Annealed Ti6Al4V substrates were cleaned in an air plasma cleaner (Harrick, PDC-001) for 2 min before being placed in a plastic dish, and submerged under 40 mL of 3-mM anhydrous methanol solution of PA-O-Br (prepared and purified as we previously reported40 (link)) at room temperature in dark for 24 h. All retrieved substrates (Ti-Br) were then annealed at 110 °C for 15 min in a vacuum oven to stabilize the bonding of the phosphonic acid group to the thin oxidized metal surface before being subjected to extensive sonication in methanol (10 min each time, twice) and vacuum drying.