Enhanced peroxidase-like activity of Mo6+ doped Co3O4 nanotubes

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Enhanced peroxidase-like activity of Mo6+ doped Co3O4 nanotubes for ultrasensitive and colorimetric L-cysteine detection Mu Gao, Xiaofeng Lu, Sihui Chen, Di Tian, Yun Zhu, and Ce Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00945 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Nano Materials

Enhanced peroxidase-like activity of Mo6+ doped Co3O4 nanotubes for ultrasensitive and colorimetric L-cysteine detection

Mu Gao, Xiaofeng Lu*, Sihui Chen, Di Tian, Yun Zhu, Ce Wang*

Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun, 130012, P. R. China.

KEYWORDS: Co3O4 nanotubes, doping, peroxidase-like activity, colorimetric detection, Lcysteine.

ABSTRACT Doping with transition metal ions has been regarded as one of the most effective means to optimize the catalytic efficiency of metal oxides. In this work, a general approach for the fabrication of Mo6+ doped Co3O4 (Mo-Co3O4) nanotubes for peroxidase mimicking has been demonstrated via an electrospinning method combined with a calcination process. The prepared Mo-Co3O4 nanotubes exhibited a higher peroxidase-like catalytic activity than pure Co3O4 nanotubes, and the highest catalytic activity is achieved when the molar fraction of Mo is 2.0%. The catalytic kinetic of Mo-Co3O4 nanotubes follows a typical Michaelis-Menten mechanism, 1 ACS Paragon Plus Environment

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representing a strong affinity to H2O2 and 3,3’,5,5’-tetramethylbenzidine (TMB) substrates. Owing to the high catalytic efficiency of the Mo-Co3O4 nanotubes, we have developed a facile and efficient strategy for the sensitive colorimetric determination of L-cysteine with a low detection limit of 24.2 nM. This detection limit is superior to many previous reported enzymelike detection systems. Furthermore, a favorable selectivity towards the determination of Lcysteine based on this approach is also achieved, showing bright prospects in environmental monitoring and biomedical analysis.

1. INTRODUCTION Over the past years, nanozymes have gained tremendous attention because of their favorable catalytic activity, excellent environmental stability, easy synthetic protocol, high efficiency for recyclability, and cheapness compared with typical natural enzymes, showing promising applications in many fields including sensing, waste water treatment, medical diagnosis, and food safety.1-4 With great potential to be efficient enzyme mimics, a variety of functional nanomaterials have been extensively studied including noble metals and their alloyed nanoparticles,5-8 transition metal oxides,9-14 chalcogenide nanomaterials,15-18 carbon nanomaterials,19-21 metal-organic framework (MOF),22 and functional polymers.23 However, the catalytic activity of these nanozymes are still relatively low, which restrict their practical applications. To promote the catalytic efficiency of nanozymes, the fabrication of hybrid nanomaterials is a meaningful object.24-36 For instance, Tao et al. has demonstrated that graphene oxide (GO)-gold nanoclusters (Au NCs) hybrid exhibited a surprisingly higher peroxidase-like activity than individual GO nanosheets and Au NCs alone over a wide pH range, which could be ascribed to their synergistic effect.24 Many other hybrid nanomaterial systems have also 2 ACS Paragon Plus Environment

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been reported to show the similar synergistic enzyme-like catalytic effect, such as Ag@carbon dots,25 Fe@Co3O4 hollow nanocages,26 FeMnO3@polypyrrole nanotubes,27 Fe3O4/nitrogen-doped carbon hybrid nanofibers,28 reduced graphene oxide/CuS hybrid nanosheets,29

Ag/MOF

hybrid,30 FePt/graphene

oxide

nanocomposites31,

CeO2-

montmorillonite nanocomposites32, and Au-loaded nanoporous Fe2O3 nanocubes33 etc. Very recently, it has been reported that doping with suitable heteroatoms is another effective route for increasing the enzyme-like catalytic activity of nanomaterials.37-39 For example, Jampaiah et al. reported that Fe-doped CeO2 nanorods possess an improved peroxidaselike activity compared with pure CeO2 nanorods, which could be attributed to the additional reaction oxygen species by the doping elements.37 However, the enhancement multiplier is still relatively low, restricting their potential applications. Therefore, it is a massive challenge to prepare novel types of heteroatom-doped nanomaterials with a significant enhancement multiplier for enzyme mimicking. Cobalt oxide (Co3O4) is one of representative transition-metal oxides with a mixedvalence spinel structure, possessing unique chemical and physical properties as well as broad applications in sensors, chemical and electrochemical catalysis, energy storage and conversion devices.40-43 Recently, Co3O4 nanomaterials were reported to possess intrinsic enzyme-like properties, showing promising application in biosensing field.44-47 For example, Mu et al. reported that Co3O4 nanoparticles possess both peroxidase-like and catalase-like

properties,

which

can

be

applied

for

the

colorimetric

glucose

determination.44 However, it is still a big challenge to promote the enzyme-like activity of Co3O4 nanomaterials for practical applications. To achieve this object, the researchers have prepared many kinds of Co3O4-based hybrid nanomaterials with a synergistic

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effect.48-54 For instance, core-shell structured Co3O4@NiO nanotubes have been reported to possess efficient peroxidase-like property, which showed a much higher catalytic activity than pristine Co3O4 nanofibers and NiO nanotubes alone.48 Co3O4/crumpled graphene (CG) microspheres have also been demonstrated to exhibit a better peroxidaselike efficiency than Co3O4 nanoparticles and CG flowers.49 However, there is no report to promote the enzyme-like property of Co3O4 nanomaterials by a simple doping approach with suitable heteroatoms. In this study, an expedient electrospinning and calcination process were applied to fabricate Mo6+ doped Co3O4 (Mo-Co3O4) nanotubes for peroxidase mimicking. Compared with the solid nanoparticles, the hollow structure of Co3O4 nanotubes merit large surface area, low density, and more active catalytic sites, which are beneficial to improve their peroxidase-like activity. In addition, the doping with transition metal ions contributes to the enhanced peroxidase-like efficiency. The prepared Mo-Co3O4 nanotubes possess a higher catalytic activity than pure Co3O4 nanotubes. The enhancement multiplier of the Mo-Co3O4 nanotubes compared with pristine Co3O4 nanotubes is definitely higher than previous reported systems of heteroatom-doped nanomaterials. According to these findings, a facile and efficient route for the colorimetric L-cysteine determination with a low detection limit and excellent selectivity has been developed.

2. EXPERIMENTAL SECTION 2.1 Chemicals Poly(vinyl pyrrolidone) (PVP) with a Mw of 1 300 000 g mol-1, L-cysteine and D-cysteine were got from Sigma-Aldrich. Molybdenylacetylacetonate (C10H14MoO6) was bought from


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Aladdin. Sodium acetate anhydrous (NaAc) and cobalt nitrate hexahydrate (Co(NO3)2•6H2O) were provided by Xilong Chemical Industry Co., Ltd. Hydrogen peroxide (H2O2), acetic acid, and the solvents including dimethylsulfoxide (DMSO), ethanol, and N,N-dimethylformamide (DMF) were supplied by Beijing Chemical Works. 3,3’,5,5’-Tetramethylbenzidine (TMB) was purchased from Sinopharm Chemical Reagent Co., Ltd. All of these chemicals were directly used in this study. 2.2 Preparation of Mo-Co3O4 nanotubes via an electrospinning method combined with a calcination process In a representative procedure, 0.63 g of PVP, 0.383 g of Co(NO3)2•6H2O and different molar fractions of molybdenyl acetylacetonate (0, 1.0, 2.0, 5.0 and 10.0%) were put into a conical flask, which contains 3.5 mL of ethanol and 3.5 mL of DMF. Then the mixture was vigorously stirred until a homogeneous and pink solution was formed. The molar fraction of molybdenyl acetylacetonate, or rather the doping ratio of Mo was calculated in accordance with the following equation: []

Mo % = [][] Next, the precursors were transferred into a glass capillary connecting to a high-voltage power supply with a setting voltage of 18 kV for electrospinning. An aluminum foil with a distance to the spinneret of around 20 cm was used as a collector to collect PVP/Co(NO3)2/molybdenyl acetylacetonate nanofibers. After the electrospinning, the flexible fibrous membrane was separated from the collector and transferred to a muffle furnance. The calcining course was conducted at 150 °C in air for 1 h with a heating speed at 1 °C min-1 to volatilize the water and solvent, followed by a calcination course at 550 °C in air for another 2 h with a heating speed at 2 °C min-1. Finally, the Mo-Co3O4 nanotubes with varied doping ratios of Mo6+ were prepared. 5 ACS Paragon Plus Environment


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2.3 Peroxidase-like property of the Mo-Co3O4 nanotubes In a representative experiment, 20 µL of Mo-Co3O4 nanotubes suspension (3 mg mL-1) as catalyst was added into 3 mL acetate buffer solution with a pH value of 4.0 consisting of 20 µL of TMB solution (15 mM in DMSO) and 20 µL of H2O2 (30%). The oxidation of TMB after 10min reaction in the spectrophotometer cell at room temperature was monitored by ultravioletvisible (UV-vis) measurement via determining its absorbance at 651 nm in a spectrum mode. To probe the influence of the environmental parameters on the catalytic property, the influences of pH values of the acetate buffer solution and the temperature were carried out. For the detection of H2O2, all of the experimental parameters were the same as those in above-mentioned reaction except that H2O2 concentration is ranging from 0 to 65 mM. With respect to the steady-state kinetic analysis, to get the kinetic parameters of H2O2, after the addition of catalyst suspension and TMB solution with the same concentrations as the abovementioned system for the peroxidase-like catalytic activity of Mo-Co3O4 nanotubes into acetate buffer solution (3 mL) with a pH value of 4.0, H2O2 with varied concentrations were added. Similarly, the H2O2 concentration was fixed at 10 mM as well as the concentrations of TMB were adjusted to explore the kinetic parameters of TMB. UV-vis measurement in time course was applied to monitor the catalytic courses. 2.4 Detection of L-cysteine L-cysteine with different concentrations were added into acetate buffer solution (3.0 mL) with a pH value of 4.0 consisting of Mo-Co3O4 nanotubes suspension, H2O2 and TMB solution with the same concentrations as the above-mentioned system for the peroxidase-like catalytic activity of Mo-Co3O4 nanotubes. The final concentration of L-cysteine was calculated to range from 0-200 µM. The absorptions of the solutions were monitored at 651 nm after 10 min.


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As for the study of the selectivity toward L-cysteine, the concentrations of L-cysteine or other amino acids as interferences were fixed at 200 µM. 2.5 Characterization The morphologies of the as-prepared samples were characterized via a field-emission scanning electron microscopy (FESEM, FEI Nova NanoSEM) manipulated at 15 kV and a transmission electron microscopy (TEM, JEOL JEM-1200 EX) manipulated at 100 kV. A FEI Tecnai G2 F20 high-resolution transmission electron microscope was applied to provide typical high-resolution TEM (HRTEM) image and energy dispersive X-ray (EDX) analysis. The crystallographic structure of the samples was investigated via a XRD (PAN-alytical B.V. Empyrean) with CuKα radiation. XPS (Thermo Scientific ESCALAB250) measurement was applied to characterize the chemical composition of the Mo-Co3O4 nanotubes. Horiba LabRAM HR Evolution apparatus with a 633 nm laser as the excitation source was used to conduct Raman characterization. UV-vis (Shimadzu UV-2501 PC spectrometer) measurement was applied to investigate the peroxidase-like properties. Agilent725 instrument was utilized to perform inductively coupled plasma atomic emission spectrometry (ICP-AES). A Quantachrome Instrument (Autosorb iQ Station 1) was employed to have access to multipoint nitrogen adsorption–desorption isotherms. Photoluminescence (PL) spectra were collected on F97Pro fluorospectro photometer (Leng guang, Shanghai).

3. RESULTS AND DISCUSSION Electrospinning technique is an efficient and universal approach to prepare functional nanomaterials with one-dimensional structures.55,56 In this study, Mo-Co3O4 nanotubes are synthesized through a traditional electrospinning combined with a calcination process.



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And tuning the doping ratios of Mo species could realize a high peroxidase-like activity of Co3O4 nanotubes. The morphologies of the resultant Mo-Co3O4 nanotubes were characterized by typcial SEM and TEM measurements. It is found in Fig. 1a-h that the Mo-Co3O4 nanotubes with doping ratios of Mo6+ less than 5.0% show uniform 1D hollow structure with diameters in the range of 120-220 nm. The formation mechanism of the hollow structure of the MoCo3O4 nanotubes through an electrospinning route with a single spinning nozzle is probable attributed to the incidental phase separation between polymer carrier and inorganic metallic compound accompanied with the evaporation of solvent during the electrospinning course. After the calcination process, the polymer core was removed, resulting in a hollow structure of Mo-Co3O4 nanotubes. This mechanism for the formation of Mo-Co3O4 nanotubes is consistent with the previous reports.57,58 The hollow structure can provide large specific surface area and plenty of active sites for catalytic reactions. It needs to be emphasized that the morphologies of nanotubes were not changed much by the dopants when the doping ratios of Mo6+ are less than 5.0%. However, the TEM images clearly show that the Mo doping reduces the thickness of the Co3O4 nanotubes. This result indicates that the Mo species shows an inhibiting effect on the growth of Co3O4 to form thin walled nanotubes. In addition, when the doping ratio of Mo6+ is increased to 10.0%, most of the products are dominated with nanobelt-like structure (Fig. 1i and j). The formation of the nanobelt-like structure of Mo-Co3O4 nanotubes with Mo doping ratio of 10.0% may be due to the a higher surface tension of the electrospinning precursors after the addition of Mo salt into the PVP solution because the coordination of Mo ions is higher than Co ions, which is similar with previous reports.59-61


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Fig. 1 SEM and TEM images of Mo-Co3O4 nanotubes with varied doping ratios of Mo6+ at (a, b) 0%, (c, d) 1.0%, (e, f) 2.0%, (g, h) 5.0% and (i, j) 10.0%.



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Fig. 2 (a) TEM image of a single Mo-Co3O4 nanotube whose doping ratio of Mo6+ is 2.0%; (b) A HRTEM image of the resultant Mo-Co3O4 nanotube; (c) SAED analysis and (d) EDX spectrum of the resultant Mo-Co3O4 nanotubes; (e) HAADF-STEM image and the EDX mapping of (f) Co, (g) Mo and (h) O elements in the Mo-Co3O4 nanotubes product.

Afterwards, HRTEM measurement was applied to characterize the detailed crystal structure and chemical composition of the Mo-Co3O4 nanotubes. A typical TEM image of a single Mo-Co3O4 nanotube with a Mo6+ doping ratio of 2.0% is shown in Fig. 2a. It is clearly depicted that the shell thickness of the well nanotube-like structure is around 12 nm. In Fig. 2b, a typical HRTEM of the Mo-Co3O4 nanotube shows clear crystal lattices, 10 ACS Paragon Plus Environment

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indicating the high crystallinity of the sample. Furthermore, two typical d-spacing values associated with the (222) and (220) planes of Co3O4 (JCPDS card no. 43-1003) can be obviously picked at approximately 0.20 and 0.29 nm, respectively. The (220), (311) and (511) diffraction disks of Co3O4 are also shown in the selected area electron diffraction (SAED) pattern, suggesting that the crystal structure of Co3O4 is not altered by the doping Mo6+ and exists in pure phase. The presence of elemental C, O, Cu, Co, Si and Mo is confirmed by the high-angle annular dark-field scanning TEM (HAADF-STEM) image and EDX spectrum of the product with a doping ratio of 2.0%. Among them, the signals of C, Cu and Si are ascribed to amorphous carbon-coated copper grids and the instrument substrate (Fig. 2d). The elemental mapping also demenstrates the presence of Co, Mo and O elements in Fig. 2e-h, further confirming the uniform distributions of Mo species in the nanotubes. The molar percentage of Mo in the sample has been further determined via ICP-AES measurement, displaying a value of 1.96%, which is accordance with the theoretical calculating ratio (2.0%).

Fig. 3 (a) XRD patterns of the resultant Mo-Co3O4 nanotubes with varied doping ratios; (b) The corresponding amplifying XRD patterns from 35.5 to 39 degrees.



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XRD measurement was further used to study the crystal structure of the Mo-Co3O4 nanotubes with varied doping ratios. From Fig. 3a, all of the samples except the one with the doping ratio of 10.0% show similar XRD diffraction peaks, which are fairly indexed to the spinel Co3O4 (JCPDS card no. 43-1003), indicating that moderate dopant couldn’t influence the crystal structure of Co3O4. However, with the increment of the content of Mo6+, the sample with a doping ratio of 10.0% exhibits another two diffraction peaks at about 23.3° and 26.5°, corresponding to the (021) and (002) planes of CoMoO4 (JCPDS 21-0868), indicating that Mo species can not completely be doped in the crystal structure of Co3O4 at a higher doping ratio. Fig. 3b shows an amplifying XRD patterns of Fig. 3a, it is obviously observed that one of the typical diffraction peaks of Co3O4 at around 37° shifts to lower degrees as the doping ratio increases from 0% to 5.0% and the shift is calculated to be 0.14°, which should be due to the difference between ionic radius of Co and doping Mo, leading to the lattice deformation of the product after the incorporation of Mo species. This result is highly consistent with the literatures.62,63 Moreover, the peak deviation of the sample with doping ratios of 10.0% is calculated to be 0.08°, which is less than that of the one with a doping ratio of 5.0%, consistent with the result that CoMoO4 has been formed with a part of Mo coexisting with Co3O4.

Fig. 4 (a) Raman spectra of the obtained Mo-Co3O4 nanotubes with varied doping ratios; (b) The corresponding amplifying Raman spectra from 630 to 720 cm-1. 12 ACS Paragon Plus Environment

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To further confirm the successful doping of Mo element, Raman spectrum analysis was applied. As shown in Fig. 4, the typical peaks of all the Mo-Co3O4 nanotubes with different doping ratios are similar and the peaks at 190, 478, 518, 617 and 687 cm-1 can be well indexed to F2g1, Eg, F2g2, F2g3 and Ag1 Raman active modes of Co3O4 nanocrystals, respectively, corresponding to the lattice vibrations of Co2+ and Co3+, proving the synthesis of Co3O4.64-66 Besides, the amplifying Raman spectra ranging from 630 to 720 cm-1 clearly exhibit that the peaks of the doped samples shift towards lower wavenumbers. It is well-known that positions of the peaks in Raman spectrum are relevant to the stress in crystalloid, to be more specific, pressure stress causes the peaks move to higher wavenumber, while tensile stress brings about a lower shift.67 Consequently, the peak shifts here suggests the tensile stress in Mo-Co3O4 nanotubes, which is generated by lattice distortion resulting from the doping of Mo species into the Co3O4 lattice with different radius from that of Co ions. In addition, the peak deviation of the sample with varied doping ratios is consistent with XRD patterns. Further information on the valence state of elements in Mo-Co3O4 nanotubes with doping ratio of 2.0% was attained from XPS measurements. Fig. 5a is the wide-scan survey spectrum, showing the existence of Mo, Co, C, and O elements. As shown in Fig. 5b, there are two predominant peaks with the binding energy of 780.1 and 796.1 eV, which are indexed to Co 2p3/2 and Co 2p1/2, respectively. In detail, according to Gaussian fitting, the predominant peaks can be deconvolved to four subpeaks with binding energy of 780.3 and 795.1 eV (Co3+) and 781.9 and 797.0 eV (Co2+), indicating the synthesis of Co3O4. Besides, two satellite peaks are presented.47,68 Fig. 5c exhibits the high resolution XPS spectrum of Mo 3d, the two effortlessly seen peaks with the binding energy of 231.9 and 234.9 eV could be indexed to Mo 3d5/2 and Mo 3d3/2, respectively, corresponding to the Mo6+ from molybdenyl acetylacetonate.69 As for the Fig.



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5d, two fitted peaks at 530.2 and 531.8 eV could be associated with lattice oxygen and oxygen species in C-OH from residual C after calcination.68 These results verify the successful synthesis of a series of Mo-Co3O4 nanotubes with varied doping ratios. Mo6+ substituted for Co3+ or Co2+ in the Co3O4 lattice or existed as amorphous species without changing the crystal structure when the doping ratios were less than 5.0%. On the contrary, CoMoO4 was generated by part of Mo and coexisted with Co3O4 in the sample with molar fraction of Mo of around 10.0%. To prove this statement, the XPS spectrum of the sample with a doping ratio of 10.0% has been further carried out. As depicted in Fig. S1, it is clearly seen that the relative intensity of the subpeaks indexed to Co2+ is higher than that of the subpeaks associated with Co3+, which is caused by the Co2+ in the CoMoO4. From Fig. S1c, both of the Mo peaks move to a higher binding energy compared with the sample with a doping ratio of 2.0%, indicating a stronger force around the Mo species due to the formation of CoMoO4. In addition, the multipoint nitrogen adsorption-desorption isotherms of Mo-Co3O4 nanotubes with Mo doping ratio of 2.0% has been studied, which is shown in Fig. S2. The Brunauer-Emmett-Teller (BET) specific surface area of the sample is reckoned to be 158.89 m2/g, which is higher than most of the solid electrospun metal oxide nanofibers.70-73 Therefore, the high specific surface area should be due to the hollow structure of the nanotubes. In addition, the most probable Barrett-Joyner-Halenda (BJH) desorption pore diameter of the sample is got to be 3.41 nm from the uneven and broadpore-size distributions (Fig. S2b). These considerable specific surface area and preponderant mesoporous structure bring a larger contact area between solution and the catalyst, which endows more active sites to promote the catalytic property.


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Fig. 5 XPS spectrum of Mo-Co3O4 nanotubes with Mo doping ratio of 2.0%: (a) full survey spectrum, (b) Co 2p, (c) Mo 3d, and (d) O 1s regions.

It is reported that Co3O4 nanomaterials such as nanoparticles, nanobelts and nanotubes with spinel structures possess peroxidase-like properties.46,74,75 However, the catalytic activities are not considerable enough. Herein, heteroatoms of Mo6+ are applied to dope with Co3O4 nanotubes to promote the peroxidase-like catalytic activity by increasing lattice defects and reducing the energy barrier for oxygen migration.38 The peroxidase-like properties of the hollow Mo-Co3O4 nanotubes with varied doping ratios are studied by monitoring the reaction process toward the oxidation of TMB with the addition of H2O2 via a UV-vis spectrometer. It is depicted in Fig. 6a that the peroxidase substrate of TMB can only be oxidized to generate a color change with the 15 ACS Paragon Plus Environment


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existences of H2O2 and hollow Mo-Co3O4 nanotubes. On the contrary, no obvious peak is picked at 651 nm in the absence of substrates or catalyst, demonstrating that the prepared Mo-Co3O4 nanotubes are advisable peroxidase mimics. Then, controlled experiments were conducted to evaluate the catalytic activities of Mo-Co3O4 nanotubes with different contents of Mo dopant (Fig. 6b). The time-dependent curve of Mo-Co3O4 nanotubes with doping ratio of 0%, or rather, the pure hollow Co3O4 nanotubes, shows nearly no absorbance at 651 nm after 10 min, indicating that the pure Co3O4 nanotubes possess a poor peroxidase-like property. With the increment of doping ratio, the catalytic activities of samples increase and reach a maximum when the molar fraction of Mo6+ is 2.0%, which is due to the fact that moderate doping can lead to more lattice defects in Co3O4, resulting in a reduced Fermi level and more active sites in catalytic process. These active sites are beneficial for the improving electron transport efficiency, making it easier for Mo-Co3O4 nanotubes to consume H2O2 to generate reactive oxygen species (ROS). Ultimately, the peroxidase-like properties are achieved. To more visually represent the influence of doping on the catalytic activities of the specimens, doping ratiodependent curve in Fig 6c is drawn based on Fig. 6b. It is obvious that the peroxidase-like property of Mo-Co3O4 nanotubes with doping ratio of 2.0% is nearly 50-fold enhancement than that of the pure hollow Co3O4 nanotubes. Hence, the hollow Mo-Co3O4 nanotubes with a doping ratio of 2.0% are chosen as the best catalyst in the subsequent experiments. Additionally, we have also compared the peroxidase-like catalytic activity of the prepared Mo-Co3O4 with a physical mixture of Co3O4 nanotubes and MoO3 materials that are prepared with the same way. As shown in Fig. S3, the physical mixture of Co3O4+MoO3 shows very few peroxidase-like activity, which is much lower than that of Mo doped Co3O4


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nanotubes, indicating that doping with Mo6+ is an efficient route to enhance the catalytic activity of Co3O4 nanotubes. Furthermore, as shown in Table S1, compared with other peroxidase and oxidase mimics from the heteroatoms-doped materials, the enhancement multiplier of catalytic property in this study is tremendous. The reproducibility was also evaluated by three consecutive experiments with samples synthesized by three different times, and the relative standard deviation (RSD) was estimated to be 2.58%, which is acceptable compared with other peroxidase mimics.76 As known to all, both natural and artificial enzymes are influenced by pH conditions. Considering this fact, the influence of pH values on the peroxidase-like activities of the as-prepared samples was studied. As show in Fig. 6d, the highest peroxidase-like activity is got at a pH value of 4.0. Hence, the pH = 4.0 is supposed to be the optimum condition in the subsequent experiments. The influence of the temperature on the catalytic activity of Mo-Co3O4 nanotubes has also been studied (Fig. S4). It was clearly observed that the Mo-Co3O4 nanotubes exhibit a temperature-dependent peroxidase-like activity. The highest peroxidase-like activity was achieved at a temperature of around 30 °C.



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Fig. 6 (a) UV-vis absorption spectra of different systems in acetate buffer solution (pH=4.0) with 10-min reaction time and their corresponding optical photographs; (b) Time-dependent absorption spectra of the systems with the addition of different types of catalysts (Mo-Co3O4 nanotubes with different doping ratios at 0%, 1.0%, 2.0%, 5.0% and 10.0%) at 651 nm; (c) Dependence of the catalytic activity of Mo-Co3O4 nanotubes on the molar fraction of Mo6+; (d) Dependence of the catalytic activity of Mo-Co3O4 nanotubes on the pH values.

It is well known that the peroxidase-like performance of the nanomaterials is attributed to the generation of the ROS via the decomposition of H2O2 by the catalyst.77 In general, the hydroxide (•OH) and superoxide (O2•−) radicals are typical ROS during the nanomaterials-based peroxidase-like reaction. To investigate which is the main radical involved in the peroxidase-like catalytic reaction for the Mo-Co3O4 nanotubes, two specific fluorescence


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probes including p-phthalic acid (PTA) for the identification of •OH and dihydroethidium (DHE) for the identification of O2•− were chosen. As shown in Fig. S5, there is almost no fluorescence emission peak in the system of Mo-Co3O4 nanotubes-H2O2-PTA, while an obvious fluorescence emission peak appeared in the Mo-Co3O4 nanotubes-H2O2-DHE system. This result demonstrates that superoxide radical contributed significantly to the excellent peroxidase-like efficiency of Mo-Co3O4 nanotubes. During the peroxidase-like reaction, the oxidation of TMB with the assistance of H2O2 is due to the transfer of the electrons from the nonbonding orbital (NBO) of TMB substrate to the lowest unoccupied molecular orbital (LUMO) of H2O2 substrate.78 In this study, the prepared pristine Co3O4 nanotubes almost could not catalyze H2O2 decomposition for the oxidation of TMB (Figure 6b and c) because the higher Fermi energy of pristine Co3O4 nanotubes hinders the electron transfer from the NBO of TMB. However, after the doping with Mo6+, the Fermi level is reduced, which has been proved by the Ultraviolet Photoelectron Spectroscopy (UPS). As shown in Fig. S6, the Fermi levels of Co3O4 nanotubes and Mo-Co3O4 nanotubes are calculated to be 5.0 and 4.5 eV, respectively. Then the lone pair electrons from the NBO of TMB can be effectively captured by the prepared Mo-Co3O4 nanotubes with reduced Fermi energy. Therefore, the original Fermi Level of Mo-Co3O4 nanotubes increases and is higher than the LUMO of H2O2, which promotes the transfer of electrons, achieving the successful formation of the oxidized TMB. This result demonstrates the mechanism of the superior catalytic property of Mo-Co3O4 nanotubes compared with the pristine Co3O4 nanotubes.



Page 20 of 36

Fig. 7 Steady-state kinetic assay of Mo-Co3O4 nanotubes. (a) The H2O2 concentration was fixed at 10 mM with varied concentrations of TMB; (b) The TMB concentration was fixed at 0.1 mM with varied concentrations of H2O2; (c) and (d) Corresponding Lineweaver–Burk reciprocal plots of (a) and (b), respectively. Other conditions were the same with the measurement of the peroxidase-like activity.

To further investigate the mechanism of the catalytic activity of Mo-Co3O4 nanotubes, the traditional Michaelis-Menten curves are shown in Fig. 7. From the Lineweaver-Burk plots, the apparent steady-state kinetic parameters can be acquired and the Michaelis-Menten formula is described below: 1

1 1 [] where v represents the initial velocity, Vmax stands for the maximal reaction velocity, [S]


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corresponds to the substrate concentration, and Km is the typical Michaelis–Menten constant. Through this formula, Km and Vmax versus TMB are calculated to be 0.0558 mM and 9.54 × 10−8 M s−1, while the Km and Vmax versus H2O2 are estimated to be 22.48 mM and 5.245 × 10−8 M s−1. There is no doubt that a lower Km value represents a stronger affinity between an enzyme and a substrate. As shown in Table 1, Km versus not only TMB but H2O2 of hollow Mo-Co3O4 nanotubes are lower than those of the Co3O4 nanoparticles (CGN),11 SiO2@Co3O4 core-shell nanorattles79 and Fe/CeO2 nanorods,36 demonstrating the better affinities of Co3O4 nanotubes to both TMB and H2O2. In addition, the Km versus TMB in this study is also lower than that of microperoxidase-11 (MP-11),80 indicating the efficient peroxidase-like property of Mo-Co3O4 nanotubes. The kinetic parameters of horseradish peroxidase (HRP) have also been added into the Table 1 in the revised manuscript. When compared with HRP, Mo-Co3O4 nanotubes show a lower Km versus TMB, reflecting to a higher affinity to the TMB substrate. However, the Km versus H2O2 substrate of the Mo-Co3O4 nanotubes is much higher than that of HRP. In other words, HRP endows a higher affinity to H2O2 due to the nature of a natural enzyme. As for the Vmax, the values of Mo-Co3O4 nanotubes and HRP are on the same order of magnitude, showing the similar catalytic performance. Table 1 Comparison of the kinetic parameters of peroxidase mimics. Catalyst

Substrate

Km [mM]

Vmax [10-8 Ms-1]

Ref.

Mo-Co3O4 nanotubes

TMB

0.0558

9.54

This work

Mo-Co3O4 nanotubes

H2O2

22.48

5.245

This work

CGN

TMB

0.12

33.2

[11]

CGN

H2O2

245

28.5

[11]

SiO2@Co3O4

TMB

0.087

0.012

[79]

SiO2@Co3O4

H2O2

25.2

0.015

[79]

Fe/CeO2 NRs

TMB

0.176

8.6

[36]

Fe/CeO2 NRs

H2O2

47.6

16.6

[36]



Page 22 of 36

MP-11

TMB

0.063

8.56

[80]

HRP

TMB

0.434

10

[1]

HRP

H2O2

3.7

8.71

[1]

On account of the considerable peroxidase-like property, and the concentrationdependent absorbance of the oxidation of TMB, the prepared Mo-Co3O4 nanotubes are applied to detect H2O2 via a simple colorimetric route. In Fig. 8a, the spectra changes of the systems, which are composed of Mo-Co3O4 nanotubes as catalysts, TMB as substrate and various concentration of H2O2, are monitored with the reaction time of 10 min. It is obviously found that the absorbance decreases along with the reduced concentration of H2O2. Fig. 8b is the dose-response curve to detect H2O2 using Mo-Co3O4 nanotubes as an artificial peroxidase based on Fig. 8a, indicating that H2O2 is well detected within the linear range of 0 to 20 µM (R2 = 0.998), and the limit of detection (LOD) is calculated to be about 0.53 µM (S/N = 3). This LOD is significantly superior to those of many other Co3O4 materials based enzyme-like systems, such as Co3O4 nanoparticles (10 µM),43 Co3O4-MMT nanoparticles (8.7 µM),54 CeO2/Co3O4/PEDOT nanofibers (1.67 µM)50 and Co3O4@NiO core-shell nanotubes (1.23 µM).47

Fig. 8 UV-vis absorption curves of the reactions system with varied concentrations of H2O2, 22 ACS Paragon Plus Environment

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other conditions were the same with the measurement of the peroxidase-like activity. (b) A doseresponse curve to detect H2O2 using Mo-Co3O4 nanotubes as peroxidase mimics. The inset shows a linear calibration plot.

As far as peroxidase-like catalytic reactions are concerned, most of them rely on the ROS which are generated from the decomposition of H2O2. During the course, L-cysteine can react with ROS, which is usually used as an inhibitor to prevent the substrates from being oxidized and color changed.81 According to the mechanism mentioned above, there is great potential of the system of Mo-Co3O4 nanotubes-H2O2-TMB to detect L-cysteine. L-cysteine, a typical semi-essential amino acid for human beings, plays an important role in various courses of cell functions.82 Hence, it is of great importance to sensitively detect L-cysteine.83 As depicted in Fig. 9a, ∆A, which equals to the difference between the absorbance of the solution without L-cysteine and that of the solution consists of different concentrations of L-cysteine, gets bigger and bigger as the concentration of L-cysteine continually increases, suggesting that the oxidation of TMB is inhibited. The corresponding optical photographs also demonstrate that the color of the solution fades as the increment of the concentration of L-cysteine. When the concentration of L-cysteine reaches to 200 µM, there is no color observed. Fig. 9b shows a linear part of Fig. 9a in the range from 0.05 to 10 µM, in which ∆A is linearly dependent on the concentration of Lcysteine. Then the LOD is estimated to be approximately 24.2 nM (S/N=3), which is superior to those of a large number of nanozymes-based colorimetric systems, such as Fe3O4 magnetic nanoparticles (0.97 µM),84 Gd(OH)3 nanorods (2.6 µM),81 hollow MnCo2O4 nanofibers (34.3 nM)85 and FeCo-carbon nanofibers (0.15 µM).86



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Fig. 9 (a) Dependence of the absorbance of the reaction system on the concentrations of L-cysteine after 10-min reaction and the relevant optical photographs; (b) The typcial linear calibration plot. ∆A: A(651 nm, absence)-A(651 nm, L-cysteine)

Furthermore, the selectivity of the system of Mo-Co3O4 nanotubes-H2O2-TMB to sensitively detect L-cysteine has also been conducted. The influence of several amino acids which often exist with L-cysteine in humans or other organisms, such as tryptophan, methionine, phenylalanine, valine, histidine and threonine, was examined. As shown in Fig. 10 and Fig. S7, it is apparent that the changes of both absorbance and the color change by the addition of other amino acids are negligible comparing with that of L-cysteine, proving an excellent selectivity of this reaction system to detect L-cysteine.


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Fig. 10 (a) The influence of the addition of L-cysteine (200 µM) or other amino acids (200 µM) on the abosorbance of the Mo-Co3O4 nanotubes-H2O2-TMB system at 651 nm after 10-min reaction, respectively. ∆A: A(651 nm, absence)-A(651 nm, amino acids); (b) The relevant optical photographs.

4. CONCLUSIONS In summary, Mo6+ doped Co3O4 nanotubes with varied doping ratios as efficient peroxidase mimics were successfully prepared via an electrospinning route along with a calcination course. A series of analysis results confirm that Mo6+ substituted for Co3+ or Co2+ in the Co3O4 lattice or existed as amorphous species without changing the crystal structure when the doping ratios were less than 5.0%. In addition, the as-prepared Mo6+ doped Co3O4 nanotubes with molar fraction of 2.0% endow a relatively best peroxidaselike property. Furthermore, the Mo6+ doped Co3O4 nanotubes show broad applications to 25 ACS Paragon Plus Environment


Page 26 of 36

sensitively detect H2O2 and L-cysteine with low LODs of 0.53 µM and 24.2 nM, respectively. This work offers a universal doping strategy to significantly enhance the peroxidase-like properties, which expand their applications in colorimetric sensing, disease diagnosis, and environmental monitoring.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: XPS spectrum of Mo-Co3O4 nanotubes with Mo doping ratio of 10.0%; Nitrogen adsorption-desorption isotherms and pore size distribution curves of Mo-Co3O4 nanotubes; Comparison of the peroxidase-like activity of Mo-Co3O4 nanotubes and Co3O4/MoO3 physical mixtures; Dependence of the peroxidase-like activity of Mo-Co3O4 nanotubes on the temperature; Specific detection of superoxide and hydroxide radicals via a fluorescence approach; UPS spectra of Co3O4 nanotubes and Mo-Co3O4 nanotubes with doping ratio of 2%; The selectivity study towards the detection of L-cysteine via UV-vis absorption spectroscopy; Comparison of the enhancement multiplier of catalytic property causing by doping in enzyme mimetics in Table S1 (PDF).

AUTHOR INFORMATION Corresponding authors: *(X.F.L.) Tel/Fax +86-431-85168292; email: [email protected]; *(C.W.) Tel/Fax +86-431-85168292; email: [email protected]. ORCID


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Xiaofeng Lu: 0000-0001-8900-9594 Ce Wang: 0000-0003-3204-5564 Notes Authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (21474043, 51773075, 51473065).

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Graphic for manuscript


Enhanced peroxidase-like activity of Mo6+ doped Co3O4 nanotubes

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