Optics and Photonics Journal, 2011, 1, 116-123
doi:10.4236/opj.2011.13020 Published Online September 2011 (http://www.SciRP.org/journal/opj)
Characterization of the Optical Properties of Heavy Metal
Ions Using Surface Plasmon Resonance Technique
Yap Wing Fen*, W. Mahmood Mat Yunus
Department of Physi c s , Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia
Received July 20, 2011; revised August 17, 2011; accepted August 27, 2011
Abstract
The aim of this research is to characterize the optical properties of heavy metal ions (Hg2+, Cu2+, Pb2+ and
Zn2+) using surface plasmon resonance (SPR) technique. Glass cover slips, used as substrates were coated
with a 50 nm gold film using sputter coater. The measurement was carried out at room temperature using
Kretchmann SPR technique. When the air medium outside the gold film is changed to heavy metal ions solu-
tion, the resonance angle shifted to the higher value for all samples of heavy metal ions solution. By our de-
veloped fitting program (using Matlab software), the experimental SPR curves were fitted to obtain the re-
fractive index of Hg2+, Cu2+, Pb2+ and Zn2+ ions solution with different concentrations. Both the real and
imaginary part of refractive index of the heavy metal ions solution increased with the concentration. The re-
sults give the basic idea such that the SPR technique could be used as an alternative optical sensor for de-
tecting heavy metal ions in solution.
Keywords: Surface Plasmon Resonance, Optical Properties, Heavy Metal Ions
1. Introduction
Heavy metals are natural components of the Earth’s crust.
It mainly includes the transition metal in periodic table.
As trace elements, heavy metals such as copper and zinc,
are essential to maintain the metabolism of the human
body. Copper is an essential component of several met-
alloenzymes, which plays a major role in the formation
and repair of extracellular matrix, and catalyzes a key
reaction in energy metabolism [1,2]. Zinc involves in a
wide variety of metabolic processes, supports a healthy
immune system, and is important for normal growth
during pregnancy, childhood and adolescence [3]. How-
ever, they can be poisoning at higher concentration.
Heavy metals are dangerous because of the potential for
bioaccumulation, which means increase in the concentra-
tion of a chemical in a biological organism over time,
compared to the chemical’s concentration in the envi-
ronment. Some of heavy metals are dangerous to health
or to the environment (e.g. mercury, lead), some may
cause corrosion (e.g. zinc, lead) and some are harmful in
other ways. Mercury poisoning causes loss of myelinated
nerve fibers, autonomic dysfunction and abnormal cen-
tral nervous system cell division [4]. Over doses of cop-
per can cause anemia, liver and kidney damage, nausea,
vomiting, abdominal pain, and gastrointestinal distress
[5]. Acute zinc toxicity in humans causes severe irrita-
tion to the gastrointestinal, as well as fever, chest pain,
chills, cough, dyspnea, nausea, muscles soreness, fatigue
and leukocytosis. Lead poisoning may cause severe
damage in brain, kidney, liver, central nervous system,
cardiovascular system, immune system, or even death
[6-8].
Surface plasmon resonance (SPR) spectroscopy is a
surface-sensitive technique that has been used to charac-
terize the thickness and refraction index of dielectric
medium at noble metal (gold) surface. For the last dec-
ade, surface plasmon resonance sensors have been exten-
sively studied. SPR technique has emerged as a powerful
technique for a variety of chemical and biological sensor
applications. The first chemical sensing based on SPR
technique was reported by Liedberg et al. (1983) [9].
SPR is an optical process in which light satisfying a
resonance condition excites a charge-density wave pro-
pagating along the interface between a metal and dielec-
tric material by monochromatic and p-polarized light
beam. The intensity of the reflected light is reduced at a
specific incident angle producing a sharp shadow (called
surface plasmon resonance) due to the resonance energy
occurs between the incident beam and surface plasmon

Y. W. FEN ET AL.117
wave [10]. SPR is regarded as a simple optical technique
for surface and interfacial studies and shows the great
potential for investigating biomolecules [11]. SPR has
been used to study the refractive index of liquid meas-
urement [12,13], pesticide detection [14] and SPR also
can be regarded as a significant tool for analyzing sac-
charides where saccharides solution commonly has a
high refractive index [15]. Optical properties of chlorine
and commercial carbonated drinks using SPR techniques
also had been reported from our laboratory by Yus-
mawati et al. (2007) [16,17].
In this paper, we investigate the optical properties of
mercury, copper, lead and zinc ions solution with differ-
ent concentration using SPR techniques. The real and
imaginary parts of refractive index of all the heavy metal
ions with different concentration were determined. These
results are important in our ongoing research on multi-
layer SPR optical sensor for heavy metal ions.
2. Theoretical Background
The SPR is a charge-density oscillation that exists at the
interface of two media with dielectric of opposite signs,
i.e. a metal and a dielectric. The interaction between a
light wave and a surface plasmon in the attenuated total
reflection method can be described using the Fresnel
theory [18,19]. In this study, we used Kretchmann con-
figuration where the metal layer (gold film) is sand-
wiched between prism and dielectric layer (heavy metal
ion sample). For this system, the reflection coefficient R,
which depends on thickness d, can be expressed as [20]


2
201 121
012
01 121
exp 2
1exp2
rr ikd
Rr rr ikd


 (1)
2
2
2π
j
j
k 



 x
k (2)
where r01 and r12 are the reflection coefficient of prism-
gold and the reflection coefficient of gold-heavy metal
ion sample, respectively. The kx is the propagation con-
stant of the evanescent wave, which is given by the fol-
lowing equation:
1
2πsin
xp
kn


 (3)
The excitation of surface plasmons occurs when the
wave vector of evanescent wave exactly matches with
that of the surface plasmons of similar frequency. The
resonance condition for surface plsmon resonance, which
depends on the refractive index of gold and the sample,
as follow:
22
12
22
12
sin
pR nn
nnn   (4)
where np, n1 and n2 are the resonance angle, refractive
index of the prism, gold layer and sample, respectively.
The refractive index of the sample is [21]:
22 2
1
2222
1
sin
sin
pR
p
R
nn
nnn


 (5)
Hence to perform calculation of refractive index of
sample, a simulation and automatic fitting program had
been developed using Matlab based on the theory as ex-
plained above.
3. Experimental
Prism with refractive index, n = 1.7786 at 632.8 nm and
the substrate, glass cover slips 24 × 24 mm with thick-
ness 0.13 - 0.16 mm were purchased from Menzel-Glaser.
The glass cover slips were cleaned using acetone to clean
off the dirt or remove fingerprint marks laid on the sur-
face of glass slides. Then gold layer were deposited us-
ing SC7640 Sputter Coater.
Mercury, copper, lead and zinc ion AAS standard so-
lutions (Merck, Germany) with a concentration of 1000
ppm were diluted accordingly to produce Hg2+, Cu2+,
Pb2+ and Zn2+ solution with concentrations 0.5, 1, 5, 10,
30, 50, 70, 100, 500 and 700 ppm.
The coated glass cover slip was attached to the prism
using index matching liquid. A cell was constructed to
hold and supply the heavy metal ions solution to the
glass cover slip with gold film, as shown in Figure 1
(based on Kretchmann configuration). An open-ended
cylindrical brass cavity with an O-ring seal was attached
to the glass cover slip that was attached to the prism. The
heavy metal solution was filled into the hollow formed
so that the laser light irradiated solution. The prism and
the cell were mounted on a rotating plate to control the
angle of the incident light.
The SPR measurement had been carried out by meas-
uring the reflected He-Ne laser beam (632.8 nm, 5 mW)
as a function of incident angle. The optical set up con-
sists of a He-Ne laser, an optical stage driven by a step-
per motor with a resolution of 0.001˚ (Newport MM
3000), a light attenuator, a polarizer and an optical
chopper (SR 540). The reflected beam was detected by a
sensitive photodiode and then processed by the lock-in-
amplifier (SR 530). The experimental setup for SPR
measurement that we used is schematically shown in
Figure 2.
4. Results and Discussion
Prior to measurement, a preliminary SPR test was carried
out for gold film in contact with deionized water. By
fitting the experimental data to the Fresnel equation

Y. W. FEN ET AL. 118
Figure 1. Structure of the cell for SPR measurement.
Figure 2. Experimental setups for angle scan surface plas-
mon resonance technique.
[18,19] using our self developed fitting program (matrix
method), the refractive index of deionized water was
obtained. Figure 3 shows that the fitting line is in good
agreement with the experimental data as compared to the
theoretical result for deionized water. The value of re-
fractive index for this fitting line is 1.3317, and the reso-
nance angle was obtained as 55.043˚.
The fitting result for refractive index of deionized wa-
ter, i.e. 1.3317, shows a good agreement with other pub-
lished values as reported by Weast et al. (1978), Webb et
al. (1989) and Ma et al. (2000) [22-24]. Thus, it shows
that the SPR measurement can be used to determine the
optical properties of the liquid sample.
Then the SPR experiment was carried out for Hg2+ so-
lutions (concentration range between 0.5 to 1000 ppm),
which was injected one after another into the cell to con-
tact with the gold film. It was observed that the reso-
nance angles determined were 55.071˚, 55.350˚, 55.467˚,
55.601˚ for Hg2+ at concentrations of 100, 500, 700 and
1000 ppm, respectively. The resonance angles remain
unchanged (same as deionized water) for the concentra-
tions less than 100 ppm. The SPR curves were shown in
Figure 4. The values of real and imaginary parts of re-
fractive index for Hg2+ solutions obtained by fitting were
notified and tabulated in Table 1.
Similarly, the above processes were applied to other
heavy metal ions solutions, Cu2+, Pb2+ and Zn2+. The re-
sonance angles obtained were 55.071˚, 55.267˚, 55.378˚,
Figure 3. Fitting experimental data to theoretical data for
gold layer in contact with deionized water. The solid line
represents the theoretical curve.
Table 1. The real and imaginary part of refractive index for
different concentration of mercury ion solutions from fit-
ting.
Concentration of
Hg (ppm)
Real Part of
Refractive Index, n
(±0.0005)
Imaginary Part of
Refractive Index, k
(±0.0002)
0.5 1.3317 0.0002
1 1.3317 0.0002
5 1.3317 0.0003
10 1.3317 0.0005
30 1.3318 0.0010
50 1.3318 0.0014
70 1.3318 0.0021
100 1.3321 0.0029
500 1.3359 0.0061
700 1.3374 0.0071
1000 1.3392 0.0089
55.490˚ for Cu2+ at concentrations of 100, 500, 700 and
1000 ppm, respectively. For Pb2+, we determined that the
resonance angles were 55.071˚, 55.326˚, 55.431˚, 55.569˚
at concentrations of 100, 500, 700 and 1000 ppm, re-
spectively. While for Zn2+, the resonance angles obtained
were 55.071˚, 55.278˚, 55.388˚, 55.501˚ at concentra-
tions of 100, 500, 700 and 1000 ppm, respectively. Also,
same as Hg2+, the resonance angles, for all Cu2+, Pb2+ and
Zn2+ with concentrations less than 100 ppm, were same
as deionized water, i.e. 55.043˚. The SPR curves for Cu2+,
Pb2+, Zn2+ were shown in Figures 5-7, respectively.

Y. W. FEN ET AL. 119
(a) (b)
Figure 4. Optical reflectance as a function of incident angle for mercury ion solutions with concentration: (a) 10 - 100 ppm,
and (b) 100 - 1000 ppm.
(a) (b)
Figure 5. Optical reflectance as a function of incident angle for copper ion solutions with concentration: (a) 10 - 100 ppm, and
(b) 100 - 1000 ppm.
The real and imaginary parts of refractive index for Cu2+,
Pb2+ and Zn2+ solutions obtained by fitting were deter-
mined and tabulated in Tables 2-4, respectively.
All the results of fitting were summarized in Figures
8(a) and (b), which shows the real and imaginary parts of
refractive index for all four heavy metal ions, respec-
tively. From the figures, we found that both of the real
and imaginary parts of refractive index for all heavy
metal ions generally increase with the increase of heavy
metal ions concentration. However, the real part of re-
fractive index for all heavy metal ions below 100 ppm
does not show the significant change. Also, we found
that the real part of refractive index is almost similar for
Hg2+, Cu2+, Pb2+ and Zn2+, while the value of the imagi-
nary part of refractive index for Cu2+ is slightly higher
than that of Hg2+, Pb2+ and Zn2+. We believe that the blue
colour of Cu2+ compared to colourless Hg2+, Pb2+ and
Zn2+ increase the absorption and hence the value of
imaginary part of refractive index.
The results also show that the SPR curves shift to the
right as the concentration of heavy metal ions increases
(for concentration 100 ppm and above). This trend was
found in all four heavy metal ions. The increase in reso-
nance angle with concentration is mainly due to the in-

Y. W. FEN ET AL. 120
(a) (b)
Figure 6. Optical reflectance as a function of incident angle for lead ion solutions with concentration: (a) 10 - 100 ppm, and (b)
100 - 1000.
(a) (b)
Figure 7. Optical reflectance as a function of incident angle for zinc ion solutions with concentration: (a) 10 - 100 ppm, and (b)
100 - 1000 ppm.
crement number of ions absorbed to the metal surface.
There is a slight difference for the shift of resonance an-
gle among different heavy metal ions. This may due to
the small difference of the size of these ions based on
periodic table. The SPR curves are almost similar for
deionized water and solution with concentration below
100 ppm. This research results are very important such
that we can use these information (real and imaginary
parts of refractive index) to further increase the sensitiv-
ity of SPR technique as an alternative optical sensor for
heavy metal ions by coating an active layer onto the gold
film. This activity is ongoing in our laboratory.
5. Conclusions
In this work, the optical properties of heavy metal ions
solution have been obtained using surface plasmon reso-
nance technique. Both the real and imaginary part of
refractive index of the heavy metal ions solution in-
creased with the heavy metal ions concentration. The
resonance angle shifted to higher values to the right with
the increasing of the concenration of heavy metal ions, t

Y. W. FEN ET AL.121
(a) (b)
Figure 8. (a) The real part of refractive index, n, as a function of Hg2+, Cu2+, Pb2+ and Zn2+ concentration. (b) The imaginary
part of refractive index, k, as a function of Hg2+, Cu2+, Pb2+ and Zn2+ concentration.
Table 2. The real and imaginary part of refractive index for
different concentration of copper ion solutions from fitting.
Concentration of
Cu (ppm)
Real Part of
Refractive Index, n
(±0.0005)
Imaginary Part of
Refractive Index, k
(±0.0002)
0.5 1.3318 0.0003
1 1.3318 0.0003
5 1.3318 0.0005
10 1.3318 0.0009
30 1.3319 0.0015
50 1.3319 0.0023
70 1.3319 0.0030
100 1.3321 0.0042
500 1.3351 0.0080
700 1.3366 0.0093
1000 1.3381 0.0108
for concentration 100 ppm and above. In this range of
concentration, the shift of resonance angle increases
linearly with concentration for all Hg2+, Cu2+, Pb2+ and
Zn2+ solution. This study also gives us the basic idea
such that the SPR technique could be used as an alterna-
tive optical sensor for detecting heavy metal ions in solu-
tion. The real and imaginary part of refractive index for
heavy metal ions that obtained from fitting (single layer)
in this study gives us the important information required
Table 3. The real and imaginary part of refractive index for
different concentration of lead ion solutions from fitting.
Concentration of
Pb (ppm)
Real Part of
Refractive Index, n
(±0.0005)
Imaginary Part of
Refractive Index, k
(±0.0002)
0.5 1.3317 0.0002
1 1.3317 0.0002
5 1.3317 0.0003
10 1.3318 0.0005
30 1.3318 0.0009
50 1.3318 0.0014
70 1.3318 0.0020
100 1.3321 0.0027
500 1.3356 0.0060
700 1.3370 0.0070
1000 1.3388 0.0086
to further increase the sensitivity of SPR optical sensor in
future research. We suggested that it can be carried out
by introducing an active layer on the gold film (multi-
layer SPR sensor).
6. Acknowledgements
The authors would like to thank the Malaysian Govern-
ment for fund support through SAGA and FRGS (No.

Y. W. FEN ET AL. 122
Table 4. The real and imaginary part of refractive index for
different concentration of zinc ion solutions from fitting.
Concentration of
Zn (ppm)
Real Part of
Refractive Index, n
(±0.0005)
Imaginary Part of
Refractive Index, k
(±0.0002)
0.5 1.3317 0.0002
1 1.3317 0.0002
5 1.3317 0.0003
10 1.3318 0.0005
30 1.3318 0.0009
50 1.3318 0.0014
70 1.3318 0.0020
100 1.3321 0.0028
500 1.3351 0.0061
700 1.3365 0.0070
1000 1.3380 0.0087
5523901) research grants. The laboratory facilities pro-
vided by the Department of Physics, Faculty of Science,
Universiti Putra Malaysia, are also acknowledged.
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