Chemosensors based on N-heterocyclic dyes: advances in sensing highly toxic ions such as CN- and Hg2

CN- and Hg2+ ions are harmful to both the environment and human health, even at trace levels. Thus, alternative methods for their detection and quantification are highly desirable given that the traditional monitoring systems are expensive and require qualified personnel. Optical chemosensors (probes) have revolutionized the sensing of different species due to their high specificity and sensitivity, corresponding with their modular design. They have also been used in aqueous media and different pH ranges, facilitating their applications in various samples. The design of molecular probes is based on organic dyes, where the key species are N-heterocyclic compounds (NHCs) due to their proven photophysical properties, biocompatibility, and synthetic versatility, which favor diverse applications. Accordingly, this review aims to provide an overview of the reports from 2016 to 2021, in which fluorescent probes based on five- and six-membered N-heterocycles are used for the detection of CN- and Hg2+ ions.

A sensitive and selective BINOL based ratiometric fluorescence sensor for the detection of cyanide ions

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻). The optical study revealed that BBCN exhibited unique spectral changes only with cyanide ions in the presence of other competing ions. Besides, an apparent fluorescent colour change from green to blue was observed. A clear linear relationship was observed between the fluorescence ratiometric ratio of BBCN and the concentration of CN⁻ with a reasonably low detection limit (LOD) of 189 nM (507 ppb). The optical response was due to the nucleophilic addition of CN⁻ to the dicyanovinyl group of the sensor, which compromises the probe's intramolecular charge transfer. This mechanism was well confirmed by Job's plot, ¹H-NMR and ESI-MS studies. BBCN showed immediate spectral response towards (1 second) CN⁻ and detection could be realized in a broad pH window. Furthermore, the practical utility of BBCN was studied by test paper-based analysis and the detection of CN⁻ in various water resources.

A highly selective, novel BINOL based sensor BBCN has been developed for the fluorescent ratiometric detection of cyanide ions (CN⁻).

Nitroreductase-responsive polymeric micelles based on 4-nitrobenzyl and AIE moieties for intracellular doxorubicin release

An amphiphilic polymer TNP demonstrated the formation of a nitroreductase-responsive DOX delivery nanoplatform with high sensitivity and selectivity.

Synthetic ratiometric fluorescent probes for detection of ions

Metal cations and anions are essential for versatile physiological processes. Dysregulation of specific ion levels in living organisms is known to have an adverse effect on normal biological events. Owing to the pathophysiological significance of ions, sensitive and selective methods to detect these species in biological systems are in high demand. Because they can be used in methods for precise and quantitative analysis of ions, organic dye-based ratiometric fluorescent probes have been extensively explored in recent years. In this review, recent advances (2015-2019) made in the development and biological applications of synthetic ratiometric fluorescent probes are described. Particular emphasis is given to organic dye-based ratiometric fluorescent probes that are designed to detect biologically important and relevant ions in cells and living organisms. Also, the fundamental principles associated with the design of ratiometric fluorescent probes and perspectives about how to expand their biological applications are discussed.

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 colorimetric and ratiometric fluorescent probe for selective recognition of cyanide in aqueous media

A simple naphthopyran derivative (L) has been synthesized as a colorimetric and ratiometric fluorescent probe for cyanide sensing in the aqueous DMSO medium and paper strips. The nucleophilic addition of CN^- to this probe blocks the π-conjugation and the intramolecular charge transfer (ICT) transition between naphthopyran and benzoindolium moieties and consequently results in remarkable color and spectral changes. Upon addition of cyanide, L displayed a ratiometric fluorescence response with a blue shift of the peak position and a noticeable color change from fuchsia to colorless within 90s. The probe exhibits high selectivity and sensitivity toward CN^- over other anions and the detection limit was calculated to be 7.56×10^-7M, which is well below the WHO cyanide standard in drinking water (1.9μM). In addition, the excitation and emission of the probe were within the visible wavelength range, which could benefit an application of the probe in an inexpensive portable cyanide sensor. The sensing ability of L has been successfully applied in real water samples. Furthermore, test strips based on L were fabricated, which can act as convenient and efficient test kits for detecting CN^-.

An easy to make chemoreceptor for the selective ratiometric fluorescent detection of cyanide in aqueous solution and in food materials

A ratiometric fluorescent receptor selectively detects cyanide in aqueous solution and food materials via deprotonation of a phenolic hydroxyl group.

A novel "turn-on" fluorescent probe based on triphenylimidazole-hemicyanine dyad for colorimetric detection of CN- in 100% aqueous solution

A new colorimetric fluorescent probe (MCy) for CN^- is designed and synthesized based on triphenylimidazole-hemicyanine dyad. The probe shows high selectivity towards CN^- in 100% aqueous solution even in the presence of other competitive anions such as F^-, Cl^-, AcO^-, NO₂^-, H₂PO₄^-, I^-, SCN^-, SO₄^2-, NO₃^-, HCO₃^-, Br^- and S^2-. Upon nucleophilic addition of CN^- to the indolium group, the probe displays large blue-shift in UV-vis spectrum and remarkable "turn-on" fluorescence owing to the disruption of intramolecular charge transfer. Meanwhile, the probe also undergoes a naked-eye discernible color change from orange to light pink in the sunlight as well as an obvious fluorescence color change from non-emissive to bright blue under a UV lamp. The detection limit of CN^- in aqueous solution can be as low as 20.6nM, which is much lower than the permissible level of CN^- in drinking water according to the WHO. In addition, the probe has been successfully used to determine CN^- in real water samples as well as to fabricate test paper kit for CN^- detection.

Nanomolar-level selective dual channel sensing of Cu2+and CN−from an aqueous medium by an opto-electronic chemoreceptor

A novel chromogenic and fluorogenic chemoreceptor exhibiting a proclivity towards Cu²⁺and CN⁻, with applications in bioimaging and molecular electronics, was developed.

A Biocompatible Colorimetric Triphenylamine- Dicyanovinyl Conjugated Fluorescent Probe for Selective and Sensitive Detection of Cyanide Ion in Aqueous Media and Living Cells

A colorimetric and turn-on fluorescent probe 1 bearing triphenylamine-thiophene and dicyanovinyl groups has been synthesized and used to detect cyanide anion via a nucleophilic addition reaction. Probe 1 exhibited prominent selectivity and sensitivity towards CN- in aqueous media, even in the presence of other anions such as S2-, HS-, SO₃2-, S₂O₃2-, S₂O₈2-, I-, Br-, Cl-, F-, NO₂-, N₃-, SO₄2-, SCN-, HCO₃-, CO₃2- and AcO-. Moreover, a low detection limit (LOD, 51 nM) was observed. In addition, good cell membrane permeability and low cytotoxicity to HeLa cells were also observed, suggesting its promising potential in bio-imaging.

A novel isophorone-based red-emitting fluorescent probe for selective detection of sulfide anions in water for in vivo imaging

A new red-emitting fluorescent probe for detection of sulfide anions in living cells (MCF-7 cells) and zebrafish was developed.