Transmission spectra for the EE AuNCs tetramer, with the 0.2 nm air gap and corner radii of 5 nm in (A), DDA calculated field distribution for the EE oriented AuNCs tetramer (B) and calculated sensitivity factor for the FF and EE oriented Ag nanocubes (C). The calculations on the electric field intensity show that the EE orientation can result in greater sensitivity in LSPR-based sensing and larger enhancement factor in SERS. The results show that the effect of plasmon tunneling in the EE-oriented NCs is even more pronounced than that of the FF-oriented NCs, due to the effect of the local topology (sharpness of the edges).The way in which gold or silver nanocubes line up affects their optical response, which could be useful in future sensing devices, according to researchers from Georgia Institute of Technology [Hooshmand et al., Nano Today (2017), doi: 10.1016/j.nantod.2017.10.009].
Gold and silver nanocubes have unique optical properties because their small size confines the oscillation of electrons in the conduction band. This collective oscillation is known as a surface plasmon. Surface plasmons in chains of nanocubes resonate at specific wavelengths, known as the localized surface plasmon resonance (LSPR). The LSPR wavelength is very responsive to any changes in the refractive index of the surroundings, indicating with great sensitivity the presence of molecules attached to or in the proximity of the nanocube chain. This makes plasmonic nanoparticles ideal for chemical and biochemical sensing. Now researchers have found that the way in which plasmonic nanocubes are arranged in chains – either face-to-face or edge-to-edge – also affects their response.
Plasmonic nanocubes have been used previously in molecular sensing applications, but their properties and sensitivity in highly ordered arrangements in different configurations have not yet been thoroughly studied before,” explains first author of the study, Nasrin Hooshmood. “Having asymmetric geometry, nanocubes can be arranged in a number of different configurations.”
The researchers used a model known as the discrete dipole approximation (DDA) to calculate the difference in plasmon resonance between face-to-face and edge-to-edge nanocube chains.
“We found that the edge-to-edge configuration results in better performance in terms of sensitivity in molecular sensing in most cases,” she says.
The team then went on to investigate what happens when nanocubes are separated by less than a nanometer. Using a modified model, the researchers found that when nanocubes are less than a nanometer apart, plasmons can tunnel through the dielectric barrier and hop from one nanocube to another. This tunneling charge transfer effect at small separation distances appears to impact the response of edge-to-edge nanocube chains more than face-to-face configurations.
“This is an important finding in practical settings, as the self-assembly of chemically synthesized nanocubes tend to form sub-nanometer gaps,” explains Hooshmood.
The team is now working on making the nanocube chains using either electron-beam lithography or a novel approach combining this technique with chemical synthesis. The nanocube chains could be used simply to detect the presence of molecules via changes in LSPR wavelength, although this would require coating with molecular probes such as antibodies. Alternatively, nanocube chains could be used to amplify the Raman ‘fingerprint’ of a few individual molecules using surface-enhanced Raman spectroscopy (SERS).
“Our results suggest that the edge-to-edge nanocube chains can be more appealing in developing efficient sensing platforms via SERS or LSPR-based sensing in comparison with the face-to-face chains,” she points out.
Jackie Y. Ying, Editor-in-Chief of Nano Today and Executive Director of the Institute of Bioengineering and Nanotechnology in Singapore, believes the work is very useful.
“This article provides fundamental insights into the coupling between plasmonic nanoparticles assembled in linear chains and its sensitivity towards orientation, type, and number of nanoparticles involved,” she says.
This article was originally published in Nano Today (2018), doi: 10.1016/j.nantod.2017.12.005.