Rice straw derived cellulose nanofibers modified with L-histidine for ultra-trace fluorometric assay of Cr(VI) and Hg(II) in aqueous medium

Through this study, an effort has been made to eliminate the problems of rice straw burning and pollution due to heavy metals by utilizing cellulose nanofibers (CNFs) derived from rice straw to develop effective fluorescent sensors for detection of chromium Cr(VI) and mercury Hg(II), concurrently. CNFs were modified using L-histidine (CHIS) to yield cellulosic fluorophore, not reported hitherto. Characterization using X-ray Powder Diffraction (XRD), Field emission scanning electron microscopy (FESEM) and Thermogravimetric analysis (TGA), indicated partial impairment of crystalline structure of cellulose with conversion to CHIS, however the sensor was thermally more stable as 40% mass residue remained even after 700 °C. Fluorescence response exhibited substantial quenching with Cr(VI) and Hg(II) ions, with detection limits of 2.236 nM and 3.97 nM, respectively. The XPS studies established strong interactions of CHIS with Hg(II) ions via imine groups and Cr(VI) ions with imine as well as amine group on CHIS resulting in substantial quenching. Further, when excited at different wavelength under fluorescence microscope, CHIS indicated suppression of fluorescence effect with adsorption of heavy metal ions. The sensor was highly selective, stable with time and pH with excellent usability up to 4 cycles. Thus, CHIS present a facile and robust platform for simultaneous detection and real time monitoring of Hg(II) and Cr(VI) metal ions.