RESUMO
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.
RESUMO
Nanocrystalline silver halides (AgX) such as AgCl, AgBr, and AgI, a class of semiconductor materials with characteristics of both direct and indirect band gaps, represent the most crucial components in traditional photographic processing. The nanocrystal surfaces provide sensitivity specks that can turn into metallic silver, forming an invisible latent image, upon exposure to light. The photographic processing implies that the AgX nanoparticles possess unique properties. First, pristine AgX nanoparticles absorb light only at low efficiency to convert surface AgX into tiny clusters of silver atoms. Second, AgX nanoparticles represent an excellent class of materials to capture electrons efficiently. Third, small metallic silver clusters can catalyze the reduction of AgX nanoparticles to Ag nanoparticles in the presence of mild reducing reagents, known as self-catalytic reduction. These properties indicate that AgX nanoparticles can be partially converted to metallic silver with high precision, leading to the formation of hybrid AgX/Ag nanoparticles. The nanosized metallic Ag usually exhibit intense absorption bands in the visible spectral region due to their strong surface plasmon resonances, which make the AgX/Ag nanoparticles a class of promising visible-light-driven photocatalysts for environmental remediation and CO2 reduction. Despite the less attention paid to their ability of capturing electrons, AgX nanoparticles might be a class of ideal electron shuttle materials to bridge light absorbers and catalysts on which electrons can drive chemical transformations. In this Account, we focus on ternary silver halide alloy (TSHA) nanoparticles, containing two types of halide ions, which increase the composition complexity of the silver halide nanoparticles. Interdiffusion of halide ions between two types of AgX at elevated temperatures has been developed for fabricating ternary silver halide alloy crystals, such as silver chlorobromide optical fibers for infrared communications. This solid state process is not feasible for synthesizing TSHA nanoparticles since it is hard to form two different types of AgX nanoparticles in direct contact. In contrast, coprecipitation of silver ions with different halide ions via colloidal chemistry represents the most promising strategy to synthesize TSHA nanoparticles. Forming uniform and phase-pure ternary silver halide nanocrystals requires that the rate ratio for precipitating both halide ions remains constant throughout the synthesis. However, the significant difference in solubility among different AgX usually leads to a nonuniform compositional distribution in the resulting nanoparticles because the halide ions corresponding to the less soluble AgX precipitate faster at the early reaction stage. This Account summarizes the methods recently developed for the successful synthesis of phase-pure TSHA nanoparticles with uniform sizes and morphologies, which involve precise control over the balanced diffusion of different halide ions to react with silver ions. Typical methods include the use of microemulsion capsules and high-viscosity solvents to lower and even the diffusion coefficients of various halide ions, thus maintaining the precipitation rates of both AgX in single nanoparticles at a constant ratio. The availability of high-quality TSHA nanoparticles provides promising opportunities to explore their new properties and applications.
