Anwendung der optischen Frequenzverdopplung in der Partikelmesstechnik (English shop)

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This work deals with the application of optical second harmonic generation (SHG) for the characterisation of particles and, in particular, of particle surfaces.

While SHG spectroscopy has long been established in the investigation of macroscopic surfaces, in particle measurement technology it is still an entirely new measurement method.

In a first step, an SHG spectrometer was set up and, over time, increasingly optimised for measurements on colloidal systems. The insights gained from systematic parameter studies were incorporated into the optimisation of the setup. Through the implementation of automated components, a large number of measurement series can now be carried out under computer control.

After it had been demonstrated in initial experiments that SHG signals can be generated at particle surfaces, the influence of the particle concentration on the quality of the measurement result was investigated. Depending on the material system investigated and the particle size considered, an optimal concentration range for the measurements was found, which can be determined not only experimentally but also numerically by means of a model developed for this purpose. In angle-resolved light scattering investigations it could furthermore be shown that even suspensions of high particle concentration, for which a measurement in the transmission setup is no longer possible, yield strong SHG signals in the backscattering direction. In addition, analogously to linear light scattering – although with a different profile – the measured values exhibited a characteristic scattered light profile for different particle sizes, which can be described approximately by a modified Rayleigh-Gans-Debye model taken from the literature. It was further shown that an optimal alignment of the detection optics depends strongly on the type and position of the sample cuvette.

As a first application of SHG spectroscopy, adsorption measurements were carried out.
It could be shown that these are possible in principle. However, both the particles and the adsorptive must fulfil certain prerequisites for this.
Using a model system that yields comparatively high signals, the influences of various parameters, such as the particle concentration and size, the ionic strength, the temperature and the pH value of the suspension, on the adsorption process were investigated. It could be shown that the dynamics of adsorption can be influenced by all parameters, whereas the maximum number of adsorbed molecules depends only on the ionic strength and the pH value. In the case of very rough surfaces or of centrosymmetric, non-absorbing adsorptive molecules, a direct measurement cannot be performed. In the second case, however, there is the alternative of characterising the adsorption behaviour via displacement adsorption.
This was successfully applied to the system Dapral on polystyrene particles. The strength of the SHG method compared with classical measurement techniques, such as the centrifuge method, lies in its high temporal resolution, which allows the process dynamics to be measured, as well as in the possibility of measuring under “extreme” ambient conditions, such as high temperatures or pressures.

A further application of the method lies in the measurement of particle surface potentials.
Here it could be shown that the SHG intensity depends on the Stern potential of the particles and thus, with suitable calibration, can be used to measure this quantity, which is otherwise not accessible by any experimental method. In contrast to the established ζ potential, the SHG signal provides information about the charge directly at the particle surface, whereby the attachment of ions is also measured.

Another important component of this work was the application of incoherent SHG, so-called hyper-Rayleigh scattering (HRS), for the characterisation of nanoscale particles. First, micellar structures composed of amphiphilic macromolecules were investigated. It was possible to observe conformational changes of the individual macromolecules of the micelles upon changing the polarity of the surrounding solvent.
Furthermore, the phase transfer of an insoluble dye – from the undissolved initial state, via migration to the micelle surfaces, through to encapsulation within the micelles – could be measured in a time-resolved manner. In both measurements, the extreme sensitivity of the molecular hyperpolarisability to the molecular conformation was the cause of the observed signal changes.

Using the model system gold, which yields comparatively high HRS signals owing to plasmon resonances, the dynamics of an agglomeration of 14 nm spheres initiated by pyridine were investigated. The increase in the nonlinear optical response observed in this process was attributed to the reduction of the symmetry of the system.
In a further experiment, the influence of the particle shape on the HRS signal was determined for the first time. Gold nanorods ripening into spheres showed a decreasing hyperpolarisability.
By measuring the particulate hyperpolarisability during the slow process of Ostwald ripening of zinc oxide spheres from about 2.0 nm to 4.5 nm in size, the relationship between particle size and hyperpolarisability – for this material system and in this size range – could be determined experimentally. In a further, time-resolved measurement it was moreover possible to investigate the dynamics of zinc oxide particle precipitation, whereby different sub-steps of the process were resolved. These two measurements constitute the first HRS measurements on the material system zinc oxide at all.

In conclusion, it can be summarised that the work carried out demonstrates the enormous potential of both SHG and HRS for particle measurement technology. Both methods provide access to properties of particles that were previously not measurable and can thereby contribute to an improved understanding of the ever more significant micro- and nanoworld. A growing importance of this measurement technique can therefore be expected in the near future. Of particular importance here will be the development of suitable theoretical models in order to facilitate the interpretation of the measurement results.