The Ratiometric Electrochemical Sensing Research Based On Covalent Organic Framework

Electrochemical sensor is a kind of sensing device by using electroactive material?enzyme,antigens,antibodies,electron mediator,etc.?or catalytic active materials referring to nano-enzymes and nano-materials as recognition element,and the electrode as exchange elements in order to achieve electrochemical response signal output.Owing to the low cost,quick response,simple equipment and easy to miniaturize,it has widely attracted researchers'attention.While the key of excellent performance sensor is to fabricate modified electrode,it mainly relies on searching for active materials and effective loading method to realize fast electron and proton transport,as well as enhance catalytic efficiency of electroactive substance,further so as to achieve high sensitivity,selectivity and stability signal.In this thesis,it was mainly aimed at designing and preparing modified electrode based on multiple redox activity covalent organic frameworks?COF?and relevant nanocomposites encapsulating electroactive molecules to develop a series of ratiometric electrochemical sensor.The resulted sensors were employed to detect glucose,hydrogen peroxide and pH.The specific work was as follows:1.Here,a kind of COF with multiple redox-active state synthesized by amine-aldehyde condensation reaction between 4,4',4''-?1,3,5-triazine-2,4,6-triyl?trianiline and 2,5-dihydroxy terethaldehyde(COF_DHTA-TTA)was used as novel electrochemical sensing and biosensing platform for the first time.The COF_DHTA-TTAHTA-TTA was demonstrated to be two-dimensional nanosheets with highly ordered conjugated structure and uniform mesopores about 3.67 nm.Electrochemical studies revealed that a ratiometric electrochemical sensor could be constructed by using COF_DHTA-TTA itself as electroactive materials to determine hydrogen peroxide(H₂O₂)and pH level based on both current and potential signals.The single electrochemical sensor could monitor H₂O₂ concentration from 5.66?M to 400?M and pH level ranging from 11.0 to 3.0.Considering that the COF_DHTA-TTA had good catalytic activity for oxygen reduction reaction in 0.2 M PBS?pH=7.0?,glucose oxidase was further loaded on COF_DHTA-TTAHTA-TTA via its nanosized pores and abundant active sites originated from N-doped skeleton,and the glucose biosensor exhibited good performances for glucose detection from1.26?M to 6.0 mM at-0.30 V,as well as from 0.60?M to 6.0 mM at-0.53 V.The work provided good examples to construct electrochemical sensors based on redox-active COF.2.A layered COF material(COF_ETTA-TPAL)with typical dual-pore sizes,was synthesizedbytheammonia-aldehydecondensationreactionbetween4,4?,4?,4?-?ethene-1,1,2,2-tetrayl?-tetraaniline and terephthalaldehyde and proposed to load double enzymes for ratiometric electrochemical biosensing for the first time.COF_ETTA-TPAL was proven to be a kind of flexible two-dimensional?2D?nanosheet with highly ordered crystalline structure and controllable dual-pore size of 3.06 nm and 0.87 nm simultaneously.COF_ETTA-TPAL could be used as suitable platform to load different enzymes.The good flexibility allowed them to be firmly attached to the electrode surface.The size of microperoxidase-11?MP-11?and glucose oxidase?GOD?were just matched well with the large and small pores of COF_ETTA-TPAL,respectively.Accordingly these two enzymes could be assembled into the pores of COF_ETTA-TPAL through intermolecular hydrogen bond between N-doped skeleton of COF_ETTA-TPAL and–COOH groups of MP-11/GOD.Considering that MP-11 could catalyze oxygen reduction in neutral condition,a ratiometric electrochemical glucose biosensor was constructed based on GOD-MP-11/COF_ETTA-TPAL/glassy carbon electrode to test the biosensing application of COF_ETTA-TPAL.It showed good selectivity,linear range from 0.017 mM to 3 mM,and detection limit of 4.97?M.The proposed ratiometric electrochemical glucose biosensor provides useful insights for the promising application of COF as platform to load enzymes/proteins for electrochemical biosensors.3.In this work,a kind of 2D COF with nanosheets structure was prepared by dehydration condensation reaction between terephthalaldehyde?TPAL?and4,4',4'',4'''-?ethane-1,1,2,2-tetrayl?tetraaniline(COF_ETTA-TPAL).Further,the electroactive molecules of ferrocenedicarboxylic acid?Fc?COOH?₂?were firstly encapsulated into COF_ETTA-TPAL and obtained 200 nm uniform nanospheres with tiny densecavitiesbeingconcentratedonthesurfaceofnanospheres(COF_ETTA-TPAL-Fc?COOH?₂).Besides,the obtained COF_ETTA-TPAL-Fc?COOH?₂nanospheres possessed smaller BET and larger pore size(281.14 m² g^-1,5.56 nm)than COF_ETTA-TPAL nanosheets(423.17 m² g^-1,3.46 nm).Considering that Fc?COOH?₂had interact with hydrogen peroxide?H₂O₂?,and H₂O₂ might undergo self-disproportionationprocessonthesurfaceofCOF_ETTA-TPAL,the COF_ETTA-TPAL-Fc?COOH?₂ nanocomposites was tried to fabricate nonenzyme H₂O₂ratiometric electrochemical sensor.The results demonstrated that the reduction peak current of generated O₂ at-0.5 V(j_{-0.5 V})gradually enhanced as detection signal whereas that of Fc?COOH?₂ around 0.45 V(j_{0.45 V})as reference signal decreased with continuously added H₂O₂.Thus,the j_{-0.5 V}/j_{0.45 V}.45 V was finally chosen as output signal to determine H₂O₂.And the proposed“on-off”ratiometric electrochemical sensor showed good catalytic and selectivity performance with wide linear range?1.1?500?M?and lower detection limit?0.33?M?.The work would contribute to exploit COF application in electro analysis yield in the future.4.Here,a kind of 2D iron-porphyrin-based COF(COF_{p-Fepor NH2-BTA})was firstly synthesized and applied for electrochemical sensing further.Firstly,the porphyrin-basedCOFmaterialwasobtainedbymonomer5,10,15,20-tetrakis?4-aminophenyl?-21H,23H-porphineinteractingwith1,3,5-benzenetricarboxaldehyde(COF_{p-por NH2-BTA}).Then,the COF_{p-Fepor NH2-BTA} product was prepared by post-modification method.And the COF_{p-por NH2-BTA} material was proved to be regular and uniform spherical particles of 1?m size,as well as possessed good crystalline structure and abundant micropores about 1.4 nm.Further,the target product of COF_{p-Fepor NH2-BTA} basically kept the original shape size and maintained crystalline structure with micropores distribution about 0.89 nm after post-modification.Electrochemical research indicated that the synthesized COF_{p-Fepor NH2-BTA} which had good redox couple and multiple proton activity owing to porphyrin-based ring cavity itself,enabled to simultaneously catalyze H₂O₂ reduction and evaluate pH using current and potential signal output,respectively.Therefore,the prepared sensor showed high electrocatalytic activity for H₂O₂ determination from6.85 nM to 7?M,the detection limit was 2.06 nM?S/N=3?,and pH test was from 3.0to 9.0.The work would be significant for porphyrin-based COF to exploit in electrochemical sensing analysis.