Mesoporous SiO₂ Nanoparticles: A Unique Platform Enabling Sensitive Detection of Rare Earth Ions with Smartphone...

Fast and sensitive detection of dilute rare earth species still represents a challenge for an on-site survey of new resources and evaluation of the economic value. In this work, a robust and low-cost protocol has been developed to analyze the concentration of rare earth ions using a smartphone camera. The success of this protocol relies on mesoporous silica nanoparticles (MSNs) with large-area negatively charged surfaces, on which the rare earth cations (e.g., Eu³⁺) are efficiently adsorbed through electrostatic attraction to enable a "concentrating effect". The initial adsorption rate is as fast as 4025 mg (g min)^-1, and the adsorption capacity of Eu³⁺ ions in the MSNs is as high as 4730 mg g^-1 (equivalent to ~ 41.2 M) at 70 °C. The concentrated Eu³⁺ ions in the MSNs can form a complex with a light sensitizer of 1,10-phenanthroline to significantly enhance the characteristic red emission of Eu³⁺ ions due to an "antenna effect" that relies on the efficient energy transfer from the light sensitizer to the Eu³⁺ ions. The positive synergy of "concentrating effect" and "antenna effect" in the MSNs enables the analysis of rare earth ions in a wide dynamic range and with a detection limit down to ~ 80 nM even using a smartphone camera. Our results highlight the promise of the protocol in fieldwork for exploring valuable rare earth resources.