Daniel T. Schwartz

Electrodeposited nanocomposite n-p heterojunctions for solid-state dye-sensitized photovoltaics

Reference LPI-ARTICLE-2000-029View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Controlled multibatch self-assembly of microdevices

A technique is described for assembly of multiple batches of micro components onto a single substrate. The substrate is prepared with hydrophobic alkanethiol-coated gold binding sites. To perform assembly, a hydrocarbon oil, which is applied to the substrate, wets exclusively the hydrophobic binding sites in water. Micro components are then added to the water, and assembled on the oil-wetted binding sites. Moreover, assembly can be controlled to take place on desired binding sites by using an electrochemical method to deactivate specific substrate binding sites. By repeatedly applying this technique, different batches of micro components can be sequentially assembled to a single substrate. As a post assembly procedure, electroplating is incorporated into the technique to establish electrical connections for assembled components. Important issues presented are: substrate fabrication techniques, electrochemical modulation by using a suitable alkanethiol (dodecanethiol), electroplating of tin and lead alloy and binding site design simulations. Finally, we demonstrate a two-batch assembly of silicon square parts, and establishing electrical connectivity for assembled surface-mount light emitting diodes (LEDs) by electroplating.

Biomineralization and Size Control of Stable Calcium Phosphate Core Protein Shell Nanoparticles: Potential for Vaccine Applications

Calcium phosphate (CaP) polymorphs are nontoxic, biocompatible and hold promise in applications ranging from hard tissue regeneration to drug delivery and vaccine design. Yet, simple and robust routes for the synthesis of protein-coated CaP nanoparticles in the sub-100 nm size range remain elusive. Here, we used cell surface display to identify disulfide-constrained CaP binding peptides that, when inserted within the active site loop of Escherichia coli thioredoxin 1 (TrxA), readily and reproducibly drive the production of nanoparticles that are 50-70 nm in hydrodynamic diameter and consist of an approximately 25 nm amorphous calcium phosphate (ACP) core stabilized by the protein shell. Like bone and enamel proteins implicated in biological apatite formation, peptides supporting nanoparticle production were acidic. They also required presentation in a loop for high-affinity ACP binding as elimination of the disulfide bridge caused a nearly 3-fold increase in hydrodynamic diameters. When compared to a commercial aluminum phosphate adjuvant, the small core-shell assemblies led to a 3-fold increase in mice anti-TrxA titers 3 weeks postinjection, suggesting that they might be useful vehicles for adjuvanted antigen delivery to dendritic cells.

Efficient dye‐sensitized charge separation in a wide‐band‐gap p‐n heterojunction

We report the fabrication and performance of a dye‐sensitized p‐n heterojunction formed from a planar interface between two wide‐band‐gap semiconductors, n‐TiO2 and p‐CuSCN, which contains an intervening monolayer of a sulforhodamine B dye. When exposed to visible light, the photoexcited dye molecules transfer electrons to the n‐TiO2 and holes to the p‐CuSCN. The absorbed‐photon‐to‐current efficiency (APCE) is ≳70% and the open circuit voltage is ≊500 mV. This heterojunction is the solid‐state analog of the dye‐sensitized photoelectrochemical interfaces used in photography and photovoltaics. The high quantum efficiency and voltage show that it is possible to simultaneously optimize both the dye/n‐type and dye/p‐type interface for efficient forward charge injection and slow charge combination in a solid‐state device.

Characterization of Ni x Fe1 − x ( 0.10 < x < 0.95 ) Electrodeposition from a Family of Sulfamate‐Chloride Electrolytes

The characteristics of a nickel sulfamate/iron chloride plating bath suitable for high rate electrodeposition of NiFe alloys are described. The effects of current density, electrolyte agitation, and Ni +2 /Fe +2 content on deposit composition and plating current efficiency are explored via stripping voltammetry using a rotating ring-disk electrode. Specific plating bath formulations and operating conditions for depositing a wide range of alloy compositions at a variety of growth rates are illustrated. Special attention is given to determination of polarization and electrolyte mixing conditions for plating Permalloy and Invar. X-ray diffraction studies are used to investigate relationships between alloy composition and crystal structure. The implications for using the plating bath to electroplate composition-modulated alloys and 3D microstructures are discussed.

Metal ion separations using electrically switched ion exchange

An electrochemical method for metal ion separations, called electrically switched ion exchange (ESIX), is described in this paper. In this method, direct oxidation and reduction of an electroactive film attached to an electrode surface is used to load and unload the film with alkali metal cations. The electroactive films under investigation are nickel hexacyanoferrates, which are deposited on the surface by applying an anodic potential to a nickel electrode in a solution containing the ferricyanide anion. Reported film preparation procedures have been modified to produce films with improved capacity and stability. Electrochemical behavior of the derivatized electrodes has been investigated with the use of cyclic voltammetry and chronocoulometry. The films show selectivity for cesium in concentrated sodium solutions. Raman spectroscopy has been used to directly monitor changes in the oxidation state of the film, and imaging experiments have demonstrated that the redox reactions are spatially homogeneous across the film.

A genetic approach for controlling the binding and orientation of proteins on nanoparticles.

Although silver nanoparticles are excellent surface enhancers for Raman spectroscopy, their use to probe the conformation of large proteins at interfaces has been complicated by the fact that many polypeptides adsorb weakly or with a random orientation to colloidal silver. To address these limitations, we sought to increase binding affinity and control protein orientation by fusing a silver-binding dodecapeptide termed Ag4 to the C-terminus of maltose-binding protein (MBP), a well-characterized model protein with little intrinsic silver binding affinity. Quartz crystal microbalance measurements conducted with the MBP-Ag4 fusion protein revealed that its affinity for silver (Kd approximately 180 nM) was at least 1 order of magnitude higher than a control protein, MBP2, containing a non-silver-specific C-terminal extension. Under our experimental conditions, MBP-Ag4 SERS spectra exhibited 2-4 fold higher signal-to-background relative to MPB2 and contained a number of amino acid-assigned vibrational modes that were either weak or absent in control experiments performed with MBP2. Changes in amino acid-assigned peaks before and after MBP-Ag4 bound maltose were used to assess protein orientation on the surface of silver nanoparticles. The genetic route described here may prove useful to study the orientation of other proteins on a variety of SERS-active surfaces, to improve biosensors performance, and to control functional nanobiomaterials assembly.

High-rate through-mold electrodeposition of thick (>200 /spl mu/m) NiFe MEMS components with uniform composition

An electrodeposition process for achieving good uniformity, growth rate, and yield in NiFe microgears is described. Microgears are electrodeposited from a mixed nickel sulfanate/iron chloride electrolyte through a 230-/spl mu/m-thick poly methylmethacrylate mold patterned using synchrotron X-ray radiation. Despite the use of a plating cell with nearly ideal wafer-scale electrolyte mixing characteristics [the uniform injection cell (UIC)], a degree of compositional variation in the microgears can arise. The composition variation is shown to be due primarily to nonuniformities in microscopic electrolyte mixing patterns within the mold. To a lesser extent, nonuniformity in the local current distribution also contributes to feature-scale composition variation. Improved composition uniformity is achieved when the plating bath is formulated to reduce the sensitivity to electrolyte agitation. Electrodeposition of MEMS components from a low-flow sensitivity electrolyte using the UIC results in NiFe growth rates greater than 60 /spl mu/m/h, yields in excess of 90% and good compositional uniformity. Analysis of mechanical properties illustrates that NiFe parts made using this technique compare favorably to typical electrodeposited MEMS components made from nickel and copper.

Single-pot biofabrication of zinc sulfide immuno-quantum dots.

Quantum dots (QDs) are a powerful alternative to organic dyes and fluorescent proteins for biological and biomedical applications. These semiconductor nanocrystals are traditionally synthesized above 200 degrees C in organic solvents using toxic and costly precursors, and further steps are required to conjugate them to a biological ligand. Here, we describe a simple, aqueous route for the one-pot synthesis of antibody-derivatized zinc sulfide (ZnS) immuno-QDs. In this strategy, easily expressed and purified fusion proteins perform the dual function of nanocrystal mineralizers through ZnS binding sequences identified by cell surface display and adaptors for immunoglobin G (IgG) conjugation through a tandem repeat of the B domain of Staphylococcus aureus protein A. Although approximately 4.3 nm ZnS wurtzite cores could be biomineralized from either zinc chloride or zinc acetate precursors, only the latter salt gives rise to protein-coated QDs with long shelf life and narrow hydrodynamic diameters (8.8 +/- 1.4 nm). The biofabricated QDs have a quantum yield of 2.5% and blue-green ensemble emission with contributions from the band-edge at 340 nm and from trap states at 460 and 665 nm that are influenced by the identity of the protein shell. Murine IgG(1) antibodies exhibit high affinity (K(d) = 60 nM) for the protein shell, and stable immuno-QDs with a hydrodynamic diameter of 14.1 +/- 1.3 nm are readily obtained by mixing biofabricated nanocrystals with human IgG.

Microfluidics without microfabrication

Microfluidic devices create spatially defined, chemically controlled environments at microscopic dimensions. We demonstrate the formation and control of microscopic hydrodynamic and chemical environments by impinging a low-intensity acoustic oscillation on a cylindrical electrode. The interaction of small-amplitude (≤203 μm), low-frequency (≤515 Hz) fluid oscillations with a submillimeter cylinder creates four microscopic eddies that circulate adjacent to the cylinder. This steady flow is known as acoustic streaming. Because the steady circulation in the eddies has closed streamlines, reagent dosed from the electrode can escape the eddies only by slow molecular diffusion. As a result, reagent dosing rates of 10 nmol/s produce eddy concentrations as high as 8 mM, without a correspondingly large rise in bulk solution composition. Imaging Raman spectroscopy is used to visualize the eddy concentration distribution for various acoustic oscillation conditions, and point Raman spectra are used to quantify eddy compositions. These results, and corresponding numerical simulations, show that each eddy acts as a microchemical trap with size determined by acoustic frequency and the concentration tuned via reagent dosing rate and acoustic amplitude. Low-intensity acoustic streaming flows can serve as microfluidic elements without the need for microfabrication.