We study the technique of photothermal microscopy by which we can detect single nano-objects by their absorption at room temperature. We optimize the sensitivity of this technique and demonstrate... Show moreWe study the technique of photothermal microscopy by which we can detect single nano-objects by their absorption at room temperature. We optimize the sensitivity of this technique and demonstrate the first optical detection of a single molecule by its absorption at room temperature. Moreover, we combine photothermal, luminescence and scattering of individual nano-objects (organic dye nanoparticles and gold nanoparticles) at single-particle level to gain insight into their radiative and nonradiative properties. Single organic nanoparticles exhibit a complex excitation power-dependent luminescence quantum yield due to singlet-singlet or singlet-triplet annihilation, and their luminescence quantum yield can be as high as 10^(__2). In contrast to organic dye nanoparticles, gold nanoparticles yield very stable optical signals. Gold nanoparticles are also easily detectable by their photoluminescence. We find that the luminescence quantum yield of single gold nanoparticles is nearly independent of their volumes and can be as high as ~10^(-5) for nanorods with a plasmon resonance of ~650 nm. We further investigate the sensitivity of a single gold nanorod to an approaching dielectric surface. We show that the nanorod exhibits significant red-shift in its plasmon resonance wavelength for distances less than 400 nm pointing the way towards the possible application of nanorods as distance sensors. Show less
Single-molecule fluorescence was invented in the 1990s and has quickly developed into an indispensable technique in the biomedical sciences and condensed-matter research. It has revolutionized... Show moreSingle-molecule fluorescence was invented in the 1990s and has quickly developed into an indispensable technique in the biomedical sciences and condensed-matter research. It has revolutionized the fields of molecular biology, imaging (super-resolution), and catalysis, to name a few. In this thesis, we will apply fluorescence enhancement by single gold nanorods to extend single-molecule studies to chromophores with low fluorescence quantum yields and to high concentrations of probe molecules. Following single-molecule trajectories, we will explore variations in the electron-transfer rates of the metalloprotein azurin both from molecule to molecule and for the same molecule as a function of time. Evidence for conformational substates will be discussed based on dynamic heterogeneity. Show less
We combine optical trapping and far-field optical detection techniques in a novel approach to study single metal nanoparticles in solution. We demonstrate the first measurements of the acoustic... Show moreWe combine optical trapping and far-field optical detection techniques in a novel approach to study single metal nanoparticles in solution. We demonstrate the first measurements of the acoustic vibrations of single gold nanoparticles optically trapped in water, and find evidence for intrinsic damping mechanisms. Additionally, we explore the potential of single gold nanorods as ultra-small mechanical actuators: we quantify the optical forces and torques on a single trapped gold nanorod (25 nm diameter and 60 nm length) and show that the rod can simultaneously exert forces and torques that would be large enough to manipulate single (macro-) molecules. We developed techniques to measure the combined translational and rotational Brownian motion of a trapped nanorod. We determine the rod's heating by the trap beam and show that translational and rotational Brownian motion of a hot particle are described by different effective temperatures and viscosities. Show less
Detecting nanoscopic objects plays an important role in nanoscience in particular, in the rapidly growing field of nanobiology. The forebear to modern super-resolution microscopy for single... Show moreDetecting nanoscopic objects plays an important role in nanoscience in particular, in the rapidly growing field of nanobiology. The forebear to modern super-resolution microscopy for single molecule investigation, is fluorescence microscopy. Fluorescence as a contrast mechanism, however, brings several restrictions. These include (1) the use of the label itself, which may introduce artifacts to the interpretation, (2) the limited photoemission caused by photobleaching and photoblinking as well as (3) low bandwidth of the emission. Fluorescence-free alternatives are thus highly desirable to overcome these limitations. Optical detection of individual proteins with high bandwidth holds great promise for understanding important biological processes on the nanoscale. In this thesis, we investigate label-free optoplasmonic detection of single proteins and particles in motion. Analysing the data provide information about the hydrodynamic volume of the diffuser and interaction such as binding events. Show less
This thesis aims to improve the detection from ultra-weak single emitter by enhancing their emission properties with plasmonic nanostructures. We exploit the wet-chemically synthesized single... Show moreThis thesis aims to improve the detection from ultra-weak single emitter by enhancing their emission properties with plasmonic nanostructures. We exploit the wet-chemically synthesized single crystalline gold nanorods (GNRs) as our basic frameworks in the whole studies, simply because of their unique optical properties, such as the intense electromagnetic fields enhancement near the tips, and the narrow, tunable resonance with light. We first explore the lower limit of fluorescence quantum yield for single-molecule detection by enhancing the fluorescence with a single gold nanorod. Later, we develop a method to synthesize end-to-end gold nanorod dimers on glass substrates with the aid of molecular linkers, and then apply these strong plasmon coupling systems to enhance the single-molecule fluorescence under two-photon excitation. Show less
