A microfluidic device using a green organic light emitting diode as an integrated excitation source

A microfluidic device using a green organic light emitting diode as anintegrated excitation source

Bo Yao,a Guoan Luo,*a Liduo Wang,bc Yudi Gao,c Gangtie Lei,b Kangning Ren,a Lingxin Chen,a

Yiming Wang,a Yan Hub and Yong Qiubc

Received 11th April 2005, Accepted 8th July 2005

First published as an Advance Article on the web 5th August 2005

DOI: 10.1039/b504959h

A simply fabricated microfluidic device using a green organic light emitting diode (OLED) and

thin film interference filter as integrated excitation source is presented and applied to fluorescence

detection of proteins. A layer-by-layer compact system consisting of glass/PDMS microchip,

pinhole, excitation filter and OLED is designed and equipped with a coaxial optical fiber and for

fluorescence detection a 300 mm thick excitation filter is employed for eliminating nearly 80%

of the unwanted light emitted by OLEDs which has overlaped with the fluorescence spectrum of

the dyes. The distance between OLED illuminant and microchannels is limited to y1 mm for

sensitive detection. The achieved fluorescence signal of 300 mM Rhodamine 6G is about 13 times

as high as that without the excitation filter and 3.5 times the result of a perpendicular detection

structure. This system has been used for fluorescence detection of Rhodamine 6G, Alexa 532 and

BSA conjugates in 4% linear polyacrymide (LPA) buffer (in 1 6 TBE, pH 8.3) and 1.4 fmol and

35 fmol mass detection limits at 0.7 nl injection volume for Alexa and Rhodamine dye have been

obtained, respectively.

Introduction

Although more and more measurement schemes besides laser

induced fluorescence (LIF),1 including electrochemistry (EC),2

chemilluminescence (CL)3/electrical chemilluminescence

(ECL),4 mass spectrumetry (MS),5 and nuclear magnetic

resonance (NMR),6 have been developed for microfluidic or

‘‘lab-on-a-chip’’ (LOC) systems in the past decade, the

performance of laser induced fluorescence detectors is still

enormously important especially in the area of life science

research. As opposed to the original, large 488 nm argon

ion lasers, small, low cost laser diodes are now commonly

being used as the excitation source as first reported by

Harrison and co-workers1 in order to produce a more compact

overall system.

Further simplification of the fluorescence detection could be

obtained by employing a light emitting diode (LED) pre-

sumably at a reduced sensitivity. Webster et al.7 presented a

monolithic device fabricated by 13 steps of lithography which

had integrated photodiodes built on a silicon substrate and

optical interference filter with a parylene based microchip.

Whitesides and co-workers8 also reported an integrated

fluorescence detection system consisting of a microavalanche

photodiode (mAPD), a thin film of polymeric colored filters

and a PDMS microchip. Blue LEDs were employed as external

light sources in both systems. Recently LEDs with a very

high output power and shorter wavelength have become

commercially available. Since LED is not very expensive and

can be driven at low power, we can even use it as a disposable

light source.

Compared with inorganic LEDs, organic light emitting

diodes (OLEDs) have a flat surface which makes it easy to

integrate with microfluidic devices and flexible to fabricate

into any size and shape by photolithography techniques. A

recent review showed high interests in this field since OLEDs

offer the potential of on-chip light source arrays with

controlled spectral characteristics and in principle are cheap

to integrate on a microchip.10 Kopelman et al. reported a

fluorescent chemical sensor platform integrating an OLED

device light-source with a fluorescent probe for an oxygen

sensor.11 Fujii and co-workers first presented an integrated

PDMS microfluidic device with a 510 nm (peak wavelength)

OLED and optical fibers.12 In order to minimize the distance

between OLED and microchannel they placed a channel cast

in PDMS directly on the rear side of the glass substrate of the

OLED. Unfortunately, no fluorescence signal of Rhodamine B

was obtained in their system and the design could be further

optimized. Kim et al. reported an advanced and compact

microchip coupled with a green OLED of 530 nm (peak

wavelength) and a PIN photodiode for fluorescence detection

which achieving a detection limit of 0.01 mM Rhodamine 6G

as reported.13 Edel and co-workers presented a polyfluorene-

based thin-film polymer light emitting diode (pLED) which

had a peak emission wavelength at 488 nm as an integrated

excitation source for microfabricated capillary electro-

phoresis.14 For fluorescein and 5-carboxyfluorescein

detection concentrations as low as 1 mM were achieved with

lock-in-amplifier equipment. Later they employed an organic

aDepartment of Chemistry, Tsinghua University, Beijing, 100084, China.E-mail: [email protected]; Fax: +86-10-62781688;Tel: +86-10-62781688bKey Lab of Organic-Optoelectronics & Molecular Engineering ofMinistry of Education, Department of Chemistry, Tsinghua University,Beijing, 100084, China. E-mail: [email protected];Fax: +86-10-62795137; Tel: +86-10-62788802cBeijing Visionox Technology Co. Ltd, Beijing, 100085, China.E-mail: [email protected]; Tel: +86-10-62968822-221

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

This journal is � The Royal Society of Chemistry 2005 Lab Chip, 2005, 5, 1041–1047 | 1041

photodiode for detecting and monitoring a peroxyoxalate

based chemiluminescence reaction.15

OLEDs as a promising light source for integrated micro-

fluidic devices and fluorescence measurements are attracting

more and more attention. However, only a few groups have

so far entered this field probably because OLEDs are not

commercially available up to now. There are still several

critically unresolved problems including spectrum purity and

intensity which also exist in LED detection systems and block

the way for a wide application. Scientists have made enormous

efforts to improve the sensitivity of LED and OLED systems

such as employing a lock-in-amplifier,7,14 a liquid core

waveguide,16 an emission interference filter7 and high sensitive

confocal structures.17 Compared with lasers, the output power

of LEDs and OLEDs is fairly low and has a wider bandwidth

of emission spectrum. So it is very important to eliminate as

much as possible the part of the excitation light which overlaps

with the emission spectrum of analytes in order to supress the

background interference and at the same time confirm that the

distance between light source and microchannels is minimized.

In our previous reported work, an argon ion laser and red

diode laser were both employed as light sources for fluores-

cence detection in microfluidics18–20 with high sensitivity yet

large size. Now we are presenting a glass/PDMS microchip

device using a green OLED which has a peak wavelength at

520 nm as the excitation source. Between the light source and

microchannel a 300 mm-thick TiO2/SiO2 interference filter is

inserted to get rid of unwanted excitation light. A conventional

photomultiplier tube and optical fiber are employed for

fluorescence detection of Rhodamine 6G and Alexa 532 dye

on the microchip. The influence of pinhole size, excitation light

filtering on detection sensitivity and stability of OLED at

different driving voltage has been studied and under optimized

conditions the obtained S/N ratio of 50 mM Rhodamine

6G and 7 mM Alexa 532 is 16.9 and 10.2 respectively. Using

this system Alexa 532 and its bovine serum albumin

(BSA) conjugates have been separated and fluorescence

detected in modified microchannels and 4% linear poly-

acrylamide (LPA) buffer.

Experimental

Reagents and protein derivation

AlexaFluor1532 carboxylic acid, succinimidyl ester (532/554

nm) was purchased from Molecular Probes (Eugene, OR,

USA). Tris(hydroxymethyl)aminomethane (Tris), bovine

serum albumin (BSA, 66 200Da), EDTA and Rhodamine 6G

(526/555 nm) were all obtained from Sigma-Aldrich (St. Louis,

MO, USA). Acrylamide monomer, and N,N,N9,N9-tetra-

methylethylenediamine (TEMED) were both bought from

Promega (Madison, WI, USA) while ammonium persulfate

(APS) was from Amresco (Solon, Oh, USA) and [c-(metha-

cryloyloxy)propyl] trimethoxysilane (MAPS) was a product

of Fluka (Buchs, Switzerland). All other chemicals were

of analytical reagent grade, and Milli-Q water (18.2 MV,

Millipore, MA, USA) was used throughout.

The running buffer of 4% (w/v) LPA in 1 6 TBE was

prepared by dissolving 1.6 g acrylamide and 0.076 g APS in

10 ml of water, slightly different from the reported work of

Schmalzing et al.21 and Gomis et al.22 Then the solution was

mixed with 20 ml of 2 6 TBE (89 mM Tris, 89 mM boric acid

and 2 mM EDTA, pH 8.3) and the volume was adjusted

to 40 ml. Immediately 24 ml of TEMED was added and the

solution was degassed with an ultrasonic bath and left

overnight at room temperature for complete polymerization.

For protein labeling, the Alexa 532 dye was stored and

handled as instructed on the web site of Molecular Probes.23 In

brief, the dye was dissolved in dimethylsulfoxide (DMSO) at

10 mg ml21 (13.8 mM) and stored at 220 uC. For derivation,

5 ml of dye was added into 45 ml of 20 mg ml21 BSA (in 0.1 M

bicarbonate buffer, pH 8.3) and immediately stirred gently

for an hour at room temperature. The conjugate solution

was then stored at 4 uC protected from light without further

purification.

Microchip fabrication and coating

The glass substrate with microchannels used in the following

experiments was designed and home-made by standard

photolithography and wet chemical etching techniques.19,24

The cover plate was a piece of 100 mm thick PDMS replica

from a flat glass wafer which was silanized in 3% (v/v)

octadecyl trichlorosilane (Sigma, St.Louis, MO, USA) in dry

toluene for 2 h beforehand.25 10 : 1 of the silicone elastomer

and curing agent (Sylgard 184, Dow Corning, Midland, MI,

USA) were mixed and poured onto the wafer after stirring and

degassing. The solution was baked in a vacuum oven at 65 uCfor 4 h. Immediately PDMS was sealed to the glass substrate

after peeling off the wafer and then the microchip was

exposed to ultraviolet light (UV) which had a peak emission

at 253.7 nm (X-30G, Spectroline, USA) and average intensity

of 1.85 mW cm22 with the PDMS side face up for 9 h for

further combination and oxidization of PDMS as described

elsewhere.26

The final chip had a cross-linked microchannel which was

70 mm deep and 100 mm wide (at half depth) with 1.5 cm of

sample channel and 3 cm of separation channel and a distance

between cross channel to detection point of 1.5 cm. 5 ml of

MAPS was added to each reservoir of S, SW and B (Fig. 1a)

and 0.1 atm of vacuum was applied to the BW reservoir and

the microchannels were soon filled with MAPS. After that it

was left at room temperature for reaction overnight. This was

followed by rinsing with methanol followed by water for 2 min

and 10 min respectively and dipping in freshly prepared

reaction buffer (3% acrylamide, 0.6% ammonium persulfate

and 0.2% TEMED in water) for 3 h as reported by Han et al.27

Finally the substrate was rinsed with water for 10 min and

dried with nitrogen for 10 min, ready for usage.

OLED fabrication

The OLEDs used in the experiments consisting of a typical p–n

diode bottom emitting structure of ITO/NPB/Alq3/Mg:Ag/Ag

were fabricated by organic molecular beam deposition on a

lithographically patterned indium tin oxide (ITO) coated glass

substrate as described previously.28–30 The ITO substrate was

routinely cleaned by ultra-sonication in acetone, ethanol,

rinsed in de-ionized water and isopropyl alcohol, and finally

irradiated in an oxygen plasma chamber. Then, the organic

1042 | Lab Chip, 2005, 5, 1041–1047 This journal is � The Royal Society of Chemistry 2005

films, 40 nm of a-napthylphenylbiphenyl (NPB) and 60 nm of

tris(8-hydroxyquinoline) aluminium (Alq3) were deposited on

the ITO substrate layer by layer in high vacuum as the hole

injection layer and the electron transport layer respectively.

After deposition of the organic layers, the top cathode was

prepared by sequential deposition of 100 nm Mg:Ag and 50 nm

Ag layers without breaking the vacuum. The sandwich

structure of OLEDs is shown in Fig. 2.

When 4.5–12 V direct current was applied to the metal

cathode and ITO anode the energy barriers between the

highest occupied molecular orbital and lowest unoccupied

molecular orbital levels were about 0.4 and 0.9 eV, respec-

tively, which were high enough to localize the holes in the NPB

layer and electrons in the Alq3 layer. Recombination of these

charges occurred across the barriers, with holes primarily

moving into Alq3. The green OLED had a 0.5 mm thick glass

substrate and an array of 250 mm 6 250 mm illuminants

controlled by parallels of deposited electrodes which emitted

an intensity of 20000 cd m22 and irradiance of 7.5 mW cm22

(at 12 V driving voltage) green fluorescence with a peak

emission at 520 nm and y60 nm bandwidth (FWHM).

Detection system

A compact OLED induced fluorescence detection system was

established for measurement of Rhodamine 6G, Alexa 532 and

its BSA conjugates. The optical set up is shown in Fig. 1a and

b. On the top of the OLED was a piece of 0.3 mm short-pass

interference filter (550 nm) designed and fabricated by Optical

Coating Center of the Film Machinery Research Institute

(Beijing, China) which consisted of 30 alternating layers of

SiO2 and TiO2 and was about 4.5 mm thick. In order to limit

the dimension of the detection point three pieces of 12 mm

thick silver foil with a 50, 100 and 200 mm pinhole were

respectively inserted between the excitation filter and micro-

chip. Above the microchip a 500 mm-core-diameter optical

fiber (Daheng Optical, Beijing, China) was inserted into a

y1 mm deep hole drilled and polished in the microchip

substrate which had y0.7 mm distance to the separation

channel coincided with the pinhole (see Fig. 1c). When 4.5–

12 V DC was applied to the anode and cathode electrodes

of OLED it was illumined and the green emission transited the

interference filter and pinhole layers in turn by which the

unwanted excitation light was removed, exciting the fluores-

cent dyes or protein derivations in the microchannels. A

perpendicular detection structure (see Fig. 1d) was also

employed here and a comparision of the results was made.

The fluorescence signals were collected by the optical fiber,

then passed through a long-pass emission filter (555 nm,

kindly presented by Beijing Yingxian Instruments, China) to

Fig. 1 Optical set-up of OLED induced fluorescence detection system: (a) detailed arrangement of each component; (b) photograph of

the microfluidics and OLED system; (c) side view of the structure with a coaxial optical fiber; (d) side view of the structure with a perpendicular

optical fiber.

Fig. 2 Bottom emitting structure of a typical p–n OLED which

consists of a layer of 40 nm NPB and 60 nm Alq3 as the hole injection

layer and electron transport layer respectively. The OLEDs were sealed

with a cover plate as protection from the air and 4.5–12 V was applied

to the ITO anode and metal cathode for luminescence.


eliminate the exciter light and other interferences focused by a

group of lens onto a confocal pinhole (400 mm id, Daheng,

China) and was finally detected by the photomultiplier tube

(PMT, CR131-01, Beijing Hamamatsu, China). Fluorescence

signals were digitized using a 400 kHz sampling frequency A/D

card (AC6111, W&W Lab, China) and a program written

with VC++ 6.0 was used for data acquisition and control of

the multi-terminal power source (Northeastern University,

Shenyang, China). Fig. 1b is a photograph of the actual

system, while c and d is its section view. The total distance

between OLED illuminants and microchannel was y1 mm

which helped to improve sensitivity of the OLED fluorescence

detection system.

Electrophoresis conditions and operation

The microchip was rinsed with water before use and 4% LPA

solution contained 1 6 TBE (pH 8.3) was used as working

buffer throughout. For microchip injections, the floating

sample loading model was employed31 in which the sample

was driven by electrophoresis as electroosmotic flow was

minimized and ignorable. Injections and separations were

performed with field strengths of 250 V cm21 and 470 V cm21

under reverse polarity for Alexa 532 and its protein conjugates

while forward polarity for Rhodamine 6G. For Alexa 532, as

an example, dye in DMSO was diluted with running buffer to

different concentrations needed for the sample solution. Before

operation, the channels were rinsed with water, then filled with

buffer. During injection (see Fig. 1a), the sample migrated

from S (grounded) to SW (500 V) with B and BW remaining

afloat for 50 s. Then the power supply switched and the sample

zone was separated during the migration from B (grounded) to

BW (1400 V) with S and SW afloat. After separation the

microchannels were immediately rinsed with water to prevent

the microchannels from blocking.

Results and discussion

Filtration of the excitation OLED emission

Although the micro photodiode and deposited interference

filter had been successfully integrated into the microfluidic

optical detection systems, LEDs were still employed as a

detached light source from the chip and detector.7,8,32

Moreover a lock-in-amplifier always had to be used in order

to achieve enough sensitivity mainly because LEDs have a

wider spectrum emission (40–60 nm)9 than lasers (5–10 nm)

and a much lower output power. Recently OLEDs have been

regarded as a promising alterative10,33 being cheap and easy to

integrate with a microchip as a thin film and flat surface and a

photolithography fabrication method similar to microfluidics.

However, as OLEDs have a wider spectrum emission (85 nm,

FWHM)14 it was imperative to eliminate the excitation light

that overlapped with the emission spectrum in order to obtain

a sensitive detection. The commercially available filters were

too thick (4–6 mm) to work for OLED fluorescence systems

because the viewing angle of OLEDs was extremely large

(about 170u) thus resulting in a decrease of luminance density

per unit area with an increase of distance from it. So the

microchannel should be in principle close to the OLED light

source as much as possible for the sake of high intensity of

excitation. A thin film (300 mm) interference filter which

blocked excitation light higher than y555 nm was designed

and fabricated for this purpose. Fig. 3 shows its spectrum and

also that of the emission filter and green OLED (with and

without excitation filter). The green OLED excited from

500 nm to 560 nm (FWHM) and there was about one fourth of

the excitation light which was able to pass through the

emission filter (blocking up to 545 nm) and overlapped with

the fluorescence signals. By inserting the excitation filter the

emission spectrum of OLED was redefined before reaching

the microchannel and the unwanted light was removed from

the excitation light (nearly 80%).

Fig. 4 shows the electropherograms of 300 mM Rhodamine

6G with three detection structures: perpendicular structure

(bottom), coaxial structure with (middle) and without (top) the

Fig. 3 Optical characteristics of the filters and emission spectrum of

OLED: excitation filter (dash dot dash); emission filter (dot); OLED

emission without excitation filter (dash) and OLED emission with

excitation filter (solid).

Fig. 4 Electropherograms of 300 mM Rhodamine 6G in 4% LPA

buffer (1 6 TBE, pH 8.3) with three detection structures: perpendi-

cular structure (bottom), coaxial structure with (middle) and without

(top) the excitation filter. Floating injection and separation performed

under forward voltage with a field strength of 250 V cm21 and

470 V cm21 respectively.


excitation filter. Since no excitation filter has been employed

with previously reported work on OLEDs, optical fiber

collected fluorescence signals perpendicular to the light source

and microchannel (see Fig. 1d) was accepted in order to

supress the background interferences by preventing the

detector from facing directly towards the OLED source,

although there was still a part of interferences that reached the

detector. When an appropriate excitation filter was designed

and used for eliminating interference excitation light a coaxial

framework as shown in Fig. 1c remarkably improves its

sensitivity by collecting more fluorescence signals and less

interferences, only if the distance between light source and

microchannel is limited as much as possible. In this experi-

ment, the achieved sensitivity of Rhodamine 6G with the

excitation filter (middle spectrum in Fig. 4) was about 13 times

as high as that without the filter (top) where a piece of 300 mm

thick glass slide was inserted instead for keeping the distance

and 3.5 times as the perpendicular detection structure

(bottom). For Rhodamine 6G, a forward voltage was applied

for injection and separation and it needed a longer migration

time (150 s) due to a weak EOF existing in the microchannels

or a small plus charge in the buffer.

However, a layer of directly deposited interference filter

onto the OLED surface was not achieved because of the

difficulties in fabrication. A thin film filter (300 mm thick)

sacrificed sensitivity to some extend, whereas, it could be kept

on being used even when the OLED did not work and was

abandoned. Furthermore, optical characteristics of the filter

were better than when deposited.7

Microchip fabrication and modification

A glass/PDMS microfluidic chip was employed in this research

because a common glass substrate available in our lab was not

less than 1.7 cm thick which resulted in a long distance

between OLED and microchannels and was disadvantageous

for high sensitivity detection. A piece of 100 mm thick PDMS

was polymerized and immediately sealed with the glass

substrate with microchannels after which the channels were

modified by MAPS. However, no methanol should be added

to the modification solution as done usually because it would

be absorbed by the PDMS material and make it fragile. When

LPA solution is polymerized in the microchip it would fill the

porous surface of PDMS and form a layer of physical coating

as well as the chemical effect of modification of glass channels.

This kind of glass/PDMS chip proved to be strong under

normal pressure as well as rinsing the microchannels and could

be used for several weeks.

Stability of OLEDs at different voltages

70 mM of Alexa 532 dye was filled into the microchannels

in order to study the influence of driving voltages of OLED

on fluorescence signals and stability. When output of the

power supply increased every 5 min from 4.5 V to 13.5 V, the

fluorescence intensity skipped upwards accordingly (see Fig. 5).

When the driving voltage was as high as 13.5 V, fluorescence

signals sharply declined 80% in 5 min which is mainly because

the working current between metal cathode and ITO anode

has exceeded its rated upper limit and excessive Joule heat

production leading to irreversible damage. Therefore, in the

following experiments 12 V of driving voltage was used as the

power supply of the OLED unless stated otherwise for the sake

of high sensitivity and several days stably output of OLEDs

could be obtained at a 12 V driving voltage.

Optimization of the pinhole

Three pieces of silver foil with about 50 mm, 100 mm and

200 mm pinholes were prepared and inserted between excita-

tion filter and microchip for confining the detection spot (see

Fig. 1a). Fig. 6 illustrates the detection of 140 mM Alexa 532

dye solution with 50 mm, 100 mm and 200 mm pinhole

respectively (from bottom to top). Two components of this dye

(hydrolytes or photolytes) were successfully separated and

fluorescently detected in the microchip. Different signal to

noise (S/N) ratios of 21.1, 132.3 and 272.5 were obtained

respectively according to the variety of the pinhole size. A

Fig. 5 Stability of OLED emission at different driving voltages

where 70 mM Alexa 532 dye in 4% LPA buffer (1 6 TBE, pH8.3)

was used as analyte and was filled into the microchannels for studying.

The power supply output increased from 4.5 V to 13.5 V every 5 min at

a step of 1.5 V.

Fig. 6 Fluorescence detection of 140 mM Alexa 532 dye in 4% LPA

buffer (1 6 TBE, pH 8.3) with 50 mm, 100 mm and 200 mm pinhole

respectively (from bottom to top); different S/N ratios of 21.1, 132.3

and 272.5 were achieved according to the increase of pinhole diameter.


pinhole with larger diameter (300 mm) was also studied in our

experiments, however, it only slightly raised the signals (less

than 10%), because the maximum width of the channel was

200 mm and the luminescent unit size of the OLED was

250 mm 6 250 mm. Therefore 200 mm diameter of pinhole

was finally selected in this system.

Performance of the OLED induced fluorescence detection

system

Under the optimized condition above, different concentrations

(from 7 mM to 700 mM) of Alexa 532 and Rhodamine 6G was

detected using this system (see Fig. 7) because these two dyes

had almost the same maximum wavelengths of absorption and

emission spectrum, 532/554 nm and 526/555 nm respectively.

Alexa 532 has recently been regarded as an ideal dye for use

with 532 nm excitation sources and has more prominent

fluorescent characteristics than Rhodamine 6G34 which is

proven by the results of Fig. 7. The concentration and mass

detection limit obtained by this system for Alexa 532 was

about 3 mM (S/N 5 3) and 1.4 fmol respectively at 0.7 nl of

sample injection volume. System sensitivity was considerably

improved compared with the previously reported work, where

about 18.5 of S/N ratio was achieved in the electrophoregram

of 10 mM fluorescein at 0.1 nl injection volume with lock-in-

amplifier equipment.

However, the result was roughly six orders of magnitude

poorer than good laser-induced fluorescence detection in

capillary electrophoresis35 because of its low irradiance and

purity. Therefore, the sensitivity of OLED induced fluores-

cence detection systems needs further improvements by

employing high performance OLED for future application.

Electrophoresis and fluorescence detection of BSA conjugates

Alexa dyes are structurally related fluorescent molecules that

are named according to the wavelength (nm) of the nearest

laser excitation. Alexa 532 ready reacts with non-protonated

aliphatic amine groups including the amine terminus of

proteins, producing stable carboxamide bonds.36 This dye

has begun to be widely used in the research of proteins,

nucleic acids and cells for biological purposes. Fig. 8 is the

electrophoresis and detection diagram of the protein deriva-

tions without further purification where the dye molecules

migrated to the detector earlier than the BSA conjugates in a

reverse electric field in 4% LPA buffer (1 6 TBE, pH 8.3). The

electropherogram of BSA conjugates in this research success-

fully correlated with that obtained by capillary electrophoresis

in 1 6 TBE (pH 8.3) buffer, while the Alexa 532 dye migrated

to the opposite direction.

Conclusions

In this study, a novel microfluidic device using a green organic

light emitting diode as excitation source was established and

the sensitivity of fluorescence detection was improved by

inserting a thin film of excitation filter which could remove the

part of excitation light which overlapped with the emission

spectrum of the dyes. Moreover the sensitivity could be further

improved if the interference filter could be deposited directly

onto the surface of OLED substrate. This OLED induced

fluorescence detection microfluidic system was applied to

electrophoresis and detection of BSA conjugates labeled with

Alexa 532. The results proved that OLEDs are promising light

sources for microfluidic fluorescence detection systems which

have a small size and are easy to integrate. However, the

intensity and stability of its fluorescence is expected to become

more powerful in the near future with enormous efforts being

directed in this area.

Compared with laser and LEDs, OLEDs are also advanta-

geous for accurate fabrication into various size and shapes

by photolithography techniques other than integration.

Therefore, it is convenient to fabricate a two dimensional

light source for multiple detection using a CCD camera.

Moreover micro detectors can also be employed in OLED

fluorescence detection systems and lead to further miniaturiza-

tion even to palm or thumb size which would resolve the

problem of a small microchip coupled with a bulky laser

Fig. 7 Detection of Rhodamine 6G and Alexa 532 dye in 4% LPA

buffer (1 6 TBE, pH 8.3) at different concentrations from 7 mM to

700 mM (n 5 3) at 0.7 nl of injection volume. For Rhodamine 6G

injection and separation conditions were the same as in Fig. 4, while

for Alexa 532 only the direction of voltage was changed.

Fig. 8 Electropherogram of BSA conjugates labeled by Alexa 532 dye

in 4% LPA buffer (1 6 TBE, pH 8.3) under optimized conditions as in

Fig. 7.


source for microfluidic fluorescence detections. We plan to

explore these avenues further.

Acknowledgements

This research was supported by National Science Foundations

of China (Grant No. 20299036 and 20475031) projects. The

authors would like to thank Dr Deqiang Zhang of Beijing

Visionox Technology Co., Ltd for OLED fabrication and

graduate students Peng Wei and Shiliang Han of Prof. Yong

Qiu’s group for helping to test optical characteristics of the

OLEDs.

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A microfluidic device using a green organic light emitting diode as an integrated excitation source

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