Novel Electrochemical Sensors For Detection Of Bisphenol A In Food Products

The safety issue of agricultural product and food arisng from bisphenol A is always attacting widespread concern, because it is widely used in food-grade packaging materials and plastic products. Traditional detection methods such as chromatography, spectro-photometry, and immunoassay are sensitive; however, more cost-effective, rapid and simple approaches are still requested by food industry. More recently, electrochemical sensor, with the advantages of simple, rapid, and suitable for field testing, provides a promising approach for the purpose, which may be a potential solution to the problem.Based on the knowledge of different fields, such as nano-materials science, electrochemistry, molecular biology, analytical chemistry, sensing technology and agricultural product and food safety, this research described different synthesis methods of nano-materials, and characterized the morphology and electrochemical properties. In addition, the conformational change, affinity and selectivity have been examined when aptamer binding its target as biological probes. Direct electrochemical sensing method based on disposable screen printed-carbon electrode (SPCE) and surface-updated carbon ionic liquid electrode (CILE) have been constructed and optimized, as well as the direct and competitive aptamer-based electrochemcial sensing methods. These sensing methods have been utilized for rapid and sensitive determination of bisphenol A (BPA), providing a new platform for on-site detecting of BPA.The main contents, results and conclusions are listed as follows:(1) A disposable electrochemical sensor based on graphene modified SPCE was constructed for rapid detection of BPA. Graphene modified SPCE was prepared by electrochemical reduction of graphene oxide (GO) on the electrode surface. The cyclic voltammetric response of GO was studied and demonstrated that the reduction potential and the optimum reduction time of GO were-0.8V and800s, respectively; the electrochemical response of different electrodes to BPA was investigated and compared by cyclic voltammetry (CV). The results indicated that the oxidation current of BPA based on graphene modified SPCE was significantly greater than that of the GO modified SPCE and bare SPCE. However, the oxidation potential of BPA was far less than the potential of GO modified SPCE; cyclic voltammetric response of BPA at different scan rates was recorded. The results suggested that the electrochemical behavior of BPA at graphene modified SPCE was a tipical adsorption controlled electrode process and the number of electron transfer in the electrochemical oxidation of BPA was two; the effect of pH values and accumulation conditions for the detection of BPA were optimized and demonstrated that the optimum pH value of PBS was7.0and the accumulation step performed at-0.6V for200s was the best; under the optimized experimental conditions, the graphene modified SPCE possessed a linear range from1～80μmol L-1and a detection limit of0.5μmol L-1; to investigate the selectivity of the constructed sensor, the possible interferents such as phenolic compounds (phenol, hydroquinone, and catechol) and common ions (Mg2+, Ca2+Fe3+, Al3+, SO42-, and Cl") were examined. The results indicated that the modification of graphene on SPCE could accelerate the electron transfer rate and enhance the electro-catalytic ability, and the graphene-based disposable electrochemical sensing method constructed an effective solution involved in nanomaterials modified metal electrodes.(2) A surface-updated electrochemical sensor based on carbon nanotube was proposed and utilized for the determination of BPA in milk samples. Electrochemical behavior of each prepared electrode (carbon nanotube/ionic liquid composite electrode, graphite powder/ionic liquid composite electrode, and bare glassy carbon electrode) was investigated using [Fe(CN)6]3*/4-as redox probe by cyclic voltammetry and the results showed that the introduction of carbon nanotube/ionic liquid composite electrode in the electrochemical sensor played a role in the increase of current response and electron transfer rate; the electrochemical response of each prepared electrode to BPA was examined and the corresponding voltammograms demonstrated that the oxidation of BPA on carbon nanotube/ionic liquid composite electrode has the maximum peak current and minimum peak potential; cyclic voltammetric response of BPA at different scan rates was recorded, and the results suggested that the electrochemical behavior of BPA was a tipical adsorption controlled electrode process and the number of electron transfer in the electrochemical oxidation of BPA was two; the effect of pH values and accumulation conditions for the detection of BPA were optimized, and under the optimized experimental conditions, the sensor possessed a linear range from1～50μmol L-1and a detection limit of0.2μmol L-1; the selectivity, stability and repeatability of the self-prepared electrode were examined. A series of concentrations of BPA were added in the milk samples and the recoveries of each sample were tested, the results were found that the sensor possessed satisfactory recoveries with a range of93.3%～106.3%and the average recovery was99.5%. The electrochemical results showed that the carbon nanotube/ionic liquid composite electrode possessed good electrochemical activity and electrocatalytic ability for the determinantion of BPA. Due to the excellent adherence of ionic liquid to carbon nanotube, the self-prepared electrode demonstrated high selectivity for detection of BPA even in the presense of foreign species, and enabled the continuously use after a simple polishing process in case of leakage, contamination and passivation. Thus, this method could provide another new way for rapid detection of BPA by direct electrochemical sensor.(3) A label-free electrochemical aptamer based sensor was exploring for BPA screening. Gold nanoparticle and graphene modified glassy carbon electrode (GCE) was prepared by electrodeposition and electroreduction, respectivity, and characterized by cyclic voltammetry; according to the relative DPV peak current change of aptasensor after reaction with0and1.0μmol L-1of BPA, respectively, the adding proportion of aptamer/MCH, the fixing time of aptamer, and BPA detection time were analyzed and optimized to obtain the maximum sensitivity of aptasensor; under the optimized experimental conditions, the electrochemical aptasensor possessed a linear range from0.01～10μmol L-1and a detection limit of5.0nmol L-1; the specificity of aptasensor was evaluated by detection BPA analogues including BPB, BP, and6F-BPA. A series of concentrations of BPA were added in the milk samples and the recoveries of each sample were tested. The results were found that the electrochemical aptasensor possessed satisfactory recoveries in liquid milk samples with a range of96～116%and the RSD was less than4.8%, and recoveries in milk powder samples with a range of90～112%and the RSD was less than5.2%. The aboved results showed that the gold nanoparticles and graphene had synergistic influence on the determination of BPA on the modified electrode. The aptamer-based electrochemical sensor improved the detection sensitivity and selectivity of BPA compared with the aboved two methods, which was more suitable to determinate BPA in complex matrices.(4) A competitive electrochemical aptasensor has been constructed and used for the determination of BPA. The feasibility of the competitive aptasensor was verified by adding different concentrations of aptamer and BPA, respectively. When a series of concentrations of aptamer were added, the adsorption amount of aptamer on the sensor surface was increased and thus the DPV current response was decreased as the concentrations of aptamer increased. When adding different concentrations of BPA on the sensor surface, there was no significant DPV current change indicating tnat BPA would not be non-specific adsorption on the modified electrode surface. When adding different concentrations of aptamer and BPA, the DPV current response was increased with the increment of BPA concentration; the adding amount of aptamer was optimized to obtain the maximum sensitivity of the competitive aptasensor. When the adding concentration of aptamer was5.0μmol L-1, the linear range of BPA detection was0.1～10μmol L-1; when the adding concentration of aptamer was2.0μmol L-1, the linear range of BPA detection was0.01～10μmol L-1; when the adding concentration of aptamer was0.5μmol L-1, the linear range of BPA detection was0.01Φ1.0μmol L-1; the specificity of aptasensor was evaluated by detecting BPA analogues including BPB, BP, and6F-BPA, and also the stability and repeatability of the competitive aptasensor was examined by thermostatic storage and repeated test. The results demonstrated that the construction of competitive electrochemical aptasensor presented a more flexible way and could provide a theoretical basis for development of an enzyme-linked aptamer kit for the detection of BPA in the future.