Azo-Dye-Functionalized Polycarbonate Membranes for Textile Dye and Nitrate Ion Removal.

Challenges exist in the wastewater treatment of dyes produced by the world's growing textiles industry. Common problems facing traditional wastewater treatments include low retention values and breaking the chemical bonds of some dye molecules, which in some cases can release byproducts that can be more harmful than the original dye. This research illustrates that track-etched polycarbonate filtration membranes with 100-nanometer diameter holes can be functionalized with azo dye direct red 80 at 1000 µM, creating a filter that can then be used to remove the entire negatively charged azo dye molecule for a 50 µM solution of the same dye, with a rejection value of 96.4 ± 1.4%, at a stable flow rate of 114 ± 5 µL/min post-functionalization. Post-functionalization, Na+ and NO3- ions had on average 17.9%, 26.0%, and 31.1% rejection for 750, 500, and 250 µM sodium nitrate solutions, respectively, at an average flow rate of 177 ± 5 µL/min. Post-functionalization, similar 50 µM azo dyes had increases in rejection from 26.3% to 53.2%. Rejection measurements were made using ultraviolet visible-light spectroscopy for dyes, and concentration meters using ion selective electrodes for Na+ and NO3- ions.

Conforming nanoparticle sheets to surfaces with Gaussian curvature.

Nanoparticle monolayer sheets are ultrathin inorganic-organic hybrid materials that combine highly controllable optical and electrical properties with mechanical flexibility and remarkable strength. Like other thin sheets, their low bending rigidity allows them to easily roll into or conform to cylindrical geometries. Nanoparticle monolayers not only can bend, but also cope with strain through local particle rearrangement and plastic deformation. This means that, unlike thin sheets such as paper or graphene, nanoparticle sheets can much more easily conform to surfaces with complex topography characterized by non-zero Gaussian curvature, like spherical caps or saddles. Here, we investigate the limits of nanoparticle monolayers' ability to conform to substrates with Gaussian curvature by stamping nanoparticle sheets onto lattices of larger polystyrene spheres. Tuning the local Gaussian curvature by increasing the size of the substrate spheres, we find that the stamped sheet morphology evolves through three characteristic stages: from full substrate coverage, where the sheet extends over the interstices in the lattice, to coverage in the form of caps that conform tightly to the top portion of each sphere and fracture at larger polar angles, to caps that exhibit radial folds. Through analysis of the nanoparticle positions, obtained from scanning electron micrographs, we extract the local strain tensor and track the onset of strain-induced dislocations in the particle arrangement. By considering the interplay of energies for elastic and plastic deformations and adhesion, we construct arguments that capture the observed changes in sheet morphology as Gaussian curvature is tuned over two orders of magnitude.

Thermomechanical Response of Self-Assembled Nanoparticle Membranes.

Monolayers composed of colloidal nanoparticles, with a thickness of less than 10 nm, have remarkable mechanical moduli and can suspend over micrometer-sized holes to form free-standing membranes. In this paper, we discuss experiments and coarse-grained molecular dynamics simulations characterizing the thermomechanical properties of these self-assembled nanoparticle membranes. These membranes remain strong and resilient up to temperatures much higher than previous simulation predictions and exhibit an unexpected hysteretic behavior during the first heating-cooling cycle. We show this hysteretic behavior can be explained by an asymmetric ligand configuration from the self-assembly process and can be controlled by changing the ligand coverage or cross-linking the ligand molecules. Finally, we show the screening effect of water molecules on the ligand interactions can strongly affect the moduli and thermomechanical behavior.

Hybrid nanostructures of well-organized arrays of colloidal quantum dots and a self-assembled monolayer of gold nanoparticles for enhanced fluorescence.

Hybrid nanomaterials comprised of well-organized arrays of colloidal semiconductor quantum dots (QDs) in close proximity to metal nanoparticles (NPs) represent an appealing system for high-performance, spectrum-tunable photon sources with controlled photoluminescence. Experimental realization of such materials requires well-defined QD arrays and precisely controlled QD-metal interspacing. This long-standing challenge is tackled through a strategy that synergistically combines lateral confinement and vertical stacking. Lithographically generated nanoscale patterns with tailored surface chemistry confine the QDs into well-organized arrays with high selectivity through chemical pattern directed assembly, while subsequent coating with a monolayer of close-packed Au NPs introduces the plasmonic component for fluorescence enhancement. The results show uniform fluorescence emission in large-area ordered arrays for the fabricated QD structures and demonstrate five-fold fluorescence amplification for red, yellow, and green QDs in the presence of the Au NP monolayer. Encapsulation of QDs with a silica shell is shown to extend the design space for reliable QD/metal coupling with stronger enhancement of 11 times through the tuning of QD-metal spatial separation. This approach provides new opportunities for designing hybrid nanomaterials with tailored array structures and multiple functionalities for applications such as multiplexed optical coding, color display, and quantum transduction.

Strong Resistance to Bending Observed for Nanoparticle Membranes.

We demonstrate how gold nanoparticle monolayers can be curled up into hollow scrolls that make it possible to extract both bending and stretching moduli from indentation by atomic force microscopy. We find a bending modulus that is 2 orders of magnitude larger than predicted by standard continuum elasticity, an enhancement we associate with nonlocal microstructural constraints. This finding opens up new opportunities for independent control of resistance to bending and stretching at the nanoscale.

Mechanical properties of self-assembled nanoparticle membranes: stretching and bending.

Monolayers composed of colloidal nanoparticles, with a thickness of less than ten nanometers, have remarkable mechanical strength and can suspend over micron-sized holes to form free-standing membranes. We discuss experiments probing the tensile strength and bending stiffness of these self-assembled nanoparticle sheets. The fracture behavior of monolayers and multilayers is investigated by attaching them to elastomer substrates which are then stretched. For different applied strain, the fracture patterns are imaged down to the scale of single particles. The resulting detailed information about the crack width distribution allows us to relate the measured overall tensile strength to the distribution of local bond strengths within a layer. We then introduce two methods by which freestanding nanoparticle monolayers can be rolled up into hollow, tubular "nano-scrolls", either by electron beam irradiation during imaging with a scanning electron microscope or by spontaneous self-rolling. Indentation measurements on the nano-scrolls yield values for the bending stiffness that are significantly larger than expected from the response to stretching. The ability to stretch, bend, and roll up nanoparticle sheets offers new possibilities for a variety of applications, including sensors and mechanical transducers.

Ion transport controlled by nanoparticle-functionalized membranes.

From proton exchange membranes in fuel cells to ion channels in biological membranes, the well-specified control of ionic interactions in confined geometries profoundly influences the transport and selectivity of porous materials. Here we outline a versatile new approach to control a membrane's electrostatic interactions with ions by depositing ligand-coated nanoparticles around the pore entrances. Leveraging the flexibility and control by which ligated nanoparticles can be synthesized, we demonstrate how ligand terminal groups such as methyl, carboxyl and amine can be used to tune the membrane charge density and control ion transport. Further functionality, exploiting the ligands as binding sites, is demonstrated for sulfonate groups resulting in an enhancement of the membrane charge density. We then extend these results to smaller dimensions by systematically varying the underlying pore diameter. As a whole, these results outline a previously unexplored method for the nanoparticle functionalization of membranes using ligated nanoparticles to control ion transport.

Influence of line tension on spherical colloidal particles at liquid-vapor interfaces.

Atomic force microscopy (AFM) imaging of isolated submicron dodecyltrichlorosilane coated silica spheres, immobilized at the liquid polystyrene- (PS-) air interface at the PS glass transition temperature, T(g), allows for determination of the contact angle θ versus particle radius R. At T(g), all θ versus R measurements are well described by the modified Young's equation for a line tension τ = 0.93 nN. The AFM measurements are also consistent with a minimum contact angle θ(min) and minimum radius R(min), below which single isolated silica spheres cannot exist at the PS-air interface.

Long reach cantilevers for sub-cellular force measurements.

Maneuverable, high aspect ratio poly(3,4-ethylene dioxythiophene) (PEDOT) fibers are fabricated for use as cellular force probes that can interface with individual pseudopod adhesive contact sites without forming unintentional secondary contacts to the cell. The straight fibers have lengths between 5 and 40 µm and spring constants in the 0.07-23.2 nN µm(-1) range. The spring constants of these fibers were measured directly using an atomic force microscope (AFM). These AFM measurements corroborate determinations based on the transverse vibrational resonance frequencies of the fibers, which is a more convenient method. These fibers are employed to characterize the time dependent forces exerted at adhesive contacts between apical pseudopods of highly migratory D. discoideum cells and the PEDOT fibers, finding an average terminal force of 3.1 ± 2.7 nN and lifetime of 23.4 ± 18.5 s to be associated with these contacts.

Improved in situ spring constant calibration for colloidal probe atomic force microscopy.

In colloidal probe atomic force microscopy (AFM) surface forces cannot be measured without an accurate determination of the cantilever spring constant. The effective spring constant k depends upon the cantilever geometry and therefore should be measured in situ; additionally, k may be coupled to other measurement parameters. For example, colloidal probe AFM is frequently used to measure the slip length b at solid/liquid boundaries by comparing the measured hydrodynamic force with Vinogradova slip theory (V-theory). However, in this measurement k and b are coupled, hence, b cannot be accurately determined without knowing k to high precision. In this paper, a new in situ spring constant calibration method based upon the residuals, namely, the difference between experimental force-distance data and V-theory is presented and contrasted with two other popular spring constant determination methods. In this residuals calibration method, V-theory is fitted to the experimental force-distance data for a range of systematically varied spring constants where the only adjustable parameter in V-theory is the slip length b. The optimal spring constant k is that value where the residuals are symmetrically displaced about zero for all colloidal probe separations. This residual spring constant calibration method is demonstrated by studying three different liquids (n-decanol, n-hexadecane, and n-octane) and two different silane coated colloidal probe-silicon wafer systems (n-hexadecyltrichlorosilane and n-dodecyltrichlorosilane).

Hydrolysis of p-nitrophenyl esters promoted by semifluorinated quaternary ammonium polymer latexes and films.

Semifluorinated polymer latexes were prepared by emulsion polymerization of 2.5-25% of a fluoroalkyl methacrylate, 25% chloromethylstyrene, 1% styrylmethyl(trimethyl)ammonium chloride, and the remainder 2-ethylhexyl methacrylate under surfactant-free conditions. The chloromethylstyrene units were converted to quaternary ammonium ions with trimethylamine. In aqueous dispersions at particle concentrations of less than 1 mg mL(-1) the quaternary ammonium ion latexes promoted hydrolyses of p-nitrophenyl hexanoate (PNPH) in pH 9.4 borate buffer and of diethyl p-nitrophenyl phosphate (Paraoxon) in 0.1 M NaOH at 30 °C with half-lives of less than 10 min. Thin 0.7-2 µm films of the latexes on glass promoted fast hydrolysis of Paraoxon but not of PNPH under the same conditions. Even after annealing the quaternary ammonium ion polymer films at temperatures well above their glass transition temperatures, AFM images of the film surfaces had textures of particles. Contact angle measurements of the annealed films against water and against hexadecane showed that the surfaces were not highly fluorinated.

Viscosity-dependent liquid slip at molecularly smooth hydrophobic surfaces.

Colloidal probe atomic force microscopy is used to study the slip behavior of 18 Newtonian liquids from two homologous series, the n-alkanes and n-alcohols, at molecularly smooth hydrophobic n-hexadecyltrichlorosilane coated surfaces. We find that the slip behavior is governed by the bulk viscosity eta of the liquid, specifically, the slip length b approximately etax with x approximately 0.33. Additionally, the slip length was found to be shear rate independent, validating the use of Vinogradova slip theory in this work.