Figure 2 . Textural characterization of surface created by titania ink: a) HRSEM cross-section of printed titania layer; b) SEM imaging of titania xerogel layer
Thus, it possible to adjust rheology properties of titania ink to optimal inkjet printability parameters for various print heads and printing systems28. This sol maintains stability over time and virtually unchanged size of the hydrodynamic radius, that allows to obtain a dense xerogel with high porosity (Figure 2 a,b). Upon the solvent evaporation the nanoparticles exhibit high crosslinking among themselves. The resulting surface is a good basis for enzymatic layer, as it has permeability and can rapidly change color reacting with hydrogen peroxide.
Optical microscopy of printed samples showed the formation of stable individual droplets with a porous structure (Figure 2 a,b). It demonstrated the possibility of good control over drop merging at higher DPI. This ensured the formation of a uniform layer with precise thickness control by layer-by-layer application starting from 200 nm to 1 µm. That is important in designing a platform for biosensor to control thickness and surface area of the reacting zone.
In addition, we used a solid wax-based ink, which was the base for hydrophobic coating boundaries on the substrate surface. Thus, using the prepared template we restricted sensitive zone.
As shown in Figure 3a, hydrophobic mask was applied only on shaded parts of template and thereby precise positioning and control of hydrophobic and hydrophilic areas for the subsequent application of the reaction components were performed. After applying the mask, we printed remaining area with hydrophilic titania ink. This approach along with the ink-jet printing has allowed us to control the reaction area and amount of the analyte reacting with the sensitive zone (Figure 3b).