With the rapid development of the industry, thousands of dyes have been used in a wide variety of fields such as printing, cosmetics, paper, agricultural products and so on (X. Chen, et al., 2018; Paliwal, Sonia, & Meher, 2021). The use of dyes brings great convenience, but also many threats. In particular, the environmental pollution and food safety caused by dyes have attracted more and more attention (Lipskikh, Korotkova, Khristunova, Barek, & Kratochvil, 2018; Senol, Gursoy, Simsek, Ozer, & Karakus, 2020). For environment, many dyes such as direct red 80 and methyl blue are used as intermediates of chemical products or dyes for products such as leather, clothes and so on. The use of these dyes will produce a lot of dye wastewater (Rafiq, et al., 2021). As a result, dye wastewater has become an important source of water pollution. Even very small amounts of dyes can reduce light transport in the water environment and thus reduce plant photosynthesis. More seriously, these dye wastewaters can lead to contamination of water, thus causing tumors, mutations and heart diseases to human (Basu & Kumar, 2015; Romdhani, et al., 2021; Salleh, Mahmoud, Karim, & Idris, 2011). For food, food dyes are found in many foods, such as meat, cereals, confectionery, beverages, and are even applied in pharmaceutics. In order to increase the marketability to consumers, food dyes such as congo red are usually used to improve the appearance of food and medicines. However, the use of food dyes in foods and medicine has historically been controversial because of the associated health consequences. It is worth noting that some foods are required to label the dye content, or in some countries, dyes are banned altogether (Oplatowska-Stachowiak & Elliott, 2017; Staples, et al., 2020). However, in order to seek profits, many unscrupulous businessmen abuse the dyes, which will lead to the excessive concentration of dyes in the food, and then cause serious problems to human health. Therefore, it is important to monitor and remove the dyes in environmental water and food samples.
Thus far, various methods such as electrochemical method (Yao, Lv, Wang, Hu, & Chen, 2022), photocatalytic degradation (Joshi, Gururani, & Gairola, 2022) and adsorption (Pishnamazi, et al., 2021; J. Q. Zhu, et al., 2019) have been developed for the removal of dyes. In these methods, adsorption has been regarded as the most promising approach for the removal of dyes due to the various advantages such as simple steps, low cost, and high efficiency (Y. Li, et al., 2022; X. H. Wang, et al., 2018). Moreover, by combining adsorption with appropriate analytical method (e.g. HPLC, UV-vis and fluorescence analysis), the concentrations of dyes in the samples such as dye wastewaters and food can be effectively monitored (Q. L. Liu, Xu, & Sun, 2021; Yu, Cheng, Li, Li, & Song, 2022). To date, to meet the demand for the effective adsorption of dyes, a variety of adsorbents such as graphene oxide, layered-double hydroxide nanomaterial, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) (El Khattabi, Mourid, Belaaouad, & Naimi, 2022; Feng, Liu, Abu-Hamdeh, Bezzina, & Malekshah, 2022; Verma, et al., 2022; Q. M. Wang, et al., 2022) have been manufactured. Specially, ascribed to the advantages including adjustable pore sizes, large specific surface area, high chemical stability, abundant functional group characteristics, COFs as adsorbents have become the focus of attention (Han, Pei, Du, & Zhu, 2022; Huang, et al., 2022; Romero, et al., 2022).
Till now, various COFs have been developed for dyes adsorption. For examples, Yan and et al have prepared carboxyl-functionalized COF (carboxyl-COF), and the carboxyl-COF can be applied to the effective adsorption of crystal violet and brilliant green from water samples (Y. Li, Yang, Qian, Zhao, & Yan, 2019). Zhou and co-workers have manufactured hierarchically porous COF and applied the COF to the adsorption and removal of triphenylmethane dyes including malachite green and crystal violet from aqueous solution (J. C. Liu, Li, Yu, & Zhou, 2021). Kuo etc. have fabricated heteroporous bifluorenylidene-based COFs for the effectively adsorption of rhodamine B with a maximum adsorption capacity of 2127 mg⋅g− 1 (EL-Mahdy, et al., 2020). It can be seen from these results that COFs has great application potential for dyes adsorption. However, existing COFs are still very limited for dyes adsorption. Most of them are used to remove dyes from water samples, and few studies have been done to detect dyes in food. In addition, most of COFs are synthesized by a solvothermal synthesis method with the freeze-pump-thaw cycle, which is relative complicated. Remarkably, synthesis method at room temperature is considered to be an effective method for the synthesis of COFs. In this method, COFs can be obtained for a period of time in solvent (e.g. water and acetonitrile) at room temperature (J. He, et al., 2022; Ma, et al., 2019). COFs with high crystallinity can be obtained by optimizing the synthesis conditions, which is conducive to the subsequent use as dyes adsorbents. Thus, it is of great value and significance to develop more COFs with room temperature synthesis method for the adsorption of dyes in water and food samples.
Inspired by above mentioned things, a simple room temperature synthesis method which can be applied to the preparation of a COF (namely, COF-TFTP) with high crystallinity, was fabricated. The resultant COF-TFTP with uniform spherical structure had high crystallinity, satisfactory surface area and rich functional group characteristics. Making use of the performance of the COF-TFTP, it was served as a potentially effective adsorbent for solid-phase extraction (SPE) (Tippins, 1988) of dyes (including congo red, direct red 80 and methyl blue, structures were shown in Figure S1). The influences parameters of adsorption and desorption of dyes were investigated in turn. Besides, the selective adsorption mechanisms of three dyes had also been studied and discussed. Finally, the practicability of COF-TFTP based SPE coupled with HPLC-DAD for extracting three dyes from food samples (e.g. pork and fish) and water samples was introduced in detail.