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Molecules 2019, 24, 2692 7 of 17 Silanes of different carbon chain lengths were also tested. Considering the higher loading amount of Ce6 molecules, silane with 18 C atoms was employed as the coating layer. Figure 2. Characterization of the NaYF4:Yb3+,Er3+,Mn2+ (0%, 10%, 20%, 30%)UCNPs. (A) and (B) are the TEM images of NaYF4:Yb3+,Er3+ and silane modified NaYF4:Yb3+,Er3+UCNPs, respectively. Color insets in (A,B) showed the schematic structures of nanoparticles. HR-TEM insets in B showed the high magnification of single nanoparticles. (C) FT-IR spectra of NaYF4:Yb3+,Er3+ and silane modified NaYF4:Yb3+,Er3+UCNPs. (D) XRD pattern of NaYF4:Yb3+,Er3+,Mn2+ (0%, 10%, 20%, 30%) UCNPs. The inset is the amplified diffraction peak of 111 plane of the NaYF4:Yb3+,Er3+,Mn2+ (10%, 20%, 30%) UCNPs. (E) Upconversion fluorescence spectra of NaYF4:Yb3+,Er3+,Mn2+ (0%, 10%, 20%, 30%) UCNPs with 980 nm excitation. The inset is the lifetime of the 2H11/2 energy level corresponding to green emission. (F) Proposed energy transfer mechanisms under the excitation of 980 nm NIR. The maximum amount of Ce6 molecules that can be loaded in the hydrophobic layer was investigated. Here, by fixing the amount of UCNPs and silane, the content of Ce6 was changed to test the stability. Herein the mass of the UCNPs was fixed at 8 mg, and the corresponding mass of Ce6 were changed from 300 to 1000 μg. In this case, the content of Ce6 molecules was not to exceed 800 μg or precipitation would occur, producing a flocculent precipitate due to the leakage of the hydrophobic molecules (data not shown). The structure of UCNPs with and without Mn doping were measured by XRD as shown in Figure 2D. Results confirmed the pure hexagonal phase NaYF4 without Mn doping. After the Mn ions were introduced into the nanocrystals, the crystal structure of the UCNPs changes from a hexagonal to a cubic phase. In addition, as the Mn ions doping increased, it could be found that the diffraction peak of 111 plane shifted toward the large angle direction (shown in inset), further illustrating the success of Mn ions doping. Note that the Mn doping do not influence the morphology and size of the UCNPs. The upconversion luminescent property of silane-coated UCNPs and Mn-doped UCNPs were investigated. Upconversion luminescence were obtained based on the anti-Stokes mechanism. The spectra of different Mn-doped samples were excited using a 980 nm continuous diode laser, where the laser power was adjusted to 1 W. From the upconversion emission spectra in Figure 2E, the green emission was located at 528 and 546 nm, and red emission was located at 660 nm, corresponding to 2H11/2, 4S3/2 and 4F9/2 excited states to the ground state 4I15/2 transition of Er3+, respectively. Yb3+ ions serve as sensitizers which can absorb 980 nm photons more efficiently and then transfer energy to the activator Er3+, thus completing the upconversion green and red emissions. Different Mn dopedPDF Image | Anti-Biofilm Property of Bioactive Upconversion Nanocomposites
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