A facile and simple approach to synthesis and characterization of methacrylated graphene oxide nanostructured polyaniline nanocomposites

A novel, scalable methacrylated graphene oxide (MeGO) nanostructured polyaniline (PANI) nanocomposite was synthesized and electrodeposited on the surface of fluorine-doped tin oxide electrode (FTOE). The two-dimensional support maintained a suitable substrate and arrayed in a conductive polymer matrix, creating an ultra-superconductive platform with extraordinary characteristics. The versatility of the nanocomposite performance was corroborated by altering the amount of MeGO coated on FTOE and changing the charge density of electro-polymerized PANI on the substrate. This exceptional nanostructure material enabled a robust platform design that demonstrated the extraordinary performance with enhanced conductivity and stability. Charge transfer resistance (Rct) was dramatically decreased from 11,000 Ω (for bare FTOE) to 65 Ω (for MeGO/PANI).


Introduction
Since the industrialization of synthetic polymers, the polymers possessed sensitive attention in various applications [1,2,[4][5][6][7][8][9][10][11]. For numerous years, there had been a vital requirement for a solid material, which could concurrently afford both processing features of polymers and ex-ceptional electrical properties of metals [12]. Conductive polymers have been used in many applications due to their great sensibility, fast response time, ability to be used at ambient temperatures and low processing costs [13,14]. Among them polyaniline [15,16], polypyrrole [17] and polythiophene [18] have been considerably developed, due to their enhanced and unique characteristics. These include good conductivity toward electric current and ability to change their respective physical and chemical properties in response to an electrical stimulation [19]. Polyaniline is a promising member of conductive polymers family and has received great attention, due to its intriguing properties such as environmental stability, high conductivity, inexpensive starting material, unique redox process, facile synthesis, tunable properties, appropriate electrochemical and environmental stability, low cost, and strong bimolecular interactions [15,16]. Nevertheless, the challenge remains to develop polymeric based nanocomposites that are amenable to the development of devices with broad conductivity and sensitivity [20][21][22].
Graphene and its derivatives are single layer sheets consisting carbon atoms [23], with SP 2 chemical bonds [4] forming a 2-D honeycomb crystal with precise atomic level organization [24,25]. Exceptional characteristics of graphene make an advanced material for several interesting applications such as facile processability, fast production in comparison with other nanoscale carbons [26] and brilliant properties including exceptionally wide porosity, zero band gap [27], superior mobility of charge carriers (200,000 cm 2 v −1 s −1 ) [28], very high specific surface area (2630 m 2 g −1 ), great flexibility [29], unique chemical stability [30], fast electron transfer, prominent conductivity and cost efficiency [31,32]. Due to superior features of graphene and its derivatives [33], some of them have remarkable attention comprising its exploitation as a functional substrate for prevention of nanoscale materials from aggregation and agglomeration [34] in catalysts [35], as a superb conductor in electrical devices and nanobiosensors, bio-acceptors in electrochemical sensing applications [36] and carriers [37] in drug delivery [38,39].
Recently, combining the carbon nanostructures with conductive polymers has attracted much attention. Among them, graphene-grafted PANI nanocomposites have been remarkably significant because of the excellent characteristics of 2D graphene nanosheets and good porosity of nanostructured PANI [40]. This strategy triggers outstanding sensitivity, improved conductivity, more selectivity and high capacity [41]. PANI and graphene possess π-conjugated electrons [30], that is why graphene is able to act as a nucleation center for the matrix, PANI, between the sheets of graphene. Therefore, they likewise establish excellent electron transfer from a reaction [42]. Consequently, improved mechanical strength and electrical conductivity are the main properties that can be expected [43] between graphene nanosheets with exceptional mechanical properties and PANI with high pseudocapacitance [44]. Many studies reported that graphenegrafted PANI composites are able to be utilized as electrode substrates. Recently, graphene/PANI composites, that have excellent electrochemistry and conductivity, are being used in several products such as nanobiosensors [45], electrochemical devices, energy storage equipment [46], etc. Recently, we have synthesized and fully characterized a graphene-grafted PANI composite as well as we developed and applied it for ascorbic acid and cancer detections. Here, we applied a new functionalization for developing graphene-grafted PANI composite.
The studies in the literature used conventional electrodes (e.g. gold and platinum), which are costly and less available. Also the modification of them is expensive and time consuming. In the present research, a simple electrode was used, which can be further amended with the nanocomposite film to increase conductivity. It is also possible to design a disposable sensor according to the structure and cost of FTOE. In summary graphene was functionalized in two stages. Then, the graphene was functionalized with a simple protocol using a low-cost method in the shortest time possible. Finally, PANI synthesized via electro-polymerization has been deposited on the obtained electrode. To the best of our knowledge, the nanocomposite consisting of MeGO and electropolymerized PANI was synthesized for the first time by a novel, simple and cost effective protocol, which can be applied in electrochemical biosensors, cardiac tissue engineering, biofuel cells and super-capacitors due to its extraordinarily high conductivity which can further increase sensitivity.

Preparation of the functionalized graphene, MeGO
Graphite oxide was prepared with the approach, modified Hummer [17]. Then, functionalized MeGO nanosheets were synthesized based on our previous study [20].

FTO modification by MeGO
For fabrication of modified electrode, FTO glass plates (8 Ω resistance) with the surface area of 0.25 cm 2 , were prepared and sequential ultrasonic cleaning performed for 10 min in acetone, ethanol, isopropanol and deionized (DI) water, and then FTO sheets were dried under Ar gas flow. Then, MeGO suspended in DI was deposited (20, 30 and 40 µL) on the FTO surface by the cast coating method and it was allowed to dry at 45 ∘ C and the optimum volume was selected (40 µL).

Characterization
The Fourier transform infrared (FTIR) spectra were recorded in the range of 400-4500 cm −1 with Shimadzu FTIR-8400s spectrometer (SHIMADZU-8400S, Japan). In order to study nanocomposite surface morphology, FE-SEM images, EDX and mapping analysis were taken by Carl Zeiss FESEM instrument model Sigma. All electrochemical studies were performed using a Potentiostat/Galvanostat model of Autolab PGSTAT 30 (Echo chemie, B. V., Netherlands). The software of this device was Nova version 1.7.8. CV curves were obtained in the potential range of −0.8 to 0.8 V with the scan rate of 100 mV s −1 . Electrochemical impedance spectroscopy (EIS) measurements were accomplished in the frequency range of 10 −1 -10 5 Hz, and the potential amplitude was 14 mV around the open circuit potential (Eocp = 0.22 V).

Electro-polymerization of aniline
Aniline electro-polymerization was shown in Figure 1. As it can be seen, three redox peaks are observed, where the first (a and a ′− ) is related to formation of cation-radicals and the second (b and b ′ ) is due to production of byproducts and intermediates. The last peaks (c and c ′ ) are related to the formation of polymer chain propagation [48].   sents the second stage of graphene oxide functionalization. The lack of epoxide ring peak proves that graphene oxide has been reduced to graphene. In the case of PANI peaks appeared in 3200-3600, 1450 and 1750 cm −1 are related to N-H bond, and C=C (PANI benzene ring). Peak in 1200 cm −1 indicates C=O bond in functionalized graphene. Spectrum peaks of PANI and MeGO/PANI exhibits some noise, because the diffuse reflectance spectroscopy (DRS) techniques have been applied to surface spectroscopy.

Microstructure analysis
FESEM analysis was performed to investigate the surface morphology of the nanocomposite and was shown in Figure 3. Graphene nanosheets can be seen in Figure 3a, where graphene nanosheets surface uniformity has changed after its modification with PANI ( Figure 3b). It clearly shows that PANI is flattened on the surface and a porous structure is established.

Electrochemical investigations by cyclic voltammetry and EIS
For electrochemical investigations cyclic voltammetry and EIS techniques were used. For this study cyclic voltammograms in 0.01 M PBS (pH 7.4) containing 5 mM K 3 Fe(CN) 6 were applied on FTO electrode, GO, MeGO, PANI, and MeGO/PANI. Also in another study, for each electrode Nyquist plots were obtained. The results of this experiment are illustrated in Figures 4 and 5 and Table 1 summarizes some important parameters extracted from these figures.
These results indicate that, by modifying the electrode conductivity using surface treatment, resistance considerably decreases and the charge transfer resistance (R ct ) losses. In Nyquist diagram, the semicircle portion observed at high frequencies corresponds to R ct . This resistance can directly be measured as the semicircle diameter. In Figure 4A, a linear part can be seen at low frequencies due to limited mass transfer. This part in Figure 5A is negligible that can be concluded with modifying the electrode surface and increasing the conductivity, upon arrival of Fe(CN) 3− 6 probe to electrode surface, electron transfer occurs with high speed and due to diffusion  layer formation and mass transfer limitation, linear part can be seen clearly. According to Figures 4 and 5, there is a complete conformity between CV curves and Nyquist plots, because with decreasing the R ct , peak height in CV increases and also, the highest charge transfer resistance (R ct ) is for bare FTO (~11,000 Ω) and at lowest (the best) is for the final modifying (MeGO/PANI) (~65 Ω), which is very lower than some previous reports. Table 2 is prepared for comparing this study with several researches. As can be seen in Table 2 R ct ME/ R ct BE or R ct PANI/ R ct BE in nanocomposite are lower than other recent studies. In MeGO/PANI, all components in modifying layer have synergistic effects by decreasing R ct , increasing electrode conductivity and surface area.  In another experiment the stability of PANI, and MeGO/PANI on the FTO surface in 0.01 M PBS containing 5 mM K 3 Fe(CN) 6 was studied by applying successive CVs ( Figure 6). As the Figure shows, when only PANI is used for FTO modification, the stability of electrode response is relatively poor, due to the gradual dissolution of PANI in aqueous medium that would reduce the redox activity. In the case when MeGO is deposited on the electrode surface the stability is very good, and when a composite of both (MeGO/PANI) is used on the electrode surface, stability is considerably better than PANI alone. These results mean that the presence of MeGO improved PANI conductivity in water remarkably. Therefore, it also increases the electrochemical stability, that can be due to stronger interaction between benzene ring and graphene sheets and groups. These tests show an excellent electrochemical stability and repeatability in a neutral solution that is very important for practical applications in biosensors [56].

Conclusions
At first, graphene was functionalized in two stages using a low-cost method in a short time. Then, PANI was electropolymerized on MeGO nanosheets-based electrode that a layer by layer nanocomposite was formed. The results achieved by the experimental and theoretical data showed that MeGO/PANI nanocomposite can act as a successful nanomaterial for electrochemical biosensors, cardiac tissue engineering, biofuel cells and super-capacitors because of its excellent conductivity, which will increase its sensitivity. In future work, the nanocomposite will be used to construct a highly sensitive biosensor for ascorbic acid diagnosis and characterization.