Pseudobilayer Vesicle Formation via Layer-by-Layer Assembly of Hydrophobically Modified Polymers on Sacrificial Substrates** By Ajay J . Khopade* and Helmuth Möhwald 1. Introduction Polymeric vesicles are stable counterparts of bilayer vesicles made up of low-molecular-weight amphiphiles, which have found enormous applications in various scientific and applied fields, for example, drug and gene delivery, [1] enzyme-encapsu- lated nanoreactors, [2] enhanced immunoassays, [3] catalysis, [4] templates for polymer and inorganic particle synthesis, [5] etc. The spontaneously formed polymeric vesicles, prepared from polymerizable amphipathic monomers, [6] hydrophobically modified polymers (HMPs), [7] or block copolymers, [8] may con- sist of varying numbers of bilayers, and vary in size and polydis- persity. [9] Tuning the number of bilayers, and the size and poly- dispersity of the vesicles, is highly desirable for precisely controlling the functional properties of the vesicles, for in- stance, the encapsulation and release of a bioactive molecule for drug delivery. Currently, a tailored number of bilayer films of amphiphiles can only be prepared using the well-known Langmuir±Blodgett (LB) technique on a planar substrate, [10] whereby the substrate is dipped and lifted through a compressed Langmuir mono- layer of a surfactant. The monolayer transfer takes place through preferential hydrophobic and hydrophilic interactions between the surfactant monolayer and the surface of the sub- strate during the dip±lift cycle. The number of dip±lift cycles regulates the number of bilayers formed. [11] The bilayers can also be prepared with a quaternized HMP, such as alkyl poly(4- vinylpyridines). [12] The LB technique, however, is not able to be used for transfering bilayers onto spherical supports, which is a primary prerequisite for preparing the vesicles of known size comprising a tailored number of concentric bilayers. The layer-by-layer (LbL) assembly of polyelectrolytes offers a vi- able alternative to the LB technique for the preparation of multilayered structures on both planar and spherical sub- strates. [13,14] LbL assembly mainly involves electrostatic inter- actions between oppositely charged polyelectrolytes and sub- strates. In addition to the electrostatic interactions, other types of molecular interactions, such as ion±dipole, dipole±dipole, and van der Waals interactions, involving molecules of solvent and hydrocarbon chains of polyelectrolytes (hydrophobic inter- actions), are also responsible for the multilayer formation. [15] For example, sodium polyacrylate is not adsorbed on hydro- phobic silica, but dodecyl polyacrylate is strongly retained. A molar ratio in alkyl chains as small as 1 % is sufficient to devel- op an effective attraction between the chains and the hydro- phobic surface. [16] This creates the possibility of making inter- digitated bilayers of HMPs on a substrate using a stepwise, electrostatic±hydrophobic-interaction-mediated adsorption. Our earlier attempts to prepare interdigitated bilayers using the LbL technique were the direct spreading of a phospholipid bilayer from charged liposomes on oppositely charged poly- electrolyte capsules, [17] and the adsorption of an amphiphile (PEGylated phospholipids; PEG: poly(ethylene glycol)) on a hydrophobized substrate. [18] Building on this prior work, we first demonstrate electrostati- cally mediated adsorption of HMPs on hydrophilic, charged planar and colloidal surfaces from aqueous solution (Scheme 1). Next, we demonstrate that the treatment of the surface with an apolar solvent, or a solvent polarity gradient, in- duces a change in the surface polarity (hydrophilic to hydropho- bic), which allows an additional HMP layer to be adsorbed from the aqueous solution, by hydrophobic interactions, to form HMP bilayers. Using decomposable colloidal cores and stabiliz- ing the HMP bilayers with additional polyelectrolyte-layer coatings allowed hollow polyelectrolyte multilayer capsules of almost the same size to be obtained after core removal. The ir- regular hydrophobic domains in the multilayer capsules, appar- 1088  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/adfm.200400500 Adv. Funct. Mater. 2005, 15, 1088±1094 ± [*] Dr. A. J. Khopade Sun Pharma Advanced Research Centre Akota, Baroda 390 020, Gujarat (India) E-mail: ajkhopade@sunpharma.com Prof. H. Möhwald Max Planck Institute of Colloid and Interfaces Am Muehlenberg 1, D-14476 Golm (Germany) [**] We thank Christine Pilz and Anne Heilig for technical assistance. AJK acknowledges the Alexander von Humboldt foundation for a fellow- ship grant. A bilayer of a hydrophobically modified polyelectrolyte, octadecyl poly(acrylamide) (PAAm), sandwiched between the layers of a hydrophilic polyelectrolyte, poly(ethyleneimine) (PEI), is prepared by the sequential electrostatic±hydrophobic±electro- static-interaction-driven self-assembly on planar and colloid substrates. This process results in a PEI/[PAAm] 2 /PEI-multilayer- coated substrate. The removal of a PAA/PEI/[PAAm] 2 /PEI-multilayer-coated decomposable colloidal template produces hollow capsules. Irregular hydrophobic domains of the [PAAm] 2 bilayer in the PEI/[PAAm] 2 /PEI-multilayer capsule are infil- trated with a lipid to obtain a uniform, distinct hydrophobic layer, imparting the capsule with a pseudobilayer vesicle structure. 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