Electrochemical Examination of the Structure of Thin Hydrogel Layers

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Electrochemical Examination of Structure of Thin Hydrogel Layers Anchored to Regular- and Microelectrode Surface Klaudia Kaniewska, Marcin Karbarz, Krzysztof Ziach, Alicja Siennicka, Zbigniew Jan Stojek, and Wojciech Hyk J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b06515 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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The Journal of Physical Chemistry

Electrochemical Examination of Structure of Thin Hydrogel Layers Anchored to Regular- and Microelectrode Surface Klaudia Kaniewska, Marcin Karbarz*, Krzysztof Ziach, Alicja Siennicka, Zbigniew Stojek, Wojciech Hyk* Faculty of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland.

* corresponding authors: Marcin Karbarz: [email protected], +48 22 5526350 Wojciech Hyk: [email protected], +48 22 5526359

ACS Paragon Plus Environment


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Abstract For the examination of hydrogel structure, thin layers of thermoresponsive gels based on poly(N-isopropylacrylamide) (pNIPA) and copolymer poly(N-isopropylacrylamide-co-sodium acrylate) (p(NIPA-co-AS)) were successfully anchored to microelectrode- and regular electrode surfaces using the electrochemically induced free radical polymerization. The obtained layers were stable and covered the entire surface of the electrodes. Electroactive probes: 1,1’-ferrocenedimethanol (Fc(CH2OH)2) and synthesized derivatives of ferrocene modified with polyethylene glycol units (Fc-PEGn) of various length (n = 4, 9, 75 and 135) were employed for studying the volume phase transition of the thin hydrogel layers and for the determination of their structural parameters. The quantitative information on the structural parameters of the hydrogel layers was derived from the obstruction model for diffusion using the voltammetrically determined diffusion coefficients for the model redox probe Fc(CH2OH)2. An approach to the determination of the effective radii of the gel openings (channels) for pNIPA and p(NIPA-co-AS) microlayers was developed. The obtained results were matched with the experimental results and allowed derivation of quantitative conclusions. The voltammograms obtained with modified electrodes in solutions containing Fc-PEG4, Fc-PEG9 and Fc-PEG75 were well-defined and of appropriate height. However, the voltammograms recorded for Fc-PEG135, the hydrodynamic radius of which exceeded the size of the gel channels, were at the baseline level.

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1. Introduction The polymer gels are extensively investigated due to their unique properties which have induced a wide range of potential applications as drug delivery systems, polymer electrolytes, superabsorbents and tissue engineering.1,2,3,4 Anchoring gel materials to electrode surface widens the possibilities to construct sensors / biosensors, actuators, logic gates and electrodetissues interfaces.5,6,7,8 The miniaturization of such systems is desired and can be reached by applying microelectrodes. Hydrogels are three dimensional networks filled with an aqueous solution. Some of the gels can undergo a fast, reversible volume phase transition (VPT) in response to a change in the environmental conditions. As a result a gel can pass from one state (swollen) to another one (shrunken) by changing content of water in the network. Gel materials represent a rather unusual way in which a large amount of liquid can be kept “solid”, and therefore gels possess properties characteristic for both the liquid and solid states. The latter property is a result of storing the mechanical energy by the polymeric network, while the former one is a consequence of density and molecular-transport properties which are similar to those of liquid media.9 The hydrogels based on N-isopropylacrylamide (pNIPA) are well characterized and are described as thermosensitive gels with amphiphilic structure; their volume phase transition occurs at circa 33 oC. This temperature is called the lower critical solution temperature (LCST).10 An addition of sodium acrylate monomer (the copolymer of NIPA and AS will be formed) makes the polymer structure more hydrophilic. The NIPA-AS copolymer gel usually has a bigger swelling ratio and the VPT occurs at a higher temperature. This is a consequence of introduction of ionized carboxylic groups into the gel network.11 The diffusional transport of probe molecules in the gel matrix depends strongly on several factors. These include: fraction of the polymer, cross-linker concentration, polymeric chain

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mobility, existence of the charged groups in the gel structure and swelling ratio.12 Diffusion of a probe molecule can be treated as a random walk of the molecule through a series of openings between the polymer chains. These openings (pores) form continuous channels for the movement of the probe species. The distribution and average diameter of openings are important characteristics of the gel structure. Their magnitudes determine directly the gel permeability and accessibility of the interior. The gel materials with nanometer-sized channels can be applied for separation of solutes by size and for control of their transport.13 On the other hand, the control of structural properties of the gel material (including size of gel openings / channels) is a necessary condition in the design of matrices for immobilization of the selected compounds, e.g. for the entrapment of enzyme molecules, and membranes in electrochemical energy storage and conversions devices.14,15 The changes in the gel opening diameter can be easily initiated by employing the shrinking and swelling process especially in the gels exhibiting the continuous volume phase transition. Its impact on the gel transport properties, and consequently the gel permeability, can be quantitatively studied by measuring the diffusion coefficient of selected electroactive probe species that is transported through the gel matrix deposited on the electrode surface. In this work an electrochemical method for the formation of pNIPA and p(NIPA-co-AS) hydrogel films on a microelectrode- and regularly-sized electrode surface, without previous functionalization, is used. The hydrogel films were anchored to the microelectrode surface via electrochemically induced free radical polymerization (EIFRP) process using the chronoamperometric technique.16 The successful application of cyclic voltammetry in the process of the formation of gel layers on the microelectrode surface is also demonstrated. Commercially

available

1,1’-ferrocenedimethanol

(Fc(CH2OH)2)

and

synthesized

polyethylene glycol ferrocene derivatives (ferrocene moieties modified with polyethylene glycol units (PEG) of various length) were employed for the study of the volume phase

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transition of thin hydrogel layers and for the determination of their structural parameters. The properties of the hydrogel layers attached to a surface may to some extent differ from those determined for the regular gels. Regular pNIPA gels have been extensively examined; however, electrochemically deposited pNIPA gel layers have different density, flexibility of chains and degree of embedded cross-linker.17 They represent another class of pNIPA polymeric systems and therefore require additional characterization. In this paper the quantitative information on the structural parameters of the hydrogel layers was obtained from the obstruction model for diffusion using the voltammetrically determined diffusion coefficients for the model redox probe species (Fc(CH2OH)2). Basing on that model a theoretical approach for the determination of the effective radii of openings (channels) for pNIPA gels and p(NIPA-co-AS) gel microlayers was developed. The results obtained agreed well with the experimental voltammetric results for the ferrocene derivatives of different size (Fc-PEGn).

2. Materials and apparatus The reagents for modification of electrode surfaces: N-isopropylacrylamide (NIPA), sodium acrylate (AS), N,N’-methylenebisacrylamide (BIS), ammonium persulfate (APS), 1,1’ferrocenedimethanol (Fc(CH2OH)2) and sodium nitrate (NaNO3) were purchased from Aldrich. All reagents and solvents for the synthesis of ferrocene derivatives were of reagent grade quality and were purchased from Aldrich. PEG9, PEG75 and PEG135 were used as mixtures of oligomers; the numbers at the name ends depict the average number of mers. All chemicals were used as received except for NIPA which was recrystallized twice from the benzene–hexane mixture (90 : 10 v/v) and dichloromethane which was distilled from CaH2. 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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Synthesis. Column chromatography was carried out using a Merck Kieselgel 60 (63–100 µm mesh size) column. TLC was carried out using Merck Kieselgel F254 plates. The NMR spectra were recorded with a Bruker Avance 300 instrument. The chemical shifts are reported in ppm and are set to solvent residue peak. The splitting pattern of multiplets is described by the following abbreviations (s – singlet, t – triplet, br – broad peak). J coupling constants values are reported in Hz. Mass spectral analyses were performed using the ESI-TOF technique (LCT mass spectrometer from Micromass) and the MALDI-TOF technique (Axima Performance mass spectrometer from Shimadzu). Electrochemical measurements. A CHI 750D Electrochemical Workstation was used in experiments. A platinum disc microelectrode of 20.0 µm in diameter and a regular platinum disc electrode of 2.0 mm in diameter were used as the working electrodes. A Ag/AgCl electrode and a platinum foil were used as the reference and the counter electrode, respectively. The concentrations of ferrocene derivatives were estimated from the UV-Vis spectra. The spectra were obtained with a Perkin-Elmer (Lambda-25) spectrometer.

3. Experimental 3.1.

Deposition of pNIPA and p(NIPA-co-5%AS) gels on electrode surface

The polymer networks were formed using the EIFRP method. The total concentration of the monomers was equal to 700 mM. The mole fractions of base monomer NIPA, AS and the cross-linker (BIS) were 0.94, 0.05 and 0.01, respectively. In the case of polymerization of pNIPA gel, the concentrations of NIPA and BIS were 0.99 and 0.01, respectively. The concentration of the initiator was 10 mM. The electrode was immersed in a 10-oC, degassed solution of the monomers; 0.2 M sodium nitrate served as the supporting electrolyte. The radicals needed for starting the polymerization reaction were formed by doing 60 cyclic

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polarizations with the platinum electrodes in a potential range from -0.1 to -1.1 V vs. the Ag/AgCl reference electrode at a scan rate of 100 mV s-1. Then, the deposited gels were purified several times by immersing them in distilled water to remove unreacted substrates.

3.2.

Synthesis and purification of PEGylated ferrocenes

For the synthesis of the ferrocene derivatives the procedure of Allali and Mamane18 was appropriately modified. Ferrocenemethanol (120 mg, 0.58 mmol) was dissolved in dry dichloromethane, then polyethylene glycol (1.1 equiv, 0.64 mmol) and aluminum trifluoromethanesulfonate (0.03 equiv., 8 mg) were added. The mixture was stirred at r.t. till the complete transformation of the ferrocene substrate; this was monitored by TLC (typically 1 h). Then, the mixture was diluted with 4 ml of DCM, washed once with 5% aqueous NaHCO3 and dried with anhydrous Na2SO4. The solvent was removed using a rotary evaporator and the crude product was purified by repetitive silica gel flash chromatography. The particular solvent systems applied in each case are given below along with the yield and the analytical data. Chromatography pure compounds were dried under high vacuum and then stored at -18 oC prior to the electrochemical experiments. Analytical data: Fc-PEG4. Product was purified upon single chromatography, solvents: gradient of ethyl acetate in hexanes (015%); yellow oil, yield 84 mg (36%). 1H NMR (300 MHz, CDCl3) δ 4.34 (Fc-CH2, s, 2H), 4.25 (Fc-H, t, J = 1.7 Hz, 2H), 4.18 – 4.10 (Fc-H, m, 7H), 3.77 – 3.56 (CH2CH2, m, 16H), 2.77 (OH, t, J = 6.1 Hz, 1H). 13C NMR (75 MHz, CDCl3) δ 83.21, 72.56, 70.60, 70.58, 70.55, 70.34, 69.57, 69.45, 68.93, 68.48, 68.44, 61.76; MS(ESI-TOF) m/z [FcPEG4+Na]+ 415.1. Fc-PEG9. PEG substrate was used as a mixture of oligomers (average MW = 400). Product was purified upon double chromatography, solvents: 1. gradient of ethyl acetate in hexanes 7



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(0100%) and 2. gradient of methanol in chloroform (05%); yellow oil; yield 130 mg (22%). 1H NMR (300 MHz, CDCl3 : DMSO-d6, 4:1) δ 4.34 (Fc-CH2, s, 2H), 4.24 (Fc-H, t, J = 1.7 Hz, 2H), 4.18 – 4.13 (Fc-H, m, 7H), 3.77 – 3.55 (CH2CH2m, 48H); 13C NMR (75 MHz, CDCl3 : DMSO-d6, 4:1) δ 83.26, 72.65, 70.57, 70.53 (br), 70.24, 69.55, 69.42, 68.91, 68.44 (br), 61.63; MS(ESI-TOF) selected m/z: [FcPEG8 + Na]+ 591.1 (5%), [FcPEG10 + Na]+ 679.2 (100%), [FcPEG12 + Na]+ 767.4 (11%). Fc-PEG75. PEG substrate was used as a mixture of oligomers (average MW = 3350). Product was purified upon triple chromatography, solvents: 1. gradient of methanol in chloroform (05%), 2. Isocratic chloroform : ethyl acetate: methanol 8 : 1 : 1, 3. Gradient of methanol in chloroform (07%); yellow solid; yield 147 mg (7%). 1H NMR (300 MHz, CDCl3) δ 4.30 (Fc-CH2, br, 2H), 4.20 (Fc-H, br, 2H), 4.10 (Fc-H, s, 7H), 3.62 (br, 336H);

13

C NMR (75

MHz, CDCl3) δ 83.35, 70.54 (br), 69.42, 68.95, 68.37. MS(MALDI-TOF) selected m/z: [FcPEG58+Na]+ 2793.55 (21%), [FcPEG65+Na]+ 3101.71 (37%), [FcPEG73+Na]+ 3454.07 (100%), [FcPEG82+Na]+ 3850.30 (44%), [FcPEG86+Na]+ 4026.27 (21%). According to MS analysis the product contained some unreacted PEG substrate. Fc-PEG135. PEG substrate was used as a mixture of oligomers (average MW = 6000). The substrate ratio was changed to 70 mg (0.32 mmol) of ferrocenemethanol, 1600 mg (0.26 mmol) of PEG135, 4.5 mg (0.03 equiv) of aluminum trifluoromethanesulfonate in 3 ml of DCM. Product was purified upon double chromatography, solvents: 1. and 2. gradient of methanol in chloroform (05%); yellow solid; yield 104 mg (6.5%). MS(MALDI-TOF) selected

m/z:

[FcPEG130+Na]+

5865.10

(22%),

[FcPEG141+Na]+

6449.69

(57%),

[FcPEG151+Na]+ 6890.69 (100%), [FcPEG160+Na]+ 7287.81 (52%), [FcPEG166+Na]+ 7553.27 (22%). According to MS analysis the product contained some unreacted PEG substrate.

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4. Results and discussion During the EIFRP process the peroxodisulphate anion was electroreduced at the platinum microelectrode / regular electrode to the corresponding radical.10 The free radicals reacted with monomers and induced the polymerization process. The cathodic wave of reduction of the initiator (APS) decreased in subsequent cyclic voltammograms, (Figure 1A, black lines). In the absence of the initiator no electrochemical signal was observed (Figure 1A, dashed line).

Figure 1. (A) Consecutive voltammograms obtained during modification of microelectrode in deoxygenated solution containing NIPA, AS, BIS and initiator (solid lines) and monomers without initiator (dashed line). Inset: corresponding voltammograms obtained during modification of regular platinum electrode in the same way. (B) SEM image of scratched pNIPA layer deposited on platinum regular electrode.

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The decrease in the cathodic wave height of the peroxodisulphate anion can be explained by the fact that after each consecutive cycle the degree of covering/blocking the electrode surface by the polymer increased. This method allowed us to form the pNIPA and p(NIPA-co-5%AS) gels on the surfaces of both: micro- and regular electrodes. The method led to covering of the entire electrode surface and to very smooth layers (Figure 1B). The changes in the steady state- and peak currents of the electrooxidation of the selected redox probe (Fc(CH2OH)2) upon temperature indicated that the obtained layers maintained their ability of undergoing the volume phase transition (Figures 2A and 2C). During the shrinking process the solution was expelled from the gel structure and a tight cover, blocking the electrode surface and limiting the diffusion of the redox probe, was formed. Temperature of the volume phase transition for copolymer p(NIPA-co-5%AS) was higher than that for pure NIPA layer due to its more hydrophilic nature. This effect can be clearly seen for the layers deposited on a regular electrode. In the case of gel microlayers (the gel layers deposited on microelectrodes) the percent of acrylic acid in the network may be relatively smaller and the temperature of volume phase transition did not change so much.

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Figure 2. Changes in voltammetric peak currents of electrooxidation of Fc(CH2OH)2 upon temperature for micro- (A) and regular electrode (C) modified with pNIPA and p(NIPA-co5%AS). Selected voltammograms for gel layers in swollen- (solid line) and shrunken state (dashed line) for micro- (B) and regular electrode (D). Solution contained 2 mM Fc(CH2OH)2 and 0.2 M NaNO3. Scan rate: 10 mV/s for microlectrode and 50 mV/s for regular electrode.

An inspection of Figure 2 reveals that magnitudes of the currents recorded at both: micro- and regular electrodes are higher for copolymer than pNIPA layers in the entire temperature range examined. Typical cyclic voltammograms of the electrooxidation of Fc(CH2OH)2 for pNIPA and p(NIPA-co-5%AS) layers on the micro- and regular electrode, below and above VPT temperature, are shown in Figures 2 B and D, respectively.

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Cycling of temperature led to consecutive processes of swelling and shrinking of the hydrogel film on the electrode surface. The shrunken gel layer swelled again after reaching a temperature of 20oC. As a result the redox probe could diffuse within the hydrogel layer and the steady state / peak currents increased. The subsequent switches between the swollen and the shrunken states of the gel (Figure 3) indicated that the hydrogel layer obtained via the electrochemical method was very stable.

Figure 3. Changes in the magnitude of the voltammetric peak height and steady-state wave height for electrooxidation of Fc(CH2OH)2 upon switching temperature. pNIPA layer on microelectrode (black circles) and regular electrode (gray circles).

Next, cyclic voltammograms were obtained for ferrocene modified with PEG units of various lengths (Fc-PEGn, where n = 4, 9, 75 and 135 EG units) in the temperature range from 20 to 55oC (Figure 4). To extract the species diffusivities from the oxidation limiting currents one needs to know exact concentrations of the redox probes employed for the voltammetric studies. Because of an unspecified amount of unreacted PEG in the Fc-PEGn products (especially for Fc-PEG75 and Fc-PEG135), the concentrations of the ferrocene derivatives

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were estimated from UV-Vis spectra.. The magnitudes of the obtained peak currents corrected for the actual concentration of the probe decreased with an increase in the redox probe size for both: pNIPA (Figure 4A) and p(NIPA-co-5%AS) layers (Figure 4B). An increase in temperature below the point of the volume phase transition of the hydrogel layer made initially the magnitude of the limiting (peak or steady-state) currents increase. This was simply due to an increase in the species diffusivities caused by increased temperature. At the temperature range where the shrinking process proceeded the gel structure became more compact – the size of openings (channels) between the polymer chains decreased and the species mobility drastically dropped. Consequently, the measured oxidation currents significantly decreased. Interestingly, it can be noticed in Figure 4 that at temperatures above the volume phase transition the magnitude of the oxidation currents tends slightly to increase but the gel layer still remains in the shrunken state. This is again due to the temperaturedriven increase in the mobility of species entrapped in the gel layer. The non-zero currents for the shrunken state of the gel layers may arise from entrapment of some fraction of the redox probe in the gel matrix.19

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Figure 4. Changes in oxidation peak currents per unit concentration of redox probe (Fc-PEGn of various length) upon temperature. Regular electrode modified with pNIPA (A) and p(NIPA-co-5%AS) (B). Voltammograms for PEGylated ferrocenes for pNIPA layers at 20 oC (C). Inset in (C): scheme of PEGylated ferrocene, where n is number of mers / units (n = 4, 9, 75 and 135). Voltammograms for Fc-PEG135 for bare- and pNIPA modified regular electrodes at 20 oC (D). Scan rate - 50 mV/s.

It is clearly seen in Figure 4 that the magnitude of the voltammetric responses for the electrodes modified with pNIPA and p(NIPA-co-5%AS) gel layers decreases gradually with increasing size of the redox probe. The cyclic voltammograms recorded for Fc-PEG4, FcPEG9, and Fc-PEG75 are well defined (Figure 4C). For the largest redox probe, Fc-PEG135, the electrochemical signal is close to zero and the layer acts as a barrier for its transport in the

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entire temperature range employed in the studies. The size of Fc-PEG135 has therefore a critical value regarding the diffusion into the hydrogel network (Figure 4D). In our previous works we have demonstrated a successful application of the obstruction model of diffusion in polyelectrolyte and polymeric gel systems.

20,21

That model allows one to

obtain quantitative characteristics of the structural parameters of the polymeric system by measuring the diffusivity of the probe species transported in the polymeric matrix. In that model, diffusion of a molecule is treated as a random walk of the molecule through a series of openings between the polymer chains. A drop in diffusion coefficient of a species in a gel medium, Dg, with respect to that in water, D0, can be expressed in terms of the probability of encountering a series of these openings.

22,23

Since only a fraction of the openings is large

enough for the movement of the probe species, their diffusion rate is reduced. According to the model the ratio Dg / D0 can be expressed as24

 π  r + r 2  s p   = exp −   D0  4  ro + rp    

Dg

(1)

where ro is the average radius of the opening between polymeric chains, and rp and rs are radii of the polymer chain and the solute, respectively. The hydrodynamic radius of a diffusing species can be estimated from the Stokes-Einstein relation: rs =

RT 6πN A ηD0

(2)

The polymer chain radius can be estimated using the following equation:25

rp =

M mν πlN A

(3)

In these equations Mm is the molecular weight of the monomer, ν is the specific volume of the polymer, l is the length of the monomer, η is the viscosity of the solution, and T is the temperature at which D0 and η were determined.

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The applicability of the obstruction model was examined for the pNIPA and p(NIPA-co5%AS) gel layers deposited on Pt microelectrodes. However, it should be emphasized that some assumptions of the model may limit its applicability for the structural characterization of the crosslinked polymeric gels. The main limitation arises from the fact that in the derivation the cylindrical geometry of the polymeric chain was assumed; that is not the case in the crosslinked polymeric gels. In addition to this, pNIPA and p(NIPA-co-5%AS) gels undergo volume phase transition induced by temperature. This may alter polymeric chain radius and therefore its magnitude cannot be treated as a constant value. The Dg to D0 ratio for a probe molecule introduced to the system can be determined straightforwardly using cyclic voltammetry at microelectrodes. The probe chosen for the studies in the polymeric gels was 1,1’-ferrocenedimethanol. The diffusion coefficient of the electroactive probe is given by equation (4). D, is directly proportional to the diffusion-limited steady-state current, Iss, recorded at a disk microelectrode of radius re, in a solution with bulk concentration of the probe cb (nel is the number of electrons in the oxidation reaction, and F is the Faraday constant). D=

I ss 4nel Fc b re

(4)

Eq. (4) was derived for disk microelectrodes that are in contact with a continuous medium, e.g. a solution, containing a redox species of bulk concentration cb. Before the voltammetric experiment the species is uniformly distributed throughout the medium. In our case, a disk microelectrode is modified with a gel layer which is in contact with the solution that can be treated as a probe-species unlimited reservoir. This implies that for an appropriately thin gel layer the transport in the solution phase may to some extent contribute to the overall steadystate transport-limited current Iss. To estimate that contribution and, consequently, to validate the applicability of eq. (4) for the determination of probe diffusivities in the gel layer attached to the microelectrode, a set of voltammetric experiments was performed in the gel phase. In 16


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those experiments a disk microelectrode was immersed into the gel medium (i.e., pNIPA and p(NIPA-co-5%AS).26,27 The gels were of the same compositions as those prepared for the electrochemical generation of the corresponding gel layers and contained a known concentration of the redox probe. The diffusion coefficients determined for Fc(CH2OH)2 at 20oC in the gel phase (Dg*) were smaller than those obtained for the gel modified microelectrodes (Dg) by less than 20% (the ratios of Dg* to Dg were equal to 0.805 and 0.810, for pNIPA and p(NIPA-co-5%AS), respectively). The differences became even smaller at increased temperature. Assuming that the physical properties of the gel layers (porosity coefficient, ability to the specific accumulation of the probe molecules, etc.) are comparable to those of the pure gel phase, the obtained numbers indicate that the major fraction of the transported mass in the electrooxidation process is located in the gel layer. Thus, Iss is mainly controlled by the transport inside the gel-layer network and may provide a good estimate of the probe diffusion coefficients in such the gel structure. The results obtained also allow one to estimate roughly the thickness of the gel layer attached to the microelectrode. Under the steady-state conditions the distance from the electrode surface where the change in the substrate concentration is less than 1% is estimated as 10re.28 The latter number and the above ratios of the diffusion coefficients suggest that the thickness of the gel layer should be comparable to or larger than the thickness of the transport layer. The mean values of Fc(CH2OH)2 diffusivities in the aqueous solutions, D0, and in the systems where the solution is in contact with pNIPA and p(NIPA-co-5%AS) microlayers, Dg, were determined voltammetrically at various temperatures from 20 to 55oC. The selected temperature range covered the temperature conditions where the volume phase transition of NIPA gels occurred. An illustration of the temperature dependence of ratio of the corresponding diffusion coefficients of Fc(CH2OH)2, Dg / D0, is provided in Figure 5.

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Figure 5. Normalized diffusion coefficient of Fc(CH2OH)2 as function of temperature for pNIPA and p(NIPA-co-5%AS) gel layers deposited on microelectrode.

Figure 5 contains two plots that illustrate the transport behavior of the redox species in pNIPA and p(NIPA-co-5%AS) gel layers deposited on Pt microelectrodes. It is easily noticed that at temperature where the volume phase transition of pNIPA gel is initiated a sudden drop in species diffusivity is observed, then for T > 34 oC a plateau is formed. Additionally, the incorporation of co-monomer AS into the pNIPA gel structure made the Dg / D0 drop smaller compared to the effect observed for the pNIPA gel layer.  r + rp  , The changes in the ratio of the effective radii of openings and the diffusing species  o r +r  s p  

presented in Figure 6, follow the line of the temperature dependence of Dg / D0. At the point where the volume phase transition is thermally induced the opening radii are smaller than the solute hydrodynamic radius.

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Figure 6. Ratio of effective radii of openings and solute as function of temperature for pNIPA and p(NIPA-co-5%AS) gel layers deposited on microelectrode.

From the physical point of view, this would prevent the probe molecules from diffusing inside the gel matrix. On the other hand, non-zero steady-state currents were observed for the redox species in the entire temperature range. This implies that the obstruction model generates invalid predictions when the gel is in the collapsed state. At a temperature range where pNIPA gels are in the swollen state, the model produces reasonable results. Therefore, it is possible to estimate the effective radius of the openings in the gel matrix for a selected temperature (e.g. 20 oC) below the point of the volume phase transition. This estimated value can be confirmed by comparing it with the magnitudes of hydrodynamic radii of all Fc-PEGn tested and their voltammetric responses at the gel modified electrodes. The hydrodynamic radii were determined from the diffusion coefficients of the probe molecules in the aqueous solution at 20 oC using the Stokes Einstein relation. The diffusion coefficients equaled 4.3·1010

, 4.0·10-10, 2.3·10-10 and 4.3·10-10 m2·s-1 and the obtained results (radii) are as follows: 0.47,

0.49, 0.68 and 1.30 nm for Fc-PEG4, Fc-PEG9, Fc-PEG75, and Fc-PEG135, respectively. The same procedure applied to ferrocenedimethanol probe yielded the hydrodynamic radius of 0.25 nm. The average radius of the polymeric chain was estimated to be around 0.45 nm. 19



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This gives the following average values of the effective radii of openings at 20 oC: 0.84 and 0.94 nm for pNIPA and p(NIPA-co-5%AS) gel microlayers, respectively. By confronting these numbers with the estimated values of hydrodynamic radii of Fc-PEGs examined in this work, one may conclude that the gel matrices (both pNIPA and pNIPA-co-5%AS) should be impermeable for the largest probe – Fc-PEG135. The diffusional transport of this probe will be drastically suppressed. This conclusion is consistent with the voltammetric data presented in the previous sections.

5. Conclusions For the examination of hydrogel structure, layers of pNIPA and copolymer p(NIPA-co5%AS) gels were successfully deposited on microelectrode- and regular electrode surfaces using electrochemical techniques. The obtained layers were stable and perfectly covered the entire surface of the electrodes. The structural characteristics of the polymer gels strongly depended on type of the monomer / monomers, concentration of the cross-linker, total concentration of monomers, and swelling degree of the gel matrix. These characteristics could be given by employing the obstruction model of diffusion of the electroactive probe species. The obstruction model combined with the transport characteristics (diffusivities) of ferrocenedimethanol provided the estimates of the pore / channel radii for the deposited gel layers. They were found to be 0.84 and 0.94 nm for pNIPA and copolymer p(NIPA-co5%AS) layers, respectively. These numbers are in agreement with the conclusions drawn from the voltammetric data obtained for the ferrocene derivatives modified with polyethylene glycol units of various length with the use of the gel modified micro- and regularly-sized working electrodes. The well-defined shapes of the voltammograms were obtained for: FcPEG4, Fc-PEG9, and Fc-PEG75. The voltammograms recorded for Fc-PEG135 were at the baseline level – the oxidation currents were close to zero. On the other hand in contrast to the

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Fc-PEG135 probe species, the Fc-PEG4, Fc-PEG9, and Fc-PEG75 probe systems could freely diffuse in the swollen gel network. Apparently the hydrodynamic radius of Fc-PEG135 exceeded the size of the gel channel size. That prevented the probe molecules from penetrating the polymer network. The obtained quantitative information on the structural parameters of the gel layers are of great importance for the development of a method for the voltammetric determination of the thickness of non-conductive porous layers.

Acknowledgement This study was supported by Grant no. 2014/15/ST5/01980 from the National Science Centre of Poland.

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TOC GRAPHIC

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Figure 1. (A) Consecutive voltammograms obtained during modification of microelectrode in deoxygenated solution containing NIPA, AS, BIS and initiator (solid lines) and monomers without initiator (dashed line). Inset: corresponding voltammograms obtained during modification of regular platinum electrode in the same way. (B) SEM image of scratched pNIPA layer deposited on platinum regular electrode. 159x232mm (300 x 300 DPI)


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Figure 2. Changes in voltammetric peak currents of electrooxidation of Fc(CH2OH)2 upon temperature for micro- (A) and regular electrode (C) modified with pNIPA and p(NIPA-co-5%AS). Selected voltammograms for gel layers in swollen- (solid line) and shrunken state (dashed line) for micro- (B) and regular electrode (D). Solution contained 2 mM Fc(CH2OH)2 and 0.2 M NaNO3. Scan rate: 10 mV/s for microlectrode and 50 mV/s for regular electrode. 299x232mm (300 x 300 DPI)



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Figure 3. Changes in the magnitude of the voltammetric peak height and steady-state wave height for electrooxidation of Fc(CH2OH)2 upon switching temperature. pNIPA layer on microelectrode (black circles) and regular electrode (gray circles). 163x157mm (300 x 300 DPI)


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Figure 4. Changes in oxidation peak currents per unit concentration of redox probe (Fc-PEGn of various length) upon temperature. Regular electrode modified with pNIPA (A) and p(NIPA-co-5%AS) (B). Voltammograms for PEGylated ferrocenes for pNIPA layers at 20 oC (C). Inset in (C): scheme of PEGylated ferrocene, where n is number of mers / units (n = 4, 9, 75 and 135). Voltammograms for Fc-PEG135 for bare- and pNIPA modified regular electrodes at 20 oC (D). Scan rate - 50 mV/s. 302x231mm (300 x 300 DPI)



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Figure 5. Normalized diffusion coefficient of Fc(CH2OH)2 as function of temperature for pNIPA and p(NIPAco-5%AS) gel layers deposited on microelectrode. 155x121mm (300 x 300 DPI)


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Figure 6. Ratio of effective radii of openings and solute as function of temperature for pNIPA and p(NIPA-co5%AS) gel layers deposited on microelectrode. 155x121mm (300 x 300 DPI)


Electrochemical Examination of the Structure of Thin Hydrogel Layers

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