misteINK: a protein nanocage-based ink with reversible, stimuli-responsive color shifts.

Abstract

Dyes exhibiting polarity-dependent color changes, known as solvatochromism, have great potential for creating sensors, smart materials, and responsive coatings. However, full-range color shifts require a technique to disperse dyes across a wide range of solvent polarities, which remains a persistent challenge. For example, hydrophobic dyes often aggregate in water, preventing effective color shifts. Although surfactants can assist in dye dispersion, they can also prevent solvent molecules from accessing the dye. To address this, we used a 60-mer protein nanocage, TIP60, with a densely pyrene-modified interior surface. The modification did not induce protein denaturation, as monitored by small-angle X-ray scattering, and greatly increased the aqueous solubility of a hydrophobic solvatochromic dye, Nile Red (NR), while preserving its fluorescence. The NR-loaded solution appeared blue, reflecting the polar environment surrounding NR. Cryogenic electron microscopy suggested that the pyrenes interacted with each other to form a binding site for NR. This interaction also contributed to thermostability of TIP60 (65 °C to 86 °C) and stability against sodium dodecyl sulfate, as observed by electrophoresis experiments. When brushed onto plain copy paper, the NR-loaded nanocage appeared bluish-purple and shifted reversibly to purplish red upon heating, returning on cooling-presumably via nanocage dissociation and reassembly. The color change was also sensitive to humidity. We term this material "misteINK", a protein-based ink with reversible temperature- and humidity-dependent color changes. These findings demonstrate that a single-step interior modification enables the rational design of protein materials for tuning dye photophysics, providing a powerful strategy for designing protein-based functional materials.