Quy Khac Ong

Synthesis and Characterization of Janus Gold Nanoparticles

When gold nanoparticles are coated with binary mixtures of dislike ligand molecules, separation in the ligand shell occurs; if the particles are smaller than a threshold size the separation is solely enthalpy driven leading to the spontaneous formation of Janus particles.

Quantitative Analysis of Scanning Tunneling Microscopy Images of Mixed-Ligand-Functionalized Nanoparticles

Ligand-protected gold nanoparticles exhibit large local curvatures, features rapidly varying over small scales, and chemical heterogeneity. Their imaging by scanning tunneling microscopy (STM) can, in principle, provide direct information on the architecture of their ligand shell, yet STM images require laborious analysis and are challenging to interpret. Here, we report a straightforward, robust, and rigorous method for the quantitative analysis of the multiscale features contained in STM images of samples consisting of functionalized Au nanoparticles deposited onto Au/mica. The method relies on the analysis of the topographical power spectral density (PSD) and allows us to extract the characteristic length scales of the features exhibited by nanoparticles in STM images. For the mixed-ligand-protected Au nanoparticles analyzed here, the characteristic length scale is 1.2 ± 0.1 nm, whereas for the homoligand Au NPs this scale is 0.75 ± 0.05 nm. These length scales represent spatial correlations independent of scanning parameters, and hence the features in the PSD can be ascribed to a fingerprint of the STM contrast of ligand-protected nanoparticles. PSD spectra from images recorded at different laboratories using different microscopes and operators can be overlapped across most of the frequency range, proving that the features in the STM images of nanoparticles can be compared and reproduced.

Characterization of Ligand Shell for Mixed-Ligand Coated Gold Nanoparticles

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.

Response to "Critical Assessment of the Evidence for Striped Nanoparticles".

Stirling et al., (10.1371/journal.pone.0108482) presented an analysis on some of our publications on the formation of stripe-like domains on mixed-ligand coated gold nanoparticles. The authors shed doubts on some of our results however no valid argument is provided against what we have shown since our first publication: scanning tunneling microscopy (STM) images of striped nanoparticles show stripe-like domains that are independent of imaging parameters and in particular of imaging speed. We have consistently ruled out the presence of artifacts by comparing sets of images acquired at different tip speeds, finding invariance of the stipe-like domains. Stirling and co-workers incorrectly analyzed this key control, using a different microscope and imaging conditions that do not compare to ours. We show here data proving that our approach is rigorous. Furthermore, we never solely relied on image analysis to draw our conclusions; we have always used the chemical nature of the particles to assess the veracity of our images. Stirling et al. do not provide any justification for the spacing of the features that we find on nanoparticles: ~1 nm for mixed ligand particles and ~ 0.5 nm for homoligand particles. Hence our two central arguments remain unmodified: independence from imaging parameters and dependence on ligand shell chemical composition. The paper report observations on our STM images; none is a sufficient condition to prove that our images are artifacts. We thoroughly addressed issues related to STM artifacts throughout our microscopy work. Stirling et al. provide guidelines for what they consider good STM images of nanoparticles, such images are indeed present in our literature. They conclude that the evidences we provided to date are insufficient, this is a departure from one of the authors’ previous article which concluded that our images were composed of artifacts. Given that four independent laboratories have reproduced our measurements and that no scientifically rigorous argument is presented to invalidate our STM images, and also given that Stirling et al. do not contest the quality of our recent STM images, we re-affirm that specific binary mixture of ligands spontaneously form features in their ligand shell that we describe as stripe-like domains ~1 nm in width.

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

Novel Sensing Strategies Based on Monolayer Protected Gold Nanoparticles for the Detection of Metal Ions and Small Molecules

Monolayer protected gold nanoparticles (AuNPs) have a huge potential for the development of innovative sensing systems for the detection of metal ions and small molecules. The organic ligand shell, primarily utilized to stabilize the gold core, can be rationally designed to promote selective interactions with a desired analyte. In addition, the outstanding physical and optical properties of AuNPs can be exploited to obtain analytically useful signals upon analyte binding. In this account, we review recent advances in AuNP-based sensing systems emphasizing on the rational design of the ligand shell for detection of heavy metal ions and small molecules. We examine various strategies of detection by evaluating their improvements in terms of sensitivity, selectivity and applicability to real samples.

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

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.The ligand shell of a nanoparticle remains difficult to resolve, as the available characterization methods provide only qualitative information. Here, the authors introduce an approach based on small-angle neutron scattering that can quantitatively reveal the organization of ligands in mixed-monolayer nanoparticles.

A review of molecular phase separation in binary self-assembled monolayers of thiols on gold surfaces*

Binary self-assembled monolayers (SAMs) on gold surfaces have been known to undergo molecular phase separation to various degrees and have been subject to both experimental and theoretical studies. On gold nanoparticles in particular, binary SAMs ligand shells display intriguing morphologies. Consequently, unexpected behaviors of the nanoparticles with respect to their biological, chemical, and interfacial properties have been observed. It is critical that the phase separation of binary SAMs be understood at both molecular and macroscopic level to create, and then manipulate, the useful properties of the functionalized surfaces. We look into the current understanding of molecular phase separation of binary SAMs on gold surfaces, represented by Au(111) flat surfaces and Au nanoparticles, from both theoretical and experimental aspects. We point out shortcomings and describe several research strategies that will address them in the future.

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

Abstract 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.