Fluorescence resonance energy transfer—a spectroscopic probe for organized surfactant media

Addressing the Superior Drug Delivery Performance of Bilosomes─A Microscopy and Fluorescence Study

The global health scenario in present times has raised human awareness about drug delivery strategies. Among colloidal drug delivery vehicles, vesicular nanocarriers such as liposomes and niosomes are popular. However, liposomes and niosomes get disrupted in the harsh environment of the gastrointestinal tract. In this context, the drug delivery community has reported the superior performance of vesicles containing bile salts, that is, bilosomes. The present work attempts to examine the structural/morphological aspects underlying the superior performance of bilosomes. Optical microscopy, electron microscopy, and light scattering give a definite proof of the enhanced stability of bilosomes compared to niosomes, both prepared from the same amphiphilic molecule. Fluorescence probing of the vesicles provides detailed insight into the bilayer characteristics and the differences between bilosomes and niosomes. Fluorescence resonance energy transfer studies lend further support to the findings that bilosomes have a more flexible bilayer structure than niosomes. The entrapment efficiency of the vesicles for the well-known antioxidant curcumin (whose bioavailability is a matter of concern due to low water solubility) was also studied. Bilosomes show higher curcumin entrapment efficiency than niosomes. For use in drug delivery, one needs to establish a trade-off between cargo/drug entrapment and release. Thus, a flexible bilayer structure is an advantage.

Macromolecular Crowding Effect on the Activity of Liposome-Bound Alkaline Phosphatase: A Paradoxical Inhibitory Action

The cytoplasm of a cell is extremely crowded, with 20-30% being large biomolecules. This crowding enforces a significant amount of the physical and chemical barrier around biomolecules, so understanding any biomolecular event within the cellular system is challenging. Unsurprisingly, enzymes show a diverse kind of catalytic behavior inside a crowded environment and thus have remained an area of active interest in the last few decades. The situation can become even more complex and exciting in the case of understanding the behavior of a membrane-bound enzyme (almost 25-30% of enzymes are membrane-bound) in such a crowded environment that until now has remained unexplored. Herein, we have particularly investigated how a membrane-bound enzyme (using liposome-bound alkaline phosphatase) can behave in a crowded environment comprising polymer molecule-like poly(ethylene glycol) (PEG) of different weights (PEG400, PEG4000, and PEG9000) and Ficoll 400. We have compared the activity using a polymer microbead conjugated enzyme and have found that liposome-bound alkaline phosphatase had much higher activity in crowded environments, showing the importance and superiority of soft-deformable particles (i.e., vesicles) over hard spheres in macro-molecularly crowded media. Interstingly, we have found a paradoxical behavior of inhibitors in terms of both their extent and pathway of inhibitory action. For instance, phosphates, known as competitive inhibitors in buffer, behave as uncompetitive inhibitors in liposome-bound enzymes in crowded media with an ∼5-fold less inhibitory effect, whereas phenyl alanine (an uncompetitive inhibitor in buffer) did not show any inhibitory potential when the enzyme was membrane-bound and in crowded media containing PEG9000 (30 wt %). Overall, this demonstration elucidates aspects of membrane-bound enzymes in crowded media in terms of both catalytic behavior and inhibitory actions and can lead to further studies of the understanding of enzymatic behavior in such complex crowded environments having a dampening effect in regular diffusive transport.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

Ligand free surface of CdS nanoparticles enhances the energy transfer efficiency on interacting with Eosin Y dye - Helping in the sensing of very low level of chlorpyrifos in water

With an aim to sense the presence of chlorpyrifos (CP) pesticide in water, fluorescence resonance energy transfer (FRET) between the chemically synthesized ligand free CdS nanocrystals (donor) and Eosin Y dye (acceptor) has been studied in presence and absence of CP in the FRET pair system. This prepared water soluble CdS nanocrystals have been characterized by Transmission Electron microscopy (TEM), which shows that CdS nanocrystals are spherical in shape with an average size of 5 nm approximately. Further, Fourier Transform Infrared Spectroscopic (FTIR) study confirms that these CdS nanocrystals are ligand free stable nanocrystals. It has been observed that this CdS nanocrystals and Eosin Y FRET pair can strongly sense the presence of chlorpyrifos (CP) pesticide in water up to a very low concentration of 10 ppb, which is the sensitivity of detection or detection limit. This FRET pair is found to be very simple and cost effective for the sensing of toxic pesticide CP.

Layer-by-layer films and colloidal dispersions of graphene oxide nanosheets for efficient control of the fluorescence and aggregation properties of the cationic dye acridine orange

Chemically derived graphene oxide (GO) nanosheets have received great deal of interest for technological application such as optoelectronic and biosensors. Aqueous dispersions of GO become an efficient template to induce the association of cationic dye namely Acridine Orange (AO). Interactions of AO with colloidal GO was governed by both electrostatic and π-π stacking cooperative interactions. The type of dye aggregations was found to depend on the concentration of GO in the mixed ensemble. Spectroscopic calculations revealed the formation of both H and J-type dimers, but H-type aggregations were predominant. Preparation of layer-by-layer (LbL) electrostatic self-assembled films of AO and GO onto poly (allylamine hydrochloride) (PAH) coated quartz substrate is also reported in this article. UV-Vis absorption, steady state and time resolve fluorescence and Raman spectroscopic techniques have been employed to explore the detail photophysical properties of pure AO, AO/GO mixed solution and AO/GO LbL films. Scanning electron microscopy was also used for visual evidence of the synthesized nanodimensional GO sheets. The fluorescence quenching of AO in the presence of GO in aqueous solution was due to the interfacial photoinduced electron transfer (PET) from photoexcited AO to GO i.e. GO acts as an efficient quenching agent for the fluorescence emission of AO. The quenching is found to be static in nature. Raman spectroscopic results also confirmed the interaction of AO with GO and the electron transfer. The formation of AO/GO complex via very fast excited state electron transfer mechanism may be proposed as to prepare GO-based fluorescence sensor for biomolecular detection without direct labeling the biomolecules by fluorescent probe.

Investigation of Fluorescence Resonance Energy Transfer between Fluorescein and Rhodamine 6G

Fluorescence Resonance Energy Transfer between two organic dyes Fluorescein and Rhodamine 6G was investigated in aqueous solution in presence and absence of synthetic clay laponite. Spectroscopic studies suggest that both the dyes were present mainly as monomer in solution. Fluorescence Resonance Energy Transfer occurred from Fluorescein to Rhodamine 6G in solutions. Energy transfer efficiency increases in presence of laponite and the maximum efficiency was 72.00% in aqueous laponite dispersion. Energy transfer efficiency was found to be pH sensitive. It has been demonstrated that with proper calibration it is possible to use the present system under investigation to sense pH over a wide range from 1.5 to 8.0.

Time resolved spectroscopy and gain studies of Fullerenes C60 and C70

The fluorescence decay time of Fullerenes C60 and C70 in pure form as well as in mixture with Coumarin C440 and Quinizarine dyes are studied. Results indicate that the decay of pure fullerenes is constant throughout the solute concentration and it is also independent of excitation wavelength, whereas in the case of mixture with dyes different behavior is noticed. We have also calculated the Stern-Volmer quenching constant and optical gain of both the fullerenes from which it is found that the optical gain is positive for Fullerene C70 only in a very narrow range of concentration.

Self-assembly of anionic gemini surfactant: fluorescence resonance energy transfer and simulation study

The interaction of dyes with a sulfonated Gemini surfactant was investigated in aqueous solution using Förster resonance energy transfer with acridine orange (AO) as a donor and rhodamine B (RhB) as an acceptor. Surface tension results showed that AO and RhB have different effects on the self-assembly of the Gemini surfactant, with AO giving a higher critical micelle concentration (cmc) and lower surface tension, while the opposite was observed for RhB. Energy transfer from AO to RhB was observed in the presence of the surfactant, and the energy transfer efficiency initially improved with increased surfactant concentration but then decreased significantly when the surfactant reached a higher concentration due to the formation of larger aggregates, which increased the average distance between AO and RhB. Dynamic light scattering demonstrated the existence of these large aggregates. Moreover, simulations using dissipative particle dynamics supported the experimental results.

Morphology-dependent energy transfer dynamics in fluorene-based amphiphile nanoparticles

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

Förster's resonance energy transfer between Fullerene C60 and Coumarin C440

The interaction between Coumarin C440 with Fullerene C60 has been studied by fluorescence and time resolved spectroscopic techniques. The Coumarin C440-Fullerene C60 pair shows Forster's resonance energy transfer (FRET) from Coumarin C440 (donor) to Fullerenes C60 (acceptor). The FRET efficiency of this pair increases with the increase of the acceptor concentration. The critical energy transfer distance (R0) at which transfer efficiency is 50% is found to be 34Ǻ. Stern-Volmer plot indicates static as well as dynamic quenching. However, the FRET studies show highest efficiency at the critical stage of dimer formation.

Study on the fluorescence resonance energy transfer between CdTe QDs and butyl-rhodamine B in the presence of CTMAB and its application on the detection of Hg(II)

The thioglycolic acid-functionalized CdTe quantum dots (QDs) were synthesized in aqueous solution using safe and low-cost inorganic salts as precursors. Fluorescence resonance energy transfer (FRET) system was constructed between CdTe QDs (donor) and butyl-rhodamine B (BRB) (acceptor) in the presence of cetyltrimethylammonium bromide (CTMAB). CTMAB micelles formed in water reduced the distance between the donor and the acceptor significantly and thus improved the FRET efficiency, which resulted in an obvious fluorescence enhancement of the acceptor. Several factors which impacted the fluorescence spectra of the FRET system were studied. The energy transfer efficiency (E) and the distance (r) between CdTe and BRB were obtained. The feasibility of the prepared FRET system as fluorescence probe for detecting Hg(II) in aqueous solution was demonstrated. At pH 6.60, a linear relationship could be established between the quenched fluorescence intensity of BRB and the concentration of Hg(II) in the range of 0.0625-2.5micromolL(-1). The limit of detection was 20.3nmolL(-1). The developed method was proved to be sensitive and repeatable to detect Hg(II) in a wide range in aqueous solutions.

Spectroscopic investigation on the interaction of ICT probe 3-acetyl-4-oxo-6,7-dihydro-12H Indolo-[2,3-a] quinolizine with serum albumins

Interaction of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ), a biologically active molecule, with model transport proteins, bovine serum albumin (BSA) and human serum albumin (HSA) have been studied using steady state and picosecond time-resolved fluorescence and fluorescence anisotropy. The polarity dependent intramolecular charge transfer (ICT) process is responsible for the remarkable sensitivity of this biological fluorophore to the protein environments. The CT fluorescence exhibits appreciable hypsochromic shift along with an enhancement in the fluorescence yield, fluorescence anisotropy (r) and fluorescence lifetime upon binding with the proteins. The reduction in the rate of ICT within the hydrophobic interior of albumins leads to an increase in the fluorescence yield and lifetime. Marked increase in the fluorescence anisotropy indicates that the probe molecule is located in a motionally constrained environment within the proteins. Micropolarities in the two proteinous environments have been determined following the polarity sensitivity of the CT emission. Addition of urea to the protein-bound systems leads to a reduction in the fluorescence anisotropy indicating the denaturation of the proteins. Polarity measurements and fluorescence resonance energy transfer (FRET) studies throw light in assessing the location of the fluorophore within the two proteinous media.

Electrochemiluminescence from successive electro- and chemo-oxidation of rifampicin and its application to the determination of rifampicin in pharmaceutical preparations and human urine

A novel electrochemiluminescence (ECL) type was proposed based on successive electro- and chemo-oxidation of oxidable analyte, which was different from both annihilation and coreactant ECL types in mechanism. Rifampicin was used as a model compound. No any chemiluminescence (CL) was produced by either electrochemical oxidation or chemical oxidation of rifampicin in KH(2)PO(4)--Na(2)B(4)O(7) (pH 6.6) buffer-dodecyl trimethyl ammonium chloride (DTAC) solution. However, an ECL was observed by electrochemical oxidization of rifampicin in the same solution in the presence of oxidant such as dissolved oxygen, activated oxygen and potassium peroxydisulfate (K(2)S(2)O(8)). The ECL was attributed to electrochemical oxidation of rifampicin to form semiquinone free radical, and then subsequently chemical oxidation of the formed radical by oxidant to form excited state rifampicin quinone. The proposed ECL type introduced additional advantages such as high selectivity, simple and convenient operation, and effective avoidance of side reaction that often took place in homogenous CL reaction, and will open a novel application field. In addition, with the ECL in the presence of K(2)S(2)O(8) as oxidant, a flow injection ECL method for the determination of rifampicin was proposed. The ECL intensity was linear with rifampicin concentration in the range of 1.0 x 10(-7) to 4.0 x 10(-5) mol l(-1) and the limit of detection (s/n=3) was 3.9 x 10(-8) mol l(-1). The proposed method was applied to the determination of rifampicin in pharmaceutical preparations and human urine.

Beta-blockers and benzodiazepines location in SDS and bile salt micellar systems. An ESR study

The work here described aimed to find out the location of the different species of two families of pharmaceutical substances, namely two beta-blockers (atenolol and nadolol) and two benzodiazepines (midazolam and nitrazepam) in synthetic (sodium dodecyl sulphate, SDS) and natural (bile salts-sodium cholate and sodium deoxycholate) micellar aggregate solutions. Electronic spin resonance spectroscopy studies were carried out, at 25 degrees C and at an ionic strength of 0.10 M in NaCl, using 5-, 12- and 16-doxylstearic acid probes (AS). The immobilization degree of solubilized stearic acid spin probes was found to vary with the position of the nitroxide group in the sequence 5-doxylstearic acid>12-doxylstearic acid>16-doxylstearic acid for SDS and 12-doxylstearic acid>5-doxylstearic acid>16-doxylstearic acid for both bile salts investigated. Therefore, from the rotational correlational time values obtained, it can be inferred that the structure of bile salt micelles is markedly different from that of SDS micelles and the results suggest that the bile salt micelles studied have similar structure independently of differences in the molecular structure of the respective bile salts. Drug location studies were performed at pH 4.0 (SDS solutions) or 7.0 (bile salt solutions) and 10.8 in order to study the effect of the drug ionisation on its relative position on micelles. The results have shown that drug location is controlled by the (i) drug hydrophilicity and acid/base properties, with the more soluble compound in water (atenolol) exhibiting smaller variation of rotational correlational time (in SDS and bile salts solutions), and with both beta-blockers exhibiting smaller deviations in the protonated forms and (ii) the bile salt monomers, with the dihydroxylic bile salt (deoxycholate) producing larger differences. The work described herein allow us to conclude that the (protonated) beta-blockers are probably located on the surface of the detergent micelles, and linked to them by means of essentially electrostatic forces, while the (neutral) benzodiazepines are probably located deeper in the interior of the micelles.

Spectral studies of N-nonyl acridine orange in anionic, cationic and neutral surfactants

The spectroscopic and photophysical properties of N-nonyl acridine orange - a metachromatic dye useful as a mitochondrial probe in living cells - are reported in water and microheterogeneous media: anionic sodium dodecylsulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB) and neutral octylophenylpolyoxyethylene ether (TX-100). The spectral changes of N-nonyl acridine orange were observed in the presence of varying amount of SDS, CTAB and TX-100 and indicated formation of a dye-surfactant complex. The spectral changes were also regarded to be caused by the incorporation of dye molecules to micelles. It was proved by calculated values K(b) and f in the following order: K(bTX-100)>K(bCTAB)>K(bSDS) and f(TX-100)>f(CTAB)>f(SDS). NAO binds to the micelle regardless the micellar charge. There are two types of interactions between NAO and micelles: hydrophobic and electrostatic. The hydrophobic interactions play a dominant role in binding of the dye to neutral TX-100. The unexpected fact of the binding NAO to cationic CTAB can be explained by a dominant role of hydrophobic interactions over electrostatic repulsion. Therefore, the affinity of NAO to CTAB is smaller than TX-100. Electrostatic interactions play an important role in binding of NAO to anionic micelles SDS. We observed a prolonged fluorescence lifetime after formation of the dye-surfactant complex tau(SDS)>tau(TX-100)>tau(CTAB)>tau(water), the dye being protected against water in this environment. TX-100 is found to stabilize the excited state of NAO which is more polar than the ground state. Spectroscopic and photophysical properties of NAO will be helpful for a better understanding of the nature of binding and distribution inside mammalian cells.

Fluorescence and dynamic light scattering studies of niosomes-membrane mimetic systems

Niosomal vesicles are more stable than liposomal vesicles due to higher chemical stability of surfactants compared to phospholipids. Niosomes have been prepared from Span20, Span80, Tween20 and Tween80. Fluorescence resonance energy transfer studies have been performed in these systems to determine donor-acceptor distances. It has been found that the fluorescence resonance energy transfer efficiency is better in niosomes compared to micelles. The formation of niosomes is guided by the hydrophile-lipophile balance value of the nonionic surfactant.

Study of Energy Transfer from 7-Amino Coumarin Donors to the Rhodamine 6G Acceptor in Lecithin Vesicles and Sodium Taurocholate−Lecithin Mixed Aggregates

The energy transfer using 7-amino coumarin dyes as the donor and rhodamine 590 (Rh6G) as the acceptor was investigated in lecithin vesicles and sodium taurocholate (NaTC)-lecithin mixed aggregates using steady-state and time-resolved fluorescence spectroscopy. All energy transfer parameters were calculated. The coumarin 153-Rh6G pair is the most efficient donor-acceptor pair as reflected by the value of k(ET). With addition of NaTC in lecithin, in the case of the coumarin 153-Rh6G pair, the energy transfer rate or efficiency does not change very much, whereas in the case of the coumarin 151-Rh6G pair, the energy transfer rate decreases 2-fold upon going from lecithin vesicles to NaTC-lecithin mixed aggregates where the molar ratio is 2.5. It is mainly due to the deeper location of coumarin 153 in the lipid bilayer or in mixed aggregates. Rotational relaxation data also support this idea.