synthesis-and-characterization-of-amphiphilic-gold-nanoparticles

Synthesis and Characterization of Amphiphilic Gold Nanoparticles

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of ...

École Polytechnique Fédérale de Lausanne

Gold nanoparticles owe a large number of their properties to their ligand shell. Indeed, many researchers routinely use mixtures of ligand molecules for their nanoparticles to impart complex property sets. It has been shown that the morphology of ligand shells (e.g., Janus, random, stripelike) leads to specific properties. Examples include wettability, solubility, protein nonspecific adsorption, cell penetration, catalysis, and cation-capturing abilities. Yet, it remains a great challenge to evaluate such morphologies in even the most fundamental terms such as dimension and shape. In this Account, we review recent progress in characterization techniques applicable to gold nanoparticles with ligand shells composed of mixed ligands. We divide the characterization into three major categories, namely, microscopy, spectroscopy, and simulation. In microscopy, we review progresses in scanning tunneling microscopy (STM), atomic force microscopy (AFM), and scanning/transmission electron microscopy. In spectroscopy, we mainly highlight recent achievements in nuclear magnetic resonance (NMR), mass spectrometry (MS), small angle neutron scattering (SANS), electron spin resonance (EPR), and adsorption based spectroscopies. In simulation, we point out the latest results in understanding thermodynamic stability of ligand shell morphology and emphasize the role of computer simulation for helping interpretation of experimental data. We conclude with a perspective of future development.

Characterization of Ligand Shell for Mixed-Ligand Coated Gold Nanoparticles.

Ligand exchange reactions are used to achieve nanoparticles coated with a mixture of ligand molecules. Currently, nothing is known on the evolution of the morphology of the ligand shell during the reaction. Here, we use a recently developed method (based on MALDI-TOF) to follow the evolution of the ligand shell composition and morphology during the reaction. We observe the expected evolution in composition and we find that the ligand shell starts as a random mixture and gradually evolves towards a patchy morphology. When the composition has reached a plateau (i.e. when the reaction is generally assumed to be finished), the ligand shell morphology keeps evolving for days, slowly approaching its equilibrium configuration.

Evolution of the Ligand Shell Morphology during Ligand Exchange Reactions on Gold Nanoparticles.

The van der Waals (vdW) interactions of n-alkanethiols (ATs) adsorbed on planar Au(111) and Au(100) surfaces and curved Au nanoparticles of different diameters are reported. By means of electrochemical measurements and molecular dynamic calculations, the increase in the average geometrical curvature of the surface influences the global interactions, that is, decreasing vdW interactions between neighboring molecules. Small NPs do not present the same electrochemical behavior as planar surfaces. The transition between nanoparticle to flat surface electrochemical response is estimated to occur at a circa 13-20 nm diameter range.

The van der Waals Interactions of n-Alkanethiol-Covered Surfaces: From Planar to Curved Surfaces.

The ligand shell (LS) determines a number of nanoparticles' properties. Nanoparticles' cores can be accurately characterized; yet the structure of the LS, when composed of mixture of molecules, can be described only qualitatively (e.g., patchy, Janus, and random). Here we show that quantitative description of the LS' morphology of monodisperse nanoparticles can be obtained using small-angle neutron scattering (SANS), measured at multiple contrasts, achieved by either ligand or solvent deuteration. Three-dimensional models of the nanoparticles' core and LS are generated using an ab initio reconstruction method. Characteristic length scales extracted from the models are compared with simulations. We also characterize the evolution of the LS upon thermal annealing, and investigate the LS morphology of mixed-ligand copper and silver nanoparticles as well as gold nanoparticles coated with ternary mixtures. Our results suggest that SANS combined with multiphase modeling is a versatile approach for the characterization of nanoparticles' LS.

Quantitative 3D determination of self-assembled structures on nanoparticles using small angle neutron scattering.

Janus, patchy, stripe-like, or random arrangements of molecules within the ligand shell of nanoparticles affect many properties. Among all existing ligand shell morphology characterization methods, the one based on mass spectroscopy is arguably the simplest. Its greatest limitation is that the results are qualitative. Here, we use a tailor-made Monte Carlo type program that fits the whole MALDI spectrum and generates a 3D model of the ligand shell. Quantitative description of the ligand shell in terms of nearest neighbor distribution and characteristic length scale can be readily extracted by the model, and are compared with the results of other characterization methods. A parameter related to the intermolecular interaction is extracted when this method is combined with NMR. This approach could become the routine method to characterize the ligand shell morphology of many nanoparticles and we provide an open access program to facilitate its use.

Zhi Luo

Institute of Materials

École Polytechnique Fédérale de Lausanne

Synthesis and Characterization of Amphiphilic Gold Nanoparticles