The quality and safety of agricultural products are strongly related to human livelihood. Thus, the government and consumers have recently paid increased attention to the quality and safety of agricultural products. The development of efficient, rapid, and sensitive analytical methods for detecting pesticides, veterinary drugs, heavy metals, mycotoxins, and environmental pollutants in agricultural products is of great significance. Owing to the complexity of many sample matrices and the low concentration of pollutants in a typical sample, appropriate sample pretreatment steps are necessary to enrich pollutants in agricultural products. Solid-phase extraction (SPE) is the most widely used sample pretreatment technology; in this technique, the adsorbent generally determines the selectivity and efficiency of the extraction process. An increasing number of novel materials have been used as SPE adsorbents. The extraction efficiency, extraction selectivity, and analytical throughput of SPE could be greatly improved by combining these novel materials with various extraction modes (e. g., solid-phase microextraction, dispersed SPE, and magnetic SPE (MSPE)) during sample preparation. Because of their large specific surface area and high affinity toward target analytes, nanomaterials are often used as SPE adsorbents, thereby greatly improving the selectivity and sensitivity of the analytical technology. More importantly, these materials have become a priority area of research on preconcentration technologies for trace compounds in agricultural products. This paper summarizes the adsorption characteristics of several new nanomaterials, including magnetic materials, carbon-based materials, metal nanomaterials (MNs), metal oxide nanomaterials (MONs), metal organic frameworks (MOFs), and covalent organic frameworks (COFs). These nanomaterials present numerous advantages, such as large specific surface areas, high adsorption capacities, and tailorable structural designs. MSPE employs magnetic materials as sorbents to afford fast dispersion and efficient recycling when applied to complex sample matrices under an external magnetic field. The use of MSPE can avoid several typical problems associated with SPE such as poor adsorbent packing and high pressure, thereby greatly simplifying the pretreatment process and providing a high flux for sample analysis. Carbon-based materials are powdered or bulk nonmetallic solid materials with carbon as the main component; carbon and nitrogen materials, mesoporous carbon, carbon nanotubes, and graphene are some examples of these materials. These materials provide large specific surface areas, abundant pore structures, good thermal stability, high mechanical strength and adsorption capacity, and controllable morphology. Pure and modified carbon nanomaterials have been successfully used to purify target analytes from agricultural products. Given their unique physical and chemical properties, MNs and MONs have attracted sig
Molecularly imprinted polymers have received wide attention from various fields owing to their pre-designable, recognition ability, and practicality. However, the disadvantages of the traditional embedding method, which include a slow recognition rate, uneven site recognition, low binding capacity, and incomplete template molecule elution, limit the development of molecular imprinting technology. Surface molecular imprinting techniques have been developed to effectively solve these problems, and different materials are used as carriers in the synthesis of molecularly imprinted polymers. Metal-organic frameworks (MOFs) show great potential as carriers. Because of their high porosity and specific surface area, MOFs can provide a large number of active sites for molecular imprinting, which can improve their detection sensitivity. The variable metal centers and organic ligands of MOF materials can also lead to multiple structures and functions. Numerous types of MOF materials have been synthesized, and the properties of these materials can be tailored by adjusting their pore size and introducing functional groups. MOFs and molecular imprinting technology can be combined to take full advantage of the specific adsorption of molecular imprinting technology and the large specific surface area and multiple active sites of MOFs, thereby expanding the application range of the resulting materials. In this paper, five aspects of the concept of MOF functionalization are discussed: introduction of special ligands, regulation of metal central sites, formation of MOF complexes, derivatization of MOFs, and sacrificial MOFs. The applications of MOF-based molecularly imprinted materials in catalysis, sample pretreatment, drug carriers, fluorescence sensors, and electrochemical sensors are also reviewed. Finally, the existing problems and future development of MOF-based molecularly imprinted materials are discussed and prospected.
Sophorolipids are secondary metabolites produced during fermentation by nonpathogenic yeasts. These molecules are amphiphilic and consist of a hydrophilic sophora sugar moiety and a hydrophobic hydroxylated fatty acid. Based on their degree of esterification, sophorolipids can be divided into the acid and lactone types. Sophorolipids are highly promising biosurfactants with good antibacterial, antiviral, and other biological activities. Moreover, they are characterized by mildness, low toxicity, and environmental friendliness. However, their composition is quite complex, and effective methods for their quality evaluation are lacking. Since sophorolipids do not absorb ultraviolet (UV) light, common UV detectors are unsuitable for fingerprint establishment. In this study, we first selected a charged aerosol detector (CAD) to establish the ultra-high performance liquid chromatography (UHPLC) fingerprint of sophorolipids. The detector had high sensitivity, good reproducibility, and excellent suitability for the detection of substances with no or weak ultraviolet absorption. We then evaluated the similarities between 17 batches of sophorolipid samples. The samples were extracted by ultrasound for 10 min in 80% ethanol aqueous solution at a liquid-solid ratio of 10∶1 (mL/g) and then separated on a Thermo Fisher Scientific Hypersil Gold chromatographic column (150 mm×2.1 mm, 1.9 μm). Separation was performed using acetonitrile-0.01% (v/v) formic acid aqueous solution as the mobile phase via gradient elution. The flow rate was 0.2 mL/min, and the column temperature was 40 ℃. The CAD was used under the following conditions: power function of 1.0, data rate of 5 Hz, filter constant of 3.6, and evaporation temperature of 45 ℃. The chromatograms and retention times of the sophorolipids were compared, and 16 common peaks with strong responses, good resolutions, and stable retention times were selected as characteristic peaks. Oleic acid was chosen as the reference peak because it achieved good separation and a strong chromatographic response in all batches of samples. UHPLC-quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS) was used to identify chromatographic peaks in the sophorolipid fingerprints. The results were combined with the retention time rule of the sophorolipids, leading to their identification based on matching with the results of the primary database, the precise relative molecular mass and fragmentation rule of secondary fragments, a self-built database, and the PubChem database. Sixteen compounds were identified, including eight acid sophorolipids, six lactone sophorolipids, and two aliphatic acids. The results of precision, repeatability, and 24 h stability tests indicated that the relative standard deviations (RSDs) of the retention times and peak areas of the 15 characteristic peaks relative to the control peak (oleic acid) were less than 3.0% (n=6). Seventeen batches of sophorolipid samples were analyzed, and the simi
Dendrobium officinale (D. officinale) and Anoectochilus roxburghii (A. roxburghii) are precious raw materials for traditional Chinese medicine. The growing demand for D. officinale and A. roxburghii cannot be met by current production techniques. Hence, the widespread artificial cultivation of D. officinale and A. roxburghii using substantial amounts of plant growth regulators (PGRs) has emerged. The excessive use of PGRs not only affects the quality and efficacy of medicinal materials but also causes a series of safety issues. Therefore, expanding research on residual PGRs in valuable Chinese medicinal materials is important to avoid the health hazards caused by these substances. Unfortunately, the identification of PGRs is challenging because of their trace and complex matrices. High performance liquid chromatography (HPLC) has become one of the mainstream analytical methods for PGR determination. An important consideration in the application of this technique to the detection of trace acidic PGRs is how to improve its accuracy and sensitivity. Three-phase hollow fiber liquid phase microextraction (3P-HF-LPME) has the advantages of a high enrichment factor, complex sample purification ability, low reagent consumption, low cost, and easy integration with chromatographic systems. Thus, the 3P-HF-LPME method overcomes the many shortcomings of traditional sample pretreatment methods. In this study, a novel, simple, and effective analytical method based on 3P-HF-LPME combined with HPLC was developed to extract, purify, enrich, and detect three trace acidic PGRs (indole-3-acetic acid, naphthyl acetic acid and indolebutyric acid) in D. officinale and A. roxburghii. The chromatographic separation conditions and 3P-HF-LPME model parameters were systematically optimized for this purpose. First, the sample solution was prepared by ultrasonication and low-temperature standing, and then adjusted to pH 3.0 using dilute hydrochloric acid. The sample solution (10 mL) and NaCl (1.50 g) were stored in a 15 mL brown extraction bottle with a built-in magnetic stirrer. Next, 30 μL of NaOH solution (pH 11.0) as the inner phase solution was injected into the inner cavity of a hollow fiber tube, which was subsequently sealed at both ends. The hollow fiber tube was soaked in n-octanol for 5 min and dried naturally to remove excess extraction solvent from its surface. Finally, the fiber tube was placed in a brown extraction bottle and stirred using a thermostatic magnetic stirrer at 40 ℃ and 1600 r/min for 2 h. After extraction, the three target analytes were separated on a Welch Ultimate XB-C18 column (250 mm×4.6 mm, 5 μm) under isocratic elution conditions using acetic acid aqueous solution and methanol (45∶55, v/v) as the eluent. The results indicated that the three PGRs showed good linearity in the range of 0.5-100.0 μg/L (coefficients of determination (r2
Asymmetrical flow field-flow fractionation (AF4), a gentle tool for the separation and characterization of particles and macromolecules, has attracted increased interest in recent years owing to its broad dynamic size range and utilization of "open channel" voids in the packing or stationary phase. A steric transition phenomenon in which the sample elution mode change from the normal mode to the steric/hyperlayer mode occurs. Accurate characterization by AF4 requires the absence of steric transition, particularly when the sample has a broad size distribution, because the effect of the combination of different modes is difficult to interpret. In this study, the relative molecular mass (M), radius of gyration (Rg), and conformation of Gastrodia elata polysaccharides (GEPs) were characterized using AF4 coupled with online multi-angle light scattering (MALS) and differential refractive index (dRI) detection (AF4-MALS-dRI). Steric transition was observed during GEP separation by AF4 owing to the broad size distribution of the molecules. This phenomenon would result in the inaccurate characterization of the GEPs in terms of M and Rg because two GEP groups of different sizes may elute together. In this study, the effects of constant and exponentially decaying cross-flow rates, sample mass concentration, and spacer thickness on steric transition were systematically investigated. The results indicated that a high GEP mass concentration (i. e., 0.75 mg/mL) can lead to steric transition. The spacer thickness affected the resolution and retention time of the GEPs and changed the steric transition point (di). An exponentially decaying cross-flow rate not only adjusted the di of the polydisperse GEP samples but also improved the GEP resolution and shortened the analysis time. The influence of steric transition was solved under the following operating conditions: injected GEP mass concentration=0.5 mg/mL; injection volume=50 μL; spacer thickness=350 μm; detector flow rate=1.0 mL/min; and cross-flow rate exponentially decayed from 0.2 to 0.05 mL/min with a half-life of 2 min. Moreover, the influence of GEP origins and ultrasound treatment time on the M and Rg distributions and conformation of GEPs were investigated under the optimized operating conditions. The results showed that the M and Rg distributions of Yunnan and Sichuan GEPs decreased with increasing ultrasound time. When the ultrasound treatment time was 15 min, the Yunnan GEPs had a loosely hyperbranched chain conformation, whereas the Sichuan GEPs had a spherical conformation. When the ultrasound treatment time was increased to 30 or 60 min, the GEPs from both Yunnan and Sichuan had a hyperbranched chain conformation, indicating that ultrasound treatment resulted in GEP degradation. Under the same extraction conditions, GEPs from Yunnan had larger M
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