Critical Assessment of Micellar Surface Charge-Dependent Disaggregation and Reaggregation of a Bis-Indole Self-Aggregate: What Should Be Our Case-Dependent Choice?

"Aggregation-caused quenching" is a deep-seated mechanism and has been widely used by the researchers as the possible basis for new sensor development. Contrast to aggregation, its turn around process, disaggregation, has gained much less consideration so far. Unfortunately, study of the further scope for reaggregation of the disaggregated probe assembly in the same solution, as and when required, is still under the rare category. The central theme of the current study is focused on this aspect. For this purpose, the effects of headgroup charge (cationic, anionic, and nonionic) and polarity of the micelles on the degree of disaggregation and subsequent emission amelioration of a synthesized bis-indole derivative, 3,3'-bisindolyl(phenyl)methane (BIPM), are studied using steady-state and time-resolved spectroscopic techniques. The relative emission yield of BIPM (φf = 0.01) is significantly enhanced in the presence of cetyltrimethylammonium bromide (φf = 0.21) and polyoxyethylene (20) sorbitan monolaurate (φf = 0.24), whereas comparatively less emission enhancement is recorded within the sodium dodecyl sulfate system (φf = 0.07). In contrast, addition of an external biophilic agent, uric acid, causes requenching of the enhanced emission because of the reaggregation of the disaggregated probes. Detailed microscopic and calorimetric studies are also adopted to investigate the disaggregation-reaggregation mechanism of BIPM associations. The study will provide prior insights about the use of suitable micellar systems for the required degree of disaggregation as well as for the modulation of emission efficiency by controlled tuning of the disaggregation-reaggregation equilibrium for similar probe associations in pure aqueous medium avoiding any chemical transformation.

Hydrogen bond directed high-fidelity optical detection of picric acid: A single driver on diverse roads towards the same destiny.

High-fidelity optical detection of picric acid (PA) is necessary due to its toxic and explosive nature. Moreover, PA is a molecule with versatile features, such as hydrogen bond or proton donor-acceptor centers, extremely low pKa, highly electron-deficient center, high water solubility, etc. These unique features and concerns stimulated us to write this review article on the small-molecule-based optical detection of PA. This article describes the hydrogen bonding triggered optical detection of PA chronologically and categorically, with a particular focus on the development of small-molecule-based fluorophores reported over the last two decades. Special attention is conferred on how the single driver, hydrogen bonding, can bring about distinct and dissimilar phenomenal changes (such as aggregation, chelation, charge transfer, electron transfer, proton transfer, energy transfer, excimer formation, etc.) on the variation of the signaling and/or the binding unit of the probe molecules upon interaction with PA to reach the single destiny of the optical detection of the explosive. Finally, conclusions are made on the key points and achievements of the optical detection of PA along with the existing potential challenges for young researchers who will develop innovations in this field. We believe that the article will be an important addition to the existing literature on PA sensing.

Disaggregation-induced resurgence of quenched emission of a self-assembled bis-indole system: photophysics, energetics, and dynamics.

Disaggregation-induced emission enhancement was studied using a self-aggregated bis-indole derivative, 3,3'-bisindolyl(phenyl)methane (BIPM), and ß-CD molecules were employed for the emission recovery. In our recent study, BIPM molecules were found to exhibit weak emission efficiency in pure water due to aggregation-caused quenching (ACQ) effects. In the present study, we employed a simple, effective, biologically benign, and sustainable strategy in an attempt to disaggregate the BIPM self-aggregates into monomers to restore their emission efficiency. The ß-CD molecules were found to be effective in disaggregating the BIPM associations through tugging the monomers from their self-associations and encapsulating them into supramolecular nanocavities. The changes in the photophysical, dynamical, and thermodynamic properties associated with the disaggregation of the probe assemblies were studied by employing steady-state and time-resolved spectroscopy, isothermal titration calorimetry, and transmission electron microscopy with support from computational studies. The detailed photophysical and thermodynamic investigations on the disaggregation of the BIPM self-associations might provide significant insights towards its suitability for diverse biological and pharmaceutical applications.

Controlled tuning of radiative-nonradiative transition via solvent perturbation: Franck-Condon emission vs. aggregation caused quenching.

Organic molecules with tunable fluorescence quantum yield are attractive for opto-electronic applications. A fluorophore with tunable fluorescence quantum yield should be a better choice for a variety of applications that demand fluorophores with different quantum yields. Here organic emitters with a continuous bell-shaped fluorescence yield profile would be promising in view of sustainability and reusability; however, fluorophores with these properties are rarely reported. A bis-indole derivative, 3,3'-bisindolyl(phenyl)methane (BIPM), was synthesised and found to undergo a unique 'rise-and-fall' profile in fluorescence yield with a compositional change of the 1,4-dioxane (DiOx)-H2O solvent system. A predominant interplay of two contrasting factors, (a) polarity and proticity induced emission enhancement and (b) aggregation caused fluorescence quenching, on either side of a crossover solvent composition (∼50% fW), resulted in a continuous bell-patterned fluorescence yield profile. Interestingly, these two factors could be observed individually or simultaneously by adjusting the H2O fraction. Detailed spectroscopic, electron microscopic and computational studies have been performed to substantiate the photophysics behind the solvent regulated modulation of fluorescence quantum yield.

A New Compound for Sequential Sensing of Picric Acid and Aliphatic Amines: Physicochemical Details and Construction of Molecular Logic Gates.

Picric acid (PA) at low concentration is a serious water pollutant. Alongside, aliphatic amines (AAs) add to the queue to pollute surface water. Plenty of reports are available to sense PA with an ultralow limit of detection (LOD). However, only a handful of works are testified to detect AAs. A new fluorescent donor-acceptor compound has been synthesized with inherent intramolecular charge transfer (ICT) character that enables selective and sensitive colorimetric quantitative detection of PA and AAs with low LODs in non-aqueous as well as aqueous solutions. The synthesized compound is based on a hemicyanine skeleton containing two pyridenylmethylamino groups at the donor and a benzothiazole moiety at the acceptor ends. The detailed mechanisms and reaction dynamics are explained spectroscopically along with computational support. The fluorescence property of the detecting compound changes due to protonation of its pyridinyl centers by PA leading to quenching of fluorescence and subsequently de-protonation by AAs to revive the signal. We have further designed logic circuits from the acquired optical responses by sequential interactions.

Hydrogen bond regulated hydrogen sulfate ion recognition: an overview.

Hydrogen sulfate possesses substantial biological importance, having a colossal impact on physiological and environmental events. Therefore, several scientific groups have devoted serious effort to the development of versatile colorimetric and fluorimetric HSO4- sensors. Along with the scope, challenges, and significance, this review emphasizes the advancement of the optical recognition of HSO4- based on hydrogen bonding during the past two decades. Moreover, hydrogen-bond-driven proton transfer, ESIPT, ICT, PET, CHEF, and TBET mechanisms that allow for the optical detection of HSO4- are also discussed concisely. The foundation of this review includes the key points of the sensing process, like the nature of spectroscopic changes, selectivity and sensitivity, naked-eye color changes, the reusability of sensors, and the in vivo detection of HSO4-, if any. Special attention is focused on the correlation between the photophysical changes and the underlying interaction mechanisms that triggered the recognition aspect.

Excitation wavelength as logic operator.

Multiple molecular logic gates were harvested on a single synthesized material, (E)-2-(2-hydroxy-3-methoxybenzylideneamino)phenol (MBAP), by combining excitation wavelength dependent multi-channel fluorescence outputs and the same chemical inputs. Interestingly, the effortless switching of logic behavior was achieved by simply tweaking the excitation wavelength and sometimes the emission wavelengths with no alteration of chemical inputs and the main device molecule, MBAP. Additionally, new generation purely optically driven memory units were designed on the same system supporting an almost infinite number of write-erase cycles since inter-conversion of memory states was completely free from chemical interferences and impurity issues. Two-way memory functions ("erase-read-write-read" and "write-read-erase-read") worked simultaneously on the same system and could be accessed by simple optical switching between two excitation and emission wavelengths. Our optically switchable device might outperform traditional multifunctional logic gates and memory devices that generally employ chemical triggers to switch functionality and memory states. These optically switchable multifunctional molecular logic gates and memory systems might drive smart devices in the near future with high energy efficiency, extended life span, structural and functional simplicity, exclusive reversibility and enhanced data storage density.

Circumstantial Overdose Management of an Efficient Cancer Cell Photosensitizer with Preclinical Evidence: A Biophysical Study.

In this article, pharmacological management of circumstantial overdose of an anticancer drug, Harmine (HM), under in vitro and in vivo conditions is described and further validated by employing in silico methods. HM, an efficient cancer cell photosensitizer, interacts extensively with nontoxic ß-cyclodextrin (ß-CD). Steady-state fluorescence studies and molecular docking analysis established differential nature of molecular inclusion depending on the relative concentrations of ß-CD. Presently, ß-CD is commonly used as a standard drug-delivery vehicle but its application for controlled drug withdrawal is rarely explored. Flow cytometric results and in vivo investigations on a zebrafish model showed that conditional overdose of preadministered drug molecules can be efficiently removed by encapsulating successfully within nontoxic ß-CDs, albeit by controlled application of the same. This is an approach to manage the cytotoxicity of a drug in a safe way that is already administered. We believe that this ß-CD-mediated withdrawal of drugs may find possible applications in controlled capturing of excess or unused drug inside living systems and reducing the unwanted toxicity associated with chemotherapeutics.

Doping Dopamine in Carbon Nanoparticles: A New Multifunctional Logic-Based Decision-Making Molecule.

A dopamine-functionalized carbon nanoparticle (CNP)-based platform is designed to reversibly control the optical signal, leading to a multifunctional logic system regulated by pH and light. pH-regulated unique reversible transformation of oxidized and reduced forms of a neurotransmitter, dopamine, conjugated with CNPs plays a decisive role in capturing the final output of the logic function. Inspired by this phenomenon, herein, we report the use of dopamine-docked CNPs to construct different molecular logic gates with an intention to develop the next-generation molecular logic gates. We could successfully construct two basic molecular logic gates, namely, YES and NOT, using one input; two modular logic gates; an IMPLICATION logic gate using two inputs; and finally a combination of OR and AND gates using three inputs. The optical response of the synthesized NP conjugate provides a fluorescence-based "Erase-Read-Write-Read" function. The proposed phenomenon may open a new concept of biochemical logic gates with fluorescence output for neuronal imaging.

A newly developed highly selective Zn2+-AcO- ion-pair sensor through partner preference: equal efficiency under solitary and colonial situation.

Unusual self-sorting of an ion-pair under highly crowded conditions driven by a synthesized intelligent molecule 2-((E)-(3-((E)-2-hydroxy-3-methoxybenzylideneamino)-2-hydroxypropyl imino)methyl)-6-methoxyphenol, hereafter HBP, is described. When a mixture of various metal salts was allowed to react with HBP, only a specific ion-pair ZnII/AcO- in the solution simultaneously reacted, resulting in high-fidelity ion-pair recognition of HBP. This phenomenon was evidenced by significant changes in the absorption spectra and huge enhancement in emission intensity of HBP. The property that one molecule preferring one particular cation-anion pair over others is a rare but interesting phenomenon. Thus, the potential to interact selectively with the targeted ion-pair resulting in the formation of a specific complex recognized HBP as a new class of molecule that might find future applications in real time and on-site monitoring and separation of new molecules.

Shipment of a photodynamic therapy agent into model membrane and its controlled release: A photophysical approach.

Harmine, an efficient cancer cell photosensitizer (PS), emits intense violet color when it is incorporated in well established self assembly based drug carrier formed by cationic surfactants of identical positive charge of head group but varying chain length, namely, dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB). Micelle entrapped drug emits in the UV region when it interacts with non-toxic ß-cyclodextrin (ß-CD). Inspired by these unique fluorescence/structural switching properties of the anticancer drug, in the present work we have monitored the interplay of the drug between micelles and non-toxic ß-CDs. We have observed that the model membranes formed by micelles differing in their hydrophobic chain length interact with the drug differently. Variation in the surfactant chain length plays an important role for structural switching i.e. in choosing a particular structural form of the drug that will be finally presented to their targets. The present study shows that in case of necessity, the bound drug molecule can be removed from its binding site in a controlled manner by the use of non-toxic ß-CD and it is exploited to serve a significant purpose for the removal of excess/unused adsorbed drugs from the model cell membranes. We believe this kind of ß-CD driven translocation of drugs monitored by fluorescence switching may find possible applications in controlled release of the drug inside cells.

A Strategic Design of an Opto-Chemical Security Device with Resettable and Reconfigurable Password Based Upon Dual Channel Two-in-One Chemosensor Molecule.

A simple strategy is proposed to design and develop an intelligent device based on dual channel ion responsive spectral properties of a commercially available molecule, harmine (HM). The system can process different sets of opto-chemical inputs generating different patterns as fluorescence outputs at specific wavelengths which can provide an additional level of protection exploiting both password and pattern recognitions. The proposed system could have the potential to come up with highly secured combinatorial locks at the molecular level that could pose valuable real time and on-site applications for user authentication.

pH triggered reversible photoinduced electron transfer to and from carbon nanoparticles.

Dopamine functionalized carbon nanoparticles (CNPs) that can act as efficient photoinduced electron donor-acceptor systems depending on the pH of the medium have been synthesized. In acidic media, dopamine on CNPs exists as hydroquinone and serves as an electron donor while under alkaline conditions the corresponding quinone form of dopamine serves as a strong electron acceptor. Application of external NADH to the system can invert the donor-acceptor roles under alkaline conditions.

FRET-based characterisation of surfactant bilayer protected core-shell carbon nanoparticles: advancement toward carbon nanotechnology.

Surfactant bilayer protected core-shell carbon nanoparticles (CNPs) have been synthesised. Förster resonance energy transfer (FRET) between the core-shell CNPs and two strategically chosen organic dyes has been exploited to characterise the protective surfactant bilayer.

Photophysical and photodynamical study of fluoroquinolone drug molecule in bile salt aggregates.

Photophysical properties of two widely used antibiotic fluoroquinolone drugs, namely Norfloxacin (NOR) and Ofloxacin (OFL) have been investigated in biomimicking environments formed by bile salts. Experimental results demonstrate that photophysical enhancement and fall of a particular prototropic species are sensitive to the excitation wavelength in bile salt aggregates. Excitation at shorter wavelengths reveals quenching of fluorescence of these fluoroquinolone with addition of sodium deoxycholate (NaDC), sodium taurocholate (NaTC) and sodium glycodeoxycholate (NaGDC). On the contrary, we observe a steady increase in the fluorescence intensity with a continuous redshift upon excitation at longer wavelength. The experimental results were rationalized in terms of the fact that, neutral and zwitterionic species of fluoroquinolone molecules in bile salt aggregates are selectively excited at shorter wavelength while the cationic form of fluoroquinolone molecules are excited at longer wavelength. The excess hydronium ions in the hydrophilic surface of bile salt aggregates convert the neutral species of NOR and OFL into cationic species causing an enhancement in the emission intensity. We found that NaGDC and NaTC because of the conjugate head group are more effective in converting the neutral species of fluoroquinolones into a cationic species than NaDC. The quenching order is in accordance with hydrophobicity indices of bile salt.

A newly developed highly selective ratiometric fluoride ion sensor: spectroscopic, NMR and density functional studies.

A new easy-to-synthesize chemosensor, 3,3'-bis(indolyl)-4-chlorophenylmethane (hereafter S), was designed, synthesized and employed as a selective optical chemosensor for fluoride ions.(1)H NMR and density functional studies on the system have been carried out to determine the nature of the interaction between S and X(-) (X = inorganic anions) responsible for the significant fluoride-induced changes in the absorption properties of S. The experimental results reveal that abstraction of an acidic proton of S by the fluoride ion, leading to the formation of anionic species, is responsible for the spectral changes. These changes allow signaling for the fluoride ion to detect and estimate the concentration of fluoride ion present even at the submicromolar level, accurate up to 2 µM. Calculations of the transition energies of S, S(-), and S···F(-) (hydrogen bonded complex) show that only S(-) is responsible for the long-wavelength absorption band in the presence of F(-).

Norharmane: old yet highly selective dual channel ratiometric fluoride and hydrogen sulfate ion sensor.

Norharmane provides a simple unexplored class of anion receptor, that allows for the ratiometric selective detection of F(-) and HSO(4)(-) ions. The presence of a strong base can easily form hydrogen bonds with the acidic hydrogen bond donor moiety and the relatively strong acid can easily protonate the basic hydrogen bond acceptor moiety, which can modulate the optical response and can detect the anions efficiently with high selectivity. In view of that, it is promising to conceive the use of these systems in various sensing applications as well as in other situations, such as anion transport and purification, where the availability of cheap and easy-to-make anion receptors, would be advantageous.

Fluorescence photoswitching of a diarylethene-perylenebisimide dyad based on intramolecular electron transfer.

A fluorescent photochromic molecule, which is composed of a photochromic diarylethene (DE) and a fluorescent perylenebisimide (PBI), was synthesized and its fluorescence photoswitching was studied. The fluorescence quantum yield of the open-ring isomer is constant irrespective of solvent polarity, while that of the closed-ring isomer decreases with an increase in the dielectric constant of solvents. Femtosecond time-resolved transient and fluorescence lifetime measurements revealed that the fluorescence quenching of the closed-ring isomer is attributed to the intramolecular electron transfer from the PBI chromophore to the DE unit.

Effect of surfactant chain length on the binding interaction of a biological photosensitizer with cationic micelles.

Steady-state and time-resolved fluorometric techniques have been exploited to study the photophysical and distribution behavior of an efficient cancer cell photosensitizer, norharmane (NHM), in well-characterized, biomimicking nanocavities formed by cationic micelles with varying surfactant chain length. Amphiphiles like dodecyl trimethyl ammonium bromide (DTAB), tetradecyl trimethyl ammonium bromide (TTAB), and cetyl trimethyl ammonium bromide (CTAB) have been used for the purpose. Emission behavior of NHM is very much dependent on the surfactant concentration as well as their hydrophobic chain length. The binding constant (K) and free-energy change (DeltaG) for the interaction of NHM with the cationic micelles have been determined from the fluorescence data. Polarity of the microenvironment around the probe has been determined in the cationic micellar environments from a comparison of the variation of fluorescence properties of the two-prototropic species of the probe in water/dioxane mixture with varying composition. Experimental results demonstrate that the variation in the cationic surfactant chain length plays an important role in promoting a specific prototropic form of the probe molecule. Fluorescence decays are biexponential in all the micelles indicating that the probe molecules are distributed between the two distinct regions of the micelles. The population of the component with a longer lifetime corresponds to the probe in the head group site, while the short-lived component comes from the probe bound to the core region of the micelles. On the basis of the lifetime measurements, the partitioning behavior of the chromophore in the head group and in the core regions in the micelles has been determined.

Application of anionic micelle for dramatic enhancement in the quenching-based metal ion fluorosensing.

We introduce a simple and efficient strategy to enhance the efficiency of a quenching-based fluorosensor for metal ions by several orders of magnitude by using commercially available anionic surfactants varying hydrophobic chain length. Anionic surfactants with a proper choice of hydrophobic chain length at their optimum concentrations are efficient to boost up the efficiency of copper ion sensor dramatically. This simple and convenient strategy is, in general, applicable to quenching-based fluorosensors, new or established, in aqueous solution. It is powerful enough to transform a virtually non-sensor fluorophore to a sensor with a commendable efficiency with the help of proper surfactant. Thus, in this communication, light has been thrown on the application of surfactants toward increasing fluorosensing efficiency of a quenching based sensor.