Rational Design of Fluorophores for In Vivo Applications - Progress in Molecular Biology and Translational Science - page 62

applications, 5 fluorophores for medical in vivo imaging, 3,4,8-11 near-IR
fluorophores, 12,13 and fluorophores for labeling of biomolecules. 14 This
chapter is particularly focused on the recent progress in the small organic
fluorophores for in vivo imaging and specifically highlights the chemical
approaches to achieve the properties of optimal fluorophores listed above.
In vivo application is defined here as an application in whole tissue or
whole body, rather than in a single cell.
Several classes of organic fluorophores are discussed here, those being
commonly used for in vivo imaging (such as cyanines, squaraines) as well
as the less commonly used ones but showing properties making them prom-
ising candidates for future use (such as boron dipyrromethenes (BDP) and
tetrapyrrolic macrocycles). Finally, two special classes of fluorophores of
growing importance for in vivo applications are discussed: self-illuminating
fluorophores and fluorophores for multicolor in vivo imaging.
Small organic molecules comprise one of the major classes of fluorophores
for in vivo applications. They offer virtually unlimited diversity of structures,
have a broad range of methods for modification of their physicochemical
properties, and are relatively easily available, inexpensive, and usually
nontoxic. 6 On the other hand, small organic fluorophores suffer from serious
imperfection of optical properties, such as low quantum yields (especially for
near-IR fluorophores), moderate excitation coefficients, and often moderate
photostability. 6 For these reasons, intensive research effort has been made to
find alternative fluorophores with improved optical properties and stability,
which has led to the development of other classes of materials suitable for
in vivo imaging. These classes of fluorophores already investigated for
in vivo applications, such as near-IR fluorescent proteins, 15,16 fluorescent
polymer nanoparticles, 17 quantum dots, 18-20 carbon dots, 21 dye-doped
nanoparticles, 22 and carbon nanotubes, 23,24 as well as commercially available
near-IR organic fluorophores, with unrevealed structures (such as Alexa
Fluors 25 ), are not discussed here. Readers interested in these topics are
referred to the reviews and original articles listed above. This review covers
the literature published approximately till the end of 2011.
2. CYANINE AND RELATED FLUOROPHORES
2.1. General characterization
Carbocyanine dyes (cyanines) remain the most prevalent fluorophores used
for in vivo imaging. The cyanine derivative indocyanine green (ICG, IR-
125) is the only near-IR fluorophore approved by the Food and Drug