Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 203

rather fast since the sensing “chemistry” is localized on the surface of the beads. On
the other hand, it is not always easy to obtain bright beads since indicators are only
loaded on the surface. Higher dye loading may even result in decreased bright-
nesses due to self-quenching.
Some bead materials possess porous structure and, therefore, have very high
surface to volume ratio. The examples include silica-gel, controlled pore glass, and
zeolite beads. These inorganic materials are made use of to design gas sensors.
Indicators are usually adsorbed on the surface and the beads are then dispersed in a
permeation-selective membrane (usually silicone rubbers). Such sensors possess high
sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the
aqueous phase since the gas contained in the pores needs to be exchanged. Porous
polymeric materials are rarer and have not been used so far in optical nanosensors.
3.3 Staining by Swelling
Polystyrene and polymethylmethacrylate beads are commonly stained by swelling
[ 8 , 9 ]. The beads are suspended in a water/methanol mixture and swelled by the
addition of a small amount of dichloromethane (where a dye is also dissolved).
Unfortunately, not all indicators show sufficient solubilities in such mixtures so
that dye loading is not very high. As was demonstrated [ 8 ], the incorporation of
hydrophobic dyes into polymer particles can be facilitated by use of surfactants.
However, these are often difficult to remove completely which may negatively
affect biological systems. Charged groups on the surface of the beads provide
additional stability toward aggregation and such materials can be usually stained
more easily. For example, carboxylated polystyrene micro- and nanospheres
(Polybead ® , Polysciences Inc.) can be stained from water/THF or water/acetone
mixtures with a high content of organic solvent. In this case solubility of lipophilic
dyes is usually not an issue. Core-shell poly(styrene- block -vinylpyrrolidone) beads
were found to be excellently suitable for staining [ 10 ]. It was demonstrated that
virtually any lipophilic oxygen indicators (such as Pt(II) and Pd(II) metalloporphyr-
ins, Ru(II) polypyridyl complexes and Ir(III) coumarins) can be embedded into the
core of the beads by swelling the particles in water/THF mixtures and by subse-
quently removing of THF at a reduced pressure [ 11 ]. Lipophilic pH indicators
(fluoresceins and 1-hydroxypyrene-3,6,8-trisulfonamides) can be stained into the
hydrophobic shell from ethanol/water mixtures [ 12 ]. Some ion sensors can also be
prepared using this versatile material.
3.4 Preparation of Stained Beads via Precipitation
Precipitation is a rather widespread method of manufacturing polymeric beads
[ 13 , 14 ] but it has only been sparsely used for the preparation of dyed beads [ 15 , 16 ].
However, this versatile method is very promising due to its simplicity. It was recently