Surface-immobilized PAMAM-dendrimers modified with cationic or anionic terminal functions: Physicochemical surface properties and conformational changes after application of liquid interface stress

Abstract

Functionalization of surfaces with highly branched dendrimer mols. has gained attractiveness for various applications because the no. of functional groups exceeds those of surfaces functionalized with self-assembled monolayers. So far, little is known about the physicochem. properties of dendrimer functionalized surfaces, esp. if the flexibility of dendrimer structure remains after covalent immobilization. Therefore, the purpose of this study was to covalently immobilize polyamidoamine (PAMAM) dendrimer mols. exhibiting terminal amine and carboxyl groups to silicon model surfaces and to explore their properties and structure at the solid-air and solid-liq. interface. The authors' results show that the surface free energy is higher for PAMAM coatings than for analogously terminated SAMs and also higher for carboxyl than amine functionalized coatings. Also, several findings suggest that conformational freedom of the dendrimers was preserved after surface immobilization. Wet compared to dry PAMAM-NH2 surfaces show reduced hydrophilicity and increased contact angle hysteresis, whereas PAMAM-COOH surfaces become more hydrophilic and showed decreased hysteresis. Streaming current measurements showed an unexpected behavior for PAMAM-COOH surfaces in that they reveal a net pos. surface charge over a wide pH range in spite of the carboxylated periphery. All of these results indicate a certain degree of masking, burrowing, back-folding and unfolding of functional groups upon environmental changes.