*2.13. Data Analysis*

The data were analyzed using SPSS 20.0 statistical analysis software (SPSS, Chicago, IL, USA). All experiments were performed three times. Statistical significance was determined as *p* < 0.05.

### **3. Results and Discussion**

#### *3.1. Morphological Observations*

Figure 2 shows SEM images of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/ Azo/β-CD. The Nps had spherical shapes and were not destroyed due to the encapsulations of Hex, Azo, and β-CD. The particle sizes, zeta-potential values, and PDIs were further measured by DLS analysis (Table 1). After Hex was loaded and the mesopore surface was grafted with Azo/β-CD, the average particle size of Nps increased from 269.8 ± 6.8 nm (BMMs) to 387.2 ± 3.8 nm (Hex@BMMs/Azo/β-CD). The highest PDI value, i.e., 0.153 ± 0.02, indicated that the Nps had better mono-dispersity and stability. Owing to the presence of -OH on the surface of the mesoporous silica, the zeta potential of BMMs was −13.93 ± 1.57 mV. After the modification of azobenzene, the zeta-potential value of BMMs/Azo increased to −8.42 ± 1.71 mV because of the neutralization of the amino groups present on the surface of the modified azobenzene. After loading Hex, the zeta-potential value of Hex@BMMs/Azo/β-CD Nps decreased to −16.97 ± 0.95 mV, due to the negative charge of Hex, indicating that the pesticide Hex was successfully loaded into the Nps.

#### *3.2. Structure and Interaction Analysis*

The structure of the Nps was also tested by XRPD and the results are shown in Figure 3A. BMMs had an obvious (100) crystal-plane diffraction peak at 2θ = 1.86◦ , which is the characteristic peak of BMMs, indicating that it had a highly ordered, double-hole structure. After grafting the modified azobenzene, BMMs/Azo exhibited the same (100) crystal-plane diffraction peak, indicating that the BMMs/Azo Nps still maintained an ordered mesoporous structure. After Hex was loaded into the BMMs, the peak intensity

decreased from 1.99◦ to 1.97◦ , and the d value moved from 44.09 to 44.85 nm. This is because the Hex in the channel reduced the scattering between the mesoporous channels and the pore wall. The strength of the XRPD peaks (2θ = 2◦ ) of the β-CD-coated Hex@BMMs/Azo Nps decreased significantly, and the shape broadened, showing that the mesoporous structure was affected, and β-CD was successfully encapsulated into the system. **Nps Mean Size (nm) PDI Zeta Potential (mV)**  BMMs 269.8 ± 6.8 0.062 ± 0.03 −13.93 ± 1.57 BMMs/Azo 367.4 ± 3.3 0.144 ± 0.01 −8.42 ± 1.71 Hex@BMMs/Azo 553.4 ± 4.1 0.123 ± 0.04 −16.77 ± 1.60 Hex@BMMs/Azo/β-CD 387.2 ± 3.8 0.153 ± 0.02 −16.97 ± 0.95

**Table 1.** The mean diameters and distributions of nanoparticles based on BMMs.

−13.93 ± 1.57 mV. After the modification of azobenzene, the zeta-potential value of BMMs/Azo increased to −8.42 ± 1.71 mV because of the neutralization of the amino groups present on the surface of the modified azobenzene. After loading Hex, the zeta-potential value of Hex@BMMs/Azo/β-CD Nps decreased to −16.97 ± 0.95 mV, due to the negative charge of Hex, indicating that the pesticide Hex was successfully loaded into the Nps.

*Coatings* **2021**, *11*, x 6 of 15

**Figure 2.** SEM images of different nanoparticles. (**A-a**,**A-b**) BMMs, (**B-a**,**B-b**) BMMs/Azo, (**C-a**,**C-b**) Hex@BMMs/Azo, (**D-a**,**D-b**) Hex@BMMs/Azo/β-CD. **Figure 2.** SEM images of different nanoparticles. (**A-a**,**A-b**) BMMs, (**B-a**,**B-b**) BMMs/Azo, (**C-a**,**C-b**) Hex@BMMs/Azo, (**D-a**,**D-b**) Hex@BMMs/Azo/β-CD.

*3.2. Structure and Interaction Analysis*  **Table 1.** The mean diameters and distributions of nanoparticles based on BMMs.


characteristic peak of BMMs, indicating that it had a highly ordered, double-hole structure. After grafting the modified azobenzene, BMMs/Azo exhibited the same (100) crystalplane diffraction peak, indicating that the BMMs/Azo Nps still maintained an ordered mesoporous structure. After Hex was loaded into the BMMs, the peak intensity decreased from 1.99° to 1.97°, and the d value moved from 44.09 to 44.85 nm. This is because the Hex in the channel reduced the scattering between the mesoporous channels and the pore wall. The strength of the XRPD peaks (2θ = 2°) of the β-CD-coated Hex@BMMs/Azo Nps decreased significantly, and the shape broadened, showing that the mesoporous structure

was affected, and β-CD was successfully encapsulated into the system.

**Figure 3.** XRPD patterns (**A**), FTIR (**B**), N2 adsorption/desorption isotherms (**C**), and BET surface areas and pore volumes (**D**) of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/Azo/β-CD. **Figure 3.** XRPD patterns (**A**), FTIR (**B**), N2 adsorption/desorption isotherms (**C**), and BET surface areas and pore volumes (**D**) of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/Azo/β-CD.

The FTIR spectra of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/Azo/β-CD were determined to evaluate the Nps' structural changes with various functional groups (Figure 3B). BMMs exhibited three characteristic peaks at 1082, 780, and 810 cm−<sup>1</sup> that were anti-symmetrical and symmetrical stretching-vibration peaks of Si–O–Si groups. The absorption bands at 1644 and 1419 cm−1 were the stretching-vibration peaks of the –CONH– group and the vibration peaks of the C=C group in the aromatic ring respectively, indicating that the modified Azo was successfully grafted onto the surface of BMMs. The characteristic absorption peak of Hex at 3227 cm−1 and the absorption peak with increasing intensity at 1082 cm−1 suggested that the pesticide Hex was adsorbed in the pores of mesoporous silica through van der Waals forces and hydrogen bonds. The loading content of Hex in Nps was also tested by HPLC, and the loading ratios of Hex (27.3%) in Hex@BMMs/Azo/β-CD were obtained. To investigate the porosity, pore surface areas, and pore volumes of the nano-delivery system, nitrogen adsorption/desorption measurements were performed. As shown in Figure 4C, the N2 adsorption/desorption isotherms of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/Azo/β-CD belong to the Langmuir IV isotherm with two hysteresis loops. The first hysteresis loop, at 0.2 < P/P0 < 0.4, rose rapidly owing to monolayer adsorption of nitrogen. The second hysteresis loop The FTIR spectra of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/Azo/β-CD were determined to evaluate the Nps' structural changes with various functional groups (Figure 3B). BMMs exhibited three characteristic peaks at 1082, 780, and 810 cm−<sup>1</sup> that were anti-symmetrical and symmetrical stretching-vibration peaks of Si–O–Si groups. The absorption bands at 1644 and 1419 cm−<sup>1</sup> were the stretching-vibration peaks of the –CONH– group and the vibration peaks of the C=C group in the aromatic ring respectively, indicating that the modified Azo was successfully grafted onto the surface of BMMs. The characteristic absorption peak of Hex at 3227 cm−<sup>1</sup> and the absorption peak with increasing intensity at 1082 cm−<sup>1</sup> suggested that the pesticide Hex was adsorbed in the pores of mesoporous silica through van der Waals forces and hydrogen bonds. The loading content of Hex in Nps was also tested by HPLC, and the loading ratios of Hex (27.3%) in Hex@BMMs/Azo/β-CD were obtained. To investigate the porosity, pore surface areas, and pore volumes of the nano-delivery system, nitrogen adsorption/desorption measurements were performed. As shown in Figure 4C, the N<sup>2</sup> adsorption/desorption isotherms of BMMs, BMMs/Azo, Hex@BMMs/Azo, and Hex@BMMs/Azo/β-CD belong to the Langmuir IV isotherm with two hysteresis loops. The first hysteresis loop, at 0.2 < P/P<sup>0</sup> < 0.4, rose rapidly owing to monolayer adsorption of nitrogen. The second hysteresis loop appeared at P/P<sup>0</sup> = 0.8–0.95, indicating that the capillary tube of the particle accumulation pore had been condensed. The corresponding pore-size distribution revealed that the Nps had a dual-model structure and two pore sizes (Figure 3C, inset). After the loading of Hex molecules and the modification of Azo/β-CD in BMMs, the shape of the adsorption isotherm remained basically unchanged compared with BMMs, indicating that the mesoporous structure of the sample still existed. However, the BET specific surface area and pore volume of Hex@BMMs/Azo/β-CD significantly decreased, implying that Hex molecules occupied a significant number of pores and surface sites of Nps (Figure 3D). These observations further suggested that the modified Azo interacted with Si–OH groups, and that Hex was successfully loaded into Hex@BMMs/Azo/β-CD Nps.

appeared at P/P0 = 0.8–0.95, indicating that the capillary tube of the particle accumulation pore had been condensed. The corresponding pore-size distribution revealed that the Nps had a dual-model structure and two pore sizes (Figure 3C, inset). After the loading of Hex molecules and the modification of Azo/β-CD in BMMs, the shape of the adsorption isotherm remained basically unchanged compared with BMMs, indicating that the mesoporous structure of the sample still existed. However, the BET specific surface area and pore volume of Hex@BMMs/Azo/β-CD significantly decreased, implying that Hex molecules occupied a significant number of pores and surface sites of Nps (Figure 3D). These observations further suggested that the modified Azo interacted with Si–OH groups, and that

The loading rate of the Hex@BMMs/Azo/β-CD Nps was determined by TG analysis. The TG curves of nanoparticles are shown in Figure 4A. The mass loss of all samples mainly included two stages. The first stage was the evaporation of water in the samples before 150 °C, and the second weight-loss peak occurred at the stage from 150 to 800 °C due to the decomposition of organic components incorporated in the samples. The weight losses of BMMs/Azo and Hex@BMMs/Azo were 8.1% and 32.8%, respectively. Hence, the loading rate of Hex@BMMs/Azo was about 24.7%, which was approximately consistent with the result of HPLC. After β-CD was grafted to the surface of Hex@BMMs/Azo Nps, the weight loss was 39.5%, proving that Hex was successfully loaded into Azo-function-

Hex was successfully loaded into Hex@BMMs/Azo/β-CD Nps.

alized BMMs and β-CD played a good encapsulation role.

**Figure 4.** TG (**A**), XPS (**B**), and element distributions (**C**) of Hex@BMMs/AZO/β-CD and the control samples. **Figure 4.** TG (**A**), XPS (**B**), and element distributions (**C**) of Hex@BMMs/AZO/β-CD and the control samples.

To analyze the chemical elements on the surface of nanoparticles, the samples were characterized by XPS. As shown in Figure 4B, the surface of BMMs nanoparticles mainly contains two elements, Si and O. The binding energies of approximately 533.08 and 104.06 The loading rate of the Hex@BMMs/Azo/β-CD Nps was determined by TG analysis. The TG curves of nanoparticles are shown in Figure 4A. The mass loss of all samples mainly included two stages. The first stage was the evaporation of water in the samples before 150 ◦C, and the second weight-loss peak occurred at the stage from 150 to 800 ◦C due to the decomposition of organic components incorporated in the samples. The weight losses of BMMs/Azo and Hex@BMMs/Azo were 8.1% and 32.8%, respectively. Hence, the loading rate of Hex@BMMs/Azo was about 24.7%, which was approximately consistent with the result of HPLC. After β-CD was grafted to the surface of Hex@BMMs/Azo Nps, the weight loss was 39.5%, proving that Hex was successfully loaded into Azo-functionalized BMMs and β-CD played a good encapsulation role.

To analyze the chemical elements on the surface of nanoparticles, the samples were characterized by XPS. As shown in Figure 4B, the surface of BMMs nanoparticles mainly contains two elements, Si and O. The binding energies of approximately 533.08 and 104.06 eV belong to O1s and Si2p, respectively. The weak emerging signal at 285.08 eV was attributed to C1s, which was a residual component after the calcination of the CTAB template. In the BMMs/AZO spectrum, the C1s peak was more intense than that of BMMs and a new signal of N1s was observed at the binding energy of 401.08 eV, confirming that Azo was successfully modified on the surface of BMMs. Furthermore, a new peak of Cl2p appeared at 201.08 eV in the spectrum of Hex@BMMs/Azo, compared with BMMs/Azo, indicating that BMMs/Azo nanoparticles were successfully loaded with Hex. The element components of Hex@BMMs/Azo/β-CD were completely consistent with those of Hex@BMMs/Azo. Due to the coating of β-CD, the thickness of the surface of Hex@BMMs/Azo increased, resulting in the weakening of the absorption peak intensity of Hex@BMMs/Azo/β-CD nanoparticles. In addition, the result of EDS analysis (Figure 4C)

showed that the elements of Si, O, C, N, and Cl were all distributed in Hex@BMMs/Azo/β-CD, which further proved the successful preparation of the nano-pesticide system. Hex@BMMs/Azo/β-CD, which further proved the successful preparation of the nano-pesticide system.

eV belong to O1s and Si2p, respectively. The weak emerging signal at 285.08 eV was attributed to C1s, which was a residual component after the calcination of the CTAB template. In the BMMs/AZO spectrum, the C1s peak was more intense than that of BMMs and a new signal of N1s was observed at the binding energy of 401.08 eV, confirming that Azo was successfully modified on the surface of BMMs. Furthermore, a new peak of Cl2p appeared at 201.08 eV in the spectrum of Hex@BMMs/Azo, compared with BMMs/Azo, indicating that BMMs/Azo nanoparticles were successfully loaded with Hex. The element components of Hex@BMMs/Azo/β-CD were completely consistent with those of Hex@BMMs/Azo. Due to the coating of β-CD, the thickness of the surface of Hex@BMMs/Azo increased, resulting in the weakening of the absorption peak intensity of Hex@BMMs/Azo/β-CD nanoparticles. In addition, the result of EDS analysis (Figure 4C) showed that the elements of Si, O, C, N, and Cl were all distributed in

*Coatings* **2021**, *11*, x 9 of 15

#### *3.3. Foliage Adhesion of Hex 3.3. Foliage Adhesion of Hex*

Adhesion experiments were conducted to prove that the Hex@BMMs/Azo/β-CD nano-delivery system had better adhesion behavior on the leaf surfaces. As shown in Figure 5, the contact-angle values of Hex@BMMs/Azo Nps and Hex@BMMs/Azo/β-CD Nps were 83.89◦ ± 0.36◦ and 71.64◦ + 0.41◦ respectively, which were obviously lower than those of Hex technical solution (103.62◦ ± 0.37◦ ) and deionized water (105.57◦ ± 0.48◦ ). The data showed that Azo/β-CD-coated Hex@BMMs microcapsules exhibited excellent adhesion properties. Compared with deionized water and technical Hex, the contact angles of the Nps were reduced because the hydroxyl groups on the surface of BMMs increased the infiltration of the nano-delivery system on the leaf surface. The smaller contact angle of Hex@BMMs/Azo/β-CD Nps was due to the properties of β-CD, including internal hydrophobicity and external hydrophilicity. After mesoporous silica was modified by β-CD, the hydrophilicity of the silica surface increased and the contact angle decreased, which improved the water solubility of the Nps and the adhesion of Nps on leaves. Adhesion experiments were conducted to prove that the Hex@BMMs/Azo/β-CD nano-delivery system had better adhesion behavior on the leaf surfaces. As shown in Figure 5, the contact-angle values of Hex@BMMs/Azo Nps and Hex@BMMs/Azo/β-CD Nps were 83.89° ± 0.36° and 71.64° + 0.41° respectively, which were obviously lower than those of Hex technical solution (103.62° ± 0.37°) and deionized water (105.57° ± 0.48°). The data showed that Azo/β-CD-coated Hex@BMMs microcapsules exhibited excellent adhesion properties. Compared with deionized water and technical Hex, the contact angles of the Nps were reduced because the hydroxyl groups on the surface of BMMs increased the infiltration of the nano-delivery system on the leaf surface. The smaller contact angle of Hex@BMMs/Azo/β-CD Nps was due to the properties of β-CD, including internal hydrophobicity and external hydrophilicity. After mesoporous silica was modified by β-CD, the hydrophilicity of the silica surface increased and the contact angle decreased, which improved the water solubility of the Nps and the adhesion of Nps on leaves.

**Figure 5.** The contact angles of Hex@BMMs/Azo (**A**), Hex@BMMs/Azo/β-CD (**B**), technical Hex (**C**), and deionized water (**D**). **Figure 5.** The contact angles of Hex@BMMs/Azo (**A**), Hex@BMMs/Azo/β-CD (**B**), technical Hex (**C**), and deionized water (**D**).

### *3.4. Photo-Responsive Property*

To verify that β-CD could self-assemble with BMMs/Azo to form a gatekeeper for the controlled-release of pesticide, we detected the UV absorption spectra of BMMs, BMMs/Azo, and BMMs/Azo/β-CD under UV irradiation. As shown in Figure 6A, com-pared with BMMs, an obvious UV absorption peak appeared at 357 nm after the modification of Azo, indicating that Azo was successfully grafted on the surface of BMMs. When β-CD was capped onto the nanoparticles' surface, the UV peak intensity was higher than that of BMMs/Azo. This was mainly because the electron cloud density of the Azoconjugated system was interfered by the higher electron cloud in the inner cavity of β-CD molecules, resulting in the decrease of electron transition energy and the increase of absorption intensity of the conjugated system, which led to the enhancement of BMMs/Azo/β-CD absorption spectrum intensity.

*3.4. Photo-Responsive Property* 

CD absorption spectrum intensity.

To verify that β-CD could self-assemble with BMMs/Azo to form a gatekeeper for the controlled-release of pesticide, we detected the UV absorption spectra of BMMs, BMMs/Azo, and BMMs/Azo/β-CD under UV irradiation. As shown in Figure 6A, compared with BMMs, an obvious UV absorption peak appeared at 357 nm after the modification of Azo, indicating that Azo was successfully grafted on the surface of BMMs. When β-CD was capped onto the nanoparticles' surface, the UV peak intensity was higher than that of BMMs/Azo. This was mainly because the electron cloud density of the Azo-conjugated system was interfered by the higher electron cloud in the inner cavity of β-CD mol-

**Figure 6.** UV–Vis spectra of BMMs/Azo/β-CD and control samples (**A**). UV–Vis spectra of Azo after different radiation: UV (365 nm) light radiation for 0, 30, and 60 min (**B**), Vis (450 nm) light irradiation for 0, 30, 60, 90, 120, 150, and 180 min after UV irradiation (120 min) (**C**). **Figure 6.** UV–Vis spectra of BMMs/Azo/β-CD and control samples (**A**). UV–Vis spectra of Azo after different radiation: UV (365 nm) light radiation for 0, 30, and 60 min (**B**), Vis (450 nm) light irradiation for 0, 30, 60, 90, 120, 150, and 180 min after UV irradiation (120 min) (**C**).

We further investigated the effect of light on the *trans/cis-*isomers of Azo molecules. The UV absorption spectra of BMMs, BMMs/Azo, and BMMs/Azo/β-CD nanoparticles were detected by UV–Vis spectroscopy. After nanoparticles were radiated by UV light, the peak at 357 nm decreased with increasing time from 0 to 60 min (Figure 6B), indicating that the UV light could induce *trans-cis* transformation of Azo molecules. Under the following Vis light radiation, the peak intensities showed contrasting trends (Figure 6C), suggesting that the Vis light radiation could induce *cis-trans* transformation of Azo molecules. Especially from 90 to 120 min, a significant transition appeared in the nano-system, implying that *cis*-Azo structures were mostly converted to *trans*-Azo structures under the irradiation of visible light, and the detached β-CD recombined with BMMs/Azo again. As a result, Azo in BMMs/Azo/β-CD Nps exhibited *trans-cis* reversible photoisomerization, which could be mutually transformed from a *trans* structure to a *cis* structure under the alternating irradiation of UV–Vis light, which was consistent with previous reports [22,33]. We further investigated the effect of light on the *trans/cis*-isomers of Azo molecules. The UV absorption spectra of BMMs, BMMs/Azo, and BMMs/Azo/β-CD nanoparticles were detected by UV–Vis spectroscopy. After nanoparticles were radiated by UV light, the peak at 357 nm decreased with increasing time from 0 to 60 min (Figure 6B), indicating that the UV light could induce *trans-cis* transformation of Azo molecules. Under the following Vis light radiation, the peak intensities showed contrasting trends (Figure 6C), suggesting that the Vis light radiation could induce *cis-trans* transformation of Azo molecules. Especially from 90 to 120 min, a significant transition appeared in the nano-system, implying that *cis*-Azo structures were mostly converted to *trans*-Azo structures under the irradiation of visible light, and the detached β-CD recombined with BMMs/Azo again. As a result, Azo in BMMs/Azo/β-CD Nps exhibited *trans-cis* reversible photoisomerization, which could be mutually transformed from a *trans* structure to a *cis* structure under the alternating irradiation of UV–Vis light, which was consistent with previous reports [22,33]. In addition, BMMs-Azo Nps were repeatedly irradiated many times, and no "fatigue" to light was observed, indicating that the nanomaterial had good reversible light-responsive capability.

#### In addition, BMMs-Azo Nps were repeatedly irradiated many times, and no "fatigue" to light was observed, indicating that the nanomaterial had good reversible light-responsive *3.5. Release Behavior*

capability.

#### 3.5.1. Effect of Light Intensity

*3.5. Release Behavior*  3.5.1. Effect of Light Intensity To investigate the controlled-release performance of Hex from Hex@BMMs-Azo-CD under UV light, Hex@BMMs-Azo-CD Nps were irradiated under different UV-light intensities (150, 300, and 500 W at 365 nm). As shown in Figure 7A, the release of Hex from the microcapsules under UV irradiation was faster than that in darkness. This was because the Hex@BMMs-Azo-CD Nps were endowed with irreversible ''gatekeeper'' systems, and Azo underwent *trans-* to *cis-*isomerization under UV irradiation. The "cap" of the pesticide-carrying non-complex that blocked the mesopores was opened, and Hex molecules To investigate the controlled-release performance of Hex from Hex@BMMs-Azo-CD under UV light, Hex@BMMs-Azo-CD Nps were irradiated under different UV-light intensities (150, 300, and 500 W at 365 nm). As shown in Figure 7A, the release of Hex from the microcapsules under UV irradiation was faster than that in darkness. This was because the Hex@BMMs-Azo-CD Nps were endowed with irreversible "gatekeeper" systems, and Azo underwent *trans-* to *cis*-isomerization under UV irradiation. The "cap" of the pesticidecarrying non-complex that blocked the mesopores was opened, and Hex molecules were released from the pores, indicating that the nano-pesticide delivery system responded to light stimulation. Compared with the control with a release rate of only 6.37% in darkness, Hex was released rapidly from the Nps within 100 min of illumination. After 360 min, the release ratios of Hex reached 41.5%, 58.3%, and 79.0% at light intensities of 150, 300, and 500 W, respectively. In addition, the Azo-modified BMMs carrier was more sensitive to UV light with increasing light intensity, and the Hex-release rate was obviously increased, which further showed that UV light was the dominant driving force for Hex release. In short, these results demonstrated that Hex could be released from Hex@BMMs-Azo-CD Nps via stimulation by UV light.

#### 3.5.2. Effect of pH

The release behaviors of Hex@BMMs-Azo-CD Nps were investigated at different pH values (4.0, 7.0, and 9.0). The sustained-release curves of pesticide at various pH values are shown in Figure 7B. The cumulative release rate gradually increased as the pH value

decreased. At pH 4, the Hex-release rate reached 66.58% at 360 min. After 360 min, the release rate of Hex@BMMs-Azo-CD Nps at pH 4 was 66.9%, and those at pH 7 and pH 9 were 58.3% and 40.4%, respectively. This was mainly due to the grafting of β-CD on the surface of the Nps. CD is relatively stable under alkaline and neutral conditions, but is easily hydrolyzed under acidic conditions. After the sealed "cap" was opened by hydrolysis, Hex molecules were easily released from the mesopores. In addition, excessive hydrogen ions in the acid solution will be added to the N=N bond of azobenzene, resulting in a pH response. sponded to light stimulation. Compared with the control with a release rate of only 6.37% in darkness, Hex was released rapidly from the Nps within 100 min of illumination. After 360 min, the release ratios of Hex reached 41.5%, 58.3%, and 79.0% at light intensities of 150, 300, and 500 W, respectively. In addition, the Azo-modified BMMs carrier was more sensitive to UV light with increasing light intensity, and the Hex-release rate was obviously increased, which further showed that UV light was the dominant driving force for Hex release. In short, these results demonstrated that Hex could be released from Hex@BMMs-Azo-CD Nps via stimulation by UV light.

were released from the pores, indicating that the nano-pesticide delivery system re-

*Coatings* **2021**, *11*, x 11 of 15

**Figure 7.** Cumulative release curves with different intensity UV light (**A**) and different pH (**B**), and kinetic fitting models of Hex@BMMs/Azo/β-CD nanoparticles (**C**). Error bars represent standard deviation from the mean (n = 3). **Figure 7.** Cumulative release curves with different intensity UV light (**A**) and different pH (**B**), and kinetic fitting models of Hex@BMMs/Azo/β-CD nanoparticles (**C**). Error bars represent standard deviation from the mean (n = 3).

3.5.2. Effect of pH 3.5.3. Release Kinetics Analysis

The release behaviors of Hex@BMMs-Azo-CD Nps were investigated at different pH values (4.0, 7.0, and 9.0). The sustained-release curves of pesticide at various pH values are shown in Figure 7B. The cumulative release rate gradually increased as the pH value decreased. At pH 4, the Hex-release rate reached 66.58% at 360 min. After 360 min, the To further elucidate the effect of pH on the mechanism of Hex sustained release from Hex@BMMs-Azo-CD NPs, we studied the release kinetics using the zero-order kinetics model, first-order kinetics model, Higuchi kinetics model, and Ritger–Peppas kinetics model (Figure 7C). Table 2 presents the values of parameters and the regression coefficients (*R* 2 ).


**Table 2.** Release parameters of HEX at different pH values.

Note: *Q* is the fractional release of pesticide, *t* is the elapsed time, *R* 2 is the high value of the linear regression coefficient, and *n* is the release exponent, where n1, n2, and n<sup>3</sup> are 0.308, 0.426, and 0.425, respectively.

Regardless of acidic or alkaline conditions, the *R* <sup>2</sup> value of the Ritger–Peppas kinetic equation was higher than those of the other three mathematical models, indicating that the Ritger–Peppas kinetic model was more suitable for the release behavior of Hex. The values of n in acidic and alkaline environments were 0.308 and 0.425 respectively, and both values, which were lower than 0.45, proved that the Hex release in Hex@BMMs/Azo/β-CD mainly followed Fick diffusion, and the concentration was the main influencing factor in the slow-release process. However, under neutral conditions, the *R* <sup>2</sup> value of the first-order kinetic equation was the highest, and the release of Hex conforms to the first-order kinetic model, implying that the release of Hex was closely related to concentration.

#### *3.6. Bioactivity Test*

Hex.

*3.7. Biosafety Evaluation* 

μg/mL, and (**f**) 500 μg/mL.

Figure 8 shows the bioactivity of Hex@BMMs/Azo/β-CD plotted against concentrations of *Rhizoctonia solani* ranging from 50 to 200 mg/L. Compared with the negative control, the inhibition rates were 40.3% (non-irradiated Nps), 60.1% (irradiated Nps), and 56.8% (technical Hex) respectively, at the Hex-as-an-active-ingredient concentration of 50 mg/L at 14 days, due to the sustained release of Hex in the nano-delivery systems. In addition, the control efficiencies of UV-irradiated samples were significantly higher than those of non-UV-irradiated samples and technical controls, indicating that UV irradiation significantly improved the release of Hex in the nano-delivery systems. This result is consistent with previous reports [23,34]. In short, the UV-stimuli-responsive Nps exhibited a better and more sustained antibacterial activity against *Rhizoctonia solani* than technical Hex. *Coatings* **2021**, *11*, x 13 of 15 significantly improved the release of Hex in the nano-delivery systems. This result is consistent with previous reports [23,34]. In short, the UV-stimuli-responsive Nps exhibited a better and more sustained antibacterial activity against *Rhizoctonia solani* than technical

**Figure 8.** Digital images (**A**) and inhibitory rates (**B**) of Hex@BMMs/Azo/β-CD nanoparticles against *Rhizoctonia solani* at 14 days. **Figure 8.** Digital images (**A**) and inhibitory rates (**B**) of Hex@BMMs/Azo/β-CD nanoparticles against *Rhizoctonia solani* at 14 days.

**Figure 9.** Biosafety evaluation of different concentrations of BMMs/Azo: (**A**) *Escherichia coli* suspension OD600, (**B**) photograph of *Escherichia coli* colony, (**C**) CCC-ESF-1 cell survival rate, and (**D**) photograph of CCC-ESF-1 cell. (**a**) Control, (**b**) 31.25 μg/mL, (**c**) 62.55 μg/mL, (**d**) 125 μg/mL, (**e**) 250

modified nano-pesticide loading system had excellent biological safety.

To further evaluate the biological safety of nanocarriers, the toxicological effects of

#### *3.7. Biosafety Evaluation 3.7. Biosafety Evaluation*

*Rhizoctonia solani* at 14 days.

Hex.

*Coatings* **2021**, *11*, x 13 of 15

To further evaluate the biological safety of nanocarriers, the toxicological effects of different concentrations of BMMs/Azo/β-CD on CCC-ESF-1 cells and *E. coli* were studied. Figure 9 shows that different concentrations of BMMs/Azo/β-CD Nps had little influence on the growth and metabolism of CCC-ESF-1 cells and *E. coli*, indicating that an Azomodified nano-pesticide loading system had excellent biological safety. To further evaluate the biological safety of nanocarriers, the toxicological effects of different concentrations of BMMs/Azo/β-CD on CCC-ESF-1 cells and *E. coli* were studied. Figure 9 shows that different concentrations of BMMs/Azo/β-CD Nps had little influence on the growth and metabolism of CCC-ESF-1 cells and *E. coli*, indicating that an Azomodified nano-pesticide loading system had excellent biological safety.

**Figure 8.** Digital images (**A**) and inhibitory rates (**B**) of Hex@BMMs/Azo/β-CD nanoparticles against

significantly improved the release of Hex in the nano-delivery systems. This result is consistent with previous reports [23,34]. In short, the UV-stimuli-responsive Nps exhibited a better and more sustained antibacterial activity against *Rhizoctonia solani* than technical

**Figure 9.** Biosafety evaluation of different concentrations of BMMs/Azo: (**A**) *Escherichia coli* suspension OD600, (**B**) photograph of *Escherichia coli* colony, (**C**) CCC-ESF-1 cell survival rate, and (**D**) photograph of CCC-ESF-1 cell. (**a**) Control, (**b**) 31.25 μg/mL, (**c**) 62.55 μg/mL, (**d**) 125 μg/mL, (**e**) 250 μg/mL, and (**f**) 500 μg/mL. **Figure 9.** Biosafety evaluation of different concentrations of BMMs/Azo: (**A**) *Escherichia coli* suspension OD600, (**B**) photograph of *Escherichia coli* colony, (**C**) CCC-ESF-1 cell survival rate, and (**D**) photograph of CCC-ESF-1 cell. (**a**) Control, (**b**) 31.25 µg/mL, (**c**) 62.55 µg/mL, (**d**) 125 µg/mL, (**e**) 250 µg/mL, and (**f**) 500 µg/mL.

#### **4. Conclusions**

In this work, we prepared a novel UV-responsive nano-pesticide delivery system by the sol–gel method. Azo was modified and grafted with BMMs, and then the fungicide Hex was loaded into mesoporous silica by adsorption. The Hex@BMMs/Azo/β-CD Nps had a uniformly spherical morphology and good dispersibility in water. The nanocomplex showed excellent photo-responsive controlled-release performance owing to the strong host–guest complex between the *trans*-Azo and β-CD, which was used to control Hex release from BMMs under UV–Vis irradiation. In addition, the release of Hex in Nps was promoted in an acidic environment by varying the pH of the structure. The Ritger– Peppas kinetic model was a better fit for the release behavior of Hex. The carrier displayed good adhesion on leaf surfaces, and was biologically benign. The sustained fungicidal efficacy against *Rhizoctonia solani* indicated that Hex@BMMs/Azo/β-CD nanoparticles could effectively improve the utilization of Hex and decrease pesticide residue. Therefore, this work provides a promising approach to reduce the risk to the environment, and promote the development of green agriculture in the future.

**Author Contributions:** Data curation, L.W.; Formal analysis, W.H.; Funding acquisition, F.Z.; Investigation, H.P. and W.L.; Supervision, F.Z.; Writing—review & editing, F.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was funded by the National Key R&D Program of China (2016YFD0200502-2).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

