of fluorescence imaging include inorganic nanoparticles, quantum dots, fluorescent
protein, and organic fluorophores. While inorganic materials account for an important
category of fluorescent contrast materials, the main focus of this chapter is limited to
organic fluorophore developments, followed by some examples of common biological applications.
2 Representative Organic Fluorophores
Organic fluorophores are widely used as contrast agents in optical microscopy.
Numerous fluorophores have been developed for fluorescence imaging. In this
section, we will summarize some commonly used organic fluorophores including
cyanines, 5-aminolevulinic acid (5-ALA), methylene blue (MB), difluoroboron
dipyrromethene (BODIPY), rhodamine, nicotinamide adenine dinucleotide
(NADH), flavin adenine dinucleotide (FAD), porphyrins, phthalocyanines (Pcs),
naphthalocyanines (Ncs), etc. Their backbone chemical structures were illustrated
in Fig. 1.
2.1 Cyanines
Cyanine dyes represent a large family of fluorescent compounds with the chemical
structure of two aromatic or heterocyclic rings connected with a polymethine chain.
Cyanines and their derivatives are a common source of organic fluorophores with
excitation wavelength in the range of 600–900 nm [17]. Among them, indocyanine
green (ICG) is a widely used fluorophore that has been approved by FDA for
medical use. Compared to other dyes such as Cy5, ICG has longer emission
wavelength so that it has deeper penetration and less autofluorescence. ICG was
initially used for medical imaging because of its minimal toxicity [18]. It has also
been used for cardiovascular function test, retinal angiography, and hepatic clearance, as well as for sentinel lymph node mapping, coronary arteriography during
cardiac bypass surgery [19], cholangiography during hepatobiliary surgery [20–22],
and blood flow measurement during aneurysm surgery [23]. Based on the characteristics of ICG, one FDA-approved intraoperative fluorescent imaging system
(Novadaq Technologies, SPY system) has been developed. The whole system
comprises an 806 nm laser to excite ICG and camera unit, monitor, central
processing unit, and laser generator. For image acquisition, the camera was positioned about 30 cm above the place of interest. An automatic distance sensor can
indicate the correct position to ensure the camera is at the right place. After injection
of ICG, the laser is activated, and image acquisition is started by a single command
to the computer. For example, in one study, Novadaq SPY system was used for
quality assessment in off-pump coronary artery bypass grafting [21]. This is based
on the fact that ICG can bind to plasma proteins and protein-ICG complex emits light
Organic Fluorescent Probes for Diagnostics and Bio-Imaging
35
protein, and organic fluorophores. While inorganic materials account for an important
category of fluorescent contrast materials, the main focus of this chapter is limited to
organic fluorophore developments, followed by some examples of common biological applications.
2 Representative Organic Fluorophores
Organic fluorophores are widely used as contrast agents in optical microscopy.
Numerous fluorophores have been developed for fluorescence imaging. In this
section, we will summarize some commonly used organic fluorophores including
cyanines, 5-aminolevulinic acid (5-ALA), methylene blue (MB), difluoroboron
dipyrromethene (BODIPY), rhodamine, nicotinamide adenine dinucleotide
(NADH), flavin adenine dinucleotide (FAD), porphyrins, phthalocyanines (Pcs),
naphthalocyanines (Ncs), etc. Their backbone chemical structures were illustrated
in Fig. 1.
2.1 Cyanines
Cyanine dyes represent a large family of fluorescent compounds with the chemical
structure of two aromatic or heterocyclic rings connected with a polymethine chain.
Cyanines and their derivatives are a common source of organic fluorophores with
excitation wavelength in the range of 600–900 nm [17]. Among them, indocyanine
green (ICG) is a widely used fluorophore that has been approved by FDA for
medical use. Compared to other dyes such as Cy5, ICG has longer emission
wavelength so that it has deeper penetration and less autofluorescence. ICG was
initially used for medical imaging because of its minimal toxicity [18]. It has also
been used for cardiovascular function test, retinal angiography, and hepatic clearance, as well as for sentinel lymph node mapping, coronary arteriography during
cardiac bypass surgery [19], cholangiography during hepatobiliary surgery [20–22],
and blood flow measurement during aneurysm surgery [23]. Based on the characteristics of ICG, one FDA-approved intraoperative fluorescent imaging system
(Novadaq Technologies, SPY system) has been developed. The whole system
comprises an 806 nm laser to excite ICG and camera unit, monitor, central
processing unit, and laser generator. For image acquisition, the camera was positioned about 30 cm above the place of interest. An automatic distance sensor can
indicate the correct position to ensure the camera is at the right place. After injection
of ICG, the laser is activated, and image acquisition is started by a single command
to the computer. For example, in one study, Novadaq SPY system was used for
quality assessment in off-pump coronary artery bypass grafting [21]. This is based
on the fact that ICG can bind to plasma proteins and protein-ICG complex emits light
Organic Fluorescent Probes for Diagnostics and Bio-Imaging
35