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Molecules 2019, 24, 2692 9 of 17 Figure 3. (A) Absorption spectrum and the upconversion spectrum of NaYF4:Yb3+,Er3+@Ce6@silane. Red emission region overlaps the absorption band of Ce6 molecules. (B) Generation of singlet oxygen generation under 980 nm irradiation. (C) Dark cytotoxicity of NaYF4:Yb3+,Er3+@Ce6@silane NPs on mouse fibroblast cell line L929: Viability of L929 cells vs. concentration of NaYF4:Yb3+,Er3+@Ce6@silane NPs after 24 h without irradiation in the dark (mean ± sd). Upconversion red light triggered PDT was first tested within biofilm experiments. The sample was irradiated with a 980 nm continuous diode laser which was adjusted to 750 J·cm−2 by tuning irradiation area for 3 min. Representative results of the live/dead analysis were performed and the results are shown in Figure 4 P. gingivalis, P. intermedia and F. nucleatum were selected as the bacteriostatic models in this work simulating early, middle and late stages of biofilm development, respectively, and colonized in plaque biofilms [27]. Live bacteria were stained as green which mainly in the control group and dead bacteria were stained red. In all three kinds of bacteria, NaYF4@Ce6@silane plays an efficient role in aPDT function. There are more and more dead bacteria in the groups with NaYF4@Ce6@silane and Mn doped NaYF4@Ce6@silane NPs under 980 nm light irradiation. The red color increases as the Mn doping increases, due to the enhanced upconversion red emission. The corresponding enhanced aPDT from Ce6 caused more and more dead bacteria. Note that the power density of the laser used in this study is strong enough for application of cell level in vitro. In addition, though the proposed periodontal bacteria locate in deepest periodontal pockets, usually 5–8 mm from the gingival margins, the power of the irradiation laser can still reach the threshold value. It is reported that the 980 nm laser can penetrate more than 0.7 cm in pork tissue without obvious reduction of power density [17,19,28]. Several kinds of UCNPs were applied to covert NIR to visible light or UV light for triggering PDT in tumor therapy or antibacterial application [29–31]. Gulzar et al. synthesized a nanocomposite based on nanographene oxide-UCNPs-Ce6 as a theranostic platform for the upconversion luminescence imaging-guided PDT/PTT of cancer [29]. The tremendous surface area of graphene oxide was allowed to house Ce6, as well as UCNPs. Remarkably, both the imaging and dual-mode treatments in this nanoplatform are stimulated by light, which unveils outstanding gains in terms of augmenting cancer killing specificity and decreasing side effects [29]. Numerous UCNPs-based nanomaterials with varieties of structures for photodynamic therapy in cancer treatment were summarized in a recent study [32]. On the other hand, for antibacterial application, Zhang et al. developed a photosensitizer (β-carboxyphthalocyanine zinc, CPZ) delivery system with UCNPs (LiYF4:Yb/Er) and polyvinylpyrrolidone (PVP) [30]. Such a near-infrared (NIR) triggered UCNPs-CPZ-PVP system significantly reduced the aggregation of CPZ and presented a high anti-infectious activity against multi-drug resistant bacteria (methicillin-resistant Staphylococcus aureus by 4.7 log and multi-drug resistant Escherichia coli by 2.1 log). Another study investigated the dual antibacterial behavior induced by the curcumin-UCNPs itself and induced by photodynamic therapy were demonstrated [31]. The results showed that nearly 100% methicillin-resistant Staphylococcus aureus was eradicated using curcumin-UCNPs under the NIR irradiation. However, to the best of our knowledge, the present study is the first report on application of near-infrared light to achieve photodynamic therapy forPDF Image | Anti-Biofilm Property of Bioactive Upconversion Nanocomposites
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