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