Abstract:

Neurotransmitters (NTs) are basic signaling chemicals used for communication between cells. The most well-known catecholamines (CAs) are epinephrine, norepinephrine, and dopamine. CAs are an important class of monoamine NTs that contain catechins and amine groups. The accurate determination of CAs in biological samples can provide essential information on potential pathogenic mechanisms. However, biological samples generally contain only trace levels of CAs. Therefore, sample pretreatment is necessary to separate and enrich CAs before instrument analysis. Dispersive solid-phase extraction (DSPE) technology combines the principles of liquid-liquid extraction and solid-phase extraction and is a useful method for purifying and enriching the target analytes in complex matrices. This method has the advantages of low solvent consumption, environmental safety, and high sensitivity and efficiency. In addition, the adsorbents used in DSPE do not need to be packed into a column and can simply be completely dispersed in the sample solution; this excellent feature greatly improves the extraction efficiency and simplifies the extraction process. Therefore, the development of new DSPE materials with high efficiency and adsorption capacity using simple preparation procedures has received wide attention from the research community. Carbon nitrides (MXenes) are a class of two-dimensional layered materials that possess good hydrophilicity, a large number of functional groups (-O, -OH, and -F), large layer spacing, different elemental compositions, excellent biocompatibility, and environmental friendliness. However, these materials have a small specific surface area and poor adsorption selectivity, which limits their applications in SPE. The separation selectivity of MXenes can be significantly improved by functional modification. Polyimide (PI) is a crosslinking product that is mainly formed by the condensation polymerization of binary anhydride and diamine. It has a unique crosslinked network structure, as well as a large number of carboxyl groups, and shows excellent characteristics. Therefore, the synthesis of new PI-functionalized Ti₃C₂T_x (Ti₃C₂T_x/PI) composites by growing a PI layer on the surface of two-dimensional MXene nanosheets in situ may not only overcome the adsorptive limitations of MXenes but also effectively improve their specific surface area and porous structure, thereby enhancing their mass transfer capacity, adsorption capacity, and selectivity.

In this study, a Ti₃C₂T_x/PI nanocomposite was fabricated and successfully applied as a DSPE sorbent to enrich and concentrate trace CAs in urine samples. The prepared nanocomposite was examined using various characterization methods, including scanning electron microscopy, Fourier transform-infrared spectroscopy, X-ray diffraction, and zeta potential analysis. The effects of the extraction parameters on the extraction efficiency of Ti₃C₂T_x/PI were also investigated in detail. The adsorption performance of Ti₃C₂T_x/PI can be described by pseudo-second-order kinetics and the Freundlich isotherm model. The adsorption process appeared to occur on the outer surface, as well as surface voids, of the nanocomposite. The adsorption mechanism of Ti₃C₂T_x/PI indicated a chemical adsorption process based on multiple electrostatic, π-π, and hydrogen-bonding interactions. The optimal adsorption conditions included an adsorbent dosage of 20 mg, sample pH of 8, adsorption and elution times of 10 and 15 min, respectively, and eluent composed of acetic acid-acetonitrile-water (5∶47.5∶47.5, v/v/v). A sensitive method for detecting CAs in urine was subsequently developed by coupling Ti₃C₂T_x/PI as a DSPE sorbent with HPLC-FLD analysis. The CAs were separated on an Agilent ZORBAX ODS analytical column (250 mm×4.6 mm, 5 μm). Methanol and an aqueous solution of 20 mmol/L acetic acid were used as the mobile phases for isocratic elution. Under optimal conditions, the proposed DSPE-HPLC-FLD method exhibited good linearity in the range of 1-250 ng/mL with correlation coefficients >0.99. The limits of detection (LODs) and limits of quantification (LOQs) were calculated based on signal-to-noise ratios of 3 and 10 and found to be in the range of 0.20-0.32 and 0.7-1.0 ng/mL, respectively. The recoveries of the method were in the range of 82.50%-96.85% with RSDs≤9.96%. Finally, the proposed method was successfully applied to the quantification of CAs in urine samples from smokers and nonsmokers, thereby indicating its applicability for determining trace CAs.

Key words: liquid chromatography (LC), dispersive solid-phase extraction (DSPE), two dimensional titanium carbide (Ti₃C₂T_x), polyimide, catecholamines (CAs)