Rapid Detection Of Residual Thiram And Thiabendazole In Fruit Juices With Surface-Enhanced Raman Spectroscopy

Pesticides are natural or synthetic compounds that are used to protect crops and fruits from pests,diseases,and weeds.Therefore,pesticides play an important role in agricultural development.However,excessive use of pesticides will cause pesticide residues exceeding standard in fruits,vegetables,and their products,and endanger human health through the food chain.Therefore,in order to ensure the safety of our food supply,it is necessary to monitor the levels of residual pesticides in food.Due to its fast detection speed,high sensitivity,and no need for complicated sample preparation,surface-enhanced Raman spectroscopy(SERS)has been widely used in the fields of environment,food,biomedicine,and others in the past few years.In this study,two composite substrates were prepared to investigate the effects of different substrates and different sample preparation methods on the SERS detection of thiram and thiabendazole in fruit juices.SERS technology combined with chemometric methods was used for the qualitative and quantitative analysis of residual thiram and thiabendazole in fruit juices.Firstly,Au@Ag nanoparticles were prepared by the seed growth method,and its optical properties and surface morphology were characterized by ultraviolet-visible spectroscopy and transmission electron microscopy.The results showed that as the Ag/Au molar ratio increasing,the particle size of Au@Ag nanoparticles also gradually increased.The average particle sizes of the three kinds of Au@Ag nanoparticles were36±5,62±7,and 85±10 nm,respectively.Secondly,using thiram and thiabendazole as probe molecules,the above three particle sizes Au@Ag nanoparticles were used as nano-substrates to study their SERS enhancement effect.According to the intensities of the characteristic peaks of the target analyte,the substrates suitable for the SERS detection of thiram and thiabendazole were selected,respectively.The results showed that the optimal sizes of Au@Ag nanoparticles for the two pesticides detection were 62±7 nm.In addition,the SERS enhancement effects for thiram and thiabendazole as affected by the ratio(v:v)of Au@Ag colloid to the pesticide standard solution was also investigated.The results indicated that the colloid-to-drug ratio of 1:2 and 1:1 led to the best SERS enhancement effect for thiram and thiabendazole,respectively.Under the optimal conditions,the minimum detectable concentrations were 2.5 ng/m L for thiram and 50 ng/m L for thiabendazole in standard solutions.In this study,two sample preparation methods were used for detection of thiram and thiabendazole in apple juice.One was directly using apple juice for the analysis,and the other was employing a modified Qu ECh ERS(quick,easy,cheap,effective,rugged,safe)method for apple juice preparation.For thiram in apple juice,the minimum detectable concentrations resulted from the two sample preparation methods were 25 ng/m L(direct sample method)and 2.5 ng/m L(modified Qu ECh ERS method),respectively.The direct sample method had no need for sample preparation and was very fast(about 10 minutes).The modified Qu ECh ERS method was fast(about 25 minutes)and efficient,and resulted in sensitive SERS detection,and the relevant partial least squares(PLS)model showed good quantitative analysis ability(R~2=0.993,RMSE=2.73 ng/m L,RPD=12.4).The minimum detection levels for thiram in apple juice based upon both sample preparation methods were much lower than the maximum residue limits specified by the Chinese standard(5000 ng/m L),indicating the feasibility of the method.However,for thiabendazole in apple juice,the direct use of apple juice for SERS analysis could not detect the pesticide at levels lower than the maximum residue limit based on the Chinese standard(3000 ng/m L),which may be due to the interference of a large number of non-target substances from apple juice for the SERS detection of thiabendazole.Therefore,the effects of some main organic components(glucose,fructose,sucrose,pectin,malic acid,and citric acid)in apple juice on the SERS detection of thiabendazole were further investigated.The results showed that both sugars and acids affected the SERS signal of thiabendazole,but the interference effect of each compound varied.Among all tested compounds,pectin had the greatest negative effect for the SERS detection of thiabendazole.Therefore,the modified Qu ECh ERS method was used for the apple juice preparation before being used for SERS detection.The minimum detectable concentration of thiabendazole in apple juice was 250 ng/m L,which was far below the maximum residue limit based on the Chinese standard,meeting test requirement.The PLS model also showed good potential for quantitative analysis(R~2=0.987,RMSE=193 ng/m L,RPD=8.82).Finally,Au@Ag nanoparticles-graphene oxide,a relatively homogeneous solid composite substrate,was synthesized by a simple self-assembly method.In order to enhance the SERS signal of thiram,the loading and aggregation state of Au@Ag nanoparticles on graphene oxide sheets were adjusted by controlling the particle size of Au@Ag nanoparticles and the concentration of graphene oxide aqueous solution.The results showed that when the particle diameter of Au@Ag nanoparticles was 62±7 nm and the concentration of graphene oxide aqueous solution was 0.1 mg/m L,the SERS enhancement effect of thiram was the best.Under the optimal conditions,the minimum detectable concentration of thiram standard solution was 10 ng/m L,and the strength of each of the four major Raman characteristic peaks for thiram had a good linear relationship with the concentration of the thiram standard solution(R~2=0.978-0.996,RMSE=123-283 ng/m L,RPD=6.77-15.6).The optimized Au@Ag nanoparticle-graphene oxide composite substrate was used to quickly detect the residues of thiram in apple juice and grape juice with SERS.The results showed that without any sample pretreatment for the juices,the minimum detectable concentrations were 50ng/m L for apple juice and 100 ng/m L for grape juice,and the strengths of four main Raman characteristic peaks of thiram all were linear related with the concentration of thiram in juices(apple juice:R~2=0.956-0.984,RMSE=240-400 ng/m L,RPD=4.79-7.96;grape juice:R~2=0.939-0.985,RMSE=234-477 ng/m L,RPD=4.01-8.17).Although the solid composite substrate was not as good as Au@Ag nanoparticle colloid in SERS enhancement effect of thiram,its nanoparticle distribution uniformity was relatively good and easy to carry.In summary,Au@Ag nanoparticles with high SERS enhancement effect for fruit juices and Au@Ag nanoparticles-graphene oxide which was a solid composite substrate with relatively good uniformity and portability were fabricated.Through optimizing the test conditions,the substrates were successfully applied for rapid detection of thiram and thiabendazole in fruit juices.The results from this study can serve as a basis for the detection of other pesticide residues.