Fingerprint spectrum matching-empowered terahertz metamaterial sensor for the ultrasensitive detection of sorbic acid

Excessive intake of sorbic acid (SA), a widely used food and beverage preservative that acts as a weakly absorbing small molecule in the terahertz band, poses potential hazards to human health. Here, we develop an asymmetric split-square metamaterial sensor and propose an ultrasensitive quantitative detection method based on fingerprint spectrum matching. Initially, a prominent SA molecular fingerprint absorption peak was identified at 1.57 THz via THz-TDS. To amplify light-matter interactions, a sharp resonance response with a high quality factor (Q = 157) was excited via structural symmetry-breaking, with its central frequency precisely locked near 1.57 THz to achieve localized near-field enhanced interaction with the SA molecular fingerprint feature. Simulation results indicated that the metasensor yielded a refractive index sensitivity of 380 GHz/RIU and a figure of merit (FoM) of 37.8. Experimentally, the resonant frequency shift exhibited an exceptional linear relationship with the logarithmic concentration (R² = 0.9929) over a broad range of 0.01-1000 mg/L, covering five orders of magnitude and delivering a theoretical limit of detection (LOD) of 0.007 mg/L based on the 3σ criterion. Comparative trials against benzoic acid, a commonly used food preservative, and citric acid, an acidity regulator, verified that the sensor possesses outstanding characteristic preferential selective responsiveness toward SA. Furthermore, spike recovery evaluations in real orange juice matrices achieved desirable yields of 95.68%-105.19%, showing no significant discrepancy (p > 0.05) compared with standard HPLC metrics. Overall, this study offers a highly efficient, label-free THz sensing platform for advanced food safety inspection of trace additives.