Unveiling the Interfacial-Wettability-Dependent Growth of Polydopamine: Transmittance Kinetics as a Rapid At-Line Quantitative Probe.

Polydopamine (PDA) deposition is often described as "substrate-independent," yet its early-stage kinetics on chemically well-defined interfaces remain insufficiently quantified. Here, we employ average visible-light transmittance as a rapid, non-destructive at-line kinetic probe to monitor PDA film formation over short timescales (1-20 min) on model hydrophilic (-OH) and hydrophobic (-CH3) surfaces. Combined characterization reveals that wettability governs the initial nucleation and growth modes of PDA and thereby determines the evolution of optical responses and film microstructure. On hydrophobic interfaces, PDA undergoes Volmer-Weber (island-like) growth, producing rapid nucleation, pronounced early transmittance attenuation, and higher surface roughness. In contrast, hydrophilic interfaces favor Frank-van der Merwe-like (layer-by-layer) growth, leading to smoother, denser films and a delayed late-stage "oxidation burst." As a result, the transmittance-time profiles establish a quantitative link between microscopic interfacial processes and macroscopic optical signals, enabling identification of the transition from nucleation-dominated to later-stage growth behavior. Functionality assays further show that coatings derived from hydrophobic substrates exhibit superior antibacterial performance (>99.9% reduction against E. coli and S. aureus), while maintaining good cytocompatibility (cell viability > 90%). This work elucidates the interfacial dependence of PDA assembly and introduces a simple, optics-based paradigm for rapid at-line process monitoring and quality control in coating technologies. Growth-mode terminology is used throughout as a descriptive analogy for the observed morphological and kinetic sequence, not as a claim of heteroepitaxial thermodynamic growth modes.