Exploring different carbon allotrope thermoplastic composites for electrochemical sensing

Composite electrodes provide an effective and cheap way to utilise a wide range of carbon materials to make electrodes. More recently thermoplastics have been widely used as the binder to make carbon composite electrodes, as varying fabrication approaches, such as 3D printing, can make complex electrode geometries. However, there is a clear need to understand how the electrochemical performance of different carbon allotrope materials varies when made into sensors. We accessed PLA thermoplastic filaments containing carbon black, graphite, graphene, multiwall carbon nanotube (MWCNT) and carbon fiber using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Electrodes were made in thin disc electrodes using molding to negate the influence of geometry on electrochemical activity and also in dome shapes using 3D printing to understand the impact of geometry. MWCNT/PLA electrodes had the lowest sensitivity for measurement of serotonin. Graphene/PLA electrodes had the greatest sensitivity and lowest limit of detection for the measurement of serotonin. However, graphene/PLA and graphite/PLA had the poorest resolution in the fabrication of dome-shaped electrodes. CB/PLA dome electrodes were the most uniform and had the best print resolution. CB/PLA electrodes also had the best stability for sustained monitoring of serotonin. Electrodes made from all carbon materials showed good electron transfer kinetics for redox probes on molded electrodes, but when dome electrodes were made using 3D printing, poor electron transfer kinetics were observed on all materials. Overall, our study highlights key differences in the electrochemical activity of different carbon allotropes and the impact fabrication of complex electrode geometries can have on performance. This interplay between geometry and electrochemical performance is critical in designing carbon electrodes for a wide range of applications.