Preparation of molecular imprinting based surface plasmon resonance and quartz crystal microbalance sensors for pesticide determination

The aim of this study is preparation of quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) sensors using molecular imprinting method for determination of organic phosphorous insecticide dimetoate and carbamated insecticide carbofuran, which are present in trace amount in environmental waters. High selectivity, accuracy, sensitivity, lower detection limits and application in environmental water samples of QCM and SPR sensors were carried out comparatively by liquid chromatography triple quadruple (tandem) mass spectrometer (LC-MS/MS). In the first stage of this study, N-metacryloyl-L-tryptophan methyl ester (MATrp), which was selected as a proper functional monomer to interact with a target molecule was characterized by FTIR-ATR. Pesticide imprinted (MIP) poly(ethylenglycol dimethacrylate-N-methacryloyl-(L)-tryptophan methyl ester) (poly(EGDMA-MATrp)) nanofilms were attached to gold surfaces of QCM and SPR sensor chips. Furthermore, non-imprinted (NIP) nanofilms were synthesized by the same method except without addition of target molecule to medium. The QCM and SPR sensor chips were characterized by atomic force microscope (AFM), elipsometer, FTIR-ATR and contact angle measurements. Thickness measurements and AFM images show that almost all nanofilms are monolayered. Then, kinetic and affinity binding of the target molecule were investigated by binding the pesticide imprinted and non-imprinted sensor chips to QCM and SPR sensor chips. The limit of detection (LOD) for dimethoate and carbofuran was calculated to be 5.35 ng/L and 4.89 ng/L for QCM, 8.37 ng/L and 7.11 ng/L for SPR, respectively. Imprinted nanofilms were found to show high sensitivity towards the target molecule than non-imprinted ones. Adsorption kinetics was determined by passing pesticide solutions at different concentrations through QCM and SPR sensor systems. Langmuir adsorption model was found as the most proper model for these affinity systems. Competitive adsorption experiments were performed to display selectivity of pesticide imprinted nanofilms. The reusability of the sensors was done by repeating once time in a week for a six week. As a case study, four different environmental waters were analyzed by adding standard 10 ng/L (spike) for each pesticide. In the second part, fragmentation ions and exact mass of the pesticides until four digits after decimal points were determined by a method prepared at a liquid chromatography mass spectroscopy ion trap / time of flight (LC-MS IT-TOF) instrument. According to these exact masses and fragmentation ions, a comprehensive method was prepared by LC-MS/MS for qualitative and quantitative analysis of the pesticides to perform validation. The limit of detection (LOD) were found to be 16.92 ng/L (R2=0.999) and 20.47 ng/L (R2=0.999) for dimetoate and carbofuran, respectively. As a case study, four different environmental waters were analyzed by adding standard 500 ng/L (spike) for each pesticide in LC-MS/MS system. To determine binding accuracy of QCM and SPR sensor systems at different concentrations, three different concentrations (10-100-1000 ng/L) were selected out of the standard solutions in the sensor systems. The samples at these concentrations were taken from the sensor systems before and after adsorption, which were then analyzed by LC-MS/MS. As a result of the analysis, the high selectivity and accuracy of the sensor systems was confirmed by LC-MS/MS. As a conclusion; the sensor chips were found to have high selectivity, accuracy, sensitivity and lower detection limits obtained from comparison experiments of molecular imprinted QCM and SPR sensors to determine pesticide in both aqueous solutions and natural source by LC-MS/MS.