An environmentally sensitive three-component hybrid microgel

Abstract

A new multifunctional microcomposite has been synthesized. It is sensitive to several medium parameters, such as temperature, pH and ionic strength, is electroactive and conductive and can be easily attached to gold surfaces. All these properties were cumulated in one microcomposite by combining three components: poly(N-isopropylacrylamide) crosslinked with N,N′-bisacryloylcystine (p(NIPA–BISS) microgel), polyaniline nanofibers and gold nanoparticles. The presence of the polyaniline nanofibers and gold nanoparticles in the composite particles led to a substantial increase in conductivity and electroactivity. Additionally, the transition from the swollen to the collapsed state further enhanced the electroactivity and conductivity of the microcomposite. The structure, morphology and swelling behavior of the microgels were investigated using SEM, TEM, TGA, DLS and Raman spectroscopy. The electrocatalytic properties of the microcomposite towards oxidation of ethanol were also found.

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1. Introduction

Additional properties of composite nanomaterials can be obtained by combining mechanically and structurally useful materials e.g. conducting materials. Furthermore, conducting polymers (CPs), e.g. polyaniline, appear to be a good environment for metal nanoparticles. Nanocomposites of PANI with noble metals, metal oxides and carbon nanostructures were found to have superior physical properties that made them chosen materials for various applications.^1,2 A number of review articles are available on the preparation of nanocomposites of PANI with nonmetals, metals and metalloids and along with exploration of their applications in catalysis, sensors and energy storage.³ Gold nanoparticles conjugated with polymers are well known as electrocatalysts for the electro-oxidation of small organic molecules and therefore they find potential applications in fuel cells. There are many reports on the use of these polymers with noble metals such as Rh, Pd, Pt and Au as electrocatalysts in many different applications.^4,5 Turning to mechanically useful and environmentally sensitive gels: “smart” microgels sensitive to external stimuli such as temperature, pH, ionic strength, presence of specific ions, solvent composition and electromagnetic radiation can react quickly to environment changes.^6–8 However, plain smart hydrogels are generally electrically nonconductive, while conductivity is required in such applications as supercapacitors, fuel cells and sensors.^9–11 The higher conductivity can be added to plain responsive microgels by incorporation of a second ingredient, such as conductive polyaniline (PANI) or polypyrrole (PPy), and metal and graphite nanoparticles.^12–14

PANI is one of the most common and interesting conducting polymers due to its good environmental stability, ease of synthesis and controllable electrical conductivity through protonation/deprotonation.¹⁵ Polymerization of aniline is usually carried out through oxidation of the monomer, where the oxidizing agents is often ammonium persulfate.¹⁶ The diameter of polyaniline nanofibers depends on the oxidation ability of the oxidant; the higher oxidation potential the larger diameter. The diameter of PANI nanofibers can be also controlled by setting appropriately the concentration of aniline.^17,18 The nanostructuring of PANI modified its properties towards various applications. The combination of gels with conducting polymers should result in adding of the properties of the components in the composites. A few approaches to the preparation of gel/conducting polymer composites have already been presented.^19,20 There are already some reports on the preparation of gel – conducting polymer micro-composites. Thermosensitive and electrically conductive composite microgels polypyrrole/poly(N-isopropylacrylamide-co-acrylic acid) were prepared by Lin et al.²¹ Polypyrrole was also incorporated into poly(N-vinylcaprolactam-co-acetoacetoxyethylmethacrylate) microgels.²² Lopez-Cabarcos et al. reported on poly(N-isopropylacrylamide) microgels interpenetrated with polypyrrole.²³ That microcomposite had a core (pNIPA) and a shell (polypyrrole) and did not exhibit the volume phase transition. Recently, we have reported on a new microcomposite gel based on cross-linked pNIPA microgel and PANI polymer. It was synthesized by using the two-phase synthesis. The obtained microcomposites underwent a rapid volume phase transition and was electroactive.²⁴

An increase in conductivity of hydrogels can be also obtained by adding metal nanoparticles to the polymer net of hydrogels.²⁵ Additionally, noble metal nanoparticles such as Pt, Au and Ag are known to exhibit catalytic properties. The pNIPA-based microgels with both thermo- and pH-responsiveness have been widely investigated.^7,26 However, the corresponding studies with respect to the hybrid microgels composed of pNIPA-based microgel and AuNP's are rarely reported. Modification of microgels with metal nanoparticles could be obtained by the microgel network formation around metal NP via the seeded precipitation polymerization which usually gives leads to a metal NP core surrounded by a polymer shell. To ensure uniform shell growth, the surface of AuNP's is usually modified by materials such as polyelectrolyte containing vinyl groups, silica and polystyrene.^27–29 Pester and co-workers introduced AuNP's into the pNIPA based microgels by heating and cooling of the microgel in an aqueous phase containing AuNP's. This approach was shown to be successful in getting further particle functionalities: antibacterial, fluorescence and catalysis.³⁰ Karg et al. demonstrated assembling of gold nanorods onto the surface of poly(NIPA-co-allylacetic acid) microgels using electrostatic interaction.³¹ Kim et al. reported on the encapsulation of AuNP's within a poly(NIPA-co-acrylic acid) microgels.³² AuNP's can also be loaded into the pNIPA microgels through in situ reduction of gold precursor by a reductant (e.g. borohydride), reduction by the groups attached to the microgel nets and fluorescent light irradiation.^33–35 On the other hand, it is well-known that gold tends to form a stable coordinative bond with a thiol group.^36–38 Shi et al. demonstrated the attachment of AuNP's to the thiol-functionalized pNIPA microgels.³⁹ Recently we have obtained new p(NIPA–BISS) microgels based on N-isopropylacrylamide cross-linked with bisacryloyl derivative of cystine using the aqueous precipitation polymerization.⁴⁰ This derivative of cystine made the microgels degradable and allowed simultaneous introduction of carboxylic groups into the polymeric network. The presence of carboxylic groups made the gels sensitive to pH, improved their stability vs. ionic strength and allowed further chemical modification of the chains.

The aim of this paper was to fabricate a new multiple stimuli-responsive hybrid microgel consisting of poly(N-isopropylacrylamide) cross-linked with a derivative of cysteine, polyaniline and Au nanoparticles. For this purpose the two-phase synthesis was used.²⁴ The complex structure and composition of the microcomposites should result in their useful properties.

2. Experimental section

2.1. Materials

N-Isopropylacrylamide (NIPA, 97%), potassium persulfate (KPS, 99.99%) and acryloyl chloride (96%) were purchased from Aldrich. Aniline, nitrobenzene (NB), tetrachloroauric acid, sodium hydroxide (NaOH, 99%), L-cystine (98.5%) and HClO₄ were purchased from POCh (Poland).

All chemicals were used as provided by manufacturer except for NIPA, which was recrystallized twice from a benzene/hexane mixture (9 : 1). All solutions were prepared using high purity water obtained from a Hydrolab/HLP purification system (water conductivity: 0.056 μS cm⁻¹). The disulfide cross-linker N,N′-bisacryloylcystine (BISS) was synthesized according to the method described previously.^40,41

2.2. Synthesis of poly(N-isopropylacrylamide) cross-linked with N,N′-bisacryloylcystine (p(NIPA–BISS))

The synthesis of the p(NIPA–BISS) microgels has been published previously.⁴⁰ They were obtained using the surfactant-free emulsion polymerization.⁴² Briefly, NIPA and BISS were dissolved in 195 mL of deionized water in a three-neck flask equipped with a magnetic stirrer (set at 1400 rpm during the entire polymerization), reflux condenser and inlet and outlet of inert gas. The total concentration of NIPA and amino-acid derivative was kept constant at 100 mM. All microgels were synthesized with the percentage of the amino acid derivative equaled 3% in the pre-gel solution.

The monomer solution was purged with argon and heated up to 70 °C in an oil bath. Then KPS (0.1 g dissolved in 5 mL of deionized and degassed water) was added to initiate the polymerization. The reaction continued for 7 h under an argon blanket. After that the solution was cooled down to room temperature. Next the microgels were purified by placing them in a dialysis tube with a 10 000 Da molecular weight cutoff (Spectra/Por® 7 Dialysis Membrane). The microgels were dialyzed against 5 L of water for two weeks at room temperature; water was changed daily. Finally the microgel solution was filtered via a syringe inline glass filter with pore size of 1–2 μm.

2.3. Preparation of poly(N-isopropylacrylamide-N,N′-bisacryloylcystine)/polyaniline–Au (p(NIPA–BISS)/PANI–Au)

The two phase synthesis of the p(NIPA–BISS)/PANI–Au microcomposites was similar to that published previously.²⁴ Purified p(NIPA–BISS) microgel solution was centrifuged (17 000 rpm, 15 min, 5 °C, an Hermle Centrifuge model Z 32 HK). Then 0.3 g of the microgel was immersed in a solution of HAuCl₄ and was twice heated (30 °C) and cooled (5 °C) alternately. The final concentration of the aniline oxidant (HAuCl₄) was 0.1 M. Finally the microgel particles were isolated by centrifuging at 17 000 rpm for 15. The obtained microgel samples were treated with 0.3 mL of 0.5 M aniline solution in nitrobenzene. After 24 h of oxidation-polymerization of aniline the obtained material was purified by repeating centrifugation and redispersion with ethanol and next with water. The microcomposite was dialyzed in a dialysis tube with a 10 000 Da molecular weight cutoff (Spectra/Por® 7 Dialysis Membrane) against 5 L of water for one week at room temperature. Water was changed daily. At the end, a homogeneous dark-green colloidal solution was obtained. A scheme of preparation of p(NIPA–BISS)/PANI–Au microcomposites is given in Fig. 1.

Fig. 1 Scheme for the preparation of p(NIPA–BISS)/PANI–Au microcomposites.

2.4. Instrumental

Dynamic light scattering. Hydrodynamic diameters of the microgel and microcomposite particles were determined using a Malvern Zetasizer instrument (Nano ZS, UK) fitted with a 4 mW He–Ne laser (λ = 632.8 nm) as the light source. Scattering angle was fixed at 173°. From the decay of the autocorrelation function provided by this instrument the corresponding diffusion coefficient was calculated. Then the hydrodynamic diameter (D_h) of the microgel particles was calculated using the Stokes–Einstein eqn (1).⁴³where D_h – hydrodynamic diameter, k – Boltzmann constant, T – temperature, η – solvent viscosity and D – diffusion coefficient.

The solutions of microgel were passed through a 1–2 μm glass-fiber membrane just before measurements. The solutions were equilibrated at selected temperatures for 5 min before measurement. The conductivity was measured using the same Malvern Zetasizer instrument. A folded capillary cell with gold electrodes was employed for this purpose.

Transmission electron microscopy (TEM). The samples for TEM were prepared by placing a drop of aqueous microgel- or microcomposite solution on a formvar-coated copper grid and allowing them to dry in air. All samples were examined using a Libra 120 microscope (Zeiss).

Scanning electron microscopy (SEM). The SEM images were taken with a Merlin (Zeiss) microscope at 3 kV. The samples were first dried completely in a hot-air oven at 50 °C and next covered with a thin layer of sputtered Au–Pd alloy to a width of approximately 3 nm using a Polaron SC7620 Mini Sputter Coater.

Raman spectroscopy. For the examination of the conducting polymer in p(NIPA–BISS)/PANI–Au microcomposites a Raman spectroscope (HR LABRAM 800 from Horiba Jobin Yvon with 632.8 nm He–Ne excitation line) was used. Polymer samples were prepared by placing a drop of an aqueous solution of microcomposite on a glass microscope slide and allowing it to dry.

Thermogravimetric analysis (TGA). Thermogravimetric curves were recorded with a TA instrument Q50 at a heating rate of 1 °C min⁻¹ under a nitrogen atmosphere.

Electrochemical measurements. Electrochemical measurements were performed using a CH Instrument, model 700D potentiostat, controlled via producer's software. To study the electrochemical properties of the p(NIPA–BISS)/PANI–Au microcomposite (signals from PANI) the three electrode system consisting of a gold electrode (ϕ = 3.0 mm) used as the working electrode, an Ag/AgCl electrode (the reference electrode) and a platinum wire (the auxiliary electrode) were employed. Before use the working disk electrode was polished with 0.3 μm Al₂O₃ powder on a wet pad. After polishing, to remove alumina oxide completely from the surface, the electrode was rinsed with a direct stream of ultrapure water. Then one drop (21 μL) of the microcomposite solution (concentration ca. 19 mg mL⁻¹) was placed on the gold electrode and left to dry. The p(NIPA–BISS)/PANI–Au microcomposite was bound/adhered to the gold electrode by chemisorption between Au and –S–S– groups. The electrodes were kept in a water-jacketed glass cell. Temperature of the cell was changed every 5 °C within the range 25–45 °C with an accuracy of 0.2 °C. 0.1 M HClO₄ solution and a scan rate of 100 mV s⁻¹ were used in voltammetric experiments. Each scan was repeated at least 3 times. The reproducibility was satisfactory and no memory effect was seen. The performance of p(NIPA–BISS)/PANI–Au microcomposites in electrooxidation of ethanol (0.5 M) in alkaline medium of 0.1 M NaOH at 25 °C was examined.

3. Results and discussion

3.1. Morphology and composition

The morphology of the obtained p(NIPA–BISS)/PANI–Au microcomposites was examined by electron microscopies. Fig. 2A and B show typical SEM images of a p(NIPA–BISS) microgel and a p(NIPA–BISS)/PANI–Au microcomposite, respectively. The plain microgel forms spherical particles with homogenous and smooth surface. The p(NIPA–BISS)/PANI–Au microcomposite also forms spherical particles but these particles possess an uneven and somewhat porous surface. In some cases the surface turned to be cauliflower like. An increase in volume of the spheres after modification of p(NIPA–BISS) with PANI confirms the presence of PANI in the microcomposite. The mean value of diameter of microcomposite particles was determined to be 690 nm and was higher than that obtained for the unmodified p(NIPA–BISS) microgel which equaled ca. 400 nm. The sizes of the microcomposite particles, calculated from the TEM and SEM micrographs, were similar. Polyaniline nanofibers apparently penetrated the microgel spheres; it is seen in the SEM images of damaged/broken microcomposite structure in Fig. 2C. An analysis of inset in Fig. 2B leads to the conclusion that AuNP's are quite evenly distributed in p(NIPA–BISS)/PANI–Au microcomposites.

Fig. 2 SEM images of p(NIPA–BISS) microgels (A) and p(NIPA–BISS)/PANI–Au microcomposites (B) and (C). Inset: TEM image of p(NIPA–BISS)/PANI–Au microcomposites.

The composition of the microcomposites was determined by doing thermogravimetric analysis (TGA). Three main steps of weight loss can be observed on thermogravimetric curves of the microcomposite (see Fig. 3). The initial slight weight loss occurring below 100 °C is ascribed to the loss of physically adsorbed water. The next weight loss seen at a temperature range between 100 and 450 °C can be ascribed to the thermal decomposition of p(NIPA–BISS) component.⁴⁴ The final step of the weight loss (above 500 °C) can be assigned to PANI decomposition.⁴⁵ According to the TGA experiments the composition of dry microcomposite can be given as: 20% of p(NIPA–BISS), 39% of PANI and 41% of Au (mass resistant to thermodecomposition).

Fig. 3 TGA analysis curve of p(NIPA–BISS)/PANI–Au microcomposite.

p(NIPA–BISS)/PANI–Au microcomposites were analyzed using Raman spectroscopy. The He–Ne laser line with a wavelength of 632.8 nm was employed for the excitation. Fig. 4 shows selected Raman spectra of p(NIPA–BISS)/PANI–Au microcomposites. They contain several characteristic peaks. The peak at 1164 cm⁻¹ represents the C–H bending vibration in the aromatic ring, whilst the band at 805 cm⁻¹ is due to C–H quinonoid deformation. The presence of phenazine, which is a byproduct of the polymerization, is indicated by the 607 cm⁻¹ band.⁴⁶ The band at 1219 cm⁻¹ (quinoid rings) is due to C–N stretching mode of single bond of the polaronic unit. The band at 1493 cm⁻¹ is assigned to C=N stretching mode of quinoid ring. The band observed at 1493 cm⁻¹ is due to C–C stretching of the benzene ring. The band at 1591 cm⁻¹ is due to the Raman-allowed phonon high frequency. The chemical bond of C–N˙⁺ is assigned to the peak located at 1337 cm⁻¹.⁴⁷ p(NIPA–BISS)/PANI–Au microcomposites showed C–N˙⁺ band due to existence of several protonation and oxidation levels.⁴⁸ The presence of all mentioned above bands confirms the formation of p(NIPA–BISS)/PANI–Au microcomposites.

Fig. 4 Raman spectra for p(NIPA–BISS)/PANI–Au microcomposite. Excitation line: 632.8 nm He–Ne.

Basing on the Raman spectra we can also estimate the content of emeraldine in the p(NIPA–BISS)/PANI–Au microcomposites. The spectra indicate the presence of both conductive polaronic (emeraldine) and fully oxidized quinoid (pernigraniline) forms of PANI. Polyaniline is conductive only in the moderately oxidized and protonated state – emeraldine. The higher intensity of the band at 1337 cm⁻¹ (the emeraldine cation radical) than that at 1493 cm⁻¹ (C=N bond in the quinoid ring of pernigraniline) indicates a substantial content of conducting emeraldine in the p(NIPA–BISS)/PANI–Au microcomposites.^49,50

3.2. Swelling behaviour

The shrinking process of the microgels and microcomposites is triggered by an increase in temperature. It is shown in Fig. 5 by plotting the temperature dependencies of the hydrodynamic diameters for the microgels and microcomposites. By analyzing these plots it can be concluded that the modification of p(NIPA–BISS) microgel with PANI–Au composite leads to: (a) an increase in microcomposite size at a temperature lower than the volume phase transition temperature, e.g. at 25 °C; at this temperature D_h equaled 875 and 1279 nm for p(NIPA–BISS) and p(NIPA–BISS)/PANI–Au, respectively, and (b) an increase in microcomposite size at a temperature higher than the volume phase transition temperature, e.g. at 40 °C; then D_h equaled 437 and 730 nm for p(NIPA–BISS) and p(NIPA–BISS)/PANI–Au, respectively. Importantly, the volume phase transition temperatures for p(NIPA–BISS) and p(NIPA–BISS)/PANI–Au were close. Both microgels and microcomposites underwent a volume phase transition from the swollen to the shrunken state at circa 34 °C. So, the obtained microcomposite materials preserved the environmental-sensitivity characteristics of the p(NIPA–BISS) matrix. Additionally, just negligible difference between the swelling behaviors of both materials suggests that the interaction of the PANI/Au system with p(NIPA–BISS) is very weak and that PANI–Au is just physically entrapped in the polymeric network of the initial microgel. The advantage of this situation is that PANI remains an independent, non-modified redox marker and AuNP's could introduce catalytic properties.

Fig. 5 Change in hydrodynamic diameter of p(NIPA–BISS) microgel and p(NIPA–BISS)/PANI–Au microcomposite in aqueous solutions as a function of temperature.

Influence of pH on the swelling behavior of the microcomposites was also investigated. The temperature dependencies of the swelling ratio were examined for selected two values of pH and are presented in Fig. 6. At alkaline pH the microgel particles, compared do an acidic pH, had bigger diameter both before and after phase transition: 1373 vs. 1079 nm and 639 vs. 497 nm at 25 °C and 45 °C, respectively. Under alkaline conditions (pH = 11.5) most of the carboxylic groups present at the cross-linker molecules were ionized and the microgels were swollen due appearance of electrostatic repulsions and an increase in the osmotic pressure between the solution and the gel microcomposites. At appropriately low pH (2.6) the diameter of microgels decreased due to the protonation of a part of the carboxylic groups.

Fig. 6 Hydrodynamic diameter as function of temperature and pH for p(NIPA–BISS)/PANI–Au microcomposite.

3.3. Electrochemical examination

The next step in the study was examination of the electrochemical properties of the p(NIPA–BISS)/PANI–Au microcomposite deposited on the Au electrode surface. Two ways of deposition were employed. In the first one a gold electrode was immersed into a microcomposite solution. In the second one a 21 μL drop of the same microcomposite solution was placed on the electrode surface and the electrode was left to dry. Interestingly, in the first approach a monolayer of the microcomposite was obtained, while the dispersion of a drop followed by drying (agglomeration of the particles), even after rinsing the modified surface with water, led to uneven, multilayer coverages, see Fig. 7.

Fig. 7 Comparison of gold electrode surfaces with layers of microcomposite prepared by: (A) immersing of electrode in a solution of microcomposite and (B) placing a drop of microcomposite solution on surface followed by subsequent drying.

Regarding the strong adhesion of p(NIPA–BISS) to the gold surface we have examined the distribution of the cross-linker in microgel p(NIPA–BISS) by using uranyl acetate. It appeared that the carboxylic groups (and therefore the crosslinker) were uniformly distributed in the microgels and were present at the surface. Further, we have also investigated the role of the crosslinker in the adhesion process. For this purpose the microgels crosslinked with BISS (N,N′-bisacryloyl derivative of cystine) and the microgels crosslinked with N,N′-methylenebisacrylamide were compared. Only in the first case the microgels strongly adhered to the gold surface. This finding was a proof of that the presence of BISS (–S–S–) played a crucial role in the attachment of the microcomposite particles to the gold electrode surface.

Typical voltammograms obtained for the p(NIPA–BISS)/PANI–Au microcomposites deposited on the gold surface (the second method) are presented in Fig. 8. They exhibit the signals that are characteristic for PANI.⁵¹ Two pairs of peaks are seen: one at ca. 0.2 V and another at 0.5 V. The first pair of peaks corresponds to the reversible electro-conversion of leucoemeraldine to emeraldine and the second pair to the emeraldine/pernigraniline couple.⁵² The magnitude of the voltammetric signals strongly depended on the swelling state of the gel. The voltammograms presented in Fig. 8A are obtained for microcomposites at various temperatures (various states in the shrinking process) in an aqueous medium containing 0.1 M HClO₄. In the most swollen state (25 °C) the polyaniline red-ox signals are small. They increase with a progress in the shrinking process and, finally, they become the biggest in the shrunken state. The change in peak current vs. temperature is shown in the inset of Fig. 8A. The major increase in current is seen in the temperature range between 30 and 35 °C where the phase transition takes place. Importantly, the return to the initial conditions preceding the phase transition fully restores the voltammetric response (see Fig. 8B and the inset). The better electrochemical response of PANI in the shrinking state is probably caused by an increase of the number of PANI fibers per unit volume of the gel and by improved conditions for the electron hopping. It should be stressed here, that PANI without AuNP's gives practically no oxidation current at the most swollen state of the pNIPA microgel.²⁴

Fig. 8 (A) Cyclic voltammograms obtained with modified gold electrode with p(NIPA–BISS)/PANI–Au microcomposites at various temperatures. Inset: dependence of peak current (peak at 0.2 V) on temperature. (B) Cyclic voltammograms obtained with modified gold electrode at selected two temperatures: 25 °C (swollen state, solid grey curve), 45 °C (shrunken state, solid black curve) and again at swollen state (25 °C, dotted curve). Inset: regularly oscillating anodic peak current (peak at 0.2 V) presented as sequence of its values for swollen state (step 1), shrunken state (step 2) and again swollen state of p(NIPA–BISS)/PANI–Au microcomposite (step 3). Concentration of p(NIPA–BISS)/PANI–Au microcomposite: 19 mg mL⁻¹; scan rate: 100 mV s⁻¹; supporting electrolyte: 0.1 M HClO₄.

To get reliable values of conductivity of the microcomposites they were centrifuged before the measurements. In this way the unwanted excess of water was removed. The measured conductivity of microcomposites was 2.0 mS cm⁻¹ at 25 °C and increased with temperature to 2.5 mS cm⁻¹ at 40 °C. It was substantially bigger than for the unmodified p(NIPA–BISS) microgel the conductivity of which was 0.1 mS cm⁻¹. To compare the contribution of PANI nanofibers and Au nanoparticles to the conductivity of the microgels we can use the previous experimental data.²⁴ They indicated that the increase in conductivity of the gel after addition of polyaniline only was circa 0.5 mS cm⁻¹. In our actual case the increase is at a level of circa 2 mS cm⁻¹. This may be treated as an indication of the contribution of Au nanoparticles to the final conductivity of the microgels.

Next, the ability of p(NIPA–BISS)/PANI–Au composite to catalyze a selected electrochemical process was examined voltammetrically. We selected the ethanol electrooxidation in alkaline medium as an important fuel-cell system. The superior activity of gold at high pH was shown by e.g. Rodriguez et al.⁵³ It was shown that in alkaline medium even carbon monoxide played an important supportive role, which resulted in high catalytic performance. In addition, the hydroxyl ions played a crucial role as the oxygen source during the electrooxidation. In Fig. 9A a comparison of voltammetric curves obtained at GC disc electrode in solutions of p(NIPA–BISS) microgel and p(NIPA–BISS)/PANI–Au microcomposites is done. As it can be seen the signals of the electrocatalytic oxidation of ethanol were only seen in the case of the GC electrode immersed in a solution containing p(NIPA–BISS)/PANI–Au material. Their shape and placement were typical for those observed for PANI–Au composite.⁵⁴ Fig. 9B presents a comparison of voltammetric curves obtained at a bare gold electrode and an Au electrode modified with p(NIPA–BISS)/PANI–Au microcomposites. In that case we observed signals of electrooxidation of ethanol for both modified and unmodified electrodes. However, for the modified electrode a substantial increase in current vs. bare Au electrode was obtained.

Fig. 9 (A) Cyclic voltammograms obtained with glassy carbon electrode (GC) in 0.1 M NaOH and 0.5 M ethanol containing either p(NIPA–BISS) microgel (solid line) or p(NIPA–BISS)/PANI–Au microcomposites (dashed line). (B) Cyclic voltammograms obtained in 0.1 M NaOH and 0.5 M ethanol using bare gold electrode (solid line) and gold electrode modified/loaded with p(NIPA–BISS)/PANI–Au microcomposites (dashed line). Concentration of p(NIPA–BISS)/PANI–Au microcomposite: 19 mg mL⁻¹.

4. Conclusions

The analysis of the synthesized products revealed that a new conducting microcomposite consisting of the environmentally sensitive poly(N-isopropylacrylamide) microgel cross-linked with N,N′-bisacryloylcystine and modified with polyaniline and Au nanoparticles was obtained. In the synthesis, the two-phase polymerization was successfully employed. Polyaniline nanofibers and AuNP's were formed in the channels of p(NIPA–BISS). Au nanoparticles, the new element in the composite, were the second product of oxidation of aniline with tetrachloroauric acid. The AuNP's stayed in the interior of p(NIPA–BISS) microgels and together with polyaniline nanofibers were evenly distributed in the entire particle volume.

Polyaniline appeared to be present in the microcomposites mainly as the moderately oxidized and protonated form – conducting emeraldine. The voltammetric response of the microcomposite strongly depended on the shrinking state of the microcomposites. The voltammetric responses of polyaniline strands were much bigger in the shrunken state of the microcomposite. Compared to the systems without Au nanoparticles²⁴ p(NIPA–BISS)/PANI–Au is much more conductive and is much more active catalytically because of the presence of AuNP's in the microgel network.

The application of BISS linker made the deposition of synthesized microparticles on gold easier. Thanks to the strong interactions between gold and sulphur atoms present in the BISS molecule the layers of p(NIPA–BISS)/PANI–Au on the gold electrode surface were strongly adhered and stayed on the surface for months. The new microcomposite deposited on the Au surface was found to be an efficient electrocatalyst for ethanol oxidation reaction in alkaline medium. In fact, it was a much stronger electrocatalyst compared to bare gold electrodes. p(NIPA–BISS)/PANI–Au present in an ethanol solution also exhibited moderate electrocatalytic properties towards oxidation of ethanol at a GC electrode.

The above properties make the new microcomposite an alternate material for a variety of applications, including switchable electrochemical systems and sensors. We believe that the multiple stimuli-responsive nature and unique nanostructure of these hybrid microgels will make them more useful in stimuli-responsive electronic devices and in electroanalysis and sensing. The facile synthesis approach reported in this paper opens up a new window for the preparation of such unique microcomposites with other noble metals and conducting polymers.

Acknowledgements

This work was supported by Grants no. UMO-2014/15/N/ST5/02937 and DEC-2013/09/B/ST5/00988 from the National Science Center of Poland, and Iuventus Grant no. IP2012 015272 from the Polish Ministry of Science and Higher Education. We thank Maciej Mazur for his assistance in TGA measurements.

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