**1. Introduction**

Bacterial infections can cause great harm to public health and have attracted considerable research attention from scientists. To date, many traditional bactericidal agents manifest a wide range of potential applications in antibacterial disinfection [1,2]. However, ordinary bactericidal agents have been shown to induce risks to the environment and encourage antibiotic resistance [3]. Contemporarily, the photocatalytic sterilization created by utilizing some reactive species produced under ultraviolet or visible light illumination has become increasingly valuable [2,4–6]. In short, photocatalytic nanocomposites harvest optical energy to produce positive and negative charge carriers that are involved in photoredox reactions [7–9]. It has been reported that TiO2 is one of the most representative and dominant photocatalysts on account of its non-toxicity and chemical stability. It, however, has a wide-band energy gap of 3.2 eV, which endows it with a high light absorption capacity within the ultraviolet (UV) light region to enhance photocatalytic efficiency with hardly any absorption within the visible region [10]; UV light and visible light account for only 5% and 45% of sunlight, respectively [10,11]. To improve the efficiency of sunlight, it would be interesting to develop TiO2-based materials that can absorb visible light. The ion doping or morphology-engineering of TiO2 nanoparticles can narrow their energy bandgap, resulting in the efficient absorption of visible light. For example, two-dimensional reduced TiO2 nanosheets with an energy bandgap of 2.86 eV have been used to efficiently inactivate bacteria under visible light irradiation [11]. Hydrothermally synthesized TiO2 nanosheets doped with N, C, and/or S also exhibit visible light absorption capacities [12–16]. However, ultraviolet and visible light can injure healthy tissue and display a short light penetration

**Citation:** Zhou, H.; He, F. Using Gd-Enhanced β-NaYF4:Yb,Er Fluorescent Nanorods Coupled to Reduced TiO2 for the NIR-Triggered Photocatalytic Inactivation of *Escherichia coli*. *Catalysts* **2021**, *11*, 184. https://doi.org/10.3390/ catal11020184

Academic Editors: Ioan Balint and Monica Pavel Received: 29 December 2020 Accepted: 26 January 2021 Published: 31 January 2021

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depth in the human body, both of which hinder their further use in in vivo antibacterial applications [17].

Recently, near-infrared (NIR) light-induced upconversion particles have been introduced to photocatalyst sterilization systems. The upconversion particles can absorb low NIR photons, convert them to higher energy photons, and emit ultraviolet or visible light [18,19]. Coupling TiO2 with upconversion particles can remarkably extend the utilization of the whole solar spectrum [20]. It has been suggested that β-NaYF4 is the most ideal upconversion matrix for the doping of different lanthanide ions, on account of its high refractive index and transparency [21]. It may act as an intermedium for transferring NIR light energy to UV-Vis light that can be absorbed by the TiO2 nanoparticles to produce oxidative holes (h+) and reductive electrons (e−). Effective e−-h+ pairs are able to react with O2, OH, and H2O in a mixed solution to generate various reactive species that are helpful for sterilization [22,23]. For instance, β-NaYF4:Yb,Er nanomaterial has been widely used in biological imaging analysis and photocatalytic applications. However, its luminous intensity is not satisfactory [24,25].

In this study, Gd-enhanced β-NaYF4:Yb,Er,Gd fluorescent nanorods with a high fluorescence intensity were coupled to reduced TiO2 nanoparticles with excellent visible light absorption abilities (UCNPs@R-TiO2 nanocomposite) using electrostatic assembly (Figure 1). As expected, the created UCNPs@R-TiO2 nanocomposite exhibits an effective photocatalytic sterilization performance against *Escherichia coli* (*E. coli*) under 980 nm NIR light illumination. The in vitro cellular cytotoxicity and antibacterial performance of the obtained UCNPs@R-TiO2 were also evaluated by MTT (3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide) assay and MALDI-TOF MS analysis, respectively.

**Figure 1.** Schematic illustration of UCNPs@R-TiO2 nanocomposite for photocatalytic sterilization under 980 nm NIR light irradiation.

### **2. Results and Discussion**
