Photoelectrochemical Sensors Using Molecular Sieve Effect And The Inner Resistance Response Characteristics

Metal-organic frameworks?MOFs?are a class of inorganic-organic hybrid materials with porous structure.Compared with the traditional inorganic porous materials,MOF has greater porosity and specific surface area.Especially,by using different metal ions and organic bridging ligands,the pore size and surface function of MOF can be easily adjusted.Recently,MOFs have been found applications in many fields such as sensors,catalysis,gas separation,and storage,among which MOFs show strong molecular selection due to the limitation of pore size.Photoelectrochemical?PEC?sensors are based on the photoelectric conversion properties of substances to determine the concentration of analytes,offering the advantages of high sensitivity,simple equipment,easy miniaturization.Hence,PEC has become a useful analytical method.In this thesis,new PEC sensors based on the moleclur size exclusion filter and catalytic effects of MOFs are designed to improve the sensitivity and selectivity for the determination of H₂O₂ and glucose.The main research works are listed below.1.Impedance response of photoelectrochemical sensor and size-exclusion filter and catalytic effects in Mn₃?BTC?₂ for the determination of H₂O₂The the photocurrent and photovoltage of a photoanode are measured in a proposed electrochemical setup.Accordingly,the nner resistance of the photoanode is obtained.Three photoanodes,including TiO₂ nanotubes array?TiO₂ NTs?,g-C₃N₄/Ti O₂ NTs and Mn₃?BTC?₂/g-C₃N₄/TiO₂ NTs?BTC=1,3,5-benzene-tricarboxylic acid?,were chosen as the models,the changes in the inner resistance of the photoanode during a PEC process are monitored.Under light excitation,the inner resistance of the photoanode is reduced remarkably,resulting in the increment in the photocurrent.It was shown the change in inner resistance of the photoanode is the main factor for the photocurrent response in a PEC sensor.The influence of the light intensity on the inner resistance is less than that on the photocurrent,due to the synchronous change in photovoltage.Hence,the inner resistance of photoanode is a useful signal indictor in PEC sensor with better stability.With increasing concentration of hole scavengers?e.g.,H₂O₂,ascorbic acid?,the steady inner resistances of the three photoanodes are decreased.In the PEC sensor with Mn₃?BTC?₂/g-C₃N₄/TiO₂ NTs,the Mn₃?BTC?₂layer is served as a size exclusion filter to prevent the diffusion of molecules with size larger than the apertures in crystals,which improves the selectivity of the PEC sensor to smaller molecules?e.g.,H₂O₂?.The catalytic effect of Mn₃?BTC?₂ to H₂O₂ enhances the PEC response sensitivity to H₂O₂.Under optimized conditions,the reciprocal of the averaged steady effective inner resistance of Mn₃?BTC?₂/g-C₃N₄/TiO₂ NTs is in a linear correlation with the logarithmic concentration of H₂O₂ in the range of 0.003 to 10?mol/L with a limit of detection of 1 nmol/L.This PEC sensor was applied for H₂O₂ determination in exhaled breath condensate samples.2.Photoelectrochemistry glucose sensor based on double mimic enzymes of alkali treatment ZIF-67 filmDetermination of glucose is one of the important content of clinical blood analysis.Usually,indirect methods are employed by using double enzymes.First,glucose reaction with dissolved oxygen to generate H₂O₂ in the presence of glucose oxidase.After then,the product of H₂O₂ as well as the related reaction products are detected by spectroscopic or electrochemical methods using horseradish oxidase as the catalyst.However,enzymes are expensive and unstable.Recently,the researches on nanomaterials-based mimic enzymes have attracted much attention.In this work,a cobalt metal organic framework?Co-MOF,ZIF-67?is found to have the function mimic peroxidase,which can catalyze the decomposition of H₂O₂ to produce free radicals.This characteristic is used to determine H₂O₂ in PEC sensor.Importantly,after treated by NaOH solution,the product of Co_xO_yH_z@ZIF-67 possesses the properties of glucose oxidase and peroxidase.Hence,Co_xO_yH_z@ZIF-67 with double mimic enzymes is promising nanomeatrial in the development of glucose biosensor.By comparing the photocurrent responses of three photosensitive electrodes,TiO₂ NTs,ZIF-67/TiO₂ NTs and Co_xO_yH_z@ZIF-67/TiO₂ NTs,to ascorbic acid,H₂O₂ and glucose,it is confirmed that ZIF-67 shows mimic peroxidase and molecular sieve effect,which is applied to H₂O₂ detection is a linear range of 5 nmol/L¹0?mol/L,with the detection limit of 2nmol/L.The double mimic enzymes catalytic properties and molecular sieve effect of Co_xO_yH_z@ZIF-67 are utilized in PEC glucose sensor.The linear range is 0.1?mol/L¹mmol/L with the detection limit is 0.03?mol/L.The proposed sensors offers the advabtages of fast response,high sensitivity,good repeatability and selectivity,showing the good application prospect of nanomaterials in the development of enzyme free biosensors.