Plasmonic color-switchable nanoparticle films change color reversibly in response to environmental cues, such as pH or temperature. However, solid-state plasmonic color-switching is challenging because strategies for nanoparticle separation and plasmonic coupling are scarce.

Wenshou Wang, Shandong University, Ji’Nan, China, Yadong Yin, University of California Riverside, USA, and colleagues have integrated plasmonic color switchable silver nanoparticle films, capped by poly(acrylic acid), with a humidity-sensitive salt. Sodium borate converts into boric acid reversibly when moisture is added or removed (pictured below), simultaneously releasing and capturing OH^– ions, respectively. The OH^– ions trigger deprotonation and protonation of the poly(acrylic acid) capping molecules, thereby changing the surface charge of the silver nanoparticles and altering their interparticle distance, which affects plasmonic coupling.

The plasmonic color-switchable films have a >100 nm plasmonic modulation range, a response time of less than 1 s, consistent performance for more than 15 months, and reversibility and stability over 1000 cycles. The researchers discovered that high-resolution patterns can be embedded in the silver films with lithography and then decrypted by exposure to the moisture in a human fingertip.

• Dynamic Color-Switching of Plasmonic Nanoparticle Films,
  Luntao Liu, Rashed Aleisa, Yun Zhang, Ji Feng, Yiqun Zheng, Yadong Yin, Wenshou Wang,
  Angew. Chem. Int. Ed. 2019, 58, 16307–16313.
  https://doi.org/10.1002/anie.201910116