Construction Of MXene-based Electrochemical Aptamer Sensor And Its Applications For Lead And Cadmium Ions Detection In Water Environment

Heavy metal pollutants are highly toxic and not easy to degrade,they can continue to accumulate in the human body through the food chain even at low concentrations,thus causing serious harm to the human internal organs and nervous system.At present,heavy metal pollutions have become a key attention problem in the fields of environmental detection,biomedicine and food safety,so as to improve the existing heavy metal ion detection technology and achieve the purpose of simple operation,sensitivity,fast and accurate and high selectivity of heavy metal ion detection.It is of great practical significance to evaluate and control environmental pollution.The electrochemical biosensor based on nucleic acid aptamer has the characteristics of specificity,sensitivity,fast and simple,and multiple detection.Because of its good biocompatibility,high conductivity and excellent electrocatalysis,MXene has shown excellent potential application value in the field of electrochemical biosensing,which has become one of the ideal electrode materials for sensor construction.It can not only greatly amplify the sensor signal,but also increase the number of aptamer fixed sites on the sensor,and achieve the sensitivity and selectivity of the electrochemical aptamer sensor.Thus,the dissertation uses nucleic acid aptamers as recognition sensing elements,synthesizes four MXene-based nanocomposites with excellent sensing performance,designs different aptamer sensing strategies,and constructs four MXene-based electrochemical aptamer sensors with Pb²⁺and Cd²⁺as detection targets,which achieves sensitivity,simple,rapid,high specificity and multiple detection.The concrete research contents and results are shown below:（1）Ti₃C₂T_x nanosheets are prepared by in-situ HF etching method（Li F/HCl）.Au NPs are modified onto the Ti₃C₂T_x surface by electrochemical deposition.Using the signal-on strategy of the reduction peak of Pb²⁺as the indicator signal,a label free electrochemical aptamer sensor was constructed based on Au NPs/Ti₃C₂T_x to construct the target itself as the indicator signal,and the selective detection of Pb²⁺was realized.The composite material combines the high conductivity,catalysis,stability and the ability of covalently connecting biological materials,which can not only provide an excellent substrate fixing material for Pb²⁺aptamer,but also serve as a platform for sensing signal amplification.The response current of the aptamer sensor showed a good linear relationship with the logarithm of Pb²⁺concentration in the range of 5×10^-7-3×10^-4 M,and its LOD is 3×10^-7M.This sensing method provides a new path for the routine detection of heavy metal ions in the environment,which also provides a new direction for the development and application of MXene-based electrochemical aptamer sensors.（2）Antimonene/Ti₃C₂T_x nanoparticle hybrid materials were prepared by ultrasonic chemical method,and a MB-labeled electrochemical aptamer sensor based on Antimonene/Ti₃C₂T_x was constructed.Since antimonene materials are more stable under natural conditions and there is no strong interaction between layers,the hybrid modification helps to improve the stability of Ti₃C₂T_x and improve its easy accumulation and aggregation due to strong van der Waals forces between layers,exposing more active sites,and the Ti₃C₂T_x nanosheets contribute to the advantages of large specific surface area and excellent conductivity.In addition,antimonene has a strong interaction with aptamer molecules,which does not require any modification,greatly reducing the cost of sensor modification and simplifying the preparation process.Using this feature,MB-labeled Pb²⁺aptamer can be directly modified on the surface of hybrid materials,and the signal-off strategy is adopted to realize the highly sensitive and highly specific detection of Pb²⁺.The prepared aptamer sensor has an excellent linear relationship in the Pb²⁺concentration range of 1×10^-12-1×10^-7 M,and its detection limit is calculated as 3.3×10^-13 M.Besides,the constructed aptamer sensor has good stability,reproducibility and high selectivity,which has been successfully applied to the Pb²⁺detection in samples of mineral water and tap water and the actual sample recovery rate is between 100.43%and116.97%,indicating it has great potential application value in the field of environmental monitoring.（3）Using the special physical and chemical properties of MXene,pyrrole monomer and chlorauric acid complex ion will gradually self-assemble with the functional groups on Ti₃C₂T_x through hydrogen bonding and electrostatic interaction,Au NPs/PPy/Ti₃C₂T_xsandwich nanostructure composite material was synthesized in situ by mechanical stirring treatment.The electrochemical aptamer sensor based on Au NPs/PPy/Ti₃C₂T_x was constructed by a competitive strategy,which was realized for the highly sensitive and selective of Pb²⁺detection.Au NPs/PPy/Ti₃C₂T_x nanocomposite maximizes the advantages of inorganic Au NPs and organic PPy materials,which not only provides an excellent fixed interface for subsequent aptamer modification,but also can be used for electrochemical signal amplification.Meanwhile,Au NPs load enables the composite to be covalently connected with biomaterials through Au-S covalent bonds,and biomolecules such as complementary chains and aptamers are gradually modified to the surface of the material.Subsequently,MB signaling molecules were modified to the DNA double-stranded structure as indicator molecules by using the property that MB can bind to DNA double-stranded by base pair insertion and electrostatic adsorption.The aptamer sensor has a good linear relationship in the Pb²⁺concentration range of 5×10^-15-1×10^-8 M,and its detection limit is as low as 1×10^-15 M,realizing the ultra-trace detection of Pb²⁺.（4）Based on the above reserach work,the MXene material was expanded from 312-phase Ti₃C₂T_x to Nb₄C₃T_x,a representative member of the 413-phase family as the substrate.Compared with the former,Nb₄C₃T_x has higher stability,especially electrochemical stability.Nb₄C₃T_x nanosheets were synthesized by direct HF etching and ultrasonic liquid phase stripping methods,and Au NPs were directly grown on Nb₄C₃T_xnanosheets by electrodeposition.In order to realize multi-target simultaneous detection,a unique dual-working area three-electrode screen printing system was designed and prepared based on previous work and combined with screen printing technology.Au NPs/Nb₄C₃T_x was used as electrode modification material for aptamer fixation and signal amplification.Pb²⁺and Cd²⁺aptamers labeled by signal molecules（MB and Fc）were modified on the electrode surface respectively,and the two aptamers were successfully fixed at the same time,and a new disposable dual-signal screen printed electrochemical aptamer sensor was constructed.The aptamer sensor has a good linear response to Pb²⁺and Cd²⁺in the range of 1×10^-10-1×10^-7 M,and its detection limits are calculated as 6.0×10^-11 M and 1.5×10^-10 M,respectively.The aptamer sensor provides a new idea and method for the development and application of the real-time detection device for heavy metal ions.