3.1 Characterization of MSNs
According to the TEM images, the three kinds of particle size of MSNs presented regular spherical, and there were obvious pore structure on the surface (Fig. 1). The hydrated particle size of MSNs samples was determined by dynamic light scattering (DLS), and the particle size of MSNs-100, MSNs-200 and MSNs-400 was mainly distributed at about 100 nm, 200 nm and 400 nm, respectively (Fig. 2). The PdI of MSNs-100, MSNs-200 and MSNs-400 were 0.107, 0.002 and 0.025, respectively. Because the PdI of MSNs-100, MSNs-200, and MSNs-400 were all less than 0.7, it indicated that MSNs-100, MSNs-200, and MSNs-400 had good dispersion in water (Table S1). Among the three kinds of particle size of MSNs, the average particle size of MSNs-100 was 112.5 nm, that of MSNs-200 was 200.1 nm, and that of MSNs-400 was 439.4 nm. However, since the particle size measured by DLS was hydrated particle size and the surface of MSNs was covered with water film, the particle size measured was larger than that observed by TEM.
The isothermal curves of the three particle sizes of MSNs were Ⅳ isothermal curves (Fig. 3). When the relative pressure was from 0.0 to 0.3, N2 was mainly adsorbed in the MSNs channels, the curve showed an upward translocation. When the relative pressure was between 0.3 and 0.8, N2 gradually covered the surface of MSNs, and the curve was relatively flat. When the relative pressure was between 0.8 and 1.0, a hysteresis loop appeared, which indicated that ordered mesopores existed in all three kinds of MSNs.
It can be seen from the results that the pore sizes of the three MSNs were all within the mesoporous range of 2 ~ 50 nm (Table S2), while the specific surface area and pore volume of MSNs-100 were larger than those of MSNs-200 and MSNs-400, indicating that MSNs-100 had a more excellent mesoporous structure. Its mesoporous structure could carry more pesticide, which was better than MSNs-200 and MSNs-400 in terms of loading capacity and sustained-release performance.
3.2 Characterization of Stm@MSNs
As shown in Fig. 4, the FTIR spectra of three MSNs with particle sizes showed that the characteristic peaks of nano-mesoporous silica materials appeared at the characteristic peaks of 804 cm− 1, 950 cm− 1, 1082 cm− 1 of MSNs-100, MSNs-200 and MSNs-400. And since the surfactant in MSNs had been removed, there was no characteristic peak of CTAB. According to FTIR, Stm@MSNs-100, Stm@MSNs-200 and Stm@MSNs-400 showed the same characteristic absorption peaks as Stm at 1082 cm− 1, 1691 cm− 1 and 1784 cm− 1, indicating that Stm was successfully loaded on the three particle sizes of MSNs.
The loss of MSNs quality occurred between 30℃and 90℃ due to evaporation of the combined water and organic solvents, and remained stable over the study temperature range thereafter. The loss of quality occurred twice in Stm@MSNs over the temperature range in the study. The initial temperatures of Stm@MSNs-100 were 28℃ and 128℃, of Stm@MSNs-200 were 29℃ and 115℃, and of Stm@MSNs-400 were 28°C and 153°C (Fig. 5). The evaporation of combined water and organic solvents in Stm@MSNs led to the first loss, while the second loss was because the discomposition of Stm loaded in Stm@MSNs at high temperature. Calculated by the method of 2.5, the loading capacity of Stm in Stm@MSNs-100 was 38%, Stm@MSNs-200 was 21% and Stm@MSNs-400 was 53%.
3.2 Release results of Stm@MSNs
By comparing the curves of release amount of Stm@MSNs with three particle sizes in the diluted solution of cucumber (Fig. 6), it could be seen that the release amount of MSNs-400 was much larger than that of Stm@MSNs-100 and MSNs-200, but the release curve of MSNs-400 tends to be flat after 20 h. The release effect was worse than Stm@MSNs-100. This was because the particle size of MSNs-400 was larger and the sphere surface area of particles was larger than that of MSNs-100, the surface of MSNs-400 and the cavity formed between particles could absorb more Stm, and the release amount of Stm@MSNs-400 was also higher. However, since the pore size and pore volume of MSNs-400 were smaller than those of MSNs-100, most Stm only adheres to the surface of the sphere and exists in the cavities formed by particles, but was not loaded into the pores of MSNs-400. In addition, a large amount of Stm was easy to fall off from the sphere surface and cavity of MSNs-400 during the release process, instead of slowly releasing from the pore channel, so that although the release amount of Stm@MSNs-400 was larger than that of MSNs-100, the release effect was not as good as that of MSNs-100 with larger pore size and pore volume.
Because the pore size and pore volume of MSNs-100 were larger than those of MSNs-200 and MSNs-400, the amount of Stm adsorbed by the Stm@MSNs-100 channel was greater than MSNs-200 and MSNs-400. Due to the binding effect of the mesoporous channel, The Stm could be released slowly from the channel, so the release effect of Stm@MSNs-100 was better than MSNs-200 and MSNs-400.
3.3 Uptake and translocation results of Stm@MSNs in cucumber plants
The concentration of Stm in cucumber plants treated by Stm@MSNs was higher, and its duration of effect was longer (Fig. 7). After 2 h of smeared treatment, the concentrations on Stm@MSNs-100 and Stm@MSNs-200 leaves were 18.91 mg kg− 1 and 17.61mg kg− 1, which were higher than those of conventional preparation (17.02 mg kg− 1). After 2 h of treatment, the water in the solution evaporated gradually. Compared with conventional preparations with larger particles, which were easier to slide off the leaves, the smaller particles Stm@MSNs-100 and Stm@MSNs-200 could be better adsorbed on the leaves to facilitate sorption by the plants, thus improving translocation ability of Stm. After 5, 10 and 14 d treatment. The mass fraction of Stm in cucumber leaves treated with Stm@MSNs-100 was 9.97, 6.64 and 3.05 mg kg− 1, respectively. The mass fractions of Stm in the leaves treated with Stm@MSNs-200 were 7.21, 3.42 and 1.84 mg kg− 1, respectively, while the mass fractions of Stm in the leaves treated with conventional preparation were 4.17, 2.43 and 0.60 mg kg− 1, respectively. As Stm was prone to degradation in the plant, other metabolites are generated. However, Stm@MSNs treatment could make Stm exist and slowly release in the MSNs channels, slow down the degradation rate of Stm and prolong the duration of efficacy.
The content of Stm in the upper leaves, lower leaves and roots of cucumber plants treated by Stm@MSNs-400 was lower than that treated by Stm@MSNs-100,Stm@MSNs-200 and conventional preparation of Stm, since its size was larger than the absorbable size of cucumber leaves (Zhao et al. 2018b). As a result, most of Stm@MSNs-400 was deposited on the surface of treated leaves and could not be absorbed by cucumber leaves and exposed to the environment (Soenen et al. 2013). However, the content of Stm in treated leaves were 0.46 and 0.14 mg kg− 1 after treatment for 10 and 14 d, indicating that there was still some Stm in treated leaves, while the anti-UV effect of MSNs made the Stm not photodegraded in the environment (Zhao et al. 2018a).