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We present a new method that combines fluorescence correlation spectroscopy (FCS) on the microsecond time scale with fluorescence antibunching measurements on the nanosecond time scale for measuring photophysical rate constants of fluorescent molecules. The antibunching measurements allow us to quantify the average excitation rate of fluorescent molecules within the confocal detection volume of the FCS measurement setup. Knowledge of this value allows us then to quantify, in an absolute manner, the intersystem crossing rate and triplet state lifetime from the microsecond temporal decay of the FCS curves. We present a theoretical analysis of the method and estimate the maximum bias caused by the averaging of all quantities (excitation rate, photophysical rates) over the confocal detection volume, and we show that this bias is smaller than 5\% in most cases. We apply the method for measuring the photophysical rate constants of the widely used dyes Rhodamine~110 and ATTO~655., Comment: 20 pages, 5 figures

We describe an exact and highly efficient numerical algorithm for solving a special but important class of convection-diffusion equations. These equations occur in many problems in physics, chemistry, or biology, and they are usually hard to treat due to the presence of a convection term, represented by a first order derivative in the spatial co-ordinate. Our algorithm reduces the convection-diffusion equation to a pure diffusion problem within a complex-valued potential which can be solved efficiently and accurately using conventional parabolic partial differential equation solvers., Comment: 6 pages, 6 figures

Ensemble averaging experiments may conceal many fundamental molecular interactions. To overcome that, a high-throughput detection of single molecules or colloidal nanodots is crucial for biomedical, nanoelectronic, and solid-state applications. One-dimensional (1D) discrete flow of nanoscale objects is an efficient approach in this direction. The development of simple and cost-effective nanofluidic devices is a critical step to realise 1D flow. This letter presents a nanofabrication technique using shadow-angle-electron-beam-deposition for a high-throughput preparation of parallel nanofluidic channels. These were used to flow and detect DNA, carbon-nanodots, and organic fluorophores. The 1D molecular mass transport was performed using electro-osmotic flow. The 1D flow behaviour was identified and analysed using two-focus fluorescence correlation spectroscopy (2fFCS). A range of flow velocities of single molecules was achieved. The transitions of single molecules or nanodots through the two foci were quantitatively analysed using confocal scanning imaging, correlative photon detection, and burst size distribution analysis. The results suggest an efficient nanofabrication technique is developed to prepare nanofluidic devices. This first demonstration of high-throughput nanochannel fabrication process and using 2fFCS-based single molecule flow detection should have a potential impact on ultra-sensitive biomedical diagnostics and studying biomolecular interactions as well as nanomaterials.

We investigate the impact of rotational diffusion on the electrodynamic coupling of fluorescent dye molecules (oscillating electric dipoles) to a tunable planar metallic nanocavity. Fast rotational diffusion of the molecules leads to a rapidly fluctuating mode density of the electromagnetic field along the molecules' dipole axis, which significantly changes their coupling to the field as compared to the opposite limit of fixed dipole orientation. We derive a theoretical treatment of the problem and present experimental results for rhodamine 6G molecules in cavities filled with low and high viscosity liquids. The derived theory and presented experimental method is a powerful tool for determining absolute quantum yield values of fluorescence., Comment: 5 pages, 3 figures, accepted by Physical Review Letters