Screen Printable, Sustainable Water-Based Functional Ink From Coal-Derived Reduced Graphene Oxide for Humidity Sensing

In this letter, we develop a sustainable water-based conductive ink using coal-derived reduced graphene oxide (C-rGO) stabilized with sodium carboxymethyl cellulose. This ink exhibits high conductivity (∼4.1 S/m) and good printability on various substrates, including polyethylene terephthalate (PET), Whatman paper, and Flexzil. Screen printing on flexible, biodegradable Flexzil substrates shows excellent adhesion and uniform morphology. The ink's humidity sensitivity was tested, displaying an 85%-resistance change over a range of 30%–80% relative humidity. The facile one-pot synthesis method ensures cost-effectiveness and scalability. This makes the C-rGO ink suitable for applications in wearable electronics, flexible sensors, and environmental monitoring, offering a sustainable alternative to traditional conductive inks.