Black diamond and black silicon for reducing marine biofilm formation.

Marine biofilms quickly colonize submerged surfaces, causing drag, reduced efficiency, and corrosion in vessels and marine infrastructure. Thus, the development of coatings that can resist bacterial adhesion and biofilm growth is essential. This study investigated two nanostructured surfaces - black silicon (bSi) and diamond-coated black silicon (black diamond, bD) - designed to physically disrupt bacterial cells using nanoscale spikes. Hydrogen- and fluorine-terminated versions of these surfaces were evaluated against 7-week-old Cobetia marina biofilms under controlled hydrodynamic conditions using flat silicon (Flat-Si) and flat diamond as controls. Nanostructured surfaces were less wettable than Flat-Si, with the contact angle of the fluorinated black diamond (bD-F) reaching 132°. Scanning Electron Microscopy confirmed that bSi and bD maintained their high-aspect-ratio nanoneedles, resisted protein adsorption, and had reduced biofilm coverage compared to flat controls. Optical Coherence Tomography revealed ∼50% thinner and less porous biofilms on the bD-F surface. Confocal Laser Scanning Microscopy analysis showed a 75% reduction in biofilm biovolume on bD-F compared to Flat-Si, with only 45% cell viability. Non-viable cells were predominantly located in inner biofilm layers, indicating a bactericidal effect. Flow cytometry supported these results, showing altered bacterial membrane potential and metabolic activity in bacteria exposed to bD surfaces. Experiments using real seawater and field immersion assays confirmed that bD surfaces maintain structural integrity and strongly reduce biofilm formation under realistic marine conditions. These findings demonstrate the antifouling and antimicrobial effects of nanostructured diamond-coated surfaces, particularly fluorine-terminated ones, for durable marine applications.