Zirconium tetrachloride (ZrCl₄), 5,5′-dicarboxylic acid, 2,2' -bipyridine (H₂BDC), glacial acetic acid, and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., LTD. (Shanghai, China). Chitosan, absolute ethanol, and acetone were purchased from Maclin Biochemical Technology Co., Ltd (Shanghai, China). All chemicals were of analytical grade and could be used directly without further purification.

Ce-UiO-67 was synthesized by a solvothermal method. First, H₂BDC (152 mg) was dissolved in 5 mL of DMF solution in a beaker, then ZrCl₄(46.6 mg) and 4,4′-bifenthalic acid (48 mg) were added to the solution in a Teflon-lined reactor, followed by an aqueous solution of cerium(IV) ammonium nitrate (3 mL, 0.5 M) drop by drop. Second, 15 mL DMF and 0.4 mL acetic acid were added to the Teflon-lined reactor and reacted in an oven for 24 h. Finally, it was cleaned twice with DMF and acetone, and white Ce@UiO-67-BPY powder was thus obtained.

The Ce@UiO-67-BPY powder was then dispersed into 80 mL DMF solution of CS (350 mg). The in situ templated growth of MOF was accomplished by heating the mixture under stirring for 40 min at 80 °C. After natural cooling, the final CS@Ce-MOF samples were collected and washed thrice with acetone and water.

The morphology of CS@Ce-MOF samples was characterized by a scanning electron microscope (SEM, Zeiss) model (S-3400, working voltage 20 KV) manufactured by Hitachi, Japan. Energy-dispersive X-ray diffraction (EDX) analysis was performed by an X-ray spectrometer attached to a scanning electron microscope. The X-ray diffractometer (XRD, Ultima IV, Rigaku Corporation, Akishima, Japan) was selected to collect the diffraction patterns of Ce-MOF and CS@Ce-MOF using Cu Kα radiation (λ = 0.15406 nm, 40 kV, 1.64 mA) in the ranges of 10°–80° of 2-theta angle. A NEXUS 6700 infrared spectrometer (FTIR) was used to characterize the chemical structure of Ce-MOF and CS@Ce-MOF in the range of 400–3,000 cm^-1. Thermogravimetric analysis (TGA) of the as-prepared samples was conducted by a thermogravimetric analyzer (Netzsch TG 209 F1 Libra, Germany), and the temperature was increased to 800°C at 10 °C/min in a nitrogen atmosphere.

The antimicrobial performances of CS@Ce-MOF were evaluated with Escherichia coli and Staphylococcus aureus (Wu et al., 2018). Luria–Bertani (LB) liquid medium was prepared by mixing distilled water, pancreatic peptone, yeast powder, and sodium chloride, and LB solid medium was prepared by mixing distilled water, pancreatic peptone, yeast powder, sodium chloride, and AGAR powder. The prepared liquid medium and solid culture were cultured at 120 °C and sterilized for 15 min. When the solid medium was cooled to about 50–60 °C, it was poured into a Petri dish and solidified and then incubated overnight (12 h) in a constant temperature stirrer (37 °C, 200 rpm). Finally, the CS@Ce-MOF suspension was added to the media of E. coli and S. aureus, respectively, and 100 μL was taken with a pipetting gun, coated onto an LB plate, and cultured at 37 °C for 24 h.

In order to test the antioxidant activity of the sample, DPPH and ABTS free radicals were selected as the elimination targets for the assay. We took 0.04 mg/mL DPPH–ethanol solution 1.5 mL, added an appropriate amount of Ce-MOF and CS@Ce-MOF suspension in 95% ethanol to make up 3 mL. It stood for 30 min, and we measured absorbance at 517 nm. R = 1 − A 1 − A 2 A 0 × 100 % . (1)

In Formula (1), R is the clearance rate, A₁ the absorbance of DPPH and ABTS solution after sample suspension, A₂ the absorbance of the sample suspension, and A₀ is the absorbance of DPPH and ABTS solution without adding the sample suspension.

The preservation effect of CS@Ce-MOF nanozyme spray coating was studied by a fruit preservation experiment. Strawberry was selected as the model fruit. The CS@Ce-MOF suspension (1 mg/mL) was sprayed three times with an airbrush with 50 μL each time on the surface of the fruit to form a film to ensure uniform coating. The fruit was then put into a pre-sterilized box at 25–30 °C with relative humidity (RH) of 65%–70%. The morphological changes and weight loss of the fruit were recorded on days 1, 3, 5, and 7. The weight loss rate of the fruit was measured gravimetrically, and the average weight loss rate of each group of fruit samples was taken as the detection index. The weight loss rate was calculated by the following formula (Pirozzi et al., 2021): WL = 1 − m 0 − m t   m 0 × 100 % . (2)

In Formula (2), m ₀ is the weight of the fruit sample on day 0 before storage and m _t is the weight of the fruit sample on day t after storage.

First, the morphologies of the synthesized UiO-67 and Ce@UiO-67 were characterized. SEM was used to demonstrate the effective preparation of UiO-67 and Ce@UiO-67. Figure 1A depicts the morphology of the UiO-67 crystals, which have a cuboidal granular structure with a uniform size of approximately 2 mm and a good regular octahedral arrangement. Figure 1B shows the microtopography of Ce-UiO-67, which also showed a relatively intact regular octahedral configuration, indicating that Ce doping penetrated deep into the skeleton rather than just surface adhesion. However, the overall crystal structure was slightly broken, indicating that the integrity of the crystals was reduced after the addition of cerium ion. Figure 1C shows the EDX energy spectrum of Ce@UiO-67, from which it is apparent that the component elements of Ce@UiO-67 are Zr, Ce, Cl, C, and O. This indicates that the Ce-UiO-67 preparation was successful. To further demonstrate the successful preparation of Ce-MOF and CS@Ce-MOF, XRD patterns of Ce-MOF and CS@Ce-MOF are shown in Figure 1D. For Ce-MOF, the main diffraction peaks are found at 2θ = 5.8°, 15.4°, and 24.2°; for CS@Ce-MOF, the main diffraction peaks are found at 2θ = 5.8°, 10.2°, 15.4°, and 30.2°. The position of the characteristic diffraction peaks of the two samples is basically the same, indicating that the crystal structure is also the same (Travlou et al., 2018; Yang et al., 2020). In addition, the samples presented a broad peak of around 5°, implying that amorphous Ce-MOF was formed on the surface of CS (Thomas et al., 2018), and in the diffractogram of CS@Ce-MOF, there is a peak at 10° that may be the CS. This furthermore revealed that the crystal structure integrities of CS@Ce-MOF were consistent with Ce-MOF, indicating the presence of Ce-MOF in the composites. The FT-IR spectra of Ce-MOF and CS@Ce-MOF are shown in Figure 1E. For Ce-MOF and CS@Ce-MOF samples, the peaks were at 1,028 cm⁻¹, 1,384 cm⁻¹, and 1,450 cm⁻¹. The appearance of the peak at 1,450 cm⁻¹ corresponded to the stretching vibrations of the coordinated Ce-O and Ce-O-C metal nodes, indicating the successful coordination of cerium ions with the H₂BDC (Ren et al., 2019). In addition, the thermal properties of CS and CS@Ce-MOF were tested by TGA at temperatures ranging from 30 °C to 800°C. For the thermogravimetric analysis in Figure 1F, the degradation temperature of CS was 220 °C in the thermograms of CS, but the decomposition temperature of CS@Ce-MOF was about 300 °C, indicating an interaction between CS and Ce-MOF. However, a weight loss of over 350°C would indicate the gradual carbonization of CS and framework collapse of Ce-MOF (Sharma et al., 2022). At the same time, it shows that CS@Ce-MOF has thermal stability within 350°C.

The optical properties of spray coating components affect the preservation of fruit; thus, we investigated the optical properties of the CS@Ce-MOF spray coating. Figure 2A shows the ultraviolet-visible transmission spectrum in the range of 300–800 nm for the CS and CS@Ce-MOF suspension. Figure 2B shows the ultraviolet-visible adsorption spectrum in the range of 300–800 nm of the CS and CS@Ce-MOF suspension. It can be observed that, in the range of visible light (400–800 nm), the CS suspension has a high transmittance of nearly 80% while the CS@Ce-MOF suspension has a transmittance of about 40%. In the range of ultraviolet light (300–400 nm), CS@Ce-MOF suspension has a lower transmittance of about 40% (Figure 2A). Meanwhile, the CS@Ce-MOF suspension has high adsorption in the range of ultraviolet light (300–400 nm) that is observed from Figure 2B, possibly due to the UV-barrier performance of Ce-MOF. The above results demonstrate that CS@Ce-MOF has good light transmittance, indicating its capability to preserve fruit against discoloration and denaturation.

The active spots on the surface of CS@Ce-MOF were characterized by XPS survey. The characteristic peaks attributed to the Ce s2p, O 1s, N 1, and C 1s regions can be seen in Figure 3A. In addition, Figures 3B,C show the high-resolution XPS spectra of Ce and O in Cs@Ce-MOF, with peaks at 88.5 eV and 530.8 eV, indicating the presence of cerium ions. The signal of O 1 s at 530.8 eV could be the Ce-O bond in the composite, revealing the successful preparation of CS@Ce-MOF (An et al., 2014)^.

In order to explore the antibacterial activity of the CS@Ce-MOF nanozyme, the antibacterial activity of CS@Ce-MOF against E. coli and S. aureus was investigated by the plate counting method (Yang et al., 2020). Figure 4A shows the number of E. coli and S. aureus colonies remaining after Ce-MOF and CS@Ce-MOF treatment. We could observe an obvious reduction of plate colonies when treated with Ce-MOF. More importantly, only a few plate colonies were found in the groups treated by CS@Ce-MOF for all these bacterial samples. In addition, separate quantification results for the plate count assay are displayed in Figures 4B,C for E. coli and S. aureus. It can be observed from Figures 4A,B that CS@Ce-MOF has better bacteriostatic performance than Ce-MOF; in particular, the bacteriostatic rate of CS@Ce-MOF to S. aureus reaches 94.5%. The results indicate that CS@Ce-MOF shows better antibacterial properties with longer lasting antibacterial duration against E. coli and S. aureus.

The CS@Ce-MOF nanozyme thus exhibited promise in combating foodborne pathogenic bacteria due to its antibacterial properties. These allow the nanozyme to efficiently degrade harmful biofilms and inhibit bacterial growth, making it an excellent candidate for food industry applications such as disinfection and preservation. Additionally, the broad-spectrum biocidal activity of the CS@Ce-MOF nanozyme makes it useful for targeting multiple types of foodborne pathogens, thus ensuring a safer food supply chain. Such antibacterial performance qualifies CS@Ce-MOF to serve as a spray coating for fruit preservation.

The oxidation of vitamins in fruit is one of the main causes of nutrient loss and deterioration (Bobasa et al., 2023). Therefore, fruit preservation coatings require excellent antioxidant properties. We tested the antioxidant properties of the prepared Cs, Ce-MOF, and CS@Ce-MOF with free-radical scavenging DPPH and ABTS (Figure 4). It can be seen from Figure 5A that, with the increase in the extract concentration, the ability of free-radical scavenging is enhanced. When the sample concentration reached 0.5 mg/mL, the DPPH free-radical clearance rate of the CS@Ce-MOF reached more than 80%. In addition, it can be observed from Figure 5B that both Ce-MOF and CS@Ce-MOF have slightly better scavenging ability for ABTS than DPPH. When the sample concentration reached 0.5 mg/mL, the DPPH free-radical clearance rate of the CS@Ce-MOF reached 85%. This result shows that the antioxidant activity of the CS@Ce-MOF suspension is very high and can be widely used in food preservation.

A fruit preservation experiment was conducted to explore the utilization of CS@Ce-MOF nanozyme for food preservation. We choose strawberry and banana, which are not easy to store, for a fruit storage experiment. Figure 6A shows the comparison of the appearance changes of strawberries under different treatments during storage. It can be observed that untreated strawberries began to show signs of decay after storage for 5 days, and, after storage for 5 days, most of the pulp was clearly rotten. Strawberries sprayed with CS@Ce-MOF nanozyme suspension looked fresher and remained fresh after storage for 5 days and only showed tiny signs of decay after 7 days of storage. As a typical fruit of respiratory menopause, water in strawberry is easily consumed and diffused to the external environment during postharvest storage due to respiration and transpiration, resulting in its reduced water content and weight. Figure 6B shows the changing trend of the weight loss rate of strawberry during storage. It is apparent that the weight-loss rate of strawberry generally rises during the whole storage cycle; the strawberry samples without any preservation treatment have serious water loss, and the weight-loss rate rises faster, reaching 43% after 7 days of storage. The weight-loss rate of strawberries treated by spraying CS@Ce-MOF nanozyme suspension increased slowly, reaching 29% after 7 days of storage, confirming the good effect of the spray in preventing fruit rot. Morphological observation of bananas at different times with various treatments is shown in Figure 6c. During the green-yellow life of banana (0–3 days of storage), CS@Ce-MOF nanozyme spray coating slowed chlorophyll degradation. During the yellow-brown life of bananas (5–7 days of storage), CS@Ce-MOF nanozyme spray coating further reduced the incidence of browning spots on the fruit surfaces compared to uncoated bananas (Deng et al., 2017). After 7 days, the weight of control bananas at room temperature dropped by more than 20% while the sprayed CS@Ce-MOF nanozyme bananas retained about 90% of original weight (Figure 6D). These results suggest that the CS@Ce-MOF nanozyme as a bio-inspired spray coating can protect the freshness of fruit during storage and can significantly delay its decay, with shelf-life extension of up to 4 days.