Abstract-Spectral Characterization and Molecular Dynamics Simulation of Pesticides Based on Terahertz Time-Domain Spectra Analyses and Density Functional Theory (DFT) Calculations

Fangfang Qu,  Lei Lin, Yong He,  Pengcheng Nie, Chengyong Cai, Tao Dong,  Yi Pan,  Yu Tang,  Shaoming Luo

http://www.mdpi.com/1420-3049/23/7/1607

This work provides the experimental and theoretical fundamentals for detecting the molecular fingerprints of six kinds of pesticides by using terahertz (THz) time-domain spectroscopy (THz-TDS). The spectra of absorption coefficient and refractive index of the pesticides, chlorpyrifos, fipronil, carbofuran, dimethoate, methomyl, and thidiazuron are obtained in frequencies of 0.1–3.5 THz. To accurately describe the THz spectral characteristics of pesticides, the wavelet threshold de-noising (WTD) method with db 5 wavelet fucntion, 5-layer decomposition, and soft-threshold de-noising was used to eliminate the spectral noise. The spectral baseline correction (SBC) method based on asymmetric least squares smoothing was used to remove the baseline drift. Spectral results show that chlorpyrifo had three characteristic absorption peaks at 1.47, 1.93, and 2.73 THz. Fipronil showed three peaks at 0.76, 1.23, and 2.31 THz. Carbofuran showed two peaks at 2.72 and 3.06 THz. Dimethoate showed three peaks at 1.05, 1.89, and 2.92 THz. Methomyl showed five peaks at 1.01, 1.65, 1.91, 2.72, and 3.20 THz. Thidiazuron showed four peaks at 0.99, 1.57, 2.17, and 2.66 THz. The density functional theory (DFT) of B3LYP/6-31G+(d,p) was applied to simulate the molecular dynamics for peak analyzing of the pesticides based on isolated molecules. The theoretical spectra are in good agreement with the experimental spectra processed by WTD + SBC, which implies the validity of WTD + SBC spectral processing methods and the accuracy of DFT spectral peak analysis. These results support that the combination of THz-TDS and DFT is an effective tool for pesticide fingerprint analysis and the molecular dynamics simulations.

Abstract-Terahertz Multivariate Spectral Analysis and Molecular Dynamics Simulations of Three Pyrethroid Pesticides

Fangfang Qu, Lei Lin,Yong He,Pengcheng Nie,Chengyong Cai,TaoDong, Yi Pan,Yu Tang,Shaoming Luo

https://link.springer.com/article/10.1007%2Fs10762-018-0519-x

The terahertz (THz) multivariate spectral characteristics and the molecular dynamics of three pyrethroid pesticides, including deltamethrin, fenvalerate, and beta-cypermethrin, were studied in this paper. THz spectra of the pesticides were measured in frequency range of 0.06–3.5 THz by using THz time-domain spectroscopy (THz-TDS). To improve the THz spectral quality, the wavelet threshold de-noising (WTD) method was used to remove spectral noise and the spectral baseline correction (SBC) method was used to remove baseline drift. Specific absorption peaks were observed in the processed THz spectra of the three pesticides. Deltamethrin showed three peaks at 0.90, 1.49, and 2.32 THz. Fenvalerate had five peaks at 1.13, 1.43, 1.61, 1.98, and 2.58 THz. Beta cypermethrin had four peaks at 1.27, 1.84, 2.12, and 2.92 THz. The density functional theory (DFT) was used to characterize the molecular dynamics and formation mechanism of the absorption peaks. Results showed that there was a good matching effect between the THz experimental spectra and the DFT quantum calculation spectra. Based on the characterized fingerprint absorption peaks, the linear addition model was used to simulate the THz spectra of mixed pesticides. The simulated spectra of multicomponent pesticides were demonstrated to be in good agreement with those obtained by THz-TDS. By analyzing the absorption peaks of THz spectra, the composition and concentration of multicomponent pesticides could be determined. The proposed strategy presented an analytical methodology for studying the THz spectral characteristics of pesticides. In addition, this work provided experimental and theoretical basis for the detection potential of pesticides in agricultural products based on THz technology.

Abstract-Experimental and Theoretical Study on Terahertz Absorption Characteristics and Spectral De-noising of Three Plant Growth Regulators

Fangfang Qu, 

Yi Pan, 

Lei Lin, 

Chengyong Cai, 

Tao Dong, 

Yong He, 

Pengcheng Nie, 

https://link.springer.com/article/10.1007%2Fs10762-018-0507-1
Accurate identification of terahertz (THz) absorption peaks of biological macromolecules is of great significance in practical applications. In this work, the experimental and theoretical fundamentals of detecting three plant growth regulators (PGRs), including 6-Benzylaminopurine (6-BA), paclobutrazol (PBZ), and maleic hydrazide (MH) were investigated by using THz time-domain spectroscopy (THz-TDS). THz absorption coefficient and refractive index in frequencies of 0.06–4 THz were obtained. The wavelet threshold de-noising (WTD) method was used to remove spectral noise and improve the signal-to-noise ratio (SNR). The density functional theory (DFT) was applied to the molecular characterization and theoretical calculation of PGRs. Experimental results showed that the three PGRs had unique characteristic absorption peaks. Based on the sym4 wavelet function and four-layer wavelet decomposition, the de-noising performance of hard threshold WTD was better than that of soft threshold WTD. The spectra processed by hard threshold de-noising achieved higher peak SNR (6-BA: 40.22, PBZ: 37.73, MH: 34.83) and lower root mean square error (6-BA: 0.41, PBZ:0.40, MH:0.54). In addition, the characteristic absorption and anomalous dispersion of 6-BA were found at 2.08 and 3.00 THz, those of PBZ were shown at 0.71, 1.30, 1.88, and 2.67 THz, and those of MH were shown at 2.34 THz. The absorption peaks in THz spectra processed by hard threshold WTD were demonstrated to be in good agreement with the simulation results of DFT. These results show the effectiveness of WTD in THz spectral de-noising and the feasibility of using THz-TDS to detect PGRs.

Abstract-nTerahertz waveguiding between parallel dielectric films

nTerahertz waveguiding between parallel dielectric films Yi Pan and Steven R. Andrews  »View Author Affiliations

Optics Express, Vol. 23, Issue 1, pp. 274-283 (2015)

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-1-274

View Full Text Article

Enhanced HTML    Acrobat PDF (2789 KB)

We report a study of terahertz waveguiding between parallel dielectric films, a system that can be viewed as a planar analogue of hollow core fibres that exploit anti-resonant reflection optical waveguiding (ARROW). With the aid of time domain waveguide mode imaging, the frequency dependent transition from ARROW to total internal reflection guiding in the individual films as the film separation is reduced and the effect on the transmission of adding variably spaced cladding layers are clearly revealed. Good agreement for the transmission, dispersion and loss is obtained with simple analytical models for film separations greater than about five wavelengths suggesting that the same models could be usefully used to predict the behavior in the case of the technologically more important cylindrical geometry.

© 2015 Optical Society of America