Graphene quantum dots functionalized β-cyclodextrin and cellulose chiral stationary phases with enhanced enantioseparation performance
Introduction
Chirality is an essential attribute of nature. The separation of enantiomers of chiral compounds continues to be of great interest due to their prevalence in the pharmaceutical industry, agrochemicals and food additives [1]. The task of enantioseparation is extremely challenging, because enantiomers are identical in most regards: they have the same densities and solubilities, boiling and melting points, electronic and vibrational frequencies, reactivities and refractivities, etc [2]. Chiral separation materials are the core to achieve enantioseparation. Cyclodextrins (CD) and their derivatives as well as cellulose derivatives are the most commonly used chiral selectors at present [[3], [4], [5], [6]]. With the rapid development of pharmaceutical industry and more and more attention to life safety, higher demands will be put forward for chiral separation. It can be expected that developing novel and effective chiral separation materials has great significance.
Carbon nanomaterials (CNM) with good physical and chemical characteristics have been widely used in various disciplines [[7], [8], [9], [10], [11], [12]]. With the development of CNMs in separation science, various CNMs have been applied to enantioseparation in recent years. Chiral single-walled carbon nanotubes and multi-walled carbon nanotubes have been directly applied for the enantioseparation of pharmaceuticals and biologicals by using them as stationary or pseudostationary phases in chromatographic separation techniques including high performance liquid chromatography (HPLC), capillary electrochromatography (CEC) and gas chromatography (GC) [13]. Collectively, functionalized carbon nanotubes have been indirectly applied in separation science by enhancing the enantioseparation of different chiral selectors [13,14]. Chiral selectors functionalized graphene oxide (GO) composites have been applied in CEC enantioseparation [[15], [16], [17], [18], [19]], however, their application in HPLC enantioseparation was relatively scarce [20,21]. In CEC, Qiu and coworkers prepared a series of chiral selectors functionalized GO nanocomposites [[15], [16], [17]] as chiral stationary phases (CSP) for chip-based open-tubular CEC. Li, Ji and coworkers prepared three types of GO-functionalized chiral affinity capillary silica monoliths [18]. Du and coworkers established a GO-modification CEC system for enantioseparation with methyl-β-CD as chiral mobile phase additives [19]. Research results demonstrated that GO possessed large surface area and abundant functional groups which were necessary for chiral selectors immobilization and could provide various interactions for enantioseparation with improved performance. In HPLC, Li and coworkers prepared cellulose derivative coated rGO-bonded silica CSP [21]. Due to the existence of rGO on the CSP, this CSP obtained stronger interaction with the analytes leading to better enantioseparation performance compared with cellulose derivative modified silica CSP. According to previous research, we may find that CNMs have great prospect in chiral separation.
Graphene quantum dots (GQD), as a new type of CNM, have been widely used in bioimaging and fluorescent sensing because of good water solubility, biocompatibility and unique fluorescence stability [22]. Recently, great efforts have been devoted to explore potential applications of GQDs in separation science, GQDs are gradually applied in GC [23], CEC [24] and HPLC [[25], [26], [27], [28], [29]] showing satisfactory separation performance. As chromatographic material, GQDs have some unique characteristics such as small dimension, high thermostability and good dispersibility, which make them easier to be immobilized on the silica support. Moreover, GQDs possess highly delocalized conjugate system of π-electron and abundant oxygen-containing groups, which enables them to provide various interactions including π-π, hydrophobic, hydrogen-bonding and hydrophilic interactions and thus to be used as multifunctional separation material. It is reasonable to believe that GQDs have great prospect in separation science. Though the separation performance of GQDs for all kinds of achiral compounds has been investigated systematically, their application in chiral separation has not been explored. We wonder if GQDs, as good chromatographic material and biocompatible nanomaterial, could be the chiral enhancement material for nature chiral selectors.
Hence, GQDs functionalized β-CD and cellulose silica composites were first prepared and applied in HPLC as CSPs to investigate the effect of GQDs on chiral separation. Ten chiral compounds were chosen as probes to evaluate the enantioseparation performance of the prepared CSPs. β-CD CSP and cellulose CSP were prepared and used as references. Generally, 3,5-dimethylphenyl isocyanate is always used as hydroxyls derivatization reagent of β-CD and cellulose to enhance their enantioseparation ability [[3], [4], [5],30,31]. Especially for cellulose CSPs, the available cellulose CSPs are cellulose derivatives-based CSPs since the poor solubility of cellulose [32,33]. To investigate the difference between GQDs and 3,5-dimethylphenyl isocyanate in abilities to enhance chiral separation performance, the enantioseparation performance of GQDs functionalized β-CD CSP was also compared with that of 3,5-dimethylphenylcarbamate derived β-CD CSP. In addition, β-CD-3,5-dimethylphenylcarbamate CSP was also functionalized by GQDs to further explore the chiral enhancement effect of GQDs to β-CD-3,5-dimethylphenylcarbamate derivative.
Section snippets
Materials and instruments
Silica gel (70 Å, 800 Å, 5 μm in diameter) was supplied by Fuji Silysia Chemical Ltd (Japan). 3-aminopropyltriethoxysilane (APTES), 3-isocyanatopropyltriethoxysilane, 3,5-dimethylphenyl isocyanate, β-CD and microcrystalline cellulose were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) were purchased from Meryer Chemical Technology Co. Ltd (Shanghai, China). All chiral chemicals
Characterization
Fig. 2 showed the FTIR spectra of Si-CD, Si-GQD-CD, Si-CDD, Si-GQD-CDD, Si-Cellulose and Si-GQD-Cellulose. Compared to Si-CD, the enhanced peak at 1731 cm−1 (CO in OCO) of Si-GQD-CD indicated the existence of GQDs since GQDs had COOH. In Si-CDD and Si-GQD-CDD, the enhanced peaks at 1731 cm−1, 1627 cm−1 and 1561/1442 cm−1 which was assigned to the characteristic absorption peak of OCO, NH and benzene ring demonstrated that β-CDs were derived with 3,5-dimethylphenyl isocyanate successfully.
Conclusions
To investigate the effect of GQDs on chiral separation, GQDs functionalized β-CD and cellulose silica composites were first prepared and applied in HPLC enantioseparation. Through comparing the enantioseparation performance of GQDs functionalized β-CD or cellulose CSPs and unmodified β-CD or cellulose CSPs, it was found GQDs could enhance the enantioseparation performance of nature β-CD, β-CD-3,5-dimethylphenylcarbamate derivative and cellulose-3,5-dimethylphenylcarbamate derivative. GQDs had
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Nos. 21405162, 21405161, 21675163 and 21702210).
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