2.6. Fabrication of Inkjet Printed Electrode

Optimization of the IJP process was performed as previously reported [35]. The manufacture of the electrodes was carried out following the sequence of steps as shown in Figure 1, in which SWCNT illustrates the strategy used for the printed electrodes with SWCNT ink and SWCNT-SNP-HRP the WE printed with bio-ink. This second strategy assumed that the enzymes functionalized in the bio-ink could not be exposed to temperatures higher than 37 °C, as this could affect the enzyme activity. As a first step (Figure 1-1), we printed the WE, CE and conductive paths with the Au nanoparticles ink, setting a drop spacing (DS) of 15 µm, which is equivalent to a printing resolution of 1693 dpi (dots per inch). Subsequently, the cartridge containing the Au ink was replaced with one containing Ag nanoparticle ink to print the pseudo reference electrodes (pRE) (Figure 1-2). In this case, the printing was done with a DS of 40 µm (resolution of 635 dpi). They were then subjected to thermal drying of 90 °C for 10 min and then to a sintering process in an oven 30 min at 120 °C (Figure 1-3). This step is where the final electrical properties of the inks are achieved. The pRE was chlorinated by cyclic voltammetry (CV) in 0.1 M HCl, scanning potential from 0 to 0.2 V against Ag/AgCl commercial RE (Metrohm, Herisau, Switzerland Germany) and Pt CE (Metrohm) at 20 mV/s to obtain a stable pRE (Figure 1-4) [36]. For the WE printed with the SWCNT ink, the next step was the printing of the WE area of 1 mm in diameter with the CNT water-based ink with a DS of 15 µm (1693 dpi) (Figure 1-5.A). Subsequently, they were subjected to a drying process of 20 min at 120 °C and then sintering at 140 °C for another 20 min (Figure 1-6.A). Afterwards, the PriElex^® SU8 dielectric ink was printed with a DS of 15 µm (1693 dpi) as a protective layer for the conductive paths and delimiting the active area for the WE and the contact pads of the electrodes (Figure 1-7.A). In this case, the curing was carried out first on a hot plate at 100 °C to evaporate the solvents and then with UV lamp exposure for 30 s, to generate the polymerization of the ink by cross-linking (Figure 1-8.A). For the electrodes printed with the bio-ink (SWCNT-SNP-HRP), after the stage of shaping the conductive paths of the electrodes, the dielectric layer was also printed with PriElex^® SU8, in the same way as previously described (Figure 1-5.B,6.B). In the last step, the bio-ink was printed on the active area of WE also with a DS of 15 µm (Figure 1-7.B), but in this case the curing was carried out at room temperature for 24 h (Figure 1-8.B). This curing strategy assumed that the bio-ink enzymes could not be subjected to higher than room temperature in order to avoid damage and losing enzymatic activity in the electrodes.

(Top) Fabrication steps of three-electrode electrochemical cell based on IJP. SWCNT: WE printed only with SWCNT and SWCNT-SNP-HRP: WE printed with bio-inks. (Bottom) Photograph of the printed electrodes steps and several all inkjet printed electrochemical sensors onto a flexible substrate.