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Scientific Report 2007-2009
Condensed matter physics and biophysics
C24. Nano-engineering of colloidal particles: Characterization of
novel supra-molecular structures with hierarchical architecture for
biotechnological applications
Systems of interacting colloidal particles and oppositely
charged polymers have recently attracted great interest,
due to their relevance in a number of biological
and technological processes, but even more to the fact
that their dynamics and out-of-equilibrium properties offer
unceasing challenges. These complexes show a rich
and fascinating phenomenology yet poorly understood.
In the last few years, in our laboratory, various novel
core-particle aggregates have been prepared by electrostatic
self-assembly of polyelectrolytes (and nanoparticles)
with oppositely charged lipid liposomes. The use
of non-covalent forces provides an efficient method to position
the polyelectrolyte chain in a well-defined supramolecular
architecture. In addition, it is possible to control
the macroscopic properties of the assembly through
an external environmental stimulus.
-potential [mV]
40
20
0
-20
-40
-60
-80
R/R 0
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.1 1
=N - /N + 10
0.1 1 10
=N - /N +
Figure 1: ζ-potential of PAA-induced lipoparticle aggregates
as a function of the molar ratio ξ = N − /N + . The charge inversion
effect changes the overall charge of the aggregates
from positive(lipoparticles in the absence of PAA) to negative,
after the adsorption in excess of PAA chains. The inset
shows the ratio R/R0 of the radius of the aggregates normalized
to the radius of the barelipoparticle as a function of the
ratio ξ = N − /N + . This behavior is typical of the reentrant
condensation effect.
In particular, we are dealing with polyelectrolyte-lipid
complexes (lipoplexes) in aqueous solutions, consisting
of linear, highly charged, anionic polyelectrolytes and
oppositely charged (cationic) liposomes. We were able
to demonstrate (by means of dynamic light scattering,
laser Doppler electrophoresis, dielectric spectroscopy
and TEM techniques) that three-dimensional structure
can be created from polyelectrolyte-coated liposome
described above. This system is characterized by the
presence of a pronounced ”charge inversion” effect that
is responsible for the formation of large equilibrium
clusters. Moreover, under certain conditions, this cluster
phase seems to undergo a gelation process, exhibiting
an aging behavior. ”Charge inversion” occurs when
at the surface of a mesoscopic charged particle more
Figure 2: AFM images taken in air of mixtures of ϵ-PLLpolystyrene
particle aggregates, induced by adding different
amount of ϵ-PLL solution. (a): below the isoelectric condition;
(b): close to the isoelectric point on the left side; (c)
close to the isoelectric point on the rightside; (d): above the
isoelectric condition. The inset in panel c) shows a detail of
a single cluster.
counterions than necessary to neutralize it collapse.
As a consequence, the resulting complex displays an
overall charge, whose sign is opposite to the one the
particle originally bears. This phenomenon, associated
with the strong lateral correlation between adsorbed
counterions, depends on counterion valence and size.
When oppositely charged macro ions of comparable
size and valence interact, as is the case of anionic polyelectrolytes
interacting with cationic liposomes, charge
inversion assumes a considerable extent (”giant” charge
inversion). Concomitant to the charge inversion, as a
print for the formation of a cluster phase, a reentrant
condensation appears (Fig. 1). Our attempt is to use
this approach to allow polyelectrolytes to adsorb onto
an oppositely charged surface of the lipid vesicle in order
to form highly structured aggregates. This new class of
micron-scaled colloids with unusual properties adds to
the array of existing hollow materials and expands the
range of possibilities with respect to technological and
drug delivery applications.
References
1. S. Zuzzi et al., Langmuir, 25, 5910, (2009)
2. C. Cametti, Chem. Phys. Lipids, 155, 63, (2008)
3. D. Truzzolillo et al., Eur. Phys. J. E., 29, 229, (2009)
4. S. Sennato et al., Langmuir, 24, 12181, (2008)
Authors
C. Cametti, S. Sennato
Sapienza Università di Roma 77 Dipartimento di Fisica