Facile synthesis of water-soluble fluorescent silver nanoclusters and HgII sensing

Article abstract of DOI:10.1021/cm1001253

A single step facile synthesis of highly emissive, water-soluble, fluorescent Ag nanoclusters has been reported using a small molecule, dihydrolipoic acid. These clusters were characterized using ultraviolet/visible (UV/vis) spectroscopy, photoluminescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), high-resolution transmission electron microscopy (HR-TEM), dynamic light scattering (DLS), and X-ray diffraction (XRD) studies. Mass spectrometric analysis shows the presence of a few atoms in nanoclusters containing only Ag₄ and Ag₅. The reported fluorescent Ag nanoclusters show excellent optical properties, including narrow emission profile, larger Stokes shift (more than 200 nm), and good photostability. Interestingly, these nanoclusters also exhibit semiconducting property. Moreover, as-prepared fluorescent Ag nanoclusters have been utilized as an indicator for selective and ultrasensitive detection of highly toxic Hg ^II ions in water, even at subnanomolar concentrations.

Full text of DOI:10.1021/cm1001253

4
364 Chem. Mater. 2010, 22, 4364–4371
DOI:10.1021/cm1001253
Facile Synthesis of Water-Soluble Fluorescent Silver Nanoclusters and
Hg Sensing
II
Bimalendu Adhikari and Arindam Banerjee*
Department of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata-700 032, India
Received January 18, 2010. Revised Manuscript Received June 3, 2010
A single step facile synthesis of highly emissive, water-soluble, fluorescent Ag nanoclusters has
been reported using a small molecule, dihydrolipoic acid. These clusters were characterized using
ultraviolet/visible (UV/vis) spectroscopy, photoluminescence spectroscopy, Fourier transform
infrared spectroscopy (FT-IR), high-resolution transmission electron microscopy (HR-TEM),
dynamic light scattering (DLS), and X-ray diffraction (XRD) studies. Mass spectrometric analysis
shows the presence of a few atoms in nanoclusters containing only Ag and Ag . The reported
4
5
fluorescent Ag nanoclusters show excellent optical properties, including narrow emission profile,
larger Stokes shift (more than 200 nm), and good photostability. Interestingly, these nanoclusters
also exhibit semiconducting property. Moreover, as-prepared fluorescent Ag nanoclusters have been
II
utilized as an indicator for selective and ultrasensitive detection of highly toxic Hg ions in water,
even at subnanomolar concentrations.
2
Introduction
and the quantum confinement effect. These nanoclusters
find possible applications in single molecular spectro-
scopy, biological labeling, optical sensing, catalysis, and
Recently, noble-metal nanoclusters have drawn consid-
erable research interest, because of their unique function in
bridging the “missing link” between atomic and nanopar-
1
ticle behavior. Noble-metal nanoclusters exhibit discrete
molecule-like electronic transition and interesting optical
properties, including fluorescence, because oftheirtinysize
3
other fields. Although gold nanoclusters have been widely
2c,d,3f,4
investigated
over the past several years, only several
methods have been developed for preparing fluorescent
silver nanoclusters. Dickson and co-workers have synthe-
sized water-soluble fluorescent Ag nanoclusters using poly-
(
amidoamine) (PAMAM) dendrimer and DNA as tem-
2
*
Author to whom correspondence should be addressed. Fax: þ91-33-
b,5
2
473-2805. E-mail: bcab@iacs.res.in.
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plates. Different sequencesof DNAtemplates havebeen
used in the formation of Ag nanoclusters and their role in
6
cluster formation has also been investigated. Kumacheva
et al. and Frey et al. have separately reported the photo-
generation of fluorescent Ag nanoclusters using poly
(
and multiarm star polyglycerol-block-poly(acrylic acid)
7
copolymer-based microgel particles as templates. Dong
(
1
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pubs.acs.org/cm
Published on Web 07/14/2010
r 2010 American Chemical Society

Article
Chem. Mater., Vol. 22, No. 15, 2010 4365
1
0
II
highly toxic Hg ion have been reported. These include
within glass using a femtosecond laser irradiation and
these are not well suited for bioapplication. Various other
irradiation techniques have been used to prepare Ag
clusters. Examples include the use of γ-ray irradiation for
the preparation of Ag clusters in aqueous solution of
1
electrochemical methods, optical methods that involve
5
II
Hg -sensitive fluorophores or chromophores, the use
16
1
7
18
of proteins, functional polymer materials, metal
3
d,19
20
nanoparticles,
and semiconductor quantum dots.
1
1
12
polymers such as polyvinylacetone and polyacrylate.
Similarly, electron irradiation, microwave irradiation, and
polychromic irradiation have been used to make Ag
clusters. The preparation of Ag clusters at room tem-
perature, using the aqueous solution of an amphiphilic
Our reported synthetic fluorescent Ag nanoclusters have
been utilized as an indicator for the selective detection of
II
II
the Hg ion in water, because Hg can solely quench the
fluorescence intensity of Ag nanoclusters selectively and
1
3
-
10
ultrasensitively, with a limit of detection of 10
M.
1
4
copolymer polyvinylacetone, has also been reported.
Although these above-mentioned methods are efficient
for the preparation of fluorescent Ag nanoclusters, all
these cases involve the costly synthesis of macromolecular
or dendrimeric templates and the sizes of these templates
are also so large that their application in labeling biomo-
Experimental Section
Materials. (()R-lipoic acid was purchased from Aldrich.
Silver nitrate (AgNO ) and sodium borohydride (NaBH ) were
3
4
purchased from Merck. The water used in all experiments was
Millipore Milli-Q grade.
4
g
lecules for the most dynamical experiments is limited.
Synthesis of Ag Clusters. In a typical experiment, 5.26 mg of
lipoic acid (LA) powder and 2 mL of Milli-Q water were placed
into a vial to prepare fluorescent Ag nanoclusters. To this
insoluble mixture, 0.24 mg of pure sodium borohydride was
Therefore, the development of a facile and convenient
procedure is needed for making fluorescent Ag nanoclus-
ters using a small molecule as a template.
4
added, so that the molar ratio of LA:NaBH was 4:1 and this
In this study, we report a new method for simplistic and
direct synthesis of highly fluorescent, water-soluble, Ag
nanoclusters using the dihydrolipoic acid (which is a
small molecule with two thiol groups) as a stabilizing
agent. This is a very convenient approach to make stable,
highly emissive, fluorescent Ag nanoclusters with control
over the cluster size. Moreover, larger Stokes shift and
good photostability are two important features of our
synthetic fluorescent Ag nanoclusters, and, interestingly,
these nanoclusters also exhibit semiconducting properties.
The highly toxic mercuric ion (Hg ), which is the most
stable form of inorganic mercury, causes serious health
and environmental problems. Hence, quick and very
II
sensitive detection of the Hg ion in water is essential.
Different analytical methods for the detection of the
was stirred well until a clear solution was observed. In this step,
lipoic acid is reduced in water to form soluble dihydrolipoic acid
(
DHLA). Now, to this freshly prepared aqueous DHLA solu-
tion, 100 μL of 25 [mM] aqueous AgNO solution was added
3
and the mixture was well-stirred for 1 min. To this mixture, a
slight excess of dilute aqueous sodium borohydride solution was
added slowly. The stirring was continued for almost 2 h. At first,
the mixture gradually became colorless to reddish pink color,
then a deep reddish color was developed. The color of this
soultion, over time (within 50 min), changed to a yellowish
orange color (see inset of Figure 1a).
II
Instrumentation. UV/vis Spectroscopy. UV/vis absorption
spectra were taken using a dilute aqueous solution of fluorescent
-
4
Ag nanoclusters (∼10 M concentration), and these spectra
were recorded using a Varian Cary 50 Bio UV-vis spectro-
photometer.
Photoluminescence (PL) Spectroscopy. Photoluminescence
PL) study was conducted using dilute aqueous solution of Ag
(
10) Dai, Y.; Hu, X.; Wang, C.; Chen, D.; Jiang, X.; Zhu, C.; Yu, B.;
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Zhu, Q.; Wang, Y.; Pang, W.; Xu, G.; Lu, F. Nanotechnology 2006, 17,
(
-
5
(
clusters at a concentration of ∼10
M. PL spectra were
recorded using a Horiba Jobin Yvon Fluoromax 3 instrument
and a 1-cm-path-length quartz cell. The slit width for the
excitation and emission was set at 5 nm. The Quantum yield
was measured by a relative comparison method using the
following equation:
1948–1953.
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12) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457–5471. (b) Ershov,
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Shchukin, D. G.; Radtchenko, I. L.; Sukhorukov, G. B. Chem-
PhysChem 2003, 4, 1101–1103.
(
2
Qt
Qs
ðIt=AtÞηt
¼
2
(
(
(
14) Wu, W.-T.; Pang, W.; Xu, G.; Shi, L.; Zhu, Q.; Wang, Y.; Lu, F.
Nanotechnology 2005, 16, 2048–2051.
ðIs=AsÞη
s
15) Wang, S.; Forzani, E. S.; Tao, N. Anal. Chem. 2007, 79, 4427–
where Q is the quantum yield, I the integral area under the PL
spectrum, η the refractive index of the solvent, and A the
absorption at the selected excitation wavelength. The subscripts
4432.
16) (a) Yang, Y.-K.; Yook, K.-J.; Tae, J. J. Am. Chem. Soc. 2005, 127,
16760–16761. (b) Nolan, E. M.; Lippard, S. J. J. Am. Chem. Soc. 2007,
1
29, 5910–5918. (c) Nazeeruddin, M. K.; Censo, D. D.; Humphry-Baker,
“t” and “s” represent the test sample and the standard sample,
respectively.
R.; Gr €a tzel, M. Adv. Funct. Mater. 2006, 16, 189–194. (d) Song, F.;
Watanabe, S.; Floreancig, P. E.; Koide, K. J. Am. Chem. Soc. 2008, 130,
1
6460–16461.
The PL lifetime was measured using a setup that involves a
Horiba Jobin Yvon Fluoromax-P time-resolved fluorometer
with multichannel scaling (MCS). The lifetime was measured
using a xenon flash lamp.
(
17) (a) Chen, P.; He, C. J. Am. Chem. Soc. 2004, 126, 728–729. (b) Ma,
L.-J.; Li, Y.; Li, L.; Sun, J.; Tian, C.; Wu, Y. Chem. Commun. 2008,
6
18) Kim, I. B.; Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 2818–2819.
19) (a) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165–
345–6347.
(
(
FT-IR Spectroscopy. Fourier transform infrared (FT-IR)
spectroscopy of dried DHLA and Ag nanocluster-DHLA
1
1
2
67. (b) Huang, C.-C.; Chang, H.-T. Chem. Commun. 2007, 1215–
217. (c) Lee, J. S; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed.
007, 46, 4093–4096. (d) Li, D.; Wieckowska, A.; Willner, I. Angew.
Chem., Int. Ed. 2008, 47, 3927–3931. (e) Xue, X.; Wang, F.; Liu, X.
J. Am. Chem. Soc. 2008, 130, 3244–3245.
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61, 1474–1477.

4
366 Chem. Mater., Vol. 22, No. 15, 2010
Adhikari and Banerjee
Figure 1. (a) Time-dependent UV/vis spectra with the progress of the formation of Ag nanoclusters. (Inset: photographs of Ag nanoclusters in water under
the exposure of daylight.) (b) Normalized photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Ag nanoclusters. (Inset:
photographs of fluorescent Ag nanoclusters in water under the exposure of UV light (excitation at 365 nm) and under exposure of fluorescence light
using a blue filter (below).) (c) Time-dependent evolution of photoluminescence spectra, relative to the progress of the formation of Ag nanoclusters.
(d) Plot of photoluminescence intensity of Ag nanoclusters versus time, relative to the progress of their formation.
nanoconjugate was performed using a Nicolet 380 FT-IR
spectrophotometer (Thermo Scientific), using a KBr pellet
technique.
High-Resolution Mass Spectrometry (HR-MS). Mass spec-
trum was recorded on a Q-Tof Micro YA263 high-resolution
spectrometer by positive-mode electrospray ionization using the
water-methanol mixture of Ag clusters solution.
High-Resolution Transmission Electron Microscopy (HR-TEM).
High-resolution transmission electron microscopy (HR-TEM)
studies of aqueous fluorescent Ag nanoclusters solution were
Current-Voltage (I-V) Measurement. A thin film of fluor-
escent Ag nanoclusters was prepared on glass substrate using
spin coating method for electrical conductivity measurements.
The film was dried well in vacuum for 2 days. The current-
voltage (I-V) measurements were carried out using Ag electro-
des in coplanar configuration using a Keithley model 6517A
Electrometer at room temperature. The thickness of the film was
determined using a surface profilometer (STYLUS, Model No.
DEKTAK 6M), and it was found to be more or less uniform in
thickness (with an average thickness of 2.45 μm). Uniformity in
thickness and absence of interstices were further checked by
atomic force microscopy of this thin film (see Figure S1a in the
Supporting Information). Electrical conductivity was measured
putting the point I = 36.6 nA, V = 0.9 V from I-V curve
(Figure S1b in the Supporting Information) using the following
equation:
-
5
carried out at a concentration of 2 ꢀ 10 M. HR-TEM images
were taken with a JEOL electron microscope operated at an
accelerating voltage of 200 kV. The as-prepared Ag nanoclusters
were dried on carbon-coated copper grids (300 mesh size) by
slow evaporation and then allowed to dry in a vacuum at 25 °C
for two days. The average size of nanoclusters was statistically
determined from the HR-TEM image by measuring the diameter
of 100 clusters using ImageJ software.
X-ray Diffraction (XRD) study. The experiment was carried
out using the dried sample of fluorescent Ag nanoclusters.
Experiment was performed using an X-ray diffractometer
(
Bruker D8 Advance), with a conventional Cu KR X-ray
˚
radiation (λ=1.5418 A) source and a Bragg diffraction setup
(
Seifert 3000P).
Dynamic Light Scattering (DLS) Study. DLS study was
1
Il
Conductivity ¼
¼
performed in MELLERS GRIOT HeNe-LASER TER-
BO-CORR CORILATOR, Model No. BI200SM-Goniometer
VER-2.0 Instrument using dilute aqueous solution of Ag clus-
F
VA
where F is the electrical registivity, I the current, l the length,
V the voltage, and A the cross-sectional area.
-
5
ters at a concentration 2 ꢀ 10 M.

Article
Chem. Mater., Vol. 22, No. 15, 2010 4367
Results and Discussion
band to the lowest unoccupied conduction band of Ag
1
nanoclusters, and this is an interband transition.
b,24
The time-dependent UV-Vis absorption spectroscopic
and PL spectroscopic measurements have been used to
monitor characteristics feature that have been observed
during the formation of Ag nanoclusters.
A time-dependent PL study was performed to probe the
kinetics of the formation of Ag nanoclusters. The time-
dependent changes in fluorescence intensity during the
progress of the formation of fluorescent Ag nanoclusters
are given in Figure 1c. It is evident from the figure that
fluorescence clusters started to form 42 min after the
beginning of the reaction (i.e., 42 min after the addition
of sodium borohydride to the aqueous mixture of silver
nitrate and dihydrolipoic acid; see the subsection “Synthe-
sis of Ag Clusters” in the Experimental Section). It is
important to note that this is the time when the absorp-
tion spectra started to split (see Figure 1a). The fluores-
cence intensity steadily increases with time and ultimately
reaches the saturation point after 97 min from the com-
mencement of the reaction. Both fluorescence and ab-
sorption spectra did not change significantly after that
time. Therefore, it can be stated that the formation of Ag
nanoclusterts was completed at 97 min. PL intensities
at different time intervals were plotted with respect to
time, and a straight line was found (see Figure 1d). This
indicates that the cluster formation kinetics follows an
apparent zero-order rate law. Concentration-dependent
fluorescence study was also carried out, and it shows
that PL intensity increases linearly with the increase in
cluster concentration (see Figure S3 in the Supporting
Information). The quantum yield (Q) of these fluorescent
Ag nanoclusters was found to be 2% in water using
Acridine yellow (Q = 0.47 in ethanol) as a reference. The
PL lifetime of these Ag nanoclusters was measured in this
study and was found to be 36.96 μs (see Figure S4 in the
Supporting Information). The observed PL decay was
fitted well with a single exponential decay.
Ultraviolet/Visible (UV-Vis) Spectroscopic Study. The
time-dependent change in UV-Vis absorption spectra dur-
ing the progress of the formation of fluorescent Ag nano-
clusters are illustrated in Figure 1a. A broad absorption
band was started to appear at ∼460 nm after 3 min from the
addition of sodium borohydride to the aqueous mixture of
silver nitrate and dihydrolipoic acid (see Figure 1a and the
subsection “Synthesis of Ag Clusters” in the Experimental
Section). This broad absorption band gradually became
sharper with the progress of the reaction, and the intensity
of this band was also increased with time up to 25 min
(Figure 1a). The absorption intensity was then steadily
diminished up to 42 min, this broad peak was started to
split: one at 435 nm with higher absorption intensity and
another at 335 nm. Another shoulder peak at ∼500 nm was
also observed after 1 h. The area under the absorption band
steadily decreased with time after 25 min. This indicates the
2
1
gradual decrease of nanoparticle size. Moreover, it has been
noticed that the absorption peak width gradually became
narrower with time, and this suggests that the size distribu-
22
tion of newly formed Ag nanoparticles is narrow. Pre-
viously, Dickson and his co-workers reported that
fluorescent silver clusters Ag -Ag show absorptions at
1
4
5
4
40 and 357 nm. Silver clusters Ag -Ag in microgel
4
9
template showed a shoulder at 330-360 nm, with an
7
a
intense peak at 490-520 nm in the absorption spectra.
Another report of microgel-templated fluorescent Ag clus-
2
3
ters exhibited a peak at 400 nm with a shoulder at 345 nm.
Therefore, in our study, the observed absorption spectra
Figure 1a) may be responsible for the formation of ultra-
These fluorescent Ag nanoclusters show interesting op-
tical properties. First, the narrow emission profile (full
width at half-maximum (fwhm) of 96 nm) of these fluor-
escent Ag nanoclusters indicates that the size distribution of
(
0
small Ag nanoparticles (Ag ).
Photoluminescence Study. Photoluminescence (PL) and
photoluminescence excitation (PLE) spectra of dilute
aqueous Ag nanoclusters colloids at room temperature are
shown in Figure 1b. The PL spectrum shows an emission
maximum at 652 nm upon excitation at 425 nm. The main
absorption peak in the UV/vis spectrum appeared at
3d
these clusters is narrow. The emission bandwidth is much
narrower than the previously reported results obtained by
3e,7a,7b,8
different research groups.
However, it is compar-
25
able to a bandwidth obtained by Ras and co-workers.
Their study demonstrated the well-controlled synthesis
of nanoclusters containing a small number of Ag atoms,
4
35 nm, with two shoulders, at 325 and 500 nm. Excitation
at either 325 or 500 nm also gives rise to an emission
maximum at 652 nm (see Figure S2 in the Supporting
Information). It is evident that no change in emission
maximum was observed by altering the excitation wave-
length. This indicates that the obtained emission is a real
luminescence from the relaxed states and it is not due to
scattering effects. It is thought that the emissive nature of
Ag nanoclusters can arise because of the fact that electrons
can travel from the submerged and quasi-continuum 5d
25
such as Ag , Ag , and Ag . Second, these Ag nanoclusters
2
3
5
show strong red luminescence. The aqueous Ag nanoclus-
ters exhibit a pronounced red photoluminescence upon the
irradiation by UV light at 365 nm. The same red photo-
luminescence is also observed under fluorescence light
using a blue filter (see inset in Figure 1b). This inherent
bright PL property can allow the imaging of a single Ag
nanocluster. It suggests that they may be used for the cell
imaging purpose. Third, these Ag nanoclusters have a very
large Stokes shift (> 200 nm). Furthermore, these emissive
nanoclusters have shown good photostability. Note that
(
(
21) Tcherniak, A.; Ha, J. W.; Dominguez-Medina, S.; Slaughter, L. S.;
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(

4
368 Chem. Mater., Vol. 22, No. 15, 2010
Adhikari and Banerjee
Figure 2. FT-IR spectra obtained from (a) dihydrolipoic acid (DHLA)
and (b) a Ag nanocluster-DHLA nanoconjugate.
the PL property of these Ag nanoclusters in water could
still be sustained for 4 h in the presence of continuous UV
irradiation. The advantage of large Stokes shift and good
photostability of these fluorescent Ag nanoclusters opens
up an opportunity to use these materials in optics and
chemical sensing.
Figure 3. Scheme illustrating the formation of a Ag
DHLA nanoconjugate.
5
nanocluster-
FT-IR Study. It is evident from FT-IR spectra (Figure 2)
-
1
that the characteristic band at 2548 cm for ν(S-H) of
dihydrolipoic acid (DHLA) disappeared, because of the
formation of a Ag nanocluster-DHLA nanoconjugate.
This clearly indicates that thiol groups of DHLA remain in
4
c
the form of the thiolate in the nanoconjugate. The pre-
-
1
sence of a band at 1564.3 cm in both DHLA and Ag
nanocluster-DHLA nanoconjugate suggests that the car-
boxylate ion remains in the free state (in unbound form).
Therefore, we believed that freshly prepared reduced lipoic
acid (DHLA) contains two free thiol groups per molecule
that are very sensitive to the surfaces of Ag nanoclusters
and these two thiol groups stabilize the fluorescent Ag
nanoclusters very well. The hydrophilic nature of the
carboxylate moiety present in the DHLA molecule at the
surface makes the Ag nanocluster-DHLA nanoconjugate
soluble in an aqueous medium (see Figure 3). Our reported
fluorescent Ag nanoclusters are very stable and last more
than 3 months in water. These clusters are not precipitated
out from water, and the optical properties (absorption and
emission) do not change significantly over this period of
Figure 4. HR-TEM image of fluorescent Ag nanoclusters, showing the
lattice fringes in the inset.
HR-TEM image of the fluorescent Ag nanoclusters are
given in Figure 4. The as-prepared Ag nanoclusters had
an average diameter of 2.44 nm. There is no formation of
larger Ag nanoparticles or aggregation, as was observed
by HR-TEM. This demonstrates that the synthetic pro-
cedure allows for control over the nanocluster size (i.e.,
the cluster size distribution; see Figure S7a in the Sup-
porting Information). The majority of these Ag clusters
are within the size range of 1-3 nm. The HR-TEM image
(see inset of Figure 4) also indicates the presence of a
3
months (see Figure S5 in the Supporting Information).
˚
Moreover, there is no substantial increase in cluster size
after keeping them in water for 3 months, and this is
evident from the HR-TEM study (see Figure S6 in the
Supporting Information). The high stability is ascribed to
the bidentate chelate effect afforded by the dithiol groups
of dihydrolipoic acid (see Figure 3).
HR-TEM, XRD, and DLS Studies. The size distribu-
tion of the fluorescent Ag nanoclusters was characterized
by two techniques, such as high resolution transmission
electron microscopy (HR-TEM) and dynamic light scat-
tering (DLS). HR-TEM was used to visualize the shape as
well as measure the diameter of the inorganic Ag clusters.
lattice plane, which has an interfringe distance of 2.34 A,
corresponding to the (111) plane of fcc Ag. Selected-area
electron diffraction study (SAED) shows the presence of
the (111) plane for silver (See Figure S7b in the Support-
ing Information). This result from the SAED and HR-
TEM analyses is consistent with that of X-ray diffrac-
tion (XRD) study. The XRD pattern for Ag nanoclusters
showed the diffraction peaks at 2θ = 38.2°, 44.2°, 64.5°,
and 77.4°, all of which are very consistent with those for
Ag (see Figure 5a). These diffraction peaks respectively
correspond to the (111), (200), (220), and (311) Miller
indices of fcc Ag. Generally, the intensity ratio between

Article
Chem. Mater., Vol. 22, No. 15, 2010 4369
Figure 5. (a) XRD pattern of dried Ag nanoclusters material and (b) DLS data of fluorescent Ag nanoclusters.
Figure 6. High-resolution mass spectroscopy (HR-MS) spectrum of fluorescent Ag nanoclusters, showing the exclusive presence of Ag
(here, L = DHLA (dihydrolipoic acid)).
4 5
and Ag clusters
3
e
the peaks corresponding to the (111) plane versus the
(
emission band. Ras et al. showed the presence of clusters
containing a small number of Ag atoms, such as Ag , Ag ,
and Ag , because it was evident from their matrix-assisted
2
200) plane is 0.40 versus 0.24. It can be mentioned in
6
1
2
this study that the intensity ratio was 2.41 versus 0.61,
which is higher than the conventional value. This obser-
vation reveals that Ag nanoclusters are primarily domi-
nated by (111) facets. The size distribution obtained from
DLS study (Figure 5b) suggests that the population of Ag
nanoclusters is almost monodisperse. The narrow size
distribution of clusters agrees well with the narrow emis-
sion bandwidth. The observed mean diameter of the
clusters is 2.5 nm, which matches the average diameter
obtained from TEM well (2.44 nm).
5
laser desorption/ionization time-of-flight (MALDI-TOF)
25
mass spectrum analysis. Mass analysis also confirms the
narrow size distribution of our Ag nanoclusters, and this
result is in good agreement with the observed narrow
emission bandwidth in the PL study (see Figure 1b).
Semiconducting Study. Another interesting feature of
these Ag nanoclusters is that they exhibit semiconducting
properties. Theoretically, it has been reported that there is
a trend to decrease the conductivity with reduction of the
2
7
High-Resolution Mass Spectral (HR-MS) Study. The
formation of nanoclusters was also confirmed by HR-MS
analysis. The spectrum (Figure 6) shows the presence of
nanoclusters containing only Ag and Ag . These two
nanoclusters exhibit their peaks both free and in associated
states with DHLA with an almost 1:1 molecular ratio. In
this regard, it can be mentioned that Tsukuda and co-
workers have reported that the stoichiometric ratio of
core size of metallic nanoparticles. Recently, Whetten
and co-workers reported, in a computational study, that a
thiolated Au nanowire can be made semiconducting by
considering the Au/thiolate interface and the electronic
4
5
2
8
configuration of the ligand-protected Au nanoclusters.
The observed electrical conductivity of our fluorescent Ag
-
5
-1
nanoclusters was 2.48 ꢀ 10 S cm at room temperature,
and this clearly indicates the transition from metal to
semiconductor at a very tiny size dimension of Ag clusters.
To the best of our knowledge, this is the first experimental
evidence that demonstrates that a conducting Ag metal
[
this ratio by analyzing the electrospray ionization (ESI)-
Au]:[ligand] is 1:1 in thiolated Au clusters. They obtained
4
c
mass data. In our study, generally no clusters other than
Ag and Ag have been observed. Dickson and co-workers
4
-1
(with an electrical conductivity of 63 ꢀ 10 S cm in the
4
5
reported the presence of Ag -Ag clusters, using matrix-
bulk state) has been transformed to semiconducting Ag
1
5
assisted laser desorption/ionization (MALDI) mass spec-
trometry, which results in the observation of a broad
(27) (a) Mikrajuddin; Shi, F. G.; Nieh, T. G.; Okuyama, K. Microele-
ctron. J. 2000, 31, 343–351. (b) Snow, A. W.; Wohltjen, H. Chem.
Mater. 1998, 10, 947–949.
(28) Jiang, D-en; Nobusada, K.; Luo, W.; Whetten, R. L. ACS Nano
2009, 3, 2351–2357.
(26) You, T.; Sun, S.; Song, X.; Xu, S. Cryst. Res. Technol. 2009, 44,
857–860.

4
370 Chem. Mater., Vol. 22, No. 15, 2010
Adhikari and Banerjee
-
5
II
Figure 7. (a) Fluorescence response of Ag nanoclusters (volume = 4 mL, concentration = 10 M) upon the successive addition of 10 μL aqueous Hg
ions with increasing concentration. (b) HR-TEM image of the aggregated Ag nanoclusters in the presence of Hg ions.
II
nanoclusters by size reduction. This type of material may
be served as a promising candidate for the development of
light-emitting diodes, solar cells, field-effect transistors,
2
9
thermoelectric materials, and applications in other fields.
Metal Ion Sensing Study. The concentration of Ag
nanoclusters in water that was used for the metal-ion sens-
-
5
ing experiment was 1 ꢀ 10 M and the metal-ion sensing
-
10
started withthe metal-ion concentration from 10 M and
-
5
it has been increased gradually to 10 M. We found that
the fluorescence emission was rapidly quenched in the
II
presence of Hg (see Figure 7 a). More and more quench-
II
ing occurs with an increase in Hg and, surprisingly,
fluorescence intensity has been decreased to zero (i.e.,
-
5
II
00% quenching) up to additions of 10 M Hg and its
1
quenching value was 87.6% up to additions of 10
-
7
M
Hg . To check the selectivity of this sensor, we also carried
Figure 8. Fluorescence quenching effect [(I
at 652 nm, up to an addition of 10 M different aqueous metal ions.
0
0
- I)/I ] of Ag nanoclusters
II
-7
I
out studies with other metal ions, such as K , Cs , Sr , Ba ,
I
II
II
II
II
II
II
II
II
II
II
II
Mg , Mn , Fe , Co , Pb , Cu , Zn , Sn , and Pd under
exactly similar conditions that were used for the detection
Agency (EPA) and by the European Union (1 ppb,
3
5 nM). To the best of our knowledge, this is the supreme
0
II
of Hg . The quenching data is presented meticulously in
II
obtainable sensitivity for the detection of Hg ions in
1
5-20
Figure S8 in the Supporting Information. For other metal
-
water, compared to other previously reported values.
The fluorescence quenching of Ag clusters may occur
7
ions, for additions of up to 10 M ions, a quenching effect
II
like that of the Hg ion was not observed. Even the addi-
II
through the Hg mediated interparticle aggregation
19
tion of more-concentrated solutions of other metal ions
cannot reduce its fluorescence intensity to zero. The rela-
tive PL quenching of fluorescent Ag nanoclusters toward
various common metal ions is presented in Figure 8. This
result suggests that our fluorescent Ag nanoclusters are
mechanism. The aggregation is conducted via the free
carboxylic acid (present in reduced lipoic acid on the sur-
face of Ag nanoclusters-ligand conjugate), which can
II
II
easily interact with Hg ions. In the presence of Hg ions,
aggregation occurred through an ion-templated chelation
19a,b
II
selective for Hg detection.
There is an excellent linear relationship between fluo-
rescence quenching and the logarithm of the concentra-
method.
aggregate formation of Ag nanoclusters upon the addition
The HR-TEM analysis shows evidence for the
II
of Hg (Figure 7b) and, in this case, particles <7 nm in size
II
-8
-5
tion of Hg ions within the range of 10 -10 M (see
Figure S9 in the Supporting Information). This suggests
that our fluorescence quenching effect of Ag nanoclusters
was never observed. This explains the nonfluorescent be-
havior of the resulted solution of the aggregated species.
The fluorescence intensity of Ag clusters also quenches to a
small amount in the presence of other metal ions (except
II
by Hg ions abide by the Stern-Volmer equation. The
limit of detection (LOD), down to 10
-
10
II
M, was reached.
Hg ). The maximum fluorescence quenching effect for
II
Hg , among all the metal ions, is due to the fact that simple
This LOD value is much lower than the maximum permis-
sible limit of mercury in the drinking water (2 ppb, 10 nM)
declared by the United States Environmental Protection
II
carboxylic acids have a much-stronger affinity toward Hg
(
30) Dynamics of Mercury Pollution on Regional and Global Scales:
Atmospheric Processes and Human Exposures Around the World;
Pirrone, N., Mahaffey, K. R., Eds.; Springer: New York, 2005.
(29) Lai, C.-H.; Lee, W.-F.; Wu, I.-C.; Kang, C.-C.; Chen, D.-Y.; Chen,
L.-J.; Chou, P.-T. J. Mater. Chem. 2009, 19, 7284–7289.

Article
Chem. Mater., Vol. 22, No. 15, 2010 4371
3
1
ions in water (log β = 17.6). The fluorescence quenching
procedure does not require high temperature or toxic
precursors. Thus, these newly prepared bright fluorescence
Ag nanoclusters hold future promise of using this material
as a good candidate for biolabeling, biosensing, and appli-
cations for other purposes.
4
effect may be interpreted another way, in terms of the
donation of electron density from the Ag nanoclusters to
the adsorbed cations. In the presence of Hg ions, the
electron donation leads to partial Ag oxidation and amal-
32
II
32
gam formation. Consequently, the fluorescent Ag nano-
clusters goes to a nonfluorescent form and this results in
a decrease in fluorescence signal of the sensor system.
However, it can be mentioned that, unlike fluorescence,
Acknowledgment. B.A. thanks the CSIR, New Delhi,
India, for financial assistance.
Supporting Information Available: AFM image of the surface
of a Ag nanocluster-DHLA nanocomposite film (Figure S1a);
current-voltage (I-V) plot of fluorescent Ag nanoclusters
(Figure S1b); PL spectra of the Ag nanocluster at different
excitation wavelengths (Figure S2); concentration-dependent
photoluminsence of Ag clusters (Figure S3); measurement of
the fluorescence lifetime of the Ag nanoclusters (Figure S4); a
comparison of the fluorescence of Ag nanoclusters just after their
synthesis and after 3 months of storage in water (Figure S5a,
absorption spectra; Figure S5b, emission spectra); HR-TEM
image of fluorescent Ag nanoclusters after 3 months of storage
II
the presence of Hg ions has a less significant effect on the
absorption spectra (see Figure S10 in the Supporting
Information), because fluorescence is more sensitive to
changes in the particle size than the absorption.
Conclusions
In summary, we have demonstrated an easy synthesis for
water-soluble, semiconducting, fluorescent Ag nanoclus-
ters using a small molecule (reduced lipoic acid) as a
template and it can readily, selectively, and very sensitively
(
Figure S6), cluster size distribution (Figure S7a) and SAED
II
image from TEM (Figure S7b); fluorescence response of Ag
nanoclusters upon the addition of different metal ions (Figure
S8); plot of fluorescence quenching effect of Ag nanoclusters
detect mercury ions (Hg ), even at subnanomolar concen-
trations. Our Ag nanoclusters have well-defined excita-
tion and emission spectra, a narrow emission profile, a
larger Stokes shift, and good photostability. Moreover, this
II
versuslog[Hg ] (Figure S9); absorption response of Ag nanoclus-
II
ters upon the addition of Hg ions (Figure S10); and expanded
view of FT-IR spectra (Figure S11) and HR-TEM image (Figure
S12) of fluorescent Ag nanoclusters. This information is available
free of charge via the Internet at http://pubs.acs.org.
(31) Morel, F. M. M. In Principles of Aquatic Chemistry; Wiley-
Interscience: New York, 1983; pp 248-249.
(32) Henglein, A. Chem. Mater. 1998, 10, 444–450.