Preparation Of Fe₃O₄-based Magnetic Nanocomposites And Their Application In Electrochemical Sensor

Fe₃O₄ NPs nanoparticles have been widely used in catalysis,biological separation,drug loading,sensing and other fields because of its super paramagnetism,peroxidase-like activity,large specific surface area and high biocompatibility.In this paper,Fe₃O₄@Pro-DNA,Fe₃O₄@HAP and Fe₃O₄@PA have been prepared by homogeneous adsorption,hydrothermal method and homogeneous precipitation method.And Fe₃O₄ have good magnetic,nano enzyme catalytic activity,nucleic acid conformation recognition properties,could be absorbed on magnetic electrode.Electrochemical sensors constructed with the above three kinds of magnetic nanomaterials have the advantage of simple operation,high sensitivity and good specificity,which were used for detection of cancer marker microRNA Let-7a（miRNA Let-7a）,antibiotic chloramphenicol（CAP）and typical heavy metal mixture of Pb²⁺,Cu²⁺and Hg²⁺,respectively.The main research work is as follows:（1）Based on the adsorption between Pro-DNA and Fe₃O₄ NPS,Fe₃O₄@Pro-DNA composites were prepared by homogeneous self-adsorption process,and the products were characterized by atomic force microscopy（AFM）and Fourier transform infrared spectroscopy（FT-IR）.The results showed that the composite material of single stranded probe DNA and Fe₃O₄ was successfully synthesized.Then Fe₃O₄@Pro-DNA was adsorbed on the surface of magnetic electrode,and the mixed system of 3,3’,5,5’-tetramethylbiphenyl and hydrogen peroxide（TMB+H₂O₂）was used as substrate for electrochemical test.The results showed that the electrocatalytic constant(k_cat)obtained was nearly 19 times higher than that of Fe₃O₄alone,and the catalytic current was significantly enhanced.When the Fe₃O₄@Pro-DNA modified electrode was hybridized with the target chain of the cancer marker miRNA Let-7a,the resulting hybrid double chain would fall off from the surface of Fe₃O₄,and the catalytic activity of the sensing interface would be weakened,resulting in the decrease of the electrochemical signal.Thus,the labeling free sensing analysis of the target nucleic acid could be realized.The quantitative analysis results show that the response signal of the sensor has a good linear relationship with the negative logarithm of the concentration in the concentration range of 1.0 f M～10 n M,and the detection limit is 0.17 f M.Pro-DNA was replaced with aptamer of chloramphenicol nucleic acid.The results showed that the sensing strategy could also be used for the determination of chloramphenicol.The linear range was 3.1 p M～31μM,and the detection limit was 0.40 p M,which indicated that the sensing strategy had good universality.（2）Fe₃O₄ coated with hydroxyapatite（HAP）（Fe₃O₄@HAP）was synthesized by hydrothermal method,and the composite was characterized by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）and scanning electron microscopy（SEM）.Then Fe₃O₄@HAP was adsorbed on the surface of magnetic electrode and mixed with Pro-DNA,miRNA Let-7a and NB for electrochemical signal test in phosphate buffer solution（PBS,p H=6.86）.The results showed that Fe₃O₄@HAP adsorbed the hybrid double-chain with NB embedded,and obvious electrochemical signal could be detected in PBS.When there is no miRNA Let-7a in the solution to be tested,it cannot hybridize to form a double strand,so NB cannot be adsorbed on the surface of Fe₃O₄@HAP,and the electrochemical signal of the sensing interface is reduced,so as to realize the sensitive detection of the target nucleic acid.The results showed that there was a good linear relationship between the electrochemical corresponding signal of the sensor and the logarithm of miRNA Let-7a concentration in the concentration range of 1 f M～100 n M,and the detection limit was 0.051 f M.In addition,the sensor has good reproducibility and stability,and has been used for the analysis of miRNA Let-7a in human serum with recovery of 94%～103%,and has the potential for analysis of actual serum samples.（3）Phytic acid（PA）coated iron oxide（Fe₃O₄）material（Fe₃O₄@PA）was prepared by homogeneous precipitation method.The material was characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,Fourier transform infrared spectroscopy（FT-IR）and X-ray diffraction（XRD）.It was proved that phytic acid coated Fe₃O₄ successfully.Then the products were adsorbed on the surface of magnetic glass carbon electrode,and the electrochemical tests were carried out in Pb²⁺,Cu²⁺and Hg²⁺solutions by adsorption stripping voltammetry.The results showed that the electrochemical reactions of heavy metal ions occurred on the electrode surface,and the obvious current signals of three heavy metals being oxidized to ions could be detected respectively.When the content of Pb²⁺,Cu²⁺and Hg²⁺in the solution decreases,the content of heavy metal ions in the sensing interface decreases,resulting in the decrease of the electrochemical signal,so as to realize the detection of the target heavy metal ions.The quantitative analysis results showed that the current of the sensing strategy showed a good linear relationship with the logarithmic（log C）concentration of the three heavy metal ions in the concentration range of 10 p M～10 m M,and the detection limits were 0.33 p M,0.65 p M,0.51 p M,respectively.And it shows good performance in the actual tap water sample.