Anion Recognition Triggered Nanoribbon-Like Self-Assembly: A Fluorescent Chemosensor for Nitrate in Acidic Aqueous Solution and Living Cells

Morphology Control and Spectral Study of the 2D and Hierarchical Nanostructures Self-Assembled by the Chiral Alanine-Decorated Perylene Bisimides

Tailoring the morphologies and optical properties of the 2D and hierarchical nanostructures self-assembled by the π-conjugated molecules is both interesting and challenging. Herein, a series of 2D ribbon-like nanostructures with single or multiple H-aggregated perylene bisimides (PBI) monolayer and hierarchical nanostructures (including straw-like, dumbbell-shaped, and rod-like nanostructures) are fabricated by solution self-assembly of three chiral alanine-decorated PBI. The influence of the solvent's dissolving capacity, the chirality of alanine, and the preparation methods on the morphologies and optical properties of the nanostructures were extensively studied. It was observed that the hierarchical nanostructures are formed by the reorganization of the 2D ribbon-like nanostructures. The size of the 2D ribbon-like nanostructures and the amount of the hierarchical nanostructures increase with the decrease in the solvent's dissolving capacity. The small chiral alanine moiety is unable to induce chirality in the nanostructures, owing to its low steric hindrance and the dominant strong π-π stacking interaction of the PBI skeleton. A weaker π-π stacking interaction and better H-aggregated arrangement of the PBI skeleton could reduce the low-wavelength fluorescence intensity. The process of heating, cooling, and aging promotes the formation of H-aggregation in the PBI skeleton. The region of spectral overlap of the PBI solutions increases with the decrease in the dissolving capacity of the solvent and the steric hindrance of the chiral alanine. This study supplies a view to tailor the morphologies and optical properties of the nanostructures, which could be used as sensors and photocatalysts.

Benzimidazolium Salt Modified Microporous Silica-Coupled Iron Oxide Nanoparticles: Material Engineered for Nitrate Removal

Today's extensive use of inorganic fertilizers in agricultural techniques has increased the concentration of nitrate in drinking water beyond safety limits, causing serious health problems in humans such as thyroidism and methemoglobinemia. Therefore, the present work describes the synthesis of a benzimidazolium salt-based fluorescent chemosensor (KG3) via a multistep synthesis which detects nitrate ions in aqueous medium. This was validated using various analytical techniques such as fluorescence spectroscopy, UV-visible spectroscopy, and electrochemical studies with a detection limit of 0.032 μM without any interference from other active water pollutants. Subsequently, KG3 is further modified with the help of iron oxide nanoparticles (Fe3O4 NPs) and silica to obtain the SiO2@Fe3O4-KG3 nanocomposite, which was immobilized over a polyether sulfone membrane and evaluated for removal of nitrate ions from groundwater with a removal efficiency of 96%. Moreover, the engineered composite membrane can serve as a solid-state fluorescence sensor to detect NO3- ions, which was demonstrated through a portable mobile-based prototype employing a hue, saturation, and value parameter model.

Precise counter anion modulation of the self-assembly behavior-endowed ultrasensitive and specific dual-mode visualization of nitrate

Pt(II) polypyridine complex-based probe exhibits promising performance in anion detection by the change of the absorption and emission properties based on supramolecular self-assembly. However, whether one can develop a modulation strategy of the counter anion to boost the detection sensitivity and anti-interference capability of the Pt(II) complex-based probe remains a big challenge. Here, an effective modulation strategy was proposed by precisely regulating the interaction energy through adjusting the type of the counter anions, and a series of probes have been synthesized by counter anion (X = Cl-, ClO4-, PF6-) exchange in [Pt(tpy)Cl]·X (tpy=2,2':6',2''-terpyridine), and thus the colorimetric-luminescence dual-mode detection toward nitrate was achieved. The optimal [Pt(tpy)Cl]·Cl probe shows superior nitrate detection performance including a limit of detection (LOD) (8.68 nM), rapid response (<0.5 s), an excellent selectivity and anti-interference capability even facing 14 common anions. Moreover, a polyvinyl alcohol (PVA) sponge-based sensing chip loaded with the probe enables the ultra-sensitive detection of nitrate particles with an ultralow detection limit of 7.6 pg, and it was further integrated into a detection pen for the accurate recognition of nitrate particles in real scenarios. The proposed counter-anion modulation strategy is expected to start a new frontier for the exploration of novel Pt(II) complex-based probes.

Active Hydrogen Free, Z-Isomer Selective Isatin Derived "Turn on" Fluorescent Dual Anions Sensor

An efficient and anions fluorescence "on-off" sensor of 1-(prop-2-yn-1-yl)-3-(quinolin-3-ylimino)indolin-2-one (PQI) has been developed for the selective sensing of dual anions of F- and NO3- ions in aqueous medium. Active hydrogen and Lewis acidic binding sites free, Z- isomer of isatin based π-conjugated quinoline exhibited excellent sensing activity against F- and NO3- ions in UV light. The fluorescence turns on the process accomplished via the PET "on-off" mechanism. The interaction between probe molecule and anions is thought to be a non-covalent interaction of the low electron density covalently bonded N-methylene moiety of propargyl isatin (-N-CH2-) of probe molecule with F- ion and the terminal acidic proton of propargyl group of isatin (-C≡C-H) with NO3- ions. The modes of anions binding with PQI and plausible mechanisms are proposed by 1H and 13C NMR titrations. The selectivity of anions sensing may be offered by the bucked structure of the Z-isomer. The calculated association constant values for PQI and F- and NO3- are ions 2.5 × 104 M-1 and 2.2 × 103 M-1, respectively, indicating strong binding interaction between the PQI and anions. The association nature of anions and probes was analyzed by a Jobs plot and the finding indicates both F- and NO3- ions are in 1:1 complexation with PQI. The limit of detection (LOD) of the probe with F- and NO3- ions is calculated and is to be 6.91 × 10-7 M and 9.93 × 10-7 M, respectively. The proposed PQI fluorophore possesses a low limit of detection (LOD) for both F- and NO3- ions which is within the WHO prescribed detection limit.

Pyrene-Based Cationic Fluorophores with High Affinity for BF4-, PF6-, and ClO4- Anions: Detection and Removal

Anions play an indispensable role in the balance and regulation of the ecological environment and human health; however, excess anions can cause serious ecological and environment problems. Therefore, the detection and removal of excess anions in aqueous solution is not only a technological problem but also crucial for environmental protection. Herein, a set of water-soluble pyrene-based cationic fluorophores were synthesized, which exhibit high sensitivity for the detection of the anions BF4-, PF6-, and ClO4- via electrostatic interactions. Such fluorescent probes exhibit "turn-on" emission characteristics even at low concentrations of anions due to anion-π+ interactions. Moreover, these fluorescence probes act as efficient precipitating agents for the removal of the BF4-, PF6-, and ClO4- anions from an aqueous environment. This work opens up new avenues for future research on pyrene-based fluorophores as turn-on fluorescence probes for anion detection and as excellent precipitating agents in environmental settings.

Fluorescent Imaging Probe Targeting Mitochondria Based on Supramolecular Host-Guest Assembly and Disassembly

Fluorescent dyes and probes play an indispensable role in bioimaging. The mitochondrion is one of the crucial organelles which takes charge of energy production and is the primary site of aerobic respiration in the cell. To illuminate mitochondria, a series of supramolecular fluorescent imaging probes were developed based on the host-guest assembly of 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPEB) derivatives and cucurbituril[6] (CB[6]). These host-guest conjugates can be efficiently internalized into cells due to their water solubility and target mitochondria according to their positive charges. In response to the intracellular microenvironments, these conjugates start dynamic disassembly. The released BPEBs show a highly hydrophobic feature, which can crystallize to form fluorescent solids that illuminate the mitochondria. The intracellular disassembly of the host-guest probes was evidenced by fluorescence lifetime imaging in situ. These smart mitochondrion-targeting fluorescent imaging probes can be available to investigate the structures and functions of mitochondria, which are of great significance in the intracellular dynamic transformation of supramolecular assemblies.

A New Cationic Fluorescent Probe for HSO3- Based on Bisulfite Induced Aggregation Self-Assembly

In comparison with the numerous studies that have centered on developing molecular frameworks for the functionalization of fluorescent materials, less research has addressed the influence of the side chains, despite such appendages contributing significantly to the properties and applications of fluorescent materials. In this work, a new series of cationic fluorescent probes with AIE characteristics have been developed, which exhibit unique sensitivity for charge-diffusion anions, namely HSO₃⁻, via the interactions of ions and the cooperation of the controllable hydrophobicity. The impact of the alkyl chain length attached at the cationic probes suggested that the fluorescent intensity and sensitivity of the probes could be partially enhanced by adjusting their aggregation tendency through the action of the hydrophobic effect under aqueous conditions. DLS and SEM images indicated that different particle sizes and new morphologies of the probes were formed in the anion-recognition-triggered self-assembly process, which could be attributed to the composite effect of electrostatic actions, Van der Waals forces and π-π stacking.

Nitrates in the environment: A critical review of their distribution, sensing techniques, ecological effects and remediation

Nitrate pollution is eminent in almost all the developing nations as a result of increased natural activities apart from anthropogenic pollution. The release of nitrates in more than critical quantities into the water bodies causes accretion impacts on living creatures, environmental receptors, and human vigour by accumulation through the food chain. Nitrates have recently acquired researchers' huge attention and extend their roots in environmental contamination of surface and groundwater systems. The presence of nitrate in high concentrations in surface and groundwater triggers several health problems, for instance, methemoglobinemia, diabetes, eruption of infectious disorders, harmfully influence aquatic organisms. Sensing nitrate is an alternate option for monitoring the distribution of nitrate in different water bodies. Here we review electrochemical, spectroscopic, and electrical modes of nitrate sensing. It is concluded that, among the various sensors discussed in this review, FET sensors are the most desirable choice. Their sensitivity, ease of use and scope for miniaturisation are exceptional. Advanced functional materials need to be designed to satiate the growing need for environmental monitoring. Different sources of nitrate contamination in ground and surface water can be estimated using different techniques such as nitrate isotopic composition, co contaminants, water tracers, and other specialized techniques. This review intends to explore the research work on remediation of nitrate from wastewater and soil using different processes such as reverse osmosis, chemical denitrification, biological denitrification, ion exchange, electrodialysis, and adsorption. Denitrification proves as a promising alternative over previously reported techniques in terms of their nitrate removal because of its high cost-effectiveness.

Selective red-emission detection for mercuric ions in aqueous solution and cells using a fluorescent probe based on an unnatural peptide receptor

The selective ratiometric red-emission detection of Hg²⁺ions in aqueous buffered solutions and live cells is still a significant challenge.

Fine Tuning of Emission Behavior, Self-Assembly, Anion Sensing, and Mitochondria Targeting of Pyridinium-Functionalized Tetraphenylethene by Alkyl Chain Engineering

Compared to the many studies that focus on the development of novel molecular frameworks pertaining to functionalized fluorescent materials, there is lesser emphasis on side chains even though they have a significant impact on the properties and applications of fluorescent materials. In this study, a series of pyridinium-functionalized tetraphenylethene salts (TPEPy-1 to TPEPy-4) possessing different alkyl chains are synthesized, and the influence of chain length on their optical performance and applications is thoroughly investigated. By changing the alkyl chain, the fluorogens exhibit opposite emission behavior in aqueous media because of their distinct hydrophobic nature, and their solid-state emission can be fine-tuned from green to red owing to their distinct molecular configuration. In addition, by increasing the chain length, the microstructure of the self-assembled fluorogens converts from microplates to microrods with various emission colors. Moreover, TPEPy-1 exhibits dual-mode fluorescence "turn-on" response toward NO₃^- and ClO₄^- in aqueous media because the anions induce the self-assembly of fluorogens. Furthermore, the fluorogens display cellular uptake selectivity while the proper alkyl chain impels the fluorogens to penetrate the cell membrane and accumulate in the mitochondria with high specificity.

Selective and Sensitive Detection of Heavy Metal Ions in 100% Aqueous Solution and Cells with a Fluorescence Chemosensor Based on Peptide Using Aggregation-Induced Emission

A fluorescent peptidyl chemosensor for the detection of heavy metal ions in aqueous solution as well as in cells was synthesized on the basis of the peptide receptor for the metal ions using an aggregation-induced emission fluorophore. The peptidyl chemosensor (1) bearing tetraphenylethylene fluorophore showed an exclusively selective turn-on response to Hg(2+) among 16 metal ions in aqueous buffered solution containing NaCl. The peptidyl chemosensor complexed Hg(2+) ions and then aggregated in aqueous buffered solution, resulting in the significant enhancement (OFF-On) of emissions at around 470 nm. The fluorescent sensor showed a highly sensitive response to Hg(2+), and about 1.0 equiv of Hg(2+) was enough for the saturation of the emission intensity change. The detection limit (5.3 nM, R(2) = 0.99) of 1 for Hg(2+) ions was lower than the maximum allowable level of Hg(2+) in drinking water by EPA. Moreover, the peptidyl chemosensor penetrated live cells and detected intracellular Hg(2+) ions by the turn-on response.

Development of an AIE based fluorescent probe for the detection of nitrate anions in aqueous solution over a wide pH range

A new type of AIE-based turn-on fluorescent probe 1 was reported to highly selective detection of NO₃⁻ anion in aqueous solution and living cells by virtue of ionic interaction.

The Synthesis and Anion Recognition Property of Symmetrical Chemosensors Involving Thiourea Groups: Theory and Experiments

The synthesis of four symmetrical compounds containing urea/thiourea and anthracene/nitrobenzene groups was optimized. N,N’-Di((anthracen-9-yl)-methylene)thio-carbonohydrazide showed sensitive and selective binding ability for acetate ion among the studied anions. The presence of other competitive anions including F⁻, H₂PO₄⁻, Cl⁻, Br⁻ and I⁻ did not interfere with the strong binding ability. The mechanism of the host-guest interaction was through multiple hydrogen bonds due to the conformational complementarity and higher basicity. A theoretical investigation explained that intra-molecular hydrogen bonds existed in the compound which could strengthen the anion binding ability. In addition, molecular frontier orbitals in molecular interplay were introduced in order to explain the red-shift phenomenon in the host-guest interaction process. Compounds based on thiourea and anthracene derivatives can thus be used as a chemosensor for detecting acetate ion in environmental and pharmaceutical samples.