Chemical-free exfoliation of biochar-derived graphene quantum dots for selective Fe³ ⁺ and Cu²⁺ fluorescence sensing

Graphene quantum dots (GQDs) were sustainably synthesized through reagent-free mechanical mixing and ultrasonic exfoliation routes from walnut shell-derived biochar prepared under air and vacuum pyrolysis environments. The influence of thermochemical history and exfoliation pathway on the structural, optical, and sensing properties of the GQDs was systematically evaluated. Air-derived biochar produced highly crystalline GQDs with particle sizes of 1.5–2.4 nm, distinct graphitic lattice fringes, low defect density, and oxygen-containing surface functionalities that contributed to colloidal stability and fluorescence activity. The synthesized GQDs exhibited excitation-dependent photoluminescence governed by both quantum confinement and surface-state-mediated emission mechanisms. Selective fluorescence quenching toward Fe³ ⁺ and Cu²⁺ ions was observed, with mechanically exfoliated GQDs exhibiting enhanced Fe³ ⁺ sensing (LOD: 0.23 ppm) and ultrasonically exfoliated GQDs demonstrating improved Cu²⁺ detection (LOD: 0.27 ppm). The sensing behaviour was attributed to surface complexation-induced charge transfer and non-radiative recombination processes. The study demonstrates an environmentally benign strategy for converting walnut shell waste into functional fluorescent nanomaterials for sustainable heavy metal sensing and environmental monitoring applications.