Historical perspective
Dispersed graphene materials of biomedical interest and their toxicological consequences

https://doi.org/10.1016/j.cis.2019.102051Get rights and content

Highlights

  • Graphene is a one-atom thick nanocarbon displaying high surface area-to-mass ratio.

  • Dispersion stable graphene is of great biomedical interest.

  • Advancement in synthetic approaches and interaction of colloidal graphene with biointerfaces are discussed.

  • Toxicological challenges on utilizing graphene for high-end biomedical applications have been reviewed.

Abstract

Graphene is one-atom thick nanocarbon displaying a unique honeycomb structure and extensive conjugation. In addition to high surface area to mass ratio, it displays unique optical, thermal, electronic and mechanical properties. Atomic scale tunability of graphene has attracted immense research interest with a prospective utility in electronics, desalination, energy sectors, and beyond. Its intrinsic opto-thermal properties are appealing from the standpoint of multimodal drug delivery, imaging and biosensing applications. Hydrophobic basal plane of sheets can be efficiently loaded with aromatic molecules via non-specific forces. With intense biomedical interest, methods are evolving to produce defect-free and dispersion stable sheets. This review summarizes advancements in synthetic approaches and strategies of stabilizing graphene derivatives in aqueous medium. We have described the interaction of colloidal graphene with cellular and sub-cellular components, and subsequent physiological signaling. Finally, a systematic discussion is provided covering toxicological challenges and possible solutions on utilizing graphene formulations for high-end biomedical applications.

Introduction

Graphene is one atom thick sheet of sp2 hybridized carbon displaying a honeycomb structure. Its six-membered ring is the building block of all other graphitic carbon materials. The rings are stacked in parallel configuration and adjacent layers are held together by weak inter-atomic van der Waals forces. Single or few-layer graphene was first derived from graphite in 2004 by scotch tape method [[1], [2], [3], [4], [5], [6], [7]]. Ever since isolation, graphene has attracted great research interest due to high mechanical strength, excellent conductivity, and large surface area to mass ratio. Structural modifications have been explored via edge functionalization and doping to customize the properties suited for specific applications [[8], [9], [10]]. With a tunable electronic structure, graphene absorbs near-infrared (NIR, 650–900 nm) radiations which have deep penetration ability (4–10 cm) and undergo efficient conversion to localized heat [11]. The light-heat conversion has shown to photo-thermally ablate solid tumors in animal models [[12], [13], [14]].

Elaborate classification of ‘graphene materials’ takes into account variables such as, composition, layer number, lateral dimension, and chemical modification. The nomenclature and description of graphene-family materials can be found elsewhere [15]. Our discussion is primarily focused on sheets existing in oxidized (GO) and reduced (rGO) forms, and graphene quantum dots (GQDs). Their properties depend on ratio of un-oxidized sp2 and sp3 oxidized regions, and can be tuned by the degree of oxidation/reduction or the use of additional purification steps. Fully oxidized GO shows low carbon-to-oxygen (C/O) ratio, along with a possibility of transition from crystalline to amorphous state [16]. Such changes can be identified in the 2D and other overtone bands of Raman spectrum. Oxygen content in sheets can be minimized through thermal treatments [17] or by refluxing the aqueous colloidal suspension under acidic conditions [18]. Comprehensive understanding on the number of oxygen functional groups can be exploited to develop high efficiency electrodes, and catalysts for reductive reactions [18].

Annual growth rate of graphene composites between 2010 and 2017 has been estimated worldwide as 61% [19]. It is likely to advance keeping in view the volume of ongoing investigations. Application of graphene in energy [20,21], water purification [22,23], material science [[24], [25], [26]], drug delivery [27,28] and tissue engineering [29] is well described in recent literatures. Drug delivery applications of graphene emerge from its hydrophobic basal plane which can be loaded with aromatic molecules via non-specific interactive forces. rGO possesses comparatively larger sp2 clusters and thus, facilitates greater π–π electron donor-acceptor interactions [30,31]. Oxygen-containing groups (carboxyl, hydroxyl, and epoxy) enable convenient modification of sheets through covalent or non-covalent approaches. These properties can be coupled with the intrinsic optothermal properties for developing multifunctional theranostics. In addition, sheets can be employed as a substrate for culturing pluripotent stem cells, mesenchymal stem cells, and neural stem cells [29,32]. Graphene-based hybrid materials, containing inorganic or organic substances, have been tested as reinforced platform for long acting depot and tissue engineering applications [33,34].

Much work has been reported on bulk production of concentrated stable graphene dispersions in organic and aqueous solvents [35,36]. However, we observe a significant knowledge gap on biosafety of colloidal graphene materials. In this review, we have summarized the advancements in synthetic approaches of graphene sheets, keeping a focus on achieving homogeneous and stable colloidal dispersion. A discussion is presented on interfacial interaction with cellular and sub-cellular components, and subsequent intracellular signaling in terms of surface characteristics of the sheets. Finally, we provide an outlook on how such cytotoxic interactions can be minimized. These insights can contribute largely while contemplating the innocuous biomedical applications of graphene.

Section snippets

Strategies for production of graphene sheets

Graphene can be obtained as single-layer, bilayer, few-layer, and the combination of these types with a range of geometry, defect density and edge profile. It is possible to achieve single layered graphene by controlling the growth kinetics. Fundamentally, synthetic techniques include either growing the sheets from small-molecule precursors (bottom up) or exfoliating the bulk graphitic materials (top-down) [37,38]. In the former method, precursors must be polycyclic aromatic compounds or other

Strategies for stabilizing graphene dispersions in aqueous media

As against to high stability in aprotic solvents [111], graphene exhibits limited aqueous dispersibility. Its quick aggregation, particularly in physiological solutions, constitutes a major obstacle to a direct biomedical application. Aggregation is typically driven by inter-sheet van der Waals forces and high surface free energy, and the resulting polydispersity vandalizes size-dependent properties of the sheets [112,113]. It can be minimized by imposing repulsive barrier among the sheets

Biosafety of graphene materials

Biosafety of graphene remains questionable due to its non-specific binding affinity for drugs, protein, nucleotides and cellular components. Cumulative toxicity of systemically available graphene is regulated by three cascading events; interaction with blood components or proteins during biodistribution, physical adsorption onto cell surface (analogous to host-pathogen interaction) and trans-cellular migration to manipulate cytosolic processes [118,[144], [145], [146]]. Magnitude of downstream

Challenges to the ecosystem

Looking at exponential rise in the demand of graphene materials for agriculture, biosensing, pollution control and remediation technologies, its commercial production is rapidly increasing [241,242]. This can be detrimental to the ecosystem since large quantity of resulting waste is discharged into water bodies. Colloidal stability and aggregation of sheets in aquatic environment follows the classical DLVO and Schulze-Hardy rule. Accordingly, changes in the counter ion and ionic strength affect

Conclusion

Graphene, the 2D-nanocarbon, is increasingly receiving interest for application in many real world technologies. With the ongoing technical advancements in synthetic methods, it can be produced cost-effectively at larger scale in comparison to other carbon materials. Its unique physical and optical properties can be leveraged to alter the very principles of electronics, health, desalination and energy sectors. Biomedical interest in graphene is expanding due to its high loading ability and

Declaration of Competing Interest

Authors declare no conflict of interest.

Acknowledgements

ST is thankful to the Science & Engineering Research Board (SERB), New Delhi, India, for funding support under ECRA scheme (#ECR/2017/000903).

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