pH-Responsive Liquid Marbles Stabilized with Chitosan-Stearic Acid-Conjugated Particles

Liquid marbles (LMs) are liquid droplets covered by hydrophobic solid particles adsorbed at liquid–gas interfaces. In this study, we designed pH-responsive LMs by using chitosan (CS) particles conjugated with stearic acid (SA) as a stabilizer, both of which are natural materials. Micrometer-sized CS-SA-conjugated particles were synthesized via a sustainable route based on solvent-free and one-pot thermal amidation between CS and SA. The resulting particles were extensively characterized on multiple scales by determining their sizes, shapes, morphologies, bulk/surface chemical compositions, hydrophilic–hydrophobic balances, and pH-responsive behaviors. The heterogeneous reactions only occurred near the particle surface, and the CS-SA-conjugated particles showed hydrophobic surfaces and pH-responsive cores. Millimeter- and centimeter-sized LMs with liquid contents ranging between 15 μL and 1.0 mL were readily prepared by rolling water droplets over a dried CS-SA particle powder bed. Stereomicroscopy studies confirmed that the CS-SA particles were adsorbed at the surfaces of the water droplets as mono- and bilayers, resulting in stable LMs. These LMs showed long-term stability (>2 h) under a water vapor atmosphere but were disrupted immediately (<2 min) when exposed to HCl vapor, with wetting of the particles by the inner aqueous solution. Here, acid vapor-induced disruption was realized with the "response cascade" concept: the initial pH stimulus led to disruption via intermediate wetting responses. We also demonstrated the use of LMs as colorimetric sensors for amines generated during food spoilage.