Construction And Application Of TiO₂ Fiber-based Heterojunction For Photoelectrochemistry Biosensor

High-performance flexible photoelectrodes,as the core component of miniaturized and portable PEC biosensor,directly affect the sensitivity and stability of PEC sensors.Electrostatic spun TiO₂ nanofiber membrane（TiO₂ NFM）combines the advantages of nanofiber membrane structure,good biocompatibility and stable photoelectric performance of TiO₂,and has a broad application prospect in flexible photoelectrodes.However,the pure TiO₂ NFM has poor mechanical properties,inferior visible light response and low photoelectric conversion efficiency,which makes it hard to satisfy the practical application in fabricating flexible photoelectrodes.In this paper,Zr-TiO₂ NFM with enhanced mechanical properties was firstly prepared by doping crystal.Then,a series of flexible TiO₂ nanofiber-based heterostructures photoactive materials were synthesized,which possessed good mechanical properties,outstanding visible light response and excellent photoelectric performance.Next,the mechanisms of photogenerated carrier separation,transport and recombination were revealed by their optical and photoelectrochemical behaviors.Meanwhile,the component regulation and photoelectric conversion mechanisms were also explored.Following that,combining with the aptamer-specific recognition function and various sensing strategies,several high-performance PEC aptasensors based on these flexible photoelectrodes were constructed and realized microcystin-LR（MC-LR）detection.The detailed research contents are as follows:（1）To improve the mechanical properties of pure TiO₂ NFM,flexible Zr-TiO₂NFM was prepared by doping crystal based on"fine crystal strengthening theory".Firstly,the surface microstructure,crystal structure,tensile properties,flexibility,and photoelectrochemical properties of Zr-TiO₂were investigated by doping different particle sizes and different concentrations of doped ions.The mechanical enhancement mechanism of doped-TiO₂ nanofibers was also explored.The results indicated that when the doping amount of Zr⁴⁺was 0.8 m L,the tensile strength of Zr-TiO₂ NFM was1.27 MPa,and the strain was increased by 1.13%,which were 60.7%and 39.5%higher than that of pure TiO₂ NFM,respectively.In addition,the photocurrent intensity was improved by 11.7 times and reached 1.64μA.（2）To address the defects of Zr-TiO₂ NFM（such as poor visible light response and easy carrier recombination）,a type-Ⅱheterojunction Mo S₂/Zr-TiO₂ NFM was constructed by coupling transition metal disulfides（TMDs）Mo S₂ nanosheets on Zr-TiO₂ NFM surface with hydrothermal reaction,improving the separation efficiency of carriers.The mechanical properties,visible light response and optoelectronic properties of Mo S₂/Zr-TiO₂ NFM were investigated.Compared with pure Zr-TiO₂ NFM,Mo S₂/Zr-TiO₂ NFM had high visible light absorption and low carrier recombination rate,and its photocurrent intensity was increased by 9.89 times and reached 16.23μA.Then,using Mo S₂/Zr-TiO₂ NFM as the flexible photoelectrode substrate and combining with the specific recognition function of aptamer,a signal-attenuating type PEC aptasensor was constructed for MC-LR detection.The linear response range was from1 pM to 100 n M,and the detection limit was as low as 0.34 pM.Furthermore,the energy band structure and state density of semiconductor materials was verified by DFT theoretical simulation and explained the photoelectric enhancement mechanism.（3）Aiming at the inhibition effect of charge repulsion on carrier space separation in typeⅡheterojunction,p-n type heterojunction Bi OBr/Zr-TiO₂ NFM was constructed by combining p-type semiconductor Bi OBr and n-type semiconductor Zr-TiO₂ NFM through the successive ion layer adsorption reaction（SILAR）method,which promoted the spatial separation of carriers under the internal electric field driving.We explored the effects of SILAR cycles on specific surface area and photoelectric properties of Bi OBr/Zr-TiO₂ NFM,and explained the mechanism of internal electric field in promoting the spatial separation of photogenerated carriers.Compared with pure Zr-TiO₂ NFM,the specific surface area of 6-Bi OBr/Zr-TiO₂ NFM was increased by 1.87times and reached 36.23 m²/g,and the photocurrent was increased to 24.2μA.Based on these results,a signal-attenuating type PEC aptasensor was constructed using 6-BiOBr/Zr-TiO₂ NFM as a flexible photoelectrode substrate for MC-LR detection with a widely linear response range of 0.5 pM～150 n M and the detection limit of 0.21 pM.（4）Due to the false positive phenomenon caused by the weakened redox properties of typeⅡheterojunction and p-n heterojunction in the signal-attenuating type PEC aptasensor,n-type semiconductor Bi OI was introduced Zr-TiO₂ NFM surface to construct S-scheme heterojunction Bi OI/Zr-TiO₂ NFM with strong redox properties by SILAR method.Compared with pure Zr-TiO₂ NFM,Bi OI/Zr-TiO₂ NFM enhanced the visible light response and inhibited the rapid recombination of carriers,which increased the photocurrent to reach 5.51μA.Using Bi OI/Zr-TiO₂ NFM as flexible photoelectrode,a signal-enhanced type PEC aptasensor was constructed and realized the highly sensitive detection of MC-LR.The detection range was 0.2 pM～220 n M,and the detection limit was as low as 0.06 pM.（5）Based on the synergic effect of photon-plasma resonance,a multi-dimensional Z-scheme heterojunction Au@Mg In₂S₄/Zr-TiO₂ NFM was constructed by combining ternary sulfide Mg In₂S₄ nanosheets and gold nanoparticles（Au NPs）with Zr-TiO₂NFM through hydrothermal reaction and photoreduction method to solve the high charge transfer barrier of carrier separation in S-scheme heterojunction.The results showed that Au@Mg In₂S₄/Zr-TiO₂ NFM possessed excellent visible and near-infrared light response,strong reoxidation-reduction properties.The photocurrent intensity was reached 11.85μA,which was 7.23 times higher than Zr-TiO₂ NFM.Then,a signal-enhanced type PEC aptasensor for MC-LR ultra-sensitive detection was constructed using Au@Mg In₂S₄/Zr-TiO₂ NFM as the photoelectrode active material and combining with MC-LR aptamer.The linear range of detection of 0.05 pM～300 n M was obtained,and the detection limit was as low as 0.01pM.