Exfoliated 2D Layered and Nonlayered Metal Phosphorous Trichalcogenides Nanosheets as Promising Electrocatalysts for CO2 Reduction.

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO2 ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh3 ) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh3 as catalysts for CO2 ECR. Unlike the layered CoPS3 and NiPS3 nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS3 nanosheets. Correspondingly, the nonlayered SnPS3 nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with -7.51 mA cm-2 at -0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS3 surface. These results provide a new avenue to understand the novel CO2 ECR mechanism of Sn-based and MPCh3 -based catalysts.

Explanation of the size dependent in-plane optical resonance of triangular silver nanoprisms.

Triangular silver nanoprisms with thicknesses of 3-5 nm and adjustable edge lengths - which can lead to nanoparticles with aspect ratios up to 1 : 50 - are quasi-two-dimensional nanoparticles. Due to high ensemble homogeneities, which are achieved by the application of a microfluid segment based preparation method, the optical properties of the silver nanoprisms can be studied directly in colloidal solution. Investigations of the shift of the longitudinal main absorption peak with varying edge length lead to a semi-empiric model in which inelastic one-photon-one-electron processes are used to explain the found absorption behavior instead of the conventional interpretation of a collective oscillation of the conduction band electrons. Independently of the inserted seed particle volumes or amounts of silver ions, all measurement series follow a linear interrelation between the spectral position of the longitudinal absorption peak and the determined edge length of the nanoprisms, which leads to the derivation of a length constant b0, which in turn can be described within the framework of the model - next to a geometry factor - exclusively by natural constants. The proposed model describes the behavior of quasi-two-dimensional noble metal nanoparticles by a dualism between the assumption of "metallic molecules" and materials with "blurred bandgaps".

Polyacrylamid/silver composite particles produced via microfluidic photopolymerization for single particle-based SERS microsensorics.

A micro-continuous-flow process was applied for the preparation of swellable polyacrylamide particles incorporating silver nanoparticles. These sensor particles are formed from a mixture of a colloidal solution of silver nanoparticles and monomer by a droplet-based procedure with in situ photoinitiation of polymerization and a subsequent silver enforcement in batch. The obtained polymer composite particles show a strong SERS effect. Characteristic Raman signals of aqueous solutions of adenine could be detected down to 0.1 µM by the use of single sensor particles. The chosen example demonstrates that the composite particles are suitable for quantitative microanalytical procedures with a high dynamic range (3 orders of magnitude for adenine).

Synergistic effects of metal nanoparticles and a phenolic uncoupler using microdroplet-based two-dimensional approach.

A droplet-based microfluidic technique for testing multiple reagent concentrations is presented. We used this experimental approach to study combined effects of gold (AuNP) and silver nanoparticles (AgNP) with the phenolic uncoupler 2,4-dinitrophenol (DNP) with respect to the growth of Escherichia coli. In order to evaluate the toxicity of binary mixtures, we first encapsulated the E. coli cells and particle mixtures inside the microdroplets using PEEK (polyetherketone) modules. Two-dimensional concentration spaces with about 500 well separated droplets were addressed. We subsequently analyzed the cell growth, the viability and the autofluorescence intensity (metabolic activity) of the bacteria with a micro-flow-through fluorometer and photometer. Dose-dependent synergistic effects were found for the binary mixture of AgNPs and DNP, which indicated a stronger interaction in the mixture than it was expected from effect summation. For the binary mixture of DNP and AuNPs in non-toxic concentrations, we found only weak synergistic effects at low DNP concentrations. Furthermore, the non-toxic tested AuNPs causes effect summation in the binary mixture with the phenolic uncoupler. In general, we demonstrated the efficiency of a droplet-based microfluidic system for fast high-throughput screenings of binary and multiple mixtures. This work also confirmed the relevance of highly resolved droplet-based assays for the miniaturization of ecotoxicological aquatic test systems.

Metal Nano Networks by Potential-Controlled In Situ Assembling of Gold/Silver Nanoparticles.

Invited for this month's cover picture is the group of Professors Michael Köhler at the Technische Universität Ilmenau. The cover picture shows an overlay of an image of a metal nanoparticle network (blue) and sets of non-spherical metal nanoparticles of different shapes (yellow). The particles can be used in plasmonic labelling, nanoparticle-based SERS-sensing and heterogeneous catalysis. Read the full text of their Full Paper at https://doi.org/10.1002/open.201900231.

Metal Nano Networks by Potential-Controlled In Situ Assembling of Gold/Silver Nanoparticles.

Non-spherical Au/Ag nanoparticles can be generated by chemical reduction of silver ions in the presence of preformed gold nanoparticles. The process of particle formation can be controlled by concentrations of ligands and reducing agent. The formation of ellipsoidal, nanorod- and peanut-shaped nanoparticles as well as of more complex fractal nanoassemblies can be explained by changes in particle surface state, electrochemical potential formation and particle-internal self-polarization effects. It is possible to create highly fractal nanoassemblies with sizes between the mid-nanometer and the lower micrometer range. The assemblies are marked by high optical absorption and complex nano-networks of very high surface-to-volume ratios and a granular base structure.

Combination of microfluidic high-throughput production and parameter screening for efficient shaping of gold nanocubes using Dean-flow mixing.

Metal nanoparticles and their special optical properties, the so-called localized surface plasmon resonance (LSPR), facilitate many applications in various fields. Due to the strong dependency of the LSPR on particle geometry, their synthesis is a challenging and time-consuming procedure especially for non-spherical shapes. In contrast, micromixers offer new experimental approaches and therefore enable the simplification of several processes. By using a zigzag micromixer (Dean-Flow-Mixer, DFM) that induces Dean-flow secondary flow patterns, we theoretically and experimentally show the mixing efficiency. Thus, we highlight the advantages of using it in the multistep synthesis of Au nanoparticles. Based on a narrow size distribution of Au nanocubes and an increased yield in combination with higher reproducibility, we depict the need for and advantage of the DFM to control the incubation times during the growth process. We further show that, by using the DFM, easy and very fast Au nanocube edge length tuning (53 nm, 58 nm, 70 nm and 75 nm) is possible by simultaneously reducing the consumption of the materials by up to 95%. We finally demonstrate the versatile abilities by using the DFM for parameter screening on examples of different halides and accessible bromide in the growth solutions. Therefore, we highlight the optimal concentration for the different growth regimes and the influences on the Au nanoparticle morphology (spheres, cubes and rods) and their defined shaping.

Nanometer precise adjustment of the silver shell thickness during automated Au-Ag core-shell nanoparticle synthesis in micro fluid segment sequences.

In this work, a wet-chemical synthesis method for gold-silver core-shell particles with nanometer precise adjustable silver shell thicknesses is presented. Typically wet-chemical syntheses lead to relatively large diameter size distributions and losses in the yield of the desired particle structure due to thermodynamical effects. With the here explained synthesis method in micro fluidic segment sequences, a combinatorial in situ parameter screening of the reactant concentration ratios by programmed flow rate shifts in conjunction with efficient segment internal mixing conditions is possible. The highly increased mixing rates ensure a homogeneous shell deposition on all presented gold core particles while the amount of available silver ions was adjusted by automated flow rate courses, from which the synthesis conditions for exactly tunable shell thicknesses between 1.1 and 6.1 nm could be derived. The findings according to the homogeneity of size and particle structure were confirmed by differential centrifugal sedimentation (DCS), scanning and transmission electron microscopy (SEM, TEM) and X-ray photoelectron spectroscopy (XPS) measurements. In UV-Vis measurements, a significant contribution of the core metal was found in the shape of the extinction spectra in the case of thin shells. These results were confirmed by theoretical calculations.

Spontaneous transformation of polyelectrolyte-stabilized silver nanoprisms by interaction with thiocyanate.

The reaction of KSCN with colloidal solutions of triangular silver nanoprisms results in a shape transformation. The reaction cannot be explained by a simple etching of the corners of the triangles, as it is described in earlier reports on the interaction of silver nanoprisms with halide anions leading to the formation of nanodisks. The reaction products after KSCN addition are spherical silver nanoparticles with a homogeneous size distribution, which display the typical short-wavelength plasmon absorption at about 410 nm. The spectral online monitoring of the reaction reflects a rather homogeneous conversion process. In some cases, isosbestic points have been observed, indicating a reaction of the initial particle type directly to the final particle type. The kinetics of the conversion process are better described by a molecular conversion, than by a process with a step-wise transport of material leading to a continuous change in the particle shape. The experimental findings suggest a two-step mechanism for the conversion: In a first (slow) step the particle is destabilized by desorption of the anionic polyelectrolyte ligand. Then the destabilized particles relax quickly in a (fast) second step to a spherical shape. This interpretation seems to have a serious impact on the understanding of non-spherical nanoparticles in general: The comparatively large triangular shaped prismatic particles in aqueous solution are stabilized by their specific electronic properties due to the interaction with one or several ligand molecules and must be understood as a molecular-analog dynamic system than as a small solid-state body.

Uncovering toxicological complexity by multi-dimensional screenings in microsegmented flow: modulation of antibiotic interference by nanoparticles.

The technique of microsegmented flow was applied for the generation of two- and higher dimensional concentration spaces for the screening of toxic effects of selected substances on the bacterium Escherichia coli at the nanolitre scale. Up to about 5000 distinct experiments with different combinations of effector-concentrations could be realized in a single experimental run. This was done with the help of a computer program controlling the flow rates of effector-containing syringe pumps and resulted in the formation of multi-dimensional concentration spaces in segment sequences. Prior to the application of this technique for toxicological studies on E. coli the accuracy of this method was tested by simulation experiments with up to five dissolved dyes with different spectral properties. Photometric microflow-through measurement of dye distribution inside the concentration spaces allowed the monitoring of microfluid segment compositions. Finally, we used this technique for the investigation of interferences of the antibiotics ampicillin and chloramphenicol towards E. coli cultures and their modulation by silver nanoparticles by measuring bacterial autofluorescence. Each concentration point in this three-dimensional concentration space was represented by 4 or 5 single segments. Thus, a high reliability of the measured dose/response relations was achieved. As a result, a complex response pattern was discovered including synergistic and compensatory effects as well as the modulation of the range of stimulation of bacterial growth by a sublethal dose of chloramphenicol by silver nanoparticles.