pH-triggered adjustable bursting of liquid marbles in water pools

Abstract

Non-sticking millimetric droplets wrapped with micron/submicron-sized liquid-repellent solid particles and aggregates are recognized as liquid marbles (LMs). LMs behave like a non-wetting solid, which enables zero-loss transportation on solid surfaces and liquid pools. The challenge is controlling the liquid-releasing behavior, especially in a liquid pool. For example, releasing the inner liquid of LMs at a specified time in adaptation to liquid pool conditions has not been achieved. In the past, different approaches were introduced to demonstrate the instant release of inner liquid in the presence of a specific stimulus, but reports on controlling the lifetime of floating LMs in the presence of a stimulus are extremely rare in the literature. In this context, an inherently porous and crystalline nanomaterial, i.e., metal organic frameworks (MOFs), is rationally transformed into a chemically reactive nanomaterial for associating both (a) dual pH-responsive hydrophobicity and (b) non-responsive superhydrophobicity via a 1,4-conjugate addition reaction. Further, the Kang–Jacobi wettability model was applied to examine the wettability transition of the modified MOFs at different pH. Hydrophobic MOFs yielded LMs with the ability to automatically release inner liquid at a pre-defined time without requiring external intervention. Furthermore, the strategic association of protonizable amines, labile coordination bonds, and the tailored hydrophobicity attributed to the dual (acidic/alkaline) stimulus-responsive adjustable lifetime of LMs in the water pool. Such LMs were utilized for demonstrating an (1) adjustable drug release, (2) on-demand release of the reactants to perform chemical reactions, and (3) rapid optode detection of NO₂⁻.