Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 197

the interferences from autofluorescence and scattering are further minimized.
Moreover, only such indicators can be made use of for subcutaneous monitoring
of important analytes such as oxygen or glucose. Finally, dyes that are suitable for
two-photon excitation with the NIR light have become increasingly popular (The
chapter of Przhonska et al. [ 127 ] addresses this issue).
Have high brightness . Brightness is commonly defined as a product of a molar
absorption coefficient and luminescence quantum yield QY. In other words, a dye

having a QY approaching unity but

e

in order of several thousands M 1 cm 1 will
be hardly of any practical use due to low brightness. For example, many cyclome-
tallated complexes of iridium(III) and platinum(II) possess strong emission, but
poorly absorb in visible. On the other hand, even a moderate emitter with QY

e

0.1

100,000 M 1 cm 1 is likely to be suitable.
Respond to the analyte of interest and preferably be inert to other species .
That is not easy and often impossible to achieve in practice. For example, any
metal- ligand luminescent complex employed for optical oxygen sensing will be
liable to thermal quenching or any pH indicator will possess higher or lower cross-
sensitivity to ionic strength. Some of these undesirable effects can be minimized by
appropriate chemical modification of an indicator (e.g., by decreasing the charge of
a pH-sensitive dye) or by making use of permeation-selective properties of a
polymer. However, most of such effects should be compensated for in an indepen-
dent measurement. Considering labels, a dye should be completely inert to any
species in order to be suitable.
Have good photostability . Photostability is particularly critical if high light
intensities are used for interrogation (such as in microscopy or fiber-optic micro-
sensors) or if the measurements are performed over a long time. Photodegradation
always affects luminescence intensity but is usually less critical in case of the decay
time measurements since this parameter can remain unaffected by photobleaching.
Possess good solubility in the polymer or have functional groups for immobilization .
It should be ensured that the dye remains in polymeric beads in the monomeric form and
neither aggregates nor leaches out into the surrounding medium. In most cases a suitable
dye is either lipophilic enough (such as e.g., many metalloporphyrins) or is rendered
lipophilic. For example, this can be achieved by choosing a bulky counterion
(e.g., BF 4 ,PF 6 , dodecylsulfate used to lipophilize cationic ruthenium(II) complexes)
or by providing a molecule with a lipophilic chain. It should be noted that hydrophilic
dyes bearing reactive groups are also commonly used for staining beads. They are either
copolymerized together with monomers or are covalently attached to the bead surface.
These dyes are often applied to design pH sensors.

but strong absorption e >

<

Fig. 1 Chemical structures of the polymers commonly used for preparation of beads: poly
(styrene- co -maleic acid) ( ¼ PS-MA); poly(methyl methacrylate- co -methacrylic acid) ( ¼ PMMA-
MA); poly(acrylonitrile- co -acrylic acid) ( ¼ PAN-AA); polyvinylchloride ( ¼ PVC); polysulfone
( ¼ PSulf); ethylcellulose ( ¼ EC); cellulose acetate ( ¼ CAc); polyacrylamide ( ¼ PAAm); poly(sty-
rene- block -vinylpyrrolidone) ( ¼ PS-PVP) and Organically modified silica ( ¼ Ormosil). PS-MA is
commercially available as an anhydride and negative charges on the bead surface are generated
during preparation of the beads