Article

Ultra-low level optical detection of mercuric ions using biogenic gold nanotriangles

Physical & Materials Chemistry Division, National Chemical Laboratory, Pune, 411 008, India.
The Analyst (Impact Factor: 4.11). 05/2012; 137(13):3083-90. DOI: 10.1039/c2an35162e
Source: PubMed

ABSTRACT

Mercury is a serious environmental pollutant known to have detrimental health effects in all life forms. Here, we report the use of biologically synthesized aqueous gold nanotriangles for sensitive and selective optical detection of femto-molar levels of mercury ions by exploiting the high amalgamation tendency of mercury metal towards gold. Aqueous chloroaurate ions were reduced using lemongrass (Cymbopogon flexuosus) leaf extract at room temperature to form gold nanotriangles. Mercuric (Hg(2+)) ions were reduced in the presence of these triangles to facilitate amalgamation and the optical properties were monitored. We observe a significant change in the longitudinal plasmon absorption band of the nanotriangles even at femto-molar concentrations of mercuric ions. High-resolution transmission electron microscopy confirms changes in particle morphology at such low concentrations. This protocol shows no sensitivity to other environmentally relevant metal ions, including Pb(2+), Zn(2+), Cd(2+), Fe(2+), Ni(2+), Sr(2+), Ca(2+), Mn(2+), and Cu(2+), confirming further that change in the optical properties of gold nanotriangles in the presence of reduced mercuric ions is solely due to the strong amalgamation tendency of mercury metal.

Full-text

Available from: Amit Singh
Ultra-low level optical detection of mercuric ions using biogenic gold
nanotriangles
Amit Singh,
*
Renu Pasrichax and Murali Sastry{
*
Received 4th February 2012, Accepted 13th April 2012
DOI: 10.1039/c2an35162e
Mercury is a serious environmental pollutant known to have detrimental health effects in all life forms.
Here, we report the use of biologically synthesized aqueous gold nanotriangles for sensitive and
selective optical detection of femto-molar levels of mercury ions by exploiting the high amalgamation
tendency of mercury metal towards gold. Aqueous chloroaurate ions were reduced using lemongrass
(Cymbopogon flexuosus) leaf extract at room temperature to form gold nanotriangles. Mercuric (Hg
2+
)
ions were reduced in the presence of these triangles to facilitate amalgamation and the optical
properties were monitored. We observe a significant change in the longitudinal plasmon absorption
band of the nanotriangles even at femto-molar concentrations of mercuric ions. High-resolution
transmission electron microscopy confirms changes in particle morphology at such low concentrations.
This protocol shows no sensitivity to other environmentally relevant metal ions, including Pb
2+
,Zn
2+
,
Cd
2+
,Fe
2+
,Ni
2+
,Sr
2+
,Ca
2+
,Mn
2+
, and Cu
2+
, confirming further that change in the optical properties of
gold nanotriangles in the presence of reduced mercuric ions is solely due to the strong amalgamation
tendency of mercury metal.
Introduction
Mercury persists in air, water, soil, and living organisms mainly
in three forms, viz. elemental, methyl mercury, and other
compounds (organic and inorganic), all being hazardous to
living organisms. In human beings, mercury poisoning has been
reported to cause severe ailment in vital organs such as the
nervous system, kidneys, heart, respiratory system, and muscles.
The major source of mercury pollution is manufacturing,
burning coal, and volcanos;
1
coal burning being the major
contributor. All different forms of mercury are finally oxidized to
form stable mercuric ions (Hg
2+
), which serve as major water and
soil contaminants.
2
A number of techniques for the accurate and
sensitive detection of mercury contamination in a variety of
samples have been developed; some of them include electro-
thermal atomic absorption spectroscopy (ETAAS),
3
cold-vapor
atomic absorption spectroscopy (CVAAS),
4
cold-vapor induc-
tively coupled plasma-mass spectrometry (CV-ICP-MS),
5,6
atomic fluorescence spectrometry (AFS),
7
neutron activation,
8
high performance liquid chromatography (HPLC),
9,10
thermometric continuous-flow sensor system,
11
surface plasmon
resonance spectroscopy,
12
and voltammetry.
13
Recently,
attempts have been made to improve upon the detection limits
using a colorimetric technique.
14–17
Coronado et al. have shown
the use of ruthenium-based complexes for sensitive detection of
mercury ions up to 20 ppb in solution.
14
They further show that
these complexes can be adsorbed on high-surface-area meso-
porous metal oxide films for the reversible detection of mercury
ions in aqueous solution. However, a majority of the methods
mentioned above suffer from the drawback of being cost inten-
sive, and involve sophisticated instrumentation and sample
processing protocols. There is clearly a need to develop rapid,
easy to use techniques for ultra-low level detection of mercuric
ions in a variety of sources. Recently, Lee et al. have elucidated
a selective protocol utilizing DNA-functionalized gold nano-
particles for colorimetric detection of Hg
2+
ions in aqueous
medium exploiting thymidine–Hg
2+
–thymidine coordination
chemistry.
17
This method shows a sensitivity up to 100 nM for
detection of Hg
2+
ions in aqueous solution.
Mercury has the unique property of amalgamating with metals
to form alloys. This tendency is particularly strong towards gold,
which has led to a detailed study of the mercury–gold amal-
gamation process.
18–22
Not surprisingly, attempts have been
made towards correlating easily detectable signatures with
a degree of gold amalgamation as a means of mercury detec-
tion.
23,24
Recently, the spectroscopic signature of nanogold in the
visible and near infrared (NIR) region of the electromagnetic
spectrum has gained attention for mercury detection resulting in
a simple, cost effective, and quick technique.
25
Kim et al.
Physical & Materials Chemistry Divisions, National Chemical Laboratory,
Pune, 411 008, India. E-mail: murali.sastry@dsm.com; am.singh@neu.edu
Electronic supplementary information (ESI) available. See DOI:
10.1039/c2an35162e
Present address: Department of Pharmaceutical Sciences, Northeastern
University, Boston, MA, USA.
x Present address: National Physical Laboratory, New Delhi, India.
{ Present address: DSM India Innovation Centre, Infinity Towers,
Gurgoan, India.
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analyzed the aggregation of functionalized gold nanoparticles by
divalent ions to detect otherwise spectroscopically silent metal
ions.
25
It has recently been shown that the changed optical
properties of amalgamated gold nanorods, resulting from
changes to their aspect ratio, can be used to detect Hg
2+
ions in
water samples at levels as low as 6.6 10
13
gL
1
.
26
In this lab, we have demonstrated the room temperature
synthesis of single crystalline gold nanotriangles by a biological
route using lemongrass leaf extract.
27
In further studies, we have
shown that the optical properties of these gold nanotriangles can
be tuned in the NIR region controlling the kinetics of their
synthesis.
28,29
These nanotriangles are found to be extremely thin
with thicknesses ranging between 15 and 25 nm and just like their
other anisotropic counterparts, possess strong surface plasmon
signatures in the NIR region. We further studied the effect of
halide ions on the morphology of these gold nanotriangles and
showed that the tight binding bromide and iodide ions tend to
distort the morphology of triangles.
28
In the work presented here,
we show that these nanotriangles can be excellent candidates for
the detection of aqueous mercuric ions at ultra-low femto-molar
concentrations. The fact that these gold nanotriangles are
extremely thin and thermodynamically unstable nanostructures
renders them prone to structural damage due to the development
of any strain in the crystal structure. Amalgamation of reduced
mercuric ions with the gold nanotriangles results in selective
disruption of particle morphology preferentially along the
particle edges and tips, these being the high energy, possible
defect sites. Any morphological changes are readily detected in
the optical properties of the amalgamated nanotriangles where
significant changes in the NIR absorption bands are observed.
Experimental section
Chemicals
Analytical reagent grade chemicals were used for all of the
experiments. All of the metal salts and sodium borohydride
(NaBH
4
) were purchased from SRL Chemicals and were used as
is. The water used in the different experiments was double
distilled de-ionized obtained from a Milli-Q Ultrafiltration
Unit.
Synthesis of gold nanotriangles and its purification
The details of the nanotriangles synthesis protocol have been
described elsewhere.
27
In a typical synthesis experiment, 100 g of
a thoroughly washed and finely cut lemongrass leaf (Cymbopo-
gon flexuosus) was boiled in 500 mL of sterile distilled water for 5
min. The obtained broth was filtered and 5 mL of this broth was
added to 45 mL of 10
3
M aqueous HAuCl
4
solution. The bio-
reduction of the AuCl
4
ions to triangular gold nanoparticles
was monitored by recording UV-vis-NIR spectra of the solution
as a function of time and was found to be complete in 6 hours.
The brownish red colored solution confirms the formation of the
triangular nanoparticles, which were then purified from the
concomitantly formed spherical gold nanoparticles by a simple
centrifugation protocol. The solution was centrifuged twice at
3000, 2000, and 1000 rpm in that order for 20 minutes each and
was resuspended every time in 5 mL of de-ionized water. This
protocol yields a brown colored gold triangle solution with
a triangular population of nearly 90% as opposed to the initial
yield of around 50% in the as prepared solution. The purified
gold nanotriangles were then used for all subsequent experi-
ments. This stringent gold nanotriangles purification protocol
also ensures the removal of any unreduced AuCl
4
ions from the
solution. The purified triangles were finally resuspended in 5 mL
of water to make a concentrated solution of the same. Atomic
absorption spectroscopy results show that the final concentration
of gold in this purified concentrated triangular gold solution was
2.38 mM.
Reaction of gold nanotriangles with Hg ions
The purified triangles were finally resuspended in 50 mL of water
to make a concentrated solution of the same. 4 mL of this
concentrated solution was then diluted to 400 mL. 4 mL aliquots
were made from this gold nanotriangles solution to which Hg
2+
ions were added in a different concentration to achieve a desired
final concentration. 10
2
M stock solution of HgCl
2
was used for
various serial dilutions and the gold nanotriangles were exposed
to mercuric ions of concentrations 10
3
M, 10
6
M, 10
9
M, 10
12
M, 10
15
M, and 10
18
M. 2.6 mM sodium borohydride was
added to each aliquot to reduce the mercuric ions in situ in the
presence of the gold nanotriangles. The final solutions were then
kept for 10 minutes at room temperature and were subsequently
used for various characterization techniques. A similar protocol
was used for reducing the 10
6
M concentration of Pb
2+
,Zn
2+
,
Cd
2+
,Fe
2+
,Ni
2+
,Sr
2+
,Ca
2+
,Mn
2+
, and Cu
2+
ions in the presence
of gold nanotriangles in the solution. As one of the negative
controls, 2.6 mM of sodium borohydride was added to the same
concentration of gold nanotriangles to monitor its effect, if any,
on the gold nanotriangles in the absence of Hg
2+
ions. As another
control, the gold nanotriangles were exposed to similar concen-
trations of Hg
2+
ions in the absence of the reducing agent and the
optical property of the solution was monitored. All spectral
measurements were performed in triplicate.
UV-vis-NIR spectroscopy measurements
The UV-vis-NIR spectra were recorded for all of the solutions
using a JASCO dual-beam spectrophotometer (V-570) operated
at a resolution of 1 nm. The UV-vis-NIR spectra of the gold
nanotriangles solution and the gold nanotriangles treated only
with 2.6 mM of sodium borohydride were taken as controls.
Transmission electron microscopy measurements
Samples for TEM imaging were prepared by drop casting the
different solutions onto the carbon coated copper grids and were
allowed to dry naturally. The TEM measurements were done on
a JEOL 1200 EX instrument at an accelerating voltage of 100 kV.
HRTEM, STEM, and EDX measurements
HRTEM measurements were performed on a Tecnai G
2
F30 S-
Twin (FEI; SuperTwin lens with Cs ¼ 1.2 mm) instrument
operated at an accelerating voltage at 300 kV having a point
resolution of 0.2 nm and lattice resolution of 0.14 nm. STEM
measurements were also performed on a Tecnai F30 transmission
electron microscope operating at 300 kV (field emission gun).
3084 | Analyst, 2012, 137, 3083–3090 This journal is ª The Royal Society of Chemistry 2012
Page 2
STEM images were recorded with a HAADF detector (image
size: 2014 2014 pixels; scan times: 5–20 s; camera length: 200
mm). An energy-dispersive X-ray spectrometer (EDAX)
attached to the Tecnai F30 was used to perform elemental
analyses at spots selected in the HAADF-STEM images. Spot
diameters in the range of 2–5 nm were applied for EDX analyses.
Program Digital Micrograph (Gatan) was used for image
processing.
Atomic force microscopy measurements
For AFM measurements, the sample was drop coated onto the
highly ordered pyrolytic graphite (HOPG) substrate and was
allowed to air dry. The topmost layer of the HOPG substrates
was removed before use to obtain a clean and atomically flat
surface. The AFM measurements were done in the contact mode
on a VEECO Digital Instruments multimode scanning probe
microscope equipped with a Nanoscope IV controller at a scan
rate of 5.086 Hz.
X-ray photoelectron spectroscopy measurement
For XPS measurements, samples were drop coated onto
a metallic copper strip and were allowed to air dry. The copper
strips were cleaned with diluted hydrochloric acid prior to sample
preparation. The measurements were done on a VG MicroTech
ESCA 3000 instrument at a pressure better than 1 10
9
Torr.
The different core levels were recorded with un-mono-
chromatized Mg Ka radiation (photon energy, 1253.6 eV) at
a pass energy of 50 eV and an electron takeoff angle (angle
between electron emission direction and surface plane) of 60
.
The overall resolution of measurement in XPS is thus 1 eV. The
core level binding energies (BEs) were aligned with reference to
the C1s BE of 285 eV.
Results and discussion
Optical spectra analysis exposure to mercury metal
The pre-synthesized gold nanotriangles were washed and purified
by centrifugation. The mercuric(
II) chloride solution was exposed
to these purified nanoparticle solutions in varying concentrations
and was reduced in situ using the borohydride solution. UV-vis-
NIR spectra were recorded for all the solutions before and after
10 min of the addition of the sodium borohydride solution. The
gold nanotriangles showed no effect of presence of the Hg
2+
ions
alone, in the absence of the reducing agent (Fig. S1†), which is
expected since Hg
2+
ions are not known to amalgamate gold. The
transmission electron microscopy analysis of these solutions was
performed to ascertain any change in the morphology of the gold
nanotriangles on exposure to Hg
2+
ions alone in the absence of
the reducing agent. However, no change was observed in the
surface texture or morphology of the gold nanotriangles exposed
to mercuric ions alone (Fig. S2, A–F†). Spectra of the purified
gold triangles as well as those that were treated only with the
reducing agent in the absence of Hg
2+
ions, were recorded as
controls. The absorption spectrum of purified gold nanotriangles
shows the characteristic peak for the transverse plasmon absor-
bance at 530 nm as well as a strong continuous absorption in the
NIR region corresponding to the longitudinal plasmon
absorption (Fig. 1A, curve 1).
25
As a negative control, the spec-
trum of the gold nanotriangles with only sodium borohydride
(reducing agent), in the absence of the Hg
2+
ions, was recorded
which does not show any change in the NIR region. The spec-
trum overlaps that of purified gold nanotriangles, suggesting that
the sodium borohydride by itself does not have any effect on the
optical properties of gold nanotriangles (Fig. 1A, curve 2).
However, the spectra of the gold nanotriangle solutions, which
are exposed to Hg
2+
ions in the presence of sodium borohydride,
show a loss in the NIR absorbance, which increases with
increasing concentrations of mercuric ions (Fig. 1A, curves 3–6).
The X-ray photoelectron spectroscopy analysis (discussed later)
confirmed that sodium borohydride reduces Hg
2+
ions to Hg(0)
at all concentrations. A high concentration of reducing agent was
used (2.6 mM) to ensure complete reduction. Thus, hereafter, the
reduced mercury will be referred to as mercury metal with
a respective concentration for simplicity. There is a significant
reduction in the NIR absorption intensity when gold nano-
triangles were exposed to 10
9
or higher concentrations of
mercury metal (curves 5–7), which could be due to a higher
degree of amalgamation. TEM imaging was performed to
observe the change in the structural integrity of the gold nano-
triangles (discussed later). The inset shows the spectra of the
solution of gold nanotriangles exposed to 10
3
M concentration
of mercury metal where we see a complete loss of NIR absorp-
tion. The ratio of the absorbance value at 1100 nm to that at
530 nm shows a clear trend in the loss of NIR absorption with
increasing concentration of the mercuric ions (Fig. 1B). It is
evident that with a decreasing concentration of mercury metal in
the solution, the ratio approaches that obtained for control-
purified triangles (R ¼ 1.82). It is important to mention that the
loss in the NIR spectral intensity depends linearly on the
concentration of the mercury metal in the concentration range of
10
6
to 10
15
M but shows a complete loss in transverse plasmon
intensity at 10
3
M. The transmission electron micrograph of the
purified gold nanoparticles solution was recorded which shows
an abundance of gold nanotriangles with a negligible number of
spherical particles (Fig. 1C).
Transmission electron microscopic analysis of triangle
morphology
TEM analysis of gold nanotriangles treated with mercury metal
reveals that the process of amalgamation of the gold nano-
triangles leads to the loss of the structural integrity as seen in the
micrographs (Fig. 2). Exposure of gold nanotriangles to 1 mM
concentration of mercury metal leads to a complete loss in the
morphology (Fig. 2A). The triangles are completely amalgam-
ated to form spherical structures, which corroborates with the
observation made from the UV-vis-NIR spectra (inset, Fig. 1A)
where a complete absence of the NIR absorption characteristic
of gold nanotriangles was observed. Besides, the longitudinal
peak characteristic for spherical gold nanoparticles at 530 nm is
also absent in the spectra due to the fact that a high concentra-
tion of mercury metal is expected to be present on the surface of
these large spherical particles. In the subsequent TEM images,
we see that the gold nanotriangles are broken to varying degrees
and have a changed topological contrast up to 1 pico-molar
concentration of mercury metal in the solution (Fig. 2B–D). The
This journal is ª The Royal Society of Chemistry 2012 Analyst, 2012, 137, 3083–3090 | 3085
Page 3
structural damage was significantly observed in the triangle
populations when they were exposed to 10
9
and higher concen-
trations of metallic mercury and the results are in agreement with
the observed spectral profile of these samples (Fig. 1A, curves 5–
7). Even at the femto-molar concentrations of mercury metal in
the solution, a significant population of the gold nanotriangles
shows the damage due to amalgamation that leads to the loss in
the optical properties even though the topology of the broken
triangles is similar to that in the control (Fig. 2E). However, at an
atto-molar concentration, a majority of the gold nanotriangles
show an intact morphology. This suggests that at such low
concentrations of mercury metal, the morphology of triangles is
not changed significantly enough to affect the optical spectra.
These observations also indicate that the triangles are probably
broken into spherical particles due to the process of amalgam-
ation from the edges or the tips of the triangles, as the topology
of gold triangles does not show any damage at low concentra-
tions. Also, it is worthwhile to note that the TEM images of the
amalgamated gold nanotriangles show a very high population of
spherical nanoparticles of high contrast, which are completely
absent in the control (Fig. 1C).
Topological analysis of the triangles
Atomic force microscopy was done on the gold nanotriangles
exposed to mercury metal to assess any change on their flat (111)
surface. The atomic force microscopic image shows that there is
no significant change in the flat surface of the triangles treated
with femto-molar concentration of mercury(
II) chloride in the
presence of a reducing agent. The control gold nanotriangles
were found to be extremely flat and truncated hexagon shown in
the AFM image of a purified triangle was 16.5 nm thick
Fig. 1 (A) UV-vis-NIR spectra of the gold nanotriangles treated with Hg
2+
ions of varying concentrations for 10 min; curves 1–6 correspond to purified
triangle solution, negative control with only 2.6 mM sodium borohydride, control with 10
15
M, 10
12
M, 10
9
M, and 10
6
M concentration of Hg
2+
ions,
and 2.6 mM sodium borohydride, respectively. The inset shows the curve corresponding to gold nanotriangles treated with 10
3
MHg
2+
ions in the
presence of 2.6 mM sodium borohydride. (B) Plot of the absorbance ratio of A
1100 nm
/A
transverse
vs. Hg
2+
ion concentration in the presence of the reducing
agent. Samples 1 and 2 correspond to the ratio of as-synthesized and sodium borohydride treated Au triangles while samples 3–8 correspond to spectra
of Au triangles exposed to 10
18
,10
15
,10
12
,10
9
,10
6
, and 10
3
M mercury in the presence of borohydride. (C) TEM image of the purified triangles
control solution. The scale bar corresponds to 500 nm.
Fig. 2 TEM images of purified gold nanotriangles exposed to varying concentrations of mercury for 10 min in the presence of the reducing agent. (A)
10
3
M concentration of Hg
2+
ions. Inset shows the representative TEM image of purified gold nanotriangle suspension, (B) 10
6
M concentration of
Hg
2+
ions, (C) 10
9
M concentration of Hg
2+
ions, (D) 10
12
M concentration of Hg
2+
ions, (E) 10
15
M concentration of Hg
2+
ions, and (F) 10
18
M
concentration of Hg
2+
ions. All scale bars correspond to 100 nm.
3086 | Analyst, 2012, 137, 3083–3090 This journal is ª The Royal Society of Chemistry 2012
Page 4
(Fig. 3A). The gold nanotriangles treated with femto-molar
concentration of mercury metal were also found to be flat with
a thickness of 23.5 nm. Although we do see a change in the
thickness in the treated triangle, it is important to note that the
lemongrass reduced gold triangles do show a good variation in
the thickness, which ranges between 15 and 25 nm. However, the
surface did not show any type of deformation due to the process
of amalgamation and was found to be flat as in the control. This
observation corroborates with the corresponding TEM image
(Fig. 2E) where we clearly see that although the triangles are
damaged at tips and edges, the flat surface of the triangle
essentially looks similar to that of the control. The arrow in the
figure points towards the deformation in the edge of the hexagon
giving it a wavy appearance, while the particle enclosed in the
circle shows a broken triangle.
Selectivity and specificity of detection
Gold nanotriangles were also subsequently exposed to other
metal ions so as to ascertain that the change in the optical
properties of the gold nanotriangles on exposure to reduced
mercury metal is due to the process of amalgamation. The optical
properties of the gold nanotriangles do not show any significant
change on exposure to other reduced metals. Fig. 4A shows the
UV-vis-NIR optical spectra of the purified gold nanotriangles,
which were exposed to 1 mM concentrations of environmentally
important metal ions (Pb
2+
,Zn
2+
,Cd
2+
,Fe
2+
,Ni
2+
,Sr
2+
,Ca
2+
,
Mn
2+
, and Cu
2+
) along with the reducing agent, where no change
was observed in the optical spectra of the gold nanotriangles. The
spectra were recorded after 10 min of exposure of gold nano-
triangles in the presence of metal ions and reducing agent (all
measurements using mercuric ions were also recorded after the
same time interval). This result confirms that the loss in the NIR
absorption peak on exposure to mercury metal for 10 min is due
to strong amalgamation tendency of mercury metal. The optical
spectra of these gold nanotriangle solutions exposed to various
metal ions in the presence of a reducing agent were also recorded
after 12 hours of reaction (data not shown). However, the optical
spectra of the gold nanotriangle solution exposed to various
reduced metals for 12 hours show a loss in the NIR absorption
peak to varying degrees. In order to understand the cause of this
change in the absorption after long exposure, gold nanotriangles
exposed to metallic lead were chosen as a model system and were
studied further. It was noted that only very high concentrations
(up to 1 mM) of the reduced lead affected the optical properties of
the triangles (Fig. 4B). In the case of lead metal, this change at
high concentrations could be attributed to reduction of metal
onto the surface of gold nanotriangles resulting in the loss of
signature of gold.
30
However, the chances of interference in
mercury detection by cross-contamination of other metal ions
are highly unlikely due to the fact that such high concentrations
of metal ions in the environment are rarely encountered. The
TEM analysis of the triangles which were treated with 1 mM
solution of lead(
II) chloride in the presence of the reducing agent
for 12 hours showed that the morphology of the triangle remains
intact, though they are covered with metallic lead on the surface
(Fig. 4C), which results in the loss of the spectral signal of gold.
This was apparent when we observed the TEM image of the gold
nanotriangles treated with 1 mM solution of lead(
II) chloride in
the presence of the reducing agent, where we observed that the
structural integrity of the triangle remains intact. However, we
do observe a cloud-like appearance around the triangles, which is
due to metallic lead deposited on their surface (Fig. 4D). Thus,
this indeed proves that the damage to the gold nanotriangles on
exposure to the mercury metal is due to the amalgamation
tendency of mercury metal.
Fig. 3 AFM images of the purified gold triangles treated with mercury
(A) control purified triangles solution and (B) purified triangles treated
with 10
15
M concentration of Hg
2+
ions in the presence of 2.6 mM
sodium borohydride. The white circle shows a broken triangle while the
arrow indicates the corrugated edge of the hexagon.
Fig. 4 (A) UV-vis-NIR spectra of the gold nanotriangles after 10 min of
treatment with different metal ions in the presence of 2.6 mM sodium
borohydride. (B) UV-vis-NIR spectra of purified gold nanotriangles
control (curve 1), control with 10
3
M (curve 2), and 10
6
M (curve 3)
concentration of PbCl
2
in the presence of 2.6 mM sodium borohydride
after 12 h of exposure. (C) TEM image of a purified triangle treated with
10
3
MPb
2+
in the presence of the reducing agent. The scale bar corre-
sponds to 1 mm. (D) TEM image of a purified triangle treated with 10
6
M
Pb
2+
in the presence of the reducing agent. The scale bar corresponds to
200 nm.
This journal is ª The Royal Society of Chemistry 2012 Analyst, 2012, 137, 3083–3090 | 3087
Page 5
Mechanism of morphological changes HRTEM analysis
All of the above data gave us a clue that the mechanism of action
of the amalgamation process on the gold triangles might be
mainly on the tips and the edges. This is fairly expected because
edges and tips are the high-energy sites with point defects in the
nanotriangles and thus the propensity of the amalgamation
process will be more there.
31
However, this was confirmed further
by high-resolution transmission electron microscopy. The
HRTEM image of the control gold nanotriangle showed uniform
edges and tips. The HRTEM micrograph in Fig. 5A and B shows
the lattice spacing to be 2.36
A, which corresponds to the (111)
plane. It has been shown previously that the gold nanotriangles
are highly (111) oriented, confirming that this plane forms the flat
surface of the triangles.
27
The spot EDX analysis from these
triangles reveals the strong signal of gold (Fig. 5C).
When the gold triangles were treated with a femto-molar
concentration of mercury metal, we observed the lattice spacing to
be 2.4
A, which corresponded to the (111) plane of fcc gold lattice.
However, the edges and tips are highly corrugated and wavy in
nature and the contrast near the edges is very high (Fig. 6A and B)
as compared to the control gold nanotriangles (Fig. 5B). One of
the triangles from the same sample showed the snap shot of finger
like projection near the edge of the triangle, which confirms that
mercury amalgamates gold triangles at the edges and tips and
breaks the amalgamated gold off the edge into small particles
(Fig. 6C). Also, when the gold triangles were treated with femto-
molar concentration of mercury metal, we observe few areas near
the edges and tips that show a darker contrast compared to the
untreated triangles. Fig. 6D shows the HRTEM image of the edge
of one such gold nanotriangle, showing regions of dark contrast.
FFT analysis was done from one such area, which has been
enclosed by the white box in the image. The inset of Fig. 6D
corresponds to the images constructed from the FFT analysis of
that area. Fig. 6E and F are the lattice images constructed by the
selective masking of the spots obtained from the FFT analysis of
the area enclosed in the white box. On measuring the lattice
spacing, it was found that they correspond to 2.34
A which
matches with the (111) plane of gold (Fig. 6E) and 2.22
A which
corresponds to the (110) plane of mercury (Fig. 6F). A similar
analysis was performed at various dark contrast regions on the
gold triangles treated with femto-molar concentration of mercury
metal and the above observation was found to be consistent. Thus,
this indicates that these dark contrast regions near the edges and
tips of the gold nanotriangles are amalgamated gold regions,
which tend to break off from the surface as shown in Fig. 6C.
The micrographs of a sample with a high concentration of
mercury metal (10
5
M) showed that most of the population of
the gold triangles was transformed into large sized spherical
particles while some triangular structures were seen with highly
damaged morphology (Fig. 6G). However, an EDX analysis
from the whole triangle or the large spherical particles did not
show any sign of presence of mercury in it. Thus, amalgamation
leads to highly corrugated edges and tips in the otherwise smooth
edged and sharp tipped triangles as seen in the control experi-
ments. When EDX analysis was done from the small spherical
particles, which appear cloud-like and are around 5–10 nm in size
(Fig. 6H), we received a very strong signature of gold as well as
mercury (Fig. 6I). This confirms that the mercury metal amal-
gamates the gold nanotriangles at the edges and tips and breaks
off amalgamated gold, leading to loss in the structural integrity
of the gold nanotriangles. A probable mechanism of amalgam-
ation of gold nanotriangles by mercury metal has been shown
schematically in Fig. S3†, which might be leading to their damage
at the edges and tips.
Chemical analysis
In order to confirm that sodium borohydride indeed reduces
Hg
2+
ions into elemental mercury, X-ray photoelectron spec-
troscopy was done for gold triangles control and gold triangles
treated with 1 mM and 1 femto-molar concentration of Hg
2+
ions
in the presence of a reducing agent. All the core level binding
energies (BE) were shift corrected according to the C1s BE at 285
eV. The Au 4f
7/2
peak in the control sample could be decomposed
into two chemically distinct peaks at 83.5 and 85.7 eV, which
correspond to Au(0) and Au(
I) oxidation states, respectively
32
(Fig. 7A). The relatively small amount of Au(I) present on the
surface of the gold nanotriangles contributes towards the high
binding energy component and is believed to stabilize the gold
nanotriangles electrostatically in solution. The gold nano-
triangles treated with 1 mM concentration of Hg
2+
ions in the
presence of a reducing agent showed a strong signal of mercury
(Fig. 7B) and a very weak signal of gold (Fig. 7B, inset). The
position of the high intensity Hg 4f
7/2
peak was found to be at
100.6 eV that corresponds to the Hg(0) oxidation state, which
confirms that mercury metal is formed by reduction. Besides, we
also observe a low binding energy peak at 98.2 eV that could be
either due to amalgamation of sodium contributed from sodium
borohydride or copper substrate, which was used for the
measurement.
32
Both sodium and copper are electropositive
compared with mercury and are capable of shifting the binding
Fig. 5 HRTEM images and spot EDX analysis of control gold nanotriangles. (A and B) HRTEM image of the gold nanotriangle control showing
uniform edges and tips. The scale bar corresponds to 5 nm in both images. (C) The spot EDX profile from an as-prepared gold nanotriangle.
3088 | Analyst, 2012, 137, 3083–3090 This journal is ª The Royal Society of Chemistry 2012
Page 6
energy of the mercury metal to a lower value. The inset shows the
shift corrected Au 4f core level from the sample and we observe
that there is a very weak signal of gold in the sample. The TEM
image of this sample (Fig. 2A) shows that the mercury has
completely destroyed the triangular morphology and the
contrast from the spherical particles indicates that mercury forms
a very thick layer on the surface of gold. With XPS being
a surface sensitive technique, the signal of gold inside the sphe-
roidal nanostructures may not be readily detected. The gold
nanotriangles treated with 1 femto-molar concentration of Hg
2+
ions in the presence of a reducing agent showed a strong signal of
gold (Fig. 7C) and a very weak signal of mercury (Fig. 7C, inset).
The spectra of Au 4f can be deconvoluted into two components
that correspond to the binding energies at 82.07 and 83.9 eV.
31
The high intensity peak at 83.9 corresponds to the Au(0)
oxidation state while the low binding energy component could be
due to the gold amalgam. The electronegativity value of mercury
is at 1.9 while that of gold is at 2.54 and thus mercury is elec-
tropositive as compared to gold, which shifts the XPS peak to
lower binding energies. The inset in Fig. 7C shows the shift
corrected Hg 4f core level spectra from the sample and we see
that the signal is very weak, suggesting that the amount of
mercury in the sample is much less. It is true as femto-molar
concentration of mercury is present in this sample.
Fig. 6 HRTEM and EDX analysis of gold nanotriangles exposed to mercury metal. (A and B) HRTEM images of the edge and tip of gold nano-
triangles exposed to 10
15
M concentration of mercury metal. (C) HRTEM image of an edge of a gold nanotriangle showing the budding off of
amalgamated gold from the triangle edge. The scale bars in A, B, and C correspond to 5 nm. (D) HRTEM image of an edge of a gold triangle exposed to
10
15
M concentration of mercury metal showing lattice planes. The scale bar corresponds to 2 nm. The inset shows the image reconstructed from the
FFT of the area enclosed in the white box in image D. (E and F) Lattice images constructed by selective masking of FFT spots from the area enclosed in
the white box in image D, showing the lattice planes corresponding to Au (111) and Hg (110) planes, respectively. (G) HRTEM image of a gold
nanotriangle exposed to a 10
5
M concentration of mercury metal. (H) HRTEM image of the spherical amalgamated gold nanoparticles. The scale bars
in G and H correspond to 100 nm. (I) EDX profile from the spherical gold nanoparticles shown in H.
Fig. 7 XPS analysis of the purified triangle treated with mercury ions in
the presence of 2.6 mM sodium borohydride. (A) Au 4f core level spectra
from the purified gold nanotriangles control. (B) Hg 4f core level spectra
from the triangles treated with 10
3
M concentration of Hg
2+
ions. The
inset shows the Au 4f core level spectra from the same sample. (C) Au 4f
core level spectra from the triangles treated with 10
15
M concentration of
Hg
2+
ions. The inset shows the Hg 4f core level spectra from the same
sample.
This journal is ª The Royal Society of Chemistry 2012 Analyst, 2012, 137, 3083–3090 | 3089
Page 7
Conclusions
In conclusion, we show a simple method of spectroscopic
detection of Hg
2+
ions in the water sample up to concentra-
tions as low as 10
15
M. Nanotriangles are thin, thermody-
namically unstable nanostructures with high energy sites at the
edges and tips and thus, their amalgamation tendency is
extremely high. The reduction of Hg
2+
in the presence of these
gold nanotriangles selectively amalgamates gold at the tips and
edges. It is noteworthy that metallic mercury atoms do actu-
ally incorporate into the lattice of the triangles and also
effectively, but actually breaks off the amalgamated gold from
the edges and the tips, eventually damaging their morphology.
This process leaves a strong spectroscopic signature in the
optical absorption characteristic of the triangles and significant
dampening in the longitudinal plasmon absorbance of the gold
triangles is observed up to femto-molar concentration levels of
mercury. We also show that this process is specific only to
mercury due to its strong tendency of amalgamation. The
method does not show any significant change in the optical
absorption spectra of the gold nanotriangles exposed to
metallic lead up to 1 mM concentration. The HRTEM analysis
clearly confirms that the action of mercury is mainly at the
tips and edges of the triangles. In addition, we also show the
budding off of amalgamated gold from the edges of the
triangles at some of the places, which further confirm the
hypothesis that the amalgamation action is mainly at the edges
and tips of the gold nanotriangles. Thus, we show the mech-
anistic aspect of the amalgamation of the gold nanotriangles
leading to loss in their structural integrity, resulting in
a changed absorption profile.
Acknowledgements
AS thanks the Council of Scientific and Industrial Research
(CSIR), Govt of India, for financial assistance. We gratefully
acknowledge Mr Anubhav Bali and Mr Abhijeet Deo for
experimental assistance.
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