Recently, it is attracted to design polymers to synthesize lower bandgap energy
probes in the NIR-II regions by D–A alternating copolymerization method. For
example, Hong et al. have reported a method for synthesizing polymer nanoparticles
(pDA) emitting in the NIR-II region peak at 1,050 nm with a quantum yield of up to
1.7%, which is much higher than SWCNTs (~0.4%) used previously [37]. In
bio-application field, this novel NIR-II fluorophore enables the imaging of dynamic
changes in blood flow at a frame rate of over 25 frames per second, which extends
the limits of temporal resolution to a previously unattainable level. However, one
drawback of this NIR-II probe is the low photostability that photodegradation upon
excitation at 808 nm for 1 h results in a decline in NIR-II fluorescence intensity of up
to 20%. In another novel research work, a highly photostable polymer nanoparticle,
namely, PDFT1032, was reported by Dr. Cheng [38]. PDFT1032 was synthesized
from the highly planar electron acceptor diketopyrrolopyrrole (DPP), which is able
to couple with many electron donors easily, and the bandgap can be tuned to obtain
the expected NIR-II emission. PDFT1032 demonstrates excellent performance in
NIR-II image-guided tumor surgery and embolization therapy at the high spatial
resolution (mm range) and high temporal resolution (425 frames per second).
3 RENPs
Rare-earth (RE) elements are the lanthanides (from La to Lu) together with Sc and Y.
Rare-earth-doped nanoparticles composed of RE ions embedded within an inorganic
crystalline host matrix have attracted considerable attention owing to their unique
optical properties that are controlled mostly through f–f electronic transitions [39–
44]. Their distinctive optical properties featuring large Stokes and anti-Stokes shifts,
narrow and multi-peak emission profiles, and long lifetime and excellent
photostability have enabled RENPs to be promising alternatives to conventional
organic fluorophores and quantum dots, and widely used in imaging and therapy, as
shown in Fig. 8.
3.1 Mechanism of NIR-II Emission
The theories of electronic properties and transitions of RE ions were mostly
established by Judd and Wybourne in 1962–1965 [45, 46]. Owing to their similar
electron configurations, trivalent RE ions have similar physical and chemical properties. The 4f
n electronic states of RE ions are separated by spin–orbit coupling and
electronic repulsion to produce a myriad of complex energy levels that allow for an
immense possibility of intra-configurational transitions as shown in Fig. 9 [47]. The
luminescence of RE ions stems mostly from the specific electronic transitions
between the ladder-like energy levels within the 4f orbitals. These f–f transitions
are shielded by 5s and 5p electrons and thus produce well-defined atomic-like
Advancements of Second Near-Infrared Biological Window Fluorophores:. . .
93
probes in the NIR-II regions by D–A alternating copolymerization method. For
example, Hong et al. have reported a method for synthesizing polymer nanoparticles
(pDA) emitting in the NIR-II region peak at 1,050 nm with a quantum yield of up to
1.7%, which is much higher than SWCNTs (~0.4%) used previously [37]. In
bio-application field, this novel NIR-II fluorophore enables the imaging of dynamic
changes in blood flow at a frame rate of over 25 frames per second, which extends
the limits of temporal resolution to a previously unattainable level. However, one
drawback of this NIR-II probe is the low photostability that photodegradation upon
excitation at 808 nm for 1 h results in a decline in NIR-II fluorescence intensity of up
to 20%. In another novel research work, a highly photostable polymer nanoparticle,
namely, PDFT1032, was reported by Dr. Cheng [38]. PDFT1032 was synthesized
from the highly planar electron acceptor diketopyrrolopyrrole (DPP), which is able
to couple with many electron donors easily, and the bandgap can be tuned to obtain
the expected NIR-II emission. PDFT1032 demonstrates excellent performance in
NIR-II image-guided tumor surgery and embolization therapy at the high spatial
resolution (mm range) and high temporal resolution (425 frames per second).
3 RENPs
Rare-earth (RE) elements are the lanthanides (from La to Lu) together with Sc and Y.
Rare-earth-doped nanoparticles composed of RE ions embedded within an inorganic
crystalline host matrix have attracted considerable attention owing to their unique
optical properties that are controlled mostly through f–f electronic transitions [39–
44]. Their distinctive optical properties featuring large Stokes and anti-Stokes shifts,
narrow and multi-peak emission profiles, and long lifetime and excellent
photostability have enabled RENPs to be promising alternatives to conventional
organic fluorophores and quantum dots, and widely used in imaging and therapy, as
shown in Fig. 8.
3.1 Mechanism of NIR-II Emission
The theories of electronic properties and transitions of RE ions were mostly
established by Judd and Wybourne in 1962–1965 [45, 46]. Owing to their similar
electron configurations, trivalent RE ions have similar physical and chemical properties. The 4f
n electronic states of RE ions are separated by spin–orbit coupling and
electronic repulsion to produce a myriad of complex energy levels that allow for an
immense possibility of intra-configurational transitions as shown in Fig. 9 [47]. The
luminescence of RE ions stems mostly from the specific electronic transitions
between the ladder-like energy levels within the 4f orbitals. These f–f transitions
are shielded by 5s and 5p electrons and thus produce well-defined atomic-like
Advancements of Second Near-Infrared Biological Window Fluorophores:. . .
93