A SERS nanocatalytic reaction and its application to quantitative analysis of trace Hg(II) with Vitoria blue B molecular probe

Abstract

Trace mercury ions can be reduced by NaH₂PO₂ to form nanomercury that can catalyze the NaH₂PO₂ reduction of HAuCl₄ to produce gold nanoparticles (AuNPs) with strong surface-enhanced Raman scattering (SERS) activity. Upon the addition of Vitoria blue B (VBB) as a molecular probe, it adsorbed on the surfaces of the AuNPs with a strong SERS peak at 1612 cm⁻¹. With an increasing concentration of Hg²⁺, the SERS effect enhanced at 1612 cm⁻¹ due to the formation of more AuNPs as substrate generated from the nanocatalytic particle reaction. This new SERS nanocatalytic indicator reaction was studied by SERS, resonance Rayleigh scattering, absorption spectrophotometry and scanning electron microscopy. Under the chosen conditions, 3.0 to 150 × 10⁻⁹ mol L⁻¹ Hg²⁺ can be analyzed quantitatively using SERS, with a detection limit of 0.8 nmol L⁻¹ Hg²⁺.

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As the economy has developed, the harm caused by toxic heavy metal pollution to the environment and human health has aroused widespread concern. Mercury has been widely recognized as one of the most hazardous pollutants and a highly dangerous element that causes great, long-term toxic effects on humans and the environment even at very low concentrations.¹ Inorganic mercury can combine with the protein sulfhydryl to inhibit the activity of the enzyme and cause disruption to the cell metabolism. Organic mercury compounds such as methylmercury have the strongest toxicity and can encroach the body’s central nervous system and cause speech and memory disorders.^2,3 Thus, the simple, rapid, sensitive, and selective detection of mercury is of great significance for biochemistry, environmental science, and medicine. At present, several methods have been reported to assay Hg²⁺, such as atomic spectrometry,^4,5 surface-enhanced Raman scattering spectroscopy,^6,7 colorimetry,^8–10 resonance Rayleigh scattering (RRS) spectroscopy,^11–13 and so on.^14–19 Atomic spectrometry is commonly used to determine trace Hg, and atomic absorption spectroscopy (AAS) is one of the best methods for Hg detection, but the instrument is expensive. Colorimetry is simple and economic, but with poor sensitivity. Surface-enhanced Raman scattering (SERS) spectroscopy is a sensitive and selective spectral technique that has been utilized to detect trace pollutants,²⁰ and most other methods are qualitative and are mainly dependent on the solid substrate used. Based on the displacement of a pre-assembled molecule with the high SERS activity of rhodamine B from a gold nanoparticle surface, with the subsequent decrease in the SERS signal,²¹ a powerful method, with detection limits below the nanomolar regime, was implemented into microfluidic devices for the online detection of Hg²⁺ in tap or waste water. A SERS method was devised for the sensitive and selective detection of Hg²⁺ ions using DNA-modified gold microshells in which tetramethyl rhodamine (TAMRA) acts as a SERS molecular probe.²² A tryptophan protected popcorn shaped gold nanomaterial based SERS probe was fabricated for the rapid, easy and highly selective recognition of Hg(II) ions at the 5 ppb level from aqueous solution.²³ A stable and reproducible nanosilver-aggregation SERS probe was developed for the quantitative analysis of trace Hg²⁺, based on the complex of 4-mercaptopyridin and Hg²⁺ that caused SERS quenching.²⁴ Compared to inorganic and enzyme catalysis amplification, nanocatalysts are easy to control, highly efficient, easy to alloy and have good stability, and have been used in nanoanalysis to improve analytical features, with spectrophotometric, fluorescence and resonance Rayleigh scattering (RRS) detection techniques.^25–30 Although many SERS methods have been developed for qualitative analysis, there are no reports about the SERS active nanocatalytic particle reaction and label-free VBB SERS methods for the quantitative analysis of trace Hg. In this article, a simple, sensitive and selective SERS active nanocatalytic indicator reaction of Hg²⁺–HAuCl₄–NaH₂PO₂ was studied, and used to determine trace mercury ions quantitatively, based on the nanomercury catalysis and the SERS effect of VBB on the surfaces of AuNP product in aqueous solution as nanosol substrate.

Materials and methods

Apparatus and chemicals

A DXR smart Raman spectrometer (Thermo Co., USA) with a laser of 633 nm and a power of 2.5 mW, and an F-7000 fluorescence spectrophotometer (Hitachi Company, Japan) were used to record the RRS intensity, with an excited and emission slit of 5.0 nm, an emission filter = 1% T attenuator, and a photoelectron multiple tube (PMT) voltage of 400 V. A 79-1 magnetic stirrer with heating (Zhongda Instrumental Plant, Jiangsu, China), a nanoparticle and zeta potential analyzer (Malvern Company, England), a JLM-6380LV scanning electron microscope (Electronic Co. Ltd, Japan), and a H-800 transmission electron microscope (Hitachi Ltd., Japan), with a point spacing of 0.45 nm, a lattice resolution of 0.204 nm, an accelerating voltage of 200 kV and a tilt angle of ±25°, were also used.

A 10 mmol L⁻¹ Hg²⁺ standard stock solution was prepared as follows: 0.2715 g HgCl₂ (National Pharmaceutical Group Chemical Reagents Company, China) was dissolved in 100 mL water. A 10 µmol L⁻¹ Hg²⁺ working solution was obtained freshly by diluting with water. 1.0 mol L⁻¹ HNO₃, 1 mol L⁻¹ HCl, 4 mmol L⁻¹ HAuCl₄, 5 mmol L⁻¹ KMnO₄, 0.2 mol L⁻¹ NH₂OH·HCl, 1% trisodium citrate and 3.75 mol L⁻¹ NaH₂PO₂ solutions were prepared. All regents were of analytical grade and the water was doubly distilled. The room temperature was 25 °C.

Preparation of the AuNPs: in a 250 mL round-bottom flask equipped with 50 mL water and put on a magnetic stirrer, 2.0 mL of the 1% trisodium citrate solution was added when the water was boiling. When the water was boiling again, 500 µL of the 1% HAuCl₄ solution was added quickly and the water was kept boiling. The solution turned red within 5 min and finally changed to brilliant red. Boiling continued for 10 min, then the heating source was removed, and the colloid solution was not stirred until it was cold. Lastly, the solution was transferred to a 50 mL volumetric flask and diluted to 50.0 mL with water. The AuNP concentration was 58 µg mL⁻¹ Au, and the size of the AuNPs was 10 nm. The AuNPs with sizes of 30 nm, 50 nm and 70 nm were prepared by the addition of different concentrations of citrate.

Procedure

60 µL of a 4.0 mmol L⁻¹ HAuCl₄ solution, 450 µL of a 1.0 mol L⁻¹ HCl solution and a certain amount of a 1.0 µmol L⁻¹ HgCl₂ solution were added to a 5 mL graduated tube successively, and mixed well. Then, 100 µL of a 3.75 mol L⁻¹ NaH₂PO₂ solution was added, and the solution was diluted to about 1.5 mL. After about 15 min, 200 µL of 1.0 × 10⁻⁵ mol L⁻¹ VBB was added, then the solution was diluted to 2.0 mL with water and mixed well. The SERS intensity at 1612 cm⁻¹ of this solution (I) and the blank solution without Hg²⁺ (I₀) was recorded. The value of ΔI = (I − I₀) was calculated.

Results and discussion

Analytical principle

Under the chosen conditions, the AuNP reaction of NaH₂PO₂–HAuCl₄ is very slow. The NaH₂PO₂ reduced Hg²⁺ to form Hg nanoparticles (HgNPs), which were characterized by laser scattering and RRS techniques, and the HgNPs exhibited catalytic activity for the reduction of HAuCl₄ by NaH₂PO₂ to generate small AuNPs. These small AuNPs also have catalytic activity for the reaction of NaH₂PO₂–HAuCl₄, which is called autocatalysis. The reduction of Hg²⁺ to Hg and Au³⁺ to Au⁺ is a fast step in the catalytic reaction because the reducer concentration of NaH₂PO₂ is high. There are strong interaction forces between the metal Hg atoms or metal Au atoms or small AuNPs, that is, metal Hg atoms, metal Au atoms and small AuNPs were rapidly aggregated to the HgNPs, AuNPs and big AuNPs respectively. When the concentration of Hg²⁺ was increased, the formation of both nanocatalysts (HgNPs and AuNPs) increased, the two steps quickened, and the catalytic reaction rate was enhanced, resulting in the formation of more AuNPs with rough surfaces, and the color of the mixing solution quickly changed from blue to red. Upon the addition of the VBB molecular probe which can adsorb on the AuNP surface, the strongest SERS peak at 1612 cm⁻¹ was exhibited. When the concentration of Hg²⁺ increased, the SERS peak enhanced due to more AuNPs forming. According to the principles of catalytic kinetic analysis and SERS, a new catalytic RRS method was established to determine trace Hg²⁺ (Fig. 1).

Fig. 1 Principles of the nanocatalytic SERS method for the detection of Hg²⁺.

Scanning electron microscopy

1.5 mL of the reaction solution was placed into a 2 mL centrifuge tube and centrifuged for 20 min (150 × 100 rpm). The supernatant was discarded and water was added up to 1.5 mL, followed by ultrasonication for 30 min. After centrifugation again, 1 mL water was added, followed by ultrasonication for 30 min. 2 µL of the sample solution was taken by pipette, dripped onto a silicon slice and allowed to dry naturally. The scanning electron microscopy (SEM) results indicated that there are AuNPs and their aggregated particles in the system, with sizes in the range of 30–800 nm, with an average size of 90 nm, and the amount of the formed particles increased when the concentration of the catalyst Hg(II) was increased (Fig. 2a and b). The electron energy spectrum showed that there are three Au energy spectral peaks at 1.70, 2.20 and 9.70 keV (Fig. 2c), and no Hg energy spectral peaks owing to the easy sublimation of Hg and trace amounts of Hg in the system.

Fig. 2 SEM and EDS results of the nanocatalytic system. a: 0.225 mol L⁻¹ HCl + 30 nmol L⁻¹ HgCl₂ + 120 µmol L⁻¹ HAuCl₄ + 0.188 mol L⁻¹ NaH₂PO₂; b: 0.225 mol L⁻¹ HCl + 150 nmol L⁻¹ HgCl₂ + 120 µmol L⁻¹ HAuCl₄ + 0.188 mol L⁻¹ NaH₂PO₂; c: EDS spectrum of the b system.

SERS spectra

The formed rough AuNP sol is of good stability and is a good SERS substrate. Several molecular probes including VBB, methyl blue, rhodamine 6G and rhodamine S were examined and the results showed that only VBB exhibited six SERS peaks at 795 cm⁻¹, 1167 cm⁻¹, 1200 cm⁻¹, 1364 cm⁻¹, 1394 cm⁻¹ and 1612 cm⁻¹. The most sensitive peak at 1612 cm⁻¹ was enhanced with increasing Hg²⁺ concentration (Fig. 3). Thus, this peak was chosen to detect Hg²⁺. Based on the nanocatalytic reaction principle and the Au/Hg alloy formation principle, the Hg(II) ions could be reduced by H₂PO₂⁻ to form Hg atoms that accumulate on the surfaces of the formed small AuNPs to produce a Au_nucleus–Hg_shell nanoalloy. Thus, the AuNP–Hg(II)–H₂PO₂⁻–VBB reaction in an acetic acid medium was examined using the SERS technique. The results indicated that the AuNP particles exhibited the strongest SERS effect, and the Hg atoms had a weak SERS effect. Thus, the SERS peak at 1612 cm⁻¹ decreased with the increasing Hg²⁺ concentration (Fig. S1†), which also demonstrated the formation of the Au_nucleus–Hg_shell nanoalloy.

Fig. 3 SERS spectra of the Hg²⁺–HAuCl₄–NaH₂PO₂ nanocatalytic system. a: 0.225 mol L⁻¹ HCl + 120 µmol L⁻¹ HAuCl₄ + 0.188 mol L⁻¹ NaH₂PO₂ + 1.0 µmol L⁻¹ VBB; b: a + 5 nmol L⁻¹ HgCl₂; c: a + 12.5 nmol L⁻¹ HgCl₂; d: a + 25 nmol L⁻¹ HgCl₂; e: a + 50 nmol L⁻¹ HgCl₂; f: a + 75 nmol L⁻¹ HgCl₂; g: a + 100 nmol L⁻¹ HgCl₂; h: a + 150 nmol L⁻¹ HgCl₂.

RRS spectra

A RRS spectrum, being obtained easily by means of the synchronous scattering technique in fluorescence spectrophotometry, is a simple and good tool to study nanoparticles in solution and their aggregation.^12,28–30 In general, increased size and aggregation cause the RRS signals to be increased. Fig. 4 shows the RRS spectra of the Hg²⁺–HAuCl₄–NaH₂PO₂ system; there are three strong RRS peaks at 295 nm, 375 nm and 530 nm. The peak at 375 nm is caused by the lamp maximal emission at 375 nm, and other peaks are owed to the RRS effect of the AuNPs. The peak at 375 nm is the strongest and the RRS intensity increased linearly with the concentration of Hg²⁺, which means that it can be used to detect 10–100 nmol L⁻¹ Hg²⁺, but the other two peaks do not have a good linear response. The results showed that when the Hg²⁺ concentration increased, the RRS peak intensity enhanced due to more AuNPs forming in the nanocatalytic system.

Fig. 4 RRS spectra of the Hg²⁺–HAuCl₄–NaH₂PO₂ catalytic system. a: 120 µmol L⁻¹ HAuCl₄ + 0.225 mol L⁻¹ HCl + 0.188 mol L⁻¹ NaH₂PO₂; b: a + 10 nmol L⁻¹ HgCl₂; c: a + 20 nmol L⁻¹ HgCl₂; d: a + 40 nmol L⁻¹ HgCl₂; e: a + 60 nmol L⁻¹ HgCl₂; f: a + 80 nmol L⁻¹ HgCl₂; g: a + 100 nmol L⁻¹ HgCl₂.

The RRS spectra of the Hg²⁺–NaH₂PO₂ system were also investigated. When the Hg²⁺ concentration increased, the RRS signal enhanced at 370 nm, as shown in Fig. S2.† This result showed that there are HgNP particles in the system because the NaH₂PO₂ reduced Hg²⁺ to Hg atoms that were aggregated to the particles. This is very significant because metal Hg sublimates easily at room temperature and its scanning electron microscopy (SEM) image can not be recorded. In addition, the AuNP–Hg(II)–H₂PO₂⁻ reaction was also examined by the RRS technique. Small AuNPs exhibited a weak RRS signal, and the results (Fig. 5a) indicated that the AuNP particles were aggregated to big loose particles with a strong RRS signal. The three RRS peaks at 375, 454 and 500 nm enhanced with the increasing Hg²⁺ concentration, which also demonstrated the formation of big Au_nucleus–Hg_shell nanoalloys.

Fig. 5 RRS spectra of the Hg²⁺–AuNPs–NaH₂PO₂ system. a: 5.7 µg mL⁻¹ AuNPs + 67.5 mmol L⁻¹ CH₃COOH + 25 mmol L⁻¹ NaH₂PO₂, the reaction time was 15 min at 60 °C; b: a + 0.25 µmol L⁻¹ HgCl₂; c: a + 0.5 µmol L⁻¹ HgCl₂; d: a + 0.75 µmol L⁻¹ HgCl₂; e: a + 1 µmol L⁻¹ HgCl₂; f: a + 1.5 µmol L⁻¹ HgCl₂; g: a + 2.0 µmol L⁻¹ HgCl₂.

Laser scattering

Under the chosen conditions, the Hg(II) ions were reduced by the reducer NaH₂PO₂ that was characterized by laser scattering (Fig. 6a), and the size distribution of the HgNP particles was in the range of 10–400 nm with an average size of 60 nm. The reaction of NaH₂PO₂–HAuCl₄ was very slow and no particles were observed by the laser scattering technique. Upon the addition of Hg(II), a large amount of the AuNPs formed in the nanocatalytic system, and the sizes were in the range of 30–800 nm with an average size of 95 nm (Fig. 6b).

Fig. 6 Laser scattering graph of the HgNP and AuNP particles. a: 0.225 mol L⁻¹ HCl + 0.188 mol L⁻¹ NaH₂PO₂ + 1000 nmol L⁻¹ Hg²⁺, the reaction time was 15 min at 60 °C; b: 0.225 mol L⁻¹ HCl + 150 nmol L⁻¹ HgCl₂ + 120 µmol L⁻¹ HAuCl₄ + 0.188 mol L⁻¹ NaH₂PO₂, the reaction time was 15 min at 60 °C.

Absorption spectra

The absorption spectra of the Hg²⁺–HAuCl₄–NaH₂PO₂ nanocatalytic system were examined. The results (Fig. S3†) showed that there are no obvious absorption peaks in the visible region, the color became light-brown slowly and the absorption increased slowly when the Hg(II) concentration increased due to AuNPs forming in the system. The red AuNP sol exhibited a SPR absorption peak at 520 nm, and the results (Fig. S4a†) indicated that the AuNP particles were aggregated to big loose AuNP clusters without the SPR peak. When the Hg²⁺ concentration increased, the two absorption peaks at about 400 and 510 nm enhanced, demonstrating the accumulation of Hg atoms on the surface of the AuNP clusters.

Catalysis of the AuNPs and HgNPs on the particle reaction

In the absence of Hg²⁺, the AuNP particle reaction between HAuCl₄ and NaH₂PO₂ is slow. Upon the addition of 10 nm AuNPs as a nanocatalyst, the particle reaction enhanced greatly, and the ΔI increased linearly with the AuNP concentration (C_AuNP) in the range of 0.1–10 µmol L⁻¹. The regression equation was ΔI = 19.1C_AuNP + 12.5, with a coefficient of 0.9849. That is, with the increasing of the C_AuNP, the SERS effect enhanced linearly due to more big AuNPs being generated from the particle reaction of HAuCl₄–NaH₂PO₂. The results (Table S1†) showed that the smaller AuNPs exhibited stronger catalysis on the HAuCl₄–NaH₂PO₂ particle reaction. Thus, the formed small AuNPs have a catalytic effect on the catalytic reaction of Hg²⁺–HAuCl₄–NaH₂PO₂. According to the procedure, a 1.0 µmol L⁻¹ HgNP sol was prepared without the addition of HAuCl₄. Using 10 µL of the 1.0 µmol L⁻¹ HgNP sol as a catalyst, it was found that it also exhibited catalysis on the HAuCl₄–NaH₂PO₂ indicator reaction.

Optimization conditions of the nanocatalytic system

HCl is a good medium for the AuNP particle reaction. We have studied the effect of HCl concentration on the detection of 0.2 µmol L⁻¹ Hg²⁺. The results indicated that the catalytic rate became fast when the concentration of HCl increased, more AuNPs formed and caused the ΔI value to increase. When the concentration of HCl was 0.225 mol L⁻¹, the value of ΔI reached its maximum (Fig. S5†). When the concentration of HCl exceeded 0.225 mol L⁻¹, the value of ΔI tended to decline, so we chose 0.225 mol L⁻¹ as the final concentration of HCl.

HAuCl₄ is an important component because it is reduced catalytically by NaH₂PO₂ to form the AuNPs. We studied the effect of HAuCl₄ concentration in the presence of 20 nmol L⁻¹ Hg²⁺ (Fig. S6†). The results indicated that the catalytic rate became fast when the concentration of HAuCl₄ increased. When the HAuCl₄ concentration was 120 µmol L⁻¹, the ΔI value reached its maximum. When the concentration of HAuCl₄ exceeded 120 µmol L⁻¹, the value of ΔI tended to decline, so we chose the concentration of 120 µmol L⁻¹ HAuCl₄ for further studies. We also studied the effect of NaH₂PO₂ concentration on the ΔI (Fig. S7†). When the concentration of NaH₂PO₂ was 0.188 mol L⁻¹, the ΔI value reached its maximum. When the NaH₂PO₂ concentration exceeded 0.188 mol L⁻¹, the value of ΔI tended to decline. Thus, we chose the concentration of 0.188 mol L⁻¹ NaH₂PO₂ for further studies.

Using the formed AuNP sol as the SERS substrate, several molecular probes such as VBB, rhodamine 6G, rhodamine B and methyl blue were examined. The results indicated that VBB exhibited strong SERS activity. The effect of the VBB concentration on the ΔI was considered. When the concentration of VBB was 1.0 µmol L⁻¹, the ΔI value was the biggest (Fig. S8†), so this concentration was selected for use. Because the blank reaction appeared slow under room temperature (25 °C), the catalytic reaction was fast, and the influence of the temperature affected the catalytic reaction weakly in the range of 20–40 °C. Thus, room temperature was chosen for use. The results indicated that the ΔI value increased when the reaction time became long, in the range of 7–15 min, and it achieved the maximum value in 15 min, after which the value of ΔI held constant. Thus, a reaction time of 15 min was chosen for use.

Influence of foreign substances

According to the procedure, the influence of coexistent metal ions on the detection of 50 nmol L⁻¹ Hg²⁺ was tested, with a relative error of ±10%. The results (Fig. S9†) showed that the common metal ions such as 100 times of Pb(II), Co(II), Ca(II), Mg(II), Cr(III), 70 times of Cd(II), 50 times of Fe(III) and Zn(II) did not enhance the Hg²⁺ catalytic reaction. This indicated that the SERS method has good selectivity for the nanocatalytic reaction.

Working curve

Under the optimal conditions, the Hg²⁺ concentration (C_Hg²⁺) and the corresponding ΔI showed a good linear relationship. The SERS analysis of the Au(III)–NaH₂PO₂–VBB system showed the best analytical features (Table S2†), with a linear range of 3.0–150 nmol L⁻¹, a regression equation of ΔI = 5.18C + 9.2, a coefficient of 0.9890, and a detection limit of 0.8 nmol L⁻¹, and was chosen for the detection of Hg(II). Compared to some reported methods for mercury ion detection,^31–34 this nanocatalytic SERS method was more sensitive and simple.

Sample analysis

We used this catalytic RRS method to measure the concentration of Hg²⁺ in water samples. Three natural water and three waste water samples were sampled with a glass sampling bottle, respectively, and filtered before pre-treatment. The water samples were pre-treated as follows. Into a tube containing 10 mL of the water sample, 0.50 mL of 1.0 mol L⁻¹ HNO₃, 0.50 mL of 1.0 mol L⁻¹ HNO₃ and 0.30 mL of 5.0 mmol L⁻¹ KMnO₄ were added to oxidize the sample completely. Then, a 2.5 mmol L⁻¹ NH₂OH·HCl solution was added to reduce the excess KMnO₄; the color appeared light pale purple which indicated that no NH₂OH·HCl existed, and the solution was diluted to 20 mL with water to obtain the sample solution. According to the procedure, no Hg²⁺ was found in the natural water samples. In the sample solutions, a known quantity of Hg²⁺ was added, then the Hg²⁺ content was measured according to the procedure, and the recovery was obtained (Table 1). The recovery was in the range of 101.5–108.7%, and the relative standard deviation (RSD) was in the range of 3–6%. This showed that this RRS assay is precise and accurate. The analytical results of the Hg levels in the waste water were in agreement with those of atomic fluorescence spectrometry (AFS).

Table 1 Analytical results for the Hg content in the water samples

Water sample	Added value (nM)	Found value (nM, n = 5)	RSD (%)	Recovery (%)	AFS results (nM)
Natural water 1	—	None detected	—	—	—
	200	216	5.0	108	—
Natural water 2	—	None detected	—	—	—
	200	203	6.0	101.5	—
Natural water 3	—	None detected	—	—	—
	300	326	3.0	108.7	—
Waste water 1	—	215 ± 8.6	4.0	—	207
	300	533	—	106	—
Waste water 2	—	186 ± 9.3	5.0	—	193
	250	453		106.8	—
Waste water 3	—	137 ± 8.1	5.9	—	131
	150	281	—	96.0	—

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Conclusions

Based on the trace Hg²⁺ catalysis of the AuNP particle reaction between NaH₂PO₂ and HAuCl₄, and VBB exhibiting a SERS effect at 1612 cm⁻¹ in the nanosol substrate, the nanomercury catalytic-SERS particle reaction was studied using the SERS and RRS techniques. A simple, sensitive and selective nanocatalytic analytical platform was developed and utilized to determine 3.0–150 nmol L⁻¹ Hg²⁺ and 10–100 nmol L⁻¹ Hg²⁺ by the SERS and RRS methods respectively. This new nanocatalytic SERS method was applied to detect Hg²⁺ in real samples, with satisfactory results.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21165005, 21267004, 21307017, 21367005, 21365011, 21467001, 21477025, 21465006, 21447006), and the Natural Science Foundation of Guangxi (no. 2013GXNSFFA019003, 2013GXNSFAA019046, 2014GXNSFAA118050, 2014GXNSFAA118059).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16110f

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