KEYWORDS: Time correlated single photon counting, Fluorescence lifetime imaging, Cameras, Microscopy, Single photon avalanche diodes, Visualization, Total internal reflection, Resonance energy transfer, Proteins, Picosecond phenomena
Camera-based time-correlated single photon counting (TCSPC) is a method where the position and the arrival time of the photons are recorded simultaneously. This has some advantages for fluorescence lifetime imaging (FLIM) with certain types of microscopy. It also allows FLIM with a macroscopic field of view.
We report on nanosecond fluorescence lifetime imaging (FLIM) combined with total internal reflection fluorescence (TIRF) microscopy based on a 40 mm diameter crossed delay line anode detector. It has a few 100 picoseconds time resolution, and is read out via three standard TCSPC boards. We apply this wide-field TCSPC detector to identify Förster Resonance Energy Transfer (FRET) in cell membrane proteins in TIR-FLIM microscopy.
In addition, we use a TCSPC single photon avalanche diode (SPAD) array as a macroscopic camera to visualise varnish removal on paintings.
The time-resolved fluorescence anisotropy of a biological sample can reveal rotational dynamics and structure of a fluorescent probe’s local environment. Here, methods are presented towards developing widefield time-resolved fluorescence anisotropy imaging (TR-FAIM) using a single photon avalanche diode (SPAD) array (QuantICAM). Such detector allows for simultaneous time-correlated single photon counting (TCSPC) measurements in each pixel with single-photon sensitivity and picosecond time resolution. Our method integrates the SPAD array with a widefield microscope, and automated rotating polarizers in the excitation and emission pathways to demonstrate TR-FAIM. We have shown the robustness of the method through spectroscopic measurements of the fluorophore PM546, and we have demonstrated the usefulness of simultaneous FLIM and TR-FAIM for studying properties of plasma membranes in live yeast cells.
KEYWORDS: Luminescence, Anisotropy, Image sensors, Biological research, Monte Carlo methods, Time correlated photon counting, Microscopy, Data acquisition, Virtual reality, Time metrology
A 128x192 SPAD array (QuantiCam) with an on-chip time-to-digital converter in each pixel is used as a camera in a single-photon time-resolved fluorescence microscope. The SPAD array introduces systematic nonlinearities and timing offset to the measured photon arrival times. This limits the fidelity of the experimental results. A Monte-Carlo algorithm was developed to transform the raw photon time-stamp stream coming from the SPAD array into a corrected virtual “photon” time-stamp stream devoid of the systematic measurement errors. This data is compatible with existing downstream data processing pipelines used in time-correlated single-photon counting. We discuss the calibration measurement, the algorithm, their performance and application to live fluorescence lifetime imaging of photosynthetic organisms.
KEYWORDS: Luminescence, Spectroscopy, Process control, Electromagnetism, Metamaterials, Composites, Biophysics, Quantum optics, Nanostructures, Resonance energy transfer
The control of photoluminescence processes, via the design of composite materials with engineered electromagnetic properties, is of great interest for the development of many application areas ranging from biophysics to quantum optical technologies. Approaches providing broadband enhancements of emission, not limited to resonant nanostructures, are particularly advantageous. We discuss how various photoluminescence processes, including conventional and dipolar-forbidden spontaneous emission, as well as Förster resonance energy transfer, are altered nearby and inside plasmonic hyperbolic metamaterials. They provide a flexible platform for engineering broadband Purcell enhancements due to their peculiar electromagnetic mode structure controlled by the nonlocal response of the metamaterial.
Here we use the combination of fluorescent molecular rotors (FMR) and fluorescence lifetime imaging (FLIM) to image matrix viscosity in live cells. We use non-cancerous, epithelial human cornea (HCE) cells as a model system. We find that mitochondrial matrix viscosity varies between individual cells and even between individual organelles, showcasing the potential of viscosity imaging for cell biology purposes.
Imaging viscosity and its spatiotemporal patterns can provide valuable insight into the underlying physical conditions of biochemical reactions and biological processes in cells and tissues. One way to measure viscosity and diffusion is the use of fluorescence recovery after photobleaching (FRAP). We combine FRAP with FLIM and time-resolved fluorescence anisotropy imaging (tr-FAIM), by acquiring time- and polarization-resolved fluorescence images in every frame of a FRAP series. This allows us to simultaneously monitor translational and rotational diffusion. This approach can be applied to measuring diffusion in homogeneous and heterogeneous environments, and in principle also allows the study of homo-FRET. Another way to measure viscosity and diffusion is through specific flexible dyes, e.g. fluorescent molecular rotors, whose fluorescence quantum yield and fluorescence lifetime depend on the viscosity of the environment, in combination with fluorescence lifetime imaging (FLIM). We show that a bodipybased fluorescent molecular rotor targeting mitochondria reports on their viscosity, which changes under physiological stimuli. Both methods can optically measure viscosity and diffusion on the micrometer scale.
We report the simultaneous combination of three powerful techniques in uorescence microscopy: Fluorescence Lifetime Imaging (FLIM), Fluorescence Anisotropy Imaging (FAIM) and Fluorescence Recovery After Photobleaching (FRAP), also called F3 microscopy. An exhaustive calibration of the setup was carried out with several rhodamine 6G (R6G) solutions in water-glycerol and from the combination of the FAIM and FRAP data, the hydrodynamic radius of the dye was directly calculated. The F3 data was analyzed with a home-built MATLAB script, and the setup is currently explored further with Green Fluorescent Protein (GFP). Some molecular dynamic (MD) simulations are currently being run in order to help with the interpretation of the experimental anisotropy data.
Fluorescence-based processes are strongly modified by the electromagnetic environment in which the emitters are placed. Hence, the design of nanostructured materials with appropriate electromagnetic properties opens up a new route in the control of, for instance, the spontaneous rate of emission or the energy transfer rate in donor-acceptor pairs. In particular, hyperbolic plasmonic metamaterials have emerged as a very flexible and powerful platform for these applications as they provide a high local density of electromagnetic states due to their peculiar mode structure which is governed by both the structural nonlocal response and the dispersion properties. Here, we will discuss an experimental and theoretical study of the influence of a hyperbolic metamaterial comprised of an array of gold nanorods on the radiative properties of quantum emitters and the energy-transfer processes between them.
KEYWORDS: Metamaterials, Plasmonics, Fluorescence resonance energy transfer, Electromagnetism, Resonance energy transfer, Luminescence, Energy transfer, Molecules, Molecular energy transfer, Biosensing
The control of the Förster resonance energy transfer (FRET) rate between molecules has recently received a lot of interest, opening opportunities in the development of sources of incoherent illumination, photovoltaics and biosensing applications. The design of nanostructured materials with appropriate electromagnetic properties, particularly with the engineered local density of electromagnetic states (LDOS), allows the enhancement of the spontaneous emission rate of emitters in their vicinity. However, the question of the influence of the LDOS on the energy transfer rate between emitters remains controversial. To date, several contradicting theoretical and experimental studies involving emitters on metallic surfaces and plasmonic metamaterials as well as in optical cavities and plasmonic antennas have been reported. In this work we study the influence of the LDOS on the energy transfer between donor-acceptor pairs placed inside the anisotropic metamaterial. The study of the emission kinetics of both the donor and the acceptor allow us to experimentally compare FRET efficiencies in different electromagnetic environments including dielectric and plasmonic substrates as well as metamaterials.
Time-correlated single photon counting (TCSPC) is a widely used, sensitive, precise, robust and mature technique to measure photon arrival times in applications such as fluorescence spectroscopy and microscopy, light detection and ranging (lidar) and optical tomography. Wide-field TCSPC detection techniques, where the position and the arrival time of the photons are recorded simultaneously, have seen several advances in the last few years, from the microsecond to the picosecond time scale. Here, we summarise some of our recent work in this field with emphasis on microsecond resolution phosphorescence lifetime imaging (PLIM) and nanosecond fluorescence lifetime imaging (FLIM) microscopy.
Spectrally resolved confocal microscopy and fluorescence lifetime imaging have been used to measure the polarity of lipid-rich regions in living HeLa cells stained with Nile red. The emission peak from the solvatochromic dye in lipid droplets is at a shorter wavelength than other, more polar, stained internal membranes, and this is indicative of a low polarity environment. We estimate that the dielectric constant, ϵ, is around 5 in lipid droplets and 25<ϵ<40 in other lipid-rich regions. Our spectrally resolved fluorescence lifetime imaging microscopy (FLIM) data show that intracellular Nile red exhibits complex, multiexponential fluorescence decays due to emission from a short lifetime locally excited state and a longer lifetime intramolecular charge transfer state. We measure an increase in the average fluorescence lifetime of the dye with increasing emission wavelength, as shown using phasor plots of the FLIM data. We also show using these phasor plots that the shortest lifetime decay components arise from lipid droplets. Thus, fluorescence lifetime is a viable contrast parameter for distinguishing lipid droplets from other stained lipid-rich regions. Finally, we discuss the FLIM of Nile red as a method for simultaneously mapping both polarity and relative viscosity based on fluorescence lifetime measurements.
Ultra-fast frame rate CMOS cameras, combined with a photon counting image intensifier, can be used for microsecond resolution wide-field time-correlated single photon counting (TCSPC) microscopy. A sequence of frames is recorded after an excitation pulse, and the number and location of photons in each frame is determined. This process is repeated until enough photons are recorded for a photon arrival time histogram in the pixels of the image. This approach combines low, nanowatt excitation power with single-photon detection sensitivity and arrival timing in many pixels simultaneously, short acquisition times in the order of seconds and allows lifetime mapping with a time resolution of ~1 microsecond. Moreover, we also show that the phosphor decay can be exploited to time the photon arrival well below the exposure time of the camera. This approach yields better time resolution and larger images than direct imaging of photon events. We show that both methods are ideal for lifetime imaging of transition metal compounds in living cells within a few seconds.
We report the use of Time-Correlated Single Photon Counting (TCSPC) in a polarization-resolved Fluorescence Lifetime Imaging (FLIM) setup for the investigation of cell membrane structural and dynamic properties. This technique allows us to study the orientation and mobility of fluorescent membrane dyes, namely di-4-ANEPPDHQ and DiO, in model bilayers of different lipid compositions. Dipole alignment and extent of rotational motion can be linked to membrane order and fluidity. Comparison of the time-resolved anisotropy decays of the two fluorescent dyes suggests that rotational motion of membrane constituents is restricted in liquid-ordered phases, and appears to be limited to the region of aliphatic tails in liquid-disordered phases. In living cells, understanding the membrane structure provides crucial information on its functional properties, such as exo- and endocytosis, cell mobility and signal transduction.
Meso-substituted boron-dipyrromethene (BODIPY-C12) was used to monitor the viscosity in cells via fluorescence
lifetime imaging (FLIM), and time-resolved fluorescence anisotropy measurements. Our results show that meso-substituted
BODIPY-C12 senses the viscosity in HeLa cells and is insensitive to the surrounding polarity. The
relationship between the fluorescence lifetime and the rotational correlation time of the dye in homogeneous solutions
agree with the combination of the Foerster Hoffmann equation and the Debye-Stokes-Einstein equation.
We present fluorescence lifetime imaging (FLIM) and fluorescence anisotropy imaging along with translational diffusion
measurements of living cells labelled with green fluorescent protein (GFP) recorded in a single experiment. The
experimental set-up allows for time and polarization-resolved fluorescence images to be measured in every frame of a
fluorescence recovery after photobleaching (FRAP) series. We have validated the method using rhodamine 123 in
homogeneous solution prior to measurements of living A431 cells labelled with cdc42-GFP, for which the FRAP
recovery exhibits an immobile fraction and the rotational mobility of the protein is hindered while the fluorescence
lifetime fairly homogeneous across the cell. By eliminating the need for sequential measurements to extract fluorescence
lifetimes and molecular diffusion coefficients we remove artefacts arising from changes in sample morphology and
excessive photobleaching during sequential experiments.
The average fluorescence lifetime of the green fluorescent protein (GFP) in solution is a function of the refractive index of its environment. We report that this is also the case for GFP-tagged proteins in cells. Using time-correlated single-photon counting (TCSPC)–based fluorescence lifetime imaging (FLIM) with a confocal scanning microscope, images of GFP-tagged proteins in cells suspended in different refractive index media are obtained. It is found that the average fluorescence lifetime of GFP decreases on addition of glycerol or sucrose to the media in which the fixed cells are suspended. The inverse GFP lifetime is proportional to the refractive index squared. This is the case for GFP-tagged major histocompatibility complex (MHC) proteins with the GFP located inside the cytoplasm, and also for GPI-anchored GFP that is located outside the cell membrane. The implications of these findings are discussed with regard to total internal reflection fluorescence (TIRF) techniques where the change in refractive index is crucial in producing an evanescent wave to excite fluorophores near a glass interface. Our findings show that the GFP fluorescence lifetime is shortened in TIRF microscopy in comparison to confocal microscopy.
An automated high-content screening microscope has been developed which uses fluorescence anisotropy imaging and fluorescence lifetime microscopy to identify Förster resonant energy transfer between eGFP and mRPF1 in drug screening assays. A wide-field polarization resolved imager is used to simultaneously capture the parallel and perpendicular components of both eGFP and mRFP1 fluorescence emission to provide a high-speed measurement of acceptor depolarization. Donor excited state lifetime measurements performed using laser scanning microscopy is then used to determine the FRET efficiency in a particular assay. A proof-of-principle assay is performed using mutant Jurkat human T-cells to illustrate the process by which FRET is first identified and then quantified by our high-content screening system.
We have used an experimental arrangement comprising two photomultipliers and time-correlated single photon counting
(TCSPC) detection to measure time and polarization-resolved fluorescence decays and images simultaneously.
Polarization-resolved measurements can provide information which may be difficult to extract from lifetime
measurements alone. The combination of fluorescence lifetime and time-resolved anisotropy in an imaging modality
with two detectors minimizes the errors arising from bleaching of a sample between consecutive measurements.
Anisotropy measurements can provide evidence of fluorescence resonance energy transfer between chemically identical
fluorophores (homo-FRET). This phenomenon is not detectable in spectral or lifetime changes, yet a lowering of the
anisotropy and a faster anisotropy decay can provide evidence for close proximity (≤ 10 nm) of adjacent fluorophores
including dimerization and oligomerization of molecules. We have used FLIM and fluorescence anisotropy to measure
variations in fluorescence lifetimes and anisotropy of GFP-tagged proteins in cells in immunological synapse samples
and also acquire images of BODIPY-stained carcinoma cells.
We demonstrateWide-Field Time-Correlated Single Photon Counting (WiFi TCSPC) imaging based on an image
intensifier and a high-speed camera running at 30,000 frames per second. The timing of photon events is thus
performed in parallel, simultaneously on every pixel. The system is applied to lanthanide lifetime measurements
and time-resolved imaging of the lanthanide complex Europium Polyoxometalate (Eu POMs). We measure a
lifetime of 2.98 ms for Eu POMs in solid state, which is in excellent agreement with the literature value.
We present a novel time-resolved photon counting imaging technique and its use in multi-dimensional luminescence
spectroscopy. By using an ultrafast camera coupled to an image intensifier on a microscope, we demonstrate
the potential of wide-field time-correlated single photon counting, with a count rate of up to 5 Mhz. This system
has the advantage of allowing the detection of single photons in parallel in every pixel. We measured the
luminescence decay of Europium Polyoxometalate (POM), and observed contrast on lifetime images of Eu-POM
on silver nanocrystals.
Fluorescence imaging techniques are powerful tools in the biological and biomedical sciences, because they are
minimally invasive and can be applied to live cells and tissues. The fluorescence emission can be characterized not only
by its intensity and position, by also by its fluorescence lifetime, polarization and wavelength. Fluorescence Lifetime
Imaging (FLIM) in particular has emerged as a key technique to image the environment and interaction of specific
proteins in living cells. Using a time-correlated single photon counting (TCSPC)-based FLIM set-up, we find that the
fluorescence lifetime of GFP-tagged proteins in cells is a function of the refractive index of the medium the cells are
suspended in. In addition, combining Fluorescence Recovery After Photobleaching (FRAP) of fluorescently labeled
proteins of different sizes in sol gels with time-resolved fluorescence anisotropy measurements, we demonstrate that we
can measure their lateral and rotational diffusion. This allows us to infer the size and connectivity of the pores in the sol
gel matrix. Moreover, wide-field photon counting imaging, originally developed for astronomical applications, is a
powerful imaging method because of its high sensitivity and excellent signal-to-noise ratio. It has a distinct advantage
over CCD-based imaging due to the ability to time the arrival of individual photons. The potential of time-resolved wide-field
photon counting imaging with a fast CMOS camera applied to luminescence microscopy is demonstrated.
The average fluorescence lifetime of GFP in solution is a function of the refractive index of its environment. Here, we
demonstrate that this also appears to be the case for GFP-tagged proteins in cells. Using TCSPC-based FLIM with a
scanning confocal microscope, we image GFP-tagged proteins in fixed cells in different media. We find that the average
fluorescence lifetime of GFP in cells is shortened, as glycerol or sucrose are added to the medium. This is the case for
GFP-tagged MHC proteins with the GFP located inside the cytoplasm, and also for GPI-anchored GFP which is located
outside the cell membrane. We observe a linear relationship between the inverse average lifetime of GFP in fixed cells
and the square of the refractive index of the medium. Implications of this phenomenon when using Total Internal
Reflection Fluorescence (TIRF) microscopy will also be discussed as a shortening of the lifetime is seen close to the
glass prism used to produce the evanescent wave in TRIF.
The fluorescence decay of the biologically important enhanced green fluorescent protein (GFP) is a function of the refractive index of its environment (Suhling et al, Biophys J 83, 3589-3595, 2002). To address the question whether this effect can be exploited to image the local environment of specific proteins in cell biology, we need to determine the distance over which the GFP fluorescence decay is sensitive to the refractive index. To this end, we employ Fluorescence Lifetime Imaging (FLIM) of GFP in buffer solution at an air and at an oil interface. This approach allows us to map the fluorescence lifetime as a function of distance from the interface. Preliminary data show that the average fluorescence lifetime of GFP increases near a buffer/air interface and decreases near a buffer/oil interface. Similar results showing the same trend are obtained using fluorescein in buffer at an oil and at an air interface. The range over which this fluorescence lifetime change occurs is found to be of the order several micrometers which is consistent with theoretical models. In addition, GFP-tagged MHC proteins in fixed cells were imaged in different refractive index media using FLIM. It appears that the average GFP fluorescence lifetime in cells is also sensitive to different refractive index environments, and is inversely proportional to the square of the refractive index.
As a precursor to applying fluorescence lifetime imaging (FLIM) to studies of intercellular communication in molecular immunology, we have investigated the fluorescence lifetime of enhanced green fluorescent protein (EGFP) in mixtures of water and glycerol using time-correlated single photon counting (TCSPC). We find that the EGFP lifetime decreases with increasing glycerol content. This is accounted for quantitatively by the refractive index dependence of the fluorescence lifetime as predicted by the Strickler Berg formula which relates the fluorescence lifetime to the absorption spectrum. The solvent viscosity has no influence on the fluorescence lifetime. We also discuss the refractive index dependence of the GFP fluorescence lifetime in more complex systems. The findings are particularly relevant for the interpretation of FLIM of GFP expressed in environments such as bacteria and cells.
We report the first read-out module for use with single- photon timing array detectors such as multi-anode MCP-PMTs. The IBH Model 5000MXR interfaces to the time-correlated single-photon counting (TCSPC) technique using a single time-to-amplitude converter. In addition to performing established multiplexing tasks, such as simultaneous acquisition of fluorescence and excitation and anisotropy, the new module enables spectral and spatial imaging of kinetic parameters such as fluorescence lifetimes and amplitudes. The system retains the inherent advantages of TCSPC with respect to picosecond time resolution and wide dynamic range, while featuring parallel data acquisition and enhanced data acquisition rates. Unlike early TTL implementations of multiplexing which were limited to four channels, our system uses an application specific integrated circuit (ASIC) which can read out the data from up to sixteen detection channels with higher reliability and less time-dispersion. The Model 5000MXR can be packaged as a NIM standard module, packaged to serve more channels or be close coupled to detector arrays for specific applications such as microscopy and lifetime based sensors. The theory, design and performance of ASIC data read-out will be described. Other applications include photon migration in tissue, time- of-flight reflectometry/mass spectrometry and nucleonics.
Nonradiative energy transfer from perylene to Co2+ and Ni2+ ions has been investigated below the phase transition in small unilamellar vesicles. In the case of cobalt, the quenching of perylene fluorescence can be described well by long-range Foerster dipole-dipole energy transfer. Although fluorescence decay data for perylene quenched by nickel also fits the Foerster model, other evidence suggests a different quenching mechanism which is more akin to the shorter-range Dexter exchange interaction.
The theory of the statistical multiplexing of single photon timing fluorescence decay data is derived for the first time and shown to be consistent with measured array data. The present theory enables single photon timing arrays to be optimised for maximum data collection rate with regard to the number of channels in an array, channel count rates and cross-channel pile-up. Both the experimental and theoretical results prove that overall data collection rates up to 35% of the source repetition rate can be achieved by means of multiplexing with a single time-to-amplitude converter.
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