Article pubs.acs.org/IECR
Synthesis and Intrinsic Peroxidase-Like Activity of Sisal-Like Cobalt Oxide Architectures Qi Wang, Suwen Liu,* Haiyan Sun, and Qifang Lu School of Material Science and Engineering, Qilu University of Technology, Jinan, 250353, People’s Republic of China S Supporting Information *
ABSTRACT: Sisal-like architectures of cobalt hydroxide carbonate precursors were prepared using Co(NO3)2·6H2O and CO(NH2)2 as raw meterials via a hydrothermal process, and the precursor could be transformed to Co3O4 nanoparticles by calcination, while maintaining the original morphology. The as-prepared sample was characterized in detail by XRD, SEM, TGDSC, FT-IR, and N2 adsorption−desorption analysis and other techniques. The XRD, TEM, and SEM analysis indicate the microarchitecture was accumulated by Co3O4 nanorods with good crystallinity. The peroxidase-like activity of the sisal-like Co3O4 nanoparticles was also investigated, showing higher activity than that of Co3O4 powder, which was obtained by calcining cobalt nitrate.
1. INTRODUCTION Co3O4, which is an important magnetic p-type semiconductor, has a myriad of technologically important applications in lithium-ion batteries,1 supercapacitor,2 solar energy absorbers,3 magnetic materials,4 gas sensing,5 electrochromic devices,6 and catalysis.7 Nanostructured Co3O4 would be expected to show superior performance in its traditional arena and also lead to other unique properties in view of its specific morphology. When the first nanoparticle-based artificial enzyme was reported, Fe3O4 nanoparticles were found to possess an intrinsic peroxidase-like activity.8 Since then, some other nanomaterials have been evaluated as peroxidase mimetics.9 Mu et al.10 demonstrated that Co3O4 nanoparticles exhibit intrinsic peroxidase-like activity. These studies show that the nanomaterial-based peroxidase mimetics take the advantages of low cost, high stability, and tunability in catalytic activities and can be potentially used in bioassays and medical diagnostics. As is known, the nature of major enzyme is a protein, which acts as a biological catalyst to catalyze a particular chemical reaction. Nature enzymes, because of their high substrate specificities and high efficiency under mild conditions, have significant practical applications in medicine, chemical industry, food processing, and agriculture. However, they bear some intrinsic drawbacks such as low stability due to denaturation, sensitivity of catalytic activity to environmental conditions and difficulty in preparation. Therefore, the construction of efficient enzyme mimetics has been an increasingly important focus for the researchers. With the development of the enzyme mimetics, many studies have been focused on peroxidase mimetics, including heme,11 porphyrin,12 and cyclodextrin,13 which have been applied in bioanalysis and other fields. There has been an explosive growth in the use of nanomaterials as catalysts that involve both homogeneous and heterogeneous catalysis.14 However, the possibility of nanomaterials as artificial enzymes, which represent another important subdivision of catalysis, had long remained unknown. Until recently, the first nanoparticlebased artificial enzyme, Fe3O4 nanoparticles, were found to possess an intrinsic peroxidase-like activity.15 The nanomateri© 2014 American Chemical Society
al-based peroxidase mimetics take the advantages of low coat, high stability, and tunability in catalytic reaction and can be potentially used in bioassays and medical diagnostics.10 Subsequently, Co3O4 nanoparticles were also reported as a class peroxidase to catalyze the reaction. In the presence of hydrogen peroxide and a substrate of horseradish peroxidase, Co3O4 nanoparticles exhibited the similar catalytic peroxidase effect. Co3O4 with a normal spinel structure, which has Co3+ occupying the octahedral sites, has a high crystal field stabilization energy, and is very stable below 800 °C in the air, is an excellent catalyst material. In recent years, many methods including solid-state reaction,16 hydro/solvothermal methods,17 hydrothermal methods,18 and pulsed plasma in liquid solution 19 have been applied to prepare Co 3 O 4 nanostructures and to adjust its chemical and physical properties. It is generally believed that special morphologies and crystallographic forms are responsible for their properties especially adsorption, catalytic and optical properties, and thus, controlling the anisotropic inorganic materials at the mesoscopic level has attracted intensive interest presently and has become a studying focus in chemical synthetic fields.20−22 Synthesis of nanostructures from suitable precursors is an available and convenient method in the synthesis of nanomaterials, which can help control morphology of the nanostructures through treating the as-obtained precursor with desired morphologies.23 Moreover, cobalt basic carbonate is a good precursor for the preparation of nano-Co3O4 functional materials, which received widespread attention.24−26 Initially, people just limited their studies to alkali carbonate synthesis and thermal decomposition behavior, and in the alkali carbonate crystal structure, thermal stability and surface property also have made considerable progress. However, Received: Revised: Accepted: Published: 7917
October 21, 2013 April 20, 2014 April 21, 2014 April 21, 2014 dx.doi.org/10.1021/ie403554v | Ind. Eng. Chem. Res. 2014, 53, 7917−7922
Industrial & Engineering Chemistry Research
Article
with the development of science and technology and special demand growth of the related fields, people have turned their attention to control of the morphology and structure of this type of material. For example, Xu et al.27 prepared the controlled morphology of cobalt basic carbonate nanorods by changing the reaction conditions; Xing28 prepared the pineal cobalt basic carbonate nanorods and nanorod aggregates with urea as a precipitating agent. In this paper, sisal-like architectures of cobalt hydroxide carbonate precursors were first synthesized through a simple hydrothermal method with cobalt nitrate as cobalt source and urea as a precipitating agent. The sisal-like architectures of cobalt hydroxide carbonate precursors can be transformed to Co3O4 by calcination without destroying the structures. The novel nanostructure of the product exhibits better intrinsic peroxidase-like activity than the powder obtained by directly calcining cobalt nitrate.
2. EXPERIMENTAL SECTION 2.1. Synthesis. All reagents were of analytical grade and were used without further purification. In a typical synthesis, 0.44 g of Co(NO3)2·6H2O and 0.18 g of CO(NH2)2 were dissolved into deionized water under magnetic stirring to form a transparent solution. The solution then was transferred to a Teflon-lined stainless steel autoclave and heated to 160 °C and maintained at that temperature for 6 h. Subsequently, the autoclave was cooled to room temperature naturally. After that, the precipitate was collected by centrifugation, washed with deionized water and ethanol three times, and dried at 70 °C for 12 h in air to obtain the precursors. The final product was obtained by calcining the precursor to 400 °C for 1 h in the muffle furnace in air with a heating rate of 1 °C/min. 2.2. Characterization. The crystal structure of the product was determined on an X-ray diffractometer (XRD, Rigaku D/ Max 2200 PC) with a graphite monochromator and Cu Kα radiation (λ = 0.15418 nm). The morphology of the product was determined using field-emission scanning electron microscopy (FE-SEM) (Toshiba, Model S4800) and transmission electron microscopy (TEM) (JEM, Model 1011). Thermogravimetric analysis (TGA) was carried out on a SDT Model Q600 thermogravimetric analyzer at a heating rate of 10 °C/min under an air atmosphere. The ultraviolet−visible (UVvis) absorption spectra were tested using a Model Lambda 35 type UV-vis spectrometer (Perkin−Elmer). 2.3. Peroxidase-Like Activity Measurement. The measurement was performed at 30 °C. First, 34 μg of catalyst was dispersed in 3 mL of acetic acid-sodium acetate buffer solution, and sonicated for 5 min. Then, 150 μL 3,3′,5,5′tetramethylbenzidine (TMB) solution (10 mg/mL, with N,Ndimethylformamide as the solvent) and 160 μL of 30% H2O2 was added as the substrates. After reacting for a certain time, the absorbance of TMB-derived oxidation product was examined at 652 nm on UV-vis spectrometer (Perkin−Elmer, Model Lambda 35). The catalytic activity was evaluated by absorbance data. Michaelis constants (Km) were determined by varying the concentrations of TMB and H2O2 under the optimal conditions.
Figure 1. SEM image and XRD pattern of precursor (a and b, respectively) and SEM image and XRD pattern of typical Co3O4 products (c and d, respectively). [The scale bars in panels (a) and (c) are 10 μm.]
Figure 2. TEM image of scattered Co3O4 nanorods. The scale bars in the inserted image is 100 nm.
precursor exhibits sisal-like morphology with the size of ca. 10 μm, and the microstructures are accumulated by nanorods. The diameter of the nanorod is ca. 125 nm from TEM image (Figure 2). From the corresponding XRD patterns of the precursor (Figure 1b), the product shows low crystallinity, which might be reasoned that the low synthesizing temperature cannot make the inner structure of the product in a long-range order. The peaks can be indexed to the orthorhombic cobalt basic carbonate phase and the peak at 35.6°, 39.5°, 41.5°, 46.3°, 55.6°, 63.8°, and 70.9° can be assigned to (300), (221), (040), (231), (340), (060), and (412) facets of orthorhombic Co(OH)x(CO3)0.5·0.11H2O (Joint Committee on Powder Diffraction Standards (JCPDS) File Card No. 48-0083).The SEM image of the pyrolysis products from the cobalt hydroxide carbonate is shown in Figure 1c, from which the products basically maintain the morphology of the precursor and the diameter and the thickness of the petals do not apparently change. XRD patterns of the pyrolysis products (Figure 1d) indicate the identical phase. No other peaks for impurities were detected and all the reflection peaks can be indexed to pure cubic phase of spinel Co3O4 (JCPDS File Card No. 42-1467).29 TEM was also used to characterize the product. From Figure 2, the nanorods are composed with nanoparticle with size of 35.7 nm. The corresponding enlarged image shows that the
3. RESULTS AND DISCUSSION 3.1. Material Characterization. Sisal-like cobalt hydroxide carbonate precursors were synthesized as described and analyzed with SEM and XRD. As shown in Figure 1a, the 7918
dx.doi.org/10.1021/ie403554v | Ind. Eng. Chem. Res. 2014, 53, 7917−7922
Industrial & Engineering Chemistry Research
Article
Figure 3. (a) TG-DSC curves of the precursor and (b) IR spectra of the precursor and the product after calcination.
Table 1. Km Values of Sisal-Like Co3O4 Nanoparticles, HRP,15 and the Co3O4 Nanoparticles Reported in the Literature10 sample
Km(H2O2)/mM
Km(TMB)/mM
sisal-like Co3O4 HRP Co3O4 nanoparticle
0.8268 3.7 140.07
0.015 13 0.434 0.037
3a, there are two weight losses. The first drop (
