Large scale synthesis of carbon nanospheres and their application as electrode materials for heavy metal ions detection

Abstract

Large-scale and monodisperse colloidal carbon nanospheres (CNSs) were synthesized with the assistance of polyoxometalates (POMs) under hydrothermal conditions. The POMs could act not only as a catalyst to promote the glucose dehydration process, but also as a stabilizer to prevent the aggregation of CNSs. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FT-IR). The diameters of CNSs from 100 nm to 900 nm can be controlled by different acidic and oxidizing properties of POMs. This method was demonstrated to be simple, reliable and reproducible. The conversion yield of CNSs prepared with the addition of phosphomolybdic acid was up to 37%. Based on the experiment, the possible mechanism for CNSs formation was proposed. After modification of CNSs with cysteine, the samples were utilized as electrode materials for sensitive heavy metal ions detection. The detection limits for lead(II) and cadmium(II) were 1 × 10⁻⁸ M and 2 × 10⁻⁷ M, respectively. The results highlight the potential application of CNSs as electrode materials for biosensors and catalysis.

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Introduction

Over the past few decades, carbon nanomaterials with tunable size and shape have been paid tremendous attention owing to their unique properties and potential applications in physics, chemistry and electronic fields.^1–5 Among them, carbon nanospheres (CNSs) have attracted considerable attention due to their excellent properties and potential applications in the fields of gas storage,^6–8 sensing,^9,10catalyst supports,^11,12lithium-ion batteries^13–15 and so on. Numerous methods have been employed for the synthesis of CNSs, such as chemical vapor deposition (CVD),^16–19pyrolysis of carbohydrate and polymers,^20–23 hydrothermal and solvothermal treatment,^24–26 ultrasonic treatment^27,28 and arc-discharge process.²⁹ However, most of these methods require complicated and expensive equipments. Some carbon precursors are toxic, expensive and non-renewable. And it is difficult to obtain the CNSs with the desired sizes. Therefore, the synthesis of CNSs with desired diameters and shapes seems to be very important and urgent for the development of micro-/nano-physics and chemistry.

In concern of the environment, economy and energy, biomass resources are now becoming important as alternative resources.^30–32Glucose is an important source of physiological energy. It can be obtained from decomposition of various renewable biomass resources, such as polysaccharose, starch and cellulose. It is also cheap, easily available experimental material. Considering all of the above advantages, the preparation of CNSs from glucose is very promising.³³ However, the CNSs formed by glucose were prone to merge together under hydrothermal conditions, and the productivity was low. These problems make it difficult to meet the demand of large scale CNSs with high quality in potential applications.

Polyoxometalates (POMs) are a valuable class of inorganic compounds,^34,35 and have been extensively investigated due to their tunability of size, charge of a polyoxometallic cage, acidic and oxidizing properties.^36,37 In the area of catalysis, they were used as both Brønsted acid catalysts and oxidation catalysts since the late 1970s. For example, Keggin-type POMs have been widely used as homogeneous and heterogeneous catalysts for the oxidation of alcohols, aldehydes, isobutyric acid, or alkanes.^38,39

In light of the merit of previous works, we believe POMs may exhibit excellent catalytic abilities for synthesis of high quality CNSs. In this work, we demonstrate the large scale synthesis (the yield of CNSs was up to 37%) of well-defined CNSs with the assistance of POMs. Three kinds of Keggin-type POMs were employed to catalyze the synthetic process, to improve the productivity of CNSs and control the diameter of CNSs from 100 to 900 nm. This method was proved to be simple, reliable and reproducible. The CNSs (100 nm in diameter) were modified with cysteine (Cys-CNSs), and utilized as electrode materials for detection of heavy metal ions (Pb²⁺, Cd²⁺), which were widely distributed in water, plants, air, and have been the fatal threat to the environment and human health due to their accumulative characters. The cysteine can especially bind heavy metals and further enhance the detection sensitivity. The result indicated that the Cys-CNSs were sensitive electrode materials.

Experimental section

Synthesis of CNSs

All reagents were of analytical grade. The preparation of CNSs was quite straightforward. In a typical procedure, 2.252 g of C₆H₁₂O₆ was dissolved in 25 mL distilled water to form 0.5 M glucose solution. Then 0.2 g of phosphotungstic acid (H₃PW₁₂O₄₀·xH₂O) was added into the glucose solution, and stirred with a magnetic blender for 5 min. The mixture was transferred to a 30 mL Teflon-sealed autoclave, and maintained at 160 °C for 8 h. After reaction, the products were collected from the autoclave, washed with absolute ethanol and distilled water consecutively, and finally dried under vacuum at 60 °C for 5 h. The conversion yields of CNSs were calculated, the equation is shown as follows:

Modification of CNSs with cysteine

The modification of CNSs with cysteine can be found elsewhere.^40,41 In a typical procedure, 5 mg of CNSs was suspended in a solution containing 0.1 M 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and 0.1 M N-hydroxysuccinimide (NHS) in 0.1 M phosphate buffer (pH 5.5)⁴² with constant agitation for 1 h. Then the products were centrifuged, washed with deionized water. The samples were resuspended into a solution containing 0.1 M cysteine in phosphate buffer overnight with constant stirring. Finally, the CNSs were collected by centrifugation and washed with deionized water. The cysteine-modified CNSs are referred to as “Cys-CNSs” in this document.

Electrode preparation

A glassy carbon electrode (GCE) was polished carefully with smaller sizes of alumina slurries (1.0 and 0.05 μm), and sonicated in 1 : 1 nitric acid, acetone, and pure water after each stage of polishing, successively. A 0.5 mg mL⁻¹ suspension of Cys-CNSs in water was sonicated for 15 minutes. 5 μL of Cys-CNSs suspension was cast on the GCE surface and left to dry under ambient conditions. And then a drop of the Nafion solution (5 μL, 0.5 wt%) was placed on the electrode surface and the solvents were left to evaporate at room temperature.

Characterization

Scanning electron microscopy (SEM) images were taken on a FEI-quanta 200F scanning electron microscope with acceleration voltage of 20 kV. Transmission electron micrographs (TEM) were taken on a FEI-Tecnai F20 (200 kV) transmission electron microscope (FEI). Fourier transform infrared spectroscopy (FT-IR) was investigated by Hyperion 3000 (Bruker). The particle size distribution of as-synthesized CNSs was performed on Malvern Zen 3690. All electrochemical measurements were performed using a CHI 750B (Shanghai CH Instruments, China) Electrochemical Workstation.

Results and discussion

Characterization of CNSs

In this paper, the CNSs were prepared from glucose with the assistance of POMs at 160 °C. The temperature is higher than the normal glycosidation temperature and would lead to polymerization and carbonization.^43,44 Typical SEM and TEM images of CNSs prepared with the addition of different POMs are shown in Fig. 1, exhibiting the perfectly spherical morphology and monodisperse diameter. Detailed TEM images, insets in Fig. 1a and b, clearly show that the CNSs prepared with the addition of phosphomolybdic acid (H₃PMo₁₂O₄₀·xH₂O) and phosphotungstic acid (H₃PW₁₂O₄₀·xH₂O) have an average diameter of 100 and 230 nm, respectively. The particle size distribution of these samples is shown in Fig. S1 (ESI†), and the Brunauer–Emmett–Teller adsorption isotherms of these samples are shown in Fig. S2 (ESI†). Interestingly, we found that the diameter of CNSs increased to 800–900 nm when using silicotungstic acid (H₄SiW₁₂O₄₀·xH₂O) as the additive, shown in Fig. 1c. That is to say, the diameter of CNSs could be controlled by different kinds of POMs. This phenomenon may be attributed to different acidic and oxidizing properties of POMs.

Fig. 1 SEM images of the obtained CNSs, 0.5 M glucose solution with the addition of (a) H₃PMo₁₂O₄₀·xH₂O, (b) H₃PW₁₂O₄₀·xH₂O acid, (c) H₄SiW₁₂O₄₀·xH₂O. The scale bars of TEM images inset in (a) and (b) are 100 nm and 200 nm, respectively.

A series of experiments were carried out to investigate the influence of POMs, concentration of glucose and reaction time. The SEM images of CNSs obtained in the absence of POMs while keeping other conditions unchanged are shown in Fig. 2a and b. It could be observed that these as-prepared CNSs were small in size, but they were prone to adhere together, forming chain structures, and at the same time, the CNSs transformed into irregular vesicles in some cases (Fig. 2b). This phenomenon could be ascribed to the formation mechanism of CNSs, which was previously reported.^33,44 As glucose aqueous solution (without additive POMs) under hydrothermal conditions, they were dehydrated and polymerized together, forming a kind of amphiphilic compounds. With the polymerization of glucose molecules, these amphiphilic compounds would form micelles, with the hydrophobic groups making up the core of the micelles and the hydrophilic hydroxyls occupying the surface. This is followed by the growth of the micelle particles through the combination of the surface hydroxyls with the nearest free molecules until the glucose molecules are exhausted. As the shape of micelles may vary under different reaction conditions, the CNSs obtained in this way would exhibit different shapes. The result indicated that the usage of POMs is critical to obtain CNSs with perfect spherical morphology and monodisperse diameter.

Fig. 2 SEM images of CNSs prepared at 160 °C, (a,b) with 0.5 M glucose but without POMs, (c) with 1 M glucose, 0.2 g H₃PW₁₂O₄₀·xH₂O, and (d) with 0.5 M glucose, 0.3 g H₃PW₁₂O₄₀·xH₂O.

Upon increasing the concentration of glucose from 0.5 M to 1 M while keeping other conditions unchanged (e.g., 160 °C, 0.2 g H₃PW₁₂O₄₀·xH₂O), the size distribution of CNSs was getting worse, shown in Fig. 2c. It could be observed that although the CNSs still maintained the spherical morphology, the diameters of CNSs ranged from 400 nm to 4 μm. In comparison, as the amount of H₃PW₁₂O₄₀·xH₂O increased from 0.2 to 0.3 g, the similar result was observed, as shown in Fig. 2d. The unexpected phenomenon may be ascribed to the change of pH value and high concentration of POM anions in the mixed solution, which accelerated the CNSs formation process. From the above experimental result, it is obvious that the proper control on the concentration of glucose and POMs (e.g., 0.5 M, 0.2 g) is very essential for the preparation of monodisperse CNSs.

The XRD pattern of the as-prepared CNSs is shown in Fig. 3. In the XRD pattern, only a wide peak at 2θ ≈ 20° was observed, indicating the highly amorphous nature of the CNSs. No diffraction peaks belonging to impurity were observed, indicating the high purity of the CNSs.

Fig. 3 XRD pattern of CNSs.

The FT-IR spectrum of CNSs is shown in Fig. 4. The peaks at 3423 and 2972 cm⁻¹ are characteristic peaks of O–H and C–H bonds, and the bands at 1703 and 1625 cm⁻¹ are attributed to C=O and C=C vibrations, respectively. The bands at 1000–1300 cm⁻¹ are due to the existence of the C–C stretching, C–OH stretching and −OH bending vibration. These residual hydroxyl groups (−OH), aldehyde groups (−CHO) and carboxyl groups (−COOH) were covalently bonded to the carbon frameworks, improving the hydrophilicity and stability of CNSs in aqueous systems, and greatly widen their potential applications in diagnostics, drug delivery and as templates for hybrid core/shell structures.^45–48

Fig. 4 FT-IR spectrum of CNSs.

Formation mechanism

According to the literature,^33,44,49,50 the transformation of glucose molecules into CNSs would involve several processes, such as dehydration, polymerization, and carbonization. First of all, the intramolecular dehydration would occur easily under hydrothermal conditions, since glucose is a polyhydroxy compound. The major product of glucose dehydration was hydroxymethylfurfural (HMF), which was composed of hydroxyl, aldehyde group and furan ring.^51–53 There were other byproducts such as levulinic acid, dihydroxyacetone, and formic acid,^54,55 which were formed by decomposition of glucose and HMF. Also, there were a few amphiphilic compounds formed by intermolecular dehydration of glucose.

Then, the hydration of the aldehyde group of HMF may result in the further elimination of formic acid,⁵⁴ and the consequent protonation of the α-position C atom with another furan ring may occur. The elimination of the hydroxyl group on HMF and followed by nucleophilic addition would generate CH₂ groups, which act as linkers between furan rings.⁵⁶ The formation of specific CH₃ and COOH end groups was previously reported.⁵⁶ These reactions of HMF that occurred at high temperature would form carbonaceous scaffolds,⁴⁹ which were composed of the mutually connected polyfuran network and cross-linked by aliphatic carbons. It is possible that the initial stages of nucleation occurred as the concentration of carbonaceous scaffolds reached a critical micelle concentration.^44,57 The “core” is decorated with aliphatic chains and carbonyl groups on the surface.

At last, the growth of the “core” is through the combination of the surface hydroxyls, aldehydes and carboxyl groups with the nearest free molecules until the molecules are exhausted.^33,44 Finally, the monodisperse CNSs were obtained.

Based on the previous reports,^36–38 the effect of POMs during the CNSs formation could be attributed to the following: (1) they could catalyze/promote the reaction process; (2) they could effectively control the diameters of CNSs, the possible mechanism is shown in Scheme 1.

Scheme 1 The possible mechanism for the formation of CNSs prepared with the addition of POMs.

In order to understand the process vividly, the appearance of resulting solutions obtained at different reaction times is shown in Fig. 5. It is obvious that colors of solution turned from colorless, orange, red to brown and reddish-black gradually. The solutions obtained at 1 h, 2 h were colorless and orange, and no precipitates were observed after centrifugation, shown in Fig. 5. The result was in accordance with our proposition: the additive POMs could accelerate the dehydration of glucose and catalyze the polymerization of HMF.⁵⁸ The orange solution obtained at 2 h indicated the initial stage of the cross-link process of HMF, since glucose, HMF levulinic acid, and formic acid were colorless in solution. The dehydration and polymerization processes occurred continuously and upon reaching a critical point, a short single burst of nucleation resulted; the nucleation occurred between 2 h and 4 h. As a result, some precipitates were observed after centrifugation, the conversion yield was calculated and shown in Fig. 5. As there was an abundance of hydroxyl, aldehyde and carboxyl groups on the surface of particles, the cross-link process between particle to particle would occur easily, which leads to the aggregation of CNSs, as shown in Fig. 2a. This situation will be changed with the addition of POMs, which could oxidize the hydroxyl and aldehyde groups to carboxyl groups. The high ratio of carboxyl groups on the surface of particles results in separation of each particle, owing to the negative charge of carboxyl groups. And this result was in accordance with the diameters of CNSs that could be tuned by POMs with different oxidizing properties. The particles accumulated molecules from solution and grew up to CNSs eventually. It is worth noting that the conversion yield of CNSs prepared with the addition of phosphomolybdic acid was up to 37% (shown in Fig. 5, curve a), significantly higher than that of H₃PW₁₂O₄₀·xH₂O (curve b) and silicotungstic acid (curve c). It means that the CNSs with a diameter of 100 nm could be obtained in a large scale, and have potential for further industrial applications.

Fig. 5 The conversion yields of CNSs with (a) H₃PMo₁₂O₄₀·xH₂O, (b) H₃PW₁₂O₄₀·xH₂O, (c) H₄SiW₁₂O₄₀·xH₂O, and appearance of resulting solution with time (inset).

Electrochemical characteristics of the electrode surface

As cysteine, a nonessential amino acid, has a very high binding constant for some toxic heavy metal ions, the modification of CNSs with cysteine could enhance the detection of heavy metal ions.^40,41 The FT-IR spectrum of Cys-CNSs is shown in Fig. 6. The characteristic C–N stretching of secondary aromatic amine appears at 1384 cm⁻¹, which is attributed to the unique amidation reaction. The FT-IR spectrum provided evidence that cysteine was covalently bonded to the CNSs. With the help of modified cysteine, the Cys-CNSs may be especially modified with heavy metals which could further enhance the detection sensitivity.

Fig. 6 FT-IR spectrum of Cys-CNSs.

A Cys-CNSs modified glassy carbon electrode (GCE) was investigated viaSWASV methods. Calibration plots for determination of Pb²⁺ and Cd²⁺ on the Cys–CNSs were achieved in 0.1 M acetate buffer (pH 4.5) under the same conditions. SWASV for different concentrations of Pb²⁺ and Cd²⁺ are shown in Fig. 7a and c, respectively. The calibration curves were linear over the range 1 × 10⁻⁸ to 1 × 10⁻⁶ M for Pb²⁺ and 2 × 10⁻⁷ M to 2 × 10⁻⁶ M for Cd²⁺ (Fig. 7b and d).

Fig. 7 SWASV analysis of Pb²⁺ and Cd²⁺ with various concentrations. (a) SWASV of Pb²⁺ accumulated on the Cys–CNSs electrode, (b) calibration plot of anodic current versus different concentrations, 1 × 10⁻⁸–1 × 10⁻⁶ M Pb²⁺, (c) SWASV of Cd²⁺ accumulated on the Cys-CNSs electrode, (d) calibration plot of anodic current versus different concentrations, 2 × 10⁻⁷–2 × 10⁻⁶ M Cd²⁺.

The calibration curves were Pb²⁺ with y = 3.202 + 1.45x (R = 0.9858, N = 7) and Cd²⁺ with y = −0.34347 + 1.68672x (R = 0.99164, N = 5). The detection limits were 1 × 10⁻⁸ M and 2 × 10⁻⁷ M for Pb²⁺ and Cd²⁺, respectively. Compared with Cd²⁺, Pb²⁺ possessed a wider linear range and lower detection limits, while the linear correlation coefficient was poorer. These results revealed that Cys-CNSs could be used to effectively detect Pb²⁺ and Cd²⁺, especially Pb²⁺.

Conclusions

In conclusion, we demonstrate that well-defined CNSs could be obtained on a large-scale with the assistance of POMs. The POMs could catalyze the CNS formation process and control the diameters. This method is proved to be reproducible, of low cost and environmentally friendly. The conversion yield of CNSs prepared with the addition of H₃PW₁₂O₄₀·xH₂O was up to 37%. The electrochemical studies show that the Cys-CNSs were sensitive electrode materials. The detection limits for Pb²⁺ and Cd²⁺ were 1 × 10⁻⁸ M and 2 × 10⁻⁷ M, respectively. The result indicated that the CNSs may hold wide potential applications for development of new biosensors and catalysis.

Acknowledgements

This work is supported by the National Basic Research Program of China (973 Program) (No. 2010CB934500), National Natural Science Foundation of China (NSFC) (No. 51132006, 91027041, 21073127, 21071104, 20801010, 20803008), A Foundation for the Author of National Excellent Doctoral Dissertation of P R China (FANEDD)(No. 200929) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1nj20756c

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