Thomas M. Hermans

Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles

Although multiple methods have been developed to detect metal cations, only a few offer sensitivities below 1 pM, and many require complicated procedures and sophisticated equipment. Here, we describe a class of simple solid-state sensors for the ultrasensitive detection of heavy-metal cations (notably, an unprecedented attomolar limit for the detection of CH(3)Hg(+) in both standardized solutions and environmental samples) through changes in the tunnelling current across films of nanoparticles (NPs) protected with striped monolayers of organic ligands. The sensors are also highly selective because of the ligand-shell organization of the NPs. On binding of metal cations, the electronic structure of the molecular bridges between proximal NPs changes, the tunnelling current increases and highly conductive paths ultimately percolate the entire film. The nanoscale heterogeneity of the structure of the film broadens the range of the cation-binding constants, which leads to wide sensitivity ranges (remarkably, over 18 orders of magnitude in CH(3)Hg(+) concentration).

Hierarchical Formation of Supramolecular Transient Networks in Water : A Modular Injectable Delivery System

A modular one-component supramolecular transient network in water, based on poly(ethylene glycol) and end-capped with four-fold hydrogen bonding units, is reported. Due to its nonlinear structural formation, this system allows active proteins to be added to the hydrogel during formation. Once implanted in vivo it releases the protein by erosion of both the protein and polymer via dissolution.

Control of Surface Charges by Radicals as a Principle of Antistatic Polymers Protecting Electronic Circuitry

Dissipating Static The accumulation of a static charge on polymers and other insulators often causes little more than a slight annoyance but it can lead to the destruction of sensitive electrical equipment. Thus, approaches are required that prevent and dissipate static electricity through improved electrical conductivity, or that ensure complete discharge before a contact with a key piece of equipment. Baytekin et al. (p. 1368) show that surface charges will colocalize with radicals on the surface of a polymer, and that the addition of free radical scavengers causes a discharge of the surface as the charges are removed. The approach was used successfully to produce coatings that protected electronic circuits from damage caused by electrostatic discharge. Removal of radicals destabilizes surface charges, providing a means for rapid dissipation of static electricity. Even minute quantities of electric charge accumulating on polymer surfaces can cause shocks, explosions, and multibillion-dollar losses to electronic circuitry. This paper demonstrates that to remove static electricity, it is not at all necessary to “target” the charges themselves. Instead, the way to discharge a polymer is to remove radicals from its surface. These radicals colocalize with and stabilize the charges; when they are scavenged, the surfaces discharge rapidly. This radical-charge interplay allows for controlling static electricity by doping common polymers with small amounts of radical-scavenging molecules, including the familiar vitamin E. The effectiveness of this approach is demonstrated by rendering common polymers dust-mitigating and also by using them as coatings that prevent the failure of electronic circuitry.

Controlled Supramolecular Oligomerization of C3‐Symmetrical Molecules in Water: The Impact of Hydrophobic Shielding

The supramolecular oligomerization of three water-soluble C(3)-symmetrical discotic molecules is reported. The compounds all possess benzene-1,3,5-tricarboxamide cores and peripheral Gd(III)-DTPA (diethylene triamine pentaacetic acid) moieties, but differ in their linker units and thus in their propensity to undergo secondary interactions in H(2)O. The self-assembly behavior of these molecules was studied in solution using circular dichroism, UV/Vis spectroscopy, nuclear magnetic resonance, and cryogenic transmission electron microscopy. The aggregation concentration of these molecules depends on the number of secondary interactions and on the solvophobic character of the polymerizing moieties. Hydrophobic shielding of the hydrogen-bonding motif in the core of the discotic is of paramount importance for yielding stable, helical aggregates that are designed to be restricted in size through anti-cooperative, electrostatic, repulsive interactions.

Non-equilibrium steady states in supramolecular polymerization

Living systems use fuel-driven supramolecular polymers such as actin to control important cell functions. Fuel molecules like ATP are used to control when and where such polymers should assemble and disassemble. The cell supplies fresh ATP to the cytosol and removes waste products to sustain steady states. Artificial fuel-driven polymers have been developed recently, but keeping them in sustained non-equilibrium steady states (NESS) has proven challenging. Here we show a supramolecular polymer that can be kept in NESS, inside a membrane reactor where ATP is added and waste removed continuously. Assembly and disassembly of our polymer is regulated by phosphorylation and dephosphorylation, respectively. Waste products lead to inhibition, causing the reaction cycle to stop. Inside the membrane reactor, however, waste can be removed leading to long-lived NESS conditions. We anticipate that our approach to obtain NESS can be applied to other stimuli-responsive materials to achieve more life-like behaviour.

Transport into Metal-Organic Frameworks from Solution Is Not Purely Diffusive**

Chemistry in motion: a combination of confocal microscopy and reaction-diffusion modeling provided a powerful toolkit with which solution transport into metal-organic framework crystals was studied. Commonly used pure diffusion models are insufficient to describe this process and, instead, it is necessary to account for the interactions of the guest molecules and the MOF scaffold.

Vortex flows impart chirality-specific lift forces

Recent reports that macroscopic vortex flows can discriminate between chiral molecules or their assemblies sparked considerable scientific interest both for their implications to separations technologies and for their relevance to the origins of biological homochirality. However, these earlier results are inconclusive due to questions arising from instrumental artifacts and/or insufficient experimental control. After a decade of controversy, the question remains unresolved-how do vortex flows interact with different stereoisomers? Here, we implement a model experimental system to show that chiral objects in a Taylor-Couette cell experience a chirality-specific lift force. This force is directed parallel to the shear plane in contrast to previous studies in which helices, bacteria and chiral cubes experience chirality-specific forces perpendicular to the shear plane. We present a quantitative hydrodynamic model that explains how chirality-specific motions arise in non-linear shear flows through the interplay between the shear-induced rotation of the particle and its orbital translation. The scaling laws derived here suggest that rotating flows can be used to achieve chiral separation at the micro- and nanoscales.

Droplets Out of Equilibrium

Magnetic droplets can be switched between static and dynamic structures. [Also see Report by Timonen et al.] Living systems create structures and functions of remarkable complexity by mastering self-assembly in different equilibrium and nonequilibrium states. Three states can be distinguished: equilibrium, nondissipative nonequilibrium (or kinetically trapped), and dissipative (or dynamic) nonequilibrium. On page 253 of this issue, Timonen et al. (1) report on a model system in which all three states are accessible (see the figure) and show how this leads to a range of well-ordered structures.

pH Oscillator Stretched in Space but Frozen in Time

Chemical oscillations are studied using a continuous-flow microfluidic system transforming the time domain of chemical oscillators into a spatial domain. This system allows one (i) to monitor the dynamics of chemical oscillators with the accuracy of vigorously stirred batch reactors but with the ease and speed of CSTRs and (ii) to rapidly screen the phase space of chemical oscillators in just one experiment versus a traditional series of batch measurements.

Self-assembly: Materials from a peptide soup

Small dipeptide fragments enzymatically combine and split to form sequences that self-assemble into nanomaterials.