An amperometric sensor was developed by depositing a film coating of hydroxyapatite (HA)/L-lysine (Lys) composite material on a glassy carbon electrode (GCE). It was applied for the detection of Nile blue A (NBA). Hydroxyapatite was obtained from snail shells and its structural properties before and after its combination with Lys were characterized using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analyses. The coupling of Lys to HA was attributed to favorable interaction between negatively charged -COO⁻ groups of Lys and divalent ions Ca²⁺ of HA. Electrochemical investigations pointed out the improvement in sensitivity of the GCE/Lys/HA sensor towards the detection of NBA in solution. The dependence of the peak current and potential on the pH, scan rate, and NBA concentration was also investigated. Under optimal conditions, the GCE/Lys/HA sensor showed a good reproducibility, selectivity, and a NBA low detection limit of 5.07 × 10⁻⁸ mol L⁻¹. The developed HA/Lys-modified electrode was successfully applied for the detection of NBA in various water samples.

Snail shells (Anadora Fulica) calcined at different temperatures were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermal analyses (TG-DTG), scanning electron microscopy (SEM) and N₂ adsorption–desorption experiments (surface area measurements were found using the coupled BET/BJH method). The principal objective was to identify different forms of calcium carbonate and calcium hydroxide in snail shells as raw materials. The calcium hydroxide thus obtained was used in the synthesis of the hydroxyapatite/L-lysine (HA/Lys) composite. The composite used to chemically modify a glassy carbon electrode (GCE) was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It appeared that the developed sensor Lys/HA/GCE facilitated electronic transfer compared to the pristine electrode. In a strongly acid medium, this surface protonated and therefore became positively charged, which allowed it to have a good affinity with [Fe(CN)₆]^3-. An application in toluidine blue (TB) electroanalysis in the phosphate buffer was carried out. Optimal sensor performances were obtained using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The performance of the sensor was determined in the concentration range 1 to 10 µM of TB, and the limit of detection (LOD) obtained by the S/N = 3 method was 2.78 × 10⁻⁷ M. The sensor was also used to detect the TB in spring water at 96.79% recovery.