Accepted Manuscript Title: One pot green synthesis of Ag, Au and Au–Ag alloy nanoparticles using isonicotinic acid hydrazide and starch Author: Sampath Malathi Tamilarasu Ezhilarasu Tamilselvan Abiraman Sengottuvelan Balasubramanian PII: DOI: Reference:
S0144-8617(14)00483-4 http://dx.doi.org/doi:10.1016/j.carbpol.2014.04.105 CARP 8880
To appear in: Received date: Revised date: Accepted date:
21-3-2014 24-4-2014 28-4-2014
Please cite this article as: Malathi, S., Ezhilarasu, T., Abiraman, T., & Balasubramanian, S.,One pot green synthesis of Ag, Au and AundashAg alloy nanoparticles using isonicotinic acid hydrazide and starch, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.04.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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One pot green synthesis of Ag, Au and Au–Ag alloy nanoparticles using isonicotinic acid hydrazide and starch
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Sampath Malathi, Tamilarasu Ezhilarasu, Tamilselvan Abiraman and
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Sengottuvelan Balasubramanian* Department of Inorganic Chemistry, University of Madras, Guindy Campus,Chennai-600
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025, Tamilnadu, India .Tel.No:+914422202794
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Graphical Abstract
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Ag Au
Au
Starch
Ag Au
M
+
O
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INH
Ag Ag
Au Ag
BH4-
OH 4-AP NO2
Au
Au-Ag alloy NPs
OH 4-NP
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HAuCl4
Au Au
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AgNO3
NH2
d
N
N H
Ag
Ag
NH2
reduction
8
cr
7
1
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Highlights Green synthesis of Ag, Au and their alloy NPs using starch as a capping agent and water as the solvent.
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Synthesis of Ag, Au and their alloy NPs using isoniazid (INH) as a reducing agent.
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The redox behaviour of alloy NPs depends on their size as well as their composition.
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The catalytic reduction of 4-nitrophenol and picric acid by metal and alloy NPs.
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When the size increases alloy NPs show bathochromic shift in their emission.
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One pot green synthesis of Ag, Au and Au–Ag alloy nanoparticles
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using isonicotinic acid hydrazide and starch
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Sampath Malathi, Tamilarasu Ezhilarasu, Tamilselvan Abiraman and
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Sengottuvelan Balasubramanian*
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Department of Inorganic Chemistry, University of Madras, Guindy Campus,
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Chennai-600 025, Tamilnadu, India .Tel.No:+914422202794
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Abstract
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Gold-silver alloy nanoparticles were synthesized via chemical reduction of
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varying mole fractions of chloroauric acid (HAuCl4) and silver nitrate (AgNO3) by
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environmentally benign isonicotinic acid hydrazide (INH) in the presence of starch
33
as a capping agent in aqueous medium. The absorption spectra of Au–Ag
34
nanoparticles show blue shift with increasing silver content indicating the
35
formation of alloy nanoparticles. When the Ag content in the alloy decreases the
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size of the nanoparticles increases and as a result of which the oxidation potential
37
also increases. The emission maximum undergoes a red shift from 443 to 614 nm.
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The nanoparticles are monodisperse and spherical with an average particle size of
39
3-18 nm. The catalytic behavior of alloy nanoparticles indicate that the rate
40
constant for the reduction of 4-nitro phenol to 4-amino phenol increases
41
exponentially from metallic Ag to metallic Au as Au content increases in the Au–
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Ag alloy nanoparticles.
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Keywords: silver, gold, alloy nanoparticles, isoniazid, cyclic voltammetry,
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4-nitrophenol
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*Corresponding author: Email id:[email protected]
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1. Introduction Metal nanoparticles show characteristic size–dependent properties different from
51
those of bulk metals with significant size effects occurring in 1–10nm diameter range.
52
(Link, Burda, Wang, & El-Sayed, 1999). Au–Ag NPs with certain bimetallic
53
compositions have been synthesized in single-phase (water) (Mallin, & Murphy, 2002) as
54
well as in bi-phasic (toluene–water) (Hostetler et al, 1998) medium.
55
nanoparticles have been synthesized by NaBH4 reduction of supported metal salts, (Kim,
56
Kim, Kim, & Yoo, 2013) citrate reduction, (Link, Wang, El-Sayed, 1999) hydrazine
57
reduction, (Chen, & Chen, 2002) laser ablation, (Liz-Marzan, L. & Philipse, 1995)
58
sputter deposition, (Okazaki et al, 2008) laser irradiation,(Chen, & Yeh, 2001)
59
sonochemical method (Anandan, Grieser, & Ashokkumar, 2008) and also by biosynthesis
60
(Senapati, Ahmad, Khan, Sastry, & Kumar, 2005). Kim et al have reported the synthesis
61
of Au–Ag alloy nanoparticles by laser ablation of bulk alloys (Lee, Han, & Kim, 2001).
62
Teo et al have synthesized 38 atom gold–silver clusters (Au18 Ag20) to investigate the
63
minimum number of atoms in a cluster required to exhibit metallic behavior (Teo,
64
Keating, & Kao, 1987).
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Au–Ag alloy
The alloy NPs with smaller sizes are often required in the field of catalysis due to
66
their higher activity (Zhang, Zhang, Wei, Du, & Li, 2009). For example, sub-5 nm Au–
67
Ag alloy NPs strongly enhanced oxidation of carbon monoxide, which could be prepared
68
by half-seeding or a templating approach (Wang, Liu, Lin, Lin, & Mou, 2005). However,
69
the complex experimental procedures of such methods restricted their practical
70
applications. Therefore, it is necessary to develop a facile method for the preparation of
71
high quality Au–Ag alloy NPs with smaller size in the solution phase.
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4-pyridine carboxylic acid hydrazide (INH) possesses significant physiological
73
activities among the three isomeric 2, 3 and 4-pyridine carboxylic acid hydrazides. INH
74
via passive diffusion rapidly permeates the bacterial cell membrane wherein the
75
hydrazide group in the molecule serves only as a carrier group (Kruger–Thiemer)
76
(Kruger–Thiemer, 1955). INH has been the accepted drug for use as TB prophylaxis in
77
immunocompetent as well as in HIV positive cases (Carosi, & Matteelli, 1996). The
78
transition metal complexes of INH exhibit interesting biological activities. The catalytic 4
Page 4 of 31
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conversion of epoxides into ninhydrins is possible with this compound (Sharghi,
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Eskandari, & Ghavami, 2004) and it has been found to act as a molecular logic gate in
81
aqueous medium (Magria, & Prasanna de Silva, 2010). The objective of this study is to develop a simple one-step and “green” approach to
83
synthesize Au, Ag and Au–Ag alloy nanoparticles by chemical reduction using
84
isonicotinic acid hydrazide (Isoniazid) and starch as the capping agent. Isoniazid is being
85
used as a first line anti-tuberculosis drug (Winder, 1956). The byproduct isonicotinic acid
86
produced in this reaction is also non-toxic and eco-friendly. The homogeneous Au–Ag
87
alloy nanoparticles prepared in this study can be used as a catalyst for the reduction of 4-
88
nitrophenol and picric acid in presence of NaBH4 for the synthesis of amino phenols. The
89
amino phenol is an important intermediate in the manufacture of many analgesic and
90
antipyretic drugs besides its application as photographic developer, corrosion inhibitor
91
and hair dyeing agent (Corbett, 1999).
2. Experimental
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2.1 Materials
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Chloroauric acid (HAuCl4.xH2O, LR, LOBA Chemie, India), silver nitrate
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(AgNO3, AR, BDH, India), 4-nitrophenol (C6H5NO3, AR, SRL, India), sodium
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borohydride (NaBH4, LR, Merck, India), picric acid (C6H3N3O7, AR, BDH, India)
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isoniazid (C6H7N3O, AR, Fluka, India) and starch [(C6H10O5)n, soluble GR, Merck, India]
99
were used as such without further purification. Milli–Q water was used throughout the
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experiment.
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2.2 Methods
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2.2.1 Synthesis of silver nanoparticles, gold nanoparticles and Au – Ag alloy
103
nanoparticles
104
20 mL aqueous solution containing starch (1%) and AgNO3 (0.1 mM) was taken
105
in a round bottom flask and 0.025 mM (2.5 mL) of freshly prepared aqueous isoniazid
106
solution was added to it. The mixture was heated to 60 ˚C with continuous stirring till the
107
solution turns yellow indicating the formation of AgNPs. Similarly a solution containing
108
HAuCl4.xH2O (0.1 mM) and starch (1%) was taken in a round bottom flask.
The
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mixture was stirred continuously at room temperature, followed by the dropwise addition
110
of isoniazid (0.025 mM). The solution turns purple indicating the formation of AuNPs.
111
Au–Ag alloy nanoparticles were synthesized by the reduction of HAuCl4.xH2O and
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AgNO3 with isoniazid in the presence of starch with varying initial Au–Ag molar ratios
113
(1:0, 0.75:0.25, 0.50:0.50, 0.25:0.75 and 0:1). The resultant 0.50:0.50 Au–Ag solution
114
consists of 10 mL of 1% starch, 2.5 mL (0.1 mM) HAuCl4.xH2O and 2.5 mL (0.1 mM)
115
AgNO3. 5.0 mL of an aqueous solution of (0.025 mM) isoniazid was added all at once
116
with vigorous stirring. The solution was allowed to stir for an additional 60 s. The color
117
of the resulting nanoparticle solution depends on the initial composition of Au–Ag alloy
118
nanoparticles.
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spectrophotometry.
resulting
nanoparticles
were
analyzed
by
UV–Visible
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Scheme 1 6
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2.2.2 Catalytic reduction of 4-nitrophenol 0.25 mL of 10 mM aqueous solution of 4-nitrophenol and 0.25 mL of 1M freshly
125
prepared aqueous solution of NaBH4 were mixed with 2.5 mL of Milli–Q water at room
126
temperature. 100 μL of 0.1 mM Au, Ag, Au–Ag alloy nanoparticles were mixed with the
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above solution and UV–Visible absorption spectra were recorded with time to monitor
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the change in the reaction mixture. The above procedure is also followed for the
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reduction of picric acid with or without the presence of the nanoparticles.
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132 133 134 135
Scheme 2
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2.2.3 Fabrication of silver, gold, and their alloy nanoparticles modified GCE The CV (cyclic voltammetry) studies were carried out on a CH Instruments
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Model CHI-1130A electrochemical workstation (USA). The three electrode cell assembly
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consists of the bare GCE as well as modified by metal/alloy nanocoating as working
138
electrode, platinum wire as counter electrode and Ag/AgCl as reference electrode. The
139
experiments were conducted in PBS (pH: 7.4) medium and 3 M KCl as supporting
140
electrolyte. The solution was purged by N2 gas to remove the dissolved oxygen. The
141
potential was scanned between 1 V and -1 V at a scan rate of 50 mVs-1. The nanoparticles
142
coated GCE electrode was fabricated individually by casting 10 μL of each nanoparticle
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solution. Then the solvent was allowed to dry at room temperature and rinsed with water
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in order to remove any loosely bound nanoparticles.
145 146
2.2.4 Characterization The aqueous solution of monometallic and bimetallic nanoparticles were
148
examined using Ultraviolet–visible spectrophotometer (Perkin–Elmer, ʎ 35, USA) in the
149
range of 200–800nm. Transmission electron microscopic (TEM) studies were carried out
150
using TECNAI, G2-MODEL, T-30 S-TWIN with an acceleration voltage of 250 kV. The
151
aqueous solution of nanoparticles was drop cast onto a carbon – coated copper grid
152
sample holder followed by evaporation at room temperature. The size distribution of the
153
nanoparticles was estimated based on images J software (version 1.47). The X-ray
154
photoelectron spectroscopy (XPS) measurements were made on a Physical Electronics-
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Quantum 2000 EAC (SPHERA 547) scanning microprobe. This system uses a focused
156
monochromatic Al Kα X-ray (1486.7eV) source for excitation, a spherical section
157
analyzer and a seven channel detection system. The fluorescence measurement was
158
carried out at room temperature using a Varian–Cary Eclipse fluorescence
159
spectrophotometer. The fluorescence measurements were carried out in the range 350-
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700nm.
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3. Results and Discussion
The green synthesis of AgNPs was achieved by chemical reduction of silver ions
164
using INH as a reducing agent and starch as a capping agent. In this study, starch is used
165
as a template for nanoparticle growth while INH serves as an environmentally benign
166
reducing agent for the reduction of silver ions. The silver nanoparticle solution shows an
167
absorption maximum at 411 nm. While mild heating is sufficient for the INH reduction
168
of silver ions, the reduction of Au3+ ions of HAuCl4 occurs at room temperature.
169
The surface plasmon absorption maximum of the purple color AuNPs, which is
170
obtained from 0.1 mM solution of Au3+ ions, is 576 nm. The reduction of Au3+ ions
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occurs fairly rapidly and more than 90% of Ag+ and Au3+ ions are reduced within 15 min
172
and 2 min respectively, after the addition of INH to the metal ion solution. The 8
Page 8 of 31
comparatively slower reduction rate of silver ions relative to that of gold ions is most
174
likely due to differences in the reduction potentials of the two metal ions, the redox
175
potential being considerably lower for Ag+ to Ag0 (Raveendran, & Wallen, 2003). INH is
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considered to be “greener” in terms of biocompatibility and starch is one of the
177
polysaccharides, which belongs to the most abundant natural and renewable biopolymers.
178
The hydroxyl groups of starch help to protect the surface of these particles in the absence
179
of which they will aggregate as a result of high surface energies. Starch serves as a
180
stabilizing template for the alloy nanoparticles (Hussain, Kumar, Hashmi, & Khan,
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2011).
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185 186
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Fig. 1
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The synthesis of Ag, Au and Au–Ag alloy nanoparticles is very simple. The
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mixtures of Ag and Au metal ion solution containing starch undergo change in color upon
188
addition of INH depending on the molar composition of Ag and Au. The AgNO3
189
solution changes from colorless to yellow upon reduction. However, as the Au content
190
increases, the intensity of the purple color increases (Vide: supporting Fig. S1). This is
191
the first report where NPs of Ag, Au and Au–Ag from Ag+ and Au3+ ions are prepared
192
using INH as a reducing agent. The Au–Ag alloy nanoparticles, as well as Ag and Au
193
nanoparticles, can be characterized by UV–Visible absorption spectroscopy. Silver and
194
gold nanoparticles are known to exhibit surface plasmon resonance bands in the visible
195
region at ~ 440 nm (Naik, Stringer, Agarwal, Jones, & Stone, 2002) and ~ 530 nm (Aryal
196
et al, 2006), respectively. The UV–Visible spectra of the Ag and Au NPs prepared from 9
Page 9 of 31
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the salts of the metals exhibit distinct peaks at 411 nm and 576 nm [Fig. 1a: spectral
198
traces (a) and (e)] respectively in the present investigation.
199
The UV–Visible spectra of the Au 0.25: Ag 0.75, Au 0.50: Ag 0.50 and Au 0.75: Ag 0.25
201
samples are reproduced in Fig. 1a: spectral traces (b), (c) and (d), which exhibit single
202
absorption maximum at 443, 481, and 513 nm respectively, thus confirming the
203
formation of alloy nanoparticles with only homogeneous composition. The spectra of the
204
alloy nanoparticles did not undergo any change for several months. The surface plasmon
205
absorption of the Au–Ag composite is linearly red-shifted ( ̴ 170nm) from that of a
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monometallic Ag in proportion to the increase in the composition.
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nanoparticles are very stable in the aqueous phase. The mole fraction of the Au–Ag
208
content indicates that the nanoparticles are alloys rather than core–shell nanoparticles or
209
mixture of monometallic nanoparticles. This is illustrated in fig. 1b in which the UV–
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Visible absorption maximum is plotted against the mole fraction of Au in the precursor
211
samples. It is known that a physical mixture of monometallic Ag and Au nanoparticles
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has two absorption peaks corresponding to the monometallic Ag and Au respectively
213
(Kim, Kim, Kim, & Yoo, 2013). (Vide: supporting information Fig. S2) The core – shell
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nanoparticles would exhibit the same two absorption peaks (Tang, Yuan, & Chai, 2006),
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where one increases in absorbance with the increase in the concentration of that
216
component with concomitant decrease in the absorbance of other component. The
217
stability of as synthesized Au and Ag nanoparticles was monitored by UV–Visible
218
spectral studies. The SPR band of starch stabilized nanoparticles did not undergo any
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change either in its position or its intensity even after two months of storage at ambient
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temperature.
Au–Ag alloy
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Fig. 2
The cyclic voltammograms of GCE modified with Au–Ag alloy nanocoating with
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the ratio Ag, Au0.75:Ag0.25, Au0.50:Ag0.50, Au0.25:Ag0.75 and Au in PBS medium (pH: 7.4)
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are reproduced in fig. 2 and supporting information (Fig. S3). The AuNPs modified GCE
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(Au) (Fig. 2a) recorded in the range 0.2 to 1.0 V shows the broad anodic peak and
228
prominent cathodic peak at 0.51 and 0.36 V respectively. In the case of Au0.50:Ag0.50
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alloy nanoparticle, the anodic peak at 0.37 V and cathodic peak at 0.15 V were observed
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(Fig. 2b). The redox behavior of the alloy NPs Au0.75:Ag0.25 and Au0.25:Ag0.75 is reported
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in supporting information. [Vide: supporting information: Fig. S3]. The oxidation and
232
reduction peaks appear at 0.34V, 0.47V and -0.37V, 0.18V respectively (Table 1). But
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the AgNPs (Ag) modified GCE exhibits the oxidation at Epa= +0.30 V and reduction at
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Epc= -0.67 V as shown in Fig. 2c. 11
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Table 1 Composition, size, cyclic voltammetry, UV–Visible, and photoluminescence Spectral data of Ag, Au and Au–Ag alloy NPs
237
Molar ratio
Size
Epa
Epc
ʎmax
ʎex
ʎem
239
of NPs
(nm)
(V)
(V)
(nm)
(nm)
(nm)
240
Ag
3.8±0.80
0.30
-0.67
411
241
Au0.25:Ag0.75
4.1±1.50
0.34
-0.37
443
242
Au0.50: Ag0.50
4.6±1.00
0.37
+0. 15
481
243
Au0.75: Ag0.25
7.0±0.50
0.47
+0. 18
513
244
Au
14.4±2.7
0.51
+0. 36
443
440
463
cr
407
475
500
563
575
614
us 576
513
nm; Nanometer, V;Volts. Epa; Oxidation potential, Epc; Reduction potential. ʎmax;Maximum wavelength, ʎex; Excitation wavelength, ʎem; Emission wavelength
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245 246 247 248
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The oxidation potential (Epa) gradually increases from 0.30 to 0.51 V and the
250
reduction potential (Epc) also gradually increases from -0.67 to 0.36 V, when the
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concentration of silver nanoparticles is reduced from 100 % to 0 % in Au–Ag alloy
252
nanoparticles (Table1). Moreover the redox potential gradually increases as the size of
253
the NPs is increased from 3.8 to 14.4 nm. These values are comparable to that of the
254
earlier report by Tominaga et al, which indicates the dependency of the redox potential
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with the size of the Au–Ag alloy NPs as well as the fall in the redox potential with
256
increasing concentration of silver (Raimondi, Scherer, Kcotz, & Wokaun, 2005). Pumera
257
et al have indicated the size dependent electrochemical behavior of silver (Giovanni, &
258
Pumera, 2012) and gold (Bonanni, Pumera, & Miyahara, 2011) nanoparticles. Compton
259
et al have demonstrated that stripping potential of Ag NPs depends on the size of the
260
nanoparticles (Jones, Campbell, Baron, Xiao, & Compton, 2008).
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The redox potential of alloy nanoparticles essentially depends on two factors: size
263
and composition. These two factors may oppose each other in some cases while they
264
have cumulative effect in some other systems. It has been found that if the size of the
265
alloy nanoparticles decreases, anodic potential increases and when the silver content 12
Page 12 of 31
decreases, the oxidation potential also increases (Tominaga et al, 2006). However, in the
267
present investigation it has been found that when the size of the nanoparticle increases,
268
the oxidation potential also increases while the reduction in silver content results in
269
increase in anodic potential. The latter behavior compares well with that of the literature
270
report, while the former exhibits a reversal in this trend. This observation confirms that
271
the Au–Ag alloy nanoparticles are composed of by homogenous mixture of AgNPs and
272
AuNPs and not by Ag and Au metal domains.
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Fig. 3
The characteristic photoluminescent behavior of Ag, Au and Au–Ag alloy
278
nanoparticles was observed by measuring fluorescence emission spectra (Fig. 3). The
279
emission maximum of Ag and AuNPs was observed at 443 nm and 614 nm respectively
280
when they were excited at 407 nm and 575 nm respectively. As the molar ratio of Ag
281
decreases, the fluorescence emission maximum undergoes a red shift from 443 to 614 nm
282
along with an increase in excitation wavelength due to synergism (Vide: supporting
283
information Fig. S4). It has also been observed that when the solutions of metal and alloy
284
NPs were excited at different wavelengths, the emission maximum did not undergo any
285
change. For example, when the solution of Ag was excited at 390, 400 and 407 nm, the
286
ʎem was found to be 443 nm while the excitation of Au0.75:Ag0.25 at 400, 420 and 440 nm 13
Page 13 of 31
resulted in ʎem at 463 nm indicating that the alloy NPs is monodisperse. This observation
288
closely resembles that of the silver nanoclusters reported earlier (Shang, Dong, 2008;
289
Ganguly, Pal, Negishi, & Pal, 2013). Millstone et al have reported the photoluminescence
290
behavior of Au/Cu NP alloys in which the fluorescence of the alloy NPs undergoes
291
bathochromic shift when the concentration of copper increases (Andolina et al, 2013).
292
This is in contrast to the observation made in the case of Au–Ag alloy nanoparticles in
293
which the decreasing concentration of Ag results in increasing red shift. The fluorescence
294
intensity of pure Ag metal is considerably reduced due to the quenching by Au with
295
increase in its concentration, which is similar to the observation made in the earlier report
296
(Ganguly, Pal, Negishi, & Pal, 2013). The fluorescence intensity also decreases with
297
decrease in the Ag concentration in the alloy NPs. The fluorescence maximum depends
298
on the applied potential and it has been demonstrated by Tafel plot (Nag et al, 2012;
299
Kaneko, Uosaki, & Kita, 1986).
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When the EPa of the system increases, the emission wavelength also undergoes a
302
red shift. When the concentration of Ag in the bimetallic alloy is reduced from 100% to
303
0% the EPa increases from +0.30 V to +0.51 V with a corresponding increase in the
304
fluorescence emission maximum from 443 nm to 614 nm. The electrochemical studies
305
have clearly established the relationship between the composition and oxidation potential
306
of the NPs.
307
potential and emission wavelength [Table 1]. The photoluminescence efficiency of
308
colloidal nanocrystals has also been reported to vary with the zeta potential especially, in
309
the case of Cd/Se colloidal nanocrystals (Nag et al, 2012).
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Similar correlation can be made between the composition, oxidation
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The crystal structure of Au, Ag and their alloy nanoparticles has been further
311
characterized by XRD analysis. The XRD pattern for the synthesized Ag, Au and their
312
alloy nanoparticles are reproduced in supporting information (vide: supporting fig. S5).
313
The XRD pattern of Au and Ag synthesized in the present investigation matches with
314
those of earlier reports(JCPDS file nos: 870597and 740526 respectively).The four peaks
315
appearing at 2θ = 37.97˚, 44.11˚, 64.35˚ and 77.27˚ correspond to the (111), (200), (220)
316
and (311) planes, respectively, of the face centered cubic Ag (or Au) particles. Since gold 14
Page 14 of 31
317
and silver have the same fcc structure and almost the same lattice constant it is difficult to
318
distinguish gold–silver alloy from a mixture of the monometallic phases from XRD
319
pattern (Shankar, Rai, Ahmad, & Sastry 2004; Chen, & Chen, 2002).
320
Table 2 XPS peak positions (BE/eV) for Au–Ag NPs Au
84.0
Ag
-
Au (0.25): Ag (0.75)
85.0
Ag (3d5/2) -
368.0 368.4
BE; Binding Energy, eV; Electron volts.
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Au (4f7/2)
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Fig. 4
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The surface alloy composition of Au–Ag alloy NPs was confirmed by X-ray
330
photoelectron spectral analysis. The XPS traces of Au–Ag alloy NPs are shown in Fig. 4.
331
The XPS data for the alloy Au
332
positions are tabulated (Table 2). Au–Ag alloy nanoparticles show both Au 4f (85.0eV)
333
and Ag 3d (368.4eV) peaks in Fig.4, confirming the bimetallic nature of alloy
334
nanoparticles. The content of Au and Ag within the X-ray penetration depth (3–5 nm) on
335
the surface is quantified by Au 4f and Ag 3d peak areas divided by the respective atomic
336
sensitive factor (ASF) 6.250 and 5.987 which indicates an average composition of 23
337
atomic% Au and 77 atomic% Ag. In general, the binding energies of alloy nanoparticles
338
exhibit a shift, compared to those of metallic Ag and Au forms, indicating that the two
339
noble metals form alloy nanoparticles rather than individual metal nanoparticles. The
340
XPS binding energies of alloy nanoparticles have been reported by several groups
341
(Srnova-Sloufova, Vickova, Bastl, & Hasslett, 2004; Kariuki et al, 2004). The shift in the
342
peak binding energy is indicative of the formation of alloys. Both Au and Ag bands have
343
been identified (Table 2) which are in agreement with the literature data (Moulder,
344
Stickle, Sobol, & Bomben, 1995). The analysis of XPS spectra confirms the presence of
345
starch (C1s) as a capping agent of the Au–Ag alloy nanoparticles, which prevents their
346
aggregation (Fig. 4). The C1s spectrum shows peaks corresponding to C (CH2) at 284.7
347
eV, C-O at 286.0 eV, O-C-O at 287.3 eV and O=C-O at 288.7 eV.
Ag
0.75
as well as metallic Ag, Au and their peak
Ac ce p
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0.25:
16
Page 16 of 31
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348
349 350
Fig. 5
The nature of the metal nanoparticles has been analyzed by TEM and selected
352
area electron diffraction pattern (SAED) studies. The representative TEM images
353
recorded from carbon coated copper grid of the AgNPs prepared by the reduction of Ag+
354
ions in starch by INH are reproduced (Fig. 5a). The particles are polydisperse in nature
355
with an average size of 3.8±0.8 nm (Fig. 5b). The SAED for AgNPs and the ring pattern
356
are indexed to the 111, 200, 220 and 311 Bragg reflections from the fcc structure of
357
silver (Fig. 5c). The EDAX spectrum of AgNPs shows peaks around 3.4 keV, which
358
correspond to the binding energies of silver thus confirming the formation of metallic
359
AgNPs (Fig. 5d).
te
Ac ce p
360
d
351
361
The TEM micrograph for AuNPs is shown in Fig. 6a. The nanoparticles are
362
spherical and monodisperse with diameter in the range of 14.4±2.7 nm (Fig. 6b). The
363
SAED for AuNPs and the ring pattern are indexed to the 111, 200, 220 and 311 Bragg
364
reflections from the fcc structure of gold (Fig. 6c). The EDAX spectrum of AuNPs is 17
Page 17 of 31
shown in fig. 6d. The peaks around1.8 and 10.8 keV correspond to the binding energies
366
of AuNPs, indicating that the resultant product is metallic AuNPs.
369 370
te
368
Fig. 6
Ac ce p
367
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365
The TEM indicates the size and shape of the alloy nanoparticles. Fig. 7a shows
371
the TEM micrograph recorded at equimolar ratio of Au–Ag alloy NPs. The average size
372
of the alloy NPs is 4.6±1.0 nm; Au: 0.50 (Fig. 7b) and they are spherical in shape (Table
373
1). The SAED pattern of Au0.50: Ag0.50 confirms that the alloy NPs formed are crystalline
374
in nature and they belong to fcc lattice (Fig. 7c). Both Au and Ag adapt an fcc structure
375
and have very close crystal lattice parameters. Hence it would be very difficult to
376
observe any shift in the position of the rings from those of individual Au or Ag NPs. The
377
EDAX analysis suggests the Au–Ag mole ratio of the sample with an elemental
378
composition of Au (50%) and Ag (50%) (Fig.7d). The size of alloy nanoparticles is less
379
than 10 nm, which is smaller than that of Ag–Au alloy prepared in aqueous solution as
380
reported earlier (Kim, Kim, Kim, & Yoo, 2013). The TEM images of Au–Ag alloy 18
Page 18 of 31
381
nanoparticles did not show either a dense core or a light shell, which is typical for core–
382
shell structure. The TEM micrographs recorded at different molar ratios of Au–Ag alloy
383
NPs and the average size of the alloy NPs are 4.1 ±1.5 nm; Au: 0.25 and 7.0±0.5 nm; Au:
384
0.75nm and they are spherical in shape (Vide: supporting fig. S6) and (Table 1).
388 389 390 391
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387
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386
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cr
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385
Fig. 7
Catalytic activity of Au, Ag, and Au–Ag alloy nanoparticles The catalytic efficiency of Au–Ag alloy nanoparticles for the reduction of mono
392
and trinitrophenol systems has been investigated. In a typical bimetallic catalytic
393
reduction in the present investigation, upon the addition of NaBH4 to the aqueous 4-
394
nitrophenol (4NP), nitrophenolate ion is formed immediately (Li et al, 2012) which
395
shows red shift in the UV–Visible absorption band from 317 to 400 nm (Fig. 8a). In the
396
absence of any catalyst, the thermodynamically favorable reduction of 4-NP (E˚ for 4-
397
NP/4-AP = -0.76 V and H3BO3/BH4- = -1.33 V versus NHE) was not observed and the
398
peak due to 4-nitrophenolate ion at 400 nm remains unaltered even for a couple of days 19
Page 19 of 31
399
(Mrinmoyee, & Tarasankar, 2011). However, immediately after adding a small amount of
400
catalyst Ag or Au into the reaction system, the intensity of the absorption band at 400
401
nm successively decreases with increasing reaction time and simultaneously a new band
402
appears at 300 nm indicating the formation of 4-AP (Fig. 8b).
ip t
403 404
cr
405
us
406 407
an
408 409
Fig. 8
411
M
410
The product conversion is fast (12 min) in the presence Au (100%) (Vide: supporting information Fig. S7)
catalyst when compared to that of Au (0%) (40 min)
413
(Vide: supporting information Fig. S8).
414
min in the presence of bimetallic alloy nanoparticles Au (50%): Ag (50%). The catalyst
415
appears to have marginal influence on the reduction of picric acid (PA) system and the
416
reduction occurs within 5 min with or without the addition of catalyst. The electron
417
withdrawing effect of the three nitro groups and resonance structure play an important
418
role in the process of strong reduction. The reduction of one of the nitro groups to amino
419
group activates the benzene ring by electron donation.
420
adding a small amount of NaBH4 to the reaction system, the disappearance of 355nm
421
peak indicates the complete reduction of nitro compound (Fig. 8c).
However, the reaction is completed within 30
422
Ac ce p
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d
412
423
reproduced in supporting information (Vide: supporting fig. S9). The rate constant of Au
424
nanoparticles is 2.884x10-3 s-1, which is significantly higher than those of Au–Ag alloy
425
nanoparticles (1.7606 x10-3 s-1, 1.3368 x10-3 s-1 and 1.0564x10-3 s-1 for Au0.75:Ag0.25,
426
Au0.50:Ag0.50 and Au0.25:Ag0.75 particles respectively) as well as that of AgNPs
427
(0.6880x10-3 s-1). The rate constant for the reduction exponentially increases from
However, immediately after
The kinetic rate constants of the Au, Au–Ag alloys and Ag nanoparticles are
20
Page 20 of 31
0.6880x10-3 s-1(Ag) to 2.884x10-3 s-1(Au) as the gold content was increased in the Au–Ag
429
alloy nanoparticles indicating that Au could be much more efficient than Ag for the
430
electron transfer from BH4- ion to 4-nitrophenol (Vide: supporting information Table:
431
S10). The reduction of 4-nitrophenol in the presence of NaBH4 and Au–Ag alloy
432
nanoparticles was investigated and it was found that the rate constant for the reduction
433
exponentially increases from Ag to Au as the Au content was increased in Au–Ag alloy
434
nanoparticles (Shin, Dohnalkova, & Lin, 2010). However, there is no definite correlation
435
between the composition of the alloy nanoparticles, size and rate. The percentage of Au
436
in Au–Ag alloy NPs and their corresponding alloy nanoparticle size can vary either
437
linearly (Mallin, & Murphy, 2002), inversely (Kariukiet al, 2004) or at random (Pal,
438
Shah, & Devi, 2007) depending on the method of preparation. The size of the Au–Ag
439
alloy NPs in the present investigation increases with the increase in the Au content in the
440
alloy NPs.
an
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428
442
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441
4. Conclusions
The rapid and green synthesis of stable silver, gold and Au–Ag alloy
444
nanoparticles has been achieved using anti-TB drug INH as a reducing agent. The
445
formation of homogeneous alloy nanoparticles was confirmed by UV–Visible absorption
446
spectroscopy, CV, fluorescence, TEM and XPS measurements. The alloy nanoparticles
447
were synthesized within a short period using environmentally benign chemicals. The
448
TEM analysis shows that the mean particle size increases from 3.8±0.8 nm for (Ag) to
449
14.4±2.7 nm for (Au) while the size of Au–Ag alloy NPs is found to be 4.1±1.5, 4.6±1.0,
450
and 7.0±0.5 nm respectively. The surface plasmon band displays a red shift with the
451
increase of Au% in the bimetallic nanoparticles, which confirms the formation Au–Ag
452
alloy nanoparticles. When the Ag content in the alloy decreases, the size of the
453
nanoparticles increases and as a result of which not only the oxidation potential increases
454
but also the emission maximum undergoes a bathochromic shift from 443 to 614 nm. The
455
rate of conversion of 4-NP to 4-AP is significantly enhanced by the bimetallic catalyst
456
when NaBH4 was employed as a reducing agent. However, the role of catalyst in the
457
reduction of picric acid was found to be only marginal.
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458 459
Acknowledgements We thank National Center for Nanoscience and Nanotechnology (NCNST),
461
University of Madras for TEM and XPS studies. The financial support by the Indian
462
Council of Medical Research (ICMR), New Delhi is gratefully acknowledged.
ip t
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466
cr
465
Supporting information
The UV–Visible spectra, cyclic voltammograms and photoluminescence spectra of Ag, Au and Au–Ag alloy nanoparticles. [Figs (S1-S9) and Table (S10)].
us
464
467
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598
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600 601 602 603 604
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606
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Ac ce p
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M
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29
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607
Table caption
608 609
Table 1: Composition, size, cyclic voltammetry, UV–Visible, and photoluminescence
610
spectral data of Ag, Au and Au–Ag alloy NPs
ip t
611
Table 1 Composition, size, cyclic voltammetry, UV–Visible, and photoluminescence
613
spectral data of Ag, Au and Au–Ag alloy NPs
614
Molar ratio
Size
Epa
Epc
ʎmax
615
of NPs
(nm)
(V)
(V)
(nm)
616
Ag
3.8±0.80
0.30
-0.67
617
Au0.25:Ag0.75
4.1±1.50
0.34
-0.37
618
Au0.50: Ag0.50
4.6±1.00
0.37
+0. 15
481
475
619
Au0.75: Ag0.25
7.0±0.50
0.47
+0. 18
513
500
563
620
Au
14.4±2.7
0.51
+0. 36
576
575
614
626
(nm)
us
(nm)
411
407
443
443
440
463
an
M
ʎem
513
d
nm; Nanometer, V;Volts. Epa; Oxidation potential, Epc; Reduction potential. ʎmax;Maximum wavelength, ʎex; Excitation wavelength, ʎem; Emission wavelength
te
625
ʎex
Ac ce p
621 622 623 624
cr
612
30
Page 30 of 31
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Scheme 2 Catalytic reduction of 4-nitrophenol
us
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Fig. 1 (a) UV–Visible absorption spectra of Au–Ag alloy nanoparticles a) Ag b) Au0.25: Ag 0.75 c) Au 0.50: Ag 0.50 d) Au 0.75: Ag 0.25 e) Au (b) A linear plot of surface plasmon bands against the mole fractions of Au atoms in alloy nanoparticles.
an
Fig. 2 Voltammograms of Ag, Au0.50:Ag0.50 and Au nanoparticles modified GCE in a PBS medium Fig. 3 Emission spectra of Ag, Au0.50:Ag0.50 and Au nanoparticles
M
Fig. 4 XPS analysis of Au–Ag alloy nanoparticles (Au0.25:Ag0.75) Fig. 5 (a) TEM micrographs of AgNPs (20nm) (b) Particle size distribution of AgNPs (c) SAED pattern of AgNPs (d) EDAX spectrum of AgNPs
d
632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660
Scheme 1 Schematic illustration of Ag, Au, Au–Ag alloy nanoparticles
Fig. 6 (a) TEM micrographs of AuNPs (50nm) (b) Particle size histogram of AuNPs (c) SAED pattern of AuNPs (d) EDAX spectrum of AuNPs
te
627 628 629 630 631
Figure Captions
Ac ce p
626
Fig. 7 (a) TEM images of Au 0.50: Ag 0.50 alloy NPs (20nm) (b) Particle size histogram of Au 0.50: Ag 0.50 alloy NPs (c) SAED pattern of Au 0.50: Ag 0.50 alloy NPs (b) EDAX spectrum of Au 0.50: Ag 0.50 alloy NPs Fig. 8 (a) UV–Visible absorption spectra of 4-nitrophenol and 4-nitrophenolate ion (b) UV–Vis absorption spectrum of 4-nitrophenol in the presence of NaBH4 and catalyst (Au 0.50: Ag 0.50) at different time intervals.(c) UV–Visible absorption spectrum of picric acid (black) and after the addition of NaBH4 (red)
661 31
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S0144-8617(14)00483-4 http://dx.doi.org/doi:10.1016/j.carbpol.2014.04.105 CARP 8880
To appear in: Received date: Revised date: Accepted date:
21-3-2014 24-4-2014 28-4-2014
Please cite this article as: Malathi, S., Ezhilarasu, T., Abiraman, T., & Balasubramanian, S.,One pot green synthesis of Ag, Au and AundashAg alloy nanoparticles using isonicotinic acid hydrazide and starch, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.04.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2
One pot green synthesis of Ag, Au and Au–Ag alloy nanoparticles using isonicotinic acid hydrazide and starch
3
Sampath Malathi, Tamilarasu Ezhilarasu, Tamilselvan Abiraman and
4
Sengottuvelan Balasubramanian* Department of Inorganic Chemistry, University of Madras, Guindy Campus,Chennai-600
6
025, Tamilnadu, India .Tel.No:+914422202794
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5
Graphical Abstract
us an
9
Ag Au
Au
Starch
Ag Au
M
+
O
12
INH
Ag Ag
Au Ag
BH4-
OH 4-AP NO2
Au
Au-Ag alloy NPs
OH 4-NP
te
11
HAuCl4
Au Au
Ac ce p
10
AgNO3
NH2
d
N
N H
Ag
Ag
NH2
reduction
8
cr
7
1
Page 1 of 31
12
Highlights Green synthesis of Ag, Au and their alloy NPs using starch as a capping agent and water as the solvent.
15 16
Synthesis of Ag, Au and their alloy NPs using isoniazid (INH) as a reducing agent.
17 18
The redox behaviour of alloy NPs depends on their size as well as their composition.
19
The catalytic reduction of 4-nitrophenol and picric acid by metal and alloy NPs.
20
When the size increases alloy NPs show bathochromic shift in their emission.
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One pot green synthesis of Ag, Au and Au–Ag alloy nanoparticles
23
using isonicotinic acid hydrazide and starch
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Sampath Malathi, Tamilarasu Ezhilarasu, Tamilselvan Abiraman and
25
Sengottuvelan Balasubramanian*
26
Department of Inorganic Chemistry, University of Madras, Guindy Campus,
27
Chennai-600 025, Tamilnadu, India .Tel.No:+914422202794
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Abstract
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Gold-silver alloy nanoparticles were synthesized via chemical reduction of
31
varying mole fractions of chloroauric acid (HAuCl4) and silver nitrate (AgNO3) by
32
environmentally benign isonicotinic acid hydrazide (INH) in the presence of starch
33
as a capping agent in aqueous medium. The absorption spectra of Au–Ag
34
nanoparticles show blue shift with increasing silver content indicating the
35
formation of alloy nanoparticles. When the Ag content in the alloy decreases the
36
size of the nanoparticles increases and as a result of which the oxidation potential
37
also increases. The emission maximum undergoes a red shift from 443 to 614 nm.
38
The nanoparticles are monodisperse and spherical with an average particle size of
39
3-18 nm. The catalytic behavior of alloy nanoparticles indicate that the rate
40
constant for the reduction of 4-nitro phenol to 4-amino phenol increases
41
exponentially from metallic Ag to metallic Au as Au content increases in the Au–
42
Ag alloy nanoparticles.
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Keywords: silver, gold, alloy nanoparticles, isoniazid, cyclic voltammetry,
45
4-nitrophenol
46
*Corresponding author: Email id:[email protected]
47 48 3
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49
1. Introduction Metal nanoparticles show characteristic size–dependent properties different from
51
those of bulk metals with significant size effects occurring in 1–10nm diameter range.
52
(Link, Burda, Wang, & El-Sayed, 1999). Au–Ag NPs with certain bimetallic
53
compositions have been synthesized in single-phase (water) (Mallin, & Murphy, 2002) as
54
well as in bi-phasic (toluene–water) (Hostetler et al, 1998) medium.
55
nanoparticles have been synthesized by NaBH4 reduction of supported metal salts, (Kim,
56
Kim, Kim, & Yoo, 2013) citrate reduction, (Link, Wang, El-Sayed, 1999) hydrazine
57
reduction, (Chen, & Chen, 2002) laser ablation, (Liz-Marzan, L. & Philipse, 1995)
58
sputter deposition, (Okazaki et al, 2008) laser irradiation,(Chen, & Yeh, 2001)
59
sonochemical method (Anandan, Grieser, & Ashokkumar, 2008) and also by biosynthesis
60
(Senapati, Ahmad, Khan, Sastry, & Kumar, 2005). Kim et al have reported the synthesis
61
of Au–Ag alloy nanoparticles by laser ablation of bulk alloys (Lee, Han, & Kim, 2001).
62
Teo et al have synthesized 38 atom gold–silver clusters (Au18 Ag20) to investigate the
63
minimum number of atoms in a cluster required to exhibit metallic behavior (Teo,
64
Keating, & Kao, 1987).
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Au–Ag alloy
The alloy NPs with smaller sizes are often required in the field of catalysis due to
66
their higher activity (Zhang, Zhang, Wei, Du, & Li, 2009). For example, sub-5 nm Au–
67
Ag alloy NPs strongly enhanced oxidation of carbon monoxide, which could be prepared
68
by half-seeding or a templating approach (Wang, Liu, Lin, Lin, & Mou, 2005). However,
69
the complex experimental procedures of such methods restricted their practical
70
applications. Therefore, it is necessary to develop a facile method for the preparation of
71
high quality Au–Ag alloy NPs with smaller size in the solution phase.
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4-pyridine carboxylic acid hydrazide (INH) possesses significant physiological
73
activities among the three isomeric 2, 3 and 4-pyridine carboxylic acid hydrazides. INH
74
via passive diffusion rapidly permeates the bacterial cell membrane wherein the
75
hydrazide group in the molecule serves only as a carrier group (Kruger–Thiemer)
76
(Kruger–Thiemer, 1955). INH has been the accepted drug for use as TB prophylaxis in
77
immunocompetent as well as in HIV positive cases (Carosi, & Matteelli, 1996). The
78
transition metal complexes of INH exhibit interesting biological activities. The catalytic 4
Page 4 of 31
79
conversion of epoxides into ninhydrins is possible with this compound (Sharghi,
80
Eskandari, & Ghavami, 2004) and it has been found to act as a molecular logic gate in
81
aqueous medium (Magria, & Prasanna de Silva, 2010). The objective of this study is to develop a simple one-step and “green” approach to
83
synthesize Au, Ag and Au–Ag alloy nanoparticles by chemical reduction using
84
isonicotinic acid hydrazide (Isoniazid) and starch as the capping agent. Isoniazid is being
85
used as a first line anti-tuberculosis drug (Winder, 1956). The byproduct isonicotinic acid
86
produced in this reaction is also non-toxic and eco-friendly. The homogeneous Au–Ag
87
alloy nanoparticles prepared in this study can be used as a catalyst for the reduction of 4-
88
nitrophenol and picric acid in presence of NaBH4 for the synthesis of amino phenols. The
89
amino phenol is an important intermediate in the manufacture of many analgesic and
90
antipyretic drugs besides its application as photographic developer, corrosion inhibitor
91
and hair dyeing agent (Corbett, 1999).
2. Experimental
94
2.1 Materials
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Chloroauric acid (HAuCl4.xH2O, LR, LOBA Chemie, India), silver nitrate
96
(AgNO3, AR, BDH, India), 4-nitrophenol (C6H5NO3, AR, SRL, India), sodium
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borohydride (NaBH4, LR, Merck, India), picric acid (C6H3N3O7, AR, BDH, India)
98
isoniazid (C6H7N3O, AR, Fluka, India) and starch [(C6H10O5)n, soluble GR, Merck, India]
99
were used as such without further purification. Milli–Q water was used throughout the
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experiment.
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2.2 Methods
102
2.2.1 Synthesis of silver nanoparticles, gold nanoparticles and Au – Ag alloy
103
nanoparticles
104
20 mL aqueous solution containing starch (1%) and AgNO3 (0.1 mM) was taken
105
in a round bottom flask and 0.025 mM (2.5 mL) of freshly prepared aqueous isoniazid
106
solution was added to it. The mixture was heated to 60 ˚C with continuous stirring till the
107
solution turns yellow indicating the formation of AgNPs. Similarly a solution containing
108
HAuCl4.xH2O (0.1 mM) and starch (1%) was taken in a round bottom flask.
The
5
Page 5 of 31
mixture was stirred continuously at room temperature, followed by the dropwise addition
110
of isoniazid (0.025 mM). The solution turns purple indicating the formation of AuNPs.
111
Au–Ag alloy nanoparticles were synthesized by the reduction of HAuCl4.xH2O and
112
AgNO3 with isoniazid in the presence of starch with varying initial Au–Ag molar ratios
113
(1:0, 0.75:0.25, 0.50:0.50, 0.25:0.75 and 0:1). The resultant 0.50:0.50 Au–Ag solution
114
consists of 10 mL of 1% starch, 2.5 mL (0.1 mM) HAuCl4.xH2O and 2.5 mL (0.1 mM)
115
AgNO3. 5.0 mL of an aqueous solution of (0.025 mM) isoniazid was added all at once
116
with vigorous stirring. The solution was allowed to stir for an additional 60 s. The color
117
of the resulting nanoparticle solution depends on the initial composition of Au–Ag alloy
118
nanoparticles.
119
spectrophotometry.
resulting
nanoparticles
were
analyzed
by
UV–Visible
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Scheme 1 6
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122 123
2.2.2 Catalytic reduction of 4-nitrophenol 0.25 mL of 10 mM aqueous solution of 4-nitrophenol and 0.25 mL of 1M freshly
125
prepared aqueous solution of NaBH4 were mixed with 2.5 mL of Milli–Q water at room
126
temperature. 100 μL of 0.1 mM Au, Ag, Au–Ag alloy nanoparticles were mixed with the
127
above solution and UV–Visible absorption spectra were recorded with time to monitor
128
the change in the reaction mixture. The above procedure is also followed for the
129
reduction of picric acid with or without the presence of the nanoparticles.
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132 133 134 135
Scheme 2
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2.2.3 Fabrication of silver, gold, and their alloy nanoparticles modified GCE The CV (cyclic voltammetry) studies were carried out on a CH Instruments
136
Model CHI-1130A electrochemical workstation (USA). The three electrode cell assembly
137
consists of the bare GCE as well as modified by metal/alloy nanocoating as working
138
electrode, platinum wire as counter electrode and Ag/AgCl as reference electrode. The
139
experiments were conducted in PBS (pH: 7.4) medium and 3 M KCl as supporting
140
electrolyte. The solution was purged by N2 gas to remove the dissolved oxygen. The
141
potential was scanned between 1 V and -1 V at a scan rate of 50 mVs-1. The nanoparticles
142
coated GCE electrode was fabricated individually by casting 10 μL of each nanoparticle
7
Page 7 of 31
143
solution. Then the solvent was allowed to dry at room temperature and rinsed with water
144
in order to remove any loosely bound nanoparticles.
145 146
2.2.4 Characterization The aqueous solution of monometallic and bimetallic nanoparticles were
148
examined using Ultraviolet–visible spectrophotometer (Perkin–Elmer, ʎ 35, USA) in the
149
range of 200–800nm. Transmission electron microscopic (TEM) studies were carried out
150
using TECNAI, G2-MODEL, T-30 S-TWIN with an acceleration voltage of 250 kV. The
151
aqueous solution of nanoparticles was drop cast onto a carbon – coated copper grid
152
sample holder followed by evaporation at room temperature. The size distribution of the
153
nanoparticles was estimated based on images J software (version 1.47). The X-ray
154
photoelectron spectroscopy (XPS) measurements were made on a Physical Electronics-
155
Quantum 2000 EAC (SPHERA 547) scanning microprobe. This system uses a focused
156
monochromatic Al Kα X-ray (1486.7eV) source for excitation, a spherical section
157
analyzer and a seven channel detection system. The fluorescence measurement was
158
carried out at room temperature using a Varian–Cary Eclipse fluorescence
159
spectrophotometer. The fluorescence measurements were carried out in the range 350-
160
700nm.
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3. Results and Discussion
The green synthesis of AgNPs was achieved by chemical reduction of silver ions
164
using INH as a reducing agent and starch as a capping agent. In this study, starch is used
165
as a template for nanoparticle growth while INH serves as an environmentally benign
166
reducing agent for the reduction of silver ions. The silver nanoparticle solution shows an
167
absorption maximum at 411 nm. While mild heating is sufficient for the INH reduction
168
of silver ions, the reduction of Au3+ ions of HAuCl4 occurs at room temperature.
169
The surface plasmon absorption maximum of the purple color AuNPs, which is
170
obtained from 0.1 mM solution of Au3+ ions, is 576 nm. The reduction of Au3+ ions
171
occurs fairly rapidly and more than 90% of Ag+ and Au3+ ions are reduced within 15 min
172
and 2 min respectively, after the addition of INH to the metal ion solution. The 8
Page 8 of 31
comparatively slower reduction rate of silver ions relative to that of gold ions is most
174
likely due to differences in the reduction potentials of the two metal ions, the redox
175
potential being considerably lower for Ag+ to Ag0 (Raveendran, & Wallen, 2003). INH is
176
considered to be “greener” in terms of biocompatibility and starch is one of the
177
polysaccharides, which belongs to the most abundant natural and renewable biopolymers.
178
The hydroxyl groups of starch help to protect the surface of these particles in the absence
179
of which they will aggregate as a result of high surface energies. Starch serves as a
180
stabilizing template for the alloy nanoparticles (Hussain, Kumar, Hashmi, & Khan,
181
2011).
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185 186
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Fig. 1
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The synthesis of Ag, Au and Au–Ag alloy nanoparticles is very simple. The
187
mixtures of Ag and Au metal ion solution containing starch undergo change in color upon
188
addition of INH depending on the molar composition of Ag and Au. The AgNO3
189
solution changes from colorless to yellow upon reduction. However, as the Au content
190
increases, the intensity of the purple color increases (Vide: supporting Fig. S1). This is
191
the first report where NPs of Ag, Au and Au–Ag from Ag+ and Au3+ ions are prepared
192
using INH as a reducing agent. The Au–Ag alloy nanoparticles, as well as Ag and Au
193
nanoparticles, can be characterized by UV–Visible absorption spectroscopy. Silver and
194
gold nanoparticles are known to exhibit surface plasmon resonance bands in the visible
195
region at ~ 440 nm (Naik, Stringer, Agarwal, Jones, & Stone, 2002) and ~ 530 nm (Aryal
196
et al, 2006), respectively. The UV–Visible spectra of the Ag and Au NPs prepared from 9
Page 9 of 31
197
the salts of the metals exhibit distinct peaks at 411 nm and 576 nm [Fig. 1a: spectral
198
traces (a) and (e)] respectively in the present investigation.
199
The UV–Visible spectra of the Au 0.25: Ag 0.75, Au 0.50: Ag 0.50 and Au 0.75: Ag 0.25
201
samples are reproduced in Fig. 1a: spectral traces (b), (c) and (d), which exhibit single
202
absorption maximum at 443, 481, and 513 nm respectively, thus confirming the
203
formation of alloy nanoparticles with only homogeneous composition. The spectra of the
204
alloy nanoparticles did not undergo any change for several months. The surface plasmon
205
absorption of the Au–Ag composite is linearly red-shifted ( ̴ 170nm) from that of a
206
monometallic Ag in proportion to the increase in the composition.
207
nanoparticles are very stable in the aqueous phase. The mole fraction of the Au–Ag
208
content indicates that the nanoparticles are alloys rather than core–shell nanoparticles or
209
mixture of monometallic nanoparticles. This is illustrated in fig. 1b in which the UV–
210
Visible absorption maximum is plotted against the mole fraction of Au in the precursor
211
samples. It is known that a physical mixture of monometallic Ag and Au nanoparticles
212
has two absorption peaks corresponding to the monometallic Ag and Au respectively
213
(Kim, Kim, Kim, & Yoo, 2013). (Vide: supporting information Fig. S2) The core – shell
214
nanoparticles would exhibit the same two absorption peaks (Tang, Yuan, & Chai, 2006),
215
where one increases in absorbance with the increase in the concentration of that
216
component with concomitant decrease in the absorbance of other component. The
217
stability of as synthesized Au and Ag nanoparticles was monitored by UV–Visible
218
spectral studies. The SPR band of starch stabilized nanoparticles did not undergo any
219
change either in its position or its intensity even after two months of storage at ambient
220
temperature.
Au–Ag alloy
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Page 10 of 31
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Fig. 2
The cyclic voltammograms of GCE modified with Au–Ag alloy nanocoating with
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the ratio Ag, Au0.75:Ag0.25, Au0.50:Ag0.50, Au0.25:Ag0.75 and Au in PBS medium (pH: 7.4)
226
are reproduced in fig. 2 and supporting information (Fig. S3). The AuNPs modified GCE
227
(Au) (Fig. 2a) recorded in the range 0.2 to 1.0 V shows the broad anodic peak and
228
prominent cathodic peak at 0.51 and 0.36 V respectively. In the case of Au0.50:Ag0.50
229
alloy nanoparticle, the anodic peak at 0.37 V and cathodic peak at 0.15 V were observed
230
(Fig. 2b). The redox behavior of the alloy NPs Au0.75:Ag0.25 and Au0.25:Ag0.75 is reported
231
in supporting information. [Vide: supporting information: Fig. S3]. The oxidation and
232
reduction peaks appear at 0.34V, 0.47V and -0.37V, 0.18V respectively (Table 1). But
233
the AgNPs (Ag) modified GCE exhibits the oxidation at Epa= +0.30 V and reduction at
234
Epc= -0.67 V as shown in Fig. 2c. 11
Page 11 of 31
235 236
Table 1 Composition, size, cyclic voltammetry, UV–Visible, and photoluminescence Spectral data of Ag, Au and Au–Ag alloy NPs
237
Molar ratio
Size
Epa
Epc
ʎmax
ʎex
ʎem
239
of NPs
(nm)
(V)
(V)
(nm)
(nm)
(nm)
240
Ag
3.8±0.80
0.30
-0.67
411
241
Au0.25:Ag0.75
4.1±1.50
0.34
-0.37
443
242
Au0.50: Ag0.50
4.6±1.00
0.37
+0. 15
481
243
Au0.75: Ag0.25
7.0±0.50
0.47
+0. 18
513
244
Au
14.4±2.7
0.51
+0. 36
443
440
463
cr
407
475
500
563
575
614
us 576
513
nm; Nanometer, V;Volts. Epa; Oxidation potential, Epc; Reduction potential. ʎmax;Maximum wavelength, ʎex; Excitation wavelength, ʎem; Emission wavelength
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245 246 247 248
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The oxidation potential (Epa) gradually increases from 0.30 to 0.51 V and the
250
reduction potential (Epc) also gradually increases from -0.67 to 0.36 V, when the
251
concentration of silver nanoparticles is reduced from 100 % to 0 % in Au–Ag alloy
252
nanoparticles (Table1). Moreover the redox potential gradually increases as the size of
253
the NPs is increased from 3.8 to 14.4 nm. These values are comparable to that of the
254
earlier report by Tominaga et al, which indicates the dependency of the redox potential
255
with the size of the Au–Ag alloy NPs as well as the fall in the redox potential with
256
increasing concentration of silver (Raimondi, Scherer, Kcotz, & Wokaun, 2005). Pumera
257
et al have indicated the size dependent electrochemical behavior of silver (Giovanni, &
258
Pumera, 2012) and gold (Bonanni, Pumera, & Miyahara, 2011) nanoparticles. Compton
259
et al have demonstrated that stripping potential of Ag NPs depends on the size of the
260
nanoparticles (Jones, Campbell, Baron, Xiao, & Compton, 2008).
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261 262
The redox potential of alloy nanoparticles essentially depends on two factors: size
263
and composition. These two factors may oppose each other in some cases while they
264
have cumulative effect in some other systems. It has been found that if the size of the
265
alloy nanoparticles decreases, anodic potential increases and when the silver content 12
Page 12 of 31
decreases, the oxidation potential also increases (Tominaga et al, 2006). However, in the
267
present investigation it has been found that when the size of the nanoparticle increases,
268
the oxidation potential also increases while the reduction in silver content results in
269
increase in anodic potential. The latter behavior compares well with that of the literature
270
report, while the former exhibits a reversal in this trend. This observation confirms that
271
the Au–Ag alloy nanoparticles are composed of by homogenous mixture of AgNPs and
272
AuNPs and not by Ag and Au metal domains.
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275 276 277
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Fig. 3
The characteristic photoluminescent behavior of Ag, Au and Au–Ag alloy
278
nanoparticles was observed by measuring fluorescence emission spectra (Fig. 3). The
279
emission maximum of Ag and AuNPs was observed at 443 nm and 614 nm respectively
280
when they were excited at 407 nm and 575 nm respectively. As the molar ratio of Ag
281
decreases, the fluorescence emission maximum undergoes a red shift from 443 to 614 nm
282
along with an increase in excitation wavelength due to synergism (Vide: supporting
283
information Fig. S4). It has also been observed that when the solutions of metal and alloy
284
NPs were excited at different wavelengths, the emission maximum did not undergo any
285
change. For example, when the solution of Ag was excited at 390, 400 and 407 nm, the
286
ʎem was found to be 443 nm while the excitation of Au0.75:Ag0.25 at 400, 420 and 440 nm 13
Page 13 of 31
resulted in ʎem at 463 nm indicating that the alloy NPs is monodisperse. This observation
288
closely resembles that of the silver nanoclusters reported earlier (Shang, Dong, 2008;
289
Ganguly, Pal, Negishi, & Pal, 2013). Millstone et al have reported the photoluminescence
290
behavior of Au/Cu NP alloys in which the fluorescence of the alloy NPs undergoes
291
bathochromic shift when the concentration of copper increases (Andolina et al, 2013).
292
This is in contrast to the observation made in the case of Au–Ag alloy nanoparticles in
293
which the decreasing concentration of Ag results in increasing red shift. The fluorescence
294
intensity of pure Ag metal is considerably reduced due to the quenching by Au with
295
increase in its concentration, which is similar to the observation made in the earlier report
296
(Ganguly, Pal, Negishi, & Pal, 2013). The fluorescence intensity also decreases with
297
decrease in the Ag concentration in the alloy NPs. The fluorescence maximum depends
298
on the applied potential and it has been demonstrated by Tafel plot (Nag et al, 2012;
299
Kaneko, Uosaki, & Kita, 1986).
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When the EPa of the system increases, the emission wavelength also undergoes a
302
red shift. When the concentration of Ag in the bimetallic alloy is reduced from 100% to
303
0% the EPa increases from +0.30 V to +0.51 V with a corresponding increase in the
304
fluorescence emission maximum from 443 nm to 614 nm. The electrochemical studies
305
have clearly established the relationship between the composition and oxidation potential
306
of the NPs.
307
potential and emission wavelength [Table 1]. The photoluminescence efficiency of
308
colloidal nanocrystals has also been reported to vary with the zeta potential especially, in
309
the case of Cd/Se colloidal nanocrystals (Nag et al, 2012).
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Similar correlation can be made between the composition, oxidation
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The crystal structure of Au, Ag and their alloy nanoparticles has been further
311
characterized by XRD analysis. The XRD pattern for the synthesized Ag, Au and their
312
alloy nanoparticles are reproduced in supporting information (vide: supporting fig. S5).
313
The XRD pattern of Au and Ag synthesized in the present investigation matches with
314
those of earlier reports(JCPDS file nos: 870597and 740526 respectively).The four peaks
315
appearing at 2θ = 37.97˚, 44.11˚, 64.35˚ and 77.27˚ correspond to the (111), (200), (220)
316
and (311) planes, respectively, of the face centered cubic Ag (or Au) particles. Since gold 14
Page 14 of 31
317
and silver have the same fcc structure and almost the same lattice constant it is difficult to
318
distinguish gold–silver alloy from a mixture of the monometallic phases from XRD
319
pattern (Shankar, Rai, Ahmad, & Sastry 2004; Chen, & Chen, 2002).
320
Table 2 XPS peak positions (BE/eV) for Au–Ag NPs Au
84.0
Ag
-
Au (0.25): Ag (0.75)
85.0
Ag (3d5/2) -
368.0 368.4
BE; Binding Energy, eV; Electron volts.
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Au (4f7/2)
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Fig. 4
328 15
Page 15 of 31
329
The surface alloy composition of Au–Ag alloy NPs was confirmed by X-ray
330
photoelectron spectral analysis. The XPS traces of Au–Ag alloy NPs are shown in Fig. 4.
331
The XPS data for the alloy Au
332
positions are tabulated (Table 2). Au–Ag alloy nanoparticles show both Au 4f (85.0eV)
333
and Ag 3d (368.4eV) peaks in Fig.4, confirming the bimetallic nature of alloy
334
nanoparticles. The content of Au and Ag within the X-ray penetration depth (3–5 nm) on
335
the surface is quantified by Au 4f and Ag 3d peak areas divided by the respective atomic
336
sensitive factor (ASF) 6.250 and 5.987 which indicates an average composition of 23
337
atomic% Au and 77 atomic% Ag. In general, the binding energies of alloy nanoparticles
338
exhibit a shift, compared to those of metallic Ag and Au forms, indicating that the two
339
noble metals form alloy nanoparticles rather than individual metal nanoparticles. The
340
XPS binding energies of alloy nanoparticles have been reported by several groups
341
(Srnova-Sloufova, Vickova, Bastl, & Hasslett, 2004; Kariuki et al, 2004). The shift in the
342
peak binding energy is indicative of the formation of alloys. Both Au and Ag bands have
343
been identified (Table 2) which are in agreement with the literature data (Moulder,
344
Stickle, Sobol, & Bomben, 1995). The analysis of XPS spectra confirms the presence of
345
starch (C1s) as a capping agent of the Au–Ag alloy nanoparticles, which prevents their
346
aggregation (Fig. 4). The C1s spectrum shows peaks corresponding to C (CH2) at 284.7
347
eV, C-O at 286.0 eV, O-C-O at 287.3 eV and O=C-O at 288.7 eV.
Ag
0.75
as well as metallic Ag, Au and their peak
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Page 16 of 31
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349 350
Fig. 5
The nature of the metal nanoparticles has been analyzed by TEM and selected
352
area electron diffraction pattern (SAED) studies. The representative TEM images
353
recorded from carbon coated copper grid of the AgNPs prepared by the reduction of Ag+
354
ions in starch by INH are reproduced (Fig. 5a). The particles are polydisperse in nature
355
with an average size of 3.8±0.8 nm (Fig. 5b). The SAED for AgNPs and the ring pattern
356
are indexed to the 111, 200, 220 and 311 Bragg reflections from the fcc structure of
357
silver (Fig. 5c). The EDAX spectrum of AgNPs shows peaks around 3.4 keV, which
358
correspond to the binding energies of silver thus confirming the formation of metallic
359
AgNPs (Fig. 5d).
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351
361
The TEM micrograph for AuNPs is shown in Fig. 6a. The nanoparticles are
362
spherical and monodisperse with diameter in the range of 14.4±2.7 nm (Fig. 6b). The
363
SAED for AuNPs and the ring pattern are indexed to the 111, 200, 220 and 311 Bragg
364
reflections from the fcc structure of gold (Fig. 6c). The EDAX spectrum of AuNPs is 17
Page 17 of 31
shown in fig. 6d. The peaks around1.8 and 10.8 keV correspond to the binding energies
366
of AuNPs, indicating that the resultant product is metallic AuNPs.
369 370
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Fig. 6
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365
The TEM indicates the size and shape of the alloy nanoparticles. Fig. 7a shows
371
the TEM micrograph recorded at equimolar ratio of Au–Ag alloy NPs. The average size
372
of the alloy NPs is 4.6±1.0 nm; Au: 0.50 (Fig. 7b) and they are spherical in shape (Table
373
1). The SAED pattern of Au0.50: Ag0.50 confirms that the alloy NPs formed are crystalline
374
in nature and they belong to fcc lattice (Fig. 7c). Both Au and Ag adapt an fcc structure
375
and have very close crystal lattice parameters. Hence it would be very difficult to
376
observe any shift in the position of the rings from those of individual Au or Ag NPs. The
377
EDAX analysis suggests the Au–Ag mole ratio of the sample with an elemental
378
composition of Au (50%) and Ag (50%) (Fig.7d). The size of alloy nanoparticles is less
379
than 10 nm, which is smaller than that of Ag–Au alloy prepared in aqueous solution as
380
reported earlier (Kim, Kim, Kim, & Yoo, 2013). The TEM images of Au–Ag alloy 18
Page 18 of 31
381
nanoparticles did not show either a dense core or a light shell, which is typical for core–
382
shell structure. The TEM micrographs recorded at different molar ratios of Au–Ag alloy
383
NPs and the average size of the alloy NPs are 4.1 ±1.5 nm; Au: 0.25 and 7.0±0.5 nm; Au:
384
0.75nm and they are spherical in shape (Vide: supporting fig. S6) and (Table 1).
388 389 390 391
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387
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386
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385
Fig. 7
Catalytic activity of Au, Ag, and Au–Ag alloy nanoparticles The catalytic efficiency of Au–Ag alloy nanoparticles for the reduction of mono
392
and trinitrophenol systems has been investigated. In a typical bimetallic catalytic
393
reduction in the present investigation, upon the addition of NaBH4 to the aqueous 4-
394
nitrophenol (4NP), nitrophenolate ion is formed immediately (Li et al, 2012) which
395
shows red shift in the UV–Visible absorption band from 317 to 400 nm (Fig. 8a). In the
396
absence of any catalyst, the thermodynamically favorable reduction of 4-NP (E˚ for 4-
397
NP/4-AP = -0.76 V and H3BO3/BH4- = -1.33 V versus NHE) was not observed and the
398
peak due to 4-nitrophenolate ion at 400 nm remains unaltered even for a couple of days 19
Page 19 of 31
399
(Mrinmoyee, & Tarasankar, 2011). However, immediately after adding a small amount of
400
catalyst Ag or Au into the reaction system, the intensity of the absorption band at 400
401
nm successively decreases with increasing reaction time and simultaneously a new band
402
appears at 300 nm indicating the formation of 4-AP (Fig. 8b).
ip t
403 404
cr
405
us
406 407
an
408 409
Fig. 8
411
M
410
The product conversion is fast (12 min) in the presence Au (100%) (Vide: supporting information Fig. S7)
catalyst when compared to that of Au (0%) (40 min)
413
(Vide: supporting information Fig. S8).
414
min in the presence of bimetallic alloy nanoparticles Au (50%): Ag (50%). The catalyst
415
appears to have marginal influence on the reduction of picric acid (PA) system and the
416
reduction occurs within 5 min with or without the addition of catalyst. The electron
417
withdrawing effect of the three nitro groups and resonance structure play an important
418
role in the process of strong reduction. The reduction of one of the nitro groups to amino
419
group activates the benzene ring by electron donation.
420
adding a small amount of NaBH4 to the reaction system, the disappearance of 355nm
421
peak indicates the complete reduction of nitro compound (Fig. 8c).
However, the reaction is completed within 30
422
Ac ce p
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412
423
reproduced in supporting information (Vide: supporting fig. S9). The rate constant of Au
424
nanoparticles is 2.884x10-3 s-1, which is significantly higher than those of Au–Ag alloy
425
nanoparticles (1.7606 x10-3 s-1, 1.3368 x10-3 s-1 and 1.0564x10-3 s-1 for Au0.75:Ag0.25,
426
Au0.50:Ag0.50 and Au0.25:Ag0.75 particles respectively) as well as that of AgNPs
427
(0.6880x10-3 s-1). The rate constant for the reduction exponentially increases from
However, immediately after
The kinetic rate constants of the Au, Au–Ag alloys and Ag nanoparticles are
20
Page 20 of 31
0.6880x10-3 s-1(Ag) to 2.884x10-3 s-1(Au) as the gold content was increased in the Au–Ag
429
alloy nanoparticles indicating that Au could be much more efficient than Ag for the
430
electron transfer from BH4- ion to 4-nitrophenol (Vide: supporting information Table:
431
S10). The reduction of 4-nitrophenol in the presence of NaBH4 and Au–Ag alloy
432
nanoparticles was investigated and it was found that the rate constant for the reduction
433
exponentially increases from Ag to Au as the Au content was increased in Au–Ag alloy
434
nanoparticles (Shin, Dohnalkova, & Lin, 2010). However, there is no definite correlation
435
between the composition of the alloy nanoparticles, size and rate. The percentage of Au
436
in Au–Ag alloy NPs and their corresponding alloy nanoparticle size can vary either
437
linearly (Mallin, & Murphy, 2002), inversely (Kariukiet al, 2004) or at random (Pal,
438
Shah, & Devi, 2007) depending on the method of preparation. The size of the Au–Ag
439
alloy NPs in the present investigation increases with the increase in the Au content in the
440
alloy NPs.
an
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428
442
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441
4. Conclusions
The rapid and green synthesis of stable silver, gold and Au–Ag alloy
444
nanoparticles has been achieved using anti-TB drug INH as a reducing agent. The
445
formation of homogeneous alloy nanoparticles was confirmed by UV–Visible absorption
446
spectroscopy, CV, fluorescence, TEM and XPS measurements. The alloy nanoparticles
447
were synthesized within a short period using environmentally benign chemicals. The
448
TEM analysis shows that the mean particle size increases from 3.8±0.8 nm for (Ag) to
449
14.4±2.7 nm for (Au) while the size of Au–Ag alloy NPs is found to be 4.1±1.5, 4.6±1.0,
450
and 7.0±0.5 nm respectively. The surface plasmon band displays a red shift with the
451
increase of Au% in the bimetallic nanoparticles, which confirms the formation Au–Ag
452
alloy nanoparticles. When the Ag content in the alloy decreases, the size of the
453
nanoparticles increases and as a result of which not only the oxidation potential increases
454
but also the emission maximum undergoes a bathochromic shift from 443 to 614 nm. The
455
rate of conversion of 4-NP to 4-AP is significantly enhanced by the bimetallic catalyst
456
when NaBH4 was employed as a reducing agent. However, the role of catalyst in the
457
reduction of picric acid was found to be only marginal.
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458 459
Acknowledgements We thank National Center for Nanoscience and Nanotechnology (NCNST),
461
University of Madras for TEM and XPS studies. The financial support by the Indian
462
Council of Medical Research (ICMR), New Delhi is gratefully acknowledged.
ip t
460
463
466
cr
465
Supporting information
The UV–Visible spectra, cyclic voltammograms and photoluminescence spectra of Ag, Au and Au–Ag alloy nanoparticles. [Figs (S1-S9) and Table (S10)].
us
464
467
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606
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607
Table caption
608 609
Table 1: Composition, size, cyclic voltammetry, UV–Visible, and photoluminescence
610
spectral data of Ag, Au and Au–Ag alloy NPs
ip t
611
Table 1 Composition, size, cyclic voltammetry, UV–Visible, and photoluminescence
613
spectral data of Ag, Au and Au–Ag alloy NPs
614
Molar ratio
Size
Epa
Epc
ʎmax
615
of NPs
(nm)
(V)
(V)
(nm)
616
Ag
3.8±0.80
0.30
-0.67
617
Au0.25:Ag0.75
4.1±1.50
0.34
-0.37
618
Au0.50: Ag0.50
4.6±1.00
0.37
+0. 15
481
475
619
Au0.75: Ag0.25
7.0±0.50
0.47
+0. 18
513
500
563
620
Au
14.4±2.7
0.51
+0. 36
576
575
614
626
(nm)
us
(nm)
411
407
443
443
440
463
an
M
ʎem
513
d
nm; Nanometer, V;Volts. Epa; Oxidation potential, Epc; Reduction potential. ʎmax;Maximum wavelength, ʎex; Excitation wavelength, ʎem; Emission wavelength
te
625
ʎex
Ac ce p
621 622 623 624
cr
612
30
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Scheme 2 Catalytic reduction of 4-nitrophenol
us
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Fig. 1 (a) UV–Visible absorption spectra of Au–Ag alloy nanoparticles a) Ag b) Au0.25: Ag 0.75 c) Au 0.50: Ag 0.50 d) Au 0.75: Ag 0.25 e) Au (b) A linear plot of surface plasmon bands against the mole fractions of Au atoms in alloy nanoparticles.
an
Fig. 2 Voltammograms of Ag, Au0.50:Ag0.50 and Au nanoparticles modified GCE in a PBS medium Fig. 3 Emission spectra of Ag, Au0.50:Ag0.50 and Au nanoparticles
M
Fig. 4 XPS analysis of Au–Ag alloy nanoparticles (Au0.25:Ag0.75) Fig. 5 (a) TEM micrographs of AgNPs (20nm) (b) Particle size distribution of AgNPs (c) SAED pattern of AgNPs (d) EDAX spectrum of AgNPs
d
632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660
Scheme 1 Schematic illustration of Ag, Au, Au–Ag alloy nanoparticles
Fig. 6 (a) TEM micrographs of AuNPs (50nm) (b) Particle size histogram of AuNPs (c) SAED pattern of AuNPs (d) EDAX spectrum of AuNPs
te
627 628 629 630 631
Figure Captions
Ac ce p
626
Fig. 7 (a) TEM images of Au 0.50: Ag 0.50 alloy NPs (20nm) (b) Particle size histogram of Au 0.50: Ag 0.50 alloy NPs (c) SAED pattern of Au 0.50: Ag 0.50 alloy NPs (b) EDAX spectrum of Au 0.50: Ag 0.50 alloy NPs Fig. 8 (a) UV–Visible absorption spectra of 4-nitrophenol and 4-nitrophenolate ion (b) UV–Vis absorption spectrum of 4-nitrophenol in the presence of NaBH4 and catalyst (Au 0.50: Ag 0.50) at different time intervals.(c) UV–Visible absorption spectrum of picric acid (black) and after the addition of NaBH4 (red)
661 31
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