3D-printed gelled electrolytes for electroanalytical applications

In this work, several gelators were employed to formulate a conducive gel phase (ionic conductivity) compatible with direct ink writing/bioprinting/robocasting (different names in the literature describe the same printing technology). The main goal of this work was to evaluate gelled phases being a mixture of background electrolyte (NaCl), redox probe (Fe(CN)₆^3-/4-), and gel precursor (guar gum, gelatine, agarose, and agar-agar). The studied concentration of gelators ranged from 0.1 to 4% depending on the employed system. Each gelator required a customized formulation protocol. We have found that guar gum exhibits the best printing properties (lack of aggregates blocking the printing nozzle) while giving the least reproducible electrochemical results (when a glassy carbon electrode was employed as the working electrode). The study of two other gelators (agarose and gelatin) indicated significant changes in the electrochemical properties of the investigated surface as their concentration and number of voltammetric scans were varied. The best electrochemical performance was obtained for agar-agar however, this was also a gelator causing the most problems during 3D printing. Finally, we have employed six screen-printed electrodes displaying approximate properties, that were further covered with a 3D-printed conductive gelled cube (direct printing over the electrode surface). We have found that such a system allowed for a surprisingly good electroanalytical response when the model redox probe (Fe(CN)₆^3-/4-) was considered. This work is a prelude to 3D-printed gel-based detection devices we are currently developing in our team.