Hydrophilic polymer/polypyrrole/graphene oxide nanosheets with different performances in electrocatalytic applications to simultaneously determine dopamine and ascorbic acid

Abstract

Hydrophilic polymers (HP) functionalized polypyrrole/graphene oxide nanosheets (HP/PPy/GO) were successfully prepared by covalently modifying polyacrylamide (PAM), polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP) on the surface of PPy/GO nanosheets containing vinyl groups. Besides significantly improving the dispersivity of PPy/GO in water, the different chemical properties among the three HP with different hydrophilic groups further resulted in the different performances of the obtained electrochemical biosensors (PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO modified glassy carbon electrodes (GCEs)) in electrocatalytic applications to simultaneously determine dopamine (DA) and ascorbic acid (AA). PAM/PPy/GO and PAA/PPy/GO modified GCEs exhibited good electrochemical responses to distinguish DA from AA in their mixture at certain concentrations, which cannot be achieved by the PVP/PPy/GO modified GCE, due to their different chemical properties among the three HP with different hydrophilic groups. Particularly, compared to PAA/PPy/GO, PAM/PPy/GO can act as a good steady electrode material with good selectivity and sensitivity that can simultaneously determine the two substrates in their mixture at lower concentrations, which may be due to the different chemical properties between the electron-donating amide groups and electron-withdrawing carboxyl groups. The different electronic effects of the amide group and carboxyl group resulted in the difference in the transmission capability of electrons released from the oxidation reaction of DA and AA. Therefore, PAM/PPy/GO modified GCE can act as a good electrochemical sensor with high sensitivity and selectivity for DA and AA.

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1. Introduction

Over the past few years, polymeric materials have attracted wide theoretical interest due to their potential practical applications in sensors,¹ particularly, as interface materials developed for biosensors,² which can be used for many different purposes to detect primary chemical and vital substances. Among them, conducting polymers (CPs), new synthetic organic materials first synthesized towards the end of 1970s,³ are considered as one of the most widely used materials to build electrochemical biosensors⁴ because their electric properties are tunable via either the reversible redox states or doping/dedoping states, which leads to a change in resistance, current or electrochemical potential.^5,6 However, conventional CPs, such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTh), usually possess poor processability due to their insolubility in water and most organic solvents as well as infusibility, arising from their strong inter- and intramolecular interactions and molecular structure of their rigid chain.^7,8 Particularly, the intrinsic hydrophobicity results in poor dispersivity in water, which may greatly limit the applications of their micro-/nanostructures in electrochemical biosensors. Therefore, several researchers have focused on the synthesis of new monomers with hydrophilic groups to improve the poor dispersivity of CPs; for instance, water-soluble fully sulfonated PANI obtained by horseradish peroxidase- and chloroperoxidase-mediated syntheses showed potential applications as a dispersing agent for nanomaterials and electroactive polymer binder for electrochemical biosensors;⁹ a molecular imprinting polymer prepared using pyrrole-phenylboronic acid (a novel electropolymerized monomer) can be used for the recognition and detection of dopamine.¹⁰ However, it is difficult to obtain the corresponding micro-/nanostructures prepared using the new monomers with hydrophilic groups via this simple method.

Recently, hydrophilic polymers (HP), such as polyacrylamide (PAM), polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP), have been modified on the surface of multi-walled carbon-nanotubes (MWNTs),^11,12 graphene (G)¹³ or hollow TiO₂ microspheres¹⁴ and CeO₂ nanoparticles¹⁵ to improve their hydrophilicity. These can be used as good electrode materials to build new electrochemical biosensors for the sensitive detection of various substances, including dopamine (DA), uric acid (UA), bisphenol A and lysozyme. HP, with different functional groups, played important roles in improving the performance of the obtained electrochemical biosensors. Herein, the abovementioned HP with different functional groups, such as PAM, PAA and PVP, were successfully modified on the surface of polypyrrole/graphene oxide nanosheets (PPy/GO) by covalent bonding. The obtained HP-functionalized polypyrrole/graphene oxide nanosheets (HP/PPy/GO), including PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO, exhibited excellent flaky textures with wrinkled forms and good dispersivity in water compared to PPy/GO. The performance of the obtained electrochemical biosensors constructed using HP/PPy/GO modified glassy carbon electrode (GCE) was evaluated by investigating their electrocatalytic application toward the simultaneous determination of DA and ascorbic acid (AA), which are two of the most important chemicals in living things and plays significant roles in regulation of human metabolism as well as in the renal, hormonal and central nervous systems.¹⁶ DA and AA are easily electrochemically oxidized, but difficult to be simultaneously detected using electrochemical techniques due to the very close or even overlapping oxidation potential in the mixed system. The performance of the three electrochemical biosensors showed significant differences in the simultaneous determination of DA and AA in the mixture, which was attributed to the difference of chemical properties among the three HP with different hydrophilic groups. Two well-defined oxidation peaks, corresponding to the oxidation of AA and DA, were clearly observed in the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) responses of the PAM/PPy/GO and PAA/PPy/GO modified GCEs, which was enough to distinguish DA from AA in their mixture at certain concentrations, but could not be achieved using the PVP/PPy/GO modified GCE. Particularly, PAM/PPy/GO can act as a good steady electrode material with good selectivity and sensitivity that can simultaneously determine the two substrates in their mixture at lower concentrations than PAA/PPy/GO, which may be due to the different chemical properties between the electron-donating amide groups and electron-withdrawing carboxyl groups. The different electronic effects of the amide group and carboxyl group result in the differences in the transmission capability of the electrons released from the oxidation reaction of DA and AA. Therefore, the PAM/PPy/GO modified GCE can act as a good electrochemical sensor for DA and AA with high sensitivity and stability.

2. Experimental

2.1 Materials

Pyrrole (Py) (≥98.0%), high-purity graphite powder (≥99.85%), allyl chloride (≥98.0%), acrylamide (AM) (≥98.5%) and acrylic acid (AA) (≥98.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd. and were of chemical grade. All the other reagents were analytical grade and used without further purification, including KMnO₄ (Tianjin Baishi Chemical Co. Ltd, ≥99.5%), FeCl₃·6H₂O (≥99.0%), H₂SO₄ (≥98.3%), KOH (≥85.0%) and NaCl (≥99.5%) (Sinopharm Chemical Reagent Co. Ltd.), N-vinylpyrrolidone (VP) (Tokyo Chemical Industry, ≥99.0%), 2,2-azobisisobutyronitrile (AIBN) (≥98.0%) (Aladdin reagent (Shanghai) Co., Ltd.), L-ascorbic acid (AA) (≥99.7%), ethanol (≥99.7%), KCl (≥99.5%), H₃PO₄ (≥85.0%), Na₂HPO₄ (≥99.0%) and NaH₂PO₄ (≥99.0%) (Tianjin Yongda Chemical Reagent Co., Ltd.), H₂O₂ (Shenyang Xinhua Reagent Factory, ≥30.0%), NaNO₃ (Tianjin Bodi Chemicals Co., Ltd., ≥99.0%), dimethyl formamide (DMF) (Tianjin Beilian Fine Chemicals Development Co., Ltd., ≥99.5%), K₄Fe(CN)₆ (Tianjin Damao Chemical Factory, ≥99.5%), K₃Fe(CN)₆ (Tianjin Chemical Reagent Factory, ≥99.5%) and dopamine hydrochloride (DA) (Alfa Aesar, ≥99.0%).

2.2 Preparation of the HP/PPy/GO nanosheets

The preparation of the HP/PPy/GO nanosheets was mainly based on our previous report on poly(ionic liquids) functionalized polypyrrole/graphene oxide (PILs/PPy/GO) nanosheets,¹⁷ which must include at least the following steps: (1) GO was prepared via a modified Hummers method;¹⁸ (2) the PPy/GO nanosheets were prepared through an in situ chemical polymerization of Py on GO under ultrasonic irradiation as reported in the literature;¹⁹ (3) the PPy/GO–CH₂–CH=CH₂ nanosheets were prepared by a substitution reaction of allyl chloride and PPy/GO in DMF with KOH as reported in the literature;²⁰ (4) the HP/PPy/GO nanosheets were prepared via the polymerization of AM, AA or VP on the surface of the PPy/GO–CH₂–CH=CH₂ nanosheets. Choosing the PAM/PPy/GO nanosheets as an example, in a typical procedure, 250 mg of AM and 5 mg AIBN were dissolved in 150 mL of ethanol. 50 mg of PPy/GO–CH₂–CH=CH₂ was added into the abovementioned solution and dispersed using ultrasound treatment for 5 min. Then, the mixture was placed in an oil bath and the reaction was carried out under reflux for 6 h with balloon protection filled with N₂ at 80 °C. The products were collected by centrifugation and then washed and redispersed with ethanol and water several times to remove any unreacted chemicals and outgrowths. Finally, the obtained black powders were dried in vacuum at 45 °C for 24 h. The reaction procedure used to prepare the HP/PPy/GO nanosheets is shown in Scheme 1.

Scheme 1 The reaction procedure used to prepare the HP/PPy/GO nanosheets.

2.3 Preparation of the HP/PPy/GO nanosheets modified GCE

The obtained HP/PPy/GO nanosheets were dispersed in ethanol to give a 1 mg mL⁻¹ black suspension. The film was prepared by dropping 3 μL of the suspension onto a clean GCE surface and then evaporating the solvent in the environment. A modified GCE was used as the working electrode.

2.4 Characterizations and apparatus

Images of the HP/PPy/GO nanosheets were obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM measurements were recorded using a Hitachi SU-8010 electron microscope with primary electron energy of 10 kV. The TEM experiments were performed using a JEM-2100 electron microscope (JEOL, Japan) at an acceleration voltage of 200 kV. The zeta-potential data of the products were obtained using a Zeta-Plus4 instrument (Brookhaven Corp., USA). Fourier transform infrared spectroscopy (FTIR) of the KBr powder-pressed pellets was carried out on a Perkin Elmer Spectrum one FTIR spectrometer (Perkin-Elmer Corp., USA). An SDT Q600 Simultaneous DSC-TGA Instrument (TA Corp., USA) was used to investigate the thermal stability of the GO, PPy/GO, PAM/PPy/GO nanosheets, PAA/PPy/GO nanosheets and PVP/PPy/GO nanosheets in the temperature range from room temperature to 700 °C under condensed N₂ at a rate of 10 °C min⁻¹. The electrochemical performance of the HP/PPy/GO nanosheets was investigated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) on a CHI660E Electrochemical Station (Shanghai CHENHUA instrument Co., Ltd.). In a three-electrode system, a modified GCE, a platinum wire and an Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. The measurements were performed in 0.05 M phosphate buffer solution (PBS) (pH = 4.0, at 20 °C) with 0.05 M NaCl. The pH value was adjusted with H₃PO₄ using a IS126 pH Meter (Shanghai InsMark Instrument Technology Co., Ltd.). The interfacial charge-transfer resistance of the samples was determined using electrochemical impedance spectroscopy (EIS) in the frequency range between 0.01 Hz and 10 000 Hz with a perturbation signal of 5 mV.

3. Results and discussion

3.1 The morphology and characterization of the HP/PPy/GO nanosheets

The SEM and TEM images of PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO, synthesized using the procedure outlined in Scheme 1, are shown in Fig. 1. Flaky textures with wrinkled forms are clearly observed in the SEM images, which are presented in Fig. 1(a), (c) and (e). In the TEM images, some nanoscrolls were evidently found in PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO (Fig. 1(b), (d) and (f), respectively), which were similar to the PILs/PPy/GO nanosheets due to their curliness.¹⁷

Fig. 1 (a) SEM and (b) TEM images of PAM/PPy/GO. (c) SEM and (d) TEM images of PAA/PPy/GO. (e) SEM and (f) TEM images of PVP/PPy/GO.

The zeta-potential data of GO, PPy/GO, PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO present a strong negative charge of nearly −28.3, −18.3, −16.7, −15 and −17.5 mV in aqueous solution, respectively. The dispersibility of HP/PPy/GO was improved when compared to PPy/GO. After ultrasound treatment for 1 h, GO, PPy/GO and HP/PPy/GO were well dispersed in the aqueous solution when they were kept for 1 min (Fig. 2(i)). However, after being kept for 20 min, PPy/GO evidently precipitated out from an aqueous solution due to the hydrophobic nature of PPy, whereas the HP/PPy/GO nanosheets remained well dispersed in the aqueous solution (Fig. 2(ii)). The sedimentation of the three types of HP/PPy/GO nanosheets were found after being kept for 3 h (Fig. 2(iv)), particularly, the PVP/PPy/GO nanosheets almost precipitated out from aqueous solution after being kept for 6 h (Fig. 2(v)). After 24 h, all of the HP/PPy/GO nanosheets completely precipitated out from aqueous solution (Fig. 2(vi)). It is well demonstrated that after modifying HP on the surface of PPy/GO, the dispersibility of HP/PPy/GO was improved significantly compared to PPy/GO in an aqueous solution.

Fig. 2 Images of the products including (a) GO, (b) PPy/GO, (c) PAM/PPy/GO, (d) PAA/PPy/GO and (e) PVP/PPy/GO dispersed in an aqueous solution after (i) 1 min, (ii) 20 min, (iii) 1 h, (iv) 3 h, (v) 6 h and (vi) 24 h.

Fig. 3(i) shows the FTIR spectra of GO, PPy/GO and the three types of HP/PPy/GO nanosheets, which can correspondingly characterize their chemical structures. The typical FTIR spectra of GO and PPy/GO are shown in Fig. 3(i)a and b, respectively, which are consistent with the previous reports.^21–25 A broad absorption band at 3450 cm⁻¹ was attributed to the –OH groups stretching vibration.²⁵ Other absorption modes for GO can be also observed in Fig. 3(i)a: at 1621 cm⁻¹ due to C=C aromatic bonding,²⁶ at 1456 cm⁻¹ assigned to the C–OH carboxyl group²⁶ and at 1120 cm⁻¹ corresponding to C–O in C–O–C.²⁷ As shown in Fig. 3(i)b, the characteristic peaks of polypyrrole at 1562 cm⁻¹ and 1461 cm⁻¹ are associated to the C–C and C–N stretching vibrations in the pyrrole ring,²⁸ respectively. The peaks at 1170, 1060 and 848 cm⁻¹ are attributed to N–C stretch bending, =C–H in-plane vibration and =C–H out-of-plane vibration,²⁹ respectively, confirming the formation of PPy in the presence of PPy/GO. Moreover, the peaks at 3420, 1665 and 1345 cm⁻¹ observed in Fig. 3(i)c correspond to the N–H stretching vibration,³⁰ the C=O stretching vibration in the –CONH₂ bonds³¹ and the C–N stretching vibration, respectively;³² the asymmetric bands at 1680 and 1560 cm⁻¹ observed in Fig. 3(i)d are related to C=O stretching of the carbonyl group³³ and carboxylate ion, respectively;³⁴ the peak at 1659 cm⁻¹ observed in Fig. 3(i)e was attributed to the C=O stretching vibrations in the pyrrolidone ring.³⁵ The broad absorption in the range of 1690–1640 cm⁻¹, which can be observed in Fig. 3(i)c–e, may be due to the formation of intermolecular hydrogen bonds between PAM, PAA and PVP. All the results indicate that HP, such as PAM, PAA and PVP, have been successfully modified on the surface of PPy/GO, forming PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO, respectively.

Fig. 3 (i) FTIR spectra and (ii) TGA curves obtained for (a) GO, (b) PPy/GO, (c) PAM/PPy/GO, (d) PAA/PPy/GO and (e) PVP/PPy/GO.

Thermogravimetric analysis (TGA) can further prove the existence of the three types of HP on the surface of the obtained PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO. Fig. 3(ii) exhibits the TGA curves obtained for GO, PPy/GO, PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO in the temperature range from room temperature to 700 °C under condensed N₂ at a rate of 10 °C min⁻¹, respectively. The typical characteristic curve of GO and PPy/GO can be clearly observed in Fig. 3(ii)a and b, respectively, which are consistent with the previous report.^17,19 However, after the modification of HP on the surface of PPy/GO, the thermostability of the obtained PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO exhibit some evident changes when compared to PPy/GO. For instance, the initial weight loss, which occurred below 110 °C due to water molecules being removed from PPy/GO was about 9.5%, but PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO showed weight losses of about 13.8%, 10.7% and 11.0% below 110 °C, respectively, which indicated that they absorbed more water than PPy/GO because of the existence of hydrophilic groups in the HP. However, PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO exhibited a small and slow weight loss in the range of 120–360 °C (Fig. 3(ii)c–e). When compared to GO and PPy/GO, the ternary composites are more stable, which proves that there are interactions in the ternary composites. Therefore, the presence of HP on the surface of the obtained PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO can be further confirmed.

3.2 The electrochemical behavior and CV response of the HP/PPy/GO nanosheets modified GCEs towards dopamine and ascorbic acid

Fig. 4 exhibits the electrochemical behaviour of the HP/PPy/GO modified GCEs in 0.05 M phosphate buffer solution (PBS) (with 0.05 M NaCl, pH = 4.0, at 20 °C) with certain concentrations of DA and AA at a scanning rate of 50 mV s⁻¹. It can be found that diverse electrochemical behaviour of the HP/PPy/GO modified GCEs resulted from different hydrophilic groups in the HP, which were modified on the surface of PPy/GO. No peaks can be observed in the CV curves for any of the HP/PPy/GO modified GCEs in 0.05 M PBS without DA and AA (Fig. 4(i)). The response current of the PAA/PPy/GO modified GCE was stronger than that of the PAM/PPy/GO modified GCE and the PVP/PPy/GO modified GCE shows the weakest response current. However, when the CV measurements were carried out in PBS only with 100 μM AA or 8 μM AA, one peak at 0.36 V due to the oxidation of AA in Fig. 4(ii)a and another peak at 0.56 V due to the oxidation of DA in Fig. 4(iii)a were clearly observed in the CV responses of PAM/PPy/GO modified GCE, respectively, which were much more prominent than that of the PAA/PPy/GO and PVP/PPy/GO modified GCE. Furthermore, Fig. 4(iv) presents the CV curves of the abovementioned three modified electrodes in 0.05 M PBS with 8 μM DA and 100 μM AA at a scanning rate of 50 mV s⁻¹. Only the CV response of PAM/PPy/GO modified GCE (Fig. 4(iv)a) exhibits two well-defined oxidation peaks at 0.38 and 0.58 V, corresponding to the oxidation of AA and DA, respectively, which was enough to distinguish DA from AA. However, for the PAA/PPy/GO and PVP/PPy/GO modified GCEs, the peak potentials for AA and DA were indistinguishable at these concentrations (Fig. 4(iv)b and c). It is worth noting that when the concentration of DA and AA significantly increased, two well-defined oxidation peaks at 0.38 and 0.58 V, corresponding to the oxidation of AA and DA, respectively, were clearly observed in the CV response of the PAA/PPy/GO modified GCE in 0.05 M PBS with 32 μM DA and 300 μM AA (Fig. 4(v)b), whose response currents were stronger than that of the PAM/PPy/GO modified GCE (Fig. 4(v)a). However, for the PVP/PPy/GO modified GCE, the peak potentials for AA and DA were still indistinguishable at this higher concentration, and only one peak at 0.40 V could be found in (Fig. 4(v)c).

Fig. 4 The cyclic voltammograms obtained for (a) PAM/PPy/GO, (b) PAA/PPy/GO and (c) PVP/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, 20 °C) at a scanning rate of 50 mV s⁻¹: (i) with 0 μM DA and 0 μM AA; (ii) with 0 μM DA and 100 μM AA; (iii) with 8 μM DA and 0 μM AA; (iv) with 8 μM DA and 100 μM AA; (v) with 32 μM DA and 300 μM AA.

In addition, the electrochemical behaviour of the bare GCE, GO modified GCE and PPy/GO modified GCE, which acts as the blank under exactly the same conditions was also investigated, as shown in Fig. 5. It was clearly found that for the bare GCE and GO modified GCE, the peak potentials for AA and DA are still indistinguishable whether the mixture is at lower or higher concentrations (Fig. 5(iv)a, b and (v)a, b). Though the CV response of PPy/GO modified GCE can be observed as two weak peaks at 0.28 V and 0.42 V in 0.05 M PBS with 32 μM DA and 300 μM AA (Fig. 5(v)c), corresponding to the oxidation of AA and DA, respectively, the potential difference between the two peaks is 0.14 V, which is not enough to distinguish DA from AA.

Fig. 5 The cyclic voltammograms obtained for (a) the bare GCE, (b) GO and (c) PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, at 20 °C) at a scanning rate of 50 mV s⁻¹: (i) with 0 μM DA and 0 μM AA; (ii) with 0 μM DA and 100 μM AA; (iii) with 8 μM DA and 0 μM AA; (iv) with 8 μM DA and 100 μM AA; (v) with 32 μM DA and 300 μM AA.

Therefore, similarly to PILs/PPy/GO,¹⁷ after the modification of certain special HPs on the surface of PPy/GO, the surface properties of the obtained HP/PPy/GO were significantly changed when compared to the original PPy/GO, which ultimately led to changes in their the electrochemical performance. It was evident that due to the different polymer molecular structures and hydrophilic groups, HPs, such as PAM and PAA played a pivotal role for the simultaneous determination of DA and AA in the mixture. Particularly, PAM/PPy/GO can act as a good electrode material with good selectivity and can simultaneously determine the two substrates in their mixtures.

Fig. 6 presents the effect of scanning rate on the oxidation peak current of DA and AA in 0.05 M PBS at certain concentrations of DA and AA, in order to investigate the kinetics of the electrode reactions at the PAM/PPy/GO and PAA/PPy/GO modified GCEs. For both modified GCEs, the oxidation peak current of DA and AA increased simultaneously along with increasing scan rate (Fig. 6(i)A and (ii)A) and all the plots of oxidation peak current at the oxidation potential of DA and AA against the scanning rate in the range of 10–100 mV s⁻¹ presented an excellent linear relationship as shown in Fig. 6(i)B and (ii)B, respectively, which indicated that the oxidation reactions of DA and AA were a typical surface-controlled process.

Fig. 6 The cyclic voltammogram obtained for the (i)-A: PAM/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, at 20 °C) with 8 μM DA and 100 μM AA and (ii)-A: PAA/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, at 20 °C) with 32 μM DA and 300 μM AA at a scanning rate of (a) 10; (b) 20; (c) 30; (d) 40; (e) 50; (f) 60; (g) 70; (h) 80; (i) 90; (j) 100 mV s⁻¹; (i)-B: a plot of peak current at (a) 0.56 V and (b) 0.36 V vs. the square root of the scanning rate (for the PAM/PPy/GO modified GCE); (ii)-B: a plot of peak current at (a) 0.58 V and (b) 0.38 V vs. the scanning rate (for the PAA/PPy/GO modified GCE).

3.3 DPV response of the PAM/PPy/GO and PAA/PPy/GO nanosheets modified GCEs for their application to the simultaneous determination of dopamine and ascorbic acid

DPV was carried out to further investigate the application of the PAM/PPy/GO and PAA/PPy/GO modified GCEs for the simultaneous determination of DA and AA in their mixture, keeping the concentration of one species constant while that of the other species was changed. Fig. 7 and 8 display the differential pulse voltammograms obtained for the PAM/PPy/GO and PAA/PPy/GO modified GCEs with different concentrations of DA ([DA]) or AA ([AA]) in the presence of another species at a certain concentration in 0.05 M PBS, respectively, as well as the corresponding plots of the peak current vs. [DA] or [AA]. Due to the essential differences between the DPV and CV test methods as well as their sensitivity differences, the oxidation potentials of AA and DA for both the PAM/PPy/GO and PAA/PPy/GO modified GCEs have changed slightly. Table 1 gives a detailed comparison of the response characteristics of the PAM/PPy/GO modified GCE with the PAA/PPy/GO modified GCE as electrochemical sensors to detect DA and AA, including the detection limit (according to the criterion of a signal-to-noise ratio = 3 (S/N = 3)), linear ranges, sensitivity, R² and relative standard deviation (RSD). Therefore, both the PAM/PPy/GO and PAA/PPy/GO modified GCEs can be applied in the simultaneous determination of DA and AA in their mixture. Though the sensitivities of the PAM/PPy/GO modified GCE for detecting DA and AA are a little lower than that of the PAA/PPy/GO modified GCE, the detection limit and RSD of the PAM/PPy/GO modified GCE for detecting DA and AA are much better than those of the PAA/PPy/GO modified GCE, which may be due to the different chemical properties of the electron-donating amide group and electron-withdrawing carboxyl group. The different electronic effects of the amide groups and carboxyl groups result in a difference in the transmission capability of the electrons released from the oxidation reaction of DA and AA. Therefore, the PAM/PPy/GO modified GCE can act as a good electrochemical sensor for DA and AA with high sensitivity and stability.

Fig. 7 (i)-A: The differential pulse voltammograms obtained for the PAM/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, at 20 °C) with 80 μM AA and different [DA]: (a) 6, (b) 7, (c) 8, (d) 9, (e) 10, (f) 11, (g) 12, (h) 13 μM; (i)-B: a plot of the peak current vs. [DA]; (ii)-A: the differential pulse voltammograms obtained for the PAM/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, 20 °C) with 16 μM DA and different [AA]: (a) 150, (b) 160, (c) 170, (d) 180, (e) 190, (f) 200, (g) 210, (h) 220 μM; (ii)-B: a plot of the peak current vs. [AA].

Fig. 8 (i)-A: The differential pulse voltammograms obtained for the PAA/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, 20 °C) with 400 μM AA and different [DA]: (a) 32, (b) 36, (c) 40, (d) 44, (e) 48, (f) 52, (g) 56, (h) 60 μM; (i)-B: plot of the peak current vs. [DA]; (ii)-A: the differential pulse voltammograms obtained for the PAA/PPy/GO modified GCE in 0.05 M PBS (with 0.05 M NaCl, pH = 4.0, 20 °C) with 30 μM DA and different [AA]: (a) 300, (b) 320, (c) 340, (d) 360, (e) 380, (f) 400, (g) 420, (h) 440 μM; (ii)-B: a plot of the peak current vs. [AA].

Table 1 A comparison of the response characteristics of the PAM/PPy/GO modified GCE with the PAA/PPy/GO modified GCE as electrochemical sensors to detect DA and AA

Materials	Detecting substance	Detection limit (μM)	Linear ranges (μM)	Sensitivity (μA μM^−1)	R^2	Relative standard deviation (RSD)
PAM/PPy/GO	DA	0.408	6–13	0.0056	0.9901	0.55%
	AA	0.659	150–220	0.0015	0.9970	1.01%
PAA/PPy/GO	DA	1.025	32–60	0.0067	0.9974	0.73%
	AA	1.962	300–440	0.0022	0.9955	1.34%

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In addition, the stability of the PAM/PPy/GO modified GCE was measured by monitoring its peak current at 0.56 and 0.36 V in the CV response to DA and AA in PBS with 8 μM DA and 100 μM AA and the peak current was found to be reduced by 6.97% and 11.17%, respectively, after 100 cycles. The stability of the PAA/PPy/GO modified GCE was measured by monitoring its peak current at 0.58 and 0.38 V in the CV response to DA and AA in PBS with 16 μM DA and 150 μM AA and the peak current was found to be reduced by 6.62% and 11.17%, respectively, after 100 cycles. These results demonstrate that PAM/PPy/GO and PAA/PPy/GO can act as good steady electrode materials for the simultaneous determination of DA and AA in their mixture.

3.4 EIS analysis

Fig. 9 presents the representative impedance spectra of the GO, PPy/GO, PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO modified GCEs using EIS to characterize their interface properties and investigate the stepwise construction process of the sensors. Fig. 9(a) illustrates the Nyquist plots of the GO modified GCE, including a small semicircle domain which implies a low electro-transfer resistance to the redox probe at the electrode interface.³⁶ After coating with PPy, the slope of the line increased dramatically (Fig. 9(b)), which indicates that PPy/GO formed high electron conduction pathways between the electrode and electrolyte. However, after modifying with HP on the surface of PPy/GO, the diameter of the impedance arc increased compared to GO due to the properties of HP in the order of PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO, indicating that the interfacial electro-transfer resistance on the HP/PPy/GO was higher than that of GO. The charge-transfer resistance (R_ct) of the GO, PPy/GO, PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO modified GCEs were 923, 168, 1089, 1369 and 1712 Ω, respectively. It is worth noting that the diameter of the impedance arc of PAM/PPy/GO is at a minimum in all of HP/PPy/GO, implying the lowest electro-transfer resistance in all of HP/PPy/GO. This may be due to the fact that amide groups with a weaker electrophilic effect in PAM are conducive to promote electron transfer at the HP/PPy/GO modified electrode interface compared to the carboxyl groups in PAA and carbonyl groups in PVP, which further demonstrates that the effect of the PAM/PPy/GO modified GCE for detecting DA and AA was better than the other two HP/PPy/GO modified GCEs.

Fig. 9 The EIS plots obtained for (a) GO, (b) PPy/GO, (c) PAM/PPy/GO, (d) PAA/PPy/GO and (e) PVP/PPy/GO in a 0.1 M KCl solution containing 2.5 mM K₃Fe(CN)₆ and 2.5 mM K₄Fe(CN)₆.

4. Conclusion

In summary, three types of HP, including PAM, PAA and PVP, were successfully modified on the surface of PPy/GO nanosheets containing vinyl groups by covalent bonding. Besides significantly improving the dispersivity of the PPy/GO in water, the obtained electrochemical biosensors constructed using PAM/PPy/GO, PAA/PPy/GO and PVP/PPy/GO modified GCEs showed different performances in the electrocatalytic application toward the simultaneous determination of DA and AA. For instance, the PAM/PPy/GO and PAA/PPy/GO modified GCEs exhibited a good electrochemical response to distinguish DA from AA in their mixture at certain concentrations, which could not be achieved using the PVP/PPy/GO modified GCE, due to the different chemical properties among the three HPs with different hydrophilic groups. Particularly, PAM/PPy/GO can act as a good steady electrode material with good selectivity and sensitivity and simultaneously determine the two substrates in their mixture at lower concentrations than PAA/PPy/GO because the different electronic effects of the electron-donating amide groups and electron-withdrawing carboxyl groups result in different transmission capability of the electrons released from the oxidation reaction of DA and AA. Therefore, PAM/PPy/GO can act as a good steady and sensitive electrode material for the development of improved sensors in simultaneous determination of DA and AA.

Acknowledgements

The financial support from the National Natural Science Foundation of China (No. 51203072, 51273087 and 21203082), the Research Fund for the Doctoral Program of Liaoning Province (No. 20131042) and the Foundation for Innovative Research Groups of Liaoning Provincial Universities (No. LT2011001) is greatly appreciated.

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