The Preparation Of Graphene And Graphene Composites For Sample Pretreatment

Sample pretreatment is an important step in the whole analytical procedures. So far, the sample pretreatment is still the bottleneck problem of sample analysis. Thus, it is important to seek new adsorbents with a high loading capacity, selective adsorption ability and fast sorption rate, especially in a complex matrix of sample.Graphene is an atomically thin honeycomb lattice of carbon atoms. In the graphene no carbon atoms is missing, stable structure, high electron mobility（2×10₅ cm² /（V·s））, high transparency, good pervious to light, huge specific surface area(2630 m-² g^-1), large conjugated system, strong hydrophobicity, good acid and alkali resistance, good thermal stability and chemical stability, and ease of functionalized modifications. Based on these excellent properties, graphene materials have already played crucial roles in various fields. Therefore, graphene and its composites have great potential applications in the sample pretreatment, and they are expected to be new types of adsorption materials with good performance in the field of sample pretreatment.In this paper, the application of graphene and its composites in the sample pretreatment process were explored and studied. The prepared graphene and its composite as solid phase adsorption materials were used in the pretreatment process of the sample including traditional Chinese medicine, fruits and vegetables, beverages etc. with different matrix. The chromatographic methods were used to detect the analytes. The specific research contents are as follows:1. Graphite oxide（GO） was prepared from graphite flakes by a modified Hummers method. GO was dispersed in ethylene glycol and ultrasonically exfoliated in a bath sonicator. Finally, the graphene was prepared of GO by a hydrothermal reaction. The morphology and functional groups of the graphene were characterized by scanning electron microscopy（SEM）, X-ray photoelectron spectroscopy（XPS）, X-ray diffractometer（XRD） and Fourier-transform infrared spectrometry（FT-IR）, respectively.Graphene was as matrix solid phase dispersion（MSPD） materials for extraction of ginsenoside Rh-₂ from the Jinxing capsule before the determination by high performance liquid chromatography（HPLC）. Several experimental parameters affecting the extraction efficiencies, including the type of eluent solvent, the amount of graphene, as well as the volume of eluent solvent were investigated and optimized in this work. Under optimized experimental conditions, the limit of detection of the method for the Rh-₂ was 0.09 μg m L^-1. Good reproducibilities were obtained with the relative standard deviations（RSDs） below 0.64 %. The recoveries of the method were in the range between 97.44 % and 101.70 %. Compared with the ultrasonic extraction on extraction and purification effect, better results were obtained, indicating MSPD method was more suitable for the extraction and determination of Ginsenoside in its products.2. Graphene encapsulated silica（GES） was prepared of GO encapsulated amino silica by a hydrothermal reduction. GO encapsulated silica was fabricated via the carboxy groups of GO which were linked to the amino groups of amino-terminated silica in aqueous solutions. The presence of silica in graphene was identified by FT-IR, SEM and XRD.GES was used as adsorbent for MSPD extraction of poly-methoxylated flavonoids from the dried leaves of Murraya panaculata（L.） Jack. In this work, the effects of adsorption and desorption conditions on the performance of GES were investigated and optimized. Under optimized experimental conditions, the LODs were in range from 0.004-0.012 μg m L-1, the repeatability of the extraction procedure was assessed by repeating the extraction in six replicates with RSD values in the satisfactory range of 1.56 %-2.42 %, and the recoveries of the four analytes ranged from 92.61 to 102.32 %, indicating that the MSPD is appropriate to the goal of the proposed method. Compared with the other adsorbents（graphene, silica gel, C18 silica, neutral alumina, diatomaceous earth） and without any adsorbents, better results were obtained. Then a method for analysis of poly-methoxylated flavonoids was established by coupling matrix solid phase dispersion extraction with ultra performance liquid chromatography-ultra violet detection（UPLC-UV）. Compared with reflux extraction（RE） and ultrasonic extraction（UE）, the proposed method was more effective with lower solvent consumption and less extraction time. The proposed MSPD method with GES as the dispersing agent was shown to be more suitable for the extraction and analysis of flavonoids in Chinese medicinal herbs.3. We synthesized graphene-Zn O, using a facile hydrothermal reduction strategy. The nanocomposites could be produced directly from graphene oxide（GO） in a facile one-step reaction, where the reduction of GO and the deposition of Zn O on graphene occur simultaneously. This method is simple, economical and environment friendly. The identity of nanocomposites was confirmed using FT-IR, XRD, SEM, transmission electron microscopy（TEM）, and thermogravimetric analysis（TGA）RGO-Zn O as adsorbent was used in solid phase dispersion extraction（DSPE） coupled with gas chromatography, the organic phosphorus pesticide residues in water, apple and cucumber samples were enriched and detected. Several experimental parameters affecting the extraction efficiencies, including the amount of the extraction time, the p H of sample solution, as well as type and volume of eluent solvent, were investigated and optimized in this work. Under the optimal experimental conditions, eight types of organophosphorus pesticides in different samples were enriched by RGO-Zn O as dispersive solid-phase extraction（DSPE） sorbent prior to their determination by gas chromatography coupled with mass spectrometry（GC-MS）. Under optimized experimental conditions, the limits of detection for the compounds were between 0.01 and 0.05 ng g^-1. Good reproducibilities were acquired with the relative standard deviations（RSDs） below 8.6 % for both intra-day and inter-day precision. The recoveries of the method were in the range from 75.0 % to 104.2 %. The performance of proposed method in this study had been compared with other previously reported methods, the results showed that the proposed method is faster, more sensitive and efficient. So, DSPE based RGO-Zn O method is more suitable for the enrichment and determination of organophosphorus pesticide residues in water, fruit and vegetable samples.4. Graphene coated Fe₃O₄ nanoparticles（Fe₃O₄@G） was prepared by covalent bonding. Firstly, the Fe₃O₄ nanoparticles was prepared by microwave synthesis, then the surface of the Fe₃O₄ magnetic nanoparticles was modified by tetraethylorthosilicate and 3-aminopropyl triethoxysilane to obtain amino functionalized Fe₃O₄ nanoparticles. Secondly, the Fe₃O₄@GO was fabricated via the carboxy groups of GO which were linked to the amino groups of anamino-Fe₃O₄ nanoparticles in aqueous solutions. Finally, Fe₃O₄@G was prepared of Fe₃O₄@GO by a hydrothermal reaction, where the GO was reduced to graphene. The presence of Fe₃O₄ was characterized by FT-IR, XRD, TGA, SEM and TEM.The Fe₃O₄@G was used as magnetic solid-phase extraction（MSPE） adsorbent for the extraction of eight organochlorine pesticides（OCPs） in orange juice samples prior to their gas chromatography mass spectrometry（GC-MS） detection. Various experimental parameters affecting the extraction efficiencies, such as the amount of Fe₃O₄@G, extraction time, the p H of the sample solution and desorption conditions were investigated. At the same time, the repeated use performance of Fe₃O₄@G material was also investigated.Under optimized experimental conditions, the limits of detection of the method for the compounds were between 0.01 and 0.05 ng m L^-1, an acceptable precision was obtained with intraday and interday RSD values within 9.6 %, and the recoveries were in the range between 73.8 % and 105.4 %, the results indicated that the developed method was effective for the determination of the OCPs in the juice with good sensitivity and reproducibility. Compared with other previously reported methods, the proposed method was simple, sensitive and rapid. So, The MSPE method with Fe₃O₄@G as magnetic solid phase adsorbent is more suitable for the enrichment and determination of organochlorine pesticide residues in beverage samples.In brief, graphene and its composites have good selectivity and excellent adsorption performance. In this work GES was first used as adsorbent for MSPD extraction of poly-methoxylated flavonoids from the traditional Chinese medicine. This study extends the application range of graphene and its composites, which can be used as solid adsorbent used in traditional Chinese medicine and traditional Chinese medicine preparations for chemical composition analysis and the analysis of pesticide residues in food samples. It provides a new research idea for looking for more suitable pretreatment method for complex matrix samples.