Construction Of SERS Substrates For Flexible Paper-based Fibers And Their Highly Sensitive Detection Of Contaminant Molecules

With the continuous development of our society,people are paying more and more attention to food safety and environmental safety.Pesticides and dyes have become important sources of contaminants in food and environmental pollution,which will not only pose a great threat to ecological and environmental systems,but also pose a serious risk to human health.However,traditional detection methods,such as mass spectrometry or chromatography methods,require complex sample pre-treatment processes,are time-consuming and cannot achieve rapid detection of large sample volumes.Surface-enhanced Raman spectroscopy（SERS）has been widely used in biomedicine,environmental testing,food science,analytical science,and explosives research due to its rapid,non-destructive,and high sensitivity.Flexible SERS substrates have outstanding advantages over traditional rigid substrates in terms of excellent mechanical properties,less susceptibility to damage,and portability,which are important for nondestructive and even in situ testing of SERS technology,and can be used as powerful analytical tools for food safety analysis in practical applications.Therefore,in this work,paper-based fibers were used as templates,and gold/silver nanoparticles were modified on the paper-based fiber support material by in situ reduction and self-assembly methods,and the substrates were functionalized.Based on this,the constructed substrates were characterized and analyzed with SERS performance analysis to achieve highly sensitive detection of aquatic antimicrobial agents,organic dye molecules and pesticides,The specific work is as follows:（1）In view of the inherent hydrophilicity and porosity of paper-based fibers,the rapid absorption and diffusion of water-soluble analytes on paper-based SERS substrates may seriously affect the sensitivity and accuracy of Raman detection of target molecules.In this work,hydrophobic CFP@PDA@Au NPs paper-based substrates were obtained by in situ synthesis of gold nanoparticles（Au NPs）on polydopamine（PDA）-modified cellulose filter paper（CFP）and functionalization of their surfaces with perfluorodecanethiol（PFDT）.When 4-aminothiophenol（4-ATP）was used as the probe molecule to test the SERS performance of the hydrophobic substrates,the substrates showed excellent sensitivity,reproducibility and stability with detection limits down to 1 n M and enhancement factors up to 2.55×10⁷ due to the hydrophobic enrichment effect of hydrophobic CFP@PDA@Au NPs substrates.More importantly,the hydrophobic CFP@PDA@Au NPs substrates can be used as excellent SERS sensors to achieve high sensitivity for methylene blue（MB）residues in lake water with a minimum detection limit of 100 n M and a relative standard deviation（RSD）value of 5.28%for the substrates.This indicates that hydrophobic CFP@PDA@Au NPs can be used as SERS sensors for target molecule detection with great potential for food safety,environmental and chemical detection.（2）Inspired by the molecular structure and superhydrophobic surface design of bionic mussel,in this work,the construction of micro-nanostructures on the surface of filter paper was achieved by self-polymerization of dopamine（DA）and in situ reduction of silver nanoparticles（Ag NPs）,and the superhydrophobic CFP@PDA@Ag NPs substrates were prepared by low surface energy modification.Unlike the conventional SERS assay,based on the superhydrophobic effect of the substrate,an aqueous solution of the sample can be dropped directly on the substrate surface for testing,and the detection limit can be as low as 10^-8 M using malachite green（MG）as the probe molecule.More importantly,the recyclable performance of the substrate was tested by using rhodamine 6G（R6G）and MG as probe molecules alternatively,which can be reused at least three times without any cleaning,with less fluctuation of SERS signal intensity.The practical application in real environments was explored by detecting crystalline violet organic dyes in water with detection limits as low as 10^-8M.This superhydrophobic SERS substrate with the advantages of simple preparation,high sensitivity,direct detection and recyclability has great promise for applications in food science,biomedicine and disease diagnosis.（3）The key to constructing ultra-sensitive SERS substrates is to increase the specific surface area to adsorb more molecules and to maximize the distribution density of"hot spots"to enhance the Raman signal.In this work,a flexible three-dimensional（3D）cellulose aerogel-based SERS substrate was developed based on the copolymerization of PDA,polyethyleneimine（PEI）,and nanocellulose（CNF）,and Ag NPs were firmly anchored to the aerogel by amino groups on the PDA and PEI molecules.Due to the unique functional groups of PDA and PEI with strong chemical affinity for various analytes（e.g.dye molecules and heavy metal ions）,the SERS substrate can actively enrich the molecules to be detected in solution by immersion adsorption.Most critically,aerogels with 3D porous network structure and high specific surface area are more favorable for depositing Ag NPs and generating dense hot spots to improve the sensitivity of detection.Using 2-Mercaptopyridine（2-Mpy）as the probe molecule,the substrate demonstrated a good sensitivity with a detection limit of 10^-7 M.The substrate was successfully applied to the detection of the pesticide residues of thiram in apple juice with a detection limit of 10^-7 M,which is lower than the allowed detection standard of the EU.This Ag NPs@CNF aerogel SERS substrate has good practical application prospects and shows potential applicability in fields such as agriculture and food safety.