Rapid Color-Switching of MnO 2 Hollow-Nanosphere Films in Dynamic Water Vapor for Reversible Optical Encryption.

Drop-casting manganese oxide (MnO 2 ) hollow nanospheres synthesized via a simple surface-initiated redox route produces thin films exhibiting angle-independent structural colors. The colors can rapidly change in response to high-humidity dynamic water vapor (relative humidity > 90%) with excellent reversibility. When the film is triggered by dynamic water vapor with a relative humidity of ≈100%, the color changes with an optimal wavelength redshift of ≈60 nm at ≈600 ms while there is no shift under static water vapor. The unique selective response originates from the nanoscale porosity formed in the shells by randomly stacked MnO 2 nanosheets, which enhances the capillary condensation of dynamic water vapor and promotes the change of their effective refractive index for rapid color switching. The repeated color-switching tests over 100 times confirm the durability and reversibility of the MnO 2 film. The potential of these films for applications in anti-counterfeiting and information encryption is further demonstrated by reversible encoding and decoding initiated exclusively by exposure to human breath.