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presence of poly(allylamine) [42] or CS (as will be shown later) [43].
Besides, the tetra-anionic form is stabilized up to pH 2.2, due to
charge neutralization by the polymer. However, when the poly-
electrolytes bear positively charged aromatic groups, aromatice
aromatic interactions between these charged aromatic groups
and aromatic counterions such as TPPS may compete with the dye
self-aggregation. In some cases, as in the presence of poly(4-
vinylpyridine) (P4VPy) this interaction becomes intense, and the
self-stacking of the dyes is inhibited [26,32,41]. This behavior has
been also observed for cationic xanthene dyes and aromatic and
aliphatic polyanions [20e22].
concentration of 10ꢀ6 M is stabilized at least up to pH 1.8 (see
Table 2). Whilst the aromatic amine density is increased, the
apparent concentration of unmodified pyrrolidones is decreased by
comparison with experiments made with P10. However, the
H2TPPS4ꢀ Soret band is also shifted to 423 nm as in the case of P10,
as can be also seen in Table 2, a fact that may indicate that this
notorious shift is mostly provided by the specific aromaticearo-
matic interaction between the dye and the aromatic amines. In this
respect, the nanoparticles formed with P25 are permeable to TPPS,
so that, a strong interaction is observed. The stabilization of the
tetra-anionic form may be due to both a higher hydrophobic en-
vironment and the charge compensation furnished by the ammo-
nium groups. The apparent VP units concentration for this
polymers is 2.7$10ꢀ4 M. Note in Fig. 9(A) that at this concentration
of pure PVP, a lower shift of the transition pH is found than in the
case of a more concentrated PVP solution, achieving a value of 3.6.
However, a higher shift of the transition pH is found for P25
comparing to the case in the presence of P10, related to the increase
on the aromatic amine density, provided that the apparent total
aromatic amine concentration has been kept constant.
Similar behavior is found in mixtures of water:DMSO 90:10.
However, contrarily to the case in the presence of P10, the stabili-
zation of the tetra-anionic form in these conditions is still higher than
in pure water, evidenced by a shift of the transition pH between the
basic and the acid form to at least pH 1.6, as can be seen in Table 2.
This could be related to swelling of DMSO inside the nanoparticles,
based on a probable preferential solvation of the polymeric func-
tional groups, that makes the environment inside the particle less
polar, and still permeable to TPPS, decreasing also the probability of
the dye to protonate. In the mixture water:DMSO 90:10, the transi-
tion pH of TPPS in the presence of 2.7$10ꢀ4 M of PVP is comparable to
that in water, taking a value of 3.6, as can be seen in Fig. 9(B).
The copolymer P10 presents 10% of its pyrrolidones substituted
with aromatic amines. As discussed above, the aromatic nature of
these substituents may increase the hydrophobia of the copolymer.
However, and in particular when the amino groups are charged,
they may also enhance the aromaticearomatic interactions with
TPPS. The low linear aromatic density of this polymer may favor its
hydration and protonation, and thus its interaction with the dye. As
aromaticearomatic interactions are short-range interactions, close
binding between both interacting aromatic residues may produce
notorious changes in the absorption characteristics of the dye, due
to both unspecific change in polarity and specific aromatice
aromatic interaction. Thus, complexation of 10ꢀ6 M of TPPS with
9$10ꢀ5 M of P10 produces a shift of 9 nm to lower energies (higher
than in pure DMSO) of the corresponding H2TPPS4ꢀ Soret band, as
can be seen in Table 2, arising a band at 423 nm, while the H4TPPS2ꢀ
Soret band is not shifted, remaining at 435 nm. Interestingly, the
transition between H2TPPS4ꢀ and H4TPPS2ꢀ decreases to pH 2.1, as
can be also seen in Table 2. Note that at the P10 concentration used
in these experiments, the apparent concentration of VP residues is
8.1$10ꢀ4 M. The behavior of TPPS in the presence of a comparable
concentration of the homopolymer PVP showed shifts of 7 nm of
the H2TPPS4ꢀ Soret band, lower than in the presence of P10 (see
Fig. 8), as well as a decrease on the transition pH to 3.3 (see
Fig. 9(A)). The higher shifts of these parameters (absorbance max-
ima and transition pH) in P10 highlight the role of the aromatic
amines of the polymers in providing specific interactions with TPPS.
When performing the study in water:DMSO 90:10, it was
observed that the H4TPPS2ꢀ Soret band of 10ꢀ6 M of TPPS moves to
437 nm in the presence of 9$10ꢀ5 M of P10 (Table 2), and the
transition pH between both tetra-anionic and di-anionic TPPS
species is shifted to 2.6, higher than in the presence of pure water as
solvent, but still much lower than for pristine TPPS. As deduced
from DLS results shown in Fig. 4, the polymeric chains are in an
extended coil form and are highly solvated. Upon preferential sol-
vation of TPPS and of the aromatic amines by DMSO, the close
binding between both species may be decreased. Besides, the less
polar solvent also contributes to decrease the basicity of the amino
groups, decreasing the probability to find them protonated. As said
before, in the mixture water:DMSO 90:10, the transition pH of TPPS
in the presence of 8.1$10ꢀ4 M of PVP is similar than in water, taking
a value of 3.3, as can be seen in Fig. 9(B).
3.6. Acidebase properties and band shift of TPPS in the presence of
P50, P75 and P100
None of these polymers are water-soluble, even at acidic condi-
tions. So, the linear aromatic amine density is too high and, as a
possible explanation, hydration of the functional groups is prevented
by means of a higher probability of these polymers to stabilize
stacked planar, non-basic structures in hydrophobic domains. How-
ever, suspensions of these polymers at a concentration of 9$10ꢀ5
M
and pH 3.5 were obtained by pouring concentrated solutions of the
polymers in pure DMSO into 9-fold water volume at the same pH, so
that a mixture water:DMSO 90:10 was obtained as solvent. As shown
in Fig. 4 and discussed in Section 3.1, the polymers where found in
the form of polydisperse microparticles. At these conditions, addition
of TPPS produced no effect concerning band shifts in comparison to
the situation in the absence of the polyelectrolytes, as can be seen in
Table 2. This suggests that the polymers are found in a rather
compact, solid form, not permeable to TPPS, with low porosity and
hardly solvated by the organic solvent, their amino groups remaining
in their basic form, decreasing the probability to undergo aromatice
aromatic interactions with TPPS. The dye may also be stabilized by
DMSO upon preferential solvation, and both entities, polymers and
dye, behave as practically independent. No effect was found con-
cerning transition pH changes in water, and weak effects are
observed in the presence of the microparticles in water:DMSO 90:10,
shifting from around 4.8 to around 4.5 in the presence of all poly-
mers, as can be seen in Fig. 9(B).
3.5. Acidebase properties and band shift of TPPS in the presence of
P25
P25 presents a higher aromatic density than P10. Its solubility in
water is decreased at neutral pH, but the polymer is apparently
readily soluble by adding HCl so that the basic functional groups
may still hydrate and protonate. This is an indication of a still low
linear aromatic amine density that provides less compact polymers,
although both in H2O at pH 1 and in mixtures water:DMSO 90:10 at
pH 3.5 polymeric aggregation results in the formation of nano-
particles, as was seen in Fig. 4. As a result of the interaction with
9$10ꢀ5 M of this polymer, the tetra-anionic form of TPPS at a
3.7. Fluorescence studies
Three dimensional graphs regarding excitation and emission
wavelengths and intensity are obtained for TPPS in the presence of
Gómez-Tardajos, Myriam
