Photocatalytic degradation of ethylene by mesoporous nano silica loaded chlorin e6

Green foods are usually harvested and stored before ripening, but excessive ethylene generated during storage can lead to the loss of vegetables and fruits. Photocatalytic oxidation provides a promising method to remove ethylene and extend the shelf life of green foods. In this study, a mesoporous nanosilica-loaded Ce6 composite nanomaterial (Ce6@SiO2) was designed and synthesized to adsorb ethylene into its pores and then degrade it under visible light illumination. The photosensitizer Ce6 in the mesopore produced large amounts of reactive oxygen species that degraded ethylene. The specific surface area was greatly increased by coating the material on the surface of a sodium alginate (SA) hydrogel membrane. This improved its degradation efficiency and facilitated the adsorption and degradation of ethylene during fruit storage. The ethylene removal capacity was studied by measuring the ethylene concentration and increases in the humidity in Tedlar gas bags. Practical applications were tested by observing color changes and surface decay of tomatoes stored with and without SA/Ce6@SiO2. The results showed the system may be used in practical commercial preservation systems.


Introduction
Ethylene released from fruits and vegetables is closely linked to the growth, ripening, and germination processes of fruits and vegetables [1]. The actual effect of ethylene is usually related to the nature of the fruit, its ripeness, the degree of exposure of the fruit, and its condition. Surveys have shown that ethylene causes significant product losses (10%-80%) for fruits and vegetables. Ethylene control strategies during postharvest ripening are essential to reduce product losses and maintain food quality [2,3]. Many fruits, vegetables, and even flowers release ethylene as a natural plant hormone that controls growth and aging [4]. In fruits such as bananas and tomatoes, ethylene is a natural hormone that accelerates their ripening and spoilage [5], even at trace concentrations (∼0.1 ppm) [6][7][8]. The removal of exogenous ethylene can be accomplished using physical adsorption, oxidation, catalysis, and biofiltration methods. Ethylene scavengers have been used to protect fruits and vegetables through inhibition, absorption, and oxidation, including ethylene inhibitors (1-MCP), ethylene absorbers (zeolites), and ethylene catalytic oxidizers (KMnO 4 , ozone, etc) [9][10][11].
Of these materials, zeolites are the most commonly used because they have large interior spaces for trapping and holding ethylene via adsorption. These properties are attributed to their three-dimensional porous structures and properties such as cation exchange, adsorption, and molecular sieving [12,13]. Physical ethylene adsorbents (e.g., activated carbon, carbon nanospheres, silica gel, and zeolites) minimize the amount of ethylene gas in storage chambers, but they can only retain ethylene in the adsorption matrix and do not completely decompose it. Moreover, the adsorption efficiency decreases significantly as the amount of adsorbed gas increases with time [14,15]. Silica-based composites have been used in many aspects of photocatalysis [16][17][18]. Therefore, we chose mesoporous silica as the carrier for ethylene adsorption. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. efficiency problems. Based on the development of new catalytic oxidation technologies, an alternative method for ethylene degradation using photosensitizers is provided [19][20][21]. Also, the use of photosensitizers to catalyze the degradation of ethylene can help solve some of the previous problems [22,23]. TiO 2 photocatalysts and UV irradiation have been used for photocatalytic ethylene degradation [24,25]. However, exposure to UV radiation can be harmful to the human eye, and long-term UV exposure can also degrade plastic packaging materials. For these reasons, UV-activated photocatalytic systems are unsuitable for commercial appliances such as freezers, which require visible wavelength-activated photocatalysts. However, there are already many photocatalytic materials that can act as catalysts under visible light irradiation [26][27][28]. Of these, Chlorin e6 (Ce6) is a photosensitizer that can remove ethylene produced during fruit ripening. It also shows antimicrobial activity. Therefore, using Ce6 as a photosensitizer and mesoporous silica as a carrier may provide a new method to photocatalytically remove ethylene to preserve fruits and vegetables. Therefore, in this paper, Ce6, a photosensitizer that could generate reactive oxygen species under visible light, was used as the drug-carrying component and then loaded onto mesoporous silica with different particle sizes to adsorb and degrade ethylene. A hydrogel synthesized by sodium alginate was chosen as the outer package due to its air permeability and auxiliary adsorption effect [29].
We used silica nanoparticles as the adsorbent and wrapped them with Ce6 as a catalyst to prepare a novel fruit preservation material to degrade ethylene (Scheme 1). The composite nanomaterial Ce6@SiO 2 was made by preparing nanoporous silica microspheres and uniformly dispersing the photosensitizer Ce6 in its mesopores [30]. Mesoporous silica acted as both an adsorbent for ethylene and a carrier for photocatalysts, enabling a modular approach to remove ethylene. Mesoporous silica is particularly suitable for the adsorption of unsaturated hydrocarbons due to its acidity. The adsorption of ethylene is a thermodynamically favorable process with an energy of −13.55 kcal mol −1 [22,31]. When adsorbed in mesopores, Ce6 degrades ethylene after irradiation by an LED lamp to generate reactive oxygen species. Since the powder form was unsuitable for practical applications, it was added to the sodium alginate hydrogel membrane surface (SA/Ce6@SiO 2 ) for simultaneous adsorption and photoactivation. The uniform dispersion of mesoporous silica on the surface of sodium alginate hydrogel greatly increased its surface area and improved the adsorption and degradation efficiency. The finished products were subjected to ethylene tests and fruit preservation tests to observe their effects. The results showed that SA/Ce6@SiO 2 degraded ethylene and has promising applications for the refrigeration and preservation of fruits and vegetables.

Preparation of SiO2 nanoparticles
CTAB (0.3 g) was dissolved in 20 ml pure water, warmed to 45°C, and magnetically stirred. Urea (3.096 g) was dissolved in 30 ml of pure water and added to the CTAB solution. The solution was stirred and warmed to 85°C, and then a drop of diluted ethanolamine was added (prepared by a drop of 2-aminoethanol in 10 ml pure water). The reaction was carried out for 15 min and then 4.2 ml of TEOS was added dropwise for 2 min The reaction mixture was stirred vigorously for 6 h. Mesoporous silica with different particle sizes was obtained by varying the reaction time. The reaction solution was cooled naturally, and the precipitate was obtained by centrifugation at 12 000 r min −1 . After that, the precipitate was washed five times with methanol, water, methanol, water, and methanol, alternately, each time at 12 000 r min −1 for 5-10 min The precipitate obtained by centrifugation was put into a 60°C oven to dry the powder, which was then calcined in a muffle furnace at 600°C for 6 h to obtain the desired mesoporous nano-silica material.

Preparation of SA/Ce6@SiO2 nano composites
The synthesized mesoporous silica powder (200 mg) was added to a beaker containing 40 ml of methanol solution and then sonicate for 1 h to dissolve it fully. Then, 20 mg of Ce6 was added to the above solution and sonicated for 1 h. The resulting Ce6@SiO 2 powder was obtained after magnetic stirring at 600 r min −1 for 48 h in the dark. Sodium alginate powder (100 mg) was dissolved in 200 ml of water, and 20 mg of calcium chloride was dissolved in 200 ml of water. The two solutions were then mixed in a circular mill and stirred well. The formed hydrogel was shaped and dried under a light-proof environment for 24 h to obtain a circular 50 mm × 50 mm sodium alginate hydrogel film. Ce6@SiO 2 powder (200 mg) was dissolved by sonication in 40 ml of methanol for 1 h and then uniformly spread on the surface of sodium alginate hydrogel film and dried at room temperature for 24 h.

Characterization
The materials to be tested were dispersed in anhydrous ethanol or an aqueous solution after sonication. The dispersion (3-5 μl) was added dropwise to the cleaned silicon wafer surface using a micropipette. After drying at room temperature, the surface morphology of the prepared nanomaterials was characterized using a field emission scanning electron microscope (HITACHI S-4800). The emission spectra of different samples in the range of 300-800 nm were measured using ultraviolet-visible (UV-vis) spectroscopy with methanol as a reference. The position of the absorption peak was used to analyze the functional groups in the sample, while changes in absorption wavelength and intensity of the nanoparticles were quantitatively analyzed. The spectra of the different samples were scanned using an infrared spectrometer (FTIR) with a wavelength range of 400-4000 cm −1 and 32 scans. This was used to analyze whether the functionalized groups were successfully modified onto the surface of the nanomaterials. A zeta potential/size analyzer was used to determine the particle size and potential of the material at pH 7. The specific surface area and pore size of SiO 2 and Ce6@SiO 2 were determined by nitrogen adsorption isotherms (BET).

Photocatalytic testing of composite granules
The photocatalytic activity of the composites was detected by using a DPBF solution as the reactive oxygen probe. First, 200 mg of Ce6@SiO 2 powder was ultrasonically dissolved in 40 ml of methanol solution. After that, dilute DPBF solution was added, and the mixed solution was left to stand in the dark for 30 min Finally, the mixture was placed 20 mm from a white LED light for irradiation. The concentration of DPBF was determined by ultraviolet-visible (UV-vis) spectroscopy at specified time intervals by extracting 2 ml samples.

Ethylene removal test
To evaluate the degradation performance of ethylene, 5 l Tedlar bags filled with diluted ethylene gas (30 ppm in the air) were used. In the initial state, the humidity inside the bag was close to 40%. After specified time intervals (up to 8 h), residual ethylene was determined by gas chromatography (GC). The composite material was placed inside the device directly under the LEDs. To test the ethylene removal performance, the Tedlar bags were continuously pumped as the gas in the bags was continuously pumped out and re-injected with an equivalent amount of 30 ppm ethylene gas after each extraction. The ethylene concentration was monitored continuously for 8 h at the same time interval. Because water is a degradation product of ethylene, the humidity inside the bag was measured using a hygrometer.

Fruit storage freshness test
During ripening, tomatoes undergo a distinct change in color from green to red. Therefore, we chose tomatoes to demonstrate that this system can maintain the freshness of fruit and to demonstrate its fruit preservation effectiveness. A photocatalytic composite (SA/Ce6@SiO 2 ) was placed at the bottom of the device, and a white LED light source was placed above the material. The tomatoes were placed evenly on a stand inside the device and left at 20°C-25°C and 45%-55% humidity for 14 days. Experiments concerning the freshness of bananas were carried out under the same experimental conditions by placing the bananas in sealed clamshell bags containing the experimental materials. All samples were illuminated using a separate power supply.

Materials and characterization
The synthesis included the surface coating of mesoporous silica and Ce6 particles, and the morphology of mesoporous silica nanoparticles with different particle sizes was first characterized by scanning electron microscopy (SEM). As shown in figure 1, the unmodified monodisperse silica microspheres (SiO 2 ) had regular spherical shapes. Nanoparticles with a particle size of 150 ± 2.3 nm ( figure 1(b)) were selected as the carriers of the material after preliminary adsorption experimental screening. Ce6 was encapsulated into the mesoporous silica by an infiltration method. Since synthesized Ce6@SiO 2 was granular, it was coated onto the surface of the sodium alginate hydrogel film (figures 1(d), (e)). This greatly increased the specific surface area of the material, which increased the possibility of the material coming into contact with ethylene. Figure 1(d) is an SEM image of the surface of the sodium alginate hydrogel. Figure 1(e) shows that the material was uniformly dispersed on the surface of the film. The successful synthesis of Ce6@SiO 2 was further analyzed by UV-vis spectroscopy ( figure 2(b)). There was no SiO 2 peak at 404 nm, which is the most obvious characteristic peak of Ce6. In contrast, Ce6@SiO 2 had an absorption peak at 404 nm, indicating that Ce6 was successfully doped into the carriers. When Ce6 was adsorbed onto SiO 2 , the main peak of Ce6@SiO 2 at 670 nm became more intense with a slight red shift, demonstrating the successful formation of Ce6@SiO 2 .
In figure 2(a), the FTIR spectra clearly show the presence of Ce6@SiO 2 and SiO 2 . The typical stretching vibrational peaks of Si-O-Si of mesoporous silica appeared at 1018 cm −1 and 956 cm −1 , which indicated that we successfully synthesized mesoporous silica nanoparticles. No characteristic peak appeared at 2800 cm −1 , which indicates that the template agent CTAB was completely removed. The color change of SiO 2 and Ce6@SiO 2 solutions showed that Ce6 was doped with SiO 2 . Ce6@SiO 2 is light green, and SiO 2 is latex-colored. Zeta potential analysis ( figure 2(d)) further demonstrated the successful formation of Ce6@SiO 2 by measuring the charge of SiO 2 , Ce6, and Ce6@SiO 2 .
The specific surface area, pore size, and pore capacity of nanomaterials will impact the adsorption of gases. When the nanocarrier had a high specific surface area with a suitable pore size, it effectively adsorbed ethylene. The specific surface area and pore size distribution of SiO 2 and Ce6@SiO 2 were determined by nitrogen adsorption-desorption (BET) isotherms. Figure 3(a) shows the nitrogen (N 2 ) adsorption-desorption isotherms and pore size distribution curves of Ce6@SiO 2 and SiO 2 . When P/P 0 < 0.2, the adsorption of nitrogen increased slowly upon increasing the partial pressure, which was caused by the unimolecular and multimolecular adsorption of nitrogen on the SiO 2 surface. When P/P 0 was in the range of 0.2-0.4, the adsorption of nitrogen increased more rapidly upon increasing the partial pressure, which was caused by the capillary coalescence of nitrogen in the sample pore channels, indicating that the SiO 2 pore channels were homogeneous. After that, between 0.4 and 0.8, a longer plateau appeared, which indicated that the adsorption of nitrogen in the capillary reached saturation. Finally, when P/P 0 reached a high value, the adsorption increased rapidly and changed only within a narrow range, which was caused by the coalescence of nitrogen between SiO 2 nanoparticles. When combined, the above features indicate a typical isotherm for mesoporous materials with hysteresis loops. The use of mesoporous silica increased the specific surface area by improving the possibility of contact with ethylene. In the same way, the composite Ce6@SiO 2 particles had the same adsorption properties, and the particle size was uniformly distributed around 150 nm ( figure 3(b)).  The above characterization techniques demonstrate the ability of the synthesized nanomaterials to solve some of the previous problems. Then, a reactive oxygen experiment was conducted to verify whether the material could degrade ethylene under visible light illumination.

Reactive oxygen experiment
The loading ratio of Ce6 to SiO 2 was 100: 1 according to the relevant literature and reactive oxygen experiments. The group with the highest oxidation efficiency (same amount of carrier) was selected by measuring different concentration ratios for active oxygen experiments. DPBF was chosen as the probe dye molecule. Figure 2(c) shows the absorption spectrum of the DPBF solution after decomposition by Ce6@SiO 2 composite particles. The catalytic performance of the Ce6@SiO 2 composite catalyst was tested under LED irradiation. After 15 min, the Ce6@SiO 2 composite reduced the concentration of DPBF by more than 50%. The above results show that the material could adsorb and degrade ethylene. Photocatalytic degradation experiments were conducted to prove its ability to adsorb and degrade ethylene.

Photocatalytic degradation under sealed conditions
The Tedlar gas bag experiment was used to simulate the presence of a stagnant atmosphere in a non-CA storage environment. The test was performed to simulate the storage of fruit under sealed conditions. The test was performed by measuring the ethylene gas residue after performing photocatalysis and changes in humidity. The catalytic activity was measured at 25°C, a relative humidity range of 40%-99%, and a volume of 5 l. The humidity in the bag gradually increased during the experiment until reaching the maximum range of the hygrometer. The obtained data were summarized in figure 4. Figures 4(a), (b) compares the residual ethylene concentration with time for Ce6@SiO 2 and SA/Ce6@SiO 2 under excess ethylene gas (30 ppm). From the graph, we could see that the degradation rate of ethylene was very fast in the first hour, and then the rate gradually stabilized and was complete after 6 h. However, the reduction in the ethylene gas concentration after the addition of sodium alginate hydrogels to the composite particles was much greater, reaching 50% ethylene decomposition after only 2 h. From the above experiments, we concluded that the sodium alginate hydrogel improved the overall catalytic performance. The homogeneous coverage of the material on its surface gave it a greater specific surface area, increasing the loading of catalytic Ce6 particles and enhancing the contact possibility with ethylene. Therefore, SA/Ce6@SiO 2 exhibited higher ethylene degradation activity than Ce6@SiO 2 .
The decomposition of ethylene occurred via the following mechanism. First, ethylene and oxygen were adsorbed onto mesoporous silica. Oxygen molecules (O 2 ) dissociated to produce singlet oxygen (O) on Ce6. Next, the C=C bond of ethylene was dissociated and reacted with atomic oxygen to produce formaldehyde (CH 2 O). The generated formaldehyde was decomposed into (CO) and atomic hydrogen (H), which reacted with atomic oxygen and was converted into carbon dioxide and water [23,32]. The ethylene decomposition process conformed to a pseudo-first-order kinetics model C= C 0 ·e -kt , where C 0 is the initial concentration of ethylene, C is the concentration of ethylene at time t after decomposition, and k is the reduction kinetic constant, which reflects the decomposition activity of the catalyst. Figure 4(d) shows variations in the ethylene concentration with time (from 0 h to 4 h), indicating the decomposition kinetics. The reduction reaction kinetic constants (K) indicated that SA/Ce6@SiO 2 exhibited better ethylene decomposition performance. The next step was to verify whether the material could extend the shelf life of fruits through actual fruit simulation preservation experiments and to verify whether it is commercially viable.

Fruit experiment
To determine the effectiveness of nanocomposites for fruit preservation, color changes of tomatoes were compared after 12 days of storage with and without nanocomposites (figure 5). Experiments were carried out without additional ethylene injection. Under these conditions, fruit ripening was achieved by self-produced ethylene. The tomatoes in the experimental group showed less red color, and the color change was not particularly noticeable after using the composite nanomaterials. Tests were conducted at room temperature to investigate the effect of nanomaterials on the freshness of tomatoes stored at room temperature.
The experiments showed that although all tomatoes were harvested simultaneously, the ripeness of the tomatoes depended on the presence of the composite nanomaterials. As shown in figure 6, quantitative measurements of tomato color using image processing software illustrate an insignificant color change in the presence of the photocatalyst. Each simulation experiment was performed without ethylene injection. Under these conditions, fruit ripening was achieved by self-produced ethylene. After 6 days, there was a significant change in the degree of discrimination of the reference points in the control group. After 14 days, the discrimination and decay of the reference points of fruits stored with the nanocomposites were greatly reduced. This demonstrated that the excellent performance of photocatalytic ethylene removal preserved the tomato's freshness.
We also used simultaneously harvested bananas to determine the effect of the composite nanomaterials. Upon extending the shelf life of the fruit, we compared changes in the bananas over 12 days of storage with and without SA/Ce6@SiO 2 (figure 7). Bananas were placed in sealed fruit preservation bags without ethylene injection. Under these conditions, overripening of the fruit was achieved by self-produced ethylene.
Comparisons were made every two days. The comparison was most obvious on day 8, but the fruit also became gradually darker and more corrupt by day 10. Since bananas very easily rot and crack, it was evident from the banana experiment that the degree of decay of bananas depended to some extent on the use of photocatalytic materials.

Conclusions
Ce6-loaded mesoporous SiO 2 (Ce6@SiO 2 ) was uniformly coated on the sodium alginate hydrogel. The composites exhibited high ethylene decomposition performance due to the large specific surface area of the carrier SiO 2 , which increased the contact possibility with ethylene. It also showed an excellent photocatalytic effect due to the loading of Ce6 particles in the pores. SA/Ce6@SiO 2 showed a larger contact area and adsorption effect after adding the sodium alginate hydrogel. Under visible light illumination, ethylene was photocatalytically degraded. In simulation experiments, SA/Ce6@SiO 2 showed a good preservation effect on fruits and vegetables and decomposed auto-generated ethylene gas. Even in a glass assembly indoors, tomatoes  were kept fresh, showing that the composite nanomaterials degraded ethylene and extended the shelf life of fruits and vegetables, even at temperatures above 25°C. This composite has commercial potential for general shelving and non-refrigerated window applications and may also be used in commercial preservation systems for fruits and vegetables. It also provides socio-economic benefits by preventing the spoilage of fruits and vegetables during their storage, thus extending their shelf life.