<p>This study examines the structure, optical and metal ion detection properties of eumelanin-thin film. X-ray and scanning electron microscopy (SEM) exhibit strong amorphous structural properties of eumelanin thin films with various microstructures, which change in volume and shape with increasing thickness. Fourier transform infrared (FT-IR) spectroscopy identifies distinctive functional groups such as O-H and N-H, and confirms the presence of carboxylic acid and phenolic groups. Optical measurements show that eumelanin thin films in the ultraviolet range have a greater absorption, with a significant red shift in the absorption edge when the film thickness increases. Important dielectric and refractive parameters of optoelectronic devices were determined and discussed. However, the metal ions detection ability of eumelanin was also investigated. The shift in the absorption spectra to the red region on interaction with metal ion like Cu, Ni and Fe was investigated to demonstrate the selectivity and sensitivity of eumelanin as a sensor material. Further, the eumelanin sensor performance was optimized utilizing the ultrasonic waves and visible irradiation effects. The relatively better results were achieved through indium doping. The constructed calibration curves for metal ion quantification have shown the eumelanin-based sensors` effectiveness in determining metal ion concentrations. The calculated Limit of Detection (LOD) and Limit of quantification (LQD) values indicate that the melanin-based sensor has the potential for detecting low levels of metal ions at concentrations within the order of 10⁻³ mg/L.</p>