*2.1. Characterization of the UCNPs@R-TiO2 Nanocomposites*

The transmission electron microscopy (TEM) image shown in Figure 2A was recorded on a Tecnai G2 F20 microscope (USA) and was used to observe the crystal morphology

and size of the β-NaYF4:Yb,Er,Gd fluorescent nanorods (UNCPs). The UNCPs were homodispersed and rod-shaped with a length of ~500 nm (Figure 2(Ba)) and a diameter of ~50 nm (Figure 2(Bb)). Moreover, the fast Fourier transform pattern indicated a (100) zone axis (Figure 2A). We found that the reduced TiO2 (R-TiO2) nanoparticles exhibited a square shape and were uniformly scattered (Figure 2C). The HRTEM (insert in Figure 2C) showed that R-TiO2 belongs to pure anatase [11]. Additionally, the TEM image in Figure 2(Da) shows that the R-TiO2 nanoparticles were successfully assembled on the UNCPs. Furthermore, we confirmed the crystal structure of the UCNPs@R-TiO2 nanocomposite and found that the average lattice spacings that can be measured are 0.521 nm and 0.352 nm (Figure 2(Db)), matching well with a (100) facet and (101) facet lattice distance for the β-NaYF4 and anatase TiO2, respectively [11,21]. From energy-dispersive X-ray spectroscopy (EDS) measurements (shown in Figure S1), the elemental composition of the UNCPs@R-TiO2 was obtained, and, as shown in the table of Figure 2(Dc), Na, Ti, Cu, Yb, F, Er, Gd, Y, and O could be detected; Cu originated from the Cu grid used for TEM measurements [26]. Taken together, these results clearly illustrate that the R-TiO2 nanoparticles were successfully assembled on the UCNPs. The FT-IR spectra of the UCNPs@R-TiO2 nanocomposite was obtained on a Nexus 670 spectrophotometer and shown in Figure S2. A strong and broad absorption band at 464 cm−<sup>1</sup> was assigned to Ti–O and O–Ti–O flexion vibration originating from the TiO2 crystals [27,28]. The wide band of around 3435 cm−<sup>1</sup> was attributed to the H–O stretching, which helped to enhance photocatalytic activity [27]. The band around 1557 cm−<sup>1</sup> was attributed to the carbonyl group (−C=O−) vibration [21]. The FT-IR spectrum analysis indicates that the R-TiO2 nanoparticles were successfully assembled onto the surface of the as-synthesized UCNPs through electrostatic attraction.

**Figure 2.** (**A**) TEM image of UCNPs, with insert showing the fast Fourier transform pattern. (**B**) Length (**a**) and diameter (**b**) analyses of (**A**). (**C**) TEM and HRTEM (high resolution TEM) images (inset) of R-TiO2. (**D**) TEM image (**a**), HRTEM image (**b**), and elemental compositions analysis (**c**) of UCNPs@ R-TiO2.

The XRD patterns were recorded on an X-ray diffractometer (D8 Advance, Brucker, Germany) with the Cu K radiation (λ = 0.155 nm) operating at 40 kV and 80 mA; see Figure 3A and Figure S3. Figure S3 shows that the UCNPs and R-TiO2 nanoparticles were pure hexagonal- (Joint Committee on Powder Diffraction Standards JCPDS 00-016-0334) and anatase-phase (JCPDS 01-021-1272), respectively. The sharp diffraction peaks indicate that the UCNPs and R-TiO2 nanoparticles were highly crystallized hexagonal-and anatasestructured. The XRD pattern of the UCNPs@R-TiO2 nanocomposite is shown in Figure 3A. And the XRD pattern analysis confirmed that the UCNPs@R-TiO2 nanocomposite had a high degree of crystallization. The XRD pattern further showed that the UCNP nanorods were in a pure hexagonal phase (JCPDS 00-016-0334), which has been previously shown to have a higher luminous efficiency than the cubic phase NaYF4 [29]. The distinct peaks at 25.3◦ of the prepared R-TiO2 nanoparticles are likely ascribed to the (101) facet of the anatase TiO2 when compared to the JCPDS 01-021-1272 database, indicating that the R-TiO2 nanoparticles were present on the surface of the UCNP fluorescent nanorods by a form of substitutional doping. Compared to the peak locations of pure hexagonal and anatase phase, we observed that all of the diffraction peaks of the UCNPs@R-TiO2 nanocomposite shifted to lower diffraction angles due to an expansion in unit-cell volume as a result of the partial substitution of Ti4+ (65 pm) by the larger Y3+ (104 pm) in the lattice [26].

As shown in Figure S4, the upconversion luminescence (UCL) intensity of the Gddoped β-NaYF4:Yb,Er (UCNPs) fluorescent nanorods recorded on a Hitachi F-7000 spectrometer was higher than that of the β-NaYF4:Yb,Er fluorescent nanorods, which was attributed to the Gd dopant [21,30]. The UCL spectra of the UCNP nanorods and the UCNPs@R-TiO2 nanocomposite were analyzed and shown in Figure 3(Ba,b). We found that, under 980 nm irradiation, the UCNP nanorods emit intense UCL emissions at 523, 542, and 658 nm, which were assigned to the 2H11/2–4I15/2, 4S3/2–4I15/2, and 4F9/2–4I15/2 transitions of Er3+ (Figure 1), respectively [21]. By contrast, the UCNPs@R-TiO2 exhibited a drastic reduction to the UCL intensity because of the energy transfer from UCNPs to R-TiO2 [20]. The absorption spectrum in the UV-Vis range (Figure 3C) was recorded on a UV-visible Cary 300 spectrophotometer and indicated that the R-TiO2 (Figure 3(Ca)) possesses a higher absorption in the visible light region than pure anatase TiO2 (Figure 3(Cb)), which was caused by the oxygen vacancies and lower bandgap of the R-TiO2 (2.8 eV) than that of TiO2 (3.2 eV) (inset in Figure 3C and Equation (S1)) [10]. The oxygen vacancies and low bandgap of the R-TiO2, which arose from the dopant of Ti3+ under an argon atmosphere, are helpful to enhance the absorption of visible light [10,11]. In addition, we observed that the emission peaks of UCNPs can match the enhanced visible light absorption of the R-TiO2 nanoparticles (see the dotted boxes in Figure 3C), based on the UV-Vis absorption spectrum data (Figure 3C). We also found that the zeta potentials of the UCNP nanorods (Figure 3(Da)) and R-TiO2 nanoparticles (Figure 3(Db)) were negative and positive, respectively, indicating that R-TiO2 can be coupled to the surface of UCNP nanorods in a solution by electrostatic attraction. As a result, the UCNPs@R-TiO2 composites possessed a positive zeta potential (Figure 3(Dc)) that helps its binding to the negatively charged surface of *E.coli* bacteria [31].
