pH-controlled reversible assembly of magnetic and plasmonic nanoparticles on gold nanorods

Overcoating gold nanorods (GNRs) with plasmonic or magnetic satellite nanoparticles (NPs) can modify the longitudinal and transverse localized surface plasmon resonance (LSPR and TSPR) through coupling with the satellite NPs. We report the use of electrostatic interactions to reversibly assemble different types and amounts of satellite NPs on GNR cores, which allows coupling with and manipulation of the LSPR and TSPR of the GNR core. Positively charged Fe₃O₄ NPs and spherical gold NPs (GNPs) functionalized with polyethylenimine (PEI) were assembled on the surface of negatively charged GNRs functionalized with bovine serum albumin (BSA). The distinct optical extinction spectra of Fe₃O₄ NPs and GNRs make straightforward quantification of the loading of satellite NPs possible. Successive overcoatings on GNRs with GNPs followed by Fe₃O₄ NPs are also demonstrated. pH is a useful parameter for controlling the assembly and loading of satellite NPs because the surface potentials on PEI- and BSA-functionalized NPs strongly depend on pH. The highest loading of satellite NPs is obtained at a pH of 10.8, which is near the isoelectric point (pH_I) of PEI (pH_I ∼11). Adjusting the pH to much higher or lower values below the pH_I of BSA (pH_I ∼5) drives disassembly. As a demonstration of the effect of disassembly, diattenuation of transmitted light is observed when GNRs coated with Fe₃O₄ NPs align in a magnetic field but vanish upon the removal of Fe₃O₄ NPs, randomizing the orientations of the GNRs.