Efficient Long-Range, Directional Energy Transfer through DNA-Templated Dye Aggregates.

The benzothiazole cyanine dye K21 forms dye aggregates on double-stranded DNA (dsDNA) templates. These aggregates exhibit a red-shifted absorption band, enhanced fluorescence emission, and an increased fluorescence lifetime, all indicating strong excitonic coupling among the dye molecules. K21 aggregate formation on dsDNA is only weakly sequence dependent, providing a flexible approach that is adaptable to many different DNA nanostructures. Donor (D)-bridge (B)-acceptor (A) complexes consisting of Alexa Fluor 350 as the donor, a 30 bp (9.7 nm) DNA templated K21 aggregate as the bridge, and Alexa Fluor 555 as the acceptor show an overall donor to acceptor energy transfer efficiency of ∼60%, with the loss of excitation energy being almost exclusively at the donor-bridge junction (63%). There was almost no excitation energy loss due to transfer through the aggregate bridge, and the transfer efficiency from the aggregate to the acceptor was about 96%. By comparing the energy transfer in templated aggregates at several lengths up to 32 nm, the loss of energy per nanometer through the K21 aggregate bridge was determined to be <1%, suggesting that it should be possible to construct structures that use much longer energy transfer "wires" for light-harvesting applications in photonic systems.

Programmed coherent coupling in a synthetic DNA-based excitonic circuit.

Natural light-harvesting systems spatially organize densely packed chromophore aggregates using rigid protein scaffolds to achieve highly efficient, directed energy transfer. Here, we report a synthetic strategy using rigid DNA scaffolds to similarly program the spatial organization of densely packed, discrete clusters of cyanine dye aggregates with tunable absorption spectra and strongly coupled exciton dynamics present in natural light-harvesting systems. We first characterize the range of dye-aggregate sizes that can be templated spatially by A-tracts of B-form DNA while retaining coherent energy transfer. We then use structure-based modelling and quantum dynamics to guide the rational design of higher-order synthetic circuits consisting of multiple discrete dye aggregates within a DX-tile. These programmed circuits exhibit excitonic transport properties with prominent circular dichroism, superradiance, and fast delocalized exciton transfer, consistent with our quantum dynamics predictions. This bottom-up strategy offers a versatile approach to the rational design of strongly coupled excitonic circuits using spatially organized dye aggregates for use in coherent nanoscale energy transport, artificial light-harvesting, and nanophotonics.

Directed Energy Transfer through DNA-Templated J-Aggregates.

Strongly coupled molecular dye aggregates have unique optoelectronic properties that often resemble those of light harvesting complexes found in Nature. The exciton dynamics in coupled dye aggregates could enhance the long-range transfer of optical excitation energy with high efficiency. In principle, dye aggregates could serve as important components in molecular-scale photonic devices; however, rational design of these coupled dye aggregates with precise control over their organization, interactions, and dynamics remains a challenge. DNA nanotechnology has recently been used to build an excitonic circuit by organizing pseudoisocyanine (PIC) dyes forming J-aggregates on the templates of poly(dA)-poly(dT) DNA duplexes. Here, the excitonic properties of the PIC J-aggregates on DNA are characterized spectroscopically in detail using poly(dA)-poly(dT) tract lengths of 24 and 48 base pairs. The excitonic properties of these DNA templated dye assemblies depend on the length and sequence of the DNA template. The incorporation of a gap of two GC base pairs between two segments of poly(dA)-poly(dT) DNA markedly reduces the delocalization of excitation in the J-aggregates. With a quantum dot (QD) as the light absorber and energy donor and using Alexa Fluor 647 (AF647) as the energy acceptor, with a DNA-templated J-aggregate in between, significant energy transfer from QD to AF647 is observed over a distance far longer than possible without the aggregate bridge. By comparing the efficiency of energy transfer through a continuous J-aggregate with the efficiency when the aggregate has a discontinuity in the middle, the effects of energy transfer within the aggregate bridge between the donor and acceptor are evaluated.

Ultrafast FRET to study spontaneous micelle-to-vesicle transitions in an aqueous mixed surface-active ionic-liquid system.

The spontaneous micelle-to-vesicle transition in an aqueous mixture of two surface-active ionic liquids (SAILs), namely, 1-butyl-3-methylimidazolium n-octylsulfate ([C4mim][C8SO4]) and 1-dodecyl-3-methylimidazoium chloride ([C12mim]Cl) is described. In addition to detailed structural characterization obtained by using dynamic light scattering, transmission electron microscopy (TEM), and cryogenic TEM techniques, ultrafast fluorescence resonance energy transfer (FRET) from coumarin 153 (C153) as a donor (D) to rhodamine 6G (R6G) as an acceptor (A) is also used to study micelle-vesicle transitions in the present system. Structural transitions of SAIL micelles ([C4mim][C8SO4] or [C12mim]Cl micelles) to mixed SAIL vesicles resulted in significantly increased D-A distances, and therefore, increased timescale of FRET. In [C4mim][C8SO4] micelles, FRET between C153 and R6G occurs on an ultrafast timescale of 3.3 ps, which corresponds to a D-A distance of about 15 Å. As [C4mim][C8SO4] micelles are transformed into mixed micelles upon the addition of a 0.25 molar fraction of [C12mim]Cl, the timescale of FRET increases to 300 ps, which suggests an increase in the D-A distance to 31 Å. At a 0.5 molar fraction of [C12mim]Cl, unilamellar vesicles are formed in which FRET occurs on multiple timescales of about 250 and 2100 ps, which correspond to D-A distances of 33 and 47 Å. Although in micelles and mixed micelles the obtained D-A distances are well correlated with their radius, in vesicles the obtained D-A distance is within the range of the bilayer thickness.

Modulation of AIE and Intramolecular Charge Transfer of a Pyrene-Based Probe for Discriminatory Detection and Imaging of Oligomers and Amyloid Fibrils.

Oligomers and amyloid fibrils formed at different stages of protein aggregation are important biomarkers for a variety of neurodegenerative diseases including Alzheimer's and Parkinson's diseases. The development of probes for the sensitive detection of oligomeric species is important for early stage diagnosis of amyloidogenic diseases. Many small molecular dyes have been developed to probe the dynamic growth of amyloid fibrils. However, there is a lack of discriminatory detection strategies to monitor the dynamics of both oligomers and amyloid fibrils based on the differential modulation of the photophysical properties of a single dye. Here we report a pyrene-based intramolecular charge transfer (ICT) dye with large Stokes shifted red-emitting aggregation induced emission (AIE) for monitoring the dynamic populations of both oligomers and fibrils during the aggregation of hen egg white lysozyme (HEWL) protein. At the early stage of protein aggregation, the accumulation of HEWL oligomers results in a rapid and substantial increase in the red AIE intensity at 660 nm. Later, as the oligomers transform into mature fibrils, the dye exhibits a distinct photophysical change. Binding of the dye to HEWL fibrils strongly suppresses the red AIE and enhances ICT emission. This is evidenced by a gradual decrease in the AIE intensity (∼660 nm) and an increase in LE (∼490 nm) and ICT (∼540 nm) emission intensities during the later stages of protein aggregation. Thus, the dye provides simultaneous measurements of the population dynamics of both HEWL oligomers and fibrils during protein aggregation based on the discriminatory modulation of AIE and ICT of the dye. The dye also enables imaging of both HEWL oligomers and fibrils simultaneously using different emission channels in super-resolution confocal fluorescence microscopy.

Dual Optical Response Strategy for the Detection of Cytochrome c Using Highly Luminescent Lanthanide-Based Nanotubular Sensor Arrays.

Here, we demonstrate a label-free dual optical response strategy for the detection of cytochrome c (Cyt c) with ultrahigh sensitivity using highly luminescent lanthanides containing inorganic-organic hybrid nanotubular sensor arrays. These sensor arrays are formed by the sequential incorporation of the photosensitizers 2,3-dihydroxynaphthalene (DHN) or 1,10-phenanthroline (Phen), and trivalent lanthanide terbium ions (Tb3+) into sodium lithocholate (NaLC) nanotube templates. Our sensing platform relies on the detection and quantification of Cyt c in solution by providing dual photoluminescence quenching responses from the nanotubular hybrid arrays in the presence of Cyt c. The large quenching of the sensitized Tb3+ emission within the DHN/Phen-Tb3+-NaLC nanotubular sensor arrays caused by the strong binding of the photosensitizers to Cyt c provides an important signal response for the selective detection of Cyt c. This long-lived lanthanide emission response-based sensing strategy can be highly advantageous for the detection of Cyt c in a cellular environment eliminating background fluorescence and scattering signals through time-gated measurements. The DHN containing nanotubular sensor arrays (DHN-NaLC and DHN-Tb3+-NaLC) provide an additional quenching response characterized by a unique spectral valley splitting with quantized quenching dip on the DHN fluorescence emission. This spectral quenching dip resulting from efficient FRET between the protein bound DHN photosensitizer and the heme group of Cyt c serves as an important means for specific detection and quantification of Cyt c in the concentration range of 0-30 µM with a low detection limit of around 20 nM.

A robust single compartment peroxide fuel cell using mesoporous antimony doped tin oxide as the cathode material.

To date, metal oxide catalysts have not been explored as cathode materials for robust and high-performance single-compartment H2O2 fuel cells due to significant non-electrochemical disproportionation losses of H2O2 on many metal oxide surfaces. Here, for the first time, we demonstrate an acidic peroxide fuel cell with antimony doped tin oxide as the cathode and widely used Ni foam as the anode material. Our constructed peroxide fuel cell records a superior open circuit potential of nearly 0.82 V and a maximum power density of 0.32 mW cm-2 with high operational stability. The fuel cell performance is further improved by increasing the ionic strength of the electrolyte with the addition of 1 M NaCl, resulting in an increased maximum power density value of 1.1 mW cm-2.

Spontaneous transition of micelle-vesicle-micelle in a mixture of cationic surfactant and anionic surfactant-like ionic liquid: a pure nonlipid small unilamellar vesicular template used for solvent and rotational relaxation study.

The micelle-vesicle-micelle transition in aqueous mixtures of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and the anionic surfactant-like ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8SO4] has been investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), surface tension, conductivity, and fluorescence anisotropy at different volume fractions of surfactant. The surface tension value decreases sharply with increasing CTAB concentration up to ∼0.38 volume fraction and again increases up to ∼0.75 volume fraction of CTAB. Depending upon their relative amount, these surfactants either mixed together to form vesicles and/or micelles, or both of these structures were in equilibrium. Fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in this system at different composition of surfactant indicates the formation of micelle and vesicle structures. The apparent hydrodynamic diameter of these large multilamellar vesicles is about ∼200 nm-300 nm obtained by DLS measurement and finally confirmed by TEM micrographs. The large multilamellar vesicles are transformed into small unilamellar ones by sonication using a Lab-line instruments probe sonicator with a diameter of ∼90-125 nm. To investigate the heterogeneity, solvent, and rotational relaxation of coumarin-153 (C-153) have been investigated in these unilamellar vesicles by using picosecond time-resolved fluorescence spectroscopic technique. The solvation dynamics of C-153 in these vesicles is found to be biexponential with average time constant ∼580 ps. This indicates the slow relaxation of water molecules in the surfactant bilayer. In accordance with solvation dynamics, fluorescence anisotropy analysis of C-153 in unilamellar vesicles also indicates hindered rotation compared to bulk water.

Modulation of the photophysical properties of 2,2'-bipyridine-3,3'-diol inside bile salt aggregates: a fluorescence-based study for the molecular recognition of bile salts.

2,2'-Bipyridine-3,3'-diol (BP(OH)(2)) has been used as a sensitive excited-state intramolecular proton transfer fluorophore to assess different bile salt aggregates as one of the potential biologically relevant host systems useful for carrying many sparingly water-soluble drug molecules. The formation of inclusion complexes, complex-induced fluorescence behavior, and their binding ability have been investigated from the modulated photophysics of BP(OH)(2) by means of photophysical techniques. The constrained hydrophobic environment provided by the aggregates significantly reduces the water-assisted nonradiative decay channels and lengthens the fluorescence lifetime of the proton-transferred DK tautomer. Both the absorption and fluorescence properties of BP(OH)(2) are found to be sensitive to the change in the structure, size, and hydrophobicity of the aggregates. Fluorescence quenching experiments were performed to gain insight into the differential distribution of the probe molecules between bulk aqueous phase and nanocavities of various aggregates. The observation of longer fluorescence lifetime and rotational relaxation time in NaDC aggregates compared to that in NaCh and NaTC aggregates indicates that the binding structures of NaDC aggregates are more rigid due to its greater hydrophobicity and larger size and therefore provide better protection to the bound guest. It is noteworthy to mention that the hydrophobic microenvironments provided by bile salt aggregates are much stronger than that provided by micelles and cyclodextrins. The accessibility of water to the aggregate-bound guest can significantly be enhanced with the addition of organic cosolvents. However, the efficiency decreases in the order of dimethylformamide, acetonitrile, and methanol.

Mechanistic Insight into the Amyloid Fibrillation Inhibition of Hen Egg White Lysozyme by Three Different Bile Acids.

Amyloid aggregation of protein is linked to many neurodegenerative diseases. Identification of small molecules capable of targeting amyloidogenic proteins has gained significant importance. Introduction of hydrophobic and hydrogen bonding interactions through site-specific binding of small molecular ligand to protein can effectively modulate the protein aggregation pathway. Here, we investigate the possible roles of three different bile acids, cholic acid (CA), taurocholic acid (TCA), and lithocholic acid (LCA) with varying hydrophobic and hydrogen bonding properties in inhibiting protein fibrillation. Bile acids are an important class of steroid compounds that are synthesized in the liver from cholesterol. Increasing evidence suggests that altered taurine transport, cholesterol metabolism, and bile acid synthesis have strong implications in Alzheimer's disease. We find that the hydrophilic bile acids, CA and TCA (taurine conjugated form of CA), are substantially more efficient inhibitors of lysozyme fibrillation than the most hydrophobic secondary bile acid LCA. Although LCA binds more strongly with the protein and masks the Trp residues more prominently through hydrophobic interactions, the lesser extent of hydrogen bonding interactions at the active site has made LCA a relatively weaker inhibitor of HEWL aggregation than CA and TCA. The introduction of a greater number of hydrogen bonding channels by CA and TCA with several key amino acid residues which are prone to form oligomers and fibrils has weakened the protein's internal hydrogen bonding capabilities for undergoing amyloid aggregation.

The chameleon-like nature of zwitterionic micelles: the effect of ionic liquid addition on the properties of aqueous sulfobetaine micelles.

We used fluorescence probing, ζ potentials, and dynamic light scattering measurements to study the interactions between the zwitterionic surfactant N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16) and three 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)]). The three ILs have the same cationic part and their anionic parts differ only in the length of the alkyl chain. The aim of our work is to offer a comparative study and establish the role of the alkyl chain length of the anion of ILs on 1) the incorporation of these anions in the zwitterionic micelles of SB-16 (selectivity of anions) and 2) the physicochemical properties of aqueous solutions of SB-16. Results show that, at lower concentrations (i.e. ≤20 mM), the different ILs modify the properties of the aqueous SB-16 solution in similar manner. All of them bring about a decrease in the critical micelle concentration (CMC) and also in size, and increase the aggregation number of the SB-16 micelles; these effects are more dramatic when [C(2)mim][C(6)SO(4)] is used as the additive rather than [C(2)mim][C(4)SO(4)] and [C(2)mim][C(2)SO(4)]. It is proposed that, in case of [C(2)mim][C(6)SO(4)], the presence of a hexyl chain on the hexylsulfate ion allows the ion to align itself with the tail part of SB-16, whereas, in the case of [C(2)mim][C(2)SO(4)], the presence of ethyl chain in the ethylsulfate ion is not sufficient to bring about a similar alignment of the ethylsulfate anion with the tail part of SB-16. This difference in the location of the anions of the ILs is responsible for the different behavior of the ILs.

Photoinduced electron transfer between various coumarin analogues and N,N-dimethylaniline inside niosome, a nonionic innocuous polyethylene glycol-based surfactant assembly.

Photoinduced electron transfer (ET) reactions between coumarin dyes and N,N-dimethylaniline have been investigated inside niosome, a nonionic innocuous polyethylene glycol (PEG)-based surfactant assembly using steady state and time-resolved fluorescence measurements. The location of coumarin dyes inside the bilayer headgroup region of niosome has been reported and it was verified by determination of the high distribution coefficient of all the dyes inside niosome compared to bulk water. Fluorescence anisotropy parameters of the dyes inside niosome are also in good correlation with the above inference about their location. Bimolecular diffusion guided rates inside niosome were determined by comparing the microviscosities inside niosome and in acetonitrile and butanol solutions and it was found that diffusion of the donor and the acceptor is much slower than the ET rates, implying insignificant role of reactant diffusion in ET reaction inside niosome. We have observed a Marcus inversion region in our restricted media, which shows maxima at lower exergonicity. Such behavior has been demonstrated by the presence of nonequilibrium solvent excited state using two dimensional ET (2DET) theory. Unusually high quenching rates of two coumarins C-152 and C-152A inside niosome were explained by the presence of a stable non-fluorescent twisted intramolecular charge transfer (TICT) state along with an emissive intramolecular charge transfer (ICT) state. Moreover, intermolecular hydrogen bonding between carbonyl oxygens of these two dyes and water in their non-emissive and emissive charge transfer states also plays a key role in their dynamical exchange with each other [G.-J. Zhao and K.-L. Han, Acc. Chem. Res., 2011].

Enhanced Catalytic Activity of a New Nanobiocatalytic System Formed by the Adsorption of Cytochrome c on Pluronic Triblock Copolymer Stabilized MoS2 Nanosheets.

The formation of nanobiohybrids through the immobilization of enzymes on functional nanomaterials has opened up exciting research opportunities at the nanobiointerfaces. These systems hold great promise for a wide range of applications in biosensing, biocatalytic, and biomedical fields. Here, we report the formation of a hybrid nanobiocatalytic system through the adsorption of cytochrome c (Cyt c) on pluronic triblock copolymer, P123 (PEO-b-PPO-b-PEO), stabilized MoS2 nanosheets. The use of pluronic polymer has helped not only to greatly stabilize the exfoliated MoS2 nanosheets but also to allow easy adsorption of Cyt c on the nanosheets without major structural changes due to its excellent biocompatibility and soft protein-binding property. By comparing the catalytic activity of the Cyt c-MoS2 nanobiohybrid with that of the free Cyt c and as-prepared MoS2 nanosheets, we have demonstrated the active role of the nanobiointeractions in enhancing the catalytic activity of the hybrid. Slight structural perturbation at the active site of the Cyt c upon adsorption on MoS2 has primarily facilitated the peroxidase activity of the Cyt c. As the MoS2 nanosheets and the native Cyt c individually exhibit weaker intrinsic peroxidase activities, their mutual modulation at the nanobiointerface has made the Cyt c-MoS2 a novel nanobiocatalyst with superior activity.

Nanotube Template-Directed Formation of Strongly Coupled Dye Aggregates with Tunable Exciton Fluorescence Controlled by Switching between J- and H-Type Electronic Coupling.

Strongly coupled dye aggregates with tailored exciton properties may find their use in developing artificial light-harvesting and optoelectronic devices. Here, we report the control of tubular pseudoisocyanine (PIC) dye J- and H-aggregate formation with tunable exciton fluorescence using lithocholic acid (LCA) as a template. The LCA-templated PIC J-aggregate nanotubes formed at a higher LCA/PIC molar ratio (≥5:1) exhibit a sharp, red-shifted absorption band (at 555 nm), intense fluorescence (at 565 nm), and shorter lifetime (200 ps), all indicating their strong superradiance properties. In contrast, the H-aggregate nanotubes formed at a lower LCA/PIC molar ratio (2:1) exhibit a significantly blue-shifted absorption band (at 420 nm) and highly red-shifted fluorescence emission (at 600 nm) with enhanced lifetime (4.40 ns). The controlled switching of the optical properties of the PIC dye aggregates achieved by controlling the LCA/PIC molar ratio could serve as an important guideline for the design of organic photo-functional materials.

Influence of the Electrochemical Properties of the Bacteriochlorophyll Dimer on Triplet Energy-Transfer Dynamics in Bacterial Reaction Centers.

Energetics, protein dynamics, and electronic coupling are the key factors in controlling both electron and energy transfer in photosynthetic bacterial reaction centers (RCs). Here, we examine the rates and mechanistic pathways of the P+HA- radical-pair charge recombination, triplet state formation, and subsequent triplet energy transfer from the triplet state of the bacteriochlorophyll dimer (P) to the carotenoid in a series of mutant RCs (L131LH + M160LH (D1), L131LH + M197FH (D2), and L131LH + M160LH + M197FH (T1)) of Rhodobacter sphaeroides. In these mutants, the electronic structure of P is perturbed and the P/P+ midpoint potential is systematically increased due to addition of hydrogen bonds between P and the introduced residues. High-resolution, broad-band, transient absorption spectroscopy on the femtosecond to microsecond timescale shows that the charge recombination rate increases and the triplet energy transfer rate decreases in these mutants relative to the wild type (WT). The increase of the charge recombination rate is correlated to the increase in the energy level of P+HA- and the increase in the P/P+ midpoint potential. On the other hand, the decrease in rate of triplet energy transfer in the mutants can be explained in terms of a lower energy of 3P and a shift in the electron spin density distribution in the bacteriochlorophylls of P. The triplet energy-transfer rate follows the order of WT > L131LH + M197FH > L131LH + M160LH > L131LH + M160LH + M197FH, both at room temperature and at 77 K. A pronounced temperature dependence of the rate is observed for all of the RC samples. The activation energy associated to this process is increased in the mutants relative to WT, consistent with a lower 3P energy due to the addition of hydrogen bonds between P and the introduced residues.

Mechanism of Triplet Energy Transfer in Photosynthetic Bacterial Reaction Centers.

In purple bacterial reaction centers, triplet excitation energy transfer occurs from the primary donor P, a bacteriochlorophyll dimer, to a neighboring carotenoid to prevent photodamage from the generation of reactive oxygen species. The BB bacteriochlorophyll molecule that lies between P and the carotenoid on the inactive electron transfer branch is involved in triplet energy transfer between P and the carotenoid. To expand the high-resolution spectral and kinetic information available for describing the mechanism, we investigated the triplet excited state formation and energy transfer pathways in the reaction center of Rhodobacter sphaeroides using pump-probe transient absorption spectroscopy over a broad spectral region on the nanosecond to microsecond time scale at both room temperature and at 77 K. Wild-type reaction centers were compared with a reaction center mutant (M182HL) in which BB is replaced by a bacteriopheophytin (Φ), as well as to reaction centers that lack the carotenoid. In wild-type reaction centers, the triplet energy transfer efficiency from P to the carotenoid was essentially unity at room temperature and at 77 K. However, in the M182HL mutant reaction centers, both the rate and efficiency of triplet energy transfer were decreased at room temperature, and at 77 K, no triplet energy transfer was observed, attributable to a higher triplet state energy of the bacteriopheophytin that replaces bacteriochlorophyll in this mutant. Finally, detailed time-resolved spectral analysis of P, carotenoid, and BB (Φ in the M182HL mutant) reveals that the triplet state of the carotenoid is coupled fairly strongly to the bridging intermediate BB in wild-type and Φ in the M182HL mutant, a fact that is probably responsible for the lack of any obvious intermediate 3BB/3Φ transient formation during triplet energy transfer.

Thermoregulated formation and disintegration of cationic block copolymer vesicles: fluorescence resonance energy transfer study.

Formation and disintegration of self-assembled nanostructures in response to external stimuli are important phenomena that have been widely explored for a variety of biomedical applications. In this contribution, we report the thermally triggered assembly of block copolymer molecules in aqueous solution to form vesicles (polymersomes) and their disassembly on reduction of temperature. A new thermoresponsive diblock copolymer of poly(N-isopropylacrylamide) poly((3-methacrylamidopropyl)trimethylammonium chloride) (PNIPA-b-PMAPTAC) was synthesized by reversible addition-fragmentation chain transfer technique. The solution properties and self-assembling behavior of the block copolymer molecules were studied by turbidimetry, temperature-dependent proton nuclear magnetic resonance, fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. Fluorescence resonance energy transfer studies between coumarin-153 (C-153, donor) and rhodamine 6G (R6G, acceptor) have been performed by steady-state and picosecond-resolved fluorescence spectroscopy to probe the structural and dynamic heterogeneity of the vesicles. The occurrence of efficient energy transfer was evident from the shortening of donor lifetime in the presence of the acceptor. The capability of the vesicles to encapsulate both hydrophobic and hydrophilic molecules and release them in response to decrease in temperature makes them potentially useful as drug delivery vehicles.

Vesicles formed in aqueous mixtures of cholesterol and imidazolium surface active ionic liquid: a comparison with common cationic surfactant by water dynamics.

The formation of stable unilamellar vesicles which hold great potential for biological as well as biomedical applications has been reported in the aqueous mixed solution of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium chloride ([C16mim]Cl) and cholesterol. To make a comparison we have also shown the formation of such stable vesicles using a common cationic surfactant, benzyldimethylhexadecylammonium chloride (BHDC) which has a similar alkyl chain length but different headgroup region to that of [C16mim]Cl. It has been revealed from dynamic light scattering (DLS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and other optical spectroscopic techniques that the micelles of [C16mim]Cl and BHDC in aqueous solutions transform into stable unilamellar vesicles upon increasing concentration of cholesterol. We find that, as the concentration of cholesterol increases, the solvation and rotational relaxation time of C153 in [C16mim]Cl/cholesterol solution as well as in BHDC/cholesterol solution gradually increases indicating a significant decrease in the hydration behavior around the self-assemblies upon micelle-vesicle transition. However, the extent of increase in solvation and rotational relaxation time is more prominent in the case of [C16mim]Cl/cholesterol solutions than in the BHDC/cholesterol system. This indicates that [C16mim]Cl/cholesterol vesicular membranes are comparatively less hydrated and more rigid than the BHDC/cholesterol vesicular bilayer.

Exploring the photophysics of curcumin in zwitterionic micellar system: an approach to control ESIPT process in the presence of room temperature ionic liquids (RTILs) and anionic surfactant.

In this manuscript, we have modulated the photophysical properties of curcumin in a zwitterionic (N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16)) micellar aggregates with addition of room temperature ionic liquids (RTILs) as well as commonly used anionic surfactant (SDS), using steady-state and time-resolved spectroscopic techniques. To modulate the photophysics, first we studied its interaction with an SB-16 micellar system, then to further exploit its photophysics, three RTILs (EmimES, EmimBS, EmimHS) with variation of alkyl chain lengths as well as SDS were used. It is observed that the rate of degradation of curcumin is drastically decreased after partitioning into the zwitterionic micellar system. It is shown that the dynamics of excited state intramolecular proton transfer (ESIPT) processes can be controlled by using those RTILs and SDS. Our study also reveals that the hindrance of nonradiative processes of curcumin, i.e., ESIPT is more pronounced in the case of RTIL containing a long alkyl chain compared to a small one. However, most interestingly the addition of long chain (dodecyl) anionic surfactant (SDS) promotes the ESIPT process of curcumin. We have also studied the effect of the addition of inorganic salt and compared the results with RTILs. The present work demonstrates an effort to decipher the photophysics of curcumin in zwitterionic micellar systems by monitoring its excited state dynamics.

Unique photophysical behavior of 2,2'-bipyridine-3,3'-diol in DMSO-water binary mixtures: potential application for fluorescence sensing of Zn2+ based on the inhibition of excited-state intramolecular double proton transfer.

In this work we have investigated the anomalous behavior of DMSO-water binary mixtures using 2,2'-bipyridine-3,3'-diol (BP(OH)2) as a microenvironment-sensitive excited-state-intramolecular-double-proton-transfer (ESIDPT) probe. Here we present results on the UV-vis absorption and fluorescence properties of BP(OH)2 in the binary solutions. DMSO-water binary mixtures at various compositions are an intriguing hydrogen bonded system, where DMSO acts to diminish the hydrogen bonding ability of water with the dissolved solutes. As a result, we observe unusual changes in the photophysical properties of BP(OH)2 with increasing DMSO content in complete correlation with the prior simulation and experimental results on the solvent structures and dynamics. The fluorescence quantum yield and fluorescence lifetime of BP(OH)2 depend strongly on the DMSO content and become maximum at very low mole fraction (∼0.12) of DMSO. The anomalous behavior at this particular region likely arises from the enhanced pair hydrophobicity of the medium as demonstrated by Bagchi and co-workers (Banerjee, S.; Roy, S.; Bagchi, B. J. Phys. Chem. B 2010, 114, 12875-12882). In addition we have also shown the utilization of BP(OH)2 as a potential Zn(2+)-selective fluorescent sensor in a 1:1 DMSO-water binary mixture useful for biological applications. We observed highly enhanced fluorescence emission of BP(OH)2 selectively for binding with the Zn(2+) metal ion. Moreover, the fluorescence emission maximum of BP(OH)2-Zn(2+) is significantly blue-shifted with a reduced Stokes shift due to the inhibition of the ESIDPT process of BP(OH)2 through strong coordination.