Acceptor Emission Research Articles

Interfacial exciplex formation in bilayers of conjugated polymers

The donor-acceptor interactions in sequential bilayer and blend films are investigated. Steady-state and time-resolved photoluminescence (PL) were measured to characterize the samples at different geometries of photoluminescence collection. At standard excitation, with the laser incidence at 45° of the normal direction of the sample surface, a band related to the aggregate states of donor molecules appears for both blend and bilayer at around 540 nm. For the PL spectra acquired from the edge of the bilayer, with the laser incidence made at normal direction of the sample surface (90° geometry), a new featureless band emission, red-shifted from donor and acceptor emission regions was observed and assigned as the emission from interfacial exciplex states. The conformational complexity coming from donor/acceptor interactions at the heterojunction interface of the bilayer is at the origin of this interfacial exciplex emission.

Near-IR Emissive Chlorin–Bacteriochlorin Energy-Transfer Dyads with a Common Donor and Acceptors with Tunable Emission Wavelength

Design, synthesis, and optical properties of a series of novel chlorin-bacteriochlorin energy transfer dyads are described. Each dyad is composed of a common red-absorbing (645-646 nm) chlorin, as an energy donor, and a different near-IR emitting bacteriochlorin, as an energy acceptor. Each bacteriochlorin acceptor is equipped with a different set of auxochromes, so that each of them emits at a different wavelength. Dyads exhibit an efficient energy transfer (≥0.77) even for chlorin-bacteriochlorin pairs with large (up to 122 nm) separation between donor emission and acceptor absorption. Excitation of the chlorin donor results in relatively strong emission of the bacteriochlorin acceptor, with a quantum yield Φf range of 0.155-0.23 in toluene and 0.12-0.185 in DMF. The narrow, tunable emission band of bacteriochlorins enables the selection of a series of three dyads with well-resolved emissions at 732, 760, and 788 nm, and common excitation at 645 nm. Selected dyads have been also converted into bioconjugatable N-succinamide ester derivatives. The optical properties of the described dyads make them promising candidates for development of a family of near-IR fluorophores for simultaneous imaging of multiple targets, where the whole set of fluorophores can be excited with the common wavelength, and fluorescence from each can be independently detected.

Donor–Acceptor Random Copolyesters containing Perylenebisimide (PBI) and Oligo(p-phenylene vinylene) (OPV) by Melt Condensation Polymerization: Energy Transfer Studies

Novel copolyesters consisting of oligo(p-phenylene vinylene) (OPV) as donor (D) and perylenebisimide (PBI) as acceptor (A) were synthesized by melt polycondensation. Photoinduced energy transfer and photoinduced charge separation in these polyesters were studied in solution as well as in the solid state. Selective excitation of OPV moiety resulted in the energy transfer with >90% efficiency from OPV to PBI chromophore in the solution state. The direct excitation of PBI in the D-A copolyester resulted in reduced fluorescence emission of acceptor, indicating electron transfer between the D and A moieties. The effect of distance between donor and acceptor on the energy transfer efficiency from donor to acceptor was studied. Compared to a physical mixture of D and A polyesters alone, the energy transfer was 4 times more efficient in the D-A copolyester, highlighting the influence of covalently linking D and A in a single polymer chain. A strong fluorescence quenching (∼ 100%) of both chromophores in solid state indicated an efficient photoinduced charge transfer after photoexcitation of either D or A. Thus, OPV-PBI main chain copolyester is an excellent system for the study of energy- and electron-transfer processes in organic semiconductor. Reactive blend of D/A copolyester was also prepared by the transesterification reaction between D and A alone copolyesters. The energy transfer efficiency from D to A moiety upon selective excitation of D chromophore in the D/A copolyester blend was ∼4 times higher compared to a physical mixture of D and A alone copolyesters, which gave direct proof for the transesterification reaction in polyester/polyester reactive blending.

Kaede for Detection of Protein Oligomerization

Photoconvertible fluorescent proteins such as Kaede are routinely used for tracking proteins, organelles, and whole cells. Kaede was the first identified photoconvertible fluorescent protein and has since become the most commonly used photoconvertible fluorescent protein in vertebrates. Kaede can be irreversibly converted from a green to a red fluorescent form upon UV/blue light irradiation and fluorescence of each form can be isolated separately by appropriate filter sets. Spectral properties of the Kaede forms allow Förster resonance energy transfer (FRET) from the green form as donor to the red form as acceptor. As a sample containing oligomerized Kaede-containing proteins is exposed to UV or blue light, FRET first increases as green Kaede is converted to red and then decreases as the green donor becomes depleted. Thus, FRET information is potentially obtained from a number of independent measurements taken as photoconversion proceeds. We demonstrate here the application of this approach to detect homo-aggregation and conformational dynamics of plant protein constructs. Structural alterations of 2-cys peroxiredoxin–Kaede were successfully detected depending on the redox state in living plant cells. Photoconversion was performed gradually and donor emission, acceptor emission, and FRET-derived sensitized acceptor emission were measured at each step of conversion. Since photoconvertible proteins have not been routinely used in plants, two plasmids have been designed to facilitate plant applications. The plasmids allow either transient expression of Kaede-containing protein constructs in plant cells or Gateway cloning and stable transformation of plants.

Fluorescence Resonance Energy Transfer in Microemulsions Composed of Tripled-Chain Surface Active Ionic Liquids, RTILs, and Biological Solvent: An Excitation Wavelength Dependence Study

In this article we have reported the fluorescence resonance energy transfer (FRET) study in our earlier characterized surface active ionic liquids (SAILs)-containing microemulsion, i.e., N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([P13][Tf2N])/[CTA][AOT]/isopropyl myristate ([IPM]) and N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide ([N3111][Tf2N])/[CTA][AOT]/[IPM] microemulsions (Banerjee, C.; Mandal, S.; Ghosh, S.; Kuchlyan, J.; Kundu, N.; Sarkar, N. J. Phys. Chem. B 2013, 117, 3927-3934). The occurrence of effective FRET from the donor, coumarin-153 (C-153) to the acceptor rhodamine 6G (R6G) is evident from the decrease in the steady state fluorescence intensity of the donor with addition of acceptor and subsequent increase in the fluorescence intensity of the acceptor in the presence of donor. The excitation wavelength dependent FRET from C-153 to R6G has also been performed to assess the dynamic heterogeneity of these confined systems. In time-resolved experiments, the significant rise time of the acceptor in the presence of the donor further confirms the occurrence of FRET. The multiple donor-acceptor (D-A) distances, for various microemulsions, obtained from the rise times of the acceptor emission in the presence of a donor can be rationalized from the varying distribution of the donor, C-153, in the different regions of the microemulsion. Time-resolved measurement reveals that with increasing excitation wavelength from 408 to 440 nm, the contribution of the faster rise component of FRET increases significantly due to the close proximity of the C-153 and R6G in the polar region of the microemulsion where occurrence of FRET is very high. Moreover, we have also studied the FRET with variation of R (R = [room temperature ionic liquids (RTILs)]/[surfactant]) and shown that the effect of excitation wavelength on FRET is similar irrespective of R values.

Observation of Selective Plasmon-Exciton Coupling in Nonradiative Energy Transfer: Donor-Selective versus Acceptor-Selective Plexcitons

We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Förster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complementary case of acceptor-selective plexciton we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor- and acceptor-selective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new techniques in FRET-based fluorescence microscopy.

Internal Calibration Förster Resonance Energy Transfer Assay: A Real-Time Approach for Determining Protease Kinetics

Förster resonance energy transfer (FRET) technology has been widely used in biological and biomedical research. This powerful tool can elucidate protein interactions in either a dynamic or steady state. We recently developed a series of FRET-based technologies to determine protein interaction dissociation constant and for use in high-throughput screening assays of SUMOylation. SUMO (small ubiquitin-like modifier) is conjugated to substrates through an enzymatic cascade. This important posttranslational protein modification is critical for multiple biological processes. Sentrin/SUMO-specific proteases (SENPs) act as endopeptidases to process the pre-SUMO or as isopeptidases to deconjugate SUMO from its substrate. Here, we describe a novel quantitative FRET-based protease assay for determining the kinetics of SENP1. Our strategy is based on the quantitative analysis and differentiation of fluorescent emission signals at the FRET acceptor emission wavelengths. Those fluorescent emission signals consist of three components: the FRET signal and the fluorescent emissions of donor (CyPet) and acceptor (YPet). Unlike our previous method in which donor and acceptor direct emissions were excluded by standard curves, the three fluorescent emissions were determined quantitatively during the SENP digestion process from onesample. New mathematical algorithms were developed to determine digested substrate concentrations directly from the FRET signal and donor/acceptor direct emissions. The kinetic parameters, kcat, KM, and catalytic efficiency (kcat/KM) of SENP1 catalytic domain for pre-SUMO1/2/3 were derived. Importantly, the general principles of this new quantitative methodology of FRET-based protease kinetic determinations can be applied to other proteases in a robust and systems biology approach.

A Novel Ionic Liquid-in-Oil Microemulsion Composed of Biologically Acceptable Components: An Excitation Wavelength Dependent Fluorescence Resonance Energy Transfer Study

In this work we have reported the formulation of a novel ionic liquid-in-oil (IL/O) microemulsion where the polar core of the ionic liquid, 1-ethyl-3-methylimidazolium n-butylsulfate ([C2mim][C4SO4]), is stabilized by a mixture of two nontoxic nonionic surfactants, polyoxyethylene sorbitan monooleate (Tween-80) and sorbitan laurate (Span-20), in a biological oil phase of isopropyl myristate (IPM). The formation of the microemulsion droplets has been confirmed from the dynamic light scattering (DLS) and phase behavior study. To assess the dynamic heterogeneity of this tween-based IL/O microemulsion, we have performed an excitation wavelength dependent fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G). The multiple donor-acceptor (D-A) distances, ∼15, 30, and 45 Å, obtained from the rise times of the acceptor emission in the presence of a donor can be rationalized from the varying distribution of the donor, C480, in the different regions of the microemulsion system. With increasing the excitation wavelength from 375 to 408 nm, the contribution of the rise component of ∼240 ps which results the D-A distance of ∼30 Å increases significantly due to the enhanced contribution of the C480 probe molecules closer to the acceptor in the ionic liquid pool of the microemulsion.

Quantitative FRET (Förster Resonance Energy Transfer) Analysis for SENP1 Protease Kinetics Determination

Reversible posttranslational modifications of proteins with ubiquitin or ubiquitin-like proteins (Ubls) are widely used to dynamically regulate protein activity and have diverse roles in many biological processes. For example, SUMO covalently modifies a large number or proteins with important roles in many cellular processes, including cell-cycle regulation, cell survival and death, DNA damage response, and stress response 1-5. SENP, as SUMO-specific protease, functions as an endopeptidase in the maturation of SUMO precursors or as an isopeptidase to remove SUMO from its target proteins and refresh the SUMOylation cycle (1,3,6,7). The catalytic efficiency or specificity of an enzyme is best characterized by the ratio of the kinetic constants, kcat/KM. In several studies, the kinetic parameters of SUMO-SENP pairs have been determined by various methods, including polyacrylamide gel-based western-blot, radioactive-labeled substrate, fluorescent compound or protein labeled substrate (8-13). However, the polyacrylamide-gel-based techniques, which used the "native" proteins but are laborious and technically demanding, that do not readily lend themselves to detailed quantitative analysis. The obtained kcat/KM from studies using tetrapeptides or proteins with an ACC (7-amino-4-carbamoylmetylcoumarin) or AMC (7-amino-4-methylcoumarin) fluorophore were either up to two orders of magnitude lower than the natural substrates or cannot clearly differentiate the iso- and endopeptidase activities of SENPs. Recently, FRET-based protease assays were used to study the deubiquitinating enzymes (DUBs) or SENPs with the FRET pair of cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) (9,10,14,15). The ratio of acceptor emission to donor emission was used as the quantitative parameter for FRET signal monitor for protease activity determination. However, this method ignored signal cross-contaminations at the acceptor and donor emission wavelengths by acceptor and donor self-fluorescence and thus was not accurate. We developed a novel highly sensitive and quantitative FRET-based protease assay for determining the kinetic parameters of pre-SUMO1 maturation by SENP1. An engineered FRET pair CyPet and YPet with significantly improved FRET efficiency and fluorescence quantum yield, were used to generate the CyPet-(pre-SUMO1)-YPet substrate (16). We differentiated and quantified absolute fluorescence signals contributed by the donor and acceptor and FRET at the acceptor and emission wavelengths, respectively. The value of kcat/KM was obtained as (3.2 ± 0.55) x10(7) M(-1)s(-1) of SENP1 toward pre-SUMO1, which is in agreement with general enzymatic kinetic parameters. Therefore, this methodology is valid and can be used as a general approach to characterize other proteases as well.

The three-dimensional context of a double helix determines the fluorescence of the internucleoside-tethered pair of fluorophores.

We report a general phenomenon of the formation of either a fluorescent or an entirely quenched oligodeoxynucleotide (ODN) duplex system by hybridizing pairs of complementary ODNs with identical chemical composition. The ODNs carried internucleoside tether-linked cyanines, where the cyanines were chosen to form a Förster's resonance energy transfer (FRET) donor-acceptor pair. The fluorescent and quenched ODN duplex systems differed only in that the cyanines linked to the respective ODNs were linked either closer to the 5'- or 3'-ends of the molecule. In either case, however, the dyes were separated by an identical number (7 or 8) of base pairs. Characterization by molecular modeling and energy minimization using a conformational search algorithm in a molecular operating environment (MOE) revealed that linking of the dyes closer to the 5'-ends resulted in their reciprocal orientation across the major groove which allowed a closely interacting dye pair to be formed. This overlap between the donor and acceptor dye molecules resulted in changes in absorbance spectra consistent with the formation of H-aggregates. Conversely, dyes linked closer to 3'-ends exhibited emissive FRET and formed a pair of dyes that interacted with the DNA helix only weakly. Induced CD spectra analysis suggested that interaction with the double helix was weaker than in the case of the closely interacting cyanine dye pair. Linking the dyes such that the base pair separation was 10 or 0 favored energy transfer with subsequent acceptor emission. Our results suggest that when interpreting FRET measurements from nucleic acids, the use of a "spectroscopic ruler" principle which takes into account the 3D helical context of the double helix will allow more accurate interpretation of fluorescence emission.

Diffusion-Enhanced FRET between Membrane Proteins

Forster resonance energy transfer (FRET) is often used to study interactions between membrane proteins. The use of long-lived (∼10−3 sec) lanthanide donors allows for a delay between excitation and detection of emission, greatly enhancing the signal-to-noise ratio. However, this long excited state lifetime also allows donor- and acceptor-labeled membrane proteins to diffuse significant distances towards each other while the donor is excited, potentially enhancing energy transfer. Here we show that diffusion enhances lanthanide-based FRET (LRET) between membrane proteins, and that this phenomenon provides information concerning membrane protein organization. We constructed tetracycline-inducible cell lines expressing SNAP-tagged glycosylphosphatidylinositol (GPI)-anchored proteins (SNAP-GPI) and β2 adrenergic receptors (SNAP-β2AR), labeled these proteins with donor (Tb2+) and acceptor fluorophores, and measured FRET while varying diffusion. The FRET signal (sensitized emission) between SNAP-GPI proteins decreased by 62 ± 3% (P<0.001; n=4; mean ± S.D.) when these proteins were immobilized with aldehyde fixation. Similarly, immobilization decreased FRET between SNAP-β2AR by 76 ± 2% (P<0.001; n=4). Fixation did not change acceptor emission after direct excitation, nor did it change donor emission in the absence of an acceptor. Similar FRET decreases were observed when mobility was decreased by crosslinking with biotin and avidin, or by decreasing temperature. The effect of decreased mobility on FRET was evident for both proteins across a wide range of expression levels. These results suggest that lateral diffusion significantly enhances FRET between membrane proteins. Because diffusion can only enhance FRET between donors and acceptors that diffuse independently (i.e. are not part of the same complex), these results further suggest that a substantial fraction of GPI-anchored proteins and β2 adrenergic receptors exist on the cell surface as rapidly-diffusing monomers.Supported by NIH grants GM078319 and GM096762.

Effect of Metal Nanoparticle Concentration on Localized Surface Plasmon Mediated Förster Resonant Energy Transfer

The influence of gold nanoparticle concentration on signatures of localized surface plasmon mediated Forster resonant energy transfer is investigated in a quantum dot–gold nanoparticle sandwich structure. At lower gold nanoparticle concentrations, localized surface plasmon mediated Forster resonant energy transfer enhancement of the acceptor emission is observed. At higher gold nanoparticle concentrations, the acceptor emission is reduced despite faster localized surface plasmon enhanced Forster resonant energy transfer rates being achieved. This is attributed to competition between localized surface plasmon mediated Forster resonant energy transfer and gold nanoparticle quenching effects.

Fabrication and characterization of highly ordered phosphorus-doped ZnO nanocombs

The high-quality phosphorus-doped ZnO nanocombs were synthesized by the simple chemical vapor deposition method without use of any catalyst. The images of field-emission scanning electron microscopy show that the phosphorus-doped ZnO nanocombs have a uniform length of 8μm with the diameter of 70nm and the cusp of the teeth of 20nm. The measurement results of selected-area electron diffraction pattern indicate that the phosphorus-doped ZnO nanocombs are single crystalline structure. The phosphorus related acceptor emission was observed in the photoluminescence spectra of phosphorus-doped ZnO nanocombs at 11K. The acceptor binding energy is estimated to be ∼127meV.

Quantitative multi-color FRET measurements by Fourier lifetime excitation-emission matrix spectroscopy

Förster resonant energy transfer (FRET) is extensively used to probe macromolecular interactions and conformation changes. The established FRET lifetime analysis method measures the FRET process through its effect on the donor lifetime. In this paper we present a method that directly probes the time-resolved FRET signal with frequency domain Fourier lifetime excitation-emission matrix (FLEEM) measurements. FLEEM separates fluorescent signals by their different phonon energy pathways from excitation to emission. The FRET process generates a unique signal channel that is initiated by donor excitation but ends with acceptor emission. Time-resolved analysis of the FRET EEM channel allows direct measurements on the FRET process, unaffected by free fluorophores that might be present in the sample. Together with time-resolved analysis on non-FRET channels, i.e. donor and acceptor EEM channels, time resolved EEM analysis allows precise quantification of FRET in the presence of free fluorophores. The method is extended to three-color FRET processes, where quantification with traditional methods remains challenging because of the significantly increased complexity in the three-way FRET interactions. We demonstrate the time-resolved EEM analysis method with quantification of three-color FRET in incompletely hybridized triple-labeled DNA oligonucleotides. Quantitative measurements of the three-color FRET process in triple-labeled dsDNA are obtained in the presence of free single-labeled ssDNA and double-labeled dsDNA. The results establish a quantification method for studying multi-color FRET between multiple macromolecules in biochemical equilibrium.

Partial Acceptor Photobleaching-Based Quantitative FRET Method Completely Overcoming Emission Spectral Crosstalks

Based on the quantitative fluorescence resonance energy transfer (FRET) method named PbFRET we reported recently, we herein developed a partial acceptor photobleaching-based quantitative FRET algorithm named B-PbFRET method. B-PbFRET overcomes not only the acceptor excitation crosstalk and donor emission spectral crosstalk but also the acceptor emission spectral crosstalk that harasses previous methods including fluorescence lifetime (FLIM), fluorescence recovery of donor after acceptor photobleaching, and acceptor sensitized emission (SE)-based methods. B-PbFRET method is implemented by simultaneously measuring the fluorescence intensity of both donor and acceptor channels at donor excitation before and after partial acceptor photobleaching, and it can directly measure the FRET efficiency (E) without any verified references. Based on the theoretical analysis of B-PbFRET, we also developed a more straightforward correction method named C-PbFRET to obtain the absolute E from the value measured by PbFRET for a given donor-acceptor pair. We validated both B-PbFRET and C-PbFRET methods by measuring the E of two linked constructs, 18AA and SCAT3 proteins, in single living cells, and our data demonstrated that both B-PbFRET and C-PbFRET methods can directly measure the absolute E of the linked constructs inside living cells under different degrees of acceptor emission spectral crosstalk.

A Limited 4 Å Radial Displacement of the S4-S5 Linker Is Sufficient for Internal Gate Closing in Kv Channels

Voltage-gated ion channels are responsible for the generation of action potentials in our nervous system. Conformational rearrangements in their voltage sensor domains in response to changes of the membrane potential control pore opening and thus ion conduction. Crystal structures of the open channel in combination with a wealth of biophysical data and molecular dynamics simulations led to a consensus on the voltage sensor movement. However, the coupling between voltage sensor movement and pore opening, the electromechanical coupling, occurs at the cytosolic face of the channel, from where no structural information is available yet. In particular, the question how far the cytosolic pore gate has to close to prevent ion conduction remains controversial. In cells, spectroscopic methods are hindered because labeling of internal sites remains difficult, whereas liposomes or detergent solutions containing purified ion channels lack voltage control. Here, to overcome these problems, we controlled the state of the channel by varying the lipid environment. This way, we directly measured the position of the S4-S5 linker in both the open and the closed state of a prokaryotic Kv channel (KvAP) in a lipid environment using Lanthanide-based resonance energy transfer. We were able to reconstruct the movement of the covalent link between the voltage sensor and the pore domain and used this information as restraints for molecular dynamics simulations of the closed state structure. We found that a small decrease of the pore radius of about 3-4 Å is sufficient to prevent ion permeation through the pore.

Study of Fluorescence Resonance Energy Transfer in Zwitterionic Micelle: Ionic-Liquid-Induced Changes in FRET Parameters

The fluorescence resonance energy transfer (FRET) using Coumarin-153 (C-153) as the donor and Rhodamine 6G (R6G) as the acceptor is studied in an aqueous solution of N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16) micelles by steady-state and picosecond time-resolved fluorescence spectroscopy. We have determined the rate of FRET (k(FRET)) from the rise of the acceptor (R6G) emission. In the absence of donor (C-153), the acceptor (R6G) displays a single-exponential decay with average lifetime of 4.77 ns, whereas in presence of donor (C-153), the acceptor (R6G) exhibits a biexponential fluorescence transient having a distinct rise component of 0.94 ns and decay component of 5.16 ns. We have carried out a comparative study of changes in FRET parameters upon addition of three different ionic liquids (ILs), 1-ethyl-3-methylimidazolium ethylsulfate [C(2)mim][C(2)SO(4)], 1-ethyl-3-methylimidazolium n-butylsulfate [C(2)mim][C(4)SO(4)], and 1-ethyl-3-methylimidazolium n-hexylsulfate [C(2)mim][C(6)SO(4)], where each ionic liquid bears the same cationic part and the anionic parts differ in the alkyl chain length only. It has been observed that with gradual addition of the ILs [C(2)mim][C(2)SO(4)], [C(2)mim][C(4)SO(4)], and [C(2)mim][C(6)SO(4)], the rise component gradually decreases and the rate of FRET (k(FRET)) gradually increases. The k(FRET) was found to be 1.06 × 10(9) s(-1) in 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C(2)mim][C(2)SO(4)], the k(FRET) increases by a factor of 1.33 (1.41 × 10(9) s(-1)), whereas with the addition of 100 mM [C(2)mim][C(6)SO(4)] it increases by a factor of 3.25 (3.45 × 10(9) s(-1)). This rapid increase in k(FRET) in the case of [C(2)mim][C(6)SO(4)] can be explained by our earlier observation ( Rao, V. G.; Ghatak, C.; Ghosh, S.; Mandal, S.; Sarkar, N. J. Phys. Chem. B2012, 116, 3690-3698 ), where we have shown that with the addition of [C(2)mim][C(6)SO(4)], C-153 moves toward the outer surface of the micelle. This movement of C-153 causes reduction in donor-acceptor distance and enhancement in FRET rate (k(FRET)). This is well-supported by the reduced donor-acceptor distance (R(DA)) observed with the addition of [C(2)mim][C(6)SO(4)]. The R(DA) was found to be 29.1 Å in 28 mM aqueous SB-16 micelles. With the addition of 100 mM [C(2)mim][C(6)SO(4)], the R(DA) decreases to 24.8 Å. With further increase in the concentration of [C(2)mim][C(6)SO(4)], the R(DA) decreases, but the time constant for the rise of acceptor emission decreases to such an extent that we are unable to observe it by our instrumental setup.

Experimental and theoretical studies of donor–acceptor scintillation from PbI2

We report on the scintillation properties of undoped PbI2, and PbI2 doped with La, Cu, Ag, and Te. X-ray luminescence spectra were recorded at 7K and the time responses for 80ps pulses of X-rays were recorded from 14K to 150K. Samples were irregularly shaped crystal pieces that ranged from 300 to 1000μm in size. We found that at 14K the undoped samples had a fast emission at 520nm (0.18eV below the band edge) and a total light output of about 40,000photons/MeV. 10% of this light appears in the first ns, which is 5 times greater than LSO and 1.5 times greater than LaBr3:Ce. 50% of the light appears in the first 100ns. The luminosity decreased dramatically as the temperature was increased above 40K, consistent with a non-radiative path that has an activation energy of 3.8meV. Doping with the donor La3+ only slightly changed these scintillation properties. Doping with the donor La3+ and the deep acceptors Cu+, Ag+, and Te2– produced primarily slow donor–acceptor emissions at peak wavelengths of 605, 653, and 800nm, respectively. The luminosity of the samples doped with La3+ and Cu+ depended strongly on the La3+ concentration, demonstrating its role as an electron donor. First-principles band structure calculations showed (1) that the donor level depth of an isolated iodine vacancy is about 0.8eV, too deep to explain the 520nm emission wavelength, and (2) that a donor–acceptor pair consisting of a spatially associated iodine vacancy donor and a lead vacancy acceptor has shallower donor and acceptor depths that are consistent with the 520nm emission wavelength.

Morphology-Dependent Energy Transfer Dynamics in Fluorene-Based Amphiphile Nanoparticles

Nanoparticles are interesting systems to study because of their large range of potential uses in biological imaging and sensing. We investigated molecular nanoparticles formed by fast injection of a small volume of molecularly dissolved fluorene-derivative amphiphilic molecules into a polar solvent, which resulted in solid spherical particles of ∼80 nm diameter with high stability. Energy transfer studies were carried out on two-component nanoparticles that contained mixtures of donor and acceptor amphiphiles of various fractions. We conducted time-resolved photoluminescence measurements on the two-component nanoparticles in order to determine whether the fundamental donor-acceptor interaction parameter (the Förster radius) depends on the acceptor concentration. The Förster radius was found to be large for very low incorporated acceptor fractions (<0.1%), but it declined with increasing concentration. These changes were concomitant with shifts in the acceptor emission and absorption circular dichroism spectra that indicated an increasing clustering of acceptors into domains as their fraction was raised. In addition, for acceptor fractions below 2% the extracted Förster radii were found to be significantly larger than predicted from donor-acceptor spectral overlap calculations, in accordance with efficient excitation diffusion within the donor matrix, aiding the overall transfer to acceptors. We conclude that energy transfer in two-component nanoparticles shows a complex interplay between phase segregation of the constituent donor and acceptor molecules and excitation diffusion within their domains.

Bio serves nano: biological light-harvesting complex as energy donor for semiconductor quantum dots.

Light-harvesting complex (LHCII) of the photosynthetic apparatus in plants is attached to type-II core-shell CdTe/CdSe/ZnS nanocrystals (quantum dots, QD) exhibiting an absorption band at 710 nm and carrying a dihydrolipoic acid coating for water solubility. LHCII stays functional upon binding to the QD surface and enhances the light utilization of the QDs significantly, similar to its light-harvesting function in photosynthesis. Electronic excitation energy transfer of about 50% efficiency is shown by donor (LHCII) fluorescence quenching as well as sensitized acceptor (QD) emission and corroborated by time-resolved fluorescence measurements. The energy transfer efficiency is commensurable with the expected efficiency calculated according to Förster theory on the basis of the estimated donor-acceptor separation. Light harvesting is particularly efficient in the red spectral domain where QD absorption is relatively low. Excitation over the entire visible spectrum is further improved by complementing the biological pigments in LHCII with a dye attached to the apoprotein; the dye has been chosen to absorb in the "green gap" of the LHCII absorption spectrum and transfers its excitation energy ultimately to QD. This is the first report of a biological light-harvesting complex serving an inorganic semiconductor nanocrystal. Due to the charge separation between the core and the shell in type-II QDs the presented LHCII-QD hybrid complexes are potentially interesting for sensitized charge-transfer and photovoltaic applications.