Abstract
In-situ X-ray reflectivity (XRR) and grazing incidence X-ray small-angle scattering (GISAXS) reveal that unfunctionalized (bare) gold nanoparticles (AuNP) spontaneously adsorb to a cationic lipid template formed by a Langmuir monolayer of DPTAP (1,2-dihexadecanoyl-3-trimethylammonium-propane) at vapor/aqueous interfaces. Analysis of the XRR yields the electron density profile across the charged-interfaces along the surface normal showing the AuNPs assemble with vertical thickness comparable to the particle size. The GISAXS analysis indicates that the adsorbed mono-particle layer exhibits short-range in-plane correlations. By contrast, single-stranded DNA-functionalized AuNPs, while attracted to the positively charged surface (more efficiently with the addition of salt to the solution), display less in-plane regular packing compared to bare AuNPs.
Highlights
Langmuir monolayers assembled with insoluble, amphiphilic organic molecules, exhibiting rich phase behaviors, have been studied extensively as a model two dimensional (2D) systems for decades[1]
In-situ X-ray reflectivity (XRR) and grazing incidence X-ray small-angle scattering (GISAXS) reveal that unfunctionalized gold nanoparticles (AuNP) spontaneously adsorb to a cationic lipid template formed by a Langmuir monolayer of DPTAP (1,2-dihexadecanoyl-3-trimethylammonium-propane) at vapor/aqueous interfaces
Using surface sensitive diffraction methods, i.e., X-ray reflectivity and grazing incidence X-ray small-angle scattering, we demonstrate that bare gold nanoparticles (AuNP) are negatively charged and spontaneously adsorb to a cationic lipid template formed by a Langmuir monolayer (in this case formed by DPTAP (1,2-dihexadecanoyl-3-trimethylammonium-propane)
Summary
Langmuir monolayers assembled with insoluble, amphiphilic organic molecules, exhibiting rich phase behaviors, have been studied extensively as a model two dimensional (2D) systems for decades[1]. Surface X-ray scattering techniques, such as specular reflectivity (XRR) and grazing incidence diffraction (GIXD), have played a decisive role in relating the phase behaviors, generally obtained from surface-pressure versus molecular area (π −A) isotherms, to the structure of the monolayer at unprecedented molecular length scales[2]. We explore the interplay of the two key components of template-directed self-assembly, AuNPs as building blocks and a cationic lipid monolayer, i.e. 1,2-dihexadecanoyl-3-trimethylammonium-propane (DPTAP), as the template. The motivation of this two-dimensional model-system study is to explore underlying control parameters that can be exploited by design to assemble regular structures from nano-particles. We demonstrate the application of synchrotron based X-ray liquid surface scattering techniques, i.e., specular reflectivity and grazing incidence scattering, and provide analytical tools to analyze similar GISAXS data in general
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