Daniel A. Higgins

Second harmonic generation measurements of molecular orientation and coadsorption at the interface between two immiscible electrolyte solutions

Abstract Resonant optical second harmonic generation (SHG) is obtained from monolayers of 4- n -octyloxybenzoic acid (OBA) and n -octyl-4-hydroxybenzoate (OHB) adsorbed at the water/1,2-dichloroethane interface. The polarization dependence of the SHG from pure monolayers of OBA and OHB indicates that the molecular axis of the OBA molecules is tilted 6° closer to the surface normal than that of the OHB molecules. The SHG from a series of mixed monolayers of OBA and OHB exhibits interference effects that demonstrate that the dipole moments of the coadsorbed molecules are oriented in opposite directions at the interface.

Optical second-harmonic generation measurements of molecular adsorption and orientation at the liquid/liquid electrochemical interface

The surface-sensitive spectroscopic technique of optical second-harmonic generation (SHG) is applied to the in situ study of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant SHG form molecules which exhibit a large non-linear optical response at a specific wavelength can be used to measure the relative surface coverage of surfactants at the ITIES as a function of the external electro-chemical parameters. In addition, the polarization dependence of the resonant surface SHG can be used to estimate the average molecular orientation of adsorbates at the liquid/liquid electrochemical interface. As an example, the adsorption of the surfactant 4-(4′-dodecyloxyazobenzene)benzoic acid at the water/1,2-dichloroethane interface is characterized as a function of applied potential, surfactant concentration and aqueous pH with in situ resonant molecular SHG measurements. An analysis of the SHG data results in the determination of the local potential and surface pH experienced by the surfactant.

Multiple diffusion pathways in Pluronic F127 mesophases revealed by single molecule tracking and fluorescence correlation spectroscopy.

Single molecule tracking (SMT) and fluorescence correlation spectroscopy (FCS) are used to investigate probe molecule diffusion within the mesophase structures of Pluronic F127 gels. Mixtures are prepared in the hexagonal, lamellar, and cubic regions of the ternary F127/water/butanol phase diagram and are doped with nanomolar concentrations of a perylene diimide dye (DTPDI). Flow aligned F127 gels comprised of hexagonally arranged cylindrical micelles exhibit distinct one-dimensional (1D) DTPDI motion in wide-field videos, with diffusion occurring parallel to the flow alignment direction. The slow 1D dye motion observed is attributed to single molecule diffusion within the viscous, hydrophobic micelle cores. FCS data acquired from the same samples reveal a bimodal distribution of diffusion coefficients with the slower component assigned to 1D motion in the micelle core and the faster component to 3D diffusion in the interconnected micelle coronas. The rate of diffusion for both components increases with decreasing F127 concentration, reflecting a decrease in gel microviscosity. SMT data from the lamellar and cubic mesophases depict isotropic 2D and 3D diffusion, respectively, and provide supporting evidence for the role of the micelle core and corona in governing diffusion. Trajectory angle distributions from 1D diffusing species in the hexagonal mesophase provide quantitative information on the alignment of the cylindrical micelles. These results, and the rare observation of misaligned trajectories, indicate the hexagonal phase is highly ordered.

Electrodeposited silicate films: importance of supporting electrolyte.

Silica and hybrid organic-inorganic films, ca. 100-200 nm thick, can be grown on glassy carbon electrodes through reactions initiated by electrogenerated hydroxide or hydronium ions in water under reductive and oxidative conditions, respectively. A variety of different alkoxysilanes (tetramethoxysilane and organoalkoxysilanes) and supporting electrolytes were used to evaluate whether film formation takes place on glassy carbon electrodes. The results of the study indicate that the acid-base properties of the supporting electrolyte are an important factor in determining whether film formation will take place. For cathodic electrodeposition, thin films can be formed using supporting electrolytes that are close to neutral, such as KCl, KNO3, and NaClO4. For anodic electrodeposition, thin films can be formed using supporting electrolytes that are acidic, such as, KH2PO4, HNO3, H2SO4, etc. The acidity/basicity effects of the electrolytes arise in part from the strong dependence of the hydrolysis and condensation rates of the silicon alkoxide precursors on pH.

Electrokinetic trapping using titania nanoporous membranes fabricated using sol-gel chemistry on microfluidic devices.

We have developed a new method for analyte preconcentration on a microfluidic device using a porous membrane fabricated via sol–gel chemistry. These porous membranes were fabricated within the channels of glass microfluidic devices exploiting laminar flow to bring an alcoholic sol–gel precursor (titanium isopropoxide in 2‐propanol) into contact with an alcohol–water solution at a channel cross intersection. These two streams reacted at the fluidic interface to form a porous titania membrane. The thickness of the membrane could be altered by changing the [H2O]. Analyte concentration was accomplished by applying a voltage across the titania membrane. The level of analyte enrichment was monitored, and enrichment factors of above 4000 in 400 s were obtained for 2,7‐dichlorofluorescein.

Single-Molecule Investigations of Morphology and Mass Transport Dynamics in Nanostructured Materials

Nanostructured materials such as mesoporous metal oxides and phase-separated block copolymers form the basis for new monolith, membrane, and thin film technologies having applications in energy storage, chemical catalysis, and separations. Mass transport plays an integral role in governing the application-specific performance characteristics of many such materials. The majority of methods employed in their characterization provide only ensemble data, often masking the nanoscale, molecular-level details of materials morphology and mass transport. Single-molecule fluorescence methods offer direct routes to probing these characteristics on a single-molecule/single-nanostructure basis. This article provides a review of single-molecule studies focused on measurements of anisotropic diffusion, adsorption, partitioning, and confinement in nanostructured materials. Experimental methods covered include confocal and wide-field fluorescence microscopy. The results obtained promise to deepen our understanding of mass transport mechanisms in nanostructures, thus aiding in the realization of advanced materials systems.

Aminoalkoxysilane reactivity in surface amine gradients prepared by controlled-rate infusion.

The reactivity of a series of substituted aminoalkoxysilanes for surface amine gradient formation has been studied using a newly developed time-based exposure method termed controlled-rate infusion (CRI). The aminoalkoxysilanes used include those that contain primary, secondary, and tertiary monoamines as well as more than one amine group (diamine and triamine). X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of a gradient in each case and to acquire detailed information on gradient composition from which kinetic data were obtained. The total area under the N 1s XPS spectra allows for the extent of amine modification to be quantitatively assessed along each gradient. The N 1s peaks actually appear as doublets, providing additional data on the level of protonation and, hence, amine basicity on the dry surface. The degree of protonation showed an interesting trend toward smaller values running from top to bottom along gradients incorporating the most basic amines. The gradient profiles, including initial steepness and extent of saturation, were shown to be highly dependent on the aminoalkoxysilane precursor employed. The highest levels of modification were achieved for the diamine and primary monoamine precursors while the more hindered amines produced lower levels of surface modification and took longer for saturation to be achieved. By fitting the gradient data to a simple first-order kinetic model, rate constants for the condensation reaction between each aminosilane and accessible surface silanol groups were obtained. The rate constants follow the trend: triamine ~ diamine > monoamine and primary > secondary > tertiary, indicating kinetic factors also play an important role in controlling surface modification. The presence of more than one amine group on the silane is concluded to enhance the rate of condensation to the surface silanol groups, while the more hindered secondary and tertiary amines slow condensation. Collectively, the results provide valuable new data on how the number of amine groups, degree of substitution, and steric hindrance influence silane reactivity with silica surfaces, amine surface coverage, and basicity along the gradient profile.

Profile Control in Surface Amine Gradients Prepared by Controlled-Rate Infusion

Surface amine gradients that exhibit a wide variety of profiles, including those that incorporate spatially distinct regions having steep and gradual variations in chemical functionality, have been prepared by the sol-gel process using a controlled-rate infusion method. In this work, a substrate that incorporates dimethyl and Si-OH groups is temporally modified with an aminoalkoxysilane (NH(2)(CH(2))(3)Si(OC(2)H(5))(3)) to build a gradient film for which the amine content changes over a 10-20 mm distance. Both X-ray photoelectron spectroscopy (XPS) and contact angle measurements confirm the presence of a chemical gradient across the surface of the film. As expected, a greater density of amine functionalities and lower contact angle were found at the bottom of the gradient relative to the top. The local steepness of the gradient was systematically controlled by changing the rate of infusion. Fast rates of infusion created gradient surfaces where the amine content changed slowly along the surface and never reached saturation, whereas slow rates of infusion formed a surface exhibiting a steep rise in amine content followed by saturation. The steepness of the gradient was also changed at predefined positions along its length by programming the rate of infusion. Gradients prepared using six-step, three-step, and two-step programmed infusion rates are shown. The data fit nicely to a kinetic model that assumes first-order kinetics. The ability to manipulate the gradient profile is particularly vital for applications that rely on mass transport and/or those that require spatial control of gradient properties.

Single molecule tracking studies of flow-aligned mesoporous silica monoliths: aging-time dependence of pore order.

Single molecule tracking (SMT) methods are employed to characterize the in-plane alignment and order of cylindrical mesopores in flow-aligned surfactant-templated silica monoliths prepared within glass microfluidic channels. The majority of dye molecules observed in wide-field fluorescence videos of these samples exhibit one-dimensional (1D) diffusive motions. Orthogonal regression analysis of these motions provides a measure of the mesopore orientation distribution function, which in turn is used to quantify the mesopore order via a two-dimensional orientational order parameter, . Mesopore organization is explored as a function of aging time between sol preparation and filling of the microfluidic channels. Channels filled well before gelation of the sol are shown to incorporate large monodomains having average pore alignment within a few degrees of the flow direction. These monodomains extend over several millimeters and yield aging-time-independent values larger than ~0.80. In contrast, channels filled near the time of sol gelation yield monoliths with misaligned pores that are also more disordered, having ≈ 0.35. The SMT results are compared to those from small-angle X-ray scattering anisotropy experiments; these data are consistent across the range of samples investigated. A model describing the aging-time dependence of sol organization is presented. These studies demonstrate that well-aligned mesoporous silica monoliths can be obtained by simple flow alignment procedures but that short sol aging times are required in order to achieve optimum pore organization.

Electric-field-induced dynamics in radial liquid crystal droplets studied by multiphoton-excited fluorescence microscopy

Time-resolved multiphoton-excited fluorescence microscopy is used to study electric-field-induced reorientation dynamics in single liquid crystal (LC) droplets within polymer-dispersed liquid crystal films. Films comprised of nematic LC dispersed in a poly(isobutyl methacrylate) matrix are characterized. An electric field is applied laterally across each droplet, using two parallel copper wires embedded in the film. Three-photon excited fluorescence images are recorded with 200 μs time resolution as the field is modulated on and off. Dramatic spatial variations in the time scales for orientational relaxation within individual droplets are observed. These effects are attributed to polymer/LC interfacial interactions and relaxation of the LC through a transient, metastable organizational state.