Facile Multistep Synthesis of ZnO-Coated β-NaYF4:Yb/Tm Upconversion Nanoparticles as an Antimicrobial Photodynamic Therapy for Persistent Staphylococcus aureus Small Colony Variants

Antibacterial treatment strategies using functional nanomaterials, such as photodynamic therapy, are urgently required to combat persistent Staphylococcus aureus small colony variant (SCV) bacteria. Using a stepwise approach involving thermolysis to form β-NaYF4:Yb/Tm upconversion nanoparticles (UCNPs) and surface ligand exchange with cetyltrimethylammonium bromide (CTAB), followed by zeolite imidazolate framework-8 (ZIF-8) coating and conversion to zinc oxide (ZnO), β-NaYF4:Yb/Tm@ZnO nanoparticles were synthesized. The direct synthesis of β-NaYF4:Yb/Tm@ZIF-8 UCNPs proved problematic due to the hydrophobic nature of the as-synthesized material, which was shown by zeta potential measurements using dynamic light scattering (DLS). To facilitate deposition of a ZnO coating, the zeta potentials of (i) as-synthesized UCNPs, (ii) calcined UCNPs, (iii) polyvinylpyrrolidone (PVP), and (iv) CTAB-coated UCNPs were measured, which revealed the CTAB-coated UCNPs to be the most hydrophilic and the better-dispersed form in water. β-NaYF4:Yb/Tm@ZIF-8 composites formed using the CTAB-coated UCNPs were then converted into β-NaYF4:Yb/Tm@ZnO nanoparticles by calcination under carefully controlled conditions. Photoluminescence analysis confirmed the upconversion process for the UCNP core, which allows the β-NaYF4:Yb/Tm@ZnO nanoparticles to photogenerate reactive oxygen species (ROS) when activated by near-infrared (NIR) radiation. The NIR-activated UCNPs@ZnO nanoparticles demonstrated potent efficacy against both Staphylococcus aureus (WCH-SK2) and its associated SCV form (0.67 and 0.76 log colony forming unit (CFU) reduction, respectively), which was attributed to ROS generated from the NIR activated β-NaYF4:Yb/Tm@ZnO nanoparticles.

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