Detection of Heavy Metal ions in water using nanoparticles

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].