Methods Towards Asymmetric Functionalization in Porous Systems

Abstract

Miniaturization is important to design complex functions or to generate complex systems containing different compartments which play a crucial role for example in “Lab-on-Chip” devices or µ-electronic sensors. Miniaturization or structuring of polymers at interfaces is usually achieved using photolithography. But lithography based structures, usually obtained using UV-light initiated polymerizations, are limited to micrometer dimensions due to Abbe’s diffraction limit. Overcoming this size limitation would open a new dimension of control in material design. Near field modes, like surface plasmons, consist of local electromagnetic fields with nanoscale dimensions and thus can offer one path for nanoscale structure design. Based on these considerations, this thesis investigates preparation and subsequent local polymer functionalization strategies for mesoporous thin silica films. Visible light initiated radical polymerizations have been selected for functionalization as they are compatible with the wavelength of near-field modes of 633 nm. In particular, dye-sensitized polymerization using methylene blue, which absorbs at the relevant wavelength, and the pH chargeable monomers of 2-(dimethylamino) ethyl methacrylate (DMAEMA) and 2-(methacryloyloxy) ethyl phosphate (MEP) have been used. To optimize the optical mesoporous film characteristics, two strategies of mesoporous film production, dip coating and gravure printing, are compared. Subsequently, the influence of irradiation energy and time on the functionalization of mesoporous silica is systematically investigated. To date achieved possibilities and limits of surface plasmon induced mesopore functionalization with respect to localized functionalization and miniaturization are discussed. As a further aspect local or asymmetric functionalization of hierarchical materials is addressed, choosing paper as strongly hierarchical material. For lab-on-chip devices, especially in medical applications, the modulation of the liquid flow in porous materials and especially in paper is of great relevance. Microfluidic paper-based devices, such as those introduced by Whitesides in 2007, are inexpensive, disposable, and therefore sustainable. However, the role of fiber nanoporosity in transportation is often neglected and difficult to control and maintain during the manufacturing process. In this work the local silica functionalization of hierarchically porous paper is shown. The amount of silica as well as the silica distribution within paper sheet was varied. A fluid-stop-barrier for water imbibition was demonstrated exclusively based on this silica functionalization. This was achieved by using a simple dip coating process, correctly adjusting the initial TEOS (silica precursor) concentration in the coating solution together with a controlled drying process. Based on these parameters the silica amount and its distribution control results into wetting properties from hydrophilic to hydrophobic including wettability gradient formation along the paper sheet cross-section. Subsequent functionalization of the mesoporous silica-coated paper with the redox-reactive polymer PFcMA adds a switchable wettability.