Study on an artificial phenomenon observed in terahertz biological imaging

Near-Field Terahertz Morphological Reconstruction Nanoscopy for Subsurface Imaging of Protein Layers

Protein layers formed on solid surfaces have important applications in various fields. High-resolution characterization of the morphological structures of protein forms in the process of developing protein layers has significant implications for the control of the layer's quality as well as for the evaluation of the layer's performance. However, it remains challenging to precisely characterize all possible morphological structures of protein in various forms, including individuals, networks, and layers involved in the formation of protein layers with currently available methods. Here, we report a terahertz (THz) morphological reconstruction nanoscopy (THz-MRN), which can reveal the nanoscale three-dimensional structural information on a protein sample from its THz near-field image by exploiting an extended finite dipole model for a thin sample. THz-MRN allows for both surface imaging and subsurface imaging with a vertical resolution of ∼0.5 nm, enabling the characterization of various forms of proteins at the single-molecule level. We demonstrate the imaging and morphological reconstruction of single immunoglobulin G (IgG) molecules, their networks, a monolayer, and a heterogeneous double layer comprising an IgG monolayer and a horseradish peroxidase-conjugated anti-IgG layer. The established THz-MRN presents a useful approach for the label-free and nondestructive study of the formation of protein layers.

Terahertz label-free detection of nicotine-induced neural cell changes and the underlying mechanisms

Nicotine exposure can lead to neurological impairments and brain tumors, and a label-free and nondestructive detection technique is urgently required by the scientific community to assess the effects of nicotine on neural cells. Herein, a terahertz (THz) time-domain attenuated total reflection (TD-ATR) spectroscopy approach is reported, by which the effects of nicotine on normal and cancerous neural cells, i.e., HEB and U87 cells, are successfully investigated in a label/stain-free and nondestructive manner. The obtained THz absorption coefficients of HEB cells exposed to low-dose nicotine and high-dose nicotine are smaller and larger, respectively, than the untreated cells. In contrast, the THz absorption coefficients of U87 cells treated by nicotine are always smaller than the untreated cells. The THz absorption coefficients can be well related to the proliferation properties (cell number and compositional changes) and morphological changes of neural cells, by which different types of neural cells are differentiated and the viabilities of neural cells treated by nicotine are reliably assessed. Collectively, this work sheds new insights on the effects of nicotine on neural cells, and provides a useful tool (THz TD-ATR spectroscopy) for the study of chemical-cell interactions.

Biomedical application of terahertz imaging technology: a narrative review

Background and Objective

Terahertz (THz) imaging has wide applications in biomedical research due to its properties, such as non-ionizing, non-invasive and distinctive spectral fingerprints. Over the past 6 years, the application of THz imaging in tumor tissue has made encouraging progress. However, due to the strong absorption of THz by water, the large size, high cost, and low sensitivity of THz devices, it is still difficult to be widely used in clinical practice. This paper provides ideas for researchers and promotes the development of THz imaging in clinical research.

Methods

The literature search was conducted in the Web of Science and PubMed databases using the keywords "Terahertz imaging", "Breast", "Brain", "Skin" and "Cancer". A total of 94 English language articles from 1 January, 2017 to 30 December, 2022 were reviewed.

Key Content and Findings

In this review, we briefly introduced the recent advances in THz near-field imaging, single-pixel imaging and real-time imaging, the applications of THz imaging for detecting breast, brain and skin tissues in the last 6 years were reviewed, and the advantages and existing challenges were identified. It is necessary to combine machine learning and metamaterials to develop real-time THz devices with small size, low cost and high sensitivity that can be widely used in clinical practice. More powerful THz detectors can be developed by combining graphene, designing structures and other methods to improve the sensitivity of the devices and obtain more accurate information. Establishing a THz database is one of the important methods to improve the repeatability and accuracy of imaging results.

Conclusions

THz technology is an effective method for tumor imaging. We believe that with the joint efforts of researchers and clinicians, accurate, real-time, and safe THz imaging will be widely applied in clinical practice in the future.

A novel frequency-dependent lattice Boltzmann model with a single force term for electromagnetic wave propagation in dispersive media

Electromagnetic wave simulation is of pivotal importance in the design and implementation of photonic nano-structures. In this study, we developed a lattice Boltzmann model with a single extended force term (LBM-SEF) to simulate the propagation of electromagnetic waves in dispersive media. By reconstructing the solution of the macroscopic Maxwell equations using the lattice Boltzmann equation, the final form only involves an equilibrium term and a non-equilibrium force term. The two terms are evaluated using the macroscopic electromagnetic variables and the dispersive effect, respectively. The LBM-SEF scheme is capable of directly tracking the evolution of macroscopic electromagnetic variables, leading to lower virtual memory requirement and facilitating the implementation of physical boundary conditions. The mathematical consistency of the LBM-SEF with the Maxwell equations was validated by using the Champman-Enskog expansion; while three practical models were used to benchmark the numerical accuracy, stability, and flexibility of the proposed method.

A New Lattice Boltzmann Scheme for Photonic Bandgap and Defect Mode Simulation in One-Dimensional Plasma Photonic Crystals

A novel lattice Boltzmann method (LBM) with a pseudo-equilibrium potential is proposed for electromagnetic wave propagation in one-dimensional (1D) plasma photonic crystals. The final form of the LBM incorporates the dispersive effect of plasma media with a pseudo-equilibrium potential in the equilibrium distribution functions. The consistency between the proposed lattice Boltzmann scheme and Maxwell’s equations was rigorously proven based on the Chapman–Enskog expansion technique. Based on the proposed LBM scheme, we investigated the effects of the thickness and relative dielectric constant of a defect layer on the EM wave propagation and defect modes of 1D plasma photonic crystals. We have illustrated that several defect modes can be tuned to appear within the photonic bandgaps. Both the frequency and number of the defect modes could be tuned by changing the relative dielectric constant and thickness of the defect modes. These strategies would assist in the design of narrowband filters.

Terahertz signal analysis and substance identification via Zernike moments

Terahertz (THz) spectra contain chemical information, along with noise and variable backgrounds. Measurement environmental changes and spectral signal differences caused by changes in the sample state can degrade the accuracy of the calibration models. This problem obviously hinders practical applications of THz spectroscopy. To tackle this problem, a three-dimensional spectrum was first self-constructed and converted into an intensity image. Zernike moments with inherently invariant properties were then used to describe the THz intensity image and extract the invariant features for further analysis. Considering the reconstruction error and computational cost, the highest order of Zernike moments and the most effective moments were selected and applied to multi-classifiers including support vector machines, naive Bayes, and regularized linear discriminant analysis. Experiments used a THz dataset collected from four chemical substances (melamine, tartaric acid, lactose, and glucose) at five thicknesses (1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm). The results confirmed the effectiveness of the proposed approach. The obtained results show that compare to traditional absorption spectrum features, Zernike moment features are less sensitive to spectral variations caused by changes in sample status. They have better feature representation ability with lower feature vector dimensions. This suggests that they can be integrated into the design of systems for THz spectral classification to increase the robustness and generalization capability of the classifier.

Label-free study on the effect of a bioactive constituent on glioma cells in vitro using terahertz ATR spectroscopy

In this work, we report that the effect of bioactive constituent on living glioma cells can be evaluated using terahertz time-domain attenuated total reflection (THz TD-ATR) spectroscopy in a label-free, non-invasive, and fast manner. The measured THz absorption coefficient of human glioma cells (U87) in cell culture media increases with ginsenoside Rg3 (G-Rg3) concentration in the range from 0 to 50 µM, which can be interpreted as that G-Rg3 deteriorated the cellular state. This is supported either by the cell growth inhibition rate measured using a conventional cell viability test kit or by the cellular morphological changes observed with fluorescence microscopy. These results verify the effectiveness of using the THz TD-ATR spectroscopy to detect the action of G-Rg3 on glioma cells in vitro. The demonstrated technique thus opens a new route to assessing the efficacy of bioactive constituents on cells or helping screen cell-targeted drugs.

A Novel Lattice Boltzmann Scheme with Single Extended Force Term for Electromagnetic Wave Propagating in One-Dimensional Plasma Medium

A one-dimensional plasma medium is playing a crucial role in modern sensing device design, which can benefit significantly from numerical electromagnetic wave simulation. In this study, we introduce a novel lattice Boltzmann scheme with a single extended force term for electromagnetic wave propagation in a one-dimensional plasma medium. This method is developed by reconstructing the solution to the macroscopic Maxwell’s equations recovered from the lattice Boltzmann equation. The final formulation of the lattice Boltzmann scheme involves only the equilibrium and one non-equilibrium force term. Among them, the former is calculated from the macroscopic electromagnetic variables, and the latter is evaluated from the dispersive effect. Thus, the proposed lattice Boltzmann scheme directly tracks the evolution of macroscopic electromagnetic variables, which yields lower memory costs and facilitates the implementation of physical boundary conditions. Detailed conduction is carried out based on the Chapman–Enskog expansion technique to prove the mathematical consistency between the proposed lattice Boltzmann scheme and Maxwell’s equations. Based on the proposed method, we present electromagnetic pulse propagating behaviors in nondispersive media and the response of a one-dimensional plasma slab to incident electromagnetic waves that span regions above and below the plasma frequency ωp, and further investigate the optical properties of a one-dimensional plasma photonic crystal with periodic thin layers of plasma with different layer thicknesses to verify the stability, accuracy, and flexibility of the proposed method.