Poly(DL-Phenylalanine) modified biosynthesized ZnO nanoparticles composite carbon paste electrode for the determination of melatonin

A new electrochemical sensor was fabricated for the selective analysis of melatonin (MEL). In this work, a novel sensing platform combining biosynthesized zinc oxide nanoparticles (ZnONPs) with electropolymerized DL-phenylalanine (DLP) was developed. The electrode was fabricated by modifying the surface of bare carbon paste electrode with biosynthesized Zinc oxide nanoparticles (BZnONPs/CPE) and further coated with an electrochemically polymerized DL-phenylalanine film (P(DLP)MZnONPs/CPE). The polymer layer enhanced the surface of the electrode and significantly improved the redox response of MEL, with the optimum peak obtained in 0.2 M phosphate buffer solution (PBS) at pH 4.0. The surface morphology and elemental composition were examined using Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX), and X ray diffraction (XRD). Electrochemical characteristics were explored through cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), and impedance spectroscopy (EIS). The scan rate results indicated that the process was diffusion controlled. The modified electrode displayed enhanced current responses across a broad concentration range for both LSV and DPV analyses. The detection limits (LOD) were calculated as 0.11 μm and 0.09 μm with corresponding limits of quantification (LOQ) were 0.37 μm and 0.30 μm using LSV and DPV methods, respectively. Notably, the developed sensor demonstrated improved analytical performance compared to conventional carbon paste electrodes, highlighting the advantage of the proposed composite configuration. The P(DLP)MZnONPs/CPE sensor also exhibited strong tolerance towards interfering metal ions and organic molecules, along with consistent reproducibility, repeatability and stability. Furthermore, it enabled the simultaneous determination of MEL, adenosine (ADN) and tryptophan (TR). This dual analyte detection capability further demonstrates the practical significance and novelty of the proposed sensing platform. The applicability of the developed electrode was verified by analysing pharmaceutical tablet samples, where precise and consistent results were obtained.