Deciphering interaction between chlorophyll functionalized carbon quantum dots with arsenic and mercury toxic metals in water as highly sensitive dual-probe sensor

Abstract

Rapid and reliable heavy metal detection in drinking water is critical in developing countries as they cause serious health hazards. In this work, a novel chlorophyll functionalized CQD (ChlCQD), is presented as a nanoprobe for the sensitive detection of As3+and Hg+ ion in aqueous solutions. These CQDs are fabricated using a one-step hydrothermal treatment of Banana leaf extract. We demonstrate that by tuning the temperature of hydrothermal synthesis from 120 °C to 230 °C we can get CQDs with different optical properties. The CQDs synthesized at 120 °C and 160 °C only are decorated with chlorophyll functional groups, while the CQDs synthesized at 200 °C and 230 °C do not have any chlorophyll groups. These chlorophyll groups give rise to a fluorescence band near the red region which gets either quenched or enhanced with the addition of metal ions. We demonstrate that the ChlCQDs synthesized at 160 °C can be utilized as a turn-off and turn-on sensor for selective detection of Hg+ ions and As3+ ions, with low limits of detection. Using surface-sensitive X-ray photoelectron spectroscopy measurements, we demonstrate that As3+ binds strongly with the carbonyl group of the chlorophyll moiety while the Hg+ ions bind very weakly to the carbon atoms of the CQDs. The theoretical calculations using Density Functional Theory (DFT) were performed to gain atomic-level insight into the interactions of As3+ and Hg+ with the surface of ChlCQD (low-temperature model surface). A strong absorption peak around 660 nm occurs due to strong overlap between As3+ and CQD surface and this peak is absent for Hg+ due to negligible overlap between orbitals of Hg+ and ChlCQD surface. As3+ and Hg+ show similar interaction with different sites on CQD (high-temperature model structure) and is consistent with observed similar experimental absorption spectra for both the metals.