Antibacterial crystalline bilayers constructed on dentin substrates via laser-assisted mineralization: characterization and functional implications

The formation of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂; HAP)-based multilayers containing antibacterial agents at different concentrations on teeth can help control infections against various bacteria in periodontal tissue. However, from a clinical perspective, rapid (within minutes) and simple construction of such multilayers on teeth has not yet been achieved. If realized, such a technique could contribute to oral health by promoting infection control. In this study, crystalline bilayers containing antibacterial agents (Ag and F) at different concentrations were rapidly constructed on dentin. This was achieved by subjecting the dentin substrates precoated with Ag(NH₃)₂F-containing light-absorbing paste to 30-s laser irradiation in a pseudo-physiological calcium phosphate solution. Transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses revealed that the upper layer on the irradiated substrate was composed of F-substituted HAP (F-HAP) rods elongated to the c-axis. The content of Ag (<50 nm in size) in the rods layer was 0.14 ± 0.03 at.%. The lower layer was composed of particulate F-HAP and CaF₂, with the Ag (<50 nm) content of 0.49 ± 0.17 at.%. X-ray photoelectron spectroscopy measurements indicated two chemical states of Ag on the irradiated substrate surface: Ag⁰, corresponding to Ag particles, and Ag^δ+, which substituted Ca²⁺ ions in the F-HAP crystals. The bilayers exhibited antibacterial activity against Streptococcus mutans, highlighting their potential for application in antibacterial dental treatment.