Nitrogen-Doped Amorphous Carbon Film for Enhanced Cu2+ Electrochemical Sensing in Marine Environments

Amorphous carbon (a-C) is a promising material for electrochemical sensing due to its wide potential window and chemical stability. However, its high resistance and limited surface activity hinder performance. Nitrogen (N) doping can improve conductivity while maintaining a low background current, but precise control of N content remains challenging. In this study, N-doped a-C films are fabricated using a high-ionization anode-layer ion source by adjusting the C₂H₂/N₂ gas ratio. The influence of N concentration on Cu²⁺ detection performance is systematically evaluated. Moderate N doping (≈12.9 at.%) facilitates the formation of sp²-hybridized carbon and nitrogen-containing functional groups, significantly enhancing electrochemical activity. The optimized electrode exhibits a wide linear detection range from 8 × 10⁻³ to 5 mM and a low detection limit of 8 × 10⁻³ mM in 3.5 wt% NaCl solution, sufficient for monitoring copper alloy crevice corrosion (≈0.1 mm). The electrodes also show excellent repeatability, reproducibility, and long-term stability. Theoretical calculations indicate that increased sp² content and C─N bonds enhance Cu²⁺ adsorption and electron transfer, thereby improving sensor performance via N doping.