Tracking the Dissolution Surface Kinetics of a Single Fluorescent Cyclodextrin Metal-Organic Framework by Confocal Laser Scanning Microscopy

The understanding of the dissolution processes of solids is important for the design and synthesis of solids in a controlled and precise manner and for predicting their fate in the aquatic environment. We report herein single-particle-based confocal laser scanning microscopy (CLSM) for tracking the dissolution surface kinetics of a single fluorescent cyclodextrin metal-organic framework (CD-MOF). As a proof of concept, CD-MOF containing fluorescein, named as CD-MOF⊃FL, was synthesized by encapsulating fluorescein into the interior of CD-MOF via a vapor diffusion method and used as a single-particle dissolution model because of its high FL efficiency and unique structure. The morphology of CD-MOF⊃FL and the distribution of fluorescein within CD-MOF⊃FL were characterized. The growth and dissolution processes of CD-MOF⊃FL at the single-particle level were visualized and quantified for the first time by recording the change of the fluorescence emission. Three processes, including nucleation, germination growth, and saturation stage, were found in the growth of CD-MOF⊃FL, and the growth kinetics followed Avrami's model. The dissolution rate at the face of a single CD-MOF⊃FL crystal was slower than that of its arris, and the dissolution rate of the CD-MOF⊃FL crystal was increased with the increase of the water amount in methanol solution. The dissolution process of the CD-MOF⊃FL crystal was a competitive process of erosion and diffusion in different methanol aqueous solutions, and the dissolution kinetics followed the Korsmeyer-Peppas model. These results offer new insights into the nature of dissolution kinetics of CD-MOF⊃FL and provide new venues for the quantitative analysis of solid dissolution and growth at the single-particle level.

A Novel Dual-Emission Fluorescence Probe Based on CDs and Eu3+ Functionalized UiO-66-(COOH)2 Hybrid for Visual Monitoring of Cu2

In this work, CDs@Eu-UiO-66(COOH)₂ (denoted as CDs-F2), a fluorescent material made up of carbon dots (CDs) and a Eu³⁺ functionalized metal-organic framework, has been designed and prepared via a post-synthetic modification method. The synthesized CDs-F2 presents dual emissions at 410 nm and 615 nm, which can effectively avoid environmental interference. CDs-F2 exhibits outstanding selectivity, great sensitivity, and good anti-interference for ratiometric sensing Cu²⁺ in water. The linear range is 0-200 µM and the limit of detection is 0.409 µM. Interestingly, the CDs-F2's silicon plate achieves rapid and selective detection of Cu²⁺. The change in fluorescence color can be observed by the naked eye. These results reveal that the CDs-F2 hybrid can be employed as a simple, rapid, and sensitive fluorescent probe to detect Cu²⁺. Moreover, the possible sensing mechanism of this dual-emission fluorescent probe is discussed in detail.