Water-Based Sustainable Ink Composed of Regenerated Silk Fibroin and Upconversion Nanoparticles for Printing/Painting on Arbitrary Surfaces

Regenerated silk fibroin derived from silkworm cocoons is considered as one of the promising sustainable materials for various fields such as biomedicine, photonics, and electronics. To date, a variety of unconventional material platforms have been demonstrated by engineering regenerated silk fibroin in various ways. In this study, we fabricate a nanocomposite composed of regenerated silk fibroin and upconversion nanoparticles (UCNPs) and demonstrate its printing/painting capabilities as an eco-friendly and biocompatible functional ink. The research is largely divided into three parts:<br/>1) securing robust RGB emission spectra under 980 nm NIR excitation through systematic dopant control of citrate-capped hydrophilic UCNPs, which can be easily dispersed in aqueous silk fibroin solution;<br/>2) selecting an appropriate auxiliary solvent that prevents the aggregation of silk fibroin and UCNPs to improve the room temperature storage and printability of ink;<br/>3) expanding the color palette through mixing RGB inks and demonstrating printing/painting capabilities of the inks on arbitrary surfaces, including 3D curved substrates.<br/>In conclusion, we developed a functional ink that can be designed with a total of six emission colors (RGBCMY), and succeeded in creating colorful patterns and tags that can be selectively revealed under NIR excitation on biological surfaces such as silkworm cocoons, pig skin, fruits, and leaves through stencil printing and brush painting. The natural product-based upconversion ink developed in this study may provide new opportunities for biocompatible and transient tattoos for medical purposes, anti-counterfeiting platforms, and sustainable nanophotonic devices.