Energy Relay from an Unconventional Yellow Dye to CdS/CdSe Quantum Dots for Enhanced Solar Cell Performance

A new design for a quasi‐solid‐state Forster resonance energy transfer (FRET) enabled solar cell with unattached Lucifer yellow (LY) dye molecules as donors and CdS/CdSe quantum dots (QDs) tethered to titania (TiO₂) as acceptors is presented. The Forster radius is experimentally determined to be 5.29 nm. Sequential energy transfer from the LY dye to the QDs and electron transfer from the QDs to TiO₂ is followed by fluorescence quenching and electron lifetime studies. Cells with a donor–acceptor architecture (TiO₂/CdS/CdSe/ZnS‐LY/S^2?‐multi‐walled carbon nanotubes) show a maximum incident photon‐to‐current conversion efficiency of 53 % at 530 nm. This is the highest efficiency among Ru‐dye free FRET‐enabled quantum dot solar cells (QDSCs), and is much higher than the donor or acceptor‐only cells. The FRET‐enhanced solar cell performance over the majority of the visible spectrum paves the way to harnessing the untapped potential of the LY dye as an energy relay fluorophore for the entire gamut of dye sensitized, organic, or hybrid solar cells.     

A Potential Carcinogenic Pyrene Derivative under Förster Resonance Energy Transfer to Various Energy Acceptors in Nanoscopic Environments

Picosecond‐resolved Förster resonance energy transfer (FRET) from various vibronic bands in benzo[a]pyrene (BP) shows a strong dependency on the spectral overlap of an energy acceptor in a confined environment. Our study on the dipolar interactions between BP and different acceptors, including ethidium (Et), acridine orange (AO), and crystal violet (CV), at the surface of a model anionic micelle revealed that the Förster distance (R₀) and the rate of energy transfer is dependent on the individual spectral overlap of the vibronic bands of BP with the absorption spectra of the different energy acceptors. The differential behavior of the vibronic bands is compared with that of different dyes [quantum dots (QDs)] in a “dye‐blend” (mixture) under FRET to an energy acceptor. Comparison of the FRET of the QDs with that of BP confirmed the independent nature of the dipolar interaction of the vibronic bands with other organic molecules, and the use of deconvolution techniques in the interpretation of the donor–acceptor (D –A) distance was also justified. We also showed that the consideration of differential FRET from the vibronic bands of BP and from the QDs in the dye‐blend is equally acceptable in theoretical frameworks including the Infelta–Tachiya model and D –A distribution analysis in nanoenvironments.     

Synthesis and spectroscopic properties of silica-dye-semiconductor nanocrystal hybrid particles

We prepared silica-dye-nanocrystal hybrid particles and studied the energy transfer from semiconductor nanocrystals (= donor) to organic dye molecules (= acceptor). Multishell CdSe/CdS/ZnS semiconductor nanocrystals were adsorbed onto monodisperse Sto?ber silica particles with an outer silica shell of thickness 2-23 nm containing organic dye molecules (Texas Red). The thickness of this dye layer has a strong effect on the energy transfer efficiency, which is explained by the increase in the number of dye molecules homogeneously distributed within the silica shell, in combination with an enhanced surface adsorption of nanocrystals with increasing dye amount. Our conclusions were underlined by comparison of the experimental results with numerically calculated FRET efficiencies and by control experiments confirming attractive interaction between the nanocrystals and Texas Red freely dissolved in solution.     

A New Robust and Highly Sensitive FRET Donor–Acceptor Pair: Synthesis,Characterization, and Application in a Thrombin Assay

The synthesis of a new, robust fluorescence‐resonance‐energy‐transfer (FRET) system is described. Its donor chromophore is derived from an N‐allyl‐substituted quinolinone attached to 4‐bromophenylalanine via Heck cross‐coupling. The resulting Fmoc‐protected derivative 11 was used as building block in solid‐phase peptide synthesis (SPPS). As FRET acceptor, a sulfonylated ruthenium(II)–bathophenanthroline complex with a peripheral COOH function was prepared for covalent attachment to target molecules. The UV/VIS absorption and emission spectra of peptides bearing only the donor (D) or acceptor (A) dye showed a good overlap of the emission band of the donor with the absorption band of the acceptor. The fluorescence spectra of a peptide bearing both dyes revealed an additional emission after excitation of the donor, which is due to indirect excitation of the acceptor via FRET. The long fluorescence lifetime of the Ru^II complex (0.53 μs) makes it well‐suited for time‐resolved measurements. As a first application of this new FRET system, the peptide 18 , with the recognition sequence for the protease thrombin, flanked by the two dyes, was synthesized and successfully cleaved by the enzyme. The change in the ratio of the fluorescence intensities could be determined.     

Comparative efficiency of energy transfer from CdSe-ZnS quantum dots or nanorods to organic dye molecules

Fluorescence resonance energy transfer (FRET) in conjugates of CdSe-ZnS semiconductor nanocrystals of different shapes (FRET donors) and an Alexa Fluor organic dye (FRET acceptors) is examined. The dye molecules are chemically conjugated with quantum dots (QDs) or nanorods (NRs) in dimethyl sulfoxide colloidal solutions, and FRET efficiency in the purified conjugates is measured. The FRET from NR to a single dye molecule is less efficient than that of the QD-dye conjugates and this effect is explained in terms of distance-limited energy-transfer rate in the case of a point-like acceptor and extended donor dipoles. However, the larger surface area of NRs allows for many more dye acceptors to be bound, and the total FRET efficiency in NR-dye conjugates approaches those of QD-dye conjugates.     

Anionic Conjugated Polyelectrolytes for FRET-based Imaging of Cellular Membrane Potential

We report a Förster resonance energy transfer (FRET)-based imaging ensemble for the visualization of membrane potential in living cells. A water-soluble poly(fluorene-cophenylene) conjugated polyelectrolyte (FsPFc10) serves as a FRET donor to a voltage-sensitive dye acceptor (FluoVolt^™). We observe FRET between FsPFc10 and FluoVolt^™, where the enhancement in FRET-sensitized emission from FluoVolt^™ is measured at various donor/acceptor ratios. At a donor/acceptor ratio of 1, the excitation of FluoVolt^™ in a FRET configuration results in a three-fold enhancement in its fluorescence emission (compared to when it is excited directly). FsPFc10 efficiently labels the plasma membrane of HEK 293T/17 cells and remains resident with minimal cellular internalization for ~ 1.5 h. The successful plasma membrane-associated colabeling of the cells with the FsPFc10-FluoVolt^™ donor-acceptor pair is confirmed by dual-channel confocal imaging. Importantly, cells labeled with FsPFc10 show excellent cellular viability with no adverse effect on cell membrane depolarization. During depolarization of membrane potential, HEK 293T/17 cells labeled with the donor-acceptor FRET pair exhibit a greater fluorescence response in FluoVolt^™ emission relative to when FluoVolt^™ is used as the sole imaging probe. These results demonstrate the conjugated polyelectrolyte to be a new class of membrane labeling fluorophore for use in voltage sensing schemes.     

Single lanthanide-doped oxide nanoparticles as donors in fluorescence resonance energy transfer experiments

We used lanthanide-ion doped oxide nanoparticles, Y(0.6)Eu(0.4)VO(4), as donors in fluorescent resonance energy transfer (FRET) experiments. The choice of these nanoparticles allows us to combine the advantages of the lanthanide-ion emission, in particular the long lifetime and the large Stokes shift between absorption and emission, with the detectability of the nanoparticles at the single-particle level. Using cyanine 5 (Cy5) organic molecules as acceptors, we demonstrated FRET down to the single-nanoparticle level. We showed that, due to the long donor lifetime, unambiguous and precise FRET measurements can be performed in solution even in the presence of large free acceptor concentrations. Highly efficient energy transfer was obtained for a large number of acceptor molecules per donor nanoparticle. We determined FRET efficiencies as a function of Cy5 concentration which are in good agreement with a multiple acceptor-multiple donor calculation. On the basis of the donor emission recovery due to acceptor photobleaching, we demonstrated energy transfer from single-nanoparticle donors in fluorescence microscopy experiments.     

Red–Green–Blue Trichromophoric Nanoparticles with Dual Fluorescence Resonance Energy Transfer: Highly Sensitive Fluorogenic Response Toward Polyanions

A red–green–blue (RGB) trichromophoric fluorescent organic nanoparticle exhibiting multi‐colour emission was constructed; the blue‐emitting cationic oligofluorene nanoparticle acted as an energy‐donor scaffold to undergo fluorescence resonance energy transfer (FRET) to a red‐emitting dye embedded in the nanoparticle (interior FRET) and to a green‐emitting dye adsorbed on the surface through electrostatic interactions (exterior FRET). Each FRET event occurs independently and is free from sequential FRET, thus the resultant dual‐FRET system exhibits multi‐colour emission, including white, in aqueous solution and film state. A characteristic white‐emissive nanoparticle showed visible responses upon perturbation of the exterior FRET efficiency by acceptor displacement, leading to highly sensitive responses toward polyanions in a ratiometric manner. Specifically, our system exhibits high sensitivity toward heparin with an extremely low detection limit.     

A novel FRET approach for in situ investigation of cellulase–cellulose interaction

A novel real-time in situ detection method for the investigation of cellulase–cellulose interactions based on fluorescence resonance energy transfer (FRET) has been developed. FRET has been widely used in biological and biophysical fields for studies related to proteins, nucleic acids, and small biological molecules. Here, we report the efficient labeling of carboxymethyl cellulose (CMC) with donor dye 5-(aminomethyl)fluorescein and its use as a donor in a FRET assay together with an Alexa Fluor 594 (AF594, acceptor)–cellulase conjugate as acceptor. This methodology was successfully employed to investigate the temperature dependency of cellulase binding to cellulose at a molecular level by monitoring the fluorescence emission change of donor (or acceptor) in a homogeneous liquid environment. It also provides a sound base for ongoing cellulase–cellulose study using cellulosic fiber.     

Self-assembled pi-nanotapes as donor scaffolds for selective and thermally gated fluorescence resonance energy transfer (FRET)

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.     

Förster resonance energy transfer in poly(methyl methacrylates) copolymers bearing donor–acceptor 1,3‐thiazole dyes

The Förster resonance energy transfer (FRET) properties in poly(methyl methacrylate) copolymers containing 2‐(pyridine‐2‐yl) thiazole dyes were studied upon systematic variation of the donor‐to‐acceptor ratio. To this end, 2‐(pyridine‐2‐yl) thiazole dyes specially designed for the usage as energy donor and acceptor molecules were incorporated within one polymer chain. Poly(methyl methacrylate) copolymers containing these donor and acceptor dyes were synthesized using the RAFT polymerization method. Copolymers with a molar mass (M_n) of nearly 10,000 g/mol were achieved with dispersity index values (?) under 1.3. The presented copolymers act as a model system for the FRET investigation. Förster resonance energy transfer properties of the copolymers are characterized by steady state as well as time resolved fluorescence spectroscopy. The results indicate that the energy transfer rates and the transfer efficiencies are tunable by variation of the donor‐acceptor‐ratio. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4765–4773     

Cascade Energy Transfer and White-Light Emission in Chirality-Controlled Crystallization-Driven Two-Dimensional Co-assemblies from Donor and Acceptor Dye-Conjugated Polylactides

Achieving predictable and programmable two-dimensional (2D) structures with specific functions from exclusively organic soft materials remains a scientific challenge. This article unravels stereocomplex crystallization-driven self-assembly as a facile method for producing thermally robust discrete 2D-platelets of diamond shape from biodegradable semicrystalline polylactide (PLA) scaffolds. The method involves co-assembling two PLA stereoisomers, namely, PY-PDLA and NMI-PLLA , which form stereocomplex (SC)-crystals in isopropanol. By conjugating a well-known Förster resonance energy transfer (FRET) donor and acceptor dye, namely, pyrene ( PY ) and naphthalene monoimide ( NMI ), respectively, to the chain termini of these two interacting stereoisomers, a thermally robust FRET process can be stimulated from the 2D array of the co-assembled dyes on the thermally resilient SC-PLA crystal surfaces. Uniquely, by decorating the surface of the SC-PLA crystals with an externally immobilized guest dye, Rhodamine-B, similar diamond-shaped structures could be produced that exhibit pure white-light emission through a surface-induced two-step cascade energy transfer process. The FRET response in these systems displays remarkable dependence on the intrinsic crystalline packing, which could be modulated by the chirality of the co-assembling PLA chains. This is supported by comparing the properties of similar 2D platelets generated from two homochiral PLLAs ( PY-PLLA and NMI-PLLA ) labeled with the same FRET pair.     

Fluorescence resonance energy transfer between quantum dot donors and dye-labeled protein acceptors

We used luminescent CdSe-ZnS core-shell quantum dots (QDs) as energy donors in fluorescent resonance energy transfer (FRET) assays. Engineered maltose binding protein (MBP) appended with an oligohistidine tail and labeled with an acceptor dye (Cy3) was immobilized on the nanocrystals via a noncovalent self-assembly scheme. This configuration allowed accurate control of the donor-acceptor separation distance to a range smaller than 100 A and provided a good model system to explore FRET phenomena in QD-protein-dye conjugates. This QD-MBP conjugate presents two advantages: (1) it permits one to tune the degree of spectral overlap between donor and acceptor and (2) provides a unique configuration where a single donor can interact with several acceptors simultaneously. The FRET signal was measured for these complexes as a function of both degree of spectral overlap and fraction of dye-labeled proteins in the QD conjugate. Data showed that substantial acceptor signals were measured upon conjugate formation, indicating efficient nonradiative exciton transfer between QD donors and dye-labeled protein acceptors. FRET efficiency can be controlled either by tuning the QD photoemission or by adjusting the number of dye-labeled proteins immobilized on the QD center. Results showed a clear dependence of the efficiency on the spectral overlap between the QD donor and dye acceptor. Apparent donor-acceptor distances were determined from efficiency measurements and corresponding F?rster distances, and these results agreed with QD bioconjugate dimensions extracted from structural data and core size variations among QD populations.     

Rate of excitation energy transfer between fluorescent dyes and nanoparticles

Because of the sensitivity of the rate of Coulomb interaction induced long range resonance energy transfer (RET) on the distance between the donor (D) and the acceptor (A) molecules, the technique of FRET (fluorescence resonance energy transfer) is popularly termed as “spectroscopic ruler” and is increasingly being used in many areas of biological and material science. For example, the phenomenon is used to monitor the in vivo separation between different (bio)polymers/units of (bio)polymers and hence the dynamics of various biomolecular processes. In this work, we examine the distance and the orientation dependence of RET in three different systems: (i) between a conjugated polymer and a fluorescent dye, (ii) between a nanometal particle (NMP) and a fluorescent dye and (iii) between two NMP. We show that in all the three cases, the rate of RET follows a distance dependence of d^−σ where exponent σ approaches 6 at large distance d (Förster type dependence) but has a value varying from 3–4 at short to intermediate distances.     

Estimating the size of laterally phase separated cholesterol domains in model membranes with Förster resonance energy transfer: a simulation study

In this work, we use two vertically-coupled square two-dimensional lattices to simulate membrane bilayers containing a uniform size distribution of cholesterol immiscible domains of a predetermined size distribution. We substitute cholesterols and phospholipids with their fluorescent analogs and calculate the efficiency of energy transfer as a function of acceptor concentration for four membrane configurations. The simulated efficiency of energy transfer as a function of acceptor concentration data is then fit with an analytical FRET model to estimate the domain size, in the same manner in which experimental FRET data is analyzed. The fitted model parameters (domain size and donor partition coefficient) are compared to the simulation inputs to test the applicability of the FRET model to estimating the size of laterally phase separated cholesterol domains. We show that the FRET model yields good size estimates for domains that range between 1 and 25 nm. We also find that the assumed fluorophore configuration in the FRET model leads to a constant under-prediction of these values. Finally, we demonstrate that when two parameters are open to the fit, the FRET model adequately predicts the donor partition coefficient in addition to the domain size.     

Fabrication of Novel Polymer Nanoparticle-Based Fluorescence Resonance Energy Transfer Systems and their Tunable Fluorescence Properties

In this study, two hydrophobic fluorescent dyes, nitrobenzoxadiazolyl (NBD) and 9-(diethylamino)benzo[a]phenoxazin-5-one (NR) with different doping ratios were incorporated into polymer nanoparticles to constitute novel polymer nanoparticle-based fluorescence resonance energy transfer (FRET) systems via a facile one-step mini-emulsion polymerization. Spectroscopic characteristics demonstrate that the two fluorophores have been successfully embedded into the nanoparticles, and the fluorescence emission intensity of the two hydrophobic dyes can be greatly enhanced in aqueous media. The as-prepared fluorescent nanoparticles also display a uniform small size (ca. 55 nm), high dye load, intense fluorescence, as well as controllable amount and ratio of the two dyes. The observed FRET efficiencies (16.0–75.2%), as well as the distance (r) between NBD (donor) and NR (acceptor), is closely correlated to the doping ratio of two dyes. Moreover, by varying the doping ratio of two dyes, the fluorescent nanoparticles would exhibit multicolor through FRET upon a single wavelength excitation, and the fluorescence emission signals of the dye-doped nanoparticles could be accurately tuned. These results indicate that the as-prepared uniform FRET-mediated nanoparticles are of high interest in multiplexed bioanalysis.     

Inverted Opal Fluorescent Film Chemosensor for the Detection of Explosive Nitroaromatic Vapors through Fluorescence Resonance Energy Transfer

This paper reports an inverted opal fluorescence chemosensor for the ultrasensitive detection of explosive nitroaromatic vapors through resonance‐energy‐transfer‐amplified fluorescence quenching. The inverted opal silica film with amino ligands was first fabricated by the acid–base interaction between 3‐aminopropyltriethoxysilane and surface sulfonic groups on polystyrene microsphere templates. The fluorescent dye was then chemically anchored onto the interconnected porous surface to form a hybrid monolayer of amino ligands and dye molecules. The amino ligands can efficiently capture vapor molecules of nitroaromatics such as 2,4,6‐trinitrotoluene (TNT) through the charge‐transfer complexing interaction between electron‐rich amino ligands and electron‐deficient aromatic rings. Meanwhile, the resultant TNT–amine complexes can strongly suppress the fluorescence emission of the chosen dye by the fluorescent resonance energy transfer (FRET) from the dye donor to the irradiative TNT–amino acceptor through intermolecular polar–polar resonance at spatial proximity. The quenching response of the highly ordered porous films with TNT is greatly amplified by at least 10‐fold that of the amorphous silica films, due to the interconnected porous structure and large surface‐to‐volume ratio. The inverted opal film with a stable fluorescence brightness and strong analyte affinity has lead to an ultrasensitive detection of several ppb of TNT vapor in air.     

Can luminescent quantum dots be efficient energy acceptors with organic dye donors?

We assessed the ability of luminescent quantum dots (QDs) to function as energy acceptors in fluorescence resonance energy transfer (FRET) assays, with organic dyes serving as donors. Either AlexaFluor 488 or Cy3 dye was attached to maltose binding protein (MBP) and used with various QD acceptors. Steady-state and time-resolved fluorescence measurements showed no apparent FRET from dye to QD. We attribute these observations to the dominance of a fast radiative decay rate of the donor excitation relative to a slow FRET decay rate. This is due to the long exciton lifetime of the acceptor compared to that of the dye, combined with substantial QD direct excitation.     

Extending spectrum response of squaraine-sensitized solar cell by Förster resonance energy transfer

To extend the spectral response region of squaraine dye (SQ)-sensitized solar cell, eosin Y (EY) is encapsulated in the SQ-sensitized nanocrystalline thin film. EY is first adsorbed on nanocrystalline TiO₂ thin film (n-TiO₂), then a thin layer of EY contained ZnO (EY-ZnO) is electrodeposited, and SQ dye is finally sensitized to form two dye-sensitized nanocrystalline thin film with a structure of n-TiO₂/EY/EY-ZnO/SQ. There is a perfect spectral overlap between the emission of EY and the absorption of SQ; EY as an energy donor simultaneously transfers both electron and hole to the energy acceptor SQ according to the Förster resonance energy transfer (FRET) process. EY shifts the spectral response edge of SQ-sensitized solar cell toward blue from 550 to 450 nm through the FRET process in this new structure. Two dye-sensitized nanocrystalline thin film demonstrates a significant enhancement in light harvesting and photocurrent generation due to the FRET process. The thickness of the EY-ZnO thin layer and spectral overlap between emission of donor dye and absorption of acceptor dye are two important factors that affect the FRET process between EY and SQ in the structure of n-TiO₂/EY/EY-ZnO/SQ.     

The photoluminescent graphene oxide serves as an acceptor rather than a donor in the fluorescence resonance energy transfer pair of Cy3.5-graphene oxide

We have systematically studied the fluorescence resonance energy transfer (FRET) efficiency between the photoluminescent graphene oxide (GO) and Cy3.5 dye by controlling the donor-acceptor distance with a double stranded DNA and demonstrated that the GO serves as an acceptor rather than a donor in this FRET system.