Transparent electrodes attract intense attention in many technological fields, including optoelectronic devices, transparent film heaters and electromagnetic applications. New generation transparent electrodes are expected to have three main physical properties: high electrical conductivity, high transparency and mechanical flexibility. The most efficient and widely used transparent conducting material is currently indium tin oxide (ITO). However the scarcity of indium associated with ITO's lack of flexibility and the relatively high manufacturing costs have a prompted search into alternative materials. With their outstanding physical properties, metallic nanowire (MNW)-based percolating networks appear to be one of the most promising alternatives to ITO. They also have several other advantages, such as solution-based processing, and are compatible with large area deposition techniques. Estimations of cost of the technology are lower, in particular thanks to the small quantities of nanomaterials needed to reach industrial performance criteria. The present review investigates recent progress on the main applications reported for MNW networks of any sort (silver, copper, gold, core-shell nanowires) and points out some of the most impressive outcomes. Insights into processing MNW into high-performance transparent conducting thin films are also discussed according to each specific application. Finally, strategies for improving both their stability and integration into real devices are presented.

Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: a Review

Au and Au-Based nanomaterials: Synthesis and recent progress in electrochemical sensor applications.

Paper-based sensors offer an affordable yet powerful platform for field and point-of-care (POC) testing due to their self-pumping ability and utility for many different analytical measurements. When combined with electrochemical detection using small and portable electronics, sensitivity and selectivity of the paper devices can be improved over naked eye detection without sacrificing portability. Herein, we review how the field of electrochemical paper-based analytical devices (ePADs) has grown since it was introduced a decade ago. We start by reviewing fabrication methods relevant to ePADs with more focus given to the electrode fabrication, which is fundamental for electrochemical sensing. Multiple sensing approaches applicable to ePADs are then discussed and evaluated to present applicability, advantages and challenges associated with each approach. Recent applications of ePADs in the fields of clinical diagnostics, environmental testing, and food analysis are also presented. Finally, we discuss how the current ePAD technologies have progressed to meet the analytical and practical specifications required for field and/or POC applications, as well as challenges and outlook.

Electrochemical paper-based devices: sensing approaches and progress toward practical applications

This review reports on recent advances in state-of-the-art detection methods for microfluidic paper-based analytical devices (μPADs). The review commences by describing the materials, fabrication methods and driving mechanisms employed in μPADs. The review then explores the main detection methods and applications of μPADs proposed over the past five years. The discussions focus on seven detection technologies, namely (1) colorimetric detection, (2) fluorescent detection, (3) electrochemical detection and photoelectrochemical detection, (4) chemiluminescent detection, (5) electrochemiluminescent detection, (6) nanoparticle-based detection, and (7) spectrometry detection. Overall, the review provides a useful insight into recent advances in the μPAD detection field and serves as a useful source of reference for further research and innovation in the field.

Detection methods and applications of microfluidic paper-based analytical devices

In recent years, gold nanoparticles have demonstrated excellent enzyme-mimicking activities which resemble those of peroxidase, oxidase, catalase, superoxide dismutase or reductase. This, merged with their ease of synthesis, tunability, biocompatibility and low cost, makes them excellent candidates when compared with biological enzymes for applications in biomedicine or biochemical analyses. Herein, over 200 research papers have been systematically reviewed to present the recent progress on the fundamentals of gold nanozymes and their potential applications. The review reveals that the morphology and surface chemistry of the nanoparticles play an important role in their catalytic properties, as well as external parameters such as pH or temperature. Yet, real applications often require specific biorecognition elements to be immobilized onto the nanozymes, leading to unexpected positive or negative effects on their activity. Thus, rational design of efficient nanozymes remains a challenge of paramount importance. Different implementation paths have already been explored, including the application of peroxidase-like nanozymes for the development of clinical diagnostics or the regulation of oxidative stress within cells via their catalase and superoxide dismutase activities. The review also indicates that it is essential to understand how external parameters may boost or inhibit each of these activities, as more than one of them could coexist. Likewise, further toxicity studies are required to ensure the applicability of gold nanozymes in vivo. Current challenges and future prospects of gold nanozymes are discussed in this review, whose significance can be anticipated in a diverse range of fields beyond biomedicine, such as food safety, environmental analyses or the chemical industry.

/pdf/gold-nanozymes-from-concept-to-biomedical-applications-fgm5kpc4t2.pdf

Gold Nanozymes: From Concept to Biomedical Applications

In this article, urchin-like palladium (Pd) nanostructures were electrochemically deposited onto pre-patterned single-walled carbon nanotube (SWCNT) thin films. X-ray photoelectron spectroscopy and X-ray diffraction techniques were used to characterize the properties of Pd nanostructures on SWCNT thin films. The spherical Pd particles and urchin-like Pd particles were synthesized at −0.5 V and +0.1 V, respectively. Scanning electron microscope images indicated that the diameters of spherical Pd particles and urchin-like Pd particles were in the range of 50–100 nm and 300–400 nm, respectively. Cyclic voltammetry was applied to investigate the urchin-like Pd/SWCNT electrode with nitrite oxidation. Results showed that the current response of urchin-like Pd/SWCNT was 1.5 times greater than that of spherical Pd/SWCNT electrodes. The effects of precursor concentration, accumulated charges, pattern size on the morphology of urchin-like Pd structures were also investigated. The urchin-like Pd NPs/SWCNT electrode displayed a suitable detection limit of 0.25 μM ( S / N  = 3) under the controlled condition (de-oxygenated buffer solution at pH 4.0) and two linear ranges with respect to nitrite concentration: one from 2 μM to 238 μM ( R 2  = 0.995) and the other from 283 μM to 1230 μM ( R 2  = 0.998). The corresponding sensitivities were 417 μA mM −1  cm −2 and 192 μA mM −1  cm −2 , respectively. The presence of K + , Na + , Cl − , PO 4 3− , NH 4 + , CH 3 COO − , Zn 2+ , Mg 2+ , Ca 2+ , Cd 2+ , CO 3 2− , NO 3 − , and SO 4 2− showed a negligible effect on the current response of nitrite determination, suggesting that developed sensors have a suitable degree of tolerance to common ions that can cause interference.

Electrochemical detection of nitrite using urchin-like palladium nanostructures on carbon nanotube thin film electrodes

Nanostructured artificial enzymes, known as nanozymes, have been considered effective alternatives for natural enzymes with the attractive advantages. Herein, bimetallic Au and Pt nanoparticles with graphene oxide (GO) were immobilized chemically on the surface of agarose microbeads. Due to the synergistic effect of the bimetallic nanoparticles and GO (Au@PtNP/GO), this hybrid nanostructure exhibited strong peroxidase-like catalytic activity toward 3,3′,5,5′-tetramethylbenzidine (TMB) substrate in the presence of H2O2. Colorimetric determination of H2O2, indicated by the blue color of the oxidized TMB within 1 min, was observed using the Au@PtNP/GO nanozymes. To demonstrate practical application of the nanozymes, the Au@PtNP/GO microbeads were packed into a film-based electrochemical point-of-care (POC) device for sensitive and rapid H2O2 detection. Upon introduction of TMB substrate solution containing H2O2, the catalytically oxidized TMB was electrochemically reduced on the electrode surfaces, resulting in a broader detection range of H2O2 (1 μM – 3 mM) and a lower LOD (1.62 μM) than achievable with the colorimetric detection method. Moreover, the developed POC devices showed the capability of accurate determination of H2O2 with strong repeatability and reproducibility in real sample test using artificial urine.

Characterization of Au@PtNP/GO nanozyme and its application to electrochemical microfluidic devices for quantification of hydrogen peroxide

We fabricated a nafion/graphene oxide-gold nanoparticle (GO-AuNP) hybrid modified indium tin oxide (ITO) electrode and proposed an electrochemical method to detect hydrogen peroxide (H2O2) using 3,3,5,5,-tetramethylbenzidine (TMB) as a redox mediator. The GO-AuNP hybrids were employed as nanozymes, which function as peroxidase mimics and show highly effective catalytic activity. Based on the high catalytic activity, enzyme mimics were entrapped on the ITO electrode to construct an electrochemical H2O2 sensor by coating nafion polymer. During the catalytic reaction, the peroxidase substrate TMB was oxidized to form the TMB oxidation product, which not only produces a blue color detected by absorbance change, but also generates an electrochemical current. As a result, both spectrophotometric and electrochemical methods were used to determine H2O2 concentration. The spectrophotometric detection displayed a linearity for H2O2 concentration from 10 μM to 5 mM (r2 = 0.989), with an estimated detection limit of 2 μM. In the electrochemical detection, the TMB peak current had a good linear relationship with H2O2 concentration from 10 nM to 10 mM, with an estimated detection limit of 1.9 nM, which was much lower than that of the spectrophotometric method result.

Graphene oxide-gold nanozyme for highly sensitive electrochemical detection of hydrogen peroxide

In the present study, we constructed a functional paper fluidic device and evaluated its electrochemical performance by analyzing acetaminophen in the presence of ascorbic acid. The device was composed of a single-walled carbon nanotube (SWCNT) electrode and nafion-modified nitrocellulose membrane. Negatively-charged nafion was employed to build up a more negative charge on the nitrocellulose membrane, and gold nanoparticles and polyglutamic acid (AuNP-PGA) were deposited on the SWCNT electrode to enhance the electrochemical performance of the device. The device had a vertical flow format in which the sample solution flowed vertically through the paper. Using the nafion-modified nitrocellulose membrane and AuNP-PGA/SWCNT film electrode as a component of the paper fluidic device, we obtained a distinguishable acetaminophen oxidation peak which was distinct from the ascorbic acid oxidation peak. The acetaminophen oxidation peak had a linear response with acetaminophen concentration, varying from 50 μM to 300 μM (r 2  = 0.992), which was broader than the standard drug dose range. The device exhibited a sensitivity of 13.3 mA/M and a detection limit of 15.0 μM. The device was stable with a relative standard deviation of 3.3% (up to 2 weeks), and the reproducibility was 1.2–5.2%. Furthermore, the fabricated device accurately measured the amount of acetaminophen in pharmaceutical samples.

Determination of acetaminophen using functional paper-based electrochemical devices

We present a simple method to fabricate flexible, transparent silver nanowire (AgNW) films. Homogenous AgNW networks were formed on a mixed cellulose ester (MCE) membrane by vacuum filtration and were easily transferable to self-adhesive poly(ethylene terephthalate). The opaque, white MCE membrane became transparent after being subjected to hot acetone vapor. The fabricated AgNW films had an average resistivity of 13 Ω/sq and a transmittance of approximately 67%. Moreover, the AgNW films showed excellent mechanical properties in repeated adhesion tests and bending tests. The AgNW films also showed good resistance against heat and NaCl solutions. AgNW films were patterned by a combination of soft lithography and a solution-based chemical etching technique. The area under the photoresist polymer maintained its conductivity and transmittance after etching, whereas increased resistance and transparency were observed in the etched area. The suitability of the patterned AgNW electrodes for electronic devices was demonstrated via a simple LED chip array. After using standard photolithography to define the working area, AgNW films were used as sensor electrodes for the electrochemical detection of hydrogen peroxide (H2O2). The AgNW sensors displayed a reasonable detection limit of 46 μM (S/N = 3), a rapid response time (within 2 s), and high sensitivity (749 μA mM−1 cm−2 and 1640 μA mM−1 cm−2). Furthermore, the AgNW sensor was resistant to other potential interfering electroactive species commonly present in physiological samples such as l -ascorbic acid, glucose, and sodium oxalate. These results indicate that the AgNW sensor is selective for electroreduction of H2O2. Additionally, the developed sensor exhibited a stable amperometric response to the reduction of H2O2 even after extended storage.

Fabrication of flexible, transparent silver nanowire electrodes for amperometric detection of hydrogen peroxide

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