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

Abstract

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.

Interfacial and Bulk Assembly of Anisotropic Gold Nanostructures: Implications for Photonics and Plasmonics

“…Given the length and thickness of AuNT (length 45 nm, thickness ≤ 23 nm). These values and the estimated thickness of the PEG corona surrounding the NSts 47,55 suggest that the PEG-AuNTs are mostly anchored face-on at the vapor/liquid interface (as illustrated in Figure 1), and consistent with the aforementioned XRR analysis. Figure 5(b) shows a similar GISAXS patterns for PEG-AuNOh, implying only shortrange order with ∼ 33 to 32 nm inter-particle distances at increased HCl concentration in the presence of PAA2k.…”

“…Both AuNTs and AuNOh are nano-sized structures (NSt) and are referred to as AuNSts in general without context. Synthesized AuNTs and AuNOh are grafted with PEG10k and PEG5k through a ligand exchange process and centrifugation as described elsewhere, 47 and are referred to as PEG-AuNTs and PEG-AuNOh, respectively.…”

“…This confirms the grafting of PEG on the facets of the AuNSts and also that the PEG-AuNSts are well dispersed in the suspensions (see more details in the SI, Figure S1). 47 Figure S2 shows the UV-visible spectra (SpectraMax M3, Molecular Devices) of PEG-AuNSts dispersed in the bulk solution. The surface plasmon resonance peaks (λ max ) of PEG-AuNTs and PEG-AuNOh are at ∼ 580 nm and ∼ 550 nm, respectively, in agreement with pre-vious reports.…”

“…The experimental protocols in the two beamlines are similar to those already described elsewhere. 24,28,30,31,47 Briefly, the measured X-ray reflectivity, R(Q z ), is a function of the vertical component of the scattering vector Q, and displayed in our figures after normalization to the calculated Fresnel reflectivity, R F , for an ideally sharp and flat surface of the bulk solution. From the reflectivity we de-termine the electron density (ED) profile, ρ(z), across the interface by refining the parameters of ρ(z) to obtain the best calculated fit to the measured reflectivity using the Parratt's recursive method, z-axis being along the surface normal.…”

“…Nevertheless, the assembly of anisotropic nanostructures remains a challenge, compared to the assembly of isotropic nanoparticles. 45,46 Here, we report on the assembly of PEG-grafted nanotriangles (NTs) and nano-octahedra (NOh) in salt or PAA suspensions at the vapor/liquid interfaces 47 and in the bulk, as illustrated in…”