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.

Exploring Various Photochemical Processes in Optical Sensing of Pesticides by Luminescent Nanomaterials: A Concise Discussion on Challenges and Recent Advancements.

Food safety is a burning global issue in this present era. The prevalence of harmful food additives and contaminants in everyday food is a significant cause for concern as they can adversely affect human health. More particularly, among the different food contaminants, the use of excessive pesticides in agricultural products is severely hazardous. So, the optical detection of residual pesticides is an effective strategy to counter the hazardous effect and ensure food safety. In this perspective, nanomaterials have played a leading role in defending the open threat against food safety instigated by the reckless use of pesticides. Now, nanomaterial-based optical detection of pesticides has reached full pace and needs an inclusive discussion. This Review covers the advancement of photoprocess-based optical detection of pesticides categorically using nanomaterials. Here, we have thoroughly dissected the photoprocesses (aggregation and aggregation-induced emission (AIE), charge transfer and intramolecular charge transfer (ICT), electron transfer and photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), hydrogen bonding, and inner filter effect) and categorically demarcated their significant role in the optical detection of pesticides by luminescent nanomaterials over the last few years.

Sequence-Specific Relay Recognition of Multiple Anions: An Interplay between Proton Donors and Acceptors.

Ratiometric detection of analyte is highly deserving since the technique is free from background correction. This work reports the design and synthesis of a pyridine-end oligo p-phenylenevinylene (OPV) derivative, 1 and its application in ratiometric dual-mode (both colorimetric and fluorogenic) recognition of dual anions, bisulfate (LOD=12.5 ppb) followed by fluoride (LOD=18.2 ppb) by sequence-specific relay (SPR) technique. The colorless probe turns brown with addition of bisulfate and again becomes colorless with the sequential addition of fluoride ion. In addition to such naked-eye color change, interestingly the ratiometric spectroscopic signals are reversible and evidently, the probe is reusable for several cycles. Besides, in presence of bisulfate, the protonated probe molecules, owing to their larger amphiphilic characteristics, formed self-assembled nanostructures. In addition to colorimetric and fluorescent changes, 1 H NMR titration and systematic DFT study evidently establish the underneath proton transfer mechanisms. Such reusable OPV-based chemosensor particularly with the capability of naked-eye recognition of dual anions using the SPR technique is seminal and possibly the first report in the literature.

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.

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.

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.

Phosphatase-like Activity of Tetranuclear Iron(III) and Zinc(II) Complexes.

Three new tetranuclear iron(III) and zinc(II) complexes, [Fe4(cpdp)2(phth)2(OH)2]·8H2O (1), [Fe4(cpdp)2(terephth)2(OH)2] (2), and [Zn4(Hcpdp)2(suc)]Br2·12H2O (3), have been synthesized as models for the active site of phosphoester hydrolases by utilizing a polydentate ligand, N, N'-bis[2-carboxybenzomethyl]- N, N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol (H3cpdp) in combination with exogeneous phthalate (phth), terephthalate (terephth), and succinate (suc). Single crystal X-ray analyses reveal that the metallic core of complex 1 consists of four distorted octahedral iron(III) ions with average intraligand Fe---Fe separation of 3.656(2) Å, while the structure 3 represents a tetranuclear metallic core containing four distorted trigonal bipyramidal zinc(II) ions with average intraligand Zn---Zn separation of 3.472(2) Å. The molecular structure of complex 2 has been optimized by the DFT method which shows that its core arrangement is similar to that of 1. Complex 1 has a very interesting centrosymmetric structure that includes two crystallographically equivalent [Fe2(cpdp)]3+ dinuclear units, connected together by a pair of syn-syn bridging phthalates and a pair of bridging hydroxides to generate a "dimer of dimers" structural motif. In complex 3, a succinate group connects two crystallographically equivalent [Zn2(Hcpdp)]2+ dinuclear units in a syn-syn bidentate manner forming a "dimer of dimers" structural design. All three complexes show phosphatase-like activity that has been examined in methanol-water (1:1; v/v) using bis( p-nitrophenyl) phosphate (BNPP) as model substrate by applying the UV-vis spectrophotometric technique. In each case, the kinetic data have been analyzed by the Michaelis-Menten approach. The order of catalytic efficiency for the conversion of substrate to product follows the trend 1 > 2 > 3 with turnover rates ( kcat) of (2.73 ± 0.13) × 10-5 for 1, (1.06 ± 0.07) × 10-5 for 2, and (2.33 ± 0.18) × 10-6 s-1 for 3. These kcat values are comparable to, albeit slightly lower than, the values reported for similar iron(III)- and zinc(II)-based model complexes in the literature. DFT calculations have been carried out to support the proposed mechanism for phosphatase-like activity.

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.

Characterization of domain-specific interaction of synthesized dye with serum proteins by spectroscopic and docking approaches along with determination of in vitro cytotoxicity and antiviral activity.

The interaction between a synthesized dye with proteins, bovine, and human serum albumin (BSA, HSA, respectively) under physiological conditions has been characterized in detail, by means of steady-state and time-resolved fluorescence, UV-vis absorption, and circular dichroism (CD) techniques. An extensive time-resolved fluorescence spectroscopic characterization of the quenching process has been undertaken in conjugation with temperature-dependent fluorescence quenching studies to divulge the actual quenching mechanism. From the thermodynamic observations, it is clear that the binding process is a spontaneous molecular interaction, in which van der Waals and hydrogen bonding interactions play the major roles. The UV-vis absorption and CD results confirm that the dye can induce conformational and micro-environmental changes of both the proteins. In addition, the dye binding provokes the functionality of the native proteins in terms of esterase-like activity. The average binding distance (r) between proteins and dye has been calculated using FRET. Cytotoxicity and antiviral effects of the dye have been found using Vero cell and HSV-1F virus by performing MTT assay. The AutoDock-based docking simulation reveals the probable binding location of dye within the sub-domain IIA of HSA and IB of BSA.