The Crucial Role of Charge in Thermoresponsive-Polymer-Assisted Reversible Dis/Assembly of Gold Nanoparticles

Dynamic control of the spacing between Au nanoparticles using nanoarchitectures incorporating the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) has the capability to induce strong color changes from the plasmon shifts. PNIPAM self-assembles on the surface of Au nanoparticles regardless of its terminal group. However, in many cases, the collapse of this PNIPAM coating at elevated temperatures fails to cause a color change, due to electrostatic and steric repulsion between the Au nanoparticles. Here, it is shown how tuning the charge repulsion between the nanoparticles is crucial to achieve large, reversible shifts of the plasmon resonances. Using NH2 terminal groups of the PNIPAM is most effective, compared with SH, COOH, and H terminations, due to their synergistic role in citrate stripping and charge neutralization. This detailed understanding of the Au-PNIPAM system is vital to enable temperature-responsive plasmonic systems with large tuning ranges, suitable for applications such as plasmonic actuators, displays, and Raman switches.