Hydrogen-bonding-induced colorimetric detection of melamine based on the peroxidase activity of gelatin-coated cerium oxide nanospheres

Xiaoyong Jin , Wenqing Yin , Gang Ni and Juan Peng *
State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, P. R. China. E-mail: pengjuan@nxu.edu.cn; Tel: +86-0951-2062004

Received 26th September 2017 , Accepted 9th January 2018

First published on 10th January 2018

Herein, we developed a simple and rapid colorimetric assay for the detection of melamine using gelatin-coated cerium oxide (Gel-CeO2) nanospheres as peroxidase mimics. Highly monodisperse Gel-CeO2 nanospheres were synthesized through a microwave-assisted hydrothermal process. The Gel-CeO2 nanospheres showed excellent peroxidase activity, which can catalyze the oxidation of ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt) by H2O2, resulting in the formation of blue oxidation products. In the presence of melamine, H2O2 will react with melamine through strong hydrogen-bonding. With the consumption of H2O2, the catalytic reaction was interrupted and the blue ABTS oxidation product solution turned pale. There was a linear relationship between the absorbance intensity of the ABTS oxidation product and the logarithm values of melamine concentrations ranging from 50 nM to 5.0 mM. Moreover, the detection limit (S/N = 3) was 5.5 nM, which is far below the regulatory level. This method was simple, rapid, sensitive and reliable, suggesting the promising practical usage of this sensing system. Finally, this method was applied to melamine detection in milk and milk powder.

1. Introduction

Melamine has been widely used in resin manufacture and other industrial fields.1 Unfortunately, melamine might be illegally added into milk and various dairy products so as to increase an apparent protein content via an incorrect high readout of total nitrogen content (66% by mass).2 As we all know, melamine has slight acute toxicity. Thus, excessive intake of it above the safety limit (2.5 ppm in the United States and European Union and 1 ppm for infant formula in China) may result in the formation of insoluble compounds (finally kidney stones), renal failure, and even death.3–5 Consequently, the development of approaches for the detection of melamine in milk products and animal feeds has become very important for public health.

Currently, a number of detection methods including gas chromatography-mass spectrometry,6 liquid chromatography,7 enzyme-linked immunosorbent assay (ELISA),8 surface enhanced Raman spectroscopy,9 nuclear magnetic resonance (NMR) spectroscopy,10 and electro-chemiluminescence11 have been used for the detection of melamine. However, these detection techniques are generally time-consuming and also require relatively expensive instrumentation and complicated analytical procedures. Therefore, it is imminently necessary to establish a simple, reliable, and low-cost method for the rapid and sensitive detection of melamine in dairy products. For this purpose, colorimetric methods have attracted considerable attention since they do not require expensive or sophisticated instruments and can be applied to real-time and on-site analysis.12,13 In addition, colour changes during the whole process are visible to the naked eye.

At present, various biomolecule functionalized silver nanoparticles (AgNPs) or gold nanoparticles (AuNPs) are used for the colorimetric detection of melamine based on the melamine-induced aggregation of AgNPs14,15 and AuNPs.16,17 Unfortunately, they often suffer from relatively poor sensitivity, selectivity, and anti-interference ability towards some common substances in milk, which limits their practical applications to some extent. Enzyme mimic-based colorimetric assays have become an increasingly important focus for researchers. A variety of inorganic nanomaterials with peroxidase-like activity including Fe3O4 nanoparticles,18,19 Co3O4 nanoparticles,20 CuO nanoparticles or nanobelts,21,22 CuS nanorods,23 NiO,24 Au@Pt nanorods,25 MnO2 nanoparticles,26 V2O5 nanowires,27 and graphene oxide28 have been employed in colorimetric biosensors.

Nanostructured CeO2 has attracted more attention because of its higher catalytic reactivity, which can be attributed to its improved redox ability, better transport properties, and larger specific surface area.29 Nanosized CeO2 also shows intrinsic enzyme activity, which has been applied to the detection of H2O2 and glucose.30–32 A mild and facile method for the synthesis of CeO2 would be favorable to its extensive applications. Dextran- or polyacrylic acid-coated CeO2 with enhanced stability and excellent catalytic properties has been prepared for biochemical applications.33,34 In this paper, gelatin-coated CeO2 nanospheres (Gel-CeO2) were prepared via a microwave-assisted hydrothermal process. The Gel-CeO2 nanospheres exhibit an intrinsic peroxidase-like activity in aqueous solution, which can be applied in the fields of biotechnology, environmental, chemistry, and so on.

Herein, we developed a novel colorimetric assay for the detection of melamine using Gel-CeO2 as a peroxidase mimic (Scheme 1). Firstly, monodisperse Gel-CeO2 nanospheres were synthesized and used to catalyze the oxidation of ABTS by H2O2, which can result in the formation of blue oxidation products and then a colourless–blue colour change of the solution. Upon addition of melamine into the H2O2–ABTS–Gel-CeO2 system, it reacts with H2O2 through hydrogen-bonding interactions to form a complex,35,36 so the formation of blue ABTS oxidation products is inhibited and the blue solution turned pale (Scheme 1B). Thus, the melamine content was determined based on the linear relationship between the absorbance of the H2O2–ABTS–Gel-CeO2 system and the logarithm value of melamine concentration. The colour changes during the whole process are visible to the naked eye without any other sophisticated instruments. In addition, our approach has been successfully applied to determine melamine in milk products with high precision and satisfactory recovery.

image file: c7ay02296d-s1.tif
Scheme 1 Schematic illustration of the detection system.

2. Experimental section

2.1. Chemicals and materials

Cerium nitrate hexahydrate (Ce(NO3)3·6H2O), gelatin, ethylene glycol, sodium acetate (NaAc), acetic acid (HAc), hydrogen peroxide (H2O2, 30%), and melamine were purchased from Beijing Chemical Company (Beijing, China). 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) was purchased from Sigma-Aldrich (USA). All these chemicals were of analytical reagent grade and used without further purification. Deionized water was used throughout the experiment.

2.2. Synthesis of Gel-CeO2 nanospheres

The Gel-CeO2 nanospheres were synthesized via a microwave-assisted hydrothermal method. Typically, 1.0 g Ce(NO3)3·6H2O was dissolved in 1 mL of deionized water. Then, 0.1 g gelatin and 30 mL of glycol were added into the solution, which was then stirred to form a uniform solution. The mixed solution was sealed and heated at 180 °C for 30 min and naturally cooled to room temperature. The resulting suspension was centrifuged for 10 min at 12[thin space (1/6-em)]000 rpm, and the pellet was washed with water three times and dried at 60 °C for 12 h. Finally, the Gel-CeO2 nanospheres were obtained.

2.3. Characterization

Absorbance spectra of Gel-CeO2 were acquired on a Cary-5000 UV-vis spectrophotometer (Varian Associates, Inc., America) using a cell with a 4 cm path length. The morphology of Gel-CeO2 was analysed on a scanning electron microscope (SEM) (JSM-6700F, JEOL, Japan) and on a transmission electron microscope (JEOL JEM-2010). Fourier transform infrared (FTIR) spectra were recorded on a Thermo Nicolet 6700 FT-IR system. The X-ray powder diffraction (XRD) pattern was obtained using a D/Max-γA rotating-anode X-ray diffractometer (Rigaku Co., Ltd., Japan). The X-ray photoelectron spectroscopy (XPS) study was performed on an ESCALAB 250 spectrometer (VG Scientific Ltd., England).

2.4. Kinetic measurements

The steady-state kinetic measurements were carried out by monitoring the absorbance at various times with a microplate reader. Catalytic experiments were performed using 5.0 mg mL−1 Gel-CeO2 in acetate buffer solution (pH 4.0) containing H2O2 and ABTS in the absence or presence of melamine. The Michaelis–Menten constants were calculated according to the Michaelis–Menten equation, v = vmax[S]/([S] + Km), where v is the reaction velocity, vmax is the maximal reaction velocity, [S] is the substrate concentration (H2O2 or ABTS) and Km is the Michaelis–Menten constant.

2.5. Application of Gel-CeO2 nanospheres to melamine detection

The pH of the detection system was optimized as follows: 40 μL of 10 mM ABTS, 20 μL of Gel-CeO2 aqueous solution (5 mg mL−1), and 50 μL of H2O2 solution (1.0 mM) were mixed in 850 μL of buffer solution (acetate buffer or PBS) with different pH. Second, the mixture was incubated at 45 °C for 20 min and kept in an ice-water bath for 10 min to stop the reaction completely. Third, 200 μL resulting solution was added into 2.8 mL water and the solution mixture was used for absorbance measurements at a wavelength of 420 nm. The concentration of Gel-CeO2 was optimized with the same procedure just using the acetate buffer (pH 4.0).

The detection of melamine was carried out using the Gel-CeO2–H2O2–ABTS system as follows: 40 μL of 10 mM ABTS, 20 μL of Gel-CeO2 aqueous solution (5 mg mL−1), and 50 μL of H2O2 solution (1.0 mM) were mixed in 850 μL of 0.2 M acetate buffer (pH 4.0). Second, the mixture was incubated with various concentrations of melamine at 45 °C for 20 min and kept in an ice-water bath for 10 min to stop the reaction completely. Third, 200 μL resulting solution was added into 2.8 mL water and the solution mixture was used for absorbance measurements at a wavelength of 420 nm.

2.6. Detection of melamine in real samples

The raw milk and milk powder were purchased from supermarkets and preferably detected on the same day. The milk and milk powder were pre-treated using the following procedure before the melamine detection step. Firstly, 5.0 mL raw milk or 5.0 mg milk powder was put into a 10 mL centrifuge tube, and 1.5 mL trichloroacetic acid (2.0 M) was added to the sample. After sonicating the sample for 10 min, the solution was centrifuged at 10[thin space (1/6-em)]000 rpm for 10 min. Then, the pH of the supernatant was adjusted to 7.0 with NaOH. After that, the supernatant was filtered through a 0.22 μm filter and the filtrate was diluted 25-fold before being used for the detection.

The melamine detection in pre-treated milk or milk powder samples was done as follows: 40 μL of 10 mM ABTS, 20 μL Gel-CeO2 aqueous solution (5 mg mL−1), and 50 μL H2O2 aqueous solution (1.0 mM) were mixed in 850 μL acetate buffer (0.2 M, pH 4.0). Then, the pre-treated milk or milk samples spiked with 6 μM melamine were incubated with the above mixture at 45 °C for 20 min and then kept in an ice-water bath for 10 min to stop the reaction completely. Finally, 200 μL resulting solution was added into 2.8 mL water and the solution mixture was used for absorbance measurements at a wavelength of 420 nm.

3. Results and discussion

3.1. Characterization of Gel-CeO2 nanospheres

Monodisperse Gel-CeO2 nanospheres were synthesized via a facile in situ coating method. The SEM image of the Gel-CeO2 products showed that they had spherical shapes with narrow size distribution and an average size of about 200 nm (Fig. 1A and C). TEM images confirmed the spherical shapes of the Gel-CeO2 products. The nanospheres were composed of very tiny nanoparticles, indicating the polycrystalline nature of ceria spheres (top-right inset in Fig. 1B). The X-ray diffraction (XRD) patterns of the products (Fig. 1D) clearly showed that all the peaks (111, 200, 220, 311, 400, and 331) can be indexed as arising from the pure phase of face-centered-cubic CeO2 nanostructures (JCPDS 65-2075).37 The FTIR spectra (Fig. 1E) showed the presence of the characteristic IR bands of gelatin, confirming the gelatin-coated surface of CeO2. As shown in Fig. 1F, the Gel-CeO2 nanospheres showed obvious absorbance around 300 nm, suggesting that the particles were nano-sized. The top-right inset in Fig. 1F was the photograph of Gel-CeO2 particles dispersed in water, and no aggregation was observed, indicating the good dispersibility of Gel-CeO2 particles in water. The XPS survey in Fig. 2A spectra of Gel-CeO2 indicated the presence of Ce, O, C, and N elements. The XPS spectrum of cerium (Fig. 2B) was complicated and split into Ce 3d3/2 and Ce 3d5/2 with multiple shake-up and shake-down satellites. The peaks at 882.4 and 885.8 eV belonged to Ce 3d3/2. These results were consistent with previously published results of CeO2 nanoparticles.38
image file: c7ay02296d-f1.tif
Fig. 1 (A) SEM and (B) TEM of Gel-CeO2 nanospheres (top-right inset: HRTEM of Gel-CeO2), (C) size distribution of Gel-CeO2 nanospheres, (D) XRD of Gel-CeO2 nanospheres, (E) FTIR spectra of gelatin (a) and Gel-CeO2 (b), (F) UV-vis spectra of Gel-CeO2 spheres (top-right inset: the photograph of Gel-CeO2 spheres dispersed in water).

image file: c7ay02296d-f2.tif
Fig. 2 (A) XPS survey spectra of Gel-CeO2 spheres and (B) XPS spectrum of Ce 3d.

3.2. Peroxidase-like activity of Gel-CeO2 nanospheres

Using ABTS as the substrate in the presence of H2O2, we investigated the peroxidase-like activity of the Gel-CeO2 nanospheres. In general, ABTS can be oxidized by H2O2 to produce a blue product by peroxidase such as horseradish peroxidase (HRP). In this experiment, the ABTS was completely oxidized by H2O2 with Gel-CeO2 nanospheres as the catalysts similar to HRP. As shown in Fig. 3A, in the absence of Gel-CeO2 (a), the initially colourless ABTS solution with H2O2 merely turned pale blue; however, in the presence of Gel-CeO2 (b) the initially colourless ABTS solution with H2O2 turned blue. These phenomena indicated that the Gel-CeO2 exhibited peroxidase-like activity. The above results were also confirmed using a spectrophotometer, which was used to monitor the absorbance of the solution mixture dynamically. The dynamic absorbance monitoring showed that the solution mixture had maximum absorbance around 420 nm (Fig. 3A). The absorbance at 420 nm increased with the extension of the reaction time (Fig. 3B), but there was no obvious change in it after 20 min, suggesting that the oxidation of ABTS by H2O2 under Gel-CeO2 catalysis reached equilibrium.
image file: c7ay02296d-f3.tif
Fig. 3 (A) Wavelength-dependent absorbance of (a) ABTS + H2O2, (b) ABTS + H2O2 + Gel-CeO2 nanospheres; (top-right inset: the photograph of two reaction system (a) ABTS + H2O2, (b) ABTS + H2O2 + Gel-CeO2 nanospheres), and (B) plots of the absorbance at 420 nm against the reaction time.

3.3. Mechanism of the detection system

Interestingly, it was found that melamine could inhibit the catalysis reaction of the system. To study the inhibition mechanism of the detection system, the steady-state kinetic curves of the ABTS–H2O2–Gel-CeO2 system in the absence and presence of melamine were obtained as shown in Fig. S1 and S2 for ABTS and H2O2, respectively. The Michaelis–Menten constant (Km) was calculated from the Michaelis–Menten equation fitting results. The Km values are directly related to the affinity of an enzyme towards the substrates, and smaller Km values indicated a higher affinity between the enzyme and the substrates and vice versa. The Km value of the ABTS–H2O2–Gel-CeO2 system is 0.21 and 0.40 for ABTS and H2O2, respectively, as shown in Table 1. With addition of melamine into the system, the Km value is 0.28 and 0.86, which is bigger than that of the ABTS–H2O2–Gel-CeO2 system without melamine. This result suggests that the melamine could inhibit the catalysis reaction in this system. It is reported that hydrogen peroxide can react with melamine to produce a stable addition compound.36 With the consumption of hydrogen peroxide, the catalytic reaction will be slowed down, resulting in the blue colour changing.
Table 1 Comparison of the kinetic parameters of the system in the absence and presence of melamine
Catalytic system K m V max
ABTS–H2O2–Gel-CeO2 0.21 0.40 2.10 8.03
ABTS–H2O2–Gel-CeO2 with melamine 0.28 0.86 1.45 7.09

3.4. Analytical performance of the detection method for melamine

Conditions including the pH of the reaction buffer and the concentration of CeO2 were investigated in detail (Fig. S3). The buffer pH 4.0 and the concentration of Gel-CeO2 (5.0 mg mL−1) were chosen as the optimal conditions for the following melamine detection experiments. The colorimetric method based on what mentioned above was used for the visual detection of melamine under the optimum conditions. As shown in Fig. 4A, the colour changed from dark blue to pale with the addition of melamine into the solutions. The absorbance intensity of the ABTS–H2O2–Gel-CeO2 system decreased gradually with increase in the concentration of melamine (Fig. 4B). The calibration curve in Fig. 4C indicated a good linear relationship between the absorbance and the concentration of melamine in the range from 5.0 × 10−8 to 5.0 × 10−3 M with a detection limit of 5.5 × 10−9 M. The linear regression equation was y = 0.7082 − 0.068x with a regression coefficient of 0.9950. These results indicated that the colorimetric method developed in this paper was able to sensitively detect melamine over a relatively wide concentration range with a low detection limit.
image file: c7ay02296d-f4.tif
Fig. 4 (A) Photograph of the H2O2–ABTS–Gel-CeO2 system as affected by melamine concentration (the concentration of H2O2 is 0.1 mM), (B) UV-vis spectra of the H2O2–ABTS–Gel-CeO2 system in the presence of various concentrations of melamine. The concentrations of melamine (a–f) were 0.05, 0.5, 5.0, 50.0, 500, and 5000 μM, respectively. (C) Calibration curve for the colorimetric detection of melamine.

Other recent studies on melamine detection using different nanoparticles including functional AgNPs14,15,42 and AuNPs16,17,39–41,43 are summarized in Table 2. The results show that our method enabled a lower detection limit than most of the recent colorimetric methods.14–17,40,42 Although lower detection limits were reported in the related studies,39,41,43 our method has a wider linear range of melamine concentrations than them. The wider linear range or lower detection limit could be attributed to the excellent catalytic properties of Gel-CeO2 particles toward the oxidation of the ABTS substrate.

Table 2 Comparison between our method and other colorimetric methods for the determination of melamine
Materials used Linear range (μM) Detection limit (μM) Reference
β-Cyclodextrin–AgNPs 0.5–1000 4.96 14
Sulfanilic acid–AgNPs 0.1–3.1 0.0106 15
Methanobactin–AuNPs 0.39–3.97 0.238 16
Citrate–AuNPs 0.396–15.8 0.396 17
AuNPs functionalized with p-chlorobenzenesulfonic acid 0.6–1.5 0.0023 39
Mb-mediated AuNP formation 0.39–3.97 0.238 40
Triton X-100 modified AuNPs 0.75–1.75 0.0051 41
Tannic acid–AgNPs 0.05–1.4 0.01 42
Cysteamine-stabilized AuNPs 0.001–0.024 0.000389 43
Gel-CeO2 nanospheres 0.05–5000 0.0055 This work

Selectivity, reproducibility, and stability of the colorimetric method. To evaluate the selectivity of our assay method, the interference of substances including some metal ions, nitrogenous compounds and proteins that might be present in real samples was investigated (Fig. S4). The results showed that even 1000 times more K+, Mg2+, and Ca2+, 50 times more casein, glycine, glucose and tyrosine, 10 times more lysine, histidine, ascorbic acid and dopamine, and an equal amount of BSA almost had no obvious influence on the absorbance intensity and cannot induce a colour change of the detection system in the absence of melamine, indicating a satisfactory selectivity of our method.

In addition, the method for the determination of melamine showed good reproducibility. The absorbance value of the ABTS–H2O2–Gel-CeO2 system was examined in the presence of 1.0 μM melamine for five replicate assays. The relative standard deviation was found to be 4.2%, suggesting the good reproducibility of the method.

The stability of the Gel-CeO2 nanospheres in a wide temperature range is crucial for extending their applications. As inorganic materials, Gel-CeO2 nanospheres were expected to be more stable than natural enzymes. For HRP, after treatment at temperatures greater than 40 °C for 2 h, the enzyme activity dramatically declined. However, Gel-CeO2 nanospheres were stable when they were incubated in a wide range of temperatures (20–90 °C) for 2 h. The stability of Gel-CeO2 nanospheres makes them suitable for a broad range of applications in the biomedicine and environmental chemistry fields.

Analysis of melamine in real samples. To verify the practical application of the assay method in real samples, we prepared raw milk and milk powder samples spiked with certain amounts of melamine. The practical samples were pre-treated according to a general procedure (see details in the Experimental section). The results are shown in Table 3. The recoveries of melamine added were in the range from 98.6 to 104.0%. Therefore, this assay method can be applied for melamine detection in real samples.
Table 3 Results of the determination of the melamine in raw milk and milk powder (n = 3)
Samples Added (μM) Found (μM) Recovery (%)
Raw milk 1 2.00 2.08 104.0
Raw milk 2 4.00 3.96 99.0
Raw milk 3 6.00 6.12 102.0
Milk powder 4 2.00 2.05 102.5
Milk powder 5 4.00 4.08 102
Milk powder 6 6.00 5.92 98.6

4. Conclusions

In summary, we developed an ABTS–H2O2–Gel-CeO2 system for colorimetric detection of melamine in dairy products. The results indicated that it was accurate, reliable, sensitive, and convenient. Using this system, dairy products containing melamine above the safety limits could be easily detected visually. The whole process was achieved in less than 1 h from the beginning of sampling to the final step of obtaining the results from the spectrophotometer. The convenient and reliable colorimetric detection of melamine content in milk and milk powder sample enables its practical application to real dairy products.

Conflicts of interest

There are no conflicts of interest to declare.


We greatly appreciate the support of the National Natural Science Foundation of China (No. 21645002, 21365016, and 21567021). This work is funded by the National First-rate Discipline Construction Project of Ningxia (NXYLXL2017A04) and Major Innovation Projects for Building First-class Universities in China's Western Region (ZKZD2017003).

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ay02296d

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