Electrodeposition of Ag nanoparticles onto carbon coated TEM gridsA direct approach to study early stages of nucleation

Copper-Decorated Titanium Electrodes: Impact of Surface Modifications of Substrate on the Morphology and Electrochemical Performance

This study investigates the effect of surface modifications of the titanium substrate on the growth of electrochemically deposited copper. These materials are intended to serve as cathodes in the electroreduction of nitrates in aqueous solutions. Surface modifications included the use of hydrogen fluoride for titanium etching and anodization to promote the growth of a structured titania nanotube array. The effect of an intermediate calcination step for the nanotubes before deposition was assessed along with a comparison to an untreated substrate electrode. The materials were comprehensively characterized by SEM, XRD, contact angle, potentiodynamic tests, EIS, and cyclic voltammetry. Their electrocatalytic ability was tested in the reduction of aqueous solutions containing nitrates. The results reveal that surface finishing impacted the shape and size of the Cu microparticles, as well as the nucleation mechanism enabling a crystal-facet-controllable synthesis. All the materials exhibited microsized copper particles with a spherical shape with some clusters. On the etched titanium surface, a high number of heterogeneous submicroscopic particles were also present. The thermally treated anodized substrate promoted the development of a combination of sparse microparticles corresponding to defect sites in amorphous titanium and the presence of a diffuse coating of octahedral nanosized particles whose growth was promoted by the tetragonal structure of anatase crystals. Electrochemical tests display reduced charge transfer resistance upon copper deposition on the modified substrates, which is indicative of the enhanced conductivity of the coated materials. Additionally, cyclic voltammetry and electrolysis experiments reveal the electrodes' potential for nitrate reduction, showing a better response for the etched titanium substrate (30% nitrate removal, after 2 h at 25 mA cm-2).

Electron Beam Transparent Boron Doped Diamond Electrodes for Combined Electrochemistry-Transmission Electron Microscopy

The majority of carbon based transmission electron microscopy (TEM) platforms (grids) have a significant sp² carbon component. Here, we report a top down fabrication technique for producing freestanding, robust, electron beam transparent and conductive sp³ carbon substrates from boron doped diamond (BDD) using an ion milling/polishing process. X-ray photoelectron spectroscopy and electrochemical measurements reveal the sp³ carbon character and advantageous electrochemical properties of a BDD electrode are retained during the milling process. TEM diffraction studies show a dominant (110) crystallographic orientation. Compared with conventional carbon TEM films on metal supports, the BDD-TEM electrodes offer superior thermal, mechanical and electrochemical stability properties. For the latter, no carbon loss is observed over a wide electrochemical potential range (up to 1.80 V vs RHE) under prolonged testing times (5 h) in acid (comparable with accelerated stress testing protocols). This result also highlights the use of BDD as a corrosion free electrocatalyst TEM support for fundamental studies, and in practical energy conversion applications. High magnification TEM imaging demonstrates resolution of isolated, single atoms on the BDD-TEM electrode during electrodeposition, due to the low background electron scattering of the BDD surface. Given the high thermal conductivity and stability of the BDD-TEM electrodes, in situ monitoring of thermally induced morphological changes is also possible, shown here for the thermally induced crystallization of amorphous electrodeposited manganese oxide to the electrochemically active γ-phase.

Controlling palladium morphology in electrodeposition from nanoparticles to dendrites via the use of mixed solvents

By changing the mole fraction of water (χwater) in the solvent acetonitrile (MeCN), we report a simple procedure to control nanostructure morphology during electrodeposition. We focus on the electrodeposition of palladium (Pd) on electron beam transparent boron-doped diamond (BDD) electrodes. Three solutions are employed, MeCN rich (90% v/v MeCN, χwater = 0.246), equal volumes (50% v/v MeCN, χwater = 0.743) and water rich (10% v/v MeCN, χwater = 0.963), with electrodeposition carried out under a constant, and high overpotential (-1.0 V), for fixed time periods (50, 150 and 300 s). Scanning transmission electron microscopy (STEM) reveals that in MeCN rich solution, Pd atoms, amorphous atom clusters and (majority) nanoparticles (NPs) result. As water content is increased, NPs are again evident but also elongated and defected nanostructures which grow in prominence with time. In the water rich environment, NPs and branched, concave and star-like Pd nanostructures are now seen, which with time translate to aggregated porous structures and ultimately dendrites. We attribute these observations to the role MeCN adsorption on Pd surfaces plays in retarding metal nucleation and growth.

High-Throughput Correlative Electrochemistry-Microscopy at a Transmission Electron Microscopy Grid Electrode

As part of the revolution in electrochemical nanoscience, there is growing interest in using electrochemistry to create nanostructured materials and to assess properties at the nanoscale. Herein, we present a platform that combines scanning electrochemical cell microscopy with ex situ scanning transmission electron microscopy to allow the ready creation of an array of nanostructures coupled with atomic-scale analysis. As an illustrative example, we explore the electrodeposition of Pt at carbon-coated transmission electron microscopy (TEM) grid supports, where in a single high-throughput experiment it is shown that Pt nanoparticle (PtNP) density increases and size polydispersity decreases with increasing overpotential (i.e., driving force). Furthermore, the coexistence of a range of nanostructures, from single atoms to aggregates of crystalline PtNPs, during the early stages of electrochemical nucleation and growth supports a nonclassical aggregative growth mechanism. Beyond this exemplary system, the presented correlative electrochemistry-microscopy approach is generally applicable to solve ubiquitous structure-function problems in electrochemical science and beyond, positioning it as a powerful platform for the rational design of functional nanomaterials.

Label-free ultrasensitive detection of breast cancer miRNA-21 biomarker employing electrochemical nano-genosensor based on sandwiched AgNPs in PANI and N-doped graphene

MicroRNAs (miRNAs) are small, endogenous, noncoding RNAs, shown to be expressed abnormally in many tumors and identified as predictive biomarkers for early diagnosis of several cancers including the breast. Therefore, the label-free and highly sensitive detection of miRNAs is of critical significance. In this work, a highly sensitive and label-free nano-genosensor is developed for the detection of miRNA-21, a known breast cancer biomarker, based on a specific architecture of nitrogen-doped functionalized graphene (NFG), silver nanoparticles (AgNPs), and polyaniline (PANI) that resulted in a remarkable effect on signal amplification. Following the successful functionalization of the nanocomposite and immobilization of the specific sequence of the aminated complementary oligonucleotide of miRNA-21, the detection was performed using differential pulse voltammetry (DPV). The oxidation peak current of the redox probe under optimal conditions was determined to monitor the event hybridization of miRNA-21 biomarker. Applying this highly sensitive and optimized nano-biosensor enabled detection of a wide dynamic range of 10 fM-10 µM with a sensitivity of 2.5 µA cm^-2 and a low detection limit of 0.2 fM. This nano-biosensor also demonstrated highly reproducible results in the analysis of blood samples, with recoveries between 94% and 107%, and could be used for early detection of breast cancer by direct detection of the miRNA-21 in real clinical samples without any need to sample preparation, RNA extraction and/or amplification.

Tracking Metal Electrodeposition Dynamics from Nucleation and Growth of a Single Atom to a Crystalline Nanoparticle

In electrodeposition the key challenge is to obtain better control over nanostructure morphology. Currently, a lack of understanding exists concerning the initial stages of nucleation and growth, which ultimately impact the physicochemical properties of the resulting entities. Using identical location scanning transmission electron microscopy (STEM), with boron-doped diamond (BDD) serving as both an electron-transparent TEM substrate and electrode, we follow this process, from the formation of an individual metal atom through to a crystalline metal nanoparticle, under potential pulsed conditions. In doing so, we reveal the importance of electrochemically driven atom transport, atom cluster formation, cluster progression to a nanoparticle, and the mechanism by which neighboring particles interact during growth. Such information will help formulate improved nucleation and growth models and promote wider uptake of electrodeposited structures in a wide range of societally important applications. This type of measurement is possible in the TEM because the BDD possesses inherent stability, has an extremely high thermal conductivity, is electron beam transparent, is free from contamination, and is robust enough for multiple deposition and imaging cycles. Moreover, the platform can be operated under conditions such that we have confidence that the dynamic atom events we image are truly due to electrochemically driven deposition and no other factors, such as electron-beam-induced movement.

Nanoparticles of methylene blue enhance photodynamic therapy

Breast cancer is the most commonly diagnosed cancer and the second leading cause of death related to cancer among women worldwide. Screening and advancements in treatment have improved survival rate of women suffering from this ailment. Novel therapeutic techniques may further reduce cancer related mortality. One of several emerging therapeutic options is Photodynamic Therapy (PDT) that uses light activated photosensitizer (PS) inducing cell death by apoptosis and/or necrosis. Nanotechnology has made contribution to improve photosensitizer for PDT, increasing the efficiency of therapy using gold and silver nanoparticles. Efforts have been done to develop better mechanism to improve PS and consequently PDT effects. In this study, we investigate the efficacy of the PDT using gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) when mixed to methylene blue (MB) in the treatment of the human breast adenocarcinoma cell line (MDA-MB-468). The MDA-MB-468 was treated in the presence of different MB concentrations with/without AuNPs or AgNPs. The colloidal solution of AgNPs showed a plasmon resonance band at 411 nm in UV-visible range and a bimodal size distribution. The results of viability analysis showed that cells treated with nanoparticles exhibited higher cytotoxicity than cells treated with only MB, improving the efficiency of the treatment in the tumor cells. The cytotoxicity effect of MB associated with AgNPs on MDA-MB-468 cell line could be related to increased reactive oxygen species production due to the release of Ag⁺ ions from nanoparticles surface, suggesting that the association between FS and AgNPs has potential as a PDT agent.

Numerical insights into the early stages of nanoscale electrodeposition: nanocluster surface diffusion and aggregative growth

Fundamental understanding of the early stages of electrodeposition at the nanoscale is key to address the challenges in a wide range of applications. Despite having been studied for decades, a comprehensive understanding of the whole process is still out of reach. In this work, we introduce a novel modelling approach that couples a finite element method (FEM) with a random walk algorithm, to study the early stages of nanocluster formation, aggregation and growth, during electrochemical deposition. This approach takes into account not only electrochemical kinetics and transport of active species, but also the surface diffusion and aggregation of adatoms and small nanoclusters. The simulation results reveal that the relative surface mobility of the nanoclusters compared to that of the adatoms plays a crucial role in the early growth stages. The number of clusters, their size and their size dispersion are influenced more significantly by nanocluster mobility than by the applied overpotential itself. Increasing the overpotential results in shorter induction times and leads to aggregation prevalence at shorter times. A higher mobility results in longer induction times, a delayed transition from nucleation to aggregation prevalence, and as a consequence, a larger surface coverage of smaller clusters with a smaller size dispersion. As a consequence, it is shown that a classical first-order nucleation kinetics equation cannot describe the evolution of the number of clusters with time, N(t), in potentiostatic electrodeposition. Instead, a more accurate representation of N(t) is provided. We show that an evaluation of N(t), which neglects the effect of nanocluster mobility and aggregation, can induce errors of several orders of magnitude in the determination of nucleation rate constants. These findings are extremely important towards evaluating the elementary electrodeposition processes, considering not only adatoms, but also nanoclusters as building blocks.

Surfactant-free electrochemical synthesis of metallic nanoparticles via stochastic collisions of aqueous nanodroplet reactors

Electrochemistry in attoliter aqueous nanodroplets: synthesis of Cu and Ag nanoparticles directly by electrode-bound and surfactant-free synthesis.

Real-time tracking of metal nucleation via local perturbation of hydration layers

The real-time visualization of stochastic nucleation events at electrode surfaces is one of the most complex challenges in electrochemical phase formation. The early stages of metal deposition on foreign substrates are characterized by a highly dynamic process in which nanoparticles nucleate and dissolve prior to reaching a critical size for deposition and growth. Here, high-speed non-contact lateral molecular force microscopy employing vertically oriented probes is utilized to explore the evolution of hydration layers at electrode surfaces with the unprecedented spatiotemporal resolution, and extremely low probe-surface interaction forces required to avoid disruption or shielding the critical nucleus formation. To the best of our knowledge, stochastic nucleation events of nanoscale copper deposits are visualized in real time for the first time and a highly dynamic topographic environment prior to the formation of critical nuclei is unveiled, featuring formation/re-dissolution of nuclei, two-dimensional aggregation and nuclei growth.

Electrochemical deposition is important for industrial processes however, tracking the early stages of metallic phase nucleation is challenging. Here, the authors visualize the birth and growth of metal nuclei at electrode surfaces in real time via high-speed non-contact lateral molecular force microscopy.

Electroactive Biofilms for Sensing: Reflections and Perspectives

Microbial electrochemistry has from the onset been recognized for its sensing potential due to the microbial ability to enhance signals through metabolic cascades, its relative selectivity toward substrates, and the higher stability conferred by the microbial ability to self-replicate. The greatest challenge has been to achieve stable and efficient transduction between a microorganism and an electrode surface. Over the past decades, a new kind of microbial architecture has been observed to spontaneously develop on polarized electrodes: the electroactive biofilm (EAB). The EAB conducts electrons over long distances and performs quasi-reversible electron transfer on conventional electrode surfaces. It also possesses self-regenerative properties. In only a few years, EABs have inspired considerable research interest for use as biosensors for environmental or bioprocess monitoring. Multiple challenges still need to be overcome before implementation at larger scale of this new kind of biosensors can be realized. This perspective first introduces the specific characteristics of the EAB with respect to other electrochemical biosensors. It summarizes the sensing applications currently proposed for EABs, stresses their limitations, and suggests strategies toward potential solutions. Conceptual prospects to engineer EABs for sensing purposes are also discussed.

Electrodeposition of Highly Porous Pt Nanoparticles Studied by Quantitative 3D Electron Tomography: Influence of Growth Mechanisms and Potential Cycling on the Active Surface Area

Nanoporous Pt nanoparticles (NPs) are promising fuel cell catalysts due to their large surface area and increased electrocatalytic activity toward the oxygen reduction reaction (ORR). Herein, we report on the influence of the growth mechanisms on the surface properties of electrodeposited Pt dendritic NPs with large surface areas. The electrochemically active surface was studied by hydrogen underpotential deposition (H UPD) and compared for the first time to high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) quantitative 3D electron tomography of individual nanoparticles. Large nucleation overpotential leads to a large surface coverage of roughened spheroids, which provide a large roughness factor (R_f) but low mass-specific electrochemically active surface area (EASA). Lowering the nucleation overpotential leads to highly porous Pt NPs with pores stretching to the center of the structure. At the expense of smaller R_f, the obtained EASA values of these structures are in the range of those of large surface area supported fuel cell catalysts. The active surface area of the Pt dendritic NPs was measured by electron tomography, and it was found that the potential cycling in the H adsorption/desorption and Pt oxidation/reduction region, which is generally performed to determine the EASA, leads to a significant reduction of that surface area due to a partial collapse of their dendritic and porous morphology. Interestingly, the extrapolation of the microscopic tomography results in macroscopic electrochemical parameters indicates that the surface properties measured by H UPD are comparable to the values measured on individual NPs by electron tomography after the degradation caused by the H UPD measurement. These results highlight that the combination of electrochemical and quantitative 3D surface analysis techniques is essential to provide insights into the surface properties, the electrochemical stability, and, hence, the applicability of these materials. Moreover, it indicates that care must be taken with widely used electrochemical methods of surface area determination, especially in the case of large surface area and possibly unstable nanostructures, since the measured surface can be strongly affected by the measurement itself.

Synthesis of dendritic silver nanostructures supported by graphene nanosheets and its application for highly sensitive detection of diazepam

In this paper, preparation, characterization and application of a new sensor for fast and simple determination of trace amount of diazepam were described. This sensor is based on Ag nanodendrimers (AgNDs) supported by graphene nanosheets modified glassy carbon electrode (GNs/GCE). The AgNDs were directly electrodeposited on the surface of electrode via potentiostatic method without using any templates, surfactants, or stabilizers. The structure of the synthesized AgNDs/GNs was characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS) techniques. The nanodendrimers with tree-like and hierarchical structures have a fascinating structure for fabrication of effective electrocatalysts. The experimental results confirmed that AgNDs/GNs/GC electrode has good electrocatalytic activity toward the reduction of diazepam. A low detection limit of 8.56 × 10^− 8 M and a wide linear detection range of 1.0 × 10^− 7 to 1.0 × 10^− 6 M and 1.0 × 10^− 6 to 20 × 10^− 6 M were achieved via differential pulse voltammetry (DPV). The proposed electrode displayed excellent repeatability and long-term stability and it was satisfactorily used for determination of diazepam in real samples (commercially tablet, injection and human blood plasma) with high recovery.

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Applying a simple, fast and cost-effective method for synthesis of silver nanodendrimers

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Characterization of AgNDs/GNs/GCE surface by SEM, EDX, XRD, EIS and CV methods

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Successful application of this sensor for diazepam determination with an excellent detection limit

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Calculation of diffusion coefficient, electron transfer coefficient and standard heterogeneous rate constant for diazepam

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Satisfactorily using of this sensor for diazepam determination in several real samples

Nucleation, aggregative growth and detachment of metal nanoparticles during electrodeposition at electrode surfaces

The nucleation and growth of metal nanoparticles (NPs) on surfaces is of considerable interest with regard to creating functional interfaces with myriad applications. Yet, key features of these processes remain elusive and are undergoing revision. Here, the mechanism of the electrodeposition of silver on basal plane highly oriented pyrolytic graphite (HOPG) is investigated as a model system at a wide range of length scales, spanning electrochemical measurements from the macroscale to the nanoscale using scanning electrochemical cell microscopy (SECCM), a pipette-based approach. The macroscale measurements show that the nucleation process cannot be modelled as either truly instantaneous or progressive, and that step edge sites of HOPG do not play a dominant role in nucleation events compared to the HOPG basal plane, as has been widely proposed. Moreover, nucleation numbers extracted from electrochemical analysis do not match those determined by atomic force microscopy (AFM). The high time and spatial resolution of the nanoscale pipette set-up reveals individual nucleation and growth events at the graphite basal surface that are resolved and analysed in detail. Based on these results, corroborated with complementary microscopy measurements, we propose that a nucleation-aggregative growth-detachment mechanism is an important feature of the electrodeposition of silver NPs on HOPG. These findings have major implications for NP electrodeposition and for understanding electrochemical processes at graphitic materials generally.