Bandgap engineering via lattice defects: Amorphous M-phosphate coatings (M = Ca, Zn, mg) achieve unprecedented NIR photothermal performance

Bacterial infections and rapid degradation remain significant challenges for biodegradable magnesium (Mg) alloy implants. While conventional photothermal antibacterial coatings often rely on exogenous agents, the intrinsic photothermal potential of phosphate materials warrants greater exploration. In this study, amorphous calcium phosphate (CaP), zinc phosphate (ZnP), and magnesium phosphate (MgP) coatings were fabricated on WE43 Mg alloys via a facile chemical deposition method utilizing ultra-low pH solutions. Characterization revealed that these amorphous coatings exhibited exceptional photothermal performance. Upon near-infrared (NIR) irradiation (808 nm) for 240 s, the maximum surface temperature of these amorphous phosphates reached 240.5 °C. The coatings maintained robust photothermal responses in PBS and ex vivo tissue models. Mechanistically, unlike well-crystallized counterparts, the amorphous phosphates possess abundant lattice defects, creating numerous defect energy levels within their bandgaps that facilitate localized photon absorption and phonon-mediated heat generation. Furthermore, the coatings provided substantial corrosion protection for the Mg substrate. This bandgap engineering strategy via structural disorder offers a shift for designing antibiotic-free, smart antibacterial biomaterials with excellent stability and biocompatibility.