Integrated Refractive Index Optical Ring
Resonator Detector for Capillary Electrophoresis
Hongying Zhu,†Ian M. White,†Jonathan D. Suter,†Mohammed Zourob,‡and Xudong Fan*,†
Department of Biological Engineering, 240D Life Sciences Center, University of Missouri-Columbia, Columbia, Missouri
65211, and Institute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QT, UK
We developed a novel miniaturized and multiplexed, on-
capillary, refractive index (RI) detector using liquid core
optical ring resonators (LCORRs) for future development
of capillary electrophoresis (CE) devices. The LCORR
employs a glass capillary with a diameter of ∼100 µm and
a wall thickness of a few micrometers. The circular cross
section of the capillary forms a ring resonator along which
the light circulates in the form of the whispering gallery
modes (WGMs). The WGM has an evanescent field
extending into the capillary core and responds to the RI
change due to the analyte conducted in the capillary, thus
permitting label-free measurement. The resonating nature
of the WGM enables repetitive light-analyte interaction,
significantly enhancing the LCORR sensitivity. This LCORR
architecture achieves dual use of the capillary as a sensor
head and a CE fluidic channel, allowing for integrated,
multiplexed, and noninvasive on-capillary detection at any
location along the capillary. In this work, we used electro-
osmotic flow and glycerol as a model system to demon-
strate the fluid transport capability of the LCORRs. In
addition, we performed flow speed measurement on the
LCORR to demonstrate its flow analysis capability. Finally,
using the LCORR’s label-free sensing mechanism, we
accurately deduced the analyte concentration in real time
at a given point on the capillary. A sensitivity of 20 nm/
RIU (refractive index units) was observed, leading to an
RI detection limit of 10-6RIU. The LCORR marries
photonic technology with microfluidics and enables rapid
on-capillary sample analysis and flow profile monitoring.
The investigation in this regard will open a door to novel
high-throughput CE devices and lab-on-a-chip sensors in
Capillary electrophoresis (CE) is a rapid, high-resolution
analytical tool and has been used extensively in many chemical
and biomedical applications,1,2ranging from genome sequencing,3
proteomics,4and clinical and environmental sample analysis5,6to
chemical cytometry.7Detection of CE is typically carried out at
the terminal end of the capillary.1,2However, in this detection
scheme, only the final separated analyte is analyzed, and the
detailed information regarding how samples move in the capillary
is completely lost.8,9To take full advantage of the high resolution
and low sample volume achieved by CE, on-capillary detection is
highly desirable.8,9As compared to end-capillary detection, on-
capillary detection can analyze sample separation by providing
vital information about the flow profile, such as flow rate and
dispersion, while also permitting detection with low sample
volume.10,11In particular, for CE modes that exhibit self-concentra-
tion and self-focusing effects, such as capillary isoelectric focusing,
on-capillary detection becomes critical due to its capability of direct
monitoring of self-focusing dynamics and fast sample analysis.12
On-capillary detection has been accomplished using UV
absorption and laser-induced fluorescence (LIF) spectroscopy;2,8,13,14
however, the sensitivity in UV absorption is generally low due to
the short light absorption path and small sample volume. In
addition, UV absorption suffers from nonlinear response to sample
concentration.9Although LIF is the most sensitive detection
technique, this method requires the presence of chromophores
that are either inherent to the analyte or introduced through
labeling. In the former case, intrinsic chromophores may not
readily fit the laser lines available in the lab, and their fluorescence
is normally weak, leading to the poor signal-to-noise ratio. In the
latter situation, labeling processes are laborious and may interfere
with the analyte’s biochemical functions.
Recently, refractive index (RI) has been used in CE for on-
capillary detection9,11-20and end-capillary detection,21as well as
* To whom correspondence should be addressed. Phone: 573-884-2543.
Fax: 573-884-9676. E-mail: firstname.lastname@example.org.
†University of Missouri-Columbia.
‡University of Cambridge.
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E.; Jones, M.; Dovichi, N. J. Anal. Chem. 2006, 78, 4097-4110.
(2) Landers, J. P. Handbook of Capillary Electrophoresis, 2nd ed.; CRC Press:
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Anal. Chem. 2007, 79, 930-937
Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
10.1021/ac061279q CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/19/2006
in microchip-based fluidic systems.22,23RI detection features a
number of advantages in comparison to UV or fluorescence
detection. First, it is noninvasive to virtually all analytes; second,
it is relatively simple to implement and provides a sensing signal
regardless of the existence of absorbing or fluorescent chro-
mophores. As a result, RI detection is considered to be a
universally applicable method in microfluidics detection.14,17,18,23
Moreover, RI detection measures the analyte concentration instead
of mass; the detection signal does not scale down with the sample
volume, which makes RI detection particularly attractive when
ultrasmall (pico-/nanoliter) detection volume is involved.17,23RI
detection has been implemented in the form of forward scatter-
ing,9,18,23backscattering,17surface plasmon resonance,21waveguide,22
and Fabry-Perot interferometry.24Detection limits ranging from
10-5to 10-9RIU have been reported.2,9,11-24
Although RI detection has been shown to be a promising
technique in CE development, miniaturization and multiplexing
of RI detectors have been a challenge.21The equipment involved
is usually bulky and highly sophisticated, which limits the RI
detection to only one or two spots along the capillary. Further-
more, due to relatively large dimensions of the detection spot on
the capillary, the resolution in flow spatial profile may be
In this article, we have developed a novel miniaturized and
multiplexed on-capillary RI detection architecture based on liquid
core optical ring resonators (LCORRs). The concept of the LCORR
is illustrated in Figure 1. The LCORR employs a piece of glass
capillary with a diameter of a few tens to a few hundreds of
micrometers and a wall thickness of a few micrometers. The
circular cross section of the capillary, shown in Figure 1B, forms
an optical ring resonator along the capillary. The ring resonator
supports the circulating mode of the light, called whispering
gallery modes (WGMs),25which can be launched by bringing the
LCORR into contact with an optical waveguide perpendicular to
the LCORR. The capillary wall is sufficiently thin (<4 µm) so that
the WGM evanescent field extends beyond the capillary interior
surface and detects the analyte in the capillary. Despite the small
physical size of the ring resonator, the resonating nature of the
WGM enables repetitive light-analyte interaction, thus signifi-
cantly enhancing the LCORR detection length and sensitivity. The
effective detection length Leffcan be characterized by the Q-factor
of the ring resonator, which determines the number of light
circulation cycles supported by the resonator,25
where λ is wavelength and n is the RI of the LCORR (n ) 1.45).
For example, for an LCORR with a Q-factor of 106and λ )
980 nm, Leffcan be as long as 10 cm. Such a long detection length
is achieved without sacrificing the CE resolution, in contrast to
other capillary designs such as “Z-shaped” and “bubble-shaped”
capillaries.2Moreover, as shown in Figure 1, the LCORR is
scalable to a 2-dimensional array, which not only is important for
high-throughput CE development, but also allows us to set up a
reference channel to reduce thermal induced noise, the biggest
limitation in RI detection.14,17
The LCORR is unique in that it achieves dual use of the
capillary as a sensor head and as a CE fluidic channel. The LCORR
belongs to the field of optical ring resonator sensors that has
recently been under intensive investigation for bio/chemical
detection.26-35As discussed in the Theory Section, these sensors
utilize RI as the sensing signal; the WGM spectral position shifts
in response to the RI change induced by either the bulk solution
change near the resonator surface or binding of the analyte to
the surface. A detection limit on the order of 10-7RIU and 10-6
(18) Swinney, K.; Markov, D.; Bornhop, D. J. Analyst. 1999, 124, 221-225.
(19) Krattiger, B; Bruin, G. J. M.; Bruno, A. E. Anal. Chem. 1994, 66, 1-8.
(20) Deng, Y.; Li, B. Appl. Opt. 1998, 37, 998-1005.
(21) Whelan, R. J.; Zare, R. N. Anal. Chem. 2003, 75, 1542-1547.
(22) Lenney, J. P.; Goddard, N. J.; Morey, J. C.; Snook, R. D.; Fielden, P. R.
Sens. Actuators, B 1997, 38-39, 212-217.
(23) Burggraf, N.; Krattiger, B.; de Moller, A. J.; de Rooij, N. F.; Manz, A. Analyst.
1998, 123, 1443-1447.
(24) Woodruff, S. D.; Yeung, E. S. Anal. Chem. 1982, 54, 2124-2125.
(25) Chang, R.; Campillo, A. Optical Processes in Microcavities; World Scientific
Pub Co., Inc.: Singapore, 1996.
(26) White, I. M.; Zhu, H.; Suter, J. D.; Hanumegowda, N. M.; Oveys, H.; Zourob,
M.; Fan, X. IEEE Sens. J., in press.
(27) Vollmer, F.; Braun, D.; Libchaber, A.; Khoshsima, M.; Teraoka, I.; Arnold,
S. Appl. Phys. Lett. 2002, 80, 4057-4059.
(28) Vollmer, F.; Arnold, S.; Braun, D.; Teraoka, I.; Libchaber, A. Biophys. J. 2003,
(29) Krioukov, E.; Greve, J.; Otto, C. Sens. Actuators, B 2003, 90, 58-67.
(30) Hanumegowda, N. M.; White, I. M.; Oveys, H.; Fan, X. Sens. Lett. 2005, 3,
(31) White, I. M.; Hanumegowda, N. M.; Fan, X. Opt. Lett. 2005, 30, 3189-
(32) Zhu, H.; Suter, J. D.; White, I. M.; Fan, X. Sensors 2006, 6, 785-795.
(33) Hanumegowda, N. M.; White, I. M.; Fan, X. Sens. Actuators, B 2006, 120,
(34) Hanumegowda, N. M.; Stica, C. J.; Patel, B. C.; White, I. M.; Fan, X. Appl.
Phys. Lett. 2005, 87, 201107-1 - 201107-3.
(35) White, I. M.; Oveys, H.; Fan, X. Opt. Lett. 2006, 31, 1319-1321.
Figure 1. (A) A conceptual illustration of multiplexed on-capillary
detection using a 2-dimensional LCORR array. A waveguide array
arranged transversely is in contact with the LCORR and launches
the WGM. (B) The cross section of an LCORR. The WGM circulates
along the LCORR circumference. Its evanescent field extends beyond
the interior surface of the LCORR and interacts with the analytes in
Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
RIU has been reported in microsphere-based ring resonator
sensors and the LCORR, respectively.26,34Moreover, as compared
to current capillary technology, the LCORR adds new function-
alities of multiplexed on-capillary detection, because the LCORR
has ring resonator sensors naturally integrated along the capillary.
As shown in Figure 1, the detection light, that is, the WGM, can
be launched externally through an array of optical waveguides at
any location along the capillary, providing a sensitive, noninvasive,
and quantitative tool to monitor the analyte flowing in the core in
real time. As shown later, the waveguide lateral size is only a few
micrometers; thus, the detection position can be defined along
the capillary with a precision down to micrometers, which
significantly increases the resolution in determination of the flow
profile in the capillary in comparison with state of the art.2,13,14
Assuming that the LCORR has an inner diameter of 100 µm and
a detection window of 10 µm (determined by the extension of
the ring resonator along the LCORR), the detection volume for
each ring resonator is estimated to be <100 pL.
In this article, we first present the underlying detection theory,
followed by a discussion on fabrication and characterization of
the LCORR. Then we report on the use of the LCORR with electro-
osmotic flow (EOF) as a model system to demonstrate its
capability for fluid transport, flow analysis, and subsequent
quantitative sample analysis. We show that the LCORR is highly
compatible with current CE technology and has great potential
for development of novel miniaturized high-throughput CE de-
The WGM of the LCORR can fully be described using Mie
theory by considering a three-layered radial structure (core, wall,
and surrounding medium).35-37The radial distribution of the
WGM electrical field of an LCORR is governed by
where Jm and Hm
Hankel function of the first kind, respectively. The RI of the core,
wall, and the surrounding medium is described by n1, n2, and n3.
The terms r1and r2represent the inner and outer radius of the
LCORR, respectively, and km,lis the amplitude of the wave vector
in a vacuum for the lth-order radial WGM. The resonant
wavelength λm,l) (2π)/(km,l) can be obtained numerically from
eq 2 by matching the boundary conditions at r1 and r2. Two
examples of the electrical field distribution of the WGM are shown
in Figure 2.
For the LCORR with the radius much larger than wavelength,
the WGM resonance condition can be approximated by a simple
(1)are the mth Bessel function and the mth
where r is the ring outer radius, and m is an integer number given
in eq 2, which represents the angular momentum term. neffis the
effective RI experienced by the WGM and is determined by the
RI of the core (sample), capillary wall, and the surrounding
medium (e.g., air).37Equation 3 shows that resonance occurs for
wavelengths when an integer multiple of that wavelength matches
The WGM has an evanescent field that extends beyond the
dielectric surface and into the core. The bulk solution change near
the interior surface or binding of molecules to the interior surface
leads to a change in neff, which in turn changes the WGM
resonance condition, as indicated by eq 3. Utilization of the shifts
in the WGM spectral position as the sensor signal enables a label-
free detection that conveys quantitative and kinetic information
about the flow in the core. We have developed an in-house
simulation tool based on eq 2 that allows us to simulate the WGM
behavior under various experimental conditions, such as wall
thickness, LCORR size, and the RI of the core.
To achieve adequate sensitivity, the WGM needs to have
sufficient evanescent exposure in the core. Figure 2 compares
the WGM radial distribution for a thick-walled (5 µm) and thin-
walled (2.5 µm) LCORR of 100-µm o.d. (outer diameter) using
Mie theory.35-37For the thick-wall case, the amount of the WGM
in the core is negligible and the WGM is, thus, insensitive to the
RI change in the core. In contrast, when the wall becomes thin,
a significant fraction of light is present in the core, resulting in a
much higher sensitivity (inset in Figure 2B).
Materials. Ethanol, glycerol (99%), sodium phosphate dibasic
(Na2HPO4, 99%), and hydrofluoric acid (HF, 48%) were purchased
from Sigma-Aldrich (St. Louis, MO). Hydrofluoric acid is a
particularly dangerous inorganic acid that should be handled with
(36) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small
Particles; John Wiley & Sons, Inc.: New York; 1998.
(37) Suter, J. D.; White, I. M.; Zhu, H.; Fan, X. Appl. Opt., in press.
(38) Knight, J. C.; Dubreuil, N.; Sandoghdar, V.; Hara, J.; Lefevre-Seguin, V.;
Raimond, J. M.; Haroche, S. Opt. Lett. 1996, 21, 698-700.
Figure 2. WGM radial distribution for (A) a thick-walled LCORR
(o.d./i.d. ) 100/90 µm) and (B) a thin-walled LCORR (o.d./i.d. ) 100/
95 µm). The dashed lines show the interior and exterior surfaces.
The structure of the LCORR is water core, n ) 1.333; glass wall,
n ) 1.45; and surrounding medium, n ) 1.0. Wavelength ) 980 nm.
The inset is the theoretically calculated WGM response to the RI
change in the core for the thick-walled LCORR (curve A) with a
sensitivity of 6 × 10-4nm/RIU and the thin-walled LCORR (curve B)
with a sensitivity of 11 nm/RIU.
BJm(km,ln2r) + CHm
(r e r1)
(1)(km,ln2r) (r1e r e r2)
(r g r2) }
Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
consideration for safety. All chemical agents were used without
purification and were prepared in the 18-MΩ water generated by
the Easypure-UV system from Barnstead (Dubuque, IA). The
glycerol solutions were prepared by initially dissolving the glycerol
in 0.001 M Na2HPO4buffer and then further diluting in Na2HPO4
to the desired concentrations. Fused-silica and aluminosilicate
glass tubes with 1.2-mm o.d. and ∼0.9-mm i.d. (inner diameter)
were purchased from Sutter Instruments (Novato, CA).
Fabrication and Characterization of the LCORR. Since the
thin-walled LCORRs are not readily available commercially, we
assembled an in-house pulling station that allowed us to fabricate
the LCORRs by rapidly stretching a fused-silica or aluminosilicate
glass tube while heating the center section, as illustrated in Figure
3. For a given initial tube size, the final LCORR o.d. is determined
by the pulling and feed-in speed, both of which are controlled by
a computer. For our experiments, the LCORR o.d. is designed to
be in the proximity of 100 µm. The pulling process was terminated
immediately when a sufficiently long LCORR was achieved. After
systematic investigation in pulling parameters and quality check
of the final size and the wall thickness using an optical microscope,
we found that the original o.d./i.d. ratio and circularity can well
be maintained after pulling. Therefore, the wall thickness of the
LCORR thus prepared was approximately 10 µm. With this
method, LCORRs of tens of centimeters in length can be made,
limited only by the pulling stage’s travel distance. For our
experiment, by cutting off both ends, only the center portion of
the original LCORR was used.
To further reduce the wall thickness to below 4 µm, various
concentrations of HF were pumped through the LCORR via a
peristaltic pump to slightly etch the LCORR interior wall. This
etching process, similar to the one used for fused-silica etching
reported in refs 35 and 39, was well-controlled and took 20-60
min. When the desired wall thickness was reached, pure water
was pumped through the LCORR to terminate the etching process.
Despite the thin wall, the final LCORRs still retain relatively strong
mechanical strength and can easily be handled without damage.
Experimental Setup and Detection Schemes. The details
of the experimental setup are schematically shown in Figure 4.
We used two LCORRs of 115-µm (LCORR no. 1) and 130-µm
(LCORR no. 2) o.d. with a length of ∼2 cm in the experiment.
Each LCORR was connected to two sample reservoirs (9 mm in
diameter and 5 mm in height) through UV-curable adhesives. Two
hundred volts from a high-voltage source from Spellman (New
York, NY) was placed across the LCORR, resulting in an electric
field of ∼100 V/cm. A digital ammeter from Omega (Stamford,
CT) was used to monitor the current passing through the LCORR.
An optical fiber taper with a diameter of ∼3 µm, fabricated by
stretching a single-mode optical fiber under flame,40was brought
in contact with the LCORR to couple the light from a 980-nm,
tunable diode laser from New Focus (San Jose, CA; spectral line
width <0.001 pm; repeatability <0.003 pm) into the WGM.
We have developed two approaches to detect the WGM
spectral position. In the first approach, a photodetector (no. 1 in
Figure 5) was used to monitor the light at the terminal end of the
optical fiber, whereas in the second approach, a photodetector
(no. 2 in Figure 5) was placed above the LCORR. In both
approaches, the laser periodically scanned in wavelength at a
constant power. As shown in Figure 5B, when the laser wave-
length matches the WGM resonance condition, the light couples
into the ring resonator and causes the measured transmission
power to drop, leaving a spectral dip at detector no. 1. In the
meantime, the light coupled into the LCORR is scattered off the
LCORR surface and can be detected as a spectral peak by detector
no. 2. Both the measured signals can be used to indicate the WGM
spectral position, which shifts in response to the RI change in
the LCORR core. The first approach is easy to implement, and
the second scheme is more suitable when multiple LCORRs are
used for high-throughput LCORR CE development.
In our experiment, since we used only one LCORR as a model
system, only detector no. 1 was employed. The laser scanning
rate was 5 Hz with a scanning range of 100 pm; the output power
was 1-2 mW. The entire measurement system was controlled
by a computer through a data acquisition card from National
Instruments (Austin, TX). The output power at detector no. 1 for
each scan was recorded for post-analysis using in-house spectral
(39) White, I. M.; Hanumegowda, N. M.; Oveys, H.; Fan, X. Opt. Express 2005,
(40) Cai, M.; Vahala, K. Opt. Lett. 2000, 25, 260-262.
Figure 3. The LCORR fabrication process.
Figure 4. (A) Schematic of EOF experimental setup with a single
capillary and a single optical fiber taper. (B) A picture of the
experimental setup. A thermal shield is used to reduce the thermal
fluctuation caused by air flow. White lines are drawn along the LCORR
and the taper to guide the eye. (C) A zoomed-in picture of the LCORR
in contact with the taper in the absence of the thermal shield.
Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
dip detection software. Figure 4B shows a picture of the actual
system. Note that a thermal shield was used to reduce the
temperature fluctuation induced by air convection. A zoomed-in
picture in Figure 4C shows the details of the taper-LCORR system.
RESULTS AND DISCUSSION
Characterization of the LCORR. The LCORR sensor utilizes
RI changes in the sample, which are detected by shifts in the
WGM resonant wavelength. To perform quantitative detection,
the magnitude of the WGM spectral shift must be calibrated to
changes in magnitude of the RI, which is the sensitivity curve for
the LCORR. We measured this by passing an ethanol-water
mixture with well-characterized RI into the LCORR34,41while
monitoring the resulting WGM spectral shift. To avoid any
potential problem in thermally induced WGM shift caused by Joule
heat generated by EOF, we connected the LCORR to a peristaltic
pump for sample delivery for this characterization. Nevertheless,
the detection principle is the same as described in the previous
Figure 6 shows the WGM obtained from LCORR no. 2 as an
example to demonstrate how to track the WGM response. The
line width ∆λ was 0.8 pm, corresponding to a Q-factor of 1.2 ×
106(Q ) λ/δλ). The WGM shifted to a longer wavelength when
the ethanol-water mixture with a higher RI was pumped into the
LCORR initially filled with pure water (n ) 1.333). The WGM of
LCORR no. 1 could be tracked in the same manner, and its
Q-factor was 2 × 105. As shown in Figure 7A, the sensorgrams
for both LCORRs were built by monitoring the evolution of the
WGM spectral position when the ethanol was pumped into the
LCORR, followed by water rinsing after each ethanol plug. Plotting
the RI change versus the respective WGM spectral shift, Figure
7B shows a good linear fit with a sensitivity of 6.6 and 20 nm/
RIU for LCORR nos. 1 and 2, respectively. These sensitivities
correspond to a wall thickness of 2.8 µm for LCORR no. 1 and
2.3 µm for LCORR no. 2, according to the in-house simulation
tool based on Mie theory.35,37
Glycerol Calibration Curve. Upon establishment of the
LCORR sensitivity, we are able to measure the RI calibration curve
for glycerol, the analyte used in our experiment. To avoid any
potential problem introduced by EOF, we used a peristaltic pump
to drive the glycerol of various concentrations into LCORR no. 2.
The sensorgram for each concentration was recorded, and the
WGM spectral shift was then converted to the RI change using
20 nm/RIU, as plotted in Figure 8.
Demonstration of Electro-osmotic Flow. To demonstrate
the flow transport capability of the LCORR, we employed EOF
with glycerol as the analyte. EOF plays an important role in
capillary electrophoresis, because it is responsible for the bulk
fluid transport. Glycerol was chosen, because it is an electrically
(41) Ghoreyshi, A. A.; Farhadpour, F. A.; Soltanieh, M.; Bansal, A. J. Membr.
Sci. 2003, 211, 193-214.
Figure 5. (A) Schemes used for WGM spectral position detection.
(B) Transmission signal from detector no. 1 and scattering signal from
detector no. 2 indicate the WGM spectral position.
Figure 6. WGM spectral position (from LCORR no. 2) shifted in
response to the refractive index change in the LCORR core. Curves
for t ) 2 and 6 s are shifted downward by 1 and 2 V, respectively, for
clarity. The Q-factor was 1.2 × 106and was determined by λ/∆λ,
where ∆λ ) 0.8 pm is the full-width-at-half-maximum of the WGM
resonance and λ ) 980 nm.
Figure 7. (A) Sensorgrams of the WGM response to various
concentrations of ethanol for LCORR nos. 1 (bottom curve) and 2
(top curve). Ethanol concentrations (v/v) are labeled in the figure. Top
curve is vertically shifted by 40 pm for clarity. (B) LCORR sensitivity
curve obtained with a linear fit of the data in A. Sensitivity is 6.6 and
20.1 nm/RIU for LCORR nos. 1 and 2, respectively.
Analytical Chemistry, Vol. 79, No. 3, February 1, 2007