Photogated Coordination Switching of Silver Nanoparticles for Reversible Coloration/Discoloration of Hydrogel

Hydrogels remain challenging for the reversible high-resolution coloration/discoloration in response to external stimuli. Herein, a photogated coordination-switching mechanism is proposed to alter the localized surface plasmon resonance of silver nanoparticles (AgNPs) inside a hydrogel network, which enables the reversible color change. The hydrogel is synthesized by free-radical crosslinking of N,N'-bisacrylylcystamine (BACA), sodium p-styrenesulfonate (NASS), and acrylamide (Am) in the presence of AgNPs. Upon light irradiation at λ = 400 nm (300 mW cm⁻²), the AgNPs selectively coordinate with the disulfide bond of BACA to result in a reddish-brown hydrogel in 30 s. In dark condition, this coordinating effect is replaced by the sulfonate of NASS and AgNPs, which brings the hydrogel into gray white. Such a photogated coordination-switching mechanism is a physical process that enables the coloration/discoloration cycles without apparent reduction in the resolution. By means of a high-precision mask, it allows writing, storing, and erasing of complicated information at the hydrogel surface, showing a great potential in informatics, cryptology, and security areas.