Development of IrO₂ bio-ink for ink-jet printing application

We develop a chemical formulation to synthesize stable IrO₂ colloidal solution as a bio-ink for ink-jet printing purpose. This wet chemical process entails Na₃IrCl₆ and NaClO as the Ir precursor and oxidizer, respectively. In addition, citric acid and sodium hydroxide are employed as the complex agent and pH adjuster. The pH value of the IrO₂ colloidal solution could be readily adjusted over a broad range without incurring undesirable precipitation. Their respective zeta potential decreases from positive to negative with increasing pH values. The IrO₂ colloidal solution demonstrates impressive chemical stability over extended storage at 5 and 65 °C. Images from transmission electron microscope indicate the average size for the IrO₂ nanoparticles to be 3.5 nm. The X-ray diffraction pattern from the heat-treated IrO₂ sample shows a polycrystalline rutile phase. The IrO₂ colloidal solution undergoes repeated filtering and washing to retrieve the IrO₂ nanoparticles. After an annealing at 450 °C for 2 h in air, the crystalline IrO₂ nanoparticles are resuspended in a 0.5 wt% aqueous medium as the bio-ink for ink-jet printing on an ITO substrate. Our method enables the synthesis of stable IrO₂ nanoparticles in a neutral solution at 25 °C, which provides a facile and effective approach over conventional ones that require alkaline environment at elevated temperature.