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Valence states effect on electrogenerated chemiluminescence of gold nanocluster Hua-Ping Peng, Mei-Li Jian, Hao-Hua Deng, Wen-Jun Wang, Zhong-Nan Huang, Kai-Yuan Huang, Ai-Lin Liu, and Wei Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02446 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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ACS Applied Materials & Interfaces

1

Valence States Effect on Electrogenerated Chemiluminescence of

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Gold Nanocluster

3

Huaping Peng†,‡ Meili Jian,†,‡ Haohua Deng,†,‡ Wenjun Wang,†,‡ Zhongnan Huang,†,‡ Kaiyuan

4

Huang,‡ Ailin Liu,†,‡ and Wei Chen*,†,‡

5

†

Department of Pharmaceutical Analysis, Fujian Medical University, Fuzhou 350004, China

6

‡

Higher Educational Key Laboratory for Nano Biomedical Technology of Fujian Province, Fujian

7

Medical University, Fuzhou 350004, China

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Huaping Peng and Meili Jian contributed equally to this work.

9

*Corresponding author. E-mail address: [email protected]

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ABSTRACT: This work elucidated the valence states effect on the electrogenerated

11

chemiluminescence

12

N-acetyl-L-cysteine-AuNCs (NAC-AuNCs) and the electrochemical reduction method for reducing

13

the AuNCs were firstly employed to this study. Results demonstrate that the electrochemical

14

reduction degree of the AuNCs depended on the reduction potential, and the enhancement of the

15

ECL signals was positively correlated with the reduction degree of AuNCs, which indicated that the

16

valence state of Au plays a vital role in the ECL performance of AuNCs. Furthermore, the proposed

17

method has been successfully extended to the chemical reduction technique and other nanoclusters.

18

Therefore, an excellent AuNC-based ECL method with various advantages, such as simple

19

preparation, less toxicity, high sensitivity and ΦECL, and excellent stability, has been proposed. This

20

approach not only opens up a new avenue for designing and developing ECL device from other

21

functional-metal based NCs, but also extends the huge potential application in the ECL sensing.

22

KEYWORDS: Au nanoclusters; Electrogenerated chemiluminescence; Electrochemical reduction;

23

Chemical reduction; Valence state.

(ECL)

performance

of

gold

nanocluster

(AuNC).

The

1

ACS Paragon Plus Environment


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INTRODUCTION

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Electrogenerated chemiluminescence (ECL) is luminescence results from excited state of

3

electrogenerated species in electrochemical redox reaction.1-3 Due to its excellent performs such as

4

low-background emission, versatility, good stability and sensitivity, ECL technologies have found

5

wide applications in the determination of various analytes.4-8 According to the types of luminophore

6

species, electrochemiluminescence systems can be classified into organic system (such as luminol),

7

inorganic system (such as Ru(bpy)32+), and quantum dot (QD) system.1 Owing to their unique

8

size-dependent electrochemical properties and regulable ECL virtues, QD ECL systems have been

9

attracted increasing attentions. However, most of the QD luminophor species, such as the

10

cadimium-containing QDs, are environmental toxicity, poor stability and biocompatibility. These

11

features limit the practical applications of the QD luminophor, especially in biosystems.9-11 Thus,

12

there is no denying the fact that developing low-toxicity or nontoxic ECL species is imperative.

13

Gold nanocluster (AuNC) has become a promising material due to its fascinating performance,

14

such as eco-friendly, high colloidal stability, superior catalytic activity, unusual photophysical

15

properties, and good biocompatibility.12-16 Although the fluorescent properties of AuNCs have been

16

widely studied, there are only a few researches about the AuNC-based ECL sensing platform for

17

analytical application.17-22 The main reasons probably lie in the weak ECL intensity and the unclear

18

mechanism. Thus, how to obtain a strong and efficient ECL signal of AuNCs becomes the key point

19

to establish AuNC-based ECL sensors.

20

To the best of our knowledge, AuNCs have different valence states of aurum, typically as Au (I)

21

and Au (0). In this paper, valence state effect of Au on the ECL performance of AuNCs was

22

investigated. For an example of N-acetyl-L-cysteine-AuNCs (NAC-AuNCs), X-ray photoelectron

23

spectroscopy (XPS), electrochemical methods, and ECL techniques were used to gain the insight 2


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into ECL mechanism of the NAC-AuNCs in K2S2O8 system. The experimental results reveal that

2

valence state of Au plays a vital role in the ECL performance. With the reduction of NAC-AuNCs

3

by either electrochemical or chemical method, obvious enhanced ECL signal was obtained. Based on

4

these results, a highly effective approach to improve the ECL signal of NAC-AuNCs has been

5

proposed, which has great potential to be used in other AuNCs and can widen the further application

6

of AuNC-based ECL sensor.

7

EXPERIMENTAL SECTION

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Materials and reagents. Chloroauric acid (HAuCl4·4H2O), N-acetyl-L-cysteine (NAC), sodium

9

borohydride (NaBH4), methionine, and NH3·H2O were purchased from Aladdin Reagent Company

10

(China). Potassium peroxydisulfate (K2S2O8), Na2HPO4, NaH2PO4, NaOH, and H2SO4 were

11

purchased from Sinopharm Chemical Reagent Company (China). Phosphate buffer solution (PB, 0.1

12

M, pH 7.4) containing 0.1 M K2S2O8 as coreactant was used in electrochemistry and ECL

13

experiments.

14

Apparatus and measurements. The transmission electron microscope (TEM) images were

15

collected with a JEM-2100 TEM instrument (JEOL, Japan). XPS studies were performed for

16

analysis of the surface composition and chemical states of the gold nanoclusters. The detachable

17

GCEs (GaossUnion Technology Co., Ltd, Wuhan) were used to measure the XPS of

18

ER-AuNC/GCE. The electrode tip was detached and used for the XPS measurement. The Au(4f) and

19

S(2p) signals were collected and analyzed using an ESCALAB 250XI electron spectrometer

20

(Thermo, USA) with monochromatic Al Kα radiation. The absorption and fluorescence spectra were

21

recorded with UV-2450 spectrophotometer (Shimadzu, Japan) and Eclipse spectrofluorometer

22

(Varian), respectively. The electrogenerated chemiluminescence was detected by a MPI-E

23

multifunctional chemiluminescent analyzer (Xi’an Rimex, China) with a three-electrode system 3



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where the modified glassy carbon electrode (GCE) was used as the working electrode, an Ag/AgCl

2

as the reference electrode, and Pt wire as the counter electrode. The ECL signals were generated by

3

step potential (SP) method with the potential steps between 0 and -2 V. The pulse periods at -2 V

4

and 0 V were 1 s and 10 s, respectively. The ECL spectra were detected by a RFAS-1 automatic

5

electrochemical luminescence spectrophotometer (Xi’an Rimex, China) with a series of optical

6

filters from 275 to 825 nm.

7

Synthesis of AuNCs. The NAC-AuNCs were synthesized in a blending manner according to our

8

previous report.23 In brief, the mixture of NaOH (0.5 M, 0.6 mL), HAuCl4 (20 mg mL-1, 0.4 mL),

9

and NAC (0.08 M, 4 mL) was incubated for 2.5 h at 37 °C. The products were purified by dialysis

10

and stored in the dark at 4 °C until use.

11

The GSH-AuNCs were prepared according the reported literature.24 The mixture of HAuCl4 (0.5

12

mL, 20 mM), GSH (0.15 mL, 100 mM), and water (4.35 mL) was incubated at 70 °C for 24 h. The

13

products were purified by dialysis and stored in the dark at 4 °C.

14

Preparation of NAC-AuNCs modified electrode. A bare GCE was polished sequentially with

15

alumina powder (1.0, 0.3 and 0.05 µm), and subsequently ultrasonically washed in 7.2 M HNO3,

16

ethanol, and water, respectively. The NAC-AuNCs modified GCE was obtained by dropping 5 µL of

17

the NAC-AuNCs solution (1.6 mg/mL, Figure S1) on the GCE surface and dried in air at room

18

temperature. The GSH-AuNCs/GCE was also prepared by the similar procedure.

19

Preparation

of

the

modified

electrodes

with

different

reduced

approaches.The

20

NAC-AuNCs/GCE was treated with electrochemical or chemical reduction methods. The

21

electrochemical reduction of NAC-AuNCs/GCE was performed by amperometric i-t technique at

22

different potentials including -0.2 V, -0.5 V, -0.8 V, -1.0 V, -1.2 V, -1.5 V, -1.7 V, -1.8 V, and -2.0

23

V for 5 min in 0.1 M PB (pH 7.4), and the electrode was recorded as ER-AuNCs/GCE. For the 4


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chemical reduction method, the NAC-AuNCs/GCE was immersed in 0.1 M NaBH4 for 5 min at

2

room temperature, and the electrode was recorded as CR-AuNCs/GCE. For the GSH-AuNCs, the

3

electrochemical reduction of GSH-AuNCs/GCE was performed by amperometric i-t technique at -2

4

V for 5 min. The chemical reduction of GSH-AuNCs/GCE was prepared similar to the

5

NAC-AuNCs/GCE. The resulting electrodes were thoroughly rinsed with water to remove excess

6

physical adsorption.

7

RESULTS AND DISCUSSION

8

Characterization of NAC-AuNCs.TEM revealed thatNAC-AuNCs were nearly spherical shape

9

with diameter of 2.5±0.5 nm (Figure S2A). The high-resolution TEM image showed lattice spacing

10

of 0.235 nm (inset of Figure S2A), which is corresponded to the (111) lattice face-centered cubic

11

Au.25 When excited at 355 nm, the NAC-AuNCs exhibited a strong emission peak centered at 650

12

nm (Figure S2B). These phenomena corresponded to our previous results, indicating the successfully

13

synthesized the NAC-AuNCs.23

14

ECL behaviors of the AuNCs. ECL behaviors of NAC-AuNCs at GCE were investigated with

15

cathodic coreactant K2S2O8. Figure 1A displays the ECL curves of bare GCE, NAC-AuNCs/GCE,

16

and ER-AuNCs/GCE by SP method with the potential step between 0 and -2 V, respectively. Weak

17

ECL emission was observed at bare GCE (Figure 1A, curve a), which was consistent with the

18

reported works.26-28 While the NAC-AuNCs/GCE had an obvious ECL emission (Figure 1A, curve

19

b), revealing that the ECL signal was generated from the NAC-AuNCs. The experimental results

20

suggested that NAC-AuNCs and S2O82- can form an ECL system (NAC-AuNCs/S2O82-), in which

21

NAC-AuNC is the luminophore and S2O82− acts as the coreactant. The ECL emission mechanism is

22

caused by the electron transfer annihilation between an anionic nanocluster radical (AuNC•−) and the

23

electrogenerated SO4•−. The possible ECL mechanism was described with the following equations:19 5



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AuNC + e− →AuNC•−

(1)

2

S2O82−+ e− → SO42− + SO4•−

(2)

3

AuNC•−+ SO4•− →AuNC* + SO42−

(3)

4

AuNC* →AuNC + hν

(4)

5

More interestingly, an obvious enhanced ECL signal was observed for ER-AuNCs/GCE, which

6

was about 30 times higher than that of NAC-AuNCs/GCE (Figure 1A, curve c). Based on these

7

results, we conceived that the reduction process of the AuNCs could play a vital role in enhancing

8

the ECL intensity in this ECL system. Furthermore, the ECL intensity of ER-AuNCs/GCE under

9

repeated SP scans in 0.1 M K2S2O8 remained at a constant value with relative standard deviation

10

(RSD) of 0.5% (Figure 1B), demonstrating that the ECL emission of ER-AuNCs/GCE was highly

11

repeatable and stable for further analytical application.

12

The ECL spectrum of ER-AuNCs was measured by a series of optical filters under SP conditions.

13

As shown in Figure 1C, ER-AuNCs had a maximum ECL emission at ~700 nm with a red-shift of

14

50 nm compared to that in the PL. This red-shift between ECL and PL emission has been also

15

observed in the previously reported ECL of CdSe nanocrystals,29 Si nanocrystals,30 and peptide

16

nanovesicles.31 We rationalize this result as the surface states role of luminophore species in the ECL

17

process. The ECL quantum efficiency (ΦECL) is described as the ratio of the number of emission

18

photons to that of chemiluminescent reactions between the redox species.32 ΦECL was obtained by

19

relative literature methods, and [Ru(bpy)3]2+ was used as a reference system.33 The ΦECL can be

20

defined by the following equation.34

21

ΦECL=Φ°ECL(IQ°f/I°Qf)

(5)

22

where Φ°ECLis the ECL efficiency of [Ru(bpy)3]2+, which is 5.0% with 1 mM [Ru(bpy)3]2+ in

23

TBAP/CAN (0.1 M), I and I° are the integrated ECL intensities of the ER-AuNCs and [Ru(bpy)3]2+, 6


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and Qf and Q°f are the passed charges for the ER-AuNCs and [Ru(bpy)3]2+, respectively.35,36 The

2

ΦECL of ER-AuNCs was calculated to be 4.11%, which was much higher than those of Mn@CdInS

3

film (2.1%)37, polyaniline-[Ru(bpy)2PIC]2+ (1.0%)38, and Mn-doped ZnS (0.3%)39. These results

4

suggested that ER-AuNCs could be an excellent and promising candidate of ECL luminophores for

5

ECL emitter and sensing.

6

Figure 1. (A) ECL-time curves of bare GCE (a), the NAC-AuNCs/GCE (b), and the

7

ER-AuNCs/GCE (c) in 0.1 M K2S2O8. Inset: the enlarged ECL-time curve of bare GCE (a). (B) ECL

8

signals of ER-AuNCs/GCE under continuous potential step between 0 and -2 V in 0.1 M K2S2O8. (C)

9

ECL spectrum of ER-AuNCs/GCE in PB (0.1 M, pH 7.4). The ER-AuNCs/GCE was prepared by

10

reduction of the NAC-AuNCs/GCE at -2 V. The ECL signals were generated by SP method with the

11

potential steps between 0 and -2 V, and the pulse periods were 10 s and 1 s, respectively.

12

The valence state effect on the ECL performance. The XPS, ECL and electrochemical

13

approaches were employed to explore our speculation that the valence state of Au played a vital role

14

in the ECL performance of AuNCs. XPS measurements were firstly performed to analyze the

15

valence states of Au and S in the NAC-AuNCs. As shown in Figure S3A, the Au 4f spectrum of the

16

untreated NAC-AuNCs could be deconvoluted to two distinct components with binding energies of

17

83.94 and 84.47 eV, which corresponded to Au(0) and Au(I), respectively. These results

18

demonstrated the coexistence of Au(0) and Au(I) in the untreated NAC-AuNCs,40,44 and the ratio of

7



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Au(0) was calculated to be 31.4%. The S (2p) XPS result of the original NAC-AuNCs revealed that

2

the asymmetric peak could be fitted by two bands centered at 162.7 eV and 164.0 eV (Figure S4A).

3

The dominant peak located at 162.7 eV was characteristic of gold surface-bounded sulfur atoms. As

4

expected, when the NAC-AuNCs were electrochemically reduced at negative potentials, the Au

5

(4f7/2) peak positions were shifted to lower binding energy with the increased electrochemical

6

reduction potential (Figure S3B to F, Table 1). It is worth noting that the peaks of the S (2p)

7

disappeared when the reduction potential was below -0.5 V (Figure S4B-J), implying that the

8

thiolates on the AuNCs surface could be fully desorbed at -0.5 V. Accordingly, the ratio of Au(0)

9

increased obviously with the reduced potentials from -0.2 to -0.5 V, while hardly increasement at the

10

potential range of -0.5~-0.8 V was observed, which was probably ascribed to the partly reduction of

11

Au(I) in the reductive desorption process of thiolate from nanocluster surface (Figure 2A). The ratio

12

of Au(0) increased sharply with the further increase of the reduction potentials from -0.8 to -1.5 V.

13

Furthermore, the Au 4f XPS spectrum showed the binding energy of 83.88 eV when the AuNCs

14

were pretreated at -1.5 V, indicating the sole existence of Au(0) (Figure S3G). Therefore, the Au(I)

15

in the NAC-AuNCs could be completely reduced by electrochemical method to the Au(0) when the

16

reduction potential was below -1.5 V.

17

Meanwhile, the effect of the reduction potential on the ECL intensities has also been investigated.

18

The ECL measurements of AuNCs were recorded with SP in 0.1 M K2S2O8. With the

19

electrochemically reduction of the NAC-AuNCs, the general trend of the increase of the ECL

20

intensity was corresponded to the ratio of the Au(0) (Figure 2B), which indicated that the increase of

21

the reductive degree of Au (I) gave rise to the enhancement of the ECL intensity. Especially, a good

22

linear relationship between the ratio of Au(0) and ECL intensity has been observed (r=0.997, Figure

23

2C), indicative of the important effect of the valence state of Au to the ECL performance of AuNCs. 8


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1

According to the previous reports, the red fluorescence of AuNCs has been suggested to be

2

originated from the Au(I) complex.23,45 In contrast, Au(0) core is main contribution of Au NCs in the

3

ECL, suggesting the different mechanisms of these two systems.

4

Table 1

5

XPS data of Au (4f7/2) of NAC-AuNCs by electrochemical reduction at different potentials. Reduction voltage (V)

6

a

Peak position (eV)

FWHM (eV)a

Area (P) (CPS·eV)

Au(0)

Au(I)

Au(0)

Au(I)

Au(0)

Au(I)

–

83.94

84.47

1.07

1.07

25350.59

55383.77

-0.2

84.19

85.28

1.22

1.15

43332.00

35830.56

-0.5

84.06

84.64

0.64

1.03

49499.00

26490.81

-0.8

83.89

84.72

1.28

1.28

50875.00

25624.16

-1.0

84.31

85.68

1.11

1.11

60473.40

18015.57

-1.2

83.56

84.41

0.78

1.05

63361.30

13217.50

-1.5

83.88

–

0.76

–

74280.71

0

-1.7

83.97

–

0.71

–

73122.30

0

-1.8

84.1

–

0.82

–

72567.05

0

-2.0

83.95

–

0.79

–

74858.64

0

FWHM：Full width at half maximum

7 8

Figure 2. Influence of the reduction potential on the ratio of Au(0) (A) and the ECL intensity (B). (C)

9

Linear relationship between the ratio of Au(0) and the ECL intensity.

10

To further demonstrate the above results, cyclic voltammetry (CV) was adopted to examine the

11

electrochemical properties of the NAC-AuNCs (Figure S5). Compared with bare GCE, the

12

NAC-AuNCs/GCE showed two reduction peaks at about -0.65 V and -1.5 V. We rationalize these 9



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peaks with the reductive desorption of thiol group from NAC-AuNCs/GCE surface (when the

2

potential is between -0.2 and -0.8 V) and the absolutely electrochemically reduction of Au(I) to Au(0)

3

(when the potential is below -0.8 V), respectively. The electrochemical results were corresponding to

4

the XPS spectrum of Au (4f) and S (2p). Therefore, the whole reduction process could be described

5

as follows:46

6

AuSR + e-→ Au (0) + RS-

(6)

7

Au(I) + e-→ Au(0)

(7)

8

In brief, the above results demonstrate that: (1) the ECL signal of the NAC-AuNCs can be

9

obviously enhanced by their electrochemical reduction; (2) the extent of the reduction of the AuNCs

10

depends on the reduction potential; (3) the desorption of the ligands has little influence on the

11

increase of the ECL signal; (4) the enhancement of the ECL signal is positively correlated with the

12

reduction degree of gold. The ECL emission process was illustrated in scheme 1. Therefore, we

13

could develop a novel high performance ECL luminophore by the proposed reduction method.

14 15

Scheme 1. Schematic illustration of the improved ECL performance.

16

To verify the versatility of the reduction method, chemical reduction of the NAC-AuNCs by

17

immersing NAC-AuNCs/GCE in 0.1 M NaBH4 for 5 min has been investigated. The XPS results

18

demonstrated that the Au(I) of NAC-AuNCs was absolutely reduced to Au(0) by the chemical

19

reduction method (Figure S6A), and the desorption of the thiol group from NAC-AuNCs was also 10


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1

observed (Figure S6B). As expected, a strong and stable ECL signal of the CR-AuNCs/GCE was

2

observed (Figure S6C).

3

On the other hand, in order to further explore the applicability and generality of this strategy for

4

other AuNCs, GSH-AuNCs was studied as another example. The XPS spectrum of Au (4f) and S

5

(2p) demonstrated that the Au(I) in the GSH-AuNCs was absolutely reduced to Au(0) both by the

6

electrochemical reduction (Figure S7) and chemical reduction (Figure S8), and the desorption of the

7

thiol group from NAC-AuNCs was also observed. As shown in Figure S6D, both of the

8

electrochemical and chemical reduction of the GSH-AuNCs modified GCE showed strong and

9

efficient ECL signals. Therefore, a general reduction approach for improving the ECL performance

10

has been developed, which could also open new routes to apply other NCs or QDs ECL in different

11

applications.

12

CONCLUSIONS

13

In summary, the valence states effect of the AuNCs to their ECL performance has been

14

investigated. Both of the electrochemical and chemical reduction methods have been employed to

15

modulate the valence state of Au in NAC-AuNCs. The results revealed that the reduction degree of

16

Au played a determinative role in the ECL property of AuNCs. The ECL intensity correlated well

17

with the reduction degree of the AuNCs. Furthermore, our system could effectively apply to the

18

other nanoclusters, such as GSH-AuNCs. Thus, a facile reduction approach for high performance

19

ECL behaviors from AuNCs has been developed. This study not only enriches the fundamental

20

study about the ECL performances of AuNCs, but also opens up the possibility for assembling

21

AuNCs based biocomposite for construction of ECL biosensors.

22

ASSOCIATED CONTENT

11



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Supporting Information

2

The effect of the concentration of the NAC-AuNCs; ECL efficiency of ER-AuNCs/GCE; TEM

3

image and photoluminescence spectra of NAC-AuNCs; XPS spectrum of Au (4f) and S (2p) of

4

NAC-AuNCs and the ER-AuNCs by electrochemical reduction at different potentials; CVs of the

5

bare GCE (black line) and the NAC-AuNCs/GCE; XPS spectrum of Au (4f) and S (2p) of

6

CR-AuNCs; ECL behaviors of the NAC-AuNCs after chemical reduction; ECL behaviors of the

7

GSH-AuNCs, and GSH-AuNCs/GCE after electrochemical and chemical reduction; XPS spectrum

8

of Au (4f) and S (2p) of GSH-AuNCs/GCE after electrochemical and chemical reduction. This

9

material is available free of charge via the Internet at http://pubs.acs.org.

10

AUTHOR INFORMATION

11

Corresponding Author

12

* E-mail: [email protected]

13

Notes

14

The authors declare no competing ﬁnancial interest.

15

ACKNOWLEDGMENT

16

This work is financial supported by the National Natural Science Foundation of China (21175023,

17

21675024, 21405015), Joint Funds for the Innovation of Science and Technology, Fujian Province

18

(2016Y9056, 2016Y9054), the Program for Innovative leading talents in Fujian Province

19

(2016B016), the Natural Science Foundation of Fujian Province (2016J01427), and the Medical

20

Innovation Project of Fujian Province (2014-CX-6).

21

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