Advancing trace liquid detection: colloidal gold-based quasi-BIC metamaterials in terahertz biosensing

Abstract

The quasi-bound states in the continuum (quasi-BIC) are uniquely attractive in the fields of nonlinear modulators, optical switches, and sensing due to their ultra-high radiative quality factor. The implementation of quasi-BIC on metamaterials can trap the energy of the electromagnetic wave whose wavelength is larger than the diffraction limit in the metamaterial without radiation leakage, leading to the development of highly sensitive terahertz (THz) biosensors. In this paper, we manipulate the interferometric coupling between multipoles by breaking the symmetry of the metal structure on the metamaterial to excite high-quality quasi-BIC resonances. In addition, we experimentally integrated colloidal gold on the proposed quasi-BIC metamaterial and combined it with microfluidics technology, in which the tip effect of colloidal gold enhances the light–substance interactions and thus improves the detection of the sensor, and realized the micro-liquid detection of imidacloprid solution with a detection limit of 1 ng mL⁻¹. We then used the continuous wavelet transform instead of the traditional Fourier transform and created a two-dimensional wavelet coefficient card that provides a more accurate method for determining solution concentration. This novel sensing platform offers the possibility to reduce the interference of water on THz signals and achieve highly sensitive detection of trace liquids via THz metamaterials, and this pioneering approach opens up a new avenue for liquid-based THz biosensing.