Silk-Based Functional Inks with Color-Tunable Light Upconversion for Printing on Arbitrary Surfaces

Abstract

A biocompatible and versatile silk-based functional ink with hydrophilic upconversion nanoparticles (UCNPs) that can be printed or painted on arbitrary substrates, including biological surfaces is demonstrated. Hydrophilic UCNPs, ready to be dispersed in aqueous silk fibroin solution, are synthesized in large quantities via facile hydrothermal synthesis using citric acid. Based on a systematic study of dopant control to achieve robust red emission from citrate-capped UCNPs, three primary colors are obtained, which serve as the basis for the expansion of the color palette. The synthesized UCNPs are homogeneously dispersed in regenerated silk fibroin solutions derived from Bombyx mori cocoons to produce water-based functional inks. Cyan, magenta, and yellow (CMY) inks are created by combining red, green, and blue (RGB) inks. Demonstrator devices, where a variety of colorful patterns and codes are revealed only under near-infrared irradiation, are realized through stencil printing or brush painting on flexible polymer films, fruit, leaves, silkworm cocoons, and pig skin. The patterns and codes are highly heat resistant (approximate to 230 degrees C) and can be easily washed off with water after use. The printable/paintable inks and functions proposed here will provide new opportunities for applications where sustainability, biocompatibility, and intentional luminescence are simultaneously required. In this study, printable and paintable silk-based functional inks with hydrophilic upconversion nanoparticles (UCNPs) are prepared via facile hydrothermal synthesis without complex ligand exchange. The developed inks, designed to emit various colors including three primary colors of light, can be patterned on arbitrary substrates, including biological surfaces, using stencil printing and brush painting. Various colorful patterns or codes that are selectively revealed only under near-infrared excitation are demonstrated.image