Development Of Magnetic Photothermal Quantitative Immunochromatographic Test Strip For Salmonella

With the improvement of living standards,dairy products represented by milk and ready-to-eat fruit and vegetable products,as an important part of a healthy diet,have gradually become indispensable foods on people’s daily table.However,milk and its products are susceptible to contamination by foodborne pathogens during actual production,processing and transportation;the ready-to-eat fruit and vegetable products will go through many processing links before eating,which are susceptible to mechanical damage,resulting in the outflow of nutrient,and the contact area with the air increases,which provides favorable conditions for the growth and reproduction of pathogenic microorganisms.As a foodborne pathogen widely distributed in nature,Salmonella is one of the important causes of human food poisoning and foodborne gastroenteritis.In recent years,poisoning incidents of milk and its products and ready-to-eat fruit and vegetable products caused by Salmonella contamination have occurred frequently,seriously endangering human health.The photothermal immunoassay sensing platform is easy to operate,regardless of time,place or professional operators,and is suitable for rapid detection in the laboratory or field testing.Compared with other types of immunoassays,the acquisition of photothermal immunoassay signal only requires an excitation light source,portable infrared thermal imager or thermometer,eliminating the need for complex and expensive instruments,and the entire detection process can be completed within 10 min.However,complex matrix components from varied food samples can easily interfere with immunoreactions during immunochromatographic assay.Magnetic nanoparticles（MNPs）can realize the separation and enrichment of targets in complex samples and eliminate the interference of sample matrix,but traditional MNPs have low photothermal conversion efficiency,which is difficult to meet the requirements of highly sensitive Salmonella detection.Theoretically,magnetic photothermal nanomaterial prepared not only retains the separation and concentration characteristics of magnetic nanomaterial,but also improves the photothermal conversion efficiency of MNPs,which provides a new solution for the highly sensitive quantitative detection of Salmonella in complex samples.In this study,Fe₃O₄ nanomaterial was first synthesized by the solvothermal method,and CuS nanoshell were then in-situ grown on the surface of Fe₃O₄nanomaterial to form Fe₃O₄@CuS composite nanomaterial.The photothermal conversion efficiency of the synthesized Fe₃O₄@CuS reaches up to 42.12%,which is much higher than that of Fe₃O₄ nanomaterial（22.97%）.In addition,the saturated magnetization strength of the obtained Fe₃O₄@CuS is 35 emu g^-1,showing good magnetic responsiveness for the separation and enrichment of analytes in complex food matrices.Under the optimal experimental conditions,the sensitive detection of Salmonella in milk was successfully achieved by using Fe₃O₄@CuS as the detection probes of immunochromatographic strip.For colorimetric detection,the quantitative regression equation is y=192.47 ln（x）-1982.09（R²=0.9848）,with a linear detection range of 5×10⁴-5×10⁶ CFU m L^-1.For photothermal detection,the quantitative regression equation is y=4.53 ln（x）-39.53（R²=0.9880）,with a linear detection range of 1×10⁴-5×10⁶ CFU m L^-1.The limit of detection（LOD）of the proposed Fe₃O₄@CuS-based immunochromatographic method for Salmonella is 5×10⁴ CFU m L^-1 for colorimetric mode and 7.7×10³ CFU m L^-1 for photothermal mode.In addition,there was no obvious cross-reaction with other non-target bacteria,showing that the developed method has good specificity.The average recoveries of within and between batches ranged from 91.25%to 103.39%,and the coefficient of variation（CV）ranged from 2.2%to 11.1%,showing good accuracy and precision.Although the above study proved that increasing the photothermal conversion efficiency of magnetic photothermal nanomaterial could improve the sensitivity of photothermal immunochromatography strip,the ability of this method to improve the detection sensitivity was relatively limited,only 10⁴ CFU m L^-1.Previous studies have shown that changing the surface structure of nanomaterial can help improve the labeling efficiency of antibody detection,thus improving the sensitivity of the detection method.Given this,herein Fe₃O₄ nanomaterials were first synthesized by the solvothermal method,Prussian blue and polydopamine nanoshells were then in-situ grown on Fe₃O₄ nanomaterial in turn to synthesize Fe₃O₄@PB@PDA composite nanomaterial.Thanks to the co-modification of Prussian blue and polydopamine nanoshells,the photothermal conversion efficiency of the obtained Fe₃O₄@PB@PDA was 45.95%,much higher than that of Fe₃O₄（24.07%）.Additionally,the saturation magnetization of the synthesized Fe₃O₄@PB@PDA was up to 40 emu g^-1,which is suitable for efficient magnetic enrichment of analytes in complex samples.Based on this,the obtained Fe₃O₄@PB@PDA was successfully used as labeling probes to construct a colorimetric/photothermal dual mode immunochromatography method for the highly sensitive detection of Salmonella in lettuce.Under the best experimental conditions,the linear detection range of the method was 1×10⁴-1×10⁶ CFU m L^-1 based on colorimetric model,and the quantitative regression equation was y=223.55 ln（x）-2000.83（R²=0.9909）.The linear detection range of the method was 1×10³-5×10⁶ CFU m L^-1 based on photothermal model,and the quantitative regression equation was y=2.07 ln（x）-11.55（R²=0.9958）.The LOD values of the Fe₃O₄@PB@PDA-based immunochromatography method for Salmonella were 5×10³ CFU m L^-1 for colorimetric detection and 4.3×10² CFU m L^-1 for photothermal detection.No obvious cross-reaction with other non-target bacteria were observed,showing good specificity.In addition,the average recoveries of within and between batches varied from 89.25%to 117.18%with the CV of 3.4%to 10.2%,showing good accuracy and precision.