pH-Responsive assembly of metal nanoparticles and fluorescent dyes by diblock copolymer micelles.

Hybrid assemblies consisting of metal nanoparticles (NPs) and fluorophores are quite interesting because the intrinsic properties of fluorophores can be engineered in the assembled structure. In this regard, we utilized the self-segregation properties of block copolymer micelles to organize metal NPs and fluorophores simultaneously in a specific arrangement. From the viewpoint of assembly methods, we first encapsulated Au NPs in the PS cores of polystyrene-block-poly(acrylic acid) (PS-PAA) micelles. Then, positively charged fluorescent dyes of rhodamine 123 (R123) were bound to the negatively charged PAA coronas by electrostatic interactions. Since carboxylic acid in the PAA block is a weak acid, the degree of R123 binding to PS-PAA micelles can be adjusted by varying the pH of the solution. Therefore, by changing the pH, we were able to control the assembly and disassembly of R123 molecules to PS-PAA micelles and the corresponding change in the fluorescence signal.