Preparation Of Cellulose SERS Substrate And Its Application In Pesticide Detection

Pesticides are extensively used in the production of agriculture.In order to pursue economic benefits,the abuse of pesticides is also common.But most pesticides are dangerous once they get into the body.Therefore,it is very important to realize the rapid monitoring of pesticides.At present,the traditional analytical methods have some limitations such as high cost and difficulty in pretreatment.Due to its high sensitivity and fast fingerprint recognition,surface-enhanced Raman scattering（SERS）spectroscopy detection technology has been developed as an effective method to detect hazardous substances in food and agricultural products in recent years.SERS amplified and analyzed molecular Raman signals through the synergistic effect of physical and chemical enhancement mechanisms.The design of SERS substrate was mainly carried out from the two basic points of improving its sensitivity and uniformity,and depended on substrate material,synthesis mode,condition optimization,nano structure and other elements.The application to the monitoring of pesticides will also consider the needs of low cost,portability,and in-situ detection.Cellulose,as the most abundant natural polymer in the world,is biodegradable,non-toxic,low-cost,soft and rich in hydroxyl and easy to modify,which has been widely used in the development of materials in various fields.Nanocellulose has many advantages such as large specific surface area,strong adsorption capacity,high reactivity,high dispersity and compatibility,as well as stabilization of nanoparticles,which is expected to be prepared into multifunctional materials after modification.Therefore,in this paper,Cellulose Nanocrystalline（CNC）and Bacterial Cellulose film（BCF）were used as raw materials to successfully prepare Dialdehyde cellulose-Ag Nanoparticles composite colloid（DAC-Ag NPs）,Dialdehyde cellulose-Ag Nanoparticles-Nylon flexible substrate（DAC-Ag NPs-Nylon）and Bacterial Cellulose film-Ag Nanoparticles flexible membrane（BCF-Ag NPs）.The substrate types cover SERS zero-dimensional substrates and two-dimensional substrates for SERS detection of three pesticides,respectively.The main research is carried out in the following three aspects:1.DAC-Ag NPs composite colloid material with good stability was prepared by selective oxidation reaction of CNC with sodium iodate（NaIO₄）and silver ammonia reaction with aldehyde group and silver ammonia solution（Ag（NH₃）₂OH）.The surface of CNC is rich in hydroxyl groups,and the aldehyde group generated after oxidation becomes the site of Ag NPs nucleation and growth.On the one hand,CNC plays a stabilizing role,and Ag NPs can be firmly and evenly attached to CNC.Hot spots can be formed between the gaps of Ag NPs.On the other hand,CNC modified with aldehyde group can play the role of reducing agent.In addition,the colloidal substrate could fully contact with the analyte molecules,and further improve the SERS sensitivity and uniformity.Eventually,the substrate was applied for the detection of diuron,and the concentration showed a good linear correlation with the intensity,with R²=0.9948.In addition,the signal uniformity RSD=8.06%,in line with the detection requirements.2.Based on DAC-Ag NPs composite colloid material,a convenient preparation idea of flexible substrate was developed.The flexible substrate with high sensitivity and uniformity can be prepared by filtering DAC-Ag NPs composite colloid material onto the filter membrane through disposable syringe.The influence of reactant concentration,filter membrane type and filter membrane aperture on SERS enhanced effect was analyzed.The results show that the substrate made of Nylon membrane has the best sensitivity and uniformity,RSD=10.47%,DAC-Ag NPs-Nylon flexible substrate material is obtained.When the base was applied to the detection of thiram,the signal was still generated at the concentration of 10^-10 M,and the characteristic peak of 1375 cm^-1 SERS signal intensity also showed a good linear relationship with the concentration,correlation coefficient R²=0.9668.The flexible substrate can be used to collect analytes directly from the surface of the material,making it feasible for in-site detection.The method is simple to operate,the substrate can meet the basic detection requirements,and has reference significance for low-cost and efficient field analysis of pesticides.3.The surface of BCF is also rich in hydroxyl groups.NaIO₄ was used to modify it with aldehyde,and then Ag NPs was reduced in situ through silver ammonia reaction to obtain BCF-Ag NPs flexible membrane material.The Ag NPs grown in situ on the surface of BCF was rose-like structure,each petal had sharp edges,and the gap between the petals grew from small to large,which could generate effective SERS"hot spots"and realize highly sensitive and reproducibility detection of Paraquat.The detection results showed that there was a good linear relationship between paraquat concentration and SERS signal intensity,R²=0.9942,RSD=9.48%.BCF-Ag NPs flexible membrane material can also be applied to the detection of paraquat samples in orange juice and gardenia leaves,which confirms the feasibility of the substrate for pesticide detection.