2.1. Characteristics of the Selected Commercial Graphene Oxide Derivatives
The physical-chemical properties of the graphene oxide derivatives selected for this study were recently determined [
4]. Microscopy analyses using AFM and TEM instruments showed that GO and GOC flakes were mostly in monolayer state and had a different size, while the analysis of their composition revealed a high similarity between both nanomaterials. In the present study, the same commercial nanomaterials’ suspensions were selected, but a new batch of the GOC material was used (for more details see the Materials and Methods section). Therefore, we decided to perform a new microscopy and spectroscopy analysis to confirm the physico-chemical properties of the new GOC sample. Surprisingly, new AFM and TEM analyses revealed that the nanoparticles of the new GOC batch were morphologically very different to the older GOC batch (GOC
o) (
Supplementary Figure S1), showing instead a high similarity in morphology and size to that observed on the monolayer GO particles (
Figure 1).
AFM topography imaging showed that both nanomaterial types have a wide lateral size distribution, ranging from the nanometric to the micrometric scale, while the flakes thickness is around 1–2 nm. Graphene oxide nanomaterials of similar characteristics have been reported to produce membrane-damaging activity in different unicellular systems [
25,
27,
28].
The FTIR spectra of GO and the new GOC batch was determined as well, and both nanomaterials showed to be very similar in their oxygen functional groups content (
Figure 2). Following the tentative assignments given in the figure, the most significant difference found between GO and GOC was that the former showed a slightly greater content in ether/alcoxy groups than the latter, which could be related with the increase in the intensity of ν(C–O) stretching modes reported by other authors [
29].
The results obtained indicate that the reproducibility in the production of commercial graphene oxide may still have relevant issues, making essential for the end user to confirm that the purchased product matches with the expected characteristics.
Since the presence of trace metal impurities in graphene derivatives, either contained in the graphite precursor or transferred by reactants used in the nanomaterial preparation, has been previously described, a trace element analysis of GO and GOC was done by inductively coupled plasma mass spectrometry (ICP-MS). As shown in
Table 1, the presence of different metallic elements was observed in GO and GOC, although the concentration of most of them was found to be low. Nevertheless, significant differences in the concentration of some of the identified metals and metalloids were observed between both nanomaterials.
Overall, the concentration of metallic elements was higher in GOC than in GO. Both nanomaterials showed to have a high content of Mn (GO: 34.700 ppm; GOC: 62.405 ppm) and K (GO: 3.770; GOC: 2.628 ppm), which suggests they were obtained through the Hummer’s method, which is the most common oxidation method currently used for GO production and known to result in residual manganese accumulation because of the use of permanganate oxidant (KMnO
4) [
30]. Additionally, ICP-MS data suggested the possible presence of S in both nanomaterials, which can be present as well in graphene oxide prepared through the Hummer´s method, being its content significantly higher in GO. However, the obtained results in case of GOC were close to the background noise. For this reason, to get further insight into the possible presence of sulfur species and the differences in their content between GO and GOC, XPS analysis was performed. Again, the obtained results indicated that S species were higher in GO (relative atomic percentage: 0.6%) than in GOC, where a reliable quantitative value could not be determined. The presence of organosulfate groups in graphene oxide is described, and suggested to be responsible for part of the reactivity of this nanomaterial, such as in the immobilization of adsorbed species [
31]. However, we could not get insights on the type of S species (e.g., organic or inorganic) present in GO or GOC.
2.2. Determination of Human Cancer Cell Line A549 Response to GO and GOC
The viability of the human cell line A549 after 24 h of exposure to 40, 80, and 160 mg L
−1 of GO and GOC was analyzed using the neutral red uptake and MTT assays. The neutral red assay is based on the ability of healthy cells to incorporate and retain the neutral red dye in their lysosomes, which is an indicator of the cell’s capacity to maintain pH gradients through the production of ATP, and thus a viability indicator. In
Figure 3, the results obtained for neutral red assay are presented. No negative effects on cell viability was observed in any of the concentrations tested for both nanomaterials, showing all the studied conditions (negative control and exposed cells) a similar percentage of viable cells.
The MTT assay is based on the ability of viable cells with active metabolism to convert MTT into a purple colored formazan product that can be measured at OD 590 nm, being this color formation a useful marker to assess cells viability. The cytotoxicity studies conducted using this assay (
Figure 4) revealed that cells exposed to GOC presented a slight decline in viability at the higher concentrations tested, being statistically significant in the case of cells exposed to 160 mg L
−1, whereas in cells incubated with GO, no significant differences were found between controls and samples.
The toxicity of graphene oxide in human cell lines has been widely investigated in different studies. However, the results and conclusions reached by them are apparently inconsistent, as evidenced by some of the recent reviews [
21,
32]. Several factors, such as the size, the surface chemistry, or the levels of impurities, critically affect the physico-chemical properties of the nanoparticles and, subsequently, the interactions with cells, which lead to differences in their inherent cytotoxicity. Moreover, the toxicity of GO varies greatly depending on the cell line and cell type exposed [
33]. In our experiments, only a slight statistically significant decrease in viability was detected in A549 cells treated with 160 mg L
−1 of GOC (less than 15% of decrease) performing the MTT assay, whereas no negative effect was detected in the NR assay. It is also important to mention that in both assays a different number of cells per well were used, being six times lower in the MTT assay. Even in this case, where the nanoparticle/cell exposure ratio was higher, both GO and GOC demonstrated to be safe in terms of cell viability. These results are in concordance with the work of Chang et al. [
34], which was performed using the same cell line. These authors described the good biocompatibility of GO, describing only a slight decrease in the viability after an exposure to high doses. In contrast, other authors observed a negative effect on the viability caused by these nanoparticles on A549 cells. Gies et al. described a size and dose dependent effect, showing a high decrease in the percentage of viable cells after 24 h of exposure to high concentrations of GO (100 and 200 mg L
−1) [
33]. Likewise, Reshma et al. showed a dose-dependent decrease in viability of cells treated with reduced GO (rGO) and PEGylated GO [
35]. These authors observed a significant reduction from concentrations of, at least, 25 mg L
−1. Mittal et al. analyzed the interaction between three graphene oxide derivatives with A549 cells [
36], observing a significant reduction of viability over 48 h of exposure even at low concentrations, whereas Hu et al. described only a mild effect in cytotoxicity of A549 cells exposed during 24 h to GO and rGO, being significantly higher in the case of the latter [
37]. This variability between the results obtained using the same cell line could be attributed to the factors explained above, such as the levels of impurities present in the nanoparticles, or even the oxidative method through which the nanoparticles were prepared, which influence their toxicological behavior [
38].
In relation to the possible induction of oxidative stress by GO and GOC, the DCFH-DA assay was used to measure the reactive oxygen species (ROS) levels on the A549 cells after contact with different concentrations of the nanomaterials.
Figure 5 shows that the ROS levels were significantly increased in A549 cells after 1 h of exposure to both nanoparticles, being this induction much higher in the case of the cells incubated with GO.
Our assays were performed using concentrations of both nanoparticle types up to 40 mg L
−1. From that concentration, we have observed that in our experimental procedure the fluorescent response may be masked by both GO and GOC, leading to an underestimation of the ROS production. Either way, our results demonstrate that the low concentrations tested in our assays are enough to produce statistically significant levels of oxidative stress after 1 h of incubation, being this much higher in the case of GO. The induction of oxidative stress after interaction with graphene oxides and their derivatives have been reported in several works using different cell lines [
39,
40,
41]. These nanomaterials can induce cellular damage through the formation of ROS by their interaction with cellular membranes. In the specific case of A549 cell line, several works have demonstrated their ability to induce ROS release. For example, Chang et al. found that GO exposure can induce oxidative stress at low concentrations [
34]. Mittal et al. observed an overproduction of ROS in A549 cells in contact with GO and their derivatives, as well as in other human lung cells such as the BEAS-2B cell line [
36]. In both studies, the times of exposure tested were longer than the times used in the present work. In any case, based on our results and in previous reports, it has been evidenced that an acute exposure of human cells to graphene oxide can induce high oxidative stress levels.
High levels of ROS can cause damage to different biomolecules of the cell, such as proteins or nucleic acids, which can lead to activation of apoptosis. In order to assess whether the levels of ROS produced by A549 cells after being exposed to GO and GOC can induce an apoptotic response, we quantified the percentages of apoptotic and necrotic cells using flow cytometry, upon the addition of different nanoparticles concentrations for 24 h. The obtained results have shown that cells treated with different GO concentrations (
Figure 6b; 40, 80, 160 mg L
−1) showed a constant 93–95% of viable cells, similar to the untreated control sample (
Figure 6a). In the case of GOC, we evidenced a stable 6–10% cell death, irrespective of the administered dose (
Figure 6b). As a positive control for the assay, we used cisplatin (a common chemotherapeutic agent) which induced over 40% cell death (
Figure 6a).
Interestingly, we found that the PI signal was decreasing in a dose-dependent manner in GO- and GOC-treated cells (
Figure 6c). However, despite the signal to noise ratio diminution for the PI staining, this did not impede the quantification of the PI
+ cell subpopulation. The PI signal decrease is probably caused by the quenching of the dye by the nanoparticles, as previously reported [
42,
43]. The quenching could be due to the energy transfer from the fluorophore to the metal [
42] or in the case of graphenes, it could be due to the excitation of an exciton too [
43]. Wu et al. found that the quenching efficiency of GO was still around 30% when the distance between dyes and GO was increased to more than 30 nm [
44].
Several studies have described the impact of graphene-based materials on different types of programmed cell death, including apoptosis [
45], in diverse cell lines, through distinct mechanisms such as caspase activation or DNA fragmentation [
46,
47]. For example, in the A549 cell line, the implication of graphene nanopores in the induction of early apoptosis was described and, at concentrations higher than 250 mg L
−1, late apoptosis was observed too [
48]. In addition, Adil et al. observed that apoptosis can be triggered by green synthesized nanocomposites of silver-decorated highly reduced graphene oxide [
49], while Mbeh et al. described that high concentrations of graphene oxide nanoribbons (100 mg L
−1) can also cause cell apoptosis [
50]. However, other authors did not find any evidence of apoptosis induction in A549 cells after treatment with GO derivatives. For instance, Chang et al. observed that, independently of dose and size, GO did not induce any apoptosis or necrosis in A549 cells [
34]. Moreover, Hu et al. described that apoptosis did not occur in A549 cells treated with GO nanosheets after a 24-h exposure with 20 and 85 mg L
−1 [
37]. Finally, Yang et al. found that the exposure to different graphene quantum dots, even at high concentration (200 mg L
−1), did not result in apoptosis induction [
51]. The results described in these latter works are in concordance with our observations, since, in spite of the fact that both GO and GOC produced oxidative stress in A549 cells, no significant increase in apoptosis was detected at concentrations up to 160 mg L
−1.
2.3. Determination of Saccharomyces Cerevisiae Cells Response to GO and GOC
The viability of
S. cerevisiae cells exposed to two different GO and GOC concentrations (160 and 800 mg L
−1) and exposure times (2 and 24 h) was assessed through colony forming units (CFU) determination. As displayed in
Figure 7, no significant differences in viability were observed in the selected exposure conditions after 2 h of exposure, except for the condition where a high GOC concentration was used. However, after 24 h, viability issues could be observed after a longer exposure time. In case of GO, the nanomaterial reduced
S. cerevisiae CFUs after an exposure of 24 h, provoking a viability loss of 36.5% when the material was present at the lower concentration and 49.7% when the material was present at the higher concentration. In contrast, GOC showed no significant influence on the yeast viability at 160 mg L
−1, although the viability loss observed at the higher concentration was very similar for both nanomaterials. The effect on
S. cerevisiae viability of non-commercial grade graphene oxide nanoparticles was also tested in a recent study, and the fungus mortality was found to be close to 20% in the presence of 600 mg L
−1 [
52]. Also, the toxicological potential of other carbon nanomaterials toward
S. cerevisiae was reported, such as multi-walled carbon nanotubes (MWCNTs) or oxidized single-walled carbon nanotubes (O-SWCNTs), which induced significant yeast mortality at 400 mg L
−1 (6.1%) and 188.2 mg L
−1 (approximately 11%) respectively [
53,
54].
To evaluate whether GO and GOC were able to induce oxidative stress in
S. cerevisiae, cells growing at exponential phase were exposed to 160 and 800 mg L
−1 of the nanomaterials, for 24 h. As shown in the
Figure 8, the oxidative stress levels were significantly increased in
S. cerevisiae in the presence of both carbon nanoparticles. Carbon derived nanomaterials have shown previously to induce oxidative stress in yeast. Non-commercial grade GO and O-SWCNT, also induced ROS with a similar concentration to the one tested here, although the exposure time tested in both cases was 24 h instead of 2 h [
52,
54]. However, the oxidative stress provoked by MWCNT in yeast seem to be lower than that observed in the present study for GO and GOC or that previously observed for other carbon derived nanoparticles [
53].
We also aimed to determine the possible genotoxic effect of the selected graphene oxide nanomaterials on
S. cerevisiae using the comet assay protocol previously described [
55]. However, because of the nanomaterials’ morphology, graphene oxide concentrations higher than 20 mg L
−1 prevented the proper visualization and analysis of the cell nuclei under the fluorescence microscope, making the comet assay an unsuitable method for the determination of genotoxiciy in yeast with two dimensional nanoparticles of a big lateral size.
2.5. Determination of GO and GOC Binding Efficiency on Different Microbial Enzymes
Biotechnological and biomedical applications of graphene oxide rely on nanomaterial-biomolecule interactions. The protein binding capacity of nanomaterials determines possible biological applications and their toxicological potential too [
60,
61]. In case of commercial GO and GOC, both nanomaterial suspensions showed a high protein loading capacity and a good potential as enzyme immobilization supports [
4]. However, their maximum protein binding capacity was not determined, and their polypeptide binding properties were determined using a single enzyme. Also, having into account that the protein binding efficiency of the new GOC lot (MKCD9594) was unknown, we decided to characterize the nanomaterial-enzyme binding efficiency of GO and GOC. In addition, to assess whether a variation on the GO and GOC oxidation state could further increase their enzyme loading capacity, the nanomaterials were partially reduced and their protein binding capacity was compared with that of the untreated nanomaterials. The partial reduction of GO and GOC was performed using a concentrated solution (50 mM) of the mild reductant mercaptoethylamine-HCl (further details are described in the Materials and Methods section). The reduction of the nanocarbon derivatives was confirmed by ATR-FTIR analysis (
Figure 10). The spectrum of GOC exhibited drastic changes after the nanomaterials’ treatment with the mercaptoethylamine-HCl. Basically, the intensity of the absorptions sharply decreased, in good agreement with the reduction of the described functional groups. In the case of rGO, an analogous trend to that shown by the rGOC spectrum was observed.
The maximum enzyme loading capacity of chemically reduced GO (rGO) and GOC (rGOC) was analyzed and compared with that of the non-modified nanoparticles, using the bacterial enzymes α-
l-rhamnosidase enzyme RhaB1, from
Lactobacillus plantarum, and the β-
d-glucosidase AbG, from
Agrobacterium sp. (strain ATCC 21400), following the immobilization protocol described previously [
4]. As displayed in
Table 2, the binding capacity of GO and GOC was different for both enzymes and significantly higher than that observed in the reduced versions of the nanoparticles.
Although π–π stacking and hydrophobic effects are considered the predominant mechanisms of protein binding with graphene-based materials, and both phenomena should be more dominant after the reduction of graphene oxide, the reduced versions of GO and GOC did not improve the enzyme binding capacity of the untreated nanomaterials. Previous studies reporting the influence of graphene oxide reduction on protein binding capacity show controversial results [
60,
62,
63,
64]. As recently described by Qi and collaborators [
64], changes on graphene-based nanomaterials’ surface properties affect as well their aggregation properties, which may become a crucial factor influencing their protein adsorption capacity. The obtained result also showed that the maximum loading capacity of GO and GOC was significantly higher for the α-rhamnosidase RhaB1. A similar result was observed when using the reduced versions. Different enzymes could exhibit different enzyme loadings and stabilities when bound to graphene oxide because of the differences in the charge status of their surface functional groups [
65].
The obtained results using distinct unicellular models and biomolecules display significant changes in the toxicological potential of GO and GOC: the former had a higher ability to induce oxidative stress in human alveolar carcinoma epithelial cells A549, and the yeast
Saccharomyces cerevisiae, while provoking a higher luminescence inhibition capacity on the bacteria
Vibrio fischeri too. Also, both products behaved differently in their enzyme binding capacity. The lateral dimension, surface structure, functional groups, purity and protein corona, strongly influence the toxicity of graphene oxide in biological systems [
66]. Since GO and GOC are distinct in terms of their apparent particle size distribution, elemental composition and in the presence of oxygen functional groups, identifying the most relevant factors determining the differences observed regarding their toxicological potential is difficult. Nevertheless, the present work contributes to have a better understanding on the biological impact and biotechnological potential of commercial grade graphene oxide.