Structurally Anisotropic Janus Particles with Tunable Amphiphilicity via Polymerization of Dynamic Complex Emulsions

Nanoparticle Surfactants at Complex Emulsion Interfaces

Using nanoparticle surfactants to stabilize the liquid-liquid interface has attracted significant attention for developing all-liquid constructs including emulsions and liquid devices. Here, an efficient strategy is demonstrated to stabilize complex emulsions that consist of multiphase droplets by using the co-assembly between the cellulose nanocrystal and amine-functionalized polystyrene. Cellulose nanocrystal surfactants (CNCSs) form and assembly in situ at the specified area of emulsion interface, showing a unique pH responsiveness due to their dynamic nature and allowing the reconfiguration of complex emulsion from encapsulated to Janus structures. Such complex emulsions can be further used as the templates to fabricate polymeric particles with hollow, semi-spherical, and spherical shapes on large scale. These findings establish a promising platform for designing intelligent soft matter that can be used in microreactors, sensors, and anisotropic materials.

Responsive Janus droplets as modular sensory layers for the optical detection of bacteria

The field of biosensor development is fueled by innovations in new functional transduction materials and technologies. Material innovations promise to extend current sensor hardware limitations, reduce analysis costs, and ensure broad application of sensor methods. Optical sensors are particularly attractive because they enable sensitive and noninvasive analyte detection in near real-time. Optical transducers convert physical, chemical, or biological events into detectable changes in fluorescence, refractive index, or spectroscopic shifts. Thus, in addition to sophisticated biochemical selector designs, smart transducers can improve signal transmission and amplification, thereby greatly facilitating the practical applicability of biosensors, which, to date, is often hampered by complications such as difficult replication of reproducible selector-analyte interactions within a uniform and consistent sensing area. In this context, stimuli-responsive and optically active Janus emulsions, which are dispersions of kinetically stabilized biphasic fluid droplets, have emerged as a novel triggerable material platform that provides as a versatile and cost-effective alternative for the generation of reproducible, highly sensitive, and modular optical sensing layers. The intrinsic and unprecedented chemical-morphological-optical coupling inside Janus droplets has facilitated optical signal transduction and amplification in various chemo- and biosensor paradigms, which include examples for the rapid and cost-effective detection of major foodborne pathogens. These initial demonstrations resulted in detection limits that rival the capabilities of current commercial platforms. This trend article aims to present a conceptual summary of these initial efforts and to provide a concise and comprehensive overview of the pivotal kinetic and thermodynamic principles that govern the ability of Janus droplets to sensitively and selectively respond to and interact with bacteria.

Dynamic In Situ Monitoring of the Salt Counter-ion Effect on Surfactant Effectiveness Using Reconfigurable Janus Emulsions

A straightforward method for visualization and quantification of surfactant effectiveness within different electrolyte environments based on using reconfigurable Janus emulsions as novel optical probes is reported. More specifically, we investigated the effect of different types and concentrations of salt counter-ions on the surfactant surface excess of commercial ionic and non-ionic surfactants, namely sodium dodecyl sulfate (SDS) and Tween 80 via in situ monitoring the morphological reconfigurations of biphasic Janus emulsions comprising hydrocarbon and fluorocarbon oils. We find that significant variations in interfacial tensions of SDS-stabilized interfaces (up to 15 mN·m^-1) can be evoked by titrating mono-, di-, and trivalent cationic counter-ions, which is coherent with the lyotropic (Hofmeister) series. In contrast, the salt counter-ion effect on the surfactant effectiveness was less pronounced for the non-ionic surfactant Tween 80 (∼3 mN·m^-1). Our results reveal a facile in situ method for monitoring the central role of electrolyte type and concentration on surfactant effectiveness and, more broadly, illustrate that Janus emulsions serve as powerful optical probes to dynamically study the properties of surfactants at liquid interfaces. We demonstrate the utility of our findings for an electro-induced morphological reconfiguration of Janus droplet morphologies by dynamically tuning Cu²⁺ concentration in solution using an electrode setup. The latter provides a unique platform for liquid-phase, real-time, and continuous tuning of Janus droplet morphologies, e.g., for their application in sensing and dynamic optical device platforms.

Morphology-Dependent Aggregation-Induced Emission of Janus Emulsion Surfactants

We report a novel stimuli-responsive fluorescent material platform that relies on an evocation of aggregation-induced emission (AIE) from tetraphenylethylene (TPE)-based surfactants localized at one hemisphere of biphasic micro-scale Janus emulsion droplets. Dynamic alterations in the available interfacial area were evoked through surfactant-induced dynamic changes of the internal droplet morphology that can be modulated as a function of the balance of interfacial tensions of the droplet constituent phases. Thus, by analogy with a Langmuir-Blodgett trough that enables selective concentration of surfactants at a liquid-gas interface, we demonstrate here a method for controllable modulation of the available interfacial area of surfactant-functionalized liquid-liquid interfaces. We show that a morphology-dependent alteration of the interfacial area can be used to evoke an optical signal, by selectively assembling synthesized TPE-based surfactants on the respective droplet interfaces. A trigger-induced increase in the concentration of TPE-based surfactants at the liquid-liquid interfaces results in an evocation of aggregation-induced emission (AIE), inducing an up to 3.9-fold increase in the measured emission intensity of the droplets.

Synthetic Strategies for Polymer Particles with Surface Concavities

Over the past decade or so, there has been increasing interest in the synthesis of polymer particles with surface concavities, which mainly include golf ball-like, dimpled and surface-wrinkled polymer particles. Such syntheses generally can be classified into direct polymerization and post-treatment on preformed polymer particles. This review aims to provide an overview of the synthetic strategies of such particles. Some selected examples are given to present the formation mechanisms of the surface concavities. The applications and future development of these concave polymer particles are also briefly discussed. This article is protected by copyright. All rights reserved.

Synthesis of Polymer Janus Particles with Tunable Wettability Profiles as Potent Solid Surfactants to Promote Gas Delivery in Aqueous Reaction Media

Janus particles exhibit a strong tendency to directionally assemble and segregate to interfaces and thus offer advantages as colloidal analogues of molecular surfactants to improve the stability of multiphasic mixtures. Investigation and application of the unique adsorption properties require synthetic procedures that enable careful design and reliable control over the particles' asymmetric chemistry and wettability profiles with high morphological uniformity across a sample. Herein, we report on a novel one-step synthetic approach for the generation of amphiphilic polymer Janus particles with highly uniform and tunable wettability contrasts, which is based on using reconfigurable bi-phasic Janus emulsions as versatile particle scaffolds. Two phase-separated acrylate oils were used as the constituent droplet phases and transformed into their solidified Janus particle replicas via UV-induced radical polymerization. Using Janus emulsions as particle precursors offers the advantage that their internal droplet geometry can be fine-tuned by changing the force balance of surface tensions acting at the individual interfaces via surfactants or the volume ratio of the constituent phases. In addition, preassembled functional surfactants at the droplet interfaces can be locked in position upon polymerization, which enables both access toward postfunctionalization reaction schemes and the generation of highly uniform Janus particles with adjustable wettability profiles. Depending on the particle morphology and wettability, their interfacial position can be adjusted, which allows us to stabilize either air bubbles-in-water or water droplets-in-air (liquid marbles). Motivated by the interfacial activity of the particles and particularly the longevity of the resulting particle-stabilized air-in-water bubbles, we explored their ability to promote the delivery of oxygen inside a liquid-phase reaction medium, namely, for the heterogeneous Au-NP-mediated catalytic oxidation of d-glucose. We observed a 2.2-fold increase in the reaction rate attributed to the increase of the local concentration of oxygen around catalysts, thus showcasing a new strategy to overcome the limited solubility of gases in aqueous reaction media.