| For colorimetry, aptamers were adsorbed on the surface of Au NPs, however, in the presence of targets, the conformation of aptamers were transited to folded state and desorbed from the surface of Au NPs to bind to targets. Consequently, Au NPs were aggregated upon adding high concentration salt and the colour changed. For Au NPs based fluorescence quenching, the chromophores conjugated to aptamer were introduced to near the Au NPs surface for fluorescence quenching. Upon the target addition, the chromophores were taken away from the Au NPs through aptamer structure-switching resulting in a recovery of fluorescence. Our study is aimed at improving colorimetry and fluorescence sensitivity for sulfadimethoxine detection by optimizing the conditions of Au NPs; improving colorimetry sensitivity for 17β-estradiol detection by optimizing the conditions of aptamer; developping colorimetry method for cyromazine detection by using self-design poly T10. The specific works are as follows:1. The effects of remnant non-Au NPs components in the colloid gold solution on the colorimetric and fluorescent methods performance were studied. There were non-Au NPs components residues in Au NPs solution. we demonstrated that the remnant sodium citrate and the reaction products of three acids play counteractive roles in Au NPs based fluorescence quenching and colorimetric aptasensor in three ways in this study. First, the remnant sodium citrate could increase the fluorescence intensity of FAM labeled on the aptamer that reduce the efficiency of Au NPs fluorescent quenching. Second, the reaction products of citric acid, HCl and ketoglutaric acid reduce the fluorescence recovery by quenching the fluorescence of FAM labeled on the aptamer dissociated from the surface of Au NPs upon addition of target. Lastly, the reaction products of three acids reduce the p H value of the colloid gold solution that reduce the sensitivity of Au NPs based colorimetric aptasensor by increasing affinity of aptamer adsorption to the surface of Au NPs. With sulfadimethoxine as model analytes, we found that water resuspended Au NPs can significantly increase the sensitivity by more than 10-fold for Au NPs based fluorescence quenching and colorimetric aptasensor.2. The effects of the lenth of aptamer on the colorimetric methods performance were studied.we split the 76 mer aptamer of 17β-estradiol into two short pieces. Our results showed that the split P1+P2 still retained the original 76 mer aptamer’s affinity and specificity but increased the detection limit by 10-fold, demonstrating that as low as 0.1ng/m L 17β-estradiol could be detected. The increased sensitivity may be caused by lower aptamer adsorption concentration and a lower affinity to the Au NPs of a short single-strand DNA(ss DNA) sequence.3. A colorimetric method was developed for CYR detection by using Au NPs and poly T10. CYR was found to bind to polythymine(poly T10) via multiple hydrogen bonding interactions. Based on this novel finding, a highly sensitive and simple colorimetric method was developed for CYR detection by using label-free gold nanoparticles(Au NPs) and poly T10.Under the optimized conditions, excellent linearity was acquired for CYR within the range of 1-500 ng/m L. In addition, the spectra and color changes of the Au NP solution were measured by spectrophotometry and observed by the naked eyes, and the results showed as low as 1 and 5 ng/m L of CYR could be detected, depending upon the measurement methods. Afterwards, cucumber was selected to investigate the sample matrix effect and a sample pretreatment procedure was developed with simple homogenization and filtration. Even after 200 times dilution, the limit of detection(LOD) and limit of quantitation(LOQ) reached 252ng/g and 500ng/g, respectively. The assay also showed a good average recovery of 83.7-104.8% with the RSD of less than 7% and good selectivity for cyromazine over other pesticides that may exist in vegetable samples. |
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