Helicoidal Patterning of Nanorods with Polymer Ligands

Abstract

Chiral packing of ligands on the surface of nanoparticles (NPs) is of fundamental and practical importance, as it determines how NPs interact with each other and with the molecular world. Herein, for gold nanorods (NRs) capped with end-grafted nonchiral polymer ligands, we show a new mechanism of chiral surface patterning. Under poor solvency conditions, a smooth polymer layer segregates into helicoidally organized surface-pinned micelles (patches). The helicoidal morphology is dictated by the polymer grafting density and the ratio of the polymer ligand length to nanorod radius. Outside this specific parameter space, a range of polymer surface structures was observed, including random, shish-kebab, and hybrid patches, as well as a smooth polymer layer. We characterize polymer surface morphology by theoretical and experimental state diagrams. The helicoidally organized polymer patches on the NR surface can be used as a template for the helicoidal organization of other NPs, masked synthesis on the NR surface, as well as the exploration of new NP self-assembly modes.

Keywords: helical structures; nanorods; patterning; polymers; self-assembly.

Figure 1.

(a) Schematic of the NR functionalized with end-grafted polymer. (b) Schematic of symmetric polymer micelle (patch) with micelle core radius, R. The core and the dimensions of the “legs” are not given to scale. (c) Theoretical diagram of the polymer morphology on the cylindrical NR segment. Hemi-spherical tips of the NRs are omitted.

Figure 2.

TEM images of PS-coated NRs in the DMF/water mixture at C_w = 4 vol% at (a-c) σ = 0.2 chain/nm² and lN_p/r (top-to-bottom) of 5 (PS-20K), 12 (PS-50K), and 29 (PS-135K), respectively, and (d-f) at lN_p/r = 14 (PS-50K) with σ (top-to-bottom) of 0.1, 0.15, and 0.2 chain/nm², respectively. Scale bars are 100 nm.

Figure 3.

Experimental diagram of polymer morphology states plotted in the lN_p/r and grafting density σ coordinates. The ratio IN_p/r was changed by varying the molecular weight of the polymer ligands from PS-20K to PS-135K and accounting for the NR radius r measured for each sample. The inset shows a fragment of the theoretical state diagram pertinent to experimental diagram.

Figure 4.

Analysis of the HP on the NR surface. (a) Histogram of polymer morphologies at σ = 0.15 chains/nm², PS-50K, and lN_p/r of 12-15. (b) Variation in the number of turns with the length of the NR. Insets show NRs with L = 153 and 226, and n = 1 and 3, respectively. The corresponding data points are represented by the hollow points. Scale bars are 100 nm.

Figure 5.

Schematics of the formation of the helicoidal polymer structures on the NR surface with increasing curvature κ = 1/r). The region outlined in red corresponds to the micelle footprint with an area A.