With the rapid advancement of the information age, electronic products equipped with liquid crystal displays (LCDs) have significantly enhanced quality of life, leading to the widespread use of diverse electronic devices [[1], [2], [3]]. Liquid crystal monomers (LCMs), as critical components of thin-film transistor liquid crystal displays (TFT-LCDs), are now ubiquitous in daily life, with their global inventory continuously rising [4]. However, within the e-waste recycling industry, LCMs are often overlooked due to their high recycling costs and low economic returns, increasing the likelihood of their release into environmental systems [[5], [6], [7]]. Consequently, the excessive release of LCMs poses significant health risks to human populations [[8], [9], [10], [11], [12]]. Given the chemical diversity of LCMs, their widespread presence in e-waste, and the PBT (persistent, bioaccumulative and toxic) properties exhibited by many LCMs [13]. It is imperative to develop specialized sampling and analytical methods to investigate the environmental concentrations and distribution patterns of LCMs for environmental protection and biosafety assurance [14].

Commonly used pretreatment methods for extracting LCMs from samples include liquid-liquid extraction (LLE) [[15], [16], [17], [18]] and solid-phase extraction (SPE) [19]. However, these methods have several disadvantages, such as requiring large sample volumes, involving complex procedures, utilizing significant amounts of organic solvents, and incurring high costs. Therefore, it is essential to create straightforward, effective, and eco-friendly pretreatment methods. Homogeneous liquid-liquid microextraction (HLLME) has emerged as a promising new pretreatment technique that effectively addresses these challenges [20]. HLLME requires a smaller sample volume and offers a straightforward operational process.

In 2003, Abbott et al. [21] first discovered and proposed the concept of deep eutectic solvents (DESs). These solvents are formed by mixing hydrogen bond acceptors (HBAs) with hydrogen bond donors (HBDs) at specific ratios, achieving 100 % atom utilization efficiency during preparation. In 2018, Fabiane Oliveira Farias et al. [22] investigated the mechanism of pH-triggered reversible phase transitions through pH-regulated DES property modification, establishing the novel concept of pH-responsive switchable deep eutectic solvents (SDESs) for the first time. The SDESs can regulate the pH of the solution system by introducing appropriate proton donors or deprotonation reagents, enabling rapid phase transition and achieving extraction equilibrium without requiring dispersing solvents. This pH-responsive characteristic significantly facilitates and simplifies the pretreatment process. Numerous studies have reported the application of SDESs as safe and environmentally friendly green solvents for pretreatment of environmental pollutant samples [[23], [24], [25], [26], [27], [28], [29]].

In this study, we developed a method combining pH-responsive SDESs with ultrasound-assisted HLLME (SDES-UA-HLLME) for the pretreatment of 7 LCMs in environmental water. Thymol and benzoic acid were selected to synthesize the SDESs, which employed as extractants during the HLLME process, and the extracted samples were analyzed using GC–MS. The addition of NaOH and HCl solutions into the sample solution acted as triggers for the hydrophilicity/hydrophobicity switch of the SDESs and the pH-switching mechanism was showed in Fig. 1. The ultrasound-assisted extraction was utilized to achieve efficient LCMs recovery from water. This work provides new a theoretical insight and practical reference for advancing the application of pH-responsive SDESs and enhancing the detection and analysis of LCMs in environmental samples.