Design And Preparation Of Two-dimensional Ultrathin Metal Organic Frameworks And Construction Of Nanosensor

The detection of biomarkers and environmental pollutants is of great significance for human health,disease diagnosis and life safety.Abnormal contents of biomarkers in vivo may cause various physiological diseases,hazardous and toxic environmental pollutants may lead to food contamination and ecological imbalance.Hence,it is utmost imperative to build a simple and fast sensing platform for accurate detection of such substances.Metal organic framework（MOF）is a promising class of functional nanomaterials composed of metal nodes and organic ligands.Two-dimensional MOF nanosheets（2D MOF NSs）have received increasing attention in the past decades owing to the unique characteristics and functionalities:ultrathin thickness,abundant accessible active sites,adjustable structure,and so forth.So far,many synthesis strategies on 2D MOF NSs have been developed,however,it remains a great challenge to develop effective ways for acquiring 2D MOFs with a uniform and controllable morphology.In this paper,two kinds of 2D MOF nanomaterials were prepared by different synthesis methods,and have been applied to construct sensors for biosensing and environmental monitoring,respectively.（1）Dextran-assisted ultrasonic exfoliation of two-dimensional metal-organic frameworks to evaluate acetylcholinesterase activity and inhibitor screening:Acetylcholinesterase（ACh E）is regarded as a biomarker of Alzheimer’s disease（AD）,and its inhibitors show great potential in AD therapy as ACh E can increase the neurotoxicity of the amyloid component that induces AD.Because of this,it is crucial and significant to develop a simple and highly sensitive strategy to monitor ACh E levels and screen highly efficient ACh E inhibitors.Herein,we synthesize an ultrathin2D metal-organic framework（MOF）based on copper-catecholate（Cu-CAT）via dextran assisted ultrasound exfoliation,followed by construction of a sensitive sensor for the monitoring ACh E and screening of its inhibitors.By adding ACh E,the acetylthiocholine（ATCh）substrate is hydrolyzed to be thiocholine（TCh）,which decreases the peroxidase-like activity of Cu-CAT nanosheets（Cu-CAT NSs）,impairing the signal reaction of 3,3′,5,5′-tetramethylbenzidine（TMB）to oxidized-TMB（ox-TMB）.In the presence of an ACh E inhibitor,the signal can be gradually restored.The newly developed sensor shows high sensitivity and selectivity for ACh E and huperzine A（HA,an effective drug for AD,an acetylcholine receptor antagonist）,as well as for AD drug discovery from traditional Chinese herbs.The limit of detection of the sensor for ACh E is 0.01 m U m L^-1 and the average IC₅₀ value of HA is30.81 n M under the optimal of catalysis conditions.Compared with the 3D bulk Cu-CAT,the current 2D Cu-CAT NSs exhibit higher peroxidase activity due to more catalytic active site exposure.This study provides a strategy to prepare an ultrathin 2D MOF with high catalytic activity and new insights for the construction of a biosensor to monitor ACh E and new AD drugs.（2）Phase transfer synthesis of two-dimensional lanthanide metal-organic framework for ratiometric fluorescence detection of Hg²⁺:The development of functional nanomaterials with excellent luminescence properties and abundant active sites is a challenge in current research.Here,we develop a novel and facile bottom-up method of phase transfer to fabricate a lanthanide bimetal organic frameworks nanosheets(2D Eu_0.5Ce_0.5-bop NSs).Firstly,europium ions(Eu³⁺)and cerium ions(Ce⁴⁺)combined with 5-boronoisophthalic acid（5-bop）to form an amorphous polymer through facile assembly coordination,then phase transfer take place in aqueous solution at room temperature to form crystalline layered nanosheets.The resulted crystal nanosheet displays dual emission of Eu³⁺and Ce⁴⁺at 619 nm and 688nm respectively under 257 nm excitation.With the addition of mercury ions(Hg²⁺),specific transmetalation reaction happens with the boric acid group,results in energy transfer from Eu³⁺to Ce⁴⁺.Therefore,the emission of Eu³⁺at 619 nm decreases and the emission of Ce⁴⁺at 688 nm increases simultaneously.The proposed ratiometric fluorescence further improves the detection accuracy,leading to the precise emission changes to Hg²⁺in a wide range of 0.5～120μM with a detection limit of 167 n M and shows preeminent selectivity over various cations.This work highlights a simple strategy for 2D bimetallic lanthanide MOF formation and promotes their application in luminescent sensors.