Water detection through Nd3+-sensitized photon upconversion in core–shell nanoarchitecture

Herein, a strategy to achieve an 808 nm excitable water probe based on an Nd3+-sensitized core–shell upconversion nanoarchitecture is presented. Specifically, the as-synthesized Yb/Er:NaGdF4@Yb/Nd:NaYF4 active-core@active-shell nanocrystals significantly enhance the Er3+ blue, green and red upconversion emissions with the assistance of spatially confined separation between the Nd3+ sensitizers and Er3+ activators and efficient energy transfer/migration of Nd3+ → Yb3+(shell) → Yb3+(core) → Er3+. To understand the exact mechanisms for water detection, core, core–shell and core–shell–shell samples were prepared and their related water-content dependent upconversion emission and decay behaviors under 980 nm and 808 nm laser excitation systematically investigated. Different to the case of the Yb/Er:NaGdF4 core, the Nd3+-sensitized core–shell product can efficiently avoid excitation attenuation (i.e., absorption of the incident laser) and show two-separated linearity with the logarithm of H2O content at a low water content and high water content intervals, respectively, owing to the combined role of Yb3+ excited state quenching and Er3+ excited state quenching. It is expected that this study could provide insights into the design of upconversion nanoparticle based water sensors.