Fluorescence And Resonance Light Scattering Technique For Detection Of Some Food Additives And Reaction Mechanism

Food safety issues that induced by the misuse of food additives have caused worldwide concern in recent years. The misuse of food additives is harmful to the pepole’s health and safety which will further affect the social stability. In this thesis, some novel fluorescence and resonance light scattering methods have been developed to the dectection of some legal or illegal food additives (e.g. tartrazine, sunset yellow FCF, Sudan Ⅰ-Ⅳ). In addition, the intereaction between Sudan Ⅰ-Ⅳ and [Cu(NH3)4]2-, the mechanism of pH-induced aggregation reaction between melamine and phosphate, and the interaction between gallic acid and ethylene imine polymer in strong alkaline solution have also been studied. The main contents and some conclusions of the thesis are as follows:1. Sensitive turn-on fluorescent detection of tartrazine based on fluorescence resonance energy transferWe introduce a sensitive, rapid, label-free and general fluorescent method for the determination of tartrazine (Tz) by competitive binding to reduced graphene oxide (rGO) against fluorescein, and the fluorescence recovery upon fluorescein desorption from rGO can be directly spectroscopically followed. It was found that the fluorescence enhancement efficiency (FEE) is proportional to the Tz concentration over the range of2.36-236.11ng mL-1. The linear regression equations were calculated as FEETz=0.0159CTz+0.0554, with the corresponding detection limit (3σ/slope) of0.53ng mL-1. The Tz in a lemon-flavored sports drink sample was assayed with satisfactory results.2. Fast and sensitive dye-sensor based on fluorescein/reduced graphene oxide complexWe report on a fast, sensitive, label-free and general dye-sensor platform for synthetic organic dyes detection by competitive adsorption on the reduced graphene oxide (rGO) against fluorescent dye (FD). Fluorescein (Fl) as fluorescence indicator and the cationic dye methylene blue (MB) as model analyte were employed to investigate the analytical feature of this assay platform. The anionic dye sunset yellow FCF (SY) was chosen as comparison analyte to test the generality of this strategy. Results show that rGO can bind Fl and quench the fluorescence by fluorescence resonance energy transfer, while MB can displace Fl fast from the Fl/rGO complex by competitive adsorption, inducing the fluorescence recovery which provides a quantitative readout for MB. Besides, this design was simply based on the competitive adsorption of rGO between dye and FD, so it could be generally applied to other dyes for label-free detection of a broad range of analytes, as evidenced by the comparison analyte SY. It was found that the fluorescence enhancement efficiency (FEE) is proportional to the dye concentration over the range of7.60-420.00ng mL-1MB and7.28-400.25ng mL-1SY, respectively. The linear regression equations were calculated as FEEMB=0.0192CMB-0.3103for MB and FEESY=0.0142CSY-0.0427for SY, with the corresponding detection limits (3σ/slope) of1.03ng mL-1and1.15ng mL-respectively. The MB in waste water and SY in an orange-flavored sports drink samples were assayed with satisfactory results.3. An ultrasensitive and selective fluorescence assay for Sudan Ⅰ and Ⅲ against the influence of Sudan Ⅱ and ⅣWe report on an ultrasensitive and selective fluorescence assay for Sudan Ⅰ and Ⅲ against the influence of Sudan Ⅱ and Ⅳ based on ligand exchange mechanism. Calcein as a fluorescence indicator and Sudan Ⅰ-Ⅳ as model analytes were employed to investigate the analytical feature of this assay platform. Results show that the fluorescence of calcein can be efficiently quenched by Cu(Ⅱ). When the ligand exchange reaction proceeds, calcein is deprived of Cu(Ⅱ) by Sudan Ⅰ and Ⅲ, resulting in the fluorescence recovery of calcein. However, the ligand exchange reaction does not happen in the presence of Sudan Ⅱ or Ⅳ due to the2-methyl steric effects, which is favorable for selective determination of Sudan Ⅰ and Ⅲ against the influence of Sudan Ⅱ and Ⅳ. It was found that the fluorescence enhancement efficiency (FEE) against the concentration of Sudan (CSudan nmol L-1) shows a linear relationship. The calibration equations are FEESudan1=0.0032CSudan1-0.02613, and FEESudanⅢ=0.0033CSudanⅢ-0.02467over the corresponding linear range of11.25-2078.29and9.441035.78nmol L-1with the correlation coefficients (R2) of0.9984and0.9955, respectively. And the detection limits (3σ/slope) are calculated to be211.3and208.5pmol L-1for Sudan Ⅰ and Ⅲ, respectively, showing ultralow detection limit. The Sudan dye in a commercial chilli powder sample was assayed with satisfactory results.4. Study on the reaction between Sudan Ⅰ-Ⅳ dye and [Cu(NH3)4]2+and its analytical applicationSudan Ⅰ-Ⅳ in [Cu(NH3)4]2-circumstances had different resonance light scattering (RLS) properties. Only Sudan Ⅱ-[Cu(NH3)4]2+complex shows enhanced RLS signal and the the RLS intensity was proportional to the concentration of Sudan Ⅱ in the range of0.06-1.20μg mL-1with correlation coefficient,0.9981. Linear regression equation was△IRLS=1488.6c-82.2(c, μg mL-1). The detection limit was2.1ng mL-1. The reaction mechanism has been investigated by a combination of RLS spectra, UV-absorption spectra, infrared spectra, and X-ray diffraction. The results showed that the copper chelates of Sudan Ⅰ and Ⅲ were formed while the ion-association complexes of Sudan Ⅱ and Ⅳ were formed in the presence of [Cu(NH3)4]2-5. Mechanism of the pH-induced aggregation reaction between melamine and phosphateThe mechanism of the pH-induced aggregation reaction between melamine (Mel) and phosphate (Ps) has been studied by a combination of resonance light scattering (RLS), FTIR, UV-visible spectra, single-crystal X-ray and powder X-ray diffraction for the first time. The results confirm that the aggregation reaction between Mel and Ps is highly pH-dependent. Changes of pH dramatically influence the mol ratio of Mel to melaminium (Hmel0) and subsequently affect the synergistic manner of hydrogen bonds, electrostatic and π-π stacking interactions, resulting in the reversibility of aggregation and dissolution process. The composition of the Mel-Ps aggregates has been characterized to be2[Hmel+]·[HPO42-]·3H2O. Mechanism analysis demonstrated that there existed two different ways for the transformation of Mel into Hmel-by considering the aggregation reaction process and conditions.6. Interaction between gallic acid and ethylene imine polymer in strong alkaline solutionThe interaction between gallic acid and PEI in strong alkaline solution has been studied by a combination of UV-visible spectra and fluorescence spectra. In basic solution (pH12.22), gallic acid undergoes fast autooxidation, leading to the production of orange red solution which is non-fluorescent. Howerer, it has been changed in the presence of PEI. A strong fluorescence gallic acid-PEI complex has been formed as a light yellow one by entrapping gallic acid inside the more hydrophobic PEI box. The UV-visible spectra showed that at pH value of12.22, gallic acid in the PEI box was in the form of p-quinone. The result of the continuous variation method showed that PEI could bind to gallic acid at a molar ratio of1:1. In addition, it is dramatical that the presence of PEI slows down the autooxidation of gallic acid compared to buffer solution only.