Supercritical Angle Fluorescence Characterization Using Spatially Resolved Fourier Plane Spectroscopy

Observation of aligned dipoles and angular chromism of exciplexes in organic molecular heterostructures

The dipole characteristics of Frenkel excitons and charge-transfer excitons between donor and acceptor molecules in organic heterostructures such as exciplexes are important in organic photonics and optoelectronics. For the bilayer of the organic donor 4,4',4''-tris[(3-methylphenyl)phenylamino]triphenylamine and acceptor 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine molecules, the exciplexes form aligned dipoles perpendicular to the Frenkel excitons, as observed in back focal plane photoluminescence images. The angular chromism of exciplexes observed in the 100 meV range indicates possible delocalization and angle-sensing photonic applications. The blue shift of the peak position and increase in the linewidth of photoluminescene spectra with increasing excitation power are caused by the repulsive aligned exciplex dipole moments with a long lifetime (4.65 μs). Electroluminescence spectra of the exciplex from organic light-emitting diodes using the bilayer are blue-shifted with increasing bias, suggesting unidirectional alignment of the exciplex dipole moments. The observation of exciplex dipole moment alignments across molecular interfaces can facilitate the controlled coupling of exciton species and increase efficiency of organic light-emitting diodes.

Single-molecule orientation localization microscopy II: a performance comparison

Various techniques have been developed to measure the 2D and 3D positions and 2D and 3D orientations of fluorescent molecules with improved precision over standard epifluorescence microscopes. Due to the challenging signal-to-background ratio in typical single-molecule experiments, it is essential to choose an imaging system optimized for the specific target sample. In this work, we compare the performance of multiple state-of-the-art and commonly used methods for orientation localization microscopy against the fundamental limits of measurement precision. Our analysis reveals optimal imaging methods for various experiment conditions and sample geometries. Interestingly, simple modifications to the standard fluorescence microscope exhibit superior performance in many imaging scenarios.

Supercritical Angle Fluorescence Microscopy and Spectroscopy

Fluorescence detection, either involving propagating or near-field emission, is widely being used in spectroscopy, sensing, and microscopy. Total internal reflection fluorescence (TIRF) confines fluorescence excitation by an evanescent (near) field, and it is a popular contrast generator for surface-selective fluorescence assays. Its emission equivalent, supercritical angle fluorescence (SAF), is comparably less established, although it achieves a similar optical sectioning as TIRF does. SAF emerges when a fluorescing molecule is located very close to an interface and its near-field emission couples to the higher refractive index medium (n2 >n1) and becomes propagative. Then, most fluorescence is detectable on the side of the higher-index substrate, and a large fraction of this fluorescence is emitted into angles forbidden by Snell's law. SAF, as well as the undercritical angle fluorescence (UAF; far-field emission) components, can be collected with microscope objectives having a high-enough detection aperture (numerical aperture >n2) and be separated in the back focal plane by Fourier filtering. The back focal plane image encodes information about the fluorophore radiation pattern, and it can be analyzed to yield precise information about the refractive index in which the emitters are embedded, their nanometric distance from the interface, and their orientation. A SAF microscope can retrieve this near-field information through wide-field optics in a spatially resolved manner, and this functionality can be added to an existing inverted microscope. Here, we describe the potential underpinning of SAF microscopy and spectroscopy, particularly in comparison with TIRF. We review the challenges and opportunities that SAF presents from a biophysical perspective, and we discuss areas in which we see potential.

In-Parallel Polar Monitoring of Chemiluminescence Emission Anisotropy at the Solid–Liquid Interface by an Optical Fiber Radial Array

Chemiluminescence (CL) detection is widely employed in biosensors and miniaturized analytical devices since it offers high detectability and flexible device design (there are no geometry requirements for the measurement cell, except the ability to collect the largest fraction of emitted photons). Although the emission anisotropy phenomenon for an emitting dipole bound to the interface between two media with different refractive index is well known for fluorescence, it is still poorly investigated for CL reactions, in which the excited-state reaction products can diffuse in solution before the photon emission event. In this paper, we propose a simple method for the real-time evaluation of the CL emission anisotropy based on a radial array of optical fibers, embedded in a poly(methyl methacrylate) semicylinder and coupled with a Charge-Coupled Device (CCD) camera through a suitable interface. The polar-time evolutions of the CL emission have been studied for catalyzing enzymes immobilized onto a solid surface (heterogeneous configuration) or free in solution (homogeneous configuration). Evidence of the anisotropy phenomenon is observed, indicating that the lifetime of the excited-state products of the enzyme-catalyzed reactions is shorter than the time required for their diffusion in solution at a distance at which the CL can be considered isotropic. These results open new perspectives in the development of CL-based miniaturized analytical devices.

Ultrasensitive Refractometry via Supercritical Angle Fluorescence

Refractometry, namely, the measurement of refractive index (RI), provides information about various sample properties such as concentrations and molecular structure. One physical phenomenon which enables precise determination of a sample's RI in a microscope is the supercritical-angle fluorescence. This effect is observed when the fluorescence from an emitter near a glass-medium interface is captured by an objective lens with a high numerical aperture. The materials' index mismatch creates a distinguishable transition in the intensity pattern at the back focal plane of the objective that changes proportionally to the RI of the media. Here, we present a refractometry approach in which the fluorophores are preattached to the bottom surface of a microfluidic channel, enabling highly sensitive determination of the RI using tiny amounts of liquid (picoliters). With this method, we attained a standard deviation of 3.1 × 10^-5 and a repeatability of 2.7 × 10^-5 RI units. We first determine the capabilities of the device for glycerol-water solutions and then demonstrate the relevance of our system for monitoring changes in biological systems. As a model system, we show that we can detect single bacteria ( Escherichia coli) and measure population growth.