They investigated the change in color of the sol from red to purple depending on the refractive indices of the solvents and found the corresponding optical characteristics to be in excellent agreement with the predictions of Mie theory. Underwood and Mulvaney described the synthesis and transfer of gold hydrosol using a polymeric comb stabilizer into various mixtures of organic solvents with different refractive indices. A plethora of theoretical investigations, computational studies, and seminal experiments by eminent research groups throughout the world have illuminated the research arena of the dielectric sensitivity of the localized surface plasmon resonances of a variety of noble-metal nanostructures in colloidal dispersions. Recent advances in synthetic strategies, characterization techniques, the fundamental understanding of relevant theories, and methodologies for simulating plasmonic nanostructures have enabled researchers to study their optical properties under a variety of microenvironmental conditions and in a diverse range of niche applications that merge physics, chemistry, biology, materials science, and engineering on the nanoscale. Under such constraints, the change in color of the nanoparticles is due to changes in the local dielectric environment arising from the specificity of interaction of the dispersed ligand-stabilized particles with the electromagnetic radiation. Therefore, for a sufficiently dilute (such that interparticle interactions can be disregarded) colloidal dispersion (devoid of any substrate), the optical properties of size-and shape-selective metallic particles are governed by the stabilizing ligand shell and the solvent medium in which the nanoparticles are dispersed. Real-time monitoring of the optical properties of metallic nanoparticles demands that the following parameters be taken into consideration: the shape and size of the particles, the presence of a supporting substrate or stabilizing ligand shell, the solvent dielectric continuum around the particles, and the electromagnetic interactions among particles that are close enough in the ensemble to influence the optical properties. The oscillation frequency is, critically, determined by four factors: the density of electrons, the effective electron mass, and the shape and size of the charge distribution. As the LSPR originates from the contributions of many electrons, the absorption and scattering cross sections of metal nanoparticles become very large, resulting in a high intensity of the plasmon resonance and its sensitivity to the local dielectric environment of the particles. The spectral position and magnitude of the LSPR is strongly dependent on the size, shape, interparticle interactions, dielectric properties, and local environment of the nanoparticles. Nanoplasmonics deals with the study of light−matter interactions that is based on the ability of small metallic particles to interact strongly with light of wavelengths significantly larger than their size. The transfer of information by surface plasmons, that is, the merging of photonics and electronics, has given birth to the concept of new physics for materials at nanoscale dimensions, referred to as plasmonics. Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal−dielectric interface. Such strong absorption induces strong coupling of the nanoparticles to the electromagnetic radiation of light and called the localized surface plasmon resonance (LSPR). The physical origin of light absorption by metal nanoparticles is the coherent oscillation of the conduction-band electrons (just as water ripples travel along the surface of a pond after a stone is thrown into it and, therefore, have been explored as an intriguing surface− wave phenomenon) induced by the interacting electromagnetic field. However, the historical landmark of scientific investigations of the significant variation of colors displayed by colloidal metal particles can be traced to the revolutionary insight of Michael Faraday in 1857 calling a solution "a beautiful ruby fluid" and noting that "a mere variation in the size of particles gave rise to a variety of resultant colors". The manifestation of unusual colors by colloidal particles has been exploited in ancient paintings and crafts as colorants in glasses, ceramics, china, and pottery and in curative and aesthetic applications. From the artistic representations in the Paleolithic cave paintings of Altamira to modern impressionism, the colors of objects play a pivotal role in human creativity. The colors in nature have fascinated humans from the ancient days of civilization.
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