Electrochemical sensing of dopamine at the surface of a dopamine grafted graphene oxide/poly(methylene blue) composite modified electrode

Abstract

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.

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1. Introduction

Graphene¹ is a single layer of graphite consisting of sp² carbon atoms packed in a 2D honeycomb lattice, which has been studied and used for remarkable applications such as in polymer nanocomposites,^2,3 for energy applications,⁴ in sensors,⁵ biomedicine,⁶ liquid crystal devices,⁷ field effect transistors,⁸ flexible materials⁹ and mechanical resonators¹⁰ due to its unique properties^11–16 which include mechanical, electrical, thermal, electronic and optical properties. The hydrophobic¹⁷ nature of graphene tends to cause irreversible agglomerations in aqueous solution due to the strong π–π stacking interactions. This explains the poor solubility of graphene in polymer matrices, which limits its applications. Graphene can be synthesized by both top-down and bottom-up approaches, which includes Chemical Vapor Deposition (CVD), micromechanical cleavage, epitaxial growth on metal surfaces and the reduction of graphite oxide and GO.^18,19 Large scale synthesis of graphene has been carried out through chemical, thermal and electrochemical reduction methods.^20,21 Among these methods, electrochemical reduction is one of the finest routes because it is a simple, time saving and eco-friendly process and through this method one can avoid the usage of environmentally harmful reductants.

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.

Scheme 1 General reaction pathway of electrochemical polymerization and the grafting of dopamine.

2. Experimental section

Materials

Graphite powder (Alfa Aesar), sodium nitrate (Sigma Aldrich), sulfuric acid (AR, Ranbaxy), potassium permanganate (Sigma Aldrich), 30% hydrogen peroxide (Alfa Aesar), monopotassium dihydrogen phosphate and dipotassium monohydrogen phosphate (Extrapure AR, SRL), methylene blue (high purity, Alfa Aesar), potassium chloride (Sigma Aldrich), potassium ferri- and ferrocyanide (SRL), dopamine hydrochloride, ascorbic acid and uric acid (98.5%, Alfa Aesar) were used in the experiments as received. In all of the experiments, distilled water with a conductivity of 36 μS cm⁻¹ was used to prepare the solutions.

Preparation of GO

Graphite oxide was prepared from graphite powder following the procedure adopted in Hummers’ method.³³ 5 g of graphite powder (99.9%) was added to 115 mL of concentrated sulfuric acid containing 2.5 g of sodium nitrate in a 500 ml beaker under vigorous stirring. Once the addition was completed, it was kept in an ice-bath and the temperature was maintained below 10 °C. At this temperature, 15 g of potassium permanganate was added to the solution mixture carefully to prevent the increase of the solution temperature beyond 20 °C. The solution was removed from the ice-bath and kept for 30 min at a temperature of 30–35 °C. A brownish yellow colored paste was formed at the end of 20 min by the elimination of gas. After 30 min, 230 mL of water was added slowly and the temperature was raised to 90 °C and maintained for 15 min. Later, this was further diluted with 700 mL of warm water and eventually the removal of residues was carried out by the addition of 30% hydrogen peroxide. Finally, a brown yellow graphite oxide solid was obtained by filtration followed by centrifugation and was dried at 60 °C for 24 h.

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.

Instrumentation

Laser Raman analysis of the fluorine doped tin oxide (FTO) coated substrate was carried out using a Renishaw Invia Raman Microscope with a He–Ne laser 633 nm (Renishaw, U.K.), and the morphology of the sample coated on the FTO plate was studied using S-3000H SEM with 30–300000× magnification (Hitachi, Japan). The absorption of the MB solution was studied using a UV-VIS-NIR Double Beam Spectrophotometer (Cary 500 Scan Model) and the FTIR spectrum of the sample was analyzed using a BRUKER TENSOR 27 FT-IR Spectrometer (Bruker Optik GmbH, Germany). The chemical composition of the surface modified by electrochemical polymerization and electrooxidative grafting of dopamine onto the FTO coated electrode was studied using XPS techniques (Multilab 2000, Thermo scientific, UK). An Al Kα radiation was used as the X-ray source with a pass energy of 20 eV to record the spectrum with a scan range of 0–1150 eV binding energy, and the pressure was about 3 × 10⁻⁸ Torr. The binding energies of all atoms in the modified surface were referenced using the 1s hydrocarbon peak at 286.4 eV and the XPS spectra was fitted with a Shirley-type background using XPS PEAK 4.1 software with a Gaussian-Lorentzien 60%/40% technique.

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.

3. Results and discussion

Electrochemical preparation of ERG/PMB composite

An ERG/PMB composite was prepared using the electrochemical polymerization method.³⁵ In this method, a drop-casted GO coated GC surface was used as a working electrode and the procedure is as follows. 10 μL of GO solution was coated onto the surface of an ultrasonically cleaned glassy carbon electrode and dried at room temperature for 4 h. The electrolytic solution was prepared with 0.026 mM MB in 0.1 M phosphate buffer solution with a pH of 7.4. Electrochemical polymerization was carried out using potential cycling between −1.5 to 1.3 V vs. SCE at a scan rate of 50 mV s⁻¹. Fig. 1(A) shows the cyclic voltammogram of the electrochemically reduced ERG/PMB composite prepared via the electrochemical polymerization method. It can be demonstrated that the position of the oxidation peak and the reduction peak moved towards higher values from the 1^st cycle to the 20^th cycle.

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 1^st and 20^th cycles of electrochemical polymerization. During the 1^st 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 1^st to 20^th cycles, which suggests that the electrochemical reduction of GO was complete.³⁸ In the 20^th 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 1^st to 20^th 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)

where A is the area of the GCE (0.07065 cm²), F is the Faraday constant (96 485.34 C mol⁻¹), Γ is the surface coverage concentration and n is the number of electrons involved in the electrochemical polymerization of MB (n = 2). The value of the charge (Q) calculated from the integration of the peak area between −0.37 to 0.48 V vs. SCE at a scan rate of 50 mV s⁻¹ is 8.314 μC. Using the above values, the Γ value calculated for the polymer film is 6.13 × 10⁻¹⁰ mole cm⁻². Finally, the Γ value demonstrates a good surface coverage of PMB film on the modified electrode surface. From this, we clearly observe that 10 μL of GO was covered by 6.13 × 10⁻¹⁰ mol cm⁻² of MB.

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 R_ct 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.

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 : 1) 2 mM L⁻¹ [Fe(CN)₆]^3−/4−.

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 C=C 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.

Fig. 3 (A) Raman spectra of (a) PMB, (b) ERG and (c) the ERG/PMB composite, (B) SEM images of (a) PMB, (b) ERG and (c) the ERG/PMB composite and (C) UV-Visible absorption spectra of MB solution (a) before potential cycling and (b) after potential cycling.

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

Electrografting of dopamine on the ERG/PMB composite modified electrode

Fig. 4(A) shows the CV curves of the 1 × 10⁻³ mol L⁻¹ dopamine in 0.1 M sulfuric acid as electrolyte. The electrografting of dopamine was performed by 10 successive cycles in the potential range of −0.2 to 1.6 V vs. SCE with a scan rate of 50 mV s⁻¹.

Fig. 4 (A) Grafting of 1 × 10⁻³ mol L⁻¹ dopamine in 0.1 M H₂SO₄ solution for 10 successive cycles at a scan rate of 50 mV s⁻¹, (inset) 1^st, 20^th, and 50^th cycles of the grafted dopamine ERG/PMB composite modified electrode and (B) 1 × 10⁻³ mol L⁻¹ dopamine with (a) the GC electrode and (b) the ERG/PMB composite modified electrode. Scan rate 50 mV s⁻¹.

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 1^st, 20^th and 50^th 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 1^st to 50^th 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 I_pa curve (6.11 μA) is greater than the slope of I_pc (−5.38 μA) vs. ν^1/2 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.

Surface chemical composition investigation

The surface chemical composition of the ERG/PMB modified electrode and the electrografted dopamine modified ERG/PMB modified electrode was analyzed using XPS, and dopamine and grafted dopamine were studied using FT-IR spectroscopy.³¹ For these studies, the FTO coated electrode was used as a working electrode which will not change the electrochemical behavior of the coated material instead of the GC electrode, and followed the same procedure described above.

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 C=O 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 ∼ C=C 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

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.

Electrocatalytic activity of the dopamine grafted ERG/PMB modified electrode towards the sensitivity of dopamine

Fig. 6 shows the electrochemical response of the bare GC, ERG, PMB, ERG/PMB and dopamine grafted ERG/PMB modified GC electrodes (A) without dopamine and (B) with 118.5 × 10⁻⁶ mol L⁻¹ dopamine in pH 7.4 phosphate buffer solution at a scan rate of 50 mV s⁻¹. In Fig. 6(A), the background current of the dopamine grafted ERG/PMB modified electrode was high and one redox peak was observed in the range of −0.2 to 0.2 V vs. SCE, demonstrating the attachment of dopamine on the surface of the ERG/PMB modified GC electrode. Fig. 6(B) shows the electrocatalytic activity of the five electrodes towards dopamine. In this, the dopamine grafted ERG/PMB modified GC electrode shows a higher oxidation current of dopamine between the −0.1 to 0.3 V vs. SCE region than the remaining modified electrodes, which explains that the surface modified with grafted dopamine is highly active.

Fig. 6 CVs of (a) GC, (b) ERG, (c) PMB (d) ERG/PMB and (e) dopamine grafted ERG/PMB modified GC electrodes (A) without dopamine, (B) with 118.57 × 10⁻⁶ mol L⁻¹ dopamine, (C) CVs of 19.96 × 10⁻⁶ mol L⁻¹, 39.84 × 10⁻⁶ mol L⁻¹, 49.75 × 10⁻⁶ mol L⁻¹, 59.64 × 10⁻⁶ mol L⁻¹, 69.5 5 × 10⁻⁶ mol L⁻¹, 79.36 × 10⁻⁶ mol L⁻¹, 89.19 × 10⁻⁶ mol L⁻¹, 99.00 × 10⁻⁶ mol L⁻¹, 108.80 × 10⁻⁶ mol L⁻¹, and 118.57 × 10⁻⁶ mol L⁻¹ dopamine at the dopamine grafted ERG/PMB modified GC electrode with a scan rate 50 mV s⁻¹, and (D) the relation between I_pa and the concentration 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 I_pa = −8.0894 + 0.59105C_D (μM) with R² = 0.99329, as shown in Fig. 6(D).

Determination of dopamine at the dopamine grafted ERG/PMB modified electrode

Dopamine was determined using DPV because of its high resolution and high current sensitivity compared to CV. Fig. 7(A) shows the DPV responses of the dopamine grafted ERG/PMB modified GC electrode toward different concentrations of dopamine from 0.0 × 10⁻⁶ mol L⁻¹ to 47.7 7 × 10⁻⁶ mol L⁻¹ in the −0.4 to 0.7 V vs. SCE region. In this, the anodic peak current of dopamine increases with each addition. Hence, by plotting the curve between peak current and concentration, two different linear curves were observed.^54,55 The first one corresponds to the lower concentration range from 5.99 − 11.98 × 10⁻⁶ mol L⁻¹ and the second one corresponds to the higher concentration range from 13.98 − 47.77 × 10⁻⁶ mol L⁻¹ with I_pa = 25.43 + 1.75C_D (R² = 0.9958) and 41.76 + 0.39C_D (R² = 0.9962). From this, we can explain that the dopamine was oxidized to dopaminoquinone and leucodopaminochrome due to the high catalytic activity of the grafted dopamine on the ERG/PMB modified surface. This also happened in the presence of AA and UA, as shown in Fig. 7(C).

Fig. 7 DPVs of (A) 0.00 × 10⁻⁶ mol L⁻¹, 5.99 × 10⁻⁶ mol L⁻¹, 7.99 × 10⁻⁶ mol L⁻¹, 9.99 × 10⁻⁶ mol L⁻¹, 11.98 × 10⁻⁶ mol L⁻¹, 13.98 × 10⁻⁶ mol L⁻¹, 15.98 × 10⁻⁶ mol L⁻¹, 19.96 × 10⁻⁶ mol L⁻¹, 23.94 × 10⁻⁶ mol L⁻¹, 27.92 × 10⁻⁶ mol L⁻¹, 31.89 × 10⁻⁶ mol L⁻¹, 35.87 × 10⁻⁶ mol L⁻¹, 39.84 × 10⁻⁶ mol L⁻¹, and 47.77 × 10⁻⁶ mol L⁻¹ dopamine on dopamine grafted ERG/PMB modified GC electrode in 0.1 M PBS (pH 7.4), (B) relationship between the oxidation peak current and the concentration of dopamine, (C) DPVs of 0.96 × 10⁻⁶ mol L⁻¹, 1.92 × 10⁻⁶ mol L⁻¹, 3.84 × 10⁻⁶ mol L⁻¹, 5.76 × 10⁻⁶ mol L⁻¹, 7.68 × 10⁻⁶ mol L⁻¹, 9.60 × 10⁻⁶ mol L⁻¹, 11.52 × 10⁻⁶ mol L⁻¹, 13.44 × 10⁻⁶ mol L⁻¹, 15.36 × 10⁻⁶ mol L⁻¹, 17.27 × 10⁻⁶ mol L⁻¹, 19.19 × 10⁻⁶ mol L⁻¹, 26.85 × 10⁻⁶ mol L⁻¹, 34.49 × 10⁻⁶ mol L⁻¹, and 42.12 × 10⁻⁶ mol L⁻¹ dopamine on dopamine grafted ERG/PMB modified GC electrode in the presence of 96.14 × 10⁻⁶ mol L⁻¹ AA and 288.43 × 10⁻⁶ mol L⁻¹ UA in 0.1 M PBS (pH 7.4), and (D) relationship between the oxidation peak current and the concentration of dopamine. Step height: 5 mV; step width: 1 s; pulse width: 200 ms; pulse height: 50 mV; and start ramp: 10 mV s⁻¹.

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.

Table 1 Comparison of sensing performances of different modified electrodes for the determination of dopamine

S. No.	Analytical method	Material used	LOD [μM]	Reference no.
1	DPV	PMB-GR/CILE	0.0056	56
2	DPV	(NG/PEI)5/GCE	0.5	57
3	Amperometry	PEDOT/RGO	0.039	58
4	LSV	PDA-RGO/Au	3.211	59
5	DPV	Dopamine grafted ERG/PMB	1.03	This work

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4. Conclusion

An ERG and PMB composite was prepared using an electrochemical redox method through potential cycling between −1.5 to 1.3 V vs. SCE. The decrease in the concentration of MB during cycling was monitored using UV-Visible absorption spectroscopy. The formation of the polymer composite was evidenced using electrochemical impedance spectroscopy, Raman and SEM. Electrografting of dopamine was done on the ERG/PMB composite modified electrode surface and the redox behavior was explained by potential values. Finally, the grafting of dopamine on the surface of the modified electrode was demonstrated using FT-IR spectroscopy and XPS analysis. The catalytic activity of the grafted dopamine was higher than that of the other modified electrodes during the sensing of dopamine in the presence of UA and AA through CV studies. The determination of dopamine and its interference with UA and AA was also studied in detail using the DPV technique with a detection limit of 1.03 × 10⁻⁶ mol L⁻¹. It is very clear from this study that the dopamine grafted ERG/PMB composite modified electrode for dopamine sensing exhibits a much lower LOD than the PDA–RGO/Au system. Hence, we have successfully explained that the formed ERG/PMB grafted dopamine composite surface acts as a catalytic platform for the sensing of dopamine. The selectivity of dopamine with a linear range in calibration was between 0.96 × 10⁻⁶ mol L⁻¹ and 7.68 × 10⁻⁶ mol L⁻¹.

Acknowledgements

Demudu Babu Gorle gratefully acknowledges UGC, New Delhi for his Research Fellowship. The authors also thank Dr Vijayamohanan K. Pillai, Director, CSIR-CECRI, for his keen interest and encouragement of our research activities. This research was financially supported by the Department of Science and Technology, New Delhi (GAP 08/13).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25541d

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