Toward sub-second solution exchange dynamics in flow reactors for liquid-phase transmission electron microscopy.

Liquid-phase transmission electron microscopy is a burgeoning experimental technique for monitoring nanoscale dynamics in a liquid environment, increasingly employing microfluidic reactors to control the composition of the sample solution. Current challenges comprise fast mass transport dynamics inside the central nanochannel of the liquid cell, typically flow cells, and reliable fixation of the specimen in the limited imaging area. In this work, we present a liquid cell concept - the diffusion cell - that satisfies these seemingly contradictory requirements by providing additional on-chip bypasses to allow high convective transport around the nanochannel in which diffusive transport predominates. Diffusion cell prototypes are developed using numerical mass transport models and fabricated on the basis of existing two-chip setups. Important hydrodynamic parameters, i.e., the total flow resistance, the flow velocity in the imaging area, and the time constants of mixing, are improved by 2-3 orders of magnitude compared to existing setups. The solution replacement dynamics achieved within seconds already match the mixing timescales of many ex-situ scenarios, and further improvements are possible. Diffusion cells can be easily integrated into existing liquid-phase transmission electron microscopy workflows, provide correlation of results with ex-situ experiments, and can create additional research directions addressing fast nanoscale processes.

Temperature-Modulated Reversible Clustering of Gold Nanorods Driven by Small Surface Ligands.

Temperature-modulated colloidal phase of plasmonic nanoparticles is a convenient playground for resettable soft-actuators or colorimetric sensors. To render reversible clustering under temperature change, bulky ligands are required, especially if anisotropic morphologies are of interest. This study showcases thermoresponsive gold nanorods by employing small surface ligands, bis (p-sulfonatophenyl) phenyl-phosphine dihydrate dipotassium salt (BSPP) and native cationic surfactant. Temperature-dependent analysis in real-time allowed to describe the structural features (interparticle distance and cluster size) as well as thermal parameters, melting and freezing temperatures. These findings suggest that neither covalent Au-S bonds nor bulky ligands are required to obtain a robust thermoresponsive system based on anisotropic gold nanoparticles, paving the way to stimuli-responsive nanoparticles with a wide range of sizes and geometries.

Light-Driven Self-Oscillation of Thermoplasmonic Nanocolloids.

Self-oscillation-the periodic change of a system under a non-periodic stimulus-is vital for creating low-maintenance autonomous devices in soft robotics technologies. Soft composites of macroscopic dimensions are often doped with plasmonic nanoparticles to enhance energy dissipation and generate periodic response. However, while it is still unknown whether a dispersion of photonic nanocrystals may respond to light as a soft actuator, a dynamic analysis of nanocolloids self-oscillating in a liquid is also lacking. This study presents a new self-oscillator model for illuminated colloidal systems. It predicts that the surface temperature of thermoplasmonic nanoparticles and the number density of their clusters jointly oscillate at frequencies ranging from infrasonic to acoustic values. New experiments with spontaneously clustering gold nanorods, where the photothermal effect alters the interplay of light (stimulus) with the disperse system on a macroscopic scale, strongly support the theory. These findings enlarge the current view on self-oscillation phenomena and anticipate the colloidal state of matter to be a suitable host for accommodating light-propelled machineries. In broad terms, a complex system behavior is observed, which goes from periodic solutions (Hopf-Poincaré-Andronov bifurcation) to a new dynamic attractor driven by nanoparticle interactions, linking thermoplasmonics to nonlinearity and chaos.

Quantification of reagent mixing in liquid flow cells for Liquid Phase-TEM.

Liquid-Phase Transmission Electron Microscopy (LP-TEM) offers the opportunity to study nanoscale dynamics of phenomena related to materials and life science in a native liquid environment and in real time. Until now, the opportunity to control/induce such dynamics by changing the chemical environment in the liquid flow cell (LFC) has rarely been exploited due to an incomplete understanding of hydrodynamic properties of LP-TEM flow systems. This manuscript introduces a method for hydrodynamic characterization of LP-TEM flow systems based on monitoring transmitted intensity while flowing a strongly electron scattering contrast agent solution. Key characteristic temporal indicators of solution replacement for various channel geometries were experimentally measured. A numerical physical model of solute transport based on realistic flow channel geometries was successfully implemented and validated against experiments. The model confirmed the impact of flow channel geometry on the importance of convective and diffusive solute transport, deduced by experiment, and could further extend understanding of hydrodynamics in LP-TEM flow systems. We emphasize that our approach can be applied to hydrodynamic characterization of any customized LP-TEM flow system. We foresee the implemented predictive model driving the future design of application-specific LP-TEM flow systems and, when combined with existing chemical reaction models, to a flourishing of the planning and interpretation of experimental observations.

Statistical thermodynamics in reversible clustering of gold nanoparticles. A first step towards nanocluster heat engines.

A statistical thermodynamics variational criterion is propounded to study thermal hysteresis in reversible clustering of gold (Au) nanoparticles. Experimentally, a transient equilibrium mapping analysis is employed to characterize it thermodynamically, further measurements being performed at the nanostructural and electrochemical levels (UV-Vis-NIR spectra, SLS/SAXS, zeta potential). Theoretically, it is successfully interpreted as a thermodynamic cycle, prompting that nanoclusters has potential to produce useful work from heat and paving the way to nanoclustering heat engines. By taking into account the virial expansion of hysteretic pressure, an entropy measure is deduced for a dilute system with given virial coefficients. This allows us to figure out the role of relevant interparticle potential parameters (i.e. surface potential, nanoparticle size, Debye's length, Hamaker energy) in both isothermal and isochoric variations at the onset of hysteresis. Application to spherical Au nanoparticles in watery salt solution (NaCl) is developed when an ad-hoc (DLVO) pairwise potential governs the second virial coefficient at the nanoscale. In particular, the variational criterion predicts a pressure drop between heating and cooling paths which is likely at the base of some energy redistribution (e.g. ordering/restructuring of electric double layers). We found an integrating factor that is able to numerically predict the existence of a critical value for the initial salt concentration maximizing the hysteretic area, and the effect of nanoparticle size on the cycle extent.

Metal-Polymer Heterojunction in Colloidal-Phase Plasmonic Catalysis.

Plasmonic catalysis in the colloidal phase requires robust surface ligands that prevent particles from aggregation in adverse chemical environments and allow carrier flow from reagents to nanoparticles. This work describes the use of a water-soluble conjugated polymer comprising a thiophene moiety as a surface ligand for gold nanoparticles to create a hybrid system that, under the action of visible light, drives the conversion of the biorelevant NAD+ to its highly energetic reduced form NADH. A combination of advanced microscopy techniques and numerical simulations revealed that the robust metal-polymer heterojunction, rich in sulfonate functional groups, directs the interaction of electron-donor molecules with the plasmonic photocatalyst. The tight binding of polymer to the gold surface precludes the need for conventional transition-metal surface cocatalysts, which were previously shown to be essential for photocatalytic NAD+ reduction but are known to hinder the optical properties of plasmonic nanocrystals. Moreover, computational studies indicated that the coating polymer fosters a closer interaction between the sacrificial electron-donor triethanolamine and the nanoparticles, thus enhancing the reactivity.

Coupling plasmonic catalysis and nanocrystal growth through cyclic regeneration of NADH.

In a typical colloidal synthesis, the molecules of the reducing agent are irreversibly oxidized during nanocrystal growth. Such a scenario is of questionable sustainability when confronted with naturally occurring processes in which reducing agent molecules are cyclically regenerated. Here we show that cofactor molecules once consumed in the nucleation and growth of metallic nanocrystals can be photoregenerated using metallic nanocrystals as photocatalysts and reused in the subsequent nucleation process. Cyclic regeneration of cofactor molecules opens up the possibilities for the sustainable synthesis of inorganic nanoparticles.

Gold nanoparticles endowed with low-temperature colloidal stability by cyclic polyethylene glycol in ethanol.

The colloidal stability of metal nanoparticles is tremendously dependent on the thermal behavior of polymer brushes. Neat polyethylene glycol (PEG) presents an unconventional upper critical solution temperature in ethanol, where phase segregation and crystallization coexist. This thermal behavior translated to a PEG brush has serious consequences on the colloidal stability in ethanol of gold nanoparticles (AuNPs) modified with PEG brushes upon cooling. We observed that AuNPs (13 nm diameter) stabilized with conventional linear PEG brushes (Mn = 6 and 11 kg mol-1) in ethanol suffer from reversible phase separation upon a temperature drop over the course of a few hours. However, the use of a polymer brush with cyclic topology as a stabilizer prevents sedimentation, ensuring the colloidal stability in ethanol at -25 °C for, at least, four months. We postulate that temperature-driven collapse of chain brushes promotes the interpenetration of linear chains, causing progressive AuNP sedimentation, a process that is unfavorable for cyclic polymer brushes whose topology prevents chain interpenetration. This study reinforces the notion about the importance of polymer topology on the colloidal stability of AuNPs.

Solution blow spinning of polymer/nanocomposite micro-/nanofibers with tunable diameters and morphologies using a gas dynamic virtual nozzle.

Uniform endless fibers are ubiquitous and their applications range from functional textiles over biomedical engineering to high-performance filtering and drug delivery systems. Here, we report a new method for the direct, reproducible fabrication of uniform polymer and composite micro-/nanofibers using a microfluidic gas flow focusing nozzle (Gas Dynamic Virtual Nozzle (GDVN)) relinquishing the need for external fiber pulling mechanisms. Compared to other methods, this technique is inexpensive, user-friendly and permits precise fiber diameter control (~250 nm to ~15 µm), high production rate (m/s-range) and direct fiber deposition without clogging due to stable, gas-focused jetting. Control over shape (flat or round) and surface patterning are achieved by simply tuning the air pressure and polymer concentration. The main thinning process happens after the polymer exits the device and is, therefore, mostly independent of the nozzle's internal geometry. Nevertheless, the lithography-based device design is versatile, allowing for precise flow-field control for operation stability as well as particle alignment control. As an example, we demonstrate the successful production of endless hematite nanocomposite fibers which highlights this technology's exciting possibilities that can lead to the fabrication of multifunctional/stimuli-responsive fibers with thermal and electrical conductivity, magnetic properties and enhanced mechanical stability.