NaYF4:Yb,Er,Nd@NaYF4:Nd Upconversion Nanocrystals Capped with Mn:TiO2 for 808 nm NIR-Triggered Photocatalytic Applications

This paper described a rational design of core/shell heterostructure consisting of upconversion nanoparticles (UCNPs) as the core and Mn-doped TiO2 (Mn:TiO2) as the shell for promising near-infrared (NIR) photocatalytic dye degradation and antibacterial applications. The initial stage was the preparation of NaYF4:Yb,Er,Nd@NaYF4:Nd (Yb/Er/Nd@Nd) active-core/active-shell UCNPs via an epitaxial growth process, which could solve the problem of intrinsic low luminescence efficiency. The UC luminescence of typically used NaYF4:18%Yb,0.5%Er,1%Nd@NaYF4:20%Nd could be enhanced by ∼20 times after coating NaYF4:20%Nd active shell around NaYF4:18%Yb,0.5%Er,1%Nd core particles, which was mainly attributed to the significant increase in the NIR absorption and efficient energy migration from shell to core. Subsequently, Mn:TiO2 nanocrystals were uniformly coated on Yb/Er/Nd@Nd to form Yb/Er/Nd@Nd@Mn:TiO2 core/shell NPs. Yb/Er/Nd@Nd acted as a NIR-to-visible transducer to sensitize Mn:TiO2 when illuminated with 808 nm NIR light. Importantly, manganese as dopant increased the photocatalytic activity, extending the photoresponse of TiO2 to the green light region and consequentially improving its harvesting efficiency of UC luminescence from Yb/Er/Nd@Nd NPs as well as playing the role of charge separators, which triggered the generation of sufficient reactive oxygen species to induce photocatalytic activity of the core/shell system. The spread-plating and MIC experiments indicated that Yb/Er/Nd@Nd@Mn:TiO2 NPs possessed excellent photocatalytic antibacterial activities against Gram-negative and Gram-positive bacteria under 808 nm laser irradiation, for example, the MIC value of E. coli was only 12.5 μg·mL–1. Meanwhile, the Nd3+/Yb3+-based UC system with 808 nm excitation could significantly minimize the overheating effect on biological cells associated with the conventional 980 nm excitation. Preliminary study on the biocompatibility of the photocatalyst showed no cytotoxic effect within a certain dose range as measured by MTT assay. A possible working mechanism of Yb/Er/Nd@Nd@Mn:TiO2 heterojunctions was proposed to understand the enhanced photochemical properties.