Sensors and Actuators B 143 (2009) 381–386
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Measurements of refractive index change due to positive ions using a surface
plasmon resonance sensor
Hyungduk Ko ∗ , Jun Kameoka, Chin B. Su
Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX 77843, United States
a r t i c l e
i n f o
Article history:
Received 5 July 2009
Received in revised form 30 August 2009
Accepted 7 September 2009
Available online 2 October 2009
a b s t r a c t
A 44-pass fiber optic surface plasmon resonance (SPR) sensor coupled with a field-assist capability for
measurement of refractive index change due to positive and negative ions is shown. The field-assist
feature forces ions to the SPR surface, causing the SPR signal response to change which reflects a decrease
or increase in refractive index depending on whether positive or negative ions are being attracted to the
surface. This technique offers the potential for the sensitive detection of cations and anions in a solution.
Keywords:
Surface plasmon resonance
Multi-pass SPR sensor
Fiber optic SPR sensor
1. Introduction
Surface plasmon resonance (SPR) sensor has been widely studied for chemical and biological sensing in the last two decades
because it allows real-time detection of biological species without labeling procedures [1–4]. It measures changes of refractive
index near the surface of a thin metal layer using surface plasmons (SPs) [5]. Fundamentally, surface plasmons (SPs) are charge
density waves that propagate along the surface of metals when incident p-polarized optical wave undergoes total internal reflection
inside the metal coated dielectric [6]. SPR is generally implemented
using the Kretschmann’s configuration [7]. At a resonance angle, the
magnitude of the evanescent optical field associated with surface
plasmon has a maximum value at the metal–dielectric interface
and the reflectivity becomes a minimum value [6,8]. A SPR sensor
utilizes this evanescent optical wave localized just above a very
thin metal surface to detect molecules. Recently, we reported the
44-pass fiber optic SPR sensor that passes the detection spot 44
times, thus enhancing sensitivity by a factor of 44 [9]. Also, we
reported the field-assist (applied voltage) 4-pass fiber optic sensor
to detect the negatively charged particles or ions [10]. In this article,
we incorporate the 44-pass feature with the field-assist method.
This technique allows the detection of the refractive index change
due to ions in a solution by attracting them to the SPR surface. The
electric field causes the charged ions to accumulate at the Au surface, leading to an effective increase in the concentration of the
∗ Corresponding author.
E-mail addresses: koh94@gmail.com (H. Ko), su@ece.tamu.edu (C.B. Su).
0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2009.09.016
© 2009 Elsevier B.V. All rights reserved.
ions at the SPR surface. We found for the first time that positive
ions contribute to a decrease in refractive index while negative ions
contribute to an increase in refractive index compare with the equilibrium solution. Because ions such as Na+ , K+ , H+ are important in
biological processes, the detection of these ions by SPR may be a
new technique for the studies of such processes.
2. Experiments
Fig. 1 shows the SPR system with the fiber recirculating loop
with the addition of a field-assist method. The principle and properties of the SPR system with the fiber recirculating loop to increase
the number of pass, and thus the detection sensitivity, were
reported in Ref. [9]. In brief, pulse generator 1 drives a laser diode
(LD) to create an optical pulse train with about 5% duty cycle and
0.13 s pulse width. The incident pulse is split into two pulses by
fiber coupler 1 (FC1). One pulse propagates toward the SPR device
by exiting port 2 of the optical circulator and the other is detected
by the photodetector (PD) as shown. There are two collimators in
the SPR unit. The optical beam exiting port 2 of the circulator is
fed to the lower collimator and propagates toward the corner cube
on the others side after reflecting off the gold surface. All fiber and
collimators are single mode as multimode fiber will support many
modes each exiting the fiber at different angles. Moreover, multimode fiber is incompatible with erbium-doped fiber amplifier
technology. Then, the retro-reflected beam from the corner cube
hits the gold surface the second time and returns to the upper
collimator. Therefore, the SPR system is a two-pass configuration
with the lower collimator functioning as an emitter and the upper
collimator functioning as a receiver (or vice versa).
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H. Ko et al. / Sensors and Actuators B 143 (2009) 381–386
Fig. 1. (a) An optical multi-pass SPR sensor system coupled with the field-assist method. Top: SPR, bottom: fiber loop, LD: laser diode, PD: photodiode, PC: polarization
controller, and FC: fiber coupler. (b) Side view of the electrode configuration.
The returned optical pulse from the SPR device is reduced in
amplitude due to SPR resonance effect and back-coupling loss,
therefore, the reduced amplitude needs to be amplified by the
erbium-doped fiber amplifier following the electro-optic modulator (EOM). The pulse eventually returns to FC1 after one round-trip
and the process repeats. Therefore, the detector (PD) detects periodic pulses that are displayed on a digital oscilloscope. The first
pulse does not pass the SPR device and is irrelevant, the 2nd pulse
passes the SPR twice and the third pulse pass four times, etc. The
period of the periodic pulses is determined by one round-trip time
around the fiber loop. The length of the delay line is not important (about 30 m in this case) as long as it guarantees that the
recirculating pulses are well separated in time.
Pulse generator 2 is gated and synchronized by pulse generator 1 to produce a synchronized pulse train with a longer pulse
width T as shown. This gated pulse applied on the RF port of the
electro-optic modulator is used for controlling its transmittance.
During the time interval T when the gated pulse is applied on
the electro-optic modulator, its transmittance is high and the fiber
loop is closed, otherwise the loop is opened with high loss preventing pulse recirculation. This leads to the periodic opening of
the EOM that prevents the fiber loop from becoming a ring laser
as the optical amplifier has gain, but at the same time limits the
total number of passes (pulse recirculation). Here, the EOM acts as
a loss-modulating optical switch with the switch closed (low loss)
when the gated electrical pulse is applied to the RF port of the EOM,
otherwise the switch is opened (high loss).
An important issue relates to the detrimental affect of interference fringes on the sensitivity of the multi-pass type of SPR of
which the multi-pass beam overlapped at the SPR surface. If the
fringe spacing (or the spot size) is smaller than the propagation
decay length of the surface plasmon, one can expect a substantial
decrease in SPR sensitivity in contrast with the case without interference effects. The propagation decay length of the plasmon wave
along the metal surface is given by 1/(2Im(ˇ)), where ˇ, the surface
plasmon wave-vector, is given by [6]
k0 = 2/, and εm and εs are the gold and analyte dielectric constant,
respectively. Using εm = (0.2 + i10.2)2 for gold and εs = 1.31592 for
water, the propagation decay length is about 250 m for a wavelength, , of 1.53 m used in this experiment. The plasmon wave
cannot freely propagate because the interference fringe spacing is
much less than 250 m. Therefore, in this work, a corner cube was
used for retro-reflection instead of a mirror, assuring that the multipass beam does not overlap. In addition, as mentioned previously,
two collimators side-by-side are used for delivering and receiving
light that passes the SPR surface instead of one collimator. The corner cube retro-reflect the light from one collimator back into the
other collimator. With this method the light spot at the SPR surface does not overlap, avoiding the possible interference effects that
occur with the use of one collimator and a mirror reflector scheme
as reported in Ref. [9]. However, if the light source has low coherence then the one collimator and one reflecting mirror scheme may
still be used if the optical path length spacing between the mirror
and the light spot at the SPR surface is longer than the coherence
length of the light source.
To apply an electric field to the analyte solution, another
Au-coated glass wafer is used as the top electrode. The bottom
electrode is the SPR surface. Poly(dimethylsiloxane) (PDMS) sheets
with 1 mm thickness are fabricated with two holes that function as
solution wells. The PDMS sheet is placed between the bottom and
top electrodes. Analyte solution is dropped into the two holes on
the PDMS sheets, and the solution is covered by the top electrode.
The bottom electrode is electrically grounded. Therefore, when a
positive voltage is applied to the top electrode, positive ions are collected onto the bottom SPR surface. The light beam hits the region
of the solution well as shown in Fig. 1(b).
In this paper a light source wavelength of 1.53 m compatible
with erbium-doped fiber amplifier technology is used. Compared to
the commonly used wavelengths of 0.67 and 0.85 m, the resonant
profile is sharper and the reflectivity dip is shallower at 1.53 m
wavelength, as verified in Ref. [11].
3. Results and discussion
ˇ = k0
εm εs
εm + εs
(1)
The one-pass SPR reflectivity profiles versus incident angle for
DI water as reference solution is shown in Fig. 2. The presence of
H. Ko et al. / Sensors and Actuators B 143 (2009) 381–386
383
Fig. 2. SPR reflectivity curve as a function of angle. The bias angle is set below the
resonant angle as depicted.
a substance with larger refractive index than the water’s refractive index shifts the resonance angle towards larger values. Thus,
by setting the bias angle below the resonance angle, the reflected
optical power increases with the solution’s index.
To investigate and calibrate the 44-pass response with respect
to changes of refractive index, various concentrations of salt solutions in DI water are prepared and measured. The corresponding
pulse signal amplitude of the 22nd pulse associated with the various salt concentrations is measured and the result plotted in Fig. 3.
Since each pulse corresponds to two passes and the total number
of pulses is 22, the total number of passes is 44. Fig. 3 shows that
the differential change in amplitude increases with the number of
passes and salt concentration. The solution’s index increases with
the increase of salt concentration, which causes the reflected optical power to increase. Since the index change can be calculated
according to the salt concentration, we can calibrate the 44-pass
response with respect to index change using salt solution. Fig. 3
shows that there is a gradual increase in the baseline signal with
time, which is due to amplified spontaneous emission associated
with the erbium-doped fiber amplifier itself. If more pass is allowed,
lasing action eventually sets in, i.e., if the EOM gate is left closed for
too long, the setup eventually becomes a ring laser, destroying the
SPR function. Thus, the ultimate number of pass is limited by lasing
effects.
The signal-to noise ratio degrades as the number of pass
increases due to increased spontaneous emission. The noise in
erbium-doped fiber system is dominated by the so-called signalspontaneous beat noise and the spontaneous-spontaneous beat
noise as described in Eqs. (7.42) and (7.43) in Ref. [13]. The
total mean square current fluctuation (ıI)2 is approximately
given by the sum of the signal–spontaneous beat noise and the
Fig. 4. (a) Normalized pulse amplitude for 44 passes versus salt concentration. (b)
Normalized pulse amplitude for 44 passes versus refractive index change corresponding to the salt concentration.
spontaneous–spontaneous beat noise [13]:
2
(ıI) = IS Isp
(2)
where IS and Isp describe the average detected signal and the spontaneous emission current, respectively. Be describes the detection
electronics bandwidth (108 Hz) and B0 describes the optical filter bandwidth (1 nm, 1.28 × 1011 Hz) placed within the fiber loop
(Fig. 1). The optical filter in the loop is very important for reducing
Isp . The shot noise contribution due to IS and Isp and the thermal
noise due to the detection electronics are neglected because they
are appreciably smaller in magnitude. The signal-to-noise ratio S/N
is given by,
S
=
N
Fig. 3. Pulse amplitude versus number of passes for DI water, 340 M salt, 680 M
and 1.02 mM salt. Note that each pulse involves passing the SPR device twice. Therefore, 22 pulses correspond to 44 passes.
Be
2 Be
+ Isp
B0
B0
IS
(ıI)
2
(3)
For the 44 passes describes in Fig. 3, the spontaneous emission
current Isp (measured voltage divided by the scope’s input resistance of 2.5 × 106 ), described by the base line, increases by a
factor of two from 2-pass to 44-pass. Thus, for example, the S/N
degrades by no more than a factor of two from 2-pass to 44-pass
for the 1.02 mM case shown in Fig. 3. Specifically, with numbers
given above, the S/N for 2-pass is 108 and 69 for 44-pass. If the gain
of the erbium-doped fiber amplifier is adjusted too high, Isp can
increase dramatically eventually leading to lasing of the ring at the
optical filter frequency and dramatically degrading the signal S/N.
However, if the gain is too low, the signal decreases as the number
of pass increases. Thus, one has to adjust the gain just right.
From Fig. 3, the normalized 44-pass pulse signal with respect to
salt concentration and its corresponding index’s change are plotted
in Fig. 4. As shown in Fig. 4(a), the normalized pulse signal increase
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H. Ko et al. / Sensors and Actuators B 143 (2009) 381–386
field in the optical regime. According to Eq. (3.71) of Ref. [14], the
response of the bounded electron to the optical field give rise to a
refractive index n that can approximately be described by,
n2 = 1 +
Fig. 5. Time-dependent SPR signals for DI water. (a) 2-Pass (2nd pulse) and (b)
44-pass (22nd pulse). The applied voltage is 0.8 V.
is linearly proportional to salt concentration. The normalized pulse
amplitude corresponding to the index change is shown in Fig. 4(b).
The conversion from salt concentration to refractive index change,
n, is by the formula: nw + n = ns x + (1 − x)nw [12], where ns and
nw are the refractive index of salt and water, respectively, and x is
the salt weight fraction. The refractive index change per the normalized pulse amplitude change, 1.13 × 10−5 gives the best fit to the
measured data in Fig. 4(b). This allows us to predict the unknown
index change of a solution.
Previously, we reported a time-dependent SPR signal response
for the attraction of negatively charged ions to the SPR surface. It
has been demonstrated that the attraction of the negatively charged
ions to the SPR surface lead to an increase in the SPR signal if the
bias angle is below the resonance angle as indicated in Fig. 2. The
SPR signal increase due to the applied field indicates an increase
of the refractive index due to the negative ions. In this article, a
time-dependent SPR signal response due to the attraction of cations
(positive ions) to the SPR surface is investigated. It will be shown
that the attraction of positive ions to the SPR surface causes the
signal to decrease indicating a decrease in the refractive index in
contrast with an increase in index for negative ions. Furthermore,
the response due to positive ions is much smaller than for negative
ions, therefore, requiring more SPR passes than Ref. [10] to enhance
sensitivity.
First, the SPR signal response to a 0.8 V step voltage for DI water
is measured as shown in Fig. 5 for both 2 passes and 44 passes. The 2pass data is taken from the amplitude change of the 2nd pulse from
among the 22 pulses (note that the first do not pass through the SPR
so is not considered). The 44-pass data is taken from the 22nd pulse.
The decrease in signal is presumably due to the attraction of the SPR
surfaces of H+ ions, which at a pH of 7 has a concentration of 10−7 M.
The signal suddenly increases abruptly at about 178 s because the
voltage is suddenly switched to the opposite polarity to attract the
negative OH− ions to the SPR surface. The attraction of negative
ion is not of interest in this paper as this has been reported in Ref.
[10]. Here, the SPR signal decrease means that the refractive index
of the solution is locally reduced in the detection region. To our
knowledge, the refractive index associated with negative and positive atomic ions have never been reported, thus, here we can only
make some qualitative conjecture as to why negative ions increase
while positive ions decrease the refractive index with respect to
the neutral environment, and that the magnitude change is much
larger for negative ions compared with positive ions. The rational is
as follows: the magnitude of the refractive index of materials in the
optical regime is described by the response of the bounded or the
free electron to an exciting optical field as described in Chapters 3
and 4 of Ref. [14]. This is because only electrons and not the atomic
ions have small enough mass to react to the fast oscillating electric
Nq2
f
ε0 me ω2 − ω2
0
(4)
Important parameters we need to make use of are the optical frequency ω, the electron density N and the resonance frequency ω0
of the particular bounded electron system. More tightly bounded
electrons give higher ω0 , which, for most materials are in the
ultraviolet frequencies (Table 4.3 of Ref. [14]). The attraction of positive ions to the detection surface merely decrease N and therefore
decrease n, while the attraction of negative ions increase N and may
also simultaneously decrease ω0 because the outer electron in the
negative ion (Cl− in NaCl, for example) is not as tightly bounded
compared with the inner electrons. Since ω0 ≫ ω, a decrease of ω0
can potentially dramatically increase n, according to Eq. (3). Thus,
the detection of negative ions increases the refractive index while
the detection of positive ions decreases the refractive index with
respect to the neutral environment in the absence of an applied
voltage. It should be noted that SPR signal change induced by the
applied voltage is a function of the refractive index change, and is
a function of the concentration of cations (positive ions) in aqueous solution. Therefore, the refractive index of a concentration of
cations in aqueous solution can be estimated because the refractive index change per the fractional signal change can be predicted
from Fig. 4(b). Moreover, 44 passes offer much better sensitivity
than 2 passes, demonstrating that our device has good sensitivity
to measure the index’s change due to the attraction of cations. The
estimated refractive index decrease using the calibration data of
Fig. 4a and b indicates that the attraction of H+ cause the refractive index to decrease by roughly 2 × 10−6 at an applied voltage
of 0.8 V.
1 M KOH and 1 M NaOH are then used as test solutions for measuring the refractive index of cations in aqueous solution to see
whether the attraction of K+ and Na+ also cause the refractive index
to decrease. These solutions are chosen because K+ and Na+ ions are
important to biological processes. The phenomenon reported here
may provide a new method for studying ion exchange across cell
membrane in cells. A time-dependent 44-pass SPR signal response
for the solution are measured and compared to DI water. For
comparison, the one-pass reflectivity versus the incident angle is
verified for the KOH and NaOH solutions, respectively, then the bias
point is set to the same voltage level as that of DI water. Since KOH
and NaOH is a strong base in aqueous solution and their degree of
electrolytic dissociation are almost equal, we assume that 1 M of
K+ and Na+ ions exist in each aqueous solution. As predicted, Fig. 6
indicates that the SPR signal is decreased for both KOH and NaOH
Fig. 6. Time-dependence of the amplitude of the 22nd pulse (44 pass) for DI water,
1 M KOH and 1 M NaOH. The applied voltage is 0.8 V.
H. Ko et al. / Sensors and Actuators B 143 (2009) 381–386
385
the applied field, the current through the solution well filled with
DI water was measured as shown in Fig. 7(a). The current was
determined by measuring the voltage across a 20 k resistor that
was connected in series with the solution wells. From Fig. 7, we
can calculate
the energy generated from the joule heating using
E = V I dt where V is the applied step voltage of 0.8 V. The calculated energy E is 6.1296 × 10−5 for 120 s. The specific heat capacity
of DI water, Csp,water is 4.186 J g−1 ◦ C−1 and the amount of water in
the solution well was 0.24 g. As a result, the increase in temperature due to the applied field is about 6.1 × 10−5 ◦ C for 120 s since the
energy needed to increase the temperature by 1 ◦ C is 1.00464 J. This
results in the change in refractive index of about −6 × 10−9 , which is
not significant with the local index change of about −10−6 at the SPR
surface due to the attraction of positive ions. In the same way, the
refractive index change in 1 M NaOH solution due to the joule heating was calculated from Fig. 7(b) and it was about −2.0 × 10−7 less
than the local index change due to the attraction of Na+ ions to the
SPR surface (specific heat capacity, Csp,1 M NaOH is 3.975 J g−1 K−1 ).
Therefore, joule heating is not a contributing factor to the results
shown in Figs. 6 and 7.
4. Conclusion
Fig. 7. Current flowing in (a) DI water and (b) 1 M NaOH upon the turn-on of the
step voltage of 0.8 V.
upon applying a positive voltage to the top electrode. This is due
to the attraction to the SPR surface of K+ and Na+ ions. The degree
of signal drop in response to the external field is proportional to
the refractive index drop of cation in aqueous solution. Moreover,
according to Fig. 4(b), the estimated refractive index decrease is of
the order of about a few times 10−6 . In Fig. 6, the fractional pulse
amplitude changes are about 0.15, 0.27 and 0.45 for DI water, 1 M
KOH solution and 1 M NaOH solution, respectively. However, these
values may vary by as much as 30%, if the experiment is repeated
by recycling the applied voltage. The variation is probably due
to residual ions or molecules sticking to the SPR surface so that
each run is not exactly the same. But the downward signal trend
upon the application of a positive voltage shown in Fig. 6 is always
the same for repeated experiments. Moreover, if more accurate
measurements were to be attempted, some reference calibration
methods or temperature controlled must also be implemented as
the refractive index of water change by about 10−4 per degree C.
Here, we only attempt to demonstrate that the attraction of positive ions to the SPR surface cause the refractive index to decrease.
We choose to apply a voltage of 0.8 V so that it is below the water
electrolysis voltage of 1.23 V. Also a lower voltage protects the
integrity of the gold surface for further use. If a higher voltage is
applied, the signal magnitude will be bigger. Additionally, the pulse
signals for the solutions increase dramatically once the reversed
voltage is applied, which is due to the attraction of OH− ions to
the SPR surface. For the case of KOH and NaOH solutions, the
stronger increase in the signal compared with DI water is shown.
This should be due to the attraction of much higher concentration
of OH− ions of 1 M of KOH or NaOH than the OH− ions of DI
water.
Joule heating due to the applied field can also cause the signal to decrease because the refractive index of water decreases
with temperature. To investigate the temperature increase due to
In conclusion, a field-assist 44-pass SPR fiber optic technique
for measurement of positive ions is demonstrated. Detection of
positive ions gives a SPR signal that indicates a decrease in the
refractive index, while the detection of negative ions gives a signal that indicates an increase in the refractive index. The detection
of positive ions by the SPR technique may be a new method for
studying ion exchange in cells. The multi-pass SPR sensor system
offers sensitivity enhancement by a factor depending on the number of passes. The technique offers a method that can measure the
refractive index with improved sensitivity.
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Biographies
Hyungduk Ko received his Ph.D. in Electrical Engineering from Texas A&M University in 2009. Currently, he is a senior researcher in Samsung LED Co. Ltd. His research
interests include nanophotonics and optics, plasmonic-assistedoptoelectronic
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H. Ko et al. / Sensors and Actuators B 143 (2009) 381–386
device, solar cells and imaging and surface plasmon resonance (SPR) fiber optical
sensors.
Jun Kameoka received his Ph.D. from Cornell University in 2002. Since 2004, he
has been an assistant professor in Texas A&M University. His research interests
include bio-nanomachining, nanostructure science and engineering, nanosensors
and molecular manipulation, micro and nanofluidics and bio-nanohybrid devices
for medical applications.
Chin B. Su received his Ph.D. in Physics from Brandeis University in 1979. In
1978–1986 he was a member of technical staff in GTE laboratories. In 1986–1987,
he was a project team leader in Rockwell International Corp. Since 1987, he has been
on the faculty of the Department of Electrical Engineering at Texas A&M University,
currently, he is a full professor in Texas A&M University. His research interest is
surface plasmon resonance sensor for bio-detection.