probe composed of 4
0 -(aminomethylphenyl)-2,2
0 :6
0 ,2
00 -terpyridine (Tpy) and
tricarbocyanine (Cy) that could detect the minor pH fluctuation in the range of
6.7–7.9 in vivo. Furthermore, real-time imaging of pH was obtained in living
HepG2 and HL-7702 cells. The probe did not exhibit autofluorescence and can
provide fluorescence contrast with high sensitivity, excellent photostability, and
remarkable cell membrane permeability [113].
3.4 Cancer Imaging
The typical distribution of phospholipid can be disrupted when cells are at the stage
of apoptosis. Therefore, targeting phosphatidylserine on the membrane surface
would be useful to evaluate the treatment efficacy. Porphyrin-ytterbium complex
was designed as a marker to distinguish tumor cells from normal cells because
the fluorescence of porphyrin ytterbium complex can strongly bind to
phosphatidylserine for tumor cell targeting [114]. Another example for cancer cell
imaging is a porphyrin derivative, a chiral porphyrazine that exhibits remarkable
accumulation of tumor cell in vivo, especially in breast tumor cells. It showed that
the high selectivity of chiral porphyrazine for tumor cell in vivo is a critical factor in
the cancer therapy [115] (see Fig. 6b). Optimized analogues of chiral porphyrazine
with enhanced photophysical properties were synthesized for applications in cancer
imaging [116]. BODIPY polymer with emission wavelength in NIR range conjugated with cancer-homing peptide residues could be employed for fluorescence
imaging of cancer cells, and compared with original polymer, the conjugate has
higher water solubility, more excellent photostability, better biocompatibility, and
specific interaction to breast tumor cells [117]. Another recent study showed that
2,4,6-trisubstituted pyridine-based fluorescent probe that is pH-dependent displayed
selectivity, photostability, and reversibility so that the HeLa cancer cells could be
distinguished from other cells in a pH range of 2.2–7.0 [118].
3.5 Bacterial Imaging
Bacterial imaging is an emerging technology that can be used in a wide variety of
fields such as food science and biomedicines. Compared to mammalian membranes,
bacterial ones have some unique characterizations that can act as targets for antimicrobial drug candidate such as peptidoglycan cell wall on bacteria and yeast [119] or
O-antigen unit of lipopolysaccharide on the outer membrane of gram-negative
bacteria [120] or anionic phospholipids (e.g., phosphatidylglycerol) [121]. Fluorescence and radiolabeling are two contrast methods for bacterial imaging. For example, zinc dipicolylamine (Zn-DPA) complexes (probe 1) were made to selectively
target the surface of bacterial cells because they have high affinity for bilayer
membrane with anionic phospholipids. Bis(Zn-DPA), probe 2, was also synthesized.
Organic Fluorescent Probes for Diagnostics and Bio-Imaging
45
0 -(aminomethylphenyl)-2,2
0 :6
0 ,2
00 -terpyridine (Tpy) and
tricarbocyanine (Cy) that could detect the minor pH fluctuation in the range of
6.7–7.9 in vivo. Furthermore, real-time imaging of pH was obtained in living
HepG2 and HL-7702 cells. The probe did not exhibit autofluorescence and can
provide fluorescence contrast with high sensitivity, excellent photostability, and
remarkable cell membrane permeability [113].
3.4 Cancer Imaging
The typical distribution of phospholipid can be disrupted when cells are at the stage
of apoptosis. Therefore, targeting phosphatidylserine on the membrane surface
would be useful to evaluate the treatment efficacy. Porphyrin-ytterbium complex
was designed as a marker to distinguish tumor cells from normal cells because
the fluorescence of porphyrin ytterbium complex can strongly bind to
phosphatidylserine for tumor cell targeting [114]. Another example for cancer cell
imaging is a porphyrin derivative, a chiral porphyrazine that exhibits remarkable
accumulation of tumor cell in vivo, especially in breast tumor cells. It showed that
the high selectivity of chiral porphyrazine for tumor cell in vivo is a critical factor in
the cancer therapy [115] (see Fig. 6b). Optimized analogues of chiral porphyrazine
with enhanced photophysical properties were synthesized for applications in cancer
imaging [116]. BODIPY polymer with emission wavelength in NIR range conjugated with cancer-homing peptide residues could be employed for fluorescence
imaging of cancer cells, and compared with original polymer, the conjugate has
higher water solubility, more excellent photostability, better biocompatibility, and
specific interaction to breast tumor cells [117]. Another recent study showed that
2,4,6-trisubstituted pyridine-based fluorescent probe that is pH-dependent displayed
selectivity, photostability, and reversibility so that the HeLa cancer cells could be
distinguished from other cells in a pH range of 2.2–7.0 [118].
3.5 Bacterial Imaging
Bacterial imaging is an emerging technology that can be used in a wide variety of
fields such as food science and biomedicines. Compared to mammalian membranes,
bacterial ones have some unique characterizations that can act as targets for antimicrobial drug candidate such as peptidoglycan cell wall on bacteria and yeast [119] or
O-antigen unit of lipopolysaccharide on the outer membrane of gram-negative
bacteria [120] or anionic phospholipids (e.g., phosphatidylglycerol) [121]. Fluorescence and radiolabeling are two contrast methods for bacterial imaging. For example, zinc dipicolylamine (Zn-DPA) complexes (probe 1) were made to selectively
target the surface of bacterial cells because they have high affinity for bilayer
membrane with anionic phospholipids. Bis(Zn-DPA), probe 2, was also synthesized.
Organic Fluorescent Probes for Diagnostics and Bio-Imaging
45