Electrochemical/Photoelectrochemical Biosensing Based On Porphyrinic Metal-Organic Framwork Composites

As a new type of porous material,metal-organic framework(MOF)has the advantages of large internal surface area,high porosity,tunable pores and easy functionalization.It has been widely studied and applied.Especially in biosensing,its excellent optoelectronic properties,chemical stability and easy biofunctionalization make it unique in detection,cell imaging and therapy.Among them,the porphyrinic MOFs with biomimetic porphyrin molecules as ligands not only has the photoelectric activity of porphyrin molecules,but also possesses many advantages of MOFs,exhibiting good photoelectric properties for broad application prospects.Based on the high porosity and photoelectrochemical activity of porphyrinic MOFs,this work utilizes MOFs to load metal nanoparticles as catalytic center or sensitize semiconductors for bioanalysis and detection,including the following two parts:1.DNA-Walker-Induced Allosteric Switch for Tandem Signal Amplification with Palladium Nanoparticles/Metal-Organic Framework Tags in Electrochemical BiosensingIn this work,a tandem signal amplification strategy based on porphyrinic MOF composites and DNA walker was proposed and used for electrochemical biosensing.Porphyrinic MOF(PCN-224)with large internal surface area and high porosity provides good opportunities to support Pd NPs,avoiding the aggregation of Pd NPs and improving the electrocatalytic activity of PCN-224.The resulting Pd NPs/PCN-224 was biologically functionalized with streptavidin(SA)to obtain the Pd/PCN-224-SA signal probe,which could efficiently electrocatalyze NaBH4 to achieve multi-electron signal amplification readout.At the same time,a DNA walker-based biosensor was constructed using the hairpin DNA containing the SA aptamer sequence as the tracks and the swing arms silenced by blockers as the DNA walker.After the addition of target DNA,the blocker would hybridize with the target DNA through a strand substitution reaction to release the swing arm.Then,the swing arm would combine with the hairpin DNA to form a recognition site for the cleavage endonuclease,and after reaction,the swing arm was released again.Thus,the cycle reciprocating,the signal amplification of the release of multiple signal molecules was available,and the conformation of the hairpin DNA after reaction turned into the stable aptamer of SA.Therefore,Pd/PCN-224-SA was able to be brought onto electrode surface via SA-aptamer biorecognition to generate the enhanced electrochemical signal,and the sensing strategy for tandem signal amplification was achieved.The electrochemical biosensor had good performance,such as wide detection range(6 orders of magnitude),low detection limit,and good single mismatch differentiation ability.Moreover,the feasibility of the biosensor was identified in serum matrixes.This tandem signal amplification strategy provided a new strategy for the construction of electrochemical biosensors based on the porphyrinic MOF composites and DNA walker.2.Porphyrinic Metal-Organic Framework Sensitized ZnO as Photocathode for Photoelectrochemical BiosensingIn this work,porphyrinic MOFs,which integrated the advantages of both the porphyrin and MOFs,was used as the sensitizer to sensitize the wide-bandgap ZnO,and then a photocathode photoelectrochemical biosensor based on this composite was successfully constructed.The ZnO/PCN-224 composites were prepared by the coordination of the abundant coordination group on porphyrinic MOF with ZnO.PCN-224 containing porphyrin ligands in the composite had a narrow band gap and was responsive to visible light,while ZnO had a wide band gap and was only responsive to ultraviolet light,so the composite could broaden the spectral absorption range and improve the utilization of light energy.At the same time,the photoexcited PCN-224 could transfer electrons to ZnO with lower conduction band level,and then electrons would transmit to the electron acceptor O2 via the conduction band of ZnO to generate singlet oxygen,which accelerated the charge transfer and greatly improved the efficiency of electron hole separation.In addition,since the electron donor with the suitable energy level could enhance the photocurrent via scavenging holes,we selected the dopamine and ascorbic acid,whose energy levels were between the HOMO and LUMO of the PCN-224 for the photoelectrochemical test.The result showed that the dopamine and ascorbic acid were oxidized by holes to inhibit the process of charge recombination,so that the photoelectric conversion efficiency was enhanced.Therefore,the electron acceptors of PCN-224 could obtain more electrons,resulting in the enhanced photocurrent.The photocathode current enabled highly sensitive photoelectrochemical biosensing.The excellent property of the photocathode biosensor was attributed to the suitable energy level matching together with MOFs high porosity and tunable structures that might facilitate the enrichment of analytical molecules.Finally,MOFs offered a new approach in the field of sensitization and photoelectrochemical biosensor.