Swelling Process in Thin Gel Layers on

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Triggering off shrinking/swelling process in thin gel layer on conducting surface by applying appropriate potential Kamil Marcisz, Andzelika Gawronska, Zbigniew Stojek, and Marcin Karbarz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00713 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Triggering off shrinking/swelling process in thin gel layer on conducting surface by applying appropriate potential Kamil Marcisza, Andzelika Gawronskaa, Zbigniew Stojeka*, Marcin Karbarza* a

Faculty of Chemistry, Biological and Chemical Research Center, University of Warsaw, 101

Żwirki i Wigury Av., PL 02-089 Warsaw, Poland. *Corresponding author. E-mail: [email protected], [email protected] Abstract: Negatively

charged,

pH-sensitive,

very

thin

gel

layers

with

accumulated

hexaammineruthenium(II)/(III) were deposited on conducting surfaces. The gel was synthesized by applying electrochemically induced free-radical polymerization method. This method allowed covering the electrode surface with an uniform and compact layer. The modified electrodes exhibited excellent current-switch on/off behavior in response to changes in pH. However, the main goal in the paper that has been achieved was the control of the layer thickness by changing the oxidation state of hexaammineruthenium. The layers could be reversibly swelled/shrinked by applying appropriate potentials.

Key words: thin gel films, poly(sodium acrylate), electroresponsive hydrogel layers, hexaammineruthenium(II)/(III), volume phase transition, ON-OFF switch behaviour

ACS Paragon Plus Environment

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1. Introduction Polymer hydrogels because of their unique structure have properties of solids and liquids. One of the most useful properties of polymeric gels is the reversible volume phase transition, i.e. the transition of the gel from the swollen state to the shrunken state or vice versa. The volume of a gel can decrease even by a factor of one thousand what makes polymer gels very attractive. The reversible phase transition occurs in response to changes in the environmental conditions, such as temperature, pH, ionic strength, presence of specific ions, pressure, solvent composition, and electromagnetic radiation.1-6 Modiﬁcation of the electrode surface with a material sensitive to changes in the environmental conditions makes the electrode response sensitive to those changes as well.7,8 The attachment of the gel to the surface of an electrode increases substantially the electrode usefulness in, e.g. surface patterning, construction of switchable sensors/biosensors and more.9-16 Crosslinked acrylic-acid polymer network forms a well-known pH-responsive hydrogel. The hydrogel swelling ratio depends on extent of protonation of carboxylic groups in the network. At pH lower than the acrylic acid pKa value (ca. 4.6) the gel is in the shrunken state. This is caused by formation of many hydrogen bonds between the carboxylic groups in the network. At pH higher than pKa the ionization degree of the carboxylic groups increases. The appearance of negative charges in the network causes an increase in osmotic pressure which plays an important role in expanding the polymer network. Additionally, the repulsive forces between the ionized carboxylic groups attached to the polymeric chains also lead to the swelling of the hydrogels.17,18 The shrinking process could be also a result of formation of complexes of stoichiometry at least 2:1 between the carboxylic groups and some cations.19,20 The expectations for further improvement of hydrogel materials strongly increase. Specifically, electrosensitive hydrogel materials that change their shape and volume in response to a change in the oxidation state of a built-in species are particularly interesting. The introduction of electroactive compounds to the polymer network results in the appearance of redox properties of the gels. In addition, in some cases, these electroactive hydrogels also become electrosensitive; it means that changes in the oxidation state of the electroactive groups in the gel lead to significant changes in their volume. In our previous study a thermoresponsive poly(N-isopropylacrylamide) (pNIPA) hydrogel with ferrocene moieties was examined. It was found that the temperature of the volume phase transition of the macrogel in its oxidized state was significantly higher than in the reduced state. As a consequence, at a specific temperature, the polymer gel could exist in one of two forms: either swollen or shrunken depending on its redox state.21 It has also been reported that the macrogels based on pNIPA and containing either ruthenium tris(2,2’-bipyridine) or ferrocene or ACS Paragon Plus Environment

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dopamine methacrylamide in the polymer network were able to change their volume in response to a change in the oxidation state of the redox group.22-24 However, in the cases of macrogels their too large size and correspondingly slow response to changes in the environmental conditions appeared to be a serious limitation in many applications. One possibility of countering these limitations is to work with very small particles of the gels (micro- and nangels) or with a thin layer of a hydrogel anchored to the surface of a substrate. For example, Mergel et al. obtained a novel, multisensitive microgel based on thermoresponsive

pNIPA.

NIPA

was

copolymerized

(dimethylamino)propyl]-methacrylamide)

neutralized

with with

positively

charged

electroactive

(N-[3-

counter-ions

(hexacyanoferrates). Size of these materials could be controlled via oxidation/reduction of the ferricyanide/ferrocyanide system.25 A similar action of ferricyanide/ferrocyanide system was reported in the work with polyelectrolyte multilayers.26 The change in oxidation state and the corresponding change in shape and volume was usually achieved by applying a chemical reaction. A possibility of attaining the reversible shape and volume change of a thin hydrogel layer by applying an appropriate potential to the electrode surface can bring new perspectives for applications. In another paper we reported on modification of electrode surface with a monolayer of thermo- and electrosensitive microgels; under particular conditions the volume was reversibly changed by an electrochemical trigger.27 In this work, we turn into thin poly(sodium acrylate) (pAS) layers placed either on bare Au disk electrodes or on electrochemical-quartz-crystal-microbalance (EQCM) Au electrodes. Positively charged hexaammineruthenium(II) and (III) were introduced into the layers. The ON-OFF switch behavior was obtained in response to a change in pH. More importantly we aimed at getting the polymer network response (shrinking/swelling) after the redox probe oxidation/reduction.

2. Experimental 2.1. Materials Sodium acrylate (97%) (AS), N,N' –methylenebisacrylamide (99.5%) (BIS), amonium persulfate (>98%) (APS), hexaammineruthenium (II) chloride (99.9%) and hexaammineruthenium (III) chloride (98%) were purchased from Aldrich. Sodium hydroxide (NaOH), hydrochloric acid (HCl) and sodium nitrate (NaNO3, 99%) were purchased from POCh. All chemicals were used as received. All solutions were prepared using high purity water obtained from a Milli-Q Plus/Millipore purification system (water conductivity: 0.056 μS cm−1).



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2.2. Electrochemical measurements All electrochemical measurements were performed using an Autlab PGSTAT128N- and an Autolab PGSTAT204 potentiostats. The second instrument was equipped with a quartz crystal microbalance module. The manufacturer's software and the three-electrode system were used in the measurements. A platinum wire and a saturated silver chloride (Ag/AgCl/sat. KCl) electrode were used as the counter and the reference electrode, respectively. An EQCM Au electrode (3.59 mm in radius), and a regular Au disk electrode (2 mm in radius) were used as the working electrodes. The electrodes were kept in a glass cell. To minimize the electric noise, the electrochemical cell was kept in a grounded, covered with aluminum-foil Faraday cage. The electrochemical impedance spectroscopy (EIS) measurements were performed using a CHInstruments model CHI 750D potentiostat. The solution contained hexaammineruthenium (II) and (III) and the measurements were done at the Ru(III)/(II) formal potential in the frequency range from 100 kHz to 0.1 Hz. An alternating voltage of 10 mV was used. The data were processed using an EIS Spectrum Analyzer. The equivalent electronic circuit selected for the fitting process included the electrolyte resistance (Rs), Warburg impedance (Zw), electron transfer resistance (Ret) and the constant phase element Q (instead of the double layer capacitance, Cdl). 2.3. EQCM measurements An Autolab PGSTAT204 potentiostat with an electrochemical quartz crystal microbalance card (EQCM) and a 6-MHz Au/TiO2 quartz-crystal resonator were used. The EQCM technique allowed us to record simultaneously voltammetric/chronoamperometric and microgravimetric curves. 2.4. SEM Measurements A scanning electron microscope (SEM, Zeiss Merlin field emission) equipped with an energy dispersive spectrometer (Bruker, EDS) was used for the examination of the modified electrode surfaces.

3. Results and discussion The Au-disk electrode- and Au-EQCM electrode surfaces were modified with a pAS gel layer through the electrochemically induced free radical polymerization.28,29 The following concentrations of the substrates were used: 693 mM sodium acrylate, 7 mM N,N′– methylenebisacrylamide (cross-linker) and 10 mM ammonium persulfate (initiator). All substrates were dissolved in 0.2 M NaNO3. The application of appropriately negative potential caused the reduction of the persulfate anions, the initiation of the polymerization reaction and simultaneous anchoring of the gel layer on the surface of the electrode. The electrochemical


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process also was monitored with the quartz crystal microbalance (QCM) technique. According to the Sauerbrey equation30:

∆𝒇 = ― 𝑨

𝟐𝒇𝟐𝟎

(1)

𝝆𝒒𝝁𝒒∆𝒎

where f0 is the quartz oscillation frequency in the fundamental mode, A is the piezoelectrically active surface area, ρq is the density of quartz (ρq = 2.648 gcm-3), and μq is the shear modulus of quartz (μq = 2.947 x 1011 g·cm-1 s-2). The frequency shift, ∆𝒇, corresponds here to the changes in the gel mass deposited on the crystal surface. A decrease in the resonant oscillation frequency of the quartz crystal indicates an increase in gel mass on the electrode surface corresponding to the polymer network growth. It should be stressed here that the Sauerbrey equation is valid only for thin rigid layers. In the case of hydrogel films, due to their viscoelastic properties, the rigorous applicability of the equations (1) is limited.31,32 Finally, the obtained QCM data gave us only qualitative information; however, that was sufficient for the aim of our research. The synthesis was usually terminated after 14 voltammetric cycles and then the total shift/drop of frequency reached circa 220 Hz. As a result, a thin pAS gel layer was obtained on the electrode surface. Typical voltammograms with simultaneously recorded quartz frequency are presented in Figs. 1A and 1B. The decrease in the resonant oscillation frequency in the quartz crystal, as a result of the mass deposition, was well correlated with the range of the potential where the electroreduction of the peroxydisulfate took place (i.e. between -0.4 and 1.0 V). After the synthesis the electrodes were washed with water to remove unbound reagents and kept in water.



A0 I / A

-200 -400 -600 -800

with initiator without initiator

-1000

B

0

-50

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-100 -150 -200

-1.2

-1.0

-0.8

-0.6 E/V

-0.4

-0.2

0.0

Figure 1. A) Voltammograms obtained during modification of electrode surface in solution containing AS monomers and crosslinker, with initiator (black solid line) and without initiator (red dashed line), with simultaneously registered frequency change, Δf, during modification (B). Arrows indicate direction of polarization and corresponding changes in frequency.

Initially, the volume phase transition from the swollen to the shrunken state through pH change was investigated for Au EQCM electrode modified with pAS. The transition was monitored with the QCM technique. The pH decrease from 9 to 2.5 caused an increase in the frequency. This was related to the transition from the swollen to the shrunken state with the accompanying removal of water from the polymer network. The return of pH to 9 caused the swelling process of the gel layer. Water was again absorbed by the polymer network. This was observed as a decrease in the quartz frequency. The obtained curves are presented in Fig 2A. We also obtained much thicker layers; however, the properties and reproducibility of the layers were not satisfactory. Fig. S1 containing CV and EQCM curves for 60 cycles of the synthesis process is presented in supporting information. In Fig. S2 the influence of pH on the frequency change of quartz crystal for EQCM Au electrode modified with pAS gel deposited in 60 cycles is presented. ACS Paragon Plus Environment

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Next, a positively charged redox probe (a equimolar mixture of hexaammineruthenium (II) and (III)) was added to the negatively charged gel layer on the Au EQCM electrode. After addition of the positively charged redox probe an increase in the frequency was observed. The electrostatic interaction between the redox probes and the negatively charged carboxylic groups caused a decrease in the swelling ratio of the gel layer. This was observed as an increase in the frequency. The corresponding frequency shift curve and scheme of this process are presented in Fig. 2B. A similar effect, the shrinking of poly(acrylic acid) macrogel, was observed in the presence of Ca2+ and Cu2+ cations, the electrostatic interactions/complex formation between the carboxylic groups and the calcium cations were used as an additional crosslinking agent in the gel.19,20

A300

pH=9

pH=2.5 pH=9 pH=2.5

pH=9

f / Hz

200 100 0 electrode modified with pAS electrode unmodified

B 300

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 100 0 Addition of redox probe

0

100

200 300 t/s

400

Figure 2. A) Frequency change of quartz crystal for EQCM Au electrode modified with pAS gel layer (black solid line) and unmodified EQCM Au electrode (red dashed line). B) Frequency change of EQCM Au electrode modified with pAS gel layer after addition of Ru(NH3)62+/3+ (to 2 mM level for both species) at pH ca. 10.

The following investigation was focused on the electrochemical response of the Au disk electrodes modified with a thin pAS gel layer. Cyclic voltammetry and electrochemical impedance spectroscopy were done in different pH. The switch between the shrunken- and ACS Paragon Plus Environment


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the swollen state of the gel was examined after addition of the redox probe. The voltammograms obtained in different pH are shown in Fig. 3A. As it can be seen, an increase in solution pH caused an increase in the current. At a low pH, where almost all of the carboxylic groups were protonated, the signals of the redox probe were practically not seen. This was apparently related to the lack of electrostatic interactions between the redox species and the polymer net, and strong limitation of the diffusion in the shrunken state. As a result the transport of the electroactive probe to the electrode surface was strongly restricted. An increase in pH caused the appearance of the voltammetric redox-probe signal. Further pH increase led to an increase in the current, and finally, at pH = 7, the current magnitude reached a plateau. As is shown in Fig. 2B the addition of Ru species to the solution, at pH circa 9 – 10, where practically all carboxylic groups are dissociated, causes the shrinking of the gel layers. However, the shrinking process was not so advances as at pH 2, what will be shown later. As it can be also seen in Fig. 3A the reduction current is bigger than the oxidation one. It can be explained by the stronger interaction

between

the

polymer

network

(ionized

carboxylic

groups)

and

hexaammineruthenium(III) cations than those with hexaammineruthenium(II) what leads to more efficient accumulation of the three valence cations in the hydrogel film. Next the electrode was placed alternatively in redox-probe solutions of different pH (2.5 and 9) and cyclic voltammograms were obtained. Typical voltammograms and the changes in the redoxprobe reduction current caused by the changes in pH are illustrated in Fig. 3B. All the consecutive curves recorded immediately after immersing the modified electrodes into the solutions of appropriate pH were of the same height. Therefore the response to the pH switch must be considered as reasonably fast and repeatable. This is a typical ON-OFF electrode behavior.


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A 0

pH = 2.0 pH = 2.5 pH = 3.0 pH = 3.5 pH = 4.0 pH = 4.5 pH = 5.0 pH 0

-100 -200

-0.8

B

100

-0.6

2

4

6

8

10

0

pH = 5.5 pH = 6.0 pH = 7.0 pH = 8.0 pH = 9.0 pH = 10 pH = 11

-50 I / A

I / A

100

-100 -150 -200

-0.4

-0.2 E/V

0.0

0.2

0.4

pH = 2.5 (1) pH = 2.5 (2) pH = 2.5 (3) pH = 11.0 (1) pH = 11.0 (2) pH = 11.0 (3)

0 pH

I / A

2.5

11.0 2.5 11.0 2.5 11.0

0

-100

-50 I / A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-100 -150

-200

-200

-0.8

-0.6

-0.4

-0.2 E/V

0.0

0.2

0.4

Figure 3. A) Cyclic voltammograms obtained for Ru(NH3)62+/3+ system at different pH at Au disk electrode modified with pAS. Inset: peak current of reduction of Ru(NH3)63+ at modified Au disk electrode in function of pH. B) Cyclic voltammograms obtained for Au disk electrode modified with pAS gel layer upon consecutive switching of pH. Inset: repeatability of reduction peak current of Ru(NH3)63+ due to consecutive switching of pH.

Next technique used to characterize the layer transformation was electrochemical impedance spectroscopy (EIS). Fig. 4A, presents typical Nyquist plots for the modified electrode at various pH. The semicircle diameter corresponds to electron transfer resistance of the hexaammineruthenium (II) and (III) electrode process. The electron transfer resistance, calculated from Nyquist plots, plotted vs. pH is presented in Fig. 4B. As it can be seen, at low pH (2-3) the electron transfer resistance was high (800 kΩ), what indicates that the electrode surface was practically blocked with the shrunken gel layer. The increase in pH from 3 to 7 caused a decrease in electron transfer resistance to ca. 4 kΩ, thus facilitated the electron transfer. At pH higher than 7 the R parameter stabilized. The current- and the electron transfer resistance changes, between the fully shrunken and swollen states, were in the range of two orders of magnitude. Other parameters of the equivalent electronic circuit selected for the ACS Paragon Plus Environment


fitting process and comparison of experimental results with fitted curves are presented in the supporting information (Table 1S and Fig. 3S).

A

pH = 2.5 pH = 3.5 pH = 4.0 pH = 4.5

Z'' / k

300

pH = 5.0 pH = 7.0

pH = 9.0 pH = 11.0

200 8

0

2

Z' / k 4 6

8

10

6 Z'' / k

100

0

4 pH = 7.0 pH = 9.0 pH = 11.0

2 0

0

100

200

300

400

500

600

Z' / k

B 800 600 R / k

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400 200 0 2

4

6

pH

8

10

12

Figure 4. A) Nyquist plots obtained in solutions of different pH for Au disk electrode modified with pAS gel layer. B) Changes in electron transfer resistance of Ru(NH3)62+/3+ system in function of pH for thin pAS gel films on the Au disk electrode and a scheme of the equivalent electronic circuit selected for the fitting process. Concentration of both Ru species in 0.2 M NaNO3: 2 mM.

Next, the influence of oxidation state of redox probe on the pAS gel layer swelling ratio was examined with cyclic voltammetry and the EQCM technique. As it can be seen in Fig. 5A, for the unmodified electrode (black dashed line) the cycling caused only a small change in the quartz crystal frequency. However, for the electrode modified with pAS thin gel layer, the reduction of Ru(NH3)63+ caused a substantial decrease in the frequency. Inversely, the oxidation process of Ru(NH3)62+ resulted in a quartz crystal frequency increase of similar magnitude. The consecutive increases and decreases in frequency are illustrated in Fig. 5B. The frequency shifts were well defined and satisfactorily fast. They were well repeatable and practically reproducible. This cyclic change in frequency was a good illustration of the reproducible swollen/shrunken ratio for the pAS gel layers. Finally, a decrease in frequency, ACS Paragon Plus Environment

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as an effect of the reduction of Ru(NH3)63+, was related to the swelling process. Then the number of the 3+ cations decreased, the electrostatic interactions between the polymer chains and the cations were weakened and the solvent was absorbed from the solution by the polymer network. As a result the mass of the gel layer increased. The oxidation of Ru(NH3)62+ caused the opposite effect. The number of 3+ cations increased, so the electrostatic interactions increased and the gel layers shrank. Since the solvent was removed from the polymer network, the mass of the hydrogel layers decreased and the frequency increased. A graphical scheme of electroinduced volume transition phenomena of pAS gel layer on the electrode surface is presented in Fig. 5C.

A

B

400

-400

-800

Oxidiazed stage

0 -50

f / Hz

I / A

0

electrode modified with pAS with Ru(NH3)6

2+/3+

-100 -150

electrode unmodified 2+/3+ electrode modified with pAS without Ru(NH3)6

Reduced stage

0

0

C f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

100

150

t/s

-50 -100 -150 -0.8

-0.6

-0.4

-0.2 E/V

0.0

0.2

0.4

Figure 5. A) Voltammograms obtained for Ru(NH3)62+/3+ system at Au EQCM electrode and simultaneously registered frequency shifts for modified with pAS (black solid line), unmodified (red dashed line) electrodes and modified with pAS and without ruthenium complex (blue dotted line). B) Quartz crystal frequency change, in function of time, during reduction and oxidation of Ru(NH3)63+ and Ru(NH3)62+, for modified with pAS (black solid line), unmodified (red dashed line) electrodes and modified with pAS and without ruthenium complex (blue dotted line), at pH ca. 10. C) Scheme of shrinking/swelling process of pAS gel layer on electrode surface caused by reduction and oxidation of Ru species.



To prove that the swelling/shrinking process can be induced by applying the appropriate potential, chronoamperometry combined with quartz frequency measurements was used. As it can be seen in Fig. 6 the reduction of Ru(NH3)63+ cations led to very fast decrease in frequency. It dropped to circa 140 Hz and remained at this level. The oxidation of Ru(NH3)62+ caused an increase in frequency. Again, the response was very fast and the frequency turned back to its initial value. This experiment proved that the obtained very thin pAS layers, in the presence of positively charged redox probe, exhibited electroresponsive properties.

I / mA

0.1 0.0 -0.1 -0.2 0

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-50 -100 -150 0

100

200 t/s

300

400

Figure 6. Double-pulse chronoamperogram and simultaneously acquired frequency change obtained for Au EQCM electrode modified with pAS gel layer.

The morphology of thin gel layers deposited onto EQCM electrode surface is seen in Fig. 7A. As it can be seen, the gel film formed a planar, thin and homogenous layer. To determine the thickness of the layers, some electrodes covered with thin gel layers were broken into small pieces. One of the electrode pieces was lyophilized (freeze dried); it’s picture is presented in Fig. 7B. The other parts of electrode were dried (the corresponding pictures are shown in Fig. 7C). On the edges of the broken electrode pieces the detached layers could be observed. We assumed that the thickness of lyophilized layers was similar to the thickness of swollen gel layers. The determined thickness of the swelled gel layers was circa 400 nm. The dried gel layers were much thinner and smoother; their thickness was circa 40 nm. ACS Paragon Plus Environment

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Figure 7. A) SEM images of EQCM electrode surface covered by pAS thin film. B) Partially detached lyophilized gel layer. C) Partially detached dried gel layer.

The thickness of thin gel layers deposited onto the EQCM electrode surface at pH 2 and pH 11, and at pH ca. 9 in the presence of Ru(NH3)63+ and Ru(NH3)62+ was also investigated. Typical SEM images of lyophilized and detached gel layers are presented in Fig. 8. The thicknesses of the gel layers at pH 2 and pH 11 were very similar to those obtained for dried gel layers and lyophilized untreated gel layers (Fig. 7B and 7C). This suggests that the states of the gel layers at pH 2 and pH 11 were close to completely shrunk and swollen states, respectively. Importantly, the determined thickness of the gel layer in the presence of Ru(NH3)63+ was ca. 140 nm and was significantly smaller than in the presence of Ru(NH3)62+, which was ca. 290 nm. We attribute the difference between Ru(II) and Ru(III) swelling behavior to the interactions of the Ru species with dissociated carboxylic group, ACS Paragon Plus Environment


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which is a hard base. Ru(III) is a harder acid and its interaction with the anion is much stronger. Similar situation exists for the interactions between Ru(III) with phosphate groups in DNA.33 Assuming that thickness of the lyophilized layers was similar to thickness of the layers in aqueous solutions, the change in thickness related to the change in pH was ca. 360 nm, while the change related to the change in the redox state of the ruthenium complex was ca. 150 nm. These data were obtained from the SEM images. The changes in f in the QCM did not reflect precisely the changes in gel mass. The use of Sauerbrey equation was limited due to the viscoelastic effect. We found that the change in thickness related to the change in pH, calculated from f, was 35.5 nm, and the change related to the change in the redox state was 18.4 nm, so the results obtained from the QCM data are severely underestimated. In the above calculations the density of the gel layer was constant and equaled 1 g mL-1.

Figure 8. SEM images of lyophilized and detached pAS gel layers at pH 2 and 11, and at pH ca. 9 in presence of Ru(NH3)63+ (2 mM) and Ru(NH3)62+ (2 mM).

4. Conclusion In conclusion, very thin and pH-sensitive gel layers based on acrylic acid sodium salt crosslinked with N,N′–methylenebisacrylamide were successfully attached to the Au disk- and Au EQCM electrode surfaces. To anchor a gel layer electrochemically on an electrode surface the induced free radical polymerization method was used. The deposition process was monitored with the quartz-crystal microbalance technique. The pH induced volume phase transition of the gel layers on the Au EQCM electrode surface was examined with the EQCM ACS Paragon Plus Environment

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technique. It was found that the shrinking process caused an increase in the quartz frequency, therefore, according to the Sauerbrey equation, the mass of the gel layer on the crystal surface decreased. The change in pH to a value higher than pKa caused the swelling process; the frequency dropped, the solvent was absorbed by the polymer network, the gel layers swelled and their mass increased. The examination of electrochemical consequences of the shrinking/swelling process, done with cyclic voltammetry and EIS, revealed that in the shrunken state, at low pH, the electron transfer to the electrode surface was really blocked. The appropriate increase in pH caused the swelling process and the electron transfer rate increased strongly. pH could be switched many times and the phase transition process was satisfactorily repeatable. Finally, the cyclic changes in the oxidation state of the Ru species caused corresponding cyclic changes in the frequency of EQCM electrodes. Apparently the polymer network interacted much stronger with the redox probe at 3+ oxidation state than with Ru(NH3)62+. Therefore the reduction process caused the swelling process, while the electrooxidation the shrinking one. This was confirmed with the SEM investigations. This novel approach to the construction of electroresponsive swelling/shrinking thin gel layers could be useful in construction of switchable microelectrochemical systems in microfluidics and as mediating matrices in biosensors. Electrochemically induced shrinking and swelling processes may be also useful in the construction of artificial muscles and in advanced drug delivery systems.

Acknowledgements This work was supported by the National Science Centre of Poland through grant number 2015/19/B/ST5/03530. Supporting Information CV and EQCM curves for 60 cycles of the pAS gel layer synthesis process, influence of pH on the frequency change of quartz crystal for EQCM Au electrode modified with pAS gel deposited in 60 cycles, parameters of the equivalent electronic circuit selected for the fitting process and comparison of experimental results with fitted curves. References (1) Ueki, T.; Yamaguchi, A.; Watanabe, M. Unlocking of Interlocked Heteropolymer Gel by Light: Photoinduced Volume Phase Transition in an Ionic Liquid From a Metastable State to an Equilibrium Phase. Chem. Commun. 2012, 48, 5133-5135. (2) Karbarz, M.; Mackiewicz, M.; Kaniewska, K.; Marcisz, K.; Stojek, Z.; Recent Developments in Design and Functionalization of Micro- and Nanostructural EnvironmentallyACS Paragon Plus Environment


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Swelling Process in Thin Gel Layers on

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