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

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.     

基于太赫兹时域光谱技术的农药残留检测方法

太赫兹时域光谱(THz-TDS)技术是近年来涌现出来的崭新的光谱测量新技术.本研究提出了一种基于THz-TDS技术农药残留检测方法,并以灭多威和乙氧氟草醚两种农药作为实验介质证明此方法的可行性.应用太赫兹时域光谱系统测得了这两种农药的时域光谱信号,利用基于菲涅尔公式的数据处理模型得到了它们在THz波段的折射率谱和吸收系数谱.从实验结果可以看出两种农药在0.2~2.0THz范围内存在明显的特征吸收峰,且差别很大.经分析认为这些吸收峰是由分子的集体振动模式以及分子间相互作用引起,它们是农药分子的指纹吸收光谱,可以应用于分子识别中.本研究证明了THz-TDS技术应用于农药残留检测的可行性,表明其在农药残留检测中具有潜在的应用价值.     

鞣花酸的太赫兹光谱研究

鞣花酸是一种存在于多种植物中的天然多酚组分,近年来越来越多的实验证实鞣花酸在抑制癌细胞增长和诱导癌细胞凋亡方面有很好的效果,同时也具有抗氧化和抗辐射等作用,在抗癌药品及保健药品中占有重要地位。因此对鞣花酸的研究有着重要的意义。太赫兹时域光谱技术（THz－TDS）是一种新型的时间分辨相干探测技术,本文利用太赫兹时域光谱技术研究了鞣花酸在0.2－2.4THz波段的光谱特性,得到了鞣花酸的吸收谱和折射率谱,并利用密度泛函理论(DFT)进行了模拟计算。研究发现,鞣花酸在该波段有光谱响应,分别在0.44、0.73、0.99、1.26、1.58、1.87和2.19THz频率处出现了反常色散现象和吸收峰。可以作为检测鞣花酸存在的定量分析依据,为药品及食品检测提供指导。     

Optical properties of selected Chinese Traditional Patent Medicines and Western Medicines terahertz range

The spectral characteristics of Sang Ju Gan Mao Granules, Gan Mao Jie Du Tablet, Fu Fang Jin Yin Hua Granules and Compound Paracetamol, and Amantadine Hydrochloride Tablets are studied by terahertz time-domain spectroscopy (THz-TDS) technologies in the spectral range of 0.2—1.7 THz. The absorption spectra and the indexes of refraction are obtained. It can be seen that the three samples of Chinese Traditional Patent Medicines have almost the same absorption peaks at 1.44 THz. However, the absorption peaks of the three western medicine samples are at 0.73, 1.01, and 1.18 THz, respectively. The large spectral differences between different drugs are strong evidences that THz time-domain spectroscopy is a useful fingerprint technique in the study of pharmaceutical compounds and the crack down on the fake medicines.     

盐酸吗啡的太赫兹光谱和理论仿真分析

采用太赫兹时域光谱(THz-TDS)技术测量了盐酸吗啡原料药、4种辅料、中间体、成药在0.2～2 THz波段的光谱.盐酸吗啡原料药特征吸收峰为0.75 THz,1.17 THz,1.76 THz,原料药到成药的THz光谱发生了改变,中间体和成药的吸收谱有非常高的一致性,说明包衣对成药的THz光谱没有显著影响.采用密度泛...     

Detection of Methomyl,a Carbamate Insecticide,in Food Matrices Using Terahertz Time-Domain Spectroscopy

The aim of this study was to investigate the feasibility of detecting methomyl, a carbamate insecticide, in food matrices (wheat and rice flours) using terahertz time-domain spectroscopy (THz-TDS). In the frequency range 0.1–3 THz, the characteristic THz absorption peaks of methomyl at room temperature were detected at 1 (33.4 cm^?1), 1.64 (54.7 cm^?1), and 1.89 (63.0 cm^?1)?THz. For detailed spectral analysis, the vibrational frequency and intensity of methomyl were calculated using solid-state density functional theory to mimic molecular interactions in the solid state. Qualitatively, the simulated spectrum was in good agreement with the experimental spectrum. Analysis of the individual absorption modes revealed that all of the features in the THz spectrum of methomyl were mainly generated from intermolecular vibrations. The peak appearing at 1 THz (33.4 cm^?1) was then selected and tested for its suitability as a fingerprint for detecting methomyl in food matrices. Its absorbance was dose-dependently distinguishable from that of wheat and rice flours. The calibration curve of methomyl had a regression coefficient of >0.974 and a detection limit of <3.74 %. Accuracy and precision expressed as recovery and relative standard deviation in interday repeatability were in the ranges 78.0–96.5 and 2.83–4.98 %, respectively. Our results suggest that THz-TDS can be used for the rapid detection of methomyl in foods, but its sensitivity needs to be improved.     

磺苄西林、舒他西林、美洛西林、替卡西林的太赫兹指纹谱

以一类常用的抗生素-青霉素类抗生素作为研究对象,选取4 种具有代表性的药品磺苄西林、舒他西林、美洛西林、替卡西林,基于太赫兹时域光谱（THz-TDS）技术,进行实验研究。通过光谱实验及理论分析,获取药品的太赫兹时域光谱,结合傅里叶变换,获得频域光谱及太赫兹吸收系数曲线。结果表明,4 种药品在0.40~1.60 THz 波段存在明显不同的吸收特征。因此,太赫兹光谱技术十分适合检测抗生素这种化学结构有微小不同的药品,并且可以清晰通过吸收峰的位置分辨出抗生素药类的种类。为国家食品药品监督管理提供一种新的可靠的检测技术,且可以以数据库的形式为药品的鉴定提供标准。     

Index of Refraction and Absorption Coefficient Spectra of Paratellurite in the Terahertz Region

Index of refraction and absorption coefficient spectra of pure paratellurite (α-TeO₂) crystal as a potential material for terahertz (THz) applications were determined in the 0.25–2 THz frequency range at room temperature by THz time domain spectroscopy (THz-TDS). The investigation was performed with beam polarization both parallel (extraordinary polarization) and perpendicular (ordinary polarization) to the optical axis [001] of the crystal. Similarly to the visible spectral range, positive birefringence was observed in the THz range as well. It was shown that the values of the refractive index for extraordinary polarization are higher and show significantly larger dispersion than for the ordinary one. The absorption coefficient values are also larger for extraordinary polarization. The measured values were fitted by theoretical curves derived from the complex dielectric function containing independent terms of Lorentz oscillators due to phonon-polariton resonances. The results are compared with earlier publications, and the observed significant discrepancies are discussed.     

Temperature Dependence of Crystal Structure and THz Absorption Spectra of Organic Nonlinear Optical Stilbazolium Material for High-Output THz-Wave Generation

A stilbazolium material comprising 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), which has a large nonlinear optical susceptibility, was studied for application in terahertz (THz)-wave generation. The temperature-dependent structure of the DAST crystal was measured by using powder X-ray diffraction from ?100 to 200 °C, indicating a volume expansion of 4.6 %. The lattice constants show anisotropic thermal expansion. Also, the temperature dependence of THz absorption spectra was measured by terahertz time-domain spectroscopy (THz-TDS) in the temperature range varying from ?80 to 88.1 °C. A strong absorption peak was found at around 1 THz, shifting slightly toward a lower frequency with increasing temperature. The temperature dependence of the THz spectra was compared with that of X-ray diffraction. The shifting of THz-vibrational frequencies of the DAST crystal suggests that the change in its lattice structure is temperature dependent.     

Characterization and Classification of Coals and Rocks Using Terahertz Time-Domain Spectroscopy

Being the key unaddressed problem in unmanned mining condition, a new method for the coal-rock interface recognition was proposed in the study. Firstly, terahertz time-domain spectroscopy (THz-TDS) was employed to measure 10 kinds of coals/rocks which were common in China. Secondly, the physical properties of coals/rocks such as absorption coefficient spectra, refractive index, and dielectric properties in THz band were studied. The different responses in THz range caused by diverse components in coals/rocks were discussed, and the dielectric property of coals/rocks in THz band was well fitted by the Lorentz model. Finally, by the means of principal component analysis (PCA), support vector machine (SVM), and THz spectral data, the recognition rate of coals/rocks reaches to 100 % and the recognition rate of different bituminous coals reaches to 97.5 %. The experimental results show that the proposed method is fast, stable, and accurate for the detection of coal-rock interface and could be a promising tool for the classification of different bituminous coals.     

太赫兹技术对二氯苯胺两种同分异构体的检测

采用太赫兹时域光谱技术测量了3,4-二氯苯胺和2,5-二氯苯胺在0.2~2 THz波段的吸收谱,3,4-二氯苯胺在1.00 THz,1.48 THz,1.88 THz处有3个吸收峰,2,5-二氯苯胺在0.83 THz,1.04 THz,1.17 THz处有3个吸收峰,吸收峰峰位强度也有明显不同。采用密度泛函理论进行理论光谱计算,理论光谱与实测光谱一致性良好。结果表明通过太赫兹时域光谱技术可以实现3,4-二氯苯胺和2,5-二氯苯胺2种同分异构体的指纹检测,为太赫兹波段有明显特征吸收峰的毒性物质检测提供了一种快速、准确的方法。     

Sharp Absorption Peaks in THz Spectra Valuable for Crystal Quality Evaluation of Middle Molecular Weight Pharmaceuticals

Middle molecular weight (MMW) pharmaceuticals (MW 400~4000) are attracting attention for their possible use in new medications. Sharp absorption peaks were observed in MMW pharmaceuticals at low temperatures by measuring with a high-resolution terahertz (THz) spectrometer. As examples, high-resolution THz spectra for amoxicillin trihydrate, atorvastatin calcium trihydrate, probucol, and α,β,γ,δ-tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluenesulfonate (TMPyP) were obtained at 10 K. Typically observed as peaks with full width at half-height (FWHM) values as low as 5.639 GHz at 0.96492 THz in amoxicillin trihydrate and 8.857 GHz at 1.07974 THz for probucol, many sharp peaks of MMW pharmaceuticals could be observed. Such narrow absorption peaks enable evaluation of the crystal quality of MMW pharmaceuticals and afford sensitive detection of impurities.     

In-Situ Measurement of Vitamin C Content in Commercial Tablet Products by Terahertz Time-Domain

Terahertz time-domain spectroscopy (THz-TDS) was applied to investigate the feasibility of in-situ measuring vitamin C content in commercial tablet products without any pretreatments. Characteristic absorption peaks of vitamin C were analyzed with quantum mechanical calculation to reveal the molecular origin of them. The peak appearing at 1.08 THz was then selected and tested for its suitability as a fingerprint signal for analyzing the vitamin C content in dietary supplement tablets. There are a couple of factors influencing THz absorbance other than concentration. Among those, the effects of tablet thickness and types of excipients in the tablet products were found to be significant, and were corrected with the calibration curve to determine vitamin C concentration in tablet forms. Furthermore, commercial tablet products in the market were analyzed using THz-TDS and the measured vitamin C contents were in good agreement with those determined using a reference method (high-performance liquid chromatography). Thus, our results suggest that THz-TDS can be used for the in-situ analysis of vitamin C in commercial tablet products.     

THz Spectroscopic Identification of Red Mineral Pigments in Ancient Chinese Artworks

Nondestructive analysis of historical objects is of significance for cultural heritage conservation. In this paper, terahertz time-domain spectroscopy (THz-TDS) was used to distinguish seven red mineral pigments used in ancient Chinese artworks. Two absorption features of natural minerals HgS and four highly resolved spectral features of mineral pigment Pb₃O₄ were observed and identified as their fingerprints in the range 0.2 to 3.0 THz, based on which the spatial distribution of individual chemical substances including cinnabar, vermilion, and red lead were clearly revealed at various frequencies using terahertz spectroscopy imaging. Moreover, a noncontact evaluation of thickness changing and dehydration of a wet painting was monitored by inferring time delay as well as signal amplitude of THz pulses transmitted through the painting. In order to demonstrate the feasibility of THz-TDS and THz imaging for authentic artworks detection, a complete set of THz analysis of two nineteenth century wall paintings discovered in the Fuchen Temple of the Forbidden City, Beijing, was performed and the results indicate that THz measurement techniques provide a noninvasive and nondestructive solution for the care, preservation, and restoration of cultural relics.     

FOX-7太赫兹波吸收特性及理论计算

为了研究新型高能钝感材料1,1-二氨基-2,2-二硝基乙烯(FOX-7)的分子结构特性,采用太赫兹时域光谱技术对FOX-7在0.2~2.3 THz波谱范围内的吸收光谱进行了探测,得到了待测样品的太赫兹吸收谱线,确定了其特征吸收峰的位置.基于密度泛函理论(DFT)对FOX-7单分子和晶体结构在小于2.4 THz范围内的吸收光谱进行了模拟计算,结果表明分子间相互作用对FOX-7吸收峰的形成起到很大的作用.完成了对实验光谱中特征吸收峰振动模式的分析和指认:1.59 THz处吸收峰的光谱特征主要由―NO2和―NH2的摆动造成,2.12 THz 处吸收峰光谱特征的产生包含―NO2和―NH2的摆动以及各自的扭动.     

利用误差逆传播神经网络法识别几种毒品的太赫兹光谱

在采用太赫兹时域光谱(THz-TDS)技术对9种常见毒品进行实验研究并得到它们在0.2～2.6THz频率范围的特征吸收谱的基础上,用误差逆传播(BP)神经网络法对9种常见毒品的太赫兹吸收光谱进行了训练及识别。首先,用9种毒品的太赫兹吸收谱训练已经建立的误差逆传播神经网络;然后,选用与训练光谱不同时间测得的9种毒品的太赫兹吸收光谱作为检测光谱,经过二阶导数预处理之后分别输入到训练好的误差逆传播神经网络中进行识别,识别率达到89%。该误差逆传播神经网络模型采用MATLAB语言编制程序。识别结果充分表明,用误差逆传播神经网络可以实现对不同种类毒品的识别和鉴定,为太赫兹光谱技术用于毒品的检测和识别提供了一种有效的方法。     

用两种不同测量技术对毒品THz光谱的研究

为了使太赫兹(THz)技术在毒品检测中变得更为实用,运用太赫兹时域光谱(THz-TDS)技术和傅里叶变换红外光谱(FTIR)技术分别研究了甲基苯丙胺(MA)、4,5-亚甲基二氧基苯丙胺(MDA)和3,4-亚甲基二氧基-N-甲氧基苯丙胺(MDMA)3种走私毒品的THz吸收光谱,获得了这些毒品不同谱宽的THz光谱,并对两类...     

维甲酸和叶酸的太赫兹光谱

利用太赫兹时域光谱技术（THz-TDS）在室温下对两种抗肿瘤维生素类药物维甲酸和叶酸在0.2～1.8 THz范围内进行了光谱测量,得到对应的吸收谱和折射率谱,发现叶酸和维甲酸在太赫兹波段都有明显的特征吸收峰,可作为其在太赫兹波段的指纹谱用于药物分子识别。同时,采用Gaussian03软件进行了单分子的密度泛函理论模拟,模拟中采用B3LYP结合高斯型基组6～31 G（d,p）,理论计算与实验结果基本吻合。另外,采用Gaussian-View3.0对叶酸和维甲酸在THz波段的特征吸收峰进行了指认。结果证明:利用太赫兹光谱技术结合密度泛函理论模拟对两种抗肿瘤维生素类药物分子识别及其新药研制具有一定的参考意义。     

基于太赫兹辐射的甘草主成分光谱分析

采用透射式太赫兹时域光谱系统测试了甘草主成分甘草酸、甘草次酸以及甘草苷的太赫兹光谱,发现甘草酸、甘草次酸以及甘草苷在0.3~1.72 THz频段内具有明显的吸收特征,此频段内它们的太赫兹吸收峰峰位接近、吸收谱谱线相似。利用量子化学方法模拟甘草酸的太赫兹吸收谱,并与实验谱进行对比指认完成对3种单质的定性分析工作。本文分别采用基于DFT和PM3模型,完成对甘草酸单分子构型的结构优化与频率计算。结果表明,两种方法得到的太赫兹模拟吸收峰与实验吸收峰基本吻合,而且基于DFT模型得到的太赫兹模拟吸收谱波形与实验谱更为接近。最后选取了甘草酸的特征吸收峰1.655 THz及其附近6个数值点的太赫兹吸收系数,将其取平均值后与浓度进行了一元线性回归拟合,拟合结果从理论上验证了甘草酸太赫兹吸收谱符合朗伯比尔定律。     

二硝基苯甲酸同分异构体的太赫兹与红外光谱特性

利用太赫兹时域光谱系统(THz-TDS)和傅里叶变换红外光谱仪(FTIR)分别研究了2,4-, 2,5-, 3,4-, 3,5-二硝基苯甲酸的吸收谱.实验结果表明, 4种同分异构体的吸收光谱在红外波段(1 400~1 800 cm-1)表现出相似性, 而在太赫兹波段(0.3~2.2 THz)却存在非常明显的区别.利用密度泛函理论(DFT)对4种物质的吸收频谱进行计算, 并根据计算结果对吸收光谱的相似性和差异性进行解释.太赫兹时域光谱技术为鉴别物质的同分异构体提供了一种可行的手段.