The present work reports the synthesis of biomass derived activated carbon and its electrochemical behaviour in different electrolytes. Ricinus communis shell (RCS) was used as a raw material in this study for the synthesis of activated carbon (AC) following a high-temperature activation procedure using potassium hydroxide as the activating agent. The physical and structural characterization of the prepared Ricinus communis shell-derived activated carbon (RCS-AC) was carried by Brunauer-Emmett-Teller analysis, X-ray diffraction analysis, Fourier Transform Infrared Spectroscopy, Raman Spectroscopy and Scanning Electron Microscopy. The synthesized AC was electrochemically characterized using various techniques such as Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) tests, and Electrochemical impedance spectroscopy (EIS) measurements in different aqueous electrolytes (KOH, H₂SO_4, and Na₂SO₄). The results show that the double layer properties of the RCS-AC material in different electrolytes are distinct. In specific, the working electrode tested in 3 M KOH showed excellent electrochemical performance. It demonstrated a specific capacitance of 137 F g⁻¹ (at 1 A g⁻¹ in 3 M KOH) and exhibited high energy and power densities of 18.2 W hkg⁻¹ and 663.4 W kg⁻¹, respectively. The observed capacitance in 3 M KOH remains stable with 97.2% even after 5000 continuous charge and discharge cycles, indicating long-term stability. The study confirmed that the synthesized RCS-derived activated carbon (RCS-AC) exhibits good stability and physicochemical characteristics, making them commercially promising and appropriate for energy storage applications.

A carbon quantum dot-based chitosan hydrogel was prepared in this work as a fluorescence sensor for the selective sensing of Hg²⁺ ions. Among the eight tested metal ions, the prepared hydrogel exhibited remarkable sensing selectivity and sensitivity toward Hg²⁺. The results demonstrated that a prominent fluorescence quenching at 450 nm was observed in the presence of Hg²⁺ with a linear response range of 0–100.0 nM and an estimated limit of detection of 9.07 nM. The as-prepared hydrogel demonstrates pH-dependent fluorescence intensity and sensitivity. The highest fluorescence intensity and sensitivity were obtained under pH 5.0. The excellent sensing selectivity could be attributed to a strong interaction between the hydrogel film and Hg²⁺ ions to form complexes, which provokes an effective electron transfer for fluorescence quenching. Results from density functional theory (DFT) calculation confirm that the interaction energies (ΔIE) of the hydrogel with three toxic metal ions (Hg²⁺, Cd²⁺, and Pb²⁺) are in the following order: Hg²⁺ > Cd²⁺ > Pb²⁺.