Research on Key Technique for Determining Pesticide Residue Level of Fruit Surface Based on SERS

Author:Liu Yu Ping

Supervisor:lv zhi wei


Degree Year:2019





It is essential for ensuring foods safety to determine whether the pesticide residue level of fruits surface meets the requirement of state-specified standard.Surface-enhanced Raman spectroscopy(SERS) has been widely applied in the pesticide detection owing to high sensitivity,excellent molecular specicity and simple sample preparation.It is the most common to use gold and silver colloids as SERS substrate because of excellent enhancement,simple preparation and low cost.However,due to poor reproducibility induced by aggregation agents,saturation effect of SERS intensity in the pesticide detection,and interference of complex matrix of fruit,the misjudgment of pesticide residue level may be caused,that is,“overstandard”is judged as“qualified”.So the reliable and inexpensive determination method of pesticide residue level of fruit surface decides the success of SERS technique.To achieve this,four key aspects are studied in this paper.Firstly,to overcome poor reproducibility of colloids SERS substrate induced by aggregation agents,a silver-assisted gold colloids SERS substrate is prepared.Because surface charge density of gold nanoparticles decreases as its size increases,the SERS substrate can be aggreagated at small scale via analyte itself.Owing to the addition of silver colloids,the enhancement performance of gold colloids is greatly boosted.The mixed colloids of gold and silver at the optimal ratio is called silver-assisted gold colloids SERS substrate.Due to no use of extra aggragation agents,the SERS substrate achieves excellent reproducibility and the relative standard deviation of SERS measurement among substrates is only 3.7%.The enhancement of the proposed SERS substrate on Raman spectroscopy of Rhodamine 6G is 9 times that of the pure gold colloids and the enhancement factor is 1.5×10~7.Secondly,the saturation effect of SERS intensity that occurs in the pesticides detection via silver-assisted gold colloids SERS substrate is studied.By respectively studying different intensity saturation effects of gold and silver colloids,it is found that pesticides can be adsorbed on the surface of silver by direct or chloride-assisted chemisorption and can only be adsorbed on the surface of gold by direct chemisorption.Due to different adsorption types,different chemical enhancement mechanisms are caused and finally different intensity saturation effects occur.For direct chemisorption,the concentration at SERS intensity saturation is almost unchanged with activating agents and the concentration is always lower than 2 ppm.Because chloride-activated chemical enhancement occurs for chloride-assisted chemisorption,the concentration at SERS intensity saturation increases with concentration of chloride and the concentration is higher than 20 ppm.However,the chloride-activated chemical enhancement is limited to specific molecules.According to SERS measurement via gold and silver colloids with different diameters,the size change of nanoparticle has little effect on the SERS intensity saturation.Thus,the intensity saturation effect in the pesticides detection is determined by the molecular adsorption types and independent of the size of silver-assisted gold colloids SERS substrate.Based on the above study,it is inferred that the most effective way to overcome SERS intensity saturation is to make the concentration determination irrelevant to SERS intensity.Then,a concentration level determination method independent of SERS intensity is developed to overcome saturation effect of SERS intensity in pesticide detection.A uniform extraction film of activated carbon nanoparticles is firstly prepared by self-assembly via air-liquid interface and the concentration level of analyte is determined by different adsorption saturation time of extraction film for different concentration of analyte.By analyzing the characteristics of SERS results in the cases of saturated and unsaturated adsorption of the extraction film dipped into Rhodamine 6G(1 ppm,2.5 ppm,5 ppm),the condition for judging adsorption saturation is summarized.In this method,the SERS intensity ratio of determinand and reference is used to judge adsorption state and SERS intensity plays no role in determining concentration level.Therefore,this method has the endogenous characteristics of overcoming intensity saturation effect and has excellent molecular specicity like SERS.Finally,determination of pesticide residue level of fruit surface is conducted by using silver-assisted gold colloids SERS substrate and the proposed determination method.The pretreatment method suitable to determine pesticide residue level of fruit surface is also studied.In the determination of thiram residue level of pear and apple surface,the detection range covers the maximum residue limit specified in 《GB2763-2016》 and the relative standard deviations are respectively 21.5%and 23.9%,which meet the requirement of spot check.It is shown that high-speed centrifugation can be used to remove the pulp in the sample and ultrapure water soaking can be used to remove the residual liquid on the surface of the extraction film.In addition,saturation effect of SERS intensity is overcome by the proposed method and the reliability of determination is boosted.About 50 min is cost for the whole determination process.