Effect of (Poly)electrolytes on the Interfacial Assembly of Poly(ethylene glycol)-Functionalized Gold Nanoparticles

Abstract

We report on the effect of interpolymer complexes (IPCs) of poly(acrylic acid) (PAA) with poly(ethylene glycol) functionalized Au nanoparticles (PEG-AuNPs) as they assemble at the vapor-liquid interface, using surface sensitive synchrotron X-ray scattering techniques. Depending on the suspension pH, PAA functions both as a weak polyelectrolyte and a hydrogen bond donor, and these two roles affect the interfacial assembly of PEG-AuNPs differently. Above its isoelectric point, we find that PAA leads to the formation of a PEG-AuNPs monolayer at the interface with hexagonal structure. In the presence of high concentration of HCl (i.e., below the isoelectric point), at which PAA forms IPCs with PEG, the hexagonal structure at the interface appears to deteriorate, concurrent with aggregation in the bulk. Thus, while electrolytic behavior of PAA induces interfacial assembly, hydrogen bonding behavior, as PAA becomes neutral, favors the formation of 3D assemblies. For comparison, we also report on the formation of PEG-AuNPs monolayers (in the absence of PAA) with strong electrolytes such as HCl, H2SO4 and NaOH that lead to a high degree of crystallinity. Disciplines

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

“…54 The lateral packing structure of the NSts on the aqueous surface is probed via GISAXS. 28,31 For the GISAXS, the incident angle of the X-ray beam, α i , is kept below the critical angle for total reflection, α c . Here, the intensity versus Q xy , the horizontal component of Q, is integrated over Q z from 0.02 to 0.1 Å −1 .…”

“…23,24 In addition to the saltmediated strategy, inter-polymer complexation (IPC), using poly(acrylic acid) (PAA) in the suspension below its isoelectric point (pI), has been recently developed for the 2D and 3D assembly of PEG-AuNPs. 30,31 Beyond traditional spherical nanoparticles, new opportunities are provided by developing strategies for self-assembly and crystallization of anisotropic nanostructures. [32][33][34][35][36][37][38][39][40][41] The advantage of using anisotropic nano-objects is their distinct properties that can be achieved by manipulating their shape and/or size.…”

Unusual Effect of Iodine Ions on the Self-Assembly of Poly(ethylene glycol)-Capped Gold Nanoparticles

“…and K 3 PO 4 . 4,5,14 By contrast, the ED profiles from PEG-AuNPs with NaI in the suspensions change significantly both in shape and magnitude as the concentration increases from 0.5 to 1.5 M. The increase in area under the peaked region of the ED is evidence for increased enrichment of PEG-AuNPs at the surface. For 0.5 M NaI, the ED profile is somewhat similar to that obtained with CsCl, albeit with broader interfaces.…”

“…[1][2][3] Adding electrolytes or polyelectrolytes to the suspension drives PEG-AuNPs to the vapor-liquid interface to form a mono-particle layer, reminiscent of a Gibbs monolayer. 1,4,5 Furthermore, beyond a threshold salt concentration (salt-specific) the surface-bound PEG-AuNPs crystallize into two dimensional (2D) hexagonal superlattices with a lattice constant that can be controlled by a specific salt and its concentration. 4,5 At sufficiently high salt concentrations, small angle X-ray scattering studies reveal that PEG-AuNPs assemble into three dimensional (3D) superlattices in the bulk suspension or precipitate.…”

“…2 In fact, the assembly of PEG-AuNPs into 2D superlattices is so robust that it can be induced at high or low pH values and also with a plethora of salts. 4,5 Based on the experimental results of various electrolyte species, it is believed that the grafted PEG is in the θ-point state 1 and that the effectiveness of ions in driving self-assembly follows to a good approximation the Jones-Dole ionic-coefficient series. 4 The grafted PEG on the surface of the AuNPs, as other surface ligands, mediates the inter-particle interactions that determine the spatial arrangement of AuNPs, such as ordered or disordered arrangement, and inter-particle spacing.…”