The optical properties of silver nanoparticles can be tuned by controlling the particle shape and size. Among the variety of shaped silver nanoparticles, silver nanocubes have received particular interest due to their distinctive morphology. By controlling reaction conditions during synthesis, highly uniform cube-shaped silver nanoparticles with unique optical, electric and chemical properties can be synthesized. Silver nanocubes have been used for a broad range of applications including plasmonic sensing, surface enhanced Raman scattering (SERS), metamaterials, catalysis, and bionanotechnology.
This module describes the physical and optical properties of silver nanocubes of different size, and their use for SERS detection and other applications.
The Effect of Cube Shape and Size on Optical Properties
The optical properties of silver nanocubes depend on their size, with the surface plasmon resonance peak of the cubes shifting to longer wavelengths as the nanocube diameter increases. Due to the cubic shape, however, silver nanocubes have optical properties that are different than similarly-sized nanospheres. For example, the silver nanocubes have sharp corners and edges that give rise to additional plasmonic modes that occur at different resonance wavelengths in the spectrum.
For nanocubes with small dimensions, such as the 40 nm-diameter nanocubes shown below, these plasmon modes are bunched together and the extinction peak of the first (highest resonance wavelength) dipole mode dominates.
As the size of the nanocubes increases, the resonance wavelengths of the higher order plasmonic modes become more separated and distinct, and increase in intensity in spectrum. As shown below, the extinction peaks of these higher plasmonic modes appear in the visible and near infrared regions of the spectrum. The cube size increase also leads to a redshift in the dipole peak position from ∼450 to ∼700 nm as the mean size of the cubes increases from 50 to 150 nm. This absorption peak becomes broad and relatively less intense as the cube size increases.
As a result of these multiple plasmonic modes, nanocubes between 60-200 nm size are bichromic, exhibiting different colors depending on whether the sample is transmitting or scattering incident light. This unique feature can be utilized to generate color properties rarely seen in other materials with applications in cosmetics, plasmonic paints, and for integration into artisan glass and jewelry. Additionally, because these color properties are nearly impossible to replicate using traditional dyes and other colored materials, silver nanocubes can be used to create a unique optical signature for use in brand protection and anti-counterfeiting applications. At nanoComposix, we can provide cube size range from 40 nm to 1 micron in diameter.
Surface Chemistry of Silver Nanocubes
Silver nanocubes are typically coated with polyvinylpyrrolidone (PVP), which can help to direct the formation of the cubic shape during synthesis by stabilizing the crystal facets on the cube faces. The PVP coating on the cubes can be displaced under certain conditions, enabling the nanocubes to be dispersed in aqueous and organic solvents via ligand exchange, or allowing for further biomolecule conjugation. By controlling the chain length and chemistry of capping ligands on the silver nanocube surface, the nanocubes have been utilized as building blocks to form 1-dimensional superstructures that have well-defined interparticle orientations such as edge–edge or face–face conformation, and tunable electromagnetic properties.
Applications of Silver Nanocubes
Silver nanocubes have been used for a broad range of applications including plasmonic sensing, metamaterials, catalysis, and bionanotechnology. The unique optical properties of the nanocubes also make them of interest for surface enhanced spectroscopic applications, which depend on the presence of strong electromagnetic fields near the particle surface.
The degree of field enhancement in a plasmonic structures strongly depends on its geometry. In the case of cubes, huge local electric-field enhancement occurs at the sharp corner sites by taking advantage of the lightening rod effect, and silver nanocubes have been used as for optical nanoantenna Surface-enhanced Raman scattering (SERS) applications.
When metal surfaces with high curvature are separated by nanoscale gaps and an electromagnetic field is localized within the gaps, generating plasmonic hotspots. These hot spots can be utilized in subwavelength focusing, surface-enhanced Raman spectroscopy and electromagnetic transparency. The optical properties of the formed plasmonic hotspots strongly depend on the geometry of the nanojunctions between the metal surfaces. In case of silver nanocubes, with sharp corners brought close to another metal surface or adjacent cubes, drastically intensified E-fields within extremely small regions drive the E-field enhancement to extreme values. Such plasmonic hotspots enable detection of SERS from a single molecule, thus providing an effective platform for ultrasensitive detection. The intensity of SERS signal was shown to be related to the sizes of the cubes, and the highest signal was observed by selecting the silver nanocubes about 100 nm size as substrates. Furthermore, by using AFM tips coated with Ag nanocubes, Tip-enhanced Raman spectroscopy (TERS) enables access to chemical information with nanoscale spatial resolution and single-molecule sensitivities.
In addition, researchers have shown that randomly depositing these silver nanocubes over a coated gold film can create a nearly perfect absorbing surface, with the absorption wavelength tunable through the visible and near-IR regions of the spectrum by tuning Ag nanocube size and coating conditions. The film-coupled nanocubes act as tiny optical antenna that can cancel out the reflectance of the metal surface.
Other applications of Ag nanocubes include serving as sacrificial templates for galvanic replacement to produce Au nanocages for drug delivery and plasmonic platform for sensing; the particles are also potential candidates for facet-selective catalysis such as epoxidation reactions.
- Akselrod, G. M., Huang, J., Hoang, T. B., Bowen, P. T., Su, L., Smith, D. R., & Mikkelsen, M. H. "Large‐Area Metasurface Perfect Absorbers from Visible to Near‐Infrared." Advanced Materials, 27(48), 8028-8034 (2015).
- Rycenga, M., Xia, X., Moran, C. H., Zhou, F., Qin, D., Li, Z. Y., & Xia, Y. "Generation of Hot Spots with Silver Nanocubes for Single‐Molecule Detection by Surface‐Enhanced Raman Scattering." Angewandte Chemie, 123(24), 5587-5591 (2011).
- Gao, B., Arya, G., & Tao, A. R. "Self-orienting nanocubes for the assembly of plasmonic nanojunctions." Nature Nanotechnology, 7(7), 433-437 (2012).
- Dill, T. J., Rozin, M. J., Palani, S., & Tao, A. R. "Colloidal Nanoantennas for Hyperspectral Chemical Mapping." ACS Nano, 10(8), 7523-7531 (2016).
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