Sonochemical-Assisted Formulation (SAF) of water-based conductive ink for sensors and biosensors on flexible and paper substrates

Herein, a new, fast, and effective method for creating versatile water-based conductive ink for developing sensors and biosensors on flexible and paper substrates is introduced. The aim is to propose a sustainable and potentially biocompatible alternative to conventional conductive carbon inks, which are formulated using toxic and pollutant solvents/components. The proposed technique, named Sonochemical-Assisted Formulation (SAF), enables the preparation of a water-based ink composed of graphite, chitosan, and glycerol within less than 1 hour. The process relies on the high-power probe sonicator's ability to dissolve chitosan in water quickly (10 min) and then properly formulate the ink through a pulsed sonication process lasting 30 minutes. This method not only mixes the ink components but also disperses, exfoliates, and reduces the size of the graphite flakes, thereby positively affecting the final performance of the printed electronic. Initially, the parameters of the SAF were thoroughly analyzed to produce printed tracks with low sheet resistance and sensors that offered the optimal balance between heterogeneous electron transfer rates and faradaic/capacitive current ratios. To understand the effect of the proposed formulation, the obtained inks were deeply characterized via contact angle studies, scanning electron microscopy, and Raman spectroscopy. These investigations were conducted on polyethylene terephthalate (PET) as model flexible substrate. In detail, sonication time, sonication power, and the amount of graphite were investigated. The optimal SAF-obtained ink was compared with conventional ink formation methods, including stirring, orbital, and coaxial formulations, demonstrating superior electrochemical performance. Eventually, the optimal ink formulation was tested for sensing and biosensing purposes on various substrates, including office and recycled paper (Rismaluce office paper, Kraft, Free Tree Bamboo, Crush Cacao, Nitrocellulose), flexible plastics (PET and Polyimide Film), and ceramic foil. Specifically, the proof of the applicability of the SAF-produced inks on these substrates is being studied for creating sensors to detect food antioxidants and for developing second-generation biosensors. Despite ongoing studies, we can confidently state that the proposed SAF ink can be applied to various substrates, resulting in a versatile and environmentally friendly alternative for the development of sustainable conductive tracks and electrochemical devices.