Synthesis and Photophysical Characterisation of Fluorescent Carbon Dots from Agricultural Biomass for Metal Ion Detection

Carbon dots, a class of zero-dimensional nanomaterials with tunable photoluminescence and exceptional biocompatibility, have emerged as a promising alternative to toxic quantum dots for chemical sensing and bioimaging applications. The synthesis of carbon dots from agricultural waste biomass addresses both the material cost and environmental sustainability dimensions, making them particularly relevant for developing country research contexts. This study synthesised fluorescent carbon dots by microwave-assisted pyrolysis of maize cob, cassava starch, and sugarcane bagasse sourced from farms in Kwara State, North Central Nigeria, and characterised their photophysical properties for application in selective detection of heavy metal ions. Carbon dots were characterised by transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, ultraviolet-visible absorption spectroscopy, and fluorescence spectroscopy. Quantum yield was measured against quinine sulphate as a standard. Selective ion sensing performance was screened for copper, cadmium, lead, mercury, iron, chromium, and zinc ions. Carbon dots from maize cob exhibited the highest quantum yield of 18.6 percent and showed a highly selective and sensitive fluorescence quenching response to copper ions with a Stern-Volmer constant of 4.8 times 10 to the power of 4 per molar and a detection limit of 0.28 nM. Interference from co-existing ions was minimal. The quenching mechanism was attributed to inner filter effect combined with photoinduced electron transfer from carbon dot surface groups to the copper ion. Application to real water samples from Kwara State rivers showed good accuracy for copper quantification by standard addition. This study represents an original contribution to sustainable nanomaterial synthesis using Nigerian agricultural waste for environmental analytical chemistry.

Keywords: carbon dots, fluorescent nanomaterials, heavy metal detection, agricultural biomass, Kwara State