Fabrication And Bioelectrocatalytic Performance Of TiO₂ Ntas-Based Enzyme Electrode

Electrochemical enzyme biosensors have been widely applied to clinical testing, environmental monitoring, food, pharmaceutical and other areas. Currently, the biggest problems in the process of preparation and application of the enzyme biosensors are the disadvantages of low bioelectrocatalytic activity, poor long-term stability, and easy inactivation of the enzyme electrode. However, such problems are closely related to the enzyme immobilization method and the supporting material for the preparation of the enzyme electrode. In this paper, we carried out the research work focused on the high efficient enzyme immobilization method and the novel supporting nanomaterials for the fabrication of the enzyme electrodes, and four TiO2 NTAs-based enzyme electrode had been synthesized. Four TiO2 NTAs-based enzyme electrode supporting nanomaterials had been designed and fabricated:titania nanotube arrays (TiO2 NTAs), titania nanotube arrays/silver nanoparticles (TiO2 NTAs/AgNPs), titania nanotube arrays/reduce graphene oxide/silver nanoparticles (TiO2 NTAs/r-GO/AgNPs) and N-doping titania nanotube arrays (N-TiO2 NTAs). Lots of enzyme electrodes with high bioelectrocatalytic activity had been fabricated based on the TiO2 NTAs-based supporting nanomaterials, the bioelectrocatalytic applications of the enzyme electrodes had been investigated. The main work are listed as follows.1. The fabrication and bioelectrocatalytic performance of TiO2 NTAs/GOx enzyme electrode. TiO2 NTAs represents an excellent material with good biocompatibility, good hydrophilicity, low cost, larger specific surface area and strong adsorptivity. Additionally, highly ordered TiO2 nanoarrays greatly assist the electron transfer. Therefore, TiO2 NTAs can be regarded as an ideal supporting material for enzyme electrode fabrication. The optimized cross-linking technique had been proposed for immobilization the glucose oxidase (GOx) on the upper surface of the enzyme electrode. Consequently, the active sites of enzyme could avoid being embedded. The active sites of enzyme were able to react with glucose efficiently for the detection of glucose. The TiO2 NTAs/GOx enzyme electrode showed a linear response to glucose over a concentration range of 0.05 to 0.65 mM. The sensitivity could reach a value of 199.61 μA mM-1 cm-2. The TiO2 NTAs/GOx enzyme electrode exhibited good bioelectrocatalytic activity, which could be used as a glucose biosensor for glucose detection.2. The fabrication and bioelectrocatalytic performance of TiO2 NTAs/AgNPs/GOx enzyme electrode. Silver is a noble metal with the superior conductivity and good biocompatibility. Additionally, the AgNPs modified enzymatic biosensors always exhibit higher sensitivity. AgNPs had been used to modify the TiO2 NTAs for enhancing the electron transfer ability of the supporting material. The enhanced electron transfer ability of the supporting material, resulting in improved bioelectrocatalytic performance of the enzyme electrode. In this study, the AgNPs was deposited on the surface of TiO2 NTAs by a chemi-deposition process for the fabrication of TiO2 NTAs/AgNPs. The diameter size of one single silver nanoparticle was in the range of 30-100 nm. Then, the optimized cross-linking technique had been used to immobilize the GOx on the upper surface of the TiO2 NTAs/AgNPs for fabricating the TiO2 NTAs/AgNPs/GOx enzyme electrode. The bioelectrocatalytic performance of the TiO2 NTAs/AgNPs/GOx enzyme electrode had been studied. The glucose biosensor based on TiO2 NTAs/AgNPs/GOx enzyme electrode exhibited a linear response to glucose in the concentration range of 0.05-0.65 mM, with a sensitivity of 207.43 μA mM-1 cm-2. The sensitivity was similar or increased slightly from 199.61 μA mM-1 cm-2 for the TiO2 NTAs/GOx enzyme electrode to 207.43 μA mM-1 cm-2 for the TiO2 NTAs/AgNPs/GOx enzyme electrode. Importantly, the well improved conductivity of TiO2 NTAs/AgNPs made it possible for the application of enzymatic fuel cell. The enzymatic fuel cell based on TiO2 NTAs/AgNPs/GOx enzyme electrode exhibited an open-circuit potential of 0.202 V, a short-circuit current density of 0.197 mA cm-2, and a maximum power density of 8.66 μW cm-2.Therefore, the TiO2 NTAs/AgNPs/GOx enzyme electrode exhibited better bioelectrocatalytic activity than TiO2 NTAs/GOx enzyme electrode, which could slightly enhance the sensitivity of the glucose biosensor and importantly be used to fabricate enzymatic fuel cell.3. The fabrication and bioelectrocatalytic performance of TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode. The aggregation of deposited nanoparticles on graphene sheets can be prevented through the strong van der Waals force between the graphene and nanoparticles. The size of the obtained nanoparticles is small accordingly. Additionally, lots of hydrophilic groups on the surface of r-GO are helpful for the immobilization of enzyme molecules. In this study, the AgNPs were chemi-deposited on r-GO-modified TiO2 NTAs, then the optimized cross-linking technique had been used to immobilize the GOx on the upper surface of the TiO2 NTAs/r-GO/AgNPs for fabrication the TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode. The r-GO modification on surface of the TiO2 NTAs was used to prevent the aggregation of chemi-deposited AgNPs. The better dispersity of AgNPs, resulting in better bioelectrocatalytic performance of the enzyme electrode. The coverage of AgNPs on the surface of TiO2 NTAs/r-GO was denser than that on the surface of TiO2 NTAs. The size of AgNPs was uniform and small, which had a range of 20-30 nm. The TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode exhibited a linear response to glucose in the concentration rangeof 0.05-0.3 mM, with a sensitivity of 257.79 μA mM-1 cm-2. Additionally, the enzymatic fuel cell based on TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode gave an open-circuit potential of 0.225 V, a short-circuit current density of 0.232 mA cm-2, and a maximum power density of 13.45 μW cm-2. The experimental results demonstrated that the rGO modification on surface of the TiO2 NTAs was an effective method for the uniform deposition of AgNPs without obvious aggregation. The detection sensitivity was promoted obviously from 207.43 μA mM-1 cm-2 for the glucose biosensor using TiO2 NTAs/AgNPs/GOx enzyme electrode to 257.79 uA mM"1 cm-2 for that using TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode. The maximum power density was promoted from 8.66 μW cm-2 for the enzymatic fuel cell using TiO2 NTAs/AgNPs/GOx enzyme electrode to 13.45 μW cm-2 for that using TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode. Thus, the TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode exhibited better bioelectrocatalytic activity than TiO2 NTAs/AgNPs/GOx enzyme electrode, which could further enhance the detection sensitivity of the glucose biosensor and the maximum power density of the enzymatic fuel cell.4. The fabrication and bioelectrocatalytic performance of N-TiO2 NTAs/GOx enzyme electrode. Element doping is often used to improve the physical and chemical properties of TiO2. N-doping TiO2 (N-Ti2) not only exhibits good hydrophilicity, but also shows higher conductivity as compared to TiO2. N-TiO2 NTA was synthesized as a supporting material to fabricate N-TiO2 NTAs/GOx enzyme electrode. Electrochemical impedance spectroscopy and contact angle measurement revealed that N-TiO2 NTA exhibited remarkably reduced charge transfer resistance and similar contact angle as compared with TiO2 NTA, presenting high electrical conductivity and good hydrophilicity. The glucose biosensor based on N-TiO2 NTAs/GOx enzyme electrode showed a detection sensitivity of 733.17 μA mM-1 cm-2 and a linear range of 0.05-0.85 mM for glucose determination. Additionally, the enzymatic fuel cell based on N-TiO2 NTAs/GOx enzyme electrode showed a maximum power density of 23.92 μW cm-2. This N-TiO2 NTAs/GOx enzyme electrode exhibited high selectivity, good reproducibility, high bioelectrocatalytic activity and long-time storage stability. The sensitivity was promoted very significantly from 199.61 μA mM-1 cm-2 for the glucose biosensor using TiO2 NTAs/GOx enzyme electrode to 733.17 μA mM-1 cm-2 for that using N-TiO2 NTAs/GOx enzyme electrode. The maximum power density was promoted significantly from 5.38 μW cm-2 for the enzymatic fuel cell using TiO2 NTAs/GOx enzyme electrode and 13.45 μW cm-2 for enzymatic fuel cell using TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrode to 23.92 μW cm-2 for that using N-TiO2 NTAs/GOx enzyme electrode. Consequently, the N-TiO2 NTAs/GOx enzyme electrode exhibited obviously better bioelectrocatalytic activity than TiO2 NTAs/GOx and TiO2 NTAs/r-GO/AgNPs/GOx enzyme electrodes, which could significantly enhance the detection sensitivity of the glucose biosensor and the maximum power density of the enzymatic fuel cell.