Graphene-Based Biosensor Technologies

Abstract

This chapter is aimed to review the state of the art related to the preparation of polymer nanocomposites having tunable graphene loadings and summarize their main peculiar features. The current open literature describes several types of so-called ‘graphene', which differ for the processes used for its obtainment, namely: expansion, exfoliation, functionalization, oxidation, oxidation/reduction, etc. Thus, very often it becomes quite difficult to discriminate among the different ‘graphene products' and avoid any misunderstanding: indeed, as an example, the use of reduced graphene oxide instead of ‘graphene' can influence the overall final properties of the polymer material, to which the nanofiller is added, in a very different way with respect to the ‘graphene' obtained through direct sonication, milling or thermal expansion. Among the up-to-date methods exploited for obtaining and utilizing this nanofiller in nanocomposite fabrication, the direct sonication from colloidal suspensions of graphite and the use of graphene nanoplatelets (defined as graphene mono- or few-layers, possibly bearing some functional groups attached to the carbon structure) are the most convenient in terms of cost and simplicity; they differ as far as the purity and the presence of process-induced defects are considered. The former method involves a simple protocol based on low energy consumption and high volume production and allows obtaining graphene substantially without any defect. In addition, such a nanofiller, obtained by direct exfoliation (i.e., without any chemical oxidation and subsequent reduction) in suitable common solvents, surfactants or monomers to eventually polymerize, can be used for the preparation of both thermoplastic and thermoset polymer nanocomposites. Graphene nanoplatelets can be obtained by exploiting different exfoliation processes (i.e. milling, sonication, thermal expansion, etc.), usually in the presence of chemically reactive compounds that are used for the functionalization of the nanofiller. Nowadays, they represent the most used type of ‘graphene' existing in the market. However, it will be shown that, despite their availability, they should be used with caution in that they can hardly be considered ‘real graphene', because of the number of defects induced by the fabrication process and still remaining in the structure. The first part of the chapter summarizes the main methods for obtaining graphene, showing advantages and drawbacks. In the second part, the morphology, as well as the thermal, physico-mechanical, electrical and rheological peculiarities of the obtained nanocomposite materials based either on graphene from direct sonication of colloidal suspensions of graphite or graphene nanoplatelets are reviewed in detail