Color and surface plasmon effects in nanoparticle systems: Case of silver nanoparticles prepared by microemulsion route

Surface plasmon resonance (SPR) in suspended metallic nanoparticles, like Ag, gives rise to fascinating colors that are different from those observed in bulk materials. Although these effects have been highlighted in many recent articles [1], [2], [3] and [4], no experimental study of color properties with respect to system parameters such as concentration and size of particles is available in literature. Theoretical studies on behavior of colloidal systems showing plasmon resonance are limited in scope and have not been applied to nanoparticle suspensions in non-aqueous solvents or complex media such as microemulsions which are commonly encountered in nanoparticle synthesis by the micellar route. In the present work, detailed characterization of color properties of silver nanoparticles (SNPs), produced by reducing silver nitrate in reverse micellar media, is carried out. The particle sizes were measured using transmission electron microscopy (TEM) and UV visible spectrophotometry. The objective quantification of color was done using the Gretag–Macbeth color matching instrument which provides measures of chroma, saturation, hue, brightness etc. The effects of particle concentration and particle size on the color of SNPs, were studied experimentally. Extensive simulations to predict the color properties of these systems within the framework of Mie theory were performed. Use of a size-dependent dielectric function for Ag nanoparticles seems essential in order to make the predictions more realistic, without the use of any fitting parameters; as the simulations done with size independent dielectric function did not explain the experimental observations correctly. The findings of this study may be summarized as: (1) the color properties of SNPs change significantly on simple dilution, (2) although color properties are quite sensitive to size of nanoparticles at constant number density, at constant mass loading they appear to be insensitive to small changes in the size of SNPs because of inverse cubic coupling between size of particles and number density at constant mass loading, and, most importantly, (3) the classical Mie theory combined with a size-dependent dielectric function, provides a theoretical framework for prediction of color in plasmonic nanoparticle systems.

(a) The variation of color of SNPs with different dilutions. Concentrations of particles are 47.7, 42.9, 38.2, 33.4, 28.6, 23.9, 21.5, 19.1, 16.7, 14.3, 13.1, 11.9, 9.5, 7.2 and 4.8 μg/m3, from left to right. (b) Experimental absorption spectra of SNPs. Figure optionsDownload as PowerPoint slideHighlights
► Examined effect of size (a) and number density (N) on color of Ag nanoparticles (SNPs).
► Color of SNPs is very sensitive to both N (at constant a) and a (at constant N).
► Simulated color properties using Mie's theory and CIE color space without any fitting.
► Size-dependent dielectric function of SNPs is key for making predictions realistic.
► At constant mass loading color appears insensitive to small changes in size of SNPs.