Ionosomes: Observation of Ionic Bilayer Water Clusters

Hydrodynamics-Controlled Single-Particle Electrocatalysis

Electrocatalysis is considered promising in renewable energy conversion and storage, yet numerous efforts rely on catalyst design to advance catalytic activity. Herein, a hydrodynamic single-particle electrocatalysis methodology is developed by integrating collision electrochemistry and microfluidics to improve the activity of an electrocatalysis system. As a proof-of-concept, hydrogen evolution reaction (HER) is electrocatalyzed by individual palladium nanoparticles (Pd NPs), with the development of microchannel-based ultramicroelectrodes. The controlled laminar flow enables the precise delivery of Pd NPs to the electrode-electrolyte interface one by one. Compared to the diffusion condition, hydrodynamic collision improves the number of active sites on a given electrode by 2 orders of magnitude. Furthermore, forced convection enables the enhancement of proton mass transport, thereby increasing the electrocatalytic activity of each single Pd NP. It turns out that the improvement in mass transport increases the reaction rate of HER at individual Pd NPs, thus a phase transition without requiring a high overpotential. This study provides new avenues for enhancing electrocatalytic activity by altering operating conditions, beyond material design limitations.

Multiphase Chemistry under Nanoconfinement: An Electrochemical Perspective

Most relevant systems of interest to modern chemists rarely consist of a single phase. Real-world problems that require a rigorous understanding of chemical reactivity in multiple phases include the development of wearable and implantable biosensors, efficient fuel cells, single cell metabolic characterization techniques, and solar energy conversion devices. Within all of these systems, confinement effects at the nanoscale influence the chemical reaction coordinate. Thus, a fundamental understanding of the nanoconfinement effects of chemistry in multiphase environments is paramount. Electrochemistry is inherently a multiphase measurement tool reporting on a charged species traversing a phase boundary. Over the past 50 years, electrochemistry has witnessed astounding growth. Subpicoampere current measurements are routine, as is the study of single molecules and nanoparticles. This Perspective focuses on three nanoelectrochemical techniques to study multiphase chemistry under nanoconfinement: stochastic collision electrochemistry, single nanodroplet electrochemistry, and nanopore electrochemistry.

Voltage-driven ion flux promotes emulsification at the water|oil interface

Emulsions are critical across a vast range of industries. Generally, emulsion synthesis is a complicated chemical process, requiring many mixed-phase systems. Here, we demonstrate that the flux of ions across the oil|water interface induces emulsification. Ion flux is achieved by a voltage-driven process, where an anode and a cathode are placed in each phase. When a current density of 2 mA cm-2 is reached across the interface, emulsification occurs. We demonstrate that emulsification can be tuned to occur in both phases, depending on the ions present. Droplet sizes are on the order of hundreds of nm and are stable for over an hour even without purposefully added surfactant. We demonstrate qualitative control over droplet size and charge based on salt content, current densities, and polarity of the interface. The process is 1000 times less energetic than ultrasonication. Our results introduce a robust and low-energy means of nanodroplet dispersion without the use of more than two phases and complex phase-transfer agents.

Observing Discrete Blocking Events at a Polarized Micro- or Submicro-Liquid/Liquid Interface

Single-entity collisional electrochemistry (SECE), a subfield of single-entity electrochemistry, enables directly characterizing entities and particles in the electrolyte solution at the single-entity resolution. Blockade SECE at the traditional solid ultramicroelectrode (UME)/electrolyte interface suffers from a limitation: only redox-inactive particles can be studied. The wide application of the classical Coulter counter is restricted by the rapid translocation of entities through the orifice, which results in a remarkable proportion of undetected signals. In response, the blocking effect of single charged conductive or insulating nanoparticles (NPs) at low concentrations for ion transfer (IT) at a miniaturized polarized liquid/liquid interface was successfully observed. Since the particles are adsorbed at the liquid/liquid interface, our method also solves the problem of the Coulter counter having a too-fast orifice translocation rate. The decreasing quantal staircase/step current transients are from landings (controlled by electromigration) of either conductive or insulating NPs onto the interface. This interfacial NP assembly shields the IT flux. The size of each NP can be calculated by the step height. The particle size measured by dynamic light scattering (DLS) is used for comparison with that calculated from electrochemical blocking events, which is in fairly good agreement. In short, the blocking effect of IT by single entities at micro- or submicro-liquid/liquid interface has been proven experimentally and is of great reference in single-entity detection.

Progress in microsphere-based scaffolds in bone/cartilage tissue engineering

Bone/cartilage repair and regeneration have been popular and difficult issues in medical research. Tissue engineering is rapidly evolving to provide new solutions to this problem, and the key point is to design the appropriate scaffold biomaterial. In recent years, microsphere-based scaffolds have been considered suitable scaffold materials for bone/cartilage injury repair because microporous structures can form more internal space for better cell proliferation and other cellular activities, and these composite scaffolds can provide physical/chemical signals for neotissue formation with higher efficiency. This paper reviews the research progress of microsphere-based scaffolds in bone/chondral tissue engineering, briefly introduces types of microspheres made from polymer, inorganic and composite materials, discusses the preparation methods of microspheres and the exploration of suitable microsphere pore size in bone and cartilage tissue engineering, and finally details the application of microsphere-based scaffolds in biomimetic scaffolds, cell proliferation and drug delivery systems.

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

Herein, we feature our recent efforts toward the development and application of nanoelectrochemistry at liquid/liquid interfaces, which are also known as interfaces between two immiscible electrolyte solutions (ITIES). Nanopipets, nanopores, and nanoemulsions are developed to create the nanoscale ITIES for the quantitative electrochemical measurement of ion transfer, electron transfer, and molecular transport across the interface. The nanoscale ITIES serves as an electrochemical nanosensor to enable the selective detection of various ions and molecules as well as high-resolution chemical imaging based on scanning electrochemical microscopy. The powerful nanoelectroanalytical methods will be useful for biological and material applications as illustrated by in situ studies of solid-state nanopores, nuclear pore complexes, living bacteria, and advanced nanoemulsions. These studies provide unprecedented insights into the chemical reactivity of important biological and material systems even at the single nanostructure level.

Monitoring Photoinduced Interparticle Chemical Communication In Situ

Monitoring interparticle chemical communication plays a critical role in the nanomaterial synthesis as this communication controls the final structure and stability of global nanoparticles (NPs). Yet most ensemble analytical techniques, which could only reveal average macroscopic information, are unable to elucidate NP-to-NP interactions. Herein, we employ stochastic collision electrochemistry to track the morphology transformation of Ag NPs in photochemical process at the single NP level. By further statistical analysis of time-resolved current transients, we quantitatively determine the dynamic chemical potential difference and interparticle communication between populations of large and small Ag NPs. The high sensitivity of stochastic collision electrochemistry enables the in situ investigation of chemical communication-dependent transformation kinetics of NPs in photochemical process, shedding light on designing nanomaterials.

Galvani Potential-Dependent Single Collision/Fusion Impacts at Liquid/Liquid Interface: Faradic or Capacitive?

A new subtype of nano-impacts by emulsion droplets via reorganization of the electric double layer (EDL) at the liquid/liquid interface (LLI) is reported. This subtype shows anodic, bipolar, and cathodic transient currents with a potential of zero charge (PZC) dependence, revealing the non-faradic characteristic of single fusion impacts. In addition, the absolute integrated mean charge is proportional to the Galvani potential at the ITIES, indicating that the EDL at the LLI may obey the discrete Helmholtz model. The exact PZC point is interpolated from the fitting curve, and the droplet size distribution is estimated from the integrated charge distribution. Moreover, the different values of E_pzc between single fusion impacts of MgCl₂ droplets and pure water droplets is due to the specific absorption between Mg²⁺ and antagonistic anion in the organic phase. The influence of the concentration of the supporting electrolyte is also investigated. The above work gives physicochemical insights into the EDL at the micropipette-supported LLI and provides potential application to measure micro/nanoscale heterogeneous media without catalytic, reactive, or charge-transfer activity via impact experiments at LLI.

Non-Covalent Reconfigurable Microgel Colloidosomes with a Well-Defined Bilayer Shell

Microgels are extremely interfacially active and are widely used to stabilize emulsions. However, they are commonly used to stabilize oil-in-water emulsions due to their intrinsic hydrophilicity and initially dispersed in water. In addition, there have been no attempts to control microgel structural layers that are formed at the interface and as a result it limits applications of microgel in advanced materials. Here, we show that by introducing octanol into poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels, octanol-swollen microgels can rapidly diffuse from the initially dispersed oil phase onto the water droplet surface. This facilitates the formation of microgel-laden interfacial layers with strong elastic responses and also generates stable inverse water-in-oil Pickering emulsions. These emulsions can be used as templates to produce microgel colloidosomes, herein termed ‘microgelsomes’, with shells that can be fine-tuned from a particle monolayer to a well-defined bilayer. The microgelsomes can then be used to encapsulate and/or anchor nanoparticles, proteins, vitamin C, bio-based nanocrystals or enzymes. Moreover, the programmed release of these substances can be achieved by using ethanol as a trigger to mediate shell permeability. Thus, these reconfigurable microgelsomes with a microgel-bilayer shell can respond to external stimuli and demonstrate tailored properties, which offers novel insights into microgels and promise wider application of Pickering emulsions stabilized by soft colloids.

Inverse W/O Pickering emulsions and reconfigurable microgelsomes with a well-defined bilayer structure are prepared from octanol-swollen PNIPAM-co-MAA microgels and the combination of binary microgels, which promise wider application of soft colloids.