Interaction of Chlorophyll with Titanium Dioxide and Iron Oxide Nanoparticles: A Temperature Dependent Fluorescence Quenching Study

This study explores the temperature-dependent interaction of chlorophyll with titanium dioxide and iron oxide nanoparticles using steady-state and time-resolved laser-induced fluorescence spectroscopy. For this work, temperature-dependent steady-state fluorescence spectra of chlorophyll and chlorophyll mixed with titanium dioxide and iron oxide nanoparticles have been recorded in spectral region from 400 to 800 nm. The scrutiny of the spectra reveals that the titanium dioxide and iron oxide nanoparticles decrease the fluorescence intensity of the chlorophyll band at 669 nm demonstrating the quenching of the chlorophyll fluorescence. The deviation from linearity and upward curvature of Stern-Volmer plot of the chlorophyll-titanium dioxide nanoparticles shows involvement of both static and dynamic quenching while the straight line plot obtained in case of the chlorophyll-iron oxide nanoparticles indicates the involvement of either static quenching or dynamic quenching. The measurements obtained from time resolved fluorescence analysis reveal that the lifetime of chlorophyll does not change significantly with increasing concentrations of the titanium dioxide and iron oxide nanoparticles. This relationship shows the predominance of static quenching mechanism in the chlorophyll-titanium dioxide and chlorophyll-iron oxide nanoparticles interaction. The binding constant for the chlorophyll-titanium dioxide nanoparticles and chlorophyll-iron oxide nanoparticles decreases with increasing temperature having only one binding site with negative values of thermodynamic quantities such as the changes in enthalpy and entropy, indicating the involvement of hydrogen bonding and van der Waals forces in the quenching process. The energy-level diagram shows that the quenching of fluorescence forces due to electron transfer from the excited state chlorophyll to the conduction band of the titanium dioxide and iron oxide nanoparticles. The results obtained in this study are useful in characterizing the photophysical properties of the semiconducting metal oxide nanoparticles with biological molecules such as chlorophyll.