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Scientific Report 2007-2009
Condensed matter physics and biophysics
C23. Polyelectrolyte-colloid complexes
as innovative multi-drug delivery systems
Different charged colloidal particles have been shown
to be able to self assemble when mixed in an aqueous
solvent with oppositely charged linear polyelectrolytes,
forming long-lived finite-size mesoscopic aggregates [1].
In fact, when a suspension of electrically charged
colloidal particles and a solution of oppositely charged
linear polyelectrolytes are mixed together, due to
the long-ranged electrostatic interactions, and to the
comparatively lower diffusivity of the bulkier particles,
the polyelectrolyte chains rapidly diffuse in the whole
solution and adsorb on the particle surface. The adsorption,
due to the repulsion between the like charged
chains, occurs in a ’correlated’ manner. The resulting
polyelectrolyte-decorated particles interact through a
potential which is the superposition of a screened
electrostatic repulsion due to the residual net charge
of the pd-particles, of the attractive forces due to the
non-uniform distribution of the surface charge, and of
dispersion forces. The net result is an adhesive effect of
the adsorbed polyelectrolytes, acting as an ’electrostatic
glue’.
Figure 1: The typical ’reentrant’ condensation of charged
colloidal particles induced by an oppositely charged polyelectrolyte.
The average hydrodynamic diameter < 2R > and
the average ζ- potential are shown as a function of polyelectrolyte
concentration (bottom) or the corresponding polymer/particle
charge ratio (top).
At increasing the polyelectrolyte content, with the
progressive reduction of the net charge of the primary
polyelectrolyte-decorated particles, larger and larger
clusters are observed. Close to the isoelectric point the
aggregates reach their maximum size, while beyond this
point any further increase of the polyelectrolyte-particle
charge ratio causes the formation of aggregates whose
size is progressively reduced (Fig. 1). This ’reentrant’
condensation behavior is accompanied by a significant
’overcharging’, or charge inversion, i.e. more polyelectrolyte
adsorbs than needed to neutralize the particle
charge and eventually the sign of the net charge of the
decorated particle is reverted.
Recently we proposed a model that takes into account
the observed phenomenology in terms of a fine balance
between long range repulsive and short range attractive
interactions, both of electrostatic nature, and van der
Waals forces [2,3].
This complex phenomenology has been observed for
different polyelectrolytes in a variety of water dispersed
colloids, such as micelle, latex particles and lipid vesicles.
Figure 2: ESI-TEM image of a typical polyelectrolyteliposome
cluster. The cluster results from the polyelectrolyte
induced aggregation of liposomes loaded with two different
concentration of a Cs salt that gives a strong contrast in
TEM images. Panel b) shows the ’Cs map’ of the aggregate.
In this image red-gold levels correspond to variations in the
Cs concentration. Bars represent 100 nm.
In the last few decades, there has been a growing
interest toward the use of delivery system for a more
effective treatment of infectious diseases and cancer,
and the interest of the scientific community increasingly
focused on designing innovative solutions based on
intra-cellular vectors. In this context, nano-technologies
based on the self-assembly of macromolecules resulting
in nano-sized complexes could play a key role, promising
a tremendous potential for developing new diagnostic
and therapeutic tools, as genuine ’nano-devices’, able to
interact with biological systems at molecular levels and
with a high degree of specificity. Polyelectrolyte-colloid
complexes appear a promising route to designing multicompartment
vectors for multi-drug delivery.
References
1. F. Bordi et al., J. Phys.: Cond. Mat. 21, 2031021 (2009).
2. S. Sennato et al., Langmuir 24, 12181 (2008).
3. D. Truzzolillo et al., Eur. Phys. J. E 29, 229 (2009).
4. S. Sennato et al., J. Phys. Chem. B 112, 3720 (2008).
Authors
F. Bordi, C. Cametti, F. Sciortino, S. Sennato 2 , D. Truzzolillo
http://phobia.phys.uniroma1.it/
Sapienza Università di Roma 76 Dipartimento di Fisica