Surface Modification of MnFe<sub>2</sub>O<sub>4</sub> Nanoparticles to Impart Intrinsic Multiple Fluorescence and Novel Photocatalytic Properties

The MnFe<sub>2</sub>O<sub>4</sub> nanoparticle has been among the most frequently chosen systems due to its diverse applications in the fields ranging from medical diagnostics to magnetic hyperthermia and site-specific drug delivery. Although numerous efforts have been directed in the synthesis of monodisperse MnFe<sub>2</sub>O<sub>4</sub> nanocrystals, unfortunately, however, studies regarding the tuning of surface property of the synthesized nanocrystals through functionalization are sparse in the existing literature. Herein, we demonstrate the emergence of intrinsic multicolor fluorescence in MnFe<sub>2</sub>O<sub>4</sub> nanoparticles from blue, cyan, and green to red, upon functionalization and further surface modification with a small organic ligand, Na-tartrate. Moreover, we have found an unprecedented photocatalytic property of the functionalized MnFe<sub>2</sub>O<sub>4</sub> nanoparticles in the degradation of a model water contaminant. Detailed characterization through XRD, TEM, and FTIR confirms the very small size and functionalization of MnFe<sub>2</sub>O<sub>4</sub> nanoparticles with a biocompatible ligand. Proper investigation through UV–visible absorption, steady-state and time-resolved photoluminescence study reveals that ligand-to-metal charge-transfer transition from the tartrate ligand to the lowest unoccupied energy level of Mn<sup>2+/3+</sup>or Fe<sup>3+</sup> of the NPs and Jahn–Teller distorted d–d transitions centered over Mn<sup>3+</sup> ions in the NPs play the key role behind the generation of multiple fluorescence from the ligand-functionalized MnFe<sub>2</sub>O<sub>4</sub> nanoparticles. VSM measurements indicates that the superparamagnetic nature of MnFe<sub>2</sub>O<sub>4</sub> nanoparticles remains unchanged even after surface modification. We believe that the developed superparamagnetic, multicolor fluorescent MnFe<sub>2</sub>O<sub>4</sub> nanopaticles would open up new opportunities as well as enhance their beneficial activities toward diverse applications.