Feasibility of Terahertz Time-Domain Spectroscopy to Detect Tetracyclines Hydrochloride in Infant Milk Powder

The Fabrication and Mechanism of a Crystalline Organic Fluorescent Probe Based on Photoinduced Electron Transfer

The response performances of the crystalline organic fluorescence probe are highly dependent on the long-range ordered arrangement of crystalline structure. Herein, a novel organic crystalline fluorescent probe with a high quantum yield was established through the rapid self-assembly of 1,2,4,5-Tetrakis (4-carboxyphenyl) benzene (H4TCPB) and DMF molecules. Each H4TCPB, which connects to four DMF molecules through hydrogen bonds, acts as the structural unit. The building units are packed by π-π, lone pair···π, and lone pair···lone pair interactions to form solid-state crystalline materials. H4TCPB·4DMF exhibits distinct blue fluorescent under UV light, while the quantum yield is as high as 89.02% and the fluorescence lifetime is 1.95 ns. The H4TCPB·4DMF nanocrystal exhibits a specific fluorescence quench sensibility to tetracycline (TC), compared with the common chemicals and ions in environmental water. Moreover, the test results can be obtained quickly and are easily visible to the naked eye. The limit of detection for TC is as low as 12 nM in an aqueous solution. Spectral analysis and density functional theory (DFT) theoretical calculations were used to explain the fluorescence quenching mechanism of H4TCPB·4DMF toward TC, which could be attributed to the photoinduced electron transfer occurring from H4TCPB·4DMF to TC. Our work enriches the database of crystalline luminescent materials and provides theoretical support for the design and mechanical studies of organic fluorescent probes.

Colorimetric Sensing of the Peroxide Number of Milk Powder Using CsPbBr3 Perovskite Nanocrystals

In this study, a wavelength-shift-based colorimetric sensing approach for the peroxide number of milk powder using CsPbBr₃ perovskite nanocrystals (CsPbBr₃ NCs) has been developed. Through the fat extraction, REDOX reactions and halogen exchange, as well as the optimized experimental conditions, a colorimetric sensing method was established to determine the peroxide number of milk powder samples. The integrated process of milk powder fat extraction and the REDOX process greatly shortened the determination time. This colorimetric method has a good linear correlation in the range of the peroxide number from 0.02 to 1.96 mmol/kg, and the detection limit was found to be 3 μmol/kg. This study further deepens the application prospect of wavelength-shift-based colorimetric sensing using CsPbBr₃ NCs.

Terahertz spectroscopy of temperature-induced transformation between glutamic acid, pyroglutamic acid and racemic pyroglutamic acid

Under heating conditions, L-Glutamic acid (L-Glu) can be dehydrated to form L-pyroglutamic acid (L-PGA), and L-PGA can racemize to form DL-PGA. Here, we characterized this transformation at different temperatures and times by terahertz time domain spectroscopy (THz-TDS). By Powder X-ray diffraction (PXRD), the validity of THz spectroscopy is verified. The results prove that the reaction rate of dehydration and racemization is significantly affected by temperature. The THz spectra divided the reactions into three stages. At 150-155 °C, the reaction changes drastically. Furthermore, we found that the absorption intensity at 0.97 and 1.55 THz has a good dependence on the reaction temperature and time, showing a non-linear relationship (R² > 0.98). Our findings suggest that the chemical transformation and reaction rate can be sensitively probed by terahertz spectroscopy, which provides a potential method for the quantitative analysis of reaction products.

Quantitative analysis of FQs antibiotics content in FMF using THz spectral and imaging technology

Rapid and accurate detection of fluoroquinolones (FQs) antibiotics residues in food substrate are of great significance to food safety. In this study, terahertz spectroscopy was employed to conduct the spectral and imaging analysis to explore its feasibility for detection concentrations of FQs in fish meal feeds (FMF). Four methods (frequency of maximum difference in frequency-domain, characteristic absorption peak, successive projections algorithm and stepwise regression) were used to selected characteristic frequencies of sample. Terahertz imaging was formed at selected characteristic frequency and back propagation neural network was used to establish quantitative evaluation models to select optimal characteristic imaging frequency. Finally, relationship between concentrations of FQs and their gray values was revealed, and each data had a small range of error bar. The results showed that terahertz spectral and imaging technique can visualize norfloxacin and enrofloxacin concentrations in FMF precisely. This study explored a brand-new visualization method for quantitative detection of FQs in FMF based on terahertz spectral and imaging technology.

Dual-Channel Probe of Carbon Dots Cooperating with Lanthanide Complex Employed for Simultaneously Distinguishing and Sequentially Detecting Tetracycline and Oxytetracycline

Tetracycline (TC) and oxytetracycline (OTC) are the most widely used broad-spectrum antimicrobial agents in tetracycline drugs, and their structures and properties are very similar, so it is a great challenge to distinguish and detect these two antibiotics with a single probe at the same time. Herein, a dual-channel fluorescence probe (SiCDs@mMIPs-cit-Eu) was developed by integrating two independent reaction sites with SiCDs-doped mesoporous silica molecular imprinting group and europium complex group into a nanomaterial. The synergistic influence of inner filter effect and "antenna effect" can be guaranteed to solve the distinction between TC and OTC. Moreover, this novel strategy can also sequentially detect TC and OTC in buffer solution and real samples with high sensitivity and selectivity. This method revealed good responses to TC and OTC ranging from 0 to 5.5 μM with a detection limit of 5 and 16 nM, respectively. Combined with the smartphone color-scanning application, the portable and cheap paper-based sensor was designed to realize the multi-color visual on-site detection of TC and OTC. In addition, the logic gate device was constructed according to the fluorescence color change of the probe for TC and OTC, which provided the application possibility for the intelligent detection of the probe.

Applications of THz Spectral Imaging in the Detection of Agricultural Products

Agricultural products need to be inspected for quality and safety, and the issue of safety of agricultural products caused by quality is frequently investigated. Safety testing should be carried out before agricultural products are consumed. The existing technologies for inspecting agricultural products are time-consuming and require complex operation, and there is motivation to develop a rapid, safe, and non-destructive inspection technology. In recent years, with the continuous progress of THz technology, THz spectral imaging, with the advantages of its unique characteristics, such as low energies, superior spatial resolution, and high sensitivity to water, has been recognized as an efficient and feasible identification tool, which has been widely used for the qualitative and quantitative analyses of agricultural production. In this paper, the current main performance achievements of the use of THz images are presented. In addition, recent advances in the application of THz spectral imaging technology for inspection of agricultural products are reviewed, including internal component detection, seed classification, pesticide residues detection, and foreign body and packaging inspection. Furthermore, machine learning methods applied in THz spectral imaging are discussed. Finally, the existing problems of THz spectral imaging technology are analyzed, and future research directions for THz spectral imaging technology are proposed. Recent rapid development of THz spectral imaging has demonstrated the advantages of THz radiation and its potential application in agricultural products. The rapid development of THz spectroscopic imaging combined with deep learning can be expected to have great potential for widespread application in the fields of agriculture and food engineering.

Tetracycline antibiotics and NH4+ detection by Zn-organic framework fluorescent probe

A fluorescent probe based on single metal-organic framework material without additional fluorophores and active sites can significantly improve the stability of the probe for detection, and has very important application value in environmental analysis and detection. In this paper, a simple and rapid fluorescence detection method was established with Zn-MOF, which realized the highly sensitive detection of tetracycline antibiotics and NH₄⁺ in water. The prepared Zn-MOF has abundant pores and can exist stably in water. When tetracycline antibiotics are present in Zn-MOF aqueous solution, based on the unique coordination ability between Zn and N, tetracycline antibiotics rich in N will be adsorbed into the pore canals of MOF, and aggregation-induced luminescence will occur. The original non-fluorescent Zn-MOF will immediately produce yellow fluorescence, realizing the detection of tetracycline antibiotics in water, with the limit of detection reaching 0.017 μM in a linear range of 0.02-13 μM. Zn-MOF is further used for the detection of tetracycline antibiotics in actual samples of milk and honey. Oxytetracycline (OTC) with the best fluorescence response of tetracycline antibiotics was coated on Zn-MOF to synthesize OTC@Zn-MOF fluorescent probe. NH₄⁺ will replace the original ligand of Zn-MOF, which will disintegrate MOF and release OTC, resulting in a fluorescence decrease. Therefore, NH₄⁺ can be detected with low limit of detection (0.038 μM) in a linear range of 0 to 3 mM. The probe is expected to be able to detect ammonia in the environment.

Research on Enhanced Detection of Benzoic Acid Additives in Liquid Food Based on Terahertz Metamaterial Devices

It is very important for human health to supervise the use of food additives, because excessive use of food additives will cause harm to the human body, especially lead to organ failures and even cancers. Therefore, it is important to realize high-sensibility detection of benzoic acid, a widely used food additive. Based on the theory of electromagnetism, this research attempts to design a terahertz-enhanced metamaterial resonator, using a metamaterial resonator to achieve enhanced detection of benzoic acid additives by using terahertz technology. The absorption peak of the metamaterial resonator is designed to be 1.95 THz, and the effectiveness of the metamaterial resonator is verified. Firstly, the original THz spectra of benzoic acid aqueous solution samples based on metamaterial are collected. Secondly, smoothing, multivariate scattering correction (MSC), and smoothing combined with first derivative (SG + 1 D) methods are used to preprocess the spectra to study the better spectral pretreatment methods. Then, Uninformative Variable Elimination (UVE) and Competitive Adaptive Reweighted Sampling (CARS) are used to explore the optimal terahertz band selection method. Finally, Partial Least Squares (PLS) and Least square support vector machine (LS-SVM) models are established, respectively, to realize the enhanced detection of benzoic acid additives. The LS-SVM model combined with CARS has the best effect, with the correlation coefficient of prediction set (R_p) is 0.9953, the root mean square error of prediction set (RMSEP) is 7.3 × 10⁻⁶, and the limit of detection (LOD) is 2.3610 × 10⁻⁵ g/mL. The research results lay a foundation for THz spectral analysis of benzoic acid additives, so that THz technology-based detection of benzoic acid additives in food can reach requirements stipulated in the national standard. This research is of great significance for promoting the detection and analysis of trace additives in food, whose results can also serve as a reference to the detection of antibiotic residues, banned additives, and other trace substances.

Low concentration noroxin detection using terahertz spectroscopy combined with metamaterial

The unreasonable use of antibiotics in poultry breeding has led to the frequent occurrence of antibiotic residues in poultry products, affecting food safety and posing a threat to human health. Accurate and rapid detection of antibiotic drug content is therefore of great significance. In this study, a new metamaterial method based on terahertz (THz) spectroscopy for the detection of the quinolone antibacterial drug noroxin in low concentrations was developed and tested. First, a new metamaterial structure was designed using a computational electromagnetics simulation tool with experimental verification. Three-dimensional full-wave electromagnetic field simulations and measured transmission spectra for the metamaterials were observed. Second, the transmission spectra of photoresists with different thicknesses (0-40 µm) on the metamaterials surface were simulated, and the transmission spectra of noroxin thin films of different concentrations on the metamaterial surface were measured. Finally, using a purchased standard sample of noroxin in ethanol (100 µg/mL) as the mother liquor, test samples of different concentrations were prepared, and experiments and multivariate data analysis were carried out. The noroxin detection limit for the method presented in this paper was determined as 0.01 µg/mL. Thus, the new metamaterial method based on terahertz spectroscopy designed in this study was shown to effectively detect low concentrations of noroxin, providing a theoretical and experimental basis for the rapid detection of quinolone antibiotic residues in food matrices.

Attenuated Total Reflection for Terahertz Modulation, Sensing, Spectroscopy and Imaging Applications: A Review

Terahertz (THz) technique has become one of the most promising analytical methods and has been applied in many fields. Attenuated total reflection (ATR) technique applied in THz spectroscopy and imaging has been proven to be superior in functionalities such as modulation, sensing, analyzing, and imaging. Here, we first provide a concise introduction to the principle of ATR, discuss the factors that impact the ATR system, and demonstrate recent advances on THz wave modulation and THz surface plasmon sensing based on the THz-ATR system. Then, applications on THz-ATR spectroscopy and imaging are reviewed. Towards the later part, the advantages and limitations of THz-ATR are summarized, and prospects of modulation, surface plasmon sensing, spectroscopy and imaging are discussed.

Terahertz spectroscopy technology as an innovative technique for food: Current state-of-the-Art research advances

With the dramatic development of source and detector components, terahertz (THz) spectroscopy technology has recently shown a renaissance in various fields such as medical, material, biosensing and pharmaceutical industry. As a rapid and noninvasive technology, it has been extensively exploited to evaluate food quality and ensure food safety. In this review, the principles and processes of THz spectroscopy are first discussed. The current state-of-the-art applications of THz and imaging technologies focused on foodstuffs are then discussed. The advantages and challenges are also covered. This review offers detailed information for recent efforts dedicated to THz for monitoring the quality and safety of various food commodities and the feasibility of its widespread application. THz technology, as an emerging and unique method, is potentially applied for detecting food processing and maintaining quality and safety.

Rapid analysis of a doxycycline hydrochloride solution by metallic mesh device-based reflection terahertz spectroscopy

Terahertz (THz) spectroscopy has the advantages of non-ionization and spectroscopic fingerprint, which can be used for biological and chemical compound analysis. However, because of the strong absorption of water in the THz region, it is still a challenge for THz waves to realize aqueous solution detection. In this study, taking a doxycycline hydrochloride (DCH) aqueous solution as the target, we proposed a THz metallic mesh device (MMD) based reflection platform for the first time for sensing. The angle characteristics of the THz MMD was investigated through numerical simulations and experimental measurements to get an optimized configuration for the platform. When the projection of THz electric field polarization onto the MMD plane gets parallel to latitudinal direction of the MMD apertures, a strong resonant surface mode can be achieved, and our proposed platform can be successfully used to detect the DCH solution with a concentration as low as 1 mg L^-1. The sensing mechanism of our platform was also explored by analyzing the influences of the immersion depth into the MMD holes and the extinction coefficient of droplets on the reflection spectra. Our work presents a rapid, low-cost, and practical platform for antibiotic solution sensing using THz radiation, which opens new avenues for the microanalysis of chemicals or biomolecules in strongly absorptive solutions in the THz region.

Determination of invert syrup adulterated in acacia honey by terahertz spectroscopy with different spectral features

BACKGROUND

Invert syrup is a common adulterant in honey falsification, thus generating risk for consumers. Most of the methods developed are tedious and time-consuming for manufactures and consumers. However, terahertz spectroscopy provides analytical information in a simple, rapid, and environmentally friendly manner. Subsequently, 3 kinds of terahertz spectroscopic characteristics data, the absorption coefficient, the slope of the absorption coefficient spectra, and the area of the absorption coefficient spectra, were employed for determination of acacia honey adulterated with invert syrup.

RESULTS

Single linear regression (SLR) models with different terahertz spectroscopic features were adopted to predict the syrup adulterant proportion in acacia honey. The best SLR model used the area of the absorption coefficient, displaying an adjusted correlation coefficient of 0.985 and a root-mean-square error of 3.201. Meanwhile, multiple linear regression (MLR) models using a successive projections algorithm for variables selection were implemented. The MLR model considered the integral area of the absorption coefficient spectra, as the inputs yielded the best result with less variables selected, higher R c 2 and R p 2 , lower root-mean-square error of calibration and prediction, as well as higher residual predictive deviation.

CONCLUSIONS

The results indicate terahertz spectroscopy combined with the integral area of the absorption coefficient spectra is reliable enough for invert syrup proportion quantification in acacia honey and is also a rapid and nondestructive determination method for other honey adulterants. © 2019 Society of Chemical Industry.

Rapid in situ analysis of l-histidine and α-lactose in dietary supplements by fingerprint peaks using terahertz frequency-domain spectroscopy

A simple, green and nondestructive method based on terahertz fingerprint peaks has been developed for rapid in situ analysis of l-histidine and α-lactose in dietary supplements. Fingerprint absorption peaks of l-histidine and α-lactose located at 0.77 and 0.53 THz could be directly used for identification and quantitation of these analytes in commercial dietary supplements. Compared with the partial least squares regression model (PLSR), the linear least squares regression (LLSR) method based on peak areas presented better performance, with the linear correlation coefficients of 0.9899 and 0.9910 for l-histidine and α-lactose, respectively. Furthermore, analysis time per sample can be shortened to less than 1 min due to the narrower spectral acquisition region. The accuracies were 94.8-110% and 98.9-110%, comparable to those of ion chromatography for l-histidine and high-performance liquid chromatography for α-lactose. The results presented great potential of the developed method for rapid in situ analysis of nutrients in dietary supplements.

Temperature-dependent terahertz vibrational spectra of tetracycline and its degradation products

Terahertz (THz) spectroscopy has emerged as an attractive technique for qualitative and quantitative detection. Analysis of these chemicals in the THz range under various temperatures can yield detailed information on molecular vibrational modes, which is of utmost importance for effective detection. Here we report the use of THz time-domain spectroscopy (THz-TDS) to measure tetracyclines hydrochloride (TCH) and its degradation products including epitetracycline hydrochloride (ETCH), anhydrotetracycline hydrochloride (ATCH), and epianhydrotetracycline hydrochloride (EATCH) over the temperature range of 4.5-300 K for the first time. The results showed that these four tetracyclines exhibited numerous distinct spectral features in frequency-dependent absorption spectra, which demonstrated the qualitative capacity of THz-TDS. Through density functional theory (DFT) calculations and analysis of temperature-dependent absorption spectra, the origin of the observed terahertz absorption peaks of these four tetracyclines were well interpreted. This study could lay the foundation for high-performance analysis of biological and chemical molecules by THz spectroscopy, which is essential for sensing application.

Europium metal-organic framework for selective and sensitive detection of doxycycline based on fluorescence enhancement

Metal-organic framework is an extended coordination compound with repeating coordination entities and voids, which shows potential applications in gas storage, separation, catalysis, and etc. This work reports the synthesis of a new functional metal-organic framework of pyromellitic acid and europium, and its application in fluorescence sensing of doxycycline, a broad-spectrum polyketide antibiotic on the basis of fluorescence turn-on method. The metal-organic framework was initially non-fluorescent, however, with the addition of doxycycline, the system exhibited significant fluorescence enhancement at 526 nm and 617 nm, where both fluorescence intensities showed good linear correlations with the doxycycline concentration, suggesting the possibility for direct two-channel detection without any sensitization materials such as metal particles or surface-modified quantum dots. More interestingly, the unique fluorescence response of the system could discriminate doxycycline from other tetracycline antibiotics with high selectivity, and a limit of detection (LOD) was estimated to be 47 nM. It has been further demonstrated that the material has been successfully applied as sensing materials for selective determination of doxycycline in real samples of fish and urine.

A Comprehensive Review on Food Applications of Terahertz Spectroscopy and Imaging

Food product safety is a public health concern. Most of the food safety analytical and detection methods are expensive, labor intensive, and time consuming. A safe, rapid, reliable, and nondestructive detection method is needed to assure consumers that food products are safe to consume. Terahertz (THz) radiation, which has properties of both microwave and infrared, can penetrate and interact with many commonly used materials. Owing to the technological developments in sources and detectors, THz spectroscopic imaging has transitioned from a laboratory-scale technique into a versatile imaging tool with many practical applications. In recent years, THz imaging has been shown to have great potential as an emerging nondestructive tool for food inspection. THz spectroscopy provides qualitative and quantitative information about food samples. The main applications of THz in food industries include detection of moisture, foreign bodies, inspection, and quality control. Other applications of THz technology in the food industry include detection of harmful compounds, antibiotics, and microorganisms. THz spectroscopy is a great tool for characterization of carbohydrates, amino acids, fatty acids, and vitamins. Despite its potential applications, THz technology has some limitations, such as limited penetration, scattering effect, limited sensitivity, and low limit of detection. THz technology is still expensive, and there is no available THz database library for food compounds. The scanning speed needs to be improved in the future generations of THz systems. Although many technological aspects need to be improved, THz technology has already been established in the food industry as a powerful tool with great detection and quantification ability. This paper reviews various applications of THz spectroscopy and imaging in the food industry.

THz Spectroscopic Investigation of Wheat-Quality by Using Multi-Source Data Fusion

In order to improve the detection accuracy for the quality of wheat, a recognition method for wheat quality using the terahertz (THz) spectrum and multi-source information fusion technology is proposed. Through a combination of the absorption and the refractive index spectra of samples of normal, germinated, moldy, and worm-eaten wheat, support vector machine (SVM) and Dempster-Shafer (DS) evidence theory with different kernel functions were used to establish a classification fusion model for the multiple optical indexes of wheat. The results showed that the recognition rate of the fusion model for wheat samples can be as high as 96%. Furthermore, this approach was compared to the regression model based on single-spectrum analysis. The results indicate that the average recognition rates of fusion models for wheat can reach 90%, and the recognition rate of the SVM radial basis function (SVM-RBF) fusion model can reach 97.5%. The preliminary results indicated that THz-TDS combined with DS evidence theory analysis was suitable for the determination of the wheat quality with better detection accuracy.

Analysis of fluoroquinolones antibiotic residue in feed matrices using terahertz spectroscopy

As antibiotic residue becomes more and more serious all over the world, a rapid and effective detection method is needed to evaluate the antibiotic residue in feed matrices to ensure food safety for consumers. In this study, three different kinds of fluoroquinolones (norfloxacin, enrofloxacin, and ofloxacin) in feed matrices were analyzed using terahertz (THz) spectroscopy, respectively. Meanwhile, pure fluoroquinolones and pure feed matrices were also measured in the same way. Then, the absorption spectra of all of the samples were extracted in the transmission mode. Pure norfloxacin has two absorption peaks at 0.825 and 1.187 THz, and they could still be observed when mixing norfloxacin with feed matrices. Also, there was an obvious and strong absorption peak for ofloxacin at 1.044 THz. However, no obvious absorption peak for enrofloxacin was observed, and only a weak absorption peak was located at 0.8 THz. Then, the different models were established with different chemometrics to identify the fluoroquinolones in feed matrices and determined the fluoroquinolones content in the feed matrices. The least squares support vector machines, Naive Bayes, Mahalanobis distance, and back propagation neural network (BPNN) were used to build the identification model with a Savitzky-Golay filter and standardized normal variate pretreatments. The results show that the excellent classification model was acquired with the BPNN combined with no pretreatment. The optimal classification accuracy was 80.56% in the testing set. After that, multiple linear regression and stepwise regression were used to establish the quantitative detection model for different kinds of fluoroquinolones in feed matrices. The optimal correlation coefficients for norfloxacin, enrofloxacin, and ofloxacin in the prediction set were obtained with multiple linear regression that combined absorption peaks with wavelengths selected by stepwise regression, which were 0.867, 0.828, and 0.964, respectively. Overall, this research explored the potential of identifying the fluoroquinolones in feed matrices using THz spectroscopy without a complex pretreatment process and then quantitatively detecting the fluoroquinolones content in feed matrices. The results demonstrate that THz spectra could be used to identify fluoroquinolones in feed matrices and also detect their content quantitatively, which has great significance for the food safety industry.

Wavelength Selection Method Based on Differential Evolution for Precise Quantitative Analysis Using Terahertz Time-Domain Spectroscopy

Quantitative analysis of component mixtures is an important application of terahertz time-domain spectroscopy (THz-TDS) and has attracted broad interest in recent research. Although the accuracy of quantitative analysis using THz-TDS is affected by a host of factors, wavelength selection from the sample's THz absorption spectrum is the most crucial component. The raw spectrum consists of signals from the sample and scattering and other random disturbances that can critically influence the quantitative accuracy. For precise quantitative analysis using THz-TDS, the signal from the sample needs to be retained while the scattering and other noise sources are eliminated. In this paper, a novel wavelength selection method based on differential evolution (DE) is investigated. By performing quantitative experiments on a series of binary amino acid mixtures using THz-TDS, we demonstrate the efficacy of the DE-based wavelength selection method, which yields an error rate below 5%.

Mechanisms and applications of terahertz metamaterial sensing: a review

Terahertz (THz) technology has attracted great worldwide interest and novel high-intensity THz sources and plasmonics are two of the most active fields of recent research. Being situated between infrared light and microwave radiation, the absorption of THz rays in molecular and biomolecular systems is dominated by the excitation of intramolecular and intermolecular vibrations. This indicates that THz technology is an effective tool for sensing applications. However, the low sensitivity of free-space THz detection limits the sensing applications, which gives a great opportunity to metamaterials. Metamaterials are periodic artificial electromagnetic media structured with a size scale smaller than the wavelength of external stimuli. They present localized electric field enhancement and large values of quality factor (Q factor) and show high sensitivity to minor environment changes. In the present work, the mechanism of THz metamaterial sensing and dry sample and microfluidic sensing applications based on metamaterials are introduced. Moreover, new directions of THz metamaterial sensing advancement and introduction of two-dimensional materials and nanoparticles for future THz applications are summarized and discussed.

Quantitative determination of Auramine O by terahertz spectroscopy with 2DCOS-PLSR model

Residues of harmful dyes such as Auramine O (AO) in herb and food products threaten the health of people. So, fast and sensitive detection techniques of the residues are needed. As a powerful tool for substance detection, terahertz (THz) spectroscopy was used for the quantitative determination of AO by combining with an improved partial least-squares regression (PLSR) model in this paper. Absorbance of herbal samples with different concentrations was obtained by THz-TDS in the band between 0.2THz and 1.6THz. We applied two-dimensional correlation spectroscopy (2DCOS) to improve the PLSR model. This method highlighted the spectral differences of different concentrations, provided a clear criterion of the input interval selection, and improved the accuracy of detection result. The experimental result indicated that the combination of the THz spectroscopy and 2DCOS-PLSR is an excellent quantitative analysis method.

THz spectral data analysis and components unmixing based on non-negative matrix factorization methods

In many situations the THz spectroscopic data observed from complex samples represent the integrated result of several interrelated variables or feature components acting together. The actual information contained in the original data might be overlapping and there is a necessity to investigate various approaches for model reduction and data unmixing. The development and use of low-rank approximate nonnegative matrix factorization (NMF) and smooth constraint NMF (CNMF) algorithms for feature components extraction and identification in the fields of terahertz time domain spectroscopy (THz-TDS) data analysis are presented. The evolution and convergence properties of NMF and CNMF methods based on sparseness, independence and smoothness constraints for the resulting nonnegative matrix factors are discussed. For general NMF, its cost function is nonconvex and the result is usually susceptible to initialization and noise corruption, and may fall into local minima and lead to unstable decomposition. To reduce these drawbacks, smoothness constraint is introduced to enhance the performance of NMF. The proposed algorithms are evaluated by several THz-TDS data decomposition experiments including a binary system and a ternary system simulating some applications such as medicine tablet inspection. Results show that CNMF is more capable of finding optimal solutions and more robust for random initialization in contrast to NMF. The investigated method is promising for THz data resolution contributing to unknown mixture identification.

Investigation of Dielectric Properties of Polymers and their Discrimination Using Terahertz Time-Domain Spectroscopy with Principal Component Analysis

Polymers are among the most commonly used materials in our everyday life. They are generally transparent to terahertz (THz) radiation, but are quite difficult to differentiate using optical techniques as few or no characteristic features exist in the spectral range of <2.0 THz for small and portable radiation systems. In this work, we report experimental measurement of refractive indices and absorption coefficients of styrene acrylonitrile (SAN) and Bakelite in the spectral range of 0.2-2.0 THz for the first time. Additionally, we demonstrate that by combining principle component analysis (PCA) with THz time-domain spectroscopy one can differentiate such polymers. In this analysis, the first three principle components PC1, PC2, and PC3 depict >94% variance with a distribution of 72.45%, 11.52%, and 9.38%, respectively.

Transformation and dehydration kinetics of methylene blue hydrates detected by terahertz time-domain spectroscopy

Three methylene blue crystalline hydrates were identified by terahertz spectroscopy according to their different THz absorption features.