## 1 Introduction  
2,4-Dichlorophenoxyoxyacetic acid (2,4-D) is a widely used plant growth regulator [1]. A small amount of it can promote crop growth and contribute to the improvement of fruit quality, and a large amount of it can help in weeding [2]. As an antialsing agent, it can enhance the disease resistance of fruits and vegetables and delay their aging. However, 2,4-D is difficult to decompose and can remain in water, crops, and soil because of its low water solubility [3; 4; 5]. Improper usage of it will affect the environment pollution and threaten the human body's immune system [6; 7; 8]. Also, long-term consumption of 2,4-D would greatly increase the incidence of cancer [9]. In view of these hazards, some countries and international organizations have made clear provisions on the residue limit of 2,4-D. Codex Alimentarius stated that the maximum residue limit of 2,4-D in food and feed varied from 0.01-400 mg/kg [10]. Therefore, it is of great significance to explore the detection method of 2,4-D. At present, the main methods for detecting 2,4-D include high-performance liquid chromatography (HPLC) [11; 12], gas chromatography (GC) [13], and enzyme-linked immunosorbent assay [14]. However, the pre-process of these methods is complicated, with low detection efficiency. Therefore, it is necessary to propose a new detection method with simple operation and high efficiency. In recent years, analysis of spectral fingerprint peaks has gradually become one of the important methods for qualitative and quantitative analysis of compounds. In the past few years, some researchers applied spectral characterization techniques to the detection of 2,4-D. [PERSON] et al. [15] applied surface-enhanced Raman scattering technology for detection of 2,4-D using laser excitation at 632.8 nm. The result showed enhancement factors of Au nanorods substrates and Au nanospheres substrates were 6.2\(\times\)10\({}^{5}\) and 5.7\(\times\)10\({}^{4}\), respectively. [PERSON] et al. [16] compared quantitative nuclear magnetic resonance spectroscopy (QNMR) with chromatographic methods for the determination of the purity of 2,4-D. The result showed that QNMR is more precise and accurate than the chromatographic methods. [PERSON] et al. [17] applied the UV-Vis spectrophotometer to detect the concentration of 2,4-D absorbed on mustard plant ash. Still, these spectroscopy technologies had some shortcomings. NMR spectroscopy mainly detected hydrogen-containing groups, which was inadequate. Raman spectra could be influenced by fluorescence, and a UV spectrophotometer could only be used to detect liquid. THz detection technology is a novel characterization technology, which has been gradually applied in many fields. The vibration and rotation frequencies of many macromolecules are in terahertz band. Therefore, they show strong absorption and dispersion characteristics in this band [18; 19; 20]. The spectral properties through resonance absorption of materials to terahertz radiation can be studied by terahertz time-domain spectroscopy (THz-TDS) technology [21; 22]. Compared with traditional spectroscopy, terahertz spectroscopy can simultaneously provide amplitude and phase information of terahertz signal [23]. Additionally, THz radiation has strong penetrability for many nonpolar materials, which can detect internal components of the tested object more adequately [24]. Researchers have made significant progress in studying and analyzing chemical composition using terahertz in the past few years, which has provided a foundation for the study of terahertz in the detection and analysis of substance components. [PERSON] et al. [25] investigated the feasibility of detecting methomyl, a carbamate in wheat and rice flours, using THz-TDS. The results showed that three absorption peaks were found in 0.1-3 THz, 1, 1.64, and 1.89 THz at room temperature, with the regression coefficient higher than 0.974, and the limit of detection of less than 3.74%. [PERSON] et al.