Colorimetric "naked eye" sensing of anions in aqueous solution

A Unique Hybridization Route to Access Hydrazylnaphthalimidols as Novel Structural Scaffolds of Multitargeting Broad-Spectrum Antifungal Candidates

This study developed a class of novel structural antifungal hydrazylnaphthalimidols (HNs) with multitargeting broad-spectrum potential via multicomponent hybridization to confront increasingly severe fungal invasion. Some prepared HNs exhibited considerable antifungal potency; especially nitrofuryl HN 4a (MIC = 0.001 mM) exhibited a potent antifungal activity against Candida albicans, which is 13-fold higher than that of fluconazole. Furthermore, nitrofuryl HN 4a displayed low cytotoxicity, hemolysis and resistance, as well as a rapid fungicidal efficacy. Preliminary mechanistic investigations revealed that nitrofuryl HN 4a could inhibit lactate dehydrogenase to decrease metabolic activity and promote the accumulation of reactive oxygen species, leading to oxidative stress. Moreover, nitrofuryl HN 4a did not exhibit membrane-targeting ability; it could embed into DNA to block DNA replication but could not cleave DNA. These findings implied that HNs are promising as novel structural scaffolds of potential multitargeting broad-spectrum antifungal candidates for treating fungal infection.

A near-infrared (NIR) organic probe for rapid naked-eye detection of fluoride ions in aqueous medium

A dihydroxy-substituted tricyanopyrroline-based near-infrared (NIR) colorimetric probe (TCP-34Hy) is developed for the effective detection of fluoride ions in aqueous medium in the presence of Ca2+ ions. The naked-eye detection (drastic color change) selectively makes the NIR colorimetric sensor more efficient. The probe can detect fluoride ions from inorganic sources (NaF/KF) as well as organic sources (TBAF) in water. The water samples from different sources, such as tap, pond, river, and sewage, have been tested successfully for fluoride ion detection using the probe (TCP-34Hy). Investigations of different kinds of soil samples (clay soil, red soil, and river sand) after pre-treatment with NaF are also productive for detection of fluoride ions in water. The probe can also detect fluoride ions in toothpaste in aqueous medium. The role of Ca2+ is discussed and explained on the basis of experimental results. Additionally, the probe (TCP-34Hy) shows high selectivity towards fluoride ion detection with a LOD of 2.3 μM (0.052 ppm) in aqueous medium.

Small Molecule Optical Probes for Detection of H2S in Water Samples: A Review

Hydrogen sulfide (H2S) is closely linked to not only environmental hazards, but also it affects human health due to its toxic nature and the exposure risks associated with several occupational settings. Therefore, detection of this pollutant in water sources has garnered immense importance in the analytical research arena. Several research groups have devoted great efforts to explore the selective as well as sensitive methods to detect H2S concentrations in water. Recent studies describe different strategies for sensing this ubiquitous gas in real-life water samples. Though many of the designed and developed H2S detection approaches based on the use of organic small molecules facilitate qualitative/quantitative detection of the toxic contaminant in water, optical detection has been acknowledged as one of the best, attributed to the simple, highly sensitive, selective, and good repeatability features of the technique. Therefore, this review is an attempt to offer a general perspective of easy-to-use and fast response optical detection techniques for H2S, fluorimetry and colorimetry, over a wide variety of other instrumental platforms. The review affords a concise summary of the various design strategies adopted by various researchers in constructing small organic molecules as H2S sensors and offers insight into their mechanistic pathways. Moreover, it collates the salient aspects of optical detection techniques and highlights the future scope for prospective exploration in this field based on the limitations of the existing H2S probes.

Evaluation of naked-eye sensing and anion binding studies in meso-fluorescein substituted one-walled calix[4]pyrrole (C4P)

In this paper, we have design, synthesized and fully characterized a new meso-fluorescein substituted one-walled calix[4]pyrrole (C4P7), obtained from simple and easily available starting materials such as fluorescein, 4-hydroxyacetophenone and pyrrole. The anion sensing studies reveal that the C4P7 system displays selective and sensitive naked-eye sensing towards fluoride, phosphate, and acetate anions with the limit of detection of 4.27 mg L-1, 6.4 mg L-1, and 5.94 mg L-1, respectively. Moreover, the C4P7 receptor displays good results of binding (host-guest, 1 : 1) towards a variety of anions. The 1 : 1 binding stoichiometry was further confirmed by means of Job's plots. TD-DFT calculations showed that the HOMO-LUMO gap decreases in all the complexes (C4P7@anions) in comparison to the free C4P7 system. The authors are of the opinion that this work may provide a good platform to explore calix[4]pyrrole chemistry in the arena of recognition/sensing of biologically significant analytes in future studies.

A new benzothiazole azo dye colorimetric chemosensor for detecting Pb2+ ion

In this work, a new benzothiazole azo dye sensor (BTS) was synthesized, and its cation binding affinity was studied using the colorimetric method, UV-vis, and ¹H NMR spectral data. The results revealed that the sensor BTS exhibits a remarkable tendency for Pb²⁺ ion to perform spontaneous visual color change from blue (BTS) to pink (BTS + Pb²⁺), without any color change in the aqueous solutions of other cations such as Hg²⁺, Cu²⁺, Al³⁺, Ni²⁺, Cd²⁺, Ag⁺_, Ba²⁺, K⁺, Co²⁺, Mg²⁺, Na⁺, Ca²⁺, Fe²⁺, and Fe³⁺ ions. The observed selectivity could be due to the formation of the complex (BTS + Pb²⁺⁾, which led to a blue shift from 586 nm (BTS) to 514 nm (BTS + Pb²⁺) in the UV spectrum. The job's plot provided the stoichiometry ratio of the complex (BTS + Pb²⁺) to be 1:1. The limit of detection (LOD) of BTS for Pb²⁺ ion sensing was obtained at 0.67 µM. Additionally, the binding constant for BTS toward Pb ²⁺ ion was studied using the Benesi-Hildebrand equation. As a result of the BTS test paper strips investigations, it was found that the synthesized sensor BTS could be used as a rapid colorimetric chemosensor for the detection of the Pb²⁺ ions in the distilled, tap, and sea waters.

Unique Metal-Ligand Interplay in Directing Discrete and Polymeric Derivatives of Isomeric Azole-Carboxylate. Varying Electronic Form, C-C Coupling, and Receptor Feature

This article dealt with the ruthenium and osmium derivatives of isomeric 1H-indazole-3-carboxylic acid/2H-indazole-3-carboxylic acid (H₂L1) and 1H-benzimidazole-2-carboxylic acid (H₂L2) along with the π-acidic bpy (bpy = 2,2'-bipyridine) and pap (pap = 2-phenylazopyridine) co-ligands. It thus extended structurally authenticated monomeric ([(bpy)₂Ru^II(HL1^-)]ClO₄ [1]ClO₄, (pap)₂Ru^II(L1^2-) 2, (bpy)₂Os^II(L1^2-) 3, (pap)₂Os^II(L1^2-) 4, (bpy)₂Ru^II(L2^2-) 5, (bpy)₂Os^II(L2^2-) 8, and (pap)₂Os^II(L2^2-) 9) and dimeric ([(bpy)₂Ru^II(μ-L2^2-)Ru^II(bpy)₂](ClO₄)₂ [6](ClO₄)₂) complexes. It also described modified L2'^2- (L2'^2- = 2,2'-bisbenzimidazolate)-bridged [(pap)₂Ru^II(μ-L2'^2-)Ru^II(pap)₂](ClO₄)₂ [7](ClO₄)₂, where L2'^2- was developed selectively with the {Ru(pap)₂} metal fragment via in situ intermolecular C-C coupling of the two units of decarboxylated benzimidazolate. Moreover, chemical oxidation (Os^II to Os^III) of (bpy)₂Os^II(L1^2-) 3 (E⁰ = 0.11 V versus SCE) and (bpy)₂Os^II(L2^2-) 8 (E⁰ = 0.12 V versus SCE) by AgClO₄ yielded unprecedented Os^III-Ag^I derived polymeric {[(bpy)₂Os^III-L1^2--Ag^I(CH₃CN)](ClO₄)₂}_n {[10](ClO₄)₂}_n and dimeric [(bpy)₂Os^III-L2^2--Ag^I(CH₃CN)](ClO₄)₂ [11](ClO₄)₂ complexes as a function of trans and cis orientations of the active N2 donor with special reference to the carboxylate O2 of L^2-, respectively. Microscopic (FE-SEM, TEM-EDX, and AFM) and DLS experiments suggested a homogeneously dispersed hollow spherical shaped morphology of {[10](ClO₄)₂}_n with an average particle size of 200-400 nm as well as its non-dissociative feature in the aprotic medium. Experimental (structure, spectroscopy, and electrochemistry) and theoretical (DFT/TD-DFT) explorations revealed a redox non-innocent feature of L^2- in the present coordination situations and the selective anion sensing (X = F^-, CN^-, and OAc^-) event of [1]ClO₄ involving a free NH group at the backface of HL1^-, which proceeded via the NH···X hydrogen bonding interaction.

Synthesis of Fluorescent, Dumbbell-Shaped Polyurethane Homo- and Heterodendrimers and Their Photophysical Properties

Fluorescent dendrimers have wide applications in biomedical and materials science. Here, we report the synthesis of fluorescent polyurethane homodendrimers and Janus dendrimers, which often pose challenges due to the inherent reactivity of isocyanates. Polyurethane dendrons (G1-G3) were synthesized via a convergent method using a one-pot multicomponent Curtius reaction as a crucial step to establish urethane linkages. The alkyne periphery of the G1-G3 dendrons was modified by a copper catalyzed azide-alkyne click reaction (CuAAC) to form fluorescent dendrons. In the reaction of the surfaces functionalized two different dendrons with a difunctional core, a mixture of three dendrimers consisting of two homodendrimers and a Janus dendrimer were obtained. The Janus dendrimer accounted for a higher proportion in the products' distribution, being as high as 93% for G3. The photophysical properties of Janus dendrimers showed the fluorescence resonance energy transfer (FRET) from one to the other fluorophore of the dendrimer. The FRET observation accompanied by a large Stokes shift make these dendrimers potential candidates for the detection and tracking of interactions between the biomolecules, as well as potential candidates for fluorescence imaging.

Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour

The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO−, SO42− HPO42−, and F−. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl−; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2.

Fabrication of stable electrospun blended chitosan-poly(vinyl alcohol) nanofibers for designing naked-eye colorimetric glucose biosensor based on GOx/HRP

In this study, designing of a stable electrospun blended chitosan (CS)-poly(vinyl alcohol) (PVA) nanofibers for colorimetric glucose biosensing in an aqueous medium was investigated. CS and PVA solutions were blended to acquire an optimum content (CS/PVA:1/4) and electrospunned to obtain uniform and bead-free CS/PVA nanofiber structures following the optimization of the electrospinning parameters (33 kV, 20 cm, and 1.2 ml.h^-1). Crosslinking process applied subsequently provided mechanically and chemically stable nanofibers with an average diameter of 378 nm. The morphological homogeneity, high fluid absorption ability (>%50), thermal (<230 °C) and morphological stability, surface hydrophilicity and degrability properties of cross-linked CS/PVA nanofiber demonstrated their great potential to be developed as an eye-readable strip for biosensing applications. The glucose oxidase (GOx) and horseradish peroxidase (HRP) was immobilized by physical adsorption on the cross-linked CS/PVA nanofiber. The glucose assay analysis by ultraviolet-visible (UV-Vis) spectrophotometry using the same enzymatic system of the proposed glucose strips in form of absorbance versus concentration plot was found to be linear over a glucose concentration range of 2.7 to 13.8 mM. The prepared naked eye colorimetric glucose detection strips, with lower detection limit of 2.7 mM, demonstrated dramatic color change from white (0 mM) to brownish-orange (13.8 mM). The developed cross-linked CS/PVA nanofiber strips, prepared by electrospinnig procedure, could be easily adapted to a color map, as an alternative material for glucose sensing. Design of a practical, low-cost, and environmental-friendly bio-based CS/PVA testing strips for eye readable detection were presented and suggested as an applicable medium for a wide range of glucose concentrations.

Squaramide-Based Self-Associating Amphiphiles for Anion Recognition

The synthesis and characterisation of two novel self-assembled amphiphiles (SSAs) SQS-1 and SQS-2 are reported. Both compounds, based on the squaramide motif, were fully soluble in a range of solvents and were shown to undergo self-assembly through a range of physical techniques. Self-assembly was shown to favour the formation of crystalline domains on the nanoscale but also fibrillar film formation, as suggested by SEM analysis. Moreover, both SQS-1 and SQS-2 were capable of anion recognition in DMSO solution as demonstrated using 1 H NMR and UV/Vis absorption spectroscopy, but displayed lower binding affinities for various anions when compared against other squaramide based receptors. In more competitive solvent mixtures SQS-1 gave rise to a colourimetric response in the presence of HPO42- that was clearly visible to the naked eye. We anticipate that the observed response is due to the basic nature of the HPO42- anion when compared against other biologically relevant anions.

Carbazole-Based Colorimetric Anion Sensors

Owing to their strong carbazole chromophore and fluorophore, as well as to their powerful and convergent hydrogen bond donors, 1,8-diaminocarbazoles are amongst the most attractive and synthetically versatile building blocks for the construction of anion receptors, sensors, and transporters. Aiming to develop carbazole-based colorimetric anion sensors, herein we describe the synthesis of 1,8-diaminocarbazoles substituted with strongly electron-withdrawing substituents, i.e., 3,6-dicyano and 3,6-dinitro. Both of these precursors were subsequently converted into model diamide receptors. Anion binding studies revealed that the new receptors exhibited significantly enhanced anion affinities, but also significantly increased acidities. We also found that rear substitution of 1,8-diamidocarbazole with two nitro groups shifted its absorption spectrum into the visible region and converted the receptor into a colorimetric anion sensor. The new sensor displayed vivid color and fluorescence changes upon addition of basic anions in wet dimethyl sulfoxide, but it was poorly selective; because of its enhanced acidity, the dominant receptor-anion interaction for most anions was proton transfer and, accordingly, similar changes in color were observed for all basic anions. The highly acidic and strongly binding receptors developed in this study may be applicable in organocatalysis or in pH-switchable anion transport through lipophilic membranes.

A molecular design for a turn-off NIR fluoride chemosensor

We designed a turn-off near-infrared fluorescent fluoride chemosensor NIR-BODIPY-Si through the density functional theory/time-dependent functional theory calculations. In the designed sensor, the tert-butyldimethylsilyloxy moiety responses to the fluoride-triggered desilylation process, and the BODIPY dye serves as fluorophore. The molecular design firstly showed that the possibility of photoinduced electron transfer is low/high in NIR-BODIPY-Si/NIR-BODIPY-O (the desilylation product), thus referring that the fluorescence sensing mechanism is a photoinduced electron transfer mechanism that quenched the sensor's fluorescence after detection of fluoride anions. Absorption and emission spectra further demonstrated that the designed sensor is a near-infrared chemosensor. The largest binding energy between NIR-BODIPY-Si and F^- suggests that the sensor has an excellent selectivity to F^- and the low barrier of the desilylation reaction accounts for the sensor's rapid response speed to F^-. We also provided the synthetic routine for the molecule sensor, with the expectation that this molecular design can shed some light on the experimentally based design procedure.

Allosteric binding properties of a 1,3-alternate thiacalix[4]arene-based receptor having phenylthiourea and 2-pyridylmethyl moieties on opposite faces

Thiacalix[4]arene receptors with two thiourea moieties and two 2-pyridyl moieties exhibited a heteroditopic dinuclear receptor with F− and Ag+ ions by a positive allosteric effect.

Multi-Responsive Polysiloxane/Poly(N-isopropylacrylamide) Interpenetrating Networks Containing Urea and Thiourea Groups

Novel interpenetrating polymer networks (IPNs) were synthesized from N-isopropylacrylamide (NIPAM) and polysiloxanes containing a urea or thiourea side group, in addition to the silanol residue, through two reactions, such as the radical gelation of NIPAM and the condensation of silanols to form a siloxane linkage. The obtained IPNs showed a typical temperature-responsive volume change in water based on the constructed poly-NIPAM gel component. In addition, the characteristic color and volume changes responding to chemical stimuli, such as acetate and/or fluoride ions, based on the introduced urea and thiourea groups onto the polysiloxane gel were observed in N,N-dimethylformamide.

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

Colorimetry is an advantageous method for detecting fluoride in drinking water in a resource-limited context, e. g., in parts of the developing world where excess fluoride intake leads to harmful health effects. Here we report a selective colorimetric chemosensor for fluoride that employs an azulene as the reporter motif and a pinacolborane as the receptor motif. The chemosensor, NAz-6-Bpin, is prepared using the Nozoe azulene synthesis, which allows for its rapid and low-cost synthesis. The chemosensor gives a visually observable response to fluoride both in pure organic solvent and also in water/alcohol binary solvent mixtures.

Substituent effect on colorimetric detection of biologically and environmentally relevant anions: Insight in real-life applications

A new set of chromogenic anion receptors R1-R4 have been synthesized with a different substituent, including electron withdrawing (nitro moiety in R1), conjugated group (naphthyl in R2), and electron donating (methyl in R3), respectively. The receptors R1-R4 exhibited very good sensitivity towards the F^- and AcO^- anions in the DMSO. In addition, R1 showed selectivity towards H₂PO₄^- ions over other tested anions. R1 especially acted as an effective sensor for sodium salts of F^-, AcO^-, AsO₂^-, and AsO₄^2- ions in an aqueous medium due to the presence of two electron-withdrawing nitro substituents, which showed hydrogen bond donor tendency and acidity of the OH proton. This result indicates that R1 is highly capable of competing with an aqueous medium to detect anions without counter Na⁺ ion interference. Interestingly, R1 displays solvatochromic property in the presence of AcO^- ions in different aprotic solvents. Additionally, the receptor R1 shows high binding affinity towards AcO^- ions in the buffer medium (DMSO: HEPES, 9:1 v/v), which displayed remarkable colour change from pale yellow to blue with a large ∆λ_{red shift} of 170 nm. The CV studies reveal the deprotonation of the -NH proton upon interaction with the AcO^- ions. The receptor R1 is subjected to practical application to sense F^- and AsO₂^- ions using the test strip. In addition, the receptor R1 proves itself as a potential applicant for the detection of F^- ions quantitatively in commercially available mouthwash.

A targetable fluorescent probe for real-time monitoring of fluoride ions in mitochondria

Fluorion are pivotal anions in biology because they play an important role in dental care, treating osteoporosis, preventing tooth decay and promoting the healthy growth of bone. Studies have shown that high levels of fluoride will lead to the inactivation of the mitochondria. Therefore, it is urgent to develop a method to detect the fluoride anions in the mitochondria. Herein, we have developed a novel mitochondrial-target fluorescent probe for detecting F^- in living cells. The probe exhibited excellent sensitivity and high selectivity for F^- over the other relative species. With changing fluoride ions, the fluorescence spectrum of the probe changed significantly with a large turn-on fluorescence signal. Cell imaging indicated that the probe can penetrate viable cell membranes and rapidly detects and images fluorion over other anions in the mitochondria.

A concise treatment of pterins: some recent synthetic and methodology aspects and their applications in molecular sensors

A concise account of pterins in chemistry and biology and their applications in molecular sensors including their optical spectroscopic properties are described. Different natural, synthetic, biological and photophysical aspects are also discussed. Synthetic access to direct functionalised pterins and a recently reported new thiophene annulation technique are described for the synthesis of Form B of molybdenum cofactor. The receptor properties of fluorescent pterin molecules including selenopyrimidines which are rarely reported for their binding of anions and neutral molecules are also of major importance in this review. For such an old and still so young, unexplored pterin system on its power to be sensitive for physical studies especially the interaction with cations, anions and neutral molecules are fascinating and research in this area is relatively new and expected to increase fast. Pterin based receptors are for the first time put into a useful review for the advantage of those who want to explore pterin and modified pterin as chromogenic and fluorogenic sensors.

Fluorescent squaramides as anion receptors and transmembrane anion transporters

A series of squaramide-based anion transporters functionalised with the 1,8-naphthalimide fluorophore has been developed for improved ionophoric activity and fluorescent imaging in cells.

Colorimetric anion sensors based on positional effect of nitro group for recognition of biologically relevant anions in organic and aqueous medium, insight real-life application and DFT studies

A new six colorimetric receptors A1-A6 were designed and synthesized, characterized by typical common spectroscopic techniques like FT-IR, UV-Visible, ¹H NMR, ¹³C NMR and ESI-MS. The receptor A1 and A2 exhibit a significant naked-eye response towards F^- and AcO^- ions in DMSO. Due to presences of the NO₂ group at para and ortho position with extended π-conjugation of naphthyl group carrying OH as a binding site. Compared to receptor A2, A1 is extremely capable of detecting F^- and AcO^- ions present in the form of sodium salts in an aqueous medium. This is owed to the occurrence of NO₂ group at para position induced in increasing the acidity of OH proton. Consequently, it easily gets deprotonated in aqueous media. The detection limit of receptor A1 was turned out to be 0.40 and 0.35ppm for F^- and AcO^- ions which is beneath WHO permission level (1.0ppm). Receptor A1 shows a solitary property of solvatochromism in different aprotic solvents in presence of AcO^- ion. Receptor A1 depicts high selectivity towards AcO^- ion in DMSO: HEPES buffer (9:1, v/v). Receptor A1 proved itself for real life application by detecting anion in solution and solid state. The binding mechanism of receptor A1 with AcO^- and F^- ions was monitored from ¹HNMR titration and DFT study.

Four-coordinated see-sawN-(aryl)-2-(propan-2-ylidene)hydrazinecarbothioamide complexes of nickel(ii), copper(ii) and zinc(ii) and their propensity for catalytic cyclisation

Four-coordinated metal complexes of two hydrazinecarbothioamide-derived ligands with divalent nickel, copper and zinc are described and the selective reactivity difference of copper(ii) to cyclise the ligand or to form a complex is described.

Robust, sensitive and facile method for detection of F-, CN- and Ac- anions

Sensing of F^-, CN^- and Ac^- is important from the viewpoint of both medically and environmentally. Particularly, sensing of the anions in 100% water by a colorimetric chemical sensor is a highly difficult task as water molecules interfere the sensing mechanism. In this regard, sensor R1, having azo and nitrophenyl groups as signaling units and thiourea as a binding site was prepared. This sensor exclusively detected CN^- ion over other testing anions in 30% aq. DMSO solution by exhibiting distinct spectral and visual color changes. However, in 15% aq. DMSO solution, R1 exhibited obvious spectral and color changes in response to F^-, CN^- and Ac^-. On the other hand, we have also designed sensor, R2, having same signaling units of R1, but a different binding site of urea group. Surprisingly, in contrast to R1, R2 exhibited obvious spectral and color changes in 5% aq. DMSO solution only. Further, economically viable "test stripes" were prepared in a facile mode to detect the CN^- in 100% aqueous solution. Such stripes can serve as a practical colorimetric probe for "in the field" detection of the ions and thus avoid additional expensive equipment.

Insights into the complexation of N-Allyl-4-(4-(N-phenylureido)benzylamino)-1,8-naphthalimide with various anions

A new urea functionalised 4-amino-1,8-naphthalimide based fluorescent anion sensor was synthesised in 64% yield over three steps. Fluorescence and ¹H NMR titrations showed that the sensor complexes strongly with acetate and dihydrogen phosphate and to a lesser extent bromide. The corresponding binding stoichiometries were examined using ¹H NMR titrations. Results show that the sensor molecule initially forms 1:1 complexes through hydrogen bonding to the urea moiety, followed by secondary complexation to form higher order host:guest stoichiometries. Specifically, oxyanions complex to the sensor via hydrogen bonding through synergistic aryl C-H and N-H anion interactions in a 1:2 sensor:oxyanion arrangement. Furthermore, 2:1 sensor:oxyanion complexes are formed through an oxyanion linkage between two urea functionalities on different host molecules. This contrasts the majority of previous reports for similar hosts, which indicate 1:1 binding stoichiometry.

'Naked-eye' detection of biologically important anions in aqueous media by colorimetric receptor and its real life applications

A colorimetric receptor R 2-[(2-Hydroxy-naphthalen-1-ylmethylene)-hydrazonomethyl]-quinolin-8-ol has been designed and synthesized with good yield and characterized by the standard spectroscopic techniques such as FT-IR, UV-Visible, ¹H NMR, ¹³C NMR and ESI-MS. The receptor R showed naked-eye detection and spectral change in the presence of F^-, AcO^- and H₂PO₄^- over other anions. Interestingly, receptor R displaying high selective recognition towards F^-, AcO^- ion with a drastic color change from pale yellow to red in dry DMSO solvent and orange in mixed solvent DMSO/H₂O (9:1, v/v). The behavior of receptor R towards F^-, AcO^- ion was investigated using UV-Vis and ¹H NMR experiment. The detailed ¹H NMR experiment result revealed that the receptor R is forming the hydrogen bonding between imine nitrogen and phenolic OH proton towards anions. The receptor R is able to detect sodium salts of flouride (NaF) and acetate (NaAcO) in aqueous medium and it exhibited dramatic color change from pale yellow to red. The receptor R demonstrated itself to be useful for real life application by detecting flouride and acetate ion in sea-water and commercially available product such as toothpaste, mouthwash and vinegar solution.

Spectral and DFT studies of anion bound organic receptors: Time dependent studies and logic gate applications

New colorimetric receptors R1 and R2 with varied positional substitution of a cyano and nitro signaling unit having a hydroxy functionality as the hydrogen bond donor site have been designed, synthesized and characterized by FTIR, ¹H NMR spectroscopy and mass spectrometry. The receptors R1 and R2 exhibit prominent visual response for F⁻ and AcO^– ions allowing the real time analysis of these ions in aqueous media. The formation of the receptor–anion complexes has been supported by UV–vis titration studies and confirmed through binding constant calculations. The anion binding process follows a first order rate equation and the calculated rate constants reveal a higher order of reactivity for AcO⁻ ions. The ¹H NMR titration and TDDFT studies provide full support of the binding mechanism. The Hg²⁺ and F⁻ ion sensing property of receptor R1 has been utilized to arrive at “AND” and “INHIBIT” molecular logic gate applications.

Design and synthesis of a new organic receptor and evaluation of colorimetric anion sensing ability in organo-aqueous medium

A new organic receptor has been designed and synthesized by the combination of aromatic dialdehyde with nitro-substituted aminophenol resulting in a Schiff base compound. The receptor exhibited a colorimetric response for F(-) and AcO(-) ion with a distinct color change from pale yellow to red and pink respectively in dry DMSO solvent and yellow to pale greenish yellow in DMSO:H2O (9:1, v/v). UV-Vis titration studies displayed a significant shift in absorption maxima in comparison with the free receptor. The shift could be attributed to the hydrogen bonding interactions between the active anions and the hydroxyl functionality aided by the electron withdrawing nitro substituent on the receptor. (1)H NMR titration and density functionality studies have been performed to understand the nature of interaction of receptor and anions. The lower detection limit of 1.12ppm was obtained in organic media for F(-) ion confirming the real time application of the receptor.

A study on the synthesis and anion recognition of a chitosan-urea receptor

Chitosan-urea receptor for sensing PO₄³⁻ and F⁻ anions.

Off-On-Off fluorescence behavior of an intramolecular charge transfer probe toward anions and CO2

The photophysical behavior of a newly developed fluorescent probe, tricyanoethylphenyl phenanthroimidazole (TCPPI) has been studied. Upon interaction of different class of anions TCPPI displayed naked-eye sensitive fluorescence "turn-on" response to detect selectively F(-) (0.98μM, 18.62ppb) and CN(-) (1.12μM, 29.12ppb) anions in acetonitrile (MeCN). Job's plot analysis revealed a 1:1 binding stoichiometry between probe and anions. The spectral data analysis and 1H NMR titration studies suggested about the affinity of F(-) and CN(-) anions with moderately acidic -NH fragment of imidazolyl unit of probe through deprotonation and H-bonding interaction. Moreover, the anion activated probe upon interaction with CO2 revived photophysical properties of probe, "On-Off-On" type fluorescence and enabled anion-induced CO2 sensing in the medium.