pubs.acs.org/Langmuir © 2010 American Chemical Society
Vesicle-Templated Layer-by-Layer Assembly for the Production of Nanocapsules Francesca Cuomo,*,† Francesco Lopez,† Maria G. Miguel,‡ and Bj€orn Lindman§ †
Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI), c/o Department of Food Technology (DISTAAM), Universit a del Molise, I-86100 Campobasso, Italy, ‡Chemistry Department, Coimbra University, 3004-535 Coimbra, Portugal, and §Physical Chemistry, Lund University, P.O. Box 124, 22100 Lund, Sweden Received February 8, 2010. Revised Manuscript Received March 8, 2010 Hollow structures on the submicrometer scale (nm) are obtained with the assembly of polyelectrolytes according to the layer-by-layer (LbL) technique. Following the LbL procedure, polymers alginate and chitosan were alternatively adsorbed on a vesicular template made of didodecyldimethylammonium bromide (DDAB). Evidence for the removal of the vesicular template entrapped in the alginate/chitosan film is presented. The removal of the vesicular template was achieved through interactions between a nonionic surfactant (Triton X100) and the double-chained surfactant forming the vesicles. The application of this approach allowed the production of hollow nanospheres with a mild procedure, avoiding the use of strong acids or other extreme working conditions that can modify the shell integrity. The obtained nanostructures were characterized by means of dynamic light scattering (DLS), the ζ potential, and scanning electron microscopy (SEM). The SEM analysis demonstrated the presence, after the core removal process, of nanocapsules indistinguishable in size and shape from the parent core-shell system. The analysis of the surface charge of the hollow nanocapsules, after the core dissolution, by ζ potential measurements, indicates good aggregate stability. DLS measurements showed that the size of the nanocapsules is on the order of hundreds of nanometers. Moreover, the size of both the core-shell and the hollow particles did not appear to be perturbed by variations in temperature or ionic strength.
1. Introduction Hollow spheres of nanometer to micrometer dimensions are considered to be an important class of materials from both scientific and biotechnological viewpoints. These systems have attracted increasing interest because of their potential applications in encapsulation and controlled release for drugs1,2 in the manufacture of advanced materials,3 in catalysis conversion,4 and so forth. However, as the literature reports, two main routes are known for the preparation of hollow spheres. One is based on polymerization in the presence of mineral nano/microparticles,5,6 and the other is based on the layer-by-layer (LbL) method. The basis of LbL assembly is the electrostatic interaction between oppositely charged species alternatively adsorbed onto an appropriate template.7-11 The deposition of polyelectrolyte layers onto colloidal particles has created the basis for the production of *Corresponding author. Phone: þ39 0874404634. Fax: þ39 0874404652. E-mail:
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core-shell particles with well-known size and composition.12-14 The core removal, from the core-shell structures, by dissolution or decomposition allowed the production of hollow capsules with different permeability properties depending on the polyelectrolyte complex used. Most of the studies on the assembly of hollow spheres based on the LbL technique employs inorganic15 or colloidal templates such as polystyrene (PS) particles,16,17 melamine formaldehyde (MF) resin particles,18 or biological cells.19-21 The core dissolution of PS particles was performed by exposure of the complexes to calcination or to solvents such as tetrahydrofuran,22 and the swelling of the capsules during core removal can lead to the rupture of the capsules. Acidic conditions (pH
