The Electrochemical Sensor Based On Nucleic Acid Aptamer For Ultrasensitive Heavy Metal Detection

Heavy metals such as lead,mercury,cadmium,nickel and chromium,which can be released into the environment through natural factors and human activities,are non-degradable,bioaccumulative and biomagnifying,and have toxic effects on the environment and human beings,therefore,they have always been the focus of environmental protection efforts in various countries.The detection of heavy metals is one of the key links in the prevention and control of heavy metal pollution.The efficient progress of detection depends on rapid and sensitive detection methods.Electrochemical aptamer sensors combine the high specificity and designability of nucleic acid aptamers with the advantages of simple operation,high sensitivity and rapid reaction of electrochemical analysis method,and are becoming the"new favorite"in the field of heavy metal detection.In this project,three electrochemical sensing platforms based on different nanocomposite materials and sensing strategies for detecting two typical heavy metals,Pb²⁺and Cd²⁺were constructed.The morphology of the synthesized materials was characterized by scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The structure and composition of the materials were analyzed and evaluated by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,and energy-dispersive spectroscopy（EDS）.The assembly process,detection performance and specificity,stability,and reproducibility of the constructed sensors were evaluated by electrochemical impedance spectroscopy（EIS）,cyclic voltammetry（CV）,and differential pulse voltammetry（DPV）.The main research contents are as follows:（1）An electrochemical aptasensor based on catalyzed hairpin assembly（CHA）and co-catalysis of Au Pd/Fe-MOF and Au/Cu₂O for the detection of Cd²⁺.In this work,a signal-attenuated electrochemical aptasensor with gold/cuprous oxide（Au/Cu₂O）nanocomposite as substrate and iron-organic metal framework-loaded gold palladium（Au Pd/Fe-MOF）as a signal probe with synergistic catalytic H₂O₂for the detection of Cd²⁺was constructed.Cu₂O prepared by in situ reduction doped with gold nanoparticles was used to modify electrodes to enhance electrical conductivity and provide gold sites for nucleic acid loading.The hairpin-shaped capture probe c DNA was fixed on the electrode by Au-S bond.After the Cd²⁺aptamer（Apt）entered the system,the CHA process was initiated to capture the signal probe HP-Au Pd/Fe-MOF consisting of the complementary hairpin chain（HP）and Au Pd/Fe-MOF,resulting in a significantly larger electrochemical signal response.After the entry of Cd²⁺,Apt preferentially bound to Cd²⁺,forming a hairpin form,and the CHA process was terminated,causing a significant signal attenuation,thus enabling the detection of Cd²⁺.The constructed electrochemical aptasensor achieved sensitive detection of Cd²⁺in the concentration range of 1×10^-4–8.9μM under optimal experimental conditions,with a detection limit as low as 2.27×10^-5μM.and had excellent specificity and good stability.（2）An electrochemical aptasensor based on Au/Ce O₂ and Au/Mo S₂ nanomaterials and DNA Walker amplification strategy for the detection of Pb²⁺.This work developed an electrochemical aptasensor based on DNA walker amplification strategy using Au/Mo S₂ as the electrode modification and Au/Ce O₂ as the signal labes.The Mo S₂ nanosheet layers were synthesized by hydrothermal method to composite with gold nanoparticles as a support platform.Au/Ce O₂ modified by streptavidin（SA）and methylene blue（MB）,which binds to biotin-modified hairpin chain（H2）in large quantities through the streptavidin-biotin（SA-Biotin）pairing system to form signal labels.Upon entry of Pb²⁺into the system,the Walker strand was driven to walk,opening the nearby hairpin-shaped capture probe（H1）,which in turn binds the signal labels SA-MB@Au/Ce O₂-H2,producing an obvious MB redox peak for the detection of Pb²⁺.With the increase of Pb²⁺concentration,the electrochemical signal was also enhanced accordingly,thus realizing the detection of Pb²⁺.Under the optimal experimental conditions,the constructed sensing platform achieved target detection in a wide concentration range with a linear range of 10^-4–10²μM and a detection limit as low as 1.57×10^-5μM.It also had good anti-interference and environmental adaptability.（3）An electrochemical aptasensor based on Au/Mxene nanocomposite and dual aptamer recognition mechanism for simultaneous detection of Pb²⁺and Cd²⁺.An electrochemical aptasensor for simultaneous detection of Cd²⁺and Pb²⁺was constructed by using Au/Mxene nanocomposite modified electrode and integrating two different sensing strategies of DNA enzyme and conformational change aptamer.Monolayer Mxene（Ti₃C₂）was obtained by etching MAX phase Ti₃Al C₂,stripping Al elements from it and ultrasonic centrifugation.Monolayer Mxene and Au NP were compounded to obtain Au/Mxene nanocomposites,which were modified on the electrode surface as a substrate to effectively improve the conductivity of the electrode.In the presence of Pb²⁺,the substrate chain（Z1）of DNA enzyme（DNAzyme）broken off,together with the catalytic chain（Z2）modified by MB from the electrode,resulting in obvious signal attenuation.After Cd²⁺entered the system,it then bound to the Cd²⁺aptamer（CAPF）,which folded into a hairpin form after binding to Cd²⁺,and the 5′-end modified ferrocene（Fc）was close to the electrode,producing an obvious signal enhancement.The simultaneous detection of Cd²⁺and Pb²⁺was achieved by two opposite signal changes.The constructed sensing platform successfully achieved the simultaneous detection of dual metals and showed good linearity in the range of 10^-4–4μM,with the detection limits as low as 3.33×10^-5μM and 5.69×10^-3μM for Pb²⁺and Cd²⁺,respectively.It provided a new platform for the simultaneous detection of multiple heavy metals.