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New Applications of Fluorescence Correlation Spectroscopy in Materials Science
(2006)
- In this work we have developed new concepts for the usage of fluorescence correlation spectroscopy. A classical FCS setup was modified in such a way that fluorescent species in aqueous as well as organic environments can be studied at varying temperature. We have synthesised a set of dye-labelled polymers that served as a well-defined system to study polymer diffusion and that was used to characterise the beam path and focal volume in environments with refractive indices different from that of water. Furthermore a new method for the labelling of ionic species was developed. The adaptation of the microscope optics to non-aqueous environments was done by replacing the present microscope objective by a multi-immersion objective. Secondly, a sample chamber was developed that was not only resistant to organic solvents in all parts but also allowed temperature control of the solution. Determining diffusion coefficients of polymers in solution and their concentrations requires the exact knowledge of shape and size of the observation volume. For this purpose we have synthesised a set of polystyrenes with molecular weights ranging from 4 to 1550 kg/mol each chain being labelled with a single dye molecule. All species were anionically polymerised in order to grant a very low polydispersity and with this a high reliability in the determination of the observation volume. This concept can be transferred to other solvents and, hence, shows an easy way to calibrate fluorescence correlation microscopes to different solvents and to investigate non-aqueous solutions. Furthermore, we have shown exemplarily for polystyrene that FCS is capable of determining the crossover between the dilute and the semi-dilute concentration regime. Dye-labelled polymer chains were mixed with unlabelled polymer chains of the same length and their mobility was measured by FCS for different mixing ratios. The change of the mobilities leads to the respective overlap concentrations, which are shown to follow a scaling law in a range of molecular weights from 4 to 1550 kg/mol. This is in excellent agreement with the predictions made by Flory and Huggins. The data shown demonstrate that FCS can measure diffusion properties in ranges that were not accessible before. Another part of this work focusses on concepts to monitor the aggregation of molecules by FCS. Taking low-molecular-weight surfactants as an example it is shown that with the help of Coulomb interaction cationic surfactants can be labelled with anionic dye molecules and vice versa. Moreover, micelle formation is observed already at concentrations slightly below the critical micelle concentration found with classical methods. This findings are in excellent agreement with the predictions made by Israeliachvili in the 1990ies. Additionally, it was demonstrated that by using insoluble dye molecules, which are incorporated by the forming aggregates, aggregate formation can be followed by FCS on a single-molecule level. This procedure was shown to work in both aqueous and organic polymer solutions. The high sensitivity of FCS permitted to determine the critical aggregation concentration of Janus micelles in THF to the very low value of around 8 mg/L. No other experimental method available today is capable of determining aggregation concentrations in such a low concentration regime. In the same way the critical aggregation concentration of block copolymer polystyrene-Amylose in THF was determined. Finally, temperature-dependent correlation curves allowed the determination of reaction constants and enthalpies. This is of particular interest in biochemical contexts, as the amount of available material can be minute. Exemplarily, the binding enthalpy of an RPA protein to a single-stranded DNA strain is determined by temperature-dependent correlation curves. The modifications made to a classical FCS setup were shown to enhance the spectrum of possible applications to new experimental fields. The methods and concepts developed in the framework of this thesis are expected to play an important role in meeting future challenges of polymer physics and microbiology.
