| Antibiotics,with the ability to kill or inhibit the growth of bacteria,have gained extensive production and application in human and veterinary medicine worldwide during the past decades.However,the booming antibiotics consumption in human and animal medicine can directly or indirectly enter into humans,animals,food,and environment,which have been emerging contaminants in the environment with much concern for their possible threats to aquatic environment and human health.Moreover,antibiotic use has increased the frequency of resistance genes,which have been an impending threat to public health all over the world.Tetracyclines are a group of broad-spectrum antibiotics,especially oxytetracycline(OTC)has been extensively used as the veterinary antibiotic to prevent bacterial infections in livestock and increase their growth rate because of its effective antimicrobial properties.Unfortunately,the abuses of tetracyclines have caused accumulation of antibiotics in environmental media and food products and posed serious implications for human health through food chain transmission.Consequently,it is urgently needed to provide a simple and effective approach to monitor OTC concentration for early warning.Unfortunately,the previously reported methods,including High Performance Liquid Chromatography(HPLC),Enzyme-linked Immunosorbent Assays(ELISA),Surface Enhanced Raman Scattering(SERS),and amperometric or antibody-based colorimetric methods,suffered from tedious sample pretreatment and analysis procedures,possible false positive signals and low specificity of tetracycline derivatives,thus failing to meet the requirements in the fields of environmental monitoring and food safety.Hence,it is imperative to develop rapid,accurate sensing method equipped with specific recognition elements.(1)A fluorescent sensing platform based on graphene oxide(GO)hydrogel was developed through a fast and facile gelation,immersion and fluorescence determination process,in which the adenosine and aptamer worked as the co-crosslinkers to connect the GO sheets and then form the three-dimensional(3D)macrostructures.The as-prepared hydrogel showed high mechanical strength and thermal stability.The optimal hydrogel had a linear response for OTC of 25-1000 ?g/L and a limit of detection(LOD)of 15.24 ?g/L.Moreover,together with the high affinity of the aptamer for its target,this assay exhibited excellent sensitivity and selectivity.According to its design principle,the as-designed hydrogel was also tested to possess the generic detection function for other molecules by simply replacing its recognition element,which is expected to lay a foundation to realize the assembly of functionalized hierarchical graphene-based materials for practical applications in analytical field.(2)The universal physisorption strategy have realized the successful detection for various targets on nanomaterial-based sensors.However,this type of "turn-on" biosensors are faced with several dilemmas,including non-specific probe displacement and relative low desorption efficiency,which have restricted their further development in emerging 2D nanosheets especially those with poor affinity towards DNA as a result of relatively high signal background.Based on the covalently-linked two-dimensional(2D)metal-organic frameworks(MOFs)nanosheets,we have realized the ultra-sensitive detection for OTC.To improve the detection performance for OTC,we have employed short-chain FAM-labelled DNA as signal reporter,and also screened a series of blocking agents to passivate the surface of MOF nanosheets and avoid the adsorption of reporter DNA.Compared with directly covalently-linked nanosensor,the nanosensor with short-chain reporter DNA and surface blocking has not only possessed intrinsic specificity as apasensor but also exhibited enhanced detection performance for OTC with a linear range of 0.50-5.00 ?g/L and a LOD of 0.40 ?g/L.Furthermore,the as-developed nanosensor could achieve detection for OTC in real samples with huge potential prospects.This strategy developed in present work shows great potential to be applied in a variety of other nanosheets-DNA sensing systems,especially favorable for the nanomaterials with poor affinity towards DNA and the targets employed long-chain aptamer as recognition element.(3)The 2D copper-based MOF nanosheets with bimetallic anchorage were successfully synthesized using a facile two-step process at room temperature and ambient pressure,denoted as Cu(HBTC)-1/Fe3O4-AuNPs nanosheets.The assynthesized 2D bimetallic MOF nanosheets displayed enhanced peroxidase-like activity with relatively high catalytic velocity and affinity for substrates compared with previously reported peroxidase mimics.Furthermore,their intrinsic peroxidase-like catalytic activity could be flexibly regulated by single-stranded DNA(ssDNA),exhibiting the enhancement of 3,3',5,5'-tetramethylbenzidine(TMB)oxidation or inhibition of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt(ABTS)oxidation due to the adsorption of ssDNA via ?-? stacking.Accordingly,on the basis of their peroxidase-like activity,our prepared 2D bimetallic immobilized MOF nanosheets achieved ultra-sensitive detection of H2O2 with a linear range of 2.86 to 71.43 nM,and comparable detection performance for glucose with a linear range of 12.86 to 257.14 p,M.By means of their controllable peroxidase-like activity,a versatile colorimetric sensing platform was developed which realized the detection of sulfadimethoxine(SDM)with a linear range of 3.57 to 357.14?g/L and the limit of detection(LOD)of 1.70 ?g/L.With the multiplexed performance for detecting various targets,our as-synthesized bimetallic MOF nanosheets hold great promise for applications in environmental monitoring,as well as bioassays by virtue of their good biocompatibility.(4)By means of the facile co-precipitation method,ZIF-8/GO biomineralized horseradish peroxidase(HRP)was successfully synthesized in aqueous phase.Based on the peroxidase catalytic activity of HRP@ZIF-8/GO,the label-free colorimetric biosensing platform was established to achieve the detection for kanamycin(KAN)and SDM,respectively.The biomimetic mineralization of ZIF-8/GO could retaine the excellent peroxidase activity of HRP.Compared with the pure ZIF-8 biomineralized HRP,the introduction of GO has increased the tolerance of HRP against extreme environments,such as high temperature,organic solvent and denaturant,also enhanced the storage stability and recycling capacity.Based on the excellent peroxidase catalytic activity of HRP@ZIF-8/GO,the ultra-sensitive detection of H2O2 was achieved,with a linear range of 6.98-178.57 nM and a detection limit of 1.33 nM.Glucose detection can be achieved by simultaneously biomimetic mineralization of glucose oxidase(GOx)and HRP,with a linear range of 0.10-8.33 mM and a detection limit of 0.10 mM.Taking advantage of the regulable peroxidase catalytic activity of HRP@ZIF-8/GO,a universal label-free colorimetric sensing platform was established to realize the specific detection of KAN and SDM,respectively.The linear detection range were 0.01-0.50 ?g/L and 1-25 ?g/L,and the detection limit were 1.45 ng/L and 0.02 ?g/L,respectively.Biomimetic mineralization of ZIF-8/GO nanocomposites have greatly improved the stability of natural enzymes.The existence of GO have also provided non-covalent binding sites for natural enzymes with nucleic acids,thus expanding its application scopes in environmental monitoring or biological detection and other fields.To conclude,in order to improve the stability and detection performance of optical biosensors,based on the excellent characteristics of graphene and metal-organic frameworks and the specific recognition capacity of functional nucleic acids,we have prepared several optical biosensing platforms to realize the detection for antibiotics,such as OTC,SDM and KAN.The establishement of these high sensitive and rapid detection biosensors for antibiotics detection will laid firm foundation for exploring rapid sensing methods for other environmental pollutants. |
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