New Indicators-based Electrochemical Aptasensor For Heavy Ions Determination

The electrochemical aptasensor is a most typical analytical techniques for heavy ions determination.However the analytical signals are mainly based on the pre-labeling electroactive probes.This on one hand increased the preparation cost and operation time of the sensor since the biojunction processes between MSA and signal molecules are sophisticated and complicated and on the other hand the analytical sensitivity of the sensor was seriousy restricted since one MSA strand could only be labeled with one signal molecule.For improving the above lack,three kinds of label-freed electrochemical biosensor for detecting of heavy ions were constructed.(1)A highly sensitive and selective electrochemical aptasensor for Pb2+was constructed based on the layer-by-layer assembly of graphene(GR)and anthraquinone-2-sulfonic acid sodium salt(AQMS)on Pb2+ aptamer(T30695)modified electrode.The mercapto-modified Pb2+ aptamer was first anchored on a gold electrode through the Au-S chemistry.Followed by,the highly conductive material of graphene was adsorbed on Pb2+ aptamer through the unique?-? stacking interaction between nucleobases and the long rang ?-conjugation of graphene.Subsequently,the electroactive aromatic anthraquinone-2-sulfonic acid sodium salt molecules were further assembled on the graphene surface also by the means of the ?-? stacking.However,upon interaction with Pb2+ ions of the aptasensor,the aptamer probe on the electrode undergone conformational switch from a single-stranded DNA form to the G-quadruplex structure,causing the GR assembled with AQMS releasing from the electrode surface into solution.As a result,the electrochemical signal of AQMS on the aptasensor was substantially reduced.Because the formed amount of G-quadruplex was dependent on the concentration of Pb2+,the electrochemical response of AQMS decreased with the increase of Pb2+ concentration,which provided a useful platform for detection of Pb2+ ions.Under the optimal experimental conditions,the attenuation of peak currents showed linear relationship with the logarithm of Pb2+concentrations over the range from 5.0×10-10 to 5.0×10-8 mol/L.The detection limit was estimated to be 6.0×10-11 mol/L.(2)We have constructed an highly sensitive and selective electrochemical aptasensor for Hg2+ by using neutral red(NR)as the electrochemical probe,which is based on the formation of a ternary complex between mercury(?)specific-aptamer(MSA),neutral red(NR)molecule and Hg2+ ion.The thiol-modified MSA was first immobilized to Au electrode through a classical assemble chemistry of Au-S bond.When the MSA was interacted with Hg2+ ions,the conformation of MSA was changed from the random coil to a hairpin-like structure due to the formation of thymine-Hg2+-thymine(T-Hg2+-T)complexes.After the electroactive NR molecules were introduced,the NR molecules were captured by sensing interface through interaclation into the space between adjacent T-Hg2+-T groups and the accompanied coordination with Hg2+.As a results,a sensitive electrochemical response corresponding to the redox of NR was observed on the biosensor.Under the optimal experimental conditions,the peak currents of NR showed a good linear relationship with the logarithm value of Hg2+concentrations over the range from 2.7×10-11 mol/L to 8.5×10-9 mol/L.The detection limit was estimated to be as low as 1.5×10-12 mol/L based on 3?.In addition,the sensor also showed excellent selectivity and good regeneration ability.(3)We have constructed an label-free aptamer-based electrochemical sensor for Pb2+ by using Alcian blue(AB)as the a G-quadruplex specific electrochemical probe for monitoring DNA structural changes.Prior to experiment,the thiol-modified aptamer was immobilized to an Au electrode through a classical assembly method by Au-S bond.In the presence of Pb2+ions,the conformation of LSA was changed from the random coil to a G-quadruplex conformation.Then the AB can ?-stack effectively on the face of the G-quadruplex because of its large ?-planar structure and the cationic groups,and consequently a sensitive electrochemical response corresponding to the redox of AB was observed.Under the optimal experimental conditions,the peak currents of AB showed a good linear relationship with the value of Pb2+ concentrations over the range from 4.9×10-6 mol/L to 3.9×10-5 mol/L.The detection limit was estimated to be 1.3J×10-7 mol/L based on 3?.It also exhibits good selectivity for Pb2+ ions over other heavy metal ions,which ensures its practical applications to real samples.