Preparation, characterization, and photophysical study of thiazine dyes within the nanotubes and nanocavities of silicate host: influence of titanium dioxide nanoparticle on the protonation and aggregation of dyes

Abstract

The photophysical properties of thionine and methylene blue have been investigated in the absence and presence of titanium dioxide nanoparticles encapsulated into the different nanoporous silicate materials: zeolite-Y, ZSM-5, and MCM-41. The titanium dioxide loaded zeolite samples were prepared by the ion-exchange method and the formation of TiO₂ nanoparticles into the zeolite host materials was ascertained by UV-visible diffuse reflectance spectroscopy and ICP-OES measurement. The crystallinity of the silicate host was found to be intact as characterized by powder X-ray diffraction, nitrogen adsorption and desorption isotherm studies and TEM. The encapsulation of TiO2 into the zeolite host leads to the protonation of the dyes, indicating that the acidity of channels increased significantly in the presence of titanium dioxide in the zeolite host. In the case of zeolite-Y, with increasing loading of titanium dioxide nanoparticles, the aggregation of dyes is found to decrease. The absorption and emission spectra of the dyes, thionine and methylene blue, encapsulated into the nanochannel of ZSM-5 show a shift as compared to the spectra of the dyes in aqueous solution indicating that the planarity of the dye was perturbed due to the confinement effect of the nanochannels. ZSM-5 host with encapsulated titanium dioxide nanoparticles shows an increase in the fluorescence intensity of thionine present in the channels that is attributed to the presence of the dye in hydrophobic channels of the ZSM-5 host. The influence of the hydrophobic environment of the host on dye in the excited state was investigated by the pico- and femtosecond time-resolved spectral studies. The N-methyl-substituted analogue of thionine, methylene blue shows a different behavior due to the bulky nature of the dye. The aggregation of methylene blue in the cavities of zeolite-Y is less prevalent and the isomeric forms of the dye are not observed in this case in the excited state of the dye in the host channel.