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Polymer waveguide backplanes for optical sensor interfaces inmicrofluidics{{

Kevin S. Lee,* Harry L. T. Lee and Rajeev J. Ram

Received 28th June 2007, Accepted 14th August 2007

First published as an Advance Article on the web 21st August 2007DOI: 10.1039/b709885p

A polymer optical backplane capable of generic luminescence detection within microfluidic chips

is demonstrated using large core polymer waveguides and vertical couplers. The waveguides are

fabricated through a new process combining mechanical machining and vapor polishing with

elastomer microtransfer molding. A backplane approach enables general optical integration with

planar array microfluidics since optical backplanes can be independently designed but still

integrated with planar fluidic circuits. Fabricated large core waveguides exhibit a loss of 0.1 dB

cm21 at 626 nm, a measured numerical aperture of 0.50, and a collection efficiency of 2.86% in an

n = 1.459 medium, comparable to a 0.50 NA microscope objective. In addition to vertical couplers

for out-of-plane collection and excitation, polymer waveguides are doped with organic dyes to

provide wavelength selective filtering within waveguides, further improving optical device

integration. With large core low loss waveguides, luminescence collection is improved and

measurements can be performed with simple LEDs and photodetectors. Fluorescein detection via

fluorescence intensity with a limit of detection (3s) of 200 nM in a 1 mL volume is demonstrated.

Phosphorescence lifetime based oxygen detection in water in an oxygen controllable microbial cell

culture chip with a limit of detection (3s) of 0.08% or 35 ppb is also demonstrated utilizing the

waveguide backplane. Single waveguide luminescence collection performance is equivalent to a

back collection geometry fiber bundle consisting of nine 500 mm diameter collection fibers.

1. Introduction

Luminescence based sensors are used for determining environ-

mental parameters such as dissolved oxygen or pH in

biological systems as well as for providing markers necessary

for cell identification and sorting. Such optical sensingtechniques can provide real time monitoring capabilities and

dynamic control without disturbing a biological systems

equilibrium or introducing contaminants into the system. As

these new lab-on-a-chip systems become more integrated, the

number of sensors required increases, leading to an increase in

size and complexity of the off-chip detection systems. While

CCD camera imaging is well suited to applications with slow

timescale (,100 Hz) intensity measurements, spatially dis-

tributed systems requiring measurements at both high speed

and low intensity, such as fluorescence lifetime or flow

cytometry, need the speed and sensitivity of photodiodes and

non-imaging optics.1,2 Fiber optics, or more generally,

waveguides are typically used to decouple the location ofmeasurement points and photodetectors, increasing design

flexibility. In addition waveguides can reduce the number of

photodetectors in these systems by routing multiple signals to

a common photodetector.

One common approach utilizing fiber optics places fibers

close to fluorescent sources to provide excitation light as well

as route collected signals to detectors with minimal loss.

However, in general, luminescence collected by a single fiber is

very low. In one instance, a 550 mm fiber performing excitation

and detection was only 0.0009% efficient in detecting

fluorescence from an oxygen sensitive ruthenium complex2

due to direct dye deposition on the small fiber. Fiber bundles

can overcome the low collection efficiency of single fibers when

collecting light from a distributed source, but typically require

many fibers, which can be expensive to assemble and not

useful when high spatial resolution is required. For example, a

fiber bundle system designed to collect fluorescence from

stained tissue samples consisted of thirty 100 mm fibers and still

required a photomultiplier tube to perform detection.3 In

addition, large fiber bundles are impractical for devices

requiring multiple sensors.

We have developed a platform of waveguide based passive

optical components utilizing the same polymer materials usedto fabricate microfluidic systems. Integration of excitation and

collection optics into microfluidic systems improves alignment

tolerances with external optical elements and also increases

system design flexibility. Many different designs have been

proposed for absorption or luminescence detection,4 including

many waveguide based geometries such as waveguide arrays,1

right angle collection,5 in-line absorption,6 evanescent cou-

pling,7 and hollow prisms.8 While each of the waveguide based

systems demonstrated optofluidic integration, all of the

designs either required waveguides to be in-plane or in direct

contact with fluid channels, or required fluidic channels to be

MIT, EECS, 32 Vassar St. 26-459, Cambridge, MA, 02139, USA.E-mail: [email protected]{ The HTML version of this article has been enhanced with colourimages.{ Electronic supplementary information (ESI) available: Maximumwaveguide width calculation, loss and roughness measurements andsystem measurements. See DOI: 10.1039/b709885p

PAPER www.rsc.org/loc | Lab on a Chip

This journal is The Royal Society of Chemistry 2007 Lab Chip, 2007, 7, 15391545 | 1539


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redesigned to accommodate waveguides. Such designs make

the layout of integrated optical systems dependent on fluidic

component locations and hinder device scalability. Previous

designs also did not consider the filters required to implement

on-chip fluorescence detection and often required lasers and

expensive detectors to detect a signal. In contrast to the strictly

planar designs demonstrated previously, waveguide back-

planes with vertical couplers allow for the integration of opticsinto fluidic systems with minimal impact on the fluidic design.

Using a polymer optical backplane, we will demonstrate a

potentially disposable high sensitivity lifetime based lumines-

cence detection system utilizing only light emitting diodes

(LED) and silicon photodetectors.

2. Design

In order to maximize the signal to noise ratio of the detection

system, stray excitation light must be minimized at the detector

and luminescence collection must be maximized. A back-

scattering collection geometry satisfies the first requirement

because only interface reflections contribute to stray light. For a

planar waveguide system, a backscattering collection geometry

can be achieved by forcing the light out of plane utilizing

waveguide vertical couplers. Due to the low index contrast of

the waveguides, the vertical couplers are metallized to maximize

efficiency. The backplane design is shown in Fig. 1a.

The second requirement, maximizing luminescence collec-

tion, depends on the location and behavior of the fluorescent

source. To demonstrate the backplane concept, we assume

little knowledge of the location of the emitting source and

optimize for collecting luminescence located 1 mm above the

backplane, or the thickness of a typical glass slide. Under this

constraint, the minimum core size of the integrated wave-

guides, which maximizes efficiency, is determined by the

distance h from the emitting source and the waveguide

numerical aperture NA = (ncore22 nclad

2)1/2 which specifies

the sine of the maximum input angle to the waveguide. With

ncore = 1.56 and nclad = 1.459 for the materials used (section 3),the NA is 0.55. Through ray geometry, eqn (1) for the

minimum width can be derived taking into account coupling

through next = nclad instead of air. This equation is derived in

the ESI.{ At h = 1 mm, the minimum core width is

approximately 800 mm.

w~2hffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

next=NA 2{1

q (1)

Sensor excitation and collection efficiency is also influenced

by the waveguide vertical coupler bend angle. While optical

backplanes developed for other interconnect applications9

focus on coupling as much power out of the waveguide as

possible, a backplane for luminescence detection focuses on

maximizing both sensor excitation and collection of sensor

emission. To meet this condition, the sensor should be spatially

illuminated where the collection waveguide is most efficient.

This occurs when the excitation and collection waveguide bend

angles result in intensity profiles which overlap at the sensor

plane.

To determine the spatial distribution of excitation light

incident on the sensor, ray tracing simulations were performed

starting with a point source located at the waveguide input

and ending at the sensor surface. For the collection efficiency

of the output waveguide, the efficiency needs to be determined

for each position on the sensor plane. Rays representing

point source emissions were placed along the sensor plane, asshown in Fig. 1a, and propagated through the collection

waveguide. The ratio of light exiting the output/filter

waveguide to light emitted by the point source determines

the collection efficiency at a particular point on the sensing

plane. By performing this simulation for points throughout

the surface plotted in Fig. 1b and 1c, a contour of the

collection area efficiency versus sensor position can be

determined. Fig. 1b and Fig. 1c show contours of the

excitation profiles (dashed lines) and point source collection

efficiencies (solid lines) at the sensing plane for 45u and 30u

vertical couplers.

For oxygen sensors used in this paper, fluorophor emission

is collected through a polystyrene encapsulating layer, 100 mmglass, and then 1 mm of polydimethylsiloxane (PDMS). For a

well centered point source above this specific material stack, a

multimode fiber or microscope objective with a 0.55 NA

achieves a collection efficiency of 3.13% into the first lens

calculated from ray optics10 ignoring reflection effects. In ray

tracing simulations, the 30u coupler, 1 mm2 collection

waveguide design presented here achieves a maximum point

source collection of 2.86%, close to the multimode fiber

maximum, suggesting that this design would be suitable for

cytometry systems using similar NA single lens collection

geometries, which achieve collection efficiencies of 4%.10

Fig. 1 (a) A side and top view of the integrated oxygen sensor chip is

shown. The waveguides are designed to excite and collect luminescence

from the sensor located 1 mm above the silvered bends. (b,c) Ray

tracing simulations of the intensity distribution for the input

waveguide excitation and point source collection efficiency of the

output waveguide at the sensing plane for 45u bends and 30u bends,

respectively. System performance increases by 200% after replacing 45u

bends with 30u bends.

1540 | Lab Chip, 2007, 7, 15391545 This journal is The Royal Society of Chemistry 2007


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While the point source collection efficiency is large, the

overall system performance also includes the excitation

efficiency. To include excitation efficiency, the system effi-

ciency is recalculated as the ratio of emission leaving the

output waveguide to excitation entering the input waveguide.

Efficiency at the sensor plane is determined by taking the

overlap of the excitation and collection areas given in Fig. 1c.

The total system efficiency resulting from a maximum angulardistribution excitation is only 0.40% with 45u couplers and a

factor of 2.8 times better at 1.14% for optimized 30u couplers.

To further improve the collection efficiency of the waveguides,

the collection waveguide can be expanded in width to

maximize collection given the large area sensor illumination.

A width expansion to 3 mm brings the system efficiency to

1.73%. While the collection efficiency of this waveguide system

is approximately half of the maximum achievable with a

microscope (3.13%), it provides a compact design, incorporat-

ing both excitation and collection.

3. Fabrication

As mentioned in section 2, the 1 mm standoff between the

sensors and waveguides requires that waveguides be fabricated

on a mm scale to perform efficient luminescence collection.

Standard lithography processes are not amenable to fabricat-

ing master molds with features of this size since mm

thicknesses require multiple spin coating and exposure steps.

In SU-8 processes, layers of 100 mm thickness already suffer

from substrate delamination due to stress.11 As a result, a

computer numerical control (CNC) machining approach is

used to fabricate the mm thick master molds necessary for

polymer molding.

The general waveguide fabrication process is shown in Fig. 2.

Polycarbonate positive master molds were machined using a

CNC mill (Sherline 2000) and a 1 mm diameter square end mill

(Microcut USA) to fabricate 800 mm 6 1 mm channels or ribs

(Fig. 2a). To make the waveguide mold as smooth as possible,

milling was performed around the waveguide structure,

defining the sidewalls only, with a first pass at 2000 rpm and

a 25 mm min21 feed speed, and a second pass removing an

extra 100 mm at 2000 rpm and a 50 mm min21 feed speed to

improve finish quality. In this way, we took advantage of the

pre-polished polycarbonate surface as one of the waveguide

walls. 30u vertical couplers are then milled with 120u full angle

drill mills (Microcut USA) at each detection location with the

same machining parameters.

Roughness due to milling was then reduced through solvent

vapor polishing where a vapor of methylene chloride diffuses

into the plastic surface and causes reflow12

(Fig. 2b). Thedegree of polishing could be controlled by varying the solvent

pressure or also by varying the exposure time. Under optimal

polishing conditions, the roughness average (Ra) from sanded

polycarbonate samples could be reduced from 1000 nm to

70 nm.13 By combining vapor polishing with CNC milling, this

method becomes viable for both optical and microfluidic

master mold fabrication.

PDMS (Dow Corning Sylgard 184) was used to replicate the

polycarbonate master molds, forming the waveguide cladding

(Fig. 2c). Since the refractive index difference between PDMS

(n = 1.459 at 600 nm via ellipsometry) and the core material,

polyurethane (NOA 71 Norland Products) (n = 1.56 given by

manufacturer), was too low to reflect light out of plane using

total internal reflection, the vertical couplers were coated with

300 nm of silver to improve reflectivity. By first applying air

plasma to modify PDMS into a glass-like layer (Fig. 2d), high

quality silver films could be deposited with e-beam evapora-

tion (Fig. 2e) on the molded PDMS cladding.14 A 100 mm sheet

of PDMS, made by spin coating onto a fluorinated silicon

wafer, was then air plasma bonded to the top cladding mold to

seal the channels and provide a flat surface for interfacing with

fluidic chips (Fig. 2f).

Once the hollow channels in PDMS were complete, the

waveguide core material was introduced. Before waveguide

core injection, all of the PDMS channels were surface treated

with air plasma to increase their surface energy. This reduces

the contact angle between liquids and cured PDMS, increasing

capillary forces as well as binding strength at the core cladding

interface.15 Without first applying a surface treatment, bubbles

formed along the sidewalls and delamination of the core from

the cladding occurred due to shrinkage after curing. In this

work, no bubbles were observed, and while 6% shrinkage

occurred, there was no sign of delamination. For all

waveguides, NOA71 was filled into the channels via direct

injection and capillary filling (Fig. 2g).

For collection waveguides with integrated filters, NOA71

was first directly mixed with an organic dye (pinacyanol

iodide) which absorbs at 600 nm in order to provide integrated

filtering of the excitation light. The dye was allowed to saturate

the prepolymer. Integrating filters into the waveguides canimprove both coupling and collection efficiency by removing

1 to 5 mm thick external filters and allowing LEDs and

detectors to be placed closer to the waveguide facets. This can

eliminate reflection losses from external filters and reduce free

space divergence losses associated with an increased path

length. Similar dye doped polymer filters have also been

produced for bulk PDMS layers.16 After the channels were

filled with core material, the waveguide array was cured under

UV illumination. While diffusive mixing between dye doped

and clear NOA71 occurred before complete cure was reached,

the dye was still able to fully absorb excitation light. No

Fig. 2 The fabrication process for making large core polymer channel

waveguides with a CNC milling process. The core preparation process

for providing integrated filter materials is not shown.



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photobleaching of the dye absorption was observed after a 24 h

UV-exposure.

After core curing, the waveguide ends were cleaved to the

appropriate size (Fig. 2h). Since cleaving elastomeric wave-

guides results in poor quality end faces, additional NOA71 was

deposited on each end and cured to repair the cut ends (Fig. 2i).

This smoothed each of the end faces and improved input

coupling dramatically.The fabricated device shown in Fig. 3 consists of a

waveguide backplane interfaced with a fluidic device capable

of controlling dissolved oxygen concentration. Design and

fabrication of this fluidic system has been previously

described.17 The fluidic device consists of 4 wells in parallel

with integrated peristaltic mixers separated by a thin gas

permeable membrane. Pressurized gas actuation of the

membrane translates into diffusion of the gas into the fluid

in the well. By placing an oxygen sensor in each well, the

oxygen concentration of the fluid can be measured and

controlled with changes in gas concentration. This microfluidic

system is ideal for testing lifetime based sensors since the

dissolved oxygen in the wells of the chip can be controlled. The

backplane contains one waveguide sensor for each well

coupled to the oxygen sensor inside the well.

The oxygen sensors used for this experiment are composed

of a mixture of platinum(II) octaethylporphine ketone (PtOEP-

K Frontier Scientific) and polystyrene deposited on glass disks.

Glass disks were etched with HF prior to deposition of the

polymer/dye solution to enhance binding of the fluorophor

film to the sensor substrate. This phosphorescent dye has a

maximum absorption at 592 nm and emits at 759 nm with a

12% quantum yield.18

4. Results and discussion

Before using the waveguide backplane for luminescencemeasurements, the fabricated system must be characterized.

We first measure straight waveguide test structures to

determine the loss, numerical aperture, and surface roughness

effects of the fabricated waveguides. We then characterize the

waveguide filters and the silvered vertical couplers.

Total system performance is then characterized, including

the input and output coupling efficiency between an LED and

the backplane. After demonstrating that luminescence collec-

tion can be performed above the noise limit of the detection

system, the backplane is integrated with a fluidic chip capable

of performing dissolved oxygen control in water and

measurements of oxygen concentration are performed.

A. Waveguide characterization

Fabricated large core waveguides (800 mm 6 1 mm) exhibited

a propagation loss of 0.10 dB cm21 at 632 nm from a HeNe

laser via the cut-back method. The measured waveguide

numerical aperture (NA) was 0.50 (50% intensity). The NA

reduction from the expected 0.55 to 0.50 in the large core

waveguides reduces the collection efficiency and most likely

results from sidewall roughness still present after the polishing

process. By assuming uniform scattering within the core and

ignoring interface effects at the waveguide ends, the scattered

light from the waveguide is also directly proportional to the

power inside the waveguide. Measurements of the scattered

light intensity versus the length of the waveguide can therefore

serve as an alternative method to measure loss. An exponential

fit to the scattered light intensity imaged using an 8-bit CCDcamera yielded a similar propagation loss as the cut-back

method of 0.13 dB cm21.

To compare with previously reported polymer waveguides,

90 mm 6 90 mm 6 70 mm waveguides were fabricated.

Perpendicular scattering power measurements yielded a

waveguide loss of 1.26 dB cm21 at 632 nm. If the majority

of loss results from sidewall scattering, then a 106 increase in

loss is expected for a 106 decrease in the width of the

waveguide core. This loss value compares well with the 1 dB

cm21 of other previously reported CNC machined wave-

guides19 but was achieved with more tolerant machining

parameters due to the additional vapor polishing step. The loss

is also comparable to 75 mm 6 50 mm 6 70 mm waveguidesfabricated by IBM with measured losses of 1.2 dB cm21 for an

NA of 0.29.20 Details of the loss measurements and numerical

aperture measurements are provided in the ESI.{

B. Dye doped filters

Filter waveguides were also fabricated to determine their

absorption and fluorescence characteristics. At maximum

solubility in NOA71, the filters containing pinacyanol iodide

(Sigma Aldrich) achieved an absorption per unit length of

71.8 dB cm21 at 590 nm and less than 0.30 dB cm21 at 760 nm

as measured with an Ocean Optics white light source and

spectrometer through a 1 mm path length of pinacyanol iodide

doped NOA71. Waveguide coupled dye autofluorescence wasmeasured by creating a 70 mm long dye doped waveguide and

injecting a 632 nm HeNe laser at the input of the waveguide.

Since the input is entirely absorbed, dye fluorescence can be

measured by measuring the waveguide output power. This

results in a 1% output power relative to the input laser power

as measured with the photodiode circuit used for actual

experiments. Pinacyanol iodide has a short lifetime (330 ps in

viscous fluids such as glycerol),21 and therefore the fluores-

cence emission adds constant intensity and phase to the

detected signal at kHz frequencies. For frequency lock-in

measurements, a filter dye with a slower lifetime would be

Fig. 3 The fabricated device is shown consisting of 4 waveguide

sensors butt coupled to photodetectors. The optical backplane rests

underneath a miniature bioreactor array containing 4 integrated

oxygen sensors. Darker sections of the waveguides result from mixing

the core polymer with organic dye and acts as a filter for the 590 nm

excitation source.



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preferable to provide both absorption based attenuation and

frequency response attenuation.

C. Silvered vertical couplers

To test the effects of air plasma on silver surface deposition

quality, 300 nm silver films were deposited on both air plasma

treated and native flat PDMS surfaces. While silver evaporated

on air plasma treated surfaces achieved reflectivities of 98% as

expected, silver evaporated on untreated PDMS surfaces

resulted in microcracks, reducing reflectivity to 90%. In

addition to silver deposition quality, bend roughness from

milling resulted in variations in surface flatness. In comparison

to flat surfaces, measurements on vertical coupler surfaces

resulted in a reduction in reflectivity from 98% to 96%, in

agreement with Monte Carlo simulations of a rough bend

incorporating measured roughness.

D. System performance

By characterizing all of the coupling, roughness, and material

losses in the experimental system, simulated system perfor-mance was found to agree with experimental system perfor-

mance. Overall efficiency from LED output to collected

luminescence was calculated to be 0.0109% while measured

results were 0.01%. A detailed description of the system losses

and characterization measurements are described in ESI.{

To compare the waveguide collection efficiency with

conventional fiber bundles, we also tested a fiber bundle with

a 1 mm core diameter PMMA excitation fiber surrounded by

0.5 mm core diameter PMMA collection fibers. Measurements

performed with the fiber bundle on the same oxygen sensor

required 9 collection fibers in order to yield a similar efficiency

of 0.0095%. Out of 9 collection fibers, a maximum efficiency of

0.0014% was measured for a single fiber.

E. Intensity detection (fluorescein fluorescence)

For intensity based fluorescence detection, a 500 mm thick 1 6

2 cm2 area microfluidic well was placed above the waveguide

detection area. Sodium fluorescein (Sigma Aldrich) in deio-

nized water was pumped through the well in increasing

concentrations starting with concentrations below the detec-

tion limit of the system. Due to the close excitation and

emission spectra, external filters were used. The input

excitation to the waveguide consisted of a filtered (Omega

Optical HQ480/406) 470 nm fiber coupled LED with an

output power of 8.6 mW. The fiber from the LED was coupled

to a second glass fiber with the excitation filter placed betweento eliminate PMMA autofluorescence, and the second fiber

was butt coupled to the waveguide. A fluorescence filter

(Omega Optical HQ510LP) was also placed between the

collection waveguide output and the silicon photodiode. The

amplifier circuit provided a gain of 3 6 106 V A21.

Fig. 4 shows the collected fluorescence intensity for different

concentrations of sodium fluorescein with the background

signal measured when DI water is in the well subtracted. From

the plot, we can determine the 3s noise level for an average of

ten 1 second acquisitions, where s is the standard deviation of

the measured intensity when the well contains no fluorescein.

From these measurements, the estimated limit of detection

(LOD) for sodium fluorescein is 200 nM with a linear

detection range up to 100 mM. With a 500 mm channel heightand an illumination area of 2 6 1 mm2 estimated in section 2,

the detection volume is 1 mL. The LOD achieved by this system

is considerably higher than other similar fluorescence detec-

tors, where an LOD of less than 0.04 nM was acheived,22,23

mainly resulting from autofluorescence of the waveguide core

material in combination with the filters used. Measurements of

the background signal show a larger detected signal than the

noise floor of the photodetector circuit by over 2 orders of

magnitude. Improvements to the LOD for fluorescein detec-

tion are possible if other core materials are used which

generate less autofluorescence.

F. Lifetime detection (oxygen sensing)

An example application replaces an array of optical fiber

bundles with the waveguide backplane for measurements of

dissolved oxygen in a microbioreactor chip.17 Sampling

oxygen quickly and accurately is necessary for feedback

control of the dissolved oxygen concentration. Due to the

high density of fluidics and control valves in this device, strictly

planar waveguides can not be implemented along side the

microfluidics. The experimental setup for measuring oxygen

concentration with the waveguide backplane consisted of four

590 nm fiber-coupled LEDs modulated at 5 kHz. To filter IR

emission from the LEDs, short pass filters (BG-39) were

placed at the end of the LED fibers and butt coupled to

secondary PMMA fibers. The second fibers were then buttcoupled to the input waveguides and four silicon photodiodes

were placed at each output guide with the option of placing an

off-chip filter (RG-9) between the output guide and the

photodiodes. Unlike the 470 nm excitation, no autofluores-

cence was detected at the photodetector with a 590 nm

excitation and RG-9 filter. The concentration of dissolved

oxygen in water was controlled by the peristaltic mixer which

mixes the liquid in the chamber with pressurized nitrogen or

pressurized air.17

The phosphorescence of the PtOEP-K dye, which composes

the oxygen sensors, is quenched by molecular oxygen, resulting

Fig. 4 Collected fluorescence intensity versus concentration at an

input power of 8.6 mW. The measurement setup is also shown. From

the standard deviation of the data for undetectable concentrations, a

limit of detection of 200 nM can be estimated for sodium fluorescein.



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in a reduction in the phosphorescence lifetime. By measuring

the phase shift of the collected phosphorescence relative to the

modulated input, a signal related to the phosphorescence

lifetime of the sensor can be determined. Each sensor exhibited

a maximum phase change of 30u with a standard deviation of

0.5u from nitrogen to air when the off-chip filter was used as

shown in Fig. 5a. Sensor calibration was performed by mixing

ratios of nitrogen and air using an electrically controlledswitching valve (Lee Co.) operating at 20 Hz. Oxygen

concentration could be varied between 0% and 21% by

changing the duty cycle of the switch between nitrogen and

air. Fitting the phase response versus frequency to a first order

model resulted in phosphorescence lifetimes ranging from

19.8 ms to 58 ms, which is comparable with previously reported

calibrations.18 The linearity of the inset in Fig. 5a shows that

the extracted lifetimes are accurately described by Stern

Volmer quenching kinetics. Using data from the frequency

response versus oxygen concentration, a calibration curve

relating phase to oxygen concentration can be determined at

specific frequencies as shown in Fig. 5b.

A step response of the oxygen concentration in water,measured by switching the peristaltic actuation gas from

nitrogen to air, is shown in Fig. 5c. Since the lifetime based

sensor measures fluorescence quenching due to oxygen, the

noise increases when the well is air mixed versus nitrogen

mixed. The noise at maximum concentration (21% oxygen)

was obtained from the standard deviation (s) of ten 1 second

acquisitions during air mixing. By converting the deviation in

phase into a deviation in concentration, the worst case

resolution (3s) for detecting oxygen in water was estimated.

For off-chip glass filtered devices, the minimal resolution was

measured at 0.4% oxygen. At 100% nitrogen, a similar

measurement results in an LOD at 0.08% oxygen. For a

dissolved oxygen concentration of 9.2 ppm in air saturated

water, this translates to an LOD of 35 ppb oxygen. This LOD

is comparable to the original work characterizing PtOEP-K18

as well as other fiber based optodes2,24 and is lower than other

waveguide oxygen sensors where LODs of 600 and 300 ppboxygen were achieved7,25 It should be noted that while

dynamic range is not affected by intensity variations due to

the ratiometric nature of the phase measurement, both the

LOD and resolution improve with increasing intensity since

the signal to noise ratio is only between 30 and 56 for air and

nitrogen, respectively. On-chip filtered waveguides measured a

worst case resolution of 5% oxygen for air saturated water, and

an LOD of 2% oxygen for nitrogen saturated water. The

decrease in resolution can be attributed to autofluorescence in

the dye doped collection waveguide which further reduces the

signal to noise ratio.

To test the waveguide sensor repeatability, each waveguide

sensor was exposed to the same set of nitrogen and oxygenmixtures and the responses of different sensors to the same

concentrations were compared. Gas mixtures were created

with the same switching valve used for calibrations. Sensors

were exposed to six different oxygen concentrations and the

measured concentration was taken as the average of 20 acquisi-

tions. Fig. 5d shows the standard deviation of the measured

concentration between the four sensors. From the data, we can

see that the off-chip filtered waveguides measure concentration

very consistently, with intersensor deviations of only 0.025%

and 0.14% oxygen concentration for nitrogen purged and air

saturated conditions. On-chip filtered waveguides performed

worse, due mainly to signal contributions from stray excitation

and dye autofluorescence. This reduced the full scale phase

range to only 6u, inflating phase deviations. Even with a

reduced phase range, on-chip filtered waveguides, using the

same calibration curve measured in Fig. 5b, measure

intersensor deviations of 0.22% and 0.53% oxygen under

nitrogen purged and air saturated conditions, respectively, for

averaged measurements.

5. Conclusions

The polymer based optical backplane fabricated in PDMS

consisted of large area waveguides, silvered vertical couplers,

and filters. A fabrication method combining CNC milling and

vapor polishing was shown to generate sufficiently smooth

surfaces for waveguiding. Measured propagation loss for800 mm 6 1 mm waveguides was 0.10 dB cm21 at 632 nm

and 30u silvered vertical coupler reflectivity was 96%. By

fabricating waveguides in a polymer backplane platform, low

cost optical detection solutions can be designed easily for

applications where CCDs are either too expensive or incapable

of detection, such as at IR wavelengths or for high frequency

measurements. Organic dye doping of waveguides provided

integrated filters, further reducing the complexity and cost of

off-chip optics. The fabricated backplane demonstrated

accurate oxygen concentration measurements via the phos-

phorescence lifetime of sensors located within a complex

Fig. 5 (a) A measured phase vs. frequency plot (dotted) is shown for

the oxygen sensors as measured with the waveguides and off-chip

filters. Curve fits (solid) using phosphorescence lifetimes ranging from

19.8 ms to 58 ms for air and nitrogen, respectively, are also shown. (b)

The inset shows the calibration curve of normalized phase vs. %

saturated oxygen concentration at 5 kHz. (c) Measured dissolved

oxygen concentration step responses from the chip shown in Fig. 3

with 4 different waveguide sensors utilizing both off-chip filters

(dashed) and on-chip filters (solid). (d) Repeatability measurements

showing deviations in measured oxygen concentration for the four

waveguides with on-chip (square) and off-chip filters (circle).



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microfluidic device with required input powers as low as 17 mW

for an SNR of 56. Simulations of the system indicate a point

source collection efficiency of 2.86%, suggesting that the

backplane could be used for fast particle fluorescence detection

systems such as flow cytometers in addition to the demon-

strated distributed area luminescence detection system.

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