Interface Template Synthesis of Zein-Based Amorphous Tio2 Composite Microcapsules with Enhanced Photocatalysis


 In order to further enhance the application field of zein-based microcapsule. Zein-based amorphous TiO2 composite microcapsules (ZTCMs) were innovatively prepared from zein, tetra butyl ortho titanate (TBOT) and PEO106PPO70PEO106 (F127) via interface template synthesis. The Effects of TBOT amount on ZTCMs structures and photo-catalytic performances were mainly investigated. Chemical structure and microstructure of the obtained composite microcapsules were characterized mainly by fourier-transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM) and energy disperse spectroscopy (EDS). The results show ZTCMs exhibited evident hollow structure with titanium dioxide (TiO2) wrapped in the outer layer. The average size of ZTCMs300 was approximately 4 µm, which increased as the increase of TBOT dosage. Significantly, ZTCMs showed excellent photo-catalytic ability on dyes, red wine and coffee alike. The degradation rate of Rhodamine B (RB) was more than 80% after irradiation for 5 h under sunlight. This study provides a facile method to fabricate natural-based photo-catalytic material, which will be a good candidate in many fields such as medicine, food packaging, leather and textile.


Introduction
In the past few decades, massive use of fossil resources has led to the rapid consumption of global oil reserves and soaring oil prices. Therefore, it's meaningful to research and develop renewable resources.
With the increasing of environmental awareness, consumers' active lifestyles and green consumption concepts have brought new challenges to daily necessities. For example, clothing is always stained by oils, juices or coffee, which can provide an easeful environment for microbial growth but a bad in uence on people's daily lives and works. Owing to the advantages of high chemical stability and photo-catalytic activity, titanium dioxide (TiO 2 ) has been widely developed to degrade organic contaminants [14][15][16].
According to researches, TiO 2 own four kinds of natural polymorphs (TiO 2 (B), brookite, anatase and rutile) and at least ve polymorphs that are produced synthetically [17]. Although the crystalline TiO 2 has a well de ned structure, a large fraction of the atoms located on the surface show structural disorder, leading to them having unique properties different to their bulk crystalline counterparts [18]. This explains why the amorphous TiO 2 , with their disordered structure can have different properties, for advanced applications, relative to the crystal structures that have well-de ned properties [19,20]. Meanwhile, amorphous TiO 2 has also captured a great deal of attention due to the characteristic of the processability at room temperature and the high speci c surface area [21,22]. So, there is a growing interest in studying the performances of amorphous TiO 2 which can possess a strong surface activity compared to crystalline TiO 2 of the same size [23,24].
Up to now, the synthesis of functional materials based on TiO 2 and zein by combining the photo-catalytic performance of TiO 2 and the lm-forming property of zein has been reported. Li et al. demonstrated a novel method to prepare composite lms by using zein, poly propylene carbonate and anatase TiO 2 , which indicated TiO 2 can endow composite lms with photo-catalytic activity [25]. Qu and co-workers reported an effective method to improve the photo-catalytic performance of composite lms by adding highly dispersible TiO 2 nanoparticles [26]. Sajed et al. created a new nano bers based on zein and sodium alginate incorporated with TiO 2 nanoparticles and betanin by using the electrospinning technique, and the nano bers were bene cial to improve the shelf life and quality of food as the food packaging [27]. However, there is few reports on the combination of zein and amorphous TiO 2 , not to mention the design and research of the microstructures of composite materials containing zein and amorphous TiO 2 .
In our previous researches, the method of interface template synthesis has been used to craft chitosancoated silica nanocapsules with a double-shelled structure, wich was proved as an effectively method for preparing organic-inorganic double-shell materials [28]. Herein, in this study, amorphous TiO 2 -coated zein microcapsules (ZTCMs) were also prepared by interface template synthesis to improve the utilization of zein and the speci c surface area of amorphous TiO 2 , and the possible synthesis strategy of ZTCMs was proposed as exhibited in Scheme 1. Firstly, the mixture was obtained after the blending, containing F127, TBOT, THF, was dropwise added into zein ethanol solution. And then, the microcapsules based on zein and F127 gradually formed with the volatilization of ethanol and THF. Meanwhile, the TBOT coated in the micelles was also gradually hydrolyzed. Finally, ZTCMs were obtained after spray drying the microcapsules emulsion. Rhodamine B (RB) was procured from Aladdin Reagents (Shanghai, China) Co., Ltd. All above chemicals were analytical grade and used without further treatment. Waterborne polyurethane was bought from Yantai Daocheng Chemical Co., Ltd. (Yantai, China). Red wine and coffee were bought from a supermarket in Xi'an, China.

Preparation of ZTCMs
Brie y, 5.0 g of zein was added into 500.0 mL of 85% (v/v) aqueous ethanol solution with magnetic stirring at 1000 rpm for 4 h at 40℃ by heating in water bath. Meanwhile, a series of TBOT blends were prepared by dissolving 0.750 g of F127 in 9.0 mL of THF and adding different volumes of TBOT, before magnetic stirring solutions at 1000 rpm for 4.0 h at ambient temperature. Afterwards, 9.1 mL of different kinds of TBOT blends were added dropwise into 100.0 mL of zein stock solution with continuous stirring at 1200 rpm for 24 h at 40℃ also by heating in water bath. Finally, all compound emulsions were stored at 4°C and powder samples were spray-dried by using a laboratory-scale spray-dryer (Spray Dryer, SD-Basic, Jia sheng technology co. LTD, Hong Kong, China) with air-inlet and air-outlet temperatures were 120℃ and 80℃, separately. In this work, zein-based amorphous TiO 2 composite microcapsules (ZTCMs) with different TBOT volumes (300.0, 400.0, 500.0, 600.0 and 700.0 µL) were termed respectively as ZTCMs 300 , ZTCMs 400 , ZTCMs 500 , ZTCMs 600 and ZTCMs 700 . At the same time, microcapsules without TBOT (ZMs) were used as the blank control. What's more, ZMs lm and ZTCMs lm were obtained by mixing 0.1 g of ZMs and ZTCMs 600 with 50.0 g of aqueous waterborne polyurethane solution respectively (Support information 1, S1).

Particle size measurement
The particle size distribution (PSD) of ZMs and ZTCMs were measured by a dynamic light scattering instrument (DLS, Master Sizer 2000; Malvern instruments Ltd.). Samples were diluted by 50 times with deionized water prior measurement to avoid multiple scattering effects. Each test was repeated in triplicate at 25 ± 0.2°C.

Fourier transform infrared spectroscopy
The infrared spectra of spray-dried samples including ZMs and ZTCMs were measured by using a fourier transform infrared spectroscopy (FTIR, Bruker, Germany) described by zhu in the wavenumber range of 500-4000 cm − 1 by using the KBr pellet method. 12

X-ray diffraction
An X-ray diffractometer (XRD, Bruker D8 Advance, Germany) was used to con rm the crystalline behavior of obtained ZMs and ZTCMs, with Cu Kα radiation at a scanning rate of 8°/min. The diffraction angle 2θ was set from 5° to 40°.

Transmission electron microscopy
The morphology and structure of ZMs and ZTCMs were investigated by a FEI transmission electron microscope (TEM, FEI Tecnai G2-F20, America) operated with an acceleration voltage of 200 kV. The freshly prepared dispersions were diluted using water, and one drop of the diluted dispersion was placed on a 200-mesh carbon-coated copper grid.

Scanning electron microscopy
The morphology of ZMs and ZTCMs were also studied by a scanning electron microscope (SEM, TESCAN-Vega 3 SBH, Czech) operated with an acceleration voltage of 10 kV. To study the morphology of microcapsules which were obtained after spray-drying, each sample was covered with a layer of gold and the sample imaging was performed in different magni cations. The surface elemental composition of ZMs and ZTCMs were also observed by this instrument equipped with an energy-dispersive spectroscopy (EDS).

Super depth-of-eld microscopy (SDOFM)
The appearance and structure of ZMs and ZTCMs were observed by a super depth of eld microscope (SDOFM, HIROX Corporation KH-8700, Japan). The AAE@Z-SMs were added dropwise on a silicon wafer before detection.

Photo-catalytic degradation of dye in the solution
As a water soluble xanthene organic dye, Rhodamine B (RB) is widely used as colourant in many industry processes [29][30][31], so it's meaningful to choose RB as the model dye. Photo-catalytic degradation experiments were performed in Xi'an, China (34º32'N, 108º55'E) in August 2019, and the sun exposure time was from 10.00 to 15.00 and the UV-light power of input solution was kept constant (10 W). The absorbance changes occurred in RB was evaluated by UV spectrophotometer (TU-1900, Beijing Purkinje General Instrument Co., Ltd). All the samples mentioned above were analyzed in a quartz cuvette with 1 cm path length over the wavelength range from 350 to 700 nm. The absorbance values of RB were recorded at the wavelength of 554 nm, meanwhile the absorbance values were converted to the corresponding concentration values on the basis of Lambert-Beer's law.
Precisely, 50.0 mL RB solution (10.0 mg/L) were poured into different glass reaction bottles which containing 25.0 mg ZMs, ZTCMs 300 , ZTCMs 400 , ZTCMs 500 , ZTCMs 600 and ZTCMs 700 , separately. At the same time, in order to verify the effects of the presence or absence of light on RB degradation, some samples were placed under light directly, while others were placed under light after wrapped in tin foil, the former was labeled (light) or (L) and the latter was labeled (dark) or (D). All experiments were tested three times, and the absorption rates and degradation rates were calculated using Eq. (1, 2).
Where RB ai and RB af represent the initial and nal RB concentration of adsorption-dissociation equilibrium, RB di and RB df are the initial and nal RB concentration of photo-catalysis degradation reaction, respectively.
In order to further examine the photo-catalytic performances of ZTCMs, three reaction cycles were carried out. Brie y, the spent ZTCMs samples were recovered via centrifugation and poured into distilled water under stirring on a magnetic stirrer. After the mixtures were ltered, the lter cakes were washed by distilled water and then were dried in a vacuum oven at 60°C. The dried again ZTCMs can be applied for the next photo-catalytic reaction. All the cycles were carried out maintaining a constant ZTCMs concentration (50% m/v). The degradation rate in each sample was obtained after irradiation for 20 h under a constant UV-light power of 10 W.

Photo-catalytic decomposition of stains on the lm
Besides, to deeper investigate the photo-catalytic activity of the prepared capsules, a similar test was also conducted by dropping 50.0 µL of red wine or coffee solution onto ZMs or ZTCMs lms, and the dimension of the two kinds of rectangular lms were 2 cm × 2 cm. These lms were exposed to UV-light (DOHO D60 10W, China) for certain time intervals to observe the photo-catalytic behaviors. Subjective stains degradation performances were evaluated by obtaining color photographs of the lms.

Particle size of ZTCMs
Controlling microcapsule size plays an important role in governing the stability of dispersion and DLS was conducted to show the particle size distribution of ZMs and ZTCMs. As shown in Fig. 1, the mean diameter of ZMs at neutral condition was about 550 nm and previous study also reported similar results [32,33]. From ZTCMs 300 to ZTCMs 600 , the particle size sharply increased from 0.9 µm to 5.2 µm. With the continuous feeding of TiO 2 , the transparency of the dispersions were continuously reduced and the turbidity of that were gradually increased (inset in Fig. 1). One suggestive reason is that, during the process, zein self-assembly formed a spherical morphology and TiO 2 adsorbed on the hydrophilic groups around zein. However, the PSD of ZTCMs 700 overlapped most that of ZTCMs 600 but the PSD of ZTCMs 700 was not uniform. ZTCMs 700 possessed a wider PSD than that of ZTCMs 600 , but the former average particle size was just slightly larger than that of the later. It's maybe that, with the increasing of TiO 2 dosage, the hydrophilic group vacancies on zein surface were reduced and a cross-linking was formed when reached adsorption saturation [18].
Meanwhile, the characteristic absorption peaks in ZMs were partially different from those appeared in ZTCMs. As provided in Fig. 2(B), the stretching vibration absorption peak and bending vibration absorption peak of O-Ti-O bond appeared at 953 cm − 1 and 842 cm − 1 proved the existence of TiO 2 in the microcapsules [35]. By comparing the spectra of ZMs and ZTCMs, the characteristic peaks of hydroxyl group were respectively shifted from 3307 cm − 1 and 3420 cm − 1 to 2876 cm − 1 and 3303 cm − 1 , implying the formation of hydrogen bonds between zein and TiO 2 .
The crystalline behaviors of as-prepared ZMs and ZTCMs were carried out by XRD as provided in Fig. 3. Obviously, not other peaks could be observed in ZMs except two broad diffraction peaks at 8° and 20°, con rming zein is an amorphous structure [36,37]. Simultaneously, two sharp diffraction peaks at 19°a nd 23° appeared in all diffraction lines of ZTCMs, this phenomenon can be illustrated by the formation of PEO crystals of F127, the similar results were also reported by other researches [38,39]. At the same, the absence of the amorphous TiO 2 related peaks may arise from their amorphous structure with low crystallinity and the overlapping of the diffraction peaks of Zein or F127, which was in good agreement with the results of Kong et al [40]. So, the ZTCMs own an amorphous structure and the micromorphology of ZTCMs have been further researched by TEM, SEM and EDS.

Micromorphology characterization of ZTCMs
To investigate the effects of TiO 2 on the morphology and size of microcapsules, the microstructural features of ZMs and ZTCMs 300 were determined by TEM. Figure 4 shows the morphology of both ZMs and ZTCMs 300 were spherical, and these phenomena were due to the self-assembly of zein when the polarity changes in the microenvironment [28]. ZMs showed smaller diameters ranging from approximately 0.5 µm to 3.0 µm than that of ZTCMs 300 , verifying the presence of TiO 2 shell can increase the diameter of ZMs. Under different contrasts, a well delimited wall demonstrated the hollow spherical structure of ZMs, while ZTCMs revealed a dense, solid and dark morphology with a microcapsule shape. The strong contrast between the dark edges and bright centers under different contrasts con rmed that zein was evenly coated by TiO 2 , and the result was also veri ed by other reports [28,41].
The morphology of ZMs and ZTCMs were also assessed by SEM. Figure 5(A) depicts that, although a few particles appeared to be collapsed, mostly-spherical particles were observed, and this result was similar to that of Fig. 4(A). Differences in PSD of ZMs could be explained that high hydrophobicity can promote the formation of smaller protein microcapsules [42]. From Fig. 5(B-F), rough surfaces of ZTCMs were observed with less-uniform diameters from approximately 2.5 µm to 7.0 µm. It could be observed that diameters of ZTCMs became larger with increasing TBOT usage, and this phenomenon further con rmed the results of DLS. It could be clearly seen from Fig. 5(F) that the diameter of ZTCMs 700 was equal to that of ZTCMs 600 , moreover, the agglomerated morphology can be observed but the boundary between zein and TiO 2 seemed not clear. It maybe that, when more and more hydrophilic TiO 2 adsorbed onto the zein microcapsules, TiO 2 will combine with each other thus to form cross-linking structure [18].
EDS was used to investigate the elements distribution based on ZTCMs 500 surface displayed in Fig. 5(G-I). The elliptical auxiliary line showed where ZTCMs 500 exist. As is shown in Fig. 5(G), elements distribution on the surface of ZTCMs 500 was homogeneous indicating an uniform structure. It can be seen from Fig. 5(H) that nitrogen was mainly concentrated in circles, con rming the existence and the morphology of zein. What's more, Fig. 5(I) presents titanium exactly has same even distribution as nitrogen, which proved TiO 2 was uniformly distributed on the surface of microcapsules. Through the results we can assume zein and TiO 2 have a stable combination, meanwhile ZTCMs 500 possessed the best uniform and stable structure.
Meanwhile, the morphology of ZMs and ZTCMs were also observed by SDOFM. As shown in Fig. 6(A), the morphology of ZMs is relatively at and the overall picture is blue-green. The particle size of ZMs is relatively small, with an average particle size of 1.4 µm. As shown in Fig. 6(B), in yellow-green and red states, ZTCMs have a relatively large particle size, with an average particle size of about 1.7 µm. At the same time, by comparing the 2D pictures, it was noted that the outline of ZMs is smooth, while that of ZTCMs is rough. The results show the size of the microcapsules increased with the addition of TiO 2 , which is consistent with the characterization results of SEM and TEM.

Photo-catalytic degradation of RB
RB has a good linear relationship between absorbance and mass concentration in the concentration range of 1-10 mg/L ( Figure S1). In order to avoid the interference of dye adsorption on RB photodegradation, a type of adsorption-dissociation equilibriums between the dye molecules and ZMs or ZTCMs were obtained (S2). The results of adsorption-dissociation equilibrium experiments were given in Fig. 7, which noted the RB absorption rates of ZTCMs increased as the increasing of TiO 2 dose. This phenomenon can be attributed to the augment of available active sites on ZTCMs surface [43].
To verify the photo-catalytic ability of ZTCMs, the effects of UV-light irradiation time on the photodegradation rate of RB was studied. As reported in Fig. 8(A) and 8(B), with the increasing of irradiation time of ZTCMs 300 , the absorbance value reduced from 0.60 at the 1st h to 0.43 at the 20th h and the concentration decreased from 8.16 to 7.38 mg/L. Meanwhile, the concentration curves of other types of ZTCMs had the similar trends. In fact, it can be illustrated that, as time prolong, the dye molecules have enough chances to adsorb on the surface of ZTCMs thus to be degraded under UV-light irradiation [43]. What's more, ZTCMs 300 almost have no ability to degrade RB in the absence of UV-light, and this result revealed ZTCMs have photo-catalytic ability only under irradiation. Figure 8(C) shows that, under UV-light, pure RB had scarcely degradation in the absence of ZTCMs. The obviously increase in degradation rates of RB is likely to arises from the increasing of TiO 2 content in ZTCMs. The color changes of RB in the inset in Fig. 8(C) also show ZTCMs had abilities of adsorption and photo-catalytic (S3). There is a good probability that increasing the dosage of TiO 2 can improve the active site and active oxygen radical concentration [44]. However, RB degradation rate of both ZTCMs 600 and ZTCMs 700 were kept at about 50% at the 20th h. Two factors can account for this observation (Scheme S1). One is the saturation of active sites by dye molecules after a speci c time [43]. Another is the contact area where ZTCMs combine with RB molecules decreased due to the accumulation of excess TiO 2 , and this reason can also be demonstrated by Figs. 5 and 6.
In order to simulate natural environment, the photo-catalytic activity of ZTCMs was further studied under sunlight. As displayed in Figure S2(A), RB's maximum absorption peaks shifted from 554 nm to 545 nm, which can be attributed to the formation of a series of N-deethylated intermediates of RB [45]. It could be seen clearly from Figure S2(B) that the concentration changes of RB degraded by ZTCMs under sunlight were similar to that under UV-light, but pure RB was still degraded about 7% even the absence of ZTCMs and the RB didn't degraded by ZTCMs 300 in the dark. We can know more details from Figure S2(C) that ZTCMs owned larger degradation rates under sun light than that under UV-light. In the meantime, RB showed more pronounced color changes under sunlight according to the inset in Figure S2(C). There are two reasons to explain this phenomenon. One is sun can provide a wider wavelength range including UVlight and a large amount of near infrared ray, so the RB molecules may be destroyed by sunlight. Another is the generation of photo-generated electrons and photo-generated holes could be promoted, thus bene ting the photo-catalytic e ciency of ZTCMs [46].
ZTCMs were recycled three times to investigate the changes of their photo-catalytic by degrading fresh RB. As represented in Fig. 9, ZTCMs 300 held dye adsorption ability in the absence of irradiation. During the rst cycle, RB degradation in ZTCMs 300 (light) was 26.5%. However, with the increasing of TiO 2 content, catalytic behaviors of ZTCMs were enhanced, RB degradation rates of both ZTCMs 600 (light) and ZTCMs 700 (light) could reached about 40%. Then, in the second and third cycles, RB degradation rates of ZTCMs300 (light) were respectively reduced to 20.4% and 19.5%, and other ZTCMs also showed relatively downward trends. The decrease in photo-catalytic e ciency can be explained by that, during recycling, ZTCMs formed hydrolysate on the surface when in contact with RB, which affected the effective contact between RB and photo-electron, photo-hole of ZTCMs [47][48][49]. The results are similar to those of Sharma et al [50], indicating the as-obtained ZTCMs, especially ZTCMs 600 , still owned a reasonable photodegradation e ciency after three cycles.

Photo-catalytic decomposition of red wine and coffee stains
In our daily life, dirt and greasiness can easily attack daily necessities' surfaces, which may cause inconvenience and troubles. Therefore, in this research, self-cleaning behaviors of ZTCMs composite lms by using red wine and coffee as stains were conducted. Pictures of red wine and coffee on composite lms before and after irradiation were listed in Table 1. It is clear to see that with the prolongation of photo-catalysis time, only slight changes in shapes and colors of droplets occurred on ZMs lms, indicating the weak photo-catalytic activity of ZMs lms. For ZTCMs lms, red wine and coffee stains gradually decomposed as the irradiation time increased from 1 h to 24 h. It was obvious that colors faded away and the outline became a bit fuzzy, suggesting a relatively higher decomposition, which was in complete agreement with the previous study [51]. The results pointed ZTCMs composite lms own expected photo-catalytic activity. To give a guidance, a feasible mechanism was used to illustrate the photo-catalytic activity of ZTCMs. Surface atoms and internal atoms have so large differences in electronic states and bond states, which makes ZTCMs possess more active sites and enables TiO 2 a higher catalytic activity [52]. When the photon energy radiated by light source was greater than or equal to the band gap energy, TiO 2 outer shell can be excited to produce photo-generated electrons (e − ) and photo-generated holes (h + ) [53].  (1-4)).

Conclusions
ZTCMs with double-shelled structure were successfully prepared via interface template synthesis. The asobtained microcapsules possessed an inner shell of zein and an outer shell of amorphous TiO 2 . This double-shell structure endowed ZTCMs with high photo-catalytic performance, and it still owned reasonable photo-degradation e ciency after three cycles of use. Most interesting, the composite lms has good self-cleaning performance and can be used to degrade red wine and coffee under UV-light. What's more, double-shelled structure of ZTCMs may also be available for imparting kinds of functions, such as keeping stable and controlling delivery of nutrients, medical drugs, avor llers and so on.
Declarations Figure 2 FTIR spectra of ZMs and ZTCMs with (A) full spectra and (B) expanded spectra at 1700-700 cm−1.       Photo-catalytic measurement of ZTCMs under UV-light for 3 cycles.

Supplementary Files
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