Lately, polymers as electrochemical catalysts have attracted intense interest. As two promising candidates, herein, electrochemically deposited polyaniline and poly(aspartic acid) thin film materials have been fabricated on glassy carbon electrodes. Through multiple characterizations, including attenuated total reflection Fourier transform infrared spectrophotometry, cyclic voltammetry, electrochemical impedance spectroscopy, and proton nuclear magnetic resonance, essential properties of these two electro-synthesized polymers, especially the capacity in electrochemically catalyzing the oxidation of electro-active isomers of hydroquinone and catechol, have been investigated. The polymer-modified electrodes present improved conductivity, and diffusion-dominated redox behaviors of hydroquinone and catechol at both modified glassy carbon electrodes are observed. In addition, quantitative proton nuclear magnetic resonance spectra provide the evident information on the electrochemically induced molecular transformation, which confirms the better electrochemical activity of poly(aspartic acid).

Amongst all catalysts, polymers prepared via electro-polymerization, a technique with high degree of control during polymerization reactions,1 have attracted many attentions. Conductive polymers, polyaniline (PANI) in particular, have always been considered promising electrode materials due to their low cost, ease of synthesis, and high conductivity.2 Currently, on the other hand, polyamino acid-modified electrodes have been extensively studied.3 In this field, poly(aspartic acid) (PASP), which is proved easy to be electrochemically polymerized on glassy carbon and chemically versatile,4 has been widely investigated. Applications of PASP-modified electrodes for electrochemical determinations of some biological molecules have been reported in detail, implying excellent electro-catalytic responses of PASP to compounds examined. Moreover, PASP is able to not only effectively promote electron transfer on electrode surface but also significantly reduce oxidation potentials during oxidation-reduction reactions.5 Hydroquinone (HQ) and catechol (CC), two kinds of dihydroxybenzene isomers extensively used as raw materials and synthetic intermediates in chemical and pharmaceutical industries,6 are typical and important electro-active molecules in fundamental electrochemical researches. Attempts have been made to study the oxidation progresses of HQ and CC in electrochemical studies.7 However, with electrochemical technique alone, it is unlikely to meet requirements of observing reaction paths, as well as evaluating oxidation efficiencies in a molecular level. As a result, a few spectrometric methods have been coupled to electrochemical experiments to obtain spectrometric information about electrolytic intermediates and products.8 In 2005, Li et al. reported a combination use of high-performance liquid chromatography and gas chromatography coupled with mass spectrometry on electrochemical degradation of phenol-derivated HQ and CC at three different types of anodes.9 In 2006, ultraviolet spectrophotometer was put into application by Leonardo et al. to follow the electrochemical degradation of phenol, p-benzoquinone and catechol.10 In recent years, nuclear magnetic resonance (NMR) have been combined with electrochemical experiments.11–17 Up till now, there have been a series of NMR-related researches on HQ oxidation mainly on account of its simplicity for setting NMR spectroelectrochemical methods with only one singlet peer molecule for four equivalent protons in 1H-NMR.18 Papers on NMR spectroelectrochemical studies dealing with CC oxidation, nevertheless, are still scant.

Herein, a comparative study on PANI and PASP as catalysts for HQ and CC electrochemical oxidation is presented. Electrochemical catalytic performances of PANI and PASP have been investigated through the electro-oxidation of HQ and CC, with 1H-NMR spectra as the monitor of reactant consumption and product generation, along with other characterization techniques of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrophotometry, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Results obtained demonstrate that PASP displays higher electro-catalytic activities in both redox systems.

HQ, CC, aniline (ANI), and aspartic acid (ASP) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals in this study were of analytical grade and used as received without further purification. Phosphate buffer solution (PBS) consisting of 0.2 M NaH2PO4-Na2HPO4 played the role of pH-adjusting supporting electrolyte, and 1 mM K3[Fe(CN)6]/K4[Fe(CN)6] was employed as the redox probe solution. Distilled water was utilized throughout. Electrochemical experiments were carried out on a CHI 660E electrochemical workstation (CH. Instruments, Shanghai Chenhua Instrument Corporation, China), with a platinum wire as the counter electrode and a silver/silver chloride (Ag/AgCl/saturated KCl) electrode as the reference one. All NMR experiments were performed on a Bruker Avance III 600 MHz spectrometer (Bruker, Biospin, Germany) using a 5-mm PABBI-Z inverse probe at 22 °C with tris(trimethylsilyl) phosphate (TMSP) as the internal standard, and one spectrum was recorded with 32 scans with each scan of ∼5 s (delay time 3 s and acquisition time 2 s). Surface morphologies of polymer thin films grown onto glassy carbon electrodes were observed on a field-emission scanning electron microscope (SEM, Hitachi SU-70, Japan) at 5 kV. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were acquired on a Fourier transform infrared spectrophotometer (Nicolet iSO10, Thermo Fisher Scientific, the U.S.A.) in the wavenumber range of 4000∼650 cm-1, with zinc selenide (ZnSe) as the reference semiconductor. X-ray diffraction (XRD) was adopted to analyze the crystallinity of PANI and PASP thin films, with two-theta scans between 5 and 90° on a Bruker-AxsD8 diffractometer with Cu-Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA. Raman spectra were recorded on a Raman spectrometer (WITec alpha 300RA, WItec, German), with samples irradiated by a 488 nm laser source (∼40 mW).

Prior to the surface modification, glassy carbon electrodes (GCEs) were carefully polished to obtain mirror-like surfaces, separately by alumina particles with diameters of 1, 0.3, and 0.05 μm. After sonication in ethanol and distilled water successively for 10 minutes (5 minutes each), the polished electrodes were rinsed with water and then dried with the help of an air blower. Following that, polymer-modified electrodes were prepared on the electrochemical workstation via cyclic voltammogram deposition, with PANI-GCE gotten in a mixture solution of ANI (0.5 M) and HClO4 (1 M) at a scanning rate of 50 mV/s from -0.2 V to 0.9 V for 10 segments, and PASP-GCE in ASP (0.01 M, pH = 7) at 100 mV/s from -1.5 V to 2.0 V for 20 segments. All parameters applied were decided with the assistance of reported papers.5,19–23

All electrochemical treatments were conducted at room temperature. Electrochemical impedance spectra in 0.1 M KCl containing K3[Fe(CN)6]/K4[Fe(CN)6] (1 mM each) were tested in the frequency range between 1 MHz and 0.1 Hz at an alternating current voltage amplitude of 5 mV, and CV measurements were performed by scanning 5 mM HQ or CC (pH 2.0) at different scanning rates. Current-time curves were recorded during the 1.5 h of electrolysis experiments by applying constant potentials on modified GCEs dipped in 5 mM HQ or CC (pH 2.0). After that, as-electrolyzed HQ solutions were collected into 5-mm NMR tubes for 1H-NMR measurements, while 1H-NMR spectra concerning CC oxidation were measured after electrolyzing 50 mM CC (pH 2.0) for 5 h to gain spectra with acceptable signal-to-noise ratios.

Before 1H-NMR tests, 10% deuterated water, containing 0.03% TMSP as the internal standard, was added into the as-collected solutions so that signal intensity normalization could be achieved quantitatively. Temperature control via the heating and cooling unit attached to the probe led to a temperature accuracy of 0.01 K, and all 1H-NMR spectra were obtained at 22 °C.

Fig. 1 shows the cyclic voltammograms of PANI and PASP growing onto the pre-polished GCEs, with PANI in 1 M HClO4 containing 0.5 M ANI at 50 mV/s from -0.2 V to 0.9 V for 10 segments, and PASP in ASP (0.01 M, pH = 7) at 100 mV/s from -1.5 V to 2.0 V for 20 segments. For either PANI or PASP, the cyclic voltammogram of the first scanning cycle barely shows oxidation peak or corresponding reduction peak. With incremental scanning cycle number, however, remarkable rise in current peaks (oxidation peak and reduction peak) can be observed. This observation depicts that electrode modification leads to extending active surface area. The SEM images of resultant films are displayed in Fig. 2. These images reveal successful growth of PANI and PASP onto glassy carbon electrodes.

FIG. 1.

Cyclic voltammograms for the fabrication of PANI (a) and PASP (b) onto GCEs.

FIG. 1.

Cyclic voltammograms for the fabrication of PANI (a) and PASP (b) onto GCEs.

Close modal
FIG. 2.

SEM images (×30k) of PANI (a) and PASP (b) thin films grown on GCEs.

FIG. 2.

SEM images (×30k) of PANI (a) and PASP (b) thin films grown on GCEs.

Close modal

The ATR-FTIR spectra over 3500∼650 cm-1 of PANI and PASP thin films in this work are given in Fig. 3. These two spectra were obtained after removing bands of bare GCE, which was tested as background substance at first. In the spectrum of PANI, the band at 1550 cm-1 corresponds to N-H in-plane bending, and the one at 1470 cm-1 is the vibrational band of benzene ring. As for N-H and C-H in benzene ring, in-plane vibration bands are found at 1280, 1230, and 1040 cm-1, with an out-of-plane band at 813 cm-1.24 On the other hand, in the spectrum of PASP, a group of absorption bands at 2930, 2850, 1050, and 863 cm-1 are observed. Bands at 2930 and 2850 cm-1 are referred to asymmetric and symmetric stretching vibrational bands of C-H,25 and the two at 1050 and 863 cm-1 might be assigned to C-N stretching and N-H out-of-plane bending, respectively. What’s more, it is noted that there is a little intensity in the hydroxyl stretching region (2500∼3800 cm-1 spectral range26), which reveals the presence of -OH in remaining -COOH. Thus, these two ATR-FTIR spectra further prove fulfilled modification treatments.

FIG. 3.

ATR-FTIR spectra of PANI and PASP thin films grown on GCEs.

FIG. 3.

ATR-FTIR spectra of PANI and PASP thin films grown on GCEs.

Close modal

As is shown in Fig. 4, XRD patterns and Raman spectra are presented to characterize the electro-deposited polymers. The obvious shouldered peaks (8∼19°) in XRD patterns indicate the semi-crystalline structures of PASP and PANI (Fig. 4a). In Raman spectra, two apparent bands – D band and G band – are observed, illustrating that carbon skeletons in both polymers are amorphous (Fig. 4b). XRD and Raman results together suggest that both PANI and PASP electrochemically deposited are semi-crystalline in nature.

FIG. 4.

XRD patterns (a) and Raman spectra (b) of PANI and PASP films.

FIG. 4.

XRD patterns (a) and Raman spectra (b) of PANI and PASP films.

Close modal

Nyquist electrochemical impedance spectra were investigated to monitor interface properties of modified GCEs, and plots are depicted in Fig. 5. Each Nyquist plot consists of an out-of-shaped semicircle in the high-frequency range, which indicates interfacial charge transfer resistance (Rct).27,28 As can be seen, Rct for bare GCE is ∼200 Ω, and the value starts to decrease remarkably when polymer thin films are prepared, implying that growth of PANI or PASP onto the pre-polished GCE will be helpful to improve the conductivity of electrode surface. Amongst all, PASP-GCE possesses the best charge transfer capability, with the smallest Rct of ∼110 Ω.

FIG. 5.

Nyquist electrochemical impedance spectra of bare GCE, PANI-GCE, and PASP-GCE measured in 0.1 M KCl containing 1mM K3[Fe(CN)6]/K4[Fe(CN)6].

FIG. 5.

Nyquist electrochemical impedance spectra of bare GCE, PANI-GCE, and PASP-GCE measured in 0.1 M KCl containing 1mM K3[Fe(CN)6]/K4[Fe(CN)6].

Close modal

In Fig. 6 cyclic voltammograms and peak current-square root of scanning rate relationship curves of modified GCEs measured in 5 mM HQ or 5 mM CC (pH 2.0) with scanning rate ranging from 200 to 500 mV/s are presented. It can be seen that redox peak currents, anodic peak current (Ip,a) and cathodic peak current (Ip,c), rise as scanning rate goes up, either at PANI-GCE or at PASP-GCE. Meanwhile, anodic peak potential shifts positively, with cathodic peak potential shifting negatively. On the other hand, good linear relationships between peak currents (Ip,a and Ip,c) and square root of scanning rate confirm that the redox reactions of HQ and CC at the surfaces of PANI-GCE and PASP-GCE are all controlled by diffusion.

FIG. 6.

Cyclic voltammograms of PANI-GCE in HQ (a), PANI-GCE in CC (b), PASP-GCE in HQ (c), and PASP-GCE in CC (d) at various scanning rates. And peak current-square root of scanning rate relationship curves of PANI-GCE in HQ (e), PANI-GCE in CC (f), PASP-GCE in HQ (g), and PASP-GCE in CC (h).

FIG. 6.

Cyclic voltammograms of PANI-GCE in HQ (a), PANI-GCE in CC (b), PASP-GCE in HQ (c), and PASP-GCE in CC (d) at various scanning rates. And peak current-square root of scanning rate relationship curves of PANI-GCE in HQ (e), PANI-GCE in CC (f), PASP-GCE in HQ (g), and PASP-GCE in CC (h).

Close modal

Fig. 7 displays current density-time curves during HQ and CC electro-oxidation at different potentials. Herein, current density refers to current, recorded in an electrochemical oxidation experiment, divided by electro-active electrode surface area figured out on the basis of Randles-Sevcik formula.7,29,30 At the test potentials, compared to PANI-GCE, PASP-GCE shows over two-fold larger current density in the electro-oxidation of HQ and CC after 200 s with a less obvious current decay afterwards, indicating the better catalytic performance of PASP towards phenolic species.

FIG. 7.

Current density-time curves for modified GCEs at potentials of 0.4 V (a), 0.5 V (b), and 0.6 V (c) in 5 mM HQ (pH 2.0); at 0.55 V (d), 0.7 V (e), and 0.8 V (f) in 5 mM CC (pH 2.0).

FIG. 7.

Current density-time curves for modified GCEs at potentials of 0.4 V (a), 0.5 V (b), and 0.6 V (c) in 5 mM HQ (pH 2.0); at 0.55 V (d), 0.7 V (e), and 0.8 V (f) in 5 mM CC (pH 2.0).

Close modal

Due to the power of 1H-NMR to quantitatively and specifically identify organic molecules, it was utilized to evaluate the electro-oxidation capacity. As is exhibited in Fig. 8, the 1H-NMR spectrum of HQ displays a singlet proton signal at around 6.63 ppm, and the signal for para-benzoquinone (p-Q, the oxidized form of HQ) appears at roughly 6.70 ppm after 1.5 h of electrochemical oxidation. In Fig. 9, the proton signal for ortho-benzoquinone (o-Q, the oxidized form of CC) appears at ∼6.97 ppm after 5 h of electrolysis, while two multiplets of CC proton signals are observed at about 6.77 and 6.69 ppm.

FIG. 8.

1H-NMR spectra of 5 mM HQ (pH 2.0) before and after 1.5 h of electro-catalytic oxidation with PANI-GCE at potentials of 0.4 V (a), 0.5 V (b), and 0.6 V (c); with PASP-GCE at 0.4 V (d), 0.5 V (e), and 0.6 V (f).

FIG. 8.

1H-NMR spectra of 5 mM HQ (pH 2.0) before and after 1.5 h of electro-catalytic oxidation with PANI-GCE at potentials of 0.4 V (a), 0.5 V (b), and 0.6 V (c); with PASP-GCE at 0.4 V (d), 0.5 V (e), and 0.6 V (f).

Close modal
FIG. 9.

1H-NMR spectra of 50 mM CC (pH 2.0) before and after 5 h of electro-catalytic oxidation with PANI-GCE at potentials of 0.55 V (a), 0.7 V (b), and 0.8 V (c); with PASP-GCE at 0.55 V (d), 0.7 V (e), and 0.8 V (f). The insets show amplified 1H-NMR spectra ranging from 6.95 to 7.05 ppm.

FIG. 9.

1H-NMR spectra of 50 mM CC (pH 2.0) before and after 5 h of electro-catalytic oxidation with PANI-GCE at potentials of 0.55 V (a), 0.7 V (b), and 0.8 V (c); with PASP-GCE at 0.55 V (d), 0.7 V (e), and 0.8 V (f). The insets show amplified 1H-NMR spectra ranging from 6.95 to 7.05 ppm.

Close modal

Further insights into the impacts of PANI-GCE and PASP-GCE on HQ and CC oxidation are provided by integrated area density comparison of p-Q and o-Q (Fig. 10). Herein, we define integrated area density as signal intensity divided by electro-active area. In this step, the approach to calculating electro-active surface area mentioned in current density calculation has been adopted, and signal intensity normalization has been achieved with TMSP signal intensity. As can be seen in Fig. 10, integrated area density increases as potential applied rises, with values at PASP-GCE growing more markedly than those at PANI-GCE. In terms of promotion in integrated area density by PASP, the most enormous increase in p-Q is observed at 0.6 V with 3.34 folds, the one in o-Q at 0.8 V with 7.46 folds. It is noticeable that, despite a ten-time-diluted concentration of HQ, p-Q signal intensity constitutes larger integrated area density than o-Q signal intensity does.

FIG. 10.

1H-NMR integrated area densities of p-Q after 1.5 h of electro-catalytic oxidation (a) and o-Q after 5 h of electro-catalytic oxidation (b) using PANI-GCE (orange) and PASP-GCE (blue) as working electrodes at different potentials.

FIG. 10.

1H-NMR integrated area densities of p-Q after 1.5 h of electro-catalytic oxidation (a) and o-Q after 5 h of electro-catalytic oxidation (b) using PANI-GCE (orange) and PASP-GCE (blue) as working electrodes at different potentials.

Close modal

As is shown in Scheme 1, PANI is well known to have various states, as a result of its molecular structure, such as leucoemeraldine base (LB, fully reduced form, x=1), emeraldine base (EB, half reduced form, 0<x<1), and pernigraniline base (PB, fully oxidized form, x=0). Generally, the structure of PANI is mainly in EB state, a mixture of benzenoid and quinoid units.31,32 If PANI is introduced into an HQ or CC system, it is expected that imino nitrogen atoms (-N=) in conjugated quinoid moieties would interact with the hydroxyl groups (-OH) in HQ or CC via hydrogen bonds, thus accelerating the oxidation of HQ or CC.33,34 On the other hand, the framework of PASP-GCE is laid out in Scheme 2, along with oxidation processes of HQ and CC with PASP as the catalyst. Apart from nitrogen atoms, oxygen atoms in amido bonds (-NH-CO-) as well as remaining carboxyl groups (-COOH) also provide locations for hydrogen bond formation. Besides, the PASP membrane electrochemically polymerized onto GCE is richly negatively charged. Those negative charges attract the electron-lacking benzene ring of HQ or CC and therefore enhance adsorption of HQ or CC onto the surface of PASP-GCE, which is consist with the results on EIS measurements: The Rct for PASP-GCE (∼110 Ω) is smaller than that for PANI-GCE (∼130 Ω), suggesting much faster electron transfer kinetics at PASP-GCE. Thirdly, one amino group (-NH2) and two carboxyl groups make ASP a tri-functional monomer, implying that cross-linking would happen during electro-polymerization so that a net-shaped three-dimensional construction would be formed in PASP. The three-dimensional structure of the PASP surface ensures widespread electro-active spots and negative charges, further improving catalytic activity.5,35–37

SCHEME 1

Structure of PANI and oxidation processes of HQ and CC with PANI as catalyst.

SCHEME 1

Structure of PANI and oxidation processes of HQ and CC with PANI as catalyst.

Close modal
SCHEME 2

Structure of PASP electro-polymerized on GCE and oxidation processes of HQ and CC with PASP as catalyst.

SCHEME 2

Structure of PASP electro-polymerized on GCE and oxidation processes of HQ and CC with PASP as catalyst.

Close modal

PANI- and PASP-modified GCEs have been successfully fabricated and the comparison on their electro-catalytic performances has been taken by combining electrochemical and spectral measurements, with HQ and CC as the electro-catalytically oxidized species. Polymer modification leads to enhanced electrode conductivity, as is shown in Nyquist electrochemical impedance spectra. In the meantime, electrochemical performances of PANI-GCE and PASP-GCE have also been studied. Cyclic voltammograms reveal diffusion-dominated redox behaviors of HQ and CC at PANI-GCE and PASP-GCE, and current density after 200 s at PASP-GCE maintains over two-fold larger than that at PANI-GCE in the electro-oxidation of HQ and CC. 1H-NMR spectra have been employed to characterize the products, where promotion in product generation by PASP has been observed, with integrated area density increasing most considerably by 3.34 folds at 0.6 V in p-Q and by 7.46 folds at 0.8 V in o-Q. All characterization results together conform better electro-catalytic effects of PASP-GCE on both HQ oxidation and CC oxidation. In summary, instead of PANI, PASP might be suggested as a more effective alternative in electrochemical researches due to its superior electro-catalytic activity.

The authors would like to acknowledge the supports from National Natural Science Foundation of China (U1632274, 21505109, 11761141010, 11475142, and 21706222), Fundamental Research Funds for the Central University (20720160074 and 20720150109), Natural Science Foundation of Fujian Province (2018J01008), and Key Science and Technique Project of Fujian Province (2017H0040).

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