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 Journal of Zhejiang University SCIENCE A 2014 Vol.15 No.8 P.634-642 http://doi.org/10.1631/jzus.A1400115

The role of humic acid in stabilizing fullerene (C60) suspensions

 Author(s):  Lu-qing Zhang1, Yu-kun Zhang1, Xiu-chun Lin1,3, Kun Yang1,2, Dao-hui Lin1,2 Affiliation(s):  1. Department of Environmental Science, Zhejiang University, Hangzhou 310058, China; more Corresponding email(s):   lindaohui@zju.edu.cn Key Words:  Fullerene, Humic acid, Colloidal stability, Natural organic matter, Nanomaterial Share this article to： More <<< Previous Article|Next Article >>>

Lu-qing Zhang, Yu-kun Zhang, Xiu-chun Lin, Kun Yang, Dao-hui Lin. The role of humic acid in stabilizing fullerene (C60) suspensions[J]. Journal of Zhejiang University Science A, 2014, 15(8): 634-642.

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volume="15",
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pages="634-642",
year="2014",
publisher="Zhejiang University Press & Springer",
doi="10.1631/jzus.A1400115"
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%A Lu-qing Zhang
%A Yu-kun Zhang
%A Xiu-chun Lin
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%A Dao-hui Lin
%J Journal of Zhejiang University SCIENCE A
%V 15
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%D 2014
%I Zhejiang University Press & Springer
%DOI 10.1631/jzus.A1400115

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T1 - The role of humic acid in stabilizing fullerene (C60) suspensions
A1 - Lu-qing Zhang
A1 - Yu-kun Zhang
A1 - Xiu-chun Lin
A1 - Kun Yang
A1 - Dao-hui Lin
J0 - Journal of Zhejiang University Science A
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%@ 1673-565X
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PB - Zhejiang University Press & Springer
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DOI - 10.1631/jzus.A1400115

Abstract:
natural organic matter (NOM) has a profound effect on the colloidal stability of discharged C60 nanoparticles in the water environment, which influences the environmental behaviors and risks of C60 and therefore merits more specific studies. This study investigates the effects of humic acid (HA), as a model NOM, on the aqueous stabilization of C60 powder and the colloidal stability of a previously suspended C60 suspension (aqu/nC60) with variations of pH values and ionic strengths. Our results reveal that HA could disperse C60 powder in water to some degree, but was unable to stably suspend them. The aqu/nC60 could remain stable at pH>4 but was destabilized at lower pH values. However, the colloidal stability of aqu/nC60 in the presence of HA was insensitive to pH 3–11, owing to the adsorption of HA onto nC60 and the increased electrosteric repulsions among nC60 aggregates. The colloidal stability of aqu/nC60, with and without HA, decreased as we increased the valence and concentration of the added cations. HA was found to mitigate the destabilization effect of Na+ on the colloidal stability of aqu/nC60 by increasing the critical coagulation concentration (CCC) of Na+, while HA lowered the CCCs of Ca2+ and La3+ probably by the bridging effect of nC60 with HA aggregates formed through the intermolecular bridging of the HA macromolecules via cation complexation at high concentrations of cations with high valences.

## 1.  Introduction

Fullerene C60 nanoparticles have triggered much attention by their unique physicochemical, electrical, and mechanical properties, which enable their numerous potential applications in electronics (Zhang et al., ), biomedicine (Colvin, ; Nakamura and Isobe, ), cosmetics (Takada et al., ), etc. C60 is inevitably released into the aquatic environment with its widespread production and usage (van Wezel et al., ), and may thus threaten aquatic organisms and human health as well through food chains (Nel et al., ; Oberdörster et al., ; Britto et al., ).

Once released into the water environment, the hydrophobic C60 will be presented as aggregates (nC60). Ubiquitous natural organic matter (NOM), such as humic acid (HA), is likely to interact with the discharged nC60 and influence the aggregation/dispersion behavior and ecotoxicity of nC60 (Bhatt and Tripathi, ; Kim et al., ). Therefore, it is essential to take into account the interactions between NOM and nC60 when elucidating the behavior of nC60 in natural water bodies.

Several studies have investigated the interaction between NOM and nC60 and the effect of NOM on the aggregation/dispersion behavior of nC60. Xie et al. () observed significant changes in the aggregate size and morphology of nC60 with the addition of NOM. Li et al. () revealed that NOM could effectively stabilize nC60 in the aqueous phase and nC60 nanoparticles may have been chemically modified under illumination. Mashayekhi et al. () attributed the increased stability of nC60 to the transformation of surface properties through the HA adsorption. The adsorption of HA can increase electrostatic and steric repulsions among nC60 aggregates, and thus facilitate dispersion and stabilization of nC60 in water (Chen and Elimelech, ; Mashayekhi et al., ; Zhang et al., ). Moreover, environmental conditions (e.g., water quality parameters) may have profound effects on the interaction between HA and nC60 (Isaacson and Bouchard, ; Qu et al., ; Yang et al., ), which has not been well examined. Chen and Elimelech () found that HA could increase the C60 nanoparticle stability in NaCl or MgCl2 electrolytes, but HA enhanced the aggregation at high concentrations of CaCl2. Therefore, water quality parameters, such as ionic strength and pH, are likely to have significant effects on the colloidal stability of C60 in the presence of HA, which merit further investigations.

This work is aimed to study the capabilities of HA in dispersing and stabilizing C60 powder in water and in maintaining the colloidal stability of nC60 suspension (aqu/nC60) as affected by variations of pH and ionic strength. The changes of zeta potentials and hydrodynamic sizes of the C60 powder and aqu/nC60 under different pH values and the impact of ionic strength on the aggregation kinetics of the aqu/nC60 in the absence and presence of HA were specifically examined.

## 2.  Materials and methods

### 2.1.  Preparation and characterization of HA

HA was purchased from Sigma-Aldrich. HA was purified before use and was characterized following the procedures detailed in Lin et al. (; ). HA of 500 mg was dissolved in 10 ml 1 mol/L NaOH solution, and adjusted to pH 7.0 using 1 mol/L HCl. The supernatant was obtained by centrifugation at 3000 r/min for 15 min, and then was diluted to 1 L using ultra-pure water and used as the stock HA solution (500 mg/L). NaN3 (200 mg/L) was added to prevent HA from microbial interference. The HA solution was scanned with an ultraviolet (UV) spectrophotometer (SHIMADZU, UV-2450, Japan).

### 2.2.  Preparation and characterization of aqu/nC60

C60 powder was purchased from Nanjing XFNANO Materials Tech Co., Ltd., China. The elemental contents were measured using an elemental analyzer (Vario ELIII, Germany). Aqueous suspension of C60 was prepared following a solvent exchange procedure modified from previous works (Deguchi et al., ; Chen and Elimelech, ; Kim et al., ). C60 powder (120 mg) was dispersed in 60 ml of high-performance liquid chromatography (HPLC)-grade toluene by stirring for several hours, forming a clear dark purple mixture. The mixture was then added into ultra-pure water at a volume ratio of 1:15, resulting in two distinct phases. The resultant mixture was sonicated with a sonifier cell disrupter (KBS-150, Kunshan, China) for more than 4 h, allowing for the evaporation of toluene. The stabilized C60 suspension (aqu/nC60) was obtained by centrifugation at 2000 r/min for 15 min, and then stored at room temperature in the dark. The morphology and aggregate size of the aqu/nC60 were examined with a transmission electron microscope (TEM; JEM-1230, JEOL, Japan). The hydrodynamic size and zeta potential were determined using a Zetasizer (Nano ZS90, Malvern Instruments, UK) at 25 °C (Tian et al., ). The aqu/C60 suspension was also scanned using the UV spectrophotometer.

### 2.3.  Suspension experiment

A suspension experiment was performed to study the effect of HA on the aqueous stabilization of C60 powder. C60 powder (10 mg) was added into 20 ml of HA solutions (pH 7.0, 10 mg/L of NaCl) with initial HA concentrations of 0, 5, 10, 20, and 40 mg/L. The mixtures were sonicated (240 W, 40 KHz) for 1 h and were subsequently kept in a thermostat shaker (150 r/min, 25 °C) for 1 d. The zeta potential and hydrodynamic size of the samples were measured right after the shaking process with the Zetasizer at 25 °C. Each concentration was run in triplicate. The absorbance at 800 nm (UV800) was considered to be capable of quantifying the stabilized multi-walled carbon nanotubes (MWCNTs) (Lin et al., ; ). Similarly, UV800 of the C60 suspension was in a positive correlation with the concentration of suspended C60 determined by a total organic carbon (TOC) analyzer (SHIMADZU, TOC-VCPH, Japan), and the HA solution had no absorbance at 800 nm (Fig. 1). Therefore, the UV800 of the supernatant of the C60-HA suspension after settling for 0–2 d was used to measure the stability of the nC60 suspensions in the presence of different concentrations of HA.

Fig.1
TEM image of the aqu/nC60 and UV-vis absorbance spectra of the aqu/nC60 (8 mg/L) and HA (10 mg/L)

### 2.4.  Effect of pH on stability of aqu/nC60 with and without HA

The stock suspension of aqu/nC60 (10 ml) was added into 22-ml glass vials containing 10 ml of ultra-pure water or 20 mg/L HA with 20 mg/L NaCl. The pH values of the suspensions were then adjusted to 3.0–11.0 with several drops of 1 mol/L NaOH or HCl solutions. The mixtures were equilibrated (150 r/min) for 1 d, and then their zeta potentials, hydrodynamic sizes, and final pH values were measured. Each pH value was run in triplicate.

### 2.5.  Aggregation kinetics of aqu/nC60 with and without HA

Aggregation kinetics of the aqu/nC60 in the presence and absence of HA were investigated under various ionic strengths using a time-resolved dynamic light scattering (DLS) technique. A portion of the stock aqu/nC60 of pH 7.0 with or without 10 mg/L HA was added into a polystyrene measurement cell and the hydrodynamic sizes were recorded by the Zetasizer at an interval of 1 min for 30 min at 25 °C. Various ionic strengths were obtained by mixing 0.5 ml of NaCl, CaCl2, or LaCl3 solutions of different concentrations into the cells containing aqu/nC60 prior to the hydrodynamic size measurements. At the early stage of aggregation, the initial aggregation rate of the aqu/nC60 can be expressed as $${\left( {\frac{{{D_h}(t)}}{{{\text{d}}t}}} \right)_{t \to 0}} \propto {k_{11}}{N_0}$$, where D h(t) is the hydrodynamic size (nm) of nC60 at time t, N 0 is the initial particle concentration (mg/L), and k 11 is the initial aggregation rate constant (Chen and Elimelech, ; Chen et al., ). The initial rate of increase in D h(t) is obtained by determining the initial slope up to the point where the hydrodynamic size reaches 1.25D h(0).

The attachment efficiency (α) under different ionic strengths was calculated to quantify the aggregation kinetics of the aqu/nC60: $\alpha = \frac{{{k_{11}}}}{{{{({k_{11}})}_{fast}}}} = \frac{{\frac{1}{{{N_0}}}{{\left( {\frac{{{\text{d}}{D_h}(t)}}{{{\text{d}}t}}} \right)}_{t \to 0}}}}{{\frac{1}{{{{({N_0})}_{fast}}}}{{\left( {\frac{{{\text{d}}{D_h}(t)}}{{{\text{d}}t}}} \right)}_{t \to 0{\text{, fast}}}}}}$, where (k 11)fast refers to k 11 in the diffusion controlled regime (Mashayekhi et al., ).

## 3.  Results and discussion

### 3.1.  Characteristics of aqu/nC60 and HA

Selected properties of the C60 powder and HA are listed in Table 1. The aggregate size of aqu/nC60 as determined using TEM was (34.8±5.9) nm (Fig. 1). The transparent yellow aqu/nC60 remained stable for several months during and after the experiments. The concentration of the stock aqu/nC60 was 8.0 mg C/L with the zeta potential of −40.8 mV, hydrodynamic size of 172 nm, and pH value of 7.0. Fig. 1 shows the UV-vis absorbance spectrum of the aqu/nC60. The aqu/nC60 had absorption peaks at 269 and 350 nm as well as a broad band between 410 and 550 nm, which were identified to be characteristics of nC60 suspension as previously reported (Hyung and Kim, ; Hwang and Li, ; Navarro et al., ).

#### Table 1

Selected properties of C60 and HA
 Material Elemental content (%) Ash (%) Acidic site (mmol/g) C H N O Total –COOH Phenolic –OH C60 99.70 0.01 HA 59.90 3.94 1.50 31.50 3.20 7.15 4.25 2.90

### 3.2.  Effect of HA on aqueous stabilization of C60

Fig. 2 shows the changes of zeta potential, hydro-dynamic size, and UV800 of the C60 powder sonicated in water against a concentration of HA. The zeta potentials of the C60 powder in the presence of 0–40 mg/L HA were all lower than −50 mV, and the electronegativity initially increased with increasing the HA concentration and then leveled off at about −65 mV (Fig. 2a). The greater electronegativity of the C60 powder in the presence of HA indicated a greater electrostatic repulsion among the C60 particles, which was evidenced by the smaller hydrodynamic sizes in the HA solutions (Fig. 2a). The hydrodynamic size of the C60 powder was (953±145) nm in the absence of HA, which however decreased to (516±31) nm in the presence of 40 mg/L HA, suggesting that HA could disperse the C60 powder to some degree in water.

Fig.2
Variations of zeta potential and hydrodynamic size (a) and UV-vis absorbance at 800 nm (UV800) (b) of C60 powder sonicated in water after settling for 0–2 d with HA
Data are presented as mean±standard deviation, n=3

The increased electrostatic repulsion between C60 aggregates accompanied by decreased hydrodynamic size with increasing HA concentrations led to the enhanced aqueous suspension of C60 in the HA solutions, which was confirmed by the increased UV800 of the C60 powder in the HA solutions. UV800 of the C60-HA suspension was positively correlated with the HA concentration after settling for 0−2 d, which was in accordance with the changes of zeta potential and hydrodynamic size against the HA concentration (Fig. 2b). However, the UV800 of the C60-HA suspensions decreased with increasing settling time. After settling for 2 d, all of the C60-HA suspensions became transparent, suggesting the severe agglomeration and precipitation of the C60 aggregates. The above result implies that after being discharged, the hydrophobic C60 powder could hardly be dispersed and stabilized in the water environment even in the presence of HA.

### 3.3.  Effect of pH on aqueous stability of aqu/nC60 with and without HA

The stability of aqu/nC60 in the absence and presence of HA was investigated under various pH values. Dramatic increases in electronegativity of the aqu/nC60 with and without HA were observed as pH increased from 3 to 6, while the increases leveled off over pH 6 (Fig. 3). This suggests that the neutralization reaction between H+ and the acidic surfaces of aqu/nC60 occurred at low pH (Yang et al., ), thereby reducing the electronegativity and electrostatic repulsion among the nC60 aggregates and consequently causing the aggregation and destabilization of the aqu/nC60 in the absence of HA. The aqu/nC60 had a lower zeta potential in the presence of 10 mg/L HA, and the differences between the zeta potentials with and without HA became more significant under acidic conditions (pH<6). HA could be adsorbed by the nC60 aggregates (Mashayekhi et al., ), and the dissociation of –COOH and phenolic –OH groups of the surface-bound HA (Table 1) could impart negative charges on the nC60 aggregates, causing the difference in the zeta potential of the aqu/nC60 with and without HA.

Fig.3
Variations of zeta potentials and hydrodynamic sizes of the aqu/nC60 with and without HA (10 mg/L)

The aqu/nC60 retained its hydrodynamic size of (186±10) nm at pH>4 with and without HA (Fig. 3). The hydrodynamic size of the aqu/nC60 in the absence of HA sharply increased to (1028±100) nm at pH=3, whereas it remained largely unchanged in the presence of HA (Fig. 3). The differences in the hydrodynamic size of the aqu/nC60 with and without HA at pH=3 could be explained by the differences in the zeta potential. At pH=3, the zeta potential of the aqu/nC60 increased to (−18±1) mV in the absence of HA while it remained lower than −30 mV in the presence of HA (Fig. 3). Colloids with absolute zeta potential >30 mV are considered to have strong electrostatic repulsions and tend to remain dispersed, while with the absolute zeta potential lower than 30 mV, they are prone to aggregation and destabilization due to the weak electrostatic repulsions among them (Lin et al., ; ). The adsorbed HA could mitigate the neutralization reaction between H+ and the acidic surfaces of the aqu/nC60, preventing the nC60 from aggregation, owing to the decreased electrostatic repulsion at low pH values. Moreover, the adsorbed HA could also enhance the aqueous stability of aqu/nC60 via the increased steric hindrance among the nC60 aggregates. From the above results, we can see that the aqu/nC60 would remain stabilized in the water environment with broad pH values, but would be subject to aggregation and destabilization at extremely low pH values; the coexisting HA is likely to help to stabilize the aqu/nC60, especially at low pH values.

### 3.4.1.  Aggregation kinetics of the aqu/nC60 in the absence of HA

Aggregation of the aqu/nC60 occurred after the addition of various concentrations of the electrolytes as evidenced by the increase in its hydrodynamic size (Fig. 4). The cations at high concentrations immediately increased the aggregate size after their addition to the aqu/nC60. Fig. 4a shows the increasing of the aggregation rate of the aqu/nC60 by increasing Na+ concentration from 50 to 400 mmol/L during 30 min. The hydrodynamic size remained constant when 50 mmol/L Na+ was added. But the aggregation rate markedly increased with the concentration of Na+ increasing from 50 to 150 mmol/L, and then remained steady with further addition of Na+. The attachment efficiencies (α) of the aqu/nC60 at each Na+ concentration were calculated according to Eq. (2) (Fig. 5a). The increase in Na+ concentration could screen the surface charge of nC60, reduce the electrostatic energy barrier, and subsequently lead to the fast aggregation in the reaction controlled regime where α linearly increased with the increasing Na+ concentration. However, once the electrolyte concentration was beyond the critical coagulation concentration (CCC), the energy barrier was completely eliminated and the diffusion controlled aggregation began. Under the diffusion controlled regime, α no longer responded to the increasing Na+ concentration. Based on the data in Fig. 5a, the CCC, the intersection of the reaction and diffusion controlled regimes, was determined to be 142 mmol/L for Na+.

Fig.4
Aggregation kinetics of the aqu/nC60 at different concentrations of NaCl (a, b), CaCl2 (c, d), and LaCl3 (e, f) in the absence (a, c, e) and presence (b, d, f) of 10 mg/L HA

Fig.5
Attachment efficiencies of the aqu/nC60 at different concentrations of NaCl (a), CaCl2 (b), LaCl3 (c), and the relationship between CCCs and cation valences (d) in the absence (solid lines) and presence (dash lines) of 10 mg/L HA
The CCCs of the eletrolytes were marked in the figures

The DLS measurement was also applied to study the aggregation kinetics of aqu/nC60 in the presence of Ca2+ and La3+. The increases in the hydrodynamic size with time were observed starting at 3 and 0.07 mmol/L of Ca2+ (Fig. 4c) and La2+ (Fig. 4e), respectively, revealing that much lower concentrations of the cations with higher valence were needed to destabilize the aqu/nC60. This is because cations with higher valence have much higher capability of eliminating the energy barrier between the dispersed nanoparticles and thereby promoting the aggregation (Chen et al., ). The increase in aggregation rate of the aqu/nC60 leveled off (i.e., α=1) with the concentrations of Ca2+ and La3+ over 5 and 0.1 mmol/L, respectively. Figs. 5b and 5c show variations of α of the aqu/nC60 with the concentrations of Ca2+ and La3+, where their CCCs were calculated to be 5.30 and 0.110 mmol/L, respectively. The ratio of CCCs of the three electrolytes NaCl:CaCl2:AlCl3 was 140:5.30: 0.110 (i.e., 1−4.78:2−4.78:3−4.78) (Fig. 5d), which was close to the ratio indicated by the Schulze-Hardy rule (i.e., 1−6:2−6:3−6) (Li and Huang, ).

Our CCC of NaCl is comparably higher than that determined by Chen and Elimelech () for fullerene nanoparticles synthesized using the same technique (120 mmol/L). Their reported CCC of CaCl2 was 4.8 mmol/L, which was close to our obtained result. It was noteworthy that the CCC values were quite similar, regardless of the 10 times higher suspension concentration which we employed, implying that the initial concentration of aqu/nC60 may play an insignificant role in its aggregation.

### 3.4.2.  Aggregation of aqu/nC60 in the presence of HA

The effects of HA on the aggregation rate of aqu/nC60 at different concentrations of NaCl, CaCl2, and LaCl3 are shown in Figs. 4b, 4d, and 4f, respectively. aqu/nC60 started to aggregate with the hydrodynamic size increasing with time at Na+ concentrations higher than 600 mmol/L and did not reach its maximum aggregation rate until 1670 mmol/L (CCC) of NaCl (Fig. 5a). Clearly, the presence of HA effectively prevented the aqu/nC60 from homoaggregation and forming bigger aggregates as affected by NaCl. This was in accordance with the result of a previous study that NOM increased the stability of nC60 suspension (Duncan et al., ). It was supposed that the adsorption of NOM including HA on the nC60 aggregates could increase electrosteric repulsions and thereby enhance stabilization of the nC60 suspension (Chen and Elimelech, ). Environmental concentrations of Na+ are generally much lower than 1670 mmol/L, therefore the Na+ effect on the stability of aqu/nC60 could be ignored in the presence of 10 mg/L HA.

The aggregation rate of aqu/nC60 started to increase at 3.2 and 0.02 mmol/L of Ca2+ (Fig. 4d) and La3+ (Fig. 4f) and the increase leveled off at 4.80 and 0.032 mmol/L of Ca2+ (Fig. 5b) and La3+ (Fig. 5c), respectively. According to Figs. 5b and 5c, the calculated CCCs were 4.90 and 0.035 mmol/L of Ca2+ and La3+, respectively, which were surprisingly lower than their respective CCCs in the absence of HA. The negative effect of HA on the stability of aqu/nC60 as affected by Ca2+ was also observed by Chen and Elimelech (). The accelerated aggregation in the presence HA was attributed to the bridging of nC60 by the HA aggregates formed through the intermolecular bridging of HA macromolecules via cation complexation at high concentrations of cations with high valences (Chen and Elimelech, ). The ratio of CCCs of the three electrolytes NaCl:CaCl2:AlCl3 in the presence of HA was 1670:4.90:0.035 (i.e., 1−8.42: 2−8.42:3−8.42) (Fig. 5d), which was also largely comparable to the precipitation by the Schulze-Hardy rule (i.e., 1−6:2−6:3−6) (Li and Huang, ) but was different from the ratio in the absence of HA. This differences were owing to the different effects of the HA on the colloidal stability of the aqu/nC60 as affected by the three electrolytes as addressed above.

## 4.  Conclusions

The effects of HA on the aqueous stabilization of C60 powder and on the colloidal stability of aqu/nC60 as affected by variations of pH and ionic strength were studied. C60 powder could be dispersed to some degree but hardly be stably suspended in the presence of HA. Low pH and high ionic strength would affect the colloidal stability of aqu/nC60. Big aggregates formed at pH<4, owing to the screening of surface charges of the aqu/nC60 at such low pH. However, the colloidal stability of aqu/nC60 in the presence of HA was insensitive to pH 3–11. It was supposed that HA could be adsorbed onto the surfaces of nC60 and the surface-bound HA increased the surface electronegativity of the nC60, and thereby mitigated the aggregation through electrosteric repulsion at low pH values. Cation valence and concentration had a profound effect on the destabilization of the aqu/nC60. The CCCs of Na+, Ca2+, and La3+ in the presence and absence of HA were exponentially decreased with the increase in cationic valence. The presence of HA could mitigate the impact of Na+ on the colloidal stability of aqu/nC60, but enhanced the destabilization effect of Ca2+ and La3+ on the aqu/C60.

* Project supported by the National Natural Science Foundation of China (No. 21337004), the National Basic Research Program (973) of China (No. 2014CB441104), the Zhejiang Provincial Natural Science Foundation of China (No. LR12B07001), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (No. 20130101110132), the Fujian Provincial Natural Science Foundation of China (No. 2014J01053), and the Fundamental Research Funds for the Central Universities (No. 2014FZA6009), China

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