Zilin Chen

Universal multilayer assemblies of graphene in chemically resistant microtubes for microextraction.

Graphene is a new kind of two-dimensional carbon nanomaterial with excellent properties and is promising for solid-phase microextraction (SPME). Plastic microtubes such as poly(tetrafluoroethylene) (PTFE) and poly(ether ether ketone) are ideal substrates for in-tube SPME. However, immobilization of graphene layers onto these materials is still a problem due to their nature of chemical resistance. In order to solve the problem, we proposed a novel method based on universal mussel-inspired polydopamine (PD) and layer-by-layer assembly of graphene in this work. To make a graphene assembly layer inside PTFE, the strategy includes two major steps. First, a PD layer is made on the PTFE surface by noncovalent interaction. Second, multilayer graphene is assembled on the PD layer by covalent interaction. By repeating these two steps, a functional graphene oxide (FGO)-modified PTFE tube with a controllable number of layers can be obtained. Morphology of the multilayer structure of graphene has been confirmed by scanning electronic microscopy. Formation of the covalent layer has also been characterized by Foourier transform infrared and X-ray photoelectron spectroscopy. It is very interesting that (FGO-PD)3-PTFE shows exceptional efficiency for SPME. Enrichment from 1082- to 2331-fold was achieved for six polyaromatic hydrocarbons (PAHs). An online SPME-HPLC-fluorescent detection method has been developed on the basis of (FGO-PD)3-PTFE. For qualitative analysis of PAHs, the method has low limits of detection of 0.05-0.1 pg/mL, which is significantly lower (up to 1000 times) than that reported in literature. The method shows wide linear range (0.3-200 pg/mL), good linearity (R(2) ≥ 0.9968), and good reproducibility (relative standard deviation < 3.4%). The method has been applied to determine PAHs in environmental samples. Good recoveries were obtained, ranging from 85.1% to 96.7%.

Mussel inspired polydopamine functionalized poly(ether ether ketone) tube for online solid-phase microextraction–high performance liquid chromatography and its application in analysis of protoberberine alkaloids in rat plasma

A novel and simple poly(ether ether ketone) (PEEK) tube-based solid-phase microextraction (SPME)-HPLC method was developed in this work. A mussel inspired polydopamine method was used to functionalize the chemically resistant surface of PEEK tube and a poly(acrylamide-ethylene glycol dimethacrylate) monolith was synthesized inside the PEEK tube and chemically bonded with its surface. The stability of the polydopamine layer was investigated and found to be well enduring to most of commonly used organic solvents and strong acidic conditions. The PEEK tube-based monolith was coupled with HPLC system through a six-port valve, and the performance of the online SPME-HPLC system was demonstrated by analyzing three protoberberine alkaloids in aqueous samples, namely jatrorrhizine (Jat), palmatine (Pal) and berberine (Ber). These analytes were well extracted and separated within 10 min, and enrichment factors of about 400 were obtained. The limit of detection of the proposed method was decreased to 0.01 ng/mL by using UV detection. Finally, the online SPME-HPLC method was applied for determining Jat, Pal and Ber in rat plasma samples. Reproducibility was obtained by evaluating the intra- and inter-day precisions with RSDs less than 6.1% and 8.5%, respectively, and recoveries of the analytes in spiked plasma samples ranged from 89.8% to 96.7%. The proposed method was also applied to determine the concentration of Jat, Pal and Ber in rat plasma after oral administration of Cortex Phellodendri Chinensis.

An electropolymerized Nile Blue sensing film-based nitrite sensor and application in food analysis.

This paper reports a poly-Nile Blue (PNB) sensing film based electrochemical sensor and the application in food analysis as a possible alternative for electrochemical detection of nitrite. The PNB-modified electrode in the sensor was prepared by in situ electropolymerization of Nile Blue at a prepolarized glassy carbon (GC) electrode and then characterized by cyclic voltammetry (CV) and pulse voltammetry in phosphate buffer (pH 7.1). Several key operational parameters affecting the electrochemical response of PNB sensing film were examined and optimized, such as polarization time, PNB film thickness and electrolyte pH values. As the electroactive PNB sensing film provides plenty of active sites for anodic oxidation of nitrite, the nitrite sensor exhibited high performance including high sensitivity, low detection limit, simple operation and good stability at the optimized conditions. The nitrite sensor revealed good linear behavior in the concentration range from 5.0x10(-7) mol L(-1) to 1.0x10(-4) mol L(-1) for the quantitative analysis of nitrite anion with a limit of detection of 1.0x10(-7) mol L(-1). Finally, the application in food analysis using sausage as testing samples was investigated and the results were consistent with those obtained by standard spectrophotometric method.

Preparation of micropipette tip-based molecularly imprinted monolith for selective micro-solid phase extraction of berberine in plasma and urine samples.

A novel berberine-imprinted polymer (MIP) monolith was prepared for extraction of berberine in aqueous medium. The MIP monolith was prepared inside a polypropylene micropipette tip by using dimethylsulfoxide as porogen, acrylamide (AA) as functional monomer and ethyleneglyol dimethacrylate (EGDMA) as cross-linker. Polymerization conditions were optimized and good permeability and selectivity was obtained when the ratio of berberine/AA/EGDMA was 1:5:30. Cross-reaction was also studied by three compounds (palmatine, coptisine, and jatrorrhizine) with similar structure. A molecularly imprinted micro solid-phase extraction (MI-μ-SPE) method was developed for selective extraction of berberine in aqueous solutions. Extraction parameters were investigated, such as sample pH value, sample flow rate, sample volume and elution solvent. By combining with HPLC/UV, MI-μ-SPE method showed a good linear range of 3-800 ng/mL with a low limit of detection limit of 1.0 ng/mL. The method was also applied for the pretreatment of berberine in human plasma and urine samples. The result showed that proteins and other biological matrix were successfully eliminated and berberine was selectively enriched. Recoveries were tested in plasma and urine samples, and calculated to be 90.6-103.2% with relative standard deviations less than 4.7%.

Polydopamine-based immobilization of zeolitic imidazolate framework-8 for in-tube solid-phase microextraction.

Zeolitic imidazolate frameworks (ZIFs), a subfamily of metal-organic frameworks (MOFs), have received considerable attention as a novel porous material for sample preparation due to their fascinating structures and unique properties. Here, we developed a novel bio-inspired polydopamine (PDA) method to immobilize ZIFs inside the chemical resistance poly(ether ether ketone) (PEEK) tube for online solid-phase microextraction (SPME). Firstly, PDA layer was assembled inside the PEEK tube. Secondly, attributing to its noncovalent adsorption and covalent reaction ability, PDA could attract and anchor ZIF-8 nutrients onto the inner surface of PEEK tube to promote the nucleation and growth of ZIF-8. The morphology and surface properties of the prepared ZIF-8-PDA-PEEK tube were characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. The ZIF-8-PDA-PEEK tube exhibits excellent extraction efficiency toward six polycyclic aromatic hydrocarbons (PAHs), with enrichment factor from 550 to 734. The developed online SPME-HPLC method shows good linearity (10-5000pg/mL) and low detection limits (0.5-5 pg/mL) for six PAHs. It also has been used to determine PAHs in environmental samples, with recoveries in the range of 82.5-98.6%.

Growth of metal–organic framework HKUST-1 in capillary using liquid-phase epitaxy for open-tubular capillary electrochromatography and capillary liquid chromatography

Much attention is being paid to applying metal-organic frameworks (MOFs) as stationary phases in chromatography because of their fascinating properties, such as large surface-to-volume ratios, high levels of porosity, and selective adsorption. HKUST-1 is one of the best-studied face-centered-cubic MOF containing nano-sized channels and side pockets for film growth. However, growth of HKUST-1 framework inside capillary column as stationary phase for capillary electrochromatography is a challenge work. In this work, we carry out the growth of HKUST-1 on the inner wall of capillary by using liquid-phase epitaxy process at room temperature. The fabricated HKUST-1@capillary can be successfully used for the separation of substituted benzene including methylbenzene, ethylbenzene, styrene, chlorobenzene, bromobenzene, o-dichlorobenzene, benzene series, phenolic acids, and benzoic acids derivates. High column efficiency of 1.5×10(5) N/m for methylbenzene was achieved. The formation of HKUST-1 grown in the capillary was confirmed and characterized by scanning electron microscopy images, Fourier transform infrared spectra and X-ray diffraction. The column showed long lifetime and excellent stability. The relative standard deviations for intra-day and inter-day repeatability of the HKUST-1@capillary were lower than 7%.

Determination of polycyclic aromatic hydrocarbons in water samples using online microextraction by packed sorbent coupled with gas chromatography–mass spectrometry

A fully automated microextraction by packed sorbents (MEPS) coupled with large volume injection gas chromatography-mass spectrometry (GC-MS) has been developed for the determination of eight polycyclic aromatic hydrocarbons (PAHs) in environmental water samples. Naphthalene (Nap), pyrene (Pyr), anthracene, acenaphthylene, phenanthrene, fluoranthene (Flr), fluorene and acenaphthene were the PAHs studied. The performance of the microextraction-GC-MS protocol was compared with solid phase extraction (SPE) and GC-MS analysis. Under optimized experimental conditions, the methods were linear for all analytes in the following ranges: 0.05-2.0 μg L(-1) (MEPS) and 0.25-10.0 μg L(-1) (SPE). The correlation coefficients (R(2)) were in the range 0.9965-0.9997 (MEPS) and 0.9978-0.9998 (SPE) for all the analytes. Limits of detection (LODs) for 2 mL samples (MEPS) ranged from 0.8 ng L(-1) to 8.2 ng L(-1). LODs for 50 mL samples (SPE) were between 4.8 ng L(-1) and 35.9 ng L(-1). The two methods were successfully applied to the determination of the 8 PAHs in environmental waters, with recoveries in the range of 70-117% (MEPS) and 72-134% (SPE) for a real spiked sample. The two sample preparation processes showed good repeatabilities with intra-day relative standard deviations below 14.0% (MEPS) and 14.6% (SPE). Nap, Flr and Pyr were found in a river water sample.

Analysis of four alkaloids of Coptis chinensis in rat plasma by high performance liquid chromatography with electrochemical detection.

A sensitive and precise high performance liquid chromatography (HPLC)-electrochemical detection (ECD) method has been developed for the simultaneous determination of four isoquinoline alkaloids including berberine, jatrorrhizine, coptisine and palmatine in Chinese medicine Coptis chinensis. The typical HPLC analysis was performed on WondaSil(®) C18-WR column (250×4.6 mm, 5 μm) with the mobile phase comprising 40 mM phosphate buffer (pH 7.0)-acetonitrile (40:60, v/v) at the flow rate of 0.8 mL min(-1). The electrochemical detection employed a three electrode system with a bare glassy carbon electrode at +1.3 V versus the Ag/AgCl reference electrode. The limits of detection (LODs) of four alkaloids ranged from 0.01 to 0.03 μmol L(-1) and the LOD of berberine was 80 times lower than LOD obtained by UV detection. The rat plasma samples were assayed after oral administration of the traditional Chinese medicine Coptis chinensis by the proposed HPLC-ECD method. The recoveries of this method were ranging from 88.0 to 116%, with the relative standard deviation lower than 3.1% for intra-day precision and 5.7% for inter-day precision. These results show that HPLC-ECD is a useful tool for the quality control of herbal medicine Coptis chinensis and also for pharmacokinetic studies.

High sensitive determination of zinc with novel water-soluble small molecular fluorescent sensor

A high sensitive method of quantitative analysis for the determination of zinc in the nutrition supplements has been developed by using a novel water-soluble fluorescent sensor HQ3: (8-pyridylmethyloxy-2-methyl-quinoline). Under the optimized condition of 67 mM phosphate buffer, pH 7.4, and 5% (v/v) DMSO, the zinc concentration showed good linear relationship with fluorescence intensity in the range of 7.5 x 10(-8) to 2.5 x 10(-5) M with the detection limit of 1.5 x 10(-8) M. HQ3 exhibited high selectivity to zinc comparing with other metal ions except for cadmium. The developed analytical method was successfully used for determining the content of zinc in a real sample of zinc gluconate solution of Sanchine.

A disposable electrochemical sensor for the determination of indole-3-acetic acid based on poly(safranine T)-reduced graphene oxide nanocomposite

A disposable electrochemical sensor for the determination of indole-3-acetic acid (IAA) based on nanocomposites of reduced graphene oxide (rGO) and poly(safranine T) (PST) was reported. The sensor was prepared by coating a rGO film on a pre-anodized graphite electrode (AGE) through dipping-drying and electrodepositing a uniform PST layer on the rGO film. Scanning electron microscopic (SEM) and infrared spectroscopic (IR) characterizations indicated that PST-rGO formed a rough and crumpled composite film on AGE, which exhibited high sensitive response for the oxidation of IAA with 147-fold enhancement of the current signal compared with bare AGE. The voltammetric current has a good linear relationship with IAA concentration in the range 1.0×10(-7)-7.0×10(-6)M, with a low detection limit of 5.0×10(-8)M. This sensor has been applied to the determination of IAA in the extract samples of several plant leaves and the recoveries varied in the range of 97.71-103.43%.