Construction And Applications Of Molecular Imprinting-Graphene/Ionic Liquid Electrochemical Sensors

In recent years, with the continuous improvement of living standards, people’s attention to food safety is growing. In March 15 this year, the web users’concern about "food safety" has been rising and still remains in the top among various consumption problems. Several kinds of food contaminants, such as tert-butylhydroquinone (TBHQ) and melamine have been chosen as the main research goal to carry out the research. Molecular imprinting technique has been used for template molecule reorganization and electrochemical technique as the main detection method. Electrochemical sensor has been constructed by combining molecular imprinting with electrochemical technology, assisted in graphene and ionic liquids, and the sensor has been applied to detect food contamination in the sample to develop new methods for the determination of food contaminants.Molecular imprinting technology is a method for synthesis polymers with specific binding sites whose shape and size are completely complementary of the target molecule. The special binding sites on molecularly imprinted polymers make them have high selectivity and high affinity to the template molecules. Compared with other recognition system, molecularly imprinted polymer obtain the promising characteristics such as high stability and good reusable in the harsh chemical and physical conditions, low cost and easy to synthesis. Therefore, molecularly imprinted polymers have attracted.more and more attention from various fields including chemical analysis and detection, separation and purification, drug delivery, catalysis and chemical sensors, etc. However, the high cross-linking of molecularly imprinted polymers (MIPs) make it difficult to extract the template molecules located inside the massive material, which can lead to an incomplete template removal, low bond capacity and slow mass transfer. The problem can be solved by the surface imprinting method. Synthetic material through surface imprinting method can make the template molecules locate in or near the surface of materials. Compared with the traditional MIPs, they have higher combining ability and faster mass transfer and combining dynamics.Many researches about synthesis and applications of molecularly imprinted polymers based on multiple hydrogen bonding have been reported. In these researches, molecules with the similar structure of uracil or thymine were usually chosen as template molecule and compounds with structure containing the unit of diaminopyridine or triazine were chosen as functional monomer. The two parts combine together by multiple hydrogen bonding and MIPs based on multiple hydrogen bonding have been developed. Gain inspiration from the above literature research, in the proposed article, using diaminopyridine or triazine derivatives as template molecules and uracil derivatives as functional monomer which exchange the location of the template molecule and functional monomer in traditional synthesis method, and molecularly imprinted polymer with specific binding sites to diaminopyridine or triazine derivatives has been synthesized. Electrochemical method is a kind of superior test method with simple operation, high sensitivity and fast response characteristics, has been widely used in medicine, biology and environmental analysis.Because MIPs are non-conductive, in order to well realize the detection performance of the proposed method in this paper, other material should be introduce to improve the electrical conductivity. Graphene has been chosen as one of the material. Graphene is a carbon atoms layer formed in honeycomb lattice. Due to its various attractive features and ultra-thin structure characteristics, it has been attracted increasingly attention. The Ultra high carrier mobility, highest thermal conductivity and excellent optical transparency make graphene to be a super electronic and optoelectronic material. Electron transfer experiment on graphene shows that graphene has unusual carrier density dependence of electrical conductivity and exceptionally high electron mobility. However, graphene is hydrophobic, and graphene sheets are easy to aggregate or even return graphite because of the van der Waals interaction and the strong π-π stack, which limit its further application. Especially in electrochemical biosensors, prevention of graphene reunion is crucial, because most of its unique properties are only associated with the single layer. In mass graphene synthesis and processing, prevention of graphene reunion is a key challenge. Ionic liquid is a good way to deal with this challenge. Due to its wide range of solubility, and the introduction of surface charge, ionic liquid can block the π-π stack between graphene layers to improve its dispersion and make full use of its high specific surface area. Research has shown that a small amount of graphene dispersion in the ionic liquid [bmim] [PF6] can greatly improve the conductivity of ionic liquids, as a result, ionic liquids was chosen as another electrode modified material.There were three systems of work in this paper. Chapter I is about the research background related to this article, the selected topic basis and research purpose of the article.In chapter Ⅱ, one novel electrochemistry-molecular imprinting sensor for determining tert-butylhydroquinone (TBHQ) in foodstuff was developed. TBHQ-imprinted core-shell nanoparticles (TICSNs) were fabricated using silica nanoparticles as core material. The silica nanoparticles were modified with (3-chloropropyl) trimethoxysilan and polyethylenimine, respectively, and polymerised to form the TICSNs with ethylene glycol dimethacrylate as cross-linker. The specific core-shell structure demonstrates extremely high specific surface area which greatly increases the effective binding sites and improves the recognition capability for model molecules.In chapter Ⅲ, a new analytical approach for detecting diaminopyridine derivatives has been constructed using a molecular imprinting-electrochemical sensor. Opposed to the conventional strategy of employing diaminopyridine as the functional monomer and uracil derivatives as the target analyte, in the current study, the 2,6-Diaminopyridine-imprinted core-shell nanoparticles was synthesized with 2,6-Diaminopyridine as the template molecule and 6-aminouracil as the functional monomer. Graphene and ionic liquid which can assist 2,6-Diaminopyridine-imprinted core-shell nanoparticles in electrochemical reaction kinetics by increasing conductivity have been introduced to form one of the electrode modified layers. The proposed analytical method has been applied in 2,6-Diaminopyridine detection in hair-dyes and demonstrated appropriate sensitivity and selectivityIn chapter Ⅳ, gained inspiration from the previous research using triazine derivatives as functional monomer and uracil derivatives as template molecules, in this paper, exchange the conventional role of functional monomer and template molecules, one new molecularly imprinted polymer based on core/shell structure with the specific recognition to melamine using 6-aminouracil as functional monomer has been developed. The molecular imprinting-electrochemical sensor has been established with the assistance of graphene and Ionic liquids and revealed high selectivity and good accuracy. The application of the senor in monitoring the concentration of melamine in dairy products received satisfied effect.In chapter Ⅴ, based on the previous work, in order to extend the application of the proposed synthesis method of MIPs, adenine-imprinted polymer based on multiple hydrogen bonding has been developed using 6AU as functional monomer and adenine as target molecule. The morphology, physical and chemical properties of adenine-imprinted polymer have been characterized by TEM and FT-IR spectrum, and its potential application has been discussed.