Alexander Konrad

Confocal sample-scanning microscope for single-molecule spectroscopy and microscopy with fast sample exchange at cryogenic temperatures.

The construction of a microscope with fast sample transfer system for single-molecule spectroscopy and microscopy at low temperatures using 2D/3D sample-scanning is reported. The presented construction enables the insertion of a sample from the outside (room temperature) into the cooled (4.2 K) cryostat within seconds. We describe the mechanical and optical design and present data from individual Photosystem I complexes. With the described setup numerous samples can be investigated within one cooling cycle. It opens the possibility to investigate biological samples (i) without artifacts introduced by prolonged cooling procedures and (ii) samples that require preparation steps like plunge-freezing or specific illumination procedures prior to the insertion into the cryostat.

Strong and Coherent Coupling of a Plasmonic Nanoparticle to a Subwavelength Fabry-Perot Resonator

A major aim in experimental nano- and quantum optics is observing and controlling the interaction between light and matter on a microscopic scale. Coupling molecules or atoms to optical microresonators is a prominent method to alter their optical properties such as luminescence spectra or lifetimes. Until today strong coupling of optical resonators to such objects has only been observed with atom-like systems in high quality resonators. We demonstrate first experiments revealing strong coupling between individual plasmonic gold nanorods (GNR) and a tunable low quality resonator by observing cavity-length-dependent nonlinear dephasing and spectral shifts indicating spectral anticrossing of the luminescent coupled system. These phenomena and experimental results can be described by a model of two coupled oscillators representing the plasmon resonance of the GNR and the optical fields of the resonator. The presented reproducible and accurately tunable resonator allows us to precisely control the optical properties of individual particles.

Spectroscopic properties of photosystem II core complexes from Thermosynechococcus elongatus revealed by single-molecule experiments

In this study we use a combination of absorption, fluorescence and low temperature single-molecule spectroscopy to elucidate the spectral properties, heterogeneities and dynamics of the chlorophyll a (Chla) molecules responsible for the fluorescence emission of photosystem II core complexes (PS II cc) from the cyanobacterium Thermosynechococcus elongatus. At the ensemble level, the absorption and fluorescence spectra show a temperature dependence similar to plant PS II. We report emission spectra of single PS II cc for the first time; the spectra are dominated by zero-phonon lines (ZPLs) in the range between 680 and 705nm. The single-molecule experiments show unambiguously that different emitters and not only the lowest energy trap contribute to the low temperature emission spectrum. The average emission spectrum obtained from more than hundred single complexes shows three main contributions that are in good agreement with the reported bands F685, F689 and F695. The intensity of F695 is found to be lower than in conventional ensemble spectroscopy. The reason for the deviation might be due to the accumulation of triplet states on the red-most chlorophylls (e.g. Chl29 in CP47) or on carotenoids close to these long-wavelength traps by the high excitation power used in the single-molecule experiments. The red-most emitter will not contribute to the fluorescence spectrum as long as it is in the triplet state. In addition, quenching of fluorescence by the triplet state may lead to a decrease of long-wavelength emission.

Variation of exciton-vibrational coupling in photosystem II core complexes from Thermosynechococcus elongatus as revealed by single-molecule spectroscopy.

The spectral properties and dynamics of the fluorescence emission of photosystem II core complexes are investigated by single-molecule spectroscopy at 1.6 K. The emission spectra are dominated by sharp zero-phonon lines (ZPLs). The sharp ZPLs are the result of weak to intermediate exciton-vibrational coupling and slow spectral diffusion. For several data sets, it is possible to surpass the effect of spectral diffusion by applying a shifting algorithm. The increased signal-to-noise ratio enables us to determine the exciton-vibrational coupling strength (Huang–Rhys factor) with high precision. The Huang–Rhys factors vary between 0.03 and 0.8. The values of the Huang–Rhys factors show no obvious correlation between coupling strength and wavelength position. From this result, we conclude that electrostatic rather than exchange or dispersive interactions are the main contributors to the exciton-vibrational coupling in this system.

One-pot synthesis of micron partly hollow anisotropic dumbbell shaped silica core–shell particles

A facile method is described to prepare micron partly hollow dumbbell silica particles in a single step. The obtained particles consist of a large dense part and a small hollow lobe. The spherical dense core as well as the hollow lobe are covered by mesoporous channels. In the case of the smaller lobe these channels are responsible for the permeability of the shell which was demonstrated by confocal imaging and spectroscopy.

Low-Cost GRIN-Lens-Based Nephelometric Turbidity Sensing in the Range of 0.1–1000 NTU

Turbidity sensing is very common in the control of drinking water. Furthermore, turbidity measurements are applied in the chemical (e.g., process monitoring), pharmaceutical (e.g., drug discovery), and food industries (e.g., the filtration of wine and beer). The most common measurement technique is nephelometric turbidimetry. A nephelometer is a device for measuring the amount of scattered light of suspended particles in a liquid by using a light source and a light detector orientated in 90° to each other. Commercially available nephelometers cost usually—depending on the measurable range, reliability, and precision—thousands of euros. In contrast, our new developed GRIN-lens-based nephelometer, called GRINephy, combines low costs with excellent reproducibility and precision, even at very low turbidity levels, which is achieved by its ability to rotate the sample. Thereby, many cuvette positions can be measured, which results in a more precise average value for the turbidity calculated by an algorithm, which also eliminates errors caused by scratches and contaminations on the cuvettes. With our compact and cheap Arduino-based sensor, we are able to measure in the range of 0.1–1000 NTU and confirm the ISO 7027-1:2016 for low turbidity values.

Polarization-dependent single-molecule spectroscopy on photosystem I

Single-molecule spectroscopy (SMS) at low temperature was used to study the spectral properties, heterogeneities and spectral dynamics of the chlorophyll a (Chl a) molecules responsible for the fluorescence emission of photosystem I (PS I). The fluorescence spectra of single PS I complexes are dominated by several red-shifted Chl a molecules categorized into red pools called C708 and C719. By polarization dependent measurements we demonstrate spectrally separate emissions corresponding to C708 and C719 in single PS I monomers and trimers. Moreover, we compared the results of SMS polarization dependent between monomeric and trimeric PS I complexes and give an estimation for the orientation between these red pools. As a consequence, we get new insight into the energy transfer towards and between the red Chl a molecules in PS I complexes.