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Molecules 2019, 24, 2692 8 of 17 samples showed no change of the peak position in the emission spectrum, but the ratios of the green/red emissions, which is dependent on the Mn doping amount. The existence of Mn2+ ions greatly influence the transition possibilities between green and red emissions of Er3+. As the Mn doping content increases from 0 to 30%, the proportion of red light gradually increases. The fine-tuning of red/green emissions could be attributed to nonradiative energy transfer from the 2H9/2 and 4S3/2 levels of Er3+ to the 4T1 level of Mn2+, followed by back-energy transfer to the 4F9/2 level of Er3+ as shown in Figure 2F, resulting in an enhanced red emission output by rational controlling the Mn2+ doping level. The energy transfer from level 4S3/2 to the Mn ion can be proved by the lifetime since the occurrence of such non-radiative transitions will reduce lifetime value. Relative to the NaYF4:Yb3+,Er3+@Ce6@silane UCNPs, all of the Mn doped UCNPs show the decreased lifetime in the green emission energy level. In addition, as the Mn doping content increases, the lifetime decreases gradually, and due to that the non-radiative transition speed is much faster than the radiation transition, as shown in the inset in Figure 2E, indicating that the more efficient energy transfer to the Mn ion happens, thus causing the further enhancement of red emission. It should be noted that this luminescence property is beneficial for the Ce6 based aPDT, because the excitation of the Ce6 molecule is located in the red region, and, in addition, the Ce6 molecule is on the surface of the UCNPs with a very close distance, thus facilitating the upconversion aPDT. The absorption spectrum of the Ce6 molecule inside the composites and the upconversion fluorescence spectrum of Mn 30% doped UCNPs are shown in Figure 3A. The absorption at the red region is the primary excitation band of the Ce6 molecule for singlet oxygen production and this region is totally overlapped with the upconversion red emission band of its UCNPs carrier. Therefore, the enhanced upconversion red emission can further improve the aPDT effect. In addition, since the Ce6 molecule is located in the hydrophilic thin layer of the NPs, this energy transfer is very efficient for stimulating the PDT function. In the present study, up to 30% Mn was selected to dope into UCNPs to realize the enhancement of red light emission. Too much foreign element doping would lead to the change of crystal lattice, following with the functional variation of UCNPs. Besides, in this doping range (10–30%), the fluorescence intensity of red light increases with Mn doping, while the overall fluorescence intensity also decreases as the Mn content further increases. Previous studies have demonstrated that the introduction of sufficient Mn2+ ions into NaYF4:Yb/Er leads to a brilliant red emission, which is brighter by as much as 15 folds than that of Mn-free sample [25]. Therefore, in this doping scale, the highest red emission intensity was obtained, which is better for the efficacy of aPDT. In addition, in such doping situation, green light was also retained, which can be further used for fluorescence imaging. Currently, the up-conversion green light as imaging signal and red light as PDT treatment source need to be further investigated. The upconversion PDT function of composite nanomaterials was tested using the singlet oxygen probe ABDA 9,10-fluorenyl-di(methylene)dimalonic acid [26]. Figure 3B shows the absorption spectra of magnetic nanocomposites under red light with an interval of 2 min. As the irradiation time increases, the absorption value of ABDA at 260 nm is reduced, indicating the generation of singlet oxygen. Furthermore, the absorption band at 400 nm showed the similar tendency, because the Ce6 molecules can also be consumed via the photodegradation of red light. It should be noted here that the detection efficiency of singlet oxygen is not very high according to the probe absorption because of the fact that the 980 nm excitation area is usually small, while the diffusion region of singlet oxygen is relatively large in the solution. This test proves that the composite material can produce singlet oxygen, indicating the successful material design and preparation. For periodontal disease treatments, the small 980 nm laser irradiation area is enough for sterilization. Dark cytotoxicity of NaYF4:Yb3+,Er3+@Ce6@silane NPs was investigated with L929 mouse fibroblast cells by CCK-8 assay, as shown in Figure 3C. The UCNPs show very good biocompatibility. The results showed that the cells were still 90% viable with the concentration of 200 μg/mL, indicating very low cytotoxicity. In this study, it should be attributed to the silane modification and the negatively-charged surface which could reduce cytotoxicity, showing great potential for new photosensitizer carriers in dental application.PDF Image | Anti-Biofilm Property of Bioactive Upconversion Nanocomposites
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