Polydopamine coating decreases longitudinal plasmon of Au nanorods: Experiment and simulations

Abstract Polydopamine (PDA) coated Au nanorods (AuNRs) have attracted great interest as a convenient platform to construct various multifunctional nanoparticles for theranostic applications. Typically, the formation of a dielectric shell on the surface of plasmonic particles results in an increase in the particle extinction and in a small red shift of the plasmonic peak. However, compared to the well-known coatings such as silica, biopolymers, and polyelectrolytes, polydopamine is light absorbing material in the visual and NIR spectral range. In this paper, we demonstrate, for the first time, unusual changes in plasmonic properties of AuNRs after PDA coating. Namely, the longitudinal plasmon resonance decreases while the transversal one increases compared to the corresponding initial values of bare AuNRs. Remarkably, the increasing and red shifting of the transversal plasmon is similar to the common changes in plasmonic properties of PDA-coated spheres. To confirm the experimental observations, we simulated the extinction, absorption and scattering spectra for AuNRs with different geometrical properties and PDA shell thicknesses by using two literature models for the refractive index of PDA. The decreasing of plasmonic peak intensity became higher with an increase in the AuNR aspect ratio and the imaginary part of PDA refractive index. Our experimental results and simulations provide a new insight into understanding the optical properties of composite plasmonic particles.