2.4 BODIPY
BODIPY
(4,4-difluoro-4-bora-3a,4a-diaza-s-indacene,
or
difluoroboron
dipyrromethene) has gained notoriety for its excellent photostability and chemical
stability and high molar absorption coefficient and fluorescence quantum yield
[31]. A wide variety of BODIPY-based fluorescent dyes have been developed, and
some of them are commercially available such as ER-Tracker Green and
LysoTracker Red. The photophysical properties of BODIPY are tuned by conjugation of various chemical residues on the difluoroboron dipyrromethene backbone.
For example, (a) 2,6-positions are available for the electrophilic substitution reactions for the introduction of bromine or iodine [32] for further synthetic modification; (b) active methyl groups at 3,5-position are subject to chemical modification
owing to their strong nucleophilic character [33]; (c) nucleophilic substitution of
leaving groups is also great modification site for a thiomethyl group, for example, at
the 8-position [34]; (d) a halogen atom could be used for further extension of the
conjugation or the structure via palladium-catalyzed coupling reaction; (e) extension
of Pi electron conjugation; and (f) modification at the boron center. Readers are
referred to these comprehensive review papers that have described the synthesis and
photophysical properties of BODIPY [35–37] (see Fig. 2c). BODIPY could be used
for many biological applications such as indicators of pH, ion, biomolecules (e.g.,
peptide, thiol, saxitoxin), reactive oxygen species and reactive nitrogen species, and
others (e.g., hydrolysis esters, biocatalytic reactions).
2.5 Rhodamine
Rhodamine is another fluorescent dye used for bio-imaging, and many rhodaminebased imaging systems have been developed. For example, a sugar-rhodamine
fluorescent probe was designed as a specific sensor for Cu
2+ , exhibiting fluorescent
color changes by naked eye. More importantly, the probe also could detect the Cu
2+
at 0.2 mg/L concentration, which is ten times lower than minimum Cu
2+ value
(2.0 mg/L) in drinking water recommended by WHO [38]. Another chemosensor
system based on the conjugation of rhodamine with quinoline was developed for the
detection of Cu
2+ and Fe
3+ in vivo with good selectivity and sensitivity [39]. In
addition, a ratiometric fluorescent probe (RIM) based on fluorescence resonance
energy transfer (FRET) was synthesized from imidazo[1,5-a] pyridine and rhodamine for the detection of HOCl. Without HOCl, the RIM displayed absorption
spectra at 360 nm, whereas, in the presence of HOCl, ring-opening form of rhodamine moiety resulted in the FRET process with imidazo[1,5-a] pyridine as donor and
rhodamine as acceptor, showing an additional absorption peak at 560 nm. The
detection range is 0–5 μM with high selectivity and sensitivity [40] (see Fig. 2d).
Another novel rhodamine derivative fluorescent probe (RDP) was synthesized
through the reaction of rhodamine B derivative with di(pyridin-2-yl)methanone in
38
X. Yang et al.
BODIPY
(4,4-difluoro-4-bora-3a,4a-diaza-s-indacene,
or
difluoroboron
dipyrromethene) has gained notoriety for its excellent photostability and chemical
stability and high molar absorption coefficient and fluorescence quantum yield
[31]. A wide variety of BODIPY-based fluorescent dyes have been developed, and
some of them are commercially available such as ER-Tracker Green and
LysoTracker Red. The photophysical properties of BODIPY are tuned by conjugation of various chemical residues on the difluoroboron dipyrromethene backbone.
For example, (a) 2,6-positions are available for the electrophilic substitution reactions for the introduction of bromine or iodine [32] for further synthetic modification; (b) active methyl groups at 3,5-position are subject to chemical modification
owing to their strong nucleophilic character [33]; (c) nucleophilic substitution of
leaving groups is also great modification site for a thiomethyl group, for example, at
the 8-position [34]; (d) a halogen atom could be used for further extension of the
conjugation or the structure via palladium-catalyzed coupling reaction; (e) extension
of Pi electron conjugation; and (f) modification at the boron center. Readers are
referred to these comprehensive review papers that have described the synthesis and
photophysical properties of BODIPY [35–37] (see Fig. 2c). BODIPY could be used
for many biological applications such as indicators of pH, ion, biomolecules (e.g.,
peptide, thiol, saxitoxin), reactive oxygen species and reactive nitrogen species, and
others (e.g., hydrolysis esters, biocatalytic reactions).
2.5 Rhodamine
Rhodamine is another fluorescent dye used for bio-imaging, and many rhodaminebased imaging systems have been developed. For example, a sugar-rhodamine
fluorescent probe was designed as a specific sensor for Cu
2+ , exhibiting fluorescent
color changes by naked eye. More importantly, the probe also could detect the Cu
2+
at 0.2 mg/L concentration, which is ten times lower than minimum Cu
2+ value
(2.0 mg/L) in drinking water recommended by WHO [38]. Another chemosensor
system based on the conjugation of rhodamine with quinoline was developed for the
detection of Cu
2+ and Fe
3+ in vivo with good selectivity and sensitivity [39]. In
addition, a ratiometric fluorescent probe (RIM) based on fluorescence resonance
energy transfer (FRET) was synthesized from imidazo[1,5-a] pyridine and rhodamine for the detection of HOCl. Without HOCl, the RIM displayed absorption
spectra at 360 nm, whereas, in the presence of HOCl, ring-opening form of rhodamine moiety resulted in the FRET process with imidazo[1,5-a] pyridine as donor and
rhodamine as acceptor, showing an additional absorption peak at 560 nm. The
detection range is 0–5 μM with high selectivity and sensitivity [40] (see Fig. 2d).
Another novel rhodamine derivative fluorescent probe (RDP) was synthesized
through the reaction of rhodamine B derivative with di(pyridin-2-yl)methanone in
38
X. Yang et al.