Coupling pH-Responsive Polymer Brushes to Electricity: Switching

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Coupling pH-Responsive Polymer Brushes to Electricity: Switching Thickness and Creating Waves of Swelling or Collapse Gary J. Dunderdale* and J. Patrick A. Fairclough Department of Chemistry, University of Sheffield, Sheffield, U.K. S Supporting Information *

ABSTRACT: Electrolysis of water is proposed as a method to couple the pH-responsive behavior of polymer brushes to an electrical stimulus. It is shown that an electrode in close proximity to a pH-responsive polymer brush can change the local solution pH, inducing either swelling or collapse of the polymer brush. By alternating the bias of the voltage applied to the electrode, either acidic or alkaline conditions can be generated, and reproducible cycles of polymer brush swelling and collapse can be achieved. It was found that the length of time which the electrical stimulus is applied to the electrodes can be as short as 10 s and that, once “switched”, polymer brushes remain in the switched state for many minutes after the electrical stimulus is turned off. In other experiments, two electrodes were positioned 10 cm apart with a pHresponsive brush in between. Under these conditions waves of either acidic or alkaline solution pH could be generated which caused a coincident wave of polymer brush swelling or collapse. These waves originate from one electrode and travel across the brush surface toward the opposite electrode with a velocity of ∼40 μm s−1.

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INTRODUCTION Polymer brushes have the potential to be useful in a wide variety of applications,1 such as drug delivery,2,3 lubrication,4,5 modification of electrode surfaces,6,7 and manipulation of small particles.8,9 This is due to the varied physical properties polymer brushes can possess. One particular class of polymer brush which is gaining increasing interest is “smart” or stimuli responsive polymer brushes. These polymer brush surfaces are able to change their physical properties in response to a range of stimuli,10−12 such as temperature,13,14 solvent,15 light,16 glucose,17 carbon dioxide,18 and acidic vapor.19 One of the most studied groups of these stimuli-responsive surfaces are polymer brushes which respond to changes in solution pH. For example, brushes of poly[2-(dimethylamino)ethyl methacrylate)] or poly(methacrylic acid) have been widely studied and found to change thickness in response to solution pH.20,21 Although these responsive polymer brushes have great potential, it is inconvenient to change the physical properties of the surface by adding or removing a chemical reagent to or from the surrounding solution, particularly if the surface properties need to be changed many times or cycled continuously between two states. Thus, research has been carried out investigating polymer brush surfaces which respond to an electrical stimulus rather than a chemical stimulus. These fall into two categories: polymer brushes, which respond to an applied electric field or electrical potential,22 and polymer brushes, which contain redox-active units which change oxidation state due to the electrical stimulus.23−25 In this article we take a different approach and show that pHresponsive brushes can be made to respond to an electrical © XXXX American Chemical Society

stimulus, by coupling the electrical stimulus and pH-responsive behavior together via the electrolysis of water. It is well-known that the electrolysis of water produces hydrogen and oxygen gas but less well-known that electrolysis results in a change in solution pH around the anode and cathode. This change in solution pH forms the mechanism for coupling the pHresponsive behavior of the polymer brush to an electrical stimulus. pH-responsive polymer gels 2 6 − 3 0 and polymer brushes20,21,31,32 have been proposed as useful in creating artificial muscles capable of doing useful work.33 Some examples of this actuation are the bending of AFM cantilevers or thin pieces of metal by stimulus responsive polymer brushes.34−36 While this simple on/off type response exhibited by these actuators is useful in many applications, another type of muscle contraction is frequently observed in naturewaves of muscle contraction. For example, a wave of muscle contraction in the oesophogus of animals gives peristalsis and allows swollowing.37 Another example is the wavelike muscle contraction observed in the flagella of sperm which gives them motility.38 These types of contractions or expansions have been recreated in synthetic stimulus-responsive gels by the incorporation of an oscillating chemical reaction.39 This procedure results in polymer gel expansions capable of moving macroscopic objects in a similar fashion to peristalsis40,41 and may lead to the development of biomimetic actuators.28,42 We Received: December 17, 2012 Revised: February 22, 2013

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Study of Propagating Waves of Swelling Using Spectroscopic Ellipsometry. An ellipsometer cell with internal dimensions of length = 10 cm, width = 1 cm, and height = 6 mm was constructed from Perspex with quartz windows inclined at an angle of 70° to the horizontal (see Figure 2 for photographs of the cell used). Platinum wire electrodes were positioned at each end of this cell through 2 mm diameter holes in the lid. A prepared polymer brush surface (∼10 cm × 1 cm) was then placed inside, and the cell and the chamber were filled with 28 mM NaNO3 solution adjusted to the desired pH using either dilute sodium hydroxide solution or dilute hydrochloric acid solution. This cell was then aligned on the ellipsometer stage with the light beam reflecting from the polymer brush substrate at the desired distance from one of the electrodes. Ellipsometric data were then captured continuously while at a known time ∼2 min after the start of data capture, a fixed voltage of 18 V was applied to the electrodes, giving a flow of current typically 1−3 mA. This voltage was then maintained for the duration of the experiment (∼10 min). The experiment was repeated several times with the ellipsometer light beam aligned at different distances away from the electrode. In all repeats the same polymer brush surface was used. Visual Analysis of pH Change in Solution. The ellipsometer cell as used above was filled with a solution of 2% v/v universal pH indicator and varying amounts of NaNO3. A fixed voltage or fixed current was then applied to the platinum electrodes, and a movie clip of the cell was captured. The position of expanding acidic or alkali regions in each frame of the movie clip was then measured using a custom-made LabView script which detects the regions of different pH based on color analysis.

have observed that electrolysis of water coupled to the pHresponsive behavior of polymer brushes is also able to create this wavelike motion of expansion or contraction and shall be investigated in this article.

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EXPERIMENTAL SECTION

Materials. All chemicals were purchased from Sigma-Aldrich and used as received. Pure water was obtained from an Elga Purelab Option Q purification system. Silicon wafers (100) were obtained from Mitsubishi Research. Preparation of Silane Initiator Monolayers. Silicon (100) wafers were cut into small pieces and cleaned by submerging in piranha solution (3:1 v/v 3 M sulfuric acid: 30% w/w hydrogen peroxide) for several hours, rinsed with water and then ethanol, and dried in an oven for several hours. The clean silicon surfaces were then functionalized with (3-aminopropyl)triethoxysilane (APTES) by placing the surfaces in a sealed tube containing a small amount of APTES. The sealed tube was then evacuated to remove air, and the surfaces became functionalized by reaction with the APTES vapor. After 30 min the functionalized surfaces were removed, washed with copious amounts of 1,4-dioxane, and then dried using a stream of nitrogen gas. The surfaces were then placed in an oven at 80 °C for 1 h to anneal the APTES monolayer. These monolayer surfaces were then placed in a 0.1 M solution of 2-bromoisobutyryl bromide in 1,4dioxane under a constant flow of nitrogen gas for 2 h, after which they were rinsed with 1,4-dioxane and blown dry in a stream of nitrogen gas. Atom Transfer Radical Polymerization of 2-(Diethylamino)ethyl Methacrylate from Silane Initiator Monolayers. 2(Diethylamino)ethyl methacrylate (10 mL, 50 mmol), 1,4-dioxane (5 mL, 57 mmol), and pentamethyldiethylenetriamine (100 μL, 480 μmol) were added to a reaction tube (Radley’s) with a cross-hair stirrer bar. An initiator functionalized silicon substrate was suspended in the reaction solution using stainless steel wire. The solution was then purged with nitrogen gas while being stirred for 30 min to remove oxygen. After 30 min copper(I) chloride (50 mg, 500 μmol) was added while the nitrogen flow was maintained. The tube was resealed and purged with nitrogen for a further 5 min, then heated to 50 °C, and left to react for up to 14 h. Polymerization was quenched by opening the tube, allowing oxygen to enter. Following polymerization substrates were rinsed with copious amounts of 1,4-dioxane and sonicated in several washings of dilute acetic acid to remove copper(II) followed by washing with water and ethanol. Spectroscopic Ellipsometry Measurements. The thickness of polymer brushes grown on surfaces was determined using a J.A. Woollam spectroscopic ellipsometer (M-2000V) operating at wavelengths of 370−998 nm at 2 nm intervals. To determine dry brush thicknesses, ellipsometric data were captured at several locations on the brush surface and were then fitted with a Cauchy layer model using the software CompleteEase. To determine brush thicknesses when submerged in aqueous solution, data were fitted with an effective medium approximation (Bruggeman) consisting of a Cauchy material (Cauchy constants A = 1.49, B = best fit minimum −0.01 maximum 0.01) and water. See Supporting Information for further details of the ellipsometry model used and fits to data in collapsed and expanded states (Figure S2 and S3, respectively). Study of Polymer Brush Thickness Switching Using Spectroscopic Ellipsometry. An ellipsometer cell with internal dimensions of 3 × 3 × 3 cm was constructed from PTFE with quartz windows inclined at an angle of 70° to the horizontal. A polymer brush surface (∼1 × 1 cm) was then positioned in the cell, and the cell was filled with 28 mM NaNO3. A platinum wire electrode was positioned close to the brush surface (∼3 mm above) and a platinum wire counter electrode far away from the surface (∼3 cm above). The ellipsometer light beam was then aligned to reflect from the brush surface underneath the electrode and ellipsometric data captured while a fixed voltage of ±5 V was periodically applied between the electrodes. Typically the amount of current flowing was measured to be ∼5 mA.

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RESULTS AND DISCUSSION Preparation of Polymer Brushes and Proof of Concept. Polymer brushes of poly[(2-diethylamino)ethyl methacrylate] (PDEAMA) were grown from silicon substrates using atom transfer radical polymerization.43,44 Measurement of the brush thickness after submersion in the polymerization solution for various amounts of time showed a linear increase in thickness with time, consistent with a controlled reversibledeactivation radical polymerization, and atomic force microscopy of the polymer brush surface in air showed a smooth homogeneous polymer layer free of defects (see Supporting Information Figure S1). In this way well-defined polymer brushes with thicknesses of up to 80 nm (measured in air) were created for investigation of their response to electrical stimuli. To investigate the response of PDEAMA brushes to different solution pH values, spectroscopic ellipsometry was used to determine the brush thickness at different pH values in aqueous solution and is shown in Figure 1. In all cases spectroscopic data were fitted well by a single nongraded layer model, and it was not necessary to include any surface roughness, indicating that the brush layer is a smooth slablike layer consistent with previous studies.20 The thickness of a weak polyelectrolyte brush at a particular pH value is an energetic balance between charge−charge repulsions, which favor an increase in thickness, and the stretching of polymer chains, which favor a decrease in thickness.45 Consequently, and as shown in Figure 1, PDEAMA brushes are thickest at low pH where tertiary amine groups in the polymer chains are mostly protonated forming cations and thinnest at high pH where the amine groups are deprotonated and neutral. The transition from thick to thin occurs gradually over several pH units approximately consistent with the calculated fraction of ionization of the polymer chains, as shown by the red line in Figure 1. A best fit is obtained with a pKb of 6.4 in close agreement to that we measured for homopolymer chains in solution by titration of 7.0 and literature sources.46 Although the transition is slightly broader B

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Figure 1. pH-responsive behavior of a poly[(2-diethylamino)ethyl methacrylate] brush. The red line shows the fraction of ionized (protonated) amine groups.

than this calculated line, it is not as broad as a thickness proportional to the fraction of ionization raised to the power 1 /3 or 2/3, as predicted by scaling theories.45 Following this investigation of the response of PDEAMA brushes to different solution pH values, it is clear that if the local pH around the polymer brush is changed, then a change in brush thickness will be observed. Namely, if the pH changes from below to above the pKb, a collapse in brush thickness will be observed. Whereas, if the pH changes from above to below the pKb, the brush will expand. In Figure 2 we show that electrolysis can be used to create this change in local pH, potentially capable of causing an expansion or collapse in brush thickness. In Figure 2 a sample cell is filled with an aqueous solution of a universal pH indicator, and 28 mM sodium nitrate is added to increase the electrical conductivity of the solution. Two pieces of platinum wire are placed in contact with the solution at either side of the sample cell (left and right) to act as electrodes. Initially, the solution pH is close to neutral and the pH indicator is green in color (Figure 2A). When a voltage of +18 V is applied to the platinum electrodes, a current flows (∼5 mA) through solution and electrolysis results. The local pH around the electrodes begins to change as visualized by a change in color of the pH indicator (Figure 2B). At the anode (right side electrode) water is split to form oxygen gas and protons (H+) which are released into the surrounding solution, consistent with eq 1. This results in a drop in the solution pH around the anode as visualized by a color change from green to red. At the cathode (left side electrode) protons in solution are consumed and so removed from solution to form hydrogen gas, consistent with eq 2. This gives an increase in the solution pH around the cathode visualized by a change in a color from green to purple. 1 H 2O → O2 + 2H+ + 2e− (1) 2

2H+ + 2e− → H 2

Figure 2. Sequential photographs of a sample cell containing solution initially at neutral pH with platinum electrodes present at each end of the cell. (A−C) Evolution of solution pH over time after a voltage of +18 V is applied. (C−E) The bias of the voltage is then reversed to be −18 V.

pH areas increases over time, resulting in a wave of high or low pH propagating through solution. Eventually after some time, the two areas of high and low pH meet approximately midway between the two electrodes, neutralization of acid and alkali occurs, and no further change is observed (Figure 2C). Following production of areas of high and low pH described above, the bias of the voltage applied to the electrodes was changed from +18 to −18 V, resulting in a change in solution pH as shown in Figure 2C−E. Because of this change in bias, the anode and cathode exchange position; i.e., the electrode on the right-hand side becomes the cathode and the electrode on the left becomes the anode. As current flows resulting in electrolysis of water, protons are consumed or produced consistent with eqs 1 and 2. Where before alkaline conditions were generated on the left of the sample cell and acidic on the right, this is now reversed due to the anode and cathode exchanging positions. Acidic conditions are now generated on the left and alkaline on the right. Following a transition in the solution pH, as shown in Figure 2D, a stable arrangement of high and low pH is generated, as shown in Figure 2E. This arrangement of solution pH in Figure 2E is equivalent but opposite to the arrangement shown in Figure 2C. To summarize this section, we have shown that PDEAMA brushes have thicknesses which depend on the solution pH, and that the solution pH can be altered using electrolysis.

(2)

As well as this useful local change in pH around the electrodes, there is further interesting behavior resulting from the electrolysis of water. Areas of high and low pH around the anode and cathode visibly appear within a few seconds of the voltage being applied (Figure 2B), and if the electrical current is allowed to continue flowing, the volume of these high- and lowC

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Figure 3. Switching of polymer brush thickness by electrolysis. (A) Schematic of the experimental setup showing the positions of the electrodes (red and blue wires) in relation to the polymer brush and ellipsometer light beam. (B) Measured thickness of the polymer brush as voltages of +5 V (pink boxes), −5 V (blue boxes), or no electrical stimulus (white boxes) are applied for various lengths of time. Thin boxes of pink (4400 s) and blue (4700 s) are pulses of 10 s. The arrow indicates several switching cycles ending in +5 V, which are not shown.

Specifically, the local pH around electrodes can be changed to be acidic or alkaline depending on the bias of the voltage applied. This change in pH is irrespective of the magnitude of the voltage, provided it is greater than the standard electrode potential of 1.23 V. Although a larger voltage is required in our experiments to result in flow of current due to the distance between electrodes and low conductivity of water. We have shown that if the voltage bias is switched, the solution pH can be cycled between acidic and alkaline. Propagating waves of acidic or alkaline conditions can be generated which originate at the electrodes and travel to a point midway between the electrodes. These changes in solution pH caused by electrolysis are potentially useful in changing the physical properties of polymer brushes and shall be investigated in the following section. Switching of Polymer Brush Thickness via Electrolysis. Figure 3A shows the experimental setup used for investigating switching of polymer brush surfaces. A PDEAMA brush surface was submerged in 28 mM sodium nitrate solution within an ellipsometer cell and a platinum electrode positioned as close as possible to the brush surface without coming into contact with the surface or blocking the ellipsometer beam (∼3 mm above the surface). A counter electrode was then positioned far away from the brush surface (∼3 cm). This setup allowed the brush thickness to be measured using ellipsometry, while an electrical stimulus was applied to the electrodes creating a localized change in pH at the brush surface. Figure 3B shows how the polymer brush thickness changed as voltages of ±5 V were applied to the electrodes resulting in a flow of current (3−5 mA). Although a smaller voltage (5 V) is used than the 18 V used in Figure 2, a similar amount of current flows due to the smaller distance between electrodes, and the same electrochemical reactions occur. Initially, the polymer brush is partially swollen by the neutral solution, and after a voltage is applied to the electrodes a change in thickness is observed. We found that the first few cycles of ±5 V were often irreproducible, probably due to convective flow in the liquid cell which transports either acidic or alkaline solution away from the electrodes, meaning that areas of high or low pH around the electrodes are dissipated by mixing and remain at neutral pH. After a short period of time ∼20 min, these changes in thickness became more reproducible, consistent with reduced convective flow and the liquid becoming stationary, allowing areas of high or low pH to be created over time around the electrodes.

As can be seen from Figure 3B, the thickness of the brush starts to change after a voltage is applied, as protons are either generated or consumed by electrolysis. When a positive voltage is applied, the electrode in close proximity to the brush surface acts as the cathode and protons are consumed, resulting in an increase in the local pH. This deprotonates tertiary amine groups within the polymer chain, neutralizing them, and causes the polymer brush to collapse in thickness. Conversely, when a negative voltage is applied, the electrode in close proximity to the brush surface acts as the anode and protons are generated by the electrolysis of water. This results in a decrease in the local pH which protonates tertiary amine groups in the polymer chain, leading to an increase in the brush thickness. Therefore, by cycling between a positive and a negative voltage, the polymer brush thickness can be cycled between thick and thin. As can be seen from Figure 3B, a sawtooth-like change in brush thickness results from the electrical stimulation, rather than the square-wave-like oscillation one may expect. This is caused by the differing rates at which the polymer brush expands and contracts in response to changes in pH.47 The response of the polymer brush is not instantaneous, requiring around 300 s to fully collapse and around 100 s to expand. These rates are slower than the response rates we have measured for polymer brushes as either acid or alkali is added to the surrounding solution but can be rationalized by the fact that it takes time for changes in pH generated from a point source such as an electrode to diffuse over the area of polymer brush measure by the ellipsometer beam (3 × 9 mm ellipse). Voltages were applied between electrodes for various different durations of time, and no discernible effect on the rate at which the polymer brush responded was observed. As shown in Figure 3B at 4400 and 4700 s, very short pulses of 10 s duration can be used to switch the polymer brush, although again a small amount of time is required to allow the change in pH to diffuse to the brush surface before any change in thickness is observed. As either hydrogen or oxygen gas is produced at electrodes, short pulses of electrical stimuli have the advantage of producing less gas and avoid the chance of a gas bubble adhering to the brush surface causing dewetting, as was sometimes observed using longer durations of electrical stimuli. Dewetting is undesirable, as it causes the polymer brush to collapse to its dry thickness and its surface properties to change from the hydrated state. It is also interesting to note that a short pulse of electrical stimuli does not result in a solution pH less acidic or less alkaline than when a longer electrical stimulus is used. This is in contrast to some other D

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Figure 4. Propagating waves of solution pH which result in waves of polymer brush swelling or collapse. (A, B) Photographs of a sample cell filled with an aqueous solution of pH indicator initially at either pH 11 or pH 3. An electrical current flows through the electrodes, resulting in a change in pH around the electrode which spreads over time. (C, D) Normalized thicknesses of polymer brushes at different distances away from either the anode or cathode over time. Dark colored plots are closest to the electrodes and light color plots furthest away. (E) Comparison of the time and duration of responses at different distances from the electrodes.

water splitting devices which can generate various solution pH values by tailoring the input of electrical work.48 Finally, we found that once a polymer brush surface is switched using the electrical stimulus described above, that it is stable for a reasonable amount of time. As can be seen in Figure 3B, once the polymer brush has responded to the local change in pH and the electrical stimulus is turned off (white boxes), the brush thickness remains close to constant for up to 10 min and does not return to its thickness at neutral pH (77 nm). This is due to the solution pH around the polymer brush continuing to be either acidic or alkaline after the electrical stimulus is removed. Although given enough time, areas of high pH and areas of low pH will mix together by diffusion, neutralize, and return the pH around the polymer brush to be

neutral, meaning the brush thickness would also return to its thickness at neutral pH. Further, we did not observe any decrease in the thickness of polymer brushes indicative of polymer degradation by the acidic or alkaline conditions generated by electrolysis over the time frame of our experiments. This is in contrast to when an electrical potential is applied to the substrate, which can result in cleavage of the polymer brush from the substrate.22 Waves of Swelling or Collapse. After showing polymer brush switching, we now investigate the wavelike change in local pH caused by the electrolysis of water. As is shown in Figure 4A, if a sample cell is filled with a solution pH initially of 11 and electrodes are positioned at each end of the cell, electrolysis of water lowers the pH around the anode to be E

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movement of the liquid inside the cell and make the motion of the wave slightly different with each repeat of the experiment. Readers are encouraged to view the animated graph of this behavior given in the Supporting Information. The time at which the polymer brush expands (as judged from the time when normalized thickness reaches 0.1) increases approximately linearly with increasing distance away from the electrode, as shown in Figure 4E, consistent with a wavelike motion. The velocity at which the change in brush thickness propagates along the surface is ∼37 μm s−1. The duration of the response, as judged from the time taken to change from a normalized thickness of 0.1 to 0.9, lasts between 30 and 80 s and is not dependent on the distance away from the electrode. Although, as mentioned earlier, the thickness calculated from ellipsometry data is an average thickness over the area of the ellipsometer beam. Calculating the time taken for the wave to travel over this area (3 mm/37 μm s−1) gives a duration of 81 s. Therefore, the duration of the response seems to be caused by the slow moving acidic solution rather than a slow response of the polymer brush to changes in solution pH. As well as creating waves of acidic conditions, electrolysis is also able to create waves of alkaline conditions as is shown in Figure 4B. It is necessary to start with an acidic solution (pH = 3), and the wave of alkalinity now propagates from the cathode toward the middle of the cell. We repeated the experiment described above for measuring waves of brush expansion to study if these waves of alkaline conditions could create a wave of polymer brush collapse. The results of this experiment are shown in Figure 4D. As can be seen, the polymer brush now starts in an expanded configuration and collapses at a certain time to become thinner. Again, the time at which the brush responds, in this case collapses rather than expands, increases as the distance away from the cathode increases. The velocity at which the wave of polymer brush collapse travels is 40 μm s−1, and the duration of the response is 30−80 s, almost identical to the velocity and duration measured for the wave of polymer brush expansion. Lastly, we investigated the velocity at which the change in solution pH propagates and whether this velocity can be tailored by changing the electrical stimulus. To do this, a sample cell was filled with an aqueous sodium nitrate (28−46 mM) solution containing universal pH indicator. Various electrical stimuli of different voltages giving different amounts of current flow were then applied to the electrodes and color changes in solution captured over time in a movie clip. To quantify the position of these color changes, and so identify the positions at which the solution pH is above or below the pKb of the polymer, a custom-made LabView script was used to find the position of color changes in the cell in each frame of the movie clip. The position of the color change from purple (pH = 11) to red (pH = 3) as the electrical stimuli was applied to the electrodes is shown in Figure 5 against time. As can be seen from Figure 5, the position of the color change initially moves rapidly away from the electrode but then slows as its position becomes closer to the midpoint between the two electrodes (distance = 50 mm). Eventually, the color change approaches the midpoint and ceases to move. A range of electrical stimuli were applied to the electrodes in an attempt to tailor the velocity of the wave, but within the ranges investigated the velocity appears to be very similar irrespective of the voltage, current, power. Therefore, it seems that the only variable which changes the velocity of the pH change in

acidic, and around the cathode the pH remains alkaline. As shown in the previous section, this acidification around the anode results in an increase in polymer brush thickness. If the electric current continues to flow, this acidic region increases in volume and propagates toward the midpoint between the electrodes and shall be the subject of investigation in this section. In Figure 4A, as the current continues to flow, the position in the sample cell where the pH is lower than the pKb of the polymer brush migrates from being close to the anode toward the middle of the sample cell. Therefore, a polymer brush submerged in this solution should respond to this wave of acidity by increasing in thickness as the local pH changes from alkaline to acidic. Polymer brush close to the anode should respond first, followed by polymer brush further away, followed by polymer brush even further away. In this fashion, a wave of polymer brush swelling should be observed which starts at the anode and travels to the middle of the sample cell. To investigate if a wave of polymer brush swelling can be created using electrolysis, spectroscopic ellipsometry was used as follows. A large area of silicon (10 × 1 cm) was functionalized with PDEAMA brush and placed in the sample cell shown in Figure 4A, then filled with an aqueous sodium nitrate solution (28 mM) at pH 11. This sample cell was aligned on the ellipsometer with the light beam reflecting from the brush surface a known distance away from the anode (14− 27 mm). The polymer brush thickness was continuously measured by ellipsometry and gave an average thickness of the area illuminated by the ellipsometer beam (3 × 9 mm elipse). As the wave of changing solution pH travels across the width of this beam, the polymer brush thickness is measured with a spatial resolution of 3 mm. At time = 0 a fixed voltage of 18 V was applied to the electrodes, resulting in the electrolysis of water and a reduction in pH around the anode. This voltage was continued to be applied until the end of the experiment resulting in a wave of acidity propagating toward the middle of the sample cell. The experiment was then repeated with the ellipsometer light beam positioned at different distances away from the anode. The results of different repeats of this experiment are shown in Figure 4C. As the measured polymer brush thickness varied by up to 5 nm from the mean value at different positions on the surface, it was necessary to normalize thicknesses to allow comparison. This was done following the equation normalized thickness =

Lt − Lmin Lmax − Lmin

(3)

where Lt is the thickness measured at time t and Lmin and Lmax are the smallest and largest measured thicknesses at any time. As can be seen, no matter what the distance away from the anode, the polymer brush initially started in a collapsed configuration consistent with being at deprotonated at pH 11 and then changed at some time to become thicker, consistent with an expanded configuration at low pH. Polymer brush positioned close to the anode swells first, followed by brush further away in a wavelike fashion. The shape of the response in normalized thickness appears slightly different at different distances away from the electrode, sometimes being fast and other times slower, and does not seem to show any clear trend with distance away from the electrode. We postulate that these slightly different responses are due to convection and gas bubbles detaching from the electrode surfaces, which cause F

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or subject the polymer brush to strong electric fields. This avoids cleavage of polymer chains from the substrate and negates the need to synthesize complex polymer brushes containing redox-active units.

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ASSOCIATED CONTENT

S Supporting Information *

An animated graph showing the wavelike expansion of a PDEAMA brush surface, a schematic reaction scheme of PDEAMA brush surface preparation, atomic force microscopy images, and ellipsometry data fitting information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: g.dunderdale@sheffield.ac.uk.

Figure 5. Evolution of waves of acidity. Colored lines indicate the position of the change in solution pH over time, as measured by visual analysis. Several different electrical stimuli were used in the ranges 6− 26 V, 2−37 mA, and 28−280 mM NaNO3. Open circles indicate the time at which the polymer brush thickness changed at different distances from the electrode (both expansion and collapse).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank EPSRC and The University of Sheffield for funding a prize postdoctoral fellowship (G.D.). Mike Weir is thanked for help with ellipsometry in the early stages of these experiments and Jonathan Howse for help with LabView.

solution is the distance of the polymer brush from the nearest electrode. Also shown in Figure 5 by black circles are the times at which polymer brush was observed to respond to the electrical stimulus at different distances from the electrodes (either expanding or collapsing). These observed responses lag behind the change in solution pH observed visually, and a limitation of this visualization technique is that the solution color may not accurately reflect the solution pH, due to pH indicator molecules migrating in the electric field between the electrodes due to electrophoresis. But we feel that a more likely explanation is due to the parabolic flow profile of color changes within the sample cell. See for example in Figure 2B the distance traveled by the acidic or alkaline conditions generated is greater in the center of the sample cell (3 mm above the bottom surface) than near the bottom or top surface. Our visual inspection measures the distance traveled by the color change at the center of the sample cell, whereas PDEAMA brushes measure the change in pH at the bottom surface of the cell, where the wave travels slower due to the shear stress caused by the nearby stationary surface.

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REFERENCES

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CONCLUSION In summary, we have shown that pH-responsive polymer brushes can be coupled to an electrical stimulus through changes in solution pH caused by the electrolysis of water. Fast switching of polymer brush thickness can be achieved by placing an electrode in close proximity to the surface and applying a continuous or short pulse of electrical stimulus. This switching can be repeated many times, allowing polymer brushes to be cycled between an expanded and collapsed state many times, without the need to add extra reagents to the surrounding solution. Waves of polymer brush expansion and contraction can also be generated by placing the brush surface in between the anode and cathode of the electrolytic cell. These waves of expansion or collapse are analogous to the wavelike muscle contractions observed in biological systems such as the flagella of sperm or peristalsis in the esophagus of animals. Unlike other research which has used electric fields or electrical potentials to cause a change in polymer brush thickness, our approach does not electrify the brush substrate G

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