Anion binding studies with anthraimidazoledione-based positional isomers: A comprehensive analysis of different strategies for improved selectivity

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.

Selective and Effective Sensing of Cyanide Ion with no Interference in Water by Phenothiazine-indolium Fused Optical Sensor

The sensor with electron donor phenothiazine-2-carbaldehyde and electron acceptor indolium carboxylic acid, is developed with an intramolecular charge transfer transition between them. The synthesized molecule senses cyanide ion in water. The cyanide ion reacts with the molecule via nucleophilic addition in the indolium ring with a noticeable purple to colorless change in the solution observed. Also with the cyanide ion interaction, the sensor exhibits change in UV-visible absorption and fluorescence spectra. While the other ion does not show spectral and visual changes when interacts with the sensor molecule. Also the interference study reveals that the molecule is highly selective towards cyanide ion. Different source of water samples confirms the CN- ion sensing efficiency of the molecule. 1:1 interaction between the molecule PTI and cyanide ion is confirmed from the results of Jobs plot, 1H NMR and HRMS. Paper strips were prepared and this can act as a simple tool to sense cyanide ion in various water samples.

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.

TDDFT Study on the ESIPT Properties of 2-(2'-Hydroxyphenyl)-Benzothiazole and Sensing Mechanism of a Derived Fluorescent Probe for Fluoride Ion

The level of fluoride ions (F-) in the human body is closely related to various pathological and physiological states, and the rapid detection of F- is important for studying physiological processes and the early diagnosis of diseases. In this study, the detailed sensing mechanism of a novel high-efficiency probe (PBT) based on 2-(2'-hydroxyphenyl)-benzothiazole derivatives towards F- has been fully investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. F- attacks the O-P bond of PBT to cleavage the dimethylphosphinothionyl group, and the potential products were evaluated by Gibbs free energy and spectroscopic analyses, which ultimately identified the product as HBT-Enol1 with an intramolecular hydrogen bond. Bond parameters, infrared vibrational spectroscopy and charge analysis indicate that the hydrogen bond is enhanced at the excited state (S1), favoring excited state intramolecular proton transfer (ESIPT). The mild energy barrier further evidences the occurrence of ESIPT. Combined with frontier molecular orbital (FMO) analysis, the fluorescence quenching of PBT was attributed to the photoinduced electron transfer (PET) mechanism and the fluorescence turn-on mechanism of the product was attributed to the ESIPT process of HBT-Enol1.

Label-free multimodal analysis of copper ions at below permissible exposure limit in the aqueous medium

An anthraimidazoledione based amphiphilic dye molecule was synthesized that shows formation of tuneable charge-transfer state in solution, susceptible to change in pH, polarity and hydrogen bonding ability of the medium. The compound also showed formation of nanoscopic self-assembled structure in water medium. The probe molecule can achieve multimodal detection (colorimetric, fluorimetric and electrochemical) of copper ions as low as 0.3 ppm in the aqueous medium. Addition of copper leads to dose-dependent ratiometric change in solution color from yellow to purple. The mechanistic investigation indicates that the coordination of copper ions was possible via simultaneous engagement of both imidazole nitrogen ends and neighbouring hydroxyl unit. Not only optical property, the changes in microenvironment also influence the selectivity as well as sensitivity of the probe molecule towards Cu2+ ions. Further, the optical probe is used for detection as well as quantification of copper ions in natural water samples without any sample pretreatment. Low-cost, reusable paper strips are developed for rapid, on-location detection of residual Cu2+ in real-life samples.

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.

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.

Tuning anion binding properties of Bis(indolyl)methane Receptors: Effect of substitutions on optical responses

Chromogenic probes based onoxidizedbis(indolyl)methanes have been synthesized with varying substituents (R = -Me [1], -OMe [2], -OH, [3]) on the central aryl ring. In addition to electronic influence, the involvement of substituents in ion-dipole and charge-assisted hydrogen bonding interactions significantly alters the solvatochromic response and pH-sensitive behavior. In polar aprotic solvents, like CH₃CN, a concentration-dependent stepwise color change was observed with F^- ions. In the case of2, a reversible hydrogen bonding interaction between the deprotonated probe and HF₂^- dimer might be responsible for that, while step-wise deprotonation caused by F^- ions could be the probable reason with3. Since the formation of HF₂^- is energetically unfavorable in a polar protic solvent, the response of 2 with F^- ions appears to be very different in EtOH medium. Interestingly, no such alteration in anion sensing behavior was noticed with3going from an aprotic to a protic solvent.

An anthraimidazoledione-based charge transfer probe for dual mode sensing of calcium ions: role of the counter ion in signal improvement

An anthraimidazoledione-based amphiphilic probe has been designed for dual-mode sensing of Ca²⁺ ions at physiological pH in a buffered medium. The compound showed deep pink color in the native state due to intramolecular charge transfer from the imidazole to the anthraquinone moiety. The addition of Ca²⁺ ions resulted changes in solution color to orange with a concomitant appearance of blue-colored fluorescence. The mechanistic investigations indicate that the Ca²⁺ ion binds to the APTRA moiety on the donor site, which leads to a blue-shift in the absorption maxima. Most importantly, because of the naked-eye response, this method does not need sophisticated visualizing instruments for the analysis. Interestingly, the counter ion showed a significant impact on the extent of Ca²⁺ sensing. The fluorescence response was large when Cs⁺ (loose ion pair) was considered as the counter ion instead of K⁺ (tight ion pair). Furthermore, the present system can achieve the detection of Ca²⁺ ions in real-life water samples and also in the presence of serum albumin protein. The high recovery values along with small standard deviations indicate the suitability of the present method in analyzing real-life samples. Finally, dye-coated paper strips were developed for rapid, on-location detection of Ca²⁺ ions.