Christopher P. Rhodes

Single molecule tracking on supported membranes with arrays of optical nanoantennas.

Coupling of the localized surface plasmons between two closely apposed gold nanoparticles (nanoantenna) can cause strong enhancements of fluorescence or Raman signal intensity from molecules in the plasmonic “hot-spot”. Harnessing these properties for practical applications is challenging due to the need to fabricate gold particle arrays with well-defined nanometer spacing and a means of delivering functional molecules to the hot-spot. We report fabrication of billions of plasmon-coupled nanostructures on a single substrate by a combination of colloid lithography and plasma processing. Controlled spacing of the nanoantenna gaps is achieved by taking advantage of the fact that polystyrene particles melt together at their contact point during plasma processing. The resulting polymer thread shadows a gap of well-defined spacing between each pair of gold triangles in the final array. Confocal surface-enhanced Raman spectroscopy imaging confirms the array is functionally uniform. Furthermore, a fully intact supported membrane can be formed on the intervening substrate by vesicle fusion. Trajectories of freely diffusing individual proteins are traced as they sequentially pass through, and are enhanced by, multiple gaps. The nanoantenna array thus enables enhanced observation of a fluid membrane system without static entrapment of the molecules.

The importance of combining disorder with order for Li-ion insertion into cryogenically prepared nanoscopic ruthenia

Cryogenically prepared RuO2 (cryo-RuO2), a material known for its ability to “self-wire” into continuous, nanoscopic electronic pathways, is proposed as an electrode for Li-ion microbatteries with three-dimensionally interpenetrated components. We determined processing guidelines that optimize Li-ion uptake in cryo-RuO2 powders by varying the solid-state structure of cryo-RuO2 with thermal processing at 50–250 °C in flowing O2(g) or Ar(g). The highly disordered structure of as-prepared cryo-RuO2 is transformed to rutile RuO2 at 200 °C in O2(g), resulting in a 60% loss of Li-ion capacity (as-prepared: 214 mA h g−1; rutile: 84 mA h g−1). In contrast, thermal processing in Ar(g) preserves structural disorder in the cryo-RuO2, even up to 250 °C. The highest Li-ion capacity occurs for the treatment that mixes order (crystallinity) with disorder: >250 mA h g−1 for cryo-RuO2 heated in oxygen to 50 °C. This study provides processing guidelines to achieve fabrication of 3-D microbattery architectures containing a nanoscopic RuO2 electrode component.

Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel–Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability

Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal-air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however, carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni-Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni(OH)2 nanosheets. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. On the basis of its unique local and extended structure, the ORR specific activity of Ni-Pt 2D nanoframes (5.8 mA cmPt-2) was an order of magnitude higher than Pt/carbon. In addition, accelerated stability testing at elevated potentials up to 1.3 VRHE showed that the metallic Ni-Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni-Pt nanoframes supports that at low overpotentials the first electron transfer is the rate-determining step, and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications.

Dynamic Organization of Myristoylated Src in the Live Cell Plasma Membrane.

The spatial organization of lipid-anchored proteins in the plasma membrane directly influences cell signaling, but measuring such organization in situ is experimentally challenging. The canonical oncogene, c-Src, is a lipid anchored protein that plays a key role in integrin-mediated signal transduction within focal adhesions and cell-cell junctions. Because of its activity in specific plasma membrane regions, structural motifs within the protein have been hypothesized to play an important role in its subcellular localization. This study used a combination of time-resolved fluorescence fluctuation spectroscopy and super-resolution microscopy to quantify the dynamic organization of c-Src in live cell membranes. Pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS) showed that a small fraction of c-Src transiently sorts into membrane clusters that are several times larger than the monomers. Photoactivated localization microscopy (PALM) confirmed that c-Src partitions into clusters with low probability and showed that the characteristic size of the clusters is 10-80 nm. Finally, time-resolved fluorescence anisotropy measurements were used to quantify the rotational mobility of c-Src to determine how it interacts with its local environment. Taken together, these results build a quantitative description of the mobility and clustering behavior of the c-Src nonreceptor tyrosine kinase in the live cell plasma membrane.

Fabrication of VB2/Air Cells for Electrochemical Testing

A technique to investigate the properties and performance of new multi-electron metal/air battery systems is proposed and presented. A method for synthesizing nanoscopic VB2 is presented as well as step-by-step procedure for applying a zirconium oxide coating to the VB2 particles for stabilization upon discharge. The process for disassembling existing zinc/air cells is shown, in addition construction of the new working electrode to replace the conventional zinc/air cell anode with a the nanoscopic VB2 anode. Finally, discharge of the completed VB2/air battery is reported. We show that using the zinc/air cell as a test bed is useful to provide a consistent configuration to study the performance of the high-energy high capacity nanoscopic VB2 anode.

Non-aqueous Electrode Processing and Construction of Lithium-ion Coin Cells.

Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational analysis. Testing of lithium-ion batteries in an academic setting has taken on several forms, but at the most basic level lies the coin cell construction. In traditional LIB electrode preparation, a multi-phase slurry composed of active material, binder, and conductive additive is cast out onto a substrate. An electrode disc can then be punched from the dried sheet and used in the construction of a coin cell for electrochemical evaluation. Utilization of the potential of the active material in a battery is critically dependent on the microstructure of the electrode, as an appropriate distribution of the primary components are crucial to ensuring optimal electrical conductivity, porosity, and tortuosity, such that electrochemical and transport interaction is optimized. Processing steps ranging from the combination of dry powder, wet mixing, and drying can all critically affect multi-phase interactions that influence the microstructure formation. Electrochemical probing necessitates the construction of electrodes and coin cells with the utmost care and precision. This paper aims at providing a step-by-step guide of non-aqueous electrode processing and coin cell construction for lithium-ion batteries within an academic setting and with emphasis on deciphering the influence of drying and calendaring.

Probing the Effect of High Energy Ball Milling on the Structure and Properties of LiNi1/3Mn1/3Co1/3O2 Cathodes for Li-Ion Batteries

Particle size plays an important role in the electrochemical performance of cathodes for lithium-ion (Li-ion) batteries. High energy planetary ball milling of LiNi1/3Mn1/3Co1/3O2 (NMC) cathode materials was investigated as a route to reduce the particle size and improve the electrochemical performance. The effect of ball milling times, milling speeds, and composition on the structure and properties of NMC cathodes was determined. X-ray diffraction analysis showed that ball milling decreased primary particle (crystallite) size by up to 29%, and the crystallite size was correlated with the milling time and milling speed. Using relatively mild milling conditions that provided an intermediate crystallite size, cathodes with higher capacities, improved rate capabilities, and improved capacity retention were obtained within 14 lm-thick electrode configurations. High milling speeds and long milling times not only resulted in smaller crystallite sizes but also lowered electrochemical performance. Beyond reduction in crystallite size, ball milling was found to increase the interfacial charge transfer resistance, lower the electrical conductivity, and produce aggregates that influenced performance. Computations support that electrolyte diffusivity within the cathode and film thickness play a significant role in the electrode performance. This study shows that cathodes with improved performance are obtained through use of mild ball milling conditions and appropriately designed electrodes that optimize the multiple transport phenomena involved in electrochemical charge storage materials. [DOI: 10.1115/1.4034755]