In this study, we synthesized pH-sensitive thiamethoxam-3-(2-aminoethylamino) propyl-bimodal mesoporous silica (P/Thi-NN-BMMs) nanoparticles (NPs). We used this bimodal mesoporous silica (BMMs) mesoporous material as a carrier based on the principle of free radical polymerization. The size of the P/Thi-NN-BMMs NPs was about 891.7±4.9 nm, with a zeta potential of about −25.7±2.5 mV. X-ray powder diffraction (XRPD) analysis, N₂-sorption measurements, and thermogravimetric (TG) analysis indicated that thiamethoxam (Thi) was loaded into the pores of the mesoporous structure and that the mesopore surface was coated with polyacrylic acid (PAA). The loading rate of P/Thi-NN-BMMs was about 25.2%. The controlled-release NPs had excellent anti-photolysis performance and storage stability. The NPs showed significant pH sensitivity, and the Thi release rate in pH 10.0 phosphate buffer was higher than those in pH 7.4 and pH 3.0 phosphate buffers. We described the sustained-release curves according to the Weibull model. The relative toxicity of P/Thi-NN-BMMs against peach aphid was 1.44 times that of commercial Thi. This provides a promising instrument for effective insect control and environment protection.

Particle size distributions of BMMs, NN-BMMs, P/NN-BMMs and P/Thi-NN-BMMs were displayed in Fig. 3. We determined particle size, polymer dispersity index (PDI), and zeta potential of BMMs, NN-BMMs, P/NN-BMMs,and P/Thi-NN-BMMs according to dynamic light scattering (DLS) using the Zetasizer Nano ZS (Malvern Instruments Ltd., Malvern, UK).

XRPD patterns, TG curves, N₂ adsorption/desorption isotherms, and BET surface areas and pore volumes of P/Thi-NN-BMMs and the control samples were displayed in Fig 4. We performed X-ray powder diffraction on an X-ray powder diffractometer (D8 ADVANCE X, Bruker/AXS, Karlsruhe, Germany) using nickel-filtered Cu Kα radiation (λ = 0.154 nm). The tube voltage and tube current were 35 kV and 35 mA, respectively, and the scan speed was 0.5 min^-1  (Fig 4A). We conducted TG analysis using a thermal analyzer (PerkinElmer, Waltham, MA, USA), under the following conditions: nitrogen atmosphere, a flow rate of 20 mLmin^-1, and a heating rate of 10 °C min^-1; the highest temperature was 800 °C. The pesticide loading content of the P/Thi-NN-BMMs nano-pesticide was calculated according to the TG curve(Fig 4B). We obtained nitrogen adsorption-desorption isotherms on an Autosorb-iQ pore analyzer (Quantachrome, Boynton Beach, FL, USA) at 77 K under continuous adsorption conditions. Brunauer-Emmett-Teller (BET) and used Barrett-Joyner-Halenda analyses to calculate the surface area, pore size, and pore volume(Fig 4C, 4D).

Photodegradation curves of P/Thi-NN-BMMs Nps and technical Thi under UV irradiation were displayed in Fig. 5. We tested the photolytic properties of P/Thi-NN-BMMs NPs with technical Thi as the control. The samples were divided equally into culture dishes and irradiated under a UV lamp (500 W) at a distance of 20 cm in a UV-light incubator (WFH-203B, Shanghai Precision Instrument Co., Ltd., Shanghai, China). At a given irradiation time (12, 24, 36, 48, 60, 72, and 96 h), we separately collected the culture dishes and analyzed the Thi contents in the samples by high-performance liquid chromatography (HPLC). A gradient of methanol–water(40:60) was programmed over 30 min at a flow rate of 1 mL min^-1 with detection at 240 nm. We performed all experiments in triplicate.

Stability of P/Thi-NN-BMMs NPs at different storage temperatures after 14 days were displayed in Fig. 6. We determined the stability of the P/Thi-NN-BMMs NPs according to CIPAC MT 46 (Miscellaneous Techniques and Impurities: MT 46 Accelerated Storage Procedure, Collaborative International Pesticides Analytical Council) and GB/T 19136–2003 (Determination of Heat Storage Stability of Pesticides, Chinese National Standard). We stored the samples at 4 ± 2 °C and 25 ± 2 °C for 7 d and 54 ± 2 °C for 14 d, and analyzed the Thi content using HPLC.

Cumulative release profiles of P/Thi-NN-BMMs with pH of 3.0, 7.4, and 10.0 were displayed in Fig. 7. we suspended 30 mg of P/Thi-NN-BMMs  in 100 mL of an ethanol:water mixture (30:70, v/v), which was used as the release medium. Then, we collected 2 mL of the sample for testing at fixed time intervals, and the system was replenished with an equal amount of fresh medium. We measured Thi concentrations using HPLC and calculated the release rate of Thi from the nano-delivery sample. The liquid-phase detection conditions included a mobile phase of methanol:water (volume ratio 40:60), injection volume of 10 μL, wavelength of 240 nm, and retention time of 4 min.

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