Chemistry, Department of

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Improved Adhesion and Compliancy of Hierarchical Fibrillar Adhesives

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-07-13
Abstract: 

The gecko relies on van der Waals forces to cling onto surfaces with a variety of topography and composition. The hierarchical fibrillar structures on their climbing feet, ranging from mesoscale to nanoscale, are hypothesized to be key elements for the animal to conquer both smooth and rough surfaces. An epoxy-based artificial hierarchical fibrillar adhesive was prepared to study the influence of the hierarchical structures on the properties of a dry adhesive. The presented experiments highlight the advantages of a hierarchical structure despite a reduction of overall density and aspect ratio of nanofibrils. In contrast to an adhesive containing only nanometer-size fibrils, the hierarchical fibrillar adhesives exhibited a higher adhesion force and better compliancy when tested on an identical substrate.

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Article
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Material Versatility Using Replica Molding for Large-Scale Fabrication of High Aspect-Ratio, High Density Arrays of Nano-Pillars

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-06-27
Abstract: 

Arrays of high aspect-ratio (AR) nano-pillars have attracted a lot of interest for various applications, such as for use in solar cells, surface acoustic sensors, tissue engineering, bio-inspired adhesives and anti-reflective surfaces. Each application may require a different structural material, which can vary in the required chemical composition and mechanical properties. In this paper, a low cost fabrication procedure is proposed for large scale, high AR and high density arrays of nano-pillars. The proposed method enables the replication of a master with high fidelity, using the subsequent replica molds multiple times, and preparing arrays of nano-pillars in a variety of different materials. As an example applied to bio-inspired dry adhesion, polymeric arrays of nano-pillars are prepared in this work. Thermoset and thermoplastic nano-pillar arrays are examined using an atomic force microscope to assess their adhesion strength and its uniformity. Results indicate the proposed method is robust and can be used to reliably prepare nano-structures with a high AR.

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Article
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Harnessing Tunable Scanning Probe Techniques to Measure Shear Enhanced Adhesion of Gecko-Inspired Fibrillar Arrays

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-12-30
Abstract: 

The hierarchical arrays of mesoscale to nanoscale fibrillar structures on a gecko’s foot enable the animal to climb surfaces of varying roughness. Adhesion force between the fibrillar structures and various surfaces is maximized after the gecko drags its foot in one direction, which has also been demonstrated to improve the adhesion forces of artificial fibrillar arrays. Essential conditions that influence the magnitude of these interactions include the lateral distance traveled and velocity between the contacting surfaces, as well as the velocity at which the two surfaces are subsequently separated. These parameters have, however, not been systematically investigated to assess the adhesion properties of artificial adhesives. We introduce a systematic study that investigates these conditions using a scanning probe microscope to measure the adhesion forces of artificial adhesives through a process that mimics the mechanism by which a gecko climbs. The measured adhesion response was different for arrays of shorter and longer fibrils. These results from 9000 independent measurements also provide further insight into the dynamics of the interactions between fibrillar arrays and contacting surfaces. These studies establish scanning probe microscopy techniques as a versatile approach for measuring a variety of adhesion properties of artificial fibrillar adhesives.

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Article
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Self-Assembly of Nanoparticles onto the Surfaces of Polystyrene Spheres with a Tunable Composition and Loading

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2015-01-09
Abstract: 

Functional colloidal materials were prepared by design through the self-assembly of nanoparticles (NPs) on the surfaces of polystyrene (PS) spheres with control over NP surface coverage, NP-to-NP spacing, and NP composition. The ability to control and fine tune the coating was extended to the first demonstration of the co-assembly of NPs of dissimilar composition onto the same PS sphere, forming a multi-component coating. A broad range of NP decorated PS (PS@NPs) spheres were prepared with uniform coatings attributed to electrostatic and hydrogen bonding interactions between stabilizing groups on the NPs and the functionalized surfaces of the PS spheres. This versatile two-step method provides more fine control than methods previously demonstrated in the literature. These decorated PS spheres are of interest for a number of applications, such as catalytic reactions where the PS spheres provide a support for the dispersion, stabilization, and recovery of NP catalysts. The catalytic properties of these PS@NPs spheres were assessed by studying the catalytic degradation of azo dyes, an environmental contaminant detrimental to eye health. The PS@NPs spheres were used in multiple, sequential catalytic reactions while largely retaining the NP coating.

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Article
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Microwave Assisted Formation of Monoreactive Perfluoroalkylsilane-based Self-Assembled Monolayers

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-12-15
Abstract: 

We demonstrate the use of microwave radiation as a tool to accelerate the formation of perfluoroalkylsilane based self-assembled monolayers (SAMs) on silicon oxide surfaces. Surface coverage of these SAMs of monoreactive perfluoroalkylsilanes increased in proportion to the duration over which the solutions were heated by microwave radiation. 

Document type: 
Article
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Simultaneous Patterning of Two Different Types of Nanoparticles into Alternating Domains of a Striped Array of a Polymer Blend in a Single Spin-Casting Step

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-11-01
Abstract: 

A fast and convenient method is developed for simultaneously patterning inorganic nanoparticles with different optical, electronic or magnetic functionality to specific surface regions, by spin-casting onto microcontact printed substrates blend solutions in which the two nanoparticle types are functionalized with surface polymer brush layers of different surface energies. The process is based on phase separation of different nanoparticles based on their immiscible brush layers during spin-casting, with the underlying surface energy heterogeneity of the patterned substrate directing the different NP types to domains of different surface energies. Here, we specifically demonstrate the simultaneous localization of cadmium sulfide quantum dots (CdS QDs), addressed with a surface layer of polystyrene (PS), and silver nanoparticles (Ag NPs), addressed with a surface layer of poly(methyl methacrylate) (PMMA), onto the non-polar and polar surface domains, respectively, of hydrophilic glass patterned with hydrophobic octadecyltrichlorosilane (OTS) stripe arrays with micron-scale periodicities. In order to prevent gelation of solvent-swollen polymer-brush coated NPs during spin casting, which effects strong kinetic constraints on phase separation and localization, PS, PMMA or PS/PMMA homopolymer blends of sufficiently high Mw were added to the NP blends to increase the free volume between approaching NPs. The process parameters were fine-tuned to obtain control over defects in the obtained patterns.

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Article
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A Proposed Mechanism of the Influence of Gold Nanoparticles on DNA Hybridization

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-06-25
Abstract: 

A combination of gold nanoparticles (AuNPs) and nucleic acids has been used in biosensing applications. However, there is a poor fundamental understanding of how gold nanoparticle surfaces influence the DNA hybridization process. Here, we measured the rate constants of the hybridization and dehybridization of DNA on gold nanoparticle surfaces to enable the determination of activation parameters using transition state theory. We show that the target bases need to be detached from the gold nanoparticle surfaces before zipping. This causes a shift of the rate-limiting step of hybridization to the mismatch-sensitive zipping step. Furthermore, our results propose that the binding of gold nanoparticles to the single-stranded DNA segments (commonly known as bubbles) in the duplex DNA stabilizes the bubbles and accelerates the dehybridization process. We employ the proposed mechanism of DNA hybridization/dehybridization to explain the ability of 5 nm diameter gold nanoparticles to help discriminate between single base-pair mismatched DNA molecules when performed in a NanoBioArray chip. The mechanistic insight into the DNA–gold nanoparticle hybridization/dehybridization process should lead to the development of new biosensors.

A combination of gold nanoparticles (AuNPs) and nucleic acids has been used in biosensing applications. However, there is a poor fundamental understanding of how gold nanoparticle surfaces influence the DNA hybridization process. Here, we measured the rate constants of the hybridization and dehybridization of DNA on gold nanoparticle surfaces to enable the determination of activation parameters using transition state theory. We show that the target bases need to be detached from the gold nanoparticle surfaces before zipping. This causes a shift of the rate-limiting step of hybridization to the mismatch-sensitive zipping step. Furthermore, our results propose that the binding of gold nanoparticles to the single-stranded DNA segments (commonly known as bubbles) in the duplex DNA stabilizes the bubbles and accelerates the dehybridization process. We employ the proposed mechanism of DNA hybridization/dehybridization to explain the ability of 5 nm diameter gold nanoparticles to help discriminate between single base-pair mismatched DNA molecules when performed in a NanoBioArray chip. The mechanistic insight into the DNA–gold nanoparticle hybridization/dehybridization process should lead to the development of new biosensors.

Document type: 
Article
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Electrochemically Active Nickel Foams as Support Materials for Nanoscopic Platinum Electrocatalysts

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2014-06-27
Abstract: 

Platinum is deposited on open-cell nickel foam in low loading amounts via chemical reduction of Pt cations (specifically, Pt2+ or Pt4+) originating from aqueous Pt salt solutions. The resulting Pt-modified nickel foams (Pt/Ni foams) are characterized using complementary electrochemical and materials analysis techniques. These include electron microscopy to examine the morphology of the deposited material, cyclic voltammetry to evaluate the electrochemical surface area of the deposited Pt, and inductively coupled plasma optical emission spectrometry to determine the mass of deposited Pt on the Ni foam substrate. The effect of potential cycling in alkaline media on the electrochemical behavior of the material and the stability of Pt deposit is studied. In the second part of this paper, the Pt/Ni foams are applied as electrode materials for hydrogen evolution, hydrogen reduction, oxygen reduction, and oxygen evolution reactions in an aqueous alkaline electrolyte. The electrocatalytic activity of the electrodes toward these processes is evaluated using linear sweep voltammetry curves and Tafel plots. The results of these studies demonstrate that nickel foams are acceptable support materials for nanoscopic Pt electrocatalysts and that the resulting Pt/Ni foams are excellent electrocatalysts for the hydrogen evolution reaction. An unmodified Ni foam is shown to be a highly active electrode for the oxygen evolution reaction.

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Article
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Ordered Porous Gold Electrodes to Enhance the Sensitivity of Enzyme-Based Glucose Sensors

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2013-12-27
Abstract: 

Glucose sensors are essential tools for diabetes patients to use in monitoring their blood glucose levels. However, to be able to detect glucose in non-invasively collected physiological fluids, such as tears and urine, the sensitivity of these glucose sensors must be significantly higher than sensors that are currently used to detect glucose concentrations in blood. Increasing the specific surface area of enzyme-based glucose sensors through the use of ordered porous gold electrodes has been shown to enhance the sensitivity of these sensors. The enzyme-based ordered porous gold glucose sensor was demonstrated to be suitable in detecting glucose concentrations ranges that are similar to those occurring in tears. Although sensitivity of the glucose sensor is enhanced, the saturation threshold of the sensor is lowered. Further optimizations of the porous gold electrodes are required to eliminate signal saturation of these improved sensors.

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Article
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Platinum Ordered Porous Electrodes: Developing a Platform for Fundamental Electrochemical Characterization

Peer reviewed: 
Yes, item is peer reviewed.
Date created: 
2013-07-09
Abstract: 

High surface area platinum electrodes with an ordered porous structure (Pt-OP electrodes) have been prepared and characterized by electrochemical methods. This study builds a foundation upon which we can seek an in-depth understanding of the limitations and design considerations to make efficient and stable Pt-OP electrodes for use in electrochemical applications. A set of Pt-OP electrodes were prepared by controlled electrodeposition of Pt through a self-assembled array of spherical particles and subsequent removal of the spherical templates by solvent extraction. The preparation method was shown to be reproducible and the resulting electrodes were found to have clean Pt surfaces and a large electrochemical surface area (A ecsa) resulting from both the porous structure, as well as the nano- and micro-scale surface roughness. Additionally, the Pt-OP electrodes exhibit a surface area enhancement comparable to commercially available electrocatalysts. In summary, the Pt-OP electrodes prepared herein show properties of interest for both gaining fundamental insights into electrocatalytic processes and for use in applications that would benefit from enhanced electrochemical response.

Document type: 
Article
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