Antibacterial and Freshness-Preserving Mechanisms of Chitosan-Nano-TiO 2 -Nano-Ag Composite Materials

: With chitosan, nano-TiO 2 and nano-Ag as raw materials, nano-TiO 2 and nano-TiO 2 -Ag were modiﬁed by a surface modiﬁer-sodium laurate. Chitosan (CTS), chitosan-nano-TiO 2 (CTS-TiO 2 ), and chitosan-nano-TiO 2 -nano-Ag (CTS-TiO 2 -Ag) composite materials and corresponding ﬁlms were prepared by a solution co-blending method. Then, the antibacterial performances of the above three types of materials against Escherichia coli , Staphylococcus aureus , and Bacillus subtilis were compared. Moreover, potato and strawberry weight loss rates, peroxidase activity, and vitamin C contents after different ﬁlm coating treatments were measured. Compared with CTS ﬁlms, the CTS-TiO 2 -Ag and CTS-TiO 2 composite ﬁlms both showed better physical properties, and both demonstrated higher antibacterial effects, especially for E . coli . Measurement of physiological indices in fruits and vegetables showed that the freshness-preserving effect of CTS-TiO 2 -Ag coating ﬁlms was the most signiﬁcant. In all, the CTS-TiO 2 -Ag coating ﬁlms can actively contribute to the storage of fruits and vegetables at room temperature, and better ensure product quality. Thus, such ﬁlms are meaningful for research and development of new fruit freshness-keeping techniques and materials. 2 , and CTS groups are 11.67%, 7.95%, and 3.71% higher, respectively (potatoes), and are 14.27%, 9.9%, and 3.70% higher, respectively (strawberry) (Figure 7). These results indicate nanomaterials coated on the surface of potatoes and strawberries can efﬁciently enhance peroxidase activity, antioxidant capacity, and food freshness, and CTS-TiO 2 -Ag is the most effective in enhancing peroxidase activity.


Introduction
With the continual rise of market demands for fruits and vegetables in recent years, the rotting and deterioration in the transportation of fruits and vegetables from mountainous areas to cities become major problems that trouble farmers and dealers. In China, the loss rates of fruits, vegetables, and other agricultural subsidiary products amid picking, transportation, storage, and other circulation steps are up to 30%. Particularly, 0.037 billion tons of fruits and vegetables are rotten on the road every year, and the direct annual economic losses of fruits and vegetables in China are up to 100 billion Yuan. Traditional freshness-keeping techniques cannot satisfy the transporting requirements of fruits and vegetables [1,2], and will cause environmental pollution and other problems that harm human health [3,4]. To decrease the waste of fruits and vegetables and improve freshness and quality, the need for finding new freshness-keeping techniques is very urgent.
Chitosan, or (1,4)-2-amino-2-deoxygen-β-D-glucosan [5], results from partial or complete deacetylation of crude polysaccharide chitin [6]. The amino groups in its molecular structure are more reactive than the acetylamino group of chitin, and therefore chitosan has many excellent biological functions, such as high biological compatability, nontoxicity, antibacterial capability, and film-forming ability [7,8]. Zhao et al. used antibacterial and reducible chitosan composites as antiseptics on strawberries and litchi, and systematically screened CTS-CS 2 -Zn and CTS-CS 2 -Ag with special antibacterial capabilities against Bacillus subtilis and Staphylococcus aureus, respectively [9]. Ding et al. synthesized RS-CTS from chitosan and salicylaldehyde and confirmed its antibacterial effects by using ultraviolet spectrometry and infrared spectroscopy [10]. Edible coatings can potentially decrease Nano-TiO 2 (0.7 g) was added into 35 mL of deionized water and ultrasonically stirred at 50 KW for 3 min (SK5210IIP, Kedao Ultrasonic Instrument Co., Ltd., Shanghai, China). Nano-Ag powder (0.118 g) was dissolved into the above solution and ultrasonically stirred for 15 min, forming a uniform solution [29].

Modification 2.3.1. Modification of Nano-TiO 2
Nano-TiO 2 (0.7 g) was added into 35 mL of deionized water and ultrasonically predispersed. Then, 0.105 g of a sodium laurate solution was added [30] and ultrasonically dispersed. After adjustment to pH 5, reaction at 40 • C proceeded for 30 min. The resulting products were filtered, washed with toluene four times, and dried, forming white nano-TiO 2 with surface lipophilicity.

Modification of Nano-Ag-TiO 2
Into the nano-Ag-TiO 2 uniform solution above, 0.1227 g of a sodium laurate solution was added and ultrasonically dispersed. After adjustment to pH 5, reaction at 40 • C proceeded for 30 min. The resulting products were filtered, washed four times with toluene, and dried, forming nano-Ag-TiO 2 with surface lipophilicity.

Preparation of Films 2.4.1. Preparation of Chitosan Films
Chitosan (2.0 g) was dissolved into 100 mL of 0.6% ice acetic acid solution (volume fraction) and stirred by magnetic stirrers for complete dissolution. After ultrasonic degassing for 15 min, the solution was stirred and degassed for three repetitions, forming a chitosan film solution. The chitosan film solution was ultrasonically degassed for 10 min and quantitatively coated onto horizontal glass plates, so the solution was flow-cast into films, which were dried under an infrared lamp. The dried films were soaked in a 1.0 mol/L NaOH solution for 30 min and neutralized by washing with abundant flow water. Then the films were carefully taken off and dried at room temperature. The single films were stored in a desiccator and marked as CTS.

Preparation of Chitosan-Nano-TiO 2 Composite Films
The activated nano-TiO 2 (0.035 g) and glycerol (2 g) were added into 100 mL of a glacial acetic acid solution with a volume fraction of 0.6%. Then, 2 g of chitosan were added and stirred by magnetic stirrers for complete dissolution. The resulting solution was ultrasonically degassed for 15 min, stirred and degassed for three repetitions. The resulting CTS-TiO 2 composite film solution was flow-cast, dried, washed, and taken off as per Section 2.4.1. The final composite film solution was marked CTS-TiO 2 .

Preparation of Nano-Ag-TiO 2 -Chitosan Composite Films
Activated nano-TiO 2 (0.035 g) was dissolved into 2 g of glycerol and added with 100 mL of a 0.6% ice acetic acid solution (volume fraction). Then, 2 g of chitosan was added and stirred by magnetic stirrers for complete dissolution. The resulting solution was ultrasonically degassed for 15 min, stirred and degassed for three repetitions. The resulting CTS-TiO 2 -Ag composite film solution was flow-cast, dried, washed, and taken off as per Section 2.4.1. The final composite film solution was marked as CTS-TiO 2 -Ag.

Antibacterial Performances
Antibacterial experiments of the samples against E. coli, B. subtilis, and S. aureus were tested by an Oxford cup method [33], and the sizes of inhibition zones after certain periods of time were recorded.
(1) Preparation of culture medium An appropriate amount of agar powder was added into a Luria-Bertani (LB) bacterial culture medium, which was sterilized in a high-pressure sterilization pot for 120 • C. Then the medium was taken out and cooled on an aseptic operation platform to 60 • C. Then each culture dish was added with an appropriate amount of the medium until the whole bottom was covered. After cooling and solidification, E. coli was inoculated [34].
(2) Procedures of antibacterial experiments All glassware and materials were autoclaved at 120 • C for 15 min to ensure sterility of the tests. The tested bacterial suspensions at concentrations of 10 7 -10 8 cfu/mL were dipped with an inoculation ring and uniformly applied three times, each time 60, to the surface of nutrient agar medium plates. The plates were covered and dried at room temperature for 5 min. Then, sterilized Oxford cups were placed onto the three coated culture dishes, which were added with CTS, CTS-TiO 2 , and CTS-TiO 2 -Ag, respectively. In each group, the Oxford cups were uniformly distributed at three directions of culture dishes. The three types of antibacterial culture dishes were put into a thermostatic incubator for 24 h of cultivation at 37 • C. Then, the inhibition zone diameter was measured, and the antibacterial ability of each type of film was evaluated.
The suspensions of the three test bacteria were 10 7 -10 8 cfu/mL. Experiments with B. subtilis and S. aureus were conducted in the same way. Mature potatoes in uniform size and without mechanical damage or plant diseases and insect pests were selected and cut into pieces, which were randomly divided into four groups (each 20 pieces). Then, the potatoes were coated with films. The pieces were soaked in corresponding solutions for 2 min and then taken out. After natural air drying, the potatoes were sent to CTS, CTS-TiO 2 , or CTS-TiO 2 -Ag film fresh-keeping experiments at room temperature, with a blank control. During the storage, all indices were measured once every 2 days, and three pieces at each time were randomly chosen from each group. Mature strawberries in uniform size and without mechanical damage or plant diseases and insect pests were selected and randomly divided into four groups (each 20 strawberries). Then, the strawberries were coated with films. The strawberries were soaked in corresponding solutions for 2 min and then taken out. After natural air drying, the strawberries were sent to CTS, CTS-TiO 2 , or CTS-TiO 2 -Ag film fresh-keeping experiments at room temperature, with a blank control. During the storage, all indices were measured once every 1 day, and three strawberries at each time were randomly chosen from each group.

Items and Methods of Measurement
The weight loss rate was measured by a weight method and calculated as follows: weight loss rate (%) = ((weight before storage − weight after storage)/weight before storage) × 100%. VC concentration was measured by UV spectrophotometry [35]. Peroxidase (POD) activity was detected by guaiacol colorimetry [36,37].

Data Processing
Experimental data were statistically analyzed on Excel and SPSS, and plotted on Origin. Each processing was repeated at least three times, and data were expressed as mean ± standard deviation of three independent experiments. Significant differences of Student t-test were set at * p < 0.05, ** p < 0.01, and *** p < 0.001. Figure 1a,b shows the SEM images of TiO 2 -Ag [38], from which the morphology and sizes of particles can be observed, where the majority of particle sizes is around 100 and 500 nm, respectively. The particles are uniform and obviously aggregate. The average size of nanoparticles is 157 nm × 273 nm. The surface of nanoparticles shows numerous dislocations, stacking faults and other defects, and increasing the surface energy and promoting mutual binding and aggregation. Clearly, the surface of TiO 2 was successfully coated with Ag, which is similar to a previous report [39]. The Ag nanoparticles mainly adhere to TiO 2 , and uniform distribution of TiO 2 also promotes and improves the broadspectrum antibacterial abilities of Ag nanoparticles. Figure 1c demonstrates the XPS spectrum of TiO 2 -Ag. The complete spectral measurement reveals peaks of TiO 2 -Ag at C1s (285.08 eV), Ag3d (367.68 eV), Ti2p (458.88 eV), and O1s (530.08 eV). C1s is ascribed to the residual carbon of the precursor, which is consistent with some experimental results reported before [40]. The peak value at Ag3d is consistent with the standard spectrum, which proves that Ag exists as elementary substance and indicates Ag was mixed in the form of elementary substance into TiO 2 . The peak of Ti2p at 458.88 eV corresponds to the electron-binding energy of Ti2p3/2 [41]. The peak of O1s at 530.08 eV is ascribed to the electron-binding energy of crystal lattice oxygen (Ti-O). XPS confirms the successful preparation of TiO 2 -Ag composite structures.

SEM, XPS and FT-IR Characterization of TiO 2 -Ag
The surface functional groups of TiO 2 and TiO 2 -Ag were detected by FT-IR ( Figure 1d). Clearly, the FT-IR spectra of the two samples show similar trends, and the absorption peaks are located very close, indicating the presence of similar surface chemical groups. The two characteristic absorption peaks of TiO 2 -Ag at 2919.31 and 2850.56 cm −1 originate from the stretching vibration of the associated carboxyl group in C-H on the surface functional groups of TiO 2 -Ag. The peak at 1713.26 cm −1 is attributed to the stretching vibration of C=O in the carboxyl group. The peak at 1467.15 cm −1 reflects the bending vibration of the methyl group. The sharp peak at 449.06 cm −1 is caused by the deformation vibration of C-C-C. The infrared spectra show that nanomaterials are rich in the carboxyl group and the hydroxyl group, which decide the ideal water solubility of nanomaterials. Since the surface of TiO 2 -Ag contains C=O and C-C-C, Ag + can combine with oxygen atoms in these groups to form stable coordination compounds. Hence, this is the theoretical basis of modifying TiO 2 by doping Ag. The surface functional groups of TiO2 and TiO2-Ag were detected by FT-IR ( Figure  1d). Clearly, the FT-IR spectra of the two samples show similar trends, and the absorption peaks are located very close, indicating the presence of similar surface chemical groups. The two characteristic absorption peaks of TiO2-Ag at 2919.31 and 2850.56 cm −1 originate from the stretching vibration of the associated carboxyl group in C-H on the surface functional groups of TiO2-Ag. The peak at 1713.26 cm −1 is attributed to the stretching vibration of C=O in the carboxyl group. The peak at 1467.15 cm −1 reflects the bending vibration of the methyl group. The sharp peak at 449.06 cm −1 is caused by the deformation vibration of C-C-C. The infrared spectra show that nanomaterials are rich in the carboxyl group and the hydroxyl group, which decide the ideal water solubility of nanomaterials. Since the surface of TiO2-Ag contains C=O and C-C-C, Ag + can combine with oxygen atoms in these groups to form stable coordination compounds. Hence, this is the theoretical basis of modifying TiO2 by doping Ag.

Reflectivity
Reflectivity refers to the percentage of reflected radiative energy accounting for the total radiative energy of an object, and higher reflectivity indicates the absorption of less solar radiation. As for films, a film with larger reflectivity absorbs less solar radiation, and the fruits and vegetables covered by this film will absorb less solar irradiation, so the rotting of fruits and vegetables is gentle. From 200-350 nm to the UV region, UV has a sterilization function, and CTS-TiO2-Ag and CTS-TiO2 have higher reflectivity compared with CTS. In the visible region, from 400-800 nm, the reflectivity of CTS-TiO2, or CTS-TiO2-Ag films is higher than that of CTS films, and that of CTS-TiO2-Ag composite films is higher Reflectivity refers to the percentage of reflected radiative energy accounting for the total radiative energy of an object, and higher reflectivity indicates the absorption of less solar radiation. As for films, a film with larger reflectivity absorbs less solar radiation, and the fruits and vegetables covered by this film will absorb less solar irradiation, so the rotting of fruits and vegetables is gentle. From 200-350 nm to the UV region, UV has a sterilization function, and CTS-TiO 2 -Ag and CTS-TiO 2 have higher reflectivity compared with CTS. In the visible region, from 400-800 nm, the reflectivity of CTS-TiO 2 , or CTS-TiO 2 -Ag films is higher than that of CTS films, and that of CTS-TiO 2 -Ag composite films is higher (Figure 2). These results indicate that, under the same conditions, the fruits and vegetables covered by CTS-TiO 2 -Ag composite films absorb less solar radiation and are not prone to oxidation or moisture loss, thus showing strong freshness-keeping ability.

Transmissivity
Chitosan films are transparent, however, CTS-TiO 2 composite films and CTS-TiO 2 -Ag composite films are non-transparent. The light transmittance of the three types of films varies to different extents at 200 to 400 nm (Figure 3), and the transmissivity rates of CTS-TiO 2 composite films and CTS-TiO 2 -Ag composite films are significantly lower than that of CTS films. These results indicate that the addition of nano-TiO 2 may alter the infiltration route of water molecules in the films and enhance the water resistance of films, increasing the moisture preserving ability [23].
( Figure 2). These results indicate that, under the same conditions, the fruits and vegetables covered by CTS-TiO2-Ag composite films absorb less solar radiation and are not prone to oxidation or moisture loss, thus showing strong freshness-keeping ability.

Transmissivity
Chitosan films are transparent, however, CTS-TiO2 composite films and CTS-TiO2-Ag composite films are non-transparent. The light transmittance of the three types of films varies to different extents at 200 to 400 nm (Figure 3), and the transmissivity rates of CTS-TiO2 composite films and CTS-TiO2-Ag composite films are significantly lower than that of CTS films. These results indicate that the addition of nano-TiO2 may alter the infiltration route of water molecules in the films and enhance the water resistance of films, increasing the moisture preserving ability [23].

Antibacterial Performance of Composite Materials
The three types of films yielded antibacterial rings in different sizes on the three types of plates ( Figure 5), indicating the antibacterial effects of the antibacterial films differ depending on the type of bacteria. In addition, Table 1 TiO2-Ag). Clearly, the same material has the strongest inhibitory effect on E. coli, followed by S. aureus and B. subtilis. Based on the above experiments, the antibacterial spectra of CTS, CTS-TiO2, and CTS-TiO2-Ag were clarified, which showed that the antibacterial spectrum of CTS-TiO2-Ag is the broadest, and this composite material also has a strong inhibiting ability toward E. coli and S. aureus [24]. In addition,

Antibacterial Performance of Composite Materials
The three types of films yielded antibacterial rings in different sizes on the three types of plates ( Figure 5), indicating the antibacterial effects of the antibacterial films differ depending on the type of bacteria. In addition, Table 1  . Clearly, the same material has the strongest inhibitory effect on E. coli, followed by S. aureus and B. subtilis. Based on the above experiments, the antibacterial spectra of CTS, CTS-TiO 2 , and CTS-TiO 2 -Ag were clarified, which showed that the antibacterial spectrum of CTS-TiO 2 -Ag is the broadest, and this composite material also has a strong inhibiting ability toward E. coli and S. aureus [24]. In addition, research by scientist has demonstrated that that film carrying the silver nanoparticles exhibits enhanced antibacterial activity against E. coli and S. aureus [43].

Weight Loss Rate
Weight loss rate is one major indicator of the freshness of fruits and vegetables [44]. Fruits and vegetables during storage will respire and evaporate, causing moisture dissipation, so the weight loss rates of fruits and vegetables will rise with the prolonging of

Weight Loss Rate
Weight loss rate is one major indicator of the freshness of fruits and vegetables [44]. Fruits and vegetables during storage will respire and evaporate, causing moisture dissipation, so the weight loss rates of fruits and vegetables will rise with the prolonging of storage. The weight loss rates of potatoes during storage are illustrated in Figure 6a. During the storage, weight loss occurred in all groups of potatoes. After 8 days of storage, the average weight loss rates were CK: 9.485%, CTS: 8.52%, CTS-TiO 2 : 7.09%, and CTS-TiO 2 -Ag: 6.62%. The weight loss rates of the experimental groups were all smaller to that of the control group, and minimized in the CTS-TiO 2 -Ag group. The weight loss rates of strawberries in the experimental groups are shown in Figure 6b, and weight loss occurred in all groups of strawberries during the storage. Water loss of strawberries during storage was very severe in the control group and the CTS group. After six days, the weight loss rate of the unpacked strawberries suddenly increased severely. This was because strawberry rotting was intensified on this day. After eight days of storage, the weight loss rates of the experimental groups and control group were CK: 11.71%, CTS: 10.56%, CTS-TiO 2 : 8.06%, CTS-TiO 2 -Ag: 7.47%, which exhibit the same trend as in the experiments on potatoes. Reportedly, the coating of fruits with films can reduce respiration and consequently lead to a decrease in ethylene production and loss of mass, which contribute to a longer conservation period [45]. The addition of AgNPs in coating films is considered an alternative to improve the mechanical, thermal, gas, and water barriers of the films [46]. storage. The weight loss rates of potatoes during storage are illustrated in Figure 6a. During the storage, weight loss occurred in all groups of potatoes. After 8 days of storage, the average weight loss rates were CK: 9.485%, CTS: 8.52%, CTS-TiO2: 7.09%, and CTS-TiO2-Ag: 6.62%. The weight loss rates of the experimental groups were all smaller to that of the control group, and minimized in the CTS-TiO2-Ag group. The weight loss rates of strawberries in the experimental groups are shown in Figure 6b, and weight loss occurred in all groups of strawberries during the storage. Water loss of strawberries during storage was very severe in the control group and the CTS group. After six days, the weight loss rate of the unpacked strawberries suddenly increased severely. This was because strawberry rotting was intensified on this day. After eight days of storage, the weight loss rates of the experimental groups and control group were CK: 11.71%, CTS: 10.56%, CTS-TiO2: 8.06%, CTS-TiO2-Ag: 7.47%, which exhibit the same trend as in the experiments on potatoes. Reportedly, the coating of fruits with films can reduce respiration and consequently lead to a decrease in ethylene production and loss of mass, which contribute to a longer conservation period [45]. The addition of AgNPs in coating films is considered an alternative to improve the mechanical, thermal, gas, and water barriers of the films [46].

Measurement of Peroxidase Activity
Peroxidase is ubiquitous in plants and is an active and adaptive enzyme in the antioxidase system. Peroxidase activity effectively reflects the internal metabolic condition, growth, development, and environmental adaptability of plants [47]. Compared with the blank control (CK), the peroxidase activities of CTS-TiO2-Ag, CTS-TiO2, and CTS groups are 11.67%, 7.95%, and 3.71% higher, respectively (potatoes), and are 14.27%, 9.9%, and 3.70% higher, respectively (strawberry) (Figure 7). These results indicate nanomaterials coated on the surface of potatoes and strawberries can efficiently enhance peroxidase activity, antioxidant capacity, and food freshness, and CTS-TiO2-Ag is the most effective in enhancing peroxidase activity.

Measurement of Peroxidase Activity
Peroxidase is ubiquitous in plants and is an active and adaptive enzyme in the antioxidase system. Peroxidase activity effectively reflects the internal metabolic condition, growth, development, and environmental adaptability of plants [47]. Compared with the blank control (CK), the peroxidase activities of CTS-TiO 2 -Ag, CTS-TiO 2 , and CTS groups are 11.67%, 7.95%, and 3.71% higher, respectively (potatoes), and are 14.27%, 9.9%, and 3.70% higher, respectively (strawberry) (Figure 7). These results indicate nanomaterials coated on the surface of potatoes and strawberries can efficiently enhance peroxidase activity, antioxidant capacity, and food freshness, and CTS-TiO 2 -Ag is the most effective in enhancing peroxidase activity.

Measurement of VC Concentration
VC concentration is a major indicator of nutrition, quality, and storage efficiency of fruits and vegetables. During storage of potatoes and strawberries, the VC concentrations in the experimental groups and control groups all decline with time; however, the VC concentrations in the experimental groups are always higher than the control groups (Figure 8). On the eighth day, the VC concentrations in the CTS-TiO2-Ag, CTS-TiO2, and CTS groups of potatoes were 55.67, 57, 62.73 mg/100 g, respectively, which an increase by 13.9%, 5.26%, and 3.0%, respectively, from that of the control group (54 mg/100 g). VC concentrations in strawberries changed in different ways. The VC concentrations in CTS-TiO2-Ag, CTS-TiO2, and CTS groups of strawberries were 14.27%, 9.87%, and 3.71% higher, compared with the control group (Figure 8b). During the whole experimental period, the VC concentrations in film-coated potatoes and strawberries were all higher than in the corresponding control groups, and the VC concentrations in the CTS-TiO2-Ag groups were always the highest, indicating that film coating can effectively inhibit the decline of VC concentrations and enhance the antioxidant capacity of fruits and vegetables, thereby ensuring food quality.  CTS-TiO 2 applied to form film-treated potatoes and strawberries; CTS-TiO 2 -Ag: CTS-TiO 2 -Ag applied to form film-treated potatoes and strawberries. Data given demonstrate the average of at least three replications (n = 10) ± SD. Asterisks within rows indicate significant differences (* p < 0.05, ** p < 0.01, *** p < 0.001), according to Student's t-test.

Measurement of VC Concentration
VC concentration is a major indicator of nutrition, quality, and storage efficiency of fruits and vegetables. During storage of potatoes and strawberries, the VC concentrations in the experimental groups and control groups all decline with time; however, the VC concentrations in the experimental groups are always higher than the control groups ( Figure 8). On the eighth day, the VC concentrations in the CTS-TiO 2 -Ag, CTS-TiO 2 , and CTS groups of potatoes were 55.67, 57, 62.73 mg/100 g, respectively, which an increase by 13.9%, 5.26%, and 3.0%, respectively, from that of the control group (54 mg/100 g). VC concentrations in strawberries changed in different ways. The VC concentrations in CTS-TiO 2 -Ag, CTS-TiO 2 , and CTS groups of strawberries were 14.27%, 9.87%, and 3.71% higher, compared with the control group (Figure 8b). During the whole experimental period, the VC concentrations in film-coated potatoes and strawberries were all higher than in the corresponding control groups, and the VC concentrations in the CTS-TiO 2 -Ag groups were always the highest, indicating that film coating can effectively inhibit the decline of VC concentrations and enhance the antioxidant capacity of fruits and vegetables, thereby ensuring food quality. Data given demonstrate the average of at least three replications (n = 10) ± SD. Asterisks within rows indicate significant differences (* p < 0.05, ** p < 0.01, *** p < 0.001), according to Student's t-test.

Measurement of VC Concentration
VC concentration is a major indicator of nutrition, quality, and storage efficiency of fruits and vegetables. During storage of potatoes and strawberries, the VC concentrations in the experimental groups and control groups all decline with time; however, the VC concentrations in the experimental groups are always higher than the control groups (Figure 8). On the eighth day, the VC concentrations in the CTS-TiO2-Ag, CTS-TiO2, and CTS groups of potatoes were 55.67, 57, 62.73 mg/100 g, respectively, which an increase by 13.9%, 5.26%, and 3.0%, respectively, from that of the control group (54 mg/100 g). VC concentrations in strawberries changed in different ways. The VC concentrations in CTS-TiO2-Ag, CTS-TiO2, and CTS groups of strawberries were 14.27%, 9.87%, and 3.71% higher, compared with the control group (Figure 8b). During the whole experimental period, the VC concentrations in film-coated potatoes and strawberries were all higher than in the corresponding control groups, and the VC concentrations in the CTS-TiO2-Ag groups were always the highest, indicating that film coating can effectively inhibit the decline of VC concentrations and enhance the antioxidant capacity of fruits and vegetables, thereby ensuring food quality. CK: Potatoes and strawberries without any treatment; CTS: CTS applied to form film-treated potatoes and strawberries; CTS-TiO2: CTS-TiO2 applied to form film-treated potatoes and strawberries; CTS-TiO2-Ag: CTS-TiO2-Ag applied to form film-treated potatoes and strawberries. Data given demonstrate the average of at least three replications (n = 3) ± SD. Asterisks within rows indicate significant differences (* p < 0.05, ** p < 0.01, *** p < 0.001), according to Student's t-test.

Conclusions
(1) Experiments show that an appropriate amount of nano-TiO 2 or nano-Ag can improve the physical properties of chitosan. Compared with chitosan films, the light transmittance of nano-Ag-TiO 2 and nano-Ag-TiO 2 -CTS composite films is enhanced, and their bonding strength, aging resistance, antibacterial ability, and water inhibition are effectively strengthened. Such improvements can decrease aerobic respiration of fruits and vegetables, preserve the flavors of fruits, and significantly prolong the fresh periods of potatoes and strawberries. (2) Comparison of antibacterial spectra among CTS, CTS-TiO 2 , and CTS-TiO 2 -Ag films shows that the CTS-TiO 2 -Ag composite has the broadest antibacterial spectrum, and the addition of nano-TiO 2 and nano-Ag significantly enhances the antibacterial ability of films. The inhibition effects of the three materials against E. coli, S. aureus, and B. subtilis were studied. All three materials most significantly resisted E. coli, followed by S. aureus, however, the inhibitory effects on B. subtilis were not significant.