Detection of Heavy Metal ions in water using nanoparticles

Detection of Heavy Metal ions in water using nanoparticles
Siriphun Ameritachot, Tanmay Bera & Joydeep Dutta*
NANOTEC Center of Excellence in Nanotechnology, School of Engineering and Technology, ISE-Building,
Asian Institute of Technology, Pathumthani 12120, Thailand
*Corresponding Author: joy@ait.ac.th
Abstract
Water pollution by toxic metals remain a serious
environmental problem and can be detrimental to plants,
animals, and human being alike. Different governments have
set up environmental laws to determine amount of heavy metal
ion in drainage, considered to be non-detrimental to the
environment. Traditional heavy metal analysis methods
require trained staff, equipment and time consuming, limiting
the application of metal ion sensing to non-specialists. The use
of surface plasmon resonance as an indicator of the ion
content can be a useful approach to disseminate the
application of continuous water quality management even in
remote sites. Following our earlier work, we report on the
enhancement of sensitivity and selectivity of sensor [1]. When
heavy metal ion is added to gold nanoparticle capped with
polymer, functionalized group in polymer attaches with metal
ion leading to the shape and size of nanoparticles being
changed, resulting in a change in surface plasmon resonance
frequency. A comparison of the optical absorption spectra of
the colloidal suspension before and after exposure to metal
ions is a good indicator of the concentration of the heavy
metal ions. Optimization of the deconvolution of the optical
absorption spectra gives a fantastic tool to follow the
agglomeration process of gold nanoparticle through the
chelation of the cations on the polymer utilized for the steric
stabilization that has been used for ion-sensing. In order to
complete the chelation of the metal ions like copper, zinc and
manganese, about 1 hour was sufficient to complete the
reaction in most of the cases. This simple metal ion sensor can
be implanted in lab-on-chip type of applications for easier
implementation.
Keywords: heavy metals gold nanoparticles, chelation, surface
plasmon resonance
1. Introduction
Environmental monitoring is becoming increasingly
critical to protect the public and the environment from toxic
contaminants and pathogens released into air, soil, and water
from toxic chemical wastes, spills, manufacturing waste and
even underground storage tanks. The United States
Environmental Protection Agency (U.S. EPA) has imposed
strict regulations on the maximum allowable concentrations of
many environmental contaminants in air and water and is
reported to be monitoring over two million underground
storage tanks containing hazardous (and often volatile)
contaminants from as early as 1992 [2]. Nanotechnology has
the potential to bring in solutions to minimize or eliminate the
use of toxic materials and the generation of undesirable by-
products, as well as, sensitively detect (and monitor) specific
polluting agents well before any major environmental
catastrophe occur. Research related to improved industrial
processes and starting material requirements, development of
new chemical and industrial procedures, and materials to
replace current hazardous constituents and processes, resulting
in reductions in energy, materials, and waste generation are
being supplemented by the application of nanotechnology to
control and predict the potential damage to the environment.
Monitoring hazardous materials with current methods are
costly and time intensive and several limitations in sampling
and testing with analytical techniques have been identified.
The time and expense involved in the detection of
environmental pollutants (i.e., sample acquisition, sample
preparation, and laboratory analysis) have led to the renewed
interest for finding newer solutions to analyze contamination
in order to prevent, seek remedial action or to destroy the
contaminants prior to the pollution of the environment. Fast
and cost-effective field-analytical technologies that can
increase the number of analyses and drastically reduce the
time required to perform them will help in prevention of
environmental catastrophe. Increasing the amount of analytical
data tends to improve the accuracy of hazardous waste site
characterization leading to a better management of the
problems by and the risk assessments can be improved by
efficient cleanup procedure [3].
The wide variation of optical properties of metal
nanoparticles with particle size and shape, particle-particle
distance, and the dielectric properties of the surrounding
solution due to the phenomenon called surface plasmon
resonance enables construction of simple but sensitive
colorimetric sensors for various analyses. The chemical
inertness and resistance to surface oxidation make production
of gold nanoparticles easier in comparison to other metal
nanoparticles.
Unlike spherical particles, light cannot polarize the
anisotropic nanoparticles homogenously and retardation
effects lead to the excitation of multimodes (higher order
modes). Therefore, several resonances are generated leading to
a broad extinction profile, or a few other low energy peaks in
the absorption spectra. The plasmon resonance of nonspherical
nanoparticles, like nanorods and nanochains, splits
into 2 modes. The first mode being perpendicular to the long
axis of rod is referred as the transverse mode. The other mode
being parallel to the long axis of rod is referred as the
longitudinal mode. Following our earlier work [1], we
improved on the sensitivity and selectivity of sensor. The test
kit must have a good metal-ion selectivity property. That
means sensor can recognize difference between metal ions.
When heavy metal ion is added to gold nanoparticle capped

with polymer, functionalized group in polymer will attach
with heavy metal ion leading to shape and size of
nanoparticles being changed, resulting in a change in surface
plasmon resonance frequency.
Table 1: Maximum concentration of heavy metal ion and
traditional analyze method [4]
Heavy
metal ion
Maximum
concentration
(ppm)
Analysis methods
Copper 2.0 Atomic Absorption
Spectrophotometry
(Direct Aspiration)
Manganese 5.0
or
Plasma Emission
Spectroscopy
Zinc 5.0 (Inductively Coupled
Plasma: ICP)
In the Table 1 the maximum tolerated concentration of
heavy metal ions in water stipulated by the Ministry of Natural
and Environment of the Royal Government of Thailand is
given. As a test case of the application of the plasmon
resonance sensor, we have considered only copper, manganese
and zinc for this study. Copper is widely used in many
products such as piping, electronic devices, structural
engineering, household products, coins, biomedical and
chemical applications. In Thailand, major industries are
working in the area of electronic assembly, so copper is the
main material for Printed Circuit Board (PCB). The process
involved in making the PCB’s lead to land and contamination
of wafer. In sufficient amounts (more than 10 mg per day) [5],
copper can be poisonous and even fatal to human organisms.
Manganese is essential to iron and steel production by virtue
of its sulfur-fixing, deoxidizing, and alloying properties.
Manganese compounds are less toxic than those of other
widespread metals such as iron, nickel and copper compounds.
However manganese is toxic in excess. Zinc is the fourth most
common metal in use, following only iron, aluminum, and
copper in annual production. Moreover, Zinc is used as part of
the containers of batteries; the most widespread such use is as
the anode in alkaline batteries. It is known that used batteries
are a major cause of zinc contamination of land and water.
In this work, the synthesized nanoparticles were capped
with chitosan, which is well known as a heavy metal-chelating
agent [6]. Chitosan has free amines in its monomer, which
gets protonated in dilute acidic media. These protonated
amines form the multiple bonding sites that are useful in
chelating heavy metals like Mn 2+ , Cu 2+ and Zn 2+ [7]. Though
chelation of heavy metal ions by chitosan has been widely
studied, relatively less attention has been given to
development of simple colorimetric sensors to detect the
presence of heavy metal ion contaminants in water. Since gold
nanoparticles are an ideal candidate for the construction of
colorimetric sensors, the electrostatic attachment of chitosan
over gold nanoparticles has been studied in order to
demonstrate a simple colorimetric sensor for indicating the
concentration of heavy metals ions (Cu 2+ , Zn 2+ and Mn 2+ ) in a
solution.
2. Materials and methods
One of the most commonly used techniques to stabilize
colloids of gold nanoparticles in aqueous system was first
described by Turkevich is based on the reduction of
choloroauric acid with trisodium citrate [8]. This technique
was continuously improved to achieve narrower particle size
distribution today.
There are many reports on the application of gold
nanoparticles capped with polymer such as polystyrene [9],
thiol [10], and chitosan [1]. Polymers serve dual-purpose, one
of providing sufficient steric or electrosteric hindrance
ensuring stability of the colloids and also to functionalize the
nanoparticles for sensing applications.
Synthesis of gold nanoparticle was based on the welldocumented
Turkevitch process. Stock solution of 5 or 50 mM
chloroauric acid (Aldrich) (Solution A), and 25 or 250 mM trisodium
citrate (Merck) (Solution B) are prepared in de-ionized
water. Solution B is employed as a reducing agent [1].
Chitosan (CTS) was used for capping the gold nanoparticle
(Aldrich, medium molecular weight). 1 wt% chitosan was
dissolved in (1 wt%) hydrochloric acid (HCl) (Merck).
Prior to measurement, the gold colloid was mixed with
prepared heavy metal ion. In our experiment, Copper acetate
(Aldrich), Copper sulphate (APS), Manganese acetate (Fluka),
and Zinc acetate (Merck) are dissolved in de-ionized water at
2000, 200 and 20 ppm. Each of final mixing reagents
contained 50 % of gold nanoparticle and 50 % of varying
concentration of heavy metal ion solution that are 100, 50, 20,
10, 5, 2, 1, 0.5, 0.2, and 0.1 ppm. The optical spectrum of
absorbance curve is measured by spectrophotometer (Ocean
Optic Model USB 2000-FLG) after mixing for varying times
from minutes to 1 hour, for ensuring that all metal ions have
reacted. Each absorbance curve reported in this work is a
representation of at least five measurements. Additionally a
reference sample was always measured during the
measurement of all optical spectra; where we used 50% of
gold nanoparticle and 50 % of de-ionized water to cross-check
the optical measurement set up prior to each separate optical
absorption measurement.
As already discussed earlier, due to the agglomeration of
spherical gold nanoparticles by the chelation of metal ions,
longitudinal plasmon resonance absorption develops. Thus the
interpretation of data is accomplished by using curve fitting
tools.
3. Results and Discussion
In Figure 1, the broad and red-shifted peak is a distinct
sign of agglomeration of gold nanoparticles. Though the signal
caused by agglomeration is apparent even with 1 mM of Cu 2+
ions, no significant difference between the signals obtained for
various concentrations, ranging from 1 mM to 5 mM could be
observed. This agglomeration is probably caused due to the
disturbance of the ionic equilibrium, resulting in loss of the
protective glutamate capping from the gold nanoparticle
surfaces. In gold colloids, it has been observed that the
transverse plasmon resonance shifts to lower energies when
the particle size increases [11].

Figure 1: Series of optical absorption spectra obtained by
exposing gold colloid to varying concentrations of Cu 2+ ions
without prior surface treatment with chitosan.
It has been reported earlier by Sugunan [1] that the gold
nanoparticles in the colloids agglomerate in linear chains upon
the disturbance of the double layer of the colloids by the
adsorption of cations. Hence in the plasmon resonance
absorption spectra the longitudinal modes change most
distinctly compared to any shifts in the transverse mode of
optical absorption, as shown in Figure 2.
wavelength shift (nm)
wavelength shift (nm)
0.5
0.4
0.3
0.2
0.1
0
-0.1
0.3
0.15
0
Transverse plasmon resonance
Gold Colloid MR3 @ 0.2 mM capped 0.1% chitosan pH6
-0.15
0 1 2
Initial Wavelength 525.399 nm
Cu#av
Mn#av
Zn#av
Cu#av (Copper Sulphate)
-0.2
0 10 20 30 40 50 60 70 80 90 100
amount of ion (ppm)
(A)
110
100
40
30
Longitudianl plasmon resonance
Gold Colloid MR3 @ 0.2 mM capped 0.1% chitosan pH6
Initial Wavelengh 564.184 nm
90
80
20
10
0
-10
70
0 1 2
60
50
40
30
20
10
0
-10
Cu#av
Mn#av
Zn#av
Cu#av (Copper Sulphate)
0 10 20 30 40 50 60 70 80 90 100
amount of ion (ppm)
(B)
Figure 2: The plasmon resonance peak shift with varied metal
ion concentration with gold colloid capped 0.1% chitosan pH
6 (MR 3; 0.2 mM) A) Transverse B) Longitudinal
In Figure 3, we have compared the result of the shift in
the longitudinal plasmon resonance in gold colloids
synthesized at two different molar rations, upon exposure to
different concentrations of copper, manganese, and zinc
cations. In either case, copper is more sensitive to changing
the longitudinal plasmon resonance peak compared to
manganese and zinc.
Figure 3: The different color with varied metal ion
concentration with gold colloid capped 0.1 % chitosan pH 6
(MR 3; 0.2 mM) (Copper acetate, Copper sulphate,
Manganese acetate, Zinc acetate and mixing ions)
4. Conclusions
In summary, a novel strategy for detection of heavy metal
ions in water has been developed employing 20 nm gold
particles capped with a biopolymer called chitosan. Polymer
capping of nanoparticles serves a two-fold purpose, that of
stabilization and surface functionalization for application as
sensors. Chitosan is widely used as a chelating agent for
removal of heavy metal contaminants in wastewater. This
property of chitosan has been effectively used to demonstrate
the detection of low concentrations of heavy metal ions like
Cu 2+ and Zn 2+ . A relatively simple characterization tool like
UV-visible absorption spectrum is found to be sufficient to
observe the concentration levels of the analyte. Further
modification of the attached chitosan molecules has the
promise to achieve high-specificity sensors for various
applications. Apart from applications in colorimetric pollution
sensors, chitosan capped gold nanoparticles may have
biology-oriented applications because it was found that
chitosan shows selectivity in attachment to certain kinds of
bacteria. Future experiments will be directed towards this
aspect of chitosan capped gold nanoparticles.

5. Acknowledgements
The authors would like to acknowledge partial financial
support from the NANOTEC Centre of Excellence in
Nanotechnology at the Asian Institute of Technology and the
National Nanotechnology Center, both belonging to the
National Science & Technology Development Agency
(NSTDA), Thailand.
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