Selective sensing of heavy metal ions using carbon dots synthesized from Azadirachta indica seeds

Abstract

There have been notable advancements in the technology associated with using waste resources to create novel and beneficial products. This study demonstrates that the kernel part of Azadirachta indica (Neem) seeds can be sustainably used for this purpose. Carbon dots (CDs) of approximately ca. 4–8 nm in size were synthesized from the kernel Azadirachta indica seeds through calcination, followed by surface modification using diethylamine, sodium methoxide, and alcohol. This produced waste seed-derived luminous surface-quaternized CDs (Ai-CDs). These CDs were used as a fluorescent nanoprobe to detect inorganic contaminants at concentrations ranging from low (5 μM) to high (120 μM), due to their strong photostability and excitation-dependent emission in aqueous solutions. Ai-CDs were used to measure the levels of Cd⁺² and As³⁺ in solution through quenching of luminescence intensity (“turn-off”), while cupric ions (Cu⁺²) selectively increased fluorescence (“turn-on”) for sensing. The current method of synthesising CDs offers quick reaction times, along with great selectivity and sensitivity. The CDs preferentially absorbed Cd²⁺ and As³⁺, causing a sharp dimming in fluorescence intensity by 27% and 30%, respectively. In contrast, for Cu⁺² and Cu⁺ the fluorescence intensity was enhanced. Consequently, this unique characteristic was utilized to exclude and identify Al³⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺, Cu²⁺, and Cu⁺ ions, with detection limits ranging from 5 μM to 120 μM. Furthermore, we demonstrated the heavy metal ion sensing activity of CDs from their salt solutions, highlighting their potential as environmentally friendly metal ion detection agents. A cell viability assay was carried out, revealing that the CDs are non-toxic.