Mesoporous silica nanoparticles with thiol groups were prepared by a surfactant-assembly sol-gel process in a solution containing deionized water (DI), tetraethylethylenediamine (THEEDA), n-cetyltrimethylammonium bromide (CTAB), te- traethylorthosilicate (TEOS), and (3-Mercaptopropyl) trimethoxysilane (MPTMS). The typical synthetic procedure is as follows: CTAB was dissolved in a mixed solution containing deionized water and THEEDA at 60˚C, and a mixture of TEOS and MPTMS was immediately added under vigorous stirring. The molar ratio of the reaction mixture was 1TEOS:0.025THEEDA:0.03MPTMS: 0.1CTAB: 206DI. After being stirred at 60˚C for 2.5 hours, the solution was allowed to stand at room temperature for 4 hours. The surfactant templates were removed by extraction in acidic ethanol rather than by calcination. Then samples were collected by centrifugation, washed, and redispersed with deionized water and ethanol several times. After being ground carefully, the sample was finally weighed and dissolved in ethanol for ultrasonic dispersion for 60 minutes [19]. The sample was named MSS-x (MSS stands for mesoporous silica spheres, x is the amount of MPTMS).

X-ray Diffractometer was used to get the XRD patterns of the prepared sulfhydryl functionalized mesoporous silica nanoparticles. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) analysis using the automatic specific surface area and pore analyzer. The pore size distribution and pore volume were obtained by the Barrett-Joyner-Halenda (BJH) method. Scanning electron microscope (SEM) images were taken by Gemini SEM 500 with an ultra- high-resolution, and transmission electron microscope (TEM) images were obtained using a transmission electron microscopy TECNAI G2 20 TEM.

Prepared MSS-x nanoparticles were transferred onto a silicon wafer with a thin gold film on the top surface. Then the wafer with a sample was put into CdCl₂ solution for 2 hours at different temperatures. The mass proportion of Cd²⁺ adsorbed by the sample of MSNPs was analyzed using an Energy Disperse Spectroscopy (EDS). Prepare Cd²⁺ ion solutions with different concentrations of CdCl₂, and repeat the adsorption test. Repeat the adsorption experiments at 60˚C.

Prepare the mixed solution saturated with Cd²⁺, Cr²⁺, and Pb²⁺ ions, repeat the adsorption test to investigate the selectivity of MSNPs.

Figure 1 shows the BET curves of the prepared sulfhydryl functionalized mesoporous silica nanoparticles. The samples were named MSS-30^a, MSS-30^b, and MSS-60, respectively. MSS-30^b is the sample prepared by the procedure described in section 2.1 with 30 μL of MPTMS, and MSS-60 was synthesized by this method but with 60 μL of MPTMS. In comparison, MSS-30^a was synthesized by the method described in reference [13] with the drug ratio of 1.0SiO₂: 0.06CTAB: 0.026THEEDA: 0.03MPTMS: 83H₂O, which means that the added MPTMS is 30 µL, CTAB is 0.1464 g, and TEOS is 1.5 mL.

Based on the BET curves above, the specific surface area, the pore volume, and the pore size of these samples were calculated and listed in Table 1.

The data in Table 1 show that all the sulfhydryl functionalized mesoporous silica samples have high specific surface areas, large pore size, and pore volume, which provide good conditions for the adsorption of heavy metal ions. Among

them, the sample MSS-30^b has the highest surface area, pore volume, and large pore size. It was prepared by the method proposed in this paper in which the molar ratio of synthetic substances and the reaction conditions of the experiment were optimized. However, as the degree of modification increased by increasing the amount of MPTMS to 60 μL, the specific surface area of the mesoporous silica nanoparticles did not increase further but decreased, as displayed in Table 1 for the sample MSS-60. The reason for this might be that the grafted sulfhydryl functional groups blocked the pores too much.

An XRD pattern for the sample MSS-30^b is shown in Figure 2. In this figure, there is a diffraction peak at (100), but the diffraction peaks (110) and (200) of MCM-4 almost disappear. The disappearance of these two peaks might be due to the pores filled with the sulfhydryl groups. From the TEM image shown in Figure 3, it can be seen clearly that the sample MSS-30^b has a honeycomb-shaped pore structure. Based on Figure 2 and Figure 3, the sample MSS-30^b can be attributed to a hexagonal structure.

Figure 4 is the SEM image of the sample MSS-30^b. As seen in the figure, the size of the mesoporous silica nanoparticles is from 20 nm to 30 nm, and the particles show good uniformity. This uniformity makes them have a better adsorption capacity for heavy metal ions in water.

Figure 5 shows the adsorption of Cd²⁺ by the sample MSS-30^b on a gold-silicon wafer in different concentrations of CdCl₂ solutions at 30˚C and 60˚C, respectively. The data in the figure is the average value of the EDS analysis values of 50 microzones. It can be seen that with the increase of concentration of CdCl₂ in the solution, the adsorption content of Cd²⁺ in the sample MSS-30^b also increases. When the temperature increases from 30˚C to 60˚C, the adsorption rises slightly. The temperature dependence of the adsorption/desorption needs to be investigated further. The slope of the fitted line shows a strong adsorption capacity of the prepared sulfhydryl functionalized mesoporous silica nanoparticles. This figure demonstrates that MSS-30^b has an excellent adsorption capacity in CdCl₂ solution with a concentration of 1.35 μg/L.

Figure 6 is the result of the adsorption of heavy metal ions on the surface of the MSS-30^b by EDS analysis after the sample was put into the mixed solution with Pb²⁺, Cr²⁺, and Cd²⁺ for two hours at room temperature. As the data in the

picture illustrated, the mass proportion of Cd²⁺ is 28.8%, which is the highest value among the mass portion of Pb²⁺ 22.1% and Cr²⁺ 18.2%. The experimental data reveal that the mercapto-functionalized mesoporous silicon MSS-30^b prepared in this paper has a good selectivity of Cd²⁺ in various heavy metal hybrid solutions due to the content of adsorbed Cd²⁺ being more prominent than other heavy metal ions.