Facile electrochemical approach to fabricate hierarchical flowerlike gold microstructures: Electrodeposited superhydrophobic surface

Amperometric Sensing of Carbon Monoxide: Improved Sensitivity and Selectivity via Nanostructure-Controlled Electrodeposition of Gold

A series of gold (Au) nanostructures, having different morphologies, were fabricated for amperometric selective detection of carbon monoxide (CO), a biologically important signaling molecule. Au layers were electrodeposited from a precursor solution of 7 mM HAuCl4 with a constant deposition charge (0.04 C) at various deposition potentials. The obtained Au nanostructures became rougher and spikier as the deposition potential lowered from 0.45 V to 0.05 V (vs. Ag/AgCl). As prepared Au layers showed different hydrophobicity: The sharper morphology, the greater hydrophobicity. The Au deposit formed at 0.05 V had the sharpest shape and the greatest surface hydrophobicity. The sensitivity of an Au deposit for amperometric CO sensing was enhanced as the Au surface exhibits higher hydrophobicity. In fact, CO selectivity over common electroactive biological interferents (L-ascorbic acid, 4-acetamidophenol, 4-aminobutyric acid and nitrite) was improved eminently once the Au deposit became more hydrophobic. The most hydrophobic Au was also confirmed to sense CO exclusively without responding to nitric oxide, another similar gas signaling molecule, in contrast to a hydrophobic platinum (Pt) counterpart. This study presents a feasible strategy to enhance the sensitivity and selectivity for amperometric CO sensing via the fine control of Au electrode nanostructures.

Antifouling strategies in advanced electrochemical sensors and biosensors

A review presented recent development of antifouling strategies in electrochemical sensors and biosensors based on the modification methods.

Rational Design of a Stimuli-Responsive Polymer Electrode Interface Coupled with in Vivo Microdialysis for Measurement of Sialic Acid in Live Mouse Brain in Alzheimer's Disease

Sensitive and selective monitoring of sialic acid (SA) in cerebral nervous system is of great importance for studying the role that SA plays in the pathological process of Alzheimer's disease (AD). In this work, we first reported an electrochemical biosensor based on a novel stimuli-responsive copolymer for selective and sensitive detection of SA in mouse brain. Notably, through synergetic hydrogen-bonding interactions, the copolymer could translate the recognition of SA into their conformational transition and wettability switch, which facilitated the access and enrichment of redox labels and targets to the electrode surface, thus significantly improving the detection sensitivity with the detection limit down to 0.4 pM. Besides amplified sensing signals, the proposed method exhibited good selectivity toward SA in comparison to potential interference molecules coexisting in the complex brain system due to the combination of high affinity between phenylboronic acid (PBA) and SA and the directional hydrogen-bonding interactions in the copolymer. The electrochemical biosensor with remarkable analytical performance was successfully applied to evaluate the dynamic change of SA level in live mouse brain with AD combined with in vivo midrodialysis. The accurate concentration of SA in different brain regions of live mouse with AD has been reported for the first time, which is beneficial for progressing our understanding of the role that SA plays in physiological and pathological events in the brain.

Bioinspired multifunctional Au nanostructures with switchable adhesion

Inspired by the self-cleaning of cicada wings, well-aligned Au-coated Ni nanocone arrays (Au@Ni NAs) have been fabricated by a simple and cheap electrodeposition method. After surface modification of n-hexadecanethiol, self-cleaning can be realized on this long-lived superhydrophobic surface with extremely low adhesive force. Switchable adhesion is obtained on its complementary porous surface. The porous Au structure is fabricated by a geometric replica of the nanocone arrays. After the same surface modification, it shows superhydrophobicity with high adhesion. The different adhesive behaviors on the two lock-and-key Au structures are ascribed to their different contact modes with a water droplet. Combining the superhydrophobic properties of the two complementary structures, they can be used to transport precious microdroplets without any loss. The bioinspired periodic Au@Ni NAs can also be potentially employed as surface-enhanced Raman scattering (SERS) substrates due to its electromagnetic enhancement effect, especially at the tips of the nanocones. Thus, superhydrophobic, SERS, long-lived, self-cleaning, microtransportation functions are realized on the basis of the two surfaces.

One-process fabrication of metal hierarchical nanostructures with rich nanogaps for highly-sensitive surface-enhanced Raman scattering

One-process fabrication of highly active and reproducible surface-enhanced Raman scattering (SERS) substrates via ion beam deposition is reported. The fabricated metal-dielectric-metal (MDM) hierarchical nanostructure possesses rich nanogaps and a tunable resonant cavity. Raman scattering signals of analytes are dramatically strengthened due to the strong near-field coupling of localized surface plasmon resonances (LSPRs) and the strong interaction of LSPRs of metal NPs with surface plasmon polaritons (SPPs) on the underlying metal film by crossing over the dielectric spacer. The maximum Raman enhancement for the highest Raman peak at 1650 cm(-1) is 13.5 times greater than that of a single metal nanoparticle (NP) array. Moreover, the SERS activity can be efficiently tailored by varying the size and number of voids between adjacent metal NPs and the thickness of the dielectric spacer. These findings may broaden the scope of SERS applications of MDM hierarchical nanostructures in biomedical and analytical chemistry.

Electrodeposition of nanoflake Pd structures: structure-dependent wettability and SERS activity

The characteristic properties of metal surfaces, i.e., wettability and surface-enhanced Raman scattering (SERS) activity, have been the subject of intensive research because of their useful applications. In the present work, we report a simple electrodeposition of nanoflake Pd structures onto clean Au surfaces without the use of additives. The fine structure of the nanoflake Pd surfaces was regulated by controlling the deposition charge, and the effect of the structural variations on the wettability and SERS activity was examined. The wettability of nanoflake Pd structures in terms of water contact angle was closely related to the fine structures of Pd deposits and their surface roughness. The SERS activity of the nanoflake Pd surfaces was highly dependent on the presence of sharp edge sites on the Pd structures. Well-defined nanoflake Pd structures prepared using a deposition charge of 0.04 C exhibited superhydrophobic natures and reproducible SERS activity. The effect of the metal surface structures on the wettability and the SERS activity demonstrated in this work provides insight into the fabrication of functional metal nanostructures.

Electrodeposition of Pt nanostructures with reproducible SERS activity and superhydrophobicity

A simple one-step electrodeposition of nanostructured Pt structures exhibiting both reproducible SERS activity and superhydrophobicity was demonstrated.

Superhydrophobicity-mediated electrochemical reaction along the solid-liquid-gas triphase interface: edge-growth of gold architectures

A superhydrophobic pillar-structured electrode leads to uncommon electrochemical behavior. The anti-wetting reaction surface restricts the contact between electrolyte and electrode to the pillar tops, as a result of trapped air pockets in the gaps between pillars. The electrochemical reaction occurs mainly at the solid/liquid/gas triphase interface, instead of the traditional solid/liquid diphase surface, yielding unique edge-growth structures - for example gold microflowers - on the top of each pillar.

Shape-diversified silver nanostructures uniformly covered on aluminium micro-powders as effective SERS substrates

Highly-sensitive, reliable and reproducible Raman-active substrates via a facile and organic-free method are reported. These intriguing hierarchical structures are formed through the uniform incorporation of silver (Ag) nanoflowers with aluminium (Al) micro-supporters. The underlying mechanism is systematically investigated, visualizing that the solvents used in galvanic displacement have a major effect on diversifying the reaction kinetics of Ag deposition. Moreover, the exploration of AgNO3 concentrations reveals a drastic transition of Ag morphologies, driven by the elimination of high-energy surfaces of Ag. In addition, the surface-modified Al@Ag structures with octadecyltrichlorosilane demonstrate both the non-wetting (contact angle = 157.2°), as well as easy droplet roll-off (contact angle hysteresis = 5.4°) characteristics, which further enables the tested targets to avoid being pinned at a static position upon detection. Finally, we find that the Ag nanoflower surfaces are corrugated with numerous nanogaps at interparticle sites, in such a way that allows the abundant active sites (referred to as "hot spots") to amplify the Raman signal, and simultaneously maintain the sound reliability and reproducibility of Raman detection. These designs along with the fabrication strategy are anticipated to benefit versatile optical, optoelectronic and energy devices.

An invisible template method toward gold regular arrays of nanoflowers by electrodeposition

A new approach, an invisible template method that is realized through controlling the interface electroconductivity of an electrode surface, is presented to synthesize gold regular arrays of nanoflowers with variable separations through further electrochemical deposition. Using polystyrene monolayer colloidal crystals as the first template, a hexagonally packed 1-hexadecanethiol pattern was self-assembled and used as an invisible template to control the interface electroconductivity. Further electrochemical deposition under appropriate conditions can easily lead to gold regular arrays of nanoflowers. This new approach demonstrates a simple route to the fabrication of novel gold micro/nanostructured arrays that may find applications as SERS active substrates, superhydrophobic materials, and so forth.

Superhydrophobic surfaces by electrochemical processes

This review is an exhaustive representation of the electrochemical processes reported in the literature to produce superhydrophobic surfaces. Due to the intensive demand in the elaboration of superhydrophobic materials using low-cost, reproducible and fast methods, the use of strategies based on electrochemical processes have exponentially grown these last five years. These strategies are separated in two parts: the oxidation processes, such as oxidation of metals in solution, the anodization of metals or the electrodeposition of conducting polymers, and the reduction processed such as the electrodeposition of metals or the galvanic deposition. One of the main advantages of the electrochemical processes is the relative easiness to produce various surface morphologies and a precise control of the structures at a micro- or a nanoscale.

Formation of nanostructured porous Cu-Au surfaces: the influence of cationic sites on (electro)-catalysis

The fabrication of nanostructured bimetallic materials through electrochemical routes offers the ability to control the composition and shape of the final material that can then be effectively applied as (electro)-catalysts. In this work a clean and transitory hydrogen bubble templating method is employed to generate porous Cu-Au materials with a highly anisotropic nanostructured interior. Significantly, the co-electrodeposition of copper and gold promotes the formation of a mixed bimetallic oxide surface which does not occur at the individually electrodeposited materials. Interestingly, the surface is dominated by Au(I) oxide species incorporated within a Cu(2)O matrix which is extremely effective for the industrially important (electro)-catalytic reduction of 4-nitrophenol. It is proposed that an aurophilic type of interaction takes place between both oxidized gold and copper species which stabilizes the surface against further oxidation and facilitates the binding of 4-nitrophenol to the surface and increases the rate of reaction. An added benefit is that very low gold loadings are required typically less than 2 wt% for a significant enhancement in performance to be observed. Therefore the ability to create a partially oxidized Cu-Au surface through a facile electrochemical route that uses a clean template consisting of only hydrogen bubbles should be of benefit for many more important reactions.

Novel method for controllable fabrication of a superhydrophobic CuO surface on AZ91D magnesium alloy

A novel method for controllable fabrication of a superhydrophobic CuO surface on AZ91D magnesium alloy is reported in this paper. Hierarchical structure composed of micro/nano-featherlike CuO was obtained by electrodeposition of Cu-Zn alloy coating and subsequently an electrochemical anodic treatment in alkaline solution. After modification with lauric acid, the surface became hydrophobicity/superhydrophobicity. The formation of featherlike CuO structures was controllable by varying the coating composition. By applying SEM, ICP-AES, and water contact angle analysis, the effects of coating composition on the surface morphology and hydrophobicity of the as-prepared surfaces were detailedly studied. The results indicated that at the optimal condition, the surface showed a good superhydrophobicity with a water contact angle as high as 155.5 ± 1.3° and a sliding angle as low as about 3°. Possible growth mechanism of featherlike CuO hierarchical structure was discussed. Additionally, the anticorrosion effect of the superhydrophobic surface was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The interface model for anticorrosion mechanism of superhydrophobic surface in corrosive medium was proposed. Besides, the mechanical stability test indicated that the resulting superhydrophobic surfaces have good mechanical stability.

Preparation of dendritic nanostructures of silver and their characterization for electroreduction

Silver nanostructures of different morphologies including well-defined dendrites were synthesized on an Au substrate by a simple surfactant-free method without using any template. The morphology of the material was investigated by field-emission transmission electron microscopy and scanning electron microscopy. The crystal nature of the dendritic nanostructure was revealed from their X-ray diffraction and electron diffraction patterns. Effects of applied potential, electrolysis time, and the solution concentration were studied. The possible formation mechanism of the dendritic morphology was discussed from the aspects of kinetics and thermodynamics based on the experiment results. The H(2)O(2) electroreduction ability of the dendritic materials was characterized. Use of silver dendrite-modified electrode as H(2)O(2) sensor was also demonstrated.

Easy route to the wettability cycling of copper surface between superhydrophobicity and superhydrophilicity

A hierarchical copper structure combining micro- and nanogaps/pores was built up on copper substrate by etching and electrodeposition. The fresh as-deposited copper was easily oxidized in air at room temperature, forming a CuO layer covering on the surface. The surface could be hydrophobized with thiol-modified fluorocarbons, after which it showed a water contact angle as high as 165° ± 2°. This surface could also regain the superhydrophilicity with a zero water contact angel after annealing at 200 °C for 10 min to desorb the low surface energy monolayer of thiol-modified fluorocarbons and reform a CuO layer again on the surface. Repeating the process of adsorption/desorption of the monolayer by modification and annealing, it was successful to fulfill the wettability cycling between superhydrophobicity and superhydrophilicity on the copper surface. The adsorption and desorption mechanism of the monolayer was discussed based on the result of surface chemistry analysis.

Laser-induced nanoscale superhydrophobic structures on metal surfaces

The combination of a dual-scale (nano and micro) roughness with an inherent low-surface energy coating material is an essential factor for the development of superhydrophobic surfaces. Ultrashort pulse laser (USPL) machining/structuring is a promising technique for obtaining the dual-scale roughness. Sheets of stainless steel (AISI 304 L SS) and Ti-6Al-4V alloys were laser-machined with ultraviolet laser pulses of 6.7 ps, with different numbers of pulses per irradiated area. The surface energy of the laser-machined samples was reduced via application of a layer of perfluorinated octyltrichlorosilane (FOTS). The influence of the number of pulses per irradiated area on the geometry of the nanostructure and the wetting properties of the laser-machined structures has been studied. The results show that with an increasing number of pulses per irradiated area, the nanoscale structures tend to become predominantly microscale. The top surface of the microscale structures is seen covered with nanoscale protrusions that are most pronounced in Ti-6Al-4V. The laser-machined Ti-6Al-4V surface attained superhydrophobicity, and the improvement in the contact angle was >27% when compared to that of a nontextured surface.

Ag dendrites with rod-like tips: synthesis, characterization and fabrication of superhydrophobic surfaces

A facile, low-cost and templateless wet-chemical method was reported for the mass synthesis of Ag dendrites with rod-like tips which were characterized by various methods. The columniform tips protruded in random directions, of which the diameters were similar. A series of experiments were carried out to explore the effect of sodium citrate on the morphology of the obtained Ag nanostructures, and it was demonstrated that the concentration of sodium citrate was important for the formation of the cylindrical tips on the Ag dendrites. A possible mechanism was proposed to explain the role of sodium citrate in the synthesis of Ag dendrites with rod-like tips. Based on the roughness caused by the Ag dendrites, a superhydrophobic surface was fabricated in a simple procedure on a glass substrate, and the contact angle of the surface was as high as 156.4°. According to similar procedures, superhydrophobic surfaces were easily attained on a DVD disc and commercial aluminium foil, indicating the as-prepared Ag dendrites can be used to fabricate superhydrophobic surfaces on various smooth substrates.

Simple electrochemical deposition of au nanoplates from au(I) cyanide complexes and their electrocatalytic activities

Nanostructured Au surfaces have unique and attractive properties as functional materials in many fields such as heterogeneous catalysis and electrocatalysis. Electrochemical deposition of Au has received much attention as a simple route for the fabrication of Au surface nanostructures. In this study, we report a simple electrodeposition of Au nanoplate structures from Au(CN)(2)(-) on Au surfaces in the absence of additives or premodification of electrode surfaces. The shape of the Au nanoplates as well as their surface structures is unique compared to other Au nanostructures electrodeposited from commonly employed AuCl(4)(-) complexes. The nanoplate Au surfaces exhibit unique electrocatalytic activities for oxygen reduction and glucose oxidation, which originate from the Au(110) and Au(100) facets present on nanoplate surfaces. A simple preparation of well-defined Au nanoplate structures would allow new opportunities in various areas utilizing Au-based substrates through further modification of Au surfaces.

Gold nanospikes formed through a simple electrochemical route with high electrocatalytic and surface enhanced Raman scattering activity

We demonstrate a simple electrochemical route to produce uniformly sized gold nanospikes without the need for a capping agent or prior modification of the electrode surface, which are predominantly oriented in the {111} crystal plane and exhibit promising electrocatalytic and SERS properties.