Lamotrigine (LTG) is frequently prescribed in conjunction with carbamazepine (CBZ) or oxcarbazepine (OXC) to alleviate the symptoms of partial-onset epilepsy [1]. The principal metabolite of CBZ, 10,11-epoxidized carbamazepine (CBZE), has demonstrated notable anticonvulsant and clinical antiepileptic properties; however, it possesses the risk of inducing neurotoxicity [2,3]. Due to the narrow therapeutic concentration range and substantial individual variation in the pharmacodynamics and pharmacokinetics of these antiepileptic drugs (AEDs), therapeutic drug monitoring is critical to achieving sufficient efficacy while avoiding life-threatening toxicity [4], [5], [6]. Immunoassays and chromatographic analysis are commonly used to determine the blood concentration of AEDs. Immunoassays are highly sensitive, but the enzymes utilized are significantly susceptible to environmental factors, and the approach cannot monitor blood levels in patients receiving coadministered medicines [7]. Chromatographic analysis is characterized by its exceptional selectivity, sensitivity, and accuracy. Moreover, it permits the concurrent determination of different AEDs. Undoubtedly, high performance liquid chromatography (HPLC) is the prevailing method utilized to quantitatively analyze biological samples, owing to apparatus availability, versatility, and detection efficiency [8,9]. However, complex biological sample matrices and low drug concentration require pretreatment prior to HPLC analysis.

Recently, a variety of adsorbent-based extraction techniques have been developed to extract AEDs from biological samples, including solid-phase extraction (SPE) [4], in-tube solid phase microextraction [10], ultrasonic-assisted dispersive micro SPE [11], magnetic SPE (MSPE) [12], and microextraction in packed syringe [13]. Among them, MSPE offers numerous appealing characteristics, including cost-effectiveness, ease of use, high preconcentration factor, and limited reliance on organic solvents. However, few studies have described the use of MSPE to extract AEDs from biological materials. Behbahani et al. [14] prepared a adsorbent by the molecular imprinting technique using Fe3O4 as the carrier and LTG as the template molecule. They used HPLC to detect LTG in urine and plasma samples, with detection limits of 0.7 and 0.5 μg/L, respectively. Similarly, Wei et al. [6] synthesized an imprinted polymer with CBZ and LTG as dual templates, and the established MSPE-HPLC method showed high selectivity for both AEDs in rat serum. Zhang et al. [12] employed dopamine-coated Fe3O4 that was subsequently modified with multi-walled carbon nanotubes to create the MSPE adsorbent. It demonstrated a rapid adsorption equilibrium time and a high adsorption capacity for the extraction of AEDs from plasma, urine, and cerebrospinal fluid. Zhang et al. [15] modified β-cyclodextrin (β-CD) on magnetic graphene oxide, showing good sensitivity, linear range and precision in extracting AEDs from plasma. These studies demonstrated the potential of MSPE for extracting AEDs. The choice of adsorbent is a key factor in improving MSPE efficiency; however, in order to comply with the principles and concepts of green analytical chemistry, most recent research has focused on the preparation of novel green adsorbents [16], [17], [18] .

Biochar (BC) is generated through the anoxic combustion of various biomass feedstocks at low temperatures. It has found applications in various research fields due to its ease of availability, cost-effectiveness and sustainability of the feedstock [19,20]. However, the application of pristine BC in sample pretreatment has received little attention due to its limited surface functional groups, small specific surface area, and imperfect structure, all of which could potentially affect the extraction reproducibility [21]. To make BC more useful in sample pretreatment, it is necessary and innovative to concurrently modify its surface properties and pore structure.

H3PO4 is considered an eco-friendly BC activation reagent because of its low decomposition temperature, low corrosiveness to equipment, and low pollutant generation[22]. Compared to pristine BC, H3PO4-activated BC (PBC) will have a substantially improved specific surface area and pore structure [21,23,24]. PBC contains numerous reactive functional groups including carboxyl, phosphorus, and hydroxyl groups, which can be easily modified by subsequent synthesis, making it a superior modification platform for adsorbents [21]. To date, no other literature has reported that PBC can be further functionalized for better performance. β-CD is a non-toxic and biodegradable macrocyclic oligosaccharide with hydrophilic outer edges and hydrophobic inner cavities. It can form inclusion complexes with organic molecules through host-guest interactions [25]. β-CD has been investigated by some researchers to modify BC [26,27], but the specific surface area was greatly reduced after modification and even lower than that of pristine BC [15]. However, no studies have examined grafting β-CD on the basis of improving the pore structure of BC.

Herein, the pore structure and surface properties of the BC generated from Zizyphus jujuba seed shells were improved by introducing H3PO4 activation and β-CD functionalization. The magnetic PBC (MPBC) was synthesized by a co-precipitation method, and its reactive hydrogen-containing functional groups were linked to β-CD (MPBCCD) using epichlorohydrin (EPI) as a cross-linking agent. The adsorption behavior of MPBCCD on four AEDs was investigated. On this basis, it was used as the MSPE adsorbent in combination with an HPLC-diode array detector (DAD) to analyze the four trace AEDs in plasma. Some variables affecting the extraction effect and the validation of the method were investigated. The potential binding mechanisms were investigated by combining the analysis of experimental results with quantum chemical theoretical calculations.