Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 195

polystyrene polybead ® microspheres from Polysciences Inc., http://www.poly-
sciences.com ) and only need to be stained with an appropriate indicator. On the
other hand, despite being time consuming, preparation of polymeric beads from
monomers is much more flexible, since many parameters (size of the beads, amount
of functional groups, tendency to swell in organic solvents, etc.) can be fine-tuned.
Some common polymers are depicted in Fig. 1 . Nonpolar polymers such as poly-
stryrene are excellent matrices for immobilization of lipophilic indicators. Such
beads can be successfully used if dispersed in other polymers but tend to aggregate
in aqueous media. Therefore, they are often provided with charged groups which
dramatically increase their stability in aqueous media. More polar polyacrylonitrile
and polymethylmethacrylate are often polymers of choice as well. Their copoly-
mers bearing carboxyl groups can be used for preparation of rather small beads
( Ø

< 50 nm). Notably, the surface of such beads is also available for further
functionalization.
Although nonpolar polymers such as polystyrene are often the best choice for the
preparation of oxygen-sensitive beads, optical temperature probes and labels cannot
be employed to design particles for sensing pH, ions, basic and acidic gasses. For
example, more polar ethylcellulose and cellulose acetate are polymers of choice for
designing carbon dioxide and ammonia sensors, respectively. The pH sensing beads
often rely on the use of hydrophilic water-swellable polymers such as cross-linked
polyacrylamide, polyurethane hydrogels, etc. Alternatively, covalent attachment of
a pH indicator to the surface of the bead can be used. Plasticized polyvinylchloride
was found to be very suitable for preparation of ion-sensitive beads.
Block polymers that are able to form core-shell particles in aqueous media are
also very promising. For example, lipophilic indicators can be embedded into the
core of a block polymer of styrene and vinylpyrrolidone. The neutral lipophilic
shell is responsible for the unique stability of the beads in water which do not
aggregate even at very high ionic strength.
Finally, some hybrid inorganic-organic materials such as Ormosil ( Organically

modified silica) are also very popular. The structure and properties of Ormosils
resemble those of silica and organic polymers.

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2.2 Dyes

Evidently, it is not possible to choose or design an “ideal” dye for an optical sensor.
However, a potential candidate should fulfill as many of the following requirements
as possible:
Be excitable in visible or NIR . Although the dyes excitable in UV can also be
used, they are much less suitable for practical applications due to several reasons:
(i) high level of background fluorescence originating from the optical components
and biological substances present in the analyzed media; (ii) high scattering of the
light; (iii) higher price and lower output of the light sources used for excitation
(such as LEDs). Red-light excitable and NIR dyes are particularly promising since