Research topics

Photoactive Inorganic Nanomaterials

   

                                                    Photoactive Inorganic Nanomaterials_Terraschke

Monitoring / governing chemical reactions


For developing strategies for a rational synthesis of solids or for discovering entirely new compounds, processes such as nucleation, crystal growth or formation of intermediates must be examined in detail during the reactions.
One method that could provide additional fundamental understanding about the mechanism of chemical reactions is the in-situ luminescence analysis of coordination sensors (ILACS, Fig. 1) applying high-resolution CCD detectors. Within this approach, emitting coordination sensors such as the lanthanide ions Eu3+, Eu2+, Ce3+ are introduced into the system under examination and monitored with the aid of in-situ luminescence measurements under real reaction conditions. Since the luminescence of these ions are highly dependent on the coordination environment, this method provides information about the changes in the coordination number, bond lengths, covalence of the ligands and symmetry of the cation site during chemical reactions. This procedure is easily available, is characterized by high sensitivity and high temporal resolution and can also be used for studying very small crystallites, ions in solution or amorphous materials. Because of these advantages, ILACS consist of a very promising strategy for complementing other in-situ characterization techniques like synchrotron-based X-ray diffraction analysis, being also integrated at the moment to the German Electron Synchrotron (DESY).
The aim of this project is to develop the ILACS technique as well as its appropriated reactors, testing its performance for monitoring the crystallization of solid materials like e.g. bioceramics and emissive oxide nanoparticles.

                                                      

Fig. 1. ILACS principle: Coordination sensors introduced to reaction system. Structural changes during formation of solids monitored by in-situ luminescence measurements.

Stimuli-responsive materials


Stimuli-responsive photofunctional nanomaterials are especially important for sensing the pH value in media, in which conventional glass electrodes cannot be used, such as determining the pH within intracellular environments. For measuring the pH value under these conditions, the Ln3+ complexes (Fig. 2) can be also used as local sensor.
Since these complexes are not able to penetrate the cell walls, they must be combined to emissive nanoparticles, which are also used as reference fluorescence signal for the pH quantification. Up to now, the stimuli-response of pH sensitive sensors are mostly determined ex-situ, exposing several samples of these compounds to discrete solutions with different pH values. Besides being very time and material consuming, this ex-situ approach is extremely disadvantageous for the development of these materials. Hence, ex-situ techniques are not able to efficiently detect time-dependent signal decrease due to possible chemical degradation of the sensor as well as the stimuli response time, which is very critical for the combination of different sensors, usually used for increasing the studied pH range.
This project aims to develop new stimuli-responsive photofunctional nanomaterials, coating pH sensitive Ln3+ complexes on emissive nanoparticle, investigating in-situ not only their synthesis and coating processes but also their pH dependence, long-time stability and stimuli response time.

              Stimuli-responsive materials         Stimuli-responsive materials_A. C. Weigt

Fig. 2: Changes in the optical properties of [Tb,Eu(phen)2(NO3)3]∙bipy in correlation with the pH value.

 

Semiconductor and plasmonic nanoparticles


The production of monodispersed semiconductor and plasmonic nanoparticles is crucially important for studying and tailoring their size-dependent optical properties and consequently for their applications in, for instance, the production of solar cells, infrared sensors or markers for cell imaging. However, the strict control over particle size of non-toxic and low cost materials remains a challenge due to lack of knowledge about the processes occurring in the early stages of the crystal formation in solutions. Usually, studies of the synthesis mechanisms of these materials are carried out removing samples from the reactor during the reaction and even sieving them for reaching the desired particle size. These samples are also generally quenched, washed and dried before their ex-situ characterization, which can modify the material structure, not reflecting therefore the real events occurring inside the reactor. For semiconductor and plasmonic nanomaterials, the emission properties strictly depend on the particle diameter, allowing to unravel the evolution of the crystal size measuring the respective shift on the respective spectra in-situ (Fig. 3). This approach is advantageous in comparison to in-situ measurements of UV/Vis absorption and reflection spectra, because it is not limited by too low or too high concentrations of the reaction solution.

 

                     ZnS nanoparticles_H. Fritzsch           ZnS NP suspension_H. Fritzsch

Fig. 3: Emission spectra recording the formation of different particle size populations during the crystallization of ZnS.

 

 

 

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