Interactions of molecules in the vicinity of gold nanoparticles

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.