Erythrosine B dye has found applications as photosensitizers in photodynamic therapy, fluorescent probes in chemistry and biology. In this study we investigated the effect of nano-droplets' concentration and size on linear and nonlinear optical characteristics of Erythrosine B (EryB) by Z-Scan, UV-Vis and spectrofluorometer techniques. Twophoton absorption coefficient (β) of EryB, was measured by use of Z-Scan technique at two molar ratios of water to AOT surfactant (W= 3 and 8) for two types of Oil (n-decane and n-heptane) and different nano-droplet concentration as well as nonlinear refractive index (n2) of EryB in bulk water. Two-photon absorption coefficient (β) was enhanced in nano-droplet as compared with bulk water. Moreover, the fluorescence spectra of EryB can be enhanced by nano-droplets (microemulsion). The excited to ground state dipole moment ratio (μe/μg) was calculated through Quantum-mechanical second order perturbation theory for different nano-droplet concentrations. The results represent that μe/μg of EryB in bulk water is higher than that of in water nano-droplets. We demonstrated that for the equal concentration of EryB in the microemulsion , the peak of fluorescence intensity varied by changing the concentration of nano-droplets. Moreover, the fluorescence peak intensity was enhanced when the continuous phase of microemulsion changed from n-decane to n-heptane.

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