Hyperbranching surface polymerization as a tool for preferential functionalization of the outer surface of mesoporous silica

A highly flexible method for selective functionalization of mesoporous silica is presented, where the key step is the preferential growth of hyperbranched poly(ethylene imine), PEI, on the outer surface of the material. Here, the monomer aziridine is polymerized in the presence of silica particles in which the surfactant used as structure-directing agent has not yet been extracted from the mesopores. Furthermore, carboxylic acid functionalities can be introduced preferentially to the outer part of the PEI layer by succinylation, and the relative concentration of carboxylic acid to amino groups in the PEI layer can be rationally controlled. This makes it possible to fine-tune the surface charge of the particles for a given application independently from the thickness of the surface layer, which is expected to offer advantages in applications where further surface functionalization is needed, in combination with molecular gate properties and/or a high dispersion stability. The introduced method also allows the synthesis of materials with antagonist (Bronsted) acidic-basic and hydrophobic-hydrophilic properties. The surface functions are accessible and covalently linked to the particle, which is important for example in order to ensure that targeting functions do not detach from the particle during targeted drug release applications, which is not necessarily the case when the surface layer is only adsorbed to the particle surface.