Acid orange 2

Acid Orange II (AO II, Sigma-Aldrich, USA) was implemented to quantify the surface amino groups of the coatings [25 ]. Micro BCA protein assay kit was used to quantify the phenolic hydroxyl/quinone groups of the coatings as the following steps. Firstly, 160 μL of BCA solution was added on the surface of the samples, and incubated at 30 °C for 90 min, then the absorbance of 150 μL reaction solution was detected at 562 nm. Infrared absorption spectra of the three coatings were obtained using a Grazing incidence attenuated total reflection Fourier transform infrared spectroscopy (GATR-FTIR) (Nicolet model 5700) in the range of 4000–400 cm⁻¹ to analyze their chemical structure. X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Electron, USA) was applied to determine their atomic composition. The instrument was operated at 12 kV × 15 mA at a pressure of 3 × 10⁻⁷ Pa. A wide-scan survey spectrum over a binding energy ranging from 0 to 1400 eV was recorded using the Al Kα excitation at a pass energy of 80 eV for estimation of the chemical elemental composition and 10 eV for high-resolution detailed scans. The system was calibrated using the C1s peak at 284.8 eV. Possible chemical structures of the coatings were determined by the MALDI-TOF MS as reported elsewhere [26 (link)].