The assembly of noble metal nanoparticles is an appealing means to control the plasmonic properties of nanostructures. Dimers are particularly interesting because they are a model system that can provide fundamental insights into the interactions between nanoparticles in close proximity. Here, we report a highly efficient and facile assembly method for dimers and other forms of assemblies. Gold nanoparticles (AuNPs) adsorbed on aminosilanized glass surfaces protect the silanes underneath the nanoparticles from hydrolysis. This masked desilanization allows us to prepare AuNP homodimers on glass slides with remarkably high yield (∼90%). The interparticle distance and, accordingly, the surface plasmon coupling are readily tuned at the molecular level using self-assembled monolayers of alkanedithiols. As the interparticle distance is reduced, the resonance surface plasmon coupling progressively redshifts, following the classical electromagnetic model. When the interparticle distance enters the subnanometer regime, however, the resonance band begins to blueshift and significantly broadens. The comparison of our observations with theoretical studies reveals that quantum tunneling effects play a significant role in the plasmonic response of AuNP dimers in the subnanometer gap region. The assembly method based on the masked desilanization is extendable to the formation of various other forms of nanoassemblies and, thus, will further our understanding of plasmonic interactions in nanoassemblies.

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The control of nanogaps lies at the heart of plasmonics for nanoassemblies. The plasmon coupling sensitively depends on the size and the shape of the nanogaps between nanoparticles, permitting fine-tuning of the resonance wavelength and near-field enhancement at the gap. Previously reported methods of molecular or lithographic control of the gap distance are limited to producing discrete values and encounter difficulty in achieving subnanometer gap distances. For these reasons, the study of the plasmon coupling for varying degrees of interaction remains a challenge. Here, we report that by using light, the interparticle distance for gold nanoparticle (AuNP) dimers can be continuously tuned from a few nanometers to negative values (i.e., merged particles). Accordingly, the plasmon coupling between the AuNPs transitions from the classical electromagnetic regime to the contact regime via the nonlocal and quantum regimes in the subnanometer gap region. We find that photooxidative desorption of alkanedithiol linkers induced by UV irradiation causes the two AuNPs in a dimer to approach each other and eventually merge. Light-driven control of the interparticle distance offers a novel means of exploring the fundamental nature of plasmon coupling as well as the possibility of fabricating nanoassemblies with any desired gap distance in a spatially controlled manner.

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The high-resolution three-dimensional solution structure of the plant toxin hordothionin-alpha obtained from korean barley was determined by using two-dimensional NMR techniques combined with distance geometry and restrained molecular dynamics. Experimentally derived restraints including 292 interproton distances from nuclear Overhauser effect measurements, 16 hydrogen bond restraints together with four disulphide bridge restraints were used as input to calculations of distance geometry and restrained molecular dynamics. Also included in the calculations were 36 phi and 17 chi 1 torsion angles obtained from 33JHN alpha and 3J alpha beta coupling constants in double quantum filtered COSY and primitive exclusive COSY experiments, respectively. The overall protein fold is similar to crambin and purothionin-alpha 1. Two alpha-helices running in opposite directions are found on the basis of 3JHN alpha and 3J alpha beta and deuterium exchange rates for backbone NH protons, and encompass residues 7-18 and 22-28. These two helices are connected by a turn and form a 'helix-turn-helix' motif. A short stretch of an anti-parallel beta-sheet exists between residues 1-4 and 31-34. the two protein termini of hordothionin-alpha are 'well-anchored'; the N-terminus of the protein is immobilized by this short beta-sheet whereas the C-terminus is 'pasted' to the carbonyl group of Cys-4 by a very stable hydrogen bond. The average root-mean-square differences for the backbone and heavy atoms after the restrained molecular dynamics calculations are 0.62 and 1.16 A respectively. These numbers represent a significant improvement over the corresponding values for the previous NMR structures of other thionins. The distance violation from the experimental interproton distances for the final structures is 0.14 for all atoms.