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This is the published version of a paper published in Lab on a Chip.
Citation for the original published paper (version of record):
Carlborg, C., Gylfason, K., Kazmierczak, A., Dortu, F., Banuls Polo, M. et al. (2010)
A packaged optical slot-waveguide ring resonator sensor array for multiplex label-free assays in
labs-on-chips.
Lab on a Chip, 10(3): 281-290
http://dx.doi.org/10.1039/b914183a
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Permanent link to this version:
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ISSN 1473-0197
Micro- & nano- uidic research for chemistry, physics, biology, & bioengineering
Carlborg
Ring resonator sensors
Zhang
Droplet pair generation
Gaver
Agent-based simulation of droplet ow
McKinney and Juncker
Brain slice microperfusion
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A packaged optical slot-waveguide ring resonator sensor array for multiplex
label-free assays in labs-on-chips†
C. F. Carlborg,‡
*
a
K. B. Gylfason,‡
a
A. Ka
zmierczak,
b
F. Dortu,
b
M. J. Ba
~
nuls Polo,
c
A. Maquieira Catala,
c
G. M. Kresbach,
d
H. Sohlstr
€
om,
a
T. Moh,
a
L. Vivien,
e
J. Popplewell,
f
G. Ronan,
f
C. A. Barrios,
g
G. Stemme
a
and W. van der Wijngaart
a
Received 15th July 2009, Accepted 29th September 2009
First published as an Advance Article on the web 12th November 2009
DOI: 10.1039/b914183a
We present the design, fabrication, and characterisation of an array of optical slot-waveguide ring
resonator sensors, integrated with microfluidic sample handling in a compact cartridge, for multiplexed
real-time label-free biosensing. Multiplexing not only enables high throughput, but also provides
reference channels for drift compensation and control experiments. Our use of alignment tolerant
surface gratings to couple light into the optical chip enables quick replacement of cartridges in the read-
out instrument. Furthermore, our novel use of a dual surface-energy adhesive film to bond a hard
plastic shell directly to the PDMS microfluidic network allows for fast and leak-tight assembly of
compact cartridges with tightly spaced fluidic interconnects. The high sensitivity of the slot-waveguide
resonators, combined with on-chip referencing and physical modelling, yields a volume refractive index
detection limit of 5 10
6
refractive index units (RIUs) and a surface mass density detection limit of
0.9 pg mm
2
, to our knowledge the best reported values for integrated planar ring resonators.
Introduction
The measurement of the optical properties of liquid samples is
one of the cornerstones of analytical chemistry. Optical
absorption spectroscopy, for example, allows probing of the
chemical bonds present in a sample, and real-time refractive
index measurement enables label-free study of binding dynamics.
Accordingly, to bring these powerful analytical tools into the
hands of a wider user base, there is a strong interest in integrating
optical sensors in labs-on-chips. Even though an abundance of
optical sensor principles has been demonstrated, very few have
successfully been integrated in complete labs-on-chips.
In this work, we present the design, fabrication, and charac-
terisation of a packaged array of optical refractive index sensors,
integrated with microfluidic sample handling in a compact
cartridge, for real-time label-free biosensing. In particular, we
address three important aspects of optical labs-on-chips: (1) on-
chip multiplexing of the optical measurement for higher
throughput and referencing, (2) the chip packaging into
a cartridge, and (3) the alignment of the chip to the read-out
instrument.
In the first section below we give background on each of the
three aspects mentioned above and explain how we improve on
each of them. Next, we describe the design and fabrication of the
sensor cartridge parts and how they are combined using our
novel packaging method. We then proceed with a description of
the necessary peripheral equipment, followed by the measure-
ment principle. In the experimental section we detail the volume
and surface sensing experiments. Finally, we discuss the results,
and provide a summary and conclusion.
Background and advances in this work
The advantages of scaling analytical chemical and biological
instruments down to a single chip have been extensively explored
in recent reviews.
1
The advantages include: automation of the
analysis, increased mobility of the instrument, shorter response
times, reduced manual sample handling, and a low cost per test.
On the other hand, downscaling the instrument limits the space
available for active temperature control components. Therefore,
space consuming environmental control should be replaced by
on-chip referencing and compensation techniques.
Integration of multiple sensors for parallel operation and on-chip
referencing
To leverage the full potential of optical analysis in labs-on-chips,
their design should allow for parallel operation of multiple
optical transducers. Parallel operation not only yields higher
throughput by multiple analyses of one sample, or simultaneous
analyses of multiple samples, but also, more importantly, it
provides reference channels for drift compensation and control
experiments. Such reference measurements are particularly
a
Microsystem Technology Laboratory, KTH—Royal Institute of
Technology, Osquldas v
€
ag 10, SE-10044 Stockholm, Sweden. E-mail:
fredrik.carlborg@ee.kth.se; Fax: +46 8 100 858; Tel: +46 8 790 7794
b
Multitel a.s.b.l., B-7000 Mons, Belgium
c
Departamento de Qu
´
ımica, Universidad Polit
ecnica de Valencia, 46022
Valencia, Spain
d
Zeptosens—A Division of Bayer (Schweiz) AG, CH-4108 Witterswil,
Switzerland
e
Institut d’Electronique Fondamentale, Universit
e Paris-Sud 11, 91405
Orsay, France
f
Farfield Group Ltd, Cheshire, UK CW1 6GU
g
Instituto de Sistemas Optoelectr
onicos y Microtecnolog
´
ıa, Universidad
Polit
ecnica de Madrid, 28040 Madrid, Spain
† Electronic supplementary information (ESI) available: Supplementary
figures (S1, S2, S3). See DOI: 10.1039/b914183a
‡ These two authors contributed equally to this work.
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important for automated labs-on-chips and chips without
temperature stabilisation.
The micro-fabrication technology developed by the micro-
electronics industry provides the means to efficiently replicate
devices over a full silicon wafer. These techniques can equally
well be used to create integrated optical circuits. One type of
integrated optical sensor, that has recently been under intense
investigation for on-chip label-free detection, is the planar
waveguide ring resonator.
2–5
Due to its small footprint and ease
of integration with other on-chip optical and fluidic functions,
the ring resonator is a particularly interesting optical sensor for
labs-on-chips.
In ring resonators, light propagates in the form of circulating
waveguide modes. The circulating waves add constructively at
those wavelengths that are divisors of the ring circumference.
These are referred to as the ring resonance wavelengths.
Even though the light is guided in the high refractive index
waveguide core, by total internal reflection at the boundary
between the core and the low index material that surrounds it,
a fraction of the light, known as the evanescent field, extends out
from the core. The evanescent field extends a few hundred
nanometres into the surrounding media, and thus the refractive
index of a sample close to the core surface influences the prop-
agation of light in the ring. In contrast to Mach–Zehnder
waveguide interferometers, the degree of interaction of light and
sample in ring resonators is not limited by the physical length of
the sensing waveguide, but rather by the number of revolutions
of light in the resonator, characterised by the quality factor Q.
Ring resonators are essentially refractive index sensors, but
since the refractive index of aqueous protein solutions is linear
with density
6
they can also be used to measure the mass of
protein binding on their surface. Thus, depending on whether the
surface of the ring is functionalised with specific chemical
receptors, ring resonators can be used either for non-specific
volume refractive index sensing or specific surface sensing. Label-
free surface sensing with single planar ring resonators has been
accomplished by multiple groups.
3,4,7
However, multiplexing of several ring resonator sensors on
a single chip, integrated with microfluidics, has, to our know-
ledge, not been reported previously. Recently, five ring sensors
were integrated on a single chip, each connected to individual
input and output optical fibres
5
but no fluidics were integrated
and light splitting was handled off-chip. Multiplex operation of
ring resonators made in glass capillaries has been suggested, but
so far only multiple detection of the same sample has been
shown.
8
The sensitivity of waveguide ring resonators, that is the shift of
resonance wavelength per refractive index or mass unit, depends
on the extent of overlap of the evanescent field with the sample.
4,9
As illustrated by the waveguide cross-section in Fig. S1,† most of
the optical power in conventional strip waveguides propagates in
the solid waveguide core, while only a small fraction propagates
in the liquid sample. The overlap can be increased by using
optical slot waveguides—a recent development in the field of
integrated optics.
10
A slot-waveguide consists of two rails of
a high index material separated by a low index slot region of sub-
wavelength width. With proper design, this double core structure
acts as one waveguide and supports only the lowest order
transverse electric (TE) and transverse magnetic (TM) modes,
with a considerable fraction of the power of the TE mode
propagating in the low index slot.
We have previously reported on volume refractive index
sensing with a single slot-waveguide ring resonator.
11
The
reported sensitivity of 212 nm resonance wavelength shift per
refractive index unit (RIU) is three times that of a recently
reported conventional strip waveguide ring resonator sensor.
4
Furthermore, we have demonstrated the utility of slot-waveguide
ring resonators for label-free surface sensing with a detection
limit of 28 pg mm
2
for bovine serum albumin antibody (anti-
BSA) captured on the waveguide surface.
12
In this work, we integrate several slot-waveguide ring reso-
nators with microfluidics on a chip, and characterise the sensor
chip by multiplex volume and surface sensing experiments.
Sample handling and packaging
One of the prerequisites for portable analysis platforms is the
integration of sample handling on-chip in order to reduce size
and simplify the analysis. To this end, manual sample and
reagent handling are replaced by a microfluidic network that
precisely handles sub-microlitre volumes.
In the design of microfluidic networks it is important to
consider the mass transport of analyte to the transducer in order
to increase its dynamic range towards fast reaction kinetics.
13
When free analyte binds to the transducer surface it needs to be
replenished from the bulk solution. If the binding reaction is
faster than the diffusion of analyte down to the transducer, the
reaction rate will be limited by the diffusional mass transport of
analyte down to the sensor. This will lead to a slower response
of the transducer and risk of underestimating the reaction rate of
the binding reaction. The relation between diffusion and binding
kinetics is defined by the Damk
€
ohler number, Da ¼ k
on
C
s0
h/D,
where k
on
is the association rate of the binding reaction, C
s0
is the
surface concentration of binding sites, h is the height of the
reaction chamber and D is the diffusion constant of the analyte.
If Da [ 1, the reaction will be completely limited by diffusion.
Advective transport of the analyte over the transducer is often
required to lift the diffusion limitation. If the convection of
analyte is faster than the diffusion, the concentration gradient
will be small at the transducer surface, and the binding reaction
rate will dominate the dynamics of the transducer read-out. The
relation between advection and diffusion of the analyte is defined
by the P
eclet number, Pe ¼ Uh/D, where U is the average flow
speed, h is the height of the channel and D is the diffusion
constant. If Pe [ 1, advection dominates the flow and the
concentration gradient at the surface is small.
To accurately determine dynamics for fast reactions and to
speed up the time-to-result, diffusion limited micro-wells are not
sufficient and a microfluidic network is required to provide the
advective transport. Moreover, to be able to run several different
samples simultaneously and reference the different transducers to
each other we need a multichannel network where single trans-
ducers can be individually addressed.
Soft lithography is an established method for high precision
fabrication of such microfluidic channels in soft polymers.
The most commonly used material is poly(dimethylsiloxane)
(PDMS), a bio-compatible, transparent, rubber-like polymer
in which features can be replicated down to nanometre
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dimensions.
14
Furthermore, a clean room is not needed for the
casting process, which helps keeping the cost low.
The properties of PDMS allow precise control of the liquid
front in pressure driven flows. The polymer surface can be acti-
vated in an oxygen plasma to expose hydroxyl groups (–OH) that
can form covalent siloxane bonds (Si–O–Si) when brought in
contact with silicon or glass substrates.
15
However, many of the
PDMS properties are unsuitable as a protective packaging
material for a cartridge,
16
in particular it deforms easily under
pressure. A hard outer shell is therefore typically needed.
Unfortunately, the rubber-like, low-energy surface of PDMS
adheres poorly to many materials used in packaging, such as
poly(methyl methacrylate) (PMMA) or poly(urethane), even
with plasma activation. Unpatterned PDMS can be bonded to
PMMA using hot embossing and heating over the glass transi-
tion temperature of PMMA,
17
but this will cause deformation of
the PMMA.
Because of its simplicity, the prevalent packaging solution for
labs-on-chips based on PDMS or other rubber-like polymers is
to clamp the soft polymer layer between the sensor chip and
a hard plastic shell. The amount of clamping is then often
adjusted with precision screws.
4
Although this method is flexible,
it is limited to low liquid throughput, because of the leakage risk
at high pressure between the PDMS and the hard plastic, in
particular at tightly spaced fluid ports. There is also a risk that
shallow channels in the soft material will be blocked if the
package is unevenly clamped. A direct bonding method, via
a thin intermediate adhesive layer without high temperature
curing, would be less complicated, ensure leak-tightness, and
eliminate the blocking risk.
Recently, a PDMS/tape composite was presented
18
that
combines the attractive properties of PDMS with the adhesive
flexibility of acrylic double sided tape. However, it requires an
intermediate thin layer of PDMS to be spun and cured on the
tape to enable adhesion to the bulk PDMS component. The
reason is that the acrylic glue adheres poorly to rubbery, low-
energy surface materials such as PDMS.
In this work we solve the adhesion problem to the packaging
material by introducing a dual surface-energy adhesive film,
where one side of the film adheres directly to the PDMS and the
other to a hard plastic shell used to package the chip.
Coupling of light into labs-on-chips and alignment with read-out
instrument
To limit chip cost and complexity, optical sensors in labs-on-
chips usually rely on off-chip light sources and detectors. Thus,
a complete measurement system must include some means of
coupling light in and out of the chip. Because of the sub-micro-
metre cross-sectional dimensions of the single-mode waveguides
used in ring resonators, coupling light into the chip is more
challenging than coupling it out, and providing enough light to
each of the integrated resonators becomes ever more challenging
as their numbers grow.
Optical fibres are well suited to transport light from an off-chip
source to the packaged chip, and light can be coupled into
a planar waveguide by aligning a cut single-mode fibre to it at the
chip edge, as illustrated in Fig. S2(A).† However, since the 7 mm
mode diameter of a single-mode fibre is much larger than the
sub-micrometre mode diameter of the on chip waveguide, only
a small fraction of the fibre mode will couple into the waveguide.
There are two ways to improve the coupling efficiency: reduce the
mode diameter of the incoming beam at the edge to match that of
the waveguide on-chip, or expand the mode diameter of the
waveguide at the edge to match that of the fibre.
The first option has been implemented with lensed fibres that
focus the transmitted beam onto the waveguide end face.
11
This
solution is, however, not practical for user replaceable cartridges,
since sub-micrometre alignment tolerances cannot be kept
between cartridge replacements. Furthermore, labour intensive
polishing of the input waveguide end face is required for efficient
coupling, thus effectively negating any potential cost benefits of
mass production with silicon micro-fabrication technology.
Surface grating couplers can be used to exploit the second
option. The mode is expanded in the plane of the chip surface by
a waveguide taper, as illustrated in Fig. S2(B).† A suitably
designed grating, etched into the expanded waveguide section
alters the propagation direction of the light, allowing the input
fibre, or free space optics, to be positioned almost perpendicular
to the surface. With the effective coupling length of the grating
adjusted to the size of the mode field from the fibre, it also
matches the out-of-plane mode sizes. In this arrangement, the
overlap of the waveguide mode and the fibre mode is much
larger, providing an improved coupling efficiency. More impor-
tantly, since the area of overlap has been scaled up quadratically,
the alignment tolerances have been greatly relaxed.
In this work, we use a fully etched input grating designed for
a high coupling efficiency, a large coupling angle tolerance, and
simple fabrication.
19
The less critical out-coupling is done by
imaging the waveguide end faces at a chip edge onto a one
dimensional (1D) photodiode array. No polishing is required for
the fabrication of the chips and the compact cartridge can be
quickly inserted into the read-out instrument and automatically
aligned by a two dimensional translation stage.
Design, fabrication, and assembly of the sensor
cartridge
The novel sensor cartridge design, utilising the dual surface-
energy adhesive film bonding method, is shown in Fig. 1. The
exploded view of the system shows the sensor cartridge posi-
tioned on the alignment platform of the read-out instrument. The
cartridge is a stack of 4 bonded layers: the optical chip rests on
a temperature controlled alignment platform in the read-out
instrument and aligns to three pins protruding from it. The
microfluidic distribution layer supplies each optical transducer
site with sample. It is bonded directly to the optical chip surface
and to the hard plastic shell by an intermediate adhesive film.
Light from a tunable laser is coupled into the optical chip from
above and collected from the long edge of the chip by imaging the
cut end faces of the 8 output waveguides on a 1D photodiode
array. Fluidic ports for sample injection are formed by steel tubes
glued into the hard plastic shell.
Optical chip
The optical chip consists of a silicon substrate, with the inte-
grated optical components etched into a silicon nitride thin-film,
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