Hyaluronic acid is a large acidic mucopolysaccharide that is commonly found in the connective tissues of animals, including joints, the vitreous body, synovial fluid, the umbilical cord, cartilage, and skin. Different molecular weight variations of hyaluronic acid exhibit distinct biological and physical properties, including plasticity, high viscoelasticity, and excellent biocompatibility. Hyaluronic acid has various applications, such as in eye preparations, supplementing missing support, regulating muscle movement of fat and dermal tissue, and improving the effects of drug treatments in slow-release preparations. Additionally, macromolecular hyaluronic acid and oligohyaluronic acid have high moisturizing and transdermal properties, making them useful for antiinflammatory and promoting tissue repair, particularly in skin burn healing and postoperative antiadhesion. Moreover, oligohyaluronic acid can serve as a targeting carrier for antitumor drugs that can be better absorbed in tumors and lymph nodes. Given its medical value in clinical medicine and other fields, the concentration change of hyaluronic acid can directly reflect the body's health status. However, due to its low concentration in the body, it is challenging to detect hyaluronic acid using traditional methods. Therefore, there is a need for rapid and highly sensitive trace detection methods of hyaluronic acid.

For this experiment, hyaluronic acid samples were obtained from Sigma Aldrich Corporation. A terahertz time-domain spectrum system (THz-TDS) (TAS7400), produced by Advantest in Japan, was used with a spectral range of 0.03-7 THz and a dynamic range of approximately 60 dB. To obtain transmission responses, 512 measurements were cumulatively averaged within the spectral range of 0.1-4.0 THz, with a resolution of 7.9 GHz. To minimize experimental error, the four repeated measurements method was employed for each sample. All experiments were performed at room temperature (~22 ℃) and with a humidity level of less than 3% to eliminate the effects of humidity and temperature on the experiment. The sensor used consisted of two-gap cut-induced split ring resonators with an asymmetry of g=13 μm for testing the hyaluronic acid sample. Standard hyaluronic acid solution was diluted into different concentrations by mixing it evenly with deionized water to obtain 0, 1, 2, 4, 8, and 16 mɡ·ml^-1 concentration diluted hyaluronic acid solution. The sensor was dried with air and 5-μL sample solution was dropped onto the metasurface using a motorized pipette. The sensor was dried for 5 min and then measured using THz-TDS at room temperature to obtain the transmission responses. To reduce error, five repeated measurements were performed for each sample solution, and after each measurement, the sensor was cleaned and dried for half an hour.

The transmission spectrum of the cut-induced asymmetric split ring resonator shows two resonances: the Fano resonance at low frequency and the dipole resonance at high frequency. The sensitivity of the resonator is influenced by the spatial overlap between the analyte and the electromagnetic field. With an increase in the asymmetric parameter g, the sensitivity of the Fano resonance decreases, while the sensitivity of the dipole resonance increases. For this experiment, a value of g=13 μm was chosen because both resonances have similar sensitivity, enabling better quantification of the hyaluronic acid solution using both resonances. As the mass concentration of the hyaluronic acid solution increased from 0 mɡ·ml^-1 to 16 mɡ·ml^-1, both resonances showed a redshift. To analyze the relationship between dual resonance frequency changes and hyaluronic acid solution concentration, we used the Hill model, which is commonly used in the biomedical field. The correlations of Hill fitting for dual resonances were found to be 0.996 and 0.994, respectively. The limit of detection is as low as 1 mɡ·ml^-1. These results demonstrate that hyaluronic acid solution can be detected quickly and accurately using the proposed method.

We present an innovative approach to detect hyaluronic acid at trace levels, using an ultrasensitive THz dual-band Fano metasurface sensor that breaks the symmetry of cut-induced split ring resonators. By carefully selecting the symmetric parameter, we achieved sensitivities of both the Fano and dipole modes that reach ~150 GHz/RIU. In our experiment, we achieve a remarkable limit of detection of 1 mɡ·ml^-1. The correlation of Hill fitting between resonance frequency shifts and hyaluronic acid concentrations is higher than 0.99, demonstrating the potential for accurate and quantitative analysis of hyaluronic acid at trace levels.