A facile synthetic approach for SiO₂@Co₃O₄ core–shell nanorattles with enhanced peroxidase-like activity

Abstract

SiO₂@Co₃O₄ core–shell nanorattles with different Co₃O₄ shell thicknesses have been successfully synthesized by the calcination of SiO₂@α-Co(OH)₂ at 500 °C. The synthetic approach is facile, economical, and requires no surface modification. The synthesized materials were thoroughly characterized using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) analysis, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and diffuse reflectance spectroscopy (DRS). SEM analysis indicates a hierarchical core–shell morphology for SiO₂@Co₃O₄ and the TEM results indicate the core–shell nanorattle morphology. Diffuse reflectance spectroscopy studies indicate that the SiO₂@Co₃O₄ core–shell nanorattles show two absorption bands in the range 420–450 nm and 700–750 nm related to ligand to metal charge transfer transitions (O²⁻ → Co²⁺ and O²⁻ → Co³⁺). The SiO₂@Co₃O₄ core–shell nanorattles act as an artificial peroxidase enzyme mimic with enhanced intrinsic peroxidase-like activity compared to pure Co₃O₄ nanoparticles and horseradish peroxidase (HRP), a natural enzyme. The SiO₂@Co₃O₄ core–shell nanorattles show higher k_cat and k_cat/K_m values compared to pure Co₃O₄ and HRP indicating their applicability as an artificial enzyme mimic in biomedicine and bio-sensing.

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1. Introduction

In recent times, scientists are paying increasing attention towards the use of metal oxide nanoparticles in various biological applications due to their versatility, sensitivity, stability, etc.^1–6 Enzymes are typically proteins that catalyze bio-chemical reactions in the cellular metabolic processes and they enhance the rate of the reactions.^7,8 Horseradish peroxidase (HRP) is a natural peroxidase enzyme which is extracted from plants and it catalyzes decomposition of peroxides and oxidation of various substrates.⁹ HRP is expensive, difficult to store, and it easily becomes inactive. Scientists have an immense interest in developing artificial enzymes (mimics) due to the inherent drawbacks of natural enzymes; they have a tendency to undergo denaturation under environmental conditions with low stability, and low redox activity.^7,10 Nanomaterials based enzyme mimics have the advantage of higher stability under harsh conditions, low cost, tunable catalytic activity and they can be potentially useful in different fields of biology.

Silica is chemically inert, stable at elevated temperatures and optically transparent. It is useful in many areas such as glass industries, ceramics, semiconductors, photovoltaic cells and it is also used as catalytic support.^11–13 Co₃O₄ is an important p-type antiferromagnetic semiconductor with a normal spinel structure.¹⁴ It is used in various applications such as anode materials in lithium ion rechargeable batteries,¹⁵ sensors,¹⁶ electrochemical devices,¹⁷ solar energy absorbers,¹⁸ supercapacitors,¹⁹ water oxidation,²⁰ and heterogeneous catalysis.²¹

In recent years, research is focused on core–shell nanoparticles with yolk–shell or nanorattle morphology. Nanorattles refer to hollow shells with solid cores with interstitial hollow space between the core and the shell. They have a large number of applications in bio-sensing, energy storage, catalysis and biomedicine.^22–25 Nanorattles exhibit interesting characteristics which are useful in multi-functional areas. One of the most explored applications of these materials is their use as nanocatalysts. Up to now, several methods have been reported for the synthesis of nanorattles and they include hydrothermal synthesis, selective etching, template-route, annealing, and Ostwald ripening.^23–28 Tan et al. have reported the synthesis of Au–Pd@SiO₂ nanorattles useful in Suzuki cross-coupling reaction.²³ Yang et al. have reported the preparation of Fe₃O₄@air@poly(vinylsilane) nanorattles and they have used them as contrast agents.²⁴ Zhang et al. have reported the preparation of Au@polyanline yolk–shell nanorattles and have used them as flexible memory device.²⁵ Xu et al. have reported the synthesis of Fe₃O₄@CuSiO₃ nanorattles and have investigated their microwave absorption properties.²⁹ In some cases, Co₃O₄ has been used as the core material and SiO₂ has been used as the shell.^22,30–32 Zong et al. have reported the confined growth of metal oxide nanocrystals on mesoporous silica spheres which act as the core material.³³

Very less attention has been paid towards the use of metal oxide nanoparticles in biological applications. For example, Mu et al. have reported the peroxidase-like activity for Co₃O₄ nanoparticles and, recently, they have also studied the effect of crystal plane of Co₃O₄ on the peroxidase-like activity.^10,34 Gao et al. have reported that Fe₃O₄ nanoparticles possess intrinsic peroxidase-like activity.³⁵ In addition to Fe₃O₄ and Co₃O₄ nanoparticles, other systems such as graphene supported Au–Pd bimetallic core–shell nanoparticles,³⁶ Au nanoparticles on citrate-functionalized graphene sheets,³⁷ Co–Al layered double hydroxides,³⁸ porous platinum nanotubes,³⁹ CuS–graphene nanosheets,⁴⁰ and Fe₃O₄ nanoparticles loaded on graphene oxide-dispersed carbon nanotubes⁴¹ have been explored for the intrinsic peroxidase-like activity. Peroxidase-like activity has also been demonstrated in Co₃O₄ nanoparticles coupled with carbon nitride nanotubes and reduced graphene oxide.^42,43 In the present study, SiO₂@Co₃O₄ core–shell nanorattles with uniform Co₃O₄ porous shell over the silica core have been successfully synthesized using a self-template route by the calcination of SiO₂@α-Co(OH)₂ at 500 °C. The synthesized materials were thoroughly characterized before examining the intrinsic peroxidase-like activity using 3,3′,5,5′-tetramethyl benzidine (TMB) as the substrate.

2. Experimental

2.1 Reagents

Tetraethyl orthosilicate (98%, ACROS®), ethanol (99.9%, AR), ammonia solution (25%, Rankem, AR), cobaltous nitrate hexahydrate (98%, Rankem, LR), urea (99.5%, Rankem, AR) and hydrogen peroxide (30%, Rankem, AR) were used as received without further purification. The peroxidase substrate, 3,3′,5,5′-tetramethyl benzidine (TMB), was received from Spectrochem chemicals, India.

2.2 Synthesis

The SiO₂@Co₃O₄ core–shell nanorattles were synthesized in two steps and the procedure is as follows.

2.2.1 Synthesis of silica spheres. Silica microspheres were synthesized according to a previous report.⁴⁴ In brief, 88 mL of ethanol was mixed with 12 mL of ammonia solution and 3.7 mL of tetraethyl orthosilicate was added drop wise under vigorous stirring. The contents were continuously stirred at room temperature for 24 h. After completion of the reaction, the contents were centrifuged, washed with ethanol several times and finally dried in an oven for overnight at 80 °C.

2.2.2 Synthesis of SiO₂@Co₃O₄ core–shell nanorattles. About 100 mg each of silica spheres was dispersed in 100 mL of aqueous cobaltous nitrate solution with different concentrations (5 mM, 10 mM and 15 mM) in a 250 mL beaker and sonicated for 3 min. After uniform dispersion of the silica spheres, 5 g of urea was added. The contents were heated at 85 °C and stirred for 6 h. During the reaction, the color of the contents changed from pink to violet indicating the formation of α-cobalt hydroxide. After 6 h, the contents were allowed to cool to room temperature, filtered using Whatman® filter paper, washed with Millipore® water several times and dried in the oven for overnight. The obtained products were calcined in a muffle furnace (Nabertherm®) at 500 °C for 3 hours (heating rate = 2° min⁻¹), under air, to get the SiO₂@Co₃O₄ core–shell nanorattles. The obtained SiO₂@Co₃O₄ samples were labelled as S1, S2, and S3, corresponding to 5 mM, 10 mM and 15 mM concentrations of cobaltous nitrate used during the preparation of SiO₂@α-Co(OH)₂ samples, respectively.

2.3 Characterization

Structural analysis of the synthesized samples was carried out using powder X-ray diffraction on a Brucker AXS D8 diffractometer over the 2θ range of 5–85° and the scan rate was 1° min⁻¹. Copper was used as the target (Cu-K_α; λ = 1.5406 Å). FT-IR spectral analysis of the samples was carried out on a Thermo Nicolet Nexus FT-IR spectrometer in the range 400–4000 cm⁻¹ using KBr pellet method. Thermogravimetric analysis patterns of the as prepared samples were recorded on a EXSTAR TG/DTA 6300 instrument, under air, over the temperature range 25–1000 °C and the heating rate was 10° min⁻¹. Morphological studies of the samples were carried out using a FEI Quanta 200F field emission scanning electron microscope (FE-SEM) operating at 15 kV. Elemental analysis was carried out using an energy dispersive X-ray analysis facility coupled with the FE-SEM instrument. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a FEI TECNAI G² transmission electron microscope. For the TEM analysis, the sample powders were dispersed in ethanol and sonicated for about 15 min. Then, one drop each of the suspensions was put on a carbon coated copper grid and allowed to dry in air. Diffuse reflectance spectroscopy (DRS) analysis was carried out on a Shimadzu UV-2450 UV-Visible spectrophotometer using the powder samples in the wavelength range 360–800 nm and BaSO₄ (Aldrich) was used as the reference. Specific surface area of the SiO₂@Co₃O₄ samples was obtained by the Brunauer–Emmett–Teller (BET) method using N₂ adsorption at 77 K on a Nova 2200e Quantachrome instrument.

2.4 Peroxidase-like redox activity and steady state kinetic analysis

To investigate the peroxidase-like redox catalytic activity of the SiO₂@Co₃O₄ core–shell nanorattles, the oxidation of 3,3′,5,5′-tetramethyl benzidine (TMB) was examined in the presence of H₂O₂. The reaction was studied at room temperature in acetic acid–acetate buffer containing SiO₂@Co₃O₄ nanorattles as the catalyst. First, 30 μg of SiO₂@Co₃O₄ catalyst was dispersed in 3 mL of 0.1 M acetate buffer and sonicated for 3 minutes. Then, 300 μL of 3 mM DMSO solution of TMB and 31 μL of 30% H₂O₂ (100 mM) were added and the absorbance values at λ_max = 652 nm were recorded upto 5 minutes in time course mode on a Shimadzu UV-2450 UV-Vis spectrophotometer. Kinetic studies were carried out using the same instrument. While keeping the TMB concentration constant, kinetic analysis was performed by varying the amount of the SiO₂@Co₃O₄ catalyst (5–60 μg per 3 mL), pH of the buffer (2–12), temperature (20–70 °C), and H₂O₂ concentration (5–200 mM). Kinetic studies were also carried out under the following conditions; 30 μg catalyst per 3 mL, pH = 5, temperature = 30 °C and TMB concentration = 0.015–0.4 mM while keeping H₂O₂ concentration as 100 mM. In another kinetic experiment, while keeping the concentration of TMB as 0.3 mM, the concentration of H₂O₂ was varied (20–200 mM). The apparent rate constant values (k_app) were evaluated from the absorbance data.^9,10 The Michaelis–Menten constant (K_m) and the maximal reaction velocity (V_max) were determined from the Lineweaver–Burk double reciprocal plots. The Michaelis–Menten equation can be expressed as

1/ν = (K_m/V_max)(1/[S]) + 1/V_max

where, ν is the initial velocity, K_m is the Michaelis–Menten constant, V_max is the maximal reaction velocity, and [S] is the concentration of the substrate (TMB).

3. Results and discussion

3.1 Material characterization

Fig. 1(a) shows the XRD patterns of as prepared SiO₂, α-Co(OH)₂, and SiO₂@α-Co(OH)₂. Silica is amorphous. All the XRD patterns show raising background which is attributed to X-ray fluorescence since Cu-K_α was used as the X-ray source during the measurements. The α-Co(OH)₂ shows reflections at 12.18°, 24.37°, 33.11°, 38.08°, 45.98°, 52.05°, and 59.71° which are attributed to (003), (006), (012), (015), (018), (100), and (110) crystal planes (JCPDS file no. 46-0605), respectively.^45,46 The first two planes in the XRD pattern of α-Co(OH)₂ are related to d-spacing between the intercalated layers (d₍₀₀₃₎ = 2d₍₀₀₆₎).⁴⁷ The XRD patterns of SiO₂@α-Co(OH)₂ samples show broad peaks of (003), (006), and (110) planes of α-Co(OH)₂ with low crystallinity.^48,49 The interplanar spacings of α-Co(OH)₂ and SiO₂@α-Co(OH)₂ samples was calculated using the (003) plane and the value is ca. 7.2 Å.

Fig. 1 (a) XRD patterns of (i) as prepared SiO₂, (ii–iv) SiO₂@α-Co(OH)₂ samples, and (v) α-Co(OH)₂. (b) XRD patterns of (i) SiO₂, (ii–iv) SiO₂@Co₃O₄ (S1, S2, and S3), and (v) Co₃O₄. All the samples were calcined at 500 °C.

Fig. 1b shows the XRD patterns of SiO₂, Co₃O₄ and SiO₂@Co₃O₄ samples (S1, S2, and S3) after calcination at 500 °C. Pure Co₃O₄ shows reflections at 19.09°, 31.26°, 36.89°, 38.47°, 44.77°, 59.19°, 65.27°, 74.26°, and 78.80° corresponding to (111), (220), (311), (222), (400), (511), (440), (620), and (622) planes of cubic Co₃O₄ (JCPDS file no. 42-1467), respectively. Since the major component in the core–shell nanorattles is SiO₂ which is amorphous, the XRD patterns for SiO₂@α-Co(OH)₂ as well as SiO₂@Co₃O₄ core–shell nanorattles (obtained on calcination) indicate poorly crystallinity. The XRD patterns for α-Co(OH)₂ (Fig. 1a(v)) and pure Co₃O₄ (Fig. 1b(v)), however, show that these samples are more crystalline compared to the other samples. Calcination does not improve the crystallinity since it only converts α-Co(OH)₂ present on SiO₂ to Co₃O₄ and Co₃O₄ exists as small nanoparticles (as evidenced by the TEM results). XRD results indicate the formation of α-Co(OH)₂ on SiO₂ spheres. On calcination at 500 °C, the formation of Co₃O₄ nanoparticles on SiO₂ is evident.

FT-IR spectra were recorded for as prepared SiO₂, α-Co(OH)₂, and SiO₂@α-Co(OH)₂ (Fig. 2a). The IR spectra for calcined SiO₂, Co₃O₄, SiO₂@Co₃O₄ (S1, S2, and S3) samples are shown in Fig. 2b. All the samples show two bands at about 3430 cm⁻¹ and 1630 cm⁻¹ assigned to stretching and bending vibrations of –OH groups of surface physisorbed water molecules. All the samples except the α-Co(OH)₂ and Co₃O₄ show bands at about 1100 cm⁻¹, 800 cm⁻¹, and 475 cm⁻¹ attributed to asymmetric, symmetric stretching, and bending vibration modes of Si–O–Si. Pure SiO₂ shows a characteristic band at 950 cm⁻¹ due to Si–OH group.⁴⁴ The SiO₂@α-Co(OH)₂ samples and α-Co(OH)₂ show IR bands at 2235 cm⁻¹, and 1387 cm⁻¹, attributed to the stretching frequencies of NCO⁻, and NO₃⁻, respectively. This indicates the presence of NCO⁻ and NO₃⁻ in between the layers.^47,50 All the samples except SiO₂ show a band at 1023 cm⁻¹ attributed to the deformation mode of –OH group.⁵¹ The α-Co(OH)₂ and SiO₂@α-Co(OH)₂ samples show a band at about 660 cm⁻¹ attributed to Co–OH vibration. Pure Co₃O₄ shows two characteristic bands attributed to Co(II)–O stretching when Co²⁺ ions are in tetrahedral coordination and Co(III)–O stretching when Co³⁺ ions are in octahedral coordination;⁵⁰ SiO₂@Co₃O₄ (S1) shows IR bands at 667 cm⁻¹ and 560 cm⁻¹, sample S2 shows bands at 667 cm⁻¹ and 576 cm⁻¹, and sample S3 shows bands at 664 cm⁻¹ and 568 cm⁻¹. In summary, FT-IR results show the evidence for the presence of NCO⁻ and NO₃⁻ ions in α-Co(OH)₂, and SiO₂@α-Co(OH)₂ and the presence of Co₃O₄ in the SiO₂@Co₃O₄ samples.

Fig. 2 (a) FT-IR spectra of (i) as prepared SiO₂, (ii) α-Co(OH)₂, (iii–v) SiO₂@α-Co(OH)₂ and (b) FT-IR spectra of (i) SiO₂, (ii) Co₃O₄, and (iii–v) SiO₂@Co₃O₄ (S1, S2, and S3) samples after calcination at 500 °C. The inset shows the magnified view of IR bands for SiO₂@Co₃O₄ samples (S1, S2, and S3).

The thermogravimetric analysis (TGA) patterns of as prepared SiO₂, α-Co(OH)₂, and SiO₂@α-Co(OH)₂ samples are shown in Fig. 3. The as prepared SiO₂ shows a total weight loss of 12.5% upto 1000 °C with two weight loss steps. The first step from 35 °C to 170 °C (weight loss = 5%) is attributed to the loss of physisorbed water molecules. The second weight loss from 350 °C to 660 °C (wt loss = ∼7.5%) is attributed to the dehydroxylation of surface hydroxyl groups.⁵² The α-Co(OH)₂ shows a total weight loss of about 35% upto 1000 °C in three steps. The weight loss upto 150 °C (∼2.5%) is related to the loss of surface physisorbed water molecules. The major weight loss (∼28.3%) observed between 220 and 400 °C is attributed to the loss of intercalated water molecules, anions, and dehydroxylation of α-Co(OH)₂ accompanied by its conversion to spinel Co₃O₄.⁵³ Another weight loss (∼4.2%) between 940 and 985 °C is associated with the thermal decomposition of Co₃O₄ to CoO.⁵⁴ The SiO₂@α-Co(OH)₂ samples (S1, S2 and S3) show weight loss features due to both SiO₂ and α-Co(OH)₂ and the overall weight loss values are 25.5%, 25.2% and 19.5%, respectively. The SiO₂@α-Co(OH)₂ samples (S1 and S2) show major weight loss (∼12.5%) between 30 and 150 °C, while SiO₂@α-Co(OH)₂ sample (S3) shows a weight loss of about 4.5% in the same temperature range. Sample S3 (SiO₂@α-Co(OH)₂ prepared using 15 mmol cobalt nitrate) shows different weight loss features compared to S1 and S2 (SiO₂@α-Co(OH)₂ samples, prepared using 5 mmol and 10 mmol cobalt nitrate, respectively). It was found from SEM studies (see Fig. 4e) that sample S3 before calcination consists of SiO₂@α-Co(OH)₂ as well as free α-Co(OH)₂ particles. Sample S3 (SiO₂@α-Co(OH)₂) shows less weight loss below 150 °C (∼4.5%) similar to α-Co(OH)₂ (∼2.5%). The low weight loss below 150 °C in these samples (α-Co(OH)₂ and S3) is attributed to strong interaction between the surface hydroxyl groups which delay the dehydroxylation process.^55,56 The low weight loss at temperature below 150 °C leads to an overall low weight loss in S3. TGA results indicate that the complete formation of spinel Co₃O₄ occurs at about 500 °C by the loss of intercalated water molecules, anions and dehydroxylation of surface hydroxyl groups of α-Co(OH)₂.

Fig. 3 TGA patterns of (a) as prepared SiO₂, (b) α-Co(OH)₂, and (c–e) SiO₂@α-Co(OH)₂ samples.

Fig. 4 FE-SEM images of (a) as prepared SiO₂ (b) α-Co(OH)₂, and (c–e) SiO₂@α-Co(OH)₂ samples.

Morphological studies of the synthesized samples were first investigated using FE-SEM. The FE-SEM images of SiO₂, α-Co(OH)₂, and SiO₂@α-Co(OH)₂ samples (all the samples before calcination) are shown in Fig. 4a–e. Pure silica shows uniform spherical particles and the diameter of the spheres is about 300 ± 20 nm; the average diameter of particles and errors were calculated using about ten particles. α-Co(OH)₂ consists of sheets and clusters. The SiO₂@α-Co(OH)₂ (S1 and S2) show that the silica spheres are completely covered with α-Co(OH)₂ sheets (petals) to form a flower-like morphology. But in the case of SiO₂@α-Co(OH)₂ sample (S3), the SiO₂ spheres are covered with α-Co(OH)₂ sheets and some uncoated particles of α-Co(OH)₂ can also be noticed. The FE-SEM images of Co₃O₄ and SiO₂@Co₃O₄ samples (i.e. after calcination) are shown in Fig. 5(a–d). Pure Co₃O₄ shows particles with petals-like morphology. The SiO₂@Co₃O₄ samples (S1 & S2) show uniform shell of Co₃O₄ over the silica core but sample S3 shows coating of Co₃O₄ shell on the SiO₂ spheres with some extra uncoated particulates of Co₃O₄. Based on UV-Vis transmittance measurements, it was found that the dispersion of the core–shell particles in water is stable for at least 6 h. It was also found that the powder obtained after drying the particles can be redispersed effectively.

Fig. 5 FE-SEM images of (a) Co₃O₄, and (b–d) SiO₂@Co₃O₄ samples (S1, S2, and S3). All the samples were calcined at 500 °C.

The elemental composition of the SiO₂@α-Co(OH)₂ and SiO₂@Co₃O₄ core–shell nanoparticles (S1, S2, and S3) was determined using EDX analysis (Table 1). The EDX results confirm the presence of silicon, oxygen and cobalt in the SiO₂@Co₃O₄ core–shell nanorattles as well as the SiO₂@α-Co(OH)₂ samples. Increase in the weight percent of cobalt in the core–shell samples is observed according to the concentration of cobaltous nitrate used during the synthesis of SiO₂@α-Co(OH)₂ samples (5 mM, 10 mM, and 15 mM).

Table 1 Elemental composition (wt%) of silicon, cobalt, and oxygen in the SiO₂@α-Co(OH)₂ and the SiO₂@Co₃O₄ core–shell nanorattles

Sample name	Weight percent
	Si	Co	O
a The concentrations of Co(NO3)2·6H2O used during the preparation of SiO2@α-Co(OH)2 samples S1, S2 and S3 were 5 mM, 10 mM and 15 mM, respectively.
SiO2	28.0 ± 1.4	—	32.5 ± 3.5
α-Co(OH)2	—	93.4 ± 1.5	6.6 ± 1.5
SiO2@α-Co(OH)2 (S1)^a	15.2 ± 1.9	17.6 ± 3.1	37.4 ± 2.3
SiO2@α-Co(OH)2 (S2)^a	13.4 ± 0.2	21.2 ± 2.6	35.1 ± 1.2
SiO2@α-Co(OH)2 (S3)^a	10.9 ± 0.5	23.6 ± 1.5	38.9 ± 1.3
Co3O4	—	83.6 ± 0.7	16.4 ± 0.3
SiO2@Co3O4 (S1)	11.1 ± 0.6	14.0 ± 0.5	37.9 ± 0.7
SiO2@Co3O4 (S2)	11.4 ± 0.2	16.0 ± 1.0	34.8 ± 0.1
SiO2@Co3O4 (S3)	9.7 ± 1.1	27.9 ± 0.3	30.3 ± 0.2

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The TEM images of SiO₂, Co₃O₄ and SiO₂@Co₃O₄ core–shell samples (S1 & S2) are shown in Fig. 6a. TEM analysis was not carried out for the SiO₂@Co₃O₄ sample S3 since the SEM analysis on this sample indicated the presence of uncoated α-Co(OH)₂ particles in the SiO₂@α-Co(OH)₂ sample (see Fig. 4e). SiO₂ shows uniform spherical particles with a diameter of about 300 ± 10 nm. The TEM image indicates the nanosized nature of Co₃O₄ particles (13.7 ± 2.1 nm). The TEM images for the SiO₂@Co₃O₄ samples (S1 & S2) show the core–shell morphology with a void space between the SiO₂ core and the Co₃O₄ shell. These type of structures are referred to as “nanorattles”.^24,29 In addition to the nanorattle morphology, the SiO₂@Co₃O₄ samples show porous hair-like morphology for the Co₃O₄ shell. The measured shell thickness values are 37 ± 3 nm, 63 ± 8 nm, for S1 and S2, respectively. The estimated void space between the core and the shell lies between 40 and 45 nm and 64 and 72 nm for the SiO₂@Co₃O₄ samples S1 and S2, respectively. The thickness of hair-like structure of the Co₃O₄ shell was found as 27 ± 3 nm and 45 ± 5 nm for the samples S1 and S2, respectively. The SAED patterns of pure Co₃O₄ and SiO₂@Co₃O₄ core–shell nanorattles (S1 & S2) are shown in Fig. 6b. The SAED patterns show spots/rings indicating polycrystalline nature of the samples. The SAED patterns could be indexed to (111), (220), (311), (400), (511), and (440) planes of cubic Co₃O₄. Pure Co₃O₄ exhibits stronger electron diffraction compared to the SiO₂@Co₃O₄ nanorattles (S1 & S2). The less crystalline nature of SiO₂@Co₃O₄ samples compared to pure Co₃O₄ suggest that the Co₃O₄ (shell) particles formed on the silica are smaller in size.

Fig. 6 (a) TEM images of (i) SiO₂, (ii) Co₃O₄, and (iii and iv) SiO₂@Co₃O₄ samples (S1, and S2). The images are obtained for all the samples after calcination at 500 °C. Scale bar is 200 nm. (b) SAED patterns of (i) Co₃O₄, and (ii and iii) SiO₂@Co₃O₄ samples (S1, and S2).

The specific surface area of pure SiO₂, Co₃O₄ and the SiO₂@Co₃O₄ nanorattles (S1 & S2) were measured using Brunauer–Emmett–Teller (BET) measurements using nitrogen gas physisorption. Pure SiO₂ and Co₃O₄ possess surface area of 34.2 and 46.5 m² g⁻¹, respectively. The SiO₂@Co₃O₄ nanorattles (S1 and S2) possess surface area of about 279 m² g⁻¹ and 268 m² g⁻¹, respectively. The higher surface area of SiO₂@Co₃O₄ nanorattles (S1 and S2) compared to pure Co₃O₄ and SiO₂ is attributed to the nanorattle morphology of SiO₂@Co₃O₄ (S1 and S2). The nanorattles consist of porous shell along with the interior void space between the core and the shell.

The optical properties of pure Co₃O₄ and SiO₂@Co₃O₄ nanorattles (S1 & S2) were studied using UV-Vis diffuse reflectance spectroscopy (Fig. 7a and b). All the samples exhibit two broad bands in the region 420–450 nm and 700–750 nm. Co₃O₄ shows absorption bands at 745 nm and 450 nm while SiO₂@Co₃O₄ samples (S1 & S2) show absorption bands at about 705 nm and 425 nm. The band at 745 nm in pure Co₃O₄ and 705 nm in SiO₂@Co₃O₄ samples (S1 & S2) are due to O²⁻ → Co³⁺ charge transfer transition and the band at 450 nm in pure Co₃O₄ and 425 nm in SiO₂@Co₃O₄ samples (S1 & S2) are due to O²⁻ → Co²⁺ charge transfer transition.^19,57 Both the SiO₂@Co₃O₄ samples (S1 & S2) show blue shift of about 40 nm with respect to the 745 nm band in pure Co₃O₄ and 25 nm with respect to the 450 nm band in pure Co₃O₄. The blue shift in the case of SiO₂@Co₃O₄ samples (S1 & S2) is attributed to quantum confinement effect and smaller particle size of Co₃O₄.^19,57 Co₃O₄ is a p-type semiconductor and the exact band gap energies are calculated using the Tauc equation which can be expressed as αhν = k (hν − E_g)ⁿ, where E_g represents the band gap, hν is the photon energy, k is the constant, α is the Kubelka–Munk function and n is dependent on the type of transition involved. In the present case, n = 1/2 gives the best fit for (αhν)^1/n versus photon energy plots suggesting direct allowed transition in Co₃O₄. The calculated band gap energy (E_g) values for Co₃O₄ is 1.70 eV and 2.43 eV and for the SiO₂@Co₃O₄ samples, the values were found to be 1.72 eV and 2.52 eV (S1) and 1.74 eV and 2.57 eV (S2). The band gap values for bulk Co₃O₄ are 1.48 eV and 2.19 eV.⁵⁸ The obtained band gap values for SiO₂@Co₃O₄ samples (S1 & S2) are in good agreement with the previously reported values for Co₃O₄ nanoparticles.^19,59

Fig. 7 (a) UV-Vis DRS spectra of (i) Co₃O₄, and (ii and iii) SiO₂@Co₃O₄ core–shell nanorattles (S1 and S2). (b) Tauc plots of (i) Co₃O₄, and (ii and iii) SiO₂@Co₃O₄ core–shell nanorattles (S1 and S2).

3.2 Proposed mechanism for the formation of SiO₂@Co₃O₄ core–shell nanorattles

For the formation of Co₃O₄ shell on the SiO₂ core, the following mechanism is proposed based on the experimental results (Scheme 1). It is proposed that an adsorption–nucleation–coalescence–anisotropic growth-self-assembly occurs.^60,61 At first, Co²⁺ ions from the solution are adsorbed on the surface of SiO₂ through electrostatic interactions. Urea decomposes to produce NH₃ and CO₂ and the released NH₃ readily reacts with the available Co²⁺ ions on the surface of SiO₂ to form a complex (i.e. Co(NH₃)_y; y ≤ 2) which reduces the concentration of free Co²⁺ ions in the solution. After attaining the optimum temperature (e.g. 85 °C), OH⁻ ions are steadily produced through the hydrolysis of urea, which is favorable for nucleation and formation of α-Co(OH)₂ on the surface of SiO₂ spheres based on the coalescence mechanism. The α-Co(OH)₂ further undergoes Ostwald ripening to produce flower-like hierarchical α-Co(OH)₂ on the surface of SiO₂.⁶¹

Scheme 1 Proposed mechanism for the formation of SiO₂@Co₃O₄ core–shell nanorattles.

Yan et al. and Yuan et al. have explained the formation of hierarchical 3D metal hydroxide flower-like structures based on thermodynamic perspective.^60,61 If one considers a single nanoplatelet of α-Co(OH)₂, the surface energy is very high. In order to reduce the overall surface energy, the formed ultrathin α-Co(OH)₂ nanoplatelets tend to aggregate which decreases the surface energy by reducing the unsatisfied bonds at the exposed areas. The formed thinner plates self-assemble spontaneously based on the coalescence mechanism to produce thicker plates. The pH would decrease and Ostwald ripening growth would dominate leading to flower-like hierarchical α-Co(OH)₂ on the surface of SiO₂. In the final step, calcination of SiO₂@α-Co(OH)₂ in air at 500 °C for 3 h leads to the formation of SiO₂@Co₃O₄ core–shell nanorattles through self-template route retaining its 3D hierarchical flower-like morphology for Co₃O₄ nanoparticles.

To understand the mechanism of formation of nanorattle structures further, time dependent TEM analysis (Fig. 8) was performed at different time intervals for SiO₂@α-Co(OH)₂ (S1) (e.g. 2 h, 4 h and 6 h) and SiO₂@Co₃O₄ (e.g. 1 h and 3 h). Initially (e.g. 2 h), α-Co(OH)₂ nanoparticles are formed around the SiO₂ spheres (Fig. 8a). The crystallites of α-Co(OH)₂ present on SiO₂ are loosely packed and the crystallites located at the outermost surface act as the nucleation seeds for the recrystallization process. As the reaction proceeds from 2 h to 4 h, inside-out Ostwald ripening process dominates which starts the hollowing process by dissolving the smaller crystallites of α-Co(OH)₂ present in the interior region. At 6 h, the outer α-Co(OH)₂ crystallites become larger which creates an interstitial space with the formation of a compact shell. Calcination of SiO₂@α-Co(OH)₂ at 500 °C leads to the formation of SiO₂@Co₃O₄ core–shell nanorattles with the retention of core–shell nanorattles morphology (Fig. 8b). The TEM images show the shrinkage of silica core as well as the Co₃O₄ shell.

Fig. 8 (a) TEM images of SiO₂@α-Co(OH)₂ samples synthesized at 2 h, 4 h and 6 h and (b) the images of SiO₂@Co₃O₄ obtained on calcination of SiO₂@α-Co(OH)₂ at 500 °C for 1 h and 3 h. The scale bar is 200 nm.

The size of the inner core (SiO₂) and shell (Co₃O₄) particles exhibits a rather larger polydispersity compared to pure SiO₂ and Co₃O₄ particles. This is explained as follows. It can be noted from TEM studies that the formation of an interface between SiO₂ core and α-Co(OH)₂ shell occurs within 2 h. After 4 h, formation of core–shell nanorattles with some extra particles of α-Co(OH)₂ occurs and core–shell nanorattles with compact α-Co(OH)₂ shell are obtained after 6 h. When SiO₂@α-Co(OH)₂ core–shell nanorattles are calcined to get the SiO₂@Co₃O₄ core–shell nanorattles, polydispersion of silica core particles with slight shrinkage occurs. Intercalated water molecules and anions (NCO⁻ and NO₃⁻) present in α-Co(OH)₂ and surface hydroxyl groups from silica are lost during the conversion of SiO₂@α-Co(OH)₂ to SiO₂@Co₃O₄ core–shell nanorattles. The polydispersity of SiO₂ spheres and Co₃O₄ shell is attributed to the different dehydroxylation rate of loss of surface hydroxyl groups and other species from silica (core) and α-Co(OH)₂ (shell).

3.3 The peroxidase-like redox catalytic activity of SiO₂@Co₃O₄ core–shell nanorattles

Enzymes are made up of proteins and they are the active species in bio-catalysis.^7,8 They possess high specificity but they can easily loose their stability in harsh environmental conditions such as high temperature, abnormal pH and in the presence of inhibitors. The lack of stability of natural enzymes provides a chance to explore artificial enzymes in bio-applications. At present, artificial enzymes are very attractive towards bio-catalysis because of their stability and high efficiency under a wide range of temperatures and acidic conditions.^10,34,62

Horseradish peroxidase (HRP), a natural enzyme, catalyzes the oxidation of substrates (e.g. amines and phenols) in the presence of H₂O₂.³⁴ Mu et al. and Gao et al. have utilized Co₃O₄ (ref. 10) and Fe₃O₄ nanoparticles³⁵ as artificial enzymes for testing the intrinsic peroxidase-like activity towards TMB. Inspired from these reports, peroxidase-like redox activity was examined using the SiO₂@Co₃O₄ core–shell nanorattles (S1, S2 and S3). TMB is a colorless peroxidase substrate which is oxidized by H₂O₂ very slowly to form a blue coloured product. In a typical experiment, the oxidation of TMB was carried out using Co₃O₄ nanoparticles and SiO₂@Co₃O₄ core–shell nanorattles (S1, S2 and S3) as the catalysts in the presence of H₂O₂. The blue colored product formed as a result of the catalytic reaction was monitored by UV-Visible spectroscopy (λ_max = 652 nm). A blank reaction was carried out with TMB and H₂O₂ in the absence of any catalyst. It was found that there is no peroxidase activity in the absence of the catalyst which confirms the role of catalyst in the peroxidase-like activity towards TMB. Although, pure Co₃O₄ nanoparticles and SiO₂@Co₃O₄ core–shell nanorattles (S1, S2 and S3) produce blue coloured product on reaction with TMB in the presence of H₂O₂, the blue colour was more intense in the case of SiO₂@Co₃O₄ core–shell nanorattles (S1, S2 and S3) compared to pure Co₃O₄.

The time dependent catalytic activity for Co₃O₄ and SiO₂@Co₃O₄ core–shell nanorattles towards TMB oxidation, as indicated by UV-Visible spectral results are shown in Fig. 9a. Their relative catalytic efficiency is shown in Fig. 9b which follows the order: S2 > S3 > S1 > Co₃O₄ > SiO₂. The activity of sample S3 is lower compared to S2 but higher than that of S1 (Fig. 9a).

Fig. 9 (a) Time dependent UV-Visible spectral results indicating the peroxidase-like activity on TMB using SiO₂, Co₃O₄, and SiO₂@Co₃O₄ core–shell nanorattles (S1, S2 and S3) as the catalyst (λ_max = 652 nm). (b) Comparison of peroxidase activity of SiO₂@Co₃O₄ core–shell nanorattles (S1, S2 and S3) with that of SiO₂ and Co₃O₄ nanoparticles.

3.4 Effect of physicochemical conditions on the peroxidase-like activity

Any catalytic reaction demands optimum conditions for maximum activity. In the present study, TMB and H₂O₂ were chosen as the reagents. Effort was made to understand the catalytic activity of SiO₂@Co₃O₄ core–shell nanorattles on various conditions such as amount of the catalyst, temperature, and pH. The amount of catalyst (SiO₂@Co₃O₄, S2) was varied from 5 μg to 60 μg per 3 mL while keeping the pH of acetate buffer at 5, temperature at 30 °C and the concentration of H₂O₂ at 100 mM. A linear relationship between the amount of catalyst and peroxidase activity was found (Fig. 10a). The catalytic activity was tested by varying the pH from 2 to 12 and also by varying the temperature between 20 and 70 °C while keeping the amount of catalyst (SiO₂@Co₃O₄, S2) at 30 μg per 3 mL, and the concentration of TMB and H₂O₂ at 0.3 mM and 100 mM, respectively. From the results (Fig. 10b and c), the optimized pH and temperature were found as 5 and 30 °C, respectively. The effect of H₂O₂ concentration on the catalytic activity of SiO₂@Co₃O₄ (S2) was also studied. By increasing the H₂O₂ concentration, the peroxidase-like activity of the SiO₂@Co₃O₄ core–shell nanorattles is steady even at higher H₂O₂ concentrations (Fig. 10d). Previous reports have indicated that higher concentration of H₂O₂ exhibits negative impact on the activity. At higher concentrations, H₂O₂ inhibits the catalytic activity of HRP due to its conversion from active form to an inactive form.^10,63 Leaching test was also performed to prove that the peroxidase-like activity of the catalysts does not result from the leaching of cobalt ions from the catalysts into the solution. To prove this, 30 μg of SiO₂@Co₃O₄ (S2) was incubated in 3 mL of acetate buffer solution (pH = 5) for 30 minutes, and then the SiO₂@Co₃O₄ sample was separated from the buffer solution by centrifugation. The catalytic activity using the resultant solution (from the leaching) was tested (3 mL of solution, 300 μL of 3 mM TMB, and 31 μL of 100 mM H₂O₂). The peroxidase activity using the leached solution was compared with that of SiO₂@Co₃O₄ sample (S2). Negligible absorbance was found as shown in Fig. 10e when the leached solution was used as the catalyst. These results conclude that the peroxidase-like activity is only due to Co₃O₄ nanoparticles and not from the leached cobalt ions.

Fig. 10 The effect of physicochemical conditions on peroxidase-like activity of SiO₂@Co₃O₄ core–shell nanorattles (S2): (a) amount of catalyst, (b) pH, (c) temperature, (d) H₂O₂ concentration, and (e) the effect of leaching.

3.5 Steady-state kinetic studies using SiO₂@Co₃O₄ core–shell nanorattles

To understand the peroxidase-like activity of SiO₂@Co₃O₄ core–shell nanorattles better, steady state kinetic analysis was carried out by choosing TMB and H₂O₂ as the reactants. The kinetic analysis was carried out by varying the concentration of TMB while keeping the H₂O₂ concentration constant, and vice versa (Fig. 11). The steady state kinetic parameters (Michaelis constant, K_m and maximal reaction velocity, V_max) were estimated using the Lineweaver–Burk double reciprocal plots^9,42 which show linear relationship between 1/[V] and 1/[S], as shown in Fig. 11c and d. The estimated kinetic parameters for the SiO₂@Co₃O₄ core–shell nanorattles (S2) were compared with that of Co₃O₄ nanoparticles and HRP (Table 2).

Fig. 11 Steady state kinetic analysis for SiO₂@Co₃O₄ core–shell nanorattles (S2) under various conditions: (a) [TMB] = 0.3 mM; [H₂O₂] = 20–200 mM, (b) [H₂O₂] = 100 mM; [TMB] = 0.015–0.4 mM, and (c & d) are the Lineweaver–Burk double reciprocal plots corresponding to conditions (a) and (b). Other conditions: 30 μg catalyst per 3 mL of 0.1 M acetate buffer, pH = 5, and temperature = 30 °C.

Table 2 Comparison of the kinetic parameters for SiO₂@Co₃O₄ core–shell nanorattles with those for Co₃O₄ and HRP

Catalyst	[E]/M^a	Substrate	Km/mM	Vmax/Ms^−1 (10^−8)	kcat/s^−1	kcat/Km (M^−1 s^−1)	Reference
a [E] = the concentration of catalyst (e.g. SiO2@Co3O4) or the concentration of enzyme (HRP).
SiO2@Co3O4 (S2)	7.54 × 10^−11	TMB	0.087 ± 0.0028	0.012 ± 0.00056	1.49 × 10^4	1.75 × 10^8	Present work
SiO2@Co3O4 (S2)	7.54 × 10^−11	H2O2	25.2 ± 1.28	0.015 ± 0.00035	1.96 × 10^4	8.08 × 10^5	Present work
Co3O4	3.43 × 10^−10	TMB	0.037	6.27	1.83 × 10^2	4.94 × 10^6	10
Co3O4	3.43 × 10^−10	H2O2	140.07	12.1	3.53 × 10^2	2.52 × 10^3	10
HRP	2.50 × 10^−11	TMB	0.434	10.0	4.00 × 10^3	9.22 × 10^6	62
HRP	2.50 × 10^−11	H2O2	3.70	8.71	3.48 × 10^3	9.40 × 10^5	62

---

The Michaelis constant (K_m), obtained from the slope of Lineweaver–Burk double reciprocal plot, is related to properties of the substrate, enzyme and the reaction conditions.⁹ A smaller K_m value indicates higher affinity between the substrate and the enzyme. The K_m value for SiO₂@Co₃O₄ core–shell nanorattles (S2) is 0.087 for TMB and 25.2 for H₂O₂, respectively. The K_m values for TMB and H₂O₂ are comparable with that of Co₃O₄–carbon nitride nanotubes⁴² (0.056 for TMB and 30.04 for H₂O₂) and Co₃O₄/reduced graphene oxide (0.19 for TMB and 24.04 for H₂O₂).⁴³ The obtained results were compared with the literature values for pure Co₃O₄ nanoparticles and HRP.^10,62 The SiO₂@Co₃O₄ core–shell nanorattles (S2) exhibit five times higher affinity for TMB than HRP and almost two times lower affinity for TMB compared to pure Co₃O₄ nanoparticles. In the case of H₂O₂, the SiO₂@Co₃O₄ core–shell nanorattles (S2) show almost six times higher affinity compared to Co₃O₄ nanoparticles and six times lower affinity compared to HRP. Higher concentration of H₂O₂ and lower concentration of TMB are the best conditions to achieve maximum peroxidase-like activity using the SiO₂@Co₃O₄ core–shell nanorattles. The higher activity of the nanorattles compared to HRP is attributed to the availability of more active sites on the surface of SiO₂@Co₃O₄ core–shell nanorattles compared to HRP, which has only one active site iron(II).^35,62

In order to compare the peroxidase-like activity of SiO₂@Co₃O₄ core–shell nanorattles (S2) with that of natural peroxidase (HRP) and pure Co₃O₄ nanoparticles, k_cat values were also estimated (Table 2). The k_cat value for SiO₂@Co₃O₄ core–shell nanorattles (S2) is much higher than that of pure Co₃O₄ and HRP. These results suggest that the SiO₂@Co₃O₄ core–shell nanorattles (S2) can be used as highly efficient peroxidase mimic compared to pure Co₃O₄ nanoparticles and HRP.^10,62 The ratio of k_cat/K_m is often referred to as specificity constant; a higher specificity constant value indicates higher peroxidase-like activity of the material. The k_cat/K_m values estimated for SiO₂@Co₃O₄ core–shell nanorattles (S2) are given in Table 2. In the present study, the SiO₂@Co₃O₄ core–shell nanorattles show higher specificity constant compared to pure Co₃O₄ and HRP for TMB. With respect to H₂O₂, the SiO₂@Co₃O₄ core–shell nanorattles (S2) show two orders of magnitude higher specificity compared to pure Co₃O₄ nanoparticles and the specificity is similar to that of HRP.⁶² These results demonstrate that the SiO₂@Co₃O₄ core–shell nanorattles show higher peroxidase-like activity compared to Co₃O₄ nanoparticles and HRP.

3.6 Proposed mechanism for the peroxidase-like activity on TMB using SiO₂@Co₃O₄ core–shell nanorattles

The proposed mechanism for the peroxidase-like activity of SiO₂@Co₃O₄ core–shell nanorattles on TMB is given in Scheme 2.^10,42,43

Scheme 2 Proposed mechanism for the peroxidase-like activity on TMB using SiO₂@Co₃O₄ core–shell nanorattles.

Neither H₂O₂ nor SiO₂@Co₃O₄ core–shell nanorattles alone was found to exhibit the peroxidase-like activity. The presence of SiO₂@Co₃O₄ and H₂O₂ together are important for the peroxidase-like activity on tetramethyl benzidine. When SiO₂@Co₃O₄, H₂O₂ and TMB are present, the TMB molecules adsorb on the surface of SiO₂@Co₃O₄ and increases the electron density by donating the lone pair electrons of the amino groups.^10,64 XPS or WAXS measurements would give more evidence on this aspect and this would be part of future studies. The SiO₂@Co₃O₄ core–shell nanorattles induce electrochemical reduction of H₂O₂ producing OH⁻ and OH˙ species which involve in the oxidation of TMB to produce TMB˙⁺ (a charge transfer complex which is blue in color). Mu et al. have reported the electrochemical reduction of H₂O₂ using pure Co₃O₄ nanoparticles.¹⁰ In the case of core–shell nanorattles, there is an assembly of Co₃O₄ nanoparticles on the surface of SiO₂ spheres. The TMB molecules undergo less steric hindrance to reach the catalytic active centers (cobalt) in the case of SiO₂@Co₃O₄ core–shell nanorattles compared to pure Co₃O₄ nanoparticles. This increases the electron density on the core–shell nanorattles and facilitate the electron transfer. The enhancement in the peroxidase-like activity of SiO₂@Co₃O₄ core–shell nanorattles compared to pure Co₃O₄ nanoparticles is due to increased surface area and also due to increase in the electron density in the case of SiO₂@Co₃O₄ core–shell nanorattles compared to pure Co₃O₄ nanoparticles. Sample S2 shows higher activity than S1 and this is due to the presence of more number of catalytic active centers in S2 compared to S1 but sample S3 consists of free Co₃O₄ nanoparticles in addition to SiO₂@Co₃O₄ nanorattles which leads to lower activity compared to S2.

4. Conclusions

A novel self-template method for the synthesis of SiO₂@Co₃O₄ core–shell nanorattles with different shell thickness has been reported. Compared to other preparation methods, this approach does not require any surface modification and the method is facile, and inexpensive. The synthesized SiO₂@Co₃O₄ core–shell nanorattles were characterized using a variety of techniques. The SiO₂@Co₃O₄ core–shell nanorattles possess enhanced intrinsic peroxidase-like activity compared to pure Co₃O₄ nanoparticles and HRP and it follows Michaelis–Menten kinetics. The enhanced peroxidase-like activity of SiO₂@Co₃O₄ core–shell nanorattles is due to the synergistic interaction between the SiO₂ core and the Co₃O₄ shell. The nanorattles show higher k_cat and k_cat/K_m values compared to pure Co₃O₄ nanoparticles and HRP suggesting their applicability as an artificial enzyme mimic in biomedicine and bio-sensing applications.

Acknowledgements

Syam Kandula thanks the Ministry of Human Resources and Development (MHRD), Govt. of India for the financial support. The authors are thankful to the Institute Instrumentation Centre, IIT Roorkee for providing the facilities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Overview image of SiO₂@Co₃O₄ core–shell nanorattles. See DOI: 10.1039/c4ra12596g

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