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Osinkina, Lidiya (2014): Interactions of molecules in the vicinity of gold nanoparticles. Dissertation, LMU München: Fakultät für Physik



Gold nanoparticles (AuNPs) can locally increase the temperature of their surrounding medium and provide regions of high field enhancement near their surface. The origin of these two effects lies in the confined oscillations of conduction band electrons called plasmons, which are excited by the resonant electromagnetic field. In this thesis heating and field enhancing properties of AuNPs are used to manipulate the interaction of molecules attached to them. Two intermolecular processes are studied: formation of DNA double strand and energy transfer between fluorescent molecules. Formation of DNA double strands near AuNPs is studied on the single-particle level. To this end, two single AuNPs with complementary DNA strands on their surface are brought into close proximity by optical trapping. The formation of DNA double strands leading to binding between two single nanoparticles is detected systematically by the change of the optical properties of AuNPs due to plasmonic coupling at small distances. Moreover, the increase of the trapping laser power slows down the specific binding by more than an order of magnitude. The observed result is explained by a semi-quantitative model where the temperature increase of the surrounding medium due to plasmonic heating is compared to the temperature required to dissociate DNA double helices. Plasmonic heating brings the system closer to the melting temperature and the formation of double strand is suppressed. Further, Foerster resonant energy transfer (FRET) between two fluorescent species attached to AuNPs is investigated. FRET is a non-radiative energy transfer leading to the decrease of fluorescence of the donor molecule and increase of fluorescence of the acceptor molecule. By measuring the fluorescence lifetime of donor and acceptor molecules near AuNPs and in free FRET pairs we quantify the influence of AuNPs on FRET. FRET efficiencies near AuNPs stay nearly as high as in the case of free FRET pairs and FRET rates in the presence of AuNPs are increased. The simulations of FRET enhancement between AuNPs suggest the presence of several regions of field enhancement and of field suppression. To fully use the potential of AuNP dimers for FRET enhancement a precise placement of molecules is required.