Stability of sub-10 nm atomic layer-deposited alumina thin films on gold electrode surfaces in aqueous electrolytes

Atomic layer-deposited (ALD) alumina (Al2O3) thin films are widely employed in sensor and device fabrication as an insulating and protective layer. However, definitive studies on the aqueous stability of sub-10 nm ALD films remain limited, which poses a challenge for evaluating their long-term performance in electrochemical applications such as sensors, batteries, and fuel cells. While bulk alumina is generally stable in near-neutral electrolytes, this may not apply to ultrathin ALD films. In this study, ALD Al2O3 films with nominal thicknesses ranging from 2.5 to 10 nm were deposited on gold electrodes via ALD and systematically evaluated for their electrochemical blocking stability in representative electrolyte solutions. The electrochemical blocking stability was assessed continuously through in situ differential pulse voltammetry (DPV) in a typical electrochemical sensing buffer containing redox probes. The samples before and after extended exposure to the electrolyte solution were further characterized using ellipsometry, atomic force microscopy, and energy-dispersive x-ray spectroscopy. The results indicate that thinner films (e.g., 2.5 nm) undergo pronounced degradation within 22 h in the tested electrolyte solution, as evidenced by the DPV signal recovering to the level of the uncoated gold electrode, whereas thicker films (e.g., 10 nm) maintain improved insulating properties after immersion. These findings provide valuable insights into the thickness-related electrochemical blocking stability of ultrathin ALD alumina films in electrochemical sensing buffers and have direct implications for the design and long-term performance of chemical and biological sensors with ultrathin ALD alumina as the insulating and protective layer, particularly in implantable and fluid-contacting applications.