Systematic approach to electrostatically induced 2D crystallization of nanoparticles at liquid interfaces

Abstract

We report an experimental demonstration of a strategy for inducing two-dimensional (2D) crystallization of charged nanoparticles on oppositely charged fluid interfaces. This strategy aims to maximize the interfacial adsorption of nanoparticles, and hence their lateral packing density, by utilizing a combination of weakly charged particles and a high surface charge density on the planar interface. In order to test this approach, we investigated the assembly of cowpea mosaic virus (CPMV) on positively charged lipid monolayers at the aqueous solution surface, by means of in situ X-ray scattering measurements at the liquid-vapor interface. The assembly was studied as a function of the solution pH, which was used to vary the charge on CPMV, and of the mole fraction of the cationic lipid in the binary lipid monolayer, which set the interface charge density. The 2D crystallization of CPMV occurred in a narrow pH range just above the particle's isoelectric point, where the particle charge was weakly negative, and only when the cationic-lipid fraction in the monolayer exceeded a threshold. The observed 2D crystals exhibited nearly the same packing density as the densest lattice plane within the known 3D crystals of CPMV. The above electrostatic approach of maximizing interfacial adsorption may provide an efficient route to the crystallization of nanoparticles at aqueous interfaces.

Crystallization, structural diversity and anisotropy effects in 2D arrays of icosahedral viruses

“…A lipid monolayer was formed from a binary mixture of cationic and neutral lipids at a fixed lipid density of 0.7 nm 2 per lipid (surface pressure 10–13 mN m −1 ), such that the interface charge density was controlled by the lipid composition. As shown previously for CPMV 28 and below for TYMV, the combination of weakly charged viruses and a highly charged lipid interface induced 2D crystallization by promoting dense lateral packing. The assembled structure was characterized by in situ grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XR).…”

“…The broad intensity modulations of these peaks along q z , the surface-normal component, arise from the form factor of TYMV, as in the case of CPMV. 28 Fig. 4 compares measured XR data from the lipid-coated interface without TYMV (solid lines) and with TYMV (circles).…”

“…The assembled structure was characterized by in situ grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XR). 28,32 The nature of the observed 2D crystals was examined further with the aid of numerical analysis of 2D packing based on model TYMV-shaped particles.…”

Role of electrostatic interactions in two-dimensional self-assembly of tobacco mosaic viruses on cationic lipid monolayers

“…As discussed above, over most of the probed range of lipid compositions, the charge density on the lipid monolayer should far exceed that on the TMV surface (<$0.1 e/nm 2 ). While the counter-ion release mechanism implies higher adsorption for weaker particle charge and higher monolayer charge density [14], adsorption is eventually limited by the electrostatic (and ultimately steric) repulsion between adsorbed particles. In addition, TMV's large size and surface curvature implies that the total area of the TMV-monolayer contacts should be much smaller than the total interfacial area.…”

“…The methods to tune the electrostatic interaction between charged building blocks include varying solution pH and ionic strength, which respectively control the net charge carried by each building block [14] and the screening of Coulomb repulsion between them [15]. The electrostatic interaction between TMVs can also depend on other parameters.…”