Joshua W. Gallaway

Kinetics of Redox Polymer-Mediated Enzyme Electrodes

Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s(-1) M(-1) when mediator and enzyme are close in redox potential to 9.4 x 10(4) s(-1) M(-1) when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase-oxygen reaction is found to be 2.4 x 10(5) s(-1) M(-1). The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (E(e)(0) = 0.82), the optimum mediator potential is 0.66 V (SHE), and a molecular structure is presented to achieve this result.

Bioelectrocatalytic hydrogels from electron-conducting metallopolypeptides coassembled with bifunctional enzymatic building blocks

Here, we present two bifunctional protein building blocks that coassemble to form a bioelectrocatalytic hydrogel that catalyzes the reduction of dioxygen to water. One building block, a metallopolypeptide based on a previously designed triblock polypeptide, is electron-conducting. A second building block is a chimera of artificial α-helical leucine zipper and random coil domains fused to a polyphenol oxidase, small laccase (SLAC). The metallopolypeptide has a helix–random-helix secondary structure and forms a hydrogel via tetrameric coiled coils. The helical and random domains are identical to those fused to the polyphenol oxidase. Electron-conducting functionality is derived from the divalent attachment of an osmium bis-bipyrdine complex to histidine residues within the peptide. Attachment of the osmium moiety is demonstrated by mass spectroscopy (MS-MALDI-TOF) and cyclic voltammetry. The structure and function of the α-helical domains are confirmed by circular dichroism spectroscopy and by rheological measurements. The metallopolypeptide shows the ability to make electrical contact to a solid-state electrode and to the redox centers of modified SLAC. Neat samples of the modified SLAC form hydrogels, indicating that the fused α-helical domain functions as a physical cross-linker. The fusion does not disrupt dimer formation, a necessity for catalytic activity. Mixtures of the two building blocks coassemble to form a continuous supramolecular hydrogel that, when polarized, generates a catalytic current in the presence of oxygen. The specific application of the system is a biofuel cell cathode, but this protein-engineering approach to advanced functional hydrogel design is general and broadly applicable to biocatalytic, biosensing, and tissue-engineering applications.

PEG, PPG, and Their Triblock Copolymers as Suppressors in Copper Electroplating

As studies of suppression during copper electrodeposition have typically involved polyethylene glycol (PEG), a deposition study was undertaken varying the chemistry of the suppressor molecule. Other polyether additives were compared to PEG in plating solutions of varying acidity, such as polypropylene glycol (PPG) and triblock copolymers of PEG and PPG, those with ethylene oxide terminal blocks termed EPE, and propylene oxide terminals termed PEP. The extent of suppression observed on a rotating disk electrode varied over -50 mV, with PEG suppressing the least and PPG the most. When both homopolymers were present, results were very near those of PEG alone. Also, a variation in Tafel slope was observed, with PPG exhibiting a more activated reaction. These results were interpreted by a significant difference in surface coverage between PEG and PPG (1.4 vs 0.25% surface availability). Using a microfluidic electrochemical cell, differences were observed in suppressor adsorption and desorption on copper under galvanostatic plating conditions. Copolymers desorbed with characteristics of both PEG and PPG. Planar copper films plated in the presence of each of the suppressors showed significant differences in surface luster and roughness, with the copolymer EPE resulting in extremely bright and smooth deposits.

Acceleration Kinetics of PEG, PPG, and a Triblock Copolymer by SPS during Copper Electroplating

Three copper-plating suppressors are examined in three-additive baths: a polyethylene glycol (PEG), a polypropylene glycol (PPG), and a triblock copolymer of the two. Bis(3-sulfopropyl)-disulfide (SPS) is found to transition each to a state of nonsuppression, i.e., accelerate each, at a rate dependent on the suppressor molecule, the SPS concentration, and, to a lesser extent, the suppressor concentration. Using a planar microscale working electrode (d = 100 μm), the kinetic currents of the plating reactions are observed without the influence of ohmic resistance, revealing far higher current densities than previously reported. Potentiostatic and galvanostatic experiments of additive adsorption at short times, t < 20 s, are compared quantitatively using a surface-blocking model to transform the data to effective surface coverage, θ EFF , vs time. A major difference is found in SPS acceleration between galvanostatic and potentiostatic experiments, with the rate of change in suppression being proportional to the current density. This results in a constant rate of change in θ EFF under constant current but a self-reinforcing rate of change in θ EFF at constant potential. A simple additive model is introduced to characterize the results.

Copper Filling of 100 nm Trenches Using PEG, PPG, and a Triblock Copolymer as Plating Suppressors

Patterned 100 nm trenches with an aspect ratio of 3.5 are filled at a nominal current density of -6.6 mA/cm 2 . A low acid copper plating bath is used with the accelerator bis(3-sulfopropyl)-disulfide (SPS) and one of three suppressor molecules: Poly(ethylene glycol) (PEG) 3350, poly(propylene glycol) (PPG) 725, and an ethylene--propylene-ethylene (EPE) oxide triblock copolymer at a molecular weight of 2000 g/mol. All suppressors result in superconformal filling, although the filling rates vary widely. EPE 2000 results in the most rapid filling, metallizing the features without voids in 5 s under some conditions. The superior performance of EPE 2000 is attributed to its high suppression strength, which is greater than either PEG 3350 or PPG 725 at the relevant plating potentials. EPE 2000 filling becomes more rapid as suppressor concentration decreases, down to 100 ppm, which is attributed to a strong correlation between EPE 2000 concentration and adsorption time. EPE 2000 performance is also improved as SPS concentration decreases, a result in contrast to literature observations on larger (500 nm) features. A simple expression is developed to demonstrate that the time scales of suppression, acceleration, and filling can account for this result.

The effect of acid on superconformal filling in 100nm trenches

A study is undertaken to determine the effect of low (10g∕L) and high (100g∕L) acid conditions on superconformal copper electroplating. The suppressor used is the surfactant P-104 at 200ppm, with the accelerator bis(3-sulfopropyl)-disulfide (SPS) at a concentration of 5–35ppm. High acid open circuit potential and polarization curves are shifted approximately −30mV from low acid, both with and without P-104; Tafel slopes are the same at ∼100mV/decade. P-104 displays the same suppression strength in both electrolytes. Patterned 100nm trenches show that the rate of high and low acid filling is essentially the same at low SPS concentration (5ppm). As SPS increases, high acid filling is improved; the effect in low acid is inverse, albeit not as strong. As suppression strength is the same in high and low acids, the dependence on SPS during filling is attributed to acceleration, which involves an interaction between suppressor and accelerator molecules.

In Situ Immersion Plating of Copper and Nickel on Aluminum Using Laser Pulses for Oxide Removal

Copper and nickel are electrodeposited onto aluminum by using a pulsed laser, which removes the native oxide in situ. The method is shown to work on aluminum simply immersed in a plating electrolyte, via exchange plating, or on aluminum with an imposed potential as part of an electrochemical cell in the electrolyte. The electrochemistry of the plating bath controls transient events following the laser strike, with anodic dissolution of aluminum dominating initially. At open circuit, it is shown that the identity of the plating metal determines the time constant for the recovery of potential transients: 5 ms for copper and 20 ms for nickel. Analogous experiments in acid with no plating metal reveal that re-formation of the aluminum oxide layer after ablation is ~ 100 times slower than exchange plating. The magnitude of the temporal voltage change resulting from ablation agrees with that calculated by means of an Evans plot. Subsequent plating on the immersion-plated regions, minutes or even hours after the immersion, yields deposits with good adhesion.