Published:June 22, 2011
r2011 American Chemical Society
dx.doi.org/10.1021/nl2014982|Nano Lett. 2011, 11, 3232–3238
Flexible Visible?Infrared Metamaterials and Their Applications
in Highly Sensitive Chemical and Biological Sensing
Xinlong Xu,†Bo Peng,†Dehui Li,†Jun Zhang,†Lai Mun Wong,‡Qing Zhang,†Shijie Wang,‡and
†Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University,
‡Institute of Materials Research & Engineering, Agency for Science, Technologies and Research, 3 Research Link, Singapore 117602
§Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
solar cells,3nanowire electronics,4and sensing circuitry.5?7Com-
pared to rigid substrates such as silicon and glass, flexible and
stretchable plastic or elastomer based substrates exhibit great
advantages of flexibility, transparency, lightweight, portability, low
hand, the recent development of transformation optics offers an
alternative arithmetic based on the topological design and manip-
ulation of light,8,9which makes flexible functional optics a promis-
ing way to control optical waves, thus leading to the integration of
functional flexible photonic devices as recently demonstrated in
strained tunability applications.10
Recently, metamaterials have been shown as an effective way
“artificial atoms” such as split ring resonators (SRRs) within a
metamaterial, one can create materials with optical properties
or composites. This has been demonstrated in microwave,11
infrared,12and terahertz regions.13Realization of electromagnetic
response for metamaterials in the visible and infrared (vis?IR)
ntegration of functional high-performance electronic devices
onto mechanically flexible and deformable substrates offers
regions may open a whole new era of photonics connected with
novel concepts and potential applications,14,15such as security
imaging, remote sensing, and switchable and transformable op-
tical frequency resonant devices.16Nevertheless it is still quite
challenging to push the metamaterial to operate in the visible
the tens of nanometers regime.
Attributing flexibility to metamaterials also has profound
interest. For example, such flexibility makes it possible to “wrap”
light-weight, transparent metamaterials around important ob-
jects as an optical cloak. The flexible metamaterials (Metaflex)
have been demonstrated mainly in microwave and terahertz
regions,17,18but with a limited access to the optical region as
discussed recently by Falco et al., who believed that electron
beam lithography (EBL) requires a rigid and flat substrate.19
Thus anindirecttransfer method was usuallyusedtoembedmeta-
materials into a flexible elastomeric matrix after the metamaterials
May 5, 2011
June 12, 2011
ABSTRACT: Flexible electronic and photonic devices have been
demonstrated in the past decade, with significant promise in low-
cost, light-weighted, transparent, biocompatible, and portable
devices for a wide range of applications. Herein, we demonstrate
in the visible?IR regime, which shows potential applications in
high sensitivity strain, biological and chemical sensing. The meta-
material structure, consisting of split ring resonators (SRRs) of
30 nm thick Au or Ag, has been fabricated on poly(ethylene
naphthalate) substrates with the least line width of ∼30 nm by
electron beam lithography. The absorption resonances can be
tuned from middle IR to visible range. The Ag U-shaped SRRs
metamaterials exhibit an electric resonance of ∼542 nm and a
magnetic resonance of ∼756 nm. Both the electric and magnetic resonance modes show highly sensitive responses to out-of-plane
resonance of 4.5 nm/nM for nonspecific bovine serum albumin protein binding and 65 nm for a self-assembled monolayer of
2-naphthalenethiol, respectively, suggesting considerable promise in flexible and transparent photonic devices for chemical and
KEYWORDS: Flexible metamaterials, split-ring resonators, strain sensor, chemical sensor, biosensor
dx.doi.org/10.1021/nl2014982 |Nano Lett. 2011, 11, 3232–3238
were fabricated onto rigid and flat substrates such as silicon and
We demonstrate in this paper that commercial flexible plastic
substrate is actually compatible for critical EBL nanofabrication
metamaterials size directly with the least line width from 80 to
30 nm. Correspondingly, the electric response resonances shift
from 975 nm (1088 nm) to 542 nm (653 nm), while the mag-
netic response resonances shift from 1687 nm (>1700 nm) to
756 nm (902 nm) for Ag (Au) metamaterials, respectively.
Due to a strong coupling in optical frequency, the electric
resonance shows an exponential dependence on the feature size.
The finite difference time-domain (FDTD) method is used to
simulate the electromagnetic response of the metamaterials,
which shows a good agreement with the measurements. We
further demonstrate that this vis?IR Metaflex exhibits a very
sensitive response to the strain, the dielectric media, and the
surface chemical and biological local environment, suggesting
photonic devices for highly sensitive strain, chemical and biolo-
gical sensing applications.
SRRs was first introduced by Pendry et al.20as an important
optical component for metamaterials which may exhibit both
commonly absent in other components such as metallic wires
and meshes. Experiments have revealed the fascinating electro-
magnetic properties of SRR combination with wires, such as
negative refractive index22and three-dimensional coupling of
SRRs.23It requires much more technological effort to push the
SRR resonances operating in the visible region than in the
microwave and terahertz regions. The geometrical parameters
that describe the U-shape SRRs demonstrated in this paper are
shown in Figure 1a. The periodicities of the unit cell along x axis
and y axis are 504 and 480 nm, respectively. In order to push the
electric response from the infrared to visible region, the whole
unit size of SRRs including the unit cell is reduced to 37.5%
(100% corresponds to original size defined in Figure 1a), which
decreases the w from 80 to 30 nm. The pattern was fabricated
using EBL on poly(ethylene naphthalate) (PEN) substrates
(Teijin DuPont Films). The PEN substrates were first sputtered
a layer of ITO with a thickness of 100 nm to decrease the
charging effect. Commercial electron beam resist PMMA (950
180 ?C.AJEOL7001F SEMequipped withananometerpattern
development, ametalfilmof Agor Au(∼30nm)witha2nm Cr
film as an adhesion layer was deposited using thermal evapora-
tion (Elite Engineering, Singapore). Figure 1b summarizes the
SEM (JEOL 7001F) images of the unit cells of the Ag SRRs for
w = 80?30 nm (from left to right, all the scale bars are 100 nm).
Figure 1c shows a typical SEM image of SRRs on PEN with a
designed w = 80 nm, where the actual width exhibits (7%
that the focus and beam alignment are very critical to achieve
good pattern fidelity.
The transmission spectra were taken on a microspectro-
photometer (Craic 2000) in the range of 300?1700 nm. To
predict the spectra thus guide us in pattern design even before
codes were performed,24with the dielectric constant of Ag based
on a Drude model obtained from Blaber et al.25The dielectric
constant of Au for calculation was obtained from ref 14. Figure 2a
displays transmission spectra as a dependence on w of SRRs for
Ag SRRs. Two main absorption peaks were identified. The one
with the shorter wavelength tuned to the visible region corre-
sponds to the electric resonance. With the increase of size, the
electric response exhibits a systematic red shift. The other
resonance in the longer wavelength has been tuned from the
mid-infrared (1687 nm) to visible region (756 nm), which
corresponds to the magnetic response and it also shows red
shift as the size of SRR increases. The magnetic response is due
to the molecular loop current when the electric polarization is
along the gap-bearing side of SRRs.12,14,21We used an unpolar-
ized light source, and thus the calculations were performed by
averaging the polarization effect for both transverse magnetic
exhibit magnetic response above the microwave region.12,13
Since the development of metamaterials, there has been an
extensive effort to push the magnetic response to a higher
frequency regime. For instance, Yen et al.13first demonstrated
the magnetic response with double ring SRRs in the terahertz
region and then Linden et al. pushed the magnetic response to
100 THz with a single ring SRR.12Enkrich et al. further pushed
the magnetic mode to the telecommunication region and de-
monstrated a record of 200 THz for the fundamental magnetic
mode and 370 THz for high order magnetic resonance for
oblique incidence.14Hence, the fundamental magnetic response
at 756 nm (396.6 THz) of our metamaterials represents the
highest frequency so far in the visible region with SRR-based
metamaterials. The weak peaks before the electric resonances in
Figure 2a on the shorter wavelength side could be the higher
order multipolar modes as suggested by Rockstuhl et al.26
values that we fabricated, suggesting a good agreement with our
Figure 1. (a) A schematic diagram of SRRs with Lx= 504 nm, Ly=
480 nm, Sx= Sy= 320 nm, w = 80 nm, and b = 128 nm. The whole unit
size and the periodicity of the unit cell were decreased accordingly in
order to push to visible regime. The actual width w is specified in the
right. (b) SEM images of the unit cells of the SRRs for w = 80?30 nm
(from left to right, scale bars 100 nm). (c) A typical SEM image (45?
tilted) of SRRs with w = 80 nm fabricated on a PEN substrate.
dx.doi.org/10.1021/nl2014982 |Nano Lett. 2011, 11, 3232–3238
experimental spectra. The slight difference is attributed to the
size distribution of the pattern resulting from fabrication. Panels
d and e of Figure 2 display the results of the experimental and
calculated transmission spectra for Au-based SRRs. The electric
response resonances shift from 1088 to 653 nm, while the
magnetic response resonances shift from >1700 nm (which is
out of our measurement region) to 902 nm. The peak positions
of the experimental spectra and simulations are extracted and
plotted in panels c and f of Figure 2 as a function of wave-
length for various w values for Ag and Au, respectively. In
Figure 2c, the circle points (black) are the experimental values
an empirical formula of λ = λ0+ a exp(w/w1). Where λ is the
resonance position, w1=30 nm is theleast width ofour patterns,
is the growth ratio demonstrating the scaling behavior. λ0
approaches the resonant wavelength of localized plasmon reso-
nance(LPR) of silver nanoparticles intherange of 400?600 nm
and is the limit of electric response when the size is further
decreased. The calculated size dependence of the resonance
position shows a similar dependence with the experiments with
the squared points (blue). Using the same formula for fitting, we
get λ0= 562 ( 21 nm and a = 31 ( 3. This empirical formula
captures the main physics embodied in our observations. Two
factors affect the scaling behavior of our SRRs metamaterials.
One is the resonance wavelength which is red-shifted with the
unit cells. From the LC resonance point of view, the resonance
frequency can be expressed as a combination of capacitance and
inductance with the formula ωLC= (LC)?1/2.12,27,28With the
increase of the unit cell size, the capacitance and inductance
other is the coupling between unit cells due to the decreasing of
periodicity. For the metallic particle?particle interaction, the
couplingbetweenthe unitcells results inanexponential decayof
resonances and blue shifts with the increasing of particle?parti-
in Figure 2a suggests that the main contribution comes from the
Recently, Falco et al.19demonstrated a Metaflex with the
electric resonance near 620 nm with a fishnet geometry, which is
less challenging to fabricate in the visible region with low quality
directly patterning on plastic substrates by EBL fabrication. To
our best knowledge, this is the first time such a short wavelength
in the visible region can be achieved for the SRR-based meta-
materials. The experimental data of electric resonance peaks in
Figure 2f show the similar dependence on the unit size for Au
metamaterials as for Ag metamaterials. The fitting for experi-
mental data yields λ0= 591 ( 36 nm and a = 37 ( 4.
The resonance peak of the magnetic response for Ag meta-
materials shows a linear dependence on size with an intercept of
approximately 177 ( 34 nm and a slope of 19 ( 1 nm as shown
in Figure 2c. The experimental point falls outside the fitting line
for magnetic resonance at 40 nm probably due to deviation of
size, especially the arms of SRRs, which will influence the
magnetic dipoles is weaker compared with the electric dipoles,
which shows exponential dependence on resonance wavelength.
From dipole?dipole interaction point of view, the quasi-static
interaction energy ΔE is written as27
ΔE ¼ γp1p2
where r is the distance between dipoles p1and p2. γ is the
interaction index, which is 1 for transverse coupling (in our case
for the magnetic resonance) and 2 for longitudinal coupling (in
our lateral coupling for electrical resonance). Considering the
electrical dipoles are stronger than the magnetic dipoles (with
sharper electric resonance compared with magnetic resonance),
the coupling of electric dipoles is stronger than that of magnetic
dipoles. Figure 2c also displays the unit size dependence of the
calculated resonance position with an intercept of 717 ( 26 nm
unit size increase and the coupling between SRRs results in the
linear dependence of the magnetic resonance peak on the unit
cellsize.Similarresultshavebeenshown forAumetamaterials as
in Figure 2f with the linear fitting.
PEN was chosen as the plastic substrate because it has a high
glass transition temperature (Tg≈ 125 ?C) than some other
PEN is also solvent, acid, and base resistant. Flexible PEN
substrate has a Young’s modulus (E) of 280 MPa (characte-
ristics of physical properties by Teijin DuPont Films Co.) and
the PEN substrates used here have thickness (t) of ∼125 μm.
The transparency in the visible and near-infrared region is over
Figure 2. (a) Experimental transmission spectra of SRRs with dif-
ferent side widths of w as shown in Figure 1 for Ag metamaterials.
(b) Calculated transmission spectra of SRRs by FDTD for Ag metama-
terials. (c) Experimental magnetic (red triangles), electric resonance
peaks (black circles), calculated magnetic (green squares), and electric
resonant peak (blue squares) as a function of w for Ag metamatierlas.
The curves in (c) are fitting curves as discussed in the text. (d)
Experimental transmission spectra of SRRs with different side widths
of w for Au metamaterials. (e) Calculated transmission spectra of SRRs
forAumetamaterials. (f)Experimental magnetic(redtriangles),electric
resonance peaks (black circles), calculated magnetic (green squares),
and electric resonance peak (blue squares) as a function of w for Au
metamaterials. The curves in (f) are fitting lines as discussed in the text.
dx.doi.org/10.1021/nl2014982 |Nano Lett. 2011, 11, 3232–3238
80%, which is adequate for transparent photonic devices. In
addition, the mechanical characteristics of PEN make it ideal to
of the out-of-plane direction.
Figure 3 demonstrates the strain sensing by PEN-based meta-
materials of Au with w= 50 nm with a deflection ratioof 1%. From
the stress δ = t/F = 4.4 ? 10?6(F is the radius of the arc after
bending), the strain σ = Eδ is approximately 1232 Pa. The electric
while the magnetic response has moved out of our measurable
region. The strain sensitivity was estimated to be ∼0.06 nm/Pa.
Tunability of metamaterials by stretching in the infrared region has
been demonstrated by Pryce et al.,10using pattern transfer of SRRs
from silicon substrate to PDMS media after EBL fabrication. Here
substrate PEN, which can also be used to integrate into a photonic
device compatible with modern optical techniques such as a remote
There are two classes of plasmon based sensing methods
reported in the literature. One is based on the surface plasmon
polariton (SPP), which has been successfully applied to bio-
LPR from the colloidal nanoparticles.34It has been shown that
the refractive index sensitivity does not exceed 100?300 nm per
RIU in visible spectral interrogation, where RIU represents
refractive index unit.35Recently, Kabashin et al.36utilized the
demonstrated an enhanced sensitivity to the refractive-index
variations of the medium between nanorods. Gu et al. also
proved that an X-shape nanohole can increase the sensitivity
due to the electric field enhancement created by LPR.37Plas-
monic nanoholes or nanorods have been used as an excellent
and glucose solution.39,40
According to the effective medium theory, the resonance
frequency of metamaterials is very sensitive to the surrounding
refractive index.36For instance, dielectric overcoating or multi-
layer dielectric sandwiching methods have been used to tune the
absorption resonance position of metamaterials in the terahertz
regime.28To investigate how metamaterials in optical frequency
spin-coated a layer of PMMA onto the metamaterials surface
PMMA in the visible and infrared regions is 1.488.41PMMA on
metamaterials will act as a good model to evaluate how the
resonances of metamaterials vary as the dielectric environment
changes,providingadirectmeasurementofthe figure of meritof
the sensitivity. We also used another photoresist (Shipley 1805)
with a refractive index of ∼1.68 in the visible and infrared
40 nm. Since Au is more inert than Ag with well established
the following sensing applications. The shift for the electric
resonance is approximately 67 nm upon PMMA coating and
that of the magnetic resonance is approximately 213 nm. The
Shipley 1805 also shows similar results with an 90 nm shift in
electric mode resonance while a 341 nm shift in magnetic
resonance mode, respectively. The sensitivity is estimated to be
137 nm/RIU and 436 nm/RIU for the electric and magnetic
responses, respectively. The resonance peak position change with
local environmental refractive index change is shown in Figure 4b.
The sensitivity of the electric response is comparable to that of the
gold nanoparticles based localized surface plasmon biosensing,35
while the sensitivity evaluated based on magnetic resonance
increases by more than twice compared with the electric response.
As a bianisotropic component,21,42SRRs show not only the
electric response due to the collective oscillations of free
electrons in metallic nanostructures, but also the artificial mag-
netic response due to the electrical field induced molecular loop
current at normal incidence. The constitutive relationship be-
tween electric component (E), magnetic component (H), elec-
D ¼ ε0εrE + ξH=c, B ¼ μ0μrH + χE=c
where εr is the relative permittivity and μr is the relative
permeability, while χ and ξ are the coupling coefficients of
Figure 3. (a) A picture of our Metaflex taken with a background of
NTU campus magazine to demonstrate the flexibility.(b) Transmission
spectra of Metaflex showing high tunability with an out-of-plane strain.
Black and red curves are the transmission spectra before and after
applying the strain, respectively.
Figure 4. (a) TransmissionspectraofMetaflex(Aufilmwithw=40nm)
dx.doi.org/10.1021/nl2014982 |Nano Lett. 2011, 11, 3232–3238
of εrchange from the microenvironments. This mechanism has
been applied for both SPP and LPR and also for the electric
resonance in SRRs.35,43On the other hand, the magnetic
response in SRRs originates from the χ, which is due to the
electrical and magnetic field coupling. χ can be modeled by a
magnetic coil with an inductance L (metal ring) coupled with a
capacitance C (the slit of the ring) and the resonance frequency
ωLC = (LC)?1/2.12,27,28Shelton et al.44suggested that the
capacitance consisted of two contributions: one is the gap
capacitance (Cg) of the SRRs, while the other is fringing-field
capacitance (Cf), which depends on the permittivity Cf? ε0
(ν)E(ν) dν and the thickness of the thin film that exists between
SRRs and the PEN substrate. The electric resonance is primarily
determined by Cf, which is highly dependent on the dielectric
the Cfbut also to the Cg. Therefore, magnetic resonance exhibits
higher sensitivity to the surrounding environment.
local effective refractive index change suggests potential applica-
tions in chemical and biological sensing. An important category
of biomolecules in biosensing is protein, which includes a variety
of biomarkers that are extremely important for disease diagnosis
and analysis.45,46To evaluate whether and how our Metaflex in
optical frequency regime detect protein molecules, we used a
well-known model protein of bovine serum albumin (BSA) with
a molecular weight of 66776 Da. The BSA powder was first
dissolved in distilled water with a concentration of 15 μM, and
then the solution was diluted to change the concentration from
the original 15 μM to 15 nM. The BSA solution was drop-casted
on the Metaflex surface, which was then dried under room
temperature. Hence, the protein molecules were nonspecifically
bound to the metamaterials surface. Transmission spectra were
taken afterward under the microspectrophotometer. After the
O2plasma treatment at 50 W for 30 s. This cleaning method was
gave identical spectra obtained from the same sample before
applying BSA. Then a different concentration sample was pre-
pared and measured.
Figure 5a displays a series of spectra of the same Metaflex
sample with different BSA concentrations. Both the magnetic
and electric response modes are significantly red-shifted. The
concentration dependence of the peak position is extracted and
plotted in Figure 5b, with red dots representing magnetic reso-
nance and black squares for electric resonance. The solid curves
are the least-squares fitting with a formula λshift= a + b[M]c,
where [M] is the concentration. The sensitivity can be approxi-
mately written as dλshift/d[M] = bc[M]c?1, for 15 nM, this can
reach 1.8 nm/nM for electric response and 4.5 nm/nM for
be further increased by metamaterial design.47For example, a
trapped resonance mode was recently demonstrated to exhibit a
high quality factor when the structural symmetry was broken,
thus higher sensitivity resulted.48
The interaction between BSA and Au metamaterials surface is
still nonspecific. Specific chemical interaction is needed to
further extend the applications of metamaterials to chemical
and biological sensing. The metamaterial Au surface can be
can be readily achievedusing thiol-terminatedfunctional groups,
e.g., thiolated biotin.49Here we further demonstrate a chemical
sensing of a monolayer of thiol-terminated molecules that are
covalently bound to Au metamaterials surface. Metamaterials-
based surface chemical sensing offers a very promising field for
sensing applications, which provides a complementary detec-
tion besides well-accepted surface-enhanced Raman scattering
Figure 5. (a) Transmission spectra of the same Metaflex sample after
different BSA concentration treatment, ranging from 15 μM to 15 nM.
The spectrum of the sample without BSA was provided as a reference.
(b) Resonance peak shift as a function of concentration with SRRs. The
solid curves are empirical fittings as discussed in the text.
Figure 6. (a) A schematic diagram of chemical sensing of 2-naphtha-
lenethiol using Metaflex. For simplicity, only one 2-naphthalenethiol
molecule was drawn on each SRR. (b) Raman spectra of 2-naphthale-
nethiol on a Metaflex sample with polarization perpendicular or parallel
as shown in (a), from 2-naphthalenethiol powder, from a clean PEN
substrate, and from a Au film on the same PEN substrate (from top to
bottom, spectra were shifted upward for clarity), respectively.
(c) Transmission spectra of SRRs with and without 2-naphthalenethiol,
the inset is the zoom-in view of the spectra around the magnetic
resonance mode. Both peak shifts due to molecule binding are labeled.
dx.doi.org/10.1021/nl2014982 |Nano Lett. 2011, 11, 3232–3238
spectra regime offers unique advantages to tune the interaction
between analyte molecules and surface plasmon, which is not
possible in thin film SPP biosensing. As a preliminary demon-
stration, 2-naphthalenethiol (Sigma-Aldrich, USA) was chosen
as a model molecule as schematically shown in Figure 6a.50To
prepare the sample, 2-naphalenethiol powder was dissolved in
ethanol with a concentration of 10 mM. The PEN-based Au
metamaterial was soaked in the solution for 24 h and then was
heavily rinsed with ethanol followed by drying with nitrogen gas.
To evaluate whether the molecule is indeedchemically bound
to metamaterials, Raman scattering spectroscopy was conducted
on the Metaflex surface using a micro-Raman spectrometer
(Horiba-JY T64,000) excited with a solid state laser (λ =
785 nm) in the backscattering configuration. The backscattered
signal was collected through a 50? objective and dispersed by a
1800 g/mm grating, and the laser power on the sample surface
was measured to be about 2.5 mW. Figure 6b shows the Raman
spectra of 2-naphthalenethiol on metamaterial with two polar-
ization configurations as shown in Figure 6a, from the powder,
from clean PEN substrate without metamaterials and from Au
film on the same chip from top to bottom, respectively. The
Raman peak around 1379.4 cm?1due to ring?ring stretching
has been chosen as a comparison.51The control spectrum from
2-naphthalenethiol powder shows a ring?ring stretching mode
at ∼1381.5 cm?1, which suggests a ∼2 cm?1red shift on the
metamaterial surface probably due to charge transfer between
gold and the molecule. Usually, chemical absorption by the thiol
molecules will modify the Raman polarizability tensor and hence
in PEN.53It is important to note that no Raman signal from
chip; hence both the thickness and reaction conditions are the
same as Metaflex, while the Raman signal from 2-naphalenethiol
molecules linked to Metaflex is exceptionally strong. This
enhancement is a typical electromagnetic enhancement in
the SERS effect.54
After that, the transmission spectra of metamaterial structures
after 2-naphthalenethiol functionalization have been taken. Com-
pared with spectra taken from bare metamaterials on PEN, the
absorption resonance peak exhibits a red shift of about 38 nm for
electric resonance mode and about 65 nm for magnetic mode as
shown clearly in Figure 6c and inset, respectively. Unlike the data
shown in Figures 4 and 5, which are due to the local change of
dielectric constant due to binding/coating of PMMA or BSA, the
significant peak shift in Figure 6 stems from only the surface
chemical modification due to a single layer of 2-naphthalenethiol,
of resonance peak of metamaterials due to a single layer chemical
molecule binding has not been well studied in the metamaterials
monolayer graphene has been observed for metamaterials gener-
ated by a focused ion beam (FIB) at the IR regime.55We believe
sensitive detection and correlation between the peak shift and the
strong SERS effect, our Metaflex photonic devices can provide
multiple channel reading for chem/biosensing applications.
In summary, SRR based Metaflex operating in the visible?IR
optical frequency regime has been experimentally realized by
EBL. We demonstrated the least line width of fabrication on
plastic substrate ∼30 nm, which exhibited the lowest electric
resonance near 542 nm and a magnetic response near 756 nm.
Flexible metamaterials, particularly the magnetic resonance of
them, exhibit high sensitivity response to strain, local dielectric
environment, and surface chemical change in the visible?IR
region, with a sensitivity as high as 436 nm/RIU, which we
attribute to the electric and magnetic field coupling in SRRs. As
two model chemical/biological sensing applications, we demon-
strate that our flexible metamaterials exhibited a sensitivity of
∼4.5 nm/nM for nonspecific BSA protein binding, and an extra-
ordinary 65 nm peak shift due to a monolayer of 2-naphthale-
nethiol molecule covalent binding. With recent progress in large
scale imprinting lithography,56our work suggests that Metaflex
operating in the visible?IR region exhibits numerous potential
applications in large scale, low-cost, transparent, and portable
photonic devices for strain and chemical/biological sensing.
Q.X. acknowledges strong support from Singapore National
Research Foundation through a NRF fellowship grant (NRF-
RF2009-06), start-up grant support (M58113004), and New
Iniative Fund (M58110100) from Nanyang Technological Uni-
PEN substrates and Dr. Daniel Aili for providing BSA sample.
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