Detection Of Residual Pesticides In Apple Juice Based On Surface-enhanced Raman Scattering Technology

Food safety is an important factor affecting the public health,but in recent years,food safety incidents caused by excessive pesticide residues in foods have frequently occurred.Chromatography based analytical methods,such as gas chromatography,liquid chromatography,gas chromatography tandem mass spectrometry and liquid chromatography tandem mass spectrometry are commonly employed for analyses of residual pesticides.These methods are quite sensitive and reliable for both quantitative and qualitative analyses if appropriately analytical procedures are followed,but normally require complicated and lengthy sample preparation.Therefore,the establishment of efficient,sensitive and rapid analysis methods for residual pesticide has been one of the hottest research area in food safety.Surface-enhanced Raman scattering(SERS)is a special optical phenomenon produced on a nanometer-scale metal surface that provides high sensitivity and good selectivity for an analyte.SERS methods are relatively easy to apply and fast in analysis as well as require no complicated sample pretreatment.SERS technology has great potential for the detection of residual pesticides in foods.The use of suitable SERS substrates is especially important for obtaining excellent enhanced signals to ensure the successful application of a SERS method.Gold core silver shell nanoparticles(Au-Ag NPs)show excellent stability as Au NPs and provide high enhancement effect as Ag NPs,while their shapes and Au to Ag ratio s could be easily adjusted.Au-Ag NPs could be an ideal SERS substrate.In this study,Au NPs with four different particle sizes were synthesized by trisodium citrate reduction method.The NPs were characterized with UV-visible spectroscopy and transmission electron microscopy.The average diameters(± standard deviation)of four Au NPs were 19±2 nm,43±4 nm,65±5 nm and 87±8 nm,respectively.Au NPs with diameters of 19±2 nm and 43±4 nm were selected as gold seeds to fabricate eight Au-Ag NPs varying in particle size and the size of Au seeds through adjusting the amount of gold seed used.The size of the Au-Ag NPs with 19 nm Au core is 35±5 nm,42±6 nm,59±10 nm and91±15 nm,while that with 43 nm Au core were 66±5 nm,78±8 nm,91±9 and 127±13 nm.Next,four Au-Ag NPs(35-91nm)with 19 nm Au core and four Au-Ag NPs(66-127 nm)with 43 nm Au core were employed as SERS substrates to analyze diquat dibromide,imidan and carbaryl.For diquat dibromide,the use of 91±15 nm Au-Ag NPs(19 nm Au core)and 78±8 nm Au-Ag NPs(43 nm Au core)led to the detection of diquat standard solution at as low as 0.1 mg/L and 0.05 mg/L,respectively;however,only the use of 78 nm Au-Ag NP(43 nm Au core)as SERS substrate allowed the detection of diquat at 0.05 mg/L or higher levels in apple juice with minimum sample preparation and it seems that non-target compounds in apple juice did not interfere much the SERS signal of diquat with 78 nm Au-Ag NPs as substrate.For imidan analysis,two selected Au-Ag NPs(42±6 nm Au-Ag NPs with 19 nm core;78±8 nm Au-Ag NPs with 43 nm core)led to similar detection limit for imidan standard solutions with minimum detected concentration of 0.05 mg/L;but both NPs could not provide sufficient enhancement effects for imidan in apple juice with minimum sample preparation(the minimum detected concentration was 0.5 mg/L in apple juice).All eight Au-Ag NPs could not provide ideal SERS enhancement effects for carbaryl standards and carbaryl in apple juice.The lowest concentration detected for carbaryl standards was 0.05 mg/L and the as-prepared Au-Ag NPs were not suitable for direct detection of carbaryl in apple juice.Finally,the effects of major components in fruit juice,including sugar(1-10% w/w,glucose,fructose and sucrose),acid(0.1-1% w/w malic acid;0.01-0.1% w/w citric acid)and pectin(0.005-0.015% w/w)on the SERS signals of diquat,imidan and carbaryl were evaluated to understand the sample matrix effect of fruit juices for SERS analysis.The results showed that with the addition of sugar,acid and pectin,the SERS intensity of diquat was decreased.The effect of each component was different and which also influenced by the type of substrate used.When 43 nm Au NPs were used as the active substrate,all tested components greatly interfered with the detection of diquat,indicating that employing as-prepared Au NPs for direct detection of diquat in apple juice was not feasible.Although the use of Au-Ag NPs for detection of imidan in apple juice was not quite satisfied,sugars,acids and pectin had minor effects on the SERS signals of Au-Ag NPs.Therefore,other minor components(such as pigments in apple juice)caused greater negative effects on SERS detection of imidan in apple juice.Sugars,acids and pectin also negatively affected the SERS detection of carbaryl in various degrees,while sugar and malic acid had greater negative impact than citric acid and pectin.In summary,the possibility of direct detection of diquat,imidan and carbaryl in apple juice with SERS method was evaluated.The enhancement effects of pesticides as affected by the sizes of Au-Ag NPs and its gold core as well as some major compounds in fruit juices were also tested.The results from this study would help promote the application of SERS technology as a routine analytical tool for the detection of residual pesticide residues in foods.