210 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS often have adverse effects on the reproducibility of the preparation protocols. Studies depicting immo- bilization of PDA liposomes on solid substrates have been also reported (Kim et al. 2003). These studies employed PDA-displaying amine moieties which were coupled to aldehyde-derivatized glass surfaces. The covalently immobilized vesicles could still be polymerized and retained their unique chromatic properties and analyte-induced color-changing capabilities (Kim et al. 2003). 3. MEMBRANES IN CHEMICAL SENSORS 3.1. CHEMICAL MEMBRANES Chemical membranes are defined here as assembled chemical systems that exhibit barrier properties, permeation, and molecular sieving. Diverse classes of materials display such properties, some widely used in sensing devices. In particular, cross-linked polymeric materials and hydrogels have attracted considerable research because of their perceived "intelligent architecture" (Peppas et al. 2006). These materials can form a variety of organized structures, such as thin films, scaffolds, colloids, and nanopar- ticles, which together underlie their versatile sensor applications. 3.1.1. Hydrogels