## 1 Introduction  
Plant growth regulators (PGRs) are synthetic organic compounds with natural plant hormone activity, which can be used to regulate the growth process of plants [1; 2]. Among them, 2,4-Dichlorophenoxyacetic acid (2,4-D), forchlorofenuron (CPPU) and indole-3-acetic acid (IAA) are widely applied in agricultural and horticultural practices. 2,4-D is a commonly used auxin analogue for inducing callus formation [3]. It effectively stimulates the growth of plants at low concentration. However, it inhibits growth or even poisons plants when the concentration is high [4]. Thus, it is widely used as a herbicide to control broad-leaved weeds and gramineous weeds [5]. CPPU is a phenyl-urea plant growth regulator with cytokin activity that can promote cell division and expansion, improve photosynthesis efficiency and enzyme activity, promote organ formation and protein synthesis, increase resistance and delay aging [6]. It has wide application prospects in agricultural production, such as the promotion of fruit growth and the prevention of fruits dropping [7]. IAA is a hormone reagent that stimulates plant growth, which can promote plant cell growth, division and vascular bundle differentiation [8]. At the same time, it plays an important regulatory role in plant rooting, sex differentiation, fruit formation, maturation, aging and shedding, and it has been widely used in agricultural production regulation and ripening of vegetables and fruits [9; 10]. With the development of new types of PGRs, the applications of PGRs are becoming increasingly widespread, and they are important measures for realizing agricultural overproduction [11]. However, due to the toxicity and harmfulness of PGRs, blind and excessive use of PGRs will cause environmental pollution and food safety issues [12]. In agricultural production, we should not only pay attention to the effects of PGRs on plants, but also the toxicity and residue of PGRs. At present, the main methods for detecting PGR residues are high-performance liquid chromatography (HPLC), gas chromatography (GC) and ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS) [13; 14; 15]. However, the pretreatment processes of these methods are complicated, and the detection efficiency is low [16]. Therefore, it is of urgent need to develop an accurate, convenient, sensitive and qualitative method for PGR detection. As an advanced technology, terahertz (THz) has received more and more attention in the field of biological sciences [17; 18; 19]. It has unique absorption characteristics for heterogeneous and biological macromolecules. In general, the vibrational absorption frequency of organic intramolecular chemical bonds is mainly in the infrared band [20; 21], apart from the weak interaction between molecules, such as hydrogen bonds, Van Der Waals force, dipole rotation and vibrational transition, and the low-frequency vibrational absorption of crystal lattices are all in the THz band (0.1-10 THz) [22]. The molecular structure and environmental information reflected by these vibrations have obvious responses in different absorption locations and absorption intensities in the THz band. Therefore, material composition, structure and other information can be analyzed through the characteristic absorption of THz radiation [23; 24]. Researchers have achieved many remarkable results in the field of chemical composition analysis of substances using THz-TDS. [PERSON] et al. [25] obtained the optical characteristics of organophosphorus pesticide acephate using THz-TDS, and absorption peaks were found at 1.58 and 2.10 THz. The calculated results by DFT showed that the absorption features were caused by the collective vibrational and torsional modes, and different absorption peaks corresponded to different vibrational modes. [PERSON] et al. [26] investigated the feasibility of detecting melamine in foodstuffs using THz-TDS. Characteristic absorption peaks of melamine were found at 2, 2.26, and 2.6 THz.