Dynamic Orientation Control of Gold Nanorods in Polymer Brushes by Their Thickness Changes for Plasmon Switching

Gold nanorods (AuNRs) have unique optical properties such as transverse and longitudinal localized surface plasmon resonance (T- and L-LSPR). As the L-LSPR absorption depends on the angle of the AuNRs to incident light and polarization, orientational control of AuNRs is a crucial issue. In spite of various techniques to control AuNR orientation, dynamic orientation tuning on a solid substrate remains challenging. Herein, dynamic changes are demonstrated in AuNR orientation in the anionic polymer (DNA) brushes via control of their thickness by salt concentration. AuNRs vertically align toward the substrates when their thickness exceeds the AuNR length. Once their thickness becomes shorter than the AuNR length, the attached AuNRs begin to tilt. The tilt reaches a maximum level close to horizontal when the thickness decreases to half that of the AuNR length. The dynamic control between the vertical (uniform) and tilted (random) orientation of the AuNRs showed not only absorption intensity changes in L-LSPR but also the switching of side-by-side plasmon coupling. The polymer brush-based system affords a novel platform for the stimuli-responsive control of AuNR orientation on the substrates via changes in the thickness of polymer brushes for actively tunable plasmonic substrates. Dynamic orientation control of gold nanorods (AuNRs) in polymer brushes on substrates is performed by shrinkage/extension of the polymers. The AuNR arrays are simply fabricated via attachment to the anionic polymer brushes. AuNR orientation on the substrates reversibly changes from vertical to horizontal in relation to changes in brush thickness, providing a novel platform for active plasmonics.image