Electrically Switchable Monostable Actuatoric Polymer-Based

Sep 11, 2018 - ... toxic dopant supplementation of the flow-through solution. Thus, our novel functional long-term stable nanovalve array offers the c...
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Electrically switchable monostable actuatoric polymer based nanovalve arrays with long-term stability Christoph Prönnecke, Marek Staude, Ronny Frank, Heinz-Georg Jahnke, and Andrea A Robitzki Nano Lett., Just Accepted Manuscript • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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Electrically switchable monostable actuatoric polymer based nanovalve arrays with long-term stability

Christoph Prönnecke, Marek Staude, Ronny Frank, Heinz-Georg Jahnke and Andrea A. Robitzki* Centre for Biotechnology and Biomedicine (BBZ), Molecular biological-biochemical Processing Technology, Deutscher Platz 5, D-04103 Leipzig, Germany *

Corresponding author: Andrea A. Robitzki E-mail: [email protected], Phone: +49 341 9731241 Fax: +49 341 9731249

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Abstract Here we present a novel electrically switchable nanovalve array based on an intrinsic conductive polymer that has the capabilities to change its volume depending on its redox state. The polymer is created by anodic deposition of a sodium dodecylbenzenesulfonate (DBS) doped polypyrrole (PPy). Optimization of the DBS doped PPy layers revealed an actuatoric performance of up to 10 % out of plane volume change. More interestingly, the electrochemical characterization revealed an actuatoric monostable polymer that could be used to fabricate nanovalve arrays that have a native opened state when no potential is applied and that can be closed when a reductive potential is applied. As a proof of concept, Atto488 labeled Biotin was used as a model compound and defined nanovalve arrays with nanopores in the range of 10 nm in diameter (opened state) were fabricated. Afterwards, we were able to successfully prove the functionality of our nanovalve array by monitoring the flowthrough rates of the Biotin-Atto488. More strikingly, we could demonstrate for the first time the robust and long-term stability of our nanovalve array without any performance loss for at least 72 hours and retention capabilities of up to 90 %. Furthermore, the demonstrated long-term stability was achieved under biocompatible conditions without the need of toxic dopant supplementation of the flow-through solution. Thus, our novel functional long-term stable nanovalve array offers the capabilities for practical applications.

Keywords nanovalve array, nanoporous aluminium oxide, actuatoric polymer, polypyrrole,

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Main In the evolving field of nanotechnology there are considerable efforts to develop actuatoric systems that can be used to build up controllable nanovalves. While there are publications demonstrating approaches based on actuatoric regulation by changing the pH1, temperature2, light3 or magnetic forces4, there are still many critical issues especially concerning performance, reproducibility and long-term stability that have prevented the use of such nanovalves in applications such as integrated lab on chip systems or compound release systems so far. In this context, we focused on the use of the conductive electroactive polymer polypyrrole (PPy) doped with anions that have the capability to change the volume in the range of 5-30 % according to published studies5-7. This offers the great advantage of regulating the nanovalve by applying a defined potential. The basic electrochemical principle for opening and closing of the nanovalve is shown in Fig. 1a. The mechanism of volume change of polypyrrole doped with stationary anions is already described in detail8. Basically, during the oxidative manufacturing process of the polymer a positive charged backbone is generated on a conductive surface (e.g. gold or platinum) within a nanoporous substrate (Fig. 1b). Charge balance is provided by an influx and integration of stationary anions (dopants) which are not able to leave the polymer matrix. This oxidized form represents the opened nanovalve. After reduction of the polypyrrole by applying a reductive potential the backbone turns neutral. Since the dopants are stationary, cations from the surrounding electrolyte (e.g. Na+) migrate into the matrix for charge balance. This is accompanied by water incorporation due to the osmotic pressure leading to swelling of the polymer and finally closing of the nanovalve. Since the nanovalve pore geometry and actuatoric performance depend on the performance of the polypyrrole layer, we initially deposited a 500 nm thick PPy/DBS ACS Paragon Plus Environment

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Page 4 of 17 layer potentiostatically (0.55 V) on a conductive path for cyclovoltammetric characterization (Fig. 2a). According to the observed main peaks, potentials above 0.2 V and below -0.2 V should be sufficient for a complete switch of the PPybackbone charge and therefore, of the corresponding volume change of the polymer matrix. To proof this, we performed three experiments on 500 nm thick PPy/DBS layers (as used for Fig. 2a) where the relative volume change (out of plane direction) was determined by an atomic force microscope (AFM) based assay7. During the monitoring of the height of the polymer layer, potentials of 0.5 V and -0.8 V were alternately applied for two minutes followed by a cycle with stepwise potential decrease or increase (0.1 V/step, every two minutes), respectively (Fig. 2b). The observed relative volume changes revealed maximum values in the range of 8-10 % for -0.8 V and 0.5 V. More interestingly, the stepwise increase of the reductive potential revealed a significant diminution of the polymer volume increase for potentials higher than -0.7 V resulting finally in a residual volume increase of 5 % at 0 V. In contrast, the step wise decrease of the oxidative potential from 0.5 V to 0 V never led to a significant loss of the polymer volume decrease capabilities. These results revealed that our polymer matrix represents a monostable system, i.e.) a potential only has to be applied continuously if the polymer volume is to increase. In contrast, to decrease the polymer volume (oxidation), the potential has only to be applied initially. Afterwards, no external potential is necessary (open cell potential). This avoids problems such as hydroxylation or loss in conductivity by over-oxidation over time, which are critical limitations for long-term stability and therefore practical applications9. Additionally, the requirement of potentials at or below -0.7 V to achieve the maximum polymer volume increase reveals the importance of the additional two small reductive peaks in Fig. 1a at -0.4 V and -0.65 V. The importance of this reductive potential range was also shown in a previous work8. ACS Paragon Plus Environment

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Page 5 of 17 Potentiostatic deposition of conductive electroactive polymers could lead to a depletion of the dopant by diffusion limitations. Since potentiodynamic deposition process is able to avoid this phenomenon10, we investigated both the potentiostatic and potentiodynamic nucleation of PPy/DBS to achieve

optimal actuatoric

performance and stability. Therefore, we fabricated in three experiments platinum conducting paths with potentiostatically deposited PPy/DBS (as described for Fig. 2b) and with PPy/DBS deposited by cyclovoltammetry (-0.5 V – +0.7 V, 0.1 V/s, 40 cycles). Depending on the deposition method, the actuatoric performance was significantly different with an average volume change of 10 % for potentiostatically and 5 % for cyclovoltammetrically deposited polypyrrole layers (Fig. 2c). Hence, all following PPy/DBS depositions were performed with constant potential (0.55 V). Furthermore, we analysed the actuatoric response characteristics in detail. The difference between constant and pulsatile potential application on a 700 nm PPy/DBS layer is exemplarily shown in Figure 2d. For the first cycle of reduction and oxidation the potential was applied constantly. The kinetic of the height trace revealed an actuatoric response time of two seconds after potential switch for both directions, which allows fast regulation of the nanovalve. Afterwards, a distinct potential and the open cell potential (OCP) were applied alternately (5 s potential, 5 s OCP). While the pulsatile potential had no effects on the actuatoric stability of the oxidized polymer, the reduced polymer revealed a fast response within seconds. In addition, an experiment was performed where the potential was applied for 60 seconds followed by OCP for 540 seconds (Supplementary Fig. 1). Once again, the use of OCP led to a rapid volume decrease of the reduced polymer (~ 50 % within 20 seconds), confirming the monostability of the actuatoric polymer. Depending on the use of our nanovalve array for specific applications, the applied potentials could theoretically be critical with regard to unwanted electrochemical side ACS Paragon Plus Environment

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Page 6 of 17 reactions. In this regard, the demonstrated monostable oxidized state is advantageous, since oxidative potentials are much more critical than reductive potentials11. Furthermore, the reduced PPy/DBS polymer provides the applied potential on its surface but due to the reduced state the conductance decreases by several decades12. Thus, electron transfer and therefore, redox reactions of molecules on the surface of the polymer becomes clearly impeded. In addition to the potentials applied to the polymer, the resulting potentials on the counter electrode must also be taken into account. Therefore, the counter electrode should ideally be electrochemically separated. We realized this using an external buffer reservoir, which isconnected via a salt bridge (see Fig. 1a). Additionally, we investigated the long-term stability of PPy/DBS layers because we found no information on the actuatoric capabilities after certain storage times. Therefore, samples were stored at 4°C or at room temperature under dry conditions for up to 2.5 years. Fortunately, the AFM based analysis revealed no loss of actuatoric capabilities for both storage conditions after 2.5 years (Supplementary Fig. 2a). Based on the results of the actuatoric capabilities and characteristics, we wanted to build up exemplarily a nanovalve array to regulate the flow-through of a small model compound. To demonstrate the possibilities on a nanoscale range, we aimed for an opened valve diameter of approx. 10 nm. Therefore, we searched for a substrate with pores in the range of 200 nm that allow polymer coatings of at least 60-70 nm after sputtering the conductive layer on the inner pore surface at the pore entrance. Since the substrate pore geometry is highly critical for the performance of the nanovalve array, we evaluated available techniques and materials. Finally, aluminium oxide structured by directed anodic oxidization13 seemed to best meet this demand and availability. In this context, we were able to identify a commercial supplier of high ACS Paragon Plus Environment

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Page 7 of 17 quality substrates (Fig. 3a, Supplementary Fig. 3). For polymer deposition, a 10-20 nm thin indium tin oxide layer (adhesion promoter) and a 20-30 nm thin platinum layer was sputtered on an aluminium oxide substrate with 13 mm in diameter. Platinum was preferred over gold because of its electrochemical stability14-15. Due to the fact that the polymer layer grows linear as a function of time on platinum coated surfaces, a simple method for tuning the pore diameter could be realized (Fig. 3a). While deposition time of around 240 s revealed the desired nanopore size, pores were totally closed after 400 s. Broken nanovalves revealed the inner structure (Fig. 3b). The penetration depth of the coating proved to be very reproducible (~ 2 µm) and the inner of the nanovalve is hollow (Fig 3b, arrows). For the quantitative analysis of the functionality of our nanovalve array, we developed a custom made two-chamber flow-through cell (Fig. 4a) with an integrated reservoir for the model compound. The electrically contacted nanovalve array (Fig. 4b) was placed between the two chambers and controlled in a three electrode setup, with the nanovalve array representing the working electrode. To demonstrate the functionality of our nanovalve array with small controllable nanopores in the range of 10 nm, we chose Biotin as a small model compound. For a sensitive quantification we used an Atto488 fluorescence labelled Biotin, which has a molecule diameter of approx. 1.21.6 nm (without hydrate shell). To avoid electrochemical side effects like oxidation of the fluorophore at the counter electrode during reduction (closing) of the nanovalve array (working electrode), the counter electrode was spatially separated from the reservoir but was still electrically connected by a salt bridge. Both chambers were filled with a physiological salt solution (0.9 % NaCl) supplemented with 1 M KCl. Depending on the opened or closed state of the nanovalve array, the Biotin-Atto488 can diffuse from the reservoir into the second (analysing) chamber and be quantitatively monitored by a fluorescence detector. ACS Paragon Plus Environment

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Page 8 of 17 Based on the findings from the electrochemical and actuatoric characterization of the PPy/DBS polymer (see Fig. 2), the nanovalve array was alternately closed for two hours by applying continuously -0.8 V and opened by applying 0.5 V for 5 seconds followed by an OCP for two hours. When the nanovalve array was opened, an increase in fluorescence was observed. More strikingly, we were able to operate our nanovalve arrays for 72 hours without any performance loss (Fig. 4c). The statistical analysis of the slopes (flow-through rates) revealed the extremely significant closing (0.3 ± 0.1 ng x ml-1 x h-1) and opening (3.1 ± 0.1 ng x ml-1 x h-1) capabilities of our nanovalve array (Fig. 4d). According to the best of our knowledge, this is the first demonstration of a long-term stable functional nanovalve array over several days. Moreover, we were able to demonstrate this performance under biocompatible conditions using only physiological compatible NaCl/KCl-solution without any addition of soluble dopants that were necessary in previous studies6-7. This is an important prerequisite for application where such supplements could interfere (e.g. in lab on chip systems) or even be harmful16-17 when used for in vivo applications such as implants. For demonstration of robust and reproducible functionality, eight independent 24 h experiments were performed under same conditions. For statistical analysis, we normalized the flow-through rates to the opening values (Fig. 4e) revealing an average retention rate of 71.5 % that was extremely significant in comparison to the open state. Since, the storage stability of the nanovalve arrays could be crucial for specific applications, the experiments were performed with nanovalve arrays that were stored up to 28 days at room temperature under dry conditions. In this context, we analysed the retention rate dependency from the storage time (Supplementary Fig. 2b). The linear regression revealed no loss of retention capabilities in the closed state within the analysed 28 days. ACS Paragon Plus Environment

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Page 9 of 17 In order to prove that the observed retention of the Biotin-Atto488 model compound is actually based on the actuatoric closing of the nanovalve and not on electrostatic effects while the reductive potential is applied (closed nanovalve), nanoporous aluminium oxide substrates with a pore diameter of 60 nm were used as a control. The conductive surface coating was sputtered in the same way as for substrates with a pore diameter of 200 nm, resulting in nanopores with a comparable diameter to the nanovalve array, but without the actuatoric polymer coating. Based on these substrates, four independent control experiments were performed and analysed as in the case of the nanovalve array experiments (Fig 4e). In contrast, to the nanovalve arrays, no Biotin-Atto488 retention rates could be observed when -0.8V was applied (4.2 %). Thus, electrostatic effects could be excluded and the actuatoric capabilities as cause for the observed nanovalve array performance was verified. Although being able to demonstrate this robust and long-term stable nanovalve performance for the first time with retention capabilities of up to 90 % for single experiments, limitations for the closing capabilities were considered. In this context, the nanoporous substrate can be pointed out. A homogeneous and accurate pore size and pore geometry over the whole array are of crucial importance. Aluminium oxide nanoporous substrates produced by anodic oxidization always exhibit natural grain boundaries that lead to pore geometry deviations as well as fusion of single adjacent pores at the grain boundaries resulting in higher diameter pores (Supplementary Fig. 3). In this regard, the manufacturer of the substrate gives an overall pore diameter deviation of 10 %, which ultimately results in partially much larger nanovalves that cannot be completely closed. However, this also means that finding or even the development of optimized (large scale) nanoporous substrates could considerably improve the already demonstrated nanovalve array performance.

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Page 10 of 17 Additionally, we considered the achieved amount of released model compound. Therefore, we performed finite element method (FEM) based simulation of a simplified nanopore model (Supplementary Fig. 4). When the model compound release was only achieved by passive diffusion, as it was the case for the performed experiments, the FEM simulation revealed an amount of approximately 25 ng/h that passed the nanopore only by passive diffusion. Our experimental results revealed compound diffusion rates of 6-18 ng/h that is a quite good value in comparison to the FEM simulation derived value. This proves that our actuatoric nanovalves do not reduce the theoretical achievable diffusion rates. If compound release rates are not sufficient for specific applications, the compound concentration could be increased. For our test setup, FEM simulations revealed compound diffusion rates of 860 ng/h for 1 mg/ml (factor of 34 in comparison to 30 µg/ml) and 4260 ng/h for 5 mg/ml (factor of 170 in comparison to 30 µg/ml). Furthermore, a pressure or specific flow rate as is common in lab on chip applications could be applied. FEM simulations revealed e.g. a compound diffusion rate of 5430 ng/h for a flow rate of 10 µl/min. Of course, increased compound concentration and application of a pressure / flow could be combined to drastically increase the released amount of compound. Thus, a FEM simulation with 5 mg/ml Bioton-Att488 solution and a flow rate of 10 µl/min resulted in 1 mg/h released compound. In conclusion, we were able to demonstrate for the first time a functional electrically controllable nanovalve array based on an actuatoric polymer that provides long-term stability for at least 72 hours without any performance loss and retention capabilities of up to 90 % under biocompatible conditions. More strikingly, the desired nanopore size can be individually adapted for application needs and the nanovalve is not limited to opened and closed states but moreover, could be accurately adjusted depending on the applied reduction potential (see Fig. 2b). In this way, applications ACS Paragon Plus Environment

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Page 11 of 17 such as molecular sieves with electrically controllable molecule exclusion sizes can be realized. Additionally, modification of the polymer redox stability could lead to shifting the actuatoric monostable character for an open nanovalve towards a bistable system or even a monostable system for a closed nanovalve which could be of interest for applications like electrically controllable drug release systems.

Acknowledgements This work was funded by the German Federal Ministry of Education and Research (BMBF), DiaImplant [Grant No 16SV5052]. The image acquisition facility (REM) was funded by the Free State of Saxony and the European Union (SMWK/EFRE (SMWK/EFRE) (Grant No 100193539).

Supporting Information Comprises detailed description of material and methods and four Supplementary Figures showing the actuatoric polymer stability depending on the applied potential, the storage stability of the polymer and nanovalve array, overview SEM images of the nanoporous Al2O3 substrates, and FEM simulation for estimation of Biotin–Atto488 diffusion rate.

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References 1. Wu, J.; Sailor, M. J., Chitosan Hydrogel-Capped Porous SiO2 as a pH Responsive Nano-Valve for Triggered Release of Insulin. Adv Funct Mater 2009, 19 (5), 733-741. 2. Bojko, A.; Andreatta, G.; Montagne, F.; Renaud, P.; Pugin, R., Fabrication of thermo-responsive nano-valve by grafting-to in melt of poly(N-isopropylacrylamide) onto nanoporous silicon nitride membranes. J Memb Sci 2014, 468, 118-125. 3. Li, H.; Tan, L.-L.; Jia, P.; Li, Q.-L.; Sun, Y.-L.; Zhang, J.; Ning, Y.-Q.; Yu, J.; Yang, Y.-W., Near-infrared light-responsive supramolecular nanovalve based on mesoporous silica-coated gold nanorods. Chem Sci 2014, 5 (7), 2804-2808. 4. van Rhee, P. G.; Rikken, R. S.; Abdelmohsen, L. K.; Maan, J. C.; Nolte, R. J.; van Hest, J. C.; Christianen, P. C.; Wilson, D. A., Polymersome magneto-valves for reversible capture and release of nanoparticles. Nat Commun 2014, 5, 5010. 5. Elisabeth, S., Microfabrication of PPy microactuators and other conjugated polymer devices. J Micromech Microeng 1999, 9 (1), 1. 6. Bay, L.; Mogensen, N.; Skaarup, S.; Sommer-Larsen, P.; Jørgensen, M.; West, K., Polypyrrole Doped with Alkyl Benzenesulfonates. Macromolecules 2002, 35 (25), 9345-9351. 7. Smela, E.; Gadegaard, N., Volume Change in Polypyrrole Studied by Atomic Force Microscopy. J Phys Chem B 2001, 105 (39), 9395-9405. 8. West, B. J.; Otero, T. F.; Shapiro, B.; Smela, E., Chronoamperometric Study of Conformational Relaxation in PPy(DBS). J Phys Chem B 2009, 113 (5), 1277-1293. 9. Ghosh, S.; Bowmaker, G. A.; Cooney, R. P.; Seakins, J. M., Infrared and Raman spectroscopic studies of the electrochemical oxidative degradation of polypyrrole. Synth Met 1998, 95 (1), 63-67. 10. Li, C. M.; Sun, C. Q.; Chen, W.; Pan, L., Electrochemical thin film deposition of polypyrrole on different substrates. Surf Coat Technol 2005, 198 (1), 474-477. 11. dos Santos, E. V.; Bezerra Rocha, J. H.; de Araujo, D. M.; de Moura, D. C.; Martinez-Huitle, C. A., Decontamination of produced water containing petroleum hydrocarbons by electrochemical methods: a minireview. Environ Sci Pollut Res Int 2014, 21 (14), 8432-41. 12. West, K.; Bay, L.; Nielsen, M. M.; Velmurugu, Y.; Skaarup, S., Electronic Conductivity of Polypyrrole−Dodecyl Benzene Sulfonate Complexes. J Phys Chem B 2004, 108 (39), 15001-15008. 13. Santos, A.; Kumeria, T.; Losic, D., Nanoporous Anodic Alumina: A Versatile Platform for Optical Biosensors. Materials 2014, 7 (6), 4297. 14. Cherevko, S.; Topalov, A. A.; Zeradjanin, A. R.; Katsounaros, I.; Mayrhofer, K. J. J., Gold dissolution: towards understanding of noble metal corrosion. RSC Adv 2013, 3 (37), 16516-16527. ACS Paragon Plus Environment

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Page 13 of 17 15. Martinez, J. G.; Otero, T. F.; Jager, E. W., Effect of the electrolyte concentration and substrate on conducting polymer actuators. Langmuir 2014, 30 (13), 3894-904. 16. Martinez-Tabche, L.; Mora, B. R.; Faz, C. G.; Castelan, I. G.; Ortiz, M. M.; Gonzalez, V. U.; Flores, M. O., Toxic effect of sodium dodecylbenzenesulfonate, lead, petroleum, and their mixtures on the activity of acetylcholinesterase of Moina macrocopa in vitro. Environ Toxicol Water Qual 1997, 12 (3), 211-215. 17. Bondi, C. A.; Marks, J. L.; Wroblewski, L. B.; Raatikainen, H. S.; Lenox, S. R.; Gebhardt, K. E., Human and Environmental Toxicity of Sodium Lauryl Sulfate (SLS): Evidence for Safe Use in Household Cleaning Products. Environ Health Insights 2015, 9, 27-32.

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Figure 1. Nanovalve array principle. a, Scheme of the electrochemical based opening and closing of the nanovalves. b, Layer composition on the nanoporous aluminium oxide substrate.

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Figure 2. Electrochemical characterisation and actuatoric performance of the doped polypyrrole based polymer. a, Cyclovoltammogram (10 cycles) derived from a deposited polypyrrole layer (500 nm). b, Actoric stability of reduced and oxidized polypyrrole layer depending on the applied potential. c, Comparison of actoric performance of polypyrrole polymer deposited by cyclovoltammetry (CV) or constant potential (CP). d, Height trace as a function of applied potential in alternation with open cell potential.

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Figure 3. Scanning electron microscopy based characterization of the nanovalve array. a, Time dependent deposition of polypyrrole on the nanoporous aluminium oxide substrate. b, Cross sections of the nanovalve array showing deposited polypyrrole tubes within the nanoporous aluminium oxide substrate as well as open channel within the polypyrrole tubes (arrows).

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Figure 4. Demonstration of nanovalve functionality. a, Scheme of self-developed flow through cell for testing of nanovalve arrays. b, Electrically contacted nanovalve array and connected flow-through cell. c, Monitoring of Biotin-Atto488 release during alternating opening (2h) and closing (2h) of the nanovalve array for 72 hours. d, Analysis of the flow-through rate of the opened and closed nanovalve array for the experiment shown in c (n = 18, mean ± SD). e, Statistical analysis of the retention rate of the nanovalve array (NVA) and the control (C) from independent experiments (mean ± SD).

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