Electrochemical method is a simple and ecofriendly alternative for graphene oxide synthesis.

El método electroquímico es una alternativa sencilla y ecológica para la síntesis de óxido de grafeno.

Health-care-associated infections, which are acquired during hospital stays, could represent a serious risk to human health [1]. The pathogens responsible for this kind of infections (also known as nosocomial) include bacteria, viruses, and fungi. The most common bacterium is Escherichia coli (E.Coli), and it causes acute urinary tract infections, urinary tract sepsis, and neonatal meningitis [2]. The World Health Organization (WHO) estimates that, in North America and Europe, nosocomial infections cause 5 % and 10 % of the hospitalizations, respectively. In Latin America and Asia, the proportion of hospitalizations due to the same cause is approximately 40 % [3].

Accordingly, the general interest in developing antibacterial materials has grown in recent years. Hybrid nanomaterials have potential in this regard because they facilitate the use of nanostructured materials with specific properties and excellent mechanical properties, as well as improved biocompatibility and antibacterial activity. This makes them good prospects in tissue engineering [4].

On the one hand, graphene oxide (GO) has attracted special attention because it is a carbon atom monolayer that forms a dense honeycomb structure containing carboxylic groups on the edges, as well as hydroxyl and epoxide groups on its two accessible sides. GO has excellent physicochemical properties, large specific surface, mechanical resistance, electrical conductivity, and stability in water [5], [6]. However, its antibacterial activity is quite controversial, and some authors have shown that it is related to the size and shape of GO [7], [8].

Nowadays, Hummers’ method is the most widely implemented strategy to synthesize GO used in bacterial applications [9], [10]. Nevertheless, it uses dangerous reagents that are harmful to the environment and humans [11]. Electrochemical exfoliation of graphite is an alternative method for obtaining graphene oxide; it is simple and requires less reagents for the synthesis process than Hummers’ method [12], [13].

On the other hand, titanium dioxide (TiO.) is considered a suitable material for medical applications due to its low toxicity, excellent thermal properties, and chemical stability. Additionally, TiO. exhibits antibacterial activity against E. Coli due to oxidative damage to the cell membrane [14], [15]. Thanks to these properties, TiO. has been used in several applications, such as microbatteries and as UV absorber in cosmetic products, anticorrosive coating, and antibacterial coatings [16], [17].

The main objective of this study was to evaluate the antibacterial activity of synthesized graphene oxide (GO), titanium dioxide (TiO.) and a TiO./GO hybrid. The combination of two types of nanoparticles with different morphologies and dimensions is a new way to produce functional hybrid materials with a synergistic improvement in material performance.

Recently, several hybrids composed of GO and conventional semiconductors demonstrated significantly enhanced photocatalytic performance during pollutant degradation. Specifically, TiO./GO is a hybrid that could be considered as a promising material photocatalysis and hydrogen evolution [18]–[20], solar mineralization [21], and others fields of engineering such as tissue engineering as hybrid filler of functional nanocomposites [22].

Escherichia coli was the model bacteria used here to evaluate the bactericidal effect of the three materials. This study was conducted to contribute to the ongoing discussion about the antibacterial properties of GO. In addition, to the best of the authors’ knowledge, the antibacterial properties of GO synthetized by the electrochemical method had not been reported before. The electrochemical method can be used as an alternative to produce GO because it is sustainable and more environmentally friendly.

Figure 1a and 1b show the X-ray diffraction pattern of TiO. nanoparticles and the Raman spectra of graphene oxide, respectively. The XRD pattern (Figure. 1a) shows anatase-phase TiO. with characteristic diffraction peaks of 20 values at about 25.3°, 37.8°, 48°, 53.9° and 55°, they are attributed to the (101), (004), (200), (105) and (211) planes, respectively. These results are in agreement with those reported by. [27] and match standard ICDD card No. 01-073-1764 for anatase.

Figure 1b shows the Raman spectra of graphene oxide, with three characteristic bands: D-band (~1339 cm^-1), G- band (~1583 cm^-1) and 2D-band (~2686 cm^-1). The G-band represents the sp. hybridization of carbon atoms [28], while the D-band is related to changes in the hybridization of carbon atoms (sp. to sp.) [29]. The change in the hybridization can be related to oxygen functionalities in graphene oxide [30]. The D/G ratio is commonly used to evaluate the number of defects in graphene oxide. The GO obtained here exhibited a D/G ratio of 0.61, which is consistent with what was found [31]. Additionally, this D/G ratio is lower than that of the GO obtained by Hummers’ method [32], [33]. The 2D/G ratio is used to estimate the number of layers in graphene oxide. In this case, the GO presented a 2D/G ratio of 1.41, which suggests the presence of bilayer graphene oxide [34].

Figure 1. Figure 1. a) XRD pattern of TiO2 nanoparticles; (b) Raman spectra of GO. Source: Own elaboration.

Figure 2 shows the FTIR spectra of TiO., GO and the TiO./GO composite obtained in this study. All the samples present a band between 3000 cm^-1 and 3377 cm^-1, which corresponds to the stretching vibration of the O-H bond. The spectrum of TiO. shows a band around 1610 cm^-1related to the bending vibration mode of the O-H bond, and characteristic bands of titanium dioxide are located at 400 cm^-1 and 800 cm^-1. The spectrum of GO shows several bands, but the one at 2939 cm^-1 is due to C-H stretching vibration, which occurs during the synthesis process [35]. The bands at 1380 cm^-1, 1220 cm^-1 and 1060 cm^-1 correspond to C-O, C-OH and C=O functional groups [36], [37]. The spectrum of TiO./GO exhibits new bands at 1690 cm^-1 and 1155 cm^-1, which are related to C=O and Ti-O-C cm^-1, respectively [38]. The presence of Ti-O-C and C=O functional groups demonstrated the interaction between oxygen functionalities of graphene oxide and TiO. nanoparticles [39].

Figure. 2. Figure. 2. FTIR of the materials synthesized in this study: TiO2/GO, GO and TiO2. Source: Own elaboration.

The thermal stability of the GO and TiO./GO was examined by TGA. Figures 3a and 3b show the TGA and DTG results of each type of sample. The weight loss below 100 °C is due to the decomposition of interstitial water [40]. Also, the weight losses between 130 °C and 250 °C correspond to hydroxyl groups (OH) and the initial degradation of carbonyl groups (C=O) [41], [42]. All the samples presented a weight loss in the thermogravimetric curve between 290 °C and 800 °C; however, the change is not significant, and it cannot be observed in the DTG curve. This weight loss is due to the partial elimination of epoxy groups (C-O) [43]. The amount of TiO. was determined at 23.2 wt. % by the weight of the inorganic residues at the end of the TGA analysis. The increase in the thermal stability of TiO./GO could be due to the presence of Ti-O-C bonds [44].

Figure. 3. Figure. 3. TGA and DTG spectra of GO and the TiO2/GO composite. Source: Own elaboration.

TGA and FTIR analyses confirmed the presence of oxygen-containing functional groups. These oxygen functionalities can be related to the oxidation of graphite by hydroxyl anions. The OH- anion attacks the sp. carbon atoms at the edge of graphite rods, causing the expansion of layers [45]. The presence of oxygenated functional groups favors the stability of GO in an aqueous medium since these groups make GO hydrophilic [46], [47].

The morphology of the materials analyzed in this study was evaluated by SEM and TEM. Figures 4a, 4b, and 4c are SEM micrographs of TiO., GO and the TiO./GO composite, respectively. Figure 4a reveals that TiO. nanoparticles have a spherical morphology. On the other hand, the micrograph of GO (Figure 4b) shows a thin graphene oxide layer; as a result, the background is visible. The micrograph of TiO./GO (Figure 4c) presents TiO. nanoparticles deposited on a GO layer. In addition, the TEM micrograph of GO (Figure 4f) confirms the exfoliation of graphite rods, which is a that involves the evolution of several gaseous species (SO., O., CO and CO.) between layers. These species come from the reduction of the SO.^-2 anion, water oxidation and carbon corrosion [48]. In addition, the TEM micrographs and the Raman analysis confirm the presence of few-layer graphene oxide.

The average diameter of the TiO. particles in the SEM images was measured by ImageJ software using more than 250 nanoparticles. Figure 4g shows the mean particle size: 24.1 ± 4.6 nm.

Figure 4. Figure 4. SEM micrographs of (a) TiO2, (b) GO and (c) the TiO2/GO composite. TEM micrographs of (d) TiO2, (e) GO and (f) the TiO2/GO composite (g) Histogram of TiO2 particle size distribution. Source: Own elaboration.

The chemical composition of the TiO./GO composite was obtained by Energy Dispersive X-ray Spectroscopy (EDS). Figure 5 shows the EDS spectrum of the hybrid nanomaterial NM and Ti; O and C were found in the sample. Additionally, sodium (Na), which can also be observed in the spectrum, is related to remnants of electrolyte (Na.SO.) used during the synthesis of graphene oxide.

Figure 5. Figure 5. Representative energy dispersive spectrum (EDS) of the TiO2/GO composite. Source: Own elaboration.

Figure 6 shows the element distribution map obtained from the TiO./GO composite. The map identified the same elements found in the EDS spectrum (C, Ti, O and Na). In addition, the maps clearly show that the nanoparticles of titanium dioxide are located on the surface of the GO layers.

Figure 6. Figure 6. Representative element distribution map of the TiO2/GO composite. Source: Own elaboration.

The antibacterial activity of the samples prepared here was determined by the macrodilution method. Figure 7 shows the bacterial viability of E. coli under different concentrations of GO, TiO. and TiO./GO. In the results, the graphene oxide sample presents an increase in the viability of E. Coli when the concentration of the treatment is higher.

Figure 7. Figure 7. Viability of E. coli subjected to GO, TiO2 and the TiO2/GO composite. Source: Own elaboration.

However, the TiO. nanoparticles show an inverse trend, i.e., the viability of E. Coli decreases as the concentration of nanoparticles increases. At the lowest concentration (i.e., 125 µg/mL), the viability of the bacteria in the TiO./GO hybrid material was 23 % and 17 % lower than in the GO and TiO. samples, respectively. Nonetheless, at higher concentrations (250, 500 and 1000 µg / mL), the antibacterial activity increased in GO samples but decreased in samples containing TiO. nanoparticles. These results indicate that each treatment follows a different trend in terms of bactericidal activity as a function of concentration.

The antibacterial activity of graphene oxide (GO) reported in several papers is controversial. Nevertheless, such activity can be explained by three main mechanisms: membrane rupture, oxidative stress and isolation of bacteria envelope [49]–[51]. The high antibacterial efficacy of graphene oxide is due to the damage it causes to cell membranes through the generation of reactive oxygen species and sharp edges of graphene oxide [49], [52], [53]. In contrast, a large number of studies indicate that this material can promote bacterial growth due to its surface functional groups and because it can act as a scaffold for bacterial attachment, proliferation, and biofilm [49], [54]–[57], which could explain the results reported in this paper.

On the other hand, TiO. nanoparticles affect biological systems since their photocatalytic activity generates potential reactive oxygen species (ROS) on their surfaces. ROS cause peroxidation of phospholipids in the cell membrane, inducing its breakdown [58], [59]. The membrane rupture causes the interruption of cellular respiration. For this reason, TiO. nanoparticles have a strong antibacterial activity, which is in line with the results reported here, where they achieved the greatest reduction in bacterial viability.

Finally, the behavior of the composite material was similar to that reported by [50] and [60] because carbon blocks the active sites on the surface of TiO.. Therefore, the contact between the TiO. surface and the bacteria can be very limited.

In addition, MIC50 is the minimum amount of drug/compound to inhibit 50 % of microorganism growth and was calculated. GO MIC50 could not be determined because bacterial viability does not decrease with increasing concentration. The MIC50 of TiO./GO and TiO. was 4575 µg/mL and 1728 µg/ mL, respectively, which confirms that TiO. shows a higher bactericidal activity than GO and TiO./GO.

This study assessed the effect of TiO., GO, and TiO./GO nanocompounds on bacterial activity against Escherichia coli. Using the hydrothermal method, TiO. nanoparticles were obtained with an anatase phase and an average size of 24.1 ± 4.6 nm. Graphene oxide was obtained through electrochemical exfoliation, a simple and environmentally friendly method that induced a lower amount of oxygen-containing functional groups on the GO surface. A 23.2 % TiO. content was evidenced in the developed hybrid nanomaterial. Furthermore, GO promoted bacterial growth due to its surface functional groups. TiO. exhibited stronger bactericidal activity than the TiO./GO compound. This behavior may be associated with the decreased contact between TiO. and bacterial cells due to the blocking of active sites on the TiO. surface by graphene sheets.