APPLIED PHYSICS LETTERS 92, 253103 共2008兲
Polarization-dependent sensing of a self-assembled monolayer
using biaxial nanohole arrays
F. Eftekhari,1,a兲 R. Gordon,1 J. Ferreira,2 A. G. Brolo,2 and D. Sinton3
1
Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia,
Canada V8P 5C2
2
Chemistry Department, University of Victoria, Victoria, British Columbia, Canada V8P 5C2
3
Department of Mechanical Engineering, University of Victoria, Victoria, British Columbia,
Canada V8P 5C2
共Received 25 March 2008; accepted 3 June 2008; published online 23 June 2008兲
We demonstrate surface plasmon resonance 共SPR兲 sensing based on the polarization-dependent
extraordinary optical transmission through a biaxial nanohole array. The biaxial array has two
periodicities in a single array that can be individually probed by varying polarization. Here, the SPR
polarization-spectral dependence is demonstrated for the detection of a self-assembled monolayer
for four sets of biaxial array periodicities. By monitoring the polarization dependence of
transmission through the nanohole arrays with biaxial periodicity, surface-sensitive refractive index
induced intensity variations may be separated from other effects such as absorption, scattering, and
intensity fluctuations, while using a single wavelength source. Biaxial sensing is useful for ongoing
microfluidic integration of nanohole SPR, where the light source is transmitted through a
microfluidic channel. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2949682兴
Surface plasmon resonance 共SPR兲 has been used widely
in sensor applications including drug discovery1 and
biosensing.2,3 The inherent dependence of resonant surface
plasmons on local refractive index changes at the surface of
a noble metal is the physical mechanism explored in SPR
sensing. Commercial SPR systems generally operate in total
internal reflection mode using the Kretschmann geometry,
where the reflection minima shift is monitored as a function
of angle or wavelength.4 Alternatively, arrays of nanoholes in
gold films provide an attractive strategy for SPR-based sensing. Nanohole SPR benefits from a collinear optical geometry, dense integration, and the possibility of multiple sensing
elements, which enables multiplexed sensing integrated in a
microfluidic environment.5 The optical coupling between
nanohole array surface absorbed molecules has been observed as a dramatic transmission resonance change, ranging
from the visible6,7 to the terahertz8 regimes. Initial demonstrations of a nanohole array based SPR sensing have shown
a sensitivity of 400 nm/RIU,5 which is an order of magnitude
lower than obtained from commercial devices based on the
Kretschmann configuration. Recent innovations to nanohole
SPR have allowed for significant improvements in both the
sensitivity9–11 and limits of detection.12 In parallel, several
strategies for microfluidic integration of nanohole SPR for a
variety of sensing applications are underway.9,13–15 Other
than SPR sensing, subwavelength hole arrays in metal films
have been explored for enhanced spectroscopy, such as IRabsorption spectroscopy,16,17 enhanced Raman spectroscopy,18–20 and fluorescence detection.21–23
For dense and cost-effective microfluidic integration of
SPR nanohole sensing, it is desirable to use a single wavelength laser diode source and either a photodiode or detector
array to detect transmission. It is more straightforward to
integrate a laser/detector pair than a broadband light source
and spectrometer at each array. The intensity change that
a兲
Electronic mail: [email protected].
comes from a SPR shift is then used to detect molecular
absorption. The drawback of this approach is that intensity
changes from absorption, reduced transmission and optical
scattering in the microfluidic channel, and intensity fluctuations 共at the source or detector兲 cannot be isolated from the
intensity changes used for SPR sensing; in short, the spectral
information is lost. To regain some of the spectral diversity,
it is possible to integrate several different periodicity arrays
in a single microfluidic channel,13 and these may be used
for real-time detection using a laser source and a twodimensional 共2D兲 detector array.14 One array may show increased transmission, while another array may show reduced
transmission. This is useful to correct for intensity fluctuations that are not associated with the SPR sensing 共as described above兲. In addition, having multiple array periodicities allows for an extended dynamic range in refractive index
detection while using a fixed wavelength 共nontunable and
inexpensive兲 optical source. The major drawback of having
multiple periodicity arrays is that each array senses a different local region of the chip.
Here we demonstrate the use of a biaxial array for SPR
detection to provide the benefits of having two periodicities
in a single array. Extraordinary transmission through nanohole array with unsymmetrical periodicity 共biaxial array兲24
and large-scale biaxial arrays of nanoholes25 have been
shown to exhibit polarization dependency. Different enhanced optical transmission resonances of the biaxial nanohole array are probed rotating the polarization. The incidence
light is polarized along x-direction to excite the transmission
resonance of periodicity a and along y-direction for periodicity b. We demonstrate the polarization dependence of the
SPR shifts for several biaxial arrays with different periodicities. We transmit the polarized light from a broadband source
and measure the transmission spectrum. For a particular
wavelength, one linear polarization shows an increased intensity, while the other shows a decreased intensity, which
allows separation of SPR sensing resonance effects from
spurious polarization-independent changes in the intensity. In
0003-6951/2008/92共25兲/253103/3/$23.00
92, 253103-1
© 2008 American Institute of Physics
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253103-2
Appl. Phys. Lett. 92, 253103 共2008兲
Eftekhari et al.
FIG. 1. Scanning electron microscope image of a biaxial array created in a
gold film. The hole diameter is 200 nm with periodicities of a = 420 nm and
b = 380 nm along the x, and y-directions, respectively.
addition, the two different periodicities of the biaxial array
can be used to extend the dynamic range of refractive index
detection from a single array.
It is well known that the x-polarization of the electric
field excites a SP that propagates in the x-direction when
scattering off of the holes, and y-polarization excites a
y-direction SP.26,27 For these SPs, the resonance wavelength
is approximately related to the array periodicity by the SP
dispersion relation and the Bragg condition. The usual expression given for this resonance 共for example, see Ref. 28兲
is modified for the case of the biaxial array as
␭max共i, j兲 =
ab
冑i2b2 + j2a2
冑
␧ d␧ m
,
␧d + ␧m
共1兲
where a is the periodicity of the array x-direction, b is the
periodicity along the y-direction, i and j are integer mode
indices, ␧m is the dielectric constant of the metal, and ␧d is
the dielectric constant of the interface medium.
Figure 1 shows a scanning electron microscope image of
a typical biaxial nanohole array. The nanohole arrays were
fabricated by focused ion beam milling of a 100 nm optically
thick commercially coated gold film on a glass slide with a
5 nm thick chromium adhesion layer. We measured the surface roughness using atomic force microscopy and found
short-range height variations between 0.5 and 1 nm, which is
a reasonable surface quality to minimize random SP scattering. The biaxial arrays were fabricated and imaged using a
FEI 235 dual-beam focused ion beam and field emission
scanning electron microscope. The gallium ion beam was set
to 30 KeV for milling and a beam current of 50 pA. The
200 nm diameter nanoholes were fabricated in two different
periodicity arrangement, the first set has 20 nm difference in
x- and y-aligned periodicities 共a and b兲 where in the second
set this parameter has been changed to 40 nm. Since the
20 nm difference did not provide sufficient spectral diversity,
here we present the results for the 40 nm arrays only. The
smallest fabricated array periodicity is shown to emphasize
the biaxial nature of these arrays.
The broadband optical spectrum from a halogen source
was collimated and focused on to each array at normal incidence using a 40⫻ microscope objective. The area of focus
of the incident light was larger than the area of each array.
FIG. 2. Zero-order transmission of light focused onto the array of nanoholes
共200 nm diameter兲 with 520 and 480 nm periodicities along the x- and
y-directions, respectively. The light was focused at normal incidence and
collected normally by a wide-core fiber optic 5 mm from the sample. The
solid and dashed curves show the transmission before and after the adsorption of the self-assembled monolayer. A possible wavelength of operation
for the laser source at 620 nm is shown with a thin vertical line.
The zeroth order transmitted light was collected using a
fiber-coupled 共200 ␮m core兲 UV-visible spectrometer at a
distance 5 mm from the sample. A polarizer was placed before the sample to obtain the x- and y-polarized incident
light, as defined with respect to the lattice, as shown in Fig.
1. The recorded transmission was normalized to the halogen
source spectrum for each polarization.
The slides containing the biaxial arrays were first
cleaned prior using a piranha solution 关3:1 共v/v兲 sulfuric
acid:H2O2兴 and an ultrasonic bath. After being characterized
optically, the slide was immersed in a 1 mM ethanoic solution of 11-mercaptoundecanoic acid 共MUA兲 for 24 h.29
MUA is well known to attach to gold surfaces to form an
organized self-assembled monolayer. The slides were washed
with ethanol and dried using a pure nitrogen stream. The
normal transmission spectra were again measured. Following
these measurements, the slides were cleaned once again, the
MUA monolayer removed, and a transmission spectrum was
obtained to ensure the repeatability of the results.
Figure 2 shows the transmission spectra of white light
through an array of 520⫻ 480 nm2 periodicity before and
after surface modification with MUA. The incident light was
polarized along x- and y-directions. The bare gold shows a
共1,0兲 resonance peak mode of gold-air interface at 632 nm
for x-polarized incident light 共for periodicity 520 nm兲 and a
共0,1兲 resonance at 604 nm for the y-polarization 共for periodicity 480 nm兲. The resonances from the glass-gold interface
are suppressed due to the lossy chromium adhesion layer.
After the metallic surface was modified with a monolayer of
MUA, a resonance shift was observed for both the x and y
incident light polarizations, as shown with the dashed curves
in Fig. 2. The shift in the SPRs are associated with the
changes in the effective dielectric constant at the metallic
interface due to the MUA adsorption.
Figure 2 demonstrates the principle of polarizationdependent biaxial sensing. The first important observation is
that the dashed curves and the solid curves intersect at a
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253103-3
Appl. Phys. Lett. 92, 253103 共2008兲
Eftekhari et al.
TABLE I. Percentage transmission intensity change after the absorption of
MUA for biaxial arrays of different periodicities. For each array, the operation wavelength is chosen between the 共1,0兲 and 共0,1兲 resonance peaks and
the transmission changes for the x- and y-polarizations are given for that
wavelength.
Periodicity 共nm兲
␭operation 共nm兲
⌬Ix %
⌬Iy %
420⫻ 380
520⫻ 480
620⫻ 580
720⫻ 680
560
620
700
790
−4.5
−15.4
−20.8
−19.5
+5.1
+10.1
+13.6
+11.7
particular wavelength. This means that a single wavelength
source at the intersection wavelength would not be able to
detect an intensity change from the MUA absorption and so
it could not be used for SPR sensing. With the additional
polarization diversity, while the one polarization may show
no intensity change, the other polarization still shows an intensity change.
To benefit from having polarization diversity while operating at a single wavelength, it is desirable to select a
wavelength of operation between the 共1,0兲 and 共0,1兲 resonance peaks. As a result, a spectral shift from molecular surface absorption will result in a transmission intensity increase for one polarization and a corresponding decrease for
the other polarization. For example, Fig. 2 shows a possible
wavelength of operation for the laser source at 620 nm with
a thin vertical line. For this wavelength, the intensity along
the y-polarization 共for periodicity 480 nm兲 is increased by
10.1% after the absorption of MUA, while the intensity
along the x-polarization 共for periodicity 520 nm兲 is decreased by 15.4%. Table I summarizes the results for different biaxial arrays with operation wavelength varying from
560 to 790 nm.
As a future experiment, we plan to integrate biaxial arrays into a microfluidic platform including a single wavelength laser source and polarization selective optics to monitor molecular adsorption to a single array in real time. By
comparing the two orthogonal polarization intensities, a differential signal will be achieved. The main advantage of this
technique is that spurious laser intensity changes that are
present in both polarizations will become separated from the
changes coming from the SPR shift provoked by surface
modification. Hence, it is not necessary to do comparison
measurements on two discrete arrays where a separate control array is required.
In conclusion, we have demonstrated a nanohole array
SPR sensor based on the polarization dependence of optical
transmission through a biaxial array. The biaxial array benefited from having two periodicities in a single array that
could be individually addressed by varying the polarization.
Thereby, the usual spectral diversity of spectrometer-based
sensing was replaced with polarization diversity while using
a single wavelength laser source and a single array. This will
allow for decoupling of the desired intensity variations of the
analyte surface adsorption from other spurious polarizationindependent intensity variations that may arise from optical
absorption or scattering in the microfluidic channel, reduced
or increased transmission through the nanohole arrays, and
intensity fluctuations at the source or detector. In addition,
the biaxial structure allows for a broader dynamic range in
refractive index detection from the distinct 共1,0兲 and 共0,1兲
resonances, while using only a single wavelength optical
source. These results will aid in the ongoing development of
nanohole SPR sensors for integration within the microfluidic
environment.
The authors would like to thank Karen L. Kavanagh, at
the NanoImaging Facility of Simon Fraser University, for
assistance with nanofabrication. Operating funding for this
work was provided through an NSERC Strategic Grant with
the BC Cancer Agency, and Micralyne Inc. Infrastructure
funding was provided by CFI and BCKDF.
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