The molecules of amino acid play a crucial role in the normal functioning of various physiological process of human body, making the detection of amino acid concentrations an essential prerequisite for clinical diagnosis, drug development, and gene therapy [1–5]. Conventional detection methods for mainstream biochemical samples involve marked the samples [6–9]. This labeling approach is complex, time-consuming, and can damage the tested samples. These factors limit the application range and testing accuracy of labeled sensors, emphasizing the importance of finding a rapid and non-destructive sensing detection method.
The terahertz frequency range is characterized by low photon energy, broadband characteristics, and specific "fingerprint spectra" for the detection of biochemical macromolecules, showing immense potential in fields such as chemistry and the detection of biological substances [10–16]. However, the limited resolution of traditional THz-TDS technology and insufficient sensitivity to detect differences in the spectral characteristics of trace samples. Therefore, it is necessary to locally enhance the intensity of the THz wave to reduce the requirements for the samples.
Electromagnetic metasurface are composed of periodically arranged subwavelength unit elements, exhibiting extraordinary properties not attainable with natural materials [17–20]. In recent years, the electromagnetic induction transparency (EIT) phenomenon and Fano resonance phenomenon in metasurface have attracted widespread attention due to their natural sensitivity to changes in the local electric field environment. Metasurface designed based on these characteristics as sensors offer higher detection sensitivity, are label-free, non-destructive, time-saving, and cost-effective, making them particularly attractive for use in biomedical and chemical sensing platforms [21–25].
However, until now, the experiments on metasurface sensors based on two resonance modes in the terahertz range have remained in the stages of sensing tests with different proportions of mixed solutions and testing on the dried samples after spraying or incubating samples [26–32]. The main reason is that the strong polarity of aqueous solutions significantly inhibits terahertz waves, and the absorption characteristics of solutes are also severely inhibited. This requires the tested substance to be thin or even dehydrated. Moreover, recent work focuses on the sensing performance of the sensor itself, with little exploration of the relationship between the substance's own "fingerprint spectrum" and the sensor resonance peak [33].
In this paper, we designed and optimized an asymmetric folded double-open ring structure based on the Fano resonance mechanism to match its resonant frequency with the absorption peak of L-Valine. A solvent mixture of water and glycerol in a volume ratio of 2:8 was proposed to reduce water absorption of terahertz waves, avoiding the disadvantages of uneven solute distribution caused by drying evaporation or spray methods. A sample chamber with a controlled liquid thickness of 15µm was fabricated. Constructed a vertical incidence THz-TDS system for easy horizontal placement of chamber. Subsequent sensing tests were conducted on solutions containing trace amounts of L-Valine and L-Phenylalanine. Experimental results revealed a pronounced frequency shift in the metasurface resonance peak with the increase in L-Valine solution concentration, demonstrating a concentration sensitivity of 9.98 GHz/mM. Similarly, an increase in the concentration of the L-Phenylalanine solution resulted in a lower frequency shift, with a concentration sensitivity of 1.88 GHz/mM.