Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 198

Be nontoxic . Low cytotoxicity is desirable but not essential for the choice of
dyes. Evidently, cytotoxicity is greatly reduced upon immobilization of dyes into
polymeric beads. However, even individual dye molecules that leach out of the
beads may undesirably affect biological systems. This also refers to other lumino-
phores such as quantum dots which are known to be highly toxic.
Virtually all classes of luminescent dyes can be potentially used for staining
polymeric beads. One can distinguish between fluorescent organic dyes and
phosphorescent metal-ligand complexes. Such fluorescent dyes as coumarins,
fluoresceins, rhodamines, seminaphthorhodafluors, phenoxazines, naphthalimides,
polyaromatic hydrocarbons and their derivatives (pyrene, perylene, perylene-
3,4,9,10-tetracarboxylic acid diimides, etc.), boradipyrroles, squaraines and cya-
nine dyes are very common (Fig. 2 ). The spectral properties of these fluorophores
can be tuned over a wide range by choosing appropriate substituents and by varying
the conjugation length. Most of these dyes possess good brightness exceeding
40,000. Modifications are done to increase photostability (e.g., by introducing
fluorine and chlorine atoms or via sulfonation), to render the dyes lipophilic
(usually by modification with an alkyl chain) or available for covalent coupling
(e.g., via a carboxyl-group). Fluoresceins, seminaphthorhodafluors and 1-hydroxy-
pyrene-3,6,8-trisulfonate (or its alkylsulfonamides) are well-known pH indicators
and are used for preparation of pH-sensitive beads. Other dye classes are mostly
used for preparation of fluorescent labels and tracers. They are also often contained
together with an indicator to provide possibility of ratiometric referencing.
Luminescent metal-ligand complexes are mostly represented by metalloporphyr-
ins and their derivatives (predominantly palladium(II) and platinum(II) complexes),
ruthenium(II) polypyridyl complexes, cyclometallated platinum(II) and iridium(III)
complexes and lanthanide chelates (mostly of terbium (III) and europium(III)), Fig. 3 .
Other luminescent complexes with rhodium(III), osmium(II), rhenium(I), etc. are used
only sparsely. Platinum(II) and palladium(II) metalloporphyrins possess very strong
absorption in the violet region but have much less efficient absorption in the green part
of the spectrum ( 20,000 M 1 cm 1 ). Although they can be used as labels, their

relatively long phosphorescence decay times (tens to hundreds microseconds) make
them particularly attractive as oxygen indicators. Most common are the complexes
with octaethylporphyrin and much more photostable meso -tetra(pentafluorophenyl)
porphyrin. Since the excitation in the red region is particularly attractive for many
applications, porphyrin derivatives excitable at longer wavelength have become
popular. These include the complexes with pophyrin-ketones and meso -porpholac-
tones (absorbing at 570-600 nm) and with tetrabenzoporphyrins. The latter are seen as
particularly strong NIR emitters (

e

760-800
nm) and the brightnesses typically exceed 40,000. Tetranaphthoporphyrins extend the
excitation window even further (

l max (abs) ¼ 610-630 nm, l max (em) ¼

670-690 nm).
The ruthenium(II) polypyridyl complexes are also popular but the brightnesses
do not exceed 15,000 and thermal quenching is rather significant. This property can
be utilized to design temperature-sensitive probes providing that the dyes are
effectively shielded from oxygen (e.g., in polyacrylonitrile beads). Despite often
very high emission quantum yields the visible absorption of cyclometallated com-
plexes of iridium(III) and platinum(II) is usually poor (

l max (abs) ¼

10,000 M 1 cm 1 ), thus, e <