Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 201

of fluorescence is a more elegant way and relies on the use of metal core covered
with a polymeric or sol-gel shell that contains an indicator [ 6 ].
Micro- and nanobeads with magnetic properties have recently become popular
since these tools can be manipulated, e.g., collected in the region of interest.
Magnetite nanoparticles are introduced in order to render the polymeric beads
magnetic. Preparation and application of magnetic beads will be discussed in
more detail in Sect 5.5 .
As in the case of bulk optodes, plasticizers can be added to modify the properties
of polymers (e.g., gas permeability). Plasticizers are mainly used to design ion-
sensitive nanobeads.
Certain biomolecules can be added into the bead or be attached to its surface.
These mostly include such recognition elements as antibodies, oligonucleotides or
other receptors such as conconovalin A. Enzymes can be used to design biosensors
(e.g., for glucose) on a microscale but this research is still in its infancy. Finally,
fluorescent proteins can be used as alternative to the dyes. The same refers to
quantum dots which can also be used in principle.
3 Fabrication Techniques
Figure 4 schematically shows common strategies of making dye-doped beads. These
are mostly suitable for preparation of both micro- and nanobeads. Some more
specific methods such as grinding and spray-drying are mostly used for
the preparation of microparticles. One can distinguish between the methods where
dye-doped polymeric beads are produced in one step (entrapment during polymeri-
zation, precipitation, and spray-drying) and the methods where undoped polymeric
beads are stained with an indicator (staining by swelling or covalent coupling to the
surface). The details of these techniques will be discussed in the following section.
3.1 Preparation of Stained Beads During Polymerization
Many commercially important polymers are produced via emulsion polymeriza-
tion. This is also one of the most common methods to produce dye-doped beads.
A dye is added to the mixture of monomers prior to initiating the polymeriza-
tion and is either noncovalently entrapped or is copolymerized. The second method
ensures that no leaching will occur from the particle but requires modification of the
dye (typically by providing it with a double bond). This method is most common for
preparation of pH-sensitive beads where a pH indicator is entrapped inside cross-
linked polyacrylamide particles. The size of the beads can be tuned over a wide
range so that preparation of both nano- and microbeads is possible. Despite
thorough washing the surfactants are rather difficult to remove completely and
their traces can influence the performance of some biological systems.