Department of Chemistry

Permanent URI for this collectionhttp://localhost:4000/handle/123456789/1924

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Start date: 2000Subject: search.filters.subject.Fluorescence

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Now showing 1 - 10 of 25
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    Mechanistic insights into the C-myc G-quadruplex and berberine binding inside an aqueous two-phase system mimicking biomolecular condensates
    (ACS, 2024-08) Patra, Satyajit
    We investigated the binding between the c-MYC G-quadruplex (GQ) and berberine chloride (BCl) in an aqueous two-phase system (ATPS) with 12.3 wt % polyethylene glycol and 5.6 wt % dextran, mimicking the highly crowded intracellular biomolecular condensates formed via liquid–liquid phase separation. We found that in the ATPS, complex formation is significantly altered, leading to an increase in affinity and a change in the stoichiometry of the complex with respect to neat buffer conditions. Thermodynamic studies reveal that binding becomes more thermodynamically favorable in the ATPS due to entropic effects, as the strong excluded volume effect inside ATPS droplets reduces the entropic penalty associated with binding. Finally, the binding affinity of BCl for the c-MYC GQ is higher than those for other DNA structures, indicating potential specific interactions. Overall, these findings will be helpful in the design of potential drugs targeting the c-MYC GQ structures in cancer-related biocondensates.
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    Luminescent triazolo-fused pyrido[3,4-b]pyrazines as novel fluorophores
    (Elsevier, 2024-12) Khungar, Bharti; Sakhuja, Rajeev
    A series of organic fluorophores based on novel [1,2,4]triazolo [4′,3':1,2]pyrido [3,4-b]pyrazine architecture were synthesized by a simple two-step protocol. Synthesized organic fluorophores exhibit blue-to-green fluorescence and their UV absorption and fluorescent emission properties were studied. The photophysical properties of synthesized derivatives were tuned by introducing functionalities of varying electronic nature on the core nucleus. The luminescence maxima of the synthesized compounds are found in the range from 427 nm to 503 nm, and the quantum yields ranging from 0.01 to 0.64 were calculated. Among various synthesized molecules, 3-(4-bromophenyl)-8,9-bis(4-methoxyphenyl)-[1,2,4]triazolo [4′,3':1,2]pyrido [3,4-b]pyrazine showed highest quantum yield (ΦF = 0.64), and was further explored for its solvatochromic, acidochromic, aggregation induced fluorescence studies along with theoretical HOMO-LUMO calculations
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    Contrasting Solid-State Fluorescence of Diynes with Small and Large Aryl Substituents: Crystal Packing Dependence and Stimuli-Responsive Fluorescence Switching
    (ACS, 2015-09) Pati, Avik K.
    There has been a significant current interest in solid state luminescence of organic molecules and their stimuli responsive fluorescence switching behavior. Although small organic derivatives with olefinic, acetylenic, phenylenevinylenic, phenyleneethynylenic spacers are widely documented as solid state emitters in the literature, the solid state photophysics of organic derivatives with “butadiyne” spacer still remains unexplored. We provide detailed investigation on the solid state fluorescence properties of a series of butadiynyl fluorophores. Replacement of a phenyl ring, which is at periphery of the butadiyne bridge, with a large moiety such as pyrenyl group furnishes contrasting emissions in the solid state. While the butadiyne bridged phenyl derivatives show a blue shift of emission maxima in the solid powder with respect to monomer spectra in solution state, the butadiyne bridged pyrenyl derivatives exhibit a red shift in the solid state. The blue shift of the emission maxima of the butadiyne bridged phenyl derivatives in the solid powder is attributed to allowed excitonic transition in aggregates with nearly parallel transition dipoles. On the other hand, formation of pyrenyl excimer accounts for the red shift of the butadiyne bridged pyrenyl derivatives in the solid powder. In addition to that, the solid state fluorescence of the pyrenyl analogues is reversibly switched between two aggregate forms through external heating and rubbing stimuli.
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    Photophysics of Diphenylbutadiynes in Water, Acetonitrile–Water, and Acetonitrile Solvent Systems: Application to Single Component White Light Emission
    (ACS, 2016-07) Pati, Avik K.
    Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile–water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (−NMe2)–pull (−CN) substituents and those are closely overlapped for non-push–pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (−NMe2)–pull (−CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push–pull diphenylbutadiyne together cover the visible region (400–700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push–pull character.
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    White Light Emission in Butadiyne Bridged Pyrene–Phenyl Hybrid Fluorophore: Understanding the Photophysical Importance of Diyne Spacer and Utilizing the Excited-State Photophysics for Vapor Detection
    (ACS, 2016-07) Pati, Avik K.
    Generation of white light emission (WLE) from a single organic fluorophore is challenging because of the need to get fluorescence covering the visible region (400–700 nm) upon excitation of the dye at near-ultraviolet wavelength. Herein, we report WLE from a butadiyne bridged pyrene–phenyl hybrid fluorophore in mixed-aqueous solvents as well as in polymer film matrices. The ability of the butadiynyl dye to emit from multiple excited states such as locally excited (LE; 400–500 nm), aggregate (excimer type; 475–600 nm), and charge transfer (CT; 500–750 nm) states spanning the emission almost throughout the visible range has made the generation of the white light to be possible. In highly polar solvent such as acetonitrile, the butadiynyl dye emits from the LE and CT states, and the WLE is achieved through a control of the dye concentration such that intermolecular CT (exciplex type) contributes along with the intramolecular CT and LE emissions. In mixed-aqueous systems such as water–acetonitrile and water–N,N-dimethylformamide, the CT emission is red-shifted (because of the high dielctric constant of water), and the contribution of the aggregate emission (originated because of the poor solvent water) is important in maintaining the relative distribution of the fluorescence intensities (LE, excimer, and CT) in the entire visible region. The significance of the diyne spacer in achieving the WLE is delineated through a control study with a single acetylenic analogue. The LE, aggregate, and CT emissions are involved in generating bluish-white light in a poly(vinyl alcohol) film matrix of the butadiynyl dye. Blue emission is noted in a poly(methyl methacrylate) (PMMA) film matrix of the dye with a major contribution from the LE and a minor contribution from the aggregate state. Exposure of the PMMA film of the dye to polar aprotic vapors assists in gaining the CT state emission such that the LE, aggregate, CT emissions cover the entire visible region to produce the WLE. This opens a new strategy for selective vapor sensing.
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    Long-Range Single-Molecule Förster Resonance Energy Transfer between Alexa Dyes in Zero-Mode Waveguides
    (ACS, 2020-03) Patra, Satyajit
    Zero-mode waveguide (ZMW) nano-apertures milled in metal films were proposed to improve the Förster resonance energy transfer (FRET) efficiency and enable single-molecule FRET detection beyond the 10 nm barrier, overcoming the restrictions of diffraction-limited detection in a homogeneous medium. However, the earlier ZMW demonstrations were limited to the Atto 550–Atto 647N fluorophore pair, asking the question whether the FRET enhancement observation was an artifact related to this specific set of fluorescent dyes. Here, we use Alexa Fluor 546 and Alexa Fluor 647 to investigate single-molecule FRET at large donor–acceptor separations exceeding 10 nm inside ZMWs. These Alexa fluorescent dyes feature a markedly different chemical structure, surface charge, and hydrophobicity as compared to their Atto counterparts. Our single molecule data on Alexa 546–Alexa 647 demonstrate enhanced FRET efficiencies at large separations exceeding 10 nm, extending the spatial range available for FRET and confirming the earlier conclusions. By showing that the FRET enhancement inside a ZMW does not depend on the set of fluorescent dyes, this report is an important step to establish the relevance of ZMWs to extend the sensitivity and detection range of FRET, while preserving its ability to work on regular fluorescent dye pairs.
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    A Fluorescence Correlation Spectroscopy, Steady-State, and Time-Resolved Fluorescence Study of the Modulation of Photophysical Properties of Mercaptopropionic Acid Capped CdTe Quantum Dots upon Exposure to Light
    (ACS, 2013-10) Patra, Satyajit
    Light-induced modulation of the fluorescence behavior of mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs) in aqueous solution is studied by a combination of fluorescence correlation spectroscopy (FCS) and steady-state and time-resolved fluorescence techniques. These investigations reveal a dramatic variation in the fluorescence properties of the QDs under exposure to light. In the FCS measurement, a large decrease in amplitude and change in shape of the correlation curves are observed with increasing excitation power. The change in the shape of the correlation curves, particularly at short lag time, e.g., a faster relaxation at high excitation power, is attributed to the increasing contribution of the off state of the QDs. Interestingly, despite this increasing contribution of the off state, which reduces the effective number of emitters in the observation volume and hence should increase the amplitude of the correlation curve, the latter actually decreases at high excitation power. This apparent contradiction is resolved by considering light-induced transformation of the dark QDs to bright QDs due to surface passivation of the QDs with increasing excitation power. Enhancement of the steady-state fluorescence intensity under light irradiation, both in aerated and deaerated environments, supports the mechanism of passivation of the surface trap states by photoadsorption of water molecules. Fluorescence lifetime data is also shown to be consistent with this light-induced surface passivation mechanism.
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    Effect of Capping Agent and Medium on Light-Induced Variation of the Luminescence Properties of CdTe Quantum Dots: A Study Based on Fluorescence Correlation Spectroscopy, Steady State and Time-Resolved Fluorescence Techniques
    (ACS, 2014-07) Patra, Satyajit
    The influence of ligand and solvent on light-induced modulation of the emission behavior of the quantum dots (QDs) has been studied for CdTe QDs capped with hexadecylamine (HDA), mercaptopropionic acid (MPA), and 1-(1-undecanethiol)-3-methyl imidazolium bromide (SMIM) in CHCl3, H2O, and [bmim][PF6] ionic liquid, respectively, using steady state and time-resolved fluorescence and fluorescence correlation spectroscopy techniques. While an aqueous solution of CdTe/MPA QDs exhibits fluorescence enhancement and a small blue shift of the emission peak (λmaxem) in the early stages of the light irradiation, such enhancement could not be observed in the case of CHCl3 solution of CdTe/HDA and [bmim][PF6] solution of CdTe/SMIM. Instead, exposure to light leads to a rapid reduction in luminescence intensity and large blue shift of λmaxem in the case of CHCl3 solution of CdTe/HDA and a very slow decrease of luminescence intensity with negligible shift of λmaxem in the case of an ionic liquid solution of CdTe/SMIM. The time-resolved fluorescence behavior of the QDs is found to be consistent with the steady state results. Fluorescence correlation spectroscopy measurements on the other hand reveal a large decrease of the amplitude of correlation at time zero [G(0)] for the aqueous solution of CdTe/MPA and negligible change in the G(0) value for the ionic liquid solution of CdTe/SMIM with increasing excitation power. The mechanism of these light-induced changes of the luminescence behavior of the QDs is investigated.
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    Diffusion of organic dyes in bovine serum albumin solution studied by fluorescence correlation spectroscopy
    (RSC, 2012-05) Patra, Satyajit
    The understanding of the transport of drugs and naturally occurring molecules in living cells and tissues requires a thorough knowledge of the diffusion behaviour of the molecular systems in these media. In this work, we studied the translational diffusion of three fluorescent molecules, electrically neutral coumarin 102 (C102), cationic rhodamine 6G (R6G) and anionic fluorescein (FL) in phosphate-buffered (pH 7) aqueous solutions of bovine serum albumin (BSA) protein in the absence and presence of common salt and urea using fluorescence correlation spectroscopy (FCS) by monitoring the fluorescence intensity fluctuations in a small confocal observation volume. The diffusion due to both free and BSA-bound molecules is observed in the case of the C102-BSA system. While no exchange between the bound and free states of the molecule is observed in this case, a rapid exchange between the two states is observed in the case of electrically charged hydrophilic dyes R6G and FL. This molecular picture, which is the first of its kind, is a reflection of a weaker binding of R6G and FL compared to C102 with the protein molecule. The binding sites of the probe molecules in BSA were identified based on the urea-induced change of diffusion of the probes in BSA.
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    Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides
    (ACS, 2019-09) Patra, Satyajit
    Single molecule detection provides detailed information about molecular structures and functions but it generally requires the presence of a fluorescent marker which can interfere with the activity of the target molecule or complicate the sample production. Detecting a single protein with its natural UV autofluorescence is an attractive approach to avoid all the issues related to fluorescence labeling. However, the UV autofluorescence signal from a single protein is generally extremely weak. Here, we use aluminum plasmonics to enhance the tryptophan autofluorescence emission of single proteins in the UV range. Zero-mode waveguide nanoapertures enable the observation of the UV fluorescence of single label-free β-galactosidase proteins with increased brightness, microsecond transit times, and operation at micromolar concentrations. We demonstrate quantitative measurements of the local concentration, diffusion coefficient, and hydrodynamic radius of the label-free protein over a broad range of zero-mode waveguide diameters. Although the plasmonic fluorescence enhancement has generated a tremendous interest in the visible and near-infrared parts of the spectrum, this work pushes further the limits of plasmonic-enhanced single molecule detection into the UV range and constitutes a major step forward in our ability to interrogate single proteins in their native state at physiological concentrations.