Studies On The Sensor Methods For Determination Of Trace Poisonous Substances In Food

Recntly, food poisoning incidents, such as San Lu milk powder and Cerasin Red,kept appearing in a high frequency. So, both the national government of the People’sRepublic of China and ordinary Chinese pay great attention to the food safety issues.However, there are many factors leading food satey incident, for example, foodadditives by human beings on purpose and environment pollution. Benzoyl peroxide（BPO） is commonly used as bleaching agent for flour to brighten flour color andshorten the fermentation process. However, BPO is hazardous to human health for itsreduction products such as benzene, phenol, free oxygen in flour cooking process.Melamine （MM） is often illegally added to protein-rich foods by unethicalmanufactures due to its high nitrogen level. Researchers have repoted that it ispossible to result in hysteresis of weight increace, disorder of Central nervous andkidney functions if one intake too much melamine. So, it is of great significance toestablish fast and efficient methods for toxic substance in food from the addition.Nanomaterials have excellent performances for its large specific surface area,richness in surface chemical active center, effective catalytic efficiency and strongadsorption capacity, et al. Furthermore, nanocomposites, which are synthesized bycombination of two or more kinds of nanoparticles via covalent bonds, coordinationbonds, ionic bons, et al., can have more novel characteristics than its singlenano-constituent. Nanocomposites materials have had a wide range of application infields of chemistry, biological cytology, electrocatalysis, biosensor, et al. In this thesis,gold nanopaiticles, quantum dots （QDs）, graphene （GO） and graphene （G） withexcellent properties were synthesized and used as basic materials for the preparationof nanocomposites by selfassembly. Chemical/biological/electrochemical biosenorswere built based on the as-prepared nanomaterials. The main contents are summarizedas followed:1. Benzoyl peroxide biosensor based on the synergistic effects of gold nanorodsand horseradish peroxidase composite membraneGold nanorods （AuNRs） and horseradish peroxidase （HRP） were assembled on toAu electrode by layer-by-layer method. Take the advantages of the large specificsurface area and biocompatibility of gold nanorods, the HRP and its biologicalactivity was well maintained on the surface of the electrode. Electrochemical catalyticproperty and sensitivity of the nanocomposites membrane were also improved because of synergistic effect between AuNRs and HRP. Benefit from the electron transferdirectly between the membrane and the electrode, the sensor showed fast response.The biosensor can be used to detect BPO in flour with a detection limit of8.5×10^-7mol L^-1.2. Benzoyl peroxide sensor based on Au/G nano-composites electrodePoly-amidoamine（PAMAM） with many–NH2group was used to synthesizePAMAM/GO nanocomposites. NH2-G was prepared by deoxidized PAMAM/NH2-GOusing NaBH4solution as the reducing agent. Au nanorods and NH2-G were assembledon to glassy carbon electrode layer-by-layer. Graphene has strong sorption of BPO,and benzoyl peroxide was oxidation. The impedance influence of BPO toPAMAM/GO/Au nanocomposites film was studied. The impedance change of theelectrode was linearly proportional to the logarithm of BPO concentration rangingfrom1.010^-3to1.010^-8mol L^-1.3. Ultrosensitive determination of trace melamine in milk powder using CdTequantum dots as fluorescence probesThe determination of trace melamine in milk powder was reported using TGAcapped CdTe quantum dots （QDs） as fluorescence probe with a detection limit of1.4×10^-11mol L^-1. The pH of the detecting system was kept at11.0. In strong alkalineaqueous solution, the selectivity and the sensitivity of the method has been greatlyimproved due to most heavy metal ions show no interference as they are in theprecipitation or anion form. Experiments showed that the–NH2of melamine can takeplace of the–SH of TGA to ligand to the Cd atom of the QDs, forming melaminecapped CdTe QDs （MM-CdTe QDs）. The TGA-CdTe QDs and MM-CdTe QDs takethe opposite charge and the fluorescence of QDs was quenched when the QDsaggregation occurred by electrostatic attraction of these two opposite chargenanoparticles.4. A novel sensor for the determination of mercury based on the impedancechange of TGA/CdTe/G nano-composites electrodeStudies had reported that mercury has strong sorption for sulfur and at the sametime graphene has strong absorption for mercury. The TGA-CdTe-G nanocompositeswere prepared to remove mercury in wast water by adsorption method. Mercuryimpedance sensor was established by dispensing TGA-CdTe-G nanocomposites ontothe glassy carbon electrode. It was found that as a result of the addition of QDs, theabsorption of graphene for mercury had been greatly improved. The impedancechange of the electrode was linearly proportional to the logarithm of mercury concentration ranging from1×10^-3–1×10^-7mol L^-1.5. Determination of trace iron in Chinese herbal medicine using CdSquantumdots as fluorescence probesWater soluble CdS quantum dots were prepared using the environmental andphysiological friendly amylum as the stabilizing agent. The relative fluorescenceintensty of the QDs is linearly propotional to the concentraton of Fe3+in the range of5×10^-8–5×10^-4mol L^-1. The probe can realize the determination of Fe in Chineseherbal medicine.