Preparation Of Novel Flexible SERS Substrates For Detection Of Thiram Residues In Fruits And Vegetables Epidermis

In recent years,people’s material living standards have risen dramatically,and with it,the need for food safety has increased day by day.As the types and quantities of pesticides used in agricultural production are gradually increasing,the food safety hazards caused by excessive harmful pesticide residues are becoming more and more prominent.Thiram（TMTD）,as a common sprayed agricultural fungicide,can easily cause great harm to human body due to improper cleaning.Surface-enhanced Raman scattering（SERS）technique is suitable for both qualitative and quantitative analysis,and has the unique advantage of"fingerprinting",which can effectively identify different molecules.At the same time,it can overcome the shortcomings of Raman scattering technology such as low sensitivity,and greatly enhance the Raman signal to achieve highly sensitive analysis.In addition,the SERS technique requires only trace samples,which can be used for nondestructive acquisition,on-site analysis and rapid detection in practice.Among them,the utilization of flexible SERS substrates provides a great boost to the application of SERS in the field of food safety.In this paper,two new flexible SERS substrates were developed by combining nanomicrosphere array template technology and ion sputtering deposition,which can efficiently detect thiram residues on the peels of a variety of fruits and vegetables,providing a reference for the research of flexible SERS substrates in the field of food safety.The specific results are as follows:1.Preparation of flexible hemisphere array SERS substrates for detection of thiram residues on the peels of fruits and vegetablesA flexible hemisphere array SERS substrate obtained by sacrificial template method was designed and prepared for the rapid and sensitive detection of thiram residues on the peels of apple,grape and cucumber,in response to the problem that the conventional rigid SERS substrates do not have sufficient contact with the actual sample surface and are difficult to be used for target collection on the peels of fruits and vegetables.Polystyrene（PS）microspheres were prepared and densely stacked PS monolayers were obtained by the LB（Langmuir Blodgett）method from the bottom up.Polydimethylsiloxane（PDMS）was added dropwise onto the PS monolayer film as a sacrificial template,cured to obtain a PDMS hemispherical depression template,coated with a rubber solution,cured and peeled to produce a rubber hemispherical array substrate.The flexible hemispherical array SERS substrate was then prepared by depositing silver nanoparticles onto its surface using ion sputtering technique.The substrate morphology was adjusted by optimizing the ion sputtering time to obtain the maximum number of"hot spots".The substrates showed excellent sensitivity with a low detection limit of 10^-12M for 4-nitrobenzenethiol（4-NTP）;good enhanced uniformity（RSD=5.01%）.It showed good linearity in the spiked detection of thiram in the epidermis of apple,grape and cucumber,and the limits of quantification for thiram were 3.6 ng/cm²,8.5 ng/cm²and 9.1 ng/cm²,respectively,meeting the needs of practical detection.2.Preparation of flexible hollow microsphere array SERS substrates for detection of thiram residues on the peels of fruits and vegetablesIn response to the problems that the solid structure of the flexible hemisphere array SERS substrate of system I is not flexible enough and the curved surface will affect the reproducibility of Raman detection,a flexible hollow microsphere array SERS substrate inspired by bubble film was designed and prepared,which was also used for the rapid and sensitive detection of thiram residues on the peels of apples,grapes and cucumbers.The densely stacked PS monolayers are transferred to a membrane filter（MF）substrate,etched with oxygen plasma and dripped with low viscosity PDMS.The PDMS was cured and the PS microspheres were dissolved to obtain the PDMS-MF substrate by using the porous and permeable properties of the MF and the negative pressure suction of the vacuum pump to make the PDMS pass through the template array and cover the microsphere surface with a thin layer.The same ion sputtering method was used to deposit silver nanoparticles onto the surface of the PDMS-MF substrate,and the flexible hollow microsphere array SERS substrate was produced by optimizing the sputtering time.Compared with the flexible hemisphere array SERS substrate of system I,this substrate has better enhanced uniformity（RSD=1.40%）and better quantitative limits of detection for thiram in the skin of apple,grape and cucumber with 1.4 ng/cm²,6.6 ng/cm²and 6.9ng/cm²,respectively,which are much lower than the MRLs for thiram in China and EU.The substrate also showed good signal reproducibility and stability in practical applications,further demonstrating the strong practical performance of the flexible hollow microsphere array SERS substrate.