Dual pH-sensitive and UCST-type thermosensitive dendrimers: phenylalanine-modified polyamidoamine dendrimers with carboxyl termini

Dendrimers are unique polymers with well-defined structures, and are useful as functional unimolecular nanoparticles. Previous reports have shown that polyamidoamine (PAMAM) dendrimers modified with hydrophobic molecules, such as amino-terminal phenylalanine (Phe), are thermosensitive at high pH. In the present study, we designed carboxyl-terminal Phe-modified PAMAM dendrimers that are thermosensitive under acidic conditions. We reacted an amino-terminal PAMAM dendrimer with various acid anhydrides, such as succinic anhydride, cyclohexanedicarboxylic anhydride, and phthalic anhydride, prior to the reaction with Phe. Interestingly, the amino-terminal Phe-modified PAMAM dendrimers exhibited LCST (lower critical solution temperature)-type thermosensitivity at approximately pH 7, but the carboxyl-terminal Phe-modified dendrimers exhibited UCST (upper critical solution temperature)-type thermosensitivity in acidic solutions. Temperature sensitivity was dependent on both pH and the anhydride modifier. We were able to separate rose bengal (a model compound) from aqueous solutions of the carboxyl-terminal Phe-modified dendrimer at low pH.