Comparative Morphological and Elemental Characterization of PDA/HAp Coatings Formed by Sequential, One-Step No-Field, and Field-Assisted Routes on Si(111) and Enamel

This study presents a comparative morphological and elemental characterization of organomineral coatings composed of polydopamine (PDA) and nanocrystalline carbonate-substituted hydroxyapatite (nano-cHAp), formed on model Si(111) substrates and human enamel specimens. Three technologically distinct routes were investigated: sequential PDA/HAp/PDA deposition without an external field (NF), one-step PDA/HAp co-deposition without applied voltage (F0, U = 0 V), and one-step PDA/HAp co-deposition under applied voltage (F, U = 50 V). X-ray diffraction was employed to verify the apatite-like structure of the initial nanocrystalline HAp and to identify drying-related contributions from the TRIS-containing medium; however, it was not used as direct evidence of the HAp phase or texture in the final coatings. SEM segmentation of selected enamel areas showed that the area fraction assigned to the morphologically continuous surface-layer class was approximately 85% for NF, 73% for F0, and 95.5% for F. Representative AFM analysis on Si(111) revealed a rough, topographically heterogeneous F0 layer (Sq ≈ 68 nm, height range ≈ 440–490 nm), a pronounced island-like NF morphology (Sq ≈ 45 nm), and a lower-amplitude F morphology (Sq ≈ 4 nm). For the F coating, scratch-edge profiling indicated an apparent step height of approximately 88 nm, while a preliminary ultrasonic retention challenge in water for 5 min at a 30 mm sample-to-probe distance preserved this apparent step height and reduced Sq from approximately 12 to 5 nm. Point-EDX-derived visualization on Ca/P-free Si(111) supported the presence of Ca/P-containing domains and indicated different spatial organization of the mineral component across the NF–F0–F sequence. FTIR and micro-Raman spectroscopy were performed for the two terminal routes, NF and F, and were interpreted as qualitative evidence of PDA- and HAp-related spectral features rather than as a comprehensive quantitative comparison of all deposition modes. Overall, the data support a transition from island-like or domain-heterogeneous coatings toward a more laterally continuous, lower-amplitude surface layer in the one-step field-assisted route. The results remain morphology-oriented and do not demonstrate clinical remineralization, HAp texture, long-term oral stability, or quantitative adhesion strength.