Influence of silver nanoparticles on the optical properties of methylene blue and curcumine dyes
Ochoo, Lawrence Otieno
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It is emerging that dyes and nanoparticles (NP) of Ag, Au and Cu have optical properties with the potential for transforming and enhancing applications of light energy in opto-electronics, solar cells and even radiotherapy on cancer. Investigations are opening up on their modes of influence, conflict of theories on their mechanisms of influence and the important parameters of influence in various systems, In this study, the influence of Ag nanoparticles (below 50nm) on the optical properties of curcumine and methylene blue dyes was investigated experimentally and theoretically. The nanoparticles of Ag were prepared by the method of photo-reduction of AgNO3 in ethanol solution and characterized by spectroscopic analysis in the UV-IR spectral range (300-900nm) using Spectro 320 analyzer, transmission electron microscopy and EDX spectroscopy. Samples of the individual dyes and mixtures of dye- silver nanoparticles were also prepared in ethanol solutions and deposited on glass substrates by spin-coating method, for the spectroscopic analysis. The influence of Ag nanoparticle size and morphology on their own optical properties and that of the dyes were investigated. The results have shown that the optical absorption of Ag nanoparticles and their influence on the dyes depend non-linearly on the nanoparticle size and shape. The results have shown enhancement on dye absorption, with the maximum enhancement factor of ~4.3, realized with Ag nanoparticle size average of 22.4 ± 3nm. A comparative analysis with the results of other studies on Au and Ag has revealed an interesting correlation about the nanoparticle sizes for optimum enhancement effect towards dyes, solar cell materials, and radiotherapy on cancer. That the most effective are the nanoparicle sizes of minimum absorption bandwidth, a size range of 15-30 nm for both metals. As a result, a new theoretical model has been proposed for the purpose of understanding the mechanisms and parameters behind the influence of metal nanoparticle size on their own absorption and in other systems. The model has shown very good qualitative and quantitative agreement with both the experimental results of this study and those of other studies, for Ag, Au and Cu. The model predicts an optimum absorption by Ag nanoparticles of the size ~30 nm and a minimum absorption bandwidth at the Ag size of ~24 nm and ~21 nm for Au. With regard to the theoretical analysis of Ag, Au and Cu the model finds weak interaction between the conduction band and interband transition absorption in silver but strong interaction in Au and Cu, which influence their optical spectra, how they interact with their environment and difference in their potential optical applications. On the Ag nanoparticles size with the highest enhancement in dye absorbance (22.4 nm), the Ag-Methylene blue dye interaction has revealed an evolving selective binding for Ag particles in the range 10-25 nm. A similar physical interaction has been reported in Ag-HIV-I and Au-cancer cell interaction and attributed to sulphur in their structures, an element also contained in Methylene blue dye. The correlation between nanoparticle sizes (10-30nm) with optimum effect in different systems is proposed to be a consequence of the electric field and dependent on the wavelength of light and the absorption bandwidth.