Hydrogel Network And Particle Surface Charge Stabilized Nanorods-based SERS Substrates For Rapid Detection Of Pesticide Residues

Food safety is of utmost importance for the reputation of the food industry and the health of every consumer.The overuse of pesticides and food additives poses significant risks to public health and safety and to ecosystem.However,traditional detection methods,such as HPLC-MS and GC-MS,are expensive,time-consuming,and environmentally unfriendly,making it challenging to achieve rapid on-site screening of pesticides.Surface-enhanced Raman Scattering（SERS）,which combines the advantages of Raman scattering and nano-optics,shows great potential in the fields of food safety,environmental detection,and biosensing.In this study,we aimed to develop novel and stable SERS methods applicable to rapid detection of food safety.Various stabilization methods have been employed to improve the sensitivity,signal stability and storage period of SERS substrates to meet the challenges of inherently unstable and prone to oxidation or aggregation of nanoparticles.Au/Agnanorods were used as effective enhancement substrates,and stimuli-responsive hydrogel materials,silicon wafers,renewable cellulose-derived materials,and polymer films were used as stabilization,support,or protection templates,combined with layer-by-layer self-assembly,organic-inorganic interfacial self-assembly,core-shell bimetallic coupling,charge theory,hydrogel network stabilization,bilayer film protection,and Self-modelling Mixture Analysis（SMA）method to stabilize Au/Agnanorods and prepare SERS active substrates with high sensitivity,stable and uniform signal,long storage period,and multi-scenario applicability.Four model molecules（rhodamine 6G,malachite green,thiram,and thiabendazole）were used as analytes to construct novel methods for SERS detection of food contaminants.The theory and mechanism of stabilization of nanoparticles were proposed,and the potential of SERS technology for multiplex detection of mixtures in practical applications,as well as the performance of in-situ extraction and nondestructive rapid detection on irregular food surfaces were explored.This study provides a basis for rapid on-site screening of food contaminants and provides a reference for promoting the commercialization and marketability of SERS technology in rapid food safety detection applications.The research content and conclusion of this study are as follows:（1）A dynamically optical and highly stable pNIPAM@Au NRs nanohybrid substrate for sensitive SERS detection of malachite green in fish fillet.In this chapter,Au NRs colloid was stabilized using stimuli-responsive materials to prepare microgel substrates.Firstly,Au NRs colloid with homogeneous morphology and photothermal properties were successfully prepared by seed growth method,and combined with poly（N-isopropylacrylamide）（pNIPAM）,a stimuli-responsive template with volume phase transition（VPT）properties,to synthesize a pNIPAM@Au NRs composite with high SERS activity,tunable plasmonic behavior and long-term stability.The results show that various optical properties of pNIPAM@Au NRs can be tuned by adjusting the Au NRs loading,laser excitation wavelength and power,and temperature.The number of"hot spots"of the composite significantly increased in the collapsed state,and the range of the longitudinal localized surface plasmon resonance（LSPR）underwent dramatically broadened and redshifted（135 nm）.The photothermal property analysis shows that the expected optical and SERS properties of pNIPAM@Au NRs composites can be obtained by using a laser with a certain power for rapid nanoscale heating to trigger the VPT of the microgel template（hydrodynamic diameter of 627±22 nm at 25 ^oC and 281±9 nm at 50^oC）.The results of the thermal-responsive SERS behavior of the composites resulting from Vis-NIR laser excitation showed that the excitation wavelength-dependent SERS efficiency can be controlled by temperature-induced swelling and collapse of the composite,and that SERS and resonance SERS（SERRS）displayed synergistic and competitive effect mechanisms under 785nm and 633 nm laser excitation,respectively.The enhancement,homogeneity and long-term stability assessment of the pNIPAM@Au NRs substrate demonstrated that it can be used as an effective SERS substrate for the quantitative detection of R6G,MG,the food additive sodium cyclamate（SC）and sodium saccharin（SS）.The limit of detection（LOD）for MG in fish tissues using this substrate was 1.58×10^-9 M（0.73 ng/g）.（2）Rapid nondestructive detection of mixed pesticides residues on fruit surface using SERS combined with self-modeling mixture analysis method.In this chapter,the interfacial self-assembled Au NRs array were stabilized using silicon wafers as carriers.Using cyclohexane as the organic phase and ethanol as the inducer,homogeneous Au NRs colloid in the aqueous phase were assembled at the cyclohexane-water interface by the organic-inorganic interfacial self-assembly method.Under the synergistic stabilization effect of interfacial tension force and Coulomb repulsion force,a uniform and dense Au NRs metallic array film was formed.Then the Au NRs array was transferred to a silicon wafer after volatilization of the organic phase,and a high-density Au NRs array SERS substrate was produced.A simple and rapid swab extraction method was developed to recover pesticides from contaminated fruit surfaces.On the surface of apple,tomato and pear,the LODs were 0.041,0.029 and 0.047ng/cm² for thiram,and 0.79,0.76 and 0.80 ng/cm² for thiabendazole,respectively.For the mixed pesticides in practical application scenarios,SMA was performed to analyze the Raman spectra of mixed pesticides to extract the individual Raman features of each component.Simultaneous qualitative and quantitative analysis of individual component in pesticide mixtures’spectra was innovatively achieved and pure components of each pesticide were presented.The recoveries ranged from 66.0-142.54%for thiram and 83.06-136.76%for thiabendazole at certain ratios,demonstrating the potential of the SERS technique combine with the SMA method for multi-analytes sensing in real-world scenarios and complex systems.（3）High-performance homogeneous carboxymethylcellulose-stabilized Au@AgNRs-CMC SERS chip for thiram detection in fruits.In this chapter,cellulose-derived material was used to stabilize Au@AgNRs colloid.Core-shell bimetallic coupling was introduced to improve the enhancement performance of SERS substrates.A large-area,high-sensitivity,stable and homogeneous Au@AgNRs-CMC substrate was constructed by electrostatic repulsion using negatively-charged Au@AgNRs as SERS active plasmonic nanomaterial,combined with negatively-charged carboxymethyl cellulose（CMC）hydrogel for nanoparticles stabilization,homogeneous dispersion and protection.The obtained Au@AgNRs-CMC SERS chip showed excellent sensitivity（EF=2.2×10⁶）,homogeneity（RSD=6.3%）,storage stability（91.7%at 3 months and>80%at 9 months）and signal reproducibility（RSD=5.0%）.For the detection of thiram in fruits containing low and abundant pigment interferents,QuECh ERS（Quick-Easy-Cheap-Effective-Rugged-Safe）method was carried out for pesticide extraction and the LODs were 42,58 and 78 ppb in methanol,apple and blueberry,respectively.（4）Flexible Au@AgNRs/CMC/qPCR film with enhanced sensitivity,homogeneity and stability for in-situ extraction and SERS detection of thiabendazole on fruits.In this chapter,a high-performance,stable and uniform Au@AgNRs/CMC/qPCR flexible film SERS substrate was constructed by synergistically stabilizing and protecting bimetallic core-shell Au@AgNRs with CMC hydrogel and fluorescent quantitative-polymerase-chain-reaction（qPCR）film.The network structure of CMC immobilized and aligned Au@AgNRs through coordination of carboxyl groups with surface Agatoms to provide intensive and stable‘hot spots’,and the qPCR bilayer film performed as carrier and barrier to protect Au@AgNRs from oxidation,humidity and optical damage and improved the robustness and stability.The synergistic stabilization mechanism of Au@AgNRs by CTAB,CMC and qPCR bilayer films and the interfacial structures were proposed.The achieved Au@AgNRs/CMC/qPCR flexible substrate showed excellent sensitivity（EF=2.1×10⁶）,uniformity（RSD=3.6%）,storage stability（>3months）and signal reproducibility（RSD=4.9%）,and good flexural stability.For in-situ extraction and nondestructive detection of thiabendazole residues on curved fruit surfaces,low LODs of 0.24 and 0.27 ppm on the surface of nectarine and lemon were obtained,respectively.