Siowling Soh

The creation of life from chemical systems remains poorly understood. Previous studies have discussed the possibility that fatty acids played a crucial role in the emergence of early primitive cells. This work discusses the idea that a droplet containing the specific combination (C16–/C18═) of a saturated fatty acid with 16 carbons (C16−) and a monounsaturated fatty acid with 18 carbons (C18═) exhibits the best migration characteristics, including Lévy flights, sustained velocity, and a large active region, among many single types or mixtures of fatty acids. Common phospholipids in many modern cells also have this specific C16–/C18═ combination as their hydrophobic tails. Hence, a match is achieved between best migration and modern cells despite vast possibilities on both sides. Lévy flights are an optimal searching strategy for scarce resources; optimal searching is critical for survival among competitors. Droplets with other fatty acids have either random or ballistic motions that are less optimal for searching. This match thus points to the possibility that the best migration of the C16–/C18═ combination may potentially be the fundamental mechanism for the evolution of a chemical into a living system.

Degradation of polymers for recycling generates mixtures of molecules. Molecular separation is needed for many applications of individual molecules. However, molecular separation is challenging, especially for recycling. This study reports on a general class of molecules ("Poly-Linkers") that act as cross-linkers for forming macroscale solid polymers and have the chemical functionality to perform molecular separation for recycling. They do not resemble typical cross-linkers; instead, they include common molecules (e.g., fatty acids) with diverse molecular structures and applications. The polymers have general and unique applications, including the simple fabrication of drug-loaded core-shell particles due to the dual roles of ibuprofen as both the drug and Poly-Linker. Besides minimal energy required for de-cross-linking, both molecular separation and formation of the macroscale polymers are driven by favorable thermodynamics and occur spontaneously without energy. Therefore, performing the entire cycle of recycling of these polymers is energetically efficient and simple.

Structured porous materials, including metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs), are being intensely developed because they can separate small molecules via precisely constructed and uniformly sized pores, despite challenges that hinder practical use. Polymers are structurally amorphous; hence, they are not considered fundamentally capable of separating different organic molecules. This manuscript reports that amorphous polymers can be prepared to separate different small organic molecules (i.e., solutes dissolved in a solvent) effectively, with very large separation factors of ~1000 and ~100,000. Effective separation involves fundamentally a delicate balance between hydrophobicity and hydrophilicity and dynamic mobility of polymeric chains to produce optimal pores. Therefore, structured pores are not fundamentally essential for molecular separation - amorphous pores are effective. These stimuli-responsive polymers have reversibly switchable states with completely opposite selectivities. They are multifunctional: they separate by polarity, size, and with opposite selectivities. Hence, they can separate many different molecules in complex solutions. These polymers are simple and inexpensive to fabricate for widespread use.

Charge separation occurs spontaneously at the solid–liquid interface, forming an electric double layer. Previous methods, including streaming current, used to harvest the constantly separated charge in micron-sized and larger systems reported negligible power output due to the fundamental limit caused by the very short nanoscale Debye length. This study reports on the phenomenon that plug flow of water that falls naturally down a millimeter-sized tube generates electricity with a high efficiency of >10% and power density of ∼100 W/m². This high power breaks the theoretical limit defined by the Debye length in macroscale channels. Plug flow generates 5 orders of magnitude more electricity than continuous flow (i.e., streaming current) and more than other technologies using falling water. Plug flow triggers a unique interfacial chemistry with large chemical potential of charge separation: the complete spatial separation of aqueous H⁺ and OH^– ions without the electric double layer. Having macroscale channels enables the energy of water from nature (e.g., rain or rivers) to be harvested freely. The simple setup lights up multiple LEDs continuously, modifies surfaces, and performs chemical reactions. Plug flow via harvesting energy from nature is a source of renewable power with many advantages for achieving sustainable societies.

What is static charge? Despite the long history of research, the identity of static charge and mechanism by which static is generated by contact electrification are still unknown. Investigations are challenging due to the complexity of surfaces. This study involves the molecular-scale analysis of contact electrification using highly well-defined surfaces functionalized with a self-assembled monolayer of alkylsilanes. Analyses show the elementary molecular steps of contact electrification: the exact location of heterolytic cleavage of covalent bonds (i.e., Si-C bond), exact charged species generated (i.e., alkyl carbocation), and transfer of molecular fragments. The strong correlation between charge generation and molecular fragments due to their signature odd-even effects further shows that contact electrification is based on cleavage of covalent bonds and transfer of ionic molecular fragments. Static charge is thus an alkyl carbocation; in general, it is an ionic molecular fragment. This mechanism based on cleavage of covalent bonds is applicable to general types of insulating materials, such as covalently bonded polymers. The odd-even effect of charging caused by the difference of only one atom explains the highly sensitive nature of contact electrification.

Polymeric molecules in a liquid medium are programmed with instructions written by specific chemical moieties for building structures. The molecules migrate to specific sites for fabricating customized shaped particles with core–shell structures.

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.