Gongke Li

Layer-by-Layer Fabrication of Chemical-Bonded Graphene Coating for Solid-Phase Microextraction

A new fabrication strategy of the graphene-coated solid-phase microextraction (SPME) fiber is developed. Graphite oxide was first used as starting coating material that covalently bonded to the fused-silica substrate using 3-aminopropyltriethoxysilane (APTES) as cross-linking agent and subsequently deoxidized by hydrazine to give the graphene coating in situ. The chemical bonding between graphene and the silica fiber improve its chemical stability, and the obtained fiber was stable enough for more than 150 replicate extraction cycles. The graphene coating was wrinkled and folded, like the morphology of the rough tree bark. Its performance is tested by headspace (HS) SPME of polycyclic aromatic hydrocarbons (PAHs) followed by GC/MS analysis. The results showed that the graphene-coated fiber exhibited higher enrichment factors (EFs) from 2-fold for naphthalene to 17-fold for B(b)FL as compared to the commercial polydimethylsioxane (PDMS) fiber, and the EFs increased with the number of condensed rings of PAHs. The strong adsorption affinity was believed to be mostly due to the dominant role of π-π stacking interaction and hydrophobic effect, according to the results of selectivity study for a variety of organic compounds including PAHs, the aromatic compounds with different substituent groups, and some aliphatic hydrocarbons. For PAHs analysis, the graphene-coated fiber showed good precision (<11%), low detection limits (1.52-2.72 ng/L), and wide linearity (5-500 ng/L) under the optimized conditions. The repeatability of fiber-to-fiber was 4.0-10.8%. The method was applied to simultaneous analysis of eight PAHs with satisfactory recoveries, which were 84-102% for water samples and 72-95% for soil samples, respectively.

Preparation and evaluation of solid-phase microextraction fiber based on molecularly imprinted polymers for trace analysis of tetracyclines in complicated samples

Molecularly imprinted polymer (MIP) is widely used in many fields because of its characteristics of high selectivity, chemical stability and easy preparation. To enhance the selectivity and applicability of solid-phase microextraction (SPME), a novel MIP-coated SPME fiber was firstly prepared by multiple co-polymerization method with tetracycline as template. It could be coupled directly to high-performance liquid chromatography (HPLC) and used for trace analysis of tetracyclines (TCs) in complicated samples. The characteristics and application of the fibers were investigated. The electron microscope provided a crosslinked and porous surface, and the average thickness of the MIP coating was 19.5 microm. Compared with the non-imprinted polymer (NIP) coated fibers, the special selectivity to tetracycline and structure-similar oxytetracycline, doxycycline, chlortetracycline were discovered with the MIP-coated fibers. The adsorption and desorption of TCs with the MIP-coated fiber could be achieved quickly. A method for the fluorimetric determination of four TCs by the MIP-coated SPME coupled with HPLC was developed. The optimized extraction conditions such as extraction solvent, desorption solvent, and stirring speed were studied. Linear ranges for the four TCs were 5.00-200 microg/L and detection limits were within the range of 1.0-2.3 microg/L. The method was applied to simultaneous multi-residue analysis of four TCs in the spiked chicken feed, chicken muscle, and milk samples with the satisfactory recoveries.

Application of ionic liquids in the microwave-assisted extraction of polyphenolic compounds from medicinal plants.

Ionic liquids (ILs) solutions as solvents were successfully applied in the microwave-assisted extraction (MAE) of polyphenolic compounds from medicinal plants. ILs, its concentration and MAE conditions were investigated in order to extract polyphenolic compounds effectively from Psidium guajava Linn. (P. guajava) leaves and Smilax china (S. china) tubers. The results obtained indicated that the anions and cations of ILs had influences on the extraction of polyphenolic compounds as well as the ILs with electron-rich aromatic pi-system enhanced extraction ability. Under the optimized conditions, the extraction yields of the polyphenolic compounds were in the range of 79.5-93.8% with one-step extraction, and meanwhile the recoveries were in the range of 85.2-103% with relative standard deviations (R.S.D.s) lower than 5.6%. Compared to conventional extraction procedures, the results suggested that the proposed method was effective and alternative for the extraction of polyphenolic compounds from medicinal plants. In addition, the extraction mechanisms and the structures of samples before and after extraction were also investigated. ILs solutions as green solvents in the MAE of polyphenolic compounds from medicinal plant samples showed a great promising prospect.

Preparation and evaluation of propranolol molecularly imprinted solid-phase microextraction fiber for trace analysis of β-blockers in urine and plasma samples

An improved multiple co-polymerization technique was developed to prepare a novel molecularly imprinted polymer (MIP)-coated solid-phase microextraction (SPME) fiber with propranolol as template. Investigation was performed for the characteristics and application of the fibers. The MIP coating was highly crosslinked and porous with the average thickness of only 25.0 microm. Consequently, the adsorption and desorption of beta-blockers within the MIP coating could be achieved quickly. The specific selectivity was discovered with the MIP-coated fibers to propranolol and its structural analogues such as atenolol, pindolol, and alprenolol. In contrast, only non-specific adsorption could be shown with the non-imprinted polymer (NIP)-coated fibers, and the extraction efficiencies of propranolol and pindolol with the MIP-coated fibers were higher markedly than that with the commercial SPME fibers. A MIP-coated SPME coupled with high-performance liquid chromatography (HPLC) method for propranolol and pindolol determination was developed under the optimized extraction conditions. Linear ranges for propranolol and pindolol were 20-1000 microg L(-1) and detection limits were 3.8 and 6.9 microg L(-1), respectively. Propranolol and pindolol in the spiked human urine and plasma samples, extracted with organic solvent firstly, could be simultaneous monitored with satisfactory recoveries through this method.

Investigation of ractopamine molecularly imprinted stir bar sorptive extraction and its application for trace analysis of β2-agonists in complex samples.

In this paper, a novel molecularly imprinted polymer (MIP) coated stir bar with ractopamine as template by glass capillary filling with magnetic core as substrate was prepared reproducibly. The ractopamine MIP coating was homogeneous and porous with the average thickness of 20.6 microm. The extraction apparatus for the stir bar was improved to avoid coating loss. The MIP-coated stir bar showed better extraction capacity and good selectivity than that of non-imprinted polymer (NIP) coated stir bar to ractopamine and its analogues. The extraction capacities of ractopamine, isoxsuprine, clenbuterol and fenoterol for MIP-coated stir bar were 3.3, 3.1, 2.8 and 2.4 times as much as that of the NIP coated stir bar, respectively. The MIP-coated stir bars could be used at least 40 times without apparent damage and kept in dried air for 8 months without reduce of extraction ability. A method for the determination of beta(2)-agonists in complex samples by MIP-coated stir bar sorptive extraction coupled with high-performance liquid chromatography (HPLC) was developed. The linear ranges were 0.5-40 microg/L for ractopamine and 1.0-40 microg/L for isoxsuprine and clenbuterol. The detection limits were within the range of 0.10-0.21 microg/L. The method was successfully applied to the analysis of beta(2)-agonists in spiked pork, liver and feed samples with the recoveries of 83.7-92.3%, 80.5-90.2% and 73.6-86.2%, respectively. The RSDs was within 2.9-8.1%. The method is very suitable for the determination of trace beta(2)-agonists in pork, liver and feed samples.

Chemical Bonding Approach for Fabrication of Hybrid Magnetic Metal–Organic Framework-5: High Efficient Adsorbents for Magnetic Enrichment of Trace Analytes

A facile and efficient strategy about the synthesis of a novel kind of hybrid magnetic metal-organic framework (MOF)-5 via chemical bonding assembly was reported. The covalent bonding established between the amino functionalized Fe3O4 nanoparticles and the surface of the metal organic framework improved the chemical stability and structure uniformity of the hybrid microcrystals. Combination of MOF-5 with Fe3O4 nanoparticles allows for facile withdrawal of the porous materials by magnetic decantation. The powder X-ray diffraction patterns of the hybrid magnetic MOF-5 showed the structure of the metal organic framework was not disturbed with the decoration of magnetic nanoparticles. The as-synthesized materials combine the favorable attributes of both magnetic characteristics of Fe3O4 nanoparticles and high porosity of metal organic framework, making them excellent candidates as adsorbents for magnetic enrichment of trace analytes. Their potential applications were explored by preconcentrating polycyclic aromatic hydrocarbons and gibberellic acids from environmental, food, and plant samples prior to gas chromatography-mass spectrometry (GC/MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), respectively. The results showed that the magnetic MOF-5 exhibited superior enrichment capacity for both of these nonpolar and polar analytes. The method demonstrated good precision (relative standard deviations (RSDs) of 1.7-9.7%), low detection limits (0.91-1.96 ng·L(-1) for polycyclic aromatic hydrocarbons and 0.006-0.08 μg L(-1) for gibberellic acid), and good linearity (correlation coefficients higher than 0.9949). The RSDs of batch-to-batch extraction were 2.9-11.2%. The magnetic MOF-5 was robust enough for repeatable use without damage of extraction performance.

Magnetic molecularly imprinted polymer beads prepared by microwave heating for selective enrichment of β-agonists in pork and pig liver samples

Novel magnetic molecularly imprinted polymer (MIP) beads using ractopamine as template for use in extraction was developed by microwave heating initiated suspension polymerization. Microwave heating, as an alternative heating source, significantly accelerate the polymerization process. By incorporating magnetic iron oxide, superparamagnetic composite MIP beads with average diameter of 80 μm were obtained. The imprinted beads were then characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis and vibrating sample magnetometer. Highly cross-linked porous surface and good magnetic property were observed. The adsorption isotherm modeling was performed by fitting the data to Freundlich isotherm model. The binding sites measured were 3.24 μmol g(-1) and 1.17 μmol g(-1) for the magnetic MIP beads and the corresponding non-imprinted magnetic beads, respectively. Cross-selectivity experiments showed the recognition ability of the magnetic MIP beads to analytes is relative to degree of molecular analogy to the template. Finally, this magnetic MIP bead was successfully used for enrichment of ractopamine, isoxsuprine and fenoterol from ultrasonically extracted solution of pork and pig liver followed by high performance chromatography with fluorescence detection. The proposed method presented good linearity and the detection limits was 0.52-1.04 ng mL(-1).The recoveries were from 82.0% to 90.0% and from 80.4% to 86.8% for the spiked pork and pig liver, respectively, with the RSDs of 5.8-10.0%. Combination of the specific adsorption property of the MIP material and the magnetic separation provided a powerful analytical tool of simplicity, flexibility, and selectivity.

Preparation of magnetic indole-3-acetic acid imprinted polymer beads with 4-vinylpyridine and β-cyclodextrin as binary monomer via microwave heating initiated polymerization and their application to trace analysis of auxins in plant tissues.

Auxin is a crucial phytohormone for precise control of growth and development of plants. Due to its low concentration in plant tissues which are rich in interfering substances, the accurate determination of auxins remains a challenge. In this paper, a new strategy for isolation and enrichment of auxins from plant tissues was obtained by the magnetic molecularly imprinted polymer (mag-MIP) beads, which were prepared by microwave heating initiated suspension polymerization using indole-3-acetic acid (IAA) as template. In order to obtain higher selective recognition cavities, an enhanced imprinting method based on binary functional monomers, 4-vinylpyridine (4-VP) and β-cyclodextrin (β-CD), was adopted for IAA imprinting. The morphological and magnetic characteristics of the mag-MIP beads were characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy and vibrating sample magnetometry. A majority of resultant beads were within the size range of 80-150μm. Porous surface morphology and good magnetic property were observed. Furthermore, the mag-MIP beads fabricated with 4-VP and β-CD as binary functional monomers exhibited improved recognition ability to IAA, as compared with the mag-MIP beads prepared with the individual monomer separately. Competitive rebinding experiment results revealed that the mag-MIP beads exhibited a higher specific recognition for the template than the non-imprinted polymer (mag-NIP) beads. An extraction method by mag-MIP beads coupled with high performance liquid chromatography (HPLC) was developed for determination of IAA and indole-3-butyric acid (IBA) in plant tissues. Linear ranges for IAA and IBA were in the range of 7.00-100.0μgL(-1) and 10.0-100.0μgL(-1), and the detection limits were 3.9 and 7.4μgL(-1), respectively. The analytical performance was also estimated by seedlings or immature embryos samples from three different plant tissues, pea, rice and wheat. Recoveries were in the range of 70.1-93.5%. The results show that the present imprinting method is a promising approach for preparation of selective adsorbents for sample preparation of auxin analysis in plant tissues.

Metal-organic framework-199/graphite oxide hybrid composites coated solid-phase microextraction fibers coupled with gas chromatography for determination of organochlorine pesticides from complicated samples

The hybrid material of a copper-based metal-organic framework (MOF-199) and graphite oxide (GO) was explored as the solid-phase microextraction (SPME) coating for the first time. This fiber was fabricated by using 3-amino-propyltriethoxysilane (APTES) as the cross-linking agent, which enhanced its durability and allowed more than 140 replicate extractions. With the incorporation of GO, the MOF-199/GO fibers with GO contents ranging from 5 to 15 wt% exhibited enhanced adsorption affinity to organochlorine pesticides (OCPs) compared to MOF or GO individually. This improvement was linked to the enhanced dispersive forces (increased volume of small pores) that provided by the dense carbon layers of GO. Combining the superior properties of high porosity of MOFs and the unique layered character of GO, the MOF-199/GO (10 wt%) fiber exhibited higher adsorption affinity to some OCPs than commercial polydimethylsiloxane (PDMS) and polydimethylsiloxane/divinylbenzene (PDMS/DVB) fibers. This new fiber was developed for headspace (HS) SPME of eight OCPs followed by GC/ECD analysis. The limits of detection were 2.3-6.9 ng/L. The relative standard deviation (RSD) for five replicate extractions using one fiber ranged from 5.3% to 8.8%. The fiber-to-fiber reproducibility was 5.2-12.8%. This method was successfully used for simultaneous determination of eight OCPs from river water, soil, water convolvulus and longan with satisfactory recoveries of 90.6-104.4%, 82.7-96.8%, 72.2-107.7% and 82.8-94.3%, respectively. These results indicated the MOF-199/GO composite provided a promising alternative in sample pretreatment.

Review of online coupling of sample preparation techniques with liquid chromatography

Sample preparation is still considered as the bottleneck of the whole analytical procedure, and efforts has been conducted towards the automation, improvement of sensitivity and accuracy, and low comsuption of organic solvents. Development of online sample preparation techniques (SP) coupled with liquid chromatography (LC) is a promising way to achieve these goals, which has attracted great attention. This article reviews the recent advances on the online SP-LC techniques. Various online SP techniques have been described and summarized, including solid-phase-based extraction, liquid-phase-based extraction assisted with membrane, microwave assisted extraction, ultrasonic assisted extraction, accelerated solvent extraction and supercritical fluids extraction. Specially, the coupling approaches of online SP-LC systems and the corresponding interfaces have been discussed and reviewed in detail, such as online injector, autosampler combined with transport unit, desorption chamber and column switching. Typical applications of the online SP-LC techniques have been summarized. Then the problems and expected trends in this field are attempted to be discussed and proposed in order to encourage the further development of online SP-LC techniques.