This literature review is an endeavor to understand that the color of the nanoparticle tends to be dependent on the size and shape of the nanoparticle and also on the dielectric constant of the adjoining medium.
Owing to their size- and shape-dependent properties metal nanoparticles have been declared as extremely attractive (El-Sayed, 2001). During the last few decades, a great deal of enthusiasm has been shown by the scientists as they belie that the optical properties of these nanoparticles are impressive. A very important and fascinating characteristic of metal nanoparticles is that size and shape of the particle strongly determine their optical properties. (Liz-Marzán, 2004) Thin Au films appear blue in conduction but in reflected light Bulk Au seems yellowish. Moving through more than a few tones of red and purple, the characteristic blue color progressively changes to orange, when the particle size is condensed down to ~3 nm. These outcomes are the consequence of changes in the alleged surface plasmon resonance (Shenhar and Rotello, 2003), which is the frequency of conduction electrons oscillating in reaction to the sporadic electric field of incident electromagnetic radiation. Nevertheless, barely the metals with free electrons; in essence; the alkali metals and Ag, Au, Cu, possess plasmon resonances in the visible spectrum, and as a result, give a boost to intense colors as such. Nanorods and ellipsoids; the elongated nanoparticles demonstrate two divergent plasmon bands interrelated with oblique and longitudinal electron oscillations. The longitudinal oscillation is incredibly responsive to the aspect ratio of the particles (Yeh, Creran, and Rotello, 2012), so even the trivial movement away from spherical geometry leads to remarkable color changes. There have been some methods that have long been acknowledged to produce marvelously colored glass by adding together gold to create reds, purples or burgundy. (Katz and Willner, 2005) These colors have been accredited to awfully finely divided colloidal gold, or as they are known today; the gold nanoparticles. On the other hand, it has been observed from a few dissimilar samples of gold nanospheres that if metal nanoparticles are inflated, their optical properties modify barely to some extent. (Mahmoud et al., 2012)
At the moment, the study field of the properties and synthesis of noble metal nanoparticles is a very dynamic area of research. There is a continuous evolution of synthetic techniques leading to further enhanced control over the shape and size of the particles produced. (Yeh, Creran, and Rotello, 2012) The intense electromagnetic fields generated by the nanoparticles and optical properties make these particles exceptionally attractive for photothermal therapeutic applications, diagnostics, and sensing in a lot of areas. The alteration in the color of the nanoparticles to the bonding of a specific molecule or signal assimilation to the surface is detected effortlessly.
Hainfeldet al.(2006) established the probability of AuNPs to augment the in vivo vascular contrast in CT imaging, and Kopelman et al. in 2009 additionally premeditated immuno-targeted AuNPs to be carefully directed at tumor specific antigens. (Popovtzer et al., 2011)
Jon et al. also made use of aptamer-conjugated AuNPs (PSMA-AuNPs) ; a prostate-specific membrane antigen (PSMA) to start a molecular CT image for the particular imaging of prostate cancer cells. (Kim, Jeong, and Jon, 2010)
Varying properties merely by altering the shape or size of the nanoparticle is appealing and will persist to be used in innovative applications in the future.