Antibiotics produced by microorganisms (such as bacteria and fungi) or through semi-synthetic and synthetic methods are a class of drugs primarily used to treat various infections caused by bacteria or fungi. The misuse and overuse of antibiotics have resulted in severe problems caused by antibiotic resistance. To ensure the effective use and management of antibiotics, one must detect their concentrations precisely. Conventional methods for detecting antibiotics require professional operational technology and complex instruments; moreover, the associated procedure is cumbersome and lengthy. Therefore, an efficient, rapid, and highly sensitive method for detecting antibiotic concentrations must be devised. Bound states in the continuum (BICs) can promote strong interactions between light and matter, which are introduced into the metasurface to achieve ultrahigh Q resonance. Combined with the advantages of terahertz technology in nondestructive testing, terahertz metasurfaces based on BICs have been widely used in biological and chemical sensing. A metasurface with BICs and a high Q-factor can significantly improve the sensitivity of a biosensor to slight environmental changes, thus providing an effective scheme for the rapid, convenient, and highly sensitive detection of antibiotics.

A metallic structure was designed on a 500-μm-thick quartz substrate. The transition from BICs to quasi-BICs was achieved by introducing asymmetric parameters. The transmitted spectra for different parameters l₁ were simulated using CST Studio with a time-domain solver. In the simulation, the x- and y-directions were set as periodic boundary conditions, and the z-direction was set as an open-boundary condition. BICs can be excited under both x- and y-polarization incidences. The far-field scattering power of the structure was analyzed via Cartesian multipole decomposition based on electromagnetic multipole theory. A series of samples with different parameters was prepared and verified using a terahertz time-domain spectroscopy (THz-TDS) system. Additionally, the refractive-index sensitivity of the sensor was simulated and analyzed. Different mass concentrations of penicillin G potassium salt were detected using these sensors. For measurement, 10 μL analytes with different concentrations were pipetted onto the sensor surface and then allowed to dry. The spectra were obtained using the THz-TDS system. After each measurement, the sensor was cleaned in ultrapure water to meticulously remove the residual analyte from the sensor surface and then dried. Prior to the next measurement, the spectrum transmitted by the cleaned sensor was measured to ensure that the cleaning process did not affect the optical response.

When x- or y-polarized terahertz waves are incident, the simulated spectra of the metasurface with BICs under different values of l₁ are as shown in Figs. 2(a) and 3(a). When l₁=l=70 μm, the spectral linewidth disappears at 1.07 THz (under x-polarized incidence) and 1.12 THz (under y-polarized incidence), thus indicating the presence of BICs with an infinite Q-factor. As l₁ increases, the spectral linewidth broadens, thus indicating a gradual increase in the radiation loss. Subsequently, the BICs transform into quasi-BICs. The resonance frequency and Q-factor of the quasi-BICs can be tuned by changing the asymmetric parameter l₁. The variation in the Q-factor with the asymmetric parameters adheres to the relationship Q∝α-2 [Figs. 2(f) and 3(e)]. For the x-polarized incidence, the excited quasi-BIC and dipole modes originate from electric quadrupole (EQ) and toroidal dipole(TD), respectively [Figs. 2(c) and (d)]. For the y-polarized incidence, the excited quasi-BIC mode is derived from electric dipole (ED) [Fig. 3(c)]. The designed sensor has a refractive index sensitivity of 210 GHz/RIU, which demonstrates its excellent sensing performance [Figs. 6(a) and (b)]. The designed sensor was used to detect different concentrations of penicillin G potassium salt, where a minimum detection mass concentration of 0.625 mg/mL is recorded [Figs. 7(a) and (b)]. The spectra transmitted by the bare and cleaned sensors after measuring different concentrations of the analyte are highly consistent [Fig. 7(d)], thus demonstrating the reusability of the device. Our design provides a rapid and effective method for the high-sensitivity detection of antibiotics.

In this study, a BIC-based metallic metasurface was designed for the detection of different antibiotic concentrations. When structural symmetry is broken, the lossless BIC transforms into a quasi-BIC with a finite Q-factor. The Q-factor of the quasi-BIC can be tuned by changing the asymmetric parameters. The results of multipole decomposition show that the quasi-BIC excited by x- and y-polarization originates from EQ and ED, respectively. A series of metasurface samples with different parameters was prepared and verified experimentally in the terahertz band. The designed sensor has a refractive-index sensitivity of 210 GHz/RIU, which renders it suitable for high-sensitivity sensing applications. Considering potassium G as an example, the sensor was used to detect different concentrations of antibiotics, and the experimental results show a minimum detection mass concentration of 0.625 mg/mL. The reusable features reduce the detection costs. The designed metasurface with BICs provides a rapid and effective method for the highly sensitive detection of antibiotic concentrations, which is expected to replace conventional detection methods in the future and exhibits potential application prospects in medical diagnosis, food safety, and environmental monitoring.