Preparation,Tunable Electrochemistry And Sensing Applications Of Copper-based Metal-organic Frameworks

Metal-organic frameworks?MOFs?,as a newly-developing multifunctional porous materials,has been widely used in hydrogen storage,gas adsorption and separation,chemical sensors,catalysis and drug release due to many unique advantages,such as large surface area,tunable morphology and pore size,varied structures and unsaturated coordination sites.At present,the study about the electrochemical sensing applications of MOFs is few.Moreover,the influence mechanisms of composition,structure and morphology on the electrochemical sensing property of MOFs have not been clearly elucidated.Therefore,systemic research about the structural design,preparation and electrochemical performance modulation of MOFs is of great novelty,which is very important for the construction of highly sensitive electrochemical sensing system.In this thesis,using Cu²⁺as the centre,copper-based MOFs with different compositions,morphologies and structures were prepared by changing the ligands and preparation parameters.A series of techniques,like electro microscope,spectrum and X-ray diffraction,were used for their characterization and analysis.On this basis,the electrochemical catalytic sensitization properties of the prepared copper-based MOFs were studied.Then,the modulation mechanism of their electrochemical sensing properties was examined,and highly sensitive electrochemical sensing systems were constructed for small biological molecules,environmental estrogens and azo colorants.The mainly research work is as follows:?1?Using 1,3,5-benzenetricarboxylic acid?H₃BTC?as the ligand and Cu?NO₃?₂·3H₂O as the center,Cu-BTC sensing materials were prepared through the interface method at room temperature.The characterizations of Fourier Transform infrared spectroscopy?FTIR?and X-ray diffraction?XRD?confirmed that the frame structures were formed.Transmission electron microscopy?TEM?and nitrogen adsorption-desorption isotherms indicated that Cu-BTC frameworks owned abundant microporous structures,with the surface area of 654.6 m²/g.The Cu-BTC frameworks were used to modify the carbon paste electrode?Cu-BTC/CPE?,and it was found that they had obvious enhancement effect on the electrochemical oxidation of diethylstilbestrol?DES?and estradiol?E2?,and greatly improved the response currents and detection sensitivity of DES and E2.Chronocoulometry and electrochemical impedance spectroscopy?EIS?were used to examine the enhancement mechanism.It was found that Cu-BTC frameworks not only improved the surface enrichment efficiency of DES and E2,but also reduced their electron transfer resistance.Based on the remarkable enhancement effect of Cu-BTC frameworks,a novel electrochemical sensing platform was developed for the simultaneous determination of DES and E2.The linear ranges were 10-1000 nM and 3-750 nM for DES and E2,with the detection limits of 2.7 nM and1.1 nM.Using this new sensing platform to analyze the lake water samples,and the recovery was over the range from 97.27%to 102.9%.?2?The electrochemical behaviors of two kinds of azo colorants?sunset yellow and tartrazine?on the surface of Cu-BTC were studied.It was found that the Cu-BTC frameworks had high accumulation ability for sunset yellow and tartrazine,and also improved their electron transfer ability.The detection parameters were optimized,and a highly sensitive and novel electrochemical method was developed.The linear range was from 0.3 to 50 nM for sunset yellow,and the detection limit was 0.05 nM.As to tartrazine,the linear range was from 1.0 to 100 nM,with the detection limit of 0.14 nM.The interference experiment indicated that 0.05 mM glucose,sucrose,caffeine,benzoic acid,Fe³⁺,Zn²⁺and Mg²⁺;0.025 mM glycine,citric acid and vitamin C;as well as 500nM amaranth,rose red,allura red and quinoline yellow had no influences on the detection of 50 nM sunset yellow and tartrazine?peak current change<5%?.The method was used for the analysis of different drink samples,and the values were compared by high-performance liquid chromatography,the relative error was less than 9%.?3?In our above work,Cu-BTC sensing materials were prepared in advance,and then used to modify the carbon paste electrode to construct electrochemical sensing platform,which existed some shortcomings.If the copper-based MOFs can be in-situ prepared on the electrode surface,it is very important for the theory and application study of MOFs in the field of electrochemical sensing.In this section,using Cu²⁺as the center,and choosing different organic carboxylic acids as the ligand,like1,3,5-benzenetricarboxylic acid?H₃BTC?,1,4-benzenedicarboxylic acid?H₂BDC?,and1,2,4,5-benzenetetracarboxylic acid?H₄BTEC?,three kinds of copper-based MOF films?Cu-BTC,Cu-BDC,and Cu-BTEC?were in-situ deposited on the surface of glassy carbon electrode?GCE?by cathodic reduction in dimethylformamide?DMF?containing triethylamine hydrochloride?Et₃NHCl?as probase source.Characterizations of FTIR,XRD and X-ray photoelectron spectra?XPS?indicated the formation of three pure phase copper-based MOFs,and the valence state of center copper was+2.Scanning electron microscopy?SEM?tests confirmed that ligands had significant effect on the morphology of copper-based MOFs.The study of cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?indicated that the number of active center,capacitance,electrochemical response area and electron transfer ability of the three copper-based MOFs were different,which was related to different secondary building units and morphology.The electrochemical behaviors of H₂O₂ on the surface of copper-based MOF sensing films were studied using linear sweep voltammetry?LSV?,chronoamperometry and rotating disk electrode?RDE?.It was found that the reduction currents and heterogeneous reaction rate constants of H₂O₂ on these films were both different,obeyed the following sequence:Cu-BTC>Cu-BDC>Cu-BTEC.The results indicated that the number of catalytic activity center and morphology of the copper-based MOFs could be tuned via changing the ligands,and moreover,their electrochemical catalytic ablily and enhancement effects also could be controlled.As a result,a new and highly sensitive electrochemical sensing plaform for H₂O₂ was developed using Cu-BTC as the sensitive film.The detection limit was 0.86?M.No interference was found in the presence of 0.5 mM glucose,sucrose,xanthine,AA,DA and UA.The sensing system was applied in the analysis of different disinfectant samples,and the recovery was in the range of 95.9%to 102.8%.?4?The study of the previous chapter indicated that the copper-based MOFs prepared using the ligand with three carboxyl groups?Cu-BTC?had superior electrochemical properties.In this section,the effects of the reduction potentials on the electrochemical properties of Cu-BTC were further investigated.A series of Cu-BTC films were prepared on the surface of GCE by controlling the deposition potentials of-1.10 V,-1.20 V,-1.30 V and-1.40 V.The characterizations using FTIR,XPS,XRD,TEM,CV,and EIS indicated that the reduction potential had no effects on the bonding properties,surface electronic states of Cu center,chemical compositions,phase purity and crystallinity of the Cu-BTC films.However,the reduction potential obviously affected the morphology and electrochemical activity of Cu-BTC films.The response behaviors of NADH were studied under the help of RDE and double potential step chronocoulometry.It was found that the electron transfer ability and accumolation ability of different Cu-BTC films for NADH showed great difference,which could lead to a large difference in their oxidation signals.Furthermore,the electrochemical behaviors of some small biological molecules?xanthine and hypoxanthine?,environmental estrogens?diethylstilbestrol and estradiol?and azo colorants?sunset yellow and tartrazine?were further studied on the Cu-BTC films prepared under different potentials,and the results were similar to NADH.Thus,by changing the deposition potentials,the modulation of the morphology and electrochemical properties of copper-based MOF films were also could be realized.Based on the reduction potential-controlled catalytic ability and enhancement effects of Cu-BTC films,a kind of electrochemical sensing platform with high sensitivity and good practicability can be developed for broader detection tragets.