Q-BIC THz Metasurface Biosensor Based on Flexible Polyimide for Solution Detection

The concept of quasi-bound states in the continuum (Q-BIC) in continuous media has garnered significant attention in fields such as sensing and optical switching. Benefiting from localized electric field enhancement, researchers can use the combination of Q-BIC and metasurfaces (MSs) to enable highly efficient and reliable sensing solutions. This article presents a Q-BIC terahertz (THz) MS biosensor integrated with a flexible polyimide (PI) capillary for concentration detection of liquid analytes. The introduction of the PI capillary effectively controls the intensity of THz wave absorption by liquids, successfully addressing the limitations of THz waves in liquid detection. The unique feature of the proposed biosensor lies in its ability to identify the concentrations of ovalbumin and hydrolyzed whey protein in the range of 1 pg/mL to 1 mg/mL by monitoring changes in modulation depth. This approach achieves excellent linear fitting, ensuring robust quantitative analysis. The sensor also demonstrates the ability to detect variations in ethanol solutions. A series of 2-D wavelet coefficient intensity maps (WCIMs) based on continuous wavelet transform (CWT) was constructed, offering a novel approach for identifying solution concentrations. The proposed sensing platform provides several advantages, including higher sensitivity and reusability. This innovative method opens new avenues for liquid-based THz biosensing.