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Molecules 2019, 24, 2692 6 of 17 Figure 1. The schematic diagram of the composite structure.Ce6 and NaYF4:Yb,Er UCNPs were combined with amphiphilic silane modification through hydrophobic-hydrophobic interaction. Hydrophobic side chain of the silane (blue) and hydrophobic group (green) on the surface of NaYF4 :Yb,Er UCNPs can attach each other forming the close coating layer. After entering into the bacteria in the area of periodontal disease, singlet oxygen (1O2) can be effectively produced under the irradiation of 980 nm irradiation as shown in the figure. Mn doping greatly enhances the red emission and then the aPDT function. As shown in Figure 2A,B the synthesized UCNPs and the silane modified UCNPs were characterized by TEM. It can be seen that the size distribution of both NPs is very uniform, about 25 nm for UCNPs and 30 nm for silane modified UCNPs. After silane coating, a thin layer of silane can be seen on the surface of the particles with a thickness of about 2–3 nm, as the black arrows point in Figure 2B. When a single nanoparticle is selected and characterized by high-power transmission electron microscopy, the thin layer of silane on the surface can be clearly examined, as indicated by the white arrow. The FTIR spectra before and after silane coating were shown in Figure 2C. It can be seen that, after silane modification with 18 carbon atoms chain, the characteristic peaks of Si-O-Si and Si-OH appear new compared to the unmodified UCNPs, indicating the successful modification of silane. Moreover, the intensity of the characteristic peak of -CH2 increases relative to the unmodified UCNPs due to the long carbon chain, which is also consistent with the experimental conditions. The zeta potential of NaYF4:Yb3+,Er3+@Ce6@silane and samples with the Mn doping percent of 10%, 20% and 30% composites are −28.5 mV, −27.8 mV, −33.5 mV, and −32.8 mV, respectively, indicating the high water solubility and stability. It should be noted that, in addition to the complexity of the oral structure, the main difficulty of this NIR triggered PDT system lies in the design of the upconversion material. In some previous construction of photodynamic systems, photosensitizers were mostly supported on the nanocarriers by physical adsorption. For example, photosensitizer zinc phthalocyanine can be loaded into the mesopores of UCNP@mSiO2 NPs [19]. Though efficient production of 1O2 was realized after these photosensitizers were loaded by physical adsorption, it still faces serious leakage problems. Therefore, the FRET efficiency becomes low in the PDT process, causing systemic toxic side effects as well. In this coating strategy, by focusing on improving the efficiency of photodynamic therapy, the UCNPs were modified with silane, achieving water solubility and biocompatibility. In addition, the stable and very thin coating layer can well encapsulate the Ce6 molecules by hydrophobic-hydrophobic interaction and the followed hydrolysis of the silane further form the close composite, avoiding the Ce6 leakage.PDF Image | Anti-Biofilm Property of Bioactive Upconversion Nanocomposites
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