Raman and SERS spectra of thiamethoxam and the Ag₃–thiamethoxam complex: an experimental and theoretical investigation

Abstract

The insecticide thiamethoxam (TMX) is one of the most important neonicotinoid pesticides. The chromatographic methods currently employed to detect TMX require multiple operational steps. This study proposes a simple method that detects TMX via surface-enhanced Raman scattering (SERS) spectroscopy with Ag nanoparticles (NPs) as the SERS active substrate. Density functional theory (DFT) was used to calculate the structures and vibrational modes of the Ag– and Ag₃–TMX complexes at the B3LYP/6-311++G(d,p)(C,H,N)/LanL2DZ(Ag) level of theory. The results reveal that the atoms in the thiazole ring all lie in the same plane, while the six-membered ring is perpendicular to the thiazole ring. Data from both Ultraviolet-visible and Raman spectroscopy indicated that TMX bonds to Ag through its nitro group, vertically. A weak intramolecular (N22–O23…H26) hydrogen bonding and Ag–O bands shift N–O symmetrical vibration to down to lower wavenumber. This was supported by the appearance of a strong 866 cm⁻¹ band in the SERS spectrum assigned to the N–O symmetrical vibration coupled with the N–N stretching vibrational mode of different excitation wavelength. Notably, a good linear relationship was observed in the TMX concentration range 1.0 × 10⁻⁶–1.0 × 10⁻⁴mol·L⁻¹ (R² = 0.9667). SERS is an extremely simple and rapid technique that requires little sample for analysis.

Keywords:

• Thiamethoxam
• SERS
• Raman spectroscopy
• electromagnetic enhancement effect
• neonicotinoid

Acknowledgments

We are grateful to Prof. Dr. Bing Zhao research team for assistance in density functional theory calculation and Raman detection.

Disclosure statement

The authors declare no conflict of interest.

Funding

The research was supported by the National Key R&D Program of China (No. 2018YFD0201004), China Postdoctoral Science Foundation (Grant No. 2018M631876), the Initial Postdoctoral Scientific Research Fund of Jilin University (Grant No. 801181010412), Modern Agricultural Industry Technology System (Grant No. CARS-04), and the Open Research Fund of National Research Center of Engineering and Technology of Tea Quality and Safety (Grant No. 2017NTQS0201).

Table A4 Calculated Raman and observed SERS vibration modes of TMX in the frequencies 400–1800 cm⁻¹

Additional information

Notes on contributors

Yu Gao

ML Xu has designed the study and Y Gao analyzed the data. Y Gao and ML Xu prepared the manuscript, whereas Y Gao, and JF Xiong conducted the experiments. All authors read and approved the final manuscript.

Meng-Lei Xu

ML Xu has designed the study and Y Gao analyzed the data. Y Gao and ML Xu prepared the manuscript, whereas Y Gao, and JF Xiong conducted the experiments. All authors read and approved the final manuscript.

Jinfeng Xiong

ML Xu has designed the study and Y Gao analyzed the data. Y Gao and ML Xu prepared the manuscript, whereas Y Gao, and JF Xiong conducted the experiments. All authors read and approved the final manuscript.