Two-dimensional carbon-based nanomaterials, including graphene oxide and graphene, are potential candidates for biomedical applications such as sensors, cell labeling, bacterial inhibition, and drug delivery. Herein, we explore the biocompatibility of graphene-related materials with controlled physical and chemical properties. The size and extent of exfoliation of graphene oxide sheets was varied by sonication intensity and time. Graphene sheets were obtained from graphene oxide by a simple (hydrazine-free) hydrothermal route. The particle size, morphology, exfoliation extent, oxygen content, and surface charge of graphene oxide and graphene were characterized by wide-angle powder X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, dynamic light scattering, and zeta-potential. One method of toxicity assessment was based on measurement of the efflux of hemoglobin from suspended red blood cells. At the smallest size, graphene oxide showed the greatest hemolytic activity, whereas aggregated graphene sheets exhibited the lowest hemolytic activity. Coating graphene oxide with chitosan nearly eliminated hemolytic activity. Together, these results demonstrate that particle size, particulate state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells. In addition, the cytotoxicity of graphene oxide and graphene sheets was investigated by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. However, appropriate alternate assessments, using the water-soluble tetrazolium salt (WST-8), trypan blue exclusion, and reactive oxygen species assay reveal that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e., whether or not aggregation occurs) and mode of interaction with cells (i.e., suspension versus adherent cell types).

We report a new, simple, hydrazine-free, high-yield method for producing single-layer graphene sheets. Graphene sheets were formed from graphite oxide by reduction with simple deionized water at 95 °C under atmospheric pressure. Over 65% of the sheets are single graphene layers; the average sheet diameter is 300 nm. We speculate that dehydration of graphene oxide is the main mechanism for oxygen reduction and transformation of C-C bonds from sp(3) to sp(2). The reduction appears to occur in large uniform interconnected oxygen-free patches so that despite the presence of residual oxygen the sp(2) carbon bonds formed on the sheets are sufficient to provide electronic properties comparable to reduced graphene sheets obtained using other methods.

The hydrophilic nature of graphene oxide sheets can be tailored by varying the carbon to oxygen ratio. Depending on this ratio, the particles can be deposited at either a water-air or a water-oil interface. Upon compression of thus-created Langmuir monolayers, the sheets cover the entire interface, assembling into a strong, compact layer of tiled graphene oxide sheets. With further compression, the particle layer forms wrinkles that are reversible upon expansion, resembling the behavior of an elastic membrane. In the present work, we investigate under which conditions the structure and properties of the interfacial layer are such that free-standing films can be obtained. The interfacial rheological properties of these films are investigated using both compressional experiments and shear rheometry. The role of surface rheology in potential applications of such tiled films is explored. The rheological properties are shown to be responsible for the efficiency of such layers in stabilizing water-oil emulsions. Moreover, because of the mechanical integrity, large-area monolayers can be deposited by, for example, Langmuir-Blodgett techniques using aqueous subphases. These films can be turned into transparent conductive films upon subsequent chemical reduction.

The effect of the addition of graphene on the glass transition temperature (T g) of polymers was investigated, first with poly­(methyl methacrylate) and then with an extensive literature review. Isotactic (i-PMMA) and atactic PMMA (a-PMMA) were blended with pristine graphene (PG) and thermally reduced graphene (TRG). A T g increase was found for a-PMMA nanocomposites made via in situ polymerization with TRG but not when a-PMMA was solvent blended with TRG. However, a T g increase was found for TRG solvent blended into i-PMMA and a smaller increase for PG with i-PMMA. Nearly all the increase occurred at the lowest loading, 0.25 wt %, with little change at increased graphene concentration. T g increases due to interfacial interactions between matrix polymers and fillers. Physical blending such as solvent processes cannot provide enough interaction at the interfaces, whereas chemical blending processes such as in situ polymerization can yield strong covalent bonds. However, i-PMMA molecules can align on graphene sheets at the interface, creating more interaction between i-PMMA and graphene than a-PMMA. Also, the T g of i-PMMA is 60 °C lower than a-PMMA, meaning that hydrogen bonds are stronger at the lower temperature. The T g increase of TRG/i-PMMA is higher than that of PG/i-PMMA due to more oxygen functionalities on TRG than on PG to act as interfacial interaction sites. A broad literature survey agrees with our PMMA results. We found no changes in T g for graphene/polymer nanocomposites synthesized via physical blending processes such as solvent or melt blending, except for blending with strongly polar polymers. In contrast, chemical blending processes such as in situ polymerization or chemically modified fillers yielded significant T g increases in graphene/polymer nanocomposites.

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