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

Functionalization of surfaces with highly branched dendrimer molecules has gained attractiveness for various applications because the number of functional groups exceeds those of surfaces functionalized with self-assembled monolayers. So far, little is known about the physicochemical properties of dendrimer functionalized surfaces, especially if the flexibility of dendrimer structure remains after covalent immobilization. Therefore, the purpose of this study was to covalently immobilize polyamidoamine (PAMAM) dendrimer molecules exhibiting terminal amine and carboxyl groups to silicon model surfaces and to explore their properties and structure at the solid–air and solid–liquid interface. Our 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. Furthermore, several findings suggest that conformational freedom of the dendrimers was preserved after surface immobilization. Wet compared to dry PAMAMNH2 surfaces show reduced hydrophilicity and increased contact angle hysteresis, whereas PAMAMCOOH surfaces become more hydrophilic and showed decreased hysteresis. Streaming current measurements showed an unexpected behavior for PAMAMCOOH surfaces in that they reveal a net positive 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.

Flexibility of polyamidoamine (PAMAM) dendrimer structure was retained after covalent immobilization to silicon model surfaces as proven by zeta potential and dynamic contact angle analysis.Figure optionsDownload high-quality image (103 K)Download as PowerPoint slideHighlights
► Surface free energy is higher for dendrimer coatings than for corresponding monolayers.
► Surfaces modified with NH2-dendrimers turn hydrophobic during wetting with water.
► Surfaces modified with COOH-dendrimers turn hydrophilic during wetting with water.
► Surface charge formation is influenced by conformational changes of dendrimer branches.
► High flexibility of dendrimer branches is maintained after surface immobilization.