Rapid Detection Of ?-lactam Antibiotic Residues In Milk Based On Smartphone Combined With Quantum Dot Fluorescence

In recent years,?-lactam antibiotics have been widely used in the livestock and poultry breeding industry,resulting in a large a mount of residues in animal-derived foods.Milk is favored by people because of its rich nutrition,easy digestion and convenient consumption.Residues of antibiotics can cause serious ly adverse reactions in the body and even death.Therefore,it is extremely necessary to realize its highly sensitive,specific,fast and simple detection.In this study,two commonly used cephalexin and ampicillin were used as the detection objects.The smartphone combined with quantum dot was used to detect the residues of ?-lactam antibiotics in milk to achieve ultra-sensitive detection of antibiotics.(1)A smartphone-combined dual-emission ratiometric fluorescence probe for specifically and visibly detecting cephalexin was first designed.In the probe,blue-emitting fluorescent carbon dots(CDs)was synthesized and covered with a layer of silica spacer.Red-emitting fluorescent CdTe QDs(r-QDs)was grafted onto the silica microspheres as an analytical probe.Then,the cephalexin antibody was covalent grafted to the ratio sensor to increase the selectivity.The ratio of fluorescence intensity(FL)of r-QDs and CDs was quenched with the increasing concentration of cephalexin.The detection method had a good linear response in the range of 1-500 ?M and the detection limit was 0.7 ?M.Then portable device based on smartphone detection was constructed according to the color change under UV lamp.The detection image was obtained through the smartphone camera,and the color picker APP installed in the smartphone captured the RGB value of the image.In addition,this method was also used to determine the amount of cephalexin in milk samples with recovery of 94.1%-102.2%.These results showed that it was a portable,simple and visible method to detect cephalexin in food analysis and environmental monitoring.(2)Based on CdTe quantum dots(Q Ds)as a fluorophore,a new open-close fluorescent aptamer sensor was proposed to detect ampicillin(AMP).The 3'end ampicillin aptamer was modified with AuNPs through Au-S bond.The 5'end of the aptamer complementary strand(cDNA)was modified with CdTe QDs.The fluorescence quenching of CdTe QDs happened when the two strands complementary hybridized.In the presence of AMP,due to the stronger binding capacity of AMP and aptamer,CdTe QDs modified cDNA were dissociated from the aptamer DNA.Meantime,an increase in fluorescence intensity was observed.In this aptamer sensor,the relative fluorescence intensity has a linear relationship with the concentration of ampicillin.The linear range for ampicillin was 0.04-20 ?M,and the detection limit was 18 nM.In addition,the sensor was used to detect ampicillin in milk samples with the recovery rate of 88.4%-99%,and the relative standard deviation is less than 1.28%.These further confirm the precise accuracy of the fluorescent sensor in the determination of ampicillin.(3)We have established two methods to detect ?-lactam antibiotics.In addition,we selected three commonly used detection methods to compare with our established methods,including high performance liquid chromatography(HPLC),enzyme-linked immunosorbent assay(ELISA),and microbial antibacterial method.HPLC detection method for ?-lactam antibiotics was as followed: acetonitrile was used to precipitate the protein,and then centrifuged,filtered and purified by the Waters Oasis PRIME HLB solid phase extraction column.HPLC was equipped with a UV detector for determination.The results showed that the recovery rate of HPLC was 67.82%-108%,and the relative standard deviation was between 2.5%-9.4%.The ELISA kit was used for the detection of?-lactam antibiotics.The results showed that the positive control recovery rate was between 103%-105%.And there was a cross-reaction between cephalexin and ampicillin.Staphylococcus aureus,Escherichia coli and Bacillus subtilis were used for microbial antibacterial method for in vitro antibacterial activity determination.The results showed that ampicillin had a certain antibacterial effect on the three test bacteria,while cephalexin has little antibacterial effect.Through comparison,it can be found that the fluorescence detection method established in this study was easy to operate,time-consuming and low in detection limit,and has good application prospects.