A new Dual-Channel Chemosensor Based on Chemodosimeter Approach for Detecting Cyanide in Aqueous Solution: a Combination of Experimental and Theoretical Studies

ICT / PET Modulated Chromogenic/Fluorogenic Detection of Cyanide Ions: A Spectroscopic and Theoretical Study

The prevalence use of cyanide and its pernicious risks to human health accentuates the prominence of cyanide detection. Concerning this, we have developed a probe VDP4 by coupling a donor (8-hydroxyquinoline-2-carbaldehyde) and an acceptor (2-(1H-benzimidazole-2-yl)acetonitrile) by Knoevenagel condensation reaction for the exclusive detection of cyanide in an aqueous solution. The VDP4 responded to CN- by changing its color to yellow and switching on its fluorescence following contact with cyanide. 1H-NMR, 13C-NMR, LC-MS, FT-IR, and DFT studies provide an attestation for the signaling mechanism for cyanide detection is the deprotonation of the -OH group combined with nucleophilic addition of cyanide at the electron-deficient vinylic carbon atom of VDP4. A very low colourimetric (86 nM) and fluorometric (44 nM) detection limit highlighted its practicability, and Job studies ratified the 1:1 binding interaction. The DFT/TDDFT studies uncovered that colour changes of VDP4 with CN- were caused by ICT variations, and PET modulations were responsible for fluorescent alterations. The spatial ICT was a key factor for the deep yellow coloration of the VDP4 in the presence of CN-. The energy level diagram of frontier molecular orbitals unveiled that Photoinduced electron transfer from the benzimidazole part to the 8-hydroxy quinoline part was liable for the frail emission of VDP4 which occluded in the presence of CN- thereby turning on the fluorescence of the fluorophore. The real-time analysis results advocated that VDP4 was the optimum choice for the qualitative and quantitative estimation of CN- in food samples.

Current Advances in Diazoles-based Chemosensors for CN- and FDetection

Advances in molecular probes have recently intensified because they are valuable tools in studying species of interest for human health, the environment, and industry. Among these species, cyanide (CN-) and fluoride (F-) stand out as hazardous and toxic ions in trace amounts. Thus, there is a significant interest in probes design for their detection with diverse diazoles (pyrazole and imidazole) used for this purpose. These diazole derivatives are known as functional molecules because of their known synthetic versatility and applicability, as they exhibit essential photophysical properties with helpful recognition centers. This review provides an overview of the recent progress (2017-2021) in diazole-based sensors for CN- and F- detection, using the azolic ring as a signaling or recognition unit. The discussion focuses on the mechanism of the action described for recognizing the anion, the structure of the probes with the best synthetic simplicity, detection limits (LODs), application, and selectivity. In this context, the analysis involves probes for cyanide sensing first, then probes for fluoride sensing, and ultimately, dual probes that allow both species recognition.

Highly Selective and Sensitive Colorimetric and Fluorescent Chemosensors for Rapid Detection of Cyanide Anions in Aqueous Medium: Investigation on Supramolecular Recognition of Tweezer‑shaped Salophenes

Three tweezer‑shaped salophenes having catechols (1), phenols (2) and anisoles (3) units in conjunction to the dipodal Schiff bases have been applied for the optical sensing of cyanide (CN¯) ions in CH3CN-H2O (7:3) as solvent of choice. Among them, compounds 1 and 2 recognized CN¯, relying on distinct color and spectral changes. They are easy-to-use probes that exhibit extremely high sensitivity (limit of detection = 1-10 nM), rapid response (5 s) and excellent selectivity. Moreover, the visual detection and concentration determination of CN¯ by solution test kits of both sensors are the advantages for the practical applications. Based on the fluorescence and NMR spectroscopy, as well as the OH¯ and reversibility experiments, the explicit effect of hydroxyl groups on sensing and as well the different recognition of 1 and 2 toward CN¯ ions was proved. While probe 1 senses CN¯ via deprotonation, probe 2 recognizes it through an intramolecular aldimine condensation cyclization, leading to formation of anions of dihydroxyquinoxaline 4. This chemodosimetry is being reported for the first time in a Schiff's base. Furthermore, the similarity of fluorescence and NMR responses of 2 and 4 toward CN¯ supports the proposed process.

3-Hydroxy-2-naphthoic hydrazide as a probe for fluorescent detection of cyanide and aluminium ions in organic and aquo-organic media and its application in food and pharmaceutical samples

The commercially available fluorophore, 3-hydroxy-2-naphthoic hydrazide (RS2), has rationally been selected for the study, which displays a rapid fluorescent response and high sensitivity for CN^- and Al(III) ions in neat DMSO and H₂O-DMSO (1:1 v/v) media. The addition of CN^- to RS2 triggers an enhancement in fluorescence at 505 nm (green fluorescence), while the addition of Al(III) increases the fluorescence of the probe with a blue-shift of emission maximum by 25 nm (bluish-green fluorescence). The probe's action was investigated by ¹H NMR titrations that indicate deprotonation of OH and NH moieties by these ions. ²⁷Al NMR of RS2-Al(III) complex suggests an octahedral geometry for the complex. The sensitivity of the fluorescent-based assays in aq. DMSO medium, 0.8 µM for CN^- and 1.9 µM for Al(III) ions are far below the limits in the World Health Organization guidelines for drinking water. RS2 detects Al(III) by the chelation-enhanced fluorescence (CHEF) mechanism. Besides, RS2 was successfully applied to detect CN^- and Al(III) ions in food materials and pharmaceutical samples, respectively.

Rational design and application of a fluorogenic chemodosimeter for selective detection of cyanide in an aqueous solution via excimer formation

A new quinone-benzothiazole imine based chemodosimeter (R) was rationally designed, synthesized and characterized using NMR and mass spectral techniques. The receptor colorimetrically senses cyanide in aq. HEPES buffer: DMF (2:8 v/v) solution with an instantaneous colour change from yellow to bluish green. An enhancement of fluorescence intensity at 429 nm with excimer formation is also observed after addition of cyanide to the receptor, which is accompanied with a colour change from yellow to blue under UV lamp. Nucleophilic addition of cyanide to imine C-atom inhibits intra-molecular charge transfer (ICT) transition, which is responsible for the excimer formation. This chemical reaction is confirmed by ¹H NMR titration. The receptor binds with two equivalents of cyanide with a binding constant of 5.55 × 10⁴ M^-1. The limit of detection (LOD) of cyanide by the receptor is found to be as low as 69 nM, which is much lower than the acceptable limit of cyanide in drinking water as set by the WHO (1.9 μM). Electrochemical studies support the termination of ICT transition upon addition of cyanide ion. Theoretical studies substantiate experimental findings and excimer formation. The receptor fluorometrically detects cyanide present in tap water and food materials such as cassava flour, almond and potato.

A new fluorene derived Schiff-base as a dual selective fluorescent probe for Cu2+ and CN. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;207:6-15. [PMID: 30195186 DOI: 10.1016/j.saa.2018.08.058]

A new fluorene based fluorogenic chemosensor, 2-[(9H-Fluoren-2-ylmethylene)-amino]-phenol (L), has been designed, synthesized, and characterized by CHN analyses and different spectroscopic methods. This turn-on fluorogenic chemosensor shows high selectivity and sensitivity toward Cu²⁺ and CN^- with low detection limits of 1.54 × 10^-9 M and 1.83 × 10^-7 M, respectively. The stoichiometry ratio of L-Cu²⁺ in solution is 1:1, by the method of Job's plot and ESI-MS. The microcrystalline solid product of the chemosensor reaction with copper is characterized as CuL₂. The χT value for CuL₂ is temperature independent at a value of 0.403 cm³ K mol^-1, which is in agreement with a mononuclear copper(II) complex with an isotropic g-value of 2.075. The fluorescence turn-on recognition process for detection of Cu²⁺ is attributed to the restricted imine isomerization and blocking of intramolecular charge transfer (ICT) quenching process in the analyte-bound sensor. The selectivity of L for Cu²⁺ is based on the chelation-enhanced fluorescence effect (CHEF) mechanism. Other interfering ions such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Ag⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Mn²⁺, Pb²⁺ and Al³⁺, show no change in the fluorescence intensity of L in the presence of Cu²⁺. Furthermore, the compound L can be used as a fluorescence and colorimetric sensor for selective detection of CN^- over a number of other anions based on the nucleophilic addition to the imine CN bond, with consequent hydrogen bond formation and electrostatic interaction of the resulting product with K⁺. The sensing mechanism for CN^- was theoretically supported by DFT calculations.

A simple, reversible, colorimetric and water-soluble fluorescent chemosensor for the naked-eye detection of Cu2+ in ~100% aqueous media and application to real samples

A simple, reversible, colorimetric and water-soluble fluorescent chemosensor ADA for the naked-eye detection of Cu²⁺ was developed. Sensor ADA showed high selectivity and sensitivity toward Cu²⁺ in ~100% aqueous media over wide pH range. Sensor ADA exhibited a red-shift in the absorption spectra from 466 to 480nm that is accompanied by significant color change from light yellow to yellowish brown instantaneously. The Cu²⁺ recognition is based on the chelation-enhanced fluorescence quenching (CHEQ) effect of the paramagnetic nature. The lowest detection limit is determined to be 15.8nM, which is much lower than the allowable level of Cu²⁺ in drinking water set by U.S. Environmental Protection Agency (~20μM) and the World Health Organization (~30μM). The 1:1 binding process was confirmed by fluorescence measurements, IR analysis and DFT studies. Moreover, sensor ADA was successfully applied for determination of trace level of Cu²⁺ with 4 reuse cycles in various water samples, which affords promising potential in ion-detection field.

A single colorimetric sensor for multiple targets: the sequential detection of Co2+and cyanide and the selective detection of Cu2+in aqueous solution

A colorimetric chemosensor was developed for simultaneous detection of Co²⁺and Cu²⁺and for sequential recognition of Co²⁺and CN⁻.

A PET-based fluorometric chemosensor for the determination of mercury(II) and pH, and hydrolysis reaction-based colorimetric detection of hydrogen sulfide

A simple fluorescent chemosensor 1 for the detection of Hg(2+) and pH was developed by a combination of 2-aminoethyl piperazine and 4-chloro-7-nitrobenz-2-oxa-1,3-diazole. The sensor 1 showed OFF-ON behavior for different colors of fluorescence in the presence of Hg(2+) and under acidic conditions, respectively, in a near-perfect aqueous solution. The turn-on fluorescence caused by inhibition of photoinduced electron transfer was explained by theoretical calculations. 1 could be used to quantify Hg(2+) in water samples, and its in vitro studies with HeLa cells showed fluorescence in the presence of Hg(2+). In addition, 1 could selectively detect S(2-) by changing its color from orange to pink in a near-perfect aqueous solution. Moreover, 1 could be used as a practical, visible test kit for S(2-).

A novel benzothiazole-based enaminone as a fluorescent probe for highly selective and sensitive detection of CN−

A novel benzothiazole derived enaminone BTP as fluorescent probe for CN⁻ recognition has been developed.

A highly selective fluorescent sensor for the detection of Al3+ and CN− in aqueous solution: biological applications and DFT calculations

A new turn-on fluorescent chemosensor 1 was developed to detect both Al³⁺ and CN⁻ and used for practical and biological applications.