A Comparative Study of Conductive 3D Printing Filaments for Electrochemical Sensing Applications Pretreated by Alumina Polishing, Electrochemical Activation, and Electrodeposition of Au Nanoparticles

3D-printed electrochemical devices have gained tremendous attention recently because they are highly customizable platforms for analysis and energy storage that can be produced using simple, inexpensive components in a wide variety of settings. 3D-printed electrochemical sensors, fabricated from carbon-loaded conductive thermoplastics, enable decentralized production of electrochemical devices that, if optimized, could be widely distributed. Achieving this goal requires a comprehensive understanding of the electrochemical behavior of these filaments. Here, we investigated how the electrochemical behavior of three commercial filaments was affected by alumina polishing, electrochemical activation in 0.5 M NaOH, and electrodepositing Au nanoparticles (NPs). The goal of this study is to characterize if/how these commonly used pretreatments affect different filaments. The study is not an exhaustive combination of all filaments and pretreatment options. We characterized the physical properties of each filament/pretreatment using thermogravimetric analysis, scanning electron microscopy, and Raman microscopy measurements. We then benchmarked the background electrochemical processes (capacitance and solvent window), the peak current response versus scan rate, and the peak potential separation of two common outer-sphere redox species (ruthenium hexamine and ferrocene methanol) for each filament under each pretreatment (i.e., nine total conditions). We subsequently investigated how the filaments responded to inner-sphere redox couples that were surface sensitive (ferrocyanide oxidation), dependent on surface adsorption (dopamine oxidation), and sensitive to surface oxides (Fe2+ oxidation). The data collectively underline the complexity of electrodes fabricated from conductive 3D printing filaments and highlight several important considerations that should be addressed when interpreting the electrochemistry of such materials. First, we present evidence that these materials behave as partially blocked electrodes, which complicates interpretations of electrochemical data. We also found that the outer-sphere electrochemical reactivity on a given filament was largely consistent regardless of pretreatment. The important variable for assessing outer-sphere electron transfer was the uncompensated resistance (Ru), which varies depending on the filament material, electrode size, and contact method. Finally, we observed that the selected filaments do not respond to pretreatments identically when tested against inner-sphere redox species, suggesting that a variety of treatments should be evaluated when assessing conductive 3D-printed filament electrodes.