Salt Mediated Self-Assembly of Poly(ethylene glycol)-Functionalized Gold Nanorods

Although challenging, assembling and orienting non-spherical nanomaterials into two- and three-dimensional (2D and 3D) ordered arrays can facilitate versatile collective properties by virtue of their shape-dependent properties that cannot be realized with their spherical counterparts. Here, we report on the self-assembly of gold nanorods (AuNRs) into 2D films at the vapor/liquid interface facilitated by grafting them with poly(ethylene glycol) (PEG). Using surface sensitive synchrotron grazing incidence small angle X-ray scattering (GISAXS) and specular X-ray reflectivity (XRR), we show that PEG-AuNRs in aqueous suspensions migrate to the vapor/liquid interface in the presence of salt, forming a uniform monolayer with planar-to-surface orientation. Furthermore, the 2D assembled PEG functionalized AuNRs exhibit short range order into rectangular symmetry with side-by-side and tail-to-tail nearest-neighbor packing. The effect of PEG chain length and salt concentration on the 2D assembly are also reported.

XRR and GISAXS data for PEG5K-AuNRs with 2 M NaCl The effect of salt concentration is further explored by increasing the concentration of NaCl up to 2 M. The first maximum in R/R F data and extracted ρ 2D ED profile show small increase of PEG-AuNRs at the interface as the concentration of NaCl increases. In spite of slightly increased ED profile at the 2 M NaCl, the thickness of ED stratum is maintained, indicating that a uniform single PEG5K-AuNR film is formed. As with the XRR, the intensity of the GISAXS linecut profile is also slightly increased at the 2 M NaCl compared to the lower concnetrations. Figure S1: (A) Normalized XRR data and (C) 2D GISAXS patterns (Q xy ,Q z ) for PEG5K-AuNRs with 2 M NaCl. Solid line of R/R F in (A) is obtained from one of the best fit electron density (ED) profile shown in (B). (D) Horizontal Q xy linecut profiles (integrated over a Q z range from 0.02 to 0.1 Å −1 ) from (C).

Estimation of surface coverage of assembled PEG-AuNRs
To determine the extent of accumulation of AuNRs at the interface, the excess electron surface density (ρ 2D ) is calculated by using equation as follows, ρ 2D of PEG5K-AuNRs and PEG2K-AuNRs are 14.4 and 56.3 e/Å 2 , respectively. ρ 2D is higher for PEG2K-AuNRs than that of PEG5K-AuNRs, indicating aqueous surface is more populated with PEG2K-AuNRs, which is consistent with the smaller lattice constants (d 1 , Next, we estimate the surface coverage of PEG-AuNRs with the maximum electron den- Figure S2: Schematic illustration of (A) PEG5K-AuNR and (B) PEG2K-AuNR layers at the vapor/liquid interface and the corresponding sectional views at the maximum electron density. sity (ED) by using a space filling model. We assume that all AuNRs are ideally packed as shown in Figure S2, and the maximum ED position is calculated. For PEG5K-AuNRs, the maximum ED position is the center of AuNRs and its corresponding sectional view is illustrated in Figure S2(A). In this plane at the maximum ED of PEG5K-AuNRs, the area fraction of AuNRs in the 2D crystalline is calculated as follows, The area fraction of AuNRs, φ = A AuN R / A 2D ,where A AuN R is the area occupied by an AuNRs and A 2D is the area of a 2D crystalline with lattice constants (a 1 , a 2 ) from GISAXS.
Using same methods as that in PEG5K-AuNRs, the surface coverage of PEG2K-AuNRs is estimated as 18.4 % and the maximum surface coverage is approximately 40.0 %, assuming that all the nanorods in nanorod suspensions with a concentration of 1.5 nM have migrated to the vapor/liquid interface. As with the result from excess electron surface density (ρ 2D ), the result from surface coverage estimation suggests that more PEG2K-AuNRs migrates and accumulates at the vapor/liquid interface than PEG5K-AuNRs. Since we have estimated the surface coverage of PEG-AuNRs by assuming an ideal 2Dassembly of PEG-AuNRs and using simple model, our estimated surface coverage values may differ from the actual values.
However, we note that it would be an effective way to compare 2D assembled PEG5K-AuNRs and PEG2K-AuNRs easily.  Hydrodynamic diamter of bare AuNRs and PEG-AuNRs Figure S4 shows the schematic illustration of hydrodynamic diameter of bare AuNRs, PEG2K-AuNRs, and PEG5K-AuNRs. The actual length of PEG polymer attached to the AuNR surface is much longer than the one depicted in Figure S4.

UV-vis spectroscopy of PEG-AuNRs.
Absorption spectra of PEG-AuNRs were acquired using Thermo Fisher NanoDrop One. A 2 µl drop of PEG-AuNR solution was placed on NanoDrop pedestal to measure the UVvis spectrum against deionized water as the reference. UV-vis spectra show a measurable red-shift from 807 to 810 nm (for bare and grafted AuNR, respectively; UV-vis spectra of bare AuNRs is provided by the manufacturer) due to surface modification, in supports of successful grafting by PEG. Figure S5: UV-VIS spectrum of PEG-AuNRs.