Structural studies of water in hydrophilic and hydrophobic mesoporous silicas: An x-ray and neutron diffraction study at 297 K

Tunable Hierarchically Porous Gadolinium-Based Metal-Organic Frameworks for Bacteria-Targeting Magnetic Resonance Imaging and In Situ Anti-Bacterial Therapy

Currently, there are no non-invasive tools to accurately diagnose deep surgical site bacterial infections before they cause significant anatomical damage in the clinic. An urgent need exists for bacteria-targeting bifunctional probes for the detection of deep bacterial infections and precise in situ treatment. Herein, the bacteria-targeting 1-borono-3,5-benzenedicarboxylic acid (BBDC) ligand and paramagnetic Gd3 + into one single metal-organic frameworks (MOFs) are integrated, synergistically realizing bacteria-specific magnetic resonance imaging (MRI) diagnosis and MRI-guided antibacterial treatment. Molecular simulations and nitrogen adsorption-desorption experiments demonstrate that a hierarchical porous structure can be constructed by tuning the Gd3 + /BBDC ratio, which endows the Gd-BBDC1.25 MOFs with an impressive longitudinal proton relaxivity of 15.81 mM-1 s-1. In particular, the bacteria-targeting boronic acid group in BBDC remained intact during the MOF synthesis, ensuring that Gd-BBDC1.25 MOFs have a unique combination of high sensitivity and specificity for bacteria. Through an in situ reduction reaction, silver nanoparticles （Ag NPs）-modified Gd-BBDC1.25 MOFs to form Ag@Gd-BBDC1.25, an interfacial Schottky heterojunction nanozyme, which enhances their peroxidase (POD)-catalyze activity. Furthermore, it is demonstrated that the bacteria-targeting Ag@Gd-BBDC1.25 bifunctional probe can image as few as 105 colony-forming units (cfu) in vivo and effectively eradicate the bacteria in situ.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Dielectric Profile and Electromelting of a Monolayer of Water Confined in Graphene Slit Pore

A monolayer of water confined between two parallel graphene sheets exists in many different phases and exhibits fascinating dielectric properties that have been studied in experiments. In this work, we use molecular dynamics simulations to study how the dielectric properties of a confined monolayer of water is affected by its structure. We consider six of the popular nonpolarizable water models-SPC/E, SPC/Fw, TIP3P, TIP3P_M (modified), TIP4P-2005, and TIP4P-2005f-and find that the in-plane structure of the water molecules at ambient temperature and pressure is strongly dependent on the water model: all the 3-point water models considered here show square ice formation, whereas no such structural ordering is observed for the 4-point water models. This allows us to investigate the role of the in-plane structure of the water monolayer on its dielectric profile. Our simulations show an anomalous perpendicular dielectric constant compared to the bulk, and the models that do not exhibit ice formation show very different dielectric response along the channel width compared to models that exhibit square ice formation. We also demonstrate the occurrence of electromelting of the in-plane ordered water under the application of a perpendicular electric field and find that the critical field for electromelting strongly depends on the water model. Together, we have shown the dependence of confined water properties on the different water structures that it may take when sandwiched between bilayer graphene. These remarkable properties of confined water can be exploited in various nanofluidic devices, artificial ion channels, and molecular sieving.

Structure of ice confined in silica nanopores

Observed anomalous thermodynamic properties of confined water such as deviations in the melting point and freezing point motivate the determination of the structure of confined water as a function of pore size and temperature. In this study, we investigate the dynamic evolution of the structure of confined ice in SBA-15 porous materials with pore diameters of 4 nm, 6 nm, and 8 nm at temperatures ranging from 183 K to 300 K using in operando Wide-Angle X-Ray Scattering (WAXS) measurements, X-Ray Partial Distribution Function (PDF) measurements, and classical Molecular Dynamics (MD) simulations. Formation of hexagonal ice structures is noted in all the three pore sizes. In silica nanopores with diameters of 4 nm, cubic ice formation is noted in addition to hexagonal ice. Longer lasting hydrogen bonds and longer residence times of the water molecules in the first coordination shell contribute to observed crystalline organization of ice in confinement. Self-diffusion coefficients of confined liquid water, predicted from classical MD simulations, are four orders of magnitude higher compared to ice formed in confinement. These experimental and simulation results provide comprehensive insights underlying the organization of confined water and ice in silica nanopores and the underlying physico-chemical interactions that contribute to the observed structures.

Crossover from localized to diffusive water dynamics in carbon nanohorns: A comprehensive quasielastic neutron-scattering analysis

Incoherent neutron scattering by water confined in carbon nanohorns was measured with the backscattering spectrometer SPHERES and analyzed in exemplary breadth and depth. Quasielastic spectra admit δ-plus-Kohlrausch fits over a wide q and T range. From the q and T dependence of fitted amplitudes and relaxation times, however, it becomes clear that the fits do not represent a uniform physical process, but that there is a crossover from localized motion at low T to diffusive α relaxation at high T. The crossover temperature of about 210 to 230 K increases with decreasing wave number, which is incompatible with a thermodynamic strong-fragile transition. Extrapolated diffusion coefficients D(T) indicate that water motion is at room temperature about 2.5 times slower than in the bulk; in the supercooled state this factor becomes smaller. At even higher temperatures, where the α spectrum is essentially flat, a few percentages of the total scattering go into a Lorentzian with a width of about 1.6μeV, probably due to functional groups on the surface of the nanohorns.

Ultraconfinement of aqueous electrolytic solutions within hydrophilic nanotubes

By means of molecular simulations we shed light on the interplay of surface, confinement and salt effects on the structure and dynamics of water and ions highly confined within a hydrophilic silica nanotube.

Model of waterlike fluid under confinement for hydrophobic and hydrophilic particle-plate interaction potentials

Molecular dynamic simulations were employed to study a waterlike model confined between hydrophobic and hydrophilic plates. The phase behavior of this system is obtained for different distances between the plates and particle-plate potentials. For both hydrophobic and hydrophilic walls, there are the formation of layers. Crystallization occurs at lower temperature at the contact layer than at the middle layer. In addition, the melting temperature decreases as the plates become more hydrophobic. Similarly, the temperatures of maximum density and extremum diffusivity decrease with hydrophobicity.

Enhanced MRI relaxivity of Gd(3+) -based contrast agents geometrically confined within porous nanoconstructs

Gadolinium chelates, which are currently approved for clinical MRI use, provide relaxivities well below their theoretical limit, and they also lack tissue specificity. Recently, the geometrical confinement of Gd(3+) -based contrast agents (CAs) within porous structures has been proposed as a novel, alternative strategy to improve relaxivity without chemical modification of the CA. Here, we have characterized and optimized the performance of MRI nanoconstructs obtained by loading [Gd(DTPA)(H(2) O)](2-) (Magnevist®) into the pores of injectable mesoporous silicon particles. Nanoconstructs with three different pore sizes were studied, and at 60 MHz, they exhibited longitudinal relaxivities of ~24 m m(-1)  s(-1) for 5-10 nm pores and ~10 m m(-1)  s(-1) for 30 - 40 nm pores. No enhancement in relaxivity was observed for larger pores sizes. Using an outer-sphere compound, [GdTTHA](3-) , and mathematical modeling, it was demonstrated that the relaxivity enhancement is due to the increase in rotational correlation times (CA adsorbed on the pore walls) and diffusion correlation times (reduced mobility of the water molecules), as the pore sizes decreases. It was also observed that extensive CA adsorption on the outer surface of the silicon particles negates the advantages offered by nanoscale confinement. Upon incubation with HeLa cells, the nanoconstructs did not demonstrate significant cytotoxicity for up to 3 days post incubation, at different particle/cell ratios. In addition, the nanoconstructs showed complete degradation after 24 h of continuous agitation in phosphate-buffered saline. These data support and confirm the hypothesis that the geometrical confinement of Gd(3+) -chelate compounds into porous structures offers MRI nanoconstructs with enhanced relaxivity (up to 6 times for [Gd(DTPA)(H(2) O)](2-) , and 4 times for [GdTTHA](3-) ) and, potentially, improved stability, reduced toxicity and tissue specificity.

Site-dipole field and vortices in confined water

We use molecular dynamics simulations to examine how the spatial patterns formed by the diffusive motion of water molecules are influenced by confinement between hydrophilic and hydrophobic surfaces. For bulk liquid water, Higo et al. found that the long-range orientational order of spatial dipole moments can form vortex-like spatial patterns for as long as 300 ps [Higo et al., Proc. Natl. Acad. Sci. USA 98, 5961 (2001)]. We perform a similar analysis for confined water and we find that the existence of vortices in these systems is dependent on both the surface separation and the surface hydrophilicity. Vortices perpendicular to the surface normal disappear when the surface separation is reduced to a thickness where the system is comprised of mostly interfacial water molecules. Vortices exist at slightly smaller separations for hydrophobic surfaces than for hydrophilic surfaces because the dipoles are less aligned at the hydrophobic surfaces. The dipole alignment that is induced by the hydrophilic surfaces is counter to the direction required by vortices.