Demudu Babu Gorle and
Manickam Anbu Kulandainathan*
Electrochemical Process Engineering Division, CSIR-Central Electrochemical Research Institute, Karaikudi, India. E-mail: manbu123@gmail.com
First published on 29th January 2016
The simultaneous electrochemical reduction of graphene oxide (GO) and oxidative polymerization of methylene blue yielding a polymer composite on a glassy carbon electrode surface is demonstrated. The stability of the reduced graphene oxide (ERG)/poly(methylene blue) (PMB) composite in buffer solution is also studied in detail. Interestingly, methylene blue initially forms a radical cation, which donates an electron to GO, then GO undergoes reduction and during the subsequent cycles, it forms the polymer composite through covalent interactions between simultaneously reduced GO and oxidized methylene blue. The formation of the polymer composite is characterized using electrochemical impedance spectroscopy, laser Raman spectroscopy, SEM and UV-Visible absorption studies. Dopamine is a neurotransmitter with primary amine and phenolic functional groups. The electrografting of dopamine onto an ERG/PMB composite modified electrode is carried out and is evaluated by FT-IR and XPS studies and the electrochemical stability of the grafted dopamine is demonstrated using CV studies. Differential pulse voltammetry studies reveal that the modified electrode shows a high selectivity and sensitivity towards the detection of dopamine in the presence of ascorbic acid (AA) and uric acid (UA) with a detection limit of 1.03 × 10−6 mol L−1 from a calibration curve with a linear range of 0.96 × 10−6 mol L−1 to 7.68 × 10−6 mol L−1. Hence, this dopamine grafted on polymer composite modified electrode provides an attractive platform for the selective sensing of dopamine in the presence of interferents.
Electropolymerization is an effective method for the synthesis of polymeric thin films of dye,22 heterocyclic compounds23 etc. Methylene blue (MB) is a cationic water-soluble phenothiazine dye. Many research groups have prepared graphene/MB polymer composites using electrochemical methods and it has been used as a catalyst in biological applications. Barsan et al. prepared phenazine polymers and used them in biosensors.24 Sun et al. fabricated poly(MB) functionalized graphene modified carbon ionic liquid electrodes for dopamine detection.25 Erçarıkcı et al. synthesized poly(MB)/graphene nanocomposite thin films for the oxidation of nitrite.26 Wojtoniszak et al. functionalized graphene oxide with MB and investigated its performance in singlet oxygen generation.27
The term electrografting refers to an electrochemical reaction between organic molecules and solid conducting materials. Furthermore, it was extended to reactions involving an electron transfer between the reagent and substrate in modified electrode surfaces. Electrografting applies to many functional groups like amines, carboxylates, alcohols, Grignard reagents, vinylics, diazonium salts, ammonium salts, phosphonium salts and sulfonium salts, etc.28,29 Electrografting is a well-established method for surface modification and is used in various applications such as sensors, biomolecules, energy storage devices and industrial applications.30 The reactive pathway in electrografting can either be oxidative or reductive depending upon the redox behavior of the molecules.
In this communication, initially we prepared an ERG/PMB composite electrochemically and demonstrated the electrografting of dopamine onto the prepared polymer composite surface, which was expected to enhance the electron transfer kinetics of the modified surface. The electrografting of dopamine follows an oxidative method through amine linkages.31 The grafting of dopamine onto the modified electrode was investigated using Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. The dopamine grafted modified electrode is a good catalyst for the sensing of dopamine through the “like recognizes like”32 principle, which was investigated using CV studies, and the interference of dopamine with UA and AA was investigated using DPV studies. Scheme 1 depicts the reaction pathway of the total system as explained in the later sections.
GO was prepared by sonicating the graphite oxide powder in water.34 For this, graphite oxide (2 mg ml−1) was dissolved in distilled water and sonicated for approximately 2 h, which finally gave a dark yellow colored solution. At the end, this was centrifuged, filtered, washed thoroughly with distilled water and dried at 100 °C for 8 h.
Cyclic voltammetry, electrochemical impedance spectroscopy and Differential Pulse Voltammetry (DPV) studies were carried out using a BAS-IM6 (Zahner Messsysteme, Germany) instrument under a three-electrode cell system. A glassy carbon electrode with 3 mm diameter was used as a working electrode. A SCE electrode and platinum wire were used as the reference and auxiliary electrodes, respectively. Prior to use, the GC electrode was cleaned using alumina paste (0.05 μ) on emery polishing paper (2/0) for 3 min. Furthermore, it was sonicated and washed thoroughly with water and ethanol. Finally, it was dried using nitrogen gas. Before each experiment, the electrolyte was deoxygenated by purging with nitrogen gas for 20 min. All experiments were done at room temperature.
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Fig. 1 (A) Electrochemical polymerization of a GO/MB modified electrode at 50 mV s−1 for 20 cycles (B) (a) 1st and (b) 20th cycles of electrochemical polymerization. |
Fig. 1(B) shows the 1st and 20th cycles of electrochemical polymerization. During the 1st cycle, well-defined reduction and oxidation peaks were observed at −0.53 (C1) and −0.19 V (A1) vs. SCE respectively. This pair of redox peaks corresponds to the reduction of GO and the oxidation of MB at the interface.26 The rapid rise of the current to a more positive potential at nearly 1.12 V (A2) vs. SCE is assigned to the formation of MB radical cations.36 After this potential value, MB undergoes electrochemical polymerization through the formation of radical cations.37 Interestingly, the reduction current of GO at −1.48 V vs. SCE decreases much less from the 1st to 20th cycles, which suggests that the electrochemical reduction of GO was complete.38 In the 20th cycle, another pair of redox peaks were formed at −0.84 (C2) and 0.05 (A3) V vs. SCE with an increase of the current with the cycles, which affects the amount of GO reduction and the formation of PMB film on the modified electrode surface.37 From this, we can clearly explain the simultaneous redox behavior of MB and GO forming an ERG/PMB composite and this behavior depends upon the concentration of MB (Fig. S1–S3 as ESI†). Similarly a poly(MB) modified GC electrode as well as an electrochemically reduced GO modified GC electrode were prepared by the same procedure (Fig. S4 and S5 as ESI†). The expected mechanism for the formation of an ERG/PMB composite by an electrochemical process is shown as Scheme S1 (ESI†).
Fig. S6 (ESI†) shows the cyclic voltammograms of the bare GC, GO and ERG coated GC electrodes in potassium phosphate buffer solution (pH 7.4) at a scan rate of 50 mV s−1 between −1.5 to 1.3 V vs. SCE. Accordingly, the GO and ERG coated electrodes exhibited one redox couple with an anodic peak at 0.5 V and a cathodic peak at −1.25 V vs. SCE, whereas the bare GC electrode showed a featureless voltammogram. The cathodic current of GO was −0.406 mA at −1.5 V, which is found to decrease with ERG, and thus it demonstrates the subsequent reduction of GO from the 1st to 20th cycles. GO started reducing to irreversible ERG, and after almost 20 cycles, the stability of the cathodic current shows the complete reduction of GO. The high anodic current observed with GO may be due to the over-oxidation of chemical impurities present in GO. After coating GO and its reductant onto the surface of the GC electrode, a high current is observed which suggests that the GO and ERG provide the conduction path for electron transfer at the electrode/electrolyte interface and are used in the electrochemical catalytic reactions as a catalyst.39 The change in the position of the redox couple towards high redox potential value and the high capacitive behavior of the modified electrode explains the improvement in the non-faradaic current of ERG due to the increase of the conductive surface area by the electrochemical reduction.40
The surface area of the polymer film formed on the GO coated GC electrode can be calculated from the surface coverage concentration of PMB.35,41 The value of the charge involved in the polymerization can be calculated from the peak area apparent in the cyclic voltammogram using the following formula;
Γ = Q/(nFA) |
The electrochemical activity of the GO coated GC electrode was characterized by the reaction with MB in buffer solution with a pH of 7.4 (Fig. S7 as ESI†). In that, the GO modified GC electrode has redox peaks at −0.53 V and 1.12 V vs. SCE, which were present as minor peaks in the bare GC electrode as shown in the inset, and also shows the high anodic and cathodic current that demonstrates the redox behaviour of MB.
The stability of the ERG/PMB composite modified electrode was checked by recording 30 cycles in the same electrolyte solution in the same potential window, which is shown in Fig. S8 (ESI†). It is noted that the changes in current are negligible after 30 cycles, which indicates the stability of the ERG/PMB composite modified electrode. Based on this result, it is very clear that the formed polymer composite material is highly stable and active and is a suitable catalytic electrode for electrochemical redox reactions.
The formation of PMB, ERG and the ERG/PMB composite was verified using electrochemical impedance spectroscopy (EIS)24,35 and CV studies (Fig. S9 as ESI†). Fig. 2 shows the impedance behavior of modified electrodes in 2 mM L−1 [Fe(CN)6]3−/4− in 0.1 M KCl solution with an amplitude of 30 mV and a frequency ranging from 106 Hz to 0.05 Hz. In this, ERG shows a semicircle at a low frequency region with an Rct of about 1.12 kΩ, which makes it difficult for electron transfer between the electrode/electrolyte interface to occurr. The semicircle at a high frequency region completely disappeared in the case of the PMB and ERG/PMB composite modified electrodes due to the porosity formed during the formation of the film, indicating that the electron transfer can be fast and diffusion-controlled.
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Fig. 2 Electrochemical impedance spectroscopy of (a) ERG, (b) PMB and (c) the ERG/PMB composite modified electrodes in 0.1 M KCl solution containing (1![]() ![]() |
Raman spectra revealed the polymerization of MB, the reduction of GO by electrochemical methods and the formation of the ERG/PMB composite.35,42 The spectra are shown in Fig. 3(A). In the Raman spectra, the D and G bands of ERG formed at 1346 cm−1 and 1604 cm−1 with approximately the same intensity indicate the presence of both sp3 and sp2 carbons with equal numbers which denotes the reduction of GO. The Raman spectrum of PMB forms peaks at 1037 cm−1, 1161 cm−1, 1304 cm−1, 1332 cm−1, 1395 cm−1, 1433 cm−1, 1477 cm−1, 1501 cm−1 and 1624 cm−1. The band at 1037 cm−1 relates to the strong C–S aromatic stretching frequency in MB and that at 1395 cm−1 and 1433 cm−1 relates to the bending vibration modes of the N–CH3 group due to polymerization. The peak at 1624 cm−1 was attributed to the CC stretching frequency in aromatic rings. The peaks of both PMB and ERG are observed in the Raman spectrum of the ERG/PMB composite with a high intensity which confirms both the polymerization and reduction of MB and GO.
The formation of product was further confirmed by SEM images.43,44 Fig. 3(B) (a) shows a clear thin layer type structure with small crystals of MB which were present on the layer of the GO surface in image (c). Image (b) shows the clear layered structure of ERG coated on the FTO plate. This confirms the polymerization of MB over the surface of GO by the electrochemical method.
The consumption in the concentration of MB during the electrochemical polymerization was shown using UV-Visible absorption spectroscopy (Fig. S10 as ESI†) as shown in Fig. 3(C) and it was noted before and after cyclic voltammetry of the same solution. In these spectra, two main absorption bands formed at 659 cm−1 and 612 cm−1 corresponding to MB and MB dimers in solution. According to Beer’s law in UV-Visible absorption spectroscopy, absorption is directly proportional to the concentration of the solution. The figure clearly shows that the decrease in the absorption intensity during the reaction indicates the decrease in concentration of MB. The difference in the concentration of MB after 20 cycles corresponds to the MB that reacted with GO during the polymerization.27,45
In the cyclic voltammogram, an anodic peak with a high oxidation current was formed at 0.384 V vs. SCE indicating the oxidation of the primary amine in dopamine to an amine radical which is strongly attached to the ERG/PMB composite material by a chemical bond due to its activity on the electrode surface.31 After the first cycle, a constant redox peak was formed in the potential region of 0.3 to 0.4 V vs. SCE with ΔE = 56 mV which indicates the attachment of incoming dopamine molecules on the surface of the grafted dopamine molecule shown in Fig. 4(A). Fig. 4(B) shows that the modified electrode has a higher oxidation current in comparison to the bare GC electrode in the presence of dopamine (Fig. S11 as ESI†), which demonstrates the activity of the coated material.
Finally, the electrode was thoroughly washed with water and the grafting of dopamine on the modified electrode was studied using cyclic voltammetry. After washing, the ERG/PMB composite modified electrode showed a higher oxidation current than the bare GC electrode (Fig. S12 as ESI†). The CV shows the potential of the redox peak at ΔE = 51 mV which is responsible for the redox behavior of dopamine. This reaction is a one electron transfer reaction which leads to the immobilization of dopamine on the modified electrode surface. Hence, one of the phenolic groups in the dopamine should undergo oxidation which is explained using the FT-IR and XPS analysis later. Furthermore, the formation of film was confirmed by the sensing of dopamine due to high electron transfer between the incoming molecules and active catalytic material through the film.
The inset of Fig. 4(A) shows the 1st, 20th and 50th cycles of dopamine grafted onto the modified electrode surface for 50 cycles in acidic solution. The stability of this material is shown to be very high as it has less current variation from the 1st to 50th cycle, because all of the cyclic voltammograms are similar and there is no change in the potential difference and current of redox peaks. The polymer composite material is highly efficient for the grafting of dopamine.
The grafting and high stability of dopamine film was investigated using the scan rate effect of cyclic voltammetry in the potential range from −0.2 to 0.8 V vs. SCE as shown in Fig. S13(A) (ESI†). In this, a well defined redox peak appears and shows an increase in the peak current with the scan rate. Fig. S13(B) (ESI†) shows the linear relationship between peak current versus the square root of the scan rate. In this, the slope of the Ipa curve (6.11 μA) is greater than the slope of Ipc (−5.38 μA) vs. ν1/2 showing that the process of grafting is oxidation with a high electron transfer, and this is confirmed by the negligible increase in the potential difference in peak separation with scan rate. All of these experiments clearly explain that the grafting of dopamine is a surface controlled process on the modified electrode surface.
Fig. S14 (ESI†) compares the FT-IR spectra of both dopamine and grafted dopamine. The intensity and vibrational frequencies of grafted dopamine are found to decrease, which implies the strong attachment of dopamine on the modified electrode surface.46,47 The evidence of the grafting was explained by the various vibrational bands. The broad absorbance band with a high intensity at 3442 cm−1 corresponds to N–H stretching in 1° amines or O–H stretching in alcohols. The band at 2960 cm−1 was attributed to C–H stretching in alkanes and the band at 1736 cm−1 corresponds to CO stretching in carbonyl compounds which confirms the grafting of dopamine on the electrode surface through oxidation. This band disappeared in the case of pure dopamine. The IR peaks at 1604 cm−1 and 1613 cm−1 are expected to be of C
C stretching in aromatics and N–H bending in 1° amines. The observations of these peaks in the dopamine spectrum with higher vibration values describes the oxidative grafting of dopamine with the modified electrode by electrochemical methods.
The surface chemical composition of the ERG/PMB modified electrode was analyzed using the XPS before and after the grafting of dopamine which is a surface sensitive technique. The XPS survey spectra of C 1s, N 1s, O 1s and S 2p elements are shown in Fig. 5. According to the XPS survey, the same elements are present before and after the grafting of dopamine. In Fig. 5(A), the deconvolution of the C 1s spectra demonstrates the following binding energy values, i.e. 284.48 eV ∼ CC or sp2 and C–C or sp3 and C–H because the peaks are not identified separately due to the resolution of the XPS spectrometer, and 285.25 eV ∼ C–N and 286.95 eV ∼ C–O with an atomic percentage of 13.92%.48 After the grafting of dopamine (Fig. 5(B)), a new peak was observed at 288.52 eV with a percentage of 7.75%, which may be identified as C
O, corresponding to the oxidation of the phenolic group in the grafted dopamine due to incoming dopamine molecules, and also 284.56 eV ∼ C can be identified as a phenolic group49 with a high atomic percentage (53.42%). The spectrum of O 1s was observed in both cases, at 530.72 eV and 531.17 eV, corresponding to the C
O group after grafting based on the literature reports.50,51 The N 1s region fitted as two peaks at 398.76 eV and 401.29 eV with a high atomic percentage demonstrates the primary and tertiary amine groups of dopamine as well as MB. This indicates that a large amount of dopamine might undergo grafting.46 The S 2p region is fitted with two peaks assigned to the SO and SO3 groups with atomic percentages of 87.78% and 12.21% respectively. Interestingly, after the grafting of dopamine, the atomic percentage of the former (46.48%) was less than the latter (53.51%). According to this, more oxygen atoms from the incoming molecules of oxidized dopamine might be reacting with sulfur in the methylene blue.52,53
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Fig. 5 XPS spectra of C 1s, N 1s, O 1s and S 2p of the ERG/PMB modified electrode (A) before the grafting of dopamine and (B) after the grafting of dopamine. |
Fig. 6(C) displays the CV curves of the dopamine grafted ERG/PMB modified GC electrode in pH 7.4 PBS containing different concentrations of dopamine. A well defined redox peak was formed in the region of −0.1 to 0.3 V vs. SCE and the peak current increased with the dopamine concentration. This was confirmed by the linear relationship between the oxidation peak current and the dopamine concentration expressed as Ipa = −8.0894 + 0.59105CD (μM) with R2 = 0.99329, as shown in Fig. 6(D).
Fig. 7(C) represents the DPV of dopamine at different concentrations where the AA and UA concentration is kept constant. Here also, the concentration of dopamine increases in the presence of AA and UA, and there is no change in the peak current of both AA and UA indicating the lack of interference with dopamine. In addition, two different linear relationships between the oxidation peak current and concentration from 0.96 × 10−6 mol L−1 to 42.12 × 10−6 mol L−1 with two regression coefficients of 0.9951 and 0.9918 were shown in Fig. 7(D). The detection limit for the determination of dopamine in the presence of AA and UA was calculated as 1.03 × 10−6 mol L−1. As shown in Table 1, the sensing performance of the dopamine grafted ERG/PMB composite modified electrode was compared with that of other electrochemical sensor systems.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25541d |
This journal is © The Royal Society of Chemistry 2016 |