Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems - Self Organized Nanostructures of Amphiphilic Block Copolymers - page 221

their periphery. They are usually chemically attached to the shell-forming blocks
[ 138 - 142 ] . The targeting groups are chemically complex structures and may have
an amphiphilic or partially hydrophobic character that could influence the targeting
effect. In other worlds, their interaction with water does not have to be favorable
and they could try to escape from the energetically unfavorable aqueous surround-
ing at the micellar periphery and bury deep within the shell close to the hydrophobic
core. When this happens, their disappearance from the uppermost part of the shell
negatively affects the drug delivery efficiency.
We addressed the above problem by studying a system of modified PS-PMA
micelles with long PMA blocks end-tagged by a strongly hydrophobic molecule in
mixtures of water with organic solvents and in purely aqueous media where elec-
trostatic effects dominate the behavior [ 93 - 96 ]. We used anthracene (An) as the
end-attached hydrophobic group, which allowed for fluorescence study. When we
started the study, it was not a priori clear what would happen when such slightly
modified micelles were dispersed in aqueous media.
Outline of Experimental Study and the Most Important Observations
For the experimental study, we used two almost identical PS-PMA samples, very
similar to those studied in the above-described study. The first sample contained
one pendant naphthalene group (Np) in between PS and PMA blocks. The second
sample was tagged by Np in the same way as the previous one, but also by one An at
the end of the PMA block. The two fluorophores, Np and An, were chosen because
they represent a suitable pair for NRET studies and they can be chemically attached
to specific sites of the studied system [ 143 ].
Experimental results were obtained by two experimental techniques, LS and
TRFS. When the micelles are formed, all attached Np molecules (potential exci-
tation energy donors) are localized at the core-shell interface. The Np fluorescence
quenching due to NRET is expected if some energy traps (An) come relatively close
to Np, i.e., to distances shorter or comparable with the Förster radius, R 0 (for the
Np-An pair, ca. 2.1 nm [ 144 ] ). Hence, the NRET study should prove whether a
fraction fraction of shell-embedded An could closely approach the core.
The time-resolved quenching of the Np emission due to Np-to-An energy trans-
fer is shown in Fig. 15 . Three transfer-affected fluorescence decays (measured in the
double-tagged system) normalized by decays in the absence of traps (measured in
the single-tagged system under identical conditions) are plotted as functions of time
in solvents differing in polarity (1,4-dioxane/water mixtures). The broken-like curve
obtained in water-rich media, consisting of a steeply decreasing part and a constant
part with almost no smoothly curved part in between, can be neither explained nor
fitted by assuming any type of a continuous monomodal distribution of Np-An dis-
tances [ 92 , 95 ]. It suggests the existence of two types of Np donors, which are
either very strongly affected by NRET, or fully unaffected. In a spherically symmet-
rical system in which all Np are uniformly distributed in a narrow spherical layer
at the core-shell interface, the concept of two types of Np molecules looks strange.