Preparation Of Single-atom Catalysts Based On Metal-organic Framework Materials For The Construction Of Electrochemical Sensors

Electrochemical sensors are considered as a promising analytical sensing detection tool due to their fast response time,ease of operation and low cost,and have been widely used in agricultural,environmental,food and biomedical analysis.The high sensitivity and selectivity of electrochemical sensors are largely dependent on the catalytic properties of the electrode materials.Single-atom catalysts(SACs)are widely used in catalytic reactions for electrochemical sensing because of their excellent catalytic properties required for electrode materials,such as maximum atom utilization efficiency,customizable catalytic activity and extremely high catalytic selectivity.The most important features of SACs are the availability of isolated metal active sites at the atomic dispersion level and the theoretical 100% atomic utilization.The metal single atoms and their coordination environment together form the metal single atomic sites that represent the active sites of SACs.The numerous dispersed metal active sites increase the efficiency of the utilization of the electrocatalytic active components,thus ensuring that the SACs exhibit high reactivity and selectivity for electrocatalytic reactions.Unfortunately,due to the extremely high specific surface energy,single atoms are easy to be sintered and aggregated into nanoclusters during the preparation process and cannot be stably dispersed on the support.Therefore,it is very important to find suitable supports for the preparation of stable and highly active SACs.In addition,suitable supports can optimize the local coordination environment and improve the electron transfer performance of SACs through "metal-support interactions"(SMSI),while the support also determines the structure of the SACs and thus affects their electrocatalytic performance.Therefore,the support is critical for SACs to maintain maximum metal dispersion and high electrocatalytic activity.Metal organic frameworks(MOFs)materials are ideal supports for the immobilization of metal single atoms due to their ultra-high specific surface area,unsaturated metal sites,abundant functional groups and ordered pore structure.Generally speaking,single metal atoms can be assembled with organic ligands of MOFs to form three-dimensional frameworks with high specific surface area,high porosity and rich structures,while the pore structure inside MOFs facilitates mass transfer analysis during electrocatalysis,thus SACs prepared with MOFs as supports can ensure the maximum electrocatalytic efficiency.In summary,in this paper,MOFs were used as anchoring support to stabilize and disperse single metal atoms in order to prepare stable and highly active SACs,and they were used as electrode sensing materials to modify glassy carbon electrodes to prepare three highly sensitive electrochemical analytical sensors for the quantitative analysis and detection of levodopa,acetaminophen(AP)and dopamine(DA),respectively,as follows:1.Preparation of SACs Pt HPCN-222 based on the hollow structure MOFs HPCN-222 for the construction of levodopa electrochemical sensorsIn this system,the hollow structure porphyrin zirconium-basedMOFs HPCN-222 as supports for the stable dispersion of single metal Pt atoms to prepare SACs Pt HPCN-222,which was then modified as signal enhancer on GCE to successfully construct a highly catalytic efficient levodopa electrochemical sensor.The high surface area of the support HPCN-222 provided a large number of anchoring sites for the load of single metal Pt atoms,which maximized the utilization rate of single metal Pt atoms.The single metal Pt atoms act as the catalytically active center of the SACs Pt HPCN-222,providing an electron transfer pathway through the jumping mechanism,thus enhancing the electrochemical catalytic signal of levodopa.The hollow porous structure of Pt HPCN-222 can promote the mass transfer and diffusion of levodopa molecules,thus improving the electrocatalytic efficiency of the prepared sensor.The strong interaction between the support HPCN-222 and the single metal Pt atoms not only stabilized the dispersion of the single metal Pt atoms,but also changed the electronic structure of the Pt atoms,thus enabling the prepared Pt HPCN-222/GCE electrochemical sensing platform to have good electrochemical activity and stability to achieve high electrocatalytic efficiency.The constructed Pt HPCN-222/GCE electrochemical sensor showed satisfactory selectivity and reasonable reproducibility in the detection of levodopa,with a wide linear detection range of 0.10 μmol/L ～ 130 μmol/L and an excellent detection limit of 3 nmol/L.Furthermore,the Pt HPCN-222/GCE electrochemical sensor constructed in this experiment accurately determined levodopa levels in human serum samples,providing an efficient single-atom catalytic platform for the clinical treatment and bioanalytical detection of Parkinson’s disease.2.Preparation of SACs CuUiO-66 based on MOFs UiO-66 with synergistic MWCNTs for the construction of acetaminophen electrochemical sensorsThe system introducedMOFs UiO-66 with excellent chemical and thermal stability as supports,and the restricted domain anchoring of single metal Cu atoms in the defective sites of UiO-66 zirconia clusters by a post-synthetic metallization method to prepare SACs CuUiO-66.To further improve the catalytic efficiency of the electrochemical sensor,MWCNTs were introduced to construct the electrochemical sensor CuUiO-66/MWCNTs/GCE to achieve highly sensitive electrochemical detection of AP.The high specific surface area of SACs CuUiO-66 can provide many active sites to enhance AP molecules adsorption.The introduction of MWCNTs can enhance electrical conductivity by increasing the rate of electron transfer.CuUiO-66 was attached to the surface of the MWCNTs,which can effectively overcome the accumulation of CuUiO-66.Similarly,CuUiO-66 supported the MWCNTs to avoide MWCNTs accumulation on the electrode surface,giving the MWCNTs better catalytic performance.The synergistic catalysis of both CuUiO-66 andMWCNTs increases the specific surface area and electron transfer rate of the electrode materials,enabling efficient electrocatalysis of AP.With the multicatalytic amplification strategy,the CuUiO-66/MWCNTs/GCE electrochemical sensors exhibited good anti-interference for the detection of AP with a detection limit of 0.024μmol/L and a linear response range of 0.10 μmol/L ～ 120 μmol/L.At the same time,the sensing platform also demonstrated excellent sensing performance in the determination of AP levels in human serum samples by standard addition method,showing great potential for practical applications and providing further opportunities for the detection of AP by electrochemical sensing analysis.3.Preparation of SACs PdMOF-525 based on porous porphyrin MOF-525 for the construction of dopamine electrochemical sensorsIn this work,porphyrin MOF-525 with porous structure were introduced as the supports,and the single metal Pd atoms were anchored in the porphyrin ring of MOF-525 by simple one-pot hydrothermal method to prepare SACs PdMOF-525 with excellent electrocatalytic activity and stability,which then use PdMOF-525 as electrode materials to build PdMOF-525/GCE electrochemical sensor for highly sensitive analysis and detection of DA.The porous structure and large specific surface area of the support MOF-525 not only accelerate the charge transfer to the electrode surface to promote the mass transfer of DA,but also generate abundant redox electroactive sites,thus improving the current response of the electrochemical sensors.The DA molecule can interact with the porphyrin ring of MOF-525 through π-π stacking and hydrogen bonding,which can promote the surface adsorption of DA and significantly enhance the electrocatalytic signal.The single metal Pd atoms were embedded in the center of the porphyrin ring of MOF-525,and the electron transfer was accelerated through the interaction of metal-support,which could improve the electrocatalytic signal of DA.The PdMOF-525/GCE electrochemical sensing platform prepared in this experiment showed a good electrochemical response for the analytical sensing detection of DA.The sensor showed a remarkably wide linear detection range(1.0 μmol/L to 120 μmol/L)and a promising detection limit(0.08 μmol/L),with reasonable detection stability over a range of interfering substances.Furthermore,the electrochemical sensor showed excellent selectivity and reasonable recovery in the detection experiment of DA recovery in human serum samples.Therefore,the PdMOF-525/GCE prepared in this work provided inspiration for the development of an efficient electrochemical sensor platform from the perspective of single atomic catalysis.It also provided reference for exploring the detection mechanism of such analytes.