Time-correlated Single Photon Counting Technique Research Articles

Near-zero distortion in TCSPC at more than one photon per excitation period: experimental validation.

The time-correlated single-photon counting (TCSPC) technique is widely renowned for its capability of reconstructing rapid and weak light signals with exceptional sensitivity and sub-picosecond timing resolution. Unfortunately, the speed of TCSPC has been historically severely limited to avoid a phenomenon known as pileup distortion. For this reason, the count rate of a classic TCSPC acquisition channel is kept below a few percent of the laser excitation rate (usually 1%-5%). In this work, we experimentally validate a novel, to our knowledge, TCSPC theory recently reported that effectively overcomes such a limitation and finally achieves high-speed operation without distortion. Exploiting a single-photon avalanche diode (SPAD), in this paper we show how to acquire additional information about the status of the system at run time, and by combining it with the classic TCSPC data histogram, we report how a count rate of approximately 60% of the excitation frequency with near-zero distortion can indeed be achieved with a commercial system.

Peering into cloud physics using ultra-fine resolution radar and lidar systems

Abstract Cloud microphysical processes, such as droplet activation, condensational growth, and collisional growth, play a central role in the evolution of clouds and precipitation. Accurate representations of these processes in numerical models are challenging partially due to incomplete understanding of them at the process-level arising from limited systematic observations. Most surface-based active remote sensors, including today’s operational cloud radars and lidars, have a resolution on the order of tens of meters. This resolution is insufficient to resolve cloud microphysical processes that manifest at finer (meter and sub-meter) scales. A new set of ultra-high-resolution ground-based radar and lidar systems have been developed to address this observational gap. The newly developed 94-GHz cloud radar has a range resolution down to 2.8 m, or a factor of 10 finer than typical radars, using a large bandwidth and quadratic phase coding techniques. The lidar has a range resolution down to 10 cm, or a factor of 100 finer than typical lidars, using a time-gated time-correlated single photon counting technique. Such high-resolution observations were previously only achievable through in situ aircraft measurements. Even then, aircraft measurements do not permit continuous long-term cloud observation as is possible with ground-based remote sensing instruments. In this study, the first-light cloud observations from the new radar and lidar systems are shown to reveal detailed cloud structures that conventional sensors could only perceive in a bulk sense, thus providing new avenues to investigate cloud microphysical processes and their impact on weather and climate.

Bismuth (Bi3+) based lead-free halide perovskitoid for light driven applications: A potential low-toxic alternative for lead (Pb2+)

Bismuth-based perovskite-like materials are recognised as feasible optoelectronic alternatives to lead-based perovskites. High toxicity and limited environmental stability of Pb-based perovskite are the principal hurdles for their practical use, whereas Bismuth (Bi3+) based materials exhibit relatively low toxicity. In this study, we fabricated (FPEA)BiI4 (FPEA = Fluorophenylethyl amine, as a hydrophobic aromatic spacer) and explored its compatibility in the field of photocatalysis and solar cells. UV–Vis, photoluminescence (PL) and time-correlated single photon counting (TCSPC) techniques were used to understand the optoelectronic properties of the material. These fabricated thin films were used towards toxic MBT remediation. Solar cells (in n-i-p configuration) were also fabricated using (FPEA)BiI4 and their photo conversion efficiency (PCE) was tested under AM1.5 illumination.

Quantifying distortion in time-correlated single photon counting: a universal parameter.

One major drawback of the classic time-correlated single photon counting (TCSPC) technique is pileup-related distortion. To keep it under a reasonable level, the maximum count rate has to be reduced, posing a serious limitation to the overall measurement speed. This means that there is an intrinsic trade-off between speed and distortion: either count rate is raised, but distortion is worsened, or distortion is minimized at the expense of speed. In both cases, it is impossible to precisely evaluate the degree of distortion introduced. Here comes our new, to the best of our knowledge, figure of merit, which is able to provide a numerical estimate of the distortion whatever the signal shape is, marking a real turning point in the way of doing TCSPC. In this article, this new parameter will be defined and its effectiveness will be demonstrated by means of mathematical computations.

Two-photon excited luminescence of sulfur quantum dots for heavy metal ion detection.

Spectrally-resolved third-order nonlinear optical properties of water-dispersed sulfur quantum dots (SQDs) were investigated in the wavelength range from 740 nm to 820 nm with the two-photon excited emission technique using a tunable femtosecond laser system. The maximum value of the two-photon absorption (TPA) cross-section (σ2) for ∼5.4 nm size SQDs was found to be 185 GM (Goeppert-Mayer unit), while the two-photon brightness (σ2 × η) was found to be 1.5 GM at 780 nm, the wavelength being in the first biological transmittance window. The TPA properties are presented here as appropriate cross-sections normalized per molecular weight which enables meaningful comparison of the nonlinear factors of the studied quantum dots with those of various nanomaterials. The optimized TPA properties of these hydrophilic colloidal SQDs may be potentially useful for detection of Fe3+ metal ions. The experimentally determined limit of Fe3+ detection for both one- and two-photon regime was 10 μmol L-1 (0.6 μg mL-1). Förster resonance energy transfer between SQDs as donors and Fe3+ metal ions as acceptors was confirmed as one of the possible detection mechanisms using a time-correlated single photon counting technique.

Assessing the overflowing pile-up effect on the photoluminescence lifetime of nanomaterials.

The tunable complex emissive states with nanosecond to microsecond lifetimes in nanomaterials, arise due to their structural heterogeneity, enabling them with a wide range of advanced optoelectronic applications. However, understanding the complex photoluminescence lifetime in these nanomaterials is critically challenged by the overflowing pile-up effect, which occurs due to the high repetition rate of the light source in the time-correlated single photon counting (TCSPC) technique. Here, we provide a quantitative lifetime analysis, especially in metal nanoclusters, metal complexes, and semiconductor quantum dots, which suggests that the same experimental parameters can mislead the lifetime data interpretation for long-ranged luminescent nanomaterials. We demonstrate that the overflowing pile-up effect could be fatal while analyzing the excited state lifetime. Furthermore, we provide the optimized parameters that could be used to get the actual lifetime data of samples. We hope that our findings will be crucial in obtaining the error-free and accurate excited state dynamics of these long-range lifetime nanomaterials.

Pulse-sampling fluorescence lifetime imaging: evaluation of photon economy.

This Letter presents an experimental study comparing the photon rate and photon economy of pulse sampling fluorescence lifetime imaging (PS-FLIm) with the conventional time-correlated single photon counting (TCSPC) technique. We found that PS-FLIm has a significantly higher photon detection rate (200 MHz) compared with TCSPC (2-8 MHz) but lower photon economy (4-5 versus 1-1.3). The main factor contributing to the lower photon economy in PS-FLIm is laser pulse variability. These results demonstrate that PS-FLIm offers 25× faster imaging speed than TCSPC while maintaining room light rejection in clinical settings. This makes PS-FLIm a robust technique for clinical applications.

Photoluminescence of high energy xenon ion irradiated magnesium aluminate spinel single crystals

Laser confocal microscopy technique (60 ps laser pulse excitation at 445 nm) and a time-correlated single photon counting (TCSPC) technique have been used to study the photoluminescence (PL) in 152 MeV Xe ion irradiated MgAl2O4 single crystals. It was shown that radiation defects produced by swift xenon ions give rise to broad luminescence band positioned in the 470–800 nm wavelength range. The measurements of the PL decay curves have evidenced that lifetime of defect-related PL signal gradually increases from 6 to 10 ns in 500–620 nm spectral range and decreases with ion fluence increasing. The decrease of the luminescence intensity observed after fluences ∼1013 cm−2 is attributed to overlapping of individual ion track core regions.

Exploring novel and fast stability or sameness evaluation tool for different categories of injectable formulations

The establishment of drug product stability and sameness is the heart of generic formulation development. For regulatory filing, various instrumental methods are used on a case basis to establish the generic and innovator product sameness in multiple aspects. Here in the present study, we explored the applicability of the Time-correlated single photon counting (TCS-PC) technique as a fast, reliable, and nondestructive method for establishing the sameness of three different categories of injectable formulations, namely, Amphotericin B liposome for injection, enoxaparin injection, and iron sucrose injection. All three category formulations were evaluated in their native and artificially induced post degradation state to identify the discrimination power of the used instrumental techniques. The degradation of materials were confirmed by high performance liquid chromatography (HPLC). Based on the product category, pre and post-degradation samples were evaluated by selective instrumental methods like differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), fluorescence spectroscopy, particle size analysis by dynamic light scattering (DLS), small angle X-ray scattering (SAXS), Raman spectroscopy, inductively coupled plasma optical-emission spectrometry (ICP-OES) and circular dichroism study. All pre and post-degradation samples were further analyzed by TCS-PC. We observed that, TCS-PC can identify the differences between the initial and post degradation samples in very less time with promising discrimination power across the different injectable formulation types. Thus TCS-PC can be used as a fast and promising stability or sameness evaluation tool for different injectable drug products.

Quantifying the interactions of micellar drug delivery system components with albumin using time-resolved fluorescent methods.

The least requirement for the bioavailability of an active pharmaceutical ingredient (API) is its solubility in aqueous media. The solubilization of poorly soluble drugs is one of the key challenges of drug formulation development. Micellar drug delivery systems (MM-DDS) are efficient in solubilizing such APIs, are well tolerated by patients upon administration, are cost efficient and easy to produce. However, the number of marketed drugs is very limited. One problem identified by a reflection paper on this topic by the European Medicines Agency (EMA) is the number of different transformations of MM-DDS upon dilution by intravenous administration. The release of an API from MM-DDS and its subsequent binding to plasma proteins such as human serum albumin (HSA) determines the fate of the API in a human body. Tracking and characterization of such binding processes in a multicomponent system opens the door to understanding the drug - blood-components interaction. In this project we develop a label free time-resolved fluorescence tracking method to investigate the interaction of small molecules (API, MM-DDS components) with albumin. In detail, the data is collected using a time-correlated single-photon counting (TCSPC) technique at 280-nm excitation. Quenching of tryptophan fluorescence through Förster resonance energy transfer (FRET) to an API reveals information about the binding process. Affinity constants and stoichiometry can be obtained by global fitting of the titration curves and confirmed by the means of support plane analysis.

Unusual similarity of DNA solvation dynamics in high-salinity crowding with divalent cations of varying concentrations.

Double-stranded DNA bears the highest linear negative charge density (2e- per base-pair) among all biopolymers, leading to strong interactions with cations and dipolar water, resulting in the formation of a dense 'condensation layer' around DNA. Interactions involving proteins and ligands binding to DNA are primarily governed by strong electrostatic forces. Increased salt concentrations impede such electrostatic interactions - a situation that prevails in oceanic species due to their cytoplasm being enriched with salts. Nevertheless, how these interactions' dynamics are affected in crowded hypersaline environments remains largely unexplored. Here, we employ steady-state and time-resolved fluorescence Stokes shifts (TRFSS) of a DNA-bound ligand (DAPI) to investigate the static and dynamic solvation properties of DNA in the presence of two divalent cations, magnesium (Mg2+), and calcium (Ca2+) at varying high to very-high concentrations of 0.15 M, 1 M and 2 M. We compare the results to those obtained in physiological concentrations (0.15 M) of monovalent Na+ ions. Combining data from fluorescence femtosecond optical gating (FOG) and time-correlated single photon counting (TCSPC) techniques, dynamic fluorescence Stokes shifts in DNA are analysed over a broad range of time-scales, from 100 fs to 10 ns. We find that while divalent cation crowding strongly influences the DNA stability and ligand binding affinity to DNA, the dynamics of DNA solvation remain remarkably similar across a broad range of five decades in time, even in a high-salinity crowded environment with divalent cations, as compared to the physiological concentration of the Na+ ion. Steady-state and time-resolved data of the DNA-groove-bound ligand are seemingly unaffected by ion-crowding in hypersaline solution, possibly due to ions being mostly displaced by the DNA-bound ligand. Furthermore, the dynamic coupling of cations with nearby water may possibly contribute to a net-neutral effect on the overall collective solvation dynamics in DNA, owing to the strong anti-correlation of their electrostatic interaction energy fluctuations. Such dynamic scenarios may persist within the cellular environment of marine life and other biological cells that experience hypersaline conditions.

Effect of Zn2+ on emodin molecules studied by time-resolved fluorescence spectroscopy and quantum chemical calculations

Emodin is a natural drug for treating neurodegenerative diseases and plays a vital role in the mitigation of nerve damage. Metal ions can modify the drug properties of emodin, where Zn2+ can synergize with the emodin molecule and enhance the drug effect of emodin. Besides, complex changes can be observed in the fluorescence intensity and fluorescence lifetime of the emodin molecule as the concentration of Zn2+ increases. Herein, the synergistic effects of ligand structural in Zn(II)-Emodin complexes and the electronic effects of metal elements on the antioxidant properties of the complexes are discussed in detail based on UV–vis absorption spectroscopy, fluorescence spectroscopy, time-correlated single photon counting (TCSPC) technique and quantum chemical calculations at the B3LYP/6-31G(d) level. The experimental results confirm that Zn2+ can coordinate with the hydroxyl groups on the emodin to make the molecule structure more rigid, thus inhibiting the non-radiative processes such as high-frequency vibrations of the emodin molecule in solution. The suppression of non-radiative processes leads to an increase in the average fluorescence lifetime of the emodin molecule, and finally results in the enhanced fluorescence intensity. The chemical softness of Zn(II)-Emodin is then confirmed to be higher than that of emodin by Gaussian calculations, indicating its higher chemical reactivity and lower stability. The stronger electron donating ability of Zn(II)-Emodin compared to emodin may explain the higher antioxidant activity of Zn(II)-Emodin, which gives it a stronger pharmacological activity. The results of this study show that emodin can well complex with Zn2+ to remove excess Zn2+ in human body and the resulting complex has better antioxidant properties, which helps to understand the role of Zn2+ in drug-metal coordination and provides guidance for the design of new drugs.

Spectral and kinetic properties of silver sulfide quantum dots in an external electric field

The effect of an external electric field on the luminescence characteristics of silver sulfide nanoparticles embedded in a film based on an optically passive dielectric matrix has been studied. The luminescence characteristics were studied using methods of optical and time-resolved spectroscopy involving the time-correlated single-photon counting technique. The morphology of the nanoparticles was studied using transmission electron microscopy. It was shown that in an external electric field, an increase in the intensity of the recombination luminescence band is observed for silver sulfide nanoparticles, together with an increase in the electronic relaxation rate. This effect is explained by the fact that the electric field enhances the transport of free holes to electron traps which are radiative recombination centers. The observed effects indicate that silver sulfide nanoparticles can be effectively used as active layers of organic light-emitting diodes, where an external field of the order of 500 kV/cm will not lead to a deterioration in their operating luminescence characteristics.

Temperature-dependent ultrafast solvation dynamics of choline chloride-based deep eutectic solvent (DES) and hydroxyl functionalized room temperature ionic liquids (RTILs): Exploring the difference in solvent response between DES and RTILs

The present study aims to provide a comprehensive understanding of the difference in solvent relaxation behavior between DESs and RTILs. For this purpose, dynamics of solvation have been investigated in a choline chloride-based DES and three hydroxyls functionalized ILs, keeping the hydroxyl functionalities same in both classes of media, so that the role of various inter/intra molecular interactions, including hydrogen bonding interaction on the solvent relaxation behavior can be understood. Fluorescence up-conversion spectroscopy (FLUPS) techniques coupled with time-correlated single-photon counting (TCSPC) techniques have been exploited to study the complete Stokes shift dynamics of a dipolar probe, coumarin 153 (C153) dissolved in these media. The solvent response functions generated from the complete dynamic Stokes shift data reveal a bimodal solvent relaxation behaviour having a very fast sub-picosecond and a relatively slower picosecond to sub-nanosecond solvation time component for both DES and hydroxyl ILs. The relatively slower solvation time component, which correlates with bulk viscosity of the concerned medium, is found to arise due to the diffusional motion of the constituents of both DES and hydroxyl ILs. However, temperature-dependent solvent relaxation measurements have revealed that at nearly isoviscous conditions, the solvent relaxation is much faster in the DES as compared to that in hydroxyl IL indicating the differences in solvent relaxation behaviour at the later part of the solvent response. Interestingly, when the early part of the dynamics is monitored, the amplitude associated with the ultrafast component for the DES increases significantly with the increase in temperature, whereas the same remains almost unchanged for the hydroxyl IL. This essentially indicates that even at shorter time scale, the process of solvent relaxation is considerably different for both DES and hydroxyl ILs. All these investigations have essentially demonstrated that despite having similar functionalities, different motions related to the solvent relaxation, operating at microscopic level in both these classes of solvents, are significantly different from each other.

Improved transient electroluminescence technique based on time-correlated single-photon counting technology to evaluate organic mobility

The transient electroluminescence (EL) technique is widely used to evaluate the carrier mobility in the field of organic light emitting diodes. The traditional analog detection strategy using oscilloscopes is generally limited since the background noise causes an underestimation of the mobility value. In this paper, we utilize time-correlated single-photon counting (TCSPC) to probe the transient EL for mobility calculation. The measurements on tris(8-hydroxyquinoline) aluminum (Alq3) show that the electron mobilities obtained using the TCSPC technique are slightly higher than those obtained from the analog method at all the investigated voltages. Moreover, the TCSPC mobilities demonstrate weaker dependence on the root of electrical field compared to the oscilloscope mobilities. These improvements are attributed to the unique principle of TCSPC, which quantifies the EL intensity by counting the number of single-photon pulses, improving its single-photon sensitivity and eliminating the negative impacts of electrical noise. These advantages make TCSPC a powerful technique in the characterization of time-resolved electroluminescence.Graphical

Size-dependent spectroscopic insight into the steady-state and time-resolved optical properties of ZnO photocatalysts

In this paper, we present a spectroscopic insight into the structural and optical properties of ZnO nanoparticles (NPs) applied for water depollution using the photocatalytic process. Samples of various sizes and shapes were synthesized using simple, cost-effective chemical methods. The steady-state optical properties were analyzed using absorption and fluorescence emission spectroscopy and the band-gap and emission energies revealed an inverse dependence on the ZnO particles size. The time-resolved fluorescence properties were studied using time-correlated single photon counting (TCSPC) technique. Three emissive states were identified for all ZnO samples, with lifetimes that showed an overall decrease with increasing the particles diameter. The insights into the photocatalytic process toward understanding the ZnO nanoparticles mediated photocatalysis for degradation of Rhodamine (RhB) and oxytetracycline (OTC) was elucidated. The increase of photocatalytic activity was due to charge carrier partition and mobility.

Enhancing Intersystem Crossing by Intermolecular Dimer-Stacking of Cyanine as Photosensitizer for Cancer Therapy

Enhancing Intersystem Crossing by Intermolecular Dimer-Stacking of Cyanine as Photosensitizer for Cancer Therapy

Laser attenuation in falling snow correlated with measurements of snow particle size distribution

We investigate transmission through falling snow using a 3D imaging lidar at 1.56-μm wavelength. The lidar is based on the time-correlated single-photon counting technique. Experimental transmission data are compared with Mie theory transmission calculations based on snow particle size distribution simultaneously measured with a laser disdrometer. The calculations were performed in two ways, using the Beer–Lambert approach where all radiation interacting with a hydrometeor is considered extinct and an approach that includes effects of the forward scattering. Comparison of these two methods shows that inclusion of the contribution from forward scattering gives better agreement between experiment and calculations than using extinction only. When comparing the results using scattering calculations with a curve fit approach based on precipitation rate, it is evident that both the Beer–Lambert approach and the forward scattering approach give a much better correlation between experiment and calculations than relying on precipitation rate, as measured with the disdrometer, only.

Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings

Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings

Single-Molecule, Non-Chromatic Discrimination in Binary Fluorophores Mixtures

SummarySingle biomolecule sensing often involves quantification of multiple fluorescent species, coupled for example to different DNAs or proteins resides. While chromatic separation of the fluorescent species is well-established and broadly used it involves several limitations, particularly when the total photon budget is limited. Here we theoretically and experimentally explore the use of time-resolved fluorescence via Time Correlated Single Photon Counting (TCSPC) to accurately quantify multiple fluorescent species. A modified maximum likelihood estimator (MLE) is introduced to include two fluorophore species, with the compensation of the instrument response. We apply this algorithm to simulated data of a simplified two fluorescent species model, as well as to experimental data of free fluorophores mixtures and doubly labelled proteins in order to estimate the errors in the two species ratios as a function of photon budget available. Moreover, focusing on the case of labeled proteins in which the ratios of the two species per molecule is a discrete variable subject to the chemical labeling numbers of the two dyes, we obtain that between 1,000 to 2,000 photons can already provide an accurate estimation of the dyes’ ratio in labelled Ovalbumin proteins. These values correspond to roughly 100 to 200 photons per fluorophore suggesting a realistic path for dyes ratio determination from a single passage of a protein in the detection volume in roughly 10 ms timescale. Our results highlight the way in which temporal limitations of the TCSPC technique could hinder its implementation in systems with fast occurring events such as translocation of biomolecules through nanopores, or fast diffusion in single-molecule burst analyses experiments and provide estimation for the desired photon integration time.