Polyphosphoniumbased bipolar membranes for rectification of ionic currents Erik O. Gabrielsson and Magnus Berggren Citation: Biomicrofluidics 7, 064117 (2013); doi: 10.1063/1.4850795 View online: http://dx.doi.org/10.1063/1.4850795 View Table of Contents: http://scitation.aip.org/content/aip/journal/bmf/7/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Tuning nano electric field to affect restrictive membrane area on localized single cell nano-electroporation Appl. Phys. Lett. 103, 233701 (2013); 10.1063/1.4833535 Role of electropores on membrane blebbing—A model energy-based analysis J. Appl. Phys. 112, 064703 (2012); 10.1063/1.4754568 DNA-based ionic conducting membranes J. Appl. Phys. 110, 033704 (2011); 10.1063/1.3610951 Ionic conduction, rectification, and selectivity in single conical nanopores J. Chem. Phys. 124, 104706 (2006); 10.1063/1.2179797 Ultrasound-mediated disruption of cell membranes. II. Heterogeneous effects on cells J. Acoust. Soc. Am. 110, 597 (2001); 10.1121/1.1376130
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
BIOMICROFLUIDICS 7, 064117 (2013)
Polyphosphonium-based bipolar membranes for rectification of ionic currents Erik O. Gabrielsson and Magnus Berggrena) Department of Science and Technology, Link€ oping University, 601 74 Norrk€ oping, Sweden (Received 16 October 2013; accepted 5 December 2013; published online 18 December 2013)
Bipolar membranes (BMs) have interesting applications within the field of bioelectronics, as they may be used to create non-linear ionic components (e.g., ion diodes and transistors), thereby extending the functionality of, otherwise linear, electrophoretic drug delivery devices. However, BM based diodes suffer from a number of limitations, such as narrow voltage operation range and/or high hysteresis. In this work, we circumvent these problems by using a novel polyphosphonium-based BM, which is shown to exhibit improved diode characteristics. We believe that this new type of BM diode will be useful for creating complex addressable ionic circuits for delivery of charged biomolecules. C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4850795] V Combined electronic and ionic conduction makes organic electronic materials well suited for bioelectronics applications as a technological mean of translating electronic addressing signals into delivery of chemicals and ions.1 For complex regulation of functions in cells and tissues, a chemical circuit technology is necessary in order to generate complex and dynamic signal gradients with high spatiotemporal resolution. One approach to achieve a chemical circuit technology is to use bipolar membranes (BMs), which can be used to create the ionic equivalents of diodes2–5 and transistors.6–8 A BM consists of a stack of a cation- and an anionselective membrane, and functions similar to the semiconductor PN-junction, i.e., it offers ionic current rectification9,10 (Figure 1(a)). The high fixed charge concentration in a BM configuration make them more suited in bioelectronic applications as compared to other non-linear ionic devices, such as diodes constructed from surface charged nanopores11 or nanochannels,12 as the latter devices typically suffers from reduced performance at elevated electrolyte concentration (i.e., at physiological conditions) due to reduced Debye screening length.13 However, a unique property of most BMs, as compared to the electronic PN-junction and other ionic diodes, is the electric field enhanced (EFE) water dissociation effect.10,14 This occurs above a threshold reverse bias voltage, typically around 1 V, as the high electric field across the ion-depleted BM interface accelerates the forward reaction rate of the dissociation of water into Hþ and OH ions. As these ions migrate out from the BM, there will be an increase in the reverse bias current. The EFE water dissociation is a very interesting effect and is commonly used in industrial electrodialysis applications,15 where highly efficient water dissociating BMs are being researched.16 Also, BMs have also been utilized to generate Hþ and OH ions in lab-on-a-chip applications.2,17 However, the EFE water dissociation effect diminishes the diode property of BMs when operated outside the 61 V window, which is unwanted in, for instance, chemical circuits and addressing matrices for delivery of complex gradients of chemical species. The effect can be suppressed by incorporating a neutral electrolyte inside the BM,10,18 for instance, poly(ethylene glycol) (PEG).2,6,7 However, as previously reported,2 the PEG volume will introduce hysteresis when switching from forward to reverse bias, due to its ability to store large amounts of charges. This was circumvented by ensuring that only Hþ and OH are present in the diode, which recombines into water within the PEG volume. Such diodes are well suited as integrated components in chemical circuits for pH-regulation, due to the in situ formed Hþ and a)
Author to whom correspondence should be addressed. Electronic mail: [email protected].
1932-1058/2013/7(6)/064117/6/$30.00
7, 064117-1
C 2013 AIP Publishing LLC V
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-2
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
FIG. 1. (a) Ionic current rectification in a BM. In forward bias, mobile ions migrate towards the interface of the BM. The changing ion selectivity causes ion accumulation, resulting in high ion concentration and high conductivity. At high ion concentration, the selectivity of the membranes fails (Donnan exclusion failure), and ions start to pass the BM. In reverse bias, the mobile ions migrate away from the BM, eventually giving a zone with low ion concentration and low conductivity. Reverse bias can cause EFE water dissociation, producing Hþ and OH- ions. (b) Chemical structures of PSS, qPVBC, and PVBPPh3. (c) The device used to characterize the BMs and the BM1A, BM2A, and BM1P designs. The BM interfaces are 50 50 lm.
OH, but are less attractive if, for instance, other ions, e.g., biomolecules, are to be processed or delivered in and from the circuit. Furthermore, a PEG electrolyte introduces additional patterning layers, making device downscaling more challenging. This is undesired when designing complex, miniaturized, and large-scale ionic circuits. Thus, there is an interest in BM diodes that intrinsically do not exhibit any EFE water dissociation, are easy to miniaturize, and that turn off at relatively high speeds. It has been suggested that tertiary amines in the BM interface are important for efficient EFE water dissociation,19–21 as they function as a weak base and can therefore catalyze dissociation of water by accepting a proton. For example, anion-selective membranes that have undergone complete methylation, converting all tertiary amines to quaternary amines, shows no EFE water dissociation,19 although this effect was not permanent, as the quaternization was reversed upon application of a current. Similar results were found for anion-selective membranes containing alkali-metal complexing crown ethers as fixed charges.21 Also, EFE water dissociation was not observed or reduced in BMs with poor ion selectivity, for example, in BMs with low fixed-charge concentration5 or with predominantly secondary and tertiary amines in the anion-selective membrane,22 as the increased co-ion transport reduces the electric field at the BM interface. However, due to decreased ion selectivity, these membranes show reduced rectification. In this work, we present a non-amine based BM diode that avoids EFE water dissociation, enables easy miniaturization, and provides fast turn-off speeds and high rectification.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-3
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
FIG. 2. Linear sweep voltammetry from 4 to þ4 V (25 mV/s) for the BM diodes. The inset shows BM1P scanning from 40 V to þ4 V (250 mV/s).
An anion-selective phosphonium-based polycation (poly(vinylbenzyl chloride) (PVBC) quaternized by triphenylphospine, PVBPPh3) was synthesized and compared to the ammonium-based polycation (PVBC quaternized by dimethylbenzylamine, qPVBC) previously used in BM diodes2 and transistors,7,8 when included in BM diode structures together with polystyrenesulfonate (PSS) as the cation-selective material (Figure 1(b)). Three types of BM diodes were fabricated using standard photolithography patterning (Figure 1(c)), either with qPVBC (BM1A and BM2A) or PVBPPh3 (BM1P) as polycation and either with (BM2A) or without PEG (BM1A and BM1P). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes covered with aqueous electrolytes were used to convert electronic input signals into ionic currents through the BMs, according to the redox reaction PEDOTþ:PSS þ Mþ þ e $ PEDOT0 þ Mþ:PSS. The rectifying behavior of the diodes was evaluated using linear sweep voltammetry (Figure 2). The BM1A diode exhibited an increase in the reverse bias current for voltages lower than 1 V, a typical signature of EFE water dissociation,10,14 which decreased the current rectification at 64 V to 6.14. No such deviation in the reverse bias current was observed for BM2A and BM1P, which showed rectification ratios of 751 and 196, respectively. In fact, for BM1P, no evident EFE water dissociation was observed even at 40 V (see inset of Figure 2). Thus, the PVBPPh3 polycation allows BM diodes to operate at voltages beyond the 61 V window with maintained high ion current rectification. The dynamic performance of the diodes was tested by applying a square wave pulse from reverse bias to a forward bias voltage of 4 V with 5–90 s pulse duration (Figure 3). To access
FIG. 3. Switching characteristics (5, 10, 20, 30, 60, or 90 s pulse) and ion accumulation (at 90 s pulse) of the BM2A and BM1P diodes. BM1A showed similar characteristics as BM1P when switched at 61V (see supplementary material).24
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-4
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
the amount of charge injected and extracted during the forward bias and subsequent turn off, the current through the device was integrated. For BM2A, we observed that the fall time increased with the duration of the forward bias pulse. This hysteresis is due to the efficient storage of ions in the large PEG volume, with no ions crossing the BM due to Donnan exclusion failure.2 As a result, during the initial period of the return to reverse bias, a large amount of charge needs to be extracted in order to deplete the BM. After a 90 s pulse, 90.6% of the injected charge during the forward bias was extracted before turn-off. This may be contrasted with BM1P, where the fall time is hardly affected by the pulse duration, and the extracted/injected ratio is only 15.4% for a 90 s pulse. For this type of BM, the interface quickly becomes saturated with ions during forward bias, leading to Donnan exclusion failure and transport of ions across the BM.4 Thus, less charge needs to be extracted to deplete the BM, allowing for faster fall times and significantly reduced hysteresis. Since the neutral electrolyte is no longer required to obtain high ion current rectification over a wide potential range, the size of the BM can be miniaturized. This translates into higher component density when integrating the BM diode into ionic/chemical circuits. A miniaturized BM1P diode was constructed, where the interface of the BM was shrunk from 50 lm to 10 lm. The 10 lm device showed similar IV and switching characteristics as before (Figure 4), but with higher ion current rectification ratio (over 800) and decreased rise/fall times (corresponding to 90%/–10% of forward bias steady state) from 10 s/12.5 s to 4 s/4 s. Since the overlap area is smaller, a probable reason for the faster switching times is the reduced amount of ions needed to saturate and deplete the BM interface. In comparison to our previous reported low hysteresis BM diode,2 this miniaturized polyphosphonium-based devices shows the same rise and fall times but increased rectification ratio. In summary, by using polyphosphonium instead of polyammonium as the polycation in BM ion diodes the EFE water dissociation can be entirely suppressed over a large operational voltage window, supporting the theory that a weak base, such as a tertiary amine, is needed for efficient EFE water dissociation.17,18 As no additional neutral layer at the BM interface is needed, ion diodes that operate outside the usual EFE water dissociation window of 61 V can be constructed using less active layers, fewer processing steps and with relaxed alignment requirement as compared to polyammonium-based devices. This enables the fabrication of ion rectification devices with an active interface as low as 10 lm. Furthermore, the exclusion of a neutral layer improves the overall dynamic performance of the BM ion diode significantly, as there is less hysteresis due to ion accumulation. Previously, the hysteresis of BM ion diodes has been mitigated by designing the diode so that only Hþ and OH enters the BM, which then recombines into water.2 Such diodes also show high ion current rectification ratio and switching speed but are more complex to manufacture, and their application in organic bioelectronic systems is limited due to the Hþ/OH production. By instead using the polyphosphonium-based BM diode, reported here, we foresee ionic, complex, and miniaturized circuits that can include charged biomolecules as the signal carrier to regulate functions and the physiology in cell systems, such as in biomolecule and drug delivery applications, and also in lab-on-a-chip applications.
FIG. 4. (a) Linear sweep voltammetry and (b) switching performance of a BM1P diode with narrow junction.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-5
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
EXPERIMENTAL SECTION
Preparation of polyphosphonium: A modified version of the synthesis reported by OrnelasMegiatto et al.23 was used. Poly(vinylbenzyl chloride) (Sigma-Aldrich) was dissolved in tetrahydrofuran (THF) (200 mg/ml) and filtered (0.2 lm polytetrafluoroethylene membrane). 1720 mg of triphenylphospine (Sigma-Aldrich) was added to 5 ml PVBC solution and heated in a water bath (60 C for 24 h), after which excess solvent was removed and the precipitate dried. 4.5 ml of dH2O and 2.46 ml of 1-propanol was added and solution filtered (0.2 lm nylon membrane), followed by further addition of 6 ml of dH2O and 8 ml of 1-propanol. Preparation of qPVBC: 76 ll of dimethylbenzylamine (Sigma-Aldrich) was mixed with 200 ll of PVBC solution and heated in water bath (50 , 1 h). The precipitate was washed in THF and acetone and dissolved in 1 ml of dH2O and 1 ml of 1-propanol. For crosslinking, 6.6 ll of a 5 M diazabicyclo[2.2.2]octane (Sigma-Aldrich) solution in ethanol was added. Manufacturing of devices: Sheets of PEDOT:PSS on polyethylene terephthalate (PET) (AGFA-Gevaert OrgaconTM F-350) were cleaned with acetone and water and baked (5 min, 110 C). Layers of Poly(methyl methacrylate) (PMMA, Sigma-Aldrich, average m.w. 120 000, 4 mg/ml in Diethyl carbonate, 4000 rpm for 30 s, 5 min 110 C bake) and Shipley S1805G2 (4000 rpm for 30 s, 5 min 110 C bake) was spincoated and the photoresist exposed and developed using a Karl Suss MA/MB 6 mask aligner and Microposit MIF319 developer. The pattern was dry-etched in a CF4/O2 plasma for 30 s, and Shipley Remover 1112 A and acetone was used as removers. Overoxidation was performed through a masking Microposit S1813G2 photoresist layer (4000 rpm, 10 min 110 C bake) by incubation in aqueous sodium hypochlorite solution (1% (vol/vol)) for 45 s, followed by Shipley Remover 1112 A remover. A 2.5 lm thick SU8-layer (SU8-2002, MicroChem) was patterned on top according to the manufacturer’s instructions. The anion-selective membranes (qPVBC and PVBPPh3 was spin coated (1500 rpm for 45 s) on top of a hard baked (1 h at 110 C) protecting Shipley S1813G2 layer and baked for 1 h at 110 C. The membranes were etched with a CF4/O2 plasma (100 s) using a PMMA (40 mg/ml in DEC, 2000 rpm for 30 s, 10 min bake 110 C) layer, and a patterned Shipley S1813G2 (4000 rpm, 30 s, 10 min 110 C bake) photoresist layer as mask, followed by clean-up with acetone and isopropanol. A layer of 10 lm thick SU8 (SU8-2010, Microchemist) was patterned (3000 rpm, 30 s) to seal or define the opening of the BMs. For BM2A, the opening was filled with the neutral electrolyte (PEG, m.w. 950–1050, Sigma-Aldrich) in cyclopentanone (500 mg/ml) by inkjet printing (Fujifilm Dimatix Material Printer 2800) and sealed using polydimethylsiloxane (Sylgard 184, 10:1 curing agent ratio, cured 1 h at 100 C). Finally, silver contacts were painted on the electrodes and baked for 10 min (100 C). Device characterization: The devices were soaked for 24 h in dH2O before measuring. 0.1 M sodium chloride in dH2O was used as electrolytes. A Keithley 2602A source meter controlled with LabVIEW was used for sourcing voltages and measuring currents at 4 Hz sampling rate.
ACKNOWLEDGMENTS
This research was financed by VINNOVA (OBOE Milj€o and AFM), the Swedish Research € Council and the Onnesj€ o foundation. The authors also acknowledge Dr. Klas Tybrandt and Dr. Roger Gabrielsson at Link€oping University for fruitful discussions. 1
G. Tarabella, F. M. Mohammadi, N. Coppede, F. Barbero, S. Iannotta, C. Santato, and F. Cicoira, Chem. Sci. 4(4), 1395 (2013); M. Berggren and A. Richter-Dahlfors, Adv. Mater. 19(20), 3201 (2007); G. G. Malliaras, Biochim. Biophys. Acta 1830(9), 4286 (2013). 2 E. O. Gabrielsson, K. Tybrandt, and M. Berggren, Lab Chip 12(14), 2507 (2012). 3 J. H. Han, K. B. Kim, H. C. Kim, and T. D. Chung, Angew. Chem., Int. Ed. 48(21), 3830 (2009). 4 J.-H. Han, K. B. Kim, J. H. Bae, B. J. Kim, C. M. Kang, H. C. Kim, and T. D. Chung, Small 7(18), 2629 (2011). 5 O. J. Cayre, T. C. Suk, and O. D. Velev, J. Am. Chem. Soc. 129(35), 10801 (2007). 6 K. Tybrandt, K. C. Larsson, A. Richter-Dahlfors, and M. Berggren, Proc. Natl. Acad. Sci. U.S.A. 107(22), 9929 (2010). 7 K. Tybrandt, E. O. Gabrielsson, and M. Berggren, J. Am. Chem. Soc. 133(26), 10141 (2011). 8 K. Tybrandt, R. Forchheimer, and M. Berggren, Nat. Commun. 3, 871 (2012). 9 B. Lovrecˇek, A. Despic´, and J. O’M Bockris, J. Phys. Chem. 63(5), 750 (1959).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-6
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
10
S. Mafe and P. Ramirez, Acta Polym. 48(7), 234 (1997). I. Vlassiouk and Z. S. Siwy, Nano Lett. 7(3), 552 (2007); M. Ali, P. Ramirez, S. Mafe, R. Neumann, and W. Ensinger, ACS Nano 3(3), 603 (2009). 12 L. J. Cheng and L. J. Guo, ACS Nano 3(3), 575 (2009); R. Karnik, C. H. Duan, K. Castelino, H. Daiguji, and A. Majumdar, Nano Lett. 7(3), 547 (2007); W. Guan, R. Fan, and M. A. Reed, Nat. Commun. 2, 506 (2011). 13 R. B. Schoch, J. Han, and P. Renaud, Rev. Mod. Phys. 80(3), 839 (2008). 14 S. Mafe, P. Ramırez, and A. Alcaraz, Chem. Phys. Lett. 294(4-5), 406 (1998). 15 C. Gavach and G. Pourcelly, Bipolar Membrane Applications (Twente University Press, Enschede, 2000). 16 N. V. D. Vegt, F. Wilhelm, M. Wessling, and H. Strathmann, in Handbook Bipolar Membrane Technology, edited by A. J. B. Kemperman (Twente University Press, Enschede, 2000), p. 79. 17 L. J. Cheng and H. C. Chang, Biomicrofluidics 5(4), 046502 (2011). 18 R. Simons and G. Khanarian, J. Membr. Biol. 38(1-2), 11 (1978); Ngoc-Ty Dang and D. Woermann, Ber. Bunsenges. phys. Chem. 97(2), 149 (1993); S. Mafe, P. Ramirez, and J. A. Manzanares, ibid. 98(2), 202 (1994); H. Strathmann, J. J. Krol, H. J. Rapp, and G. Eigenberger, J. Membr. Sci. 125(1), 123 (1997). 19 R. Simons, Desalination 28(1), 41 (1979). 20 R. Simons, Nature 280(5725), 824 (1979); R. Simons, Electrochim. Acta 30(3), 275 (1985). 21 I. Rubinstein, A. Warshawsky, L. Schechtman, and O. Kedem, Desalination 51(1), 55 (1984). 22 T. Hosono and A. Tanioka, Polymer 39(18), 4199 (1998); A. Tanioka and K. Shimizu, Bull. Soc. Sea Water Sci., Jpn. 51(4), 205 (1997), available at https://www.jstage.jst.go.jp/article/swsj1965/51/4/51_205/_article. 23 C. Ornelas-Megiatto, P. R. Wich, and J. M. J. Frechet, J. Am. Chem. Soc. 134(4), 1902 (2012). 24 See supplementary material at http://dx.doi.org/10.1063/1.4850795 for switching characteristics of BM1A. 11
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
BIOMICROFLUIDICS 7, 064117 (2013)
Polyphosphonium-based bipolar membranes for rectification of ionic currents Erik O. Gabrielsson and Magnus Berggrena) Department of Science and Technology, Link€ oping University, 601 74 Norrk€ oping, Sweden (Received 16 October 2013; accepted 5 December 2013; published online 18 December 2013)
Bipolar membranes (BMs) have interesting applications within the field of bioelectronics, as they may be used to create non-linear ionic components (e.g., ion diodes and transistors), thereby extending the functionality of, otherwise linear, electrophoretic drug delivery devices. However, BM based diodes suffer from a number of limitations, such as narrow voltage operation range and/or high hysteresis. In this work, we circumvent these problems by using a novel polyphosphonium-based BM, which is shown to exhibit improved diode characteristics. We believe that this new type of BM diode will be useful for creating complex addressable ionic circuits for delivery of charged biomolecules. C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4850795] V Combined electronic and ionic conduction makes organic electronic materials well suited for bioelectronics applications as a technological mean of translating electronic addressing signals into delivery of chemicals and ions.1 For complex regulation of functions in cells and tissues, a chemical circuit technology is necessary in order to generate complex and dynamic signal gradients with high spatiotemporal resolution. One approach to achieve a chemical circuit technology is to use bipolar membranes (BMs), which can be used to create the ionic equivalents of diodes2–5 and transistors.6–8 A BM consists of a stack of a cation- and an anionselective membrane, and functions similar to the semiconductor PN-junction, i.e., it offers ionic current rectification9,10 (Figure 1(a)). The high fixed charge concentration in a BM configuration make them more suited in bioelectronic applications as compared to other non-linear ionic devices, such as diodes constructed from surface charged nanopores11 or nanochannels,12 as the latter devices typically suffers from reduced performance at elevated electrolyte concentration (i.e., at physiological conditions) due to reduced Debye screening length.13 However, a unique property of most BMs, as compared to the electronic PN-junction and other ionic diodes, is the electric field enhanced (EFE) water dissociation effect.10,14 This occurs above a threshold reverse bias voltage, typically around 1 V, as the high electric field across the ion-depleted BM interface accelerates the forward reaction rate of the dissociation of water into Hþ and OH ions. As these ions migrate out from the BM, there will be an increase in the reverse bias current. The EFE water dissociation is a very interesting effect and is commonly used in industrial electrodialysis applications,15 where highly efficient water dissociating BMs are being researched.16 Also, BMs have also been utilized to generate Hþ and OH ions in lab-on-a-chip applications.2,17 However, the EFE water dissociation effect diminishes the diode property of BMs when operated outside the 61 V window, which is unwanted in, for instance, chemical circuits and addressing matrices for delivery of complex gradients of chemical species. The effect can be suppressed by incorporating a neutral electrolyte inside the BM,10,18 for instance, poly(ethylene glycol) (PEG).2,6,7 However, as previously reported,2 the PEG volume will introduce hysteresis when switching from forward to reverse bias, due to its ability to store large amounts of charges. This was circumvented by ensuring that only Hþ and OH are present in the diode, which recombines into water within the PEG volume. Such diodes are well suited as integrated components in chemical circuits for pH-regulation, due to the in situ formed Hþ and a)
Author to whom correspondence should be addressed. Electronic mail: [email protected].
1932-1058/2013/7(6)/064117/6/$30.00
7, 064117-1
C 2013 AIP Publishing LLC V
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-2
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
FIG. 1. (a) Ionic current rectification in a BM. In forward bias, mobile ions migrate towards the interface of the BM. The changing ion selectivity causes ion accumulation, resulting in high ion concentration and high conductivity. At high ion concentration, the selectivity of the membranes fails (Donnan exclusion failure), and ions start to pass the BM. In reverse bias, the mobile ions migrate away from the BM, eventually giving a zone with low ion concentration and low conductivity. Reverse bias can cause EFE water dissociation, producing Hþ and OH- ions. (b) Chemical structures of PSS, qPVBC, and PVBPPh3. (c) The device used to characterize the BMs and the BM1A, BM2A, and BM1P designs. The BM interfaces are 50 50 lm.
OH, but are less attractive if, for instance, other ions, e.g., biomolecules, are to be processed or delivered in and from the circuit. Furthermore, a PEG electrolyte introduces additional patterning layers, making device downscaling more challenging. This is undesired when designing complex, miniaturized, and large-scale ionic circuits. Thus, there is an interest in BM diodes that intrinsically do not exhibit any EFE water dissociation, are easy to miniaturize, and that turn off at relatively high speeds. It has been suggested that tertiary amines in the BM interface are important for efficient EFE water dissociation,19–21 as they function as a weak base and can therefore catalyze dissociation of water by accepting a proton. For example, anion-selective membranes that have undergone complete methylation, converting all tertiary amines to quaternary amines, shows no EFE water dissociation,19 although this effect was not permanent, as the quaternization was reversed upon application of a current. Similar results were found for anion-selective membranes containing alkali-metal complexing crown ethers as fixed charges.21 Also, EFE water dissociation was not observed or reduced in BMs with poor ion selectivity, for example, in BMs with low fixed-charge concentration5 or with predominantly secondary and tertiary amines in the anion-selective membrane,22 as the increased co-ion transport reduces the electric field at the BM interface. However, due to decreased ion selectivity, these membranes show reduced rectification. In this work, we present a non-amine based BM diode that avoids EFE water dissociation, enables easy miniaturization, and provides fast turn-off speeds and high rectification.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-3
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
FIG. 2. Linear sweep voltammetry from 4 to þ4 V (25 mV/s) for the BM diodes. The inset shows BM1P scanning from 40 V to þ4 V (250 mV/s).
An anion-selective phosphonium-based polycation (poly(vinylbenzyl chloride) (PVBC) quaternized by triphenylphospine, PVBPPh3) was synthesized and compared to the ammonium-based polycation (PVBC quaternized by dimethylbenzylamine, qPVBC) previously used in BM diodes2 and transistors,7,8 when included in BM diode structures together with polystyrenesulfonate (PSS) as the cation-selective material (Figure 1(b)). Three types of BM diodes were fabricated using standard photolithography patterning (Figure 1(c)), either with qPVBC (BM1A and BM2A) or PVBPPh3 (BM1P) as polycation and either with (BM2A) or without PEG (BM1A and BM1P). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes covered with aqueous electrolytes were used to convert electronic input signals into ionic currents through the BMs, according to the redox reaction PEDOTþ:PSS þ Mþ þ e $ PEDOT0 þ Mþ:PSS. The rectifying behavior of the diodes was evaluated using linear sweep voltammetry (Figure 2). The BM1A diode exhibited an increase in the reverse bias current for voltages lower than 1 V, a typical signature of EFE water dissociation,10,14 which decreased the current rectification at 64 V to 6.14. No such deviation in the reverse bias current was observed for BM2A and BM1P, which showed rectification ratios of 751 and 196, respectively. In fact, for BM1P, no evident EFE water dissociation was observed even at 40 V (see inset of Figure 2). Thus, the PVBPPh3 polycation allows BM diodes to operate at voltages beyond the 61 V window with maintained high ion current rectification. The dynamic performance of the diodes was tested by applying a square wave pulse from reverse bias to a forward bias voltage of 4 V with 5–90 s pulse duration (Figure 3). To access
FIG. 3. Switching characteristics (5, 10, 20, 30, 60, or 90 s pulse) and ion accumulation (at 90 s pulse) of the BM2A and BM1P diodes. BM1A showed similar characteristics as BM1P when switched at 61V (see supplementary material).24
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-4
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
the amount of charge injected and extracted during the forward bias and subsequent turn off, the current through the device was integrated. For BM2A, we observed that the fall time increased with the duration of the forward bias pulse. This hysteresis is due to the efficient storage of ions in the large PEG volume, with no ions crossing the BM due to Donnan exclusion failure.2 As a result, during the initial period of the return to reverse bias, a large amount of charge needs to be extracted in order to deplete the BM. After a 90 s pulse, 90.6% of the injected charge during the forward bias was extracted before turn-off. This may be contrasted with BM1P, where the fall time is hardly affected by the pulse duration, and the extracted/injected ratio is only 15.4% for a 90 s pulse. For this type of BM, the interface quickly becomes saturated with ions during forward bias, leading to Donnan exclusion failure and transport of ions across the BM.4 Thus, less charge needs to be extracted to deplete the BM, allowing for faster fall times and significantly reduced hysteresis. Since the neutral electrolyte is no longer required to obtain high ion current rectification over a wide potential range, the size of the BM can be miniaturized. This translates into higher component density when integrating the BM diode into ionic/chemical circuits. A miniaturized BM1P diode was constructed, where the interface of the BM was shrunk from 50 lm to 10 lm. The 10 lm device showed similar IV and switching characteristics as before (Figure 4), but with higher ion current rectification ratio (over 800) and decreased rise/fall times (corresponding to 90%/–10% of forward bias steady state) from 10 s/12.5 s to 4 s/4 s. Since the overlap area is smaller, a probable reason for the faster switching times is the reduced amount of ions needed to saturate and deplete the BM interface. In comparison to our previous reported low hysteresis BM diode,2 this miniaturized polyphosphonium-based devices shows the same rise and fall times but increased rectification ratio. In summary, by using polyphosphonium instead of polyammonium as the polycation in BM ion diodes the EFE water dissociation can be entirely suppressed over a large operational voltage window, supporting the theory that a weak base, such as a tertiary amine, is needed for efficient EFE water dissociation.17,18 As no additional neutral layer at the BM interface is needed, ion diodes that operate outside the usual EFE water dissociation window of 61 V can be constructed using less active layers, fewer processing steps and with relaxed alignment requirement as compared to polyammonium-based devices. This enables the fabrication of ion rectification devices with an active interface as low as 10 lm. Furthermore, the exclusion of a neutral layer improves the overall dynamic performance of the BM ion diode significantly, as there is less hysteresis due to ion accumulation. Previously, the hysteresis of BM ion diodes has been mitigated by designing the diode so that only Hþ and OH enters the BM, which then recombines into water.2 Such diodes also show high ion current rectification ratio and switching speed but are more complex to manufacture, and their application in organic bioelectronic systems is limited due to the Hþ/OH production. By instead using the polyphosphonium-based BM diode, reported here, we foresee ionic, complex, and miniaturized circuits that can include charged biomolecules as the signal carrier to regulate functions and the physiology in cell systems, such as in biomolecule and drug delivery applications, and also in lab-on-a-chip applications.
FIG. 4. (a) Linear sweep voltammetry and (b) switching performance of a BM1P diode with narrow junction.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-5
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
EXPERIMENTAL SECTION
Preparation of polyphosphonium: A modified version of the synthesis reported by OrnelasMegiatto et al.23 was used. Poly(vinylbenzyl chloride) (Sigma-Aldrich) was dissolved in tetrahydrofuran (THF) (200 mg/ml) and filtered (0.2 lm polytetrafluoroethylene membrane). 1720 mg of triphenylphospine (Sigma-Aldrich) was added to 5 ml PVBC solution and heated in a water bath (60 C for 24 h), after which excess solvent was removed and the precipitate dried. 4.5 ml of dH2O and 2.46 ml of 1-propanol was added and solution filtered (0.2 lm nylon membrane), followed by further addition of 6 ml of dH2O and 8 ml of 1-propanol. Preparation of qPVBC: 76 ll of dimethylbenzylamine (Sigma-Aldrich) was mixed with 200 ll of PVBC solution and heated in water bath (50 , 1 h). The precipitate was washed in THF and acetone and dissolved in 1 ml of dH2O and 1 ml of 1-propanol. For crosslinking, 6.6 ll of a 5 M diazabicyclo[2.2.2]octane (Sigma-Aldrich) solution in ethanol was added. Manufacturing of devices: Sheets of PEDOT:PSS on polyethylene terephthalate (PET) (AGFA-Gevaert OrgaconTM F-350) were cleaned with acetone and water and baked (5 min, 110 C). Layers of Poly(methyl methacrylate) (PMMA, Sigma-Aldrich, average m.w. 120 000, 4 mg/ml in Diethyl carbonate, 4000 rpm for 30 s, 5 min 110 C bake) and Shipley S1805G2 (4000 rpm for 30 s, 5 min 110 C bake) was spincoated and the photoresist exposed and developed using a Karl Suss MA/MB 6 mask aligner and Microposit MIF319 developer. The pattern was dry-etched in a CF4/O2 plasma for 30 s, and Shipley Remover 1112 A and acetone was used as removers. Overoxidation was performed through a masking Microposit S1813G2 photoresist layer (4000 rpm, 10 min 110 C bake) by incubation in aqueous sodium hypochlorite solution (1% (vol/vol)) for 45 s, followed by Shipley Remover 1112 A remover. A 2.5 lm thick SU8-layer (SU8-2002, MicroChem) was patterned on top according to the manufacturer’s instructions. The anion-selective membranes (qPVBC and PVBPPh3 was spin coated (1500 rpm for 45 s) on top of a hard baked (1 h at 110 C) protecting Shipley S1813G2 layer and baked for 1 h at 110 C. The membranes were etched with a CF4/O2 plasma (100 s) using a PMMA (40 mg/ml in DEC, 2000 rpm for 30 s, 10 min bake 110 C) layer, and a patterned Shipley S1813G2 (4000 rpm, 30 s, 10 min 110 C bake) photoresist layer as mask, followed by clean-up with acetone and isopropanol. A layer of 10 lm thick SU8 (SU8-2010, Microchemist) was patterned (3000 rpm, 30 s) to seal or define the opening of the BMs. For BM2A, the opening was filled with the neutral electrolyte (PEG, m.w. 950–1050, Sigma-Aldrich) in cyclopentanone (500 mg/ml) by inkjet printing (Fujifilm Dimatix Material Printer 2800) and sealed using polydimethylsiloxane (Sylgard 184, 10:1 curing agent ratio, cured 1 h at 100 C). Finally, silver contacts were painted on the electrodes and baked for 10 min (100 C). Device characterization: The devices were soaked for 24 h in dH2O before measuring. 0.1 M sodium chloride in dH2O was used as electrolytes. A Keithley 2602A source meter controlled with LabVIEW was used for sourcing voltages and measuring currents at 4 Hz sampling rate.
ACKNOWLEDGMENTS
This research was financed by VINNOVA (OBOE Milj€o and AFM), the Swedish Research € Council and the Onnesj€ o foundation. The authors also acknowledge Dr. Klas Tybrandt and Dr. Roger Gabrielsson at Link€oping University for fruitful discussions. 1
G. Tarabella, F. M. Mohammadi, N. Coppede, F. Barbero, S. Iannotta, C. Santato, and F. Cicoira, Chem. Sci. 4(4), 1395 (2013); M. Berggren and A. Richter-Dahlfors, Adv. Mater. 19(20), 3201 (2007); G. G. Malliaras, Biochim. Biophys. Acta 1830(9), 4286 (2013). 2 E. O. Gabrielsson, K. Tybrandt, and M. Berggren, Lab Chip 12(14), 2507 (2012). 3 J. H. Han, K. B. Kim, H. C. Kim, and T. D. Chung, Angew. Chem., Int. Ed. 48(21), 3830 (2009). 4 J.-H. Han, K. B. Kim, J. H. Bae, B. J. Kim, C. M. Kang, H. C. Kim, and T. D. Chung, Small 7(18), 2629 (2011). 5 O. J. Cayre, T. C. Suk, and O. D. Velev, J. Am. Chem. Soc. 129(35), 10801 (2007). 6 K. Tybrandt, K. C. Larsson, A. Richter-Dahlfors, and M. Berggren, Proc. Natl. Acad. Sci. U.S.A. 107(22), 9929 (2010). 7 K. Tybrandt, E. O. Gabrielsson, and M. Berggren, J. Am. Chem. Soc. 133(26), 10141 (2011). 8 K. Tybrandt, R. Forchheimer, and M. Berggren, Nat. Commun. 3, 871 (2012). 9 B. Lovrecˇek, A. Despic´, and J. O’M Bockris, J. Phys. Chem. 63(5), 750 (1959).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
064117-6
E. O. Gabrielsson and M. Berggren
Biomicrofluidics 7, 064117 (2013)
10
S. Mafe and P. Ramirez, Acta Polym. 48(7), 234 (1997). I. Vlassiouk and Z. S. Siwy, Nano Lett. 7(3), 552 (2007); M. Ali, P. Ramirez, S. Mafe, R. Neumann, and W. Ensinger, ACS Nano 3(3), 603 (2009). 12 L. J. Cheng and L. J. Guo, ACS Nano 3(3), 575 (2009); R. Karnik, C. H. Duan, K. Castelino, H. Daiguji, and A. Majumdar, Nano Lett. 7(3), 547 (2007); W. Guan, R. Fan, and M. A. Reed, Nat. Commun. 2, 506 (2011). 13 R. B. Schoch, J. Han, and P. Renaud, Rev. Mod. Phys. 80(3), 839 (2008). 14 S. Mafe, P. Ramırez, and A. Alcaraz, Chem. Phys. Lett. 294(4-5), 406 (1998). 15 C. Gavach and G. Pourcelly, Bipolar Membrane Applications (Twente University Press, Enschede, 2000). 16 N. V. D. Vegt, F. Wilhelm, M. Wessling, and H. Strathmann, in Handbook Bipolar Membrane Technology, edited by A. J. B. Kemperman (Twente University Press, Enschede, 2000), p. 79. 17 L. J. Cheng and H. C. Chang, Biomicrofluidics 5(4), 046502 (2011). 18 R. Simons and G. Khanarian, J. Membr. Biol. 38(1-2), 11 (1978); Ngoc-Ty Dang and D. Woermann, Ber. Bunsenges. phys. Chem. 97(2), 149 (1993); S. Mafe, P. Ramirez, and J. A. Manzanares, ibid. 98(2), 202 (1994); H. Strathmann, J. J. Krol, H. J. Rapp, and G. Eigenberger, J. Membr. Sci. 125(1), 123 (1997). 19 R. Simons, Desalination 28(1), 41 (1979). 20 R. Simons, Nature 280(5725), 824 (1979); R. Simons, Electrochim. Acta 30(3), 275 (1985). 21 I. Rubinstein, A. Warshawsky, L. Schechtman, and O. Kedem, Desalination 51(1), 55 (1984). 22 T. Hosono and A. Tanioka, Polymer 39(18), 4199 (1998); A. Tanioka and K. Shimizu, Bull. Soc. Sea Water Sci., Jpn. 51(4), 205 (1997), available at https://www.jstage.jst.go.jp/article/swsj1965/51/4/51_205/_article. 23 C. Ornelas-Megiatto, P. R. Wich, and J. M. J. Frechet, J. Am. Chem. Soc. 134(4), 1902 (2012). 24 See supplementary material at http://dx.doi.org/10.1063/1.4850795 for switching characteristics of BM1A. 11
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 155.33.16.124 On: Tue, 25 Nov 2014 07:07:27
