Poly(2-vinyl pyridine) Brush Grafting Density Controls Gold Nanorod Adsorption and Orientation

Gold nanorods (AuNRs) possess orientation-dependent optical properties, making them promising for plasmonic applications. Poly(2-vinyl pyridine) (P2VP) brushes can serve as a tunable scaffold for controlling AuNR adsorption and orientation through modifications of both grafting density and pH. Planar glass substrates grafted with P2VP brushes were exposed to citrate-modified AuNRs, which were negatively charged, to examine how brush structure and environment affect alignment. P2VP brushes with increasing grafting densities were prepared by varying annealing time and characterized by ellipsometry. Substrates were immersed in 200 pM AuNR (14 × 45 nm) solutions at pH 4.0 and 6.2 for 1–96 h, and ultraviolet–visible spectroscopy measured adsorption and orientation. At neutral pH (6.2), higher grafting densities favored horizontally oriented AuNRs that assembled end-to-end, as indicated by red-shifted longitudinal surface plasmon resonance (LSPR) peaks. At acidic pH (4.0), stronger electrostatic attraction between protonated P2VP and negatively charged AuNRs led to more random orientations. Overall, longer immersion times increased adsorption. These results show that P2VP brush density and pH can tune anisotropic nanoparticle alignment for responsive plasmonic materials.