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Nanomaterials have unqiue properties not found in their bulk counterparts, such as hydrophobicity/hydrophilicity, size-dependent color absorption, mechanical strength, lightness, and more. 
These properties make nanotechnology one of the most important frontiers of modern science, with applications including medical diagnosis, tissue engineering, energy harvesting, water purification, chemical sensing, self-cleaning films, smart fabrics, and more.
In bulk, metals shine because free electrons on the surface oscillate collectively when interacting with light.
On the nanoscale, some metals--known as plasmonic nanoparticles--have unique optical properties that can be finely tuned by changing their size, shape, and arrangement. 
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Electron Delocalization: a surface plasmon is characterized as a surface charge density wave at the metal surface.
(Image taken from NanoHybrids: Advanced Imaging Solutions.)
The free electrons in plasmonic nanoparticles vibrate at a specific frequency dependent on those factors, and only light at the same frequency is absorbed--this is known as localized surface plasmon resonance.
This phenomenon has exploited to create chemical sensors, biological agents, efficient solar cells, photo-thermal treatments, and more. 
Plasmonic nanoparticles also exhibit a unique phenomenon when in close proximity with another plasmonic nanoparticle--coupling. When nanoparticles get within around one diameter's length of each other, their resonance starts to hybridize, either red-shifting or blue-shifting the peak resonance wavelength. 
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Single gold nanosphere absorbance spectrum in green, plasmon coupling absorbance spectrum in red.
(Image taken from NanoHybrids: Advanced Imaging Solutions.)
In my research on colorimetric strain sensing, I exploit this coupling phenomenon. My novel nanomaterial integrates Au nanocubes into a flexible nanostructured substrate. The nanostructured surface changes color when stretched due to the properties of structural color. This change synergizes with the change in peak resonance wavleength of the Au nanocubes when they are decoupled upon stretching.