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Ultrafine and well dispersed silver nanocrystals on 2D
2
nanosheets: synthesis and application as a multifunctional
3
material for electrochemical catalysis and biosensing
4 Tao Gao,a Dawei Yang,a Limin Ning,a Lin Lei,a Zonghuang Ye,a Genxi Li a, b, *
5
0F
a
6 7
8
State Key Laboratory of Pharmaceutical Biotechnology, Department of Biochemistry, Nanjing University, Nanjing 210093, China
b
Laboratory of Biosensing Technology, School of Life Sciences, Shanghai University,
9
Shanghai 200444, P R China
10
*Address correspondence to [email protected]
*
Corresponding author at Department of Biochemistry, Nanjing University, Nanjing
210093, China. Tel: +86 25 83593596. Fax: +86 25 83592510. 1
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Abstract
2
The strong coupling of inorganic nanocrystals with 2D nanosheet to produce
3
function-enhanced nano-materials with uniform size, dispersion, and high coverage
4
density has long been an interest to scientists from various research fields. Here, a
5
simple and effective method has been described to fabricate ultrafine and well
6
dispersed silver nanocrystals (AgNCs) on graphene oxide (GO), based on a
7
facial-induced co-reduction strategy. The synthesized nanohybrid has shown uniform
8
and well dispersed AgNCs (2.9 ± 1.4 nm), individually separated GO sheets, as well
9
as highly covered surface (5250 nanocrystals per square micrometer), indicating the
10
formation of a high-quality GO-based nanohybrid. Moreover, this material shows
11
excellent catalytic activity for oxygen reduction reaction (ORR) and exhibits
12
enhanced signal readout for molecular sensing, demonstrating the potential
13
application of this newly synthesized inorganic hybrid with strong synergistic
14
coupling effects on advanced functional systems.
15
Keywords: hybrid 2D nanosheets, graphene oxide, oxygen reduction reaction,
16
electrochemical sensing
17
2
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1. Introduction
2
Taking advantages of abundant oxygen functional groups, large surface area, as
3
well as excellent aqueous dispersibility and stability,1,2 2D-structured GO is recently
4
emerging as a promising platform for GO-based functional materials.3-6 Among the
5
fabricated materials, those involving inorganic nanocrystals (metal, metal oxide,
6
semiconductor, and magnetic nanoparticles) are of particular interest owing to their
7
exceptional structural properties7 and intrinsic catalytic characteristics.8-10 Such
8
hybrids have led to a variety of promising applications, such as electrocatalysis,11-14
9
photocatalysis,15 energy conversion and storage,16-21 and biosensing.22-25
10
Synthesis of high-quality inorganic nanomaterials is the prerequisite of excellent
11
performance for their possible applications.26,27 To date, several methods have been
12
developed to integrate inorganic nanocrystals onto the surface of GO. They can be
13
generally classified as ex situ hybridization strategy and in situ crystallization strategy
14
according to the synthetic routes.6,11 The ex situ hybridization strategy usually
15
involves the pre-modifications of GO or nanocrystals, followed by physical
16
mixture.14,28 Although this strategy enables us to select inorganic nanocrystals with
17
desired properties in advance, it sometimes suffers from low coverage density and
18
non-uniform dispersion of the nanocrystals, together with the stacking and
19
agglomeration of GO-based nanosheets after physical mixing.1,2 The in situ
20
crystallization strategy means the direct nucleation and growth of inorganic
21
nanocrystals on GO flakes.11,16,29 Disappointedly, the strategy is not satisfactory either, 3
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because it is confronted with the failure of selective nucleation and growth of
2
inorganic nanocrystals on GO sheets over self-seeding nucleation in the solution.
3
Besides, effective control of the size, dispersibility, and coverage density of the
4
anchored inorganic nanocrystals is also challenging.5 These obstacles have greatly
5
posed limitations on the synthesis and application of these GO-based inorganic
6
materials.
7
Herein, we describe a facial-induced co-reduction strategy for the synthesis of
8
ultrafine, uniform and well dispersed silver nanocrystals (AgNCs) on individually
9
separated GO sheets (GO-Ag). Based on the surface-controlled reaction, AgNCs can
10
nucleate, crystallize and grow well on GO sheets. In addition, the particle size and
11
coverage density can be controlled by adjusting the initial concentration of AgNO3.
12
Moreover, the synthesized GO-Ag hybrid can exhibit significant Raman-scattering
13
enhancement for signal amplification, excellent electrochemical catalytic activity for
14
oxygen reduction reaction (ORR) and enhanced electrochemical signal readout for
15
various analytes, including drugs (doxorubicin, mitoxantrone and salicylic acid),
16
proteins (cytochrome c and myoglobin) and protein enzyme (horseradish peroxidase).
17
Therefore, the synthesized GO-Ag hybrid can be a promising functional material for
18
the fabrication of advanced platforms for catalysis, sensing, etc.
19
2. Experimental Section
20
Materials and reagents: Chemically exfoliated GO was obtained from Nanjing
21
XFNANO Materials Tech Co., Ltd. and was further purified by ultrasonic wash and 4
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centrifugation filtering. Silver nitrate (AgNO3, ≥99.8%), hydroquinone (HQ, ≥99%),
2
doxorubicin, mitoxantrone hydrochloride, salicylic acid, cytochrome c, myoglobin
3
and horseradish peroxidase (HRP) were purchased from Sigma-Aldrich. Other
4
chemicals were all of analytical grade and were used without further purification. All
5
solutions were prepared with deionized water, which was purified with the Milli-Q
6
purification system (Bedford, MA) to a specific resistance of 18.2 MΩ.
7
Synthesis of GO-Ag hybrid: Before the synthesis, the obtained GO was purified to
8
remove large flakes and impurities. Firstly, GO flakes were dispersed in deionized
9
water (1.5 mg mL-1) and sonicated in a bath-type sonicator (SB-80, Scientz, China)
10
for 60 min. Then, the solution was centrifuged at 1000 rpm for 5 min to remove large
11
precipitates. After that, GO sheets were washed several times by centrifugation
12
filtration and finally dried at 80 °C. For the synthesis of GO-Ag hybrid, 400 µL of 0.4
13
mM HQ solution was added to a 500 µL GO dispersion (0.05 mg mL-1). After
14
agitation, 100 µL designed concentration of AgNO3 solution were added. The mixture
15
was immediately sonicated for 2 min. Then, it was brought to a water bath at 50 °C
16
for further reaction in the dark. After 20 min, centrifugation filtering were carried out
17
repeatedly to eliminate free Ag+ and HQ molecules that were not bound to form the
18
GO-Ag hybrid. The synthesized hybrid could be readily re-dispersed well in
19
deionized water, and they can be stored at room temperature with high stability.
20
Characterizations: All the samples were dispersed in deionized water for
21
characterization. The hydrodynamic diameters (DH) of the purified GO sheets were 5
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determined by dynamic light scattering (DLS) by using a particle size analyzer
2
(Brookhaven 90Plus, USA) under the spread sheet mode. The scattering angle was
3
15° and the experimental temperature was 25 °C. Transmission electron microscopes
4
(TEM) and high-resolution transmission electron microscopy (HRTEM) observations
5
were carried out on Hitachi H7650 and JEOL 2200FS microscopes with accelerating
6
voltages of 80 kV and 200 kV, respectively. The TEM specimens were prepared by
7
dispersing the GO-Ag hybrid on 400 mesh size copper grids. Measurements of Raman
8
spectra were carried out by using a JY LabRam HR800 UV micro-Raman system
9
with a 514.5 nm line of Ar+ laser as the excitation source. The X-ray photoelectron
10
spectroscopy (XPS) analysis was performed on a UIVAC-PHI5000 VersaProbe
11
scanning microprobe device. The specimens for XPS were deposited onto silicon
12
wafers before characterization. Ultraviolet–visible (UV-vis) spectra were recorded by
13
a Shimadzu UV-2450 spectrophotometer. The changes of pH value of the reaction
14
solution were recorded by a Delta 320 pH meter (Mettler Toledo, Switzerland) at 25
15
°C.
16
Electrochemical
measurements:
All
electrochemical
measurements
were
17
performed on a CHI660D Potentiostat (CH Instruments, Chenhua, Shanghai) work
18
station at room temperature. Cyclic voltammograms (CVs), electrochemical
19
impedance spectra (EIS) and square wave voltammograms (SWVs) were recorded by
20
using a conventional three-electrode system, which included a pre-modified glass
21
carbon electrode as the working electrode (3 mm in diameter), a saturated calomel 6
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electrode (SCE, saturated KCl) as the reference electrode and a platinum pillar as the
2
counter electrode, respectively.
3
In the electrochemical experiments for ORR, the modification procedures of
4
working electrodes were as follows: 10 µL of the GO-Ag hybrid (2.0 mg mL-1 in a
5
25% ethanol solution) was dropped on the surface of polished GC electrode. After the
6
hybrids were dried and stuck to the electrode surface under an infrared lamp, 5 µL of
7
a nafion solution (about 0.5% in a 25% ethanol solution) was dropped as a binder.
8
The modified electrode was then dried and brought to measurements immediately.
9
The CVs for ORR were recorded by scanning the potential from -0.8 to 0.2 V (versus
10
SCE) at a scan rate of 50 mV s-1. The electrolyte was 0.1 M KOH, which was bubbled
11
with ultra-high purity O2 or N2 for 30 min and the entire experimental procedures
12
were under N2 or O2 atmosphere.
13
The GO modified GC electrode (GO/GC) and GO-Ag modified GC electrode
14
(GO-Ag/GC) for electrochemical sensing were prepared by casting 10 µL of GO
15
suspension (25 µg mL-1) and GO-Ag hybrid suspension (25 µg mL-1) on electrode
16
surface and dried under an infrared lamp. For the analysis of proteins, GC, GO/GC
17
and GO-Ag/GC electrodes were further coated with 5 µL protein solution (1.0 mg
18
mL-1), then air-dried at 4°C, followed by dropping 2 µL of 0.5% nafion solution as a
19
binder. These electrodes were then transferred to 0.1 M phosphate buffer solution
20
(PBS, pH 7.0) before measurements. The electrochemical response of different
21
analytes on GC, GO/GC and GO-Ag/GC electrodes were recorded by SWVs. The 7
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experimental parameters were as follows: step potential, 5 mV; frequency, 10 Hz;
2
amplitude, 20 mV. The supporting electrolyte was 0.1 M PBS (pH 7.0), which was
3
bubbled with ultra-high purity N2.
4
3. Result and Discussion
5
GO sheets usually suffer from self-crispation and stacked structures among these
6
large-area, flexible and ultrathin basal planes due to hydrogen bonding
7
interaction.1,30-32 Such interaction may significantly affect distribution of the reactants
8
to be functionalized on these sheets.33 Therefore, before the synthesis of GO-Ag
9
hybrid, two complementary methods have been used to get high-quality GO sheets in
10
aqueous solution. First, GO sheets with large basal plane are removed by
11
ultrasonication and centrifugation filtration. The TEM images in Fig. S1 in Electronic
12
Supplementary Information show a significant size change of GO from large sheets
13
into small ones after ultrasonication and centrifugation filtration. Second, initially
14
stacked GO structures are conversed into individually stretched sheets after the
15
addition of hydroquinone (HQ). As shown in Fig. 1a and b, the average
16
hydrodynamic diameter (HD) of GO sheets has changed from 490.4 nm to 820.2 nm
17
after its incubation with HQ. This may indicate that the addition of HQ maintains the
18
wide stretch of GO sheets in aqueous solution (Fig. 1c).
19
On the individually separated GO sheets, GO-Ag hybrid has been fabricated. Fig.
20
2a shows the UV-vis spectra of the reaction solution. A characteristic absorption peak
21
of AgNCs can be displayed after the reaction, indicating the formation of AgNCs. The 8
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insets further shows that the adsorption peaks of the solution have a red-shift of 32 nm
2
and a blue-shift of 27 nm compared with that of colloid AgNCs and GO, respectively.
3
These shifts may indicate the formation of GO-Ag hybrid. Interestingly, we find that
4
neither HQ nor GO can separately reduce Ag+ into Ag0, unless combined use of the
5
two species (Fig. 2b). This observation reveals that GO may help to perform the
6
reduction capacity of HQ in a surface-confined manner. To find a clue in this
7
observation, we have monitored the pH changes during the reaction. As shown in Fig.
8
3a, the reaction can be divided into three stages according to the variation of reaction
9
rates, which is calculated from the changes of hydrogen ion concentration ([H+]). At
10
the first stage, from 1 to 3 min, a small increase of [H+] is observed after the addition
11
of AgNO3. We attribute this to the cation exchange of Ag+ with protons on GO
12
sheets34,35 at the begining of nucleation. At the second stage from 3 to 15 min, a fast
13
reaction rate is observed, indicating the surface-anchored small AgNCs may act as a
14
catalyzer36,37 to co-reduce Ag+ and GO along with the oxidation of HQ molecules. At
15
the third stage, after 15 min, the reaction slowly comes to a stop as the result of
16
consumption of HQ. The proposed nucleation stages may be further confirmed by the
17
polycrystal structure of AgNCs, among which there’s a core siting on one side (Fig.
18
5a). HRXPS has been further used to analyze different chemical states of carbon
19
atoms in GO and GO-Ag hybrid to reveal the reaction process. As shown in Fig. 3b,
20
different chemical states of carbon are easily observed, which appear at 284.8 eV for
21
C=C/C–C, 286.8 eV for C–O, and 289.0 eV for O–C=O bonds, respectively. After 9
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incubation GO with HQ for 30 min, the content of the C–O group keeps almost the
2
same (from initial 42.4% to 41.2%). However, after the formation of GO-Ag hybrid,
3
the content of C–O group decreases significantly to 27.2%. This may imply that GO
4
can provide the reactive sites for the nucleation and growth of AgNCs, then induce
5
the co-reduction of GO and Ag+ by HQ, and finally facilitate the formation of GO-Ag
6
hybrid (Fig. 3c).
7
In order to get high-quality GO-Ag hybrid, the initial concentration of AgNO3 has
8
been optimized. As shown by the inserted photograph in Fig. S2, with the increased
9
concentration of AgNO3 from 0 mM to 15 mM, the reaction solution shows a gradual
10
colour change. The corresponding UV-vis spectra also show enhanced adsorption
11
peaks of GO-Ag hybrid. However, the hybrid tends to precipitate when the
12
concentration of AgNO3 is higher than 9 mM. This observation indicates a higher
13
loading of silver nanocrystals on GO sheets. TEM images is further used to confirm
14
the morphological changes of GO-Ag hybrid. As shown in Fig. 4, with the increased
15
concentration of AgNO3, larger AgNCs (average size increases from 2.9±1.4 to 27.4
16
±10.6 nm) are formed and cover almost the whole surface of GO sheets. When the
17
concentration is increased to be 9 mM, uneven AgNCs are formed, which may impair
18
the stability of this material in aqueous solution after most of the hydrophilic groups
19
on colloidal GO flakes are covered by AgNCs.30 Based on these observations, it is
20
concluded that AgNCs can be effectively anchored on GO sheets. Moreover, the
10
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particle size and coverage density can be controlled by adjusting the initial
2
concentration of AgNO3 from 0 mM to 9 mM.
3
The fine details of the synthesized GO-Ag hybrid have been characterized. HRTEM
4
image in Fig. 5a shows that AgNCs are confined to the surface of 2D-structured
5
nanosheet and are evenly dispersed with uniform size, which is calculated to be 2.9±
6
1.4 nm. Also, the surface of GO is highly covered with AgNCs (5250 per square
7
micrometer). In the inset, the lattice spacing of Ag0 reveals periodicities of 0.20 nm
8
and 0.24 nm from separates sites, corresponding to the (200) and (111) plane of Ag0,
9
respectively. For further analysis of the GO-Ag hybrid, Raman spectroscopy is used.
10
As shown in Fig. 5b, the intensity of D-band (1350 cm-1) and G-band (1595 cm-1) for
11
GO has been enhanced significantly after coupling of AgNCs, which arises from the
12
increased Raman scattering cross sections of high-density metallic nanogaps.31
13
Moreover, a low enhancement factor for GO-Ag hybrid indicates the formation of
14
charge transfer complexes between AgNCs and GO sheet.40 Full XPS survey
15
spectrum in Fig. 5c has clearly defined the existence of carbon (72.40%), oxygen
16
(25.82%) and silver (1.78%). HRXPS analysis also shows the 3d3/2 and 3d5/2 doublet
17
of silver, whose core levels are located at 368.2 eV and 374.2 eV, respectively. These
18
peaks suggest the successful fabrication of AgNCs on GO. The above properties
19
indicate the formation of high-quality GO-based nanohybrid. A comparison with the
20
previously reported hybrids may further show the superior properties of this
21
synthesized GO-Ag hybrid (see Table S1 in Electronic Supplementary Information). 11
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To demonstrate the potential applications of this material in functional systems,
2
catalytic activity and sensing ability of GO-Ag hybrid have been examined. Ag-based
3
materials are recently investigated as electrocatalysts for ORR in alkaline fuel cells
4
because of their thermodynamical and electrochemical stability, higher tolerance to
5
methanol crossover, and lower cost than commercial Pt/C catalyst.40-42 So, firstly, the
6
electrocatalytic activity of this synthesized material for ORR has been characterized
7
by cyclic voltammetry. As shown in Fig. 6a, in contrary to the cyclic voltammogram
8
(CV) in N2 saturated electrolyte, the CV in O2 saturated electrolyte shows an obvious
9
reduction current peak from ORR. The reduction current density has reached −1.06
10
mA cm−2 at −0.46 V, indicating relative high activity towards ORR when compared to
11
GO nanosheets and comercial Pt/C (Fig. S4). Moreover, in the durability tests (Fig.
12
6b) and methanol-tolerant experiments (Fig. 6c), the hybrid still remain high
13
electroactivity towards ORR. These results indicates that this synthesized GO-Ag
14
hybrid may provide potential applications in fuel cells as an efficient platinum-free
15
catalyst with high catalytic activity and low cost.
16
Since the electrochemical response is highly sensitive to the physicochemical
17
properties of an electrode surface,43 electrochemical performance of the working
18
electrode modified with GO-Ag hybrid has been examined. By using 5 mM
19
Fe(CN)63-/4- redox probe as the indicator, experimental results obtained by cyclic
20
voltammetry and electrochemical impedance spectra techniques have all confirmed
21
the enhanced electron transfer ability of the working electrode by this material (see 12
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Fig. S3 in Electronic Supplementary Information). This property may contribute
2
much to the application of this material for electrochemical sensing. In addition, to
3
further test its sensing ability, drugs (doxorubicin, mitoxantrone and salicylic acid),
4
protein (cytochrome c and myoglobin) and protein enzyme (HRP) are chosen as the
5
representative analytes to study their electrochemical responses on GO-Ag modified
6
glassy carbon (GC) electrode. Fig. 7 shows the electrochemical responses of these
7
molecules at the electrode surfaces of GC (black curve), GO/GC (blue curve) and
8
GO-Ag/GC (red curve), respectively. As expected, GO-Ag/GC electrodes exhibit
9
higher current peaks, as well as increased cathodic potentials (or decreased anodic
10
potential) for the reduction (Fig. 7a, b, d-f) (or oxidation (Fig. 7c)) of the analytes.
11
This is because the hybrid GO-Ag nanomaterial may provide large electro-active
12
surface for molecule absorption, so it may facilitate the electron transfer at the sensing
13
interface, and consequently give rapid and enhanced current responses. Therefore,
14
GO-Ag can be used as a new electrode hybrid material for the fabrication of superior
15
electrochemical sensors.
16 17
4. Conclusion
18
In summary, we have developed a simple and effective method to prepare
19
high-quality GO-Ag hybrid. The AgNCs on GO surfaces are uniform and
20
well-dispersed, which have shown many improved properties compared to previously
21
reported materials. Meanwhile, we have presented the inductive effects of GO on 13
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preparing inorganic nanocrystals on 2D nanosheets, which is based on the
2
surface-induced co-reduction of Ag+ and GO by HQ. This not only makes it possible
3
to understand the surface-confined reactivity and intrinsic kinetic electron transfer
4
properties during the functionalization of GO, but also provides a new route to
5
prepare other GO-based inorganic hybrids with improved performance in fuel cells,
6
catalysis, energy conversion and storage, as well as electrochemical sensors.
7 8
Acknowledgements
9
This work is supported by the National Natural Science Foundation of China (Grant
10
No. 21235003), and the National Science Fund for Distinguished Young Scholars
11
(Grant No. 20925520).
12
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DOI: 10.1039/C4NR04283B
Nanoscale
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21. H. Wang and H. Dai, Chem. Soc. Rev., 2013, 42, 3088-3113.
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22. Y. Liu, X. Dong and P. Chen, Chem. Soc. Rev., 2012, 41, 2283-2307.
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23. S. Wu, Q. He, C. Tan, Y. Wang and H. Zhang, Small, 2013, 9, 1160-1172.
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24. M. Liu, H. M. Zhao, S. Chen, H. T. Yu and X. Quan, ACS Nano, 2012, 6,
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3142-3151.
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25. Zhang, Y. M. Guo, Y. L. Xianyu, W. W. Chen, Y. Y. Zhao and X. Y. Jiang, Adv.
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Mater., 2013, 25, 3802-3819.
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26. J. N. Tiwari, K. Nath, S. Kumar, R. N. Tiwari, K. C. Kemp, N. H. Le, D. H. Youn,
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J. S. Lee and K. S. Kim, Nat. Commun., 2013, 4, 2221.
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27. Q. Wang, X. Guo, L. Cai, Y. Cao, L. Gan, S. Liu, Z. Wang, H. Zhang and L. Li,
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Chem. Sci., 2011, 2, 1860-1864.
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28. J. B. Liu, S. H. Fu, B. Yuan, Y. L. Li and Z. X. Deng, J. Am. Chem. Soc., 2010,
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132, 7279-7281.
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29. I. V. Lightcap, T. H. Kosel and P. V. Kamat, Nano Lett., 2010, 10, 577-583.
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30. N. V. Medhekar, A. Ramasubramaniam, R. S. Ruoff and V. B. Shenoy, ACS
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Nano, 2010, 4, 2300-2306.
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31. L. Wu, L. Liu, B. Gao, R. Munoz-Carpena, M. Zhang, H. Chen, Z. Zhou and H.
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Wang, Langmuir, 2013, 29, 15174-15181.
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32. C. J. Shih, S. Lin, R. Sharma, M. S. Strano and D. Blankschtein, Langmuir, 2012,
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28, 235-241.
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33. B. J. Hong, O. C. Compton, Z. An, I. Eryazici and S. T. Nguyen, ACS Nano,
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2012, 6, 63-73.
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34. J. Balapanuru, J. X. Yang, S. Xiao, Q. L. Bao, M. Jahan, L. Polavarapu, J. Wei, Q.
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H. Xu and K. P. Loh, Angew. Chem. Int. Ed., 2010, 49, 6549-6553.
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35. G. K. Ramesha, A. V. Kumara, H. B. Muralidhara and S. Sampath, J. Colloid
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Interface Sci., 2011, 361, 270-277.
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36. T. H. James, J. Am. Chem. Soc., 1939, 61, 648-652.
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37. A. Panáček, R. Prucek, J. Hrbáč, T. j. Nevečná, J. Šteffková, R. Zbořil and L.
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Kvítek, Chem. Mater., 2014, 26, 1332-1339.
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38. G. Lu, H. Li, S. X. Wu, P. Chen and H. Zhang, Nanoscale, 2012, 4, 860-863.
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Car, Nano Lett., 2008, 8, 36-41.
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Nanoscale Accepted Manuscript
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Figures and Captions:
2 3 4 5 6
Fig. 1. The average hydrodynamic diameters of GO sheets (0.05 mg mL-1): (a) in the absence of HQ; (b) in the presence of 0.2 mM HQ in aqueous solution. (c) Proposed scheme for the separation of GO sheets by HQ.
7 8 9 10 11 12 13
Fig. 2. (a) The UV-vis spectra of GO (black curve), AgNCs (blue curve) and GO-Ag hybrid (red curve) in aqueous solution. Inserted curves show absorption peak shifts of the synthesized GO-Ag hybrid compared to GO and AgNCs, respectively. (b) The UV-vis spectra of reaction solutions in the presence (red curve) and absence (red curve) of 0.1 mM HQ, respectively. The blue curve shows the UV-vis spectra of the reaction solution in the absence of GO.
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Fig. 3. (a) The pH values of HQ, GO and their mixture in aqueous solution. The right part shows plots of dynamic pH value changes of the reaction solution and corresponding H+ concentration changes against time. (b) Typical XPS spectra of C1s on GO before and after the addition of HQ, as well as on GO-Ag hybrid (from left to right). (c) A scheme proposed for the probable growth stages of AgNCs on GO sheets.
19
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1 2 3 4 5
Fig. 4. Typical TEM iamges (a-f) of GO-Ag hybrid prepared from different concentrations of AgNO3 (0, 3, 6, 9, 12 and 15 mM). The inserted histograms show the corresponding size distributions of AgNCs anchored on GO sheets.
6 20
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Fig. 5. (a) Typical HRTEM image of GO-Ag hybrid. The concentration of AgNO3 is 3 mM; scale bar is 50 nm. The histogram shows the size distribution of AgNCs (2.9±1.4 nm). The inset: HRTEM image of an individual AgNC on GO sheets. The lattice spacing are (1) 0.20 nm and (2) 0.24 nm from separates sites, scale bar is 2 nm. (b) Typical Raman spectra of GO and GO-AgNP hybrid. (c) Typical XPS full spectra of GO-Ag hybrid. (d) HRXPS spectra of Ag 3d.
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8 9 10 11 12 13
Fig. 6. (a) Cyclic voltammograms of the GO-Ag/GC electrodes in 0.1 M KOH solution saturated with N2 and O2, respectively. (b) Cyclic voltammograms for the durability test of GO-Ag hybrid for ORR, from 1st to 100th cycle. (c) Cyclic voltammograms to show the influence of 0.5 M methanol over GO-Ag/GC electrode. The poteintial scan rate is 50 mV s-1.
21
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1 2 3 4 5 6
Fig. 7. Square wave voltammograms of drugs (a-c: doxorubicin, mitoxantrone and salicylic acid), proteins (d-e: cytochrome c and myoglobin) and protein enzyme (f: HRP) obtained on GC (black curves), GO/GC (blue curves) and GO-Ag/GC (red curves) electrodes. The supporting electrolyte is 0.1 M PBS, pH 7.0. The step potential is 5 mV, scanned from left to right.
22
Nanoscale Accepted Manuscript
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DOI: 10.1039/C4NR04283B
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. Gao, D. Yang, L. Ning, L. Lei, Z. Ye and G. Li, Nanoscale, 2014, DOI: 10.1039/C4NR04283B.
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Ultrafine and well dispersed silver nanocrystals on 2D
2
nanosheets: synthesis and application as a multifunctional
3
material for electrochemical catalysis and biosensing
4 Tao Gao,a Dawei Yang,a Limin Ning,a Lin Lei,a Zonghuang Ye,a Genxi Li a, b, *
5
0F
a
6 7
8
State Key Laboratory of Pharmaceutical Biotechnology, Department of Biochemistry, Nanjing University, Nanjing 210093, China
b
Laboratory of Biosensing Technology, School of Life Sciences, Shanghai University,
9
Shanghai 200444, P R China
10
*Address correspondence to [email protected]
*
Corresponding author at Department of Biochemistry, Nanjing University, Nanjing
210093, China. Tel: +86 25 83593596. Fax: +86 25 83592510. 1
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DOI: 10.1039/C4NR04283B
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Abstract
2
The strong coupling of inorganic nanocrystals with 2D nanosheet to produce
3
function-enhanced nano-materials with uniform size, dispersion, and high coverage
4
density has long been an interest to scientists from various research fields. Here, a
5
simple and effective method has been described to fabricate ultrafine and well
6
dispersed silver nanocrystals (AgNCs) on graphene oxide (GO), based on a
7
facial-induced co-reduction strategy. The synthesized nanohybrid has shown uniform
8
and well dispersed AgNCs (2.9 ± 1.4 nm), individually separated GO sheets, as well
9
as highly covered surface (5250 nanocrystals per square micrometer), indicating the
10
formation of a high-quality GO-based nanohybrid. Moreover, this material shows
11
excellent catalytic activity for oxygen reduction reaction (ORR) and exhibits
12
enhanced signal readout for molecular sensing, demonstrating the potential
13
application of this newly synthesized inorganic hybrid with strong synergistic
14
coupling effects on advanced functional systems.
15
Keywords: hybrid 2D nanosheets, graphene oxide, oxygen reduction reaction,
16
electrochemical sensing
17
2
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1. Introduction
2
Taking advantages of abundant oxygen functional groups, large surface area, as
3
well as excellent aqueous dispersibility and stability,1,2 2D-structured GO is recently
4
emerging as a promising platform for GO-based functional materials.3-6 Among the
5
fabricated materials, those involving inorganic nanocrystals (metal, metal oxide,
6
semiconductor, and magnetic nanoparticles) are of particular interest owing to their
7
exceptional structural properties7 and intrinsic catalytic characteristics.8-10 Such
8
hybrids have led to a variety of promising applications, such as electrocatalysis,11-14
9
photocatalysis,15 energy conversion and storage,16-21 and biosensing.22-25
10
Synthesis of high-quality inorganic nanomaterials is the prerequisite of excellent
11
performance for their possible applications.26,27 To date, several methods have been
12
developed to integrate inorganic nanocrystals onto the surface of GO. They can be
13
generally classified as ex situ hybridization strategy and in situ crystallization strategy
14
according to the synthetic routes.6,11 The ex situ hybridization strategy usually
15
involves the pre-modifications of GO or nanocrystals, followed by physical
16
mixture.14,28 Although this strategy enables us to select inorganic nanocrystals with
17
desired properties in advance, it sometimes suffers from low coverage density and
18
non-uniform dispersion of the nanocrystals, together with the stacking and
19
agglomeration of GO-based nanosheets after physical mixing.1,2 The in situ
20
crystallization strategy means the direct nucleation and growth of inorganic
21
nanocrystals on GO flakes.11,16,29 Disappointedly, the strategy is not satisfactory either, 3
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because it is confronted with the failure of selective nucleation and growth of
2
inorganic nanocrystals on GO sheets over self-seeding nucleation in the solution.
3
Besides, effective control of the size, dispersibility, and coverage density of the
4
anchored inorganic nanocrystals is also challenging.5 These obstacles have greatly
5
posed limitations on the synthesis and application of these GO-based inorganic
6
materials.
7
Herein, we describe a facial-induced co-reduction strategy for the synthesis of
8
ultrafine, uniform and well dispersed silver nanocrystals (AgNCs) on individually
9
separated GO sheets (GO-Ag). Based on the surface-controlled reaction, AgNCs can
10
nucleate, crystallize and grow well on GO sheets. In addition, the particle size and
11
coverage density can be controlled by adjusting the initial concentration of AgNO3.
12
Moreover, the synthesized GO-Ag hybrid can exhibit significant Raman-scattering
13
enhancement for signal amplification, excellent electrochemical catalytic activity for
14
oxygen reduction reaction (ORR) and enhanced electrochemical signal readout for
15
various analytes, including drugs (doxorubicin, mitoxantrone and salicylic acid),
16
proteins (cytochrome c and myoglobin) and protein enzyme (horseradish peroxidase).
17
Therefore, the synthesized GO-Ag hybrid can be a promising functional material for
18
the fabrication of advanced platforms for catalysis, sensing, etc.
19
2. Experimental Section
20
Materials and reagents: Chemically exfoliated GO was obtained from Nanjing
21
XFNANO Materials Tech Co., Ltd. and was further purified by ultrasonic wash and 4
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centrifugation filtering. Silver nitrate (AgNO3, ≥99.8%), hydroquinone (HQ, ≥99%),
2
doxorubicin, mitoxantrone hydrochloride, salicylic acid, cytochrome c, myoglobin
3
and horseradish peroxidase (HRP) were purchased from Sigma-Aldrich. Other
4
chemicals were all of analytical grade and were used without further purification. All
5
solutions were prepared with deionized water, which was purified with the Milli-Q
6
purification system (Bedford, MA) to a specific resistance of 18.2 MΩ.
7
Synthesis of GO-Ag hybrid: Before the synthesis, the obtained GO was purified to
8
remove large flakes and impurities. Firstly, GO flakes were dispersed in deionized
9
water (1.5 mg mL-1) and sonicated in a bath-type sonicator (SB-80, Scientz, China)
10
for 60 min. Then, the solution was centrifuged at 1000 rpm for 5 min to remove large
11
precipitates. After that, GO sheets were washed several times by centrifugation
12
filtration and finally dried at 80 °C. For the synthesis of GO-Ag hybrid, 400 µL of 0.4
13
mM HQ solution was added to a 500 µL GO dispersion (0.05 mg mL-1). After
14
agitation, 100 µL designed concentration of AgNO3 solution were added. The mixture
15
was immediately sonicated for 2 min. Then, it was brought to a water bath at 50 °C
16
for further reaction in the dark. After 20 min, centrifugation filtering were carried out
17
repeatedly to eliminate free Ag+ and HQ molecules that were not bound to form the
18
GO-Ag hybrid. The synthesized hybrid could be readily re-dispersed well in
19
deionized water, and they can be stored at room temperature with high stability.
20
Characterizations: All the samples were dispersed in deionized water for
21
characterization. The hydrodynamic diameters (DH) of the purified GO sheets were 5
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determined by dynamic light scattering (DLS) by using a particle size analyzer
2
(Brookhaven 90Plus, USA) under the spread sheet mode. The scattering angle was
3
15° and the experimental temperature was 25 °C. Transmission electron microscopes
4
(TEM) and high-resolution transmission electron microscopy (HRTEM) observations
5
were carried out on Hitachi H7650 and JEOL 2200FS microscopes with accelerating
6
voltages of 80 kV and 200 kV, respectively. The TEM specimens were prepared by
7
dispersing the GO-Ag hybrid on 400 mesh size copper grids. Measurements of Raman
8
spectra were carried out by using a JY LabRam HR800 UV micro-Raman system
9
with a 514.5 nm line of Ar+ laser as the excitation source. The X-ray photoelectron
10
spectroscopy (XPS) analysis was performed on a UIVAC-PHI5000 VersaProbe
11
scanning microprobe device. The specimens for XPS were deposited onto silicon
12
wafers before characterization. Ultraviolet–visible (UV-vis) spectra were recorded by
13
a Shimadzu UV-2450 spectrophotometer. The changes of pH value of the reaction
14
solution were recorded by a Delta 320 pH meter (Mettler Toledo, Switzerland) at 25
15
°C.
16
Electrochemical
measurements:
All
electrochemical
measurements
were
17
performed on a CHI660D Potentiostat (CH Instruments, Chenhua, Shanghai) work
18
station at room temperature. Cyclic voltammograms (CVs), electrochemical
19
impedance spectra (EIS) and square wave voltammograms (SWVs) were recorded by
20
using a conventional three-electrode system, which included a pre-modified glass
21
carbon electrode as the working electrode (3 mm in diameter), a saturated calomel 6
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electrode (SCE, saturated KCl) as the reference electrode and a platinum pillar as the
2
counter electrode, respectively.
3
In the electrochemical experiments for ORR, the modification procedures of
4
working electrodes were as follows: 10 µL of the GO-Ag hybrid (2.0 mg mL-1 in a
5
25% ethanol solution) was dropped on the surface of polished GC electrode. After the
6
hybrids were dried and stuck to the electrode surface under an infrared lamp, 5 µL of
7
a nafion solution (about 0.5% in a 25% ethanol solution) was dropped as a binder.
8
The modified electrode was then dried and brought to measurements immediately.
9
The CVs for ORR were recorded by scanning the potential from -0.8 to 0.2 V (versus
10
SCE) at a scan rate of 50 mV s-1. The electrolyte was 0.1 M KOH, which was bubbled
11
with ultra-high purity O2 or N2 for 30 min and the entire experimental procedures
12
were under N2 or O2 atmosphere.
13
The GO modified GC electrode (GO/GC) and GO-Ag modified GC electrode
14
(GO-Ag/GC) for electrochemical sensing were prepared by casting 10 µL of GO
15
suspension (25 µg mL-1) and GO-Ag hybrid suspension (25 µg mL-1) on electrode
16
surface and dried under an infrared lamp. For the analysis of proteins, GC, GO/GC
17
and GO-Ag/GC electrodes were further coated with 5 µL protein solution (1.0 mg
18
mL-1), then air-dried at 4°C, followed by dropping 2 µL of 0.5% nafion solution as a
19
binder. These electrodes were then transferred to 0.1 M phosphate buffer solution
20
(PBS, pH 7.0) before measurements. The electrochemical response of different
21
analytes on GC, GO/GC and GO-Ag/GC electrodes were recorded by SWVs. The 7
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experimental parameters were as follows: step potential, 5 mV; frequency, 10 Hz;
2
amplitude, 20 mV. The supporting electrolyte was 0.1 M PBS (pH 7.0), which was
3
bubbled with ultra-high purity N2.
4
3. Result and Discussion
5
GO sheets usually suffer from self-crispation and stacked structures among these
6
large-area, flexible and ultrathin basal planes due to hydrogen bonding
7
interaction.1,30-32 Such interaction may significantly affect distribution of the reactants
8
to be functionalized on these sheets.33 Therefore, before the synthesis of GO-Ag
9
hybrid, two complementary methods have been used to get high-quality GO sheets in
10
aqueous solution. First, GO sheets with large basal plane are removed by
11
ultrasonication and centrifugation filtration. The TEM images in Fig. S1 in Electronic
12
Supplementary Information show a significant size change of GO from large sheets
13
into small ones after ultrasonication and centrifugation filtration. Second, initially
14
stacked GO structures are conversed into individually stretched sheets after the
15
addition of hydroquinone (HQ). As shown in Fig. 1a and b, the average
16
hydrodynamic diameter (HD) of GO sheets has changed from 490.4 nm to 820.2 nm
17
after its incubation with HQ. This may indicate that the addition of HQ maintains the
18
wide stretch of GO sheets in aqueous solution (Fig. 1c).
19
On the individually separated GO sheets, GO-Ag hybrid has been fabricated. Fig.
20
2a shows the UV-vis spectra of the reaction solution. A characteristic absorption peak
21
of AgNCs can be displayed after the reaction, indicating the formation of AgNCs. The 8
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insets further shows that the adsorption peaks of the solution have a red-shift of 32 nm
2
and a blue-shift of 27 nm compared with that of colloid AgNCs and GO, respectively.
3
These shifts may indicate the formation of GO-Ag hybrid. Interestingly, we find that
4
neither HQ nor GO can separately reduce Ag+ into Ag0, unless combined use of the
5
two species (Fig. 2b). This observation reveals that GO may help to perform the
6
reduction capacity of HQ in a surface-confined manner. To find a clue in this
7
observation, we have monitored the pH changes during the reaction. As shown in Fig.
8
3a, the reaction can be divided into three stages according to the variation of reaction
9
rates, which is calculated from the changes of hydrogen ion concentration ([H+]). At
10
the first stage, from 1 to 3 min, a small increase of [H+] is observed after the addition
11
of AgNO3. We attribute this to the cation exchange of Ag+ with protons on GO
12
sheets34,35 at the begining of nucleation. At the second stage from 3 to 15 min, a fast
13
reaction rate is observed, indicating the surface-anchored small AgNCs may act as a
14
catalyzer36,37 to co-reduce Ag+ and GO along with the oxidation of HQ molecules. At
15
the third stage, after 15 min, the reaction slowly comes to a stop as the result of
16
consumption of HQ. The proposed nucleation stages may be further confirmed by the
17
polycrystal structure of AgNCs, among which there’s a core siting on one side (Fig.
18
5a). HRXPS has been further used to analyze different chemical states of carbon
19
atoms in GO and GO-Ag hybrid to reveal the reaction process. As shown in Fig. 3b,
20
different chemical states of carbon are easily observed, which appear at 284.8 eV for
21
C=C/C–C, 286.8 eV for C–O, and 289.0 eV for O–C=O bonds, respectively. After 9
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incubation GO with HQ for 30 min, the content of the C–O group keeps almost the
2
same (from initial 42.4% to 41.2%). However, after the formation of GO-Ag hybrid,
3
the content of C–O group decreases significantly to 27.2%. This may imply that GO
4
can provide the reactive sites for the nucleation and growth of AgNCs, then induce
5
the co-reduction of GO and Ag+ by HQ, and finally facilitate the formation of GO-Ag
6
hybrid (Fig. 3c).
7
In order to get high-quality GO-Ag hybrid, the initial concentration of AgNO3 has
8
been optimized. As shown by the inserted photograph in Fig. S2, with the increased
9
concentration of AgNO3 from 0 mM to 15 mM, the reaction solution shows a gradual
10
colour change. The corresponding UV-vis spectra also show enhanced adsorption
11
peaks of GO-Ag hybrid. However, the hybrid tends to precipitate when the
12
concentration of AgNO3 is higher than 9 mM. This observation indicates a higher
13
loading of silver nanocrystals on GO sheets. TEM images is further used to confirm
14
the morphological changes of GO-Ag hybrid. As shown in Fig. 4, with the increased
15
concentration of AgNO3, larger AgNCs (average size increases from 2.9±1.4 to 27.4
16
±10.6 nm) are formed and cover almost the whole surface of GO sheets. When the
17
concentration is increased to be 9 mM, uneven AgNCs are formed, which may impair
18
the stability of this material in aqueous solution after most of the hydrophilic groups
19
on colloidal GO flakes are covered by AgNCs.30 Based on these observations, it is
20
concluded that AgNCs can be effectively anchored on GO sheets. Moreover, the
10
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particle size and coverage density can be controlled by adjusting the initial
2
concentration of AgNO3 from 0 mM to 9 mM.
3
The fine details of the synthesized GO-Ag hybrid have been characterized. HRTEM
4
image in Fig. 5a shows that AgNCs are confined to the surface of 2D-structured
5
nanosheet and are evenly dispersed with uniform size, which is calculated to be 2.9±
6
1.4 nm. Also, the surface of GO is highly covered with AgNCs (5250 per square
7
micrometer). In the inset, the lattice spacing of Ag0 reveals periodicities of 0.20 nm
8
and 0.24 nm from separates sites, corresponding to the (200) and (111) plane of Ag0,
9
respectively. For further analysis of the GO-Ag hybrid, Raman spectroscopy is used.
10
As shown in Fig. 5b, the intensity of D-band (1350 cm-1) and G-band (1595 cm-1) for
11
GO has been enhanced significantly after coupling of AgNCs, which arises from the
12
increased Raman scattering cross sections of high-density metallic nanogaps.31
13
Moreover, a low enhancement factor for GO-Ag hybrid indicates the formation of
14
charge transfer complexes between AgNCs and GO sheet.40 Full XPS survey
15
spectrum in Fig. 5c has clearly defined the existence of carbon (72.40%), oxygen
16
(25.82%) and silver (1.78%). HRXPS analysis also shows the 3d3/2 and 3d5/2 doublet
17
of silver, whose core levels are located at 368.2 eV and 374.2 eV, respectively. These
18
peaks suggest the successful fabrication of AgNCs on GO. The above properties
19
indicate the formation of high-quality GO-based nanohybrid. A comparison with the
20
previously reported hybrids may further show the superior properties of this
21
synthesized GO-Ag hybrid (see Table S1 in Electronic Supplementary Information). 11
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To demonstrate the potential applications of this material in functional systems,
2
catalytic activity and sensing ability of GO-Ag hybrid have been examined. Ag-based
3
materials are recently investigated as electrocatalysts for ORR in alkaline fuel cells
4
because of their thermodynamical and electrochemical stability, higher tolerance to
5
methanol crossover, and lower cost than commercial Pt/C catalyst.40-42 So, firstly, the
6
electrocatalytic activity of this synthesized material for ORR has been characterized
7
by cyclic voltammetry. As shown in Fig. 6a, in contrary to the cyclic voltammogram
8
(CV) in N2 saturated electrolyte, the CV in O2 saturated electrolyte shows an obvious
9
reduction current peak from ORR. The reduction current density has reached −1.06
10
mA cm−2 at −0.46 V, indicating relative high activity towards ORR when compared to
11
GO nanosheets and comercial Pt/C (Fig. S4). Moreover, in the durability tests (Fig.
12
6b) and methanol-tolerant experiments (Fig. 6c), the hybrid still remain high
13
electroactivity towards ORR. These results indicates that this synthesized GO-Ag
14
hybrid may provide potential applications in fuel cells as an efficient platinum-free
15
catalyst with high catalytic activity and low cost.
16
Since the electrochemical response is highly sensitive to the physicochemical
17
properties of an electrode surface,43 electrochemical performance of the working
18
electrode modified with GO-Ag hybrid has been examined. By using 5 mM
19
Fe(CN)63-/4- redox probe as the indicator, experimental results obtained by cyclic
20
voltammetry and electrochemical impedance spectra techniques have all confirmed
21
the enhanced electron transfer ability of the working electrode by this material (see 12
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Fig. S3 in Electronic Supplementary Information). This property may contribute
2
much to the application of this material for electrochemical sensing. In addition, to
3
further test its sensing ability, drugs (doxorubicin, mitoxantrone and salicylic acid),
4
protein (cytochrome c and myoglobin) and protein enzyme (HRP) are chosen as the
5
representative analytes to study their electrochemical responses on GO-Ag modified
6
glassy carbon (GC) electrode. Fig. 7 shows the electrochemical responses of these
7
molecules at the electrode surfaces of GC (black curve), GO/GC (blue curve) and
8
GO-Ag/GC (red curve), respectively. As expected, GO-Ag/GC electrodes exhibit
9
higher current peaks, as well as increased cathodic potentials (or decreased anodic
10
potential) for the reduction (Fig. 7a, b, d-f) (or oxidation (Fig. 7c)) of the analytes.
11
This is because the hybrid GO-Ag nanomaterial may provide large electro-active
12
surface for molecule absorption, so it may facilitate the electron transfer at the sensing
13
interface, and consequently give rapid and enhanced current responses. Therefore,
14
GO-Ag can be used as a new electrode hybrid material for the fabrication of superior
15
electrochemical sensors.
16 17
4. Conclusion
18
In summary, we have developed a simple and effective method to prepare
19
high-quality GO-Ag hybrid. The AgNCs on GO surfaces are uniform and
20
well-dispersed, which have shown many improved properties compared to previously
21
reported materials. Meanwhile, we have presented the inductive effects of GO on 13
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preparing inorganic nanocrystals on 2D nanosheets, which is based on the
2
surface-induced co-reduction of Ag+ and GO by HQ. This not only makes it possible
3
to understand the surface-confined reactivity and intrinsic kinetic electron transfer
4
properties during the functionalization of GO, but also provides a new route to
5
prepare other GO-based inorganic hybrids with improved performance in fuel cells,
6
catalysis, energy conversion and storage, as well as electrochemical sensors.
7 8
Acknowledgements
9
This work is supported by the National Natural Science Foundation of China (Grant
10
No. 21235003), and the National Science Fund for Distinguished Young Scholars
11
(Grant No. 20925520).
12
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Figures and Captions:
2 3 4 5 6
Fig. 1. The average hydrodynamic diameters of GO sheets (0.05 mg mL-1): (a) in the absence of HQ; (b) in the presence of 0.2 mM HQ in aqueous solution. (c) Proposed scheme for the separation of GO sheets by HQ.
7 8 9 10 11 12 13
Fig. 2. (a) The UV-vis spectra of GO (black curve), AgNCs (blue curve) and GO-Ag hybrid (red curve) in aqueous solution. Inserted curves show absorption peak shifts of the synthesized GO-Ag hybrid compared to GO and AgNCs, respectively. (b) The UV-vis spectra of reaction solutions in the presence (red curve) and absence (red curve) of 0.1 mM HQ, respectively. The blue curve shows the UV-vis spectra of the reaction solution in the absence of GO.
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Fig. 3. (a) The pH values of HQ, GO and their mixture in aqueous solution. The right part shows plots of dynamic pH value changes of the reaction solution and corresponding H+ concentration changes against time. (b) Typical XPS spectra of C1s on GO before and after the addition of HQ, as well as on GO-Ag hybrid (from left to right). (c) A scheme proposed for the probable growth stages of AgNCs on GO sheets.
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Fig. 4. Typical TEM iamges (a-f) of GO-Ag hybrid prepared from different concentrations of AgNO3 (0, 3, 6, 9, 12 and 15 mM). The inserted histograms show the corresponding size distributions of AgNCs anchored on GO sheets.
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Fig. 5. (a) Typical HRTEM image of GO-Ag hybrid. The concentration of AgNO3 is 3 mM; scale bar is 50 nm. The histogram shows the size distribution of AgNCs (2.9±1.4 nm). The inset: HRTEM image of an individual AgNC on GO sheets. The lattice spacing are (1) 0.20 nm and (2) 0.24 nm from separates sites, scale bar is 2 nm. (b) Typical Raman spectra of GO and GO-AgNP hybrid. (c) Typical XPS full spectra of GO-Ag hybrid. (d) HRXPS spectra of Ag 3d.
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Fig. 6. (a) Cyclic voltammograms of the GO-Ag/GC electrodes in 0.1 M KOH solution saturated with N2 and O2, respectively. (b) Cyclic voltammograms for the durability test of GO-Ag hybrid for ORR, from 1st to 100th cycle. (c) Cyclic voltammograms to show the influence of 0.5 M methanol over GO-Ag/GC electrode. The poteintial scan rate is 50 mV s-1.
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Fig. 7. Square wave voltammograms of drugs (a-c: doxorubicin, mitoxantrone and salicylic acid), proteins (d-e: cytochrome c and myoglobin) and protein enzyme (f: HRP) obtained on GC (black curves), GO/GC (blue curves) and GO-Ag/GC (red curves) electrodes. The supporting electrolyte is 0.1 M PBS, pH 7.0. The step potential is 5 mV, scanned from left to right.
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DOI: 10.1039/C4NR04283B
