| dc.description.abstract | Gold nanorods, with their unique plasmonic properties, have a wide range of applications
in nanotechnology. When trapped in an optical tweezer, they can function
as nanomotors, converting energy into motion at the nanoscale, serving as highly
sensitive detector of movement, driven by scattering-induced optical torques. The
development of artificial nanomotors hold promise for applications in DNA manipulation,
nanolithography and environmental remediation to name a few. This thesis
aims to investigate the dynamics of gold nanorods within an optical trap as a function
of position along the optical axis of a laser beam in various media using video
microscopy and analysis of back-scattered light. The nanorods ability to act as
highly sensitive sensors for detecting nanoscale motion, particularly in single bacteria,
is investigated. Since their motion is influenced by interactions within the optical
trap and their rotational speed varies with height in the beam profile, calibrating
the rotational speed variations allows for the detection of subtle fluctuations caused
by bacterial interactions. When bacteria interacts with the nanorods, they shift the
nanorods’ position within the beam, enabling precise detection of the nanomotion.
The optical setup included circularly polarized laser tweezers, dark-field illumination,
a photon multiplier tube collecting the scattered light and video microscopy
for real-time measurements of the dynamics of the gold nanorods.
The study revealed that the surrounding medium affects nanoparticle behavior in
different ways: in lysogeny broth (LB), biomolecule binding increases the effective
particle size and friction, slowing rotation motion. In phosphate buffered saline
(PBS), ions confine particles closer to the surface, while milli-Q water (MQ), lacking
ions, allows for greater freedom of movement relative to the interacting surface.
Moreover, the higher stiffness measured in MQ is associated with the absence of
ions and Coulomb screening, enhancing movement in the z-direction and indicating a
more stable trapping environment in the xy-plane. Additionally, unexpected results
present how the temperature rises gradually as the focal point is approached, likely
due to reflections from the laser on the glass surface. The system demonstrates high
sensitivity for detecting bacterial motion, with a sensitivity ranging of 0.5 Hznm−1
to 0.6 Hznm−1, exceeding the standard deviations of the fluctuations. The results
and conclusions presented in this thesis provide insights that could contribute to
future applications like detecting bacterial activity. | sv |
