Nitrate is an ionic nutrient that can flow into rivers, lakes, and seas through agricultural fertilizers, organic nitrogen-containing soils, industrial and domestic wastewater, as well as other sources, leading to the eutrophication of water bodies. Therefore, the detection of nitrate content is an important indicator of water quality. The main detection methods for nitrate include ion chromatography and spectrophotometry. Ion chromatography requires professional instruments, resulting in a high cost, whereas spectrophotometry requires pretreatment of the sample, resulting in a complicated and time-consuming process. Surface-enhanced Raman spectroscopy (SERS) has high sensitivity, does not require pretreatment of the sample, and does not experience interference from water. Thus, it is applicable to detect substances in aqueous solution. Presently, nitrate detection in water using SERS has low detection limits, which cannot meet the national environmental quality standards for groundwater. In this study, a composite SERS substrate is reported with positively charged cysteamine modified around negatively charged gold nanoparticles. The reported substrate can directly detect nitrate while reaching the detection standards of Class-I water.
In this paper, Au-cysteamine composite SERS substrates were prepared using a combination of chemical reduction and self-assembly methods. First, gold nanoparticles were prepared through the reduction of chloroauric acid using trisodium citrate. Second, cysteamine, having strong coordination with metals and charged groups, was used to functionally modify the surrounding negatively charged gold nanoparticles to increase the affinity of gold nanoparticles to nitrate anions and improve the signal-detection sensitivity. Finally, the morphology of the cysteamine and AuNPs-cysteamine composite SERS substrates were characterized by scanning electron microscopy (SEM), and the effects of cysteamine stacking density and gold nanoparticle density on the performance of the substrates were investigated to improve the signal sensitivity to nitrate anions.
The AuNPs-cysteamine composite SERS substrate prepared in this paper has high sensitivity to nitrate anions in water and can meet the detection standard for Class-I water in the national groundwater environmental quality standard. SEM showed a coral-like network formed by the cysteamine soaked in deionized water, which had a uniform distribution and was tightly and uniformly bound to the gold nanoparticles (Fig. 2). The self-assembled coverslip with cysteamine was immersed in 20 mL of gold sol, and the bound cysteamine and gold nanoparticles were more uniformly distributed, which produced more hotspots (Fig. 3). The AuNPs-cysteamine composite SERS substrate performed better in experiments when soaked in deionized water than otherwise, owing to the uniform distribution of cysteamine. The AuNPs-cysteamine composite SERS substrate prepared by the same batch of cysteamine modified with 20 mL of gold sol had moderate detection performance owing to the content of gold nanoparticles, whereas the best performance was obtained after binding with cysteamine (Fig. 4). The detection limit of the AuNPs-cysteamine composite SERS substrate prepared by self-assembly using cysteamine soaked in deionized water and 20 mL of gold sol was 0.01 mg/L (Fig. 5) with an enhancement factor of 2.14×105 (Fig. 6), and the relative standard deviation (RSD) of the potassium nitrate signal on eight different AuNPs-cysteamine composite SERS substrates was 10.36% (Fig. 7), which meets the detection standard of Class-I water.
In this paper, an AuNPs-cysteamine composite SERS substrate for the detection of nitrate in water is prepared. To increase the affinity of gold nanoparticles toward nitrate anions, gold nanoparticles are prepared by chemical reduction, and the signal-detection sensitivity is improved by functionalizing the surrounding negatively charged gold nanoparticles with positively charged cysteamine. The experimental results show that the best enhancement performance is obtained when AuNPs-cysteamine composite SERS substrate is prepared by soaking coverslips with 10-3 mol/L cysteamine for 3 h using deionized water and then modifying 20 mL of gold sol, with an enhancement factor of 2.14×105. The RSD of potassium nitrate SERS signal on eight different AuNPs-cysteamine composite SERS substrates was 10.36%; moreover, the detection limit for standard nitrate solution is 0.01 mg/L. Therefore, the development of AuNPs-cysteamine composite SERS substrate has laid the foundation for the detection of nitrate nitrogen in water by Raman spectroscopy and has a great prospect of application.
