Hai V H Krishna Prasad

Nanoparticles have different colors due to the phenomenon of surface plasmon resonance. Plasmons are waves of electron density that exist on the surface of metal nanoparticles. The size and shape of a nanoparticle, as well as the material it is composed of, can affect the frequency of the plasmons and thus alter the color of the nanoparticles.

V H Krishna Prasad Many scholars were concerned with this problem at the end of the 1800's and beginning of 1900's. A classic example is the color of gold sols (others were the color of sky and the rainbow). The color arises from the (preferential) scattering of light when it interacts with matter. Gustav Mie (fully) explained it in his classic paper of 1908. For a little on the man and Mie theory view this webinar (registration required):

The life of Gustav Mie and the development of the Lorenz-Mie solution to Maxwell’s equations (free registration required)

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W140225Gustav-mie-maxwell.html

At 16:54 & 47:50 in the talk you'll see the plot Mie gave in his paper of 'optical resonance' showing the (calculated) maxima in wavelengths for different sized Au particles (note that μ μ is nm in modern language). He summed all terms to indicate when resonance could happen (see his diagram at ~ 37:14 in the talk). You can replicate some of this with (freeware) Philip Laven's MiePlot which is also mentioned in the talk.

Hello

you can see these articles:

https://www.sciencedirect.com/topics/materials-science/optical-property-of nanomaterials

Nanoparticles: Properties, applications and toxicities

Optical Properties and Spectroscopy of Nanomaterials- J. Am. Chem. Soc. 2010, 132, 10, 3637–3638 Publication Date:February 24, 2010

https://doi.org/10.1021/ja101350v

Hi,

Nanoparticles of different size made of the same base material can have different colors because of a phenomenon known as plasmon resonance. Plasmon resonance is the collective oscillation of free electrons in a metal nanoparticle in response to the electromagnetic field of light.

When a beam of light interacts with a metal nanoparticle, the oscillation of the electrons in the nanoparticle can be excited, causing the absorption and scattering of light at certain wavelengths. The wavelength of light that is absorbed or scattered depends on the size, shape, and composition of the nanoparticle, as well as the refractive index of the surrounding medium.

For example, as the size of a metal nanoparticle decreases, the plasmon resonance peak shifts to shorter wavelengths (i.e., higher energy), resulting in a blue shift in the color of the nanoparticle. This is because smaller nanoparticles have a higher density of free electrons and a higher surface-to-volume ratio, leading to stronger plasmon resonance and increased absorption at shorter wavelengths.

Conversely, as the size of a metal nanoparticle increases, the plasmon resonance peak shifts to longer wavelengths (i.e., lower energy), resulting in a red shift in the color of the nanoparticle. This is because larger nanoparticles have a lower density of free electrons and a lower surface-to-volume ratio, leading to weaker plasmon resonance and increased absorption at longer wavelengths.

Therefore, the color of a metal nanoparticle depends on the size, shape, and composition of the nanoparticle, as well as the refractive index of the surrounding medium. This can be exploited in various applications, such as in colorimetric sensors, where the color change of metal nanoparticles can be used to detect the presence of analytes or changes in the surrounding environment.

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