Efficient Surfactant-Mediated Photovoltaic Manipulation of fL-Scale Aqueous Microdroplets for Diverse Optofluidic Applications on LiNbO3 Platform

The electrodeless biocompatible manipulation of femtoliter-scale aqueous microdroplets remains challenging. The appropriate isolation of electrostatic charges from femtoliter-scale aqueous microdroplets is crucial for electrodeless optoelectronic manipulation based on space-charge-density modulation. Here, surfactant-mediated photovoltaic manipulation is proposed, where the surfactant layers self-assembled at the water-oil and oil-Lithium niobate interfaces are employed to isolate photovoltaic charges. The reduced electrostatic attenuation, remarkable hydrophobicity, and strong electrical breakdown suppression of the surfactant layers enable the stable and swift manipulation of femtoliter-scale aqueous microdroplets using µW-level laser in oil media. By virtue of the surfactant-mediated photovoltaic manipulation, a controllable merging/touching/detaching switch of aqueous microdroplets by adjusting the laser illumination intensity and position is realized and the cascading biochemical operations and microreactions of aqueous microdroplets and microdroplet strings are demonstrated. To demonstrate its potential in photonic Micro-Electro-Mechanical-System assemblies, the end coupling of a focused-laser-beam into a ZnO microrod leveraging the refraction effect occurring at the water/oil interface is demonstrated. Moreover, because of the selective permeability of the droplet-interface-bilayer developed between the touching microdroplets, in situ adjustment of the size of the microdroplets and the fluorescent solute contained in the microdroplets are achieved, aiming at constructing multicomponent fluorescent microdroplets with tunable whispering-gallery-mode characteristics.

Engineering Polymeric Nanofluidic Membranes for Efficient Ionic Transport: Biomimetic Design, Material Construction, and Advanced Functionalities

Design elements extracted from biological ion channels guide the engineering of artificial nanofluidic membranes for efficient ionic transport and spawn biomimetic devices with great potential in many cutting-edge areas. In this context, polymeric nanofluidic membranes can be especially attractive because of their inherent flexibility and benign processability, which facilitate massive fabrication and facile device integration for large-scale applications. Herein, the state-of-the-art achievements of polymeric nanofluidic membranes are systematically summarized. Theoretical fundamentals underlying both biological and synthetic ion channels are introduced. The advances of engineering polymeric nanofluidic membranes are then detailed from aspects of structural design, material construction, and chemical functionalization, emphasizing their broad chemical and reticular/topological variety as well as considerable property tunability. After that, this Review expands on examples of evolving these polymeric membranes into macroscopic devices and their potentials in addressing compelling issues in energy conversion and storage systems where efficient ion transport is highly desirable. Finally, a brief outlook on possible future developments in this field is provided.

Enhanced Cell Osteogenesis and Osteoimmunology Regulated by Piezoelectric Biomaterials with Controllable Surface Potential and Charges

Bone regeneration is a well-orchestrated process involving electrical, biochemical, and mechanical multiple physiological cues. Electrical signals play a vital role in the process of bone repair. The endogenous potential will spontaneously form on defect sites, guide the cell behaviors, and mediate bone healing when the bone fracture occurs. However, the mechanism on how the surface charges of implant potentially guides osteogenesis and osteoimmunology has not been clearly revealed yet. In this study, piezoelectric BaTiO₃/β-TCP (BTCP) ceramics are prepared by two-step sintering, and different surface charges are established by polarization. In addition, the cell osteogenesis and osteoimmunology of BMSCs and RAW264.7 on different surface charges were explored. The results showed that the piezoelectric constant d₃₃ of BTCP was controllable by adjusting the sintering temperature and rate. The polarized BTCP with a negative surface charge (BTCP-) promoted protein adsorption and BMSC extracellular Ca²⁺ influx. The attachment, spreading, migration, and osteogenic differentiation of BMSCs were enhanced on BTCP-. Additionally, the polarized BTCP ceramics with a positive surface charge (BTCP+) significantly inhibited M1 polarization of macrophages, affecting the expression of the M1 marker in macrophages and changing secretion of proinflammatory cytokines. It in turn enhanced osteogenic differentiation of BMSCs, suggesting that positive surface charges could modulate the bone immunoregulatory properties and shift the immune microenvironment to one that favored osteogenesis. The result provides an alternative method of synergistically modulating cellular immunity and the osteogenesis function and enhancing the bone regeneration by fabricating piezoelectric biomaterials with electrical signals.

Evaluation of Topical and Subconjunctival Injection of Hyaluronic Acid-Coated Nanoparticles for Drug Delivery to Posterior Eye

Posterior eye diseases, such as age-related macular degeneration and diabetic retinopathy, are difficult to treat due to ineffective drug delivery to affected areas. Intravitreal injection is the primary method for posterior eye drug delivery; however, it is usually accompanied by complications. Therefore, an effective and non-invasive method is required. Self-assembling nanoparticles (NPs) made from gelatin-epigallocatechin gallate (EGCG) were synthesized (GE) and surface-decorated with hyaluronic acid (HA) for drug delivery to the retinal/choroidal area. Different HA concentrations were used to prepare NPs with negative (GEH-) or positive (GEH+) surface charges. The size/zeta potential and morphology of the NPs were characterized by a dynamic light scattering (DLS) system and transmission electron microscope (TEM). The size/zeta potential of GEH+ NPs was 253.4 nm and 9.2 mV. The GEH- NPs were 390.0 nm and -35.9 mV, respectively. The cytotoxicity was tested by adult human retinal pigment epithelial cells (ARPE-19), with the results revealing that variant NPs were non-toxicity at 0.2-50 µg/mL of EGCG, and that the highest amount of GEH+ NPs was accumulated in cells examined by flowcytometry. Topical delivery (eye drops) and subconjunctival injection (SCI) methods were used to evaluate the efficiency of NP delivery to the posterior eyes in a mouse model. Whole eyeball cryosections were used to trace the location of fluorescent NPs in the eyes. The area of fluorescent signal obtained in the posterior eyes treated with GEH+ NPs in both methods (eye drops: 6.89% and SCI: 14.55%) was the greatest when compared with other groups, especially higher than free dye solution (2.79%). In summary, GEH+ NPs can be transported to the retina by eye drops and SCI; in particular, eye drops are a noninvasive method. Furthermore, GEH+ NPs, characterized by a positive surface and HA decoration, could facilitate drug delivery to the posterior eye as a useful drug carrier.

Aqueous dispersion and tuning surface charges of polytetrafluoroethylene particles by bioinspired polydopamine-polyethyleneimine coating via one-step method

We propose a surface modification of poorly dispersive polytetrafluoroethylene (PTFE) particles via bioinspired polydopamine-polyethyleneimine (PDA-PEI) which conferred PTFE particles a uniform dispersion in aqueous medium. With increasing dopamine concentration in the reaction solution, dispersity of PTFE particles improved and the surface charges of particles changed from negative to positive due to an increase of surface coverage of PDA-PEI layers. Simplicity of the method here outlines an attractive route for surface modification of inert surfaces useful for large-scale applications.

A Conceptual Framework for Integrating Cellular Protein Folding, Misfolding and Aggregation

How proteins properly fold and maintain solubility at the risk of misfolding and aggregation in the cellular environments still remains largely unknown. Aggregation has been traditionally treated as a consequence of protein folding (or misfolding). Notably, however, aggregation can be generally inhibited by affecting the intermolecular interactions leading to aggregation, independently of protein folding and conformation. We here point out that rigorous distinction between protein folding and aggregation as two independent processes is necessary to reconcile and underlie all observations regarding the combined cellular protein folding and aggregation. So far, the direct attractive interactions (e.g., hydrophobic interactions) between cellular macromolecules including chaperones and interacting polypeptides have been widely believed to mainly stabilize polypeptides against aggregation. However, the intermolecular repulsions by large excluded volume and surface charges of cellular macromolecules can play a key role in stabilizing their physically connected polypeptides against aggregation, irrespective of the connection types and induced conformational changes, underlying the generic intrinsic chaperone activity of cellular macromolecules. Such rigorous distinction and intermolecular repulsive force-driven aggregation inhibition by cellular macromolecules could give new insights into understanding the complex cellular protein landscapes that remain uncharted.

Surface Charges Control the Structure and Properties of Layered Nanocomposite of Cellulose Nanofibrils and Clay Platelets

The interfacial bonding and structure at the nanoscale in the polymer-clay nanocomposites are essential for obtaining desirable material and structure properties. Layered nanocomposite films of cellulose nanofibrils (CNFs)/montmorillonite (MTM) were prepared from the water suspensions of either CNFs bearing quaternary ammonium cations (Q-CNF) or CNFs bearing carboxylate groups (TO-CNF) with MTM nanoplatelets carrying net surface negative charges by using vacuum filtration followed by compressive drying. The effect of the ionic interaction between cationic or anionic charged CNFs and MTM nanoplatelets on the structure, mechanical properties, and flame retardant performance of the TO-CNF/MTM and Q-CNF/MTM nanocomposite films were studied and compared. The MTM nanoplatelets were well dispersed in the network of TO-CNFs in the form of nanoscale tactoids with the MTM content in the range of 5-70 wt %, while an intercalated structure was observed in the Q-CNF/MTM nanocomposites. The resulting TO-CNF/MTM nanocomposite films had a better flame retardant performance as compared to the Q-CNF/MTM films with the same MTM content. In addition, the effective modulus of MTM for the TO-CNF/MTM nanocomposites was as high as 129.9 GPa, 3.5 times higher than that for Q-CNF/MTM (37.1 GPa). On the other hand, the Q-CNF/MTM nanocomposites showed a synergistic enhancement in the modulus and tensile strength together with strain-to-failure and demonstrated a much better toughness as compared to the TO-CNF/MTM nanocomposites.

Charge Trapping-Based Electricity Generator (CTEG): An Ultrarobust and High Efficiency Nanogenerator for Energy Harvesting from Water Droplets

Strategies toward harvesting energy from water movements are proposed in recent years. Reverse electrowetting allows high efficiency energy generation, but requires external electric field. Triboelectric nanogenerators, as passive energy harvesting devices, are limited by the unstable and low density of tribo-charges. Here, a charge trapping-based electricity generator (CTEG) is proposed for passive energy harvesting from water droplets with high efficiency. The hydrophobic fluoropolymer films utilized in CTEG are pre-charged by a homogeneous electrowetting-assisted charge injection (h-EWCI) method, allowing an ultrahigh negative charge density of 1.8 mC m^-2 . By utilizing a dedicated designed circuit to connect the bottom electrode and top electrode of a Pt wire, instantaneous currents beyond 2 mA, power density above 160 W m^-2 , and energy harvesting efficiency over 11% are achieved from continuously falling water droplets. CTEG devices show excellent robustness for energy harvesting from water drops, without appreciable degradation for intermittent testing during 100 days. These results exceed previously reported values by far. The approach is not only applicable for energy harvesting from water droplets or wave-like oscillatory fluid motion, but also opens up avenues toward other applications requiring passive electric responses, such as diverse sensors and wearable devices.

Electrically Controlled Localized Charge Trapping at Amorphous Fluoropolymer-Electrolyte Interfaces

Charge trapping is a long-standing problem in electrowetting on dielectric, causing reliability reduction and restricting its practical applications. Although this phenomenon is investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at the three-phase contact line (TPCL) region by using three detecting methods-local contact angle measurements, electrowetting (EW) probe, and Kelvin probe force microscopy. Moreover, it is demonstrated that this EW-assisted charge injection (EWCI) process can be utilized as a simple and low-cost method to deposit charges on fluoropolymer surfaces. Charge densities near the TPCL up to 0.46 mC m^-2 and line widths of the deposited charge ranging from 20 to 300 µm are achieved by the proposed EWCI method. Particularly, negative charge densities do not degrade even after a "harsh" testing with a water droplet on top of the sample surfaces for 12 h, as well as after being treated by water vapor for 3 h. These findings provide an approach for applications which desire stable and controllable surface charges.

High-Throughput Identification and Screening of Single Microbial Cells by Nanobowl Array

High-throughput screening and fast identification of single bacterial cells are crucial for clinical diagnosis, bioengineering, and fermentation engineering. Although single-cell technologies have been developed extensively in recent years, the single-cell technologies for bacteria still need further exploration. In this study, we demonstrate an identification and screening technology for single bacterial cells based on a large-scale nanobowl array, which is well-ordered and size-adjustable for use with different kinds of bacteria. When the culture medium with monodispersed bacteria was placed on the nanobowl array, it successfully enabled loading of single bacterium into a single nanobowl. Because of the limitative size and depth of the nanobowls, mixture of different bacteria species could be screened according to their sizes. In addition, with the help of a low electrical current, the bacteria can be further screened according to their intrinsic surface charges. If combined with micromanipulation technology, high-throughput single bacterial selection can be achieved in future.

Two-Dimensional Ti3C2Tx MXene Membranes as Nanofluidic Osmotic Power Generators

Salinity-gradient is emerging as one of the promising renewable energy sources but its energy conversion is severely limited by unsatisfactory performance of available semipermeable membranes. Recently, nanoconfined channels, as osmotic conduits, have shown superior energy conversion performance to conventional technologies. Here, ion selective nanochannels in lamellar Ti₃C₂T_x MXene membranes are reported for efficient osmotic power harvesting. These subnanometer channels in the Ti₃C₂T_x membranes enable cation-selective passage, assisted with tailored surface terminal groups, under salinity gradient. A record-high output power density of 21 W·m^-2 at room temperature with an energy conversion efficiency of up to 40.6% is achieved by controlled surface charges at a 1000-fold salinity gradient. In addition, due to thermal regulation of surface charges and ionic mobility, the MXene membrane produces a large thermal enhancement at 331 K, yielding a power density of up to 54 W·m^-2. The MXene lamellar structure, coupled with its scalability and chemical tunability, may be an important platform for high-performance osmotic power generators.

Self-Organized Ferroelectric Domains Controlled by a Constant Bias from the Atomic Force Microscopy Tip

The ferroelectric polarization switching along an external electric field is most important for the applications of ferroelectric memories and piezoelectric sensors and actuators; however, the depolarization commonly occurs randomly and cannot be controlled exactly until now. Here, a tip bias introduces the polarization switching and a ∼10 μm-scale domain in a triglycine sulfate crystal, and then the polarization backswitching as a special depolarization introduces a series of ordered granular domains along a line being parallel to the c axis and through the tip which divides the original domain to two similar parts. Such backswitching is controlled by the surface charge change as a result of the interplay among polarization charges, mobile H⁺ ions at the surface, and the strong crystal anisotropy. The self-organized ferroelectric domains offer us a new freedom to design novel ferroelectric or piezoelectric devices in future.

Surface Charges on CdSe-Dot/CdS-Rod Nanocrystals: Measuring and Modeling the Diffusion of Exciton-Fluorescence Rates and Energies

By performing spectroscopic single-particle measurements at cryogenic temperatures over the course of hours, we study both the spectral diffusion as well as the diffusion of the decay rates of the fluorescence emission of core/shell CdSe/CdS dot/rod nanoparticles. A special analysis of the measurements allows for a correlation of data for single neutral excitons only, undisturbed by the possible emission of other excitonic complexes. We find a nearly linear dependency of the fluorescence decay rate on the emission energy. The experimental data are compared to self-consistent model calculations within the effective-mass approximation, in which migrating point charges set onto the surface of the nanoparticles have been assumed to cause the temporal changes of optical properties. These calculations reveal a nearly linear relationship between the squared electron-hole wave function overlap, which is linked to the experimentally determined fluorescence rate, and the exciton emission energy. Within our model, single migrating surface charges are not sufficient to fully explain the measured rather broad ranges of emission rates and energies, while two-and in particular negative-surface charges close to the core of the DR induce large enough shifts. Importantly, for our nanoparticle system, the surface charges more strongly affect the hole wave function than the electron wave function and both wave functions are still localized within the dot-like core of the nanoparticle, showing that the type-I character of the band alignment between core and shell is preserved.

Scalable Graphene-Based Membranes for Ionic Sieving with Ultrahigh Charge Selectivity

Nanostructured graphene-oxide (GO) laminate membranes, exhibiting ultrahigh water flux, are excellent candidates for next generation nanofiltration and desalination membranes, provided the ionic rejection could be further increased without compromising the water flux. Using microscopic drift-diffusion experiments, we demonstrated the ultrahigh charge selectivity for GO membranes, with more than order of magnitude difference in the permeabilities of cationic and anionic species of equivalent hydration radii. Measuring diffusion of a wide range of ions of different size and charge, we were able to clearly disentangle different physical mechanisms contributing to the ionic sieving in GO membranes: electrostatic repulsion between ions and charged chemical groups; and the compression of the ionic hydration shell within the membrane's nanochannels, following the activated behavior. The charge-selectivity allows us to rationally design membranes with increased ionic rejection and opens up the field of ion exchange and electrodialysis to the GO membranes.

Determination of the Optimal Shell Thickness for Self-Catalyzed GaAs/AlGaAs Core-Shell Nanowires on Silicon

We present a set of experimental results showing a combination of various effects, that is, surface recombination velocity, surface charge traps, strain, and structural defects, that govern the carrier dynamics of self-catalyzed GaAs/AlGaAs core-shell nanowires (NWs) grown on a Si(111) substrate by molecular beam epitaxy. Time-resolved photoluminescence of NW ensemble and spatially resolved cathodoluminescence of single NWs reveal that emission intensity, decay time, and carrier diffusion length of the GaAs NW core strongly depend on the AlGaAs shell thickness but in a nonmonotonic fashion. Although 7 nm AlGaAs shell can efficiently suppress the surface recombination velocity of the GaAs NW core, the influence of the surface charge traps and the strain between the core and the shell that redshift the luminescence of the GaAs NW core remain observable in the whole range of the shell thickness. In addition, the band bending effect induced by the surface charge traps can alter the scattering of the excess carriers inside the GaAs NW core at the core/shell interface. If the AlGaAs shell thickness is larger than 50 nm, the luminescence efficiency of the GaAs NW cores deteriorates, ascribed to defect formation inside the AlGaAs shell evidenced by transmission electron microscopy.

Macromolecule-assisted de novo protein folding

In the processes of protein synthesis and folding, newly synthesized polypeptides are tightly connected to the macromolecules, such as ribosomes, lipid bilayers, or cotranslationally folded domains in multidomain proteins, representing a hallmark of de novo protein folding environments in vivo. Such linkage effects on the aggregation of endogenous polypeptides have been largely neglected, although all these macromolecules have been known to effectively and robustly solubilize their linked heterologous proteins in fusion or display technology. Thus, their roles in the aggregation of linked endogenous polypeptides need to be elucidated and incorporated into the mechanisms of de novo protein folding in vivo. In the classic hydrophobic interaction-based stabilizing mechanism underlying the molecular chaperone-assisted protein folding, it has been assumed that the macromolecules connected through a simple linkage without hydrophobic interactions and conformational changes would make no effect on the aggregation of their linked polypeptide chains. However, an increasing line of evidence indicates that the intrinsic properties of soluble macromolecules, especially their surface charges and excluded volume, could be important and universal factors for stabilizing their linked polypeptides against aggregation. Taken together, these macromolecules could act as folding helpers by keeping their linked nascent chains in a folding-competent state. The folding assistance provided by these macromolecules in the linkage context would give new insights into de novo protein folding inside the cell.

Surface charges and optical characteristic of colloidal cubic SiC nanocrystals

Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CHx groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.

Chaperoning roles of macromolecules interacting with proteins in vivo

The principles obtained from studies on molecular chaperones have provided explanations for the assisted protein folding in vivo. However, the majority of proteins can fold without the assistance of the known molecular chaperones, and little attention has been paid to the potential chaperoning roles of other macromolecules. During protein biogenesis and folding, newly synthesized polypeptide chains interact with a variety of macromolecules, including ribosomes, RNAs, cytoskeleton, lipid bilayer, proteolytic system, etc. In general, the hydrophobic interactions between molecular chaperones and their substrates have been widely believed to be mainly responsible for the substrate stabilization against aggregation. Emerging evidence now indicates that other features of macromolecules such as their surface charges, probably resulting in electrostatic repulsions, and steric hindrance, could play a key role in the stabilization of their linked proteins against aggregation. Such stabilizing mechanisms are expected to give new insights into our understanding of the chaperoning functions for de novo protein folding. In this review, we will discuss the possible chaperoning roles of these macromolecules in de novo folding, based on their charge and steric features.