Nanotopography modulates intracellular excitable systems through cytoskeleton actuation.

Cellular sensing of most environmental cues involves receptors that affect a signal-transduction excitable network (STEN), which is coupled to a cytoskeletal excitable network (CEN). We show that the mechanism of sensing of nanoridges is fundamentally different. CEN activity occurs preferentially on nanoridges, whereas STEN activity is constrained between nanoridges. In the absence of STEN, waves disappear, but long-lasting F-actin puncta persist along the ridges. When CEN is suppressed, wave propagation is no longer constrained by nanoridges. A computational model reproduces these experimental observations. Our findings indicate that nanotopography is sensed directly by CEN, whereas STEN is only indirectly affected due to a CEN-STEN feedback loop. These results explain why texture sensing is robust and acts cooperatively with multiple other guidance cues in complex, in vivo microenvironments.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores.

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Correction: Extracting accurate information from triplet-triplet annihilation upconversion data with a mass-conserving kinetic model.

Correction for 'Extracting accurate information from triplet-triplet annihilation upconversion data with a mass-conserving kinetic model' by Abhishek Kalpattu et al., Phys. Chem. Chem. Phys., 2022, 24, 28174-28190, https://doi.org/10.1039/D2CP03986A.

Beyond the electrical double layer model: ion-dependent effects in nanoscale solvent organization.

The nanoscale organization of electrolyte solutions at interfaces is often described well by the electrical double-layer model. However, a recent study has shown that this model breaks down in solutions of LiClO4 in acetonitrile at a silica interface, because the interface imposes a strong structuring in the solvent that in turn determines the preferred locations of cations and anions. As a surprising consequence of this organisation, the effective surface potential changes from negative at low electrolyte concentration to positive at high electrolyte concentration. Here we combine previous ion-current measurements with vibrational sum-frequency-generation spectroscopy experiments and molecular dynamics simulations to explore how the localization of ions at the acetonitrile-silica interface depends on the sizes of the anions and cations. We observe a strong, synergistic effect of the cation and anion identities that can prompt a large difference in the ability of ions to partition to the silica surface, and thereby influence the effective surface potential. Our results have implications for a wide range of applications that involve electrolyte solutions in polar aprotic solvents at nanoscale interfaces.

Contractility, focal adhesion orientation, and stress fiber orientation drive cancer cell polarity and migration along wavy ECM substrates.

Contact guidance is a powerful topographical cue that induces persistent directional cell migration. Healthy tissue stroma is characterized by a meshwork of wavy extracellular matrix (ECM) fiber bundles, whereas metastasis-prone stroma exhibit less wavy, more linear fibers. The latter topography correlates with poor prognosis, whereas more wavy bundles correlate with benign tumors. We designed nanotopographic ECM-coated substrates that mimic collagen fibril waveforms seen in tumors and healthy tissues to determine how these nanotopographies may regulate cancer cell polarization and migration machineries. Cell polarization and directional migration were inhibited by fibril-like wave substrates above a threshold amplitude. Although polarity signals and actin nucleation factors were required for polarization and migration on low-amplitude wave substrates, they did not localize to cell leading edges. Instead, these factors localized to wave peaks, creating multiple "cryptic leading edges" within cells. On high-amplitude wave substrates, retrograde flow from large cryptic leading edges depolarized stress fibers and focal adhesions and inhibited cell migration. On low-amplitude wave substrates, actomyosin contractility overrode the small cryptic leading edges and drove stress fiber and focal adhesion orientation along the wave axis to mediate directional migration. Cancer cells of different intrinsic contractility depolarized at different wave amplitudes, and cell polarization response to wavy substrates could be tuned by manipulating contractility. We propose that ECM fibril waveforms with sufficiently high amplitude around tumors may serve as "cell polarization barriers," decreasing directional migration of tumor cells, which could be overcome by up-regulation of tumor cell contractility.

Gating ion and fluid transport with chiral solvent.

The development of modern membranes for ionic separations and energy-storage devices such as supercapacitors depends on the description of ions at solid interfaces, as is often provided by the electrical double layer (EDL) model. The classical EDL model ignores, however, important factors such as possible spatial organization of solvent at the interface and the influence of the solvent on the spatial dependence of the electrochemical potential; these effects in turn govern electrokinetic phenomena. Here we provide a molecular-level understanding of how solvent structure can dictate ionic distributions at interfaces using a model system of a polar, aprotic solvent, propylene carbonate, in its enantiomerically pure and racemic forms, at a silica interface. We link the interfacial structure to the tuning of ionic and fluid transport by the chirality of the solvent and the salt concentration. The results of nonlinear spectroscopic experiments and electrochemical measurements suggest that the solvent exhibits lipid-bilayer-like interfacial organization, with a structure that is dependent on the solvent chirality. The racemic form creates highly ordered layered structure that dictates local ionic concentrations, such that the effective surface potential becomes positive in a wide range of electrolyte concentrations. The enantiomerically pure form exhibits weaker ordering at the silica surface, which leads to a lower effective surface charge induced by ions partitioning into the layered structure. The surface charge in silicon nitride and polymer pores is probed through the direction of electroosmosis that the surface charges induce. Our findings add a new dimension to the nascent field of chiral electrochemistry, and emphasize the importance of including solvent molecules in descriptions of solid-liquid interfaces.

Excitable systems: A new perspective on the cellular impact of elongate mineral particles.

We investigate how the geometry of elongate mineral particles (EMPs) in contact with cells influences esotaxis, a recently discovered mechanism of texture sensing. Esotaxis is based on cytoskeletal waves and oscillations that are nucleated, shaped, and steered by the texture of the surroundings. We find that all EMPs studied trigger an esotactic response in macrophages, and that this response dominates cytoskeletal activity in these immune cells. In contrast, epithelial cells show little to no esotactic response to the EMPs. These results are consistent with the distinct interactions of both cell types with ridged nanotopographies of dimensions comparable to those of asbestiform EMPs. Our findings raise the question of whether narrow, asbestiform EMPs may also dominate cytoskeletal activity in other types of immune cells that exhibit similar esotactic effects. These findings, together with prior studies of esotaxis, lead us to the hypothesis that asbestiform EMPs suppress the migration of immune cells and activate immune signaling, thereby outcompeting signals that would normally stimulate the immune system in nearby tissue.

Extracting accurate information from triplet-triplet annihilation upconversion data with a mass-conserving kinetic model.

Triplet-triplet annihilation upconversion (TTA-UC) is a process that shows promise for applications such as energy-harvesting and light-generation technologies. The irradiance dependent performance of TTA-UC systems is typically gauged using a graphical analysis, rather than a detailed model. Additionally, kinetic models for TTA-UC rarely incorporate mass conservation, which is a phenomenon that can have important consequences under experimentally relevant conditions. We present an analytical, mass-conserving kinetic model for TTA-UC, and demonstrate that the mass-conservation constraint cannot generally be ignored. This model accounts for saturation in TTA-UC data. Saturation complicates the interpretation of the threshold irradiance Ith, a popular performance metric. We propose two alternative figures of merit for overall performance. Finally, we show that our model can robustly fit experimental data from a wide variety of sensitized TTA-UC systems, enabling the direct and accurate determination of Ith and of our proposed performance metrics. We employ this fitting procedure to benchmark and compare these metrics, using data from the literature.

Polymeric Ligand-Mediated Regioselective Bonding of Plasmonic Nanoplates and Nanospheres.

Nanoparticle (NP) clusters are attractive for many applications, but controllable and regioselective assembly of clusters remains challenging. This communication reports a strategy to precisely assemble Ag nanoplates (NP-As) and Au nanospheres (NP-Bs) grafted with copolymer ligands into defined ABx clusters with controlled coordination number (x) and orientation of the NPs. The directional bonding of shaped NPs relies on the stoichiometric reaction of complementary reactive groups on copolymer ligands. The x value of NP clusters can be tuned from 1 to 4 by varying the number ratio of reactive groups on single NP-Bs to NP-As. The regioselective bonding of nanospheres to the edge or face of a central nanoplate is governed by the steric hindrance of copolymeric ligands on the nanoplate. The clusters exhibit distinctive plasmonic properties that are dependent on the bonding modes of NPs. This study paves a route to fabricating nanostructures with high precision and complexity for applications in plasmonics, catalysis, and sensing.

Extracting Information on Linear and Nonlinear Absorption from Two-Beam Action Spectroscopy Data.

Two-beam action (2-BA) spectroscopies are a recently developed class of techniques for determining the order(s) of absorption (one-photon, two-photon, etc.) that contribute to an observable signal. When only a single order of absorption is present, 2-BA spectroscopies allow for the determination of that order from data obtained at a single value of the observable. It has been shown previously that when two orders of absorption are present, they can be determined unambiguously from measurements made at several values of the observable. However, this latter approach cannot be used for single-valued observables, such as a polymerization threshold. Here we develop a theoretical comparison between conventional methods that determine the order(s) of absorption using logarithmic plots and 2-BA-based techniques. We also explore how 2-BA plots arising from two orders of absorption deviate from a plot with a single, noninteger exponent. We demonstrate that these deviations can usually be used to identify the two orders of absorption and their relative contributions to the signal on the basis of measurements made at a single value of the observable.