Efficient visible light-harvesting film with multi-channel sterilization behavior for ultra-persistent freshness of perishable products
Introduction
The microbial spoilage of perishable products such as fruits and vegetables accounts for the largest percentage of food loss globally due to their high microbial susceptibility, especially in underdeveloped rural areas that lack proper storage facilities [1], [2], [3]. Given the limitations of economic development in these areas, it is urgent to develop a low-cost, easy-to-operate, and minimal infrastructure-required strategy for the efficient control of microorganisms in fruits and vegetables. Statistical reports have confirmed that these regions possess higher solar radiation than the global average, so sterilization technology driven by solar radiation is the best choice [4]. Although direct sunlight sterilization hardly involves any additional capital investment, the long processing time and low bactericidal efficiency limit its widespread application. The addition of photo-responsive materials can significantly enhance the solar sterilization performance by triggering their photothermal or photocatalytic sterilization effect. The former is based on the principle that photothermal agent can generate a large amount of heat when irradiated with visible or NIR light, and thus direct heat conversion endows it with broad-spectrum physical bactericidal efficiency and relatively brief treatment [5], [6]. The latter relies on the generation of reactive oxygen species (ROS) by photocatalysts under UV or visible light irradiation, producing a non-selective chemodynamic bactericidal effects without harmful by-products [7]. However, for such photoactive sterilization, either too high temperature or excessive ROS accumulation will cause damage to food quality. In contrast, integrated technologies of photothermal and photocatalytic sterilization have showed upgraded antibacterial activity through the synergy between their mechanisms, accompanied by shorter duration of action, lower equilibrium temperature, appropriate ROS production, and wide light absorption [8], [9]. In this regard, this integrated technology may provide a promising direction for the efficient inactivation of microorganisms in fruits and vegetables using sunlight while ensuring safety and quality.
So far, a range of photo-responsive materials from organic polymers to inorganic crystalline compounds have been explored [10], [11], [12], [13]. Among them, titanium dioxide (TiO2) and carbon-based nanomaterials have been successfully used in food systems due to their excellent chemical stability and proven biosafety [14], [15]. However, the rapid recombination of photogenerated charge carriers and narrow light adsorption range hinder pristine TiO2 from high photocatalytic ability under visible light [16]. In regard to the reinforcement strategies, constructing semiconductor composites is among the most popular, in which the enhancement principle is to accelerate the transport rate of photogenerated charge carriers and suppress the recombination of photogenerated electron-hole pairs by coupling TiO2 with a cocatalyst [17]. Furthermore, nitrogen-rich environments can also effectively improve the photoactivity of TiO2 by broadening its light absorption range [18], [19]. For carbon-based nanomaterials, special hybrid orbital and electron cloud distribution endow them with excellent photothermal properties [20]. In particular, porous carbon materials possess higher photothermal conversion efficiency compared to traditional carbon nanotubes and graphite-based nanoagents due to the special regulation of optical reflection and thermal storage by the microporous structure [21]. More importantly, the excellent electrical conductivity and strong visible light absorption makes it an ideal cocatalyst [18], [22]. Thus, coupling TiO2 with porous nitrogen-rich nanocarbon is expected to achieve functional synergy while making up for the aforementioned defects of TiO2. In addition, considering the inevitable residue of powdered photo-responsive materials, an ideal loading matrix is the premise of their application to food systems. Recently, natural biopolymer films based on polysaccharide, protein or cellulose have showed great potential for food preservation by virtue of their safe source, biodegradability, and tailorability of structure and function [23], [24]. Moreover, incorporating functional fillers is a versatile means to functionalize such polymer films and enhance their physicochemical properties [25], [26]. Therefore, natural biopolymer film may be an ideal grafting platform for the application of integrated technologies of photothermal and photocatalytic sterilization in food systems.
Inspired by the above discussion, we propose to construct a photothermal-photocatalytic sterilization nanosystem with sufficient visible light utilization to achieve high-efficiency preservation of perishable agricultural products with the help of suitable carrying media. A cubic TiO2-anchored porous nitrogen-rich nanocarbons (TPNC) was elaborately fabricated by successively annealing the MOF precursor (NH2-MIL-125(Ti)) composed of central Ti and aminoterephthalic acid ligand in different atmospheres, in which the organic ligands provided abundant N and C sources and the Ti component was oxidized to TiO2. Unlike traditional TiO2/C composites, the prepared TPNC retained the unique structural properties of MOFs such as ingenious component assembly, large specific surface area, random porosity, and well-exposed active sites, which is responsible for its upgraded photothermal and photocatalytic activities. Further, the TPNC nanocube was incorporated into the chitosan (CS) film matrix as filler to obtain a series of TPNC-CS nanocomposite films. The TPNC-CS films were expected to possess the following favorable properties: (i) the integrated photocatalytic and photothermal effects of TPNC and the inherent antibacterial property of CS can endow TPNC-CS films with multi-channel sterilization behavior under visible light triggering, and (ii) TPNC-CS films can significantly prolong the fresh-keeping period of fruits and vegetables. For verification, various characterization techniques were used to confirm the successful preparation of TPNC and to analyze the physicochemical properties of TPNC-CS films. The antibacterial capacity of TPNC-CS films was systematically explored by the spread plate method and a kumquat storage model was established for the evaluation of their preservation performance. The safety was assessed by hemolysis and cytotoxicity assays. Finally, the related antibacterial mechanisms were also discussed in depth.
Section snippets
Synthesis and characterization of TPNC nanocube
The NH2-MIL-125(Ti) precursor was prepared based on a reported solvothermal method with some modifications [27]. In detail, 2.25 mmol of Ti(OC4H9)4 and 9 mmol of NH2-H2BDC were dissolved into 30 mL of the mixed system of DMF and CH3OH (9:1, v/v) with vigorous stirring. Obtained solution was charged into a 50 mL of Teflon-line autoclave and subsequently kept at 150 °C for 3 days. Then, the yellow precipitate was collected by centrifugation and washed 3 times with DMF and methanol, respectively.
Synthesis and characterization of TPNC hybrids
The preparation of cube-like TPNC was implemented through a modified carbonization-oxidation process on the bright yellow MOF precursor (NH2-MIL-125(Ti)) (Fig. 1a). In detail, the carbonization process (800 °C, Ar) promotes the conversion of organic ligands to nanocarbons, and the central metal Ti undergoes a subsequent oxidation process (380 °C, O2) to form TiO2. The final TPNC hybrids appeared as a black powder (Fig. S1). A series of physicochemical characterizations were carried out to
Conclusions
In summary, we successfully introduced the visible light-triggered photocatalytic and photothermal synergistic sterilization into food systems by integrating nanotechnology and membrane technology, thereby realizing the regulation of the fresh-keeping period of fruits and vegetables by sunlight. The photoresponsive TPNC was prepared through high temperature carbonization and oxidation of NH2-MIL-125(Ti) precursor and incorporated into CS film matrix as filler. Compared with the neat CS film,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (2018YFE0127000), National Natural Science Foundation of China (21675127), Shaanxi Provincial Science Fund for Distinguished Young Scholars (2018JC-011), Qinghai Special Project of Innovation Platform for Basic Conditions of Scientific Research of China (2020-ZJ-T05). Authors would like to thank Yayun Hu (Instrument Shared Platform of College of Food Science & Engineering, of Northwest A&F University, China)
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