Fruit packaging film is essential for fruit preservation. It protects the fruit from external microbial infections and environmental influences, while also helping to reduce water loss and its oxidation, thereby resulting in longer preservation [1,2]. Currently, traditional fruit packaging materials mainly rely on oil-based plastics like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) [3]. These films are convenient with certain preservation. However, the non-degradability and extensive use of these plastics have led to “white pollution” and resource waste, causing significant damage to the ecological environment [4]. In addition, plastic films such as polylactide (PLA) are degradable but cannot be recycled [5]. Thus, there is an immediate requirement to develop novel antibacterial and preservation packaging materials characterized by recyclability, greenness, economy and efficiency [6].

Bio-based polymers are considered promising substitutes due to their non-toxic nature, ease of recyclability, and environmental friendliness. In particular, chitosan (CS) has garnered great attention in fruit packaging and preservation owing to its lack of toxicity, eco-friendly, and cost-effective nature, combined with its excellent film-forming and bactericidal properties [7]. However, single-component CS film demonstrates inadequate mechanical performance and antibacterial activity, which limits their large-scale practical applications [8]. Therefore, combining CS with other polymers is being explored as a possible approach to enhance the film's mechanical properties and modify its gas microenvironment [9]. Polyvinyl alcohol (PVA) possesses several advantages compared to other polymers, including good chemical stability, strong mechanical properties, biodegradability, and low air permeability [10]. In addition, the introduction of PVA chains can fully interpenetrate with CS chains to form a strong intermolecular network structure. Bonilla et al. developed a composite film by blending CS with PVA, avoiding environmental pollution [11]. Wang et al. fabricated an antibacterial composite film of CS/PVA for air filtration purposes [12]. These research works proved the feasibility of CS and PVA as packaging materials with good mechanical properties and environmental friendliness. However, the low antibacterial activity of CS/PVA composite film limits its practical application.

The integration of photocatalytic technology and packaging materials has emerged as a prominent area of research in the field of fruit preservation. The principle of photocatalytic preservation is to inhibit the growth of microorganisms in fruits through the action of special substances such as reactive oxygen species (ROS) produced by photocatalysts, so as to prolong the freshness period of fruits [13]. Some photocatalytic nanoparticles have been applied to fresh-keeping film and show better photocatalytic antibacterial activity and freshness preservation performance, such as ZnO [14], CuO [15], g-C3N4 [16], AgO [17], and TiO2 [18]. Among these photocatalysts, TiO2 has become a promising material in photocatalysis due to its simple synthesis, stable physical and chemical properties, and green economy. Furthermore, TiO2 is considered non-toxic and is approved by both the US Food and Drug Administration (USFDA) and the Chinese National Standard (GB 2760-2014) for application as a food additive and food contact material [19]. However, TiO2 possesses a broad bandgap of approximately 3.2 eV, rendering it only capable of absorbing ultraviolet light and exhibiting a suboptimal utilization rate of solar energy [20]. There are further issues present, including suboptimal separation efficiency between photogenerated electrons and holes, which severely limits its practical application in the field of photocatalysis. To overcome these difficulties and maximize the photocatalytic activity of TiO2, researchers have proposed many modification strategies for TiO2 and achieved some excellent results [21]. So far, about nine popular TiO2 modification strategies have been reported, including element doping [22], surface plasmon resonance [23], building heterojunctions [24], crystal face control [25], built-in electric fields [26], co-catalysts [27], morphology control [28], oxygen vacancy construction [29], and surface frustrated Lewis acid-base pair design [30]. Note that building heterojunctions has the advantages of being economic, efficient, and easy to operate, and has received widespread attention. Xie et al. successfully synthesized ternary heterojunction photocatalysts based on Ag2O-TiO2-Bi2WO6 (ATB). Subsequently, they fabricated ATB/PVA composite films and evaluated their performance in the photodegradation of ethylene for banana preservation [31]. This study demonstrates the potential of heterojunction photocatalysts for fruit preservation applications. Compared to conventional heterostructures, Z-scheme heterojunctions have been demonstrated to offer fascinating advantages in charge separation and transport with applications in fruit preservation.

Recently, graphitic carbon nitride (g-C3N4) has gained prominence as a non-metallic semiconductor material. g-C3N4 is a low-cost functional substance, which has been widely used in the field of food packaging due to its abundant source, easy preparation and stable physical and chemical properties [32,33]. More importantly, g-C3N4 is non-toxic and also has an innate photodynamic sterilisation effect with appropriate intensity [34]. Many studies have reported that g-C3N4 can be safely used for fruit preservation [35]. For example, Ni et al. prepared CS/negatively charged g-C3N4 self-activated biomimetic composite films for preserving tangerines by a one-step electrostatic self-assembly method [36]. Our previously prepared g-C3N4/CS/PVA composite film also showed good antibacterial freshness preservation effect on strawberries [37]. Therefore, g-C3N4 can be considered for modification and enhancement of TiO2. Wu et al. have developed a photocatalyst for nitrogen fixation that uses heterojunctions of carbon nitride and oxygen vacancy titanium dioxide [38]. Wang et al. successfully fabricated a 3D/2D direct Z-scheme heterojunction of TiO2/g-C3N4 with enhanced charge carrier separation. This innovative heterojunction structure was specifically designed for efficient photocatalytic H2 evolution [39]. However, the study of applying Z-scheme heterojunctions in fresh-keeping film to improve photocatalytic antibacterial activity and freshness preservation performance has rarely been reported.

Herein, we successfully prepared Z-scheme heterojunction g-C3N4-TiO2 microsphere by one-step calcination and constructed g-C3N4-TiO2/CS/PVA green renewable composite films via the casting method. The structures and properties of g-C3N4-TiO2 microsphere and g-C3N4-TiO2/CS/PVA composite films were characterized, and the composite films were evaluated for their antibacterial and strawberry freshness preservation effects under LED light. The recyclability and regenerability of the composite films were also researched. This work provides guidance in the area of green and low-cost preservation and packaging of fruits, which improves the resource utilization efficiency without generating secondary pollution and promotes the development of photocatalytic technology in the field of preservation.