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(<i>N</i>-isopropylacrylamide) poly­((3-methacrylamidopropyl)­trimethylammonium chloride) (PNIPA-<i>b</i>-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

Integrating Highly Luminescent Lanthanides with Strongly Coupled Dye J‑Aggregates on Nanotubes for Efficient Cascade Energy Transfer

Photoluminescent one-dimensional hybrid nanostructured materials having outstanding inorganic–organic advantages are gaining significant attention on account of their intriguing applications in nanoscale optoelectronic devices, (bio)­sensors, and energy harvesting and conversion technologies. Here, we first report on the development of highly photoluminescent lanthanide organic hybrid nanotubular assemblies through in situ incorporation of a trivalent lanthanide ion, terbium (Tb3+), along with organic photosensitizers 2,3-dihydroxynaphthalene (DHN) or 1,10-phenanthroline (Phen) into the self-assembled nanotubes of sodium lithocholate (NaLC). Both the photosensitizers (DHN/Phen) are effective in sensitizing intense narrow emission peaks of Tb3+ on the nanotubes. Next, we utilize these luminescent lanthanides containing hybrid nanotubular assemblies as templates for spontaneous integration of strongly coupled pseudoisocyanine (PIC) dye J-aggregates with a sharp J-band absorption at 555 nm and strong fluorescence emission at 570 nm. The presence of the significant spectral overlap between the luminescence peak of Tb3+ at 545 nm and the J-aggregate absorption band results in efficient cascade energy transfer from photosensitizers to Tb3+ to the coherently coupled PIC dye J-aggregates. These NaLC nanotube-templated photosensitizer-Tb3+-J-aggregate hybrid systems have great potential for sensing and optoelectronic applications

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

Roles of Viscosity, Polarity, and Hydrogen-Bonding Ability of a Pyrrolidinium Ionic Liquid and Its Binary Mixtures in the Photophysics and Rotational Dynamics of the Potent Excited-State Intramolecular Proton-Transfer Probe 2,2′-Bipyridine-3,3′-diol

The room-temperature ionic liquid [C₃mpyr]­[Tf₂N] and its binary mixtures with methanol and acetonitrile provide microenvironments of varying viscosity, polarity, and hydrogen-bonding ability. The present work highlights their effects on the photophysics and rotational dynamics of a potent excited-state intramolecular double-proton-transfer (ESIDPT) probe, 2,2′-bipyridine-3,3′-diol [BP­(OH)₂]. The rotational diffusion of the proton-transferred diketo (DK) tautomer in [C₃mpyr]­[Tf₂N] ionic liquid was analyzed for the first time from the experimentally obtained temperature-dependent fluorescence anisotropy data using Stokes–Einstein–Debye (SED) hydrodynamic theory and Gierer–Wirtz quasihydrodynamic theory (GW-QHT). It was found that the rotation of the DK tautomer in neat ionic liquid is governed solely by the viscosity of the medium, as the experimentally observed boundary-condition parameter, <i>C</i>_obs, was very close to the GW boundary-condition parameter (<i>C</i>_GW). On the basis of photophysical studies of BP­(OH)₂ in IL–cosolvent binary mixtures, we suggest that methanol molecules form hydrogen bonds with the cationic counterpart of the DK tautomers, as evidenced by the greater extent of the decrease in the fluorescence lifetime of BP­(OH)₂ upon addition of methanol compared to acetonitrile. It is also possible for the methanol molecules to form hydrogen bonds with the constituents of the RTIL, which is supported by the lesser extent of the decrease in the viscosity of the medium upon addition of methanol, leading to a less effective decrease in the rotational relaxation time compared to that observed upon acetonitrile addition

Tuning the Probe Location on Zwitterionic Micellar System with Variation of pH and Addition of Surfactants with Different Alkyl Chains: Solvent and Rotational Relaxation Studies

In this manuscript, we have modulated the location of an anionic probe, Coumarin-343 (C-343) in a zwitterionic (<i>N</i>-hexadecyl-<i>N</i>,<i>N</i>-dimethylammonio-1-propanesulfonate (SB-16)) micellar system by three different approaches. The effect of addition of the surfactant sodium dodecyl sulfate (SDS) and the room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium octylsulfate (EmimOs) and <i>N</i>,<i>N</i>-dimethylethanol hexanoate (DAH), to the micellar solution has been studied. The effect of pH variation has been studied as well using solvent and rotational measurements. Migration of the anionic probe, C-343, from the palisade layer of SB-16 micelle to the bulk water has been observed to varying extents with the addition of SDS and EmimOs. The effect is much more pronounced in the presence of SDS and can be ascribed to the presence of the long alkyl (dodecyl) chain on SDS which can easily orient itself and fuse inside the SB-16 micelle and facilitate the observed migration of the probe molecule. This phenomenon is confirmed by faster solvation and rotational relaxation of the investigated probe molecule. The analogous fusion process is difficult in case of EmimOs and DAH because of their comparatively smaller alkyl (octyl and hexanoate) chain. However, the direction of C-343 migration is reversed with the decrease of pH of the SB-16 micellar medium. An increase in the average solvation and rotational relaxation time of the probe in acidic medium has been observed. Since experimental conditions are maintained such that the probe molecules and the zwitterionic SB-16 micelles remain oppositely charged, the observed results can be attributed to the increased electrostatic interaction (attractive) between them. Temperature dependent study also supports this finding

Modulation of Photophysics and Photodynamics of 1′-Hydroxy-2′-acetonaphthone (HAN) in Bile Salt Aggregates: A Study of Polarity and Nanoconfinement Effects

The modulation of the photophysical properties of 1′-hydroxy-2′-acetonaphthone (HAN) upon encapsulation into the hydrophobic nanocavities of different bile salt aggregates has been investigated for the first time using steady-state and time-resolved fluorescence spectroscopy. Because HAN is very sensitive to the polarity of the microenvironment in which it is confined, we performed a comparative study on the excited-state binding dynamics of HAN using three different bile salts of varying hydrophobicity. The encapsulation of HAN into the bile salt aggregates led to an enhanced fluorescence intensity along with a significant blue shift in the emission maxima that was highly sensitive to the confined microenvironment. Using HAN as a sensitive fluorophore to probe the nanocavities of bile salt aggregates in aqueous solution, we found different mechanisms of probe encapsulation depending on the degree of hydrophobicity of the nanocavities, which results in a difference in the alteration of the spectral behavior. A sharp increase in the fluorescence quantum yield near the cmc was observed, followed by saturation for all three bile salt aggregates. However, maximum fluorescence quantum yield in NaDC aggregates can be rationalized by maximum partitioning of HAN into the more hydrophobic and rigid environment provided by NaDC aggregates. Moreover, the alteration of the spectral behavior with increasing concentration of bile salts strikingly differs from that observed previously in the presence of conventional surfactants. Time-resolved fluorescence measurements further elucidated how the probe molecules interact with the aggregates. Longer fluorescence lifetime and anisotropy values clearly indicate the caging of the tautomers of HAN into the hydrophobic nanocavities of bile salt aggregates. This work further demonstrates the changes in the fluorescence properties of HAN with structural changes of bile salt aggregates induced by the addition of salt and organic cosolvent

Solvation Dynamics and Rotational Relaxation Study Inside Niosome, A Nonionic Innocuous Poly(ethylene Glycol)-Based Surfactant Assembly: An Excitation Wavelength Dependent Experiment

Excitation wavelength dependence of solvation and rotational relaxation dynamics has been investigated inside niosome, a biologically stable, nontoxic to our body, multilamellar vesicle system, by using steady state and time-resolved fluorescence spectroscopy to explore the heterogeneity of such a system. Red edge excitation shifts (REES) of 7 nm for Coumarin-153 (C-153) and 11 nm for C-480 were observed with change in λ_ex. Average solvation dynamics is composed of two types of slow components and one fast component. There are two distinct restricted regions, one at the bilayer headgroup region and the other on the two extreme surfaces, which are responsible for the slow components. An unaltered fast component is reported for the segmental chain dynamics of poly(ethylene glycol) (PEG) located at the headgroup region of niosome. The trend in λ_ex dependence obtained for C-153 is found to be similar to that obtained for C-480. Such hindered solvation is attributed to the presence of a strong H-bonding environment of water molecules in the headgroup region, and movement of these highly bound water molecules along with a hydrated oxyethyelene moiety control the observed slow relaxation

Photophysics of 3,3′-Diethyloxadicarbocyanine Iodide (DODCI) in Ionic Liquid Micelle and Binary Mixtures of Ionic Liquids: Effect of Confinement and Viscosity on Photoisomerization Rate

The dynamics of photoisomerization of 3,3′-diethyloxadicarbocyanine iodide (DODCI) has been investigated inside micellar environment formed by a surfactant-like ionic liquid, 1-butyl-3-methylimidazolium octyl sulfate ([C₄mim]­[C₈SO₄]) and also in binary mixture of another ionic liquid, <i>N</i>,<i>N</i>,<i>N</i>-trimethyl-<i>N</i>-propyl ammonium bis­(trifluoromethanesulfonyl) imide, ([N₃₁₁₁]­[Tf₂N]) with methanol, acetonitrile, and <i>n</i>-propanol by using steady-state and picosecond time-resolved fluorescence spectroscopy. The entrapment of DODCI into the [C₄mim]­[C₈SO₄] micellar environment led to the enhanced fluorescence intensity along with ∼13 nm red shift in the emission maxima. A sharp increase in the fluorescence quantum yield (Φ) and the lifetime (τ_<i>f</i>) near the critical micelle concentration (cmc) range is observed followed by saturation at higher concentration. As a result of partitioning of the probe molecules in the micellar phase from water, the nonradiative rate constant (<i>k</i>_nr) of DODCI decreases 2.7 times than in water. The retardation of isomerization rate is due to high microviscosity of the micellar system compared to bulk water. In order to understand how the rate of isomerization depends on polarity as well as viscosity, we have measured isomerization rate in neat [N₃₁₁₁]­[Tf₂N] and its mixtures with polar solvents, like methanol, acetonitrile, and <i>n</i>-propanol. The addition of methanol and <i>n</i>-propanol increases the polarity, but viscosity of the medium decreases. The nonradiative rate constant that represents the rate of photoisomerization decreases with the addition of the polar solvent in [N₃₁₁₁]­[Tf₂N]. Complete analysis of all the experimental results indicate that viscosity is the sole parameter that regulates the rate of photoisomerization. Temperature-dependent <i>k</i>_nr are used to determine the activation energy (<i>E</i>_a) in 100 mM [C₄mim]­[C₈SO₄] solution and neat [N₃₁₁₁]­[Tf₂N] system

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. <i>J. Phys. Chem. B</i> <b>2010</b>, <i>114</i>, 12875–12882). In addition we have also shown the utilization of BP­(OH)₂ as a potential Zn²⁺-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 Zn²⁺ metal ion. Moreover, the fluorescence emission maximum of BP­(OH)₂-Zn²⁺ is significantly blue-shifted with a reduced Stokes shift due to the inhibition of the ESIDPT process of BP­(OH)₂ through strong coordination