Time-correlated Single-photon Counting Spectroscopy Research Articles

Waste toner–derived carbon dots for bioimaging and photodynamic therapy

Printer toner, which accounts for a large fraction of electrical and electronic wastes, poses serious environmental and health risks but cannot be efficiently recycled because of its high chemical durability and complexity. Specifically, only 20–30% of waste toner is recycled, while much of the remainder ends up in landfills and thus contributes to air/soil pollution and respiratory disease development. Herein, we report the one-pot hydrothermal processing of waste toner into well-dispersed and uniform-sized carbon dots featuring abundant surface defects (e.g., radicals and dangling bonds), strongly interacting with UV (200–400 nm) light, and exhibiting intense photoluminescence in both UV (300 nm) and visible (400–500 nm) ranges. In particular, ethanolamine is used to chemically disintegrate the carbon black cores of toner particles into nanoparticles and simultaneously passivate the nanoparticle surface. According to the results of time-correlated single photon counting spectroscopy measurements, UV- and visible-region photoluminescence originates from amorphous carbon structures in the core and oxygen/nitrogen-containing functional groups on the surface, respectively. Finally, in view of their excellent visible-region optical properties and the ability to generate reactive oxygen species under laser irradiation, the waste toner–derived carbon dots are demonstrated to be promising agents for bioimaging and photodynamic therapy, respectively.

Correlating Broadband Photoluminescence with Structural Dynamics in Layered Hybrid Halide Perovskites.

The emission of white light from a single material is atypical and is of interest for solid-state lighting applications. Broadband light emission has been observed in some layered perovskite derivatives, A2PbBr4 (A = R-NH3+), and correlates with static structural distortions corresponding to out-of-plane tilting of the lead bromide octahedra. While materials with different organic cations can yield distinct out-of-plane tilts, the underlying origin of the octahedral tilting remains poorly understood. Using high energy resolution (e.g., quasi-elastic) neutron scattering, this contribution details the rotational dynamics of the organic cations in A2PbBr4 materials where A = n-butylammonium (nBA), 1,8-diaminooctammonium (ODA), and 4-aminobutyric acid (GABA). The organic cation dynamics differentiate (nBA)2PbBr4 from (ODA)PbBr4 or (GABA)2PbBr4 in that the larger spatial extent of dynamics of nBA yields a larger effective cation radius. The larger effective volume of the nBA cation in (nBA)2PbBr4 yields a closer to ideal A-site geometry, preventing the out-of-plane tilt and broadband luminescence. In all three compounds, we observe hydrogen dynamics attributed to rotation of the ammonium headgroup and at a time scale faster than the white light photoluminescence studied by time-correlated single photon counting spectroscopy. This supports a previous assignment of the broadband emission as resulting from a single ensemble, such that the emissive excited state experiences many local structures faster than the emissive decay. The findings presented here highlight the role of the organic cation and its dynamics in hybrid organic-inorganic perovskites and white light emission.

Dynamic synthesis of CdTe NRs: Diameter dependent tuning of PL quenching efficiency for sensitive organic vapor detection

Cadmium telluride (CdTe) is one of the well-explored semiconductors in the history of material science research, because of its innumerable benefits in all dimensional nanostructures. However, no endeavor has been dedicated as of now to understanding the size-dependent tuning of nonradiative emission efficiency and how it influences the sensitive detection of organic vapors (ethanol and acetone) through photoluminescence (PL) quenching mechanism. Herein, the aspect ratio of CdTe nanorods (NRs) is varied by changing its diameter while keeping the axial length constant through electrodeposition into lab made anodic aluminium oxide (AAO) membranes. The growth kinetics and thermodynamics involved while electrodeposition of CdTe NR in each distinct pore diameter of AAO membrane is studied and reasoned. The surface morphology of variable diameter AAO membranes and NRs, respectively fabricated under various electrochemical parameters are observed through a field emission scanning electron microscope. The relation between NRs dimension and structural parameters like d-spacing, crystallite size, band area ratio, and exciton-phonon ratio are extracted from X-ray diffraction and Resonance Raman spectroscopy. The strongly luminescent CdTe NRs, coupled with their chemical durability and organic molecule affinity displayed a sensitive response of ~12 and 6 ppm to ethanol and acetone vapors in the range of 10–100 ppm as a PL sensor. The carrier lifetime measurement through time-correlated single photon counting spectroscopy endorses the role of increased recombination density with a decrease in the diameter of NRs.

High-Efficiency Excitation Energy Transfer in Biohybrid Quantum Dot-Bacterial Reaction Center Nanoconjugates.

Reaction centers (RCs) are the pivotal component of natural photosystems, converting solar energy into the potential difference between separated electrons and holes that is used to power much of biology. RCs from anoxygenic purple photosynthetic bacteria such as Rhodobacter sphaeroides only weakly absorb much of the visible region of the solar spectrum, which limits their overall light-harvesting capacity. For in vitro applications such as biohybrid photodevices, this deficiency can be addressed by effectively coupling RCs with synthetic light-harvesting materials. Here, we studied the time scale and efficiency of Förster resonance energy transfer (FRET) in a nanoconjugate assembled from a synthetic quantum dot (QD) antenna and a tailored RC engineered to be fluorescent. Time-correlated single-photon counting spectroscopy of biohybrid conjugates enabled the direct determination of FRET from QDs to attached RCs on a time scale of 26.6 ± 0.1 ns and with a high efficiency of 0.75 ± 0.01.

Semi-autonomous real-time programmable fluorescence lifetime segmentation with a digital micromirror device.

Time-correlated single-photon counting (TCSPC) is the gold standard for performing lifetime spectroscopy in biological assays. Traditional fluorescence lifetime imaging (FLIM) using laser scanning microscopes are inherently slow due to point scanning all pixels in the field-of-view. Wide-field implementations of TCSPC spectroscopy using microchannel plates benefit from particularly fast acquisition times at the expense of temporal resolution, and are fundamentally limited by photon counting rates. Here, we introduce programmable lifetime imaging (PLI), combining the advantages of wide-field imaging using total internal reflection excitation with state-of-the-art TCSPC detector technology for accurate lifetime determination in an object-oriented manner using a digital micromirror device (DMD). The fluorescent emission is projected onto the DMD to facilitate the sequential segmentation of fluorescence from individual objects in the field-of-view, allowing for both image acquisition and fluorescence lifetime determination of the assay. The sensitivity of PLI is demonstrated by manually segmenting fluorescence from fixed cell assays. We also demonstrate an automated implementation of PLI, using a camera as a feedback mechanism to segment fluorescence produced by emitting objects of interest in the imaging field-of-view, highlighting the advantages of measurement only in areas where valuable information exists. As a result, PLI is able to reduce acquisition time of fluorescence lifetime data by at least an order of magnitude compared to laser scanning implementations.

Spectroscopic Characterization and Mechanistic Studies on Visible Light Photoredox Carbon–Carbon Bond Formation by Bis(arylimino)acenaphthene Copper Photosensitizers

Currently, the most popular molecular photosensitizers used for synthetic organic chemistry and energy applications are still the noble-metal-based ruthenium and iridium complexes that usually require expensive metal and ligand precursors. In contrast, bis(arylimino)acenaphthene (Ar-BIAN) is an established redox noninnocent π-accepting ligand that is easily assembled in one condensation step from affordable and commercially available precursors. Herein, we have developed a series of Ar-BIAN CuI complexes as visible-light-harvesting photosensitizers. Notably, one of these panchromatic, homoleptic Ar-BIAN CuI complexes exhibits a radiative recombination lifetime that is longer than diffusion-controlled reactions, as observed by time-correlated single-photon counting spectroscopy. Ar-BIAN CuI facilitates visible-light-promoted atom-transfer radical addition reactions via carbon–carbon bond formation with CBr3 radicals in good yields of up to 75%. Steady-state and transient absorption spectroscopic measuremen...

Measurement of the lifetime of the 7sS1/22 state in atomic cesium using asynchronous gated detection

We report a measurement of the lifetime of the cesium $7s\,^2S_{1/2}$ state using time-correlated single-photon counting spectroscopy in a vapor cell. We excite the atoms using a Doppler-free two-photon transition from the $6s\,^2S_{1/2}$ ground state, and detect the 1.47$\mu$m photons from the spontaneous decay of the $7s\,^2S_{1/2}$ to the $6p\,^2P_{3/2}$ state. We use a gated single photon detector in an asynchronous mode, allowing us to capture the fluorescence profile for a window much larger than the detector gate length. Analysis of the exponential decay of the photon count yields a $7s\,^2S_{1/2}$ lifetime of 48.28$\pm$0.07ns, an uncertainty of 0.14%. These measurements provide sensitive tests of theoretical models of the Cs atom, which play a central role in parity violation measurements.

Spectroscopic Study of a Photoactive Antibacterial Agent: 2,3-Distyrylindole.

Optical properties and fluorescence decay dynamics of a photoactive indole based antibacterial chromophore system, 2,3-distyrylindole (23DSI), were investigated using various spectroscopic characterization techniques. Experimental studies were done by utilizing steady-state UV-vis spectroscopy, steady-state fluorescence spectroscopy, time-resolved fluorescence upconversion spectroscopy, and time-correlated single-photon counting spectroscopy. Our studies show that the 23DSI molecule has a multiphoton absorption property as indicated by two- and three-photon absorption in the both the solution and the solid phases. The ultrafast time-resolved fluorescence upconversion studies show that this molecule undergoes a fast decay process with an average time constant of 34 ps, a single exponential decay, and an average fluorescence lifetime of 1 ns. The compound 23DSI did not show any signs of singlet oxygen production. The density functional theory (DFT) calculations showed that the 23DSI molecule has conjugated electron densities that are responsible for multiphoton absorption. The chlorine-substituted styryl groups, attached to the central indole ring facilitate the excellent electron delocalization within the molecule. This optimal electron delocalization, combined with the good electron conjugation in the 23DSI molecule is important for efficient multiphoton absorption and is in excellent agreement with experimental observations. Both the optical spectrum and emission spectrum using DFT calculations are also surprisingly well matched with the experimentally measured UV-vis spectrum and the emission spectrum, respectively. Combined experimental and theoretical studies suggest that excited electrons initially relax to the singlet state (S1) by internal conversion (IC) and subsequently relax back to their ground state by emitting absorbed energy as fluorescence emission. The outstanding multiphoton absorption capabilities of this 23DSI molecule support its potential application in both biological imaging and photodynamic inactivation (PDI).

Luminescent water-soluble cycloplatinated complexes: Structural, photophysical, electrochemical and chiroptical properties

The luminescent square-planar Pt(II) complexes of the type [Pt(C^N)(N^N)]+X-, where (C^N) is the cyclometallated 2-phenylpyridine ligand, (N^N) a chiral diamine and BF4− or CH3COO− as counterion (X−) have been synthesized. The chiral diamine Campy has been used, resulting in a mixture of regioisomers. The complexes were characterized and their structures in solution were determined by a combination of different spectroscopic and chiroptical techniques (NMR, UV–Vis, ECD, IR, VCD and CPL) associated with electrochemical, DFT and TDDFT studies. Remarkably, the acetate derivatives proved to be soluble and stable in water for at least a month. The electrochemical behaviour of the [Pt(C^N)(N^N)]BF4 was investigated and the LUMO energies estimated. The photophysical properties of the new compounds were deeply investigated in water or dimethyl sulfoxide solution by UV–Vis, steady state emission and time-correlated single-photon counting spectroscopy.

Luminescent chiral ionic Ir(III) complexes: Synthesis and photophysical properties

Three homologous series of luminescent octahedral ionic Ir(III) complexes (1–12) with a dual stereogenic center of general formula Δ,Λ (R,S)[(ppy)2Ir(R-campy)]X, where ppy=2-phenylpyridine, R-campy=2-methyl-5,6,7,8-tetrahydroquinolin-8-amine (Me-campy) or 8-amino-5,6,7,8-tetrahydroquinolines (H-campy) and as counterions X−=Cl− or CH3COO− have been synthesized and characterized. The NMR characterization of each complex highlighted the diastereoisomeric purity and the absolute configuration has been confirmed by Electronic Circular Dichroism spectroscopy. The absorption and the luminescence properties of the compounds in solution and in solid state have been investigated by UV–vis, steady-state emission and time-correlated single-photon counting spectroscopy. The obtained results from the 12 compounds highlight the difficult to correlate photophysical properties in solution to the stereochemistry, while excited states decay studies of the solid state samples indicate a correlation between photophysics and packing mode which is affected by the different stereochemistry.

Spectroscopic and Nonlinear Optical Properties of 3, (4-tert-butylphenoxy) Phthalocyanine Constitutional Isomers

Introduction Organic material with second-order nonlinear optical (NLO) material properties are useful in applications such as optical data processing or storage devices and can be easily processed and integrated into optical devices [1, 2]. Organic materials such as phthalocyanines (Pcs) exhibit large and fast nonlinearities, which can be fine-tuned by rational modification of the molecular structure [3, 4, 5]. Pc molecules are known to have large third-order optical nonlinearities which arise from the highly delocalised two dimensional 18-electron system [6, 7, 8, 5]. Pcs have been intensively investigated for their NLO properties. In most studies, NLO properties of tetra substituted Pcs have been studied as a mixture of isomers [9]. Aims In this work four constitutional isomers of unmetalated tert-butylphenoxy phthalocyanine were separated and their NLO properties measured, in order to identify which isomer contributes the most to the measured NLO response.This work presents spectroscopic and nonlinear optical properties of unmetalated 3, (4-tert-butylphenoxy) phthalocyanine constitutional isomers. Second order nonlinear polarizability, second order hyperpolarizability and third order imaginary susceptibility (Im[χ(3)]) values were determined for monomeric and aggregated constitutional isomers. Z-scan technique is employed in determining the second order NLO property. Spectroscopic and photophysical properties of the isomers, such as fluorescence lifetime and rotational correlation lifetimes are determined. Magnetic circular dichroism (MCD), time correlated single photon counting spectroscopy (TCSPC), ultra-violet visible (UV/vis) spectroscopy, Infra-red (IR) spectroscopy and density functional theory (DFT) are some of the techniques used to characterise physical properties of the isomers. References G. de la Torre, P. Va´zquez, F. Agullo´-Lo´pez, T. Torres, J. Mater. Chem., 1998, 8, 1671-1683.T. Verbiest, S. Houbrechts, M. Kauranen, K. Clays, A. Persoons, J. Mater. Chem., 1997, 7, 2175-2189J. M. Fox, T. J. Katz, S. V. Elshocht, T. Verbiest, M. Kauranen, A. Persoons, T. Thongpanchang, T. Krauss, L. Brus, J. Am. Chem. Soc.,1999, 121, 3453-3459E. M. Maya, A. W. Snow, J. S. Shirk, R. G. S. Pong, S. R. Flom, G. L. Roberts, J. Mater. Chem., 2003, 13, 1603-1613G. de la Torre, P. Vazquez, F. Agullo-Lopez and T. Torres, J. Mater. Chem., 1998, 8, 1671-1683.J. Britton, M. Durmus ̧ S. Khene, V.Chauke, T. Nyokong, J. Porphyrins Phthalocyanine, 2013, 17, 692-702.C. Mkhize, J. Britton, T. Nyokong, Polyhedron, 2014, 81, 607-613.J. Britton, M. Durmuş, V. Chauke, T. Nyokong, Journal of Molecular Structure, 2013, 1054-1055, 209-214.C. Nitschke, S. M. O’Flaherty, M. Kroll, J. J. Doyle, W. J. Blau, Chemical Physics Letters, 2004, 383, 555-560.

Systematic study of defect-related quenching of NV luminescence in diamond with time-correlated single-photon counting spectroscopy

We report on the systematic characterization of photoluminescence (PL) lifetimes in NV- and NV0 centers in 2 MeV H+ implanted type Ib diamond samples by means of a time correlated single photon counting (TCSPC) microscopy technique. A dipole-dipole resonant energy transfer model was applied to interpret the experimental results, allowing a quantitative correlation of the concentration of both native (single substitutional nitrogen atoms) and ion-induced (isolated vacancies) PL-quenching defects with the measured PL lifetimes. The TCSPC measurements were carried out in both frontal (i.e. laser beam probing the main sample surface along the same normal direction of the previously implanted ions) and lateral (i.e. laser beam probing the lateral sample surface orthogonally with respect to the same ion implantation direction) geometries. In particular, the latter geometry allowed a direct probing of the centers lifetime along the strongly nonuniform damage profiles of MeV ions in the crystal. The extrapolation of empirical quasi-exponential decay parameters allowed the systematic estimation of the mean quantum efficiency of the centers as a function of intrinsic and ion-induced defect concentration, which is of direct relevance for the current studies on the use of diamond color centers for photonic applications.

Calcium-Dependent Operation of the Human BK Channel Gating Ring Apparatus

The large cytoplasmic C-terminal domain (CTD) of the human BK channel forms a gating ring structure composed by two tandem RCK domains serving as a signal transducer for intracellular Ca2+ and other ligands. However, the mechanism of the gating ring's operation remains unknown. We have used steady-state and time-resolved spectroscopy in combination with dynamic light scattering to the Ca-induced conformational changes of the purified CTD of human BK channel. The CTD domain of the human BK channel assembles as a tetrameric gating ring structure (MW∼310 kDa) with a hydrodynamic radius (RH) ∼10.5 nm. In the presence of 35 μM free Ca2+, RH reversibly decreases to ∼7.5 nm. The modulation of the gating ring hydrodynamic shape suggests that it undergoes Ca2+-induced conformational transitions, which we further, characterized using time-correlated single-photon counting spectroscopy. Increasing free [Ca2+] up to 35 μM shortened the average Trp fluorescence lifetime (τavg) of the wild-type gating ring from ∼2.6ns to ∼1.9ns, while the neutralization of the high-affinity Ca-binding site within RCK2 (Ca bowl, D894-898N) attenuated the effect of Ca2+. Steady-state fluorescence analysis revealed that these ligand-dependent structural rearrangements of the gating ring possess strong divalent cation selectivity. The gating ring exhibited i) high-affinity Ca2+-binding (Khalf_1 ∼0.3μM and Khalf_2 ∼4μM); ii) significantly lowered affinity for Mg2+ (Khalf ∼200μM) and iii) no Ba2+ sensitivity up to 13mM, consistent with the lack of Ba2+-dependent BK channel activation. Interestingly, Ca2+ bowl neutralization eliminated Mg2+ sensing up to 12mM. In summary, under physiologically-relevant conditions, these ligand-induced conformational transitions are strongly ion-specific and associated with changes in the hydrodynamic properties of the BK gating ring and likely represent the ligand-induced molecular events underlying channel activation.

Critical flocculation concentrations, binding isotherms, and ligand exchange properties of peptide-modified gold nanoparticles studied by UV-visible, fluorescence, and time-correlated single photon counting spectroscopies.

Protocols for modifying gold nanoparticles with peptide-bovine serum albumin (BSA) conjugates are described within. The resulting constructs were characterized using a number of techniques including static fluorescence spectroscopy and time-correlated single photon counting spectroscopy (TCSPC) in order to quantify peptide-BSA binding isotherms, exchange rates, critical flocculation concentrations, and the composition of mixed peptide-BSA monolayers on gold nanoparticles. TCSPC has proven to be a powerful technique for observing the microenvironment of protein-gold nanoparticle conjugates because it can distinguish between surface-bound and solution-phase species without the need for separation steps. Full characterization of the composition and stability of peptide-modified metal nanoparticles is an important step in their use as intracellular delivery vectors and imaging agents.

Natively fluorescent isoflavones exhibiting anomalous Stokes’ shifts

The fluorescence behaviour of 19 flavonoids was studied. Three isoflavones (formononetin (F), ononin (FG) and daidzein (D)) were found to exhibit large Stokes’ shifts, possibly due to a change of the structure of the molecule from non-planar in the S 0 state to planar in the S 1 state. Lifetime measurements were carried out using time-correlated single photon counting spectroscopy to further characterise the mechanism. These large shifts provide a high selectivity, so that fluorescent isoflavones can be readily detected in plant samples by means of reversed-phase LC with fluorescence detection. Attention has to be paid to possibly fluorescent impurities in flavonoid standards, as was observed for daidzin (DG), which is not fluorescent itself, but has a fluorescent isomer. To distinguish between the two compounds, LC with fluorescence and MS detection was used to separate and identify the impurity.

Measurement of the lifetime of the atomic cesium 5^2D_5/2 state with diode-laser excitation

The lifetime of the atomic cesium 5(2)D(5/2) state was measured with time-correlated single-photon counting spectroscopy. Ground-state cesium atoms were excited with a diode laser by use of an electric quadrupole transition. Analysis of the exponential decay of the cascade photons from the 6(2)P(3/2) state yields a 5(2)D(5/2) lifetime of 1225(12) ns.

Structural measurements of perylene clusters with cyclopropane via rotational coherence spectroscopy

Rotational coherence experiments using picosecond time-correlated single-photon counting spectroscopy have measured the polarized fluorescence time profiles of van der Waals aggregates of perylene with up to three cyclopropane molecules. Three structural isomers have been confirmed at the 2:1 level of aggregation, one of which contains a contacted cyclopropane dimer. The other two are symmetrically distinct isomers of a (1|1) species. The 3:1 aggregate also exhibits two resolvable structures. The structures of all complexes studied are shown to be generated by the off-center adsorption site of the cyclopropane molecule, evidently dictated by short-range repulsive forces.

ChemInform Abstract: Fluorescence Quenching of Some Aromatic Amines by Ethylene Trithiocarbonate in Solution.

The interaction of some aromatic amines such as o-aminobenzoic acid, methyl anthranilate, and o-m-p-toluidines with very dilute solutions of ethylene trithiocarbonate (ETTC) have been studied employing steady-state fluorescence quenching measurements and time-correlated single-photon counting spectroscopy. Electronic excitation transfer and static quenching is assumed to describe the higher bimolecular rate constants for the toluidine–ETTC systems. In addition, the possible interpretation of the quenching mechanism is discussed in relation to the electron-transfer between colliding species.

Fluorescence quenching of some aromatic amines by ethylene trithiocarbonate in solution

The interaction of some aromatic amines such as o-aminobenzoic acid, methyl anthranilate, and o-m-p-toluidines with very dilute solutions of ethylene trithiocarbonate (ETTC) have been studied employing steady-state fluorescence quenching measurements and time-correlated single-photon counting spectroscopy. Electronic excitation transfer and static quenching is assumed to describe the higher bimolecular rate constants for the toluidine–ETTC systems. In addition, the possible interpretation of the quenching mechanism is discussed in relation to the electron-transfer between colliding species.