D. Keith Roper

Separation of biomolecules using adsorptive membranes

Abstract The efficient recovery of labile biomolecules requires rapid, reliable separation processes using mild conditions. Adsorptive membranes are available in a range of chemistries and geometries which permit their application as clarification, concentration, fractionation and purification tools in a biorecovery sequence. Available devices exhibit low backpressure, short residence times and high volumetric throughputs relative to conventional chromatographic packed beds. Non-uniform flow, dead volumes and backmixing observed in some adsorptive membrane systems preclude them from achieving substantial improvements in resolution relative to conventional packed beds. Improvements in design and operation of these systems should increase their separation performance tenfold. Adsorptive separations using affinity, ion-exchange and hydrophobic membranes are reviewed.

Gold nanoparticles reduced in situ and dispersed in polymer thin films: optical and thermal properties.

Optical and thermal activity of plasmon-active nanoparticles in transparent dielectric media is of growing interest in thermal therapies, photovoltaics and optoelectronic components in which localized surface plasmon resonance (LSPR) could play a significant role. This work compares a new method to embed gold nanoparticles (AuNPs) in dense, composite films with an extension of a previously introduced method. Microscopic and spectroscopic properties of the two films are related to thermal behavior induced via laser excitation of LSPR at 532 nm in the optically transparent dielectric. Gold nanoparticles were incorporated into effectively nonporous 680 μm thick polydimethylsiloxane (PDMS) films by (1) direct addition of organic-coated 16 nm nanoparticles; and (2) reduction of hydrogen tetrachloroaurate (TCA) into AuNPs. Power loss at LSPR excitation frequency and steady-state temperature maxima at 100 mW continuous laser irradiation showed corresponding increases with respect to the mass of gold introduced into the PDMS films by either method. Measured rates of temperature increase were higher for organic-coated NP, but higher gold content was achieved by reducing TCA, which resulted in larger overall temperature changes in reduced AuNP films.

Electroless Gold Island Thin Films: Photoluminescence and Thermal Transformation to Nanoparticle Ensembles

Electroless gold island thin films are formed by galvanic replacement of silver reduced onto a tin-sensitized silica surface. A novel approach to create nanoparticle ensembles with tunable particle dimensions, densities, and distributions by thermal transformation of these electroless gold island thin films is presented. Deposition time is adjusted to produce monomodal ensembles of nanoparticles from 9.5 +/- 4.0 to 266 +/- 22 nm at densities from 2.6 x 1011 to 4.3 x 108 particles cm-2. Scanning electron microscopy and atomic force microscopy reveal electroless gold island film structures as well as nanoparticle dimensions, densities, and distributions obtained by watershed analysis. Transmission UV-vis spectroscopy reveals photoluminescent features that suggest ultrathin EL films may be smoother than sputtered Au films. X-ray diffraction shows Au films have predominantly (111) orientation.

Estimating plate heights in stacked-membrane chromatography by flow reversal

Abstract A novel method to quantitatively evaluate chromatographic separation performance is presented. Distortion of effluent profiles by non-uniform flow is eliminated by reversing flow direction during the experiment. A semiempirical approach accounts for extracolumn effects on peak shape. Effluent profiles are fitted to a closed-form mathematical description of the experiment to estimate plate height values. Plate heights obtained from two stacked-membrane columns decrease from 3.3 μm at 0.52 cm/min to 0.59 μm at 3.8 cm/min, in agreement with dispersive theory. Performance estimates based on moments or bandwidth measurement are less reliable than this method when non-uniform flow or extracolumn effects are significant.

Geometric effects on far-field coupling between multipoles of nanoparticles in square arrays

Metallic nanoparticles organized in regular arrays exhibit an extraordinary spectral feature that arises from electromagnetic coupling between localized surface plasmons and constructive interference from diffracted far-field radiation. A rapid semianalytical description of coupling between dipoles and scattering modes is applied to examine the influence of nanoparticle size, dielectric, and interparticle separation on the occurrence, resonant wavelength, and intensity of the extraordinary spectral feature. Introducing a dynamic polarizability that includes higher-order electric poles into the description accurately characterizes plasmon resonances of larger particles. Previously unrecognized patterns and periodic variations in the extraordinary feature were observed to result from modulations in polarizability, as well as from interference of scattered modes that were distinguishable for the first time using the rapid semianalytic solution. Streamlined rational design of metamaterials with optimum optical properties using the rapid semianalytic coupled dipole approximation is considered.

Electron energy loss spectroscopy of gold nanoparticles on graphene

Plasmon excitation decay by absorption, scattering, and hot electron transfer has been distinguished from effects induced by incident photons for gold nanoparticles on graphene monolayer using electron energy loss spectroscopy (EELS). Gold nano-ellipses were evaporated onto lithographed graphene, which was transferred onto a silicon nitride transmission electron microscopy grid. Plasmon decay from lithographed nanoparticles measured with EELS was compared in the absence and presence of the graphene monolayer. Measured decay values compared favorably with estimated radiative and non-radiative contributions to decay in the absence of graphene. Graphene significantly enhanced low-energy plasmon decay, increasing mode width 38%, but did not affect higher energy plasmon or dark mode decay. This decay beyond expected radiative and non-radiative mechanisms was attributed to hot electron transfer, and had quantum efficiency of 20%, consistent with previous reports.

Periodic nanotemplating by selective deposition of electroless gold island films on particle-lithographed dimethyldichlorosilane layers.

Uniform hexagonal arrays of diverse nanotemplated metal structures were formed via selective electroless gold plating on particle-lithographed dimethyldichlorosilane layers. Surface-associated water at silica bead interstices was shown to correlate with the formation of silane rings with outer ring diameters ranging from 522.5+/-29.7 to 1116.9+/-52.6 nm and/or spherical gold nanoparticles with diameters from 145.5+/-20.2 to 389.1+/-51.1 nm in the array. Reproducibility and millimeter-size scalability of the array were achieved without the need for expensive and sophisticated lithography or metal deposition equipment. The formation of each structure was explained on the basis of the silanization mechanism and microscopic characterization, as well as dimensional analysis of the nanostructures. This new, facile, and versatile method enables fine fabrication of regular metal nanoparticle array platforms to improve optical and plasmonic features in nanoelectronics and nanophotonic devices.

Thermodynamics of Optoplasmonic Heating in Fluid-Filled Gold-Nanoparticle-Plated Capillaries

Dynamic and equilibrium thermal behavior of plasmon-heated gold/silica capillary nanocomposite during evaporative cooling by water or butanol is accurately described at centimeter length scales by continuum optoplasmonic thermodynamics for continuous-wave laser irradiation of 15-50 mW. Gold nanoparticles randomly distributed on the capillary via electroless plating exhibited a composite extinction cross section of 66.74 ± 0.72% of the area of the laser spot, more than 2-fold larger than the physical cross-section of the AuNPs. The extinction cross-section of the AuNPs capillary was invariant for incident laser powers of 15-150 mW and was reduced slightly in the presence of butanol and water due to absorption peak-shifting to lower energies. Introducing composite thermal parameters into the optoplasmonic thermodynamic relation extended its ability to predict heat transfer to laser powers of 100 and 150 mW for water and butanol, respectively. Nonlinear behaviors such as exponential thermal profiles caused by limited thermal conductivity and film boiling are identified at higher laser powers and prevent further extension of the relation. Mathematical reduction of temperature and time variables of the mathematical description shows it accounts for all measured thermodynamic effects when the aforementioned nonlinear behaviors are not present. This confirms that extraordinary thermal transport observed in some nanocomposites are absent for AuNP/silica systems in the given ranges, which allows a macroscale, continuum approach to describe thermal transport.

Optical attenuation of plasmonic nanocomposites within photonic devices

Plasmonic nanostructures enable microscopic optical manipulation such as light trapping in photonic devices. However, integration of embedded nanostructures into photonic devices has been limited by tractability of nanoscale and microscale descriptions in device architectures. This work uses a linear algebraic model to distinguish geometric optical responses of nanoparticles integrated into dielectric substrates interacting with macroscopic back-reflectors from absorptive and nonlinear plasmonic effects. Measured transmission, reflection, and attenuation (losses) from ceramic and polymer composites supporting two- and three-dimensional distributions of gold nanoparticles, respectively, are predictable using the model. A unique equilateral display format correlates geometric optical behavior and attenuation to nanoparticle density and back-reflector opacity, allowing intuitive, visual specification of density and opacity necessary to obtain a particular optical performance. The model and display format are useful for facile design and integration of plasmonic nanostructures into photonic devices for light manipulation.

Asymmetric reduction of gold nanoparticles into thermoplasmonic polydimethylsiloxane thin films.

Polymer thin films containing gold nanoparticles (AuNPs) are of growing interest in photovoltaics, biomedicine, optics, and nanoelectromechanical systems (NEMs). This work has identified conditions to rapidly reduce aqueous hydrogen tetrachloroaurate (TCA) that is diffusing into one exposed interface of a partially cured polydimethylsiloxane (PDMS) thin film into AuNPs. Nanospheroids, irregular gold (Au) networks, and micrometer-sized Au conglomerates were formed in a ∼5 μm layer at dissolved TCA contents of 0.005, 0.05, and 0.5 mass percent, respectively. Multiscale morphological, optical, and thermal properties of the resulting asymmetric AuNP-PDMS thin films were characterized. Reduction of TCA diffusing into the interface of partially cured PDMS film increased AuNP content, robustness, and scalability relative to laminar preparation of asymmetric AuNP-PDMS thin films. Optical attenuation and thermoplasmonic film temperature due to incident resonant irradiation increased in linear proportion to the order of magnitude increases in TCA content, from 0.005 to 0.05 to 0.5 mass percent. At the highest TCA content (0.05 mass percent), an asymmetric PDMS film 52-μm-thick with a 7 μm AuNP-containing layer was produced. It attenuated 85% of 18 mW of incident radiation and raised the local temperature to 54.5 °C above ambient. This represented an increase of 3 to 230-fold in photon-to-heat efficiency over previous thermoplasmonic AuNP-containing systems.