Green synthesis of fluorescent carbon quantum dots from Eleusine coracana and their application as a fluorescence ‘turn-off’ sensor probe for selective detection of Cu2+

Abstract

A facile, rapid and frugal approach was developed for the preparation of highly fluorescent carbon quantum dots (CQDs) by pyrolysis of Finger millet ragi (Eleusine coracana) as a carbon source. The optical properties of as-synthesized CQDs were analyzed by using UV-Visible and fluorescence spectroscopies. Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) showed the formation of CQDs that predominantly consists of carbonyl and hydroxyl groups which can provide more adsorption sites. The X-ray diffraction (XRD), High-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy indicated that the synthesized CQDs made of amorphous graphitic carbon like structure. The size distribution was in the range of 3–8 nm with a high abundance of 6 nm particles. The CQDs exhibited bright blue fluorescence emission (wavelength ∼425 nm) under UV-light with an excitation wavelength of 340 nm. Our study indicated that Cu2+ strongly quenched the fluorescent intensity of CQDs compared to other metal ions (Al3+, Ca2+, Mg2+, Ni2+, Pb2+, Mn2+, Cr3+ and Hg2+). There was a linear correlation between the quenching efficiency vs. concentration of Cu2+ ions in the range from 0 to 100 μM (R2 = 0.9918). Furthermore, CQDs have been applied in sensing of Cu2+ in real water samples with a detection limit (LOD) of 10 nM. In addition, density functional theory (DFT) calculations revealed that there was a strong interaction between CQDs with divalent metal ions whereas trivalent metal ions adsorbed weakly. Particularly, Ni2+ and Cu2+ formed a very strong bonding with CQDs compared to other divalent ions. The metal-CQDs complex structure was reported with optimized highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) energy gap. The nature of interaction and optical properties of the COOH-CQD’s have been studied with metal ion adsorbed on the surface. Charge analysis corroborated that there was a significant charge transfer between CQDs with Ni2+ and Cu2+. Our study revealed that Cu2+ preferentially get adsorbed on aromatic CC (π-bond) of CQDs whereas other divalent metals form σ-bond(s) with the CQDs. This observation further confirmed by the HOMO-LUMO band gap (Eg) and Frontier molecular orbital analysis.