Dynamic Light-Activated Control of Local ChemicalConcentration in a Fluid

Hyundoo Hwang and Je-Kyun Park*

Department of Bio and Brain Engineering, College of Life Science and Bioengineering, KAIST, 335 Gwanhangno,Yuseong-gu, Daejeon 305-701, Republic of Korea

This article reports a method for controlling the chemicalconcentration in a localized region of a fluid using opto-electrofluidic mechanisms. Optoelectrofluidic fluores-cence microscopy (OFM), in which an optoelectrofluidicdevice is integrated into a conventional fluorescencemicroscope, allows both modulation and detection of localchemical concentration in an easy and simple way. Here,we present the first experimental investigation of theconcentration change of polysaccharides, proteins, andfluorophores, due to the frequency-dependent ac electro-kinetics and electrostatic interactions in an optoelectrof-luidic device. The dynamic modulation of the local con-centration of biomolecules such as dextran and serumalbumin was demonstrated in a temporal and spatialmanner. This OFM can be a useful tool for controlling thelocal chemical concentration in several chemical andbiological applications.

The local concentration of molecules can affect the activitiesand assembly of them, modulating whole chemical processes ina biological system. A dynamic control of the chemical concentra-tion in a specifically localized region of a sample fluid with a rapidand noncontact method has been difficult, although such atechnology can be valuable for several biological studies, includingmacromolecular crowding on protein folding kinetics1-3 andcellular chemotaxis in a chemical gradient.4,5 Recently, an opticalheating has been applied for the dynamic modulation of chemicalconcentration in an aqueous droplet due to its shrinkage andexpansion.6

Optoelectrofluidics, which is also known as optically inducedelectrokinetics, refers to the motion of particles or moleculesand their interactions with an optically induced electric field andsurrounding fluid.7 In 2000, this concept has been demonstratedthrough the electrophoresis of colloidal particles using an ultra-violet light pattern projected onto an indium tin oxide (ITO)

electrode.8 After that, most of the researchers have startedapplying a photoconductive material deposited on a metal plateelectrode to induce a nonuniform electric field, which results inan electrokinetic motion of particles and fluids, only with a weakwhite light source7,9-13 or a conventional laser source.14-17

Although optically induced dielectrophoresis (DEP) has frequentlybeen employed to manipulate individual microparticles, light-induced ac electroosmosis (ACEO) has recently attracted a greatdeal of attention because of its capability to rapidly manipulatenanoparticles or molecules, which are too small to manipulateusing DEP force proportional to the particle volume.17 Recently,a new optoelectrofluidic technology that combined both severalelectrokinetic mechanisms and electrostatic interactions for therapid and selective concentration of microparticles has beenreported.7

In the previous studies, conventional display devices such asa digital micromirror device and a liquid crystal display haveusually been applied to generate and control the light pattern andelectric field gradient.7,9-14,17 However, the display-based opto-electrofluidic platform has a limitation for detecting signalsinduced from the light-matter interactions, including fluorescenceand Raman scattering, as manipulating them. This limitation isdue to the characteristic of the optoelectrofluidic device, in whicha photoconductive material is used to optically induce an electricfield. However, the photoconductivity of the optoelectrofluidicdevice was advantageous for our objective that is dynamicmodulation of chemical concentration. We could dynamicallycontrol the local chemical concentration using optically inducedelectrokinetics and electrostatic molecular interactions and detectthe concentration change at the same time with a single lightsource for fluorescence excitation. This concept is almost the samewith that described in the previous study about the optically

* To whom correspondence should be addressed. E-mail: jekyun@kaist.ac.kr.Phone: +82-42-350-4315. Fax: +82-42-350-4310.

(1) Minton, A. P. Biophys. J. 2005, 88, 971–985.(2) Laurent, T.; Ogston, A. Biochem. J. 1963, 89, 249–253.(3) Berg, B. v. d.; Ellis, R. J.; Dobson, C. M. EMBO J. 1999, 18, 6927–6933.(4) Meili, R.; Ellsworth, C.; Lee, S.; Reddy, T. B. K.; Ma, H.; Firtel, R. A. EMBO

J. 1999, 18, 2092–2105.(5) Jeon, N. L.; Baskaran, H.; Dertinger, S. K. W.; Whitesides, G. M.; Van De

Water, L.; Toner, M. Nat. Biotechnol. 2002, 20, 826–830.(6) Jeffries, G. D. M.; Kuo, J. S.; Chiu, D. T. Angew. Chem., Int. Ed. 2007, 46,

1326–1328.(7) Hwang, H.; Park, J.-K. Lab Chip 2009, 9, 199–206.

(8) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56–59.(9) Choi, W.; Kim, S.-H.; Jang, J.; Park, J.-K. Microfluid. Nanofluid. 2007, 3,

217–225.(10) Hwang, H.; Choi, Y.-J.; Choi, W.; Kim, S.-H.; Jang, J.; Park, J.-K. Electro-

phoresis 2008, 29, 1203–1212.(11) Hwang, H.; Kim, J.-J.; Park, J.-K. J. Phys. Chem. B 2008, 112, 9903–9908.(12) Hwang, H.; Oh, Y.; Kim, J.-J.; Choi, W.; Kim, S.-H.; Jang, J.; Park, J.-K. Appl.

Phys. Lett. 2008, 92, 024108.(13) Choi, W.; Nam, S.-W.; Hwang, H.; Park, S.; Park, J.-K. Appl. Phys. Lett. 2008,

93, 143901.(14) Chiou, P. Y.; Ohta, A. T.; Wu, M. C. Nature 2005, 436, 370–372.(15) Hoeb, M.; Radler, J. O.; Klein, S.; Stutzmann, M.; Brandt, M. S. Biophys. J.

2007, 93, 1032–1038.(16) Neale, S. L.; Mazilu, M.; Wilson, J. I. B.; Dholakia, K.; Krauss, T. F. Opt.

Express 2007, 15, 12619–12626.(17) Chiou, P. Y.; Ohta, A. T.; Jamshidi, A.; Hsu, H. Y.; Wu, M. C. J.

Microelectromech. Syst. 2008, 17, 525–531.

Anal. Chem. 2009, 81, 5865–5870

10.1021/ac901047v CCC: $40.75 2009 American Chemical Society 5865Analytical Chemistry, Vol. 81, No. 14, July 15, 2009Published on Web 06/04/2009

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induced ACEO.17 However, only the simple concentration ofnanoparticles and DNA molecules was performed, and the in-depthinvestigations into the phenomena were not enough.

In this article, we investigate the behavior of molecules underthe optically induced electric field against various factors anddemonstrate a simple method for dynamic control of localchemical concentration in a sample fluid using optoelectrofluidicfluorescence microscopy (OFM), in which an optoelectrofluidicdevice is integrated into a conventional fluorescence microscope.The local concentration change of molecules was determinedaccording to the applied ac signal, the type of molecules, and theinitial background concentration of the sample solution. On thebasis of these results, the effects of optically induced electrokineticmechanisms and electrostatic interaction forces at the molecularlevel on the change of local chemical concentration were inves-tigated. Our experimental results are quite different from theprevious notion about the optoelectrofluidic molecular concentra-tion. In addition, a dynamic control of local concentration ofmacromolecules such as dextran and serum albumin, which haveoften been applied as a crowding agent, was demonstrated usingOFM. The frequency-dependent behavior of molecules were firstinvestigated and applied for dynamic temporal modulation of localmolecular concentration.

EXPERIMENTAL SECTIONDevice Fabrication. To fabricate an optoelectrofluidic device,

glass substrates coated with an ITO were purchased fromSamsung-Corning Precision Glass (Korea). Triple layers of a 50-nm-thick heavily doped hydrogenated amorphous silicon (a-Si:H), a 1-µm-thick intrinsic a-Si:H, and a 20-nm-thick silicon nitridewere deposited sequentially on the ITO-coated glass substrate bythe plasma enhanced chemical vapor deposition (PECVD) method.After the fabrication of a 30-µm-thick photoresist spacer on a bareITO-coated glass substrate, a 10 µL droplet of sample solutionwas sandwiched between the bare ITOsglass substrate and thea-Si:H-deposited substrate and a wrapping wire was connected forapplying a voltage. This fabrication process is almost the samewith that of the conventional optoelectrofluidic devices which havebeen reported in previous studies.7,9-13

Sample Preparation. Several target molecules, includingfluorescein isothiocyanate (FITC)-labeled dextran, fluorescein-conjugated bovine serum albumin (BSA), fluorescein, and bis-benzimide, were purchased from Sigma-Aldrich (Milwaukee,WI). FITC-dextran was diluted with deionized water into 1, 5, 10,30, 50, 75, and 100 µM solutions. Fluorescein-BSA, fluorescein,and bisbenzimide were diluted into 10 µM solutions. The sampleswere stored in an ice box to keep its temperature until injectinginto the optoelectrofluidic device.

System Configuration. A fabricated optoelectrofluidic devicewas put on the stage of an epi-fluorescence microscope (BA400T;Martin Microscope Company, SC) after injecting a liquid samplecontaining target molecules. A light for excitation of fluorescencesignal was projected from a mercury lamp through an irisdiaphragm for controlling the light pattern and an objective lensfor focusing the light pattern onto the photoconductive layer ofthe optoelectrofluidic device. At the same time, an ac voltage wasapplied across the liquid chamber by a function generator(AGF3022; Tektronix, OR). The fluorescence signal from thesample solution was detected by a charge-coupled device (CCD)

camera (DS-U1; Nikon Instruments Inc., NY) and analyzed witha conventional image analysis program (NIS-Elements; NikonInstruments Inc.). Figure 1 shows a schematic diagram of theOFM system, in which an optoelectrofluidic device was utilizedfor the dynamic control of local molecular concentration using aconventional fluorescence microscope.

RESULTS AND DISCUSSIONLight-Activated Molecular Concentration. When a light for

the excitation of fluorescence from molecules was projected ontothe optoelectrofluidic device, the number of electron-hole pairsat the partially illuminated area was significantly increased. As aconsequence, a nonuniform electric field was formed in the samplesolution, resulting in the optically induced electrokinetic flows andelectrostatic interactions around the light pattern which induceconvergence or divergence of molecules. This OFM system makesboth modulation and detection of local molecular concentrationeasily possible by applying the light source for both fluorescenceexcitation and electric field activation.

When we applied a voltage and projected a light pattern tothe optoelectrofluidic device, a sudden change of fluorescencesignal due to the change of molecular concentration in theilluminated area was detected (Figure 2a). With application of avoltage of 10 Vpp at 1 kHz, the ACEO flow was induced by themovements of closely packed ions at the electric double layeron the optically induced electrode surface due to the tangentialelectric field (Et) with a slip velocity defined as

⟨vslip⟩t )12

λD

ηRe(σqEt*)

where λD, η, and σq are the Debye length, the fluid viscosity,and the charges contained in the electric double layer,respectively.7,18 The ACEO flow converging into the illuminatedarea forms a stagnation point, where the flow velocity approacheszero, near the center of the area, and FITC-dextran molecules inthe bulk fluid were concentrated into the point. This local chemical

(18) Probstein, R. F. Physicochemical Hydrodynamics: An Introduction, 2nd ed.;Wiley and Sons, Inc.: New York, 1994.

Figure 1. Schematic diagram of optoelectrofluidic fluorescencemicroscopy (OFM) for tuning the local molecular concentration. Thecross section (A-A′) of the optoelectrofluidic device was alsoillustrated.

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concentration of molecules in the illuminated area was saturatedinto a specific constant value within a few seconds. Figure 2bshows the rapid change of local chemical concentration accordingto the amplitude of the applied voltage. The time for the changeof FITC-dextran concentration was almost independent of theamplitude of the applied voltage, while the value of steady-stateconcentration depended not only on the amplitude of the appliedvoltage but also on various parameters, including the applied acfrequency, the type of molecules, and the bulk chemical concen-tration of the solution.

Frequency-Dependent Concentration Changes. Figure 3shows the change of fluorescence signal of FITC-dextran fordifferent ac frequencies such as 100, 10, and 1 kHz according tothe time. The fluorescence intensity was maintained in a certainvalue with application of a voltage. The fluorescence intensityprofiles of FITC-dextran at 1 and 10 kHz had always triangularand square shapes, respectively (Figure S1 in the SupportingInformation), although the steady-state magnitude of the fluores-cence signal was changed proportionally to the amplitude of theapplied ac signal. When we turned off the applied voltage, theconcentration was rapidly changed into a specific value and thenslowly recovered due to the diffusion transport of target molecules.From these results, we concluded that the electrostatic interactionsamong the molecules due to their polarization in the opticallyinduced electric field must also affect the behavior of molecules,since the electrohydrodynamic flow by ACEO cannot raise thesephenomena for itself. For example, the first rapid decrement ofmolecular concentration as soon as the voltage is turned off, before

the second slow recovery due to the diffusion, might be due tothe sudden disappearance of the electric field-induced attractionsbetween the FITC-dextran molecules. The positive DEP forcescan also affect the molecular concentration, but the effect of DEPwill be described later.

Effect of Molecular Type. Figure 4a shows the microscopicpictures of the FITC-dextran solution in the OFM for different acfrequencies from 100 Hz to 100 kHz. All the pictures were takenat the steady state, which is about 3 s after the voltage was applied.When an ac signal of 1 kHz frequency was applied, FITC-dextranconcentrated into the illuminated area with high density, forminga triangular cross-sectional profile of fluorescence intensity asmentioned above. As the frequency increased from 1 to 10 kHz,the concentration of FTIC-dextran was significantly decreased.

Figure 2. (a) Microscopic pictures showing the change of thechemical concentration of FITC-dextran in the illuminated region ofa sample fluid after application of a voltage of 10 Vpp at 1 kHz. (b)Temporal change of the chemical concentration according to theamplitude of the applied voltage at 1 kHz.

Figure 3. Change of chemical concentration of FITC-dextran in theilluminated area against the applied ac frequency with application ofa voltage of 10 Vpp (black/white triangle: on/off).

Figure 4. Microscopic pictures of (a) FITC-dextran and (b) bisben-zimide solutions against the ac frequencies from 100 Hz to 100 kHz.(c) Fluorescence intensity according to the type of molecules. Thefluorescence intensity was normalized to the average intensity valueat the no voltage condition as a control.

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This was due to the weaker ACEO flow around the illuminatedarea at the high-frequency range. The weak focusing flow, whichis slightly stronger than the Brownian motion of molecules at 10kHz, decreased the amount of concentrated molecules in theilluminated area. As a consequence, the weak dipole attractionamong the molecules due to the low-volume fraction of themmakes the resulting chemical concentration in the illuminated areabecome lower.19

When we increased the applied frequency to 100 kHz, therewas no significant change of the chemical concentration comparedto that of the no voltage condition. This was due to the frequencycharacteristics of ACEO, which is generally activated at the low-frequency range below 10 kHz. On the other hand, the positiveDEP can be activated and move the molecules at the high-frequency around 100 kHz.15 Nevertheless, the molecular con-centration in the illuminated area at 100 kHz was not muchdifferent from that at the no voltage condition in this experiment.It might be due to the weak strength of the positive DEP actingon the FITC-dextran molecules comparable to the Brownianmotion of the molecules. We could determine the effect of theweak DEP force in the experiments for dynamic temporalmodulation of the local molecular concentration using a consecu-tive frequency change later.

In the previous studies using the optically induced ACEO, theelectrohydrodynamic flow always concentrated the molecules intothe illuminated area.17 According to our experimental results,however, the local chemical concentration of FITC-dextran couldbe decreased at a few hundred Hertz. It has been shown to bedriven by the electric field-induced flow due to the lateral electricfield component within the electric double layer at the low-frequency range where the Faradaic reaction becomes consider-able.20 In addition, the weaken dipole interaction forces amongthe molecules could also help the depletion of molecules at theilluminated area.

In the case of bisbenzimide, the molecular concentration inthe illuminated area was decreased into a specific steady-statevalue even in the high-frequency range above 1 kHz as shown inFigure 4b. These results are quite different from the previousnotion that the molecules are always concentrated and trappedby the optically induced ACEO vortices into the illuminated area.17

To investigate these phenomena more deeply, we observedthe optoelectrofluidic change of local chemical concentration withvarious types of molecular solutions. The frequency-dependentconcentration change showed a different tendency according tothe type of molecules. In the case of bisbenzimide and fluorescein,which are small fluorophores, their local concentration decreasedas the ac frequency decreased from 100 to 1 kHz, while theconcentration of FITC-dextran and fluorescein-BSA, which aremacromolecules, had a maximum value at 1 kHz and decreasedas the frequency increased to 100 kHz (Figure 4c). On the basisof the experimental results for four different types of molecules,we inferred that the size of molecules is one of the importantparameters to determine the tendency of the chemical concentra-tion change in the illuminated area at the high-frequency rangeabove 1 kHz. The smaller molecules such as fluorophores, of

which the size is approximately 1 nm,21 have a larger diffusioncoefficient and a smaller dipole interaction energy than relativelylarger molecules such as dextran and BSA, of which the size isabout 22 nm.22 In the case of small molecules, therefore, thedispersion forces due to the Brownian motion would be moredominant than the concentration forces due to the electrostaticdipole interactions and DEP, resulting in the interference withthe molecular aggregation in the illuminated area. When weapplied a frequency of 100 Hz, all the molecules rapidly disap-peared from the illuminated area. The mechanisms regarding thesignificant decrement of molecular concentration, which wasindependent of the molecular type, in the low-frequency rangebelow 1 kHz is considered to have different physical origin fromthat in the high-frequency range above 1 kHz. The causes whichinduced the difference between the absolute fluorescence intensityof each molecule could not be completely determined. However,it might be caused by the difference of physical and chemicalproperties of each molecule.

Electrostatic Aggregation of Molecules. We also determinedthe electrostatic aggregation of FITC-dextran molecules, whichwas mentioned above as a dominant factor for molecular concen-tration changes. When we projected a light pattern of larger area,we could clearly observe the aggregation of molecules due to thedipole-dipole interactions with application of a voltage of 10 Vpp

at 1 kHz (Figure 5). The aggregated FITC-dextran moleculeswere dispersed as soon as the applied voltage turned off afterabout 14 s. This electrostatic aggregation of molecules under anelectric field have also been observed using DNA molecules in acapillary tube for electrophoresis.19 The physical mechanism ofour experimental observation might be much the same with thatof the previous study using DNA molecules. The electrostaticinteractions among the molecules became dominant near thestagnation point, where the ACEO flow can be negligible.Therefore, the molecules concentrated by ACEO flow into thestagnation point must be aggregated by the electrostatic attractionforces with application of a voltage of 1 kHz.

Effect of Bulk Chemical Concentration. We measured theconcentration of FITC-dextran which could be increased by theoptoelectrofluidics according to its initial concentration, which

(19) Mitnik, L.; Heller, C.; Prost, J.; Viovy, J. L. Science 1995, 267, 219–222.(20) Fagan, J. A.; Sides, P. J.; Prieve, D. C. Langmuir 2004, 20, 4823–4834.

(21) Yildiz, A.; Forkey, J. N.; McKinney, S. A.; Ha, T.; Goldman, Y. E.; Selvin,P. R. Science 2003, 300, 2061–2065.

(22) Grybos, J.; Marszałek, M.; Lekka, M.; Heinrich, F.; Troger, W. Hyperf.Interact. 2004, 159, 323–329.

Figure 5. Aggregation of FITC-dextran molecules near the centerof the light pattern due to the induced dipole of the molecules underthe optically induced electric field. The voltage condition was at 10Vpp at 1 kHz. The aggregated molecules were dispersed as soon asthe voltage turned off.

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means the bulk concentration of the sample droplet (Figure 6).The maximum molecular concentration with application of avoltage of 10 Vpp at 1 kHz frequency was proportional to theinitial concentration. The local concentration of moleculesconverged on a specific amount with a very fast response time.The time that the molecular concentration reaches the steadystate was not affected by both the magnitude and frequency ofthe operating voltage signal, while the steady-state concentra-tion of the molecules and the fluorescence intensity profileswere affected.

According to these results, we concluded that the moleculesmust be in a dynamic state and not be accumulated into theilluminated area. If the molecules concentrated by the opticallyinduced ACEO flows are accumulated, the maximum concentra-tion must have a constant value independent of the applied voltageand the initial concentration of the sample solution. However, theconcentration was saturated rapidly into a specific value, whichis proportional to the initial concentration and the applied ac signaland depends on the physical and chemical properties of themolecules.

The electrostatic interaction of the molecules could alsoaccount for this concentration-dependent phenomenon. If theamount of FITC-dextran molecules in the fluid is too small, thedipole attraction among the molecules might not be strong enoughto keep them in the illuminated area as well as the amount of themolecules, which exist around the light pattern and can beconcentrated into it, is also insufficient.23

Dynamic Control of FITC-Dextran Concentration. On thebasis of the frequency-dependent change of the local chemicalconcentration, dynamic control of the FITC-dextran concentrationwas performed as shown in Figure 7a. As switching the frequencyof the applied ac signal to 1, 10, and 100 kHz with an intervaltime of 10 s, the maximum fluorescence intensity was measuredaccording to the time. In this temporal modulation, the differencesof the molecular concentration against the applied ac frequencyincreased as the amplitude of the ac signal increased from 6 Vpp

to 10 Vpp. Under the 6 Vpp, the change of the fluorescence signaldue to the molecular movements was too small to detect overthe signal noise, since the ACEO flow and the dipole attraction

among the molecules were induced enough to concentrate themolecules. The response time at the frequency for increasingmolecular concentration, i.e., from 100 to 10 kHz and from 10to 1 kHz, was shorter than for decreasing it, i.e., from 1 to 10kHz and from 10 to 100 kHz. This was due to the diffusion ofthe concentrated molecules, which is a much slower transportmechanism than the optically induced ACEO flows.

The chemical concentration at the frequency of 100 kHz wasalso dependent on the amplitude of the applied ac signal, althoughthe change of local chemical concentration due to the opticallyinduced ACEO did not appear at 100 kHz according to Figure 4a.This phenomenon was due to the effect of a weak positive DEPforce, which could be induced by an optically induced nonuniformelectric field, as mentioned above.15 Under the consecutiveconcentration and release processes, the molecules, which werealready concentrated into the illuminated area at 1 and 10 kHz,would also be affected by the weak DEP force trapping thembefore they were diffused out from the area. Therefore, thepositive DEP as well as the ACEO flows and the electrostaticinteractions must affect the molecular behavior in the OFM.

Figure 7b shows the spatial control of the local chemicalconcentration of FITC-dextran using OFM. With application of avoltage of 10 Vpp at 1 kHz, FITC-dextran molecules were rapidlyconcentrated into a specific area by being exposed to the lightsource for fluorescence excitation. When we observed the(23) Du, J.-R.; Juang, Y.-J.; Wu, J.-T.; Wei, H.-H. Biomicrofluidics 2008, 2, 044103.

Figure 6. Plot of the maximum concentration of FITC-dextranmodulated by the optoelectrofluidics according to the initial concentra-tion of the sample droplet with application of a voltage of 10 Vpp at 1kHz. A linear regression line was also represented.

Figure 7. (a) Temporal control of the local concentration of FITC-dextran with changing the frequency and amplitude of the applied acsignal with an interval of 10 s (red/blue/green triangle: 100/10/1 kHzfrequency). (b) Spatial control of the local concentration of FITC-dextran was also possible as the light pattern was controlled withapplication of a voltage of 10 Vpp at 1 kHz (dotted circle: light pattern).

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sample after moving the light pattern and opening the irisdiaphragm, the molecular concentration at the localized areaalong the trajectory of moved light was significantly increased.Finally, the concentrated molecules were diffused out accordingto the time.

Dynamic Control of Fluorescein-BSA Concentration. Fluo-rescein-conjugated BSA, which is one of the most widely usedcrowding agents for protein folding and aggregation research,1,3

was also applied as a target molecule to control its local concentra-tion in a temporal and spatial manner as shown in parts a and bof Figure 8, respectively. The fluorescence intensity profiles ofBSA against the ac frequency showed very similar tendency tothat of dextran. However, the time for diffusing out of BSA waslonger than that of 10 kDa dextran. It was due to the smallerdiffusion coefficient of BSA (5.1 × 10-11 m2 s-1), of which themolecular weight is 65 kDa.24

We could also trap an aggregate, which is shown in Figure8b, using the optoelectrofluidic vortices. However, in this experi-ment, the moving velocity of the light pattern was too fast totransport the aggregate. The trapped aggregate could be manipu-lated, only when we moved the light pattern with much slowervelocity (data not shown). This phenomenon is in contradiction

to the results from the previous study, which emphasized thatthe trapping and transport of nanosized particles without los-ing them is possible using the light-induced ACEO.17 Accordingto these results, however, it is too difficult to trap and transportmolecules without any losses because of their dynamic behaviorand electrostatic interactions. We concluded that the trapping andtransport of molecules using the optically induced ACEO flow ispossible, but the dynamic control of local chemical concentrationis the more correct function of this OFM technique. As in thecase of colloidal particles, some molecules may be permanentlytrapped and assembled in a very thin layer on the surface of theoptically induced virtual electrode7 but the further studies for itsverification are necessary.

CONCLUSIONSDynamic control of the local chemical concentration was

demonstrated using a simple optoelectrofluidic platform, wherea conventional fluorescence microscope was combined with anoptoelectrofluidic device. In this OFM system, the light sourcefor fluorescence excitation was applied for both detection andmodulation of molecular concentration in a sample fluid. We firstinvestigated the frequency-dependent change of the local chemicalconcentration using the OFM. The optoelectrofluidic change ofthe local chemical concentration was dependent on the type ofmolecules and the bulk concentration as well as the applied acsignal. The investigation of dipole-dipole attraction among themolecules in the optoelectrofluidic device was also performed.Temporal control of the local molecular concentration has beendemonstrated by applying the frequency-dependency of severalelectrokinetic mechanisms and electrostatic interactions of bio-molecules such as FITC-dextran and fluorescein-BSA. Spatialcontrol of their concentration at the localized area was alsoperformed by controlling the light pattern.

This OFM system will be utilized as a useful tool in severalapplications such as molecular patterning and self-assembly aswell as studies about protein folding kinetics, cellular chemotaxis,molecular aggregation, and diffusion kinetics. Moreover, it canbe applied for the analysis of micro/nanoparticles and moleculesbased on their size, structure, and permittivity, which affect theirmass transfer and electrokinetic characteristics.

ACKNOWLEDGMENTSupport for this work from the Nano/Bio Science and Tech-

nology Program (Grant 2005-01291) and the National ResearchLaboratory (NRL) Program (Grant R0A-2008-000-20109-0) fundedby the Korea government (MEST) is gratefully acknowledged.The authors also thank the Chung Moon Soul Center forBioInformation and BioElectronics, KAIST, and the TFT-LCDResearch Center, Kyung Hee University, Korea.

SUPPORTING INFORMATION AVAILABLEFluorescence intensity profiles of FITC-dextran according to

the applied ac frequency. This material is available free of chargevia the Internet at http://pubs.acs.org.

Received for review May 14, 2009. Accepted May 18,2009.

AC901047V(24) Krouglova, T.; Vercammen, J.; Engelborghs, Y. Biophys. J. 2004, 87, 2635–

2646.

Figure 8. (a) Temporal control of the local concentration offluorescein-BSA as the frequency of the applied ac signal is changed(red/blue/green triangle: 100/10/1 kHz frequency). (b) Spatial controlof the local concentration of fluorescein-BSA was also possible asthe light pattern was controlled with application of a voltage of 10 Vpp

at 1 kHz (dotted circle: light pattern).

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