Development of fluorescent carbon nanoparticles from Madhuca longifolia flower for the sensitive and selective detection of Cr(6+): a collective experimental–computational approach

Herein, blue-emitting carbon nanoparticles (CNPs) were synthesized using the Madhuca longifolia flower for the highly selective and sensitive detection of Cr(6+) ions in aqueous media using a simple, green, and cost-effective approach, and computational experiments were also performed. The prepared CNPs were well-dispersed in water with an average diameter of 12 nm and functionalized with carbonyl, hydroxyl and carboxylic acid groups. The decrease in the fluorescence intensity of the CNPs with an increase in the content of Cr(6+) provided an important signal for the sensitive and selective detection of Cr(6+) in aqueous media. The limit of detection for Cr(6+) in an aqueous medium was found to be 103 ppb, which is more sensitive in comparison with the previously reported study. Furthermore, the validation of the proposed higher sensing feature and more selective nature of the CNPs towards Cr(6+) was also explained using an in silico approach. The results from the theoretical calculations based on the DFT approach demonstrated that the binding energy (BE) of the CNPs with three transition metal (TM) cations (Cr(6+), Fe(3+), and Hg(2+)) follows the order of Cr(6+) > Fe(3+) > Hg(2+), where the Cr(6+) TM cation associated with the CNPs possesses the highest valence state, showing the highest sensing feature and highest selectivity among the investigated ions, as expected. The metal ions associated with the CNPs having a higher charge and a smaller radius indicated a higher BE and larger degree of deformation of the CNPs. Moreover, to achieve new insights into the structural, stability/energetics, and electronic features, some useful tools, such as NCI-plot, HOMO–LUMO gap, MESP, and QTAIM analysis were employed, which facilitated noteworthy outcomes.