**1. Introduction**

Indoor air pollution has been described as the most significant environmental cause of death globally, accounting for an estimated 3.8–4.3 million premature deaths each year over the past decade [1]. In major cities around the world, people spend more than 90% of their time in confined indoor environments. There is evidence that short-term exposure of human subjects to air pollution may exacerbate asthma and lead to hospitalizations, whereas long-term exposure to air pollution is repeatedly associated with a higher incidence of cardiovascular and respiratory diseases, birth defects, and neurodegenerative disorders. Fungi are ubiquitous and are a serious threat to public health in indoor environments [2].

Fungi can grow on almost all natural and synthetic materials, especially if they are hygroscopic or wet. As common indoor building materials, inorganic [3], wood-based [4], and bamboo-based materials [5] could serve as good growth substrates for fungi. In recent years, bamboo has received considerable attention because of its high strength, fast growth, renewability, and carbon sequestration potential [6]. All types of bamboo products have

**Citation:** Li, J.; Ma, R.; Wu, Z.; He, S.; Chen, Y.; Bai, R.; Wang, J. Visible-Light-Driven Ag-Modified TiO<sup>2</sup> Thin Films Anchored on Bamboo Material with Antifungal Memory Activity against *Aspergillus niger*. *J. Fungi* **2021**, *7*, 592. https:// doi.org/10.3390/jof7080592

Academic Editor: Kamel A. Abd-Elsalam

Received: 29 June 2021 Accepted: 15 July 2021 Published: 23 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

been developed and used in interior construction, decoration, and furniture materials worldwide. Nevertheless, bamboo is highly vulnerable to fungal attacks, especially during the rainy season. Therefore, efficient and environmentally friendly methods for fungi inhibition are highly desirable.

Semiconductor photocatalysis has been considered one of the most promising technologies for environmental purification, as additional chemical compounds such as strong oxidants are not introduced into the environment, and energy consumption is much lower than that of other advanced oxidation technologies [7]. Among semiconductors, TiO<sup>2</sup> has proven to be the most suitable photocatalyst because of its abundance, chemical stability, nontoxicity, and low cost [8]. Nevertheless, TiO<sup>2</sup> can harvest only ultraviolet (UV) light and has a high recombination rate of electron–hole pairs, leading to low photocatalytic efficiency [9]. The key issue for TiO2-based photocatalysts is tuning their photoactive range toward the visible light region (λ > 400 nm); thus, more solar energy can be used. To overcome this problem, we recently conducted studies to enhance photocatalytic efficiency and antifungal activities, such as decorating ZnO nanoparticles (NPs) on TiO<sup>2</sup> film or doping Fe3+ into TiO<sup>2</sup> films [10,11].

Photocatalysis requires a continuous light source to facilitate redox reactions. From the practical application perspective, we do not want antifungal photocatalysts to be constantly exposed to light. Presently, increasing efforts are being made to develop photocatalysts for photocatalytic reactions in both light and dark conditions, termed "round-the-clock photocatalysis" or "memory catalysis" [12]. Energy-storage substances such as carbon nanotubes [13], C3N<sup>4</sup> [14], Se [15], Bi [16], WO<sup>3</sup> [17] and MoO<sup>3</sup> [18] have been developed for catalytic memory reactions. In addition to these nanomaterials, Ag NPs can also store electrons because of their capacitive activity [12]. The capacitive nature of Ag NPs impedes the charge transfer of trapped electrons out of their surface. Kamat et al. [19] suggested that electron storage depends on the amount of Ag deposited on TiO<sup>2</sup> NPs. Choi et al. [20] investigated a sequential photocatalysis-dark reaction, where organic pollutants were degraded on Ag/TiO<sup>2</sup> under UV irradiation and the storage of electrons in Ag/TiO2, which were then used to reduce Cr(VI) in the post-irradiation period. Liu et al. [21] and Jiao et al. [22] presented a new strategy to improve the catalytic memory activity of Ag/TiO<sup>2</sup> for organic contaminant removal under UV light. In addition, Ag nanomaterials are widely used as antimicrobials [23]. In addition to their toxicity, they could produce a synergistic antibacterial effect with other nanomaterials such as TiO<sup>2</sup> [24]. Chen et al. [25] also showed that the size of the Ag nanostructure is a critical factor in antibacterial capacity. Despite research in this area, few studies have focused on the use of energy-storing photocatalysts for mildew control, let alone under visible light conditions.

In this study, a photoactivated antifungal coating with catalytic memory activity was assembled on the surface of a hydrophilic bamboo by first anchoring anatase TiO<sup>2</sup> thin films and then decorating Ag NPs. Different characterization methods were used to analyze the structural and optical properties of Ag-modified TiO<sup>2</sup> thin films grown on the bamboo surface. The Ag/TiO<sup>2</sup> composite films grown on the bamboo surface produced a synergistic antifungal mechanism under both light and dark conditions. Remarkably, post-illumination catalytic memory was observed for ATB in the dark after visible light was removed, as demonstrated by the inhibition of *A. niger* spores. The mechanisms involved in the antifungal processes of Ag/TiO<sup>2</sup> under both dark and visible-light conditions are discussed and proposed.

#### **2. Materials and Methods**

### *2.1. Materials*

Air-dried moso bamboo (*Phyllostachys edulis* (Carr.) J.Houz.) specimens with dimensions of 50 mm (longitudinal) × 20 mm (tangential) × 5 mm (radial) were purchased from Zhejiang YoYu Corporation. All chemicals used in the experiments were of analytical reagent grade. Potato dextrose agar (PDA; 1 L of water, 6 g potato, 20 g dextrose, and 20 g agar, pH = 5.6) was obtained from Qingdao Hope Bio-Technology Co., Ltd. Deionized

water was prepared using a Milli-Q Advantage A10 water purification system (Millipore, Billerica, MA, USA) and used throughout all experiments.
