Colorimetric Detection of Co2+ Ion Using Silver Nanoparticles with ...

Article
pubs.acs.org/Langmuir
Colorimetric Detection of Co 2+ Ion Using Silver Nanoparticles with
Spherical, Plate, and Rod Shapes
Hwa Kyung Sung, Seung Yeon Oh, Chulhwan Park,* and Younghun Kim*
Department of Chemical Engineering, Kwangwoon University, Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea
*S Supporting Information
ABSTRACT: A highly sensitive colorimetric sensing platform for the selective
trace analysis for Co 2+ ions is reported, based on glutathione (GSH)-modified
silver nanoparticles (AgNP). The shape of metallic nanoparticles used in
colorimetric detection, using the unique optical properties of plasmonic
nanoparticles, is almost spherical. Therefore, in this work we attempted to
investigate the selective detection of heavy metal ion (Co 2+ ), with the shape of
AgNPs (nanosphere, nanoplate, and nanorod). GSH-AgNP with spherical shape
shows a high sensitivity for all of the metal ions (Ni 2+ ,Co 2+ ,Cd 2+ ,Pb 2+ , and
As 3+ ) but poor selective recognition for target metal ions. Whereas, AgNPs
solution containing rod-type GSH-AgNP has a special response to Co 2+ , and its
selective detection might be based on the cooperative effect of CTAB and GSH.
Therefore, Co 2+ ion could be selectively recognized using rod-type GSH-AgNPs.
■ INTRODUCTION
One of the most important characteristics of metal nanoparticles
(NPs) is their localized surface plasmon resonance
(LSPR), which exhibits sensitivity to their size, shape,
composition, and dielectric constant. 1 This unique optical
property in the visible spectral region originates from the
excitation of the collective oscillation of conducting electrons of
metal NPs. 2 In particular, the plasmon properties of NPs
depend strongly on the interparticle distance between pairs of
(aggregated) NPs compared to the monodispersed state. 3 This
distance-dependent LSPR absorption of metal NPs has been
emerging as the basis of useful colorimetric sensors for
detecting various chemicals due to its extreme simplicity and
low cost.
The development of highly sensitive and selective analytical
tools for heavy metal ions is of great importance to avoid their
cytotoxicity effects. Therefore, this colorimetric method, which
can be observed by the naked eye, is appropriate as an on-site
method for real-time detection of target heavy metal ions due
to its simple configuration and portability to on-site. To date,
various metal NPs have been used as colorimetric detectors for
heavy metal ions in the aqueous phase. In particular, gold (Au)
and silver nanoparticles (AgNPs) offer excellent LSPR
properties, exhibiting strong and well-defined color and easy
visualization of color change. 2−9
Visual detection has been based on the well-known metal−
ligand coordination, where metal and ligand act as electronic
acceptor and donor, respectively. A simple colorimetric assay
employing peptide and AuNPs with 10 nm diameter for the
detection of Hg 2+ and Pb 2+ ions, based on the metal ions−
peptide complex inducing the aggregation of AuNPs, was
reported by Slocik et al. 4 A highly selective detection for Co 2+
in the presence of other metal ions (Hg 2+ ,Na 2+ ,Cu 2+ , and so
on) was evaluated by thioglycollic acid-functionalized AuNPs,
which were stabilized with cetyltriammonium bromide
(CTAB). 5 AuNPs modified with ammonium group-terminated
thiols were prepared to selectively detect Hg 2+ ion, via
abstraction of thiols induced to aggregate AuNPs. 6
Although AgNPs have been used less extensively than AuNPs
in colorimetric assays, AgNPs also have good applicability to
the detection of heavy metal ions, based on color change
between the dispersed and aggregated ones. Triazocarboxyl
AgNPs show a cooperative effect on the recognition of Co 2+
over other metal ions tested, 3 and mercaptopyridineglutathione
(GSH)-modified AgNP was used as a colorimetric
detector of As 3+ ion. 7
As described above, the metal NPs show excellent selectivity
and sensitivity as colorimetric sensors, and metal NPs used in
sensor assay are mostly of the spherical type. 8,9 The LSPR
absorption of metal NPs was changed also, with the particle’s
shape. Therefore, in this work we investigated the selective
detection of heavy metal ion (herein, Co 2+ ) with the shape of
AgNPs (nanospherical, nanoplate, and nanorod). It is not easy
to prepare different shaped AgNPs with the same stabilizer, and
thus three different AgNPs are synthesized via the seedmediated
method, using different stabilizers. 10−12
In general, the bifunctionalization of metal NPs was
introduced in order to obtain selective detection of target
metal ions in the presence of others. 3,5,7 Therefore, GSH was
here used as a common functional material to selectively detect
the target metal ion (Co 2+ ) compared to Ni 2+ ,Cd 2+ ,Pb 2+ , and
As 3+ ions.
Received: April 15, 2013
Revised: June 24, 2013
Published: June 25, 2013
© 2013 American Chemical Society 8978 dx.doi.org/10.1021/la401408f | Langmuir 2013, 29, 8978−8982

■Langmuir
EXPERIMENTAL SECTION
Preparation of AgNPs of Different Shapes. Ag seed was
prepared by dissolving 0.1 mL of 18 mM AgNO 3 and 0.1 mL of 17
mM trisodium citrate (TSC) in 20 mL of deionized (DI) water. To
this solution, 0.6 mL of 10 mM NaBH 4 solution was added dropwise
under vigorous stirring. This citrate-stabilized Ag seed with 3−4 nm
size yield was kept in a dark place at room temperature before usage.
The growth solution was prepared by the reported method for each
AgNP.
Spherical AgNP (AgNP-S) was prepared using polymer stabilizer. 10
85 mg of AgNO 3 and 83 mg of PVP (polyvinylpyrrolidone) were
added into 10 mL of EG (ethylene glycol). The resulting solution was
heated at 185 °C for 20 min, and particles were separated by
centrifugation, and washed with DI water several times, to remove the
rest of the EG. Washed particles were mixed with 19.4 mL of DI water
and 0.6 mL of 10 mM NaBH 4 .
To synthesize silver nanoplate (AgNP-P), 11 the growth solution was
prepared by dissolving 0.1 g of TSC, 0.14 mL of 18 mM AgNO 3 , and
0.1 mL of 100 mM L-ascobic acid (AA) in 20 mL of DI water. After 0.4
mL of Ag seed was added to the growth solution, the mixture was
allowed to stand without stirring until the color changed to blue.
Silver nanorod (AgNP-R) stabilized with CTAB was prepared by
the reported method. 12 In brief, 0.25 mL of Ag seed was added to a
mixture of 10 mL of 25 mM CTAB, 0.125 mL of 10 mM AgNO 3 , and
0.125 mL of 100 mM AA, followed by the addition of 1 M NaOH,
until the color changed to pale blue.
Preparation of GSH-AgNPs. As shown in Figure 1, three different
shaped AgNPs were obtained. To recognize the target metal ion, the
Figure 1. Schematic diagram of the preparation of AgNPs with
different shapes.
surface of the AgNPs should be functionalized. GSH can bind to
AgNPs easily through Ag−S bonds, 8 and thus COO − and/or NH 3+
groups of GSH-modified AgNPs could bind to positive ions via the
cooperative effect. 5 Therefore, GSH was selected as the functional
material to recognize the target metal ion. 0.025 g of GSH was added
into as-made AgNPs solution, and the pH of the resulting mixture was
adjusted to 8.0 using 1 M NaOH. Upon the addition of GSH to
AgNPs, aggregation of the AgNPs was observed. 13 Finally, GSH-
AgNP-X was obtained after neutralization of the AgNPs solution.
Detailed analytical conditions for the preparation of AgNPs are
summarized in Table S1.
Colorimetric Detection and Characterizations. The colorimetric
detection of heavy metal ions was performed at room
temperature. The resulting solutions’ concentrations of GSH-AgNP-
S, -P, and -R are 70, 100, and 20 ppm, respectively. A volume of 3 mL
of GSH-AgNP-X solution was added to 2 mL of different
concentrations of Ni 2+ , Co 2+ , Cd 2+ , Pb 2+ , and As 3+ ions (5−700
Article
μM). The stock solution of metal ions was adjusted to under pH 4 in
order to maintain their ionic state. The concentration, ranging from 1
to 100 ppm, was prepared by using serial dilution of the stock solution.
The morphology of AgNPs was analyzed using transmission electron
microscopy (TEM, JEM-1010, JEOL) and UV−vis spectra (UV-
18000, Shimadzu).
■ RESULTS AND DISCUSSION
The chelating sulfur-containing GSH ligands bind to the
AgNPs surface through Ag−S bonds. Each metal ion (M n+ ) can
bind with GSH-AgNPs through an M−S linkage; however,
there is no free SH group available for binding with M 2+ ions.
The remaining functional moiety, COO − groups of GSHmodified
AgNPs, could bind to positive ions, and thus positive
ions can bind with two or three GSH-modified AgNPs through
a M−O linkage or complex. 7,9,13 In the case of mercaptobenzoic
acid (MBA)-modified AgNPs, the capped AgNPs are
linked together by carboxylate−M n+ −carboxylate coordinative
couplings. 14,15 Therefore, colorimetric detection herein is based
on the fact that the GSH-AgNPs undergo aggregation due to
the formation of chelating complex between the metal ions and
COO − groups, namely, ion-templated chelation.
The addition of GSH with NaOH does not change the color
of AgNPs, which indicates that there is no aggregation. GSH is
linked with AgNPs through an Ag−S linkage, and as a result,
there are two free carboxyl groups and one amine group, which
can be used for functionalization with metal ions. Although
Cd 2+ ,Cu 2+ , and Zn 2+ are well-known to bind to the amine
group, the NH 2 group in the GSH-AgNPs is already protonated
+
to NH 13 3 due to the experimental condition at pH 8. As a
result, the carboxyl groups are the only binding site.
GSH-AgNP-S with spherical shape shows 30 nm of diameter;
it is well-dispersed in aqueous phase, and its characteristic peak
in UV−vis spectroscopy is about 400 nm. When we added
metal ions to GSH-AgNP-S, the AgNPs underwent aggregation
due to the formation of strong chelating complex via
carboxylate ions. As shown in Figure 2, the presence of metal
ions led to red-shift of the peak at 400 nm and emergence of a
new peak at about 550 nm in the UV−vis spectra. This red-shift
might be due to the change of local refractive index on the
AgNPs surface caused by the specific binding of GSH-AgNPs
with metal ions and the interparticle interaction resulting from
the AgNPs assembly. 13 Therefore, aggregation of AgNPs in the
presence of metal ions yields both a substantial shift in the
plamon band energy to longer wavelength and a red color
change.
Quantitative analysis was performed by adding different
concentrations of metal ions into the GSH-AgNP-S solution
and monitoring the absorption peak in the UV−vis spectra.
The UV−vis absorbance ratio (A 550 /A 400 ) increased linearly
with the concentration (5−400 μM) of metallic cations (Figure
S1). The determination coefficient (R 2 ) for all of the Ni 2+ ,
Co 2+ , Cd 2+ , Pb 2+ , and As 3+ ions is high, at 0.99. A linear
correlation between the absorbance ratio and concentrations of
metal ions makes it suitable for the quantitative determination
of target metal ions in aqueous solutions.
Although the colorimetric sensitivity of GSH-AgNP-S for
various metal ions is excellent, the color changes of individual
metal ions are similar to each other, regardless of the ion type.
That is, the colorimetric selectivity of GSH-AgNP-S is very
poor, but GSH-AgNP-S is applicable to a universal colorimetric
sensor for various metal ions. Therefore, we examined the
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Figure 2. (a) TEM image of GSH-AgNP-S in the absence of metal
ions and (b) TEM image of GSH-AgNP-S in the presence of 10 ppm
Co 2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-
S in the presence of Ni 2+ ,Co 2+ ,Cd 2+ ,Pb 2+ , and As 3+ ions. Inset
pictures in (c) show the color change of GSH-AgNP-S in the presence
of 10 ppm metallic cations.
different shaped AgNPs, for colorimetric detection of metal
ions.
As shown in Figure 3a, GSH-AgNP-P showed ca. 40 nm size
and formed a distinctive blue solution. As previously reported,
they have a highly stable morphology in the aqueous phase. 11
As compared to the absorbance peak in GSH-AgNP-S, GSH-
AgNP-P has three plasmon peaks in the UV−vis spectra
(Figure 3c). Three distinctive peaks are assigned to the out-ofplane
quadrupole resonance (the first peak), the in-plane
quadrupole resonance (the second peak), and the in-plane
dipole plasmon resonance (the third peak). 16 However, after
the addition of 100 ppm metal ions in GSH-AgNP-P solution,
all of the plasmon peaks were eliminated, and the color of
solution was transparent, as shown in Figure 3c. For 1−10 ppm
of metal ions, the solution color of GSH-AgNP-P was very
slightly changed, so it could not be recognized by the naked
eye. As shown in Figure 3b, GSH-AgNPs-P in the presence of
metal ions found a few, and showed smaller size, as compared
to GSH-AgNP-P in the absence of metal ions.
In our previous report, 11 a strong oxidizing agent readily
takes electrons from AgNPs and releases silver ions (Ag + ) into
the solution. This ion release results in a transformation of the
AgNPs shape and size, and thus metal nanoparticles could
finally be fully ionized. Citrate-stabilized AgNPs might be easy
oxidized chemically in low pH, namely by the addition of high
concentration of metal ion solutions. Consequently, it is not
Figure 3. (a) TEM image of GSH-AgNP-P in the absence of metal
ions and (b) TEM image of GSH-AgNP-P in the presence of 100 ppm
Co 2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-
P in the presence of Ni 2+ ,Co 2+ ,Cd 2+ ,Pb 2+ , and As 3+ ions. Inset
pictures in (c) show the color change of GSH-AgNP-P in the presence
of 100 ppm metallic cations.
easy to find the GSH-AgNP-P in the TEM image (Figure 3b)
due to the presence of few nanoparticles in solution. Therefore,
GSH-AgNP-P is not suitable as a colorimetric sensor for
detecting metal ions.
Finally, rod-type AgNPs were prepared, and modified with
GSH, to evaluate the colorimetric selectivity and sensitivity for
metal ions. As shown in Figures 4a and 4c, rod-type AgNPs
with above 400 nm size have a longitudinal peak at 750 nm in
the UV−vis spectra, and it is noted that AgNPs have directional
growth. Because of low yield for AgNP-R, very small spherical
AgNPs under 10 nm size coexisted with rod-type AgNPs
(Figure S2a). After mixing between GSH and AgNP-R solution,
spherical AgNPs with 100 nm as well as rod-type AgNPs were
found which were grown from small spherical seed in AgNP-R
solution. GSH could act as a reducing as well as a capping
agent, 17 and thus small spherical particles were regrown to
larger particles. After addition of metal ions into the GSH-
AgNP-R solution, the absorbance peaks in the UV−vis spectra
were not changed, except for Co 2+ . When Co 2+ ion was mixed
with GSH-AgNP-R, the solution color changed dramatically,
from pale blue to dark green. Even though the main peak at 750
nm in the UV−vis spectra was maintained, its UV absorbance
band was largely changed. In the UV−vis spectra, a new
shoulder peak from 300 to 550 nm emerged, and the
absorption intensity of the main peak for the longitudinal
band decreased. It should be noted that smaller particles in
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Figure 4. (a) TEM image of GSH-AgNP-R in the absence of metal
ions and (b) TEM image of GSH-AgNP-R in the presence of 10 ppm
Co 2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-
R in the presence of Ni 2+ ,Co 2+ ,Cd 2+ ,Pb 2+ , and As 3+ ions. Inset
pictures in (c) show the color change of GSH-AgNP-R in the presence
of 10 ppm metallic cations.
length than the rod-type AgNPs were formed in the presence of
Co 2+ solution. As shown in Figure 4b, many spherical AgNPs
with ca. 150 nm diameter, as well as rod-type AgNPs, were
exhibited in the TEM image, and all of the spherical and rodtype
AgNPs aggregated by formation of coordination
compounds between Co 2+ and functional groups (Figure 5
and Figure S3).
Although carboxyl-modified AgNPs can respond to many
transition metal ions, only GSH-CTAB modified AgNPs have a
special response to Co 2+ . By a report by Bala and co-worker, 15
the metal diacetate cohesive energies and respective metal−
acetate bond energy of Co 2+ are higher than those of other
metal ions. The metal−acetate bond energies of Co 2+ ,Cd 2+ ,
Figure 5. A strategy for Co 2+ detection using a GSH-functionalized
CTAB-stabilized AgNPs (the scheme was modified from ref 5).
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and Pb 2+ are 180, 16, and 168 kJ/mol, respectively, but that of
Ni 2+ is 212 kJ/mol. Specifically, the bonding energy of
carboxyl−Co 2+ is high, but lower than that of Ni 2+ . Therefore,
we found the other reason for the selective recognition of Co 2+ .
It is well-known that each GSH molecule contains amine and
carboxylate functionalities that provide coupling possibilities for
further cross-linking to other molecules of sensing interest. 17 In
addition, it is reported that thioglycollic acid (TGA)-functionalized
CTAB-modified AuNPs can selectively detect Co 2+ ions. 5
The surface modification system of that was similar to our case.
Although their suggested mechanism is not yet clear, we can
understand the selective recognition of Co 2+ ion, based on the
cooperative effect of CTAB and GSH. The GSH was absorbed
on the surface of AgNPs through the Ag−S bond, and AgNPs
were aggregated in the presence of Co 2+ due to binding with
chelating ligands, and CTAB separated from the AgNPs surface
because CTAB and GSH had a cooperative effect on the
recognition of Co 2+ . It could be explained that coordination
compounds were formed by Co 2+ with carboxyl groups (Figure
5). Namely, coordination compounds with Co 2+ formed
aggregates between each AgNPs.
Aggregation of nanoparticles by the abstraction of stabilizer
was also found in another example. AuNPs stabilized with thiol
groups successively recognized the Hg 2+ ion by the abstraction
of the thiols group from the AuNPs that led to the aggregation
of AuNPs. 6 This mechanism is helpful in understanding the
formation of aggregates of AgNPs after the addition of Co 2+
solution. Because CTAB was separated from GSH-AgNP-R, the
stability of AgNPs was reduced, and Ag + ion could be released
from the AgNPs in the presence of low pH (pH 3.61 for 1 ppm,
pH 2.72 for 10 ppm, and pH 1.83 for 100 ppm of Co 2+
solution). Metal silver was readily oxidized in oxygen contained
solution under low pH, and Ag + ion was released by
oxidation: 15 2Ag + 1 / 2 O 2 +H 2 O → 2Ag + + 2OH − . Jin et al.
discovered that normal room light stimulated colloidal silver
nanocrystals to re-form into larger nanoprisms, without
addition of Ag + . 18 Therefore, Ag + ion release results in a
transformation of AgNP shape, and finally, spherical AgNPs, as
the most stable form, were grown by Ostwald ripening.
■ CONCLUSIONS
GSH-modified AgNPs with spherical, plate, and rod shapes
were prepared and evaluated for the detection of metal ions by
colorimetric sensing. The carboxyl group in GSH has high
affinity to the transition metal ions, and thus several metallic
cations were selected as target ions. Colorimetric detection is
based on the fact that GSH-AgNPs undergo aggregation due to
the formation of chelating complex between metal ions and
COO − groups. Spherical GSH-AgNP-S was highly sensitive to
all metal ions but did not show selective detection. Meanwhile,
GSH-AgNP-P with plate-type NPs in the presence of metal
ions was ionized, and a few particles were found in the TEM
image. In particular, GSH-AgNP-P was not suitable as a
colorimetric sensor for metal ions. Finally, GSH-AgNP-R with
rod-type NPs was prepared and tested for the same metal ions.
The results showed that GSH-AgNP-R solution has high
sensitivity to only Co 2+ ion. Because CTAB and GSH had a
cooperative effect on the recognition of Co 2+ , CTAB separated
from the GSH-AgNP-R surface, and the reducing stability of
AgNPs led to them being reformed from rod-type to spherical
shape. Therefore, we found that GSH-AgNP-S is applicable to a
universal colorimetric sensor for various metal ions, and GSH-
AgNP-R has high selectivity for the Co 2+ ion.
dx.doi.org/10.1021/la401408f | Langmuir 2013, 29, 8978−8982

■Langmuir
ASSOCIATED CONTENT
*S Supporting Information
UV−vis absorbance ratio for GSH-AgNP-S, TEM images of
AgNP-R, particle size distribution of GSH-AgNP-X, and
analytical condition for metal detection. This material is
■available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel +82-2-940-5768; fax +82-2-941-5769; e-mail korea1@kw.
ac.kr (Y.K.), chpark@kw.ac.kr (C.P.).
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the Research Grant of
Kwangwoon University in 2013 and the National Research
Foundation of Korea (NRF-2010-0007050).
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