Characterisation of laser fabricated graphene materials and their application in electrochemical sensing

Abstract

Graphene has been publicised as the electrode material of the future due to its large surface area and excellent electrical properties. However when considering electrochemical sensors, true monolayer graphene is of limited use owing to its basal nature which results in electrochemical performance akin to basal plane pyrolytic graphite. The future of electrochemical sensing requires electrodes with performance superior to edge plane pyrolytic graphite the ‘gold standard’ of carbon electrodes. Such electrodes must not only be simple and cost effective to produce but also capable of high sensitivity and precision. Here, two new materials are presented which demonstrate excellent electrochemical responses whilst being amenable to disposable point of care sensors, akin to the planar three electrode screen printed predecessors. Laser scribed graphene and laser induced graphene both rely on a simple laser reduction method to fabricate electrodes. Laser scribed graphene, utilises the laser within Lightscribe enabled DVD drives to thermally reduce graphene oxide to a multi-layered graphene material. The expanded surface area and low oxygen content of 6.5 % result in electrochemical performance surpassing that of edge plane pyrolytic graphite. Laser induced graphene employs a CO2 laser to reduce Kapton to a highly porous graphene material. It also retains low levels of oxygen (10 %) making it an interesting prospect for electrochemical sensing. These materials have been extensively characterised physicochemically and electrochemically. Regarding electrochemistry both inner- and outer-sphere redox probes were used in comparative studies with conventional carbon based electrodes. Here the graphene electrodes demonstrated enhanced performance compared to other carbon electrodes. The heterogeneous electron transfer rate of laser scribed graphene was calculated as 0.02373 cm s−1, compared with edge plane pyrolytic graphite at 0.002601 cm s−1 and basal plane pyrolytic graphite at 0.00033 cm s−1. Electrochemical performance of such materials is clearly influenced by small changes in the oxygen content of the material but most importantly by the morphology of the electrode surface. Ability to detect biologically relevant molecules dopamine, ascorbic acid, uric acid and NADH was then investigated. Laser scribed graphene and laser induced graphene demonstrated successful simultaneous detection of dopamine, ascorbic acid and uric acid which was not achieved with edge plane pyrolytic graphite or glassy carbon electrodes. Detection limits of 0.17 μM were achieved for detection of dopamine with laser scribed 3 graphene electrodes comparing well with the literature. In conclusion both laser scribed graphene and laser induced graphene electrodes have demonstrated exceptional electrochemical behaviour with promise for future use in disposable point of care electrochemical sensors.