Sphere-like CoS with nanostructures as peroxidase mimics for colorimetric determination of H₂O₂ and mercury ions

Abstract

CoS, which was prepared using a facile solvothermal method, and characterized using various analytical techniques, was demonstrated for the first time to exhibit intrinsic peroxidase-like activity. CoS catalyzed the oxidation of the peroxidase substrate 3,3′,5,5′-tetramethylbenzidine in the presence of H₂O₂ rendering the reaction solution blue, which is the principle used for H₂O₂ sensing. The mechanism of the CoS catalysis was also investigated. Under optimal conditions, the linearity of H₂O₂ detection ranged from 0.05 to 0.8 mM (R² = 0.9965). More importantly, using the reducibility of glutathione and the strong affinity to thiol compounds exhibited by mercury ions (Hg²⁺), a novel “off–on” colorimetric sensor for Hg²⁺ determination was designed using CoS as a peroxidase mimics. The analytical platform for Hg²⁺ determination was developed, ranging from 0.25 to 3 μM (R² = 0.9965). The limit of detection was 0.1 μM. CoS exhibited several advantageous features including good chemical stability, ease of preparation, simple purification, and excellent reproducibility.

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Introduction

Natural enzymes have received wide ranging attention for more than 200 years, and play an important role in biochemistry and biosensors, because they exhibit excellent substrate specificity and efficiency under mild conditions. However, there are some drawbacks limiting their practical application, such as low stability and declining catalytic activity under harsh conditions. Moreover, their preparation and handling are expensive and time-consuming.¹ Therefore, in recent years, much effort has been dedicated to the development of enzyme mimics with catalytic activity similar to that of natural enzymes, such as superoxide dismutase, catalase, peroxidase, oxidase, esterase, nuclease, ferroxidase, phosphatase, and protease.²

In particular, the advantages of nanomaterial-based enzyme mimics, have received much attention, such as, their large surface-to-volume ratio, ease of preparation, and relatively simple procedures for storage and separation. Nanomaterial-based enzyme mimics have been studied extensively for various applications, including biosensors, immunoassays, and sensing.³ Fe₃O₄ magnetic nanoparticles were first found to possess intrinsic peroxidase-like activity by the Yan's group.⁴ Since then, various nanomaterials such as metal–organic frameworks (MIL-53(Fe),⁵ MIL-68,⁶ MIL-100,⁶ and HKUST-1 (ref. 7)), metallic oxide nanoparticles (CeO₂,⁸ V₂O₅,⁹ CuO,¹⁰ MnO₂,¹¹ and Co₃O₄ (ref. 12)), metallic and bimetallic nanoparticles (Au,¹³ Ag,¹⁴ Pt,¹⁵ and Au@Pt),¹⁶ carbon-based nanomaterials (graphene oxide,¹⁷ carbon nanotubes,¹⁸ and carbon dots¹⁹), bimetallic oxides (ZnFe₂O₄,²⁰ CoFe₂O₄,²¹ and FeWO₄ (ref. 22)) have been reported to exhibit intrinsic enzyme mimetic activity and have been used for colorimetric sensing of H₂O₂, Hg²⁺, glucose, and other species.²³

In recent years, metal sulfide nanostructures have attracted much attention due to their excellent optical and electronic properties, and wide ranging of applications in catalysis.²⁴ For example, FeS,²⁵ CdS,²⁶ and CuS²⁷ have all been shown to possess peroxidase-like activity. Cobalt sulfides are of interest as semiconductor materials, due to their advanced electronic properties, with applications in energy storage, lithium-ion batteries, solar cells and more.²⁸ The simplest cobalt sulfide, CoS, has been investigated in many fields due to its advantages. Qu et al.²⁹ prepared a plate-like CoS electrocatalyst for use in the hydrogen evolution reaction. Wang et al.³⁰ developed a CoS spindle as an efficient counter electrode for dye-sensitized solar cells. Jiao et al.³¹ studied the electrochemical properties of flower-like CoS in lithium ion batteries. Long et al.³² found that electrodeposited Co-S film catalyzed both electrochemical and photoelectrochemical hydrogen generation from water. Li et al.³³ reported that CoS nanowires were successfully synthesized and used in supercapacitors. However, to our knowledge, there have been no investigations into the peroxidase-like activity of CoS in the enzyme mimics field. To exploit a new application of CoS is attractive prospect in nanoscience and nanotechnology.

In this work, CoS was found to exhibit intrinsic peroxidase mimetic activity for the first time. A facile and green solvothermal method was used for CoS preparation. The as-synthesized CoS catalyzed the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ to produce a blue color change in the reaction solution. Glutathione (GSH) successfully caused a decline in blue color generation due to its strong radical restoration ability. Mercury ions (Hg²⁺) exhibited a strong affinity for the thiolated compounds. Based-on these phenomena, using CoS as a peroxidase mimic offered a simple colorimetric sensor for use in H₂O₂ detection, and a novel “off–on” colorimetric sensor designed for Hg²⁺ determination. CoS demonstrated a great potential for applications in biosensors, medical diagnosis, and environmental conservation.

Experimental

Chemicals and materials

All chemicals were obtained from commercial sources as analytical grade reagents (unless otherwise stated) and used without further purification. TMB, ABTS and OPD were purchased from TCI (Shanghai, China). Calcium chloride (CaCl₂), acetic acid (HAc), sodium chloride (NaCl), H₂O₂ (30 wt%), lead chloride (PbCl₂), mercury chloride (HgCl₂), zinc chloride (ZnCl₂), and sodium acetate (NaAc) were obtained from Shantou Xilong Chemical Co. Ltd. (Guangdong, China). Nickel nitrate (Ni(NO₃)₂), chromic nitrate (Cr(NO₃)₃), ferrous nitrate (Fe(NO₃)₂), cadmium chloride (CdCl₂), tert-butyl alcohol, silver nitrate (AgNO₃), magnesium sulfate (MgSO₄), and ammonium chloride (NH₄Cl) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). HRP and GSH were from Sigma-Aldrich (St. Louis, MO, USA). Ultrapure water was produced by a Millipore purification system (Bedford, MA, USA) and used to prepare all aqueous solutions.

Apparatus

Absorption spectra were obtained using a model Cary 60 spectrophotometer (Agilent, USA). Powder X-ray diffraction (XRD) patterns for CoS were obtained using a D/max 2550 VB/PC diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 0.15418 nm) from 5 to 90°. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo-ESCALAB 250XI (Thermo, USA) instrument with non-monochromated Al Kα 1486.6 eV radiation. Energy corrections were performed using the C 1s peak (284.7) as reference. Scanning electron microscopy (SEM) images were recorded on a field-emission microscope (JSM 7600F). Transmission electron microscopy (TEM) was performed using a Tecnai G20 (FEI, USA). The inductively coupled plasma mass spectrometry (ICP-MS) was recorded by Flexar/NexlON300X (PerkinElmer, USA). High-resolution transmission electron microscope (HRTEM) was obtained using a FEI Fecnai G2 F20 S-Twin transmission electron microscope, operating at an accelerating voltage of 200 kV.

Synthesis of CoS

CoS was synthesized according to the procedure of Wu et al.²⁸ with slight modifications. Typically, the cobalt source was cobalt chloride hexahydrate (CoCl₂·6H₂O) and the sulfur source was thiourea (CN₂H₄S). The mixed solution was prepared by dissolving CoCl₂·6H₂O (594.8 mg) into a mixture solution of ultrapure water (20 mL) and ethylene glycol (5 mL), and then thiourea (761.2 mg) was added. After stirring for 10 min, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, and heated for 15 h at 180 °C. Finally, the autoclave was cooled to room temperature, and the black powder was collected by centrifugation and washed several times with ultrapure water and ethanol to remove unreacted reagents. The black powder of CoS was dried for 12 h at 50 °C under vacuum conditions.

Mimetic peroxidase activity tests

All reactions were monitored in time scan mode at 652 nm using a Cary 60 spectrophotometer. Kinetic measurements were implemented by monitoring the absorbance change at 652 nm. Typical catalytic tests: used 0.5 mg mL⁻¹ CoS or 1.25 ng mL⁻¹ HRP, 0.15 mM TMB, and 20 mM H₂O₂ for CoS, or 2 mM H₂O₂ for HRP, as the substrates in a reaction volume of 2 mL. Kinetic constants were calculated using the Lineweaver–Burk plots of the double reciprocal of the Michaelis–Menten equation 1/ν = V_max[S]/(K_m + [S]), where ν is the initial velocity, V_max is the maximum reaction velocity, [S] is the concentration of the substrate, and K_m is the Michaelis constant.⁴

Determination of H₂O₂ and Hg²⁺ using CoS

H₂O₂ determination was conducted as follows: CoS (0.5 mg mL⁻¹), TMB (0.15 mM), and HAc–NaAc buffer (1.8 mL, 0.2 M, pH 3.5) were added to 5 mL centrifuge tube. Then H₂O₂ (100 μL) was added at different concentrations. Subsequently, the solutions were incubated for 10 min at room temperature (25 °C). The absorption spectra of the solutions were measured using a model Cary 60 spectrophotometer.

Hg²⁺ determination was performed as follows: at room temperature, Hg²⁺ solutions (50 μL) at different concentrations, and GSH (0.1 mM) were added to NaAc–HAc buffer (1.7 mL, 0.2 M, pH 3.5). After incubation for 5 min, TMB (0.15 mM) and CoS (0.5 mg mL⁻¹) were add. The final volume of the mixed solution was 2 mL, which was then incubated for another 10 min. Lastly, a model Cary 60 spectrophotometer was used to measure the absorption spectra of the mixed solutions.

Determination of real samples

Li River water and tap water samples were analyzed as real samples without further treatment. Li River water and tap water samples (15 mL each) were diluted to 30 mL with acetate buffer (0.4 M, pH 3.5). A 3.5 mL aliquot of these solutions was added to a 5 mL centrifugal tube, and spiked with GSH (0.1 mM) and Hg²⁺ solutions at various concentrations. After incubation for 5 min, TMB (0.15 mM) and CoS (0.5 mg mL⁻¹) were added. The final volume was 4 mL, which was then incubated for another 10 min at 25 °C. The mixed solutions were analyzed using the proposed method and the percentage recovery values were obtained.

Results and discussion

Characterization of CoS

XRD patterns of the as-prepared CoS were shown in Fig. 1a. As can be seen, the main diffraction peaks appeared at 2θ values of 30.6°, 35.3°, 46.9°, 54.4°, and 74.6°, which coincide with the (100), (101), (102), (110), and (202) surfaces of hexagonal CoS (JCPDS card No. 65-3418, cell = 3.368 × 5.17, α = 90°, β = 90°, γ = 120°). No impure peaks were observed, which proved that the product was highly crystalline and pure. Significantly, the crystal structure of CoS remained very well intact after catalytic reaction, five recycle sensing H₂O₂ and five recycle sensing Hg²⁺. A typical Raman scattering spectrum was used to further study the CoS structure (Fig. 1b). The four remarkable peaks at 476, 520, 606, and 685 cm⁻¹ corresponded with the E_g, F¹_2g, F²_2g, and A_1g modes of CoS, respectively.³⁴

Fig. 1 (a) XRD patterns of the as-prepared CoS (black line), after catalytic reaction (green line), after five recycle sensing H₂O₂ (blue line), after five recycle sensing Hg²⁺ (yellow line). (b) Raman spectrum of the as-prepared CoS.

X-ray photoelectron spectra (XPS) of CoS, as well as the CoS Co 2p, and S 2p spectra were studied (Fig. 2). As shown in Fig. 2a, all peaks could be attributed to Co, S, C, and O elements. The appearance of the O element peak (531.7 and 533.0 eV) in the sample might be attributed to the oxygen of the hydroxide ions and a small amount of physically adsorbed water molecules.³⁵ There are two strong peaks at about 793.1 and 778.1 eV with a weak peak at about 780.2 eV, which are ascribed to the Co 2p_1/2 and Co 2p_3/2 binding energies (Fig. 2b), these results corresponds to Co²⁺ of CoS.^36–40 In Fig. 2c, peaks could be observed at 161.5 and 162.6 eV in the region of S 2p due to the S 2p_3/2 and S 2p_1/2 binding energies, which corresponds to S⁻² of CoS.^38–40 The observation of a weak peak at 168.5 eV is associated with sulfur oxides.^41,42

Fig. 2 (a) XPS analysis of as-prepared CoS, (b) and (c) show narrow range XPS scans for the Co and S signals, respectively.

The energy dispersive X-ray spectrometry (EDS) spectrum and elemental mapping images were shown in Fig. 3. The EDS spectrum revealed that S and Co were found in the as-prepared CoS. The atom ratio of S and Co elements in CoS was nearly 1 : 1 (Fig. 3a). The elemental mapping images revealed that S and Co were uniformly distributed in the as-prepared CoS (Fig. 3b). Furthermore, the as-prepared sample was also investigated using inductively coupled plasma mass spectrometry (ICP-MS) (Table S1†), which indicated that the Co composition was close to the theoretical value, suggesting that CoS had been successfully synthesized.

Fig. 3 (a) The EDS spectrum, (b) elemental mapping images of S and Co in CoS. The atom ratio of S and Co elements in CoS was around 1 : 1.

The CoS was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Fig. 4). SEM images of as-prepared CoS and the CoS after sensing Hg²⁺ were composed of sphere-like nanostructures, with no nanoflakes on the surface (Fig. 4a–d). The CoS retains its shape after sensing Hg²⁺. The TEM image further demonstrated that the as-prepared CoS exhibited sphere-like nanostructures (Fig. 4e). The calculated lattice spacing were 0.252 nm and 0.298 nm, which respectively matched well with the (101) and (100) planes of the as-prepared CoS (JCPDS card No. 65-3418) (Fig. 4f).

Fig. 4 (a and b) The SEM images of as-prepared CoS, (c and d) the SEM images of CoS after sensing Hg²⁺, (e) the TEM image and (f) HRTEM of as-prepared CoS.

Peroxidase-like activity of CoS

The peroxidase-like activity of CoS was investigated using the catalytic oxidation of TMB as a peroxidase substrate in the presence of H₂O₂. TMB is a typical chromogenic substrate. First, the peroxidase-like activity of CoS was examined. As shown in Fig. 5a, the CoS/H₂O₂ and CoS/TMB systems were colorless with no absorbance at 652 nm. However, the H₂O₂/TMB system exhibited a light blue color and very weak absorption at 652 nm. However, in the presence of CoS, the H₂O₂/TMB system produced a typical blue color reaction and a strong absorption peak was observed at 652 nm. H₂SO₄ was also used to test the catalytic activity of CoS. As shown in Fig. 5b, the absorption peak at 652 nm gradually declined with increasing H₂SO₄ concentration. This was similar to that observed for the peroxidase activity of natural enzymes, in which that blue color was quenched by adding various concentrations of H₂SO₄.⁴ In order to further characterize the peroxidase-like activity of CoS, ABTS and OPD were also investigated as peroxidase substrates (Fig. S1†). ABTS and OPD changed from colorless to green and orange, respectively. ABTS and OPD also had absorption maxima at 414 nm and 450 nm, respectively. These results confirmed that CoS exhibited catalytic properties. In contrast with Co₃O₄, the catalytic activity of CoS has rarely been studied making, this discovery particularly interesting. Moreover, the reaction rate was dependent on the reaction time and amount of CoS (Fig. S2†), further confirming the peroxidase-like activity of CoS.

Fig. 5 (a) UV-vis spectra of TMB solution (a), H₂O₂ solution (b), TMB–H₂O₂ (c), TMB–H₂O₂–CoS (d) in pH 3.5 acetate buffer at 25 °C ([TMB]: 0.15 mM; [H₂O₂]: 20 mM; [CoS]: 0.5 mg mL⁻¹). Inset: photographs of the aqueous solution. (b) Effect of H₂SO₄ concentration on TMB oxidation.

It was important to determine whether this catalytic activity was caused by intact CoS or cobalt ions leaching into the acidic solution. To study this, CoS was incubated in the standard NaAc–HAc buffer (0.2 M, pH 3.5) for 10 min (the reaction time needed to reach maximum catalytic activity). Then, CoS was removed by centrifugal separation (14000 rpm, 2 min). The catalytic activity of CoS was compared with that in the leaching solution under the same conditions. As shown in Fig. 6a, CoS had good activity, but the leaching solution had no activity. This result indicated that the catalytic activity originated from intact CoS. In order to determine the CoS catalytic mechanism, tert-butyl alcohol was applied as a typical ˙OH radical capture reagent to the CoS/TMB/H₂O₂ reaction system (Fig. 6b). tert-Butyl alcohol can react rapidly with ˙OH and terminate radical chain reactions by generating inert intermediate radicals.⁴³ Experimental results revealed that the absorbance intensity gradually decreased as tert-butyl alcohol concentration increased from 0 to 400 mg mL⁻¹, which indicated that the peroxidase-like activity of CoS in the catalytic oxidation of TMB in the presence of H₂O₂ originated from H₂O₂ decomposition generating the ˙OH radicals.

Fig. 6 (a) Time dependent absorbance evolution at 652 nm for TMB oxidation system in the presence of CoS and leaching solution. (b) Effect of tert-butyl alcohol concentration on TMB oxidation.

Optimization of experimental conditions

The catalytic activity of CoS was similar to that of HRP, and was dependent on pH, temperature and H₂O₂ concentration. As shown in Fig. S3,† different pH values (2.5–6), the temperatures (20–60 °C), and H₂O₂ concentrations (0.1–200 mM) were investigated. These results were compared with those of HRP under the same condition. The optimum pH and temperature for CoS were 3.5 and 25 °C, respectively. And the best pH and temperature for HRP were 4 and 35 °C, respectively. These values for CoS and HRP were similar, therefore, pH 3.5 and 25 °C were selected as the optimal conditions for studying CoS catalytic activity. As shown in Fig. S3c,† CoS needed a higher concentration of H₂O₂ than HRP to reach maximum catalytic activity. Nevertheless, further increasing H₂O₂ concentration restricted the catalytic activities of both CoS and HRP. To accurately achieve maximum CoS activity, the reaction time and amount of CoS were also investigated. Relative activity gradually increased with reaction time, reaching a maximum value at 10 min, which was used in all subsequent tests (Fig. S3e†). The amount of CoS also affected catalytic activity. The relative activity rapidly increased, and then decreased, as CoS concentration was gradually increased (Fig. S3d†). A CoS concentration of 0.5 mg mL⁻¹ was chosen as the standard condition for subsequent analysis.

Kinetic analysis of CoS as peroxidase mimics

Under the optimal conditions, the mechanism of catalytic activity and the kinetic parameters of CoS were investigated using enzyme kinetics theory and methods with H₂O₂ and TMB as substrates. Within a certain range of substrate concentrations, typical Michaelis–Menten curves were obtained for CoS and HRP (Fig. 7). Kinetic parameters were acquired by changing the concentration of one substrate while the other remained the same. The Michaelis–Menten constant (K_m) and the Maximum initial velocity (V_max) were calculated according to the Lineweaver–Burk plot.⁴ The kinetic parameters of CoS and HRP are listed in Table S2.† The K_m value for CoS with TMB as the substrate was 0.41 mM, which was slightly lower than that of HRP. Moreover, the V_max value for CoS was higher than that of HRP. These results suggested that CoS had a higher catalytic activity in the reaction with TMB than HRP. The K_m value of CoS with H₂O₂ as a substrate was 7.15, which was higher than that of HRP. This result revealed that CoS had a lower affinity for H₂O₂ than HRP. This result agreed with the observation that a higher concentration of H₂O₂ was required to obtain maximum catalytic activity from CoS. The kinetic parameters of CoS also compared to other metal oxide and metal sulphide nanostructures (Table S2†). The kinetic parameters of CoS showed a very comparable affinity for TMB and H₂O₂.

Fig. 7 Steady-state kinetic analyses using Michaelis–Menten model and Lineweaver–Burk model (insets) for CoS by (a and c) varying the TMB concentration with a fixed H₂O₂ concentration and (b and d) varying the H₂O₂ concentration with a fixed TMB concentration.

Determination of H₂O₂ using CoS–TMB system

A simple and rapid colorimetric method for the determination of H₂O₂ was developed based on the catalytic activity of CoS (Fig. 9a). The dependency of CoS catalytic activity on H₂O₂ concentration was discussed above, suggesting that the CoS–TMB system could be applied to H₂O₂ determination. Under optimal conditions, the changes in absorbance curves at different H₂O₂ concentrations were investigated (Fig. 8a). The concentration of H₂O₂ ranged from 0.05 to 1.2 mM. The linearity ranged of 0.05 to 0.8 mM (R² = 0.9965) (Fig. 8b), and the limit of detection for H₂O₂ was estimated to be 0.02 mM using the formula, S/N = ((average_sample − average_blank)/SD_blank). Sample concentrations consistent with 3 < S/N < 5 condition were defined as the limits of detection.⁴⁴ The linearity and limits of detection of this colorimetric method and those of other nanomaterials-based colorimetric methods are listed in Table S3.†

Fig. 8 H₂O₂ detection based on CoS peroxidase-like activity. (a) Effect of H₂O₂ on absorption spectra in the CoS–TMB system. (b) Linear calibration plot for H₂O₂, where ΔA = A_{(H2O2,652 nm)} − A_{(blank,652 nm)}.

Mechanism and colorimetric determination of Hg²⁺ in aqueous solution

The production of TMB cation radical (TMB⁺) was detected in the three systems (CoS, CoS–GSH, and CoS–GSH–Hg²⁺) at 652 nm under optimal reaction conditions. As shown in Fig. S4,† the color changed from deep blue to a very light blue, while the absorption peak was decreased dramatically in the presence of GSH. However, the color returned to deep blue and the absorption peak reappeared, in the presence of Hg²⁺. Based on CoS catalytic activity, the reduction properties of GSH, and the strong affinity of Hg²⁺ for thiol compounds, a simple and novel “off–on” colorimetric method was developed for Hg²⁺ detection. The sensing method in this work is described in Fig. 9a. The sensor was in the “off” state, when GSH was added to the CoS system, because GSH could restore TMB⁺ to the original colorless TMB molecule by its thiol functionality. These results were similar to some reported reactions,^45,46 the reaction equations were shown in Fig. 9b. When GSH bonded to Hg²⁺, the sensor was in the “on” state due to the strong affinity between Hg²⁺ and GSH.⁴⁵ This phenomenon was believed to be due to the Hg²⁺–thiol complexes, which had high stability constants,^47,48 and the reaction equations were shown in Fig. 9c. The changes in absorbance curves were studied at various Hg²⁺ concentrations (Fig. 10). The linearity ranged from 0.25 to 3 μM and the limit of detection was estimated to be 0.1 μM based on the same formula as previously. The results of Hg²⁺ detection using this and previously reported nanomaterials-base methods are summarized in Table S4.† Compared to other nanomaterial-based colorimetric detection methods for Hg²⁺, the above colorimetric method showed a very comparable detection limit.

Fig. 9 (a) Schematic illustration of colorimetric sensing of H₂O₂ and Hg²⁺ using CoS catalyzed reactions, (b and c) the reaction equations illustration of colorimetric sensing process.

Fig. 10 Hg²⁺ determination using CoS as peroxidase mimics. (a) Effect of Hg²⁺ on absorption spectra in the CoS–GSH system. (b) Curve for Hg²⁺ determination in the range 0.25–3 μM, where ΔA = A_{(Hg²⁺,652 nm)} − A_{(blank,652 nm)}.

Selectivity of the Hg²⁺ colorimetric determination method

To test the selectivity of Hg²⁺ determination control experiments were performed under standard conditions using Na⁺, Cr³⁺, NH₄⁺, Zn²⁺, Mg²⁺, Ca²⁺, Ni²⁺, Fe³⁺, Cd²⁺, and Pb²⁺. These ions could potentially disturb Hg²⁺ detection. As shown in Fig. 11, there was no apparent signal interference under the same Hg²⁺ concentration (3 μM). Therefore, ion interference did not influence Hg²⁺ determination.

Fig. 11 Detected selectivities for Hg²⁺ determination with other ions at the same concentration based-on CoS peroxidase-like activity.

Determination of Hg²⁺ in real samples

To test the feasibility of the above colorimetric approach, two real water samples were tested under optimal experiment conditions. The resulting detection and recovery values are listed in Table 1, with Hg²⁺ recovery values at the three spiked levels ranged from 85.94 to 106.04%. The relative standard deviation (RSD) values of the measurements were obtained at each concentration level. These findings clearly indicated that this colorimetric sensor could be applied to detection in real samples.

Table 1 Results of recovery values from the analysis of Hg²⁺ in real water samples

Samples	Original amount (μM)	Added (μM)	Found (μM)	Recovery (%)	RSD (%, n = 3)
a N.D.: not detected.
Li River water	N.D.^a	0.5	0.4620	92.40	1.42
		1	1.0511	105.11	0.57
		2	2.1208	106.04	0.42
Tap water	N.D.^a	0.5	0.4274	85.94	2.39
		1	0.8734	88.34	2.68
		2	1.9648	98.24	0.78

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Reutilization assays

The reutilization of CoS was studied under the optimal conditions. The blue colored solutions were detected using a spectrophotometer at 652 nm after 10 min reacting with CoS. After each detection, CoS was separated by centrifugation (14 000 rpm, 2 min) and washed several times with water and ethanol, respectively. As shown in Fig. 12, there was not obvious decline in CoS catalytic activity after five cycles, indicating that CoS was highly catalytically stable and reproducible.

Fig. 12 Five CoS catalytic cycles.

Conclusions

In summary, CoS was prepared using a simple solvothermal method. The catalytic activity of CoS was investigated for the first time and similar to HRP, was shown to be dependent on pH, temperature, and H₂O₂ concentration. The catalysis was shown to coincide with typical Michaelis–Menten kinetics. CoS has several advantages, such as low cost, simple preparation, good stability, and unexceptionable reproducibility. CoS catalyzed the oxidation of TMB to generate a color-change reaction with H₂O₂. GSH and Hg²⁺ possessed a strong radical restoration ability and strong affinity for thiolated compounds, respectively. Based-on these findings, a facile and cheap method for H₂O₂ detection, and a novel “off–on” colorimetric sensor for the determination of Hg²⁺ were developed. The “off–on” colorimetric sensor was also used for the determination of real samples.

Acknowledgements

The financial support from the National Natural Science Foundation of China (21365005), Guangxi Natural Science Foundation of China (2014GXNSFGA118002) and Guangxi Pharmaceutical Industry Talent Highland Project (1414) is gratefully acknowledged.

Notes and references

1. W. Chen, J. Chen, Y.-B. Feng, L. Hong, Q.-Y. Chen, L.-F. Wu, X.-H. Lin and X.-H. Xia, Analyst, 2012, 137, 1706–1712.
2. X. Wang, Y. Hu and H. Wei, Inorg. Chem. Front., 2016, 3, 41–60.
3. L. Su, W. Qin, H. Zhang, Z. U. Rahman, C. Ren, S. Ma and X. Chen, Biosens. Bioelectron., 2015, 63, 384–391.
4. L. Z. Gao, J. Zhuang, L. Nie, J. B. Zhang, Y. Zhang, N. Gu, T. H. Wang, J. Feng, D. L. Yang, S. Perrett and X. Y. Yan, Nat. Nanotechnol., 2007, 2, 577–583.
5. L. Ai, L. Li, C. Zhang, J. Fu and J. Jiang, Chem.–Eur. J., 2013, 19, 15105–15108.
6. J.-W. Zhang, H.-T. Zhang, Z.-Y. Du, X. Wang, S.-H. Yu and H.-L. Jiang, Chem. Commun., 2014, 50, 1092–1094.
7. H. Tan, Q. Li, Z. Zhou, C. Ma, Y. Song, F. Xu and L. Wang, Anal. Chim. Acta, 2015, 856, 90–95.
8. A. Asati, S. Santra, C. Kaittanis, S. Nath and J. M. Perez, Angew. Chem., Int. Ed., 2009, 48, 2308–2312.
9. R. André, F. Natálio, M. Humanes, J. Leppin, K. Heinze, R. Wever, H.-C. Schröder, W. E. G. Müller and W. Tremel, Adv. Funct. Mater., 2011, 21, 501–509.
10. W. Chen, J. Chen, A.-L. Liu, L.-M. Wang, G.-W. Li and X.-H. Lin, ChemCatChem, 2011, 3, 1151–1154.
11. Y. Wan, P. Qi, D. Zhang, J. Wu and Y. Wang, Biosens. Bioelectron., 2012, 33, 69–74.
12. J. Yin, H. Cao and Y. Lu, J. Mater. Chem., 2012, 22, 527–534.
13. Y. J. Long, Y. F. Li, Y. Liu, J. J. Zheng, J. Tang and C. Z. Huang, Chem. Commun., 2011, 47, 11939–11941.
14. H. Jiang, Z. Chen, H. Cao and Y. Huang, Analyst, 2012, 137, 5560–5564.
15. L. Chen, N. Wang, X. Wang and S. Ai, Microchim. Acta, 2013, 180, 1517–1522.
16. J. Liu, X. Hu, S. Hou, T. Wen, W. Liu, X. Zhu, J.-J. Yin and X. Wu, Sens. Actuators, B, 2012, 166–167, 708–714.
17. Y. Song, K. Qu, C. Zhao, J. Ren and X. Qu, Adv. Mater., 2010, 22, 2206–2210.
18. Y. Song, X. Wang, C. Zhao, K. Qu, J. Ren and X. Qu, Chem.–Eur. J., 2010, 16, 3617–3621.
19. W. Shi, Q. Wang, Y. Long, Z. Cheng, S. Chen, H. Zheng and Y. Huang, Chem. Commun., 2011, 47, 6695–6697.
20. L. Su, J. Feng, X. Zhou, C. Ren, H. Li and X. Chen, Anal. Chem., 2012, 84, 5753–5758.
21. K. Zhang, W. Zuo, Z. Wang, J. Liu, T. Li, B. Wang and Z. Yang, RSC Adv., 2015, 5, 10632–10640.
22. T. Tian, L. Ai, X. Liu, L. Li, J. Li and J. Jiang, Ind. Eng. Chem. Res., 2015, 54, 1171–1178.
23. H. Wei and E. Wang, Chem. Soc. Rev., 2013, 42, 6060–6093.
24. Y. Gu, Y. Xu and Y. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 801–806.
25. Z. Dai, S. Liu, J. Bao and H. Ju, Chem.–Eur. J., 2009, 15, 4321–4326.
26. S. K. Maji, A. K. Dutta, S. Dutta, D. N. Srivastava, P. Paul, A. Mondal and B. Adhikary, Appl. Catal., B, 2012, 126, 265–274.
27. W. He, H. Jia, X. Li, Y. Lei, J. Li, H. Zhao, L. Mi, L. Zhang and Z. Zheng, Nanoscale, 2012, 4, 3501–3506.
28. D. He, D. Wu, J. Gao, X. Wu, X. Zeng and W. Ding, J. Power Sources, 2015, 294, 643–649.
29. J. Li, X. Zhou, Z. Xia, Z. Zhang, J. Li, Y. Ma and Y. Qu, J. Mater. Chem. A, 2015, 3, 13066–13071.
30. G. Wang and S. Zhuo, Phys. Chem. Chem. Phys., 2013, 15, 13801–13804.
31. Q. Wang, L. Jiao, H. Du, W. Peng, Y. Han, D. Song, Y. Si, Y. Wang and H. Yuan, J. Mater. Chem., 2011, 21, 327–329.
32. Y. Sun, C. Liu, D. C. Grauer, J. Yano, J. R. Long, P. Yang and C. J. Chang, J. Am. Chem. Soc., 2013, 135, 17699–17702.
33. S.-J. Bao, C. M. Li, C.-X. Guo and Y. Qiao, J. Power Sources, 2008, 180, 676–681.
34. P. Qu, Z. Gong, H. Cheng, W. Xiong, X. Wu, P. Pei, R. Zhao, Y. Zeng and Z. Zhu, RSC Adv., 2015, 5, 106661–106667.
35. S. Xiong, J. S. Chen, X. W. Lou and H. C. Zeng, Adv. Funct. Mater., 2012, 22, 861–871.
36. B. You, N. Jiang, M. Sheng and Y. Sun, Chem. Commun., 2015, 51, 4252–4255.
37. F. Bai, H. Huang, C. Hou and P. Zhang, New J. Chem., 2016, 40, 1679–1684.
38. G. Valiulienė, A. Žielienė and J. Vinkevičius, J. Solid State Electrochem., 2002, 6, 396–402.
39. C.-Y. Lin, D. Mersch, D. A. Jefferson and E. Reisner, Chem. Sci., 2014, 5, 4906–4913.
40. Y. Sun, L. Yang, M. Wei, G. Chen, G. Che, J. Yang, Z. Wang and Z. Gao, J. Mater. Sci.: Mater. Electron., 2016, 27, 1457–1462.
41. Y. Su, Y. Zhang, X. Zhuang, S. Li, D. Wu, F. Zhang and X. Feng, Carbon, 2013, 62, 296–301.
42. J.-Y. Lin and S.-W. Chou, RSC Adv., 2013, 3, 2043–2048.
43. G. V. Buxton, C. L. Greenstock, W. P. Heiman and A. B. Ross, J. Phys. Chem. Ref. Data, 1988, 17, 513–886.
44. L. Guo, Y. Xu, A. R. Ferhan, G. Chen and D.-H. Kim, J. Am. Chem. Soc., 2013, 135, 12338–12345.
45. Z. Mohammadpour, A. Safavi and M. Shamsipur, Chem. Eng. J., 2014, 255, 1–7.
46. X. Liu, Q. Wang, Y. Zhang, L. Zhang, Y. Su and Y. Lv, New J. Chem., 2013, 37, 2174–2178.
47. M. Ravichandran, Chemosphere, 2004, 55, 319–331.
48. H.-C. Chang, Y.-F. Chang, N.-C. Fan and J. A. Ho, ACS Appl. Mater. Interfaces, 2014, 6, 18824–18831.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16619a

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