Photoelectrochemical Biosensors Based On Novel Sensing Strategy

Photoelectrochemical?PEC?biosensor is a newly rised and rapidly developed biosensing technology in recent years,based on photoinduced electron-transfer processes at electrode/solution interfaces to generate photocurrent response.The PEC detection possesses high sensitivity,the reduced background signals,low cost,simplicity and easy miniaturization,based on the combination of electrochemical and optical methods.Despite the PEC method has shown many advantages and there have been many reports on the PEC biosensors in recent years,the photoelectric conversion efficiency and detection strategy of PEC method still needs to be further studied and extended compared with the electrochemical and optical methods.Thus,it is great importance to search efficient photoactive species with super photoelectric conversion efficiency and to develop new biosensing strategy for the construction of PEC biosensing platform.In this dissertation,through preparing new photoactive materials and exploring new biosensing strategies,a series of novel PEC biosensors with high sensitivity were successfully constructed and applied in alkaline phosphatise,caspase-3,prion and thrombin assay.?1?Novel photoactive materials,nitrogen-doped porous carbon-zinc oxide?NPC-ZnO?nanopolyhedra,were prepared by direct carbonization of zeolitic imidazolate framework materials?ZIF-8 nanopolyhedra?in a nitrogen atmosphere.The morphology,structure and PEC properties were characterized by scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,Raman spectroscopy,nitrogen adsorption-desorption method,and PEC methods.The results showed that the obtained NPC-ZnO nanopolyhedra had rhombic dodecahedron morphology with uniform particle size of about 100 nm and high surface area of 609.2 m² g^-1.Under visible light irradiation,the NPC-ZnO nanopolyhedra showed better PEC performance than ZnO nanorod and ZIF-8nanopolyhedra in aqueous media with dissolved oxygen and ascorbic acid?AA?.Taking alkaline phosphatase?ALP?as a model,a NPC-ZnO nanopolyhedra-based PEC sensor was developed and showed good performance for ALP assay with a wide linear response range from 2 to 1500 U L^-1 and a low detection limit of 1.7 U L^-1.Moreover,the PEC sensor possessed acceptable selectivity,reproducibility and stability.The prepared NPC-ZnO nanopolyhedra provide a new photoactive material for construction of PEC sensors and may have promising applications in PEC assay of heavy metal ions,organic pollutants and biomolecules.?2?A new label-free and blocker-free PEC sensor based on bifunctional CC-DEVD-peptide modified-NPC-ZnO/CdS hybrids was developed for sensitive assay of apoptotic protease caspase-3.The NPC-ZnO/CdS hybrids were used as the photoactive materials and exhibited super PEC activity.Then the CC-DEVD-peptide is immobilized successively on the ITO/NPC-ZnO/CdS surface through Cd-S bond.Two cysteines in CC-DEVD-peptide not only make CC-DEVD-peptide be immobilized stably on the surface of ITO/NPC-ZnO/CdS electrode,but also result in a flat-lying of CC-DEVD-peptide onto the surface of the NPC-ZnO/CdS hybrids with the help of the electrostatic interaction between negatively-charged CC-DEVD-peptide and positively-charged NPC-ZnO/CdS hybrids.This made CC-DEVD-peptide was not only a specific cleaving substrate of caspase-3,but also a super blocker to prevent the non-specific adsorption occurred on the surface of the NPC-ZnO/CdS hybrids.In the presence of caspase-3,the caspase-3 could specifically recognize and cleave the C-terminus of DEVD,leading to the washing-away of peptide from the surface of the NPC-ZnO/CdS hybrids and the increase of the photocurrent.The photocurrent response is linear with the concentration of caspase-3from 0.2 to 20 ng mL^-1 with a low detection limit of 0.14 ng mL^-1.Based on its simple structure of the sensing platform and high sensitivity,the developed method has promising applications in the diagnosis of apoptosis-associated diseases and biomedicine.?3?Taking zeolitic imidazolate framework materials?ZIF-67 polyhedra?as the precusor,p-type semiconducting cobalt trioxide-gold?Co₃O₄-Au?polyhedra were synthesized and used as the signal amplifier to construct a sensitive PEC sensor for caspase-3 activity assay.Here,the n-type semiconductor Bi₂S₃-modified ITO slice was used as the photoelectrode.After immobilization of the biotin-DEVD-peptide onto the Bi₂S₃ surface,the streptavidin-labelled Co₃O₄-Au polyhedra were introduced to the sensing platform via the specific interaction between biotin and streptavidin.The Co₃O₄-Au polyhedra could not only quench the photocurrent of the Bi₂S₃ because of the competitive absorption of exciting light and the consumption of electron donors,but also act as peroxidase mimetics to produce catalytic precipitate.Additionally,the steric hindrance effect from the Co₃O₄-Au polyhedra would decrease the PEC response of the Bi₂S₃.Ingeniously,the precipitates could not only deposit on the ITO electrode to decrease the photocurrent of PEC sensor,but also act as electron acceptors to scavenge the photo-generated electrons of Co₃O₄-Au polyhedra,leading to enhanced quenching ability of the Co₃O₄-Au polyhedra.When caspase-3 existed,caspase-3 could specifically recognize and cleave the biotin-DEVD-peptide,resulting in the reduced introduction of Co₃O₄-Au polyhedra and the increase of PEC response.Based on the multifunctional Co₃O₄-Au polyhedra,caspase-3 was detected sensitivily with a linear range from 0.5 to 50 ng mL^-1 and limit of detection down to 0.10 ng mL^–1.The proposed Co₃O₄-Au polyhedra act as a new signal amplifier to construct PEC sensing platform and have potential applications in bioanalysis,disease diagnostics,and clinical biomedicine.?4?A new PEC immunosensor has been developed based on hemin-induced switching of photocurrent direction for prion?PrP^C?assay.In the presence of PrP^C,nitrogen-doped porous carbon-hemin polyhedra labelled with secondary antibody?NPC-hemin-Ab2?were introduced onto the CdS-chitosan?CdS-CS?nanoparticles-modified ITO electrode via the antigen-antibody specific recognition.Because of the matched energy level between CdS and hemin,the high-efficiency switch of photocurrent direction of the ITO/CdS-CS photoelectrode from anodic to cathodic photocurrent was observed even at very low concentration?0.4 aM?of PrP^C.Through changing the specific antibody,this method can be easily expanded to high pathogenic PrP^Sc assay.Such low detectable limit is very useful in the early diagnosis and screening of prion diseases.?5?A photocurrent direction self-switching PEC biosensing strategy based on a direct Z-scheme CdS/hemin system has been successfully developed for PrP^C assay.A cathodic photocurrent of the CdS-CS nanoparticles-modified ITO electrode was obtained through voltage-stimulated switching of the photocurrent direction.While the target PrP^C existed,hemin was introduced into the sensing platform via the antigen-antibody specific recognition.Based on the well-matched energy level of CdS and hemin,a new direct Z-scheme CdS/hemin system was formed with self-switching photocurrent direction of small cathodic to big anodic photocurrents.The developed PEC sensor showed a wide linear response range from 4 to 1000 aM with a low detection limit of 0.53 aM,and exhibited excellent reproducibility,acceptable stability and satisfactory selectivity.Moreover,this biosensing platform could clearly distinguish whether the signal change was caused by PrP^C or other interference factors and avoid a false negative or positive detection result.The photocurrent direction self-switching Z-scheme CdS/hemin system may open a new promising approach for PEC bioanalysis.?6?A highly selective and sensitive PEC biosensor was developed based on based on a novel switching of indirect Z-scheme NPC-ZnO/Au/CdS to direct Z-scheme CdS/hemin systems,driving the conversion of cathodic to anodic photocurrents.Taking the detection of thrombin?TB?as example,the indirect Z-scheme NPC-ZnO/Au/CdS system were served as photoactive materials and exhibited excellent cathodic photocurrent,whereas Au NPs between NPC-ZnO and CdS were not only used as an energy harvester to collect the incident light,but served as an electron-transporting mediator to promote electron from conduction band of NPC-ZnO to valence band of CdS,thus achieving the efficient separation of electron-hole pairs.After immobilization of the TB aptamer?TBA1?onto the NPC-ZnO/Au/CdS polyhedra surface via Cd-S bond,the hemin/G-quadruplex was introduced to the sensing platform through the HCR-based double stranded DNA polymers,and a direct Z-scheme CdS/hemin system was formed owe to the well-matched energy levels between CdS and hemin,leading to a big anodic photocurrent which was opposite to that of NPC-ZnO/Au/CdS polyhedra.Based on the switching of indirect Z-scheme to direct Z-scheme systems driving the inversion of photocurrent polarity,TB was detected sensitively with a linear range from 0.5 fM to 100 pM and limit of detection down to 16 aM.Also,the developed PEC biosensing platform showed excellent selectivity.Coupling Z-scheme system switching with photocurrent polarity inversing may open a new way to construct PEC biosensors which may have potential prospect in early diagnosis and screening of diseases and bioanalysis.