quickly growing optical imaging technique, NIR-II imaging demonstrates high
promise to diagnose diseases, monitor disease development, and guide surgery and
so on.
In this chapter, firstly we will briefly introduce the mechanism of NIR-II fluorescence emission. Secondly, we will summarize the synthesis method, including the
chemical methods for synthesis donor–acceptor (D–A) structures and the luminescence characteristics, size, morphology, and surface characteristics of brightemission NIR-II fluorophores. Thirdly, the use of emerging NIR-II fluorophores
applications in single NIR-II imaging, multimodal imaging, and theragnosis will be
highlighted. The future perspective and new challenges of NIR-II fluorophores are
discussed in the final section of this chapter.
2 Organic Dyes
Organic dyes have been used to detect and visualize structures and processes in
biological samples. Today, many of the favored NIR-II organic dyes have a fluorescent component that can be detected with extraordinary sensitivity and selectivity.
To date, several types of organic NIR-II fluorophores with excellent emission
performance are synthesized, such as SMDs, SMDCs, and SMDNPs. SMD NIR-II
fluorophore is first reported in 2015, named CH1055 [8]. Most of the SMDs are
hydrophobic and need to be encapsulated into surfactant (e.g., PEG, liposome) or
conjugated with protein to form NIR-II SMD complexes or SMD nanoparticles,
named as SMDCs and SMDNPs, respectively. They have different degrees of
brightness in the NIR-II emission and rapid excretion, and they can be manufactured
under current good manufacturing practice (cGMP) condition. Because of these
advantage factors, they are highly attractive and promising NIR-II fluorophores for
future clinical applications.
2.1 Mechanism of NIR-II Emission
Briefly, fluorescence occurs when an orbital electron of a molecule, or atom, relaxes
to its ground state after emitting a photon from an excited singlet state. The
fluorescence process can be summarized as follows: first, excitation of a fluorophore
through the absorption of light energy; second, a transient excited lifetime with some
loss of energy; and third, return of the fluorophore to its ground state, accompanied
by the emission of light. Generally, due to the energy loss during the transient
excited lifetime, the light emitted is always of a longer wavelength than the light
energy absorbed. Taking the SMD CH1055 as an example, it is composed of
conjugated aromatic units, with a donor–acceptor–donor (D–A–D) structure and a
benzobisthiadiazole core. The energy gap can be reduced through π-spacers by
attaching of strong electron donors. The spatial configuration of strong electron84
S. He and Z. Cheng
promise to diagnose diseases, monitor disease development, and guide surgery and
so on.
In this chapter, firstly we will briefly introduce the mechanism of NIR-II fluorescence emission. Secondly, we will summarize the synthesis method, including the
chemical methods for synthesis donor–acceptor (D–A) structures and the luminescence characteristics, size, morphology, and surface characteristics of brightemission NIR-II fluorophores. Thirdly, the use of emerging NIR-II fluorophores
applications in single NIR-II imaging, multimodal imaging, and theragnosis will be
highlighted. The future perspective and new challenges of NIR-II fluorophores are
discussed in the final section of this chapter.
2 Organic Dyes
Organic dyes have been used to detect and visualize structures and processes in
biological samples. Today, many of the favored NIR-II organic dyes have a fluorescent component that can be detected with extraordinary sensitivity and selectivity.
To date, several types of organic NIR-II fluorophores with excellent emission
performance are synthesized, such as SMDs, SMDCs, and SMDNPs. SMD NIR-II
fluorophore is first reported in 2015, named CH1055 [8]. Most of the SMDs are
hydrophobic and need to be encapsulated into surfactant (e.g., PEG, liposome) or
conjugated with protein to form NIR-II SMD complexes or SMD nanoparticles,
named as SMDCs and SMDNPs, respectively. They have different degrees of
brightness in the NIR-II emission and rapid excretion, and they can be manufactured
under current good manufacturing practice (cGMP) condition. Because of these
advantage factors, they are highly attractive and promising NIR-II fluorophores for
future clinical applications.
2.1 Mechanism of NIR-II Emission
Briefly, fluorescence occurs when an orbital electron of a molecule, or atom, relaxes
to its ground state after emitting a photon from an excited singlet state. The
fluorescence process can be summarized as follows: first, excitation of a fluorophore
through the absorption of light energy; second, a transient excited lifetime with some
loss of energy; and third, return of the fluorophore to its ground state, accompanied
by the emission of light. Generally, due to the energy loss during the transient
excited lifetime, the light emitted is always of a longer wavelength than the light
energy absorbed. Taking the SMD CH1055 as an example, it is composed of
conjugated aromatic units, with a donor–acceptor–donor (D–A–D) structure and a
benzobisthiadiazole core. The energy gap can be reduced through π-spacers by
attaching of strong electron donors. The spatial configuration of strong electron84
S. He and Z. Cheng