Well-defined polyindole–Au NPs nanobrush as a platform for electrochemical oxidation of ethanol

Chemicals and methods

The polyindole (PIN) was electrochemically synthesized from acetonitrile (analytical grade, POCH) solution containing monomer and 0.1 M LiClO₄ (≥98 %, Sigma-Aldrich) as the basic electrolyte. Electropolymerization was carried out on a GC electrode, which was previously polished to a mirror with diamond paste and washed several times with Milli-Q grade water and acetone (analytical grade, POCH). Polyindole was synthesized from 0.5 M indole (≥99 %, Sigma-Aldrich) monomer solution on (i) bare GC electrode and (ii) in the polycarbonate (PC) membrane with 200 nm size of pores (Whatman) stuck to GC electrode. Polyindole was electropolymerized within 10 cyclic voltammetry cycles in the potential range from 0 to 1.3 V with a sweep rate of 10 mV/s. After polymerization, the electrode was gently soaked three times with acetonitrile and Milli-Q grade water to remove monomer and basic electrolytes from the polymer pores. Additionally, the GC/PC/PIN electrode was immersed for a few seconds in chloroform (analytical grade, POCH) to superficially etch the PC membrane and reveal the polymer brush formed in the pores. Then, the electrode was immersed in the water and dried in air.

After polymer synthesis, gold nanoparticles (AuNPs) were formed from a 5 µL droplet of 4 mg/mL gold (III) chloride HAuCl₄ (51.5 % Au, Fluka) solution pipetted on the polymer. The Au formed spontaneously onto the polymer surface, while the electrode was left for 30 min under the fume hood to let the droplet soak into the polymer membrane and dry. Next, the electrode was washed several times with water and dried for 5 min in the air.

Electrooxidation of ethanol (≥98 %, POCH) was performed in 0.1 M NaOH (analytical grade, POCH) aqueous solution.

Instrumentation

Electrochemical measurements were carried out in the three-electrode system by using an Autolab potentiostat/galvanostat equipped with GPS software (Eco-ChemieNetherlanden). A glassy carbon disc electrode was used as a working electrode, 3 M silver chloride electrode, and platinum wire were employed as a reference electrode and an auxiliary electrode respectively.

Electrochemical impedance measurements were performed with Autolab potentiostat/galvanostat by FRA software in the frequency range from 0.1 Hz to 100 kHz with 10 mV rms sinusoidal voltage amplitude.

The SEM images were taken with an FE-SEM Merlin (Zeiss, Germany) instrument controlled by the manufacturer’s software. The device was working at a low beam voltage (3 kV) and a low sample current (15–30 pA) to give the highest possible resolution and detailed structure imaging, and diminish the charging effect of examined hybrids.

The elemental analysis (EDS – energy dispersive X-ray spectrometry) was conducted with a multichannel XFlash Detector 5010 125eV device (Quantax, Bruker) coupled with FE-SEM Merlin (Zeiss) microscope using 15 kV electron beam energy.

X-ray Diffraction was employed for the structural characterization of PIN film using the Bruker D8 GADDS system with CuKα radiation. 1D XRD patterns were obtained by the integration of raw 2D data over azimuthal angle in the range 10°–50° 2-theta degree.

Electropolymerization

Polymerization of the indole was carried out with hard-template synthesis using elastic polycarbonate (PC) membranes. Initially, the aluminum oxide membranes (AAO) were proposed for that synthesis, however, electrooxidation studies are conducted in alkali media, which causes degradation of the AAO membrane. Therefore, the polymerization was performed in the PC elastic membranes that are resistant to a solution of pH above 8.

The electropolymerization was performed with cyclic voltammetry technique on the GC electrode and on the PC membrane that was stuck to the GC electrode. Figure 1a presents CV curves recorded during electropolymerization carried out onto the GC covered with PC membrane (GC/PC) for 0.5 M monomer concentration. The measurement was conducted at a scan rate of 10 mV/s. As can be seen in Fig. 1a there is an oxidation process above 0.9 V corresponding to the monomer oxidation [33]. Moreover, the redox peaks characteristic for polyindole are well visible on cyclic voltammetry plots in the potential range between 0.1 and 0.8 V (Fig. 1a, inset plot). The current densities related to these redox processes increase with the number of CV scans confirming the polymer growth on the electrode surface. Figure 1b shows the electrochemical setup in which the GC/PC electrode was placed in the Teflon tube and sealed with silicone o-rings. The electropolymerization was performed within 10 cycles, which, based on our previous studies with AAO membrane, was found as the optimal polymer deposition condition to ensure tight and uniform PIN nanobrush formation.

Fig. 1:

(a) CV curves for electropolymerization from 0.5 M monomer solution containing 0.1 M LiClO₄ at 10 mV/s. Measurements were performed vs. Ag|AgCl|KCl reference electrode, (b) electrochemical setup, and (c) scheme of membrane onto the working electrode.

Morphology studies

The morphology of the deposited polymer was investigated with scanning electron microscopy (SEM) after the dissolution of the membrane in the chloroform media, so that the structure of the polymer is revealed. The diameter of each polymer wire is determined by the diameter of the pores in the PC membrane. The concentration of monomer for polymerization was 0.5 M and the scan rate 10 mV/s, where polymerization was performed for 10 CV cycles. Experimental conditions, as mentioned above, were chosen based on the results of the optimization in our previous studies in AAO membranes [17].

SEM images show rod-like structures forming a PIN brush (Fig. 2 a). PIN rods are well-defined and uniform in size and shape on the whole surface area (Fig. 2b). However, in some places, they tend to collapse due to the drying process (Fig. 2c).

Fig. 2:

The SEM images of polyindole wires synthesized from the bath containing 0.5 M indole solution with 0.1 M LiClO₄ electrolyte in ACN with 10 scans and sweep rate of 10 mV/s. Scale is (a) 10 µm, (b) 100 nm, and (c) 1 µm scale.

In the next step, the PIN nano-brush was modified with gold nanostructures. A freshly prepared PIN nano-brush was immersed in the HAuCl₄ solution for 1 min. Polyindole was able to reduce AuCl₄⁻ ions to metallic form while it was further oxidized. Figure 3 shows typical SEM images obtained for PIN nanobrush modified with gold nanoparticles. As can be seen, each rod was evenly covered with small gold nanocrystals.

Fig. 3:

Polyindole nanobrush modified with gold nanoparticles when PIN was (a) in pores of PC membrane, (b) membrane was partially dissolved. Image (c–d) present sample in different scale revealing regular nanobrush-like structure.

Crystalline structure

After the synthesis of polyindole brush modified with Au nanoparticles and SEM studies, the composite was characterized using X-ray diffraction to confirm the structure of Au NPs. The results presented in Fig. 4 show two reflection peaks at 36.5°, characteristic for the Au (111) plane, and 44.2° characteristic for gold (200), confirming the formation of Au NPs onto the PIN nanobrush. Another reflection peak located at 19° confirms the polymerization of indole, while the following peak at 27° denotes the partial crystalline structure of PIN. The high intensity of reflection peaks reveals good crystallinity of gold NPs into PIN. The obtained data are in good agreement with the literature [11, 34, 35] and preliminary data for pure PIN investigated within our previous studies [10].

Fig. 4:

XRD analysis of PIN/Au nanobrush. Inset shows the XRD pattern characteristic to polyindole.

Electrooxidation of ethanol

In this work, the electrocatalytic activity of novel hybrid Au–polyindole composites was evaluated by studying the ethanol electrooxidation in alkaline media. Initially, the measurements were performed in 0.1 M NaOH solution containing various concentrations of ethanol at (i) bare glassy carbon (GC) electrode, (ii) GC covered with polyindole formed in and without membrane, and (iii) Au electrode to make sure that the electrocatalytic activity comes from proposed hybrid composites. After that, CV measurements were conducted for the same electrochemical conditions for Au NPs-polyindole composites.

The inset plot in Fig. 5a presents the cyclic voltammograms measured for pure GC electrode in the absence and presence of ethanol in the potential range from −0.2 to 0.3 V. It reveals that bare GC electrode is not an active material towards ethanol oxidation because there are no significant differences between CV curves recorded in solution with and without ethanol addition. In turn, results obtained for the bare gold electrode, displayed in Fig. 5c, confirmed its significant activity towards alcohol electrooxidation by increasing the current density with the ethanol concentration. The ethanol oxidation is noticeable at potentials higher than 0.1 V with a peak of current density ca. 2 mA/cm² at 0.3 V. After that, the electrocatalytic properties of polyindole were examined. The results recorded for the GC electrode covered with PIN deposit (Fig. 5a) show that the current density slightly rises with increasing ethanol concentration and there appears a minor anodic peak ca. 0.2 V suggesting only slight electrooxidation of ethanol. Following electrochemical experiments were performed for polyindole deposited in the PC membrane (GC/PC/PIN) as the nanobrush structure. CV curves presented in Fig. 5b reveal a characteristic peak at 0.2 V corresponding to the ethanol electrooxidation, but the current densities are much higher in comparison to the PIN prepared without membrane. It confirms that enlargement of the PIN surface significantly increases the current response of ethanol electrooxidation. Therefore, the GC/PC/PIN electrodes were chosen for further modification by Au nanoparticles.

Fig. 5:

Cyclic voltammograms recorded for (a) GC/PIN, where the inset was recorded for GC electrode, (b) GC/PC/PIN deposited in the membrane, and (c) bare gold electrode in 0.1 M NaOH solution without and with various ethanol concentrations. Measurements were conducted with a scan rate of 20 mV/s at room temperature. The concentration of the ethanol in the solution are: 0.5, 1, 1.5, 2, 3, 4, and 10 mM.

Moreover, comparing cyclic voltammograms presented in Fig. 5 an important conclusion should be pointed out here: although, the current density recorded GC/PC/PIN electrode was much lower than for gold electrode, the potential at which forward anodic peak occurred was more cathodic compared to gold, which means that the electrode covered by polyindole exhibits some electrocatalytic properties towards ethanol oxidation.

Next, the electrocatalytic activity of polyindole nanobrush decorated with Au NPs towards ethanol electrooxidation was explored in alkaline media. As displayed in Fig. 6 the current density increases with increasing the ethanol concentration in the base solution and reaching for 10 mM ethanol concentration ca. 4 mA/cm² at peak potential (0.2 V) which is more than 20 times higher than the current density corresponding for CG/PC/PIN electrode without modification by Au NPs and twice higher than for the bare gold electrode. It should be stressed here, that such impressive improvement of current density values is a result of the synergetic effect of active surface enhancement by utilized PIN nanobrush and its further decoration by Au NPs, which is well known as catalytic material.

Fig. 6:

Cyclic voltammograms were recorded for GC/PC/PIN/Au, while the Au(III) solution was placed on the PIN inside pores. Measurements were performed at 0.1 M NaOH with different amount of ethanol addition with 20 mV/s at (a) 22 °C, (b) 30 °C, (c) 40 °C, and (d) 50 °C.

During the cathodic (reverse) scan of CV, the anodic current peak occurs. This phenomenon is elucidated in the literature as the non-equilibrium behavior of Au atoms in the lattice after the reduction of blocking oxides so the sites exhibiting high energy can act as electrocatalytic centers [36]. This reverse anodic peak is seen especially for the higher concentration of ethanol, however, in this work, the research was focused on the lower concentration of EtOH, below 10 mM, showing the ability of the examined system to detect this alcohol in aqueous media. The hybrid proposed in this paper reveals selectivity towards EtOH. Within the preliminary data the CV curved for electrooxidation of methanol were recorded, and the peaks for MetOH were shifted cathodically at about 0.15 V and the peaks were much broader than for EtOH. Moreover, the absence of the hysteresis loop in the high potential region indicates that the oxidation process of adsorbed species is negligible [37], [38]. Among the studied systems, including bare GC, gold nanoparticles, GC/PIN in the membrane and without, GC/PIN/AuNps, the last modified electrode showed the highest current densities corresponding to ethanol oxidation and showed lower (more cathodic) onset potential, which means that such hybrid composite exhibits better electrocatalytic activity than its compounds alone.

To get insight more into the kinetics of the reaction ethanol electrooxidation, cyclic voltammograms were recorded for GC/PIN/Au composites towards EtOH oxidation at various temperatures and results are presented in Fig. 6. It is visibly shown that the current densities increased with temperature rise and the potential onset was more negative at higher temperatures. It suggests that the EtOH process requires lower energy activation at a higher temperature because of the higher catalytic activity of composites.

According to the literature the electrooxidation of ethanol has been widely studied in alkali media at various electrodes such as Au [39], Ni–Ru [40], Pt [41], Pd, and composites like Pd–Ni [42], Pd₈₅Ni₁₀Bi₅ nanoparticles on Vulcan electrode [43]. Compared our results with literature data concerning the ethanol electrooxidation, one can notice that current densities recorded for noble metal electrodes are much higher e.g. 6–15 mA/cm² [41], [44], however, electrooxidation is recorded for much higher EtOH concentrations than we examined in our work.

In comparison to platinum, gold exhibits lower electrocatalytic activity towards ethanol electrooxidation, especially in acidic media. However, in alkaline solutions, that reaction can occur at the Au electrode at potentials even about 0.0 V or at higher potential values [45], [46], [47]. Tremiliosi-Filho et al. [39] widely studied the electrooxidation of ethanol that takes place at the gold electrode. They reported an oxidation wave related to ethanol oxidation at gold between 1 and 1.3 V vs. RHE. The Authors found that the main product of ethanol electrooxidation in alkaline media was acetate. It can be assumed that ethanol reacts with gold via hydroxyl species occurred at the Au surface by the following steps (Scheme 3) [36]:

Scheme 3:

Possible steps for ethanol oxidation in alkaline media.

Our proposed hybrid composites achieved lower current density related to ethanol electrooxidation compared to literature data concerning bare gold electrode [47], however, the onset potential was more cathodic and ethanol electrooxidation occurred at lower (more cathodic) potentials which proved the electrocatalytic activity of our material.

The stability and durability of Au–polyindole composites were examined by the chronoamperometry technique. First, the trend of current density changing towards EtOH concentration (addition of EtOH to NaOH solution) was studied. As shown in Fig. 7a, the current density rises when that alcohol is added. The potential applied to the electrode was about 0.21 V and was chosen based on the CV curve presented in Fig. 6a, where the anodic peak corresponding to the oxidation of EtOH takes place.

Fig. 7:

i–t Curves recorded for GC/PC/PIN/Au electrode in 0.1 M NaOH after addition of (a) 10 µL (0.5 mM) doses of EtOH and (b) 100 µL (10 mM) of EtOH. Measurements were performed at 22 °C.

The following experiment was performed with just one addition of EtOH into the alkali media, while the i–t curve was measured for 1 h. As can be seen in Fig. 7b the plateau after 1 h polarization proves that the electrode remained active and was not poisoned by the products of EtOH oxidation.

All of presented electrochemical research shows that the Au–polyindole nanobrush composites can be regarded as promising electrocatalytic material as a platform for ethanol detection. The electrochemical impedance measurements were utilities to examine the charge transfer processes during ethanol oxidation at polyindole nanobrush and hybrid materials. The Nyquist plots in Fig. 8 display a branch of semicircles recorded for various electrodes in 0.1 M NaOH containing 10 mM of ethanol at an applied potential of 0.2 V that is related to the forward anodic peak potential (Fig. 6). The diameter of the semicircle denotes that the charge transfer resistance equals the polarization resistance [48]. Thus, the lower the semicircle diameter is, the better electrocatalytic activity is exhibited by the examined material.

Fig. 8:

Impedance plots for GC, PIN, and hybrid nanobrush PIN/Au were performed at 10 mM EtOH in 0.1 M NaOH. The potential applied to the electrode was 0.2 V vs. Ag/AgCl.

As shown in Fig. 8, for Au–polyindole composite the smallest semicircle was observed, which confirms that it possesses better electrocatalytic properties than all the other examined electrodes. For GC and GC covered by polyindole larger semicircles were recorded than for composites, indicating the higher charge transfer resistance towards ethanol oxidation, which means that this reaction does not take place or only occurs slightly with minor effect. Impedance data are in full agreement with cyclic voltammetry measurements (Fig. 5) confirming the high catalytic features of proposed composites.