Insights into the Process and Mechanism of Polyethylenimine-Initiated Dopamine Polymerization for Precisely Regulating Their Dispersibility and Photothermal Properties

Polydopamine (PDA), as a biocompatible polymer derived from mussels, possesses strong adhesive capability and high photothermal conversion efficiency, which have already been employed in cancer diagnosis and treatment, drug/gene delivery, biosensing, antibacterial, etc. Although PDA could be spontaneously polymerized under mild alkaline conditions, the polymerization mechanism concerning the morphological characteristics, dispersion stability, and photothermal response under different initiation systems remains unclear. This study systematically investigated the influence of different initiation systems, specifically comparing the macromolecular initiator (polyethylenimine, PEI) with typical small molecule initiators (NaOH, NH3·H2O, Tris), on the polymerization behavior, dispersion stability, and photothermal performance of PDA nanoparticles. Comprehensive characterization demonstrated that small-molecule initiators led to the formation of aggregated spherical morphologies with an average size exceeding 186.9 nm, whereas the macromolecular system facilitated a well-dispersed network structure and enabled precise control over the particle size of PDA, tuning it within the range of approximately 158.4 to 11.2 nm. Furthermore, the effects of the DA/PEI mass ratio, molecular weight of PEI, initial temperature, dosage of synthesized PDA, and laser power on the photothermal performance were systematically investigated, with macromolecular PEI serving as both initiator and structural scaffold. When the mass ratio of DA/PEI was 1:1, the molecular weight of PEI was 600 g/mol, the initial temperature was 25 °C, and under 808 nm laser irradiation (2.0 W cm–2), the synthesized PEI600-PDA reached a high photothermal conversion efficiency of 52.95%. DFT simulations revealed that PEI initiates the expansion of donor–acceptor structures, achieving relaxation attenuation through electron–phonon coupling, thereby enhancing the efficiency of light absorption and conversion. Moreover, PEI600-PDA presented the relatively low toxicity toward both normal and cancer cells under the laser off state, as well as the selective inhibitory effect on cancer cells via the laser irradiation. These findings elucidate the mechanism behind PEI’s superiority in achieving the exceptional dispersion stability, excellent biocompatibility, and high photothermal efficiency, providing a critical foundation for the rational design of PDA-based photothermal materials.