Terahertz waves have gained attention owing to their potential for various bio-applications, including broadband spectroscopy, imaging, and sensing. For development of practical applications in clinical fields, highly sensitive molecular detection is essential. Notably, the molecular sensitivity of optical sensing can be enhanced by utilizing localized electromagnetic waves in metallic optical resonators. In this study, we theoretically design terahertz (THz) metamaterial sensors based on the Fano resonance, enabling highly sensitive and selective molecular sensing without additional molecular labeling. The metamaterial sensors consisting of symmetry-broken unit resonators are designed to support Fano resonances with strongly localized electromagnetic waves through coupling of the resonators. The key concept behind the design is to match the Fano resonance with the characteristic resonance in the target molecules, leading to pronounced changes in the transmission spectra of THz waves through the metamaterial with the analytes. We numerically demonstrate the sensing performance by considering steroids as the target molecules and explain the sensing mechanism using both temporal coupled-mode theory and FDTD numerical simulations.

BACKGROUND: Near-infrared fluorescence indocyanine green lymphangiography, a primary modality for detecting lymphedema, which is a disease due to lymphatic obstruction, enables real-time observations of lymphatics and reveals not only the spatial distribution of drainage (static analysis) but also information on the lymphatic contraction (dynamic analysis). METHODS: We have produced total lymphatic obstruction in the upper limbs of 18 Sprague-Dawley rats through the dissection of proximal (brachial and axillary) lymph nodes and 20-Gy radiation (dissection limbs). After the model formation for 1 week, 9 animal models were observed for 6 weeks using near-infrared fluorescence indocyanine green lymphangiography by injecting 6-μL ICG-BSA (indocyanine green-bovine serum albumin) solution of 20-μg/mL concentration. The drainage pattern and leakage of lymph fluid were evaluated and time-domain signals of lymphatic contraction were observed in the distal lymphatic vessels. The obtained signals were converted to frequency-domain spectrums using signal processing. RESULTS: The results of both static and dynamic analyses proved to be effective in accurately identifying the extent of lymphatic disruption in the dissection limbs. The static analysis showed abnormal drainage patterns and increased leakage of lymph fluid to the periphery of the vessels compared with the control (normal) limbs. Meanwhile, the waveforms were changed and the contractile signal frequency increased by 58% in the dynamic analysis. Specifically, our findings revealed that regular lymphatic contractions, observed at a frequency range of 0.08 to 0.13 Hz in the control limbs, were absent in the dissection limbs. The contractile regularity was not fully restored for the follow-up period, indicating a persistent lymphatic obstruction. CONCLUSIONS: The dynamic analysis could detect the abnormalities of lymphatic circulation by observing the characteristics of signals, and it provided additional evaluation indicators that cannot be provided by the static analysis. Our findings may be useful for the early detection of the circulation problem as a functional evaluation indicator of the lymphatic system.