Interfacial Polydiacetylene at Nanoemulsion Droplet: A PEG Hydrogel Platform from Thermochromic to Hydrogen-Responsive Colorimetric Films

Polydiacetylene (PDA)-based materials are attractive colorimetric indicators because their pi-conjugated backbones undergo distinct blue-to-red transitions in response to external stimuli. While PDA-polymer hybrid films have been widely explored for thermochromic and mechanochromic responses, extending them to chemical gas sensing requires architectures that couple an analyte-responsive element to the PDA colorimetric readout within a single, structurally stable film. Here, we report a poly(ethylene glycol) (PEG) hydrogel film containing PDA-functionalized nanoemulsion droplets. A polymerizable anionic diacetylene emulsifier was synthesized to electrostatically stabilize oil droplets while localizing diacetylene units at the oil–water interface. Nanoemulsions were prepared by high-pressure homogenization using an oil phase as the dispersed phase and an aqueous continuous phase containing the emulsifier and a photocrosslinkable PEG precursor. By tuning emulsifier concentration, PEG precursor content, and oil volume fraction, stable nanoemulsions were obtained and locked in a PEG hydrogel network by 365 nm UV crosslinking; cross-sectional SEM confirmed droplets embedded within the films. Subsequent 254 nm UV irradiation drove topochemical polymerization of the interfacial diacetylenes, producing blue-phase PDA at the droplet interfaces. As a baseline, thermal stimulation produced a clear blue-to-red transition, confirming that the interfacial PDA forms and responds as expected. Building on this platform, we incorporate Pt/ceria catalytic nanoparticles into the oil core to realize a hydrogen-responsive film, in which catalytic oxidation of hydrogen locally heats the droplet and triggers the interfacial PDA transition for naked-eye hydrogen detection. By co-localizing the catalyst within the oil core and the PDA transducer at the droplet interface, this architecture offers a versatile polymer platforms for stimuli-responsive optical sensing of gases.