New trends in the functionalization of metallic gold: from organosulfur ligands to N-heterocyclic carbenes.

In this Tutorial Review, we describe the development of new ligands for functionalizing and stabilizing metallic gold in the form of planar gold surfaces and gold nanoparticles (NPs). Starting from the state-of-the-art of organosulfur ligands, we describe the gold-sulfur bond formation and the nature of the resulting interface. In addition, we explain methods to prepare ordered monolayers on planar surfaces and stable ligand shells around NPs, illustrating important pioneering studies and examples of current research. Moreover, we highlight recent advancement in functionalizing gold by N-heterocyclic carbenes (NHCs), a promising alternative ligand class regarding stability and variable design strategies. We discuss the chemistry of the carbene-gold bond and report on advantages of this new ligand. Additionally, selected examples of current research illustrate the formation of ultra-stable self-assembled monolayers of NHCs on gold surfaces as well as the preparation of NHC-stabilized gold NPs.

Kinetic control in the temperature-dependent sequential growth of surface-confined supramolecular copolymers.

We report the sequential growth of supramolecular copolymers on gold surfaces, using oppositely charged dendritic peptide amphiphiles. By including water-solubilising thermoresponsive chains in the monomer design, we observed non-linear effects in the temperature-dependent sequential growth. The step-wise copolymerisation process is characterised using temperature dependent SPR and QCM-D measurements. At higher temperatures, dehydration of peripheral oligoethylene glycol chains supports copolymer growth due to more favourable comonomer interactions. Both monomers incorporate methionine amino acids but remarkably, desorption of the copolymers via competing sulphur gold interactions with the initial monomer layer is not observed. The surface-confined supramolecular copolymers remain kinetically trapped on the metal surface at near neutral pH and form viscoelastic films with a tuneable thickness.

Arylazopyrazoles as Light-Responsive Molecular Switches in Cyclodextrin-Based Supramolecular Systems.

A simple and high yield synthesis of water-soluble arylazopyrazoles (AAPs) featuring superior photophysical properties is reported. The introduction of a carboxylic acid allows the diverse functionalization of AAPs. Based on structural modifications of the switching unit the photophysical properties of the AAPs could be tuned to obtain molecular switches with favorable photostationary states. Furthermore, AAPs form stable and light-responsive host-guest complexes with ß-cyclodextrin (ß-CD). Our most efficient AAP shows binding affinities comparable to azobenzenes, but more effective switching and higher thermal stability of the Z-isomer. As a proof-of-principle, we investigated two CD-based supramolecular systems, containing either cyclodextrin vesicles (CDVs) or cyclodextrin-functionalized gold nanoparticles (CDAuNPs), which revealed excellent reversible, light-responsive aggregation and dispersion behavior. To conclude, AAPs have great potential to be incorporated as molecular switches in highly demanding and multivalent photoresponsive systems.

Impact of the Nanoscale Gap Morphology on the Plasmon Coupling in Asymmetric Nanoparticle Dimer Antennas.

Coupling of plasmon resonances in metallic gap antennas is of interest for a wide range of applications due to the highly localized strong electric fields supported by these structures, and their high sensitivity to alterations of their structure, geometry, and environment. Morphological alterations of asymmetric nanoparticle dimer antennas with (sub)-nanometer size gaps are assigned to changes of their optical response in correlative dark-field spectroscopy and high-resolution transmission electron microscopy (HR-TEM) investigations. This multimodal approach to investigate individual dimer structures clearly demonstrates that the coupling of the plasmon modes, in addition to well-known parameters such as the particle geometry and the gap size, is also affected by the relative alignment of both nanoparticles. The investigations corroborate that the alignment of the gap forming facets, and with that the gap area, is crucial for their scattering properties. The impact of a flat versus a rounded gap structure on the optical properties of equivalent dimers becomes stronger with decreasing gap size. These results hint at a higher confinement of the electric field in the gap and possibly a different onset of quantum transport effects for flat and rounded gap antennas in corresponding structures for very narrow gaps.

Kinetically Controlled Sequential Growth of Surface-Grafted Chiral Supramolecular Copolymers.

We report a facile strategy to grow supramolecular copolymers on Au surfaces by successively exposing a surface-anchored monomer to solutions of oppositely charged peptide comonomers. Charge regulation on the active chain end of the polymer sufficiently slows down the kinetics of the self-assembly process to produce kinetically trapped copolymers at near-neutral pH. We thereby achieve architectural control at three levels: The ß-sheet sequences direct the polymerization away from the surface, the height of the supramolecular copolymer brushes is well-controlled by the stepwise nature of the alternating copolymer growth, and 2D spatial resolution is realized by using micropatterned initiating monomers. The programmable nature of the resulting architectures renders this concept attractive for the development of customized biomaterials or chiral interfaces for optoelectronics and sensor applications.

Sequential surface modification of au nanoparticles: from surface-bound Ag(I) complexes to Ag(0) doping.

Gold nanoparticles (Au NPs) with tailor-made structures and properties are highly desirable for applications in catalysis and sensing. In this context, surface modifications of Au NPs are of particular relevance. Herein, we present a sequential surface modification of Au NPs with Ag(I) coordination complexes, which can be converted into Ag(0)-doped Au NPs by simple ligand-exchange reaction. The key innovative element of this surface modification is a multifunctional bioxazoline-based ligand that brings coordinated Ag(I) into close proximity to the particle surface.

Immobilization of liposomes and vesicles on patterned surfaces by a peptide coiled-coil binding motif.

Patchy surfaces: An azide-terminated self-assembled monolayer was patterned with the peptide sequence (EIAALEK)(3) by using microcontact printing. This sequence forms stable coiled-coil heterodimers with the complementary peptide (KIAALKE)(3). By introducing this peptide to the surface of phospholipid liposomes and cyclodextrin vesicles, liposomes and vesicles can be immobilized at the patterned surface.

Preventing the Capillary-Induced Collapse of Vertical Nanostructures.

Robust processes to fabricate densely packed high-aspect-ratio (HAR) vertical semiconductor nanostructures are important for applications in microelectronics, energy storage and conversion. One of the main challenges in manufacturing these nanostructures is pattern collapse, which is the damage induced by capillary forces from numerous solution-based processes used during their fabrication. Here, using an array of vertical silicon (Si) nanopillars as test structures, we demonstrate that pattern collapse can be greatly reduced by a solution-phase deposition method to coat the nanopillars with self-assembled monolayers (SAMs). As the main cause for pattern collapse is strong adhesion between the nanopillars, we systematically evaluated SAMs with different surface energy components and identified H-bonding between the surfaces to have the largest contribution to the adhesion. The advantage of the solution-phase deposition method is that it can be implemented before any drying step, which causes patterns to collapse. Moreover, after drying, these SAMs can be easily removed using a gentle air-plasma treatment right before the next fabrication step, leaving a clean nanopillar surface behind. Therefore, our approach provides a facile and effective method to prevent the drying-induced pattern collapse in micro- and nanofabrication processes.

Electrochemically driven host-guest interactions on patterned donor/acceptor self-assembled monolayers.

Here, on ITO//Au patterned substrates SAMs of ferrocene (Fc) on the Au regions and of anthraquinone (AQ) on the ITO areas are prepared, exhibiting three stable redox states. Furthermore, by selectively oxidizing or reducing the Fc or AQ units, respectively, the surface properties are locally modified. As a proof-of-concept, such a confinement of the properties is exploited to locally form host-guest complexes with ß-cyclodextrin on specific surface regions depending on the applied voltage.