Biology Reference
In-Depth Information
O
N
O
Ph
Ph
N
N
Ph
Ph
O
O
N
SQ1
SQ2
l
abs
= 637 nm
l
abs
= 641 nm
= 257,000 M
-
1
cm
-
1
e
= 280,000 M
-
1
cm
-
1
e
l
em
= 660 nm
l
em
= 652 nm
F
f
= 0.45
F
f
= 0.20
Solvent: THF/H
2
O
Ref. 109
Solvent: DMSO
Ref. 108
Chart 3.2 Structures and optical properties of representative squaraine dyes.
nanoparticles made from organic polymers: poly(lactic-
co
-glycolic acid)
93,94
and poly-(
DL
-lactic-
co
-glycolic acid)-poly(vinyl alcohol) systems,
95
lipid
nanoparticles,
96
self-assembled polyethylene glycol-phospholipid
nanoparticles,
97
self-assembled polyallylamine hydrochloride-dihydrogen
phosphate nanoaggregates,
98,99
poly-(
g
-glutamic acid),
100
low-density
lipoprotein (LDL) nanoparticles,
101
liposomes,
102
frozen ionic liquid
nanoparticles,
103
perfluorocarbon nanoparticles,
104
and peptosomes.
105
Many of these systems have been examined
in vivo
.Encapsula ionof
cyanine fluorophores in nanoparticles seems to be an excellent method for
improving their chemical and optical properties, and given the significant
progress in the formulation of a variety of self-assembled nanostructures,
this direction is likely to be intensively investigated in the future.
3. SQUARAINES
Squaraines (squarilium dyes) are polymethine fluorophores containing
a hydroxyoxocyclobutene core with electron-donating substituents on both
sides.
106,107
Squaraines possess a strong, narrow absorption band in the red
and near-IR spectral region, and in many cases, they are intensely
fluorescent. The structures and optical properties of the most
representative squaraines are presented in
Chart 3.2
.
108,109
The absorption
maxima, extinction coefficients, and fluorescence quantum yields of
squaraines depend on the nature of aromatic groups flanking the
oxabutene core. Synthesis of symmetrical squaraines is relatively
straightforward and entails one-step condensation of commercially
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