Journal of Materials Science, Vol.55, No.24, 10284-10302, 2020
A versatile route to edge-specific modifications to pristine graphene by electrophilic aromatic substitution
This work presents a general method for producing edge-modified graphene using electrophilic aromatic substitution. Five types of edge-modified graphene were created from graphene/graphite nanoplatelets sourced commercially and produced by ultrasonic exfoliation of graphite in N-methyl-2-pyrrolidone. In contrast to published methods based on Friedel-Crafts acylation, this method does not introduce a carbonyl group that may retard electron transfer between the graphene sheet and its pendant groups. Graphene sulphonate (G-SO3-) was prepared by chlorosulphonation and then reduced to form graphene thiol (G-SH). The modifications tuned the graphene nanoparticles' solubility: G-SO3- was readily dispersible in water, and G-SH was dispersible in toluene. The synthetic utility of the directly attached reactive moieties was demonstrated by creating a "glycographene" through radical addition of allyl mannoside to G-SH. Chemical modifications were confirmed by FT-IR and XPS. Based on XPS analysis of edge-modified GNPs, G-SO3- and G-SH had a S:C atomic ratio of 0.3:100. XPS showed that a significant amount of carbon sp(2) character remained after functionalisation, indicating little modification to the conductive basal plane. The edge specificity of the modifications was visualised on edge-modified samples of graphene produced by chemical vapour deposition (CVD): scanning electron microscopy of gold nanoparticles attached to G-SH samples, epifluorescence microscopy of a glycographene bioconjugate with a fluorescently tagged lectin, and quenched stochastic optical reconstruction microscopy (qSTORM) of thiol-reactive fluorophores on CVD G-SH samples. Microelectrochemistry of unmodified CVD graphene and dye-modified CVD G-SH showed no statistically significant difference in interfacial electron transfer rate (k(0)). This platform synthesis technology can allow pristine graphene, rather than graphene oxide or its derivatives, to be used in applications that require the superior mechanical or electronic properties of pristine graphene, including theranostics and tissue engineering.