From tobacco waste to functional nanobiochar: a sustainable carbon platform for sensitive mercury detection

In this work, three biomass wastes, orange peel (OP), beer bagasse (BB), and tobacco waste (TT), were investigated as sustainable precursors for the production of biochar (BC) and nanobiochar (NBC) via hydrothermal carbonization (HTC) followed by microwave-assisted treatment. A comparative physicochemical characterization by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), and photoluminescence (PL) demonstrated that the composition of the feedstock plays a crucial role in determining the physicochemical properties of the resulting nanocarbons. Among the investigated materials, tobacco-derived nanobiochar (TT-NBC) exhibited the highest degree of structural organization, as confirmed by Raman spectroscopy through the lowest ID/IG ratio (0.71), indicating enhanced graphitization and improved sp² carbon domain formation. TT-NBC was selected as the most promising candidate for electrochemical applications and subsequently functionalized with diethylenetriamine (DETA) and methyl-arginine (ArgOMe) to introduce nitrogen-rich chelating sites for metal ion detection. The functionalized materials were deposited onto screen-printed carbon electrodes (SPCEs) and the modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), while Hg²⁺ detection was performed by square-wave anodic stripping voltammetry (SWASV). The modified electrodes exhibited enhanced electron transfer, reduced charge-transfer resistance, and excellent analytical performance. The calibration curves displayed two distinct linear concentration ranges, with the highest analytical sensitivity observed between 1 and 6 nM. Within this range, the SPCE/TT-NBC-DETA sensor achieved a sensitivity of 74.11 μA nM⁻¹ cm⁻² with a limit of detection (LOD) of 0.30 nM, whereas the SPCE/TT-NBC-ArgOMe sensor exhibited a sensitivity of 47.02 μA nM⁻¹ cm⁻² and a LOD of 0.55 nM. The improved sensing performance is attributed to the synergistic effect between the conductive graphitic framework of TT-NBC and the strong metal-binding ability of the introduced amine- and guanidinium-based functionalities. Overall, this work demonstrates, for the first time, that tobacco-derived nanobiochar can be successfully exploited as a sustainable nitrogen-rich platform for electrochemical Hg²⁺ sensing. The proposed strategy combines biomass valorization, microwave-assisted nanostructuring, and selective surface functionalization to produce high-performance sensing interfaces, providing an effective alternative for the development of sustainable electrochemical sensors based on waste-derived carbon materials.