Temporal Multilevel Luminescence Anticounterfeiting through Scattering Media

Optical anticounterfeiting, typically using luminescent materials to encode and unveil hidden patterns, plays an essential role in countering fraud in trademark, document security, food industry, and public safety. However, this technique is often realized through color-encoded fashion and in the visible range, preventing high-order encryption as well as visualization through scattering layers. Here, we describe a set of shortwave infrared (SWIR)-emitting lanthanide-doped nanoparticles with precisely controlled luminescence lifetime, which can be utilized as temporary codes for multilevel anticounterfeiting through opaque layers. To achieve this, we devise a core/shell/shell/shell structure of NaYF₄:Yb³⁺/Er³⁺ @ NaYbF₄ @ NaYF₄ @ NaYF₄:Nd³⁺, in which the inert NaYF₄ shell acts as an energy-retarding layer to regulate energy flow from the outmost light-harvesting layer to the inner core domain to produce long-lived SWIR luminescence at 1532 nm. A precise control of the NaYF₄ layer thickness enables yielding a precisely defined lifetime tunable between ∼3 and 10 ms, yet without compromising luminescence intensities. Importantly, optical patterns of these lifetime-encoded core/multishell nanoparticles are able to dynamically show a multitude of secured images in the time domain at defined time points through opaque plastic and biomimetic intralipid layers (about half a centimeter thick). Our temporal optical multiplexing results, demonstrated here in multilevel anticounterfeiting, have implications for optical data storage, biosensing, diagnostics, and nanomedicine.