Electrochemiluminescence DNA Biosensors Based On Quantum Dots And Their Application In Environmental Analysis

Because heavy metals,even if their amount are extremely small,easily accumulates persistently in living organisms and seriously harms the health of organisms and humans.This feature has caused widespread concern about heavy metal pollution.Chloramphenicol（CAP）is a highly effective broad-spectrum antibiotic.CAP has very serious toxic and side effects on the human body when taken in excess,such as aplastic anemia,gray infant syndrome,etc.Therefore,a new highly sensitive and selective rapid detection technology for heavy metals and anti-biotic residues is of great significance for environmental protection and food safety.In this paper,new types of nanomaterials such as molybdenum disulfide and gold nanoparticles,as well as DNA probes and aptamers were used to construct the electrochemiluminescence（ECL）biosensors for detecting heavy metals and chloramphenicol with high sensitivity,good selectivity,fast response,and stable performance.The main research contents are as follows:（1）Core-shell quantum dots（QDs）CdSe@CdS were synthesized.Positively charged Polycationic poly（diallyldimethylammonium chloride）（PDDA）and negatively charged QDs were adsorbed on the two-dimensional nanomaterial Mo S₂,in sequence,to form the Mo S₂–QDs composites,which were employed as matrices for immobilizing thymine-rich DNA sequences（DNA₁）.The gold nanoparticles（Au NPs）were combined with another type of thymine-rich DNA sequences（DNA₂）and glucose oxidase（GOD）to prepare the GOD–Au NP–DNA₂ conjugates.The composites were characterized via UV-visible absorption spectroscopy,fluorescence and transmission electron microscopy.A thymine–Hg²⁺–thymine complex was formed through Hg²⁺mismatching with DNA₁ and DNA₂.GOD was,therefore,modified onto the electrode surface.GOD catalyzed the reduction of dissolved oxygen by glucose to produce hydrogen peroxide,which was a co-reactant of the QDs.The electrochemiluminescence signal of the biosensor increased linearly with the increase of Hg²⁺concentration over the range from 1.0×10-¹² to 1.0×10^-6 mol/L with a detection limit of 1×10^-13 mol/L.The reproducibility,stability and specificity of the biosensor was also studied.The biosensor was used to determine Hg²⁺in real sample,e.g.water,milk,soil and satisfactory results were obtained.（2）In this paper,CdS quantum dots were synthesized.Positively charged PDDA and negatively charged CdS quantum dots were sequentially adsorbed on the surface of graphene oxide（GO）.The nano-luminescent materials GO-QDs were prepared and characterized by UV-Vis absorption spectroscopy,fluorescence spectroscopy and transmission electron microscopy.Biotin-avidin-labeled horseradish peroxidase（HRP）was used to prepare an enzyme-linked aptamer system which used to detect chloramphenicol.Biotin modified at the end of the chloramphenicol aptamer could be specific for avidin-labeled HRP.HRP can thereby consume hydrogen peroxide and can weaken the electrochemiluminescence intensity.Chloramphenicol in solution can specifically bind to the aptamer to form a composite,HRP also immobilized the electrode surface so that the electrochemiluminescence signal was significantly enhanced.The electrochemiluminescence intensity of the sensor showed a linear correlation with the logarithm of chloramphenicol concentration.The linear range was1.0×10^-12～1.0×10^-7 mol/L,the detection limit is 0.5 pmol/L.The reproducibility,stability and specificity of the sensor were also studied.The sensor was used for the determination of chloramphenicol in milk,real durg samples and satisfactory results were obtained.（3）Nano-composite electroluminescent materials were prepared using natural chitosan（CS）functionalized CdS quantum dots.The synthesized composites were characterized by transmission electron microscope and scanning electron microscope.The chloramphenicol aptamer and the amino-modified aptamer complementary strand hybridize to double-stranded ds DNA.When hemin is inserted into the double-strand and binds to the surface of the electrode,biocatalytic precipitation（BCP）occurs,which means that hemin catalyzes the oxidation of 4-chloro-1-naphthol by H₂O₂ in solution to produce a bio-insoluble precipitate that quenches the electroluminescent signal of quantum dots.When the aptamer biosensor is immersed in a chloramphenicol solution,chloramphenicol will induce the aptamer to leave the electrode surface and bind with the aptamer,and at this time,hemin will also leave the surface of the electrode.Thereby significantly enhancing the electrochemiluminescence signal.The electrochemiluminescence intensity of the sensor has a certain linear correlation with the concentration of chloramphenicol.The linear range is 1.0×10^-11～1.0×10^-7 mol/L,and the detection limit is 5 pmol/L.The reproducibility,stability and specificity of the sensor were also studied.The sensor was used to determine chloramphenicol in milk,and satisfactory results were obtained.