Aggregation Induced Fluorescence Enhancement Of Copper Nanocluster Probes For Tetracycline Detection

Tetracycline（TC）is commonly used to treat bacterial infections,but its excessive use can lead to a threat to the growth of plants as well as microorganisms in the environment,and can also result in a risk to human health through the food chain cycle.Currently,the main detection methods for TC are high performance liquid chromatography,capillary electrophoresis-mass spectrometry and fluorescence spectrometry.Among them,fluorescence spectrometry has been successfully applied to the detection of TC in actual samples such as food samples（milk,honey and chicken）,water samples（tap water and lake water）,biological samples（serum and urine）and tablets due to the advantages of fast analysis,good selectivity and high sensitivity.Nanoclusters（NCs）are a new type of nanomaterials which consisting of several or even thousands of atoms.Nanoclusters exhibite the molecular-like properties due to the spliting of their own energy levels caused by strong quantum confinement effect on free electrons in the size range of nanoclusters,which usually smaller than 3 nm.NCs such as gold nanoclusters（Au NCs）and silver nanoclusters（Ag NCs）have the advantages of long fluorescence lifetime,large Stokes shift,good biocompatibility,and easily modified surfaces,which are suitable for developing sensing systems that can selectively detect ultra-trace analytes.In recent years,copper nanoclusters（CuNCs）prepared by using non-precious copper metal have become a hot research topic,but the preparation,quantum properties,luminescence properties and applications of CuNCs need to be more widely and deeply explored.In this thesis,three novel fluorescence-enhanced nanoprobes were prepared based on CuNCs,based on the AIE sensing strategy of electrostatic interaction and ion-induced aggregation-enhanced emission,and the detection method of TC was established and successfully applied to the detection of TC in actual water samples.The AIE performance and TC detection mechanism of three fluorescent probes were further discussed.The main studies are as follows.（1）CTAB aggregation induced fluorescence enhancement of TA-CuNCs for the detection of tetracycline:A novel fluorescence-enhanced nanoprobe（TA-CuNCs-CTAB）was constructed based on the electrostatic interaction between copper nanoclusters（TA-CuNCs）with tannic acid（TA）as the template and cetyltrimethylammonium bromide（CTAB）.TEM,FT-IR and XPS were used to show the successful preparation of TA-CuNCs-CTAB probe.It was found that TC could effectively quench the fluorescence of TA-CuNCs-CTAB probe,and the detection method of TC was established.Firstly,the detection conditions were optimized as follows:CTAB（1.2 mg/m L）,PBS buffer solution（p H=4）and reaction time（25 min）.Under the optimal conditions,TC showed good linearity in the ranges of 5-50μmol/L（R²=0.9954）and 50-130μmol/L（R²=0.9987）with a detection limit of 65 nmol/L.The selectivity and anti-interference ability of the method were further investigated to achieve the detection of TC in lake and tap water with recoveries ranging from 94.2%to 103.3%and relative standard deviations（RSD）below 4.9%.Finally,the mechanism of TC detection was investigated based on experiments such as UV-vis and fluorescence lifetime,and the detection mechanism was the internal filtration effect（IFE）.This work provides a new idea for the aggregation-induced emission of CuNCs based on electrostatic interaction.（2）SDS aggregation induced fluorescence enhancement of BPEI-CuNCs for the detection of tetracycline:In order to further improve the fluorescence performance of CuNCs,using dendrimers as templates can provide a stable scaffold for the formation of CuNCs to avoid their aggregation or dispersion in solution,and can provide multiple binding sites for the response of TC and improve the response time of CuNCs to TC.A novel fluorescence-enhanced nanoprobe（BPEI-CuNCs-SDS）was constructed based on the electrostatic interaction between copper nanoclusters（BPEI-CuNCs）with branched polyethyleneimine（BPEI）as the template and sodium dodecyl sulfate（SDS）.TEM,FT-IR and XPS were used to show the successful preparation of BPEI-CuNCs-SDS probe.It was found that TC could effectively quench the fluorescence of BPEI-CuNCs-SDS probe,and the detection method of TC was established.Firstly,the detection conditions were optimized as follows:SDS（0.6 mg/m L）,PBS buffer solution（p H=3）and reaction time（5 min）.Under the optimal conditions,TC showed good linearity in the ranges of 0.5-60μmol/L（R²=0.9974）and 60-140μmol/L（R²=0.9925）with a detection limit of 105.3 nmol/L.In contrast,the detection range and limit of detection of TC based on BPEI-CuNCs were 2-40μmol/L（R²=0.9940）and 340.1nmol/L,respectively.The detection sensitivity of TC was improved by 3-fold due to the formation of BPEI-CuNCs-SDS probe.The selectivity and anti-interference ability of the method were further investigated to achieve the detection of TC in lake and tap water with recoveries ranging from 90.1%to 109.5%.Finally,the mechanism of TC detection was investigated based on experiments such as UV-vis and fluorescence lifetime,and the detection mechanism was fluorescence resonance energy transfer（FRET）.This work improves the response time for TC detection by changing the template molecules for synthesizing CuNCs,which is expected to provide a new idea for rapid detection of TC in the field.（3）Eu³⁺aggregation induced fluorescence enhancement of His-CuNCs for the detection of tetracycline:The first two systems were implemented by electrostatic interaction to promote the AIE sensing strategy,and the AIE sensing strategy of ion-induced aggregation-enhanced emission is also one of the main methods to improve the emission efficiency of CuNCs.A novel fluorescence-enhanced nanoprobe(His-CuNCs-Eu³⁺)was constructed based on the coordination of copper nanoclusters（His-CuNCs）with L-histidine（His）as the template and lanthanide metal ions Eu³⁺.TEM,FT-IR and XPS were used to show the successful preparation of His-CuNCs-Eu³⁺probe.It was found that TC could effectively quench the fluorescence of His-CuNCs-Eu³⁺probe,and the detection method of TC was established.Firstly,the detection conditions were optimized as follows:disodium hydrogen phosphate citric acid buffer（p H=6）and reaction time（2 min）.Under the optimal conditions,TC showed good linearity in the ranges of 0.1-70μmol/L（R²=0.9916）and 70-110μmol/L（R²=0.9970）with a detection limit of 100 nmol/L.The selectivity and anti-interference ability of the method were further investigated to achieve the detection of TC in lake and tap water with recoveries ranging from 98.2%to 104.4%and and relative standard deviations（RSD）below 2.0%.Finally,the mechanism of TC detection was investigated based on experiments such as UV-vis and fluorescence lifetime,and the detection mechanism was based on FRET and the stronger affinity between TC and Eu³⁺that significantly quench the fluorescence of His-CuNCs-Eu³⁺.