Nanoscopic Pores Research Articles

Capture CO2 from Ambient Air Using Nanoconfined Ion Hydration

AbstractWater confined in nanoscopic pores is essential in determining the energetics of many physical and chemical systems. Herein, we report a recently discovered unconventional, reversible chemical reaction driven by water quantities in nanopores. The reduction of the number of water molecules present in the pore space promotes the hydrolysis of CO32− to HCO3− and OH−. This phenomenon led to a nano‐structured CO2 sorbent that binds CO2 spontaneously in ambient air when the surrounding is dry, while releasing it when exposed to moisture. The underlying mechanism is elucidated theoretically by computational modeling and verified by experiments. The free energy of CO32− hydrolysis in nanopores reduces with a decrease of water availability. This promotes the formation of OH−, which has a high affinity to CO2. The effect is not limited to carbonate/bicarbonate, but is extendable to a series of ions. Humidity‐driven sorption opens a new approach to gas separation technology.

Phenomenon of the ousting of a monatomic ion from its hydration shell in flat nanopores

The structure and stability of hydrate shells of singly charged sodium and chlorine ions are studied by computer simulations under the conditions of nanoscopic flat pores with the use of the previously proposed detailed force field model containing polarization interactions, transferring charge effects as well as manybody interactions of covalent type. It is found that the effect of ousting a monatomic ion from its hydration shell, which has previously been observed by independent authors in bulk vapor, is also reproduced persistently in nanoscopic pores. Whereas the ousting of the ion from its hydration shell in bulk vapor is accompanied by the loss of thermodynamic stability of the system and at sufficiently high vapor pressure causes avalanche-like condensation, under the conditions of a nanoscopic pore the thermodynamic stability is retained. The obtained data show that the ousting of the ion from its hydration shell is a universal phenomenon covering the majority, if not all, of monatomic and, possibly, some of molecular ions.

On the rewriting of ultrashort pulse-induced nanogratings.

In this study, we report on the erasure and rewriting of nanogratings by femtosecond laser pulses in the bulk of fused silica. To map the structural processes during rewriting, a combination of optical retardance measurement, small angle X-ray scattering, and scanning electron microscopy was used. The results reveal that already few pulses lead to erasure and formation of anisotropic structures. Repetitive rewriting favors the formation of nanoscopic pores, which increases the optical retardance of nanogratings for large pulse numbers.

Visualization and thermodynamic encoding of single-molecule partition function projections.

Ensemble averaging of molecular states is fundamental for the experimental determination of thermodynamic quantities. A special case occurs for single-molecule investigations under equilibrium conditions, for which free energy, entropy and enthalpy at finite temperatures are challenging to determine with ensemble averaging alone. Here we report a method to directly record time-averaged equilibrium probability distributions by confining an individual molecule to a nanoscopic pore of a two-dimensional metal-organic nanomesh, using temperature-controlled scanning tunnelling microscopy. We associate these distributions with partition function projections to assess real-space-projected thermodynamic quantities, aided by computational modelling. The presented molecular dynamics-based analysis is able to reproduce experimentally observed projected microstates with high accuracy. By an in silico customized energy landscape, we demonstrate that distinct probability distributions can be encrypted at different temperatures. Such modulation provides means to encode and decode information into position-temperature space.

Nanoscopic Confinement through Self-Assembly: Crystallization within Micellar Cores Exhibits Simple Gibbs-Thomson Behavior

It is well known that liquids confined to small nanoscopic pores and droplets exhibit thermal behavior very different from bulk samples. Less is known about liquids spontaneously confined through self-assembly into micellar structures. Here we demonstrate, using a very well-defined n-alkyl-poly(ethylene oxide) polymer system with a tunable structure, that n-alkane(s) forming 2-3nm small micellar cores are affected considerably by confinement in the form of melting point depressions. Moreover, comparing the reduction in melting points, ΔT_{m}, determined through volumetric and calorimetric methods with the micellar core radius, R_{c}, obtained from small-angle x-ray scattering, we find excellent agreement with the well-known Gibbs-Thomson equation, ΔT_{m}∼R_{c}^{-1}. This demonstrates that the reduced size, i.e., the Laplace pressure, is the dominant parameter governing the melting point depression in micellar systems.

Characterizing Solvent Dynamics in Nanoscopic Silica Sol–Gel Glass Pores by 2D-IR Spectroscopy of an Intrinsic Vibrational Probe

Porous sol–gel matrices were synthesized with IR-active Si–H vibrational chromophores integrated into the silica network. The Si–H vibrational mode was found to be highly accessible to solvents within the nanoscopic pores. Vibrational solvatochromism of the silane vibration was controlled largely by interactions between infiltrating solvents and the oxygen and hydroxide sites in the silica. Exchanging solvents in the silica matrices produced reversible solvatochromic shifts in some cases but led to irreversible shifts when strongly interacting solvents were tested, suggesting that a layer of solvent was not exchangeable. 2D-IR spectroscopy was used to monitor spectral diffusion and extract the homogeneous line widths of the Si–H mode for a range of infiltrating solvents as well as solvent-free aerogel samples. It was demonstrated that the silane vibration is sensitive to the nature of the infiltrating solvent, making these vibrationally active sol–gel films a general platform for solvent dynamics in nanoscopic confined volumes.

Solid-State Nanopores for Nanoparticle Sensing

In recent years, single particle detection techniques based on driving nanoparticles through nanoscopic pores with a voltage or pressure had been the subject of numerous studies. Since particles with high charge, such as DNA and proteins, have been widely used for research, there was little information about the translocation behavior of nanosized particles which with low charge. However, nanoparticles include virus and colloids are important samples for nanopore sensing technology. In this study, we employed the solid-state nanopore to sense nanoparticles as fundamental study. Nanopore with diameter of 185 nm has been used to detect the nanoparticles with compared low zeta potential of-16 mv in different voltages. And simulations of electric field strength was made by using COMSOL Multiphysics to assist analyze the translocation behavior. The result suggests that the nanoparticles could transport the nanopore under the large electric field. By rising up the bias voltage could favor the detection of the nanoparticles.

Label-free detection of DNA hybridization using a porous poly-Si ion-sensitive field effect transistor

We report an ultra-sensitive, label-free detection system for DNA hybridization using a porous poly-Si gate ion-sensitive field effect transistor. Various physical and chemical characteristics of the sensor system were optimized, along with parameters affecting the immobilization of a label-free single-strand DNA and the specific hybridization of a complementary sequence on the silicon oxide layer, for quantitative detection down to the femtomole level. X-ray photoelectron spectroscopy (XPS) was employed to investigate the nature of the modifications to the porous layer by covalent bonding. Due to the utilization of nanoscopic pores at the gate region, the Debye screening length is no longer a limitation for macromolecule detection. In addition to its compatibility with the CMOS fabrication process, this sensor is a promising device for the label-free electrical detection of DNA pairs or other biomolecules at low concentrations.

Resolution dependence of petrophysical parameters derived from X-ray tomography of chalk

X-ray computed tomography data from chalk drill cuttings were taken over a series of voxel dimensions, ranging from 320 to 25 nm. From these data sets, standard petrophysical parameters (porosity, surface area, and permeability) were derived and we examined the effect of the voxel dimension (i.e., image resolution) on these properties. We found that for the higher voxel dimensions, they are severely over or underestimated, whereas for 50 and 25 nm voxel dimension, the resulting values (5%–30% porosity, 0.2–2 m2/g specific surface area, and 0.06–0.34 mD permeability) are within the expected range for this type of rock. We compared our results to macroscopic measurements and in the case of surface area, also to measurements using the Brunauer-Emmett-Teller (BET) method and found that independent of the degree of compaction, the results from tomography amount to about 30% of the BET method. Finally, we concluded that at 25 nm voxel dimension, the essential features of the nanoscopic pore network in chalk are captured but better resolution is still needed to derive surface area.

Nanoscale Assembly of Proteins into Amyloid Oligomers, Pores and Fibrils

Protein amyloid aggregates are implicated in a variety of debilitating human disorders such as Alzheimer's, Parkinson's and prion diseases. The transition from a normal functional protein to an abnormal misfolded form involves a profound conformational change that serves as the key step in amyloid assembly leading to nanoscopic oligomers, pores and fibrils. Using Raman spectroscopy in combination with atomic force microscopy (Figure 1a,b), we have been able to delineate the key structural transitions during amyloid formation (Bhattacharya et al. J. Phys. Chem. Lett. 2013,4, 480-485). Moreover, the underlying molecular mechanism by which amyloids are involved in inducing cellular toxicity remains elusive because the conventional optical microscopy does not allow one to monitor these processes directly at a high spatial resolution due to the diffraction-limit. Using near-field scanning fluorescence microscopy, we have been able to image the fibrils far beyond the diffraction-limit and interrogate individual fibrils by simultaneously monitoring both nanoscale topography and fluorescence brightness along the length of the fibrils (Figure 1c,d). Our nanoscale imaging results provide structural underpinnings of the supramolecular packing within the nanoscopic amyloids (Dalal & Bhattacharya et al. J. Phys. Chem. Lett. 2012, 3, 1783-1787).View Large Image | View Hi-Res Image | Download PowerPoint Slide

Micropipette Contact Technique as a Tool to Reveal, Characterize, and Modify Nanopore Electrodes

AbstractThis paper reports on the use of a micropipette contact method (MCM) to reveal and characterise single submicrometer‐sized pores randomly distributed within a thin insulating alumina layer (250 nm thickness) that coats a platinum film substrate (Pt/Al2O3). Moveable micropipette probes, with orifice radii of 40–50 μm, filled with electrolyte solutions, are brought in contact with the sample surface to form a microcell and allow electrochemical reactions to be confined within a limited contact region. The aqueous solutions inside the micropipettes contain Ru(NH3)63+ or Ag+ ions and cyclic voltammograms (CVs) are obtained for several locations of 500×500 μm2 above the Pt/Al2O3 sample. The presence of nanoscopic pores is revealed by sigmoidally shaped volatmmograms, which are recorded when Ru(NH3)63+ is used as redox probe. From the steady‐state limiting current and the theoretical equation that applies for recessed microelectrodes, the radius of each platinum disk at the bottom of the pore is evaluated. In this way, pore sizes spanning from about 50 nm to 350 nm are revealed. Similar information is obtained by the use of the MCM with the pipette filled with Ag+ ions. In this case, CVs typical for nucleation and growth of metallic silver are recorded. The volume of a single pore is evaluated from the charge involved in the anodic stripping process of the metal deposited. Potential step measurements and relevant current (I) against time (t) transients allow establishing that only a single nucleus can be grown in each submicometer‐sized pore. Moreover, a diffusion coefficient value of 1.41×10−5 cm2 s−1 for Ag+ is evaluated from the experimental I vs. t1/2 linear plot. Finally, the measurements performed at the Pt/Al2O3 sample are compared with those obtained by using a plain platinum disk electrode 3 mm diameter as substrate for MCM measurements.

Multi‐Length Scale Porous Polymers

Thin films with porosities spanning from the nanoscopic to the macroscopic are obtained by combining breath figures (BFs), micrometer‐sized surface cavities arising from the condensation of water on the surface of a film as solvent evaporates rapidly, with the nanoscopic morphology inherent to block copolymers. Using chloroform as a solvent for polystyrene‐b‐poly methyl methacrylate (PS‐b‐PMMA) block copolymers (BCPs), micrometer‐sized pores arise from the formation of the BFs, while nanoscopic pores are generated by the removal of the PMMA by deep UV‐irradiation, which also crosslinks the PS. Solvent retention, though, limits its utility. This is overcome using PS‐b‐poly(n‐butyl methacrylate) dissolved in dichloromethane where, again, multi‐length scales of porosity are achieved by a selective removal of one component of BCPs. Arrays of nanopores on the surface of a film can also be obtained by swelling the hydrophilic component block of PS‐b‐poly(ethyleneoxide) (PEO) with water vapor, under controlled humidity. Simultaneously, large pores can be obtained by macrophase separation between BCPs and water, which leads to multi‐length scale porous films.

Reduction in thermal conductivity of Bi thin films with high-density ordered nanoscopic pores

We prepared two-dimensional Bi thin films with high-density ordered nanoscopic pores by e-beam evaporation of Bi metal. For this structure, we used polystyrene beads ranging from 200 to 750 nm in diameter as an etch mask. The typical hole and neck sizes of the Bi thin films with approximately 50 nm in thickness on SiO2/Si substrates were in the range of 135 to 490 nm and 65 to 260 nm, respectively. By measuring the thermal characteristics through a 3ω technique, we found that the thermal conductivities of nanoporous Bi thin films are greatly suppressed compared with those of corresponding bulk materials. With a decrease in pore size to approximately 135 nm, the thermal conductivity decreased significantly to approximately 0.46 W/m·K at 300 K.

Translocation dynamics of a semiflexible chain under a bias: Comparison with tension propagation theory

We study translocation dynamics of a semi-flexible polymer through a nanoscopic pore in two dimensions (2D) using Langevin dynamics simulation in presence of an external force inside the pore. We observe that for a given chain length N the mean first passage time (MFPT)〈τ〉 increases for a stiffer chain. By repeating the calculation for various chain lengths N and bending rigidity parameter κb we calculate the translocation exponent α (〈τ〉 ∼ Nα). For chain lengths N and bending rigidity κb considered in this paper we find that the translocation exponent satisfies the inequality α < 1 + ν, where is the equilibrium Flory exponent for a given chain stiffness, as previously observed in various simulation studies for fully flexible chains. We observe that the peak position of the residence time W(s) as a function of the monomer index s shifts at a lower s-value with increasing chain stiffness κb. We also monitor segmental gyration 〈Rg(s)〉 both at the cis and trans side during the translocation process and find that for κb ≠ 0 the late time cis conformations are nearly identical to the early time trans conformations, and this overlap continues to increase for stiffer chains. Finally, we try to rationalize dependence of various quantities on chain stiffness κb using Sakaue’s tension propagation (TP) theory [Phys. Rev. E 76, 021803 (2007)] and Brownian Dynamics Tension Propagation (BDTP) theory due to Ikonen et al. [Phys. Rev. E 85 051803 (2012); J. Chem. Phys. 137 085101 (2012)] originally developed for a fully flexible chain to a semi-flexible chain.

Driven translocation of a semi-flexible chain through a nanopore: A Brownian dynamics simulation study in two dimensions

We study translocation dynamics of a semi-flexible polymer chain through a nanoscopic pore in two dimensions using Langevin dynamics simulation in presence of an external bias F inside the pore. For chain length N and stiffness parameter κb considered in this paper, we observe that the mean first passage time <τ> increases as <τ(κb)>~<τ(κb=0)>lp(aN) , where κb and lp are the stiffness parameter and persistence length, respectively, and aN is a constant that has a weak N dependence. We monitor the time dependence of the last monomer xN(t) at the cis compartment and calculate the tension propagation time (TP) ttp directly from simulation data for <xN(t)> ~ t as alluded in recent nonequlibrium TP theory [T. Sakaue, Phys. Rev. E 76, 021803 (2007)] and its modifications to Brownian dynamics tension propagation theory [T. Ikonen, A. Bhattacharya, T. Ala-Nissila, and W. Sung, Phys. Rev. E 85, 051803 (2012); and J. Chem. Phys. 137, 085101 (2012)] originally developed to study translocation of a fully flexible chain. We also measure ttp from peak position of the waiting time distribution W(s) of the translocation coordinate s (i.e., the monomer inside the pore), and explicitly demonstrate the underlying TP picture along the chain backbone of a translocating chain to be valid for semi-flexible chains as well. From the simulation data, we determine the dependence of ttp on chain persistence length lp and show that the ratio ttp∕<τ> is independent of the bias F.

Nanoscopic Amyloid Pores Formed via Stepwise Protein Assembly.

Protein aggregation leading to various nanoscale assemblies is under scrutiny due to its implications in a broad range of human diseases. In the present study, we have used ovalbumin, a model non-inhibitory serpin, to elucidate the molecular events involved in amyloid assembly using a diverse array of spectroscopic and imaging tools such as fluorescence, laser Raman, circular dichroism spectroscopy, and atomic force microscopy (AFM). The AFM images revealed a progressive morphological transition from spherical oligomers to nanoscopic annular pores that further served as templates for higher-order supramolecular assembly into larger amyloid pores. Raman spectroscopic investigations illuminated in-depth molecular details into the secondary structural changes of the protein during amyloid assembly and pore formation. Additionally, Raman measurements indicated the presence of antiparallel β-sheets in the amyloid core. Overall, our studies revealed that the protein conformational switch in the context of the oligomers triggers the hierarchical assembly into nanoscopic amyloid pores. Our results will have broad implications in the structural characterization of amyloid pores derived from a variety of disease-related proteins.

Elasticity of the Lipid Bilayer Edge. Trends in Line Tension

The line tension or excess free energy per unit length of a bilayer edge is an essential measure of the toughness of a lipid bilayer and its ability to support nanoscopic pores. Well-converged values for the line tension of model pure lipid bilayer edges are important for evaluation of the tendency of additives to stabilize or destabilize the edge. Experimental measurements of bilayer edge line tension are challenging, and new approaches are still under development. In this study we report trends for line tensions and microscopic details of the lipid bilayer edge from a series of atomistic simulations of phosphatidylcholine lipids with varying degree of saturation and tail lengths. The simulation line tensions we obtain are higher than those reported from experiments. The choice of force-field on the resulting ribbon properties was investigated and found to not affect the results. The energetics of edge formation as an area expansion perturbation to the bilayer state was also explored and explains the simulation line tensions within a factor of two.

Differential Cell Adhesion on Mesoporous Silicon Substrates

Porous silicon (PSi) is a promising material in several biomedical applications because of its biocompatibility and biodegradability. Despite the plethora of studies focusing on the interaction of cells with micrometer and submicro geometrical features, limited information is available on the response of cells to substrates with a quasi-regular distribution of nanoscopic pores. Here, the behavior of four different cell types is analyzed on two mesoporous (MeP) silicon substrates, with an average pore size of ∼5 (MeP1) and ∼20 nm (MeP2), respectively. On both MeP substrates, cells are observed to spread and adhere in a larger number as compared to flat silicon wafers. At all considered time points, the surface density of the adhering cells nd is larger on the PSi substrate with the smaller average pore size (MeP1). At 60 h, nd is from ∼1.5 to 5 times larger on MeP1 than on MeP2 substrates, depending on the cell type. The higher rates of proliferation are observed for the two neuronal cell types, the mouse neuroblastoma cells (N2A) and the immortalized human cortical neuronal cells (HCN1A). It is speculated that the higher adhesion on MeP1 could be attributed to a preferential matching of the substrate topography with the recently observed multiscale molecular architecture of focal adhesions. These results have implications in the rational development of PSi substrates for supporting cell adhesion and controlling drug release in implants and scaffolds for tissue engineering applications.

Surface and confinement effects on the dielectric relaxation of a monohydroxy alcohol

2-ethyl-1-hexanol (2E1H) was confined to the surface of a collagen matrix at various concentration levels c. Dielectric spectroscopy revealed that upon decreasing c, the alcohol's prominent hydrogen-bond mediated Debye-like relaxation broadens and turns nonexponential. This destabilization of the supramolecular association is accompanied by an increasing relative strength of the structural relaxation in 2E1H up to a point beyond which the two processes are merged when the solvent molecules are sufficiently diluted. These results demonstrate that the contribution of the Debye-like relaxation can be completely suppressed and concomitantly the limit of a simple, nonassociating liquid is reached. Confinement of the alcohol in a monolithic glass with nanoscopic pores subjected to different internal surface treatments is also demonstrated to bear a large impact on the relative strengths of the two processes.

Surface Design for Controlled Crystallization: The Role of Surface Chemistry and Nanoscale Pores in Heterogeneous Nucleation

Current industrial practice for control of primary nucleation (nucleation from a system without pre-existing crystalline matter) during crystallization from solution involves control of supersaturation generation, impurity levels, and solvent composition. Nucleation behavior remains largely unpredictable, however, due to the presence of container surfaces, dust, dirt, and other impurities that can provide heterogeneous nucleation sites, thus making the control and scale-up of processes that depend on primary nucleation difficult. To develop a basis for the rational design of surfaces to control nucleation during crystallization from solution, we studied the role of surface chemistry and morphology of various polymeric substrates on heterogeneous nucleation using aspirin as a model compound. Nucleation induction time statistics were utilized to investigate and quantify systematically the effectiveness of polymer substrates in inducing nucleation. The nucleation induction time study revealed that poly(4-acryloylmorpholine) and poly(2-carboxyethyl acrylate), each cross-linked by divinylbenzene, significantly lowered the nucleation induction time of aspirin while the other polymers were essentially inactive. In addition, we found the presence of nanoscopic pores on certain polymer surfaces led to order-of-magnitude faster aspirin nucleation rates when compared with surfaces without pores. We studied the preferred orientation of aspirin crystals on polymer films and found the nucleation-active polymer surfaces preferentially nucleated the polar facets of aspirin, guided by hydrogen bonds. A model based on interfacial free energies was also developed which predicted the same trend of polymer surface nucleation activities as indicated by the nucleation induction times.