Controlled Transport of Droplets Using Conducting Polymers

Aug 17, 2009 - Australia, and ‡National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland. Received March 8, 2009. Revised Manus...
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Controlled Transport of Droplets Using Conducting Polymers Jennifer A. Halldorsson,† Shannon J. Little,† Dermot Diamond,‡ Geoffrey Spinks,† and Gordon Wallace*,† †

ARC Centre of Excellence for Electromaterials Science and Intelligent Polymer Research Institute, University of Wollongong, AIIM Building, Innovation Campus, Squires Way, Fairy Meadow, NSW 2522, Australia, and ‡National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland Received March 8, 2009. Revised Manuscript Received July 9, 2009 The controlled transport and delivery of dichloromethane through platinum mesh coated with dodecylbenzenesulfonate-doped polypyrrole is demonstrated upon in situ electrochemical redox switching. Droplets of dichloromethane were observed to pass freely through the mesh upon reduction of the polymer as a result of the release of the surfactant dopant into the dichloromethane and the change in the surface energy of the polymer. Planar and liquid-filled tube configurations are investigated. These concepts are envisaged to prove useful for fluid control in microfluidic devices, in the preparation of microparticles for drug delivery, and in the development of organic microreactors.

Introduction The control of fluids on the microscale is of critical importance in many applications including microfluidic systems,1-5 controlled drug delivery,6-8 and organic microreactors.9,10 In particular, the delivery of small volumes of liquids is crucial in inkjet printing and applications where rare or expensive reagents are used. Microfluidic devices have typically been composed of tiny interconnected microchannels through which continuous streams of liquid are moved. However, channel-based microfluidic devices often have problems associated with leakage and bonding of the device components.11 Droplet- or surface-based configurations have been increasingly investigated because of their ease of manufacture (planar surface compared to microfabricated channels) and the simplicity of introducing sample fluids into the device.10,12-14 The beneficial scaling effect of surface tension forces upon the miniaturization of device dimensions lends itself well to being exploited for fluid control in microfluidic devices.15 Because of their attractive mechanical properties and processability in addition to their ability to be reversibly switched between oxidized (conducting) and reduced (insulating) forms, *Corresponding author. E-mail: [email protected].

(1) Whitesides, G. M. Nature 2006, 442, 368. (2) Fair, R. B. Microfluid. Nanofluid. 2007, 3, 245. (3) Reyes, D. R.; Iossifidis, D.; Auroux, P. A.; Manz, A. Anal. Chem. 2002, 74, 2623. (4) Stone, H. A.; Stroock, A. D.; Ajdari, A. Ann. Rev. Fluid Mech. 2004, 36, 381. (5) Beebe, D. J.; Mensing, G. A.; Walker, G. M. Ann. Rev. Biomed. Eng. 2002, 4, 261. (6) Low, L. M.; Seetharaman, S.; He, K. Q.; Madou, M. J. Sens. Actuators, B 2000, 67, 149. (7) Xu, H.; Wang, C.; Wang, C. L.; Zoval, J.; Madou, M. Biosens. Bioelectron. 2006, 21, 2094. (8) Zafar Razzacki, S.; Thwar, P. K.; Yang, M.; Ugaz, V. M.; Burns, M. A. Adv. Drug Delivery Rev. 2004, 56, 185. (9) DeWitt, S. H. Curr. Opin. Chem. Biol. 1999, 3, 350. (10) Dubois, P.; Marchand, G.; Fouillet, Y.; Berthier, J.; Douki, T.; Hassine, F.; Gmouh, S.; Vaultier, M. Anal. Chem. 2006, 78, 4909. (11) Yussuf, A. A.; Sbarski, I.; Solomon, M.; Tran, N.; Hayes, J. P. J. Mater. Proc. Technol. 2007, 189, 401. (12) Becker, H.; Locascio, L. E. Talanta 2002, 56, 267. (13) Fair, R. B.; Khlystov, A.; Tailor, T. D.; Ivanov, V.; Evans, R. D.; Griffin, P. B.; Srinivasan, V.; Pamula, V. K.; Pollack, M. G.; Zhou, J. IEEE Design Test Comput. 2007, 24, 10. (14) Cho, S. K.; Moon, H. J.; Kim, C. J. J. Microelectromech. Syst. 2003, 12, 70. (15) Darhuber, A. A.; Troian, S. M. Ann. Rev. Fluid Mech. 2005, 37, 425.

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conducting organic polymers such as polypyrrole have found widespread use in recent years. The redox switching of these materials is often accompanied by a dramatic change in properties such as the polymer’s wettability and surface energy. Conducting polymers are well suited for fluid control because of this change in wettability, which arises as a result of the dopant incorporation and expulsion associated with switching between conducting and nonconducting forms. Wettability switch devices based on polyaniline and polythiophene have been reported by Berggren and co-workers16-18 as capable of moving water drops on their surfaces toward the more wettable polymer upon electrochemical redox switching. We have recently described the novel mechanism by which the shape and dimensions of organic liquids (typically dichloromethane, DCM) may be controlled upon in situ electrochemical oxidation and reduction of polypyrrole films coated onto a platinum sheet.19 The controlled release of surfactant dopant molecules (namely dodecylbenzenesulfonate, DBS) into the DCM droplet upon reduction of the polypyrrole (PPyDBS) and the subsequent incorporation of the surfactant dopant back into the polymer upon oxidation caused a dramatic spreading and contraction of DCM drops. Drops were observed to go from a contracted, spherical cap-like shape to an elongated “pancakelike” shape when the polymer was switched between oxidized and reduced states, respectively. This shape change was accompanied by concurrent reversible switching in the droplet contact angle between ca. 120° in the oxidized state and ca. 150° in the reduced state. The exchange of DBS between the polymer substrate and the DCM droplet dramatically alters the balance between surface forces and gravity, causing the droplet to flatten and spread when DBS enters the DCM droplet. The process was observed to be reversible for many hundreds of cycles, although the extent of change diminished with repeated cycling. Herein we report the (16) Isaksson, J.; Tengstedt, C.; Fahlman, M.; Robinson, N.; Berggren, M. Adv. Mater. 2004, 16, 316. (17) Robinson, L.; Hentzell, A.; Robinson, N.; Isaksson, J.; Berggren, M. Lab Chip 2006, 6, 1277. (18) Isaksson, J.; Robinson, N. D.; Berggren, M. Thin Solid Films 2006, 515, 2003. (19) Halldorsson, J. A.; Wu, Y.; Brown, H. R.; Spinks, G. M.; Wallace, G. G. Submitted to Langmuir.

Published on Web 08/17/2009

DOI: 10.1021/la900835w

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Figure 1. Electrochemical cell configuration for redox switching of PPyDBS in an aqueous electrolyte. Droplets of DCM were introduced onto the surface of a PPyDBS-coated Pt mesh using a syringe.

Figure 2. Electrochemical cell configuration for dispensing DCM from a glass tube via redox switching of a PPyDBS-coated Pt mesh in an aqueous electrolyte.

extension of this phenomenon to a mesh configuration, enabling the controlled transport of droplets through the polymer-coated mesh upon electrochemical redox switching of PPyDBS. We also consider using this phenomenon in a microdosing device.

Experimental Section PPyDBS was deposited on platinum mesh (wire diameter 0.06 mm, active surface area 120 mm2/cm2 for the mesh) to a charge density of 100 or 500 mC/cm2 by the application of 1 mA/cm2 for 100 or 500 s, respectively, in a three-electrode cell configuration containing 0.2 M pyrrole (Py) and 0.2 M sodium dodecylbenzenesulfonate (NaDBS). The platinum mesh was typically 1  2 cm2 in dimensions, with the active surface area of the mesh and the connecting wire determined to be 2.5 cm2. The counter electrode was a cylindrical Pt mesh encircling the working electrode. A Ag/AgCl (in 3 M KCl) reference electrode was used throughout these studies. Electrochemical redox switching was carried out in a square glass cell (2  2  2 cm3 in dimensions) containing a 0.1 M NaNO3 electrolyte and a Pt mesh counter electrode that was placed along the bottom edge of the cell (Figure 1). Drops of DCM (ca. 1 to 5 μL) were gently dispensed onto the surface of the polymer-coated mesh using a syringe after the mesh was immersed in the surrounding aqueous electrolyte. The working electrode potential was controlled to between +0.6 and -0.8 V, and the position of the DCM drops through the mesh was monitored using the video camera capability of a DataPhysics OCA20 goniometer. Pulse lengths for the application of oxidizing and reducing potentials were typically several seconds, though this was varied from less than a second to tens of seconds. To test the tube-dispensing configuration, a PPyDBS-coated mesh was prepared as described above, except that the mesh was wrapped around the end of a glass tube (tube diameter 5 mm, dosing hole diameter 1 mm) prior to deposition of the polymer. A platinum wire was connected to the mesh using spot welding. The mesh was secured in place around the tube using parafilm or an Oring. The tube/mesh was placed in a glass cell containing 0.1 M NaNO3 electrolyte using a three-electrode cell configuration as shown in Figure 2. The potential was controlled to between +0.6 and -0.8 V. The effect of the applied potential on the position of the DCM at the entrance of the tube and the volume of DCM passed through the mesh before droplet motion ceased was monitored.

Results and Discussion A platinum mesh coated galvanostatically with PPyDBS to a charge densitiy of 100 or 500 mC/cm2 was placed in a conventional three-electrode cell containing an aqueous electrolyte, as detailed in the Experimental Section. SEM images of a PPyDBScoated platinum mesh and an uncoated platinum mesh are shown in Figure 3. DCM drops, typically 1-5 μL in volume, were observed to adopt an approximately spherical cap shape with a 11138 DOI: 10.1021/la900835w

Figure 3. SEM image of PPyDBS deposited onto a Pt mesh using a charge density of 500 mC/cm2; the scale bar is 20 μm. (Inset) SEM image of a bare Pt mesh; the scale bar is 24 μm.

Figure 4. Video images of a DCM droplet on a PPyDBS-coated Pt mesh in a three-electrode cell configuration containing 0.1 M NaNO3 electrolyte (a) upon application of an oxidizing potential (+0.6 V vs Ag/AgCl) and (b) after application of a reducing potential for several seconds (-0.8 V vs Ag/AgCl). Refer to the Supporting Information for the original video. The scale bar is 1 mm.

contact angle of ca. 120° (Figure 4a) upon placement on the PPyDBS-coated mesh, consistent with analogous results on a solid platinum plate.19 Other chlorinated solvents, including trichloroethane, chloroform, and dichloroethane, were all found to exhibit similar shape changes to dichloromethane upon redox switching of PPyDBS. Other liquids such as glycerol, ethylene glycol, and some ionic liquids were tested but did not exhibit any shape changes.19 The DCM drops used throughout this study were observed to pass freely through the mesh upon the application of a reducing potential, with many smaller drops forming on the underside of the mesh (Figure 4b). Depending on the length of time for which the polymer was poised at a reducing potential, one of two scenarios occurred. First, if the reducing potential was applied for a prolonged period of time (several seconds or more), then the smaller drops passing through reassimilated into larger drops on the underside of the mesh. These drops continued to grow in size until they reached a sufficient weight such that the Langmuir 2009, 25(18), 11137–11141

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Figure 5. Schematic showing the change in DCM drop shape upon reversible incorporation and expulsion of minute amounts of DBS- dopant into and from PPyDBS upon oxidation and reduction in an external electrolyte (not shown), respectively.

gravitational force overcame the capillary forces that adhered the drop to the mesh, causing the drops to detach. In the other scenario, the application of an oxidizing potential prior to the drop detaching resulted in the droplet moving back up through the mesh and returning to its original shape and position. This response in droplet shape change was instantaneous upon redox switching and could be repeated reproducibly for many hundreds of cycles at varying pulse lengths, which varied from ca. 0.5 s to tens of seconds (Supporting Information). Dynamic fluid control through a mesh has been reported previously by Song et al.20 for the flow of water through a poly(N-isopropylacrylamide)-modified nanostructured copper mesh. However, the method used was markedly different from that described here because the flow was controlled by exploiting the amplified change in the water contact angle due to the nanostructuring of the surface. In the present case, the flow of dichloromethane through the mesh was mediated by the incorporation and release of surfactant from the polymer upon redox switching.19 The proposed mechanism for the shape change observed in dichloromethane (DCM) droplets resting on polypyrrole films doped with dodecylbenzenesulfonate (PPyDBS) upon application of oxidizing and reducing potentials (+0.6 V and -0.8 V vs Ag/AgCl, respectively) is detailed in Figure 5 and described in detail in reference 19. Upon reduction, sodium ions are incorporated into the polymer for charge neutrality, increasing the polymer wettability and decreasing the affinity of DCM for the polymer, leading to an increase in the contact angle. The surfactant near the interface between the polymer and DCM either at the actual surface or close to the surface of the polymer moves into the drop, thereby causing the DCM-electrolyte interfacial tension to decrease and the droplet to spread. Droplets of DCM resting on a bare Pt mesh were not observed to change shape upon pulsing the potential. However, if millimolar quantities of DBS were added to the DCM prior to its placement on the bare Pt mesh, then a small amount of the droplet was able to pass through because of a decrease in droplet surface tension and a reduction in capillary effects caused by the presence of the DBS surfactant. To investigate further the role of DBS in droplet transport, the concentration of DBS necessary to enable the transport of DCM droplets through a bare Pt mesh was quantified. Adding small (20) Song, W.; Xia, F.; Bai, Y.; Liu, F.; Sun, T. L.; Jiang, L. Langmuir 2007, 23, 327.

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increments of a NaDBS/DCM solution to a DCM drop resting on the Pt mesh surface gave a critical surfactant concentration of 0.013 M NaDBS for droplet transport. In our redox-driven experiments, a 5 μL DCM drop was in contact with approximately 0.024 mm3 of PPyDBS containing approximately 50% (by weight) DBS. Release of all of the DBS would give a concentration of ∼0.01 M DBS in DCM, which is close to the critical concentration needed for droplet movement on a bare Pt mesh. Droplet movement during the reduction of PPyDBS occurred almost immediately, however, well before all of the DBS could be released. Consequently, it is likely that changes in the PPy surface energy during reduction also contribute to the transport of DCM droplets. The extent of droplet shape change and subsequent movement through the polymer-coated mesh was dependent on the potential pulse length. DCM droplets changed dimensions less dramatically when using 1 s redox-switching pulses and remained resting on the top of the mesh because there was not sufficient time to pass through using such short pulse lengths. The application of longer reducing potentials (e.g., 5 s redox pulses) enabled the drops to move through the mesh as a result of the increased amount of DBS entering the droplets upon reduction of the polymer. Note that new drops placed in the same position took several cycles of switching from oxidized to reduced for drops to start moving through the mesh. This suggests that sufficient DBS must enter the DCM from the polymer before the droplet may pass through. Droplet movement through the mesh was observed to be reversible. Reduction of the PPyDBS released DBS into the DCM drop and altered the surface energy so that DCM droplets could pass through the mesh (Supporting Information).19 Subsequent oxidation of the PPy reabsorbed the DBS from the DCM and shifted the PPy surface energy, which resulted in the droplet moving against gravitational forces back to its original position. If the drops were allowed to detach from the mesh, then eventually the DCM shape-change effect ceased because of the depletion of DBS from the polymer via the DCM transported through the mesh. The opaque appearance of the accumulated DCM drops that had passed through the polymer-coated mesh was consistent with the presence of DBS in the DCM droplets. The use of thicker PPyDBS films (charge density of 500 mC/cm2) dramatically increased the switching lifetime for DCM drops as compared to that for thinner films (100 mC/cm2 charge density), presumably because of the larger reservoir of DBS dopant in the thicker polymer coatings. DCM drops resting on the polymer-coated mesh were observed to exhibit a slight shape change after extended redox cycling (ca. 2000 cycles); however, the effect had diminished greatly, and the movement was more like a slight adjustment in shape. This is presumably due to excess NaDBS in the DCM droplet, which may have prevented a large momentum change in the droplet. The influence of the electrolyte composition on the observed fluid movement was investigated by studying the electrochemistry (via cyclic voltammetry (CV)) of PPyDBS deposited on a glassy carbon disk electrode. Four different electrolytes-potassium chloride (KCl), tetrabutylammonium chloride (TBACl), sodium perchlorate (NaClO4) and sodium nitrate (NaNO3)-were investigated at a concentration of 0.1 M in all cases with all electrolytes being degassed via N2 purging prior to use. Cyclic voltammograms were obtained at a scan rate of 50 mV/s between an upper potential limit of þ0.6 V and a lower potential limit of -0.8 V as shown in Figure 6 for the different electrolytes. A redox couple at ca. -0.55 to -0.65 and -0.35 V (vs Ag/AgCl) due to the incorporation of Naþ into the polymer and the expulsion of DOI: 10.1021/la900835w

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Figure 6. Cyclic voltammograms of PPyDBS on a glassy carbon disk recorded in various electrolytes between the potential limits þ0.6 and -0.8 V; the second cycle is shown. All potentials are recorded vs the Ag/AgCl reference electrode.

Naþ into the electrolyte, respectively,21 was observed in all electrolytes except TBACl, which showed little response over the entire potential range. The peak at ca. -0.55 to -0.65 V has also been attributed to the expulsion of small anions into the electrolyte, whereas a shoulder in the anodic scan centered at ca. 0 V (vs Ag/AgCl) has been reported to be associated with the insertion of anions into the polymer from the electrolyte.21 Because DBS- is a relatively immobile anion, the charge-transfer characteristics of PPy are likely to be dominated by the cation. The large TBAþ cation cannot easily intercalate into the polymer, restricting the reduction of the polymer in the TBACl electrolyte. The transport of DCM through the mesh upon redox switching was found to depend upon the electrolyte used, with the quickest and most prolonged fluid movement occurring in 0.1 M NaNO3. This was presumably due to the higher mobility of the sodium ion compared to that of TBAþ and Kþ ions. NaNO3 electrolyte was therefore used in subsequent studies. Controlled droplet transport using PPyDBS was also investigated as a portable dosing system. The dosing device used a PPyDBS-coated mesh secured around the end of a narrow glass tube with a 1-mm-diameter opening at one end. The meshcovered tube was placed in the electrochemical cell containing 0.1 M NaNO3 electrolyte and was filled with DCM (Figure 7a). Prior to the application of a potential, the DCM was observed to dispense freely from the tube; however, the flow of DCM ceased upon applying an oxidizing potential (þ0.6 V) to the polymer. Hence, in subsequent experiments PPyDBS was held in the oxidized state prior to filling the tube with DCM. DCM was then observed to dispense freely from the tube only upon application of a reducing potential (-0.8 V) and to withdraw back into the tube upon oxidation of the polymer. This effect was immediate and reversible (Figure 7b). When drops of DCM became too large, the application of an oxidizing pulse was not sufficient to withdraw them back into the tube and the drops detached from the mesh. (See the video in Supporting Information.) A micropump configuration is envisaged whereby the glass tube containing the DCM is connected to another tube containing the immiscible liquid to be delivered, with aliquots dispensed upon application of a reducing potential to the polymer. Aliquots as small as 65 nL were able to be dispensed from and retracted into the tube, rendering this system useful for applications where such small delivery volumes are necessary, (21) Skaarup, S.; Bay, L.; Vidanapathirana, K.; Thybo, S.; Tofte, P.; West, K. Solid State Ionics 2003, 159, 143.

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Figure 7. Video images showing the control of DCM flow from a glass tube covered with a PPyDBS-coated Pt mesh in a threeelectrode cell containing 0.1 M NaNO3 electrolyte. (a) Droplets of DCM dispensed from the tube upon application of a reducing potential (-0.8 V vs Ag/AgCl) and (b) DCM droplets returning to the tube upon application of an oxidizing potential (þ0.6 V vs Ag/ AgCl). Arrows indicate the direction of droplet motion upon application of the potential. The scale bar is 1 mm. Refer to the Supporting Information for the original video.

such as for rare or expensive materials. A further decrease in the minimum dispensing volume may be achieved with optimization of the mesh dimensions. Gravitational forces were significant in the operation of the microdosing device. The PPyDBS-coated mesh in the oxidized state was capable of withstanding ca. 0.2 mL of DCM in the tube in the oxidized state before the liquid was freely expelled, which equates to a pressure of ca. 20 Pa that the oxidized polymer was able to withstand. If the DCM level was higher than 0.2 mL, then it dispensed from the tube freely until it reached the ca. 0.2 mL mark. The level to which the tube was filled with DCM affected the rate at which it was dispensed, with the stream dispensing more quickly when there was a greater amount of DCM (i.e., greater pressure) in the tube. In addition to this, the vertical position of the tube tip in the electrochemical cell was found to affect the rate at which droplets were dispensed during the application of a constant reducing potential. When the tube was positioned close to the electrolyte-air interface, drops of DCM were expelled quickly whereas when the tube was positioned closer to the bottom of the cell (ca. 1 cm deeper into the electrolyte) the drops were dispensed much more slowly as a result of the higher opposing water pressure at this depth. At least 1000 μL of DCM was passed through the tube, with effective dispensing control still exhibited by the polymer. This was quite surprising because the area of polymer controlling the fluid position was very small (ca. 0.1 mm2). It is believed that because the DCM was moving through so quickly as a result of the height of fluid in the tube, the DBS was not depleted from the polymer as quickly and hence the fluid-transport effect was controlled for a longer period of time.

Conclusions We have demonstrated the transport and control of the delivery of organic droplets, in particular, dichloromethane, through a polypyrrole-coated mesh upon in situ electrochemical redox switching. DCM was observed to pass freely through the PPyDBS-coated mesh upon application of negative potentials due to the release of DBS surfactant into the drop and the change in PPy surface energy. These changes shifted the balance between surface and gravitational forces so that the DCM passed through the mesh. This principle was extended from a planar configuration to a tube configuration, which enabled the position of droplet delivery to be precisely designated. These concepts could prove useful for fluid control in microfluidic devices and in the Langmuir 2009, 25(18), 11137–11141

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preparation of microparticles for drug delivery,22 where the presence of the surfactant in the droplet may prove beneficial in preventing droplet coalescence.23 It is envisaged that these principles could also be extended to develop an organic microreactor, whereby reactants are delivered in dichloromethane droplets and transported along tracks, as controlled by the oxidation state of the polymer. (22) Brown, D. M. Drug Delivery Systems in Cancer Therapy; Humana Press: Totowa, NJ, 2004. (23) Malmsten, M. Surfactants and Polymers in Drug Delivery; Marcel Dekker: New York, 2002.

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Acknowledgment. We are grateful to the Australian Research Council for continuing financial support. The technical insights and invaluable discussions provided by Yanzhe Wu are gratefully acknowledged. Supporting Information Available: Videos detailing the transport of dichloromethane through a PPyDBS-coated mesh in planar and tube configurations upon oxidation and reduction. This material is available free of charge via the Internet at http://pubs.acs.org.

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