A femtosecond study of excitation-wavelength dependence of solvation dynamics in a vesicle

Does Viscosity Decoupling Guarantee Dynamic Heterogeneity? A Clue through an Excitation and Emission Wavelength-Dependent Time-Resolved Fluorescence Anisotropy Study

Traditionally, deviation from Stokes-Einstein-Debye (SED) relation in terms of viscosity dependence of medium dynamics, i.e., τ x ∝ ( η T ) p with p ≠ 1, is taken as a signature of dynamic heterogeneity. However, it does not guarantee medium heterogeneity, as the decoupling may also originate from the deviation of the basic assumption of SED. Here, we developed a method to find a stronger relation between viscosity decoupling (p ≠ 1) and dynamic heterogeneity in terms of rotational motion. Our approach exploited the fact that in heterogeneous media, a solvatochromic probe will be solvated to a different extent at different microdomains (subpopulations), and photoselection of these subpopulations can be achieved by excitation or emission wavelength-dependent measurements. We hypothesized that the dynamics of a homogeneous system might show viscosity decoupling, but the extent of decoupling at different excitations (or at different emissions) should not be different. On the other hand, in a heterogeneous medium, this extent of viscosity decoupling (p-value) should be different at different excitations (or at different emissions). As proof of concept, we investigated three versatile solvent media: squalane (viscous molecular liquid), 1-ethyle-3-methylimidazolium ethyl sulfate ionic liquid (IL), and [0.78 acetamide + 0.22 LiNO3] deep eutectic solvent (DES). We found that squalane is homogeneous, although it shows fractional viscosity dependence (p ≠ 1). Interestingly, mild heterogeneity in IL and significant heterogeneity in the DES were observed. Overall, we conclude that the difference in the p-value as a function of excitation (or emission) wavelength-dependent might be a superior way for the detection of dynamic heterogeneity.

Dodecahedron CsPbBr3 Perovskite Nanocrystals Enable Facile Harvesting of Hot Electrons and Holes

This Letter reports the facile harvesting of hot carriers (HCs) in a composite of 12-faceted dodecahedron CsPbBr₃ nanocrystal (NC) and a scavenger molecule. We recorded ∼3.3 × 10¹¹ s^-1 HC cooling rate in NC when excited with ∼1.4 times the band gap energy (E_g), increasing to >3 × 10¹² s^-1 in the presence of scavengers at high concentration due to the HC extractions. Since the observed intrinsic charge transfer rate (∼1.7 × 10¹² s^-1) in our NC-scavenger complex is about an order of magnitude higher than the HC cooling rate (∼3.3 × 10¹¹ s^-1), carriers are harvested before their cooling. Further, a fluorescence correlation spectroscopy study reveals NC tends to form a quasi-stable complex with a scavenger molecule, ensuring charge transfer completed (τ_ct ≈ 0.6 ps) much before the complex breaks apart (>600 μs). The overall results of our study highlight the promise shown by 12-faceted NCs and their implications in modern applications, including hot carrier solar cells.

Facet Engineering for Decelerated Carrier Cooling in Polyhedral Perovskite Nanocrystals

We report here the hot carrier (HC) cooling time scales within polyhedral CsPbBr₃ nanocrystals (NCs) characterized by different numbers of facets (6 to 26) utilizing a femtosecond upconversion setup. Interestingly, the observed cooling time scale slows many-fold (>10 times) upon opening the new facets on the NC surface. Furthermore, a temperature-dependent study reveals that cooling in multifaceted NCs is polaron mediated, where newly opened polar facets and the soft lattice of CsPbBr₃ NCs play pivotal roles. Our hallmark result of slow cooling in polyhedral NCs renders an excellent opportunity for harvesting high-energy carriers by a carefully chosen molecular system. To this end, employing the hole scavenger molecule aniline, we successfully extracted hot holes from optically pumped NCs. We believe that several intriguing properties of the polyhedral NCs, including rapid polaron formation, defect-tolerant nature, and the capability of soft lattice to support slow diffusion of charge carriers, resulted in decelerated cooling.

Subpicosecond Hot Hole Transfer in a Graphene Quantum Dot Composite with High Efficiency

Extraction of hot carriers is of prime importance because of its potential to overcome the energy loss that limits the efficiency of an optoelectronic device. Employing a femtosecond upconversion setup, herein we report a few picoseconds carrier cooling time of colloidal graphene quantum dots (GQDs) is at least an order of magnitude slower compared to that in its bulk form. A slower carrier cooling time of GQDs compared to that of the other semiconductor quantum dots and their bulk materials is indeed a coveted property of GQDs that would allow one easy harvesting of high energy species employing a suitable molecular system as shown in this study. A subpicosecond hot hole transfer time scale has been achieved in a GQD-molecular system composite with high transfer efficiency. Our finding suggests a dramatic enhancement of the efficiency of GQD based optoelectronic devices can possibly be a reality.

A Comparative Study on DMSO-Induced Modulation of the Structural and Dynamical Properties of Model Bilayer Membranes

Modulating the structures and properties of biomembranes via permeation of small amphiphilic molecules is immensely important, having diverse applications in cell biology, biotechnology, and pharmaceuticals, because their physiochemical and biological interactions lead to new pathways for transdermal drug delivery and administration. In this work, we have elucidated the role of dimethyl sulfoxide (DMSO), broadly used as a penetration-enhancing agent and cryoprotective agent on model lipid membranes, using a combination of fluorescence microscopy and time-resolved fluorescence spectroscopy. Spatially resolved fluorescence lifetime imaging microscopy (FLIM) has been employed to unravel how the fluidity of the DMSO-induced bilayer regulates the structural alteration of the vesicles. Moreover, we have also shown that the dehydration effect of DMSO leads to weakening of the hydrogen bond between lipid headgroups and water molecules and results in faster solvation dynamics as demonstrated by femtosecond time-resolved fluorescence spectroscopy. It has been gleaned that the water dynamics becomes faster because bilayer rigidity decreases in the presence of DMSO, which is also supported by time-resolved rotational anisotropy measurements. The enhanced diffusivity and increased membrane fluidity in the presence of DMSO are further ratified at the single-molecule level through fluorescence correlation spectroscopy (FCS) measurements. Our results indicate that while the presence of DMSO significantly affects the 1,2-dimyristoyl-rac-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-rac-glycero-3-phosphatidylcholine (DPPC) bilayers, it has a weak effect on 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG) vesicles, which might explain the preferential interaction of DMSO with the positively charged choline group present in DMPC and DPPC vesicles. The experimental findings have also been further verified with molecular dynamics simulation studies. Moreover, it has been observed that DMSO is likely to have a differential effect on heterogeneous bilayer membranes depending on the structure and composition of their headgroups. Our results illuminate the importance of probing the lipid structure and composition of cellular membranes in determining the effects of cryoprotective agents.

Modulation of Membrane Fluidity to Control Interfacial Water Structure and Dynamics in Saturated and Unsaturated Phospholipid Vesicles

The structure and dynamics of interfacial water in biological systems regulate the biochemical reactions. But, it is still enigmatic how the behavior of the interfacial water molecule is controlled. Here, we have investigated the effect of membrane fluidity on the structure and dynamics of interfacial water molecules in biologically relevant phopholipid vesicles. This study delineates that modulation of membrane fluidity through interlipid separation and unsaturation not only mitigate membrane rigidity but also disrupt the strong hydrogen bond (H-bond) network around the lipid bilayer interface. As a result, a disorder in H-bonding between water molecules arises several layers beyond the first hydration shell of the polar headgroup, which essentially modifies the interfacial water structure and dynamics. Furthermore, we have also provided evidence of increasing transportation through these modulated membranes, which enhance the membrane mediated isomerization reaction rate.

Dynamics of the vesicles composed of fatty acids and other amphiphile mixtures: unveiling the role of fatty acids as a model protocell membrane

Fundamental research at the interface of chemistry and biology has the potential to shine light on the question of how living cells can be synthesized from inanimate matter thereby providing plausible pathways for the emergence of cellular life. Compartmentalization of different biochemical reactions within a membrane bound water environment is considered an essential first step in any origin of life pathway. It has been suggested that fatty acid-based vesicles can be considered a model protocell having the potential for change via Darwinian evolution. As such, protocell models have the potential to assist in furthering our understanding of the origin of life in the laboratory. Fatty acids, both by themselves and in mixtures with other amphiphiles, can form different self-assembled structures depending on their surroundings. Recent studies of fatty acid-based membranes have suggested likely pathways of protocell growth, division and membrane permeabilisation for the transport of different nutrients, such as nucleotides across the membrane. In this review, different dynamic processes related to the growth and division of the protocell membrane are discussed and possible pathways for transition of the protocell to the modern cell are explored. These areas of research may lead to a better understanding of the synthesis of artificial cell-like entities and thus herald the possibility of creating new form of life distinct from existing biology. Graphical Abstract Table of Content (TOC) only.

Selective Self-Assembly of 5-Fluorouracil through Nonlinear Solvent Response Modulates Membrane Dynamics

Controllable self-assembly and understanding of the interaction between single metabolite fibrils and live-cell membranes have paramount importance in providing minimal treatment in several neurodegenerative disorders. Here, utilizing the nonlinear nature and peculiar hydrogen bonding behavior of the dimethyl sulfoxide (DMSO)-water mixture, the selective self-assembly of a single metabolite 5-fluorouracil (5-FU) is achieved. A direct correlation between water availability and selective self-assembly of 5-FU is ratified from the excited-state dynamics. The specific fibrillar structures of 5-FU exhibit a great potential to modulate live cell membrane fluidity and model membrane lipid distribution. After 5-FU fibril addition, a disorder of H-bonded water molecules arises several layers beyond the first hydration shell of the polar headgroups, which essentially modifies interfacial water structure and dynamics. Overall, our results shed light on the role of solvent to govern specific self-assembly and also lay the foundation accounting for the earlier stage of several diseases and multidrug resistance.

Antagonist Effects of l-Phenylalanine and the Enantiomeric Mixture Containing d-Phenylalanine on Phospholipid Vesicle Membrane

One of the congenital flaws of metabolism, phenylketonuria (PKU), is known to be related to the self-assembly of toxic fibrillar aggregates of phenylalanine (Phe) in blood at elevated concentrations. Our experimental findings using l-phenylalanine (l-Phe) at millimolar concentration suggest the formation of fibrillar morphologies in the dry phase, which in the solution phase interact strongly with the model membrane composed of 1,2-diacyl-sn-glycero-phosphocholine (LAPC) lipid, thereby decreasing the rigidity (or increasing the fluidity) of the membrane. The hydrophobic interaction, in addition to the electrostatic attraction of Phe with the model membrane, is found to be responsible for such phenomena. On the contrary, various microscopic observations reveal that such fibrillar morphologies of l-Phe are severely ruptured in the presence of its enantiomer d-phenylalanine (d-Phe), thereby converting the fibrillar morphologies into crushed flakes. Various biophysical studies, including the solvation dynamics experiment, suggest that this l-Phe in the presence of d-Phe, when interacting with the same model membrane, now reverts the rigidity of the membrane, i.e., increases the rigidity of the membrane, which was lost due to interaction with l-Phe exclusively. Fluorescence anisotropy measurements also support this reverse rigid character of the membrane in the presence of an enantiomeric mixture of amino acids. A comprehensive understanding of the interaction of Phe with the model membrane is further pursued at the single-molecular fluorescence detection level using fluorescence correlation spectroscopy (FCS) experiments. Therefore, our experimental conclusion interprets a linear correlation between increased permeability and enhanced fluidity of the membrane in the presence of l-Phe and certifies d-Phe as a therapeutic modulator of l-Phe fibrillar morphologies. Further, the study proposes that the rigidity of the membrane lost due to interaction with l-Phe was reinstated-in fact, increased-in the presence of the enantiomeric mixture containing both d- and l-Phe.

Modulation of Membrane Fluidity Performed on Model Phospholipid Membrane and Live Cell Membrane: Revealing through Spatiotemporal Approaches of FLIM, FAIM, and TRFS

We have elucidated the role of unsaturated fatty acid in the in vitro model phospholipid membrane and in vivo live cell membrane. Fluorescence microscopy and time-resolved fluorescence spectroscopy have been employed to uncover how modulation of vesicle bilayer fluidity persuades structural transformation. This unsaturation induced structural transformation due to packing disorder in bilayer has been delineated through spatially resolved fluorescence lifetime imaging microscopy (FLIM) and fluorescence polarization or anisotropy imaging microscopy (FPIM/FAIM). Structure-function relationship of phospholipid vesicle is also investigated by monitoring intervesicular water dynamics behavior, which has been demonstrated by temporally resolved fluorescence spectroscopy (TRFS) techniques. Nevertheless, it has also been manifested from this study that loss of rigidity in bilayer breaks down the strong hydrogen bond (H-bond) network around the charged lipid head groups. The disruption of this H-bond network increases the bilayer elasticity, which helps to evolve various kinds of vesicular structure. Furthermore, the significant influence of unsaturated fatty acid on membrane bilayer has been ratified through in vivo live cell imaging.

Self-Assembly of Amphiphiles into Vesicles and Fibrils: Investigation of Structure and Dynamics Using Spectroscopy and Microscopy Techniques

Amphiphiles are a class of molecules which are known to assemble into a variety of nanostructures. The understanding and applications of self-assembled systems are based on what has been learned from biology. Among the vast number of self-assemblies, in this article, we have described the formation, characterization, and dynamics of two important biologically inspired assemblies: vesicles and fibrils. Vesicles, which can be classified into several categories depending on the sizes and components, are of great interest due to their potential applications in drug delivery and as nanoscale reactors. The structure and dynamics of vesicles can also mimic the complex geometry of the cell membrane. On the other hand, the self-assembly of proteins, peptides, and even single amino acids leads to a number of degenerative disorders. Thus, a complete understanding of these self-assembled systems is necessary. In this article, we discuss recent work on vesicular aggregates composed of phospholipids, fatty acids, and ionic as well as nonionic surfactants and single amino acid-based fibrils such as phenylalanine and tyrosine. Beside the characterization, we also emphasize the excited-state dynamics inside the aggregates for a proper understanding of the organization, reactivity, and heterogeneity of the aggregates.

Optically Controlled Electron-Transfer Reaction Kinetics and Solvation Dynamics: Effect of Franck-Condon States

Experimental results for optically controlled electron-transfer reaction kinetics (ETRK) and nonequilibrium solvation dynamics (NESD) of Coumarin 480 in DMPC vesicle show their dependence on excitation wavelength λ_ex. However, the celebrated Marcus theory and linear-response-theory-based approaches for ETRK and NESD, respectively, predict both of the processes to be independent of λ_ex. The above said lacuna in these theories prompted us to develop a novel theory in 1D space, where the effect of innumerable Franck-Condon states is included through λ_ex. The present theory not only sheds light on the origin of failure of the existing theories but also gives the correct trend for the effect of λ_ex on ETRK and NESD. More importantly, the calculated results of NESD are in excellent agreement with the experimental results for different values of λ_ex. The new theory will therefore advance the knowledge of scientific community on the dynamics of photoinduced nonequilibrium processes.

Ultrafast solvation dynamics and charge transfer reactions in room temperature ionic liquids

In this perspective, we review the recent studies concerning the liquid structure and solvation dynamics of ionic liquids, and their influence upon electron transfer and charge transfer reactions.

Solvation dynamics under a microscope: single giant lipid vesicle

Picosecond spectroscopy under a confocal microscope is employed to study solvation dynamics of coumarin 153 (C153) inside a single giant lipid vesicle (1,2-dilauroyl-sn-glycero-3-phosphocholine, DLPC) of diameter 20 μm. Fluorescence correlation spectroscopy (FCS) indicates that the diffusion coefficient (D(t)) of the probe (coumarin153, C153) in the immobilized vesicle displays a wide distribution from ~3 to 21 μm(2) s(-1). The distribution of D(t) suggests that the microenvironment of the probe (C153) is highly heterogeneous and the local friction is different for probe molecules in different regions. The values of D(t) is significantly smaller than that for the same dye in bulk water (550 μm(2) s(-1)). This suggests that the probe is located in the interface or membrane region rather than in the water pool of the vesicle. The solvation time of C153 in different regions of the lipid vesicle varies between 750 to 1200 ps. This result clearly shows that a confocal microscope is able to resolve the spatial heterogeneity in local friction (i.e., D(t)) and solvation dynamics within a lipid vesicle.

Solvation oscillations and excited-state dynamics of 2-amino- and 2-hydroxy-7-nitrofluorene and its 2'-deoxyriboside

Push-pull substituted fluorenes are considered for use as dynamic solvation probes in polynucleotides. Their fluorescence band is predicted (by simulations) to show weak spectral oscillations on the subpicosecond time scale depending on the nucleotide sequence. The oscillations reflect the local far-infrared spectrum of the environment around the probe molecule. A connection is provided by the continuum theory of polar solvation which, however, neglects molecular aspects. We examine the latter using acetonitrile solution as a test case. A collective librational solvent mode at 100 cm(-1) is observed with 2-amino-7-nitrofluorene, 2-dimethylamino-7-nitrofluorene, 2-hydroxy-7-nitrofluorene, and its 2'-deoxyriboside. Different strengths of the oscillation indicate that rotational friction of nearby acetonitrile molecules depends on the solute structure or that H bonding is involved in launching the librational coherence. Polar solvation in methanol is used for comparison. With hydroxynitrofluorenes, the observation window is limited by intersystem crossing for which rates are reported. A prominent excited-state absorption band of nitrofluorenes at 430 nm can be used to monitor polar solvation. Structural and electronic relaxation pathways are discussed with the help of quantum chemical calculations.

Ultrafast Fluorescence Resonance Energy Transfer in the Micelle and the Gel Phase of a PEO−PPO−PEO Triblock Copolymer: Excitation Wavelength Dependence

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in the micelle and the gel phase of a triblock copolymer, (PEO)20-(PPO)70-(PEO)20 (Pluronic P123 (P123)) by picosecond and femtosecond emission spectroscopy. The time constants of FRET were obtained from the rise time of the acceptor (R6G) emission. In a P123 micelle, FRET occurs in multiple time scales: 2.5, 100, and 1700 ps. In the gel phase, three rise components are observed: 3, 150, and 2600 ps. According to a simple Förster model, the ultrafast (2.5 and 3 ps) components of FRET correspond to donor-acceptor distance RDA=13 +/- 2 A. The ultrafast FRET occurs between a donor and an acceptor residing at close contact at the corona (PEO) region of a P123 micelle. With increase in the excitation wavelength (lambdaex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET ( approximately 3 ps) increases from 13% to 100% in P123 micelle and from 1% to 100% in P123 gel. It is suggested that at lambdaex = 435 nm, mainly the highly polar peripheral region is probed where FRET is very fast due to close proximity of the donor and the acceptor. The 100 and 150 ps components correspond to RDA = 25 +/- 2 A and are ascribed to FRET from C480 deep inside the micelle to an acceptor (R6G) in the peripheral region. The very long component of FRET (1700 ps in micelle and 2600 ps component in gel) may arise from diffusion of the donor from outside the micelle to the interior followed by fast FRET.

Excitation Wavelength Dependence of Solvation Dynamics in a Supramolecular Assembly: PEO−PPO−PEO Triblock Copolymer and SDS

The triblock copolymer (PEO)20-(PPO)70-(PEO)20 (P123) forms a supramolecular aggregate with sodium dodecyl sulfate (SDS). The solvation dynamics and anisotropy decay of coumarin 480 (C480) in different regions of a P123-SDS aggregate are studied through variation of the excitation wavelength (lambdaex) using femtosecond upconversion. In a P123 micelle, because of the drastic differences in polarity between the hydrophilic corona region (PEO block) and the hydrophobic PPO core, C480 exhibits a pronounced red edge excitation shift (REES) of emission maximum by 24 nm. In the P123-SDS aggregate, SDS penetrates the core of the P123 micelle. This increases the polarity of the core and reduces the difference in the polarity between the core and the corona region. In a P123-SDS aggregate, the REES is much smaller (5 nm) which suggests a reduced difference between the core and the corona. Solvation dynamics in a P123 micelle displays a bulklike ultrafast component (<0.3 and 1 ps) in the PEO corona region, a 200 ps component arising from dynamics of polymer segments, and a very long component (5000 or 3000 ps) due to the highly restricted PPO core. In a P123-SDS aggregate, at lambdaex = 375 and 405 nm, the solvation dynamics is found to be faster than that in P123 micelle. In this case, the component (3000 ps) arising from the core region is faster than that (5000 ps) in P123 micelle. In both P123 micelle and P123-SDS aggregate, the relative contribution of the core region decreases and that of the corona region increases with an increase in lambdaex. At lambdaex = 435 nm, which probes the hydrophilic corona, the solvation dynamics for both P123 micelle and P123-SDS aggregate are almost similar.

Ultrafast fluorescence resonance energy transfer in a reverse micelle: Excitation wavelength dependence

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to fluorescein 548 (F548) in a sodium dioctyl sulfosuccinate (AOT) reverse micelle is studied by picosecond and femtosecond emission spectroscopy. In bulk water, at the low concentration of the donor (C480) and the acceptor (F548), no FRET is observed. However, when the donor (C480) and the acceptor (F548) are confined in a AOT reverse micelle very fast FRET is observed. The time constants of FRET were obtained from the rise time of the emission of the acceptor (F548). In a AOT microemulsion, FRET is found to occur in multiple time scales--3, 200, and 2700 ps. The 3 ps component is assigned to FRET in the water pool of the reverse micelle with a donor-acceptor distance, 16 A. The 200 ps component corresponds to a donor-acceptor distance of 30 A and is ascribed to the negatively charged acceptor inside the water pool and the neutral donor inside the alkyl chains of AOT. The very long 2700 ps component may arise due to FRET from a donor outside the micelle to an acceptor inside the water pool and also from diffusion of the donor from bulk heptane to the reverse micelle. With increase in the excitation wavelength from 375 to 405 nm the relative contribution of the FRET due to C480 in the AOT reverse micelle (the 3 and 200 ps components) increases.