Mesoporous silica nanoparticles (MSNPs), possession of pore diameter in the range of 2 - 50 nm, have unique properties, such as easily tunable particle size, high stability and rigid framework, easily tunable and uniform pore size, high pore volume and greater surface area, bifunctional surfaces, and simple fabrication. MSNPs have attracted considerable attention because of their potential applications in drug release systems, catalysis, chromatography, and adsorption/ desorption of heavy metal ions or dyes in waste water    . The features of greater surface area and bifunctional surfaces permit the functionalization of surfaces of MSNPs with different molecules or functional groups to enhance the selectivity in the application of adsorption/desorption of heavy metal ions . The functionalization can be achieved by the grafting method (post- synthesis modification) or the co-condensation process (one-pot synthesis)    . Yeung  prepared the ordered mesoporous silica adsorbents by grafting amino- and carboxylic-containing functional groups to remove acid blue 25 and Methylene blue dyes from water. Murakami  synthesized amino- functionalized mesoporous silica coated with the temperature-responsive copolymer, poly(N-isopropyl acrylamide-co-acrylamide) (P(NIPAM-co-Am)), and investigated the temperature dependence of the adsorption/desorption of the methyl orange (MO) anions. Chen and He  synthesized functional chiral porous silica nanorods by a one-step process. They showed that the chiral nanorods could be used as a highly selective adsorbent for Hg2+ ions in the wastewater. Li and Mao  synthesized monodispersed mesoporous silica particles and functionalized the MSNPs by co-condensation of tetraethyloxane silane and mercaptopropylsilane. The products showed a good performance in the removal of Au3+ and Hg2+.
So far, only rare reports have been given to show the adsorption of heavy metal ion Cd2+ by mesoporous silica . As we all know, Cd2+ is a highly toxic heavy metal ion, which is bioaccumulative, nondegradable, carcinogenic, and can cause danger to humans even at very low concentrations . Cd2+ can be commonly found in industrial wastewater, including mining, smelting, electroplating, nuclear, and other industries . According to the World Health Organization, the maximum permissible level of cadmium in drinking water is 0.005 mg/L . If ingested beyond the limit, it would lead to carcinogenesis, renal dysfunction, and lung damage in humans . Considering the terrible harm of Cd2+ to human health, the detection and removal of Cd2+ in industrial wastewater are essential.
In this paper, low-cost and high-stability sulfhydryl functionalized mesoporous silica nanoparticles were successfully prepared by an improved soft template method. The functionalization results of mesoporous silica and the ability to absorb heavy metal ions were studied. The test results show that the sulfhydryl functionalized mesoporous silica particles have good selectivity and a low detection limit for Cadmium ions Cd2+.
2. Materials and Methods
2.1. Preparation of Materials
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 . 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.
2.3. Adsorption of Heavy Metal Ions Experiments
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.
3. Results and Discussion
3.1. Nanoparticle Size, Surface Area, and Pore Size
Figure 1 shows the BET curves of the prepared sulfhydryl functionalized mesoporous silica nanoparticles. The samples were named MSS-30a, MSS-30b, and MSS-60, respectively. MSS-30b 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-30a was synthesized by the method described in reference  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
Table 1. Pore structure data of sulfhydryl functionalized mesoporous silicon.
(a) (b) (c)
Figure 1. BET curves of (a) MSS-30a; (b) MSS-30b; (c) MSS-60.
them, the sample MSS-30b 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-30b 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-30b has a honeycomb-shaped pore structure. Based on Figure 2 and Figure 3, the sample MSS-30b can be attributed to a hexagonal structure.
Figure 4 is the SEM image of the sample MSS-30b. 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.
3.2. Adsorption of Heavy Metal Ions
Figure 5 shows the adsorption of Cd2+ by the sample MSS-30b 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-30b 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-30b has an excellent adsorption capacity in CdCl2 solution with a concentration of 1.35 μg/L.
Figure 2. XRD image of sample MSS-30b.
Figure 3. TEM image of sample MSS-30b.
Figure 4. SEM image of sample MSS-30b.
Figure 5. Adsorption of Cd2+ by MSS-30b.
Figure 6 is the result of the adsorption of heavy metal ions on the surface of the MSS-30b 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
Figure 6. EDS analysis of adsorption of heavy metal ions by sample MSS-30b.
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-30b 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.
Sulfhydryl functionalized mesoporous silica nanoparticles have been prepared successfully by an improved simple soft template method. The specific surface area of the MSNPs is as high as 926 m2/g because of the high cavities in the material’s porosity. The mesoporous silica nanoparticles display high adsorption of Cd2+ by complexation between Cd2+ ions and thiol groups. The quantitative determination of Cd2+ was observed by energy dispersive spectroscopy, and the detection accuracy of Cd2+ is estimated to be 1.35 µg/L in the water. The experiment also showed that the prepared mercapto-functionalized mesoporous silica in this paper has a good selectivity of Cd2+ compared with other heavy metals, like Pb2+ and Cr2+. In this study, the utilization of sulfhydryl functionalized mesoporous silica nanoparticles might pave a new way to detect or remove Cd2+ in surface sewage.
This research was supported by the National Natural Science Foundation of China (Grant No. 61771137).
The authors would like to thank Professor Xiao Xie at Southeast University for his suggestion and help take the sample’s TEM images.
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