Construction Of Cellulose-based Fluorescent Sensors For Food Safety Inspection And Their Sensing Mechanisms

Food might be contaminated by some harmful substances during the production,processing and storage,such as pathogenic microorganisms,foodborne pathogens,heavy metals,pesticide residues,food additives,chemical pollutants,etc,which seriously threaten human health.In this study,we constructed a series of chemical reaction-based fluorescent probes,and some of them were fabricated into fluorescent test strips for rapid detection of food hazards,providing a simple and reliable tool for rapid inspection of food safety in the field.Fluorescent test strips have been widely used in medical,environmental and food safety detection due to the advantegy of high sensitivity,good selectivity,good portability,simple operation and low cost.Fluorescent test strips mainly determine the analytes through the interaction of fluorescent probe loaded on the test strips with the analyte to produce changes in fluorescent signals.Therefore,the design of fluorescent probe according to the specific chemical properties of the analyte is the key to the construction of fluorescent test strips.This paper main research work and findings are as follows:（1）Construction of biogenic amine responsive fluorescent test paper and investigation of its sensing mechanism.Herein,we designed a highly sensitive fluorescent probe Cl-BDP for the detection of volatile biogenic amines generated from the spoilage of meat.The fluorescent probe Cl-BDP underwent a specific nucleophilic substitution reaction and molecular isomerization reaction with biogenic amines,producing significant fluorescence color change from green to blue.The fluorescent probe showed a fast response（～2 min）,high selectivity,low detection limit（5.1 n M）,and significant fluorescence color change for biogenic amines.The chemical structure of product from the reaction of Cl-BDP with biogenic amines was studied by NMR spectroscopy and X-ray single crystal diffractometer.The precise chemical structure of the product was obtained,and the sensing mechanism was proposed.The probe was fabricated into a portable fluorescent test strip for nondestructive visualization of meat freshness.This fluorescent test paper is superior to conventional instruments in terms of sensitivity,selectivity,detection efficiency and operating procedures.Therefore,this work provides a portable tool for non-destructive visualization of meat freshness in the field.（2）Construction of biogenic amine-responsive multicolor fluorescent test strips and rational modulation of its sensing performance.To determine volatile biogenic amines from the spoilage of meat,we constructed a series of ratiometric fluorescent probes using cyanovinyl as the recognition group.These probes underwent an aza-Michael Addition-Elimination cascade reaction with biogenic amines,producing significant ratiometric fluorescence changes.By introducing strong electron-withdrawing groups on cyano-vinyl groups,the reaction rate and sensitivity of the probes can be improved.Fluorescent probe PTCN showed a significant ratiometric fluorescence response to cadaverine with high sensitivity（LOD=46 n M）,fast response（<15 s）and good selectivity.PTCN was fabricated into a low-cost,portable fluorescent test strip for the detection of gaseous cadaverine.As the concentration of cadaverine vapor increases,the fluorescence of this test paper gradually changes from red to cyan,which facilitates naked eye identification.In addition,a good linear relationship between the RGB blue-to-red ratios（B/R）and the concentrations of cadaverine vapor was obtained,which can be used to quantitatively determine the level of cadaverine vapor.This fluorescent test strip has been successfully used to monitor the freshness of meat under different temperature.Therefore,this work developed a simple,rapid,non-contact and visual tool for monitoring the freshness of meat.（3）Construction of sulfur dioxide activated near infrared fluorescent probe and investigation of its application.A fluorescent probe DCQN with quinoline moiety was synthesized from quinoline-3-carbaldehyde and 2-（3,5,5-trimethylcyclohex-2-enylidene）malononitrile,which specifically reacts with sulfite in a 1,2-nucleophilic addition reaction,producing a high contrast chromogenic effect along with strong near-infrared fluorescence.The fluorescent probe has good selectivity,low detection limit（24 n M）,fast response（<5 s）,large Stokes shift,high fluorescence efficiency as well as strong NIR fluorescence emission.The NIR fluorescent probe can overcome the background fluorescence interference of the foodstuff,and was successfully used to determine sulfite in sugar with simple and fast operation,high sensitivity,good accuracy（recovery>92%）and low error（RSD<5%）.To obtain a portable sensor device,we fabricated this fluorescent probe as a cellulose hydrogel and applied it to on-site visualize the content of sulfite in sugar.The fluorescent probe can in situ monitor active SO₂ in living organisms with fast response,and thus has been successfully used to image sulfite in MCF-7 cells,zebrafish and mice.