Construction Of Metal Nanoclsusters As Fluorescent Probes With Aggregation-induced Emission Prorerty For Analyticalapplication

Metal nanoclusters（MNCs）are an emerging nanomaterial with the performance of aggregation-induced emission（AIE）.The synthesis method is environmental friendly and the reaction conditions are mild.Meanwhile,MNCs show excellent properties such as water solubility,good biocompatibility,low toxicity,large Stokes shift,ultra-small size and adjustable emission wavelength,thereby making up for the drawbacks of organic aggregation-induced emission materials and attracting tremendous attentions in a wide range of applications,such as detection of various targets in drug,food and environmental,bioimaging,disease diagnosis and photoelectric devices.Metal cations,as a common inducer,can effectively and quickly trigger the AIE properties of MNCs.Besides,metal cations can be used as the reaction recognition site of the analytes to regulate aggregation and disaggregation states of MNCs,thus achieving the goals of analysis and detection.Therefore,coupling the diversity of metal ions participating in chemical reactions with AIE effect,four fluorescent probes with three modes（on,off and ratio types）were constructed and used to detect five targets in practical samples.The specific studies are as follows:1.Using glutathione（GSH）and N-acetyl-L-cysteine（NAC）as both reducing agents and ligands,three MNCs including GSH-Cu NCs,NAC-Cu NCs and NAC-Cu@Au NCs were successfully prepared by one-step"bottom-up"reduction method;and their properties were characterized by the UV-Vis absorption spectrum,FT-IR infrared spectrum,TEM and XPS.Then,the AIE properties of rare earth metal ions(RE³⁺)on GSH-Cu NCs and NAC-Cu NCs,and Al³⁺on NAC-Cu@Au NCs were systematically studied and verified.Finally,the fluorescence quantum yields（QYs）and stabilities of the three MNCs before and after aggregation were compared.2.Two fluorescence probes of GSH-Cu NCs/Ce³⁺and Ce⁴⁺/NAC-Cu NCs were constructed to detect 2,6-pyridine dicarboxylic acid（DPA）and ascorbic acid（AA）in"turn-off"and"turn-on"modes,respectively,which use the various properties of Ce³⁺,including coordination capacity,reducing ability,and inducing MNCs for AIE performance.For the probe of GSH-Cu NCs/Ce³⁺,coordination affinity between Ce³⁺and DPA is stronger than Ce³⁺and GSH,making disaggregation of GSH-Cu NCs/Ce³⁺and quenched the fluorescence,thus realizing detection of DPA in"turn-off"mode.The detection showed a good linear relationship over the range of 0.3-18μM,with low detection limit（LOD）of 0.19μM,showing good selectivity.DPA was detected in water samples and serum.For the sensor of Ce⁴⁺/NAC-Cu NCs,"turn-on"detection of AA was realized by two steps.Firstly,Ce⁴⁺reacted with AA to produce Ce³⁺.Secondly,the resultant Ce³⁺induced NAC-Cu NCs aggregation and enhanced fluorescence intensity.Then,a good linear relationship in the range of 4-60μM,LOD of 0.26μM,and a good recovery were obtained.At last,AA in commercially available soft drinks was detected indirectly.3.Using folic acid and terbium nitrate as the precursors to fabricate terbium doped quantum dots（Tb@CDs）,and strong coordination capacity of Tb³⁺and fluorescence property of CDs were preserved.Besides,Tb³⁺can induce AIE effect of GSH-Cu NCs.Thus,ratiometric fluorescent probe of Tb@CDs/GSH-Cu NCs was constructed for detection of PO₄^3-.After mixing of the two nanomaterials,Tb³⁺coordinated with GSH to induce AIE performance of GSH-Cun Cs with orange-red emission at 630 nm.Meanwhile,blue emission of Tb@CDs at 454 nm drops slightly due to the energy resonance transfer effect（FRET）.When PO₄^3-added in,routes of both AIE and FRET were blocked based on acid-base reaction of PO₄^3-and Tb³⁺,resulting in fluorescence quenched at 630 nm and fluorescence recover at454 nm,thus realizing ratiometric detection of PO₄^3-.The linear relationship was obtained over the range 2-16μM,with LOD of 0.32μM,and showed good recovery.This design strategy made full use of the superior properties of CDs,Tb³⁺and MNCs,which enriches the material system of AIE type MNCs for designing sensors.4.NAC-Cu@Au NCs were used as the first probe to detect Al³⁺in the"turn-on"mode in neutral solution based on AIE mechanism.Two linear relationships between concentration of Al³⁺and fluorescence intensity were obtained:0.1-10μM and 10-90μM,and the lower of the detection limit（LOD）was 0.15μM.Then,based on the coordination reaction between Norfloxacin（NOR）and Al³⁺,NAC-Cu@Au NCs/Al³⁺was used as the second sensor to detect NOR in real samples of Norfloxacin capsules,urine and serum in the“ratio”mode.Orange-red emission of NAC-Cu@Au NCs/Al³⁺at 630 nm was used as a reference signal,and blue emission of NOR-Al³⁺at 436 nm was used as the response signal.A good linear relationship was obtained in the range of 0.02-16μM,with LOD as low as 38.3 n M.Tandem detection of dual targets was realized by only one probe.This work shows great potential in constructing MNCs as a sensor for multifunctional detection by combinatorial principles,which is cost-saving and economical.In summary,four kinds of fluorescent probes were constructed based on the diversity of metal ions participating in chemical reactions and the diversity of quantum dots,followed by applying to analyse and detect DPA,AA,PO₄^3-,Al³⁺and NOR in environmental,drug and food samples.These design strategies broaden the application scope of AIE-MNCs and enrich the probe construction system.