Metal Nanoclusters Encapsulated In Metal-organic Frameworks And Applications In Fluorescence Detection

Metal nanoclusters（MNCs）,usually consisting of a few to several hundred metal atoms and protected by organic ligands,are widely used because their average size is only 1-10 nm and they can accurately detect heavy metal ions and small biological molecules.However,MNCs suffer from weak fluorescence intensity and tend to aggregate and form precipitates in solution,which in turn leads to poor reproducibility in practical applications.In order to improve these problems,in recent years,research on the application of the confinement effect of metal-organic frameworks（MOFs）and the aggregation-induced luminescence（AIE）effect of MNCs to enhance the fluorescence intensity of MNCs has attracted much attention.Based on this,MNCs were encapsulated in zeolitic imidazolate material（ZIF-8）to rapidly synthesize composites with good luminescence performance and dispersion and used for the detection of copper ions(Cu²⁺)and glutamate（L-Glu）with good detection results,as follows:（1）Short-lived methionine-protected gold nanoclusters（Met-Au NCs）have relatively low toxicity,but suffer from disadvantages such as low fluorescence quantum yields and not easy synthesis,and are not widely used.To further improve the fluorescence performance of Met-Au NCs,Met-Au NCs were encapsulated in ZIF-8/PEI.Due to the confinement effect of ZIF-8and the AIE effect of Met-Au NCs themselves,improved luminescence properties of Met-Au NCs,increasing the quantum yield from 1.14%to 4.28%and extending the lifetime from a few tens of nanoseconds to 824 ns.Meanwhile,polyethyleneimine（PEI）has good hydrophilicity,which further improves the dispersibility and fluorescence intensity of the composites.And ZIF-8 has a good ability to adsorb metal ions,which allows the composites to accumulate Cu²⁺in the solution system in a relatively short time.Therefore,the synthesized Met-Au NCs@ZIF-8/PEI has a wide detection range and a low detection limit for the detection of Cu²⁺.The linear response range for detecting Cu²⁺was 0.3-37μM,and the detection limit was 0.062μM.（2）Gold-silver nanoclusters（Au Ag NCs）possess higher fluorescence intensity and better stability and interference resistance due to the doping of silver atoms.Meanwhile,glutathione（GSH）has a relatively large molecular structure and contains more polar functional groups,and its binding to Au（I）is more stable.We therefore prepared GSH-Au Ag NCs and further encapsulated them in ZIF-8 to achieve doubly enhanced fluorescence,the quantum yield of the composite increased from 4.8%to 12.7%after coating ZIF-8,while the fluorescence lifetime was extended from 2.54μs to 3.7μs.In addition,the synthesis time of GSH-Au Ag NCs@ZIF-8 is short,which overcomes the drawback of lengthy synthesis time of metal nanocluster composites.The composite can accumulate Cu²⁺more strongly,triggering a significant fluorescence burst effect.With the addition of L-Glu,the fluorescence intensity of the composite is enhanced due to the strong binding between Cu²⁺and L-Glu,resulting in an“on-off-on’’fluorescence detection method for the continuous detection of Cu²⁺and L-Glu.The linear range of Cu²⁺detection was 0-2μM with a detection limit of 0.005μM,while the detection range of L-Glu was 0-400μM with a detection limit of 0.24μM.