Andrea Knauer

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).

Continuous-Microflow Synthesis and Morphological Characterization of Multiscale Composite Materials Based on Polymer Microparticles and Inorganic Nanoparticles

This paper presents a new route to the synthesis of uniform and size-controlled inorganic/organic composite microparticles by means of microreaction technology. Au-nanoparticles in the range of 3 to 14 nm are synthesized by reduction of tetrachloroauric acid, while ZnO-nanoparticles (200–2000 nm) are synthesized in a continuous-flow twostep process using microtube arrangements for microsegmented flow. Both inorganic nanoparticles have a wellcontrolled size and narrow size distribution. Upon surface modification, the nanoparticles are then mixed on one hand with an acrylate-based monomer and, on the other hand, with an aqueous solution of acrylamide. Both solutions were then emulsified into uniform core-shell droplets by means of a capillary-based microfluidic device. Droplet’s shell was hardened through UV-induced polymerization, whereas the core led to a hydrogel upon thermal-induced polymerization. Core-shell polymer microparticles (200–300 μm) with inorganic nanoparticles selectively incorporated into the core and the shell are thus obtained as proven by extensive morphological characterizations using electronic and optical microscopies.

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.

Controlling formation and assembling of nanoparticles by control of electrical charging, polarization, and electrochemical potential

Abstract The well-known concept of electrical charging for the stabilization of colloidal solutions is extended to a general concept for explanation of behavior of nanoparticles during their formation, in their response behavior on chemical or electrochemical stress, and in particle/particle interactions. This concept helps to understand the formation of polynuclear sphere-like plasmonic nanoparticles of gold, the surprising high stability of extended flat silver nanoprisms, and their switch-like transformation as well as the formation of special types of nanoparticle assemblies. It can be used for controlling the size of nanoparticles, for the synthesis of composite micro and nanoparticles, and for the generation of different classes of nonspherical polymer particles by particle/particle interactions during the process of particle growth. The reported experimental examples were mainly obtained by microfluidic experiments, which supplied a high homogeneity of the product particles. This high homogeneity is an excellent basis for studying the nanoparticle behavior in analogy to molecular processes and helps to get new insights in to the importance of electrical effects for nanoparticle synthesis and nanoparticle assembling.

Single-Photon-Single-Electron Transition for Interpretation of Optical Spectra of Nonspherical Metal Nanoparticles in Aqueous Colloidal Solutions

Noble metal nanoparticles—especially shape anisotropic particles—have pronounced resonances in the optical spectrum. These sensitive absorption modes attract great interest in various fields of application. For nonspherical particles, no analytic description of the absorption spectra according to the commonly used Mie theory is possible. In this work, we present a semi-empirical approach for the explanation of the optical spectra of shape anisotropic particles such as silver nanoprisms and gold nanorods. We found an interpretation of the optical absorption spectra which is based on a single-photon-single-electron transition. This model is in a better agreement with the basic assumptions of quantum mechanics than the electrodynamic model of a localized surface plasmon excitation. Based on microfluidically obtained Ag nanoprisms and Au nanorods with very high ensemble homogeneities, dependencies between the geometrical properties of the shape anisotropic noble metal nanoparticles and the spectral position of the longitudinal absorption mode could be derived, which show that the assumption of a composed relative permittivity and the inclusion of the Rydberg constant is sufficient to describe the optical properties of the shape anisotropic particles. Within the scope of the measuring accuracy, the calculations furthermore lead to the value of the refractive index of the particle-surrounding medium.