Nano-confined Environment Research Articles

Mutual Relationships of Nanoconfined Hexoses: Impacts on Hydrodynamic Radius and Anomeric Ratios.

Although all hexose sugars share the same chemical formula, C6H12O6, subtle differences in their stereochemical structures lead to their various biological roles. Due to their prominent role in metabolism, hexose sugars are commonly found in nanoconfined environments. The complexity of authentic nanoconfined biological environments makes it challenging to study how confinement affects their behavior. Here, we present a study using a common model system, AOT reverse micelles, to study hexose sugars in nanoconfinement. We examine how reverse micelles affect the hexoses, how the hexoses affect reverse micelle formation, and the differences between specific hexoses: glucose, mannose, and galactose. We find that addition of glucose, mannose or galactose to reverse micelles that already contain water leaves their size smaller or nearly unchanged. Introducing aqueous hexose solution yields reverse micelles smaller than those prepared with the same volume of water. We use 1H NMR to show how the nanoconfined environment impacts hexose sugars' anomeric ratios. Nanoconfined mannose and galactose display smaller changes in their anomeric ratios compared to glucose. These conclusions may provide insights about the biological roles of each hexose when studied under a more authentic nanoconfined system.

An enzymatic continuous-flow reactor based on a pore-size matching nano- and isoporous block copolymer membrane

Continuous-flow biocatalysis utilizing immobilized enzymes emerged as a sustainable route for chemical synthesis. However, inadequate biocatalytic efficiency from current flow reactors, caused by non-productive enzyme immobilization or enzyme-carrier mismatches in size, hampers its widespread application. Here, we demonstrate a general-applicable and robust approach for the fabrication of a high-performance enzymatic continuous-flow reactor via integrating well-designed scalable isoporous block copolymer (BCP) membranes as carriers with an oriented and productive immobilization employing material binding peptides (MBP). Densely packed uniform enzyme-matched nanochannels of well-designed BCP membranes endow the desired nanoconfined environments towards a productive immobilized phytase. Tuning nanochannel properties can further regulate the complex reaction process and fortify the catalytic performance. The synergistic design of enzyme-matched carriers and efficient enzyme immobilization empowers an excellent catalytic performance with >1 month operational stability, superior productivity, and a high space-time yield (1.05 × 105 g L−1 d−1) via a single-pass continuous-flow process. The obtained performance makes the designed nano- and isoporous block copolymer membrane reactor highly attractive for industrial applications.

Investigation of the effect of CO2 on asphaltene deposition and flow mechanism under nano-confined environment

Through the molecular dynamic simulation, the effect of CO2 on asphaltene deposition behavior and fluid flow mechanism under nano-confined conditions has been investigated. The dynamic of asphaltene deposition indicates that the maximum deposition effect is achieved when the CO2 mass fraction exceeds 40%. The solvent composition is a crucial factor in asphaltene aggregation and deposition morphology. The competition between CO2 and asphaltene molecules for surface adsorption is the main factor for the detachment of aromatic core from the surface, which further determines the deposition structure of the surface asphaltene. The flow simulation demonstrates that deposited asphaltene molecules adhere to the pore surface, resulting in a negative slip boundary condition. Additionally, the increase in CO2 concentration further reduces the pore permeability, which is attributed to asphaltene volume swelling, CO2 adsorption, and the change in asphaltene structure. The volume correction method based on Hagen-Poiseuille formula cannot accurately predict the nanopore permeability under asphalt deposition, while the prediction of pore permeability using the thickness of the adhesive layer introduced by MD results has a relative error less than 3%. This study provides a theoretical basis for predicting the nanopore permeability under asphaltene deposition.

(Invited) Electrochemical Characterization of Catalyst/Ionomer Interfaces

Electrochemical energy conversion devices such as polymer-electrolyte fuel cells, water-splitting or carbon dioxide reduction electrolyzers, utilize ionically-conductive polymers (ionomers) in their electrode structure where an ionomer forms interfaces with the catalysts. The ionomers exist as nanometer-thick film to cover the catalyst particles and act as a binder while enabling the transport of active species necessary for the reactions. While the nature of species and operational environment differs across these devices, their overall efficiency and performance is influenced by the electrode architecture, where the interfaces and interactions between the ionomers and catalysts play an important role. In hydrogen technologies, for example, ionomer thin films bind the catalyst sites and carbon supports to facilitate transport of ionic (e.g. protons), liquid (water) and gaseous (e.g., oxygen) species under a nano-confined environment. Such confinement effects not only change the ionomer structure and properties, usually with a tendency to increase transport resistances, but also amplify the impact of the electrocatalyst-ionomer interactions in the electrodes, thereby reducing the catalyst activity. Thus, understanding the behavior of nano-confined ionomers and their interactions with the catalyst surfaces is key for mitigating the transport resistances in catalyst ionomers and improving the electrode performance and cell efficiency.This talk will provide insights into electrochemical characterization of catalyst ionomers by focusing on varying chemistries of perfluorosulfonated cation-exchange ionomers with additional examples of anion-exchange ionomers for emerging technologies. The hydration and transport properties of proton-exchange catalyst-ionomers will be examined with the effects of chemistry, processing and thickness (degree of confinement), along with their impact on the morphological changes governing the film function. Through a systematic variation of material chemistries and environmental parameters, primary factors impacting the structure-property relationship of catalyst ionomers are described along with a discussion of secondary factors altering the cation-anion interactions. Then, the interplay between the effects of chemistry and ion exchange (capacity) on modifying the transport function and the strength of ionomer-catalyst interactions will be presented. The results will be collated to elucidate the underlying origins of the transport resistances occurring in electrodes, and to develop materials and processing strategies for their mitigation for improved performance and efficiency in electrochemical energy devices.

Construction of a biomimetic core-shell PDA@Lac bioreactor from intracellular laccase as a nano-confined biocatalyst for decolorization

Enzymes immobilized on the surface of the carriers are difficult to maintain their conformation and high activity due to the influence of the external harsh environments. A biomimetic core-shell PDA@Lac bioreactor was constructed by depositing polydopamine (PDA) on the surface of the recombinant Escherichia coli with CotA laccase gene, and releasing intracellular laccase into the PDA shell using ultrasound to break the cell wall of the bacteria. The bioreactor provided a nano-confined environment for the laccase and accelerated the mass and electron transfer in the volume-confined space, with a 2.77-fold increase in Km compared with the free laccase. Since there was no barrier of the cell wall, the crystal violet dye can enter the bioreactor to participate in the enzymatic reaction. As a result, PDA@Lac achieved excellent decolorization performance even without ABTS as an electron mediator. Moreover, the cytoplasmic solution retained in the PDA shell promoted the enzyme's tolerance to pH, temperature and harsh environments. In addition to PDA encapsulation, carbonyl and –NH2 groups of PDA were bound covalently with –NH2 and –COOH on the laccase in the PDA@Lac, resulting in an extremely high laccase loading of 817.59 mg/g. Also, the relative activity of the bioreactor maintained approximately 75% after 10 cycles of reuse. In addition, the protection of the PDA shell increased the resistance of laccase to UV irradiation. This work provides a novel method of laccase immobilization for application in wastewater treatment.

Designing plasmonically integrated nanoreactors for efficient catalysis

AbstractPlasmonically coupled nanoreactors capable of harnessing light energy and efficiently transforming it to perform chemical reactions are in significant demand in the field of catalysis. The development of unique solution‐phase synthesis techniques for engineering intricate nanoarchitectures by introducing multiple functionalities can dramatically improve the efficacy of these nanostructures. In this context, the precise modulation of the nanostructural features and the integration of other components with the plasmonic nanoparticles within an isolated nanoconfined reaction environment is the current state of the art for their synthesis. This account highlights our achievements in designing different synthesis strategies and elucidating the mechanistic pathways involved in the development of plasmonically integrated nanoreactors. The range of applications of these plasmonic nanoreactors, which employ plasmon‐induced energy for catalytic transformations, is also discussed.

Functional hydrogel for fast, precise and inhibition-free point-of-care bacteria analysis in crude food samples

In this work, instead of performing nucleic acid amplification in the bulk solution, we report a nanoporous hydrogel with controlled release function for rapid, precise, and inhibition-free nucleic acid analysis in crude food samples. The cross-linked PEG hydrogel with nanoporous structures possesses adsorption, release, separation, restriction and self-cleaning abilities. When digital loop-mediated isothermal amplification (LAMP) was performed inside this hydrogel, the surrounding nanostructure act as a temporary reservoir for reagents storage and release them on demand during or after amplification. Meanwhile, the restricted nanoconfined environment of hydrogel also favor the enzymatic amplification process. Thus, an enhanced signal readout, robust anti-inhibition, faster amplification rate, more products yields and specific amplification without primer-dimers were obtained. Moreover, direct amplification in untreated complex food sample was successfully performed inside hydrogel without any sample pretreatment, while conventional droplets digital LAMP failed for detection. Absolute quantification of Escherichia coli and Salmonella typhi directly in fresh fruit and vegetables was achieved within 20 min, with high precision and sensitivity down to single cell. This novel lab-on-hydrogel concept has an enormous potential for future molecular diagnostic assays, and can be also applied for other point-of-care assays.

(Invited) Non-Monotonic Temperature Dependence of Hydroxide Diffusion in Model Anion Exchange Membranes Based on Functionalized Nano-Confined Environments

Anion exchange membrane (AEM) based fuel cells constitute some of the cleanest and lowest-cost emerging electrochemical device technologies. Recently, nano-confined structures have been used as a model to study membrane functionalities in recognition of the fact that obtaining high hydroxide conductivity in confined environments remains a key hurdle to realizing the full potential of AEM fuel cells. A key design principle for optimizing AEM device performance is a molecular-level understanding of the effect of temperature on hydroxide and water diffusion in confined environments with different arrangements of functional cation groups. For this purpose, we present fully atomistic ab initio molecular dynamics (AIMD) simulations of model AEMs with different hydration levels and cation spacings at temperatures ranging from 275 to 445 K. We find that the OH- diffusion coefficient changes non-monotonically with increasing temperature. While this trend is consistent for all systems studied, the specific reasons for this behavior were observed to depend on the hydration level. Our results will help guide the design of nanoconfined environments and the choice of optimal operating temperatures for achieving high hydroxide conductivity in AEM fuel cells.

Characterisation of nano-assemblies inside mesopores using neutron scattering*

Adsorption of molecular and nanoscale matter in mesoporous materials is important in filtration and chromatography as well as membrane processes. However, the morphology and distribution of self-assembled structures of adsorbate formed within nanoconfined environments is largely unknown. Small Angle Neutron Scattering (SANS) with porous matrix matching the neutron scattering length density of the solvent has the potential to provide detailed information on the self-assemblies formed in pore spaces. However, analysis of such SANS profiles remains a challenge. In this study, we extend the SANS analysis method previously developed by Findenegg and co-workers to include interparticle correlations, providing a qualitative characterisation of the adsorption state of surfactants and nanoparticles in the cylindrical pores of silica nanomaterials. We find that the pore filling fraction and the self-assembled state of the adsorbate governs the scattering profile. We apply the model to two materials namely, triethyleneglycol monohexyl ether (C6E3) surfactant and gold nanoparticles adsorbed in SBA-15 mesoporous silica. In contrast to the C6E3 system, Bragg scattering dominates the diffuse scattering in gold nanoparticle system, which we attribute to the immobile internal structure of the nanoparticles. This new SANS analysis method has potential to elucidate structures of soft matter nanoassemblies inside mesopores.

Modelling Water Behaviour in Hydrophobic Gates of Ion Channels

Ion channels are a key class of membrane proteins as they enable ions and water to pass into and out of cells. Some of these ion channels possess a hydrophobic gate, which enables the channel to switch between wetted and de-wetted states without sterically occluding the channel. The transition between wetting and de-wetting can be determined as a function of both the radius and hydrophobicity of the channel lining. Molecular dynamics simulations are a powerful tool for exploring this parameter space. However, due to the complexity of modelling bulk water interactions and the translation of these to nano-confined environments, the level of theory chosen for modelling the water is an important consideration. We present a study of how different techniques for modelling water affect simulations of hydrophobic gating in selected ion channels.

NMR Intermolecular Dipolar Cross-Relaxation in Nanoconfined Fluids.

Spin relaxation, a defining mechanism of nuclear magnetic resonance (NMR), has been a prime method for determining three-dimensional molecular structures and their dynamics in solution. It also plays key roles for contrast enhancement in magnetic resonance imaging (MRI). In bulk solutions, rapid Brownian molecular diffusion modulates dipolar interactions between a spin pair from different molecules, resulting in very weak intermolecular relaxations. We show that in fluids confined in nanospace or nanopores (nanoconfined fluids) the correlation of dipolar coupling between spin pairs of different molecules is greatly enhanced by the nanopore constraint boundaries on the molecular diffusion, giving rise to an enhanced correlation for the spin pair. As a result, the intermolecular dipolar interaction behaves cooperatively, which leads to a large intermolecular dipolar relaxation rate and opposite in sign to the bulk solution. We found that the classical NMR relaxation theory fails to capture these observations in a nanoconfined fluid environment. Hence, we developed a formal theory and experimentally confirmed that enhanced correlation and cooperated relaxation are ubiquitous in nanoconfined fluids. The newly discovered phenomenon and the developed NMR method reveal new applications in a broad range of synthesized and naturally occurring materials in the field of nanofluidics to study molecular dynamics and structure as well as for MRI image enhancement.

Water Layering Affects Hydroxide Diffusion in Functionalized Nanoconfined Environments.

In functionalized nanoconfined environments of the type employed in the study of anion exchange membranes (AEMs), a unique set of water layers forms as a result of the presence of cations and the proximity of the waters to the edges of the confining volume. In this work, we employ fully atomistic ab initio molecular dynamics in order to provide a clear picture of the solvation patterns and diffusion mechanisms of the hydroxide ion within each water layer. We find that each water layer supports a particular dominant coordination pattern for the hydroxide ion and that these solvation complexes differ among the layers. As these solvation structures affect the rate of hydroxide diffusion, it is suggested that different water layers can either promote or suppress diffusion. We believe the results presented in this work elucidate water layer features that influence hydroxide transport and can provide a guide for engineering AEMs with high hydroxide conductivity.

Tunneling effects in confined gold nanoparticle hydrogenation catalysts.

Clean surface gold nanoparticles (AuNPs) of ∼6.6 nm that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al2O3) displayed hydrogenation pathways in the reduction of trans-cinnamaldehyde distinct from those imprinted directly onto γ-Al2O3. Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and via heterolytic cleavage for IL-free supported AuNPs. Higher negative apparent entropy (ΔSapp) values were obtained for the IL-confined AuNPs compared to the non-hybrid catalyst (Au/γ-Al2O3), suggesting a decrease in the number of microstates induced by the nano-confined environment. High kinetic isotope effect (KIE) values (kH/kD = 2.5-2.9 at 273 K) and Arrhenius convex curves were observed. Furthermore, differences of 5.6 and 6.2 kJ mol-1 between the apparent activation energies of the deuteration and hydrogenation reactions (E-E) associated with pre-exponential factor ratios (AD/AH) of 4.6 and 5.1 provided strong evidence of the possible involvement of a tunneling pathway in the case of the confined AuNPs.

Proton Diffusion through Bilayer Pores.

The transport of protons through channels in complex environments is important in biology and materials science. In this work, we use multistate empirical valence bond simulations to study proton transport within a well-defined bilayer pore in a lamellar Lβ phase lyotropic liquid crystal (LLC). The LLC is formed from the self-assembly of dicarboxylate gemini surfactants in water, and a bilayer-spanning pore of radius of approximately 3-5 Å results from the uneven partitioning of surfactants between the two leaflets of the lamella. Local proton diffusion within the pore is significantly faster than diffusion at the bilayer surface, which is due to the greater hydrophobicity of the surfactant/water interface within the pore. Proton diffusion proceeds by surface transport along exposed hydrophobic pockets at the surfactant/water interface and depends on the continuity of hydronium-water hydrogen bond networks. At the bilayer surface, there is a reduced fraction of the "Zundel" intermediates that are central to the Grotthuss transport mechanism, whereas the fraction of these species within the bilayer pore is similar to that in bulk water. Our results demonstrate that the chemical nature of the confining interface, in addition to confinement length scale, is an important determiner of local proton transport in nanoconfined aqueous environments.

Studies on electrostatic interactions within model nano-confined aqueous environments of different chemical nature.

We study the potential of mean force for pairs of parallel flat surfaces with attractive electrostatic interactions by employing model systems functionalized with different charged, hydrophobic and hydrophilic groups. We study the way in which the local environment (hydrophobic or hydrophilic moieties) modulates the interaction between the attractive charged groups on the plates by removing or attracting nearby water and thus screening or not the electrostatic interaction. To explicitly account for the role of the solvent and the local hydrophobicity, we also perform studies in vacuo. Additionally, the results are compared to that for non-charged plates in order to single out and rationalize the non-additivity of the different non-covalent interactions. Our simulations demonstrate that the presence of neighboring hydrophobic groups promote water removal in the vicinity of the charged groups, thus enhancing charge attraction upon self-assembly. This role of the local hydrophobicity modulating electrostatic interactions is consistent with recent qualitative descriptions in the protein binding context.

A molecular dynamics investigation into the adsorption behavior inside {001} kaolinite and {1014} calcite nano-scale channels: the case with confined hydrocarbon liquid, acid gases, and water

A set of molecular dynamics simulations was conducted, as the first comparative study of the adsorption behavior of liquid hydrocarbon/acid gases/water molecules over { 10bar{1}4} calcite surface and {001} octahedral kaolinite surface in nano-confined slit. According to atomic z-density profiles, hydrocarbon molecules have higher tendency towards the { 10bar{1}4} calcite surface than the {001} octahedral kaolinite surface. In addition, water molecules form stronger adsorption layer over calcite surface than kaolinite. In contrast, acid gas molecules have higher tendency towards kaolinite surface than calcite. This behavior was spotted within nanometer-sized slit pores. The results also point to reduction in self-diffusion coefficient of molecules with strong adsorption over mineral surfaces in nano-confined environment.

Molecular Dynamics Simulations of Hydroxyapatite Nanopores in Contact with Electrolyte Solutions: The Effect of Nanoconfinement and Solvated Ions on the Surface Reactivity and the Structural, Dynamical, and Vibrational Properties of Water

Hydroxyapatite, the main mineral phase of mammalian tooth enamel and bone, grows within nanoconfined environments and in contact with aqueous solutions that are rich in ions. Hydroxyapatite nanopores of different pore sizes (20 Å ≤ H ≤ 110 Å, where H is the size of the nanopore) in contact with liquid water and aqueous electrolyte solutions (CaCl2 (aq) and CaF2 (aq)) were investigated using molecular dynamics simulations to quantify the effect of nanoconfinement and solvated ions on the surface reactivity and the structural and dynamical properties of water. The combined effect of solution composition and nanoconfinement significantly slows the self-diffusion coefficient of water molecules compared with bulk liquid. Analysis of the pair and angular distribution functions, distribution of hydrogen bonds, velocity autocorrelation functions, and power spectra of water shows that solution composition and nanoconfinement in particular enhance the rigidity of the water hydrogen bonding network. Calculation of the water exchange events in the coordination of calcium ions reveals that the dynamics of water molecules at the HAP–solution interface decreases substantially with the degree of confinement. Ions in solution also reduce the water dynamics at the surface calcium sites. Together, these changes in the properties of water impart an overall rigidifying effect on the solvent network and reduce the reactivity at the hydroxyapatite-solution interface. Since the process of surface-cation-dehydration governs the kinetics of the reactions occurring at mineral surfaces, such as adsorption and crystal growth, this work shows how nanoconfinement and solvation environment influence the molecular-level events surrounding the crystallization of hydroxyapatite.

High performance asymmetric V2O5-SnO2 nanopore battery by atomic layer deposition.

Here we report the high performance and cyclability of an asymmetric full cell nanopore battery, comprised of V2O5 as the cathode and prelithiated SnO2 as the anode, with integrated nanotubular Pt current collectors underneath each nanotubular storage electrode, confined within an anodized aluminium oxide (AAO) nanopore. Enabled by atomic layer deposition (ALD), this coaxial nanotube full cell is fully confined within a high aspect ratio nanopore (150 nm in diameter, 50 μm in length), with an ultra-small volume of about 1 fL. By controlling the amount of lithium ion prelithiated into the SnO2 anode, we can tune the full cell output voltage in the range of 0.3 V to 3 V. When tested as a massively parallel device (∼2 billion cm-2), this asymmetric nanopore battery array displays exceptional rate performance and cyclability: when cycled between 1 V and 3 V, capacity retention at the 200C rate is ∼73% of that at 1C, and at 25C rate only 2% capacity loss occurs after more than 500 charge/discharge cycles. With the increased full cell output potential, the asymmetric V2O5-SnO2 nanopore battery shows significantly improved energy and power density over the previously reported symmetric cell, 4.6 times higher volumetric energy and 5.2 times higher power density - an even more promising indication that controlled nanostructure designs employing nanoconfined environments with large electrode surface areas present promising directions for future battery technology.

Thermally modulated nanostructure of poly(ε-caprolactone)–POSS multiblock thermoplastic polyurethanes

A series of multiblock polyurethanes with alternating sequence structures of a poly(ε-caprolactone) (PCL) segment of 2600 or 3600 g/mol and a polyhedral oligomeric silsesquioxane (POSS) segment with multiple POSS moieties (TPU2.6k_1-x or TPU3.6k_1-x, respectively; the molar ratio of PCL:POSS is 1: x; x = 2, 3, or 4) were synthesized through two-step polymerization to assure quantitative conversion of reactants. Differential scanning calorimetry and simultaneous wide- and small-angle X-ray scattering measurements were performed to study the nanostructures of those samples. The multiblock and alternating sequence structures provided nano-confined environments for PCL and POSS domains, which significantly suppressed crystallinity of the PCL phase, while nano-sized crystallites were formed in the POSS phase. The samples in series TPU2.6k and TPU3.6k were also proven to display either lamellar, cubic, or cylindrical hexagonal phase-separated nanostructures depending on the molecular weight of the PCL segment, as well as the PCL/POSS ratio. It was also found that repeated thermal cycling under a nitrogen atmosphere low enough in temperature not to alter molecular weight caused larger and more ordered PCL and POSS crystalline structure to form for the TPU3.6k series. Apparent reconfiguration of the PCL and POSS moieties along the backbone by exchange reactions associated with reversibility of urethane bonds led to increases in PCL and POSS block lengths in the TPU chains. We envision an opportunity of future research and applicability in the areas of tailored toughness and rigidity in biodegradable polymer coatings, devices with enlarged data storage capacity, drug delivery systems and tissue engineering.

Periodic Behavior of Lanthanide Coordination within Reverse Micelles

Trends in lanthanide(III) (Ln(III)) coordination were investigated within nanoconfined solvation environments. Ln(III) ions were incorporated into the cores of reverse micelles (RMs) formed with malonamide amphiphiles in n-heptane by contact with aqueous phases containing nitrate and Ln(III); both insert into pre-organized RM units built up of DMDOHEMA (N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide) that are either relatively large and hydrated or small and dry, depending on whether the organic phase is acidic or neutral, respectively. Structural aspects of the Ln(III) complex formation and the RM morphology were obtained by use of XAS (X-ray absorption spectroscopy) and SAXS (small-angle X-ray scattering). The Ln(III) coordination environments were determined through use of L(3)-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure), which provide metrical insights into the chemistry across the period. Hydration numbers for the Eu species were measured using TRLIFS (time-resolved laser-induced fluorescence spectroscopy). The picture that emerges from a system-wide perspective of the Ln-O interatomic distances and number of coordinating oxygen atoms for the extracted complexes of Ln(III) in the first half of the series (i.e., Nd, Eu) is that they are different from those in the second half of the series (i.e., Tb, Yb): the number of coordinating oxygen atoms decrease from 9O for early lanthanides to 8O for the late ones--a trend that is consistent with the effect of the lanthanide contraction. The environment within the RM, altered by either the presence or absence of acid, also had a pronounced influence on the nitrate coordination mode; for example, the larger, more hydrated, acidic RM core favors monodentate coordination, whereas the small, dry, neutral core favors bidentate coordination to Ln(III). These findings show that the coordination chemistry of lanthanides within nanoconfined environments is neither equivalent to the solid nor bulk solution behaviors. Herein we address atomic- and mesoscale phenomena in the under-explored field of lanthanide coordination and periodic behavior within RMs, providing a consilience of fundamental insights into the chemistry of growing importance in technologies as diverse as nanosynthesis and separations science.