Rise-time of FRET-acceptor fluorescence tracks protein folding

Waveguided energy transfer in pseudo-two-dimensional systems

Resonance energy transfer (RET) is an important and ubiquitous process whereby energy is transferred from a donor chromophore to an acceptor chromophore without contact via Coulombic coupling. There have been a number of recent advances exploiting the quantum electrodynamics (QED) framework for RET. Here, we extend the QED RET theory to investigate whether real photon exchange can allow for excitation transfer over very long distances if the exchanged photon is waveguided. To study this problem, we consider RET in two spatial dimensions. We derive the RET matrix element using QED in two dimensions, consider an even greater confinement by deriving the RET matrix element for a two-dimensional waveguide using ray theory, and compare the resulting RET elements in 3D and 2D and for the 2D waveguide. We see greatly enhanced RET rates over long distances for both the 2D and 2D waveguide systems and see a great preference for transverse photon mediated transfer in the 2D waveguide system.

Intense up-conversion luminescence from Dy3+-doped multi-component telluroborate glass matrix: Role of CuO nanoparticles embedment

This paper reports an intense up-conversion luminescence from Dy³⁺-doped strontium-telluro-alumino-magnesium-borate glasses for the first time. The samples were made via the melt-quenching method and characterized to determine the influence of various CuO nanoparticles contents change on their up-conversion emission traits. Absorption spectral data were used to calculate the Judd-Ofelt intensity parameters. The sample without CuO nanoparticles revealed two intense photoluminescence up-conversion emission peaks at 478 and 570 nm. In addition, CuO nanoparticles-activated sample displayed about 1.4-fold up-conversion emission intensity improvement due to strong light absorption in the visible to the infrared region at 799 nm excitation. The stimulated emission cross-section of the CuO nanoparticles-activated glasses was increased from 102.4 × 10^-23 to 1301.1 × 10^-23 cm² (nearly 10-fold amplification) while the branching ratio was reduced to 66.9%. Thus, CuO nanoparticles as an additive in the current glass matrix enhanced the up-conversion emission and strengthened the associated nonlinear optical properties. CIE 1931 color matching revealed the influence of CuO in modifying the up-conversion color coordinates, thereby improving the white color purity. The achieved up-conversion emission coupled with the color tunability of the proposed glasses may be advantageous for the up-conversion UV tunable laser making.

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

Artificial Light-Harvesting System with White-Light Emission in a Bicontinuous Ionic Medium

Artificial light-harvesting systems (ALHSs), which are closely related to Förster resonance energy transfer (FRET), are among the most attractive scientific topics during the past few decades. Specifically, binary ALHSs that are composed of a fluid donor and acceptor have a simplified composition and high number density of the donor units. However, largely due to the difficulty in obtaining a fluid donor, investigation of these systems is still quite limited, especially for the ionic systems. Herein, we report a new type of binary ALHS using an ionic naphthalimide (NPI) derivative as a donor, which shows greatly improved photoluminescence for its bicontinuous liquid structure. When blending with an acceptor such as rhodamine 6G or trans-4-[4-(dimethylamino)styryl]-methylpyridinium iodide, efficient FRET was confirmed by both experimental results and molecular dynamics simulations, with an energy transfer efficiency up to ∼90%. Tunable color, including white-light emission, was achieved by tuning the acceptor/donor ratio, opening the door for a variety of applications such as light-emitting diodes and photoluminescent inks.

Competition between Photoinduced Electron Transfer and Resonance Energy Transfer in an Example of Substituted Cytochrome c-Quantum Dot Systems

Colloidal quantum dots (QDs) are nanoparticles that are able to photoreduce redox proteins by electron transfer (ET). QDs are also able to transfer energy by resonance energy transfer (RET). Here, we address the question of the competition between these two routes of QDs’ excitation quenching, using cadmium telluride QDs and cytochrome c (CytC) or its metal-substituted derivatives. We used both oxidized and reduced versions of native CytC, as well as fluorescent, nonreducible Zn(II)CytC, Sn(II)CytC, and metal-free porphyrin CytC. We found that all of the CytC versions quench QD fluorescence, although the interaction may be described differently in terms of static and dynamic quenching. QDs may be quenchers of fluorescent CytC derivatives, with significant differences in effectiveness depending on QD size. SnCytC and porphyrin CytC increased the rate of Fe(III)CytC photoreduction, and Fe(II)CytC slightly decreased the rate and ZnCytC presence significantly decreased the rate and final level of reduced FeCytC. These might be partially explained by the tendency to form a stable complex between protein and QDs, which promoted RET and collisional quenching. Our findings show that there is a net preference for photoinduced ET over other ways of energy transfer, at least partially, due to a lack of donors, regenerating a hole at QDs and leading to irreversibility of ET events. There may also be a common part of pathways leading to photoinduced ET and RET. The nature of synergistic action observed in some cases allows the hypothesis that RET may be an additional way to power up the ET.

Spectroscopic Evidence of Energy Transfer in BODIPY-Incorporated Nano-Porphyrinic Metal-Organic Frameworks

Metal–organic frameworks (MOFs) represent a class of solid-state hybrid compounds consisting of multitopic organic struts and metal-based nodes that are interconnected by coordination bonds, and they are ideal for light harvesting due to their highly ordered structure. These structures can be constructed with chromophore organic ligands structures for the purpose of efficient light harvesting. Here, we prepared porphyrin-based nano-scaled MOFs (nPCN-222) with BODIPY and I₂BODIPY photosensitizers by incorporating BODIPY/I₂BODIPY into nPCN-222 (nPCN-BDP/nPCN-I₂BDP) and demonstrated resonance energy transfer from the donor (BODIPY/I₂BODIPY) to the acceptor (nPCN-222) resulting in greatly enhanced fluorescence of nPCN-222, as visually manifested by time-resolved and space-resolved fluorescence imaging of the nano-scaled MOFs.

Gradient heterostructure perovskite single crystals enable the improvement of radiative recombination for scintillator application

Halide funnel-typed perovskite materials have been synthesized by simply solution-processed method. This gradient heterostructure materials have been investigated that helpful for charges radiative recombination and suppress non-radiative recombination in the interfaces.

Reply to the 'Comment on ""Where does the fluorescing moiety reside in a carbon dot?"- Investigations based on fluorescence anisotropy decay and resonance energy transfer dynamics"' by H. C. Joshi, Phys. Chem. Chem. Phys., 2019, 21, DOI: 10.1039/c9cp00136k

The claim that the analysis regarding resonance energy transfer should have been made using different equations than those that we have used is negated based on the following points: (1) we are well aware of the equations the author has provided in his comment. The equation (eqn (3) mentioned below) that the author has written is undoubtedly too simple to describe the complex system delineated in our original paper. This particular equation is perhaps OK for simple dye (donor and acceptor) systems; however, such a simple equation is never enough for nanoparticle/quantum dot systems. (2) Another equation suggested by the author in his comment (eqn (2)) contains a parameter called donor concentration in excited state. We have categorically described in page 6-7 of our original paper why it is difficult to measure the donor concentration accurately even in the ground state. When the donor concentration can't be known accurately it can't be used in the suggested equation. (3) Donor-acceptor distance calculated by eqn (3)/Table 1 provided by the author deviates more than 100% from the distance that is physically feasible. Such kinds of problems are well documented in the literature. (4) One of the papers cited by the author in his comment and many other published papers clearly mention that in the case when all donor molecules/particles do not take part in the resonance energy transfer process or the stoichiometry of a donor-acceptor complex is not known or deviates strongly from 1 : 1, especially in quantum dots or any other nanomaterial system, it is not possible to extract accurate dynamical information related to RET from donor decay. Instead risetime of acceptor yields much more accurate information. Such situations do arise in our system as well.

Comment on ""Where does the fluorescing moiety reside in a carbon dot?"- Investigations based on fluorescence anisotropy decay and resonance energy transfer dynamics" by A. Das, D. Roy, C. K. De and P. K. Mandal, Phys. Chem. Chem. Phys., 2018, 20, 2251

In a recent paper published in Physical Chemistry Chemical Physics (Phys. Chem. Chem. Phys., 2018, 20, 2251-2259), Förster resonance energy transfer (FRET) between carbon dots and rhodamine 123 has been reported. The FRET rates and hence donor acceptor distances estimated from acceptor rise time for various acceptor concentrations appear to be incorrect. In the present comment need for a correct analysis in the case of such a scenario is addressed.

Metal Halide Perovskite Nanosheet for X-ray High-Resolution Scintillation Imaging Screens

Scintillators, which are capable of converting ionizing radiation into visible photons, are an integral part of medical, security, and commercial diagnostic technologies such as X-ray imaging, nuclear cameras, and computed tomography. Conventional scintillator fabrication typically involves high-temperature sintering, generating agglomerated powders or large bulk crystals, which pose major challenges for device integration and processability. On the other hand, colloidal quantum dot scintillators cannot be cast into compact solid films with the necessary thickness required for most X-ray applications. Here, we report the room-temperature synthesis of a colloidal scintillator comprising CsPbBr₃ nanosheets of large concentration (up to 150 mg/mL). The CsPbBr₃ colloid exhibits a light yield (∼21000 photons/MeV) higher than that of the commercially available Ce:LuAG single-crystal scintillator (∼18000 photons/MeV). Scintillators based on these nanosheets display both strong radioluminescence (RL) and long-term stability under X-ray illumination. Importantly, the colloidal scintillator can be readily cast into a uniform crack-free large-area film (8.5 × 8.5 cm² in area) with the requisite thickness for high-resolution X-ray imaging applications. We showcase prototype applications of these high-quality scintillating films as X-ray imaging screens for a cellphone panel and a standard central processing unit chip. Our radiography prototype combines large-area processability with high resolution and a strong penetration ability to sheath materials, such as resin and silicon. We reveal an energy transfer process inside those stacked nanosheet solids that is responsible for their superb scintillation performance. Our findings demonstrate a large-area solution-processed scintillator of stable and efficient RL as a promising approach for low-cost radiography and X-ray imaging applications.

A One Donor-Two Acceptor Lipid Bilayer FRET Assay Based on Asymmetrically Labeled Liposomes

The fusion of two opposing membranes is essential in biological functions such as fertilization, viral entry, membrane trafficking and synaptic transmission. Before the membrane bilayers are fully connected, at some stage a hemifusion intermediate-when the outer leaflets are merged but not the inner leaflets-is formed. However, the position of hemifusion in the energy landscape and the duration of it vary and have not been fully mapped out. To date, there has not been a way to differentiate lipid mixing of the two leaflets directly in a single experiment. Herein we demonstrate labeling of the outer and inner leaflets with different fluorophores, which can be distinguished by their fluorescence lifetimes. As a proof of concept, the asymmetrically labeled liposomes were used as acceptor liposomes in a novel one donor-two acceptor Förster resonance energy transfer (FRET) assay to monitor membrane fusion reactions mediated by the synaptic proteins soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) in microfluidic devices. Initial hemifusion was clearly indicated by the acceptor fluorescence lifetime originating solely from FRET acceptors on the outer leaflet (Oregon Green 488; τ_Fl ∼ 4.8 ns). Progression to full fusion was then indicated by the significantly increasing lifetime contribution from acceptors on the inner leaflet (nitrobenzoxadiazole; τ_Fl ∼ 6.7 ns). The new labeling strategy creates many possibilities in the design of bulk and single-molecule experiments.

Triangulating Nucleic Acid Conformations Using Multicolor Surface Energy Transfer

Optical ruler methods employing multiple fluorescent labels offer great potential for correlating distances among several sites, but are generally limited to interlabel distances under 10 nm and suffer from complications due to spectral overlap. Here we demonstrate a multicolor surface energy transfer (McSET) technique able to triangulate multiple points on a biopolymer, allowing for analysis of global structure in complex biomolecules. McSET couples the competitive energy transfer pathways of Förster Resonance Energy Transfer (FRET) with gold-nanoparticle mediated Surface Energy Transfer (SET) in order to correlate systematically labeled points on the structure at distances greater than 10 nm and with reduced spectral overlap. To demonstrate the McSET method, the structures of a linear B-DNA and a more complex folded RNA ribozyme were analyzed within the McSET mathematical framework. The improved multicolor optical ruler method takes advantage of the broad spectral range and distances achievable when using a gold nanoparticle as the lowest energy acceptor. The ability to report distance information simultaneously across multiple length scales, short-range (10-50 Å), mid-range (50-150 Å), and long-range (150-350 Å), distinguishes this approach from other multicolor energy transfer methods.

Gradual Folding of an Off-Pathway Molten Globule Detected at the Single-Molecule Level

Molten globules (MGs) are compact, partially folded intermediates that are transiently present during folding of many proteins. These intermediates reside on or off the folding pathway to native protein. Conformational evolution during folding of off-pathway MGs is largely unexplored. Here, we characterize the denaturant-dependent structure of apoflavodoxin's off-pathway MG. Using single-molecule fluorescence resonance energy transfer (smFRET), we follow conversion of unfolded species into MG down to denaturant concentrations that favor formation of native protein. Under strongly denaturing conditions, fluorescence resonance energy transfer histograms show a single peak, arising from unfolded protein. The smFRET efficiency distribution shifts to higher value upon decreasing denaturant concentration because the MG folds. Strikingly, upon approaching native conditions, the fluorescence resonance energy transfer efficiency of the MG rises above that of native protein. Thus, smFRET exposes the misfolded nature of apoflavodoxin's off-pathway MG. We show that conversion of unfolded into MG protein is a gradual, second-order-like process that simultaneously involves separate regions within the polypeptide.

Macrocrystals of Colloidal Quantum Dots in Anthracene: Exciton Transfer and Polarized Emission

In this work, centimeter-scale macrocrystals of nonpolar colloidal quantum dots (QDs) incorporated into anthracene were grown for the first time. The exciton transfer from the anthracene host to acceptor QDs was systematically investigated, and anisotropic emission from the isotropic QDs in the anthracene macrocrystals was discovered. Results showed a decreasing photoluminescence lifetime of the donor anthracene, indicating a strengthening energy transfer with increasing QD concentration in the macrocrystals. With the anisotropy study, QDs inside the anthracene host acquired a polarization ratio of ~1.5 at 0° collection angle, and this increases to ~2.5 at the collection angle of 60°. A proof-of-concept application of these excitonic macrocrystals as tunable color converters on light-emitting diodes was also demonstrated.