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".

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.

Differentiation of liquid analytes in gel films by permeability-modulated double-layer chemo-chips.

A double-layer chemo-chip for the characterization of liquid analytes by rapid fluorimetric imaging is described. The chemo-chip consists of an array of polymeric micro-spots prepared on a glass slide. Each spot is composed of a thin indicator layer made of PVA doped with an immobilized fluorescence dye and a top layer polymer spot with different permeation properties. The analytes can be differentiated by variations in the optical response rate of the indicator dye after its application. Consequently, different cross-linker concentrations were applied using the Nano-Plotter((TM)) which formed top layers of varying permeability. The chemo-chips were tested with the aqueous solutions of two model liquids (aqueous solutions of malonic acid and phenanthroline hydrochloride). It was found that the transition time of response had changed considerably (up to a factor of about 10) depending on different local cross linking degrees. This has resulted in time-dependent fluorescent patterns of the fluorescence images of the micro-array. The response was fast and the transition times were in the range between a few seconds to 30 s.