Tuning Sensing Efficacy of Oligo(phenylenevinylene) Based Chromogenic Probes: Effect of Alkyl Substituents on Metal Ion Detection at Micelle-Water Interface

In this work, we conducted a comparative analysis of the metal ion sensing capabilities of two pyridine-end oligo p-phenylenevinylene compounds featuring different alkyl substituents (-C4H9 and -C16H33) within a micelle medium. Our findings revealed a correlation between the positioning of the probe molecules within the micelle and the length of the alkyl chains, impacting their self-assembly tendencies and optical characteristics. The compound with shorter alkyl chains demonstrated a superior affinity towards Hg2+ ions, whereas exposure to the compound with longer alkyl substituent resulted in a color-changing response with both Cu2+and Hg2+ ions. Intriguingly, the sensitivity towards Hg2+ ions heightened with increasing alkyl chain length. This trend persisted in non-polar solvents like THF. The capacity to modulate sensing efficacy solely by adjusting the length of the alkyl chains represents a relatively uncommon occurrence in the existing literature. This discovery suggests promising prospects for engineering sensory devices equipped with adaptable sensitivity.

Multifaceted Applications of Luminescent Metalloporphyrin Derivatives: Fluorescence Turn-On Sensing of Nicotine and Antimicrobial Activity

In this study, we designed and synthesized metalloporphyrin derivatives (with Ni and Zn) specifically intended for the fluorescence detection of nicotine in aqueous solutions. Our results showcased a notable selectivity for nicotine over other naturally occurring food toxins, exhibiting an exceptional sensitivity with a limit of detection as low as 7.2 nM. Through mechanistic investigations (1H NMR, FT-IR, etc.), we elucidated the binding mechanism, revealing the specific interaction between the pyridine ring of nicotine and the metal center, while the N atom pyrrolidine unit engaged in the hydrogen bonding with the side chain of the porphyrin ring. Notably, we observed that the nature of the metal center dictated the extent of interaction with nicotine; particularly, Zn-porphyrin demonstrated a superior response compared to Ni-porphyrin. Furthermore, we performed the quantitative estimation of nicotine in commercially available tobacco products. Additionally, we conducted the antibacterial (Staphylococcus aureus and Escherichia coli) and antifungal (Candida albicans) activities of the porphyrin derivatives.

Surfactant-induced alterations in optoelectronic properties of perylene diimide dyes: modulating sensing responses in the aqueous environment

The compartmentalization effect of microheterogeneous systems, like surfactant aggregates, showcases altered optoelectronic properties of a perylene diimide-based chromogenic dye (PDI-Ala) compared to bulk water. The relatively hydrophobic microenvironment, poor hydration, and exceptionally large local concentration of dye molecules in the confined environment affect their interaction with target analytes. This realization intrigued us to investigate if micellization can modify the sensing properties (selectivity, sensitivity, response kinetics, output signal, etc.) of the encapsulated dye molecules in the aqueous medium. Response comparisons of PDI-Ala to the ionic analyte (Fe3+) and biomolecule (heparin) in aqueous and surfactant-bound states highlighted significant variations. Fe3+ interaction exhibited a "turn-off" fluorescence response in a water medium, while surfactant-bound conditions triggered "turn-on" fluorescence, enhancing selectivity at the micelle-water interface. Conversely, the native probe showed no interaction with heparin in water but displayed a turn-on fluorescence response in cetyltrimethylammonium bromide (CTAB) micelles, indicating the transformation of a silent molecule into a turn-on fluorescence sensor. This study underscores the influence of micellar environments on dye molecules, altering the sensing responses and selectivity toward analytes, crucial for applications in understanding cellular pathways and toxicity mechanisms.

Tuning sensing efficacy of anthraimidazoledione-based charge transfer dyes: nitro group positioning impact

Anthraimidazoledione-based optical sensors have been designed by varying the position of the nitro functional group. All three positional isomers showed highly colored, photostable optical signals owing to intramolecular charge transfer interactions. Despite having the same anion-binding site (imidazole unit), the selectivity and sensitivity of the compounds depend on the positioning of the nitro group. The selectivity was fairly good for the meta isomer, followed by the ortho and para isomers, respectively. In contrast, the sensitivity towards anions followed a completely opposite trend, with the para isomer being the most sensitive one towards anions. Interestingly, the color changing response along the turn-on fluorescence signal was observed only with CN- ions in a semi-aqueous environment. Though the introduction of water as a co-solvent could improve the selectivity, the sensitivity was found to be slightly less than that observed in pure organic medium. Mechanistic studies indicated hydrogen bonding interactions between the imidazole -NH proton and cyanide, which further facilitated the extent of intramolecular charge transfer.

Biogenic Polymer-Based Fluorescent Assemblies: Versatile Platforms for Ultrasensitive ATP Detection and Enzyme Assay

Here, we investigated the optical properties of biocompatible supramolecular assemblies formed through electrostatic interactions between anionic fluorescent dyes and biogenic polymers. The dynamic equilibrium between the monomeric form (fluorescent) and aggregates (nonfluorescent) of dye molecules is responsible for the stimuli-responsive behavior of these polymer composites, which can respond to changes in pH, temperature, and ionic strength. Furthermore, we employed supramolecular assemblies for the purpose of turn-on fluorescence sensing of adenosine triphosphate (ATP) at physiological pH. Notably, no interference was observed even in the presence of well-known competing analytes such as pyrophosphate. In addition to its outstanding selectivity, the present system can detect ATP at concentrations as low as 4.8 nM. The superior detection capabilities are achieved through multiple interactions with biogenic polymers, involving the adenine ring, ribose unit (through hydrogen bonding), and phosphate groups (via charge pairing) of ATP. Given the remarkable sensitivity to ATP, we have applied the present system for the detection of a dephosphorylating enzyme, alkaline phosphatase.

AIE active fluorescent organic nanoparticles based optical detection of Cu2+ ions in pure water: a case of aggregation-disaggregation reversibility

An AIE-active pyrene-terpyridine derivative, (4'-(pyren-1-yl)-2,2':6',2''-terpyridine) (1) was found to form nanoaggregate in an aqueous medium. The probe involved hydrogen bonding with solvent molecules that modulated the charge transfer behavior and consequently resulted in different spectroscopic behavior due to the formation of fluorescent organic nanoparticles (FONs). In the presence of Cu2+ ions, FONs displayed a ratiometric red shift of the absorption band (360 to 420 nm) accompanied by a prominent naked-eye color change from colorless to light yellow. With a gradual increase in water content, 1 displayed a huge red shift of the emission band (430 to 475 nm) denoting its switching from monomer to FONs. In the presence of Cu2+, the 475 nm emission band of FONs gradually diminished, facilitating the micromolar scale detection of Cu2+ (LOD = 8.57 μM) in a 100% aqueous medium with a fluorogenic color change from cyan to dark. The SEM and DLS data indicated the cation-induced disaggregation of FONs, which was further confirmed by mass spectral analysis and electron paramagnetic resonance measurement. In addition, the high selectivity of FONs towards Cu2+ ions over other potential cations and the 2 : 1 (1-Cu2+) binding stoichiometry were also determined. Moreover, the spectroscopic behavior of the monomeric amphiphilic probe was well supported by extensive DFT study. Such detection of Cu2+ ions in pure aqueous medium denoting an aggregation-disaggregation event is very rare in the literature.

Fluorescent Nanoassembly of Tetrazole-Based Dyes with Amphoteric Surfactants: Investigation of Cyanide Sensing and Antitubercular Activity

Tetrazole-based easily synthesizable fluorogenic probes have been developed that can form self-assembled nanostructures in the aqueous medium. Though the compounds could achieve detection of cyanide ions in apolar solvents, such as, THF, significant interference was observed from other basic anions, such as F-, AcO-, H2PO4-, etc. On the other hand, a highly specific response was observed for CN- ions in the aqueous medium. However, the sensitivity was so poor that it could hardly be useful for real-life sample analysis. Interestingly, the co-assembly of such probe molecules with hydroxyethyl-anchored amphoteric surfactants could drastically improve the sensitivity toward CN- ions in water without dampening their excellent selectivity. Also, it was observed that the degree of fluorescence response for CN- ions depends on the nature of the polyaromatic scaffolds (naphthyl vs anthracenyl), the nature of the surfactant assembly (micelle vs vesicle), etc. The mechanistic investigation indicates the hydrogen bonding interaction between the tetrazole -NH group and cyanide ions in the aqueous medium, which can effectively change the electronics of the tetrazole unit, resulting in alteration in the extent of charge transfer interaction. Then, the biocompatible composite materials (dye-surfactant assemblies at different ratios) were tested for antituberculosis activity. Fortunately, in a few cases, the compositions were found to be as effective as the commercially available antituberculosis drug, ethambutol.

Improved Analytical Performance of an Amphiphilic Probe upon Protein Encapsulation: Spectroscopic Investigation along with Computational Rationalization

An easily synthesizable pyrene-based amphiphilic probe (Pybpa) has been developed, which exhibited no responses with metal ions in the pure aqueous medium despite possessing a metal ion-chelating bispicolyl unit. We believe that spontaneous aggregation of Pybpa in aqueous medium makes the ion binding unit not accessible to the metal ions. However, the sensitivity and selectivity of Pybpa toward Zn²⁺ ions drastically improve in the presence of serum albumin protein, HSA. The differences in the microenvironment inside the protein cavity, in terms of local polarity, and conformational rigidity might be attributing factors for that. The mechanistic investigations also suggest that there might be the involvement of polar amino acid residues that take part in coordination with Zn²⁺ ions. Pybpa shows no detectable spectroscopic changes with Zn²⁺ ions in aqueous medium in the absence of HSA. However, it can effectively recognize Zn²⁺ ions in the protein-bound form. Moreover, the photophysical behavior of Pybpa and its zinc complex have been investigated with DFT and docking studies. Noteworthy, such an unusual sensing aspect of Zn²⁺ exclusively in the protein-bound state and particularly in aqueous medium is truly rare and innovative.

Dual-Mode Multiple Ion Sensing via Analyte-Specific Modulation of Keto-Enol Tautomerization of an ESIPT Active Pyrene Derivative: Experimental Findings and Computational Rationalization

A pyrene-based e xcited - state intramolecular proton transfer (ESIPT) active probe PMHMP was synthesized, characterized, and employed for the ppb-level, dual-mode, and high-fidelity detection of Cu²⁺ (LOD: 7.8 ppb) and Zn²⁺ ions (LOD: 4.2 ppb) in acetonitrile medium. The colorless solution of PMHMP turned yellow upon the addition of Cu²⁺, suggesting its ratiometric, naked-eye sensing. On the contrary, Zn²⁺ ions displayed concentration-dependent fluorescence rise till a 0.5 mole fraction and subsequent quenching. Mechanistic investigations indicated the formation of a 1:2 exciplex (Zn²⁺:PMHMP) at a lower concentration of Zn²⁺, which eventually turned into a more stable 1:1 (Zn²⁺:PMHMP) complex with an additional amount of Zn²⁺ ions. However, in both cases, it was observed that the hydroxyl group and the nitrogen atom of the azomethine unit were involved in the metal ion coordination, which eventually altered the ESIPT emission. Furthermore, a green-fluorescent 2:1 PMHMP-Zn²⁺ complex was developed and additionally employed for the fluorimetric analysis of both Cu²⁺ and H₂PO₄ ^- ions. The Cu²⁺ ion, owing to its higher binding affinity for PMHMP, could replace the Zn²⁺ ion from the preformed complex. On the other hand, H₂PO₄ ^- formed a tertiary adduct with the Zn²⁺-complex, leading to a distinguishable optical signal. Furthermore, extensive and organized density functional theory calculations were performed to explore the ESIPT behavior of PMHMP and the geometrical and electronic properties of the metal complexes.

Monitoring Local pH of Membranous Aggregates via Ratiometric Color Changing Response

A series of oxidized di(indolyl)arylmethanes (DIAM) with polyaromatic signaling moieties have been designed for monitoring local pH at interfacial region of surfactant aggregates, such as micelles and vesicles etc. The oxidized DIAMs show changes in solution color from red to yellow when incorporated in cationic surfactants (at pH 7.4) and yellow to reddish pink when exposed to negatively-charged surfactants (at pH 5.0). The changes in surface charge can influence the interfacial pH (distinct from bulk pH of the medium) of the surfactant aggregates. The mechanistic studies indicate the red-shifted absorption maxima observed in the presence of anionic amphiphiles (acidic local pH) was originated from the protonated species. On the contrary, maxima in the blue region, triggered by positively charged amphiphiles (basic local pH), can be attributed to the zwitterionic species. Such prototropic equilibrium affects charge transfer states of the molecules along with their self-assembly properties. Thus, it is evident that probes can predict as well as quantify the local pH change using the pseudophase ion exchange formalism. Also, the probes can detect the presence of anionic amphiphiles even when bound to phospholipid membranes.

A Simple Method for Continuous Synthesis of Bicelles in Microfluidic Systems

Bicelle has great potential for drug delivery systems due to its small size and biocompatibility. The conventional method of bicelle preparation contains a long process and harsh conditions, which limit its feasibility and damage the biological substances. For these reasons, a continuous manufacturing method in mild conditions has been demanded. Here, we propose a novel method for DMPC/DHPC bicelle synthesis based on a microfluidic device without heating and freezing processes. Bicelles were successfully prepared using this continuous method, which was identified by the physicochemical properties and morphologies of the synthesized assemblies. Experimental and analytical studies confirm that there is critical lipid concentration and critical mixing time for bicelle synthesis in this microfluidic system. Furthermore, a linear relation between the actual composition of bicelle and initial lipid ratio is deduced, and this enables the size of bicelles to be controlled.

A Tunable Palette of Molecular Rotors Allows Multicolor, Ratiometric Fluorescence Imaging and Direct Mapping of Mitochondrial Heterogeneity

Environment-sensitive molecular probes offer the potential for a comprehensive mapping of the complex cellular milieu. We present here a radically new strategy of multiplexing highly sensitive, spectrally tuned fluorescent dyes for sensing cellular microenvironment. To achieve this multicolor, ratiometric cellular imaging, we first developed a series of highly sensitive, tunable molecular rotors for mitochondrial imaging, with emission wavelengths spanning the visible spectrum. These fluorogenic merocyanine dyes are all sensitive to solvent viscosity despite distinctive photophysical features. Our results show that merocyanine dyes can show a rotor-like behavior despite significant changes to the conventional donor-acceptor or push-pull scaffolds, thereby revealing conserved features of rotor dye chemistry. Developing closely related but spectrally separated dyes that have distinct response functions allows us to do ″two-color, two-dye″ imaging of the mitochondrial microenvironment. Our results with multidye, combinatorial imaging provide a direct visualization of the intrinsic heterogeneity of the mitochondrial microenvironment. The overall mitochondrial microenvironment (including contributions from local membrane order) as reported through two-color fluorescence ″ratio″ changes of multiplexed rotor dyes shows dynamic heterogeneity with distinct spatiotemporal signatures that evolve over time and respond to chemical perturbations. Our results offer a powerful illustration of how multiplexed dye imaging allows the quantitative imaging of mitochondrial membrane order and cellular microenvironment.

Diphenyl-1-pyrenylphosphine: photo-triggered AIE/ACQ transition with remarkable third-order nonlinear optical signal change

A propeller-like pyrene derivative is reported, which exhibits a unique photo-triggered AIE/ACQ transition with a remarkable third-order nonlinear optical signal change.

A biomimetic self-assembled cobaloxime@CdS/rGO hybrid for boosting photocatalytic H2 production

A biomimetic CoPe@CdS/rGO hybrid that self-assembles via the integration of a molecular cobalt catalyst and CdS nano-semiconductor on reduced graphene oxide was constructed for boosting photocatalytic H2 production. Photoinduced electron transfer from CdS/rGO to the molecular catalyst occurs and a long-lived charge-separation state forms for high H2 production.

Effect of substitution on the excited state photophysical and spectral properties of boron difluoride curcumin complex dye and their derivatives: A time dependent-DFT study

The optical, charge transport and electronic properties of boron difluoride curcumin (BFC) complex have been explored using the DFT (Density Functional Theory) method and B3LYP functional with the combination of 6-31 + G(d,p) as a basis set. The influence of substitution with various electron releasing and withdrawing groups on the above properties is analyzed and discussed in this work. The results reveal that the BFC complex on additional electron releasing substitution experiences redshifts in the optical transitions, and this is correlated with the dipole moment, NBO charges, HOMO-LUMO energy gap. Further, the absorption (λ_abs) and emission (λ_ems) spectra of substituted and unsubstituted BFCs are calculated using Time-Dependent Density Functional Theory (TD-DFT). The results show that the electron releasing groups strongly influence the absorption and emission spectra of BFC. Electron releasing groups in BFC derivatives generate the wavelength shift (Bathochromic), but the electron-withdrawing groups in BFC don't affect the λ_abs and λ_ems when compare to its original (parent) compound. The output of the research work strongly recommends that the amino, phenyl and N, N'-dimethylamino derivatives are potential candidates to act as fluorescent materials due to enhance the emission behavior of BFC and also can be used as an electron/charge transport material for organic light-emitting diodes (OLEDs).

Modulation of Excited-State Proton-Transfer Dynamics inside the Nanocavity of Microheterogeneous Systems: Microenvironment-Sensitive Förster Energy Transfer to Riboflavin

The excited-state proton-transfer efficiency of a tetraarylpyrene derivative, 1,3,6,8-tetrakis(4-hydroxy-2,6-dimethylphenyl)pyrene (TDMPP), was investigated thoroughly in the presence of various surfactant assemblies, such as micelles and vesicles. The confined microheterogeneous environments can significantly retard the extent of the excited-state proton-transfer process, resulting in a distinguishable optical signal compared to that in the bulk medium. Physical characteristics of the surfactant assemblies, such as order, interfacial hydration, and surface charge, influence the proton transfer process and allow multiparametric sensing. A higher degree of interfacial hydration facilitates the proton-transfer process, while the positively charged head groups of the surfactants specifically stabilize the anionic form of the probe (TDMPP-O*). Furthermore, Forster energy transfer from the probe to riboflavin was studied in a phospholipid membrane, wherein the relative ratio of the neutral versus anionic forms (TDMPP-OH/TDMPP-O*) was found to influence the extent of energy transfer. Overall, we demonstrate how an ultrafast photophysical process, that is, the excited-state proton transfer, can be influenced by the microenvironment.

Simultaneous Detection of Cu2+ and Hg2+ via Two Mutually Independent Sensing Pathways of Biimidazole Push-Pull Dye

An easy-to-synthesize, biimidazole push-pull dye has been designed, comprising two mutually independent analyte binding sites. It has been found that Hg²⁺ coordinates with the compound via thiophene residue and inhibits the charge-transfer (CT) process, which transforms the yellow-colored solution colorless. On the other hand, an unusually large bathochromic shift is observed in CT band upon addition of Cu²⁺, accompanied by a change in the color from yellow to red. A rather surprising observation is made from mechanistic studies, where it indicates that Cu²⁺ catalyzes the formation of 6-imino-5,6-dihydropyrrolo[3,4- d]imidazole-4(3 H)-one (IPIMO) derivative. This strongly affects the charge-transfer state of the compound as well as its polarizability. Most importantly, this is the first report where IPIMO formation reaction has been exploited for sensing of a metal ion. Further, the system was employed for screening of both of these metal ions in wastewater samples. Recovery values ranging from 93.3 to 105.0% confirm the suitability of the present method for estimating trace level of metal ions in complex matrices. In addition, inexpensive on-site detection systems were developed using paper strips.