Porous Gold Films Research Articles

THE PREPARATION OF NANOPARTICLE FILMS BASED ON LIGHT WELDING TECHNOLOGY

Continuous improvements in scientific research methods have led to increasingly in-depth technological applications of nanomaterials. Currently, research on nanomaterials is no longer limited to analyzing their physical properties, but also focuses on ways to achieve efficient and low-cost nanomaterial synthesis. Nanoparticle films (NFs) are a type of nanomaterial that can be applied in multiple fields. In this study, we investigated ways to improve the self-assembly technology at the water-vapor interface using light welding (LW) technology in order to enhance the mechanical strength and achieve low-cost nanomaterial preparation of high conductivity and flexible metal NFs. The results showed that the resistance of NFs significantly decreased after improving the self-assembly technology at the water-vapor interface using the LW technology. After 60 minutes of light treatment, the resistance of silver NFs decreased by about 53 Ω and the resistance of the gold NFs decreased by about 9 Ω. After photo-welding treatment, the catalytic activity of the self-supporting porous gold film was the highest with a peak current density of 24.6 μA/cm<sup>2</sup>. The results obtained in this research study can be employed to improve the preparation technology of fluid self-assembled nanomaterials. LW technology can be utilized to achieve low-cost manufacturing of nanomaterials with high strength and expand the application field of NFs.

Electrodeposition Behavior of Al-Au Alloys in AlCl3-NaCl-KCl-Aucl Molten Salt

Quartz Crystal Microbalance (QCM) has been used as a highly sensitive mass sensor. This study aims to form porous gold thin films on QCM elements and apply them to highly sensitive gas sensors by attaching selective media. Physical vapor deposition and chemical etching are known methods for forming porous Au thin films. In this study, the electrochemical formation of porous gold films in AlCl3-NaCl-KCl molten salts containing AuCl was investigated. There are few reports about electrodeposition and dissolution potentials of Au in the molten salt electrolyte. As a first step in this study, voltammogram measurements and constant potential electrolysis were performed in the electrolyte, and electrodeposition potentials of Au and Al-Au alloys were discussed.An electrolyte in this study is a mixture of 61 mol% AlCl3- 26 mol% NaCl- 13 mol% KCl molten salt. The mixture salt was melted at 443 K and AuCl was added to the molten salt. A glassy carbon plate (GC) and a pure Al wire were used as the working electrode and reference electrode, respectively. The GC plate and Au mesh were used as the counter electrode for voltammogram measurement and constant potential electrolysis, respectively. Voltammogram measurements were carried out at potential range from -0.1 to 2.4 V vs. Al/Al(Ⅲ). Constant potential electrolysis was performed in the range from -0.2 to 0.3 V with an electric charge density of 40 C cm-2. The electrodeposits were observed by scanning electron microscopy (SEM) and determined the composition of Al and Au by energy dispersive X-ray spectroscopy (EDS).Au ions concentration in the electrolyte was 2.5 mmol dm-3. In voltammogram measurements, a small cathodic current was observed at about 0.8 V, then gradually increased from 0.2 V. In the potential lower than 0 V, a drastical increase of cathodic current was observed, which was attributed to Al electrodeposition. When the potential was reversed, large anodic current flowed from 0 V and a small current peak was also observed at 0.2 V. These were considered to correspond to the dissolution of Al and Al-Au alloys, respectively. Furthermore, a relatively large anodic current peak was observed at about 2 V, which was expected to be due to the dissolution of Au. From the results of constant potential electrolysis, the Au concentration in the electrodeposits at -0.1 V and -0.2 V was less than 1 at%. At 0 V, the Au concentration in the electrodeposit increased sharply to 84 at%, and at 0.3 V it increased moderately to 89 at%. From SEM observation, the morphology of the electrodeposits at -0.1 V and -0.2 V was dense, while that at potentials more than 0 V was sponge-like.AcknowledgementsThis work was supported by JST SICORP and JPMJSC2108.

Elaboration and characterization of porous ultrathin gold films grown by ion beam assisted deposition

The growth of ultrathin gold layers on a silicon substrate is performed with an unconventional ion beam assisted deposition. In this setup, evaporated gold flow and ion beam flux are perpendicular. 5 and 10 nm thick gold were deposited on tilted samples with the assistance of an argon ion beam at energies of 70 and 150 eV. We explored the evolution of surface morphology, mainly roughness and porosity, with ion bombardment for two gold thicknesses. We have also extrapolated Young’s modulus of these ultrathin layers by means of high-frequency acoustic measurements in the 90–260 MHz range. We also investigated the evolution of wettability of our structures with ion bombardment as well as their electrical and optical responses.

Self-Powered Detection of Glucose by Enzymatic Glucose/Oxygen Fuel Cells on Printed Circuit Boards.

Monitoring glucose levels in physiological fluids can help prevent severe complications associated with hypo- and hyper-glycemic events. Current glucose-monitoring systems require a three-electrode setup and a power source to function, which can hamper the system miniaturization to the patient discomfort. Enzymatic fuel cells (EFCs) offer the opportunity to develop self-powered and minimally invasive glucose sensors by eliminating the need for an external power source. Nevertheless, practical applications demand for cost-effective and mass-manufacturable EFCs compatible with integration strategies. In this study, we explore for the first time the use of gold electrodes on a printed circuit board (PCB) for the development of an EFC and demonstrate its application in saliva. To increase the specific surface area, the PCB gold-plated electrodes were modified with porous gold films. At the anode, glucose oxidase is immobilized with an osmium redox polymer that serves as an electron-transfer mediator. At the cathode, bilirubin oxidase is adsorbed onto the porous gold surface with a blocking agent that prevents parasitic reactions while maintaining the enzyme catalytic activity. The resulting EFC showed a linear response to glucose in phosphate buffer within the range 50 μM to 1 mM, with a sensitivity of 14.13 μA cm–2 mM–1. The sensor was further characterized in saliva, showing the linear range of detection of 0.75 to 2 mM, which is within the physiological range, and sensitivity of 21.5 μA cm–2 mM–1. Overall, this work demonstrates that PCBs are suitable platforms for EFCs, paving the way for the development of fully integrated systems in a seamless and miniaturized device.

Porous Gold: A New Frontier for Enzyme-Based Electrodes.

Porous gold (PG) layers modified electrodes have emerged as valuable enzyme support to realize multiple enzyme-based bioelectrochemical devices like biosensors, enzymatic fuel cells (EFCs), smart drug delivery devices triggered by enzyme catalyzed reactions, etc. PG films can be synthesized by using different methods such as dealloying, electrochemical (e.g., templated electrochemical deposition, self-templated electrochemical deposition, etc.) self-assembly and sputter deposition. This review aims to summarize the recent findings about PG synthesis and electrosynthesis, its characterization and application for enzyme-based electrodes used for biosensors and enzymatic fuel cells (EFCs) development.

An Excellent Electrochemical Sensor Based on Highly Porous Gold Film Modified Gold Electrode for Detecting Quercetin in Food and Medicine

Quercetin is one of the most important nutrients in human daily diet, the accurate and efficient analytical approaches for quercetin is highly required as it’s closely related to many biological activities. In this paper, the three-dimensional highly porous gold film (hp-Au) modified gold electrode (Au) was prepared by a facile two steps self-templating method, it was found that hp-Au possess high surface-to-volume ratio and excellent conductivity. Combining with the multiple merits of hp-Au, the hp-Au/Au showed an excellent detection performance towards quercetin compared with bare electrode and Au nanoparticles modified gold electrode (AuNPs/Au), the detection limit was 3.9 nM in a wide concentration range from 20 nM to 100 μM. Meantime hp-Au/Au also showed decent stability, satisfied selectivity and desirable reproducibility. Moreover, the practical application of this sensor was verified by detecting QR in drink or food and medicine samples and the satisfactory recoveries of 97.43% to 101.59% and low RSD value (n = 3) no more than 3% were obtained.

Lamellar nanoporous gold thin films with tunable porosity for ultrasensitive SERS detection in liquid and gas phase.

Lamellar nanoporous gold thin films, constituted of a stack of very thin layers of porous gold, are synthesized by chemical etching from a stack of successively deposited nanolayers of copper and gold. The gold ligament size, the pore size and the distance between lamellas are tunable in the few tens nanometer range by controlling the initial thickness of the layers and the etching time. The SERS activity of these lamellar porous gold films is characterized by their SERS responses after adsorption of probe bipyridine and naphtalenethiol molecules. The SERS signal is investigated as a function of the bipyridine concentration from 10-14 mol L-1 to 10-3 mol L-1. The higher SERS response corresponds to an experimental detection limit down to 10-12 mol L-1. These performance is mainly attributed to the specific nanoporous gold architecture and the larger accessible surface to volume ratio. The lamellar nanoporous gold substrate is explored for sensitive SERS detection of dimethyl methylphosphonate (DMMP), a surrogate molecule of the highly toxic G-series nerve agents. The resultant nanostructure facilitates the diffusion of target molecules through the nanopores and their localization at the enhancing metallic surface leading to the unequivocal Raman signature of DMMP at a concentration of 5 parts per million.

Fabrication of Porous Gold Film Using Graphene Oxide as a Sacrificial Layer.

An original and simple fabrication process to produce thin porous metal films on selected substrates is reported. The fabrication process includes the deposition of a thin layer of gold on a substrate, spin coating of a graphene oxide dispersion, etching the gold film through the graphene oxide layer, and removing the graphene oxide layer. The porosity of the thin gold film is controlled by varying the etching time, the thickness of the gold film, and the concentration of the graphene oxide dispersion. Images by scanning electron and metallurgical microscopes show a continuous gold film with random porosity formed on the substrate with a porosity size ranging between hundreds of nanometers to tens of micrometers. This general approach enables the fabrication of porous metal films using conventional microfabrication techniques. The proposed process is implemented to fabricate electrodes with patterned porosity that are used in a microfluidic system to manipulate living cells under dielectrophoresis. Porous electrodes are found to enhance the magnitude and spatial distribution of the dielectrophoretic force.

Variable Thickness Porous Anodic Alumina/Metal Film Bilayers for Optimization of Plasmonic Scattering by Nanoholes on Mirror.

Continuously variable thickness porous anodic aluminum oxide (PAAO) films were obtained using electrochemical oxidation of bulk aluminum sheet while both electrodes were simultaneously withdrawn from the electrolyte solution. The thickness gradient was controlled by the withdrawal rate (1–10 mm/min range) and thickness variation demonstrated from below 50 nm to above 1 micrometer. The thickness increased linearly with the sample lateral coordinate, whereas the nanopore structure (diameter and interpore distance) remained unchanged. Effects of the initial pore growth and capillary forces are discussed. The presented method can be used for tuning optimal PAAO thickness for optical and other applications as exemplified by finding maximum plasmonic scattering in structured Al–PAAO–Au multilayers. Enhanced scattering from porous gold film separated by a specific-thickness PAAO layer from aluminum mirror surface is demonstrated.

Electroplating of porous gold films for SERS analysis of heme derivatives

Gold porous films with an 2D inverse opal structure were prepared via electrochemical deposition for further application as SERS-active substrates for the on-chip analysis of heme B derivatives. The thickness of the deposited film was controlled through deposition charge while the grain size was dependent significantly on electrolyte content. A semiquantitative surface analysis of film composition and phase analysis of the gold films were performed by XPS and XRD methods while the observed local, Mie and Bragg plasmon resonance modes were discussed based on UV–vis reflectance data.

Selective electrochemical CO2 reduction over highly porous gold films

Electrocatalytic reduction of CO2 to CO is usually subject to the competitive reduction of H+ to hydrogen.

(Invited) Gold Nanobridge Enhanced Fiber Optic Sensor

A wide variety of optical sensors have been used for gas detection. Hollow core waveguides have shown particular usefulness due to the larger interaction volume of the optical signal with the gas species through direct absorption instead the lower interaction volume of evanescent field spectroscopy. In particular porous capillary tube waveguides (PCTW) have been of particular interest because of their very fast response time. One limitation, in particular, is the increased loss due to the nearly free space transmission through the hollow core. PCTWs have been successfully tested on a variety of gases and across a wide temperature range by our group previously. In this paper, we present the results for a fiber optic sensor structure which is composed of gold nanobridges. The gold nanobridges are formed by coating the inside of a microcapillary glass tube with gold. The glass tube is phase separated and one of the phases is removed by a leaching process. This leaves a three dimensionally interconnected porous phase and a three dimensionally interconnected glass phase which is coated with gold. The structure was analyzed for topology and chemistry using a scanning electron microscope. SEM micrographs of the microcapillary tubes both before and after phase separation were obtained. In addition, EDAX was used to confirm the presence of the gold coating on the samples after the leaching process. The porous gold film was found to be composed of approximately 100nm ligaments in an interconnected network and the glass had approximately 30nm pores around these ligaments. The process to produce these gold coated nanoporous elements is abbreviated here due to word count restriction for this abstract. Porous capillary tubes were produced from a phase separable glass composition. This type of glass produces 3-dimensionally interconnected phases through a compositional phase separation process induced by heat treatment. Porosity is created when the secondary phase or soluble phase is removed by immersion in a dilute acid solution that dissolves and leaches out the secondary phase. Capillary tubes to be plated were drawn with an inner diameter of 170-220 µm and an outer diameter range of 400-500 µm. Dimensions were selected to provide an adequate ID for plating and then the insertion of the lead-in and lead-out fibers. Drawn capillary tubes were plated using a commercially available electroless gold plating solution. Plating was accomplished after fibers were annealed to induce phase separation in order to minimize thermal changes to the gold film that the necessary heat treatment would cause. The inner diameter of the capillary tube was coated by flowing the plating solutions through the tubes. Plating was carried out per the manufacturer’s directions with the exception that the solutions were in a flowing state compared to the static application recommended. Plating occurred in a three step process with the first step being a sensitizing solution, then a brief rinse, followed by a combined mixture of an activated gold salt and a reducing agent. Flow rates of the plating solution ranged from 0.5 mL/min-2mL/min depending on tube ID. After plating, the tube and gold film was air dried followed by acid leaching of the secondary phase. Optical transmission spectra were characterized over the spectral range from 1520-1570 using a commercially available spectrometer which contained a swept laser interrogator. A Micron Optics Si-720 Component Testing System (CTS) and SMF-28 lead-in and lead-out fibers were used in the transmission testing. The apparatus constructed for the transmission tests consisted of the CTS unit with the swept laser being used as the light source connected to a SMF optical fiber as the lead-in fiber. A MMF was used for the lead out fiber to the oscilloscope which was attached to a photodetector for detection. The transmission spectrums were studied as a function of the microcapillary tubing length. By comparing the transmission spectrums of the microcapillaries before gold coating and after gold coating, it was shown that the gold coated microcapillaries had higher transmission than the uncoated for each length studied. The gold coated nanobridge enhanced fiber optic sensor system was tested in a gas cell using acetylene gas. Test concentrations ranging from 10% Acetylene down to 100ppm were obtained using a Sierra Instruments gas mixing system. From the optical spectrums measured, it was shown that the absorption peaks associated with the acetylene gas are clearly visible in the optical spectrums. The absorption peak strengths were in the range from 5-10dB depending upon which line in the acetylene absorption spectrum one is considering and what the gas concentration was. Figure 1

Surface plasmons in porous gold films

The surface plasmon resonance effects in porous gold (por-Au) films—nanocomposite porous films containing an ensemble of disordered gold nanoparticles—have been investigated by modulation-polarization spectroscopy. Por-Au films have been obtained by pulsed laser deposition (using a direct particle flow from an erosion torch formed by a YAG:Nd3+ laser in argon). The spectral and angular dependences of the polarization difference ρ(λ, θ) of internal-reflection coefficients of s- and p-polarized radiation in the Kretschmann geometry and the spectral dependences of isotropic reflection angles at ρ(θ) = 0 are measured. Two types of surface plasmon resonance are found: one occurs on isolated nanoparticles (dipole and multipole modes), and the other is due to the dipole–dipole interaction of neighboring nanoparticles. The frequency of electron plasma oscillations for the nanoparticle ensemble and the frequencies and decay parameters of resonances are determined. Dispersion relations for the radiative and nonradiative modes are presented. The negative sign of the dispersion branch of nonradiative modes of dipole–dipole interaction is explained by the spatial dispersion of permittivity. The relationships between the formation conditions of the films, their structure, and established resonance parameters (determining the resonant-optical properties of films) are discussed.

Surface Plasmon's Dispersion Properties of Porous Gold Films.

Nanostructure porous films with arrays of gold nanoparticles (Au NPs) have been produced by pulsed laser deposition. Dispersion properties of surface plasmons have been studied by the modulation-polarization spectroscopy technique. The dispersion relations for radiative modes and two types of non-radiative modes of localized and propagating surface plasmons were obtained. The branches of propagating modes were characterized by negative group velocity caused by spatial dispersion of dielectric function. The propagating modes are caused by dipole-dipole interactions between adjacent Au NPs. The frequencies and relaxation parameters of surface plasmon resonances and the plasma frequencies for Αu NPs were obtained. The relation between the surface plasmon’s properties and formation conditions of films with arrays of Αu NPs is discussed.

Porous Gold Films Fabricated by Wet-Chemistry Processes

Porous gold films presented in this paper are formed by combining gold electroless deposition and polystyrene beads templating methods. This original approach allows the formation of conductive films (2 × 106 (Ω·cm)−1) with tailored and interconnected porosity. The porous gold film was deposited up to 1.2 μm on the silicon substrate without delamination. An original zirconia gel matrix containing gold nanoparticles deposited on the substrate acts both as an adhesion layer through the creation of covalent bonds and as a seed layer for the metallic gold film growth. Dip-coating parameters and gold electroless deposition kinetics have been optimized in order to create a three-dimensional network of 20 nm wide pores separated by 20 nm thick continuous gold layers. The resulting porous gold films were characterized by GIXRD, SEM, krypton adsorption-desorption, and 4-point probes method. The process is adaptable to different pore sizes and based on wet-chemistry. Consequently, the porous gold films presented in this paper can be used in a wide range of applications such as sensing, catalysis, optics, or electronics.

Ambient Filtration Method To Rapidly Prepare Highly Conductive, Paper-Based Porous Gold Films for Electrochemical Biosensing.

Thin gold films offer intriguing material properties for potential applications including fuel cells, supercapacitors, and electronic and photonic devices. We describe here an ambient filtration method that provides a simple and novel way to generate rapidly porous and thin gold films without the need for sophisticated instruments, clean-room environments, and any postgrowth process or sintering steps. Using this approach, we can fabricate highly conductive gold films composed of gold nanoparticles layered atop a matrix of metallic single-walled carbon nanotubes on mixed cellulose ester filter paper within 20 min. These hybrid films (thickness ∼40 nm) exhibit fast electron transfer and excellent electrocatalytic properties that are similar to purchased gold films, but with a larger electroactive surface that lends itself to more sensitive analyte detection. We used the neurotransmitters dopamine and serotonin as benchmark analytes to demonstrate that our hybrid gold films can clearly discriminate the presence of both molecules in a mixture with resolution that greatly exceeds that of either purchased gold slides or electrodeposited gold films. Importantly, we postulate that this new approach could readily be generalized for the rapid fabrication of films from various other metals under ambient conditions, and could also be used as a prelude to transferring the resulting films onto glass or other flexible substrates.

Study of flow rate induced measurement error in flow-through nano-hole plasmonic sensor.

Flow-through gold film perforated with periodically arrayed sub-wavelength nano-holes can cause extraordinary optical transmission (EOT), which has recently emerged as a label-free surface plasmon resonance sensor in biochemical detection by measuring the transmission spectral shift. This paper describes a systematic study of the effect of microfluidic field on the spectrum of EOT associated with the porous gold film. To detect biochemical molecules, the sub-micron-thick film is free-standing in a microfluidic field and thus subject to hydrodynamic deformation. The film deformation alone may cause spectral shift as measurement error, which is coupled with the spectral shift as real signal associated with the molecules. However, this microfluid-induced measurement error has long been overlooked in the field and needs to be identified in order to improve the measurement accuracy. Therefore, we have conducted simulation and analytic analysis to investigate how the microfluidic flow rate affects the EOT spectrum and verified the effect through experiment with a sandwiched device combining Au/Cr/Si3N4 nano-hole film and polydimethylsiloxane microchannels. We found significant spectral blue shift associated with even small flow rates, for example, 12.60 nm for 4.2 μl/min. This measurement error corresponds to 90 times the optical resolution of the current state-of-the-art commercially available spectrometer or 8400 times the limit of detection. This really severe measurement error suggests that we should pay attention to the microfluidic parameter setting for EOT-based flow-through nano-hole sensors and adopt right scheme to improve the measurement accuracy.

Two-dimensional colloidal crystal assisted formation of conductive porous gold films with flexible structural controllability

Two-dimensional (2D) colloidal crystals of polystyrene (PS) particles were used as a structure-controlling template to fabricate conductive Au films with an ordered array of nanoholes. The fabrication mainly involved the functionalization of the supporting substrate with polyelectrolyte (PE) functional layers, self-assembly of Au nanoparticles, and electroless deposition of gold. The self-assembly of Au nanoparticles and electroless deposition of gold were macroscopically monitored using ultraviolet–visible (UV–vis) spectroscopy based on the changes in both the extinction spectra of Au nanoparticles and the optical responses of ordered arrays of PS particles. By scanning electron microscopy (SEM) characterization, it was found that Au nanoparticles were assembled into a film structure with orderly dispersed nanoholes and the deposition of gold was confined to the preformed Au nanoparticle films. During the formation of Au films, PE layer structure, Au nanoparticle size and heating treatment applied to the PS template could influence the structures of conductive porous Au films such as the hole diameter, film thickness, and hole diameter/wall thickness ratio (D/W). In addition, this paper also described electrochemical cyclic voltammetry (CV) employed to demonstrate the porosity of the ultimate Au films.

Porous Gold Films-A Short Review on Recent Progress.

Porous gold films have attracted increasing interest over the last ten years due to the unique properties of high specific surface area and electrical conductivity combined with chemical stability and ability to alter the surface chemistry. Several methods have been developed to synthesize porous gold films such as de-alloying, templating, electrochemical, and self-assembling. These porous gold films are used in diverse fields, for example, as electrochemical and Raman sensors or for chemical catalysis. Here, we provide a short review on the progress of porous gold films over the past ten years, including the synthesis and applications of such films.

Electroassisted codeposition of sol-gel derived silica nanocomposite directs the fabrication of coral-like nanostructured porous gold.

Herein, we report on a one-step coelectrodeposition method to form gold-silica nanocomposite materials from which high surface area nanostructured gold electrodes can be produced. The as-prepared Au-SiO2 films possess an interconnected three-dimensional porous framework with different silica-gold ratios depending on the deposition solutions and parameters. Chemical etching of the nanocomposite films using hydrofluoric acid resulted in the formation of nanostructured porous gold films with coral-like structures and pores in the nanometer range. The cross-linkage of the gold coral branches resulted in the generation of a porous framework. X-ray photoelectron spectroscopy confirms the complete removal of silica. Well-controlled surface area enhancement, film thickness, and morphology were achieved by manipulating the deposition parameters, such as potential, time, and gold ion and sol-gel monomer concentrations in the deposition solution. An enhancement in the surface area of the electrode up to 57 times relative to the geometric area has been achieved. The thickness of the as-prepared Au-SiO2 nanocomposite films is relatively high and varied from 8 to 15 μm by varying the applied deposition potential while the thickness of the coral-like nanostructured porous gold films ranged from 0.22 to 2.25 μm. A critical sol-gel monomer concentration (CSGC) was determined at which the deposited silica around the gold coral was able to stabilize the coral-like gold nanostructures, while below the CSGC, the coral-like gold nanostructures were unstable and the surface area of the nanostructured porous gold electrodes decreased.