The World Health Organization reported that 4.2 million deaths every year were a direct result of exposure to ambient air pollution (NO 2 , SO 2 , NH 3 , CO 2 , CO, CH 4 ). There is a well-demonstrated global need for high sensitivity, low cost and low energy consumption miniaturised gas sensors to be deployed in a dense network and to be used in an attempt to pinpoint and avoid high pollution hot spots. The high sensitivity of graphene to the local environment has shown to be highly advantageous in sensing applications, where ultralow concentrations of adsorbed molecules induce a significant response on the electronic properties of graphene. This is commonly attributed to the π electrons of graphene, being directly exposed to the surrounding environment. The unique electronic structure makes graphene the 'ultimate' sensing material for applications in environmental monitoring and air quality. In this review, we present the frontiers of graphene-based sensors considering both electrical and optical methods of detection and discuss the topical progress in an attempt to establish whether graphene can be considered as the ideal sensing material. We pay special attention to the optimization of the sensor performance, using various graphene hybrids and doping mechanisms. Furthermore, we present the recent developments in other 2D material-based sensors that have followed in the wake of graphene. We discuss the benchmarked parameters of graphene sensors, such as sensitivity, selectivity, response/recovery time and detection limit, and compare them with other 2D materials as well as existing state-of-the-art sensors currently being used in the field. We also perform a brief market analyses for the environmental sensing industry as well as provide a Strengths-Weaknesses-Opportunities-Threats analysis of graphene technology for environmental sensing.

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Solutions of negatively charged graphene (graphenide) platelets were produced by intercalation of nanographite with liquid potassium-ammonia followed by dissolution in tetrahydrofuran. The structure and morphology of these solutions were then investigated by small-angle neutron scattering. We found that >95 vol % of the solute is present as single-layer graphene sheets. These charged sheets are flat over a length scale of >150 Å in solution and are strongly solvated by a shell of solvent molecules. Atomic force microscopy on drop-coated thin films corroborated the presence of monolayer graphene sheets. Our dissolution method thus offers a significant increase in the monodispersity achievable in graphene solutions.

Chemical charging of single-walled carbon nanotubes (SWCNTs) and graphenes to generate soluble salts shows great promise as a processing route for electronic applications, but raises fundamental questions. The reduction potentials of highly-charged nanocarbon polyelectrolyte ions were investigated by considering their chemical reactivity towards metal salts/complexes in forming metal nanoparticles. The redox activity, degree of functionalisation and charge utilisation were quantified via the relative metal nanoparticle content, established using thermogravimetric analysis (TGA), inductively coupled plasma atomic emission spectroscop y (ICP-AES) and X-ray photoelectron spectroscopy (XPS). The fundamental relati onship between the intrinsic nanocarbon electronic density of states and Coulombic effects during charging is highlighted as an important area for future research.