MODIFICATION OF GRAPHENE OXIDE SURFACE BY DOPAMINE HYDROCHLORIDE

Polydopamine is an organic substance that is almost universally applicable to cover all kinds of materials, including nanomaterials. Considering the possible use of polydopamine in the water purification process, this article focuses on the change of selected properties of the chosen material (graphene oxide and the hybrid compound graphene oxide-C60) after being coated with polydopamine. Attention was paid to changes in thermal stability and surface morphology. In particular, changes in surface morphology could positively affect the sorption capabilities of the original materials in relation to organic pollutants.


INTRODUCTION
Dopamine (DA) is a biologically active substance from the catecholamine group. It is a neurotransmitter that is naturally produced in the nervous system of humans and animals. At the same time, it can be easily synthetized in large quantities. It is easy to prepare polydopamine (PDA) from dopamine and other catecholamines, [1,2]. It is currently being studied in detail for its unique properties, which are very similar to the adhesive proteins of marine shells. Adhesive proteins allow mussels to interact with the substrate in moist environments, and they are rich in 3,4-dihydroxyphenylalanine (DOPA) as well as cysteine and lysine [1,4]. The catechol side chain of DOPA is responsible for the strong bond between the surface of a bivalve leg and the substrate, as well as for the rapid solidification of the adhesive proteins. Catechol forms strong covalent and non-covalent interactions with the surface [5]. The synergistic effect of catechol and amine moiety further enhances biomolecular adhesion [1,3]. These strong interactions with surfaces are also generated by PDA. Therefore, PDA is seen as an organic substance that is almost universally applicable to cover all kinds of materials. The thickness of the PDA layer can be influenced by the reaction conditions (reaction time, temperature, pH) [1,5,6]. Following PDA application, new functional groups (mainly hydroxyl or other, depending on the pH) appear on the surface of a material increasing its hydrophilicity. These functional groups can be subsequently used for further surface modifications [3]. PDA demonstrates considerable chemical and environmental stability [7]. Pyrolysis at temperatures around 800 °C produces a material with a graphene-like structure, where a nitrogen atom is irregularly embedded in the carbon skeleton. The obtained material is electrically conductive [1,6]. PDA can be applied in many fields due to its potential to undergo further surface modifications and its ability to adhere to organic and inorganic substances. In biomedicine, for example, it is used to prepare nanocapsules for drug delivery or to reduce the immunogenicity of materials by improving their hydrophilicity and biocompatibility [8]. PDA can be widely used in wastewater treatment to remove heavy metals, organic pollutants, dyes and radioactive isotopes [9,[10][11][12].
Our experiments focused specifically on the potential applications in water treatment. Graphene oxide (GO) [13,14] and its hybrid compounds [15,16] demonstrate sorption capabilities. It can be assumed that the reaction with DA will increase these abilities as a result of surface modification. Since we consider using both materials for sorption of organic pollutants present in water, it is necessary to have characteristics of both the input materials and of the materials with modified surfaces. For the pilot characterization we have chosen to use FTIR, TGA, DSC and microscopic methods (optical microscopy, TEM).

EXPERIMENTAL PART AND RESULTS
GO was prepared by oxidating finely ground graphite (0.025 mm) according to the classical Hummers method (H2SO4, NaNO3, H2O2, KMnO4, HCl) [17]. The oxidation product was repeatedly centrifuged until a negative reaction to sulfate ions was obtained. The prepared GO (0.389 g) was subsequently used for the reaction with dopamine hydrochloride (0.309 g). PDA was synthesized via DA self-polymerization. The reaction was carried out in water (30 ml) with an addition of hydrogen peroxide (30%; 0.4 ml). The suspension was agitated for 19 h and the pH was adjusted to 8-9 using NaHCO3. The resulting suspension was washed with water and ethanol on a filter and then dried at 55 °C on a Petri dish. The hybrid compound GO-C60 (3:1) was prepared by the simultaneous oxidation of graphite with fullerene C60 in a weight ratio of 3:1. The procedure is described in Roupcová's dissertation [18]. The obtained GO-C60 (3:1) (0.350 g) was used for the reaction with dopamine hydrochloride (0.3 g). The reaction was carried out in water (30 ml) with an addition of hydrogen peroxide (30%; 0.4 ml) for 4 h. The pH was adjusted to 8-9 using NaHCO3. The obtained suspension was washed with water and ethanol on a filter and then dried at 55 °C on a Petri dish.

Characterization of materials by FTIR
Fourier transform infrared spectroscopy (FTIR) of GO and GO-PDA in KBr pellets was conducted on Digilab Excalibur FTS 3000 MX model spectrometer (USA) with a scanning range of 4000-600 cm -1 . The obtained spectra are presented in Figure 1. Although the spectrum of the GO-PDA product has different absorbance intensities, it also features characteristic vibration values for oxygen functional groups (1720 cm -1 , 1575 cm -1 , 1390 cm -1 , 1210 cm -1 , a broad band in the region  3050 to 3450 cm -1 ). No significant absorbance bands were identified in the spectrum for C-N or N-H bonds which should be present in the broad band ( 3050 to 3500 cm -1 ). This is caused by the overlapping of contributions of different functional groups with similar absorption energies. The obtained results are consistent with those published by other teams [19,20]. Insignificant peaks are visible at 1500 cm -1 and 1680 cm -1 which correspond to other characteristic bands for N-H and C-N bonds, respectively.

TDA and DSC analysis of GO and GO-PDA
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Mettler Toledo STAR e , temperature range 25 -750 °C (20 K/min, Air 50 ml/min). The decomposition was examined in air atmosphere. The thermal decomposition curves of GO are shown in Figure 2, and of GO-PDA in Figure 3.
The GO decomposition showed one endothermic effect with the maximum at 77 ⁰C and two continuous exothermic effects [with the maximums at 236 ⁰C and 537 ⁰C]. The overall thermal effect of the composition is 13003 J/g. The thermal decomposition of the GO and DA product shown in Figure 2 starts from 90 ⁰C with a slight exothermic effect which reaches the maximum at 207 ⁰C. From this maximum to 440 ⁰C there is a steady 9.73% decline, and the second exothermic effect begins with the maximum at 566 ⁰C. Its shape is a typical Gaussian curve. The weight loss of the sample at the second exothermic effect is 63.22 % and the overall thermal effect is 14590 J/g.

Figure 2
Thermal analysis data measured for GO (TGA/DSC curves)

TDA and DSC analysis of hybrid compounds GO-C60 and GO-C60 -PDA
In her thesis Roupcová reports that the hybrid compound GO-C60 (3:1) decomposes with two exothermic effects. The maximum weight loss (51.1 %) occurs during the first exothermic effect (197 -205 °C) with the maximum at 205 °C. The second exothermic effect occurs in the range 281-491 °C with the maximum at 390 °C (18 % weight loss) [14]. The thermal decomposition of GO-C60 -PDA occurred essentially in two continuous exothermic effects (Figure 4). The maximum weight loss (48.1 %) occurred during the first part of the exothermic effect in the temperature range 215-380 °C. The second part of the exothermic effect followed in the 380-561 ⁰C range, with three maximums (430 °C, 456 °C and 470 °C) and with a cumulative weight loss of 32.7 %. The results described above indicate that the PDA coating increased the thermal stability of the original GO-C60.

Morphology
Microscopic images (Twist Digital Microscope, Learning Resources) of the dried suspension of GO and GO-PDA on a Petri dish show different surface morphologies of GO and GO after reaction with DA (GO-PDA) even at low magnification (20x) (Figure 5). The PDA coating increased the surface roughness. Transmission electron microscopy (TEM) (HT7800, Hitachi, accelerating voltage 100 kV) was used to investigate the GO-PDA and GO-C60 -PDA morphology. Nanoparticles present in the filtrate after filtration of prepared suspensions were used for the investigation. TEM image of GO-PDA and GO-C60 -PDA are shown in Figure 6. It is obvious that nanoparticles of both materials are mostly spherical in shape and form aggregates

DISCUSSION AND CONCLUSION
The FTIR spectrum of GO contains well-resolved bands, which correspond to vibrations of individual types of bonds in the compound. The spectrum of GO-PDA is quite different. It is formed by broad bands that overlap. These bands correspond to different bond types and oxidation states, different cyclic structures (both heterocyclic and aromatic), and to a variety of bridging bonds and positions. The reason is the PDA structure.
The PDA polymer consists of several various monomer units, which are diversely grouped and interconnected and are in different oxidation states [1]. The results of the GO thermal analysis are consistent with those obtained by other research teams [20,21]. The curves suggest that the reaction with DA resulted in a significant reduction of the epoxy bonds (C-O-C) present in both the initial GO and GO-C60. The PDA covering also increased thermal stability of both the initial materials. The PDA covering resulted in a change of the surface morphology. As reported by Bogdan et al. [22], the PDA coating is always made up of two components. The first is a quasi-flat film with intrinsic roughness in the order of 1 nm. The second consists of PDA aggregates lying on top of this film. Larger aggregates can be easily separated by sonication. Therefore sonication may be used to achieve a more even covering and to reduce roughness [22]. Since the GO used in our experiments was not sonicated, we assume that this fact may have influenced the newly formed surface structures. Different monomer units or PDAs could have intercalated between the oxidized carbon layers of the initial graphite. As reported by previous research, thickness of the PDA layer can be influenced by reaction conditions (reaction time, temperature, pH) [1,5,6]. For example, Jia et al. [23] reported that the PDA layer may be up to 50 nm thick. Bogdan et al. [22] therefore suggest that morphological characteristics of PDA coverings should be evaluated based on an agreed standardized protocol. Based on the results of the performed experiments, we assume that the coverings of GO and of the hybrid compound GO-C60 changed their sorption properties. The introduction of novel functional groups will allow the formation of new non-covalent bonds with the surface of materials. Therefore the simultaneous use of initial materials and materials with a modified surface could expand the application potential of GO and its reduced forms.