An integrated system capable of microfluidic actuation, detection and sensing is described which combines evanescent wave sensing with thermocapillary manipulation. Liquid droplets or streams transported across the beam path of a planar thin film waveguide, which encapsulates the microheater array, induce attenuation of the propagating waveguide modes. The attenuated signal is used to monitor droplet location, dye concentration in aqueous solutions and reaction kinetics for enzymatic hydrolysis of the sugar X-galactose by beta-galactosidase

Troian, S. M.

Valentino, J. P.

Wagner, S.

English

Caltech Authors - Main

MICROFLUIDIC DETECTION AND ANALYSIS BY INTEGRATION OF 
EVANESCENT WAVE SENSING WITH THERMOCAPILLARY ACTUATION 
J. P. Valentino', S. M Troian', andS. Wagner' 
'Department of Electrical Engineering, Princeton University, Princeton, NJ, USA 
2Department of Chemical Engineering, Princeton University, Princeton, NJ, USA 
ABSTRACT 
An integrated system capable of microfluidic actuation, 
detection and sensing is described which combines 
evanescent wave sensing with .thennocapillary 
manipulation. Liquid droplets or streams transported across 
the beam path of a planar thin film waveguide, which 
encapsulates the microheater array, induce attenuation of the 
propagating waveguide modes. The attenuated signal is 
used to monitor droplet location, dye concentration in 
aqueous solutions and reaction kinetics for 'enzymatic 
hydrolysis of the sugar X-galactose by beta-galactosidase. 
1. INTRODUCTION 
Current development of various surface tension-driven 
microfluidic devices (1 -5 J requires new and innovative 
methods for in-situ droplet detection and analysis. Given 
that portability is a major design goal for most microfluidic 
devices, such sensors should be directly incorporated into 
the chip to minimize size requirements. Sensor integration 
can also provide additional benefits such as reducing 
fabrication and packaging costs. 
The location, volume and composition of droplets can be 
detected in a number of ways. For thermocapillary 
actuation by embedded microheater arrays, the resistor rise 
time upon activation can be used to extract droplet volume, 
given the thermal diffusivity of the overlying liquid sample 
[l]. Non-uniform heating, however, can cause undesired 
liquid migration away from the sensing regions by 
thermocapillary forces. A second method relies on 
miniaturized coplanar capacitive sensors responsive to 
changes in the dielectric constant of the ,overlying liquid 
medium [6,7]. These sensors, however, can be hampered by 
large mutual capacitive effects and charge buildup within 
the droplet. Planar optical waveguides, by contrast, offer 
the possibility of non-intrusive detection and analysis. The 
main challenge, however, rests with direct integration of the 
optical coupling elements and planar waveguide with the 
device etements used for fluidic transport and mixing. The 
design described provides high sensitivity and minimal 
additional fabrication and design steps for open format 
microfluidic devices. 
Previous work on integration of evanescent wave sensing 
with liquid diagnostics has focused primarily on enclosed 
flow cells or enclosed microfluidic channels [X-1 11. In these 
studies, the liquid sample is positioned to flow continuously 
over the entire surface of the guided beam. For open 
microfluidic devices based on droplet manipulation, sample 
detection requires closed loop control of droplet position as 
well as composition. While other groups have demonstrated 
the use of planar waveguides to launch and collect light 
across enclosed fluidic channels [ 12,131, the device layouts 
are not easily adaptable to open format chips. We present in 
this paper the design and fabrication of a planar optical 
waveguide coupled with a surface tension driven 
microfluidic system that can detect droplet location, 
measure sample concentration, and monitor on-chip droplet 
reactions. 
2. THEORETICAL BACKGROUND 
Integrated Optical Waveguide Sensors 
For the planar waveguide shown in Fig. 1 and nliglrid = HI, 
light propagates by total internal reflection in an optically 
dense thin film of refractive index n2 situated in-between a 
supporting substrate and a sample (superstrate) of lower 
refractive indices, n3 and nl ,  respectively. Light can only 
propagate at specific angles I? corresponding to solutions of 
the eigenvalue equation [ 141 
where d is the waveguide thickness and m = 0, 1, 2. .  , is the 
mode number. For p-polarized light (i.e. transverse 
magnetic [TM] modes) of wavelength 5 
The reflections at each interface give rise to an electric 
field, known as an evanescent wave, whose amplitude 
decays exponentially with distance within the lower 
refractive index medium. The propagation length is 
normally confined to distances much less than 100 nm. 
Analyte concentration in the superstrate can therefore be 
measured by monitoring absorption of the evanescent wave 
energy. The absorbance A is given by 
where E is the sample's molar absorptivity, c is the molarity, 
N is the total number of reflections at the 
waveguidehperstrate interface, and L is the effective 
evanescent path length. Both N and L are a function of 
waveguide refractive index as well as mode number and 
their product yields a useful comparison to absorbance 
measurements by conventional spectrophotometers. In 
addition to absorption, the signal output can also be 
modulated by changes in the superstrate rehctive index, as 
for example in Fig. 1, where an overlying droplet covers a 
portion of the waveguide surface with a different refractive 
index, nliquid. The discontinuity in the superstrate refractive 
A = E c N L  (3) 
1 - 
T n3 n2 ' "I~"3r"liquid 
Figure 1:  Illustration of total internal reflection in thin film 
plmar waveguide with guiding angle t? 
index leads to reflective losses of the incident light as well 
as attenuation of the higher order modes [ 151. 
Thermocapillary Flow 
The droplets on the chip surface are transported using 
thermocapillary flow [1,2]. This actuation method uses a 
series of embedded thin film resistors to create a gradient in 
surface tension across the droplet surface by locally heating 
the trailing end. The induced thermocapillary shear stress, 
z = (dy /aT)VT (where T is the local temperature), pulls 
the droplet away from the heated regions toward cooler 
portions of the pathway. The liquid is confined laterally 
fiom uncontrolled spreading by lithographically patterning 
the chip surface with perfluorinated silanized self- 
assembling monolayers into hydrophobic and hydrophilic 
regions. For the studies presented here, the droplet height to 
length ratio was approximately 0.05, and the droplets were 
transported by thermal gradients of approximately 7 "Cimm. 
3. DESIGN AND FABRZCATION 
Figure 2 shows the device cross-section and experimental 
setup. The supporting substrate consisted of a 0.7 mm thick 
Corning 2737 glass slide. Thin-film titanium heaters (50 
nm) having a length, width and spacing equal to 0.8 mm, 3 
mm, and 60 pm, respectively, were patterned on the glass 
surface by electron beam evaporation. A 2 pm CycloteneTM 
BCB polymer (n = 1.56) was then spun onto the surface and 
cured at 250°C for 2 hours. The BCB polymer is used to 
planarize the device surface in order to reduce waveguide 
scattering and absorption caused by the non-unifonn surface 
with discrete heaters. A SiN, waveguiding layer (800 nm, n 
= 1.83) was then deposited using plasma enhanced chemical 
vapor deposition. Finally, the waveguide surface was 
treated with a hydrophobic monolayer and patterned to 
create 2 mm wide hydrophilic pathways for lateral 
confinement of droplets [ 1,2]. 
The fabricated chip was placed on an x-y microscope 
translation stage. A pair of in-coupling and out-coupling 
prisms (SFL57 glass, n = 1.84) was secured to the 
waveguide surface by applying pressure to their top faces 
with Delrin@ tipped spring plungers. Prism couplers are 
more amenable to lab trials than grating couplers in that no 
fabrication is required and coupling efficiency is typically 
50% or higher [16]. The waveguide parameters for the 
sensor (effective refractive index, number of modes, and 
Droplet n = 1.33 - 1.42 
800 nm SiNx n =  1.84 
0.7 mm Cominq 1737 glass 
L - - - - - v  
Liquid Droplet 
Figure 2: (a) Device cross-section (not to scale). (b) 
Experimental setup: HeNe laser light is prism-coupled into 
and out df SiN, film of microfluidic device; photodiode 
output used to monitor changes in attenuation as droplet 
traverses optical beam path. 
incident angle for each mode) were determined via the 
prism coupling method [17J For this device, three modes 
of propagation are allowed. TM-polarized 632.8 nm He-Ne 
laser light was then coupled into a single mode polarization- 
preserving fiber that was mounted to a goniometer stage 
capable of vertical and horizontal translation. The stage was 
rotated to enswe that the light from the fiber was incident on 
the prism face at an angle equal to the coupling angle of the 
m = 1 mode of the waveguide. Due to scattering, some of 
the coupled light also propagates in the m = 0 and 2 modes. 
The decoupled output was then measured via a 
photodetector at the face of the out-coupling prism. A lock- 
in amplifier was used to display the output voltage, which is 
directly proportional to the detected photocurrent. 
4. RESULTS AND DISCUSSION 
Figure 3 shows the photodiode response of an experiment 
performed for two separate 500 nl droplets of dodecane. 
The first droplet was eIectronically programed to pass 
over the guided beam at a speed of 250 p d s .  Detection of 
the droplet is indicated by the sharp dip in voltage. The 
second drop was then moved into position directly above 
the beam, held in place for three minutes by thermocapillary 
trapping [2], and finally transported away. As evident in 
the figure, the guided beam intensity dropped as the liquid 
was positioned over the beam path, remained at this lower 
level for three minutes and then almost completely resumed 
its baseline value, 
Using the BeamPROP waveguide modeling software, we 
attempted to simulate the attenuation effects caused by a 
rectangular liquid element located over a guided beam with 
similar aspect ratios, waveguide thickness and rehctive 
indices as in the experiment. Figure 4 shows an example of 
the simulated waveguide output power normalized to the 
731 
0 200 400 600 800 
Time (sec) 
Figure 3: Photodiode response to traversal of two 500 nl 
droplets of dodecane across beam path. The just  droplet 
passes at 250 ph while the second droplet is held for 3 
minutes over the beam and then moved may. 
I .o 
A 
r 
II 0.8 
E 
v - 0.6 
1 
Q 
.w 
0.4 
L 
d) 
0 
L 
3 0.2 
0.0 
0.0 0.2 0.4 0.6 0.8 1.0 
tiquid - nair n 
Figure 4: Normalized power output for m= 1 mode vs. 
refractive index difference for2 mm wide droplet (nair =l). 
launched power for the m = 1 mode. The refractive index of 
the sample liquid element was vaned from 1 to 2 in steps of 
0.1. As seen in Fig. 4, the attenuation caused by a 
modification of the superstrate refiactive index is negligible 
in the range of interest, i.e. 1.30 - 1.45. These simulations 
therefore suggest that other mechanisms, like scattering 
losses due to the wedge profile at the droplet edges, are 
responsible for the large (up to 30 %) attenuation ratios 
obtained in experiment. Different wedge profiles would 
also account for the discrepancy in maximum attenuation 
obtained for each droplet as seen in Fig. 3. 
To demonstrate the use of evanescent sensing for liquid 
concentration measurements, 400 nl droplets of aqueous 
solutions of FD&C Blue 1 dye (molar absorptivity = 1.38 x 
lo" M" cm-') were transported across the beam path at a 
speed of 250 pmls. The absorption of evanescent wave 
energy as the droplet traverses the beam is directly 
proportional to the dye concentration. The normalized 
voltage response, VNO, for droplets of different dye 
concentration is plotted in Fig. 5, where Vis the photodiode 
voltage output as the droplet is transported past the beam 
0.90F---- .75 
0.45 1 I 
a= 
0.30' I " " I 
0 400 800 1200 1600 
Concentration (pM) 
Figure 5:  Absorbance response of sensor vs. concentration 
for water droplets containing the +e FR&C Blue 1 .  Each 
data point is an averuge of three measurements. 
and YO is the reference voltage measured with no liquid 
present. The response can be increased by coupling to a 
higher order mode as well as by optimizing the waveguide 
thickness and refractive index. These studies indicate that 
the dye does not adhere to the waveguide surface since the 
signal output is observed to recover its baseline value after 
the droplet has passed. The limit of detection of 
concentration was calculated using three times the root 
mean square of signal noise and in this case was equivalent 
to about 3.8 pM. 
I 
0 750 1500 2250 
Time (s) 
Figure 6: Sensor response to enzymatic breakdown of X-gal 
at T = 22 "c. Rate of reaction estimated f iom dope of 
initial voltage drop. 
We also used absorbance measurements to monitor the 
kinetics of an enzyme reaction on the chip surface. 
Specifically, the sugar X-galactose (X-gal) was hydrolyzed 
into the products galactose and 4-Cl-3-Br indigo, using the 
enzyme beta-galactosidase @gal) as a catalyst [18]. 4-Cl- 
3-Br indigo is a deep blue precipitate [19], which strongly 
absorbs the 6323 nm line of the He-Ne laser. The 
microfluidic chip was programmed to merge one 400 nl 
droplet of a phosphate buffer soIution containing 1 unit of 
p-gal with another 400 nl drop of 200 mM X-gal in 
dimethylsulfoxide at 22 "C. The photodetector response is 
732 
shown in Fig. 6. The abrupt attenuation of the guided light, 
highlighted by the sloped dashed line, represents the initial 
reaction rate. During this period, the enzyme is primarily 
converting X-gal to the precipitate. As more precipitate is 
produced, the enzyme also catalyzes the reverse reaction 
until equilibrium is attained and the attenuation is constant. 
While not shown here, preliminary studies indicate that a 
higher operating temperature will lead to an enhanced 
absorption rate as more precipitate is produced during the 
initial reaction. The initial noisy attenuation observed in the 
experiment is primarily caused by the optical effects 
incurred during positioning and mekging of the droplets over 
the beam path. 
5. CONCLUSION 
The studies presented here demonstrate the successful 
integration of evanescent wave sensing with thennocapillary 
actuation as a non-invasive sensor for droplet location and 
composition. Similar integration can be achieved with other 
surface tension driven microfluidic devices based on 
electrowetting-on-dielectric [3,4] or dielectrophoretic 
pumping [5], which rely on open transport of droplets on the 
surface of glass or silicon wafers. CompIete device 
integration is possible by replacing the prism couplers with 
grating couplers and combining the chip with solid-state 
light sources and detectors, 
6. ACKNOWLEDGEMENTS 
This work was funded by the NASA Microgravity Fluid 
Physics Program and the US Army TACOM-ARDEC. JF’V 
wishes to thank Dr. P. K. Tien of Bell Labs for helpfbl 
discussions. SMT gratehlly acknowledges the Moore 
Distinguished Scholar Program at Caltech, where she is 
currently on leave, for the generous hospitality and financial 
support extended during her stay. 
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733 


Microfluidic detection and analysis by integration of evanescent wave sensing with thermocapillary actuation

A Monolithic Silicon Optoelectronic Transducer as A Real-Time Affinity Biosensor”,

Automated OnChip Droplet Dispensing With Volume Control By Electrowetting Actuation and Capacitance Metering”,

Capacitive Sensing of Droplets For Microfluidic Devices Based On Thennocapillary Actuation”,

Chip Vision-Optics for Microchips”,

Dielcctrophoretic Liquid Droplet Actuation and Nanodroplet Formation”,

Electrowetting-Based Actuation of Liquid Droplets For Microfluidic Applications”,

Integration of Polymer Waveguides for Optical Detection In Microfabricated Chemical Analysis Systems”,

ism Coupling Efficiencies By Direct Measurement of the Coupling Gap in Integrated Optical Structures”,

Substrates for Cytochemical Demonstration of Enzyme Activity.

Surface-Tension-Driven Microactuation Based on Continuous Electrowetting”,

Thennocapillary Actuation of Droplets on Chemically Patterned Surfaces by Programmable Microheater Arrays”,

https://authors.library.caltech.edu/4967/1/VALmems05.pdf

Microfluidic detection and analysis by integration of evanescent wave sensing with thermocapillary actuation

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