Plant growth regulators (PGRs) contain synthetic hormone analogues and endogenous plant hormones [1]. They can not only regulate the growth and development of plants but also mediate environmental stress with a minute quantity [2], [3], [4], [5]. Therefore, it is of great significance to trace the changes of PGRs to reveal the metabolic process of plants. According to their physiological function, PGRs can be divided into 9 categories including auxins (Auxs), gibberellins (GAs), cytokinins (CKs), abscisic acid (ABA), ethylene, brassinosteroids (BRs), salicylic acid (SA), jasmonates (JAs) and growth retardants (Table S1). Auxs are a group of PGRs, which are derivatives of indole or acetic acid. Indole acetic acid (IAA) and indole butyric acid (IBA) are examples of endogenous Auxs characterized by indolic acids, which can be distinguished by a variable side chain. Besides, naphthalene acetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichloro phenoxy propionic acid (2,4,5-TP) and 4-chlorophenoxyacetic acid (4-CPA) originated from the way of synthesis are also used to classifying as Auxs. Auxs contain the chemical group carboxylic acid, which confers acidic properties. GAs belongs to a class of diterpenoid compounds, with over a hundred species discovered, but only some of them have physiological effects in regulating plant growth and development. Active GAs such as GA1, GA3, GA4, GA7 have identical structural characteristics of 4, 10-lactone and a C6 carboxylic acid group [6]. Natural CKs are a series of adenine derivatives where the hydrogen atom at the N6 -position is substituted by various groups. Based on the structure of the N6-substituents, CKs can be grouped into isopentenyl-type CKs (such as isopentenyl, trans-zeatin, cis-zeatin, dihydrozeatin, etc.) and aromatic-type CKs (benzyladenine). Artificially synthesized CKs include isoprene-type kinetin, aromatic-type 6-benzyladenine (6-BA), p-chlorophenoxy acetic acid (CPPU) and thidiazuron (TDZ), etc. There are also synthetic CKs that are not related in structure, such as phenylurea-type cytokinins N, N-diphenylurea and some phenylurea derivatives with high physiological activity [7]. Generally, typical strigolactones (SLs) present a tetracyclic skeleton (A, B, C, and D rings) in which the tricyclic lactone (ABC-ring) is connected to an α, β-unsaturated furanone moiety (D ring) via an enol ether linkage. The basic structure of BRs consists of a steroid nucleus with a C17 side chains. Brassinolide is a representative BR. Other analogues are mainly different in the substituents of C2 and C3 in ring A, the deoxidation or ketonization of C6 in ring B, and the conformation and hydroxyl modification of the side chain. According to the properties of the oxygen-containing functional groups in the B-ring, they can be classified as lactone-type, ketone-type, and deoxy-type. 24-epibrassinolide is a widely applied artificially synthesized BRs. BRs are neutral hydrophobic steroids and lack of ionizable functional groups, thus leading to poor ionization efficiency and cause the inherent hurdle for the accurate analysis of BRs by MS [8]. JAs are phospholipid-derived compounds mainly consisting of jasmonic acid, methyl jasmonate, jasmonoyl-isoleucine, 12-oxo-phytodienoic acid and other cyclopentanone derivatives [9]. Plant growth inhibitors are exogenous synthetic compounds, including (a) onium-type compounds, such as chlormequat chloride, etc. (b) triazole compounds characterized by a cyclic structure containing three nitrogen atoms, chlorophenyl groups and carbon side chains, e.g., paclobutrazol (PBZ), uniconazole (UCZ). (c) structural mimics of 2-oxoglutaric acid. (d) 16, 17-dihydroGAs [10,11].

There are several obstacles to obtaining a stable and reliable detection of PGRs. First, there is a big discrepancy in the concentration of plant hormones among plants. Additionally, the levels of endogenous PGRs can vary significantly even within different regions of the same plant [12]. Second, some PGRs exhibit instability. For example, the lactone structures within the C ring and D ring triggered the instability of SLs under acidic or alkaline conditions. Thirdly, the polarity of various PGRs is quite different. According to the functional groups and structure features, ABA, IAA, JA, SA, GAs, and CKs belong to moderately polar compounds. BRs and CLs are compounds with weak polarity. Peptide hormones are amphoteric compounds with strong polarity. Finally, PGRs exhibit varying physicochemical properties. The pKa values of PGRs exhibit diversity due to their structural variations, as illustrated in Table S1. The presence of carboxylic acid structure in ABA, IAA, JA, SA, and GAs imparts them with acidic properties, while CKs exhibit alkaline characteristics due to the N-atoms present in their structures. SLs and BRs are neutral compounds lacking ionizable functional groups which pose challenges to ionization efficiency in mass spectrometry [13]. The above-mentioned problems caused by the inherent physical and chemical properties of PGRs have caused obstacles in extraction, separation, and detection with mass spectrometry. There is often a trade-off between a broad range of adsorption and complete extraction of PGRs.

Solid-phase extraction (SPE) is widely used in the PGRs pretreatment process due to its high selectivity and suitability for low concentration samples [14]. It has noteworthy advantages such as low solvent consumption, short time and flexible adsorbent selection [15,16]. In recent years, researchers have paid great attention to the miniaturization and automation technology development of PGRs. Several new technologies have been derived, including dispersed solid phase extraction (d-SPE), magnetic solid phase extraction (MSPE), pipette tip solid phase extraction (PT-SPE), solid phase microextraction (SPME), and so on. The choice of adsorbent in the SPE process is the core of good adsorption capacity and extraction efficiency of target compounds [17]. Despite the potential advantages of commonly used commercial adsorbents (including C18, C8, hydrophilic and lipophilic balance resin (HLB), mixed/cation exchange resin (MCX), mixed/anion exchange resin (MAX), etc.), there are still some limitations of the traditional adsorbents. Classic sorbents employed in SPE such as surface-modified silica become unstable if the pH value of aqueous samples is beyond the range of 2-8 due to silicon dissolving under alkaline conditions and hydrolyzing under acidic conditions [18,19]. The limited recognition and adsorption capacity also lead to an unsatisfying adsorption efficiency. Evidence shows that the loading efficiency of commercial sorbents (MWCNTS, C18, Si-SCX, MCX) is not comparable to β-CD/MAA-MIPs adsorbents in the adsorption of PGRs [20]. Moreover, the commercial SPE systems are disposable which is not environmentally friendly [21]. With the continuous development of functional materials, efforts are made to solve the mentioned problems. Materials are designed with a stability property that can be performed at a wide range of pH [22,23]. On the other hand, introducing specific functional groups (e.g. -NH2, -NO2, -OH, -COOH, -SH, etc.) on the surface of the adsorbent material is favored for the improvement of the adsorption performance [24,25]. Then, π-π stacking, electrostatic attraction, hydrophobic interactions, and intermolecular hydrogen bonding generated between the adsorbent and PGRs further assist the sorbent selectivity by specific recognitions. The reusability of sorbents is an essential feature in an ideal sorbent which is in accordance with the principle of green chemistry. In this regard, thiol functionalized nanofiber pads and multi-walled nanotube-covalent organic backbone composites exhibit excellent reusability and stability with no significant decrease in the extraction efficiency of PGRs after repeated use [24,26]. In other cases, the porous polymer monolithic column material is proved to be capable of recycling up to 600 times as an online solid phase extraction sorbent [27].

Up to now, significant improvements are made in the pretreatment of PGRs before liquid chromatography-mass spectrometry (LC-MS) analysis. It is worth noting that in order to meet the requirements of sensitive analysis of PGRs, the current functional materials are committed to modifying the surface of materials with specific functional groups to increase the adsorption capacity and specific recognition capacity of PGRs [28], [29], [30]. On the other hand, to cope with the instability of PGRs, a special focus is devoted to the magnetic materials with a fast and simple separation process, which simplifies the pretreatment steps and reduces the loss [31,32]. Hence, this review was conducted to introduce the selection of the extraction phase during PGRs pretreatment based on LC-MS and analyze the adsorption mechanism and performance. Also, we focus on the challenges and future trends in the synthesis and application of functional materials for PGRs analysis. All the data in this review were collected from Web of science academic databases, and the keywords used are "solid phase extraction", the specific functional materials and "plant growth regulator" (Fig. 1).