Eliminating Flooding-related Issues in Electrochemical CO2-to-CO Converters: Two Lines of Defense.

By using silver (Ag) in nanostructured (nanowire, nanosphere, etc.) or thin-layer form as a catalyst for electrochemical CO2 reduction, very high CO-forming selectivity of almost 100% can be achieved. Supported by gas diffusion layers (GDLs),  the reactant CO2 in the gas phase can approach and potentially access active Ag sites, which allows current densities in the range of a few hundred mA cm-2 to be reached. Yet, the stability of gas diffusion electrode (GDE) based electrochemical CO2-to-CO converters is far from perfect, and the activity of GDE cathodes, especially when operated at high current densities, often significantly decays during electrolyses after no more than a few hours. The primary reason of stability losses in GDE-based CO2-to-CO electrolysers is flooding: that is, the excess wetting of the GDE that prevents CO2 from reaching Ag catalytic sites. In the past years, the authors of this paper at Empa and at the University of Bern, cooperating with other partners of the National Competence Center for Research (NCCR) on Catalysis, took different approaches to overcome flooding. While opinions differ with regard to where the first line of defense in protecting GDEs from flooding should lie, a comparison of the recent results of the two groups gives unique insight into the nature of processes occurring in GDE cathodes used for CO2 electrolysis.

Feasibility of SERS-Active Porous Ag Substrates for the Effective Detection of Pyrene in Water.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutants that are typically released into the environment during the incomplete combustion of fossil fuels. Due to their relevant carcinogenicity, mutagenicity, and teratogenicity, it is urgent to develop sensitive and cost-effective strategies for monitoring them, especially in aqueous environments. Surface-enhanced Raman spectroscopy (SERS) can potentially be used as a reliable approach for this purpose, as it constitutes a valid alternative to traditional techniques, such as liquid and gas chromatography. Nevertheless, the development of an SERS-based platform for detection PAHs has so far been hindered by the poor adsorption of PAHs onto silver- and gold-based SERS-active substrates. To overcome this limitation, several research efforts have been directed towards the development of functionalized SERS substrates for the improvement of PAH adsorption. However, these strategies suffer from the interference that functionalizing molecules can produce in SERS detection. Herein, we demonstrate the feasibility of label-free detection of pyrene by using a highly porous 3D-SERS substrate produced by an inductively coupled plasma (ICP). Thanks to the coral-like nanopattern exhibited by our substrate, clear signals ascribable to pyrene molecules can be observed with a limit of detection of 23 nM. The observed performance can be attributed to the nanoporous character of our substrate, which combines a high density of hotspots and a certain capability of trapping molecules and favoring their adhesion to the Ag nanopattern. The obtained results demonstrate the potential of our substrates as a large-area, label-free SERS-based platform for chemical sensing and environmental control applications.

Stereometric and fractal analysis of granulated silver films used in thin-film hybrid structures.

Silver nanostructures are of interest to be used in hybrid thin-film structures with various materials. In this work, we analyse 3D AFM images of granulated silver film nanostructures prepared by thermal evaporation. The advanced AFM data study aims to understand how film thickness and postannealing affect the nanostructure morphology changes. For the first time, the evaluation of surface statistical parameters and fractal geometry were used to characterise the nanostructure morphology of Ag island films. The samples with gravimetric thickness of 2, 4 and 10 nm were analysed before and after annealing at 200°C for 10 min. The statistical processing revealed the essential variation of parameters with Ag film thickness increment and as a postannealing result. The nonmonotonic variation of surface roughness, skewness and fractal dimensions were found. It is caused by the features of the film growth process with the thickness increment and thermally activated diffusion of Ag nanoparticles during annealing. LAY DESCRIPTION: Silver nanostructures are of great scientific interest due to their unique properties and wide applicability. In this work, we analyse microscopic 3D images of silver nanostructures deposited on quartz in vacuum chamber. Such thin-film nanostructures consisted of silver nanoparticles attached to the surface of quartz substrate. We aimed at studying how effective film thickness and postannealing affect the nanostructure shapes and forms. Effective thickness is the equivalent thickness of a smooth film. For this purposes we used special parameters called statistical parameters and fractal geometry to investigate nanostructures. The samples with effective thickness of 2, 4 and 10 nm were analysed before and after annealing at 200°C for 10 min. It has been shown, that effective Ag film thickness and postannealing results in variation of nanostructures forms. We have shown that the variation of statistical parameters were nonmonotonic. It is caused by the features of the film growth process with the thickness increment and thermally activated diffusion of Ag nanoparticles during annealing.

Plasmonic Nanostructures Grown from Reacting Droplet-In-Microwell Array on Flexible Films for Quantitative Surface-Enhanced Raman Spectroscopy in Plant Wearable In Situ Detection.

Plant wearable detection has garnered significant interest in advancing agricultural intelligence and promoting sustainable food production amidst the challenges of climate change. Accurately monitoring plant health and agrochemical residue levels necessitates qualities such as precision, affordability, simplicity, and noninvasiveness. Here, a novel attachable plasmonic film is introduced and designed for on-site detection of agrochemical residues utilizing surface-enhanced Raman spectroscopy (SERS). By functionalizing a thin polydimethylsiloxane film with silver nanoparticles via controlled droplet reactions in micro-well arrays, a plasmonic film is achieved that not only maintains optical transparency for precise analyte localization but also conforms closely to the plant surface, facilitating highly sensitive SERS measurements. The reliability of this film enables accurate identification and quantification of individual compounds and their mixtures, boasting an ultra-low detection limit ranging from 10-16 to 10-13 m, with mini mal relative standard deviation. To showcase its potential, on-field detection of pesticide residues on fruit surfaces is conducted using a handheld Raman spectrometer. This advancement in fabricating plasmonic nanostructures on flexible films holds promise for expanding SERS applications beyond plant monitoring, including personalized health monitoring, point-of-care diagnosis, wearable devices for human-machine interface, and on-site monitoring of environmental pollutants.

Real-Time Monitoring of the In Situ Microfluidic Synthesis of Ag Nanoparticles on Solid Substrate for Reliable SERS Detection.

A sharpened control over the parameters affecting the synthesis of plasmonic nanostructures is often crucial for their application in biosensing, which, if based on surface-enhanced Raman spectroscopy (SERS), requires well-defined optical properties of the substrate. In this work, a method for the microfluidic synthesis of Ag nanoparticles (NPs) on porous silicon (pSi) was developed, focusing on achieving a fine control over the morphological characteristics and spatial distribution of the produced nanostructures to be used as SERS substrates. To this end, a pSi membrane was integrated in a microfluidic chamber in which the silver precursor solution was injected, allowing for the real-time monitoring of the reaction by UV-Vis spectroscopy. The synthesis parameters, such as the concentration of the silver precursor, the temperature, and the flow rate, were varied in order to study their effects on the final silver NPs' morphology. Variations in the flow rate affected the size distribution of the NPs, whereas both the temperature and the concentration of the silver precursor strongly influenced the rate of the reaction and the particle size. Consistently with the described trends, SERS tests using 4-MBA as a probe showed how the flow rate variation affected the SERS enhancement uniformity, and how the production of larger NPs, as a result of an increase in temperature or of the concentration of the Ag precursor, led to an increased SERS efficiency.

Analyte-Induced Desert Rose-like Ag Nanostructures for Surface-Enhanced Raman Scattering-Based Biomolecule Detection and Imaging.

Biomolecule detection based on surface-enhanced Raman scattering (SERS) for application to biosensors and bio-imaging requires the fabrication of SERS nanoprobes that can generate strong Raman signals as well as surface modifications for analyte-specific recognition and binding. Such requirements lead to disadvantages in terms of reproducibility and practicality, and thus, it has been difficult to apply biomolecule detection utilizing the advantages of the SERS phenomenon to actual clinically relevant analysis. To achieve reproducible and practical SERS signal generation in a biomolecule-specific manner without requiring the synthesis of nanostructures and their related surface modification to introduce molecules for specific recognition, we developed a new type of SERS probe formed by enzyme reactions in the presence of Raman reporters. By forming unique plasmonic structures, our method achieves the detection of biomolecules on chips with uniform and stable signals over long periods. To test the proposed approach, we applied it to a SERS-based immunohistochemistry assay and found successful multiplexed protein detection in brain tissue from transgenic mice.

Dendritic Forest-Like Ag Nanostructures Prepared Using Fluoride-Assisted Galvanic Replacement Reaction for SERS Applications.

Dendritic forest-like Ag nanostructures were deposited on a silicon wafer through fluoride-assisted galvanic replacement reaction (FAGRR) in aqueous AgNO3 and buffered oxide etchant. The prepared nanostructures were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, inductively coupled plasma-optical emission spectroscopy, a surface profiler (alpha step), and X-ray diffraction. Additionally, the dendritic forest-like Ag nanostructures were characterized using surface-enhanced Raman scattering (SERS) when a 4-mercaptobenzoic acid (4-MBA) monolayer was adsorbed on the Ag surface. The Ag nanostructures exhibited intense SERS signal from 4-MBA because of their rough surface, and this intense signal led to an intense local electromagnetic field upon electromagnetic excitation. The enhancement factor for 4-MBA molecules adsorbed on the Ag nanostructures was calculated to be 9.18 × 108. Furthermore, common Raman reporters such as rhodamine 6G, 4-aminothiolphenol, 5,5'-dithiobis-2-nitrobenzoic acid, and carboxyfluorescein (FAM) were characterized on these dendritic forest-like Ag nanostructures, leading to the development of an ultrasensitive SERS-based DNA sensor with a limit of detection of 33.5 nM of 15-mer oligonucleotide.

Evolution of Ag nanostructures created from thin films: UV-vis absorption and its theoretical predictions.

Ag-based plasmonic nanostructures were manufactured by thermal annealing of thin metallic films. Structure and morphology were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). SEM images show that the formation of nanostructures is influenced by the initial layer thickness as well as the temperature and the time of annealing. The Ag 3d and Ag 4d XPS spectra are characteristic of nanostructures. The quality of the nanostructures, in terms of their use as plasmonic platforms, is reflected in the UV-vis absorption spectra. The absorption spectrum is dominated by a maximum in the range of 450-500 nm associated with the plasmon resonance. As the initial layer thickness increases, an additional peak appears around 350 nm, which probably corresponds to the quadrupole resonance. For calculations leading to a better illustration of absorption, scattering and overall absorption of light in Ag nanoparticles, the Mie theory is employed. Absorbance and the distribution of the electromagnetic field around the nanostructures are calculated by finite-difference time-domain (FDTD) simulations. For calculations a novel approach based on modelling the whole sample with a realistic shape of the nanoparticles, instead of full spheres, was used. This led to a very good agreement with the experiment.

Various Silver Nanostructures on Sapphire Using Plasmon Self-Assembly and Dewetting of Thin Films.

Silver (Ag) nanostructures demonstrate outstanding optical, electrical, magnetic, and catalytic properties and are utilized in photonic, energy, sensors, and biomedical devices. The target application and the performance can be inherently tuned by control of configuration, shape, and size of Ag nanostructures. In this work, we demonstrate the systematical fabrication of various configurations of Ag nanostructures on sapphire (0001) by controlling the Ag deposition thickness at different annealing environments in a plasma ion coater. In particular, the evolution of Ag particles (between 2 and 20 nm), irregular nanoclusters (between 30 and 60 nm), and nanocluster networks (between 80 and 200 nm) are found be depended on the thickness of Ag thin film. The results were systematically analyzed and explained based on the solid-state dewetting, surface diffusion, Volmer-Weber growth model, coalescence, and surface energy minimization mechanism. The growth behavior of Ag nanostructures is remarkably differentiated at higher annealing temperature (750 °C) due to the sublimation and temperature-dependent characteristic of dewetting process. In addition, Raman and reflectance spectra analyses reveal that optical properties of Ag nanostructures depend on their morphology.

One step 'dip' and 'use' Ag nanostructured thin films for ultrahigh sensitive SERS Detection.

A simple one step galvanic displacement method which involves dipping of the silicon substrate in the AgNO3/HF solution and using it for SERS application without any further process is demonstrated. The size and shape of the Ag nanoparticles changes as the deposition time is increased. Initially the shape of the particles was nearly spherical and as it grows, becomes oblong and then coalesce to form a discontinuous film with vertically grown hierarchical Ag nanostructures. The sizes of the deposited particles were in the ranges from 30nm to a discontinuous film. It also demonstrated a highly sensitive chemical detection by surface-enhanced Raman scattering of rhodamine 6G dye, down to 10(-16)M concentration. Prepared samples were able to detect lower concentrations of Melamine. Discontinuous thin films with hierarchical Ag nanostructures were obtained for 5min Ag deposition. The formation of Hot spots between the discontinuous islands and also along the hierarchical structures is responsible for the high SERS enhancement. This simple one step, fast, non-lithographic and cost effective method can be applied for various label free detection of analytes of importance.