Quantifying the Selective Modification of Au(111) Facets via Electrochemical and Electroless Treatments for Manipulating Gold Nanorod Surface Composition

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

Thermal Stability of Thiolated DNA SAMs in Buffer: Revealing the Influence of Surface Crystallography and DNA Coverage via In Situ Combinatorial Surface Analysis

The thermal stability of thiol based DNA SAMs prepared on gold surfaces is an important parameter that is correlated to sensor lifetime. The thermal stability of DNA SAMs was evaluated in aqueous buffer through the use of fluorophore labeled DNA, a single crystal gold bead electrode, and microscopy. The stability of different crystallographic regions on the electrode was studied for thermal treatments up to 95 °C for 90 min. Using a in situ combinatorial surface analytical measurement showed that the crystallography of the underlying gold surface played a significant role, with the square or rectangular lattices (e.g., 110, 100, 210) having the highest stability. Surfaces with hexagonal lattices (e.g., 111, 311, 211) were less stable toward thermal treatments. These crystallographic trends were observed for both high and low coverage DNA SAMs. High coverage DNA SAMs were the most stable, with stability decreasing with decreasing coverage on average. Increasing DNA SAM coverage appears to slow the kinetics of thermal desorption, but the coordination to the underlying surface determined their relative stability. Preparing the DNA SAMs under nominally similar conditions were found to create surfaces that were similar at room temperature, but had significantly different thermal stability. Optimal DNA sensing with these surfaces most often requires low coverage DNA SAMs which results in poor thermal stability, which is predictive of a poor shelf life, making optimization of both parameters challenging. Furthermore, the crystallographically specific results should be taken into account when studying the typically used polycrystalline substrates since the underlying surface crystallography maybe different for different samples. It appears that preparing DNA SAMs with low coverage and significant thermal stability will be challenging using the current SAM preparation procedures.

Electrodepositing DNA Self-Assembled Monolayers on Au: Detailing the Influence of Electrical Potential Perturbation and Surface Crystallography

The preparation of DNA self-assembled monolayers (SAMs) on single-crystal gold bead electrodes using an applied potential is evaluated with in situ electrochemical fluorescence microscopy. Applying a constant deposition potential or a square-wave potential perturbation during the formation of DNA SAMs is compared for two different modification methods: DNA SAM formation on a clean gold surface followed by alkythiol backfilling (as is typically done in the literature) or via thiol-exchange on an alkylthiol-modified gold surface. DNA SAMs prepared from a chloride-containing deposition buffer were not significantly different when using either square-wave potential perturbation or at a constant applied potential even when considering different surface crystallographies. Greater variations were observed when applying more positive potentials for both DNA thiol-exchange and DNA adsorption on clean Au. Our results suggest that using either a constant potential or a square-wave potential perturbation for 5 min both create defects by weakening the gold-thiol interaction. When the deposition is performed with the adsorption of chloride ions from the electrolyte, the electrodeposition results in a similar increase in DNA coverage when compared to depositions performed at open circuit potentials.

Direct Mapping of Heterogeneous Surface Coverage in DNA-Functionalized Gold Surfaces with Correlated Electron and Fluorescence Microscopy

The characterization of biofunctionalized surfaces such as alkanethiol self-assembled monolayers (SAMs) on gold modified with DNA or other biomolecules is a challenging analytical problem, and access to a routine method is desirable. Despite substantial investigation from structural and mechanistic perspectives, robust and high-throughput metrology tools for SAMs remain elusive but essential for the continued development of these devices. We demonstrate that scanning electron microscopy (SEM) can provide image contrast of the molecular interface during SAM functionalization. The high-speed, large magnification range, and ease of use make this widely available technique a powerful platform for measuring the structure and composition of SAM surfaces. This increased throughput allows for a better understanding of the nonideal spatial heterogeneity characteristic of SAMs utilized in real-world conditions. SEM image contrast is characterized through the use of fluorescently labeled DNA, which enables correlative SEM and fluorescence microscopy. This allows identification of the DNA-modified regions at resolutions that approach the size of the biomolecule. The effect of electron beam irradiation dose is explored, which leads to straightforward lithographic patterning of DNA SAMs with nanometer resolution and with control over the surface coverage of specifically adsorbed DNA.