Photoelectrochemical Biosensor Based on Co3O4 Nanoenzyme

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Photoelectrochemical Biosensor Based on Co3O4 Nanoenzyme Coupled with PbS Quantum Dots for Hydrogen Peroxide Detection Panpan Wang, Ling Cao, Yu Chen, Ying Wu, and Junwei Di ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00165 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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ACS Applied Nano Materials

Photoelectrochemical Biosensor Based on Co3O4 Nanoenzyme Coupled with PbS Quantum Dots for Hydrogen Peroxide Detection Panpan Wang, Ling Cao, Yu Chen, Ying Wu, Junwei Di* College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou, 215123, PR China KEYWORDS: Co3O4; PbS; nanozyme; hydrogen peroxide; photoelectrochemical biosensor

ABSTRACT: A novel kind of hydrogen peroxide (H2O2) photoelectrochemical (PEC) sensor was constructed based on Co3O4 and PbS nanomaterials modified indium tin oxide (ITO) photoelectrode. The Co3O4 nanoparticles, as mimic enzyme of catalase (CAT), can catalyze H2O2 to generate oxygen (O2) in suit. Then the electron acceptor of O2 enhances the cathodic photocurrent of the photoelectrode. The PEC sensor exhibited high sensitivity because of the formation of p-p type heterostructure between PbS and Co3O4 semiconductors. The photocurrent enhancement can be used to detect concentration of H2O2. The calibration plot was linear in the range from 5 to 250 μM and the detection limit was estimated to be 1.2 μM. The results demonstrated the possibility of nanozyme application in PEC biosensors and the substitution of Co3O4 nanozyme for the natural enzyme.

1. Introduction Hydrogen peroxide (H2O2) is considered toxic for living organism because it is a reactive molecule in vivo and plays an important part in the regulation of various biological processes in normal cell functions or disease progressions1-3. Many enzymes, including glucose oxidase, alcohol oxidase, urate oxidase and lactate oxidase, generate H2O2 in the enzymatic reaction process. It also widely used in food industry, textile treatment, paper production, environmental engineering, and medical analysis. Hence, a large amount of research work was devoted to detecting H2O2 4-7.

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Enzyme electrodes are widely used to detect H2O2 due to their obvious advantage of sensitivity and selectivit4-5. Many electrochemical sensors were developed by using the heme proteins, such as horseradish peroxidase (HRP)8-9, myoglobin (Mb)10, hemoglobin (Hb)11, catalase (CAT)12 and cytochrome c (cyt.c)13 . In recent years, the photoelectrochemical (PEC) sensing is developed as a promising analytical method based on coupling photo-irradiation with electrochemical detection14-15. In comparison with conventional electrochemical method, PEC sensors possess potentially high sensitivity due to its separation for excitation and detection16. Several PEC sensors for detection of H2O2 have also been designed by using HRP17-19. However, natural enzymes are high cost, poor stability, and the critical operating conditions, which limits the application of enzyme-based biosensors20. Nanozyme, as a new type of mimetic enzyme, has recently drawn a great deal of attention in biochemical analysis, disease diagnosis, biochemical engineering, and food testing due to its low cost, simple preparation, and adjustable catalytic activity21-25. Compared to natural enzymes, nanozyme-based biosensor has several distinct advantages, such as stability, sustainability, and robustness to bad conditions. Several nanozymes, such as CoxNi1−xFe2O4, N-doped Graphene/ZnFe2O4, and Graphene oxide-gold nanomaterials, have been used in H2O2 detection with colorimetric and electrochemical methods26-29. Among nanozymes, Co3O4 nanoparticles (Co3O4 NPs) exhibit multi-enzyme activities at different pH conditions. Under acidic medium, Co3O4 NPs exhibit mimetic peroxidase ability to catalytically oxidize the substrate by H2O2. In neutral or basic solutions, Co3O4 NPs act as mimetic CAT to decompose H2O2 and produce O2. Moreover, Co3O4 NPs is a low-toxic p-type semiconductor with excellent physicochemical properties, and extensively applied in lithium cells, catalyzers and PEC devices30-35. It is critical to select appropriate materials to possess the superior properties in the PEC strategy. Co3O4 has a narrow band gape of 2.07 eV. Thus, it is a suitable candidate as PEC materials excited by visible light. However, the electron-hole pairs of Co3O4 NPs separated slowly under irradiation with light, which limited its application36. Hybrid nanomaterials are considered to be able to integrate various functions. PbS, as a p-type semiconductor, has an attractive small band gap, big exciton Bohr radius, and powerful confinement effects on charge carriers37. Due to these attractive characteristics, we propose a new type of PEC sensor based on PbS/Co3O4 nanoparticle hybrid modified indium tin oxide (ITO) photoelectrode for detection of


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H2O2. The Co3O4 NPs served as CAT mimic enzyme to catalytically decompose H2O2 into water and oxygen. The PbS quantum dots (PbS QDs) was PEC activity for the production of oxygen. Moreover, the p-p type heterostructure improved the photocurrent response. It was demonstrated that the proposed biosensor had a sensitive response to H2O2. Compared with Co3O4 nanoparticles amperometric sensor for H2O2 detection33,

38-39,

this PEC sensor utilized PbS-

CO3O4 heterostructure to directly detect oxygen generated in the suit at the electrode surface. Thus, its measurement is carried out at low polarized potential of -0.2 V (vs. SCE). We expect this new procedure to generate much interest in development and fabrication of many other exquisite nanozyme-based PEC sensors.

2. Experimental details 2.1. Reagents NH3·H2O, Pb(NO3)2, KCl, HCl and NaOH were obtained from Guoyao Chemical Reagent Co. Ltd. China. Co(NO3)2·6H2O was purchased from Jiangsu Qiangsheng Co., Ltd (Suzhou, China). Na2S·9H2O and poly (diallyl dimethyl ammonium chloride) (PDDA) were obtained from Aladdin Chemical Reagent (Shanghai, China). Dimercaptosuccinic acid (DMSA, 98%) was purchased from Shanghai Dibai Biological Reagent (Shanghai, China). ITO glass (1.1mm) was purchased from Suzhou NSH Electronics Co. Ltd. (Suzhou, China). H2O2 was purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd (Shanghai China). The supporting electrolyte solution was 0.1 M Tris-HCl (pH 7.4). All of the chemicals used in this work were of analytical grade. All ultrapure water (18.2 MΩ · cm) were prepared by a Hitech water purification system.

2.2. Apparatus Scanning electron microscopy (SEM) images and elemental analysis of this image region were obtained on S-4700 SEM (Japan Hitachi). The X-ray powder diffraction (XRD) was performed on a MiniFlex 300/600 XRD (Japan). Transmission electron microscope (TEM) images were obtained by Tecnai G20 (FEI, U.S.A.). The UV-vis absorption spectra of PbS QDs


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and Co3O4 NPs were performed on a Cary 60 UV-vis spectrophotometer (Agilent, U.S.A.). Xray photoelectron spectroscopy (XPS) was obtained using ESCALAB 220i-XL (Thermo Scientific). The linear sweep voltammetry (LSV) experiments were taken in 0.2 M Na2SO4 solution saturated with nitrogen gas using the RST5200 electrochemical workstation at 100 mV /s. All of the PEC measurements were performed on a RST5200 electrochemical workstation (Suzhou Risetest Electronics Co., Ltd.) with three-electrode system, where a modified ITO photoelectrode as a working electrode, platinum wire was used as a counter electrode and saturated calomel electrode (SCE) was used as reference electrode. Illumination area was a circle with a radius of 0.25. A LED with a central wavelength of 470 nm was employed as an excited light source (PEAC 200A, Tianjin Ida, China), and the light energy is 20.4 mW.

2.3. Synthesis of TGA-Stabilized PbS QDs TGA-capped PbS QDs were prepared according to the previous report with a slight modification40. Typically, 40 μL TGA was injected into 50 mL of 3.0 mM Pb(NO3)2 solution in a 100 mL three-necked flask under strong magnetic stirring. High purity nitrogen was bubbled throughout the solution for 30 minutes. During this period, 1.0 M NaOH was added in the solution to adjust pH value to about 10. Then 4 mL of 0.012 M of Na2S was injected slowly and the mixture became brown immediately. Finally, the solution was kept at room temperature for 4 hours under stirring.

2.4. Synthesis of DMSA-Modified Co3O4 NPs The DMSA-modified Co3O4 NPs were synthesized according to the previous report with slight modification31. Typically, Co(NO3)2 solution was first obtained by dissolution of Co(NO3)2 ·6H2O in water. Then, [Co(NH3)6]3+ solution was prepared by addition of H2O2 (30%) in the mixture of Co(NO3)2 and NH4·H2O solution in stirring for 15 min at 60 °C. Next, 0.5 mmol of Co(NO3)2 and 1 mmol of [Co(NH3)6]3+ solution was mixed for 3 h at 60 °C with stirring. After cooling down, solid product of Co3O4 was obtained by centrifugation and rinsed with water.


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The pH 2.0−3.0 of Co3O4 suspension was prepared by sonication of the precipitate for 10 min and addition of 0.1 M HCl. Then DMSA in DMSO solution was added under stirring, following sonication for 2 h and subsequently stirring for another 5 h. After centrifugated and rinsed with water, solid product was obtained. Next, The product was dissolved in water. 0.1 M of NaOH was used to adjust pH to 10. The mixed liquid was sonicated for 5 min to obtain clean solution. On this basis, 0.1 M HCl was used to adjust the pH value of the solution to 8.0, and dialysis method was used to remove impurities. Finally, the Co3O4 nanomaterials modified by DMSA were obtained by 0.22 μm membrane filter and stored at 4 °C for later use.

2.5. Preparation of ITO/PbS/Co3O4 photoelectrode The immobilization of PbS QDs and Co3O4 NPs were based on electrostatic interaction. The ITO glass (5 × 0.5 cm) were ultrasonically washed with dilute aqueous ammonia, ethanol and water, respectively, and then dried with N2 gas. These clean ITO slices were alternately immersed in mixture of 2% PDDA - 0.5 M NaCl solution and the as-fabricated PbS QDs solution for 10 min, respectively. After each soak, the modified electrodes were rinsed carefully with water. The multilayer of (PDDA/PbS)3 modified electrodes were fabricated by repeating the process three times. Then, the PbS modified electrodes were alternately immersed in 2% PDDA 0.5 M NaCl mixture and DMSA-modified Co3O4 solution for 30 min to obtain ITO/PbS/Co3O4 electrodes.

Scheme1 The fabrication process of ITO/PbS/Co3O4 electrode 2.6. PEC detection of H2O2


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For H2O2 detection, a LED lamp with a wavelength of 470 nm was employed as an excited light source41. All of the PEC experiments were carried out at an applied potential of -0.2 V (vs SCE). High-purity nitrogen was used for degassing for 30 min before the experiment, and the nitrogen atmosphere was retained over the solution during the whole experiment. The ITO/PbS/Co3O4 electrode was immersed in 10 mL of 0.1 M Tris-HCl (pH 7.4) solution and recorded its photocurrent. Then the photocurrent was recorded again after addition of H2O2 into the solution. The enhancement of photocurrent intensity was employed to detect concentration of H2O2.

3. Results and discussion 3.1. Morphological and structural characterization The morphology and structure of the materials were investigated by TEM, SEM and XRD. Figure 1 exhibits the TEM of images of the as-fabricated TGA-capped PbS QDs and DMSAcapped Co3O4 NPs, which were ball-shaped nanoparticles with a diameter of about 5 nm and 20 nm, respectively. The crystal structures of the as-synthesized PbS QDs, Co3O4 NPs and hybrid ITO/PbS/Co3O4 film were characterized by XRD, which are given in Figure S1. The XRD pattern of the PbS QDs was described in our previous report41. The XRD pattern of Co3O4 showed peaks at 31.14°，36.51°，44.68°，59.31° and 65.41° accorded with the (220)，(311)， (400)，(511) and (404) planes respectively, which is consistent with the standard XRD patterns of cubic Co3O4 (JCPDS No. 03-065-3103). In addition, with respect to the pattern of ITO/PbS/Co3O4 hybrid nanofilm, two sharp peaks at 31.17° and 36.11° were attributed to the characteristic peaks of PbS and Co3O4, indicating the successful integration of both nanoparticles. UV-vis absorption spectra were also used to characterize the PbS QDs and Co3O4/PbS composites. A broad absorption range was observed in Figure S2. For Co3O4/PbS composites, the absorption intensity in the wide range increased due to addition of Co3O4 in the PbS QDs solution, suggesting that it is suitable as a photoactive material.


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A

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Figure 1 TEM images of PbS QDs (A) and Co3O4 NPs (B).

3.2. Characterization of ITO/PbS/Co3O4 electrode Figure 2A shows the SEM image of the ITO/PbS/Co3O4 electrode. Obviously, the ITO/PbS/Co3O4 nanofilm exhibited a porous structure composed of densely interconnected nanoparticles. This porous nanostructure was capable to provide a large surface area at the modified photoelectrode, which facilitates the effective separation of photoexcited electron-hole pairs. In order to further explore the distribution of PbS QDs and Co3O4 NPs on the ITO surface, the element analysis of SEM image region was presented in Figure S3. The peaks from Pb and Co appeared in the EDX spectrum, which demonstrated the existence of Pb and Co element. The elemental mapping analysis of this area of the SEM image was performed to verify the uniform of nanomaterials. As shown in Figure 2B, the nanofilm formed by electrostatic adsorption has very uniform element distribution on its surface, which is conducive to the improvement of the photoelectric properties. The chemical composition and valence state of the ITO/PbS/Co3O4 electrode was also investigated by XPS. The C 1s binding energy of 284.6 eV was used as the internal marked standard to determine peak positions. In the full-scan XPS spectra (Figure 3A),


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Figure 2 SEM image (A) and elemental mapping (B) of the ITO/PbS/Co3O4 electrode. the existence of elements Co, O, Pb, C and S can be confirmed. Pb 4f, S 2P, Co 2p and O 1S are displayed in the relevant high-resolution XPS spectra. The Pb 4f peaks are shown in Figure 3B, and the peaks located at 137 and 142 eV were corresponding to Pb 4f7/2 and Pb 4f5/2. In the Figure 3C, a peak at ~162 eV was assigned to the combination of S 2p1/2 and S 2p3/2, which was ascribed to the PbS compound 42. Figure 3D shows the XPS spectrum of Co 2p. There are two distinct peaks at 779 eV and 794 eV, corresponding to the Co 2p3/2 and Co 2p1/243. Figure 3E shows the XPS spectrum of O 1s at 531 eV, belonging to the bridging O in Co3O443.


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Figure 3 Full-scan XPS spectrum of the as-fabricated ITO/PbS/Co3O4 nanofilm (A) and highresolution XPS spectra of Pb 4f, S 2p, Co 2p and O 1s, respectively (B-E).


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3.3. PEC Behavior of ITO/PbS/Co3O4 electrode The photocurrent performance ITO/Co3O4, ITO/PbS and ITO/PbS/Co3O4 electrodes were determined in Tris-HCl buffer solution with saturation of air at a polarized potential of -0.2 V under 470 nm irradiation source (Figure 4A). Sole PbS QDs is no PEC response to H2O2 44. The ITO/Co3O4 electrode had almost no photoelectric response (curve a). This may be ascribed to the slow separation of photo-induced electron-hole pairs36. The photocurrent intensity of ITO/PbS electrode was about 0.95 μA (curve b), whereas the response of the ITO/PbS/Co3O4 photoelectrode increased greatly to ~2.6 μA. Figure 4B exhibits the photocurrent intensities at the ITO/PbS/Co3O4 photoelectrode in the solution deaerated by nitrogen, 100 μM H2O2 and airsaturated. There were huge instantaneous current at the beginning of illumination. According to the previous report45, this phenomenon was on account of the sharp separation of electron-hole pairs, the following steady photocurrent suggest the capture of build-up holes in surface states inducing an electron flux associated with recombination. All the calculation of photocurrent was stable part. The photocurrent intensity of the ITO/PbS/Co3O4 electrode increased markedly from ~0.7 μA in N2-saturated system up to ~2.6 μA in air-saturated system. Moreover, its photocurrent intensity also largely increased after 0.1 mM H2O2 was added in the photocell. 5000

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Figure 4 Photocurrent responses of (a) ITO/Co3O4, (b) ITO/PbS, and (c) ITO/PbS/Co3O4 electrode in Tris-HCl buffer solution with saturation of air (A); Photocurrent responses of ITO/PbS/Co3O4 photoelectrode in (a) saturation of nitrogen, (b) 100 μM H2O2, and (c) airsaturated pH 7.4 Tris-HCl buffer solution (B).


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Scheme 2 Schematic representation of proposed PEC mechanism at the ITO/PbS/Co3O4 photoelectrode

The possible mechanism of the ITO/PbS/Co3O4 electrode is proposed in Scheme 1. The conduction potential (CB) and valence potential (VB) of PbS QDs were estimated to -0.84 V and +0.34 V (Figure S4A and 4B), the CB and VB potential of PbS QDs were -0.64 V and +0.54 V versus normal hydrogen electrode (NHE), which was consistent with the previous literature46. The CB and VB potential of Co3O4 NPs were estimated to -0.30 V and +2.02 V (Figure S4 C and D), corresponding to previous works for the CB and VB potential (vs NHE) of -0.10 V and +2.22 V47. After the Co3O4 NPs were assembled on the PbS layer, the p-type PbS matched the energy band position of the p-type Co3O4 to form a p-p type heterostructure48. Under the excitation of 470-nm light, the PbS QDs and Co3O4 NPs composites absorbed the matched energy, generating electron-hole pairs. Holes of Co3O4 NPs were transferred to the VB of PbS QDs due to its low energy lever, enhancing the cathodic photocurrent intensity. It is well known that oxygen is good electron acceptor to remove the photoexcited electrons for p-type semiconductors due to inhibition of electron-hole recombination. Thus, the cathodic photocurrent increased in the presence of oxygen46. Furthermore, in neutral and basic solution, the Co3O4 NPs exhibit mimic CAT activity and catalytically decompose H2O2 to produce O2 in suit through the following reactions.31 Therefore, the cathodic photocurrent intensity of ITO/PbS/Co3O4 photoelectrode enhanced in the presence of H2O2 in the system. This is the basis for quantitative detection of H2O2. We also examine the mixture system of PbS and Co3O4 nanoparticles (Figure S5). The photocurrent was small and no marked change was observed after addition of H2O2.


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2Co3+ + H2O2+ 2OH-→ 2Co2+ + 2H2O + O2

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2Co2++H2O2 →2Co3+ + 2OH-

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3.4 Optimization of experimental conditions The effect of polarized potential is investigated in the PEC process. Figure S6A shows the photocurrent intensity of ITO/PbS/Co3O4 electrode at applied potentials ranging from 0.1 V to 0.4 V. The photocurrent response increased as the applied potential fell from 0.1 to -0.4 V, which was consistent with the characteristics of the p-type semiconductor. The more negative applied potential has the disadvantage of the interference of other substance in the sample. Considering these two aspects, we selected -0.2 V as applied potential in the following experiments. The PEC current intensity is also dependent on the pH value of system. The enhancement of photocurrent at ITO/PbS/Co3O4 electrode increased with pH value from 5.6 to 9.1 (Figure S6B). This was attributed to the multi-enzyme activity of Co3O4 NPs. In neutral and basic solution, Co3O4 NPs act as a CAT-like enzyme which can catalyze the reaction of H2O2 into oxygen31. Thus, the photocurrent intensity of ITO/PbS/Co3O4 electrode enhances quickly because the production of oxygen is an excellent electron acceptor. However, the high pH environment is not conducive to the detection of biological systems. Therefore, pH 7.4 of Tris-HCl was used in the detection solution.

3.5 Analytical Performance Under the optimal conditions, the response current of ITO/PbS/Co3O4 electrode raised with the increasing concentration of H2O2 (Figure 5). There is a good linear relation between the increment of photocurrent (ΔI = I - I0) in the H2O2 concentration range from 5 to 250 μΜ. The linear regression equation was ΔI (nA) = 0.799+11.68C with a correlation coefficient of 0.9974. The detection limit was estimated to be 1.2 μM (S/N =3). The comparison with the results from previous reports was illustrated in Table S1. The higher sensitivity may result from the direct decomposition of H2O2 at the interface, and the in-situ generated electron acceptor (oxygen)


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accelerates photocurrent amplification. Moreover, compared to other PEC H2O2 sensor, this nanozyme-based sensing platform is more facile and requires no complex functionalization process for the sensing interface. 500

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Figure 5 Photocurrent responses of the ITO/PbS/Co3O4 photoelectrode to different concentrations of H2O2: 0, (b) 5, (c) 10, (d) 25, (e) 50, (f) 100, (g) 250, (h) 500 μΜ (A). The represent photocurrent changes (ΔI = I - I0) in the concentration range of 5−500 μΜ (B).

The reproducibility of this system was determined in 100 μM H2O2 using five freshly prepared ITO/PbS/Co3O4 electrodes with the same procedure, and a RSD of 7.3% was obtained, indicating good reproducibility of the PEC bioanalysis platform. The response of ITO/PbS/Co3O4 photoelectrode exhibited no obvious variation after storage at 4 ºC for 4 weeks. Common interfering molecules including glucose, citric acid (CA), uric acid (UA), ascorbic acid (AA), and dopamine (DA), were examined (Figure 6). Comparison with the H2O2, the photocurrent responses of these species were less than 7%. The excellent selectivity can be attributed that the application of nanozyme can change the target response molecule from H2O2 to O2 (electron acceptor)49. Moreover, the effect of the mixture of H2O2 and all interfering species mentioned was also investigated. It was 93.4% of the response compared with no interfering species, confirming the good selectivity of this PEC bioanalysis platform.


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Figure 6 Selectivity investigations for H2O2. The concentration of H2O2 was 50 μM, the interfering compounds were glucose and citric acid of 1000 μM, uric acid, ascorbic acid, and dopamine of 500 μM, and the concentrations of 50 μM of agents mentioned above together with 50 μM H2O2. 4. Conclusions A p-p type PbS/Co3O4 NPs heterostructure was constructed and used in the application of a PEC biosensor based on Co3O4 NPs nanozyme. Co3O4 nanoparticles were coupled well onto the PbS QDs layer, and the whole system was high oxygen sensitivity in the PEC process. Furthermore, Co3O4 nanoparticles could also act as mimic CAT enzyme, which catalyzed H2O2 to generate O2 in the suit. In comparison with natural enzyme electrodes, the nanozyme based biosensors exhibit benefits of good stability, mass preparation, and simple storage. This strategy is a good potential for development of various other PEC biosensors based on nanozymes coupling with PEC active materials.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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XRD, UV-vis spectra , the element analysis, cathodic and anodic linear potential scan for determining CB and VB position, conditional optimization, comparison with different methods (file type, PDF). AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] ORCID 0000-0002-6426-7201 Notes The authors declare no competing ﬁnancial interest ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21475092) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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TOC A novel ITO/PbS/Co3O4 photoelectrode was fabricated successfully. Co3O4 nanoparticles as a mimic enzyme of catalase could catalyze H2O2 to generate O2 in suit. The nanozyme-based PEC sensor was satisfactorily used to detect concentration of hydrogen peroxide.


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Photoelectrochemical Biosensor Based on Co3O4 Nanoenzyme

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