Erkin Seker

Nanoporous Gold: Fabrication, Characterization, and Applications

Nanoporous gold (np-Au) has intriguing material properties that offer potential benefits for many applications due to its high specific surface area, well-characterized thiol-gold surface chemistry, high electrical conductivity, and reduced stiffness. The research on np-Au has taken place on various fronts, including advanced microfabrication and characterization techniques to probe unusual nanoscale properties and applications spanning from fuel cells to electrochemical sensors. Here, we provide a review of the recent advances in np-Au research, with special emphasis on microfabrication and characterization techniques. We conclude the paper with a brief outline of challenges to overcome in the study of nanoporous metals.

The fabrication of low-impedance nanoporous gold multiple-electrode arrays for neural electrophysiology studies

Neural electrodes are essential tools for the study of the nervous system and related diseases. Low electrode impedance is a figure of merit for sensitive detection of neural electrical activity and numerous studies have aimed to reduce impedance. Unfortunately, most of these efforts have been tethered by a combination of poor functional coating adhesion, complicated fabrication techniques, and poor fabrication repeatability. We address these issues with a facile method for reliably producing multiple-electrode arrays with low impedance by patterning highly adherent nanoporous gold films using conventional microfabrication techniques. The high surface area-to-volume ratio of self-assembled nanoporous gold results in a more than 25-fold improvement in the electrode-electrolyte impedance, where at 1 kHz, 850 kOmega impedance for conventional Au electrodes is reduced to 30 kOmega for nanoporous gold electrodes. Low impedance provides a superior signal-to-noise ratio for detection of neural activity in noisy environments. We systematically studied the effect of film morphology on electrode impedance and successfully recorded field potentials from rat hippocampal slices. Here, we present our fabrication approach, the relationship between film morphology and impedance, and field potential recordings.

Fabrication and Characterization of Flexible and Tunable Plasmonic Nanostructures

We present a novel method to fabricate flexible and tunable plasmonic nanostructures based on combination of soft lithography and nanosphere lithography, and perform a comprehensive structural and optical characterization of these structures. Spherical latex particles are uniformly deposited on glass slides and used as molds for polydimethylsiloxane to obtain nanovoid structures. The diameter and depth of the nanostructures are controlled by the size of the latex particles. These surfaces are coated with a thin Ag layer for fabrication of uniform plasmonic nanostructures. Structural characterization of these surfaces is performed by SEM and AFM. Optical properties of these plasmonic nanostructures are evaluated via UV/Vis absorption spectroscopy, dark field microscopy, and surface–enhanced Raman spectroscopy (SERS). Position of the surface plasmon absorption depends on the diameter and depth of the nanostructures. SERS enhancement factor (measured up to 1.4 × 106) is dependent on the plasmon absorption wavelength and laser wavelength used in these experiments.

Effect of Nanoporous Gold Thin Film Morphology on Electrochemical DNA Sensing

Advances in materials science and chemistry have led to the development of a wide range of nanostructured materials for building novel electrochemical biosensors. A systematic understanding of the challenges related to electrode morphology involved in designing such sensors is essential for developing effective biosensing tools. In this study, we use nanoporous gold (np-Au) thin film electrode coatings with submicrometer thicknesses, as a model system to investigate the influence of nanostructuring on DNA-methylene blue (MB) interactions and their application to DNA biosensors. The interaction of single- and double-stranded DNA immobilized onto morphologically different np-Au films with MB was electrochemically interrogated via square wave voltammetry (SWV). The electrochemical signal from these electrodes in response to MB decayed progressively with each SWV scan. The decay rate was governed by accessibility of the electrochemically active np-Au surface by the analyte. The optimum frequency for extracting the maximum signal via SWV was influenced by the film morphology, where the optimum frequency was lower for the nanoporous morphology with lower density of molecular access points into the porous coating. Overall, the np-Au electrodes exhibited a 10-fold enhancement in probe grafting density and approximately 10-fold higher electrochemical current upon probe-target hybridization as compared to the planar Au electrodes. The np-Au electrodes enabled sensitive detection with a dynamic range of 10 to 100 nM that shifts by 1 order of magnitude for coarsened np-Au morphology due to increased target penetration into the porous network and hence enhanced hybridization efficiency. These findings provide insight into the influence of nanostructuring on the transport mechanisms of small molecules and nucleic acids, and yield an understanding of diverse sensor performance parameters such as DNA grafting density, hybridization efficiency, sensitivity and dynamic range.

Nanoporous Gold as a Neural Interface Coating: Effects of Topography, Surface Chemistry, and Feature Size

Designing neural interfaces that maintain close physical coupling of neurons to an electrode surface remains a major challenge for both implantable and in vitro neural recording electrode arrays. Typically, low-impedance nanostructured electrode coatings rely on chemical cues from pharmaceuticals or surface-immobilized peptides to suppress glial scar tissue formation over the electrode surface (astrogliosis), which is an obstacle to reliable neuron-electrode coupling. Nanoporous gold (np-Au), produced by an alloy corrosion process, is a promising candidate to reduce astrogliosis solely through topography by taking advantage of its tunable length scale. In the present in vitro study on np-Aus interaction with cortical neuron-glia co-cultures, we demonstrate that the nanostructure of np-Au achieves close physical coupling of neurons by maintaining a high neuron-to-astrocyte surface coverage ratio. Atomic layer deposition-based surface modification was employed to decouple the effect of morphology from surface chemistry. Additionally, length scale effects were systematically studied by controlling the characteristic feature size of np-Au through variations in the dealloying conditions. Our results show that np-Au nanotopography, not surface chemistry, reduces astrocyte surface coverage while maintaining high neuronal coverage and may enhance neuron-electrode coupling through nanostructure-mediated suppression of scar tissue formation.

Biofouling-Resilient Nanoporous Gold Electrodes for DNA Sensing

Electrochemical nucleic acid sensors are promising tools for point-of-care diagnostic platforms with their facile integration with electronics and scalability. However, nucleic acid detection in complex biological fluids is challenging as biomolecules nonspecifically adsorb on the electrode surface and adversely affect the sensor performance by obscuring the transport of analytes and redox species to the electrode. We report that nanoporous gold (np-Au) electrodes, prepared by a microfabrication-compatible self-assembly process and functionalized with DNA probes, enabled detection of target DNA molecules (10-200 nM) in physiologically relevant complex media (bovine serum albumin and fetal bovine serum). In contrast, the sensor performance was compromised for planar gold electrodes in the same conditions. Hybridization efficiency decreased by 10% for np-Au with coarser pores revealing a pore-size dependence of sensor performance in biofouling conditions. This nanostructure-dependent functionality in complex media suggests that the pores with the optimal size and geometry act as sieves for blocking the biomolecules from inhibiting the surfaces within the porous volume while allowing the transport of nucleic acid analytes and redox molecules.

Mitigation of tensile failure in released nanoporous metal microstructures via thermal treatment

The authors have developed a method to fabricate released microstructures of nanoporous Au (np-Au) by dealloying a Au–Ag alloy film patterned over a sacrificial Al layer. Doubly clamped bridges fail during dealloying owing to large tensile stresses induced during the dealloying process. Thermal treatments of released microstructures prior to dealloying generate sufficient compressive stress to induce plastic buckling. This buckling compensates the tensile stresses generated during the dealloying process, thus mitigating fracture of the np-Au.

Particles and microfluidics merged: perspectives of highly sensitive diagnostic detection

AbstractThere is a growing need for diagnostic technologies that provide laboratories with solutions that improve quality, enhance laboratory system productivity, and provide accurate detection of a broad range of infectious diseases and cancers. Recent advances in micro- and nanoscience and engineering, in particular in the areas of particles and microfluidic technologies, have advanced the “lab-on-a-chip” concept towards the development of a new generation of point-of-care diagnostic devices that could significantly enhance test sensitivity and speed. In this review, we will discuss many of the recent advances in microfluidics and particle technologies with an eye towards merging these two technologies for application in medical diagnostics. Although the potential diagnostic applications are virtually unlimited, the most important applications are foreseen in the areas of biomarker research, cancer diagnosis, and detection of infectious microorganisms. FigureThere is a growing need for diagnostic technologies that provide laboratories with solutions that improve quality, enhance laboratory system productivity, and provide accurate detection of a broad range of infectious diseases and cancers. In this review, we will discuss many of the recent advances in microfluidics and particle technologies with an eye towards merging these two technologies for application in medical diagnostics such as microfluidic device to monitor molecular secretions in real-time as demonstrated in this figure.

Microfabrication-Compatible Nanoporous Gold Foams as Biomaterials for Drug Delivery

Nanoporous gold is a promising material for multi-functional bio-interfaces with its well-characterized surface chemistry, compatibility with conventional micropatterning techniques, electrical conductivity, and high effective surface-area for molecular release. This paper demonstrates its biocompatibility and capability for drug release to modulate cellular response.

PDMS-Glass Interface Adhesion Energy Determined Via Comprehensive Solutions for Thin Film Bulge/Blister Tests

This paper provides comprehensive relationships for pressure, deflection, energy release rate, and phase angle for bulge testing, which are valid for all combinations of the testing length-scales (film thickness, debond size, and bulge height) and materials. These solutions can be used to design experiments that vary relative contributions of opening and sliding displacements at the crack tip by modulating the film thickness, debond size, and bulge volume. Their closed-form nature greatly facilitates property extraction via regression, e.g., modulus from experimental pressure/deflection data or interface toughness from debond size/injected volume data. This is illustrated using experiments to quantify the interfacial adhesion energy between an initially dry polydimethylsiloxane-glass interface via bulge testing under controlled volume injection. The results indicate that the mode-mixity has no effect on the energy required for debonding, which suggests that wetting of the crack faces behind the debonding front eliminates friction. DOI: 10.1115/1.4000428