning imaging such as fluorescence confocal microscopy.…”

“…If fast scanning is performed, the integration time of a single pixel could be as low as 10 μs, which is not suitable for a long-lifetime luminescence dye [ 3 ]. However, the current organic HBA-induced anti-Stokes fluorescence dyes in use, such as indocyanine green (ICG), suffer from aggregation-induced quenching which hinders further improvement of brightness [ 14 ]. Fortunately, aggregation-induced emission (AIE) dyes were first proposed in 2001, which avoided the aggregation-caused quenching effect and emitted bright fluorescence in the aggregate or solid state [ 30 ].…”