Fluorescence-based ratiometric sensors as emerging tools for CN- detection: Chemical structures, sensing mechanisms and applications

Hazardous cyanide anions (CN-) are increasingly threatening the environment and human health due to their widespread use in industry and many other fields. Over the past three decades, a large number of probes have been reported to sensitively and selectively detect this toxic anion, while a rather limited number of ratiometric fluorescent probes have been developed. The ratiometric probes have significant potential in bio-imaging and biomedical applications because of the ability to detect CN- in a quick, convenient and affordable way. In this review, we introduce 42 ratiometric fluorescent probes reported in the past 6 years (2018-2023) for CN- detection. Our description includes the chemical structures, photo-physical properties, CN- sensing mechanisms, solution color changes, limits of detection (LODs) and/or various applications of these chemical probes. This review provides guidelines for design and development of a new ratiometric probe for effective CN- detection.

Ratiometric orange fluorescent and colorimetric highly sensitive imidazolium-bearing naphthoquinolinedione-based probes for CN- sensing in aqueous solutions and bio-samples

The widespread use of cyanide (CN^-) in industry results in contamination of various effluents such as drain, lake, and tap water, an imminent danger to the environment and human health. We prepared naphthoquinolinedione (cyclized; 1-5) and anthracenedione (un-cyclized) probes (6-7) for selective detection of CN^-. The addition of CN^- to the probe solutions (1-5) resulted in a color change from pale green to orange under 365 nm illumination. The nucleophilic addition of CN^- to C2 of the imidazolium ring of the probes is responsible for selective CN^- detection. Among all probes, 1 gave the lowest fluorescence-based LOD of 0.13 pM. In contrast, the un-cyclized probes (6 and 7) were substantially inferior to the cyclized counterparts (1 and 2, respectively) for detecting a trace amount of CN^-. The notably low LOD displayed by probe 1 was maintained in the detection of CN^- in real food samples, human fluids, and human brain cells. This is the first report studying imidazolium-bearing naphthoquinolinedione-based probes for CN^- sensing in 100% water.

A unique triple-channel fluorescent probe for discriminative detection of cyanide, hydrazine, and hypochlorite

Herein, the first triple-channel fluorescent probe, TTB, excited at the same wavelength (λ_ex = 360 nm) in the same sensing medium for the detection and discrimination of cyanide, hydrazine, and hypochlorite, is disclosed. While a fluorescent white color appeared (λ_em = 470 nm) with the addition of cyanide ion into the probe solution, upon addition of hydrazine and hypochlorite, green (λ_em = 503 nm) and orange (λ_em = 585 nm) fluorescent colors, respectively, were observed. A naked-eye detection for the three ions was documented. With the appearance of orange color, a mega Stokes shift of 175 nm was observed. The probe exhibited excellent selectivity and lower detection limits of 0.24 μM, 4.1 nM and 0.27 μM, and dynamic ranges of 0.0-2.0 μM, 0.0-0.05 μM and 0.0-2.0 μM for cyanide, hydrazine and hypochlorite, respectively. The sensing mechanism was investigated through computational studies before and after the addition of cyanide, hypochlorite, and hydrazine, applying density functional theory (DFT), along with the calculation of optical properties by time-dependent DFT (TD-DFT) method. The results were found to be in good agreement with the experimental values. Remarkably, the probe, TTB, successfully detected cyanide, hydrazine, and hypochlorite in complex water samples. Moreover, the detection of cyanide was successfully performed in apricot kernels, as well as hypochlorite in fruits and vegetables.

Photophysical and anion sensing properties of a triphenylamine-dioxaborinine trimeric compound

Herein, we report the synthesis and photophysical characterization of the novel tris(4-(2,2-difluoro-6-methyl-2H-1λ³,3,2λ⁴-dioxaborinin-4-yl)phenyl)amine trimeric probe (A2) via the reaction between triphenylamine (1), acetic anhydride, and BF₃·OEt₂ implying the twelve new bond formation in a one-pot manner. This highly fluorescent compound in solution (φ up to 0.91 at 572 nm) and solid state (φ = 0.24 at 571 nm) showed a better solvatofluorochromism than its analog monomeric A1 due to symmetry-broken charge transfer, which is consistent with high solvent dipolarity (SdP) response in Catalán's multiparametric regression. Notably, A2 had a high sensibility and selectivity for CN^- or F^- in solution (LODCN^-/F^- = 0.18/0.70 μM), and CN^- can be discriminated from F^- by the reaction of A2 with 3.0 equiv. of CN^-. In addition, A2 was impregnated on filter paper to prepare test strips that were applied to naked-eye qualitative sensing of CN^- or F^-. Finally, the octupolar system in A2 allows for better action of two-photon excitation cross-section values when compared with that of the dipolar structure in A1. These findings provide further information for the design of new efficient two-photon absorption dyes.

Selective visualization of cyanide in food, living cells and zebrafish by a mitochondria targeted NIR-emitting fluorescent probe

Cyanide is a highly toxic substance, and the detection of cyanide in the environment and food samples is critical to public health care. Herein, we rationally designed a mitochondria-targeted near-infrared fluorescent probe BTC for ratiometric monitoring of CN^- in water, food, living cells, and zebrafish. BTC exhibits a remarkable colorimetric ratiometric fluorescence response to CN^- with high selectivity, low detection limit (54.3 nM), and large Stokes shift. The cyanide sensing mechanism was demonstrated by NMR and ESI-MS analysis and density functional theory (DFT). More importantly, BTC was used for efficient naked-eye colorimetric detection of CN^- in sprouting potatoes, almonds, and ginkgo fruit samples. Further, the BTC is capable of situ tracking and imaging cyanide in mitochondria of SMMC-7721 cells and in zebrafish via dual emission channels, and was prepared into a kit for convenient and visual on-site sensing of cyanide in food samples.

Quantitatively analysis and detection of CN- in three food samples by a novel nopinone-based fluorescent probe

Cyanide (CN^-) is one of the most lethal chemical substance and exists in the organisms and environment. Due to the CN^- and CN^--containing chemicals being widely applied in industrial fields and threatening human health, the sensitive and selective detection techniques towards CN^- are still essential. Based on this, a "turn-on" fluorescent probe 2-(4-(5,5-dimethyl-4,5,6,7-tetrahydro-3H-4,6-methanobenzo[d]imidazol-2-yl)styryl)-3-ethylbenzo[d]thiazol-3-ium iodide (NCy) was designed and synthesized for monitoring CN^-. NCy had a distinguishable color change towards CN^- from colorless to yellow under 365 nm UV-light. NCy possessed the merits including low LOD (75 nM), good selectivity, and wide suitable pH range (4-10). The sensing mechanism of NCy towards CN^- was proved by HRMS, ¹H NMR titration and DFT analysis. Furthermore, the probe NCy was successfully utilized in detecting endogenous CN^- in three food samples (green potato, cassava, and bitter almond) quantitatively. In bioimaging aspect, NCy was also successfully applied in detecting the exogenous CN^- in living zebrafish.

A benzothiazole-based new fluorogenic chemosensor for the detection of CN- and its real-time application in environmental water samples and living cells

Since the cyanide ion is used in a wide range of industries and is harmful to both human health and the environment, a number of research efforts are dedicated to creating fluorescence sensors for the detection of cyanide (CN-). Herein, for the fluorescence detection of CN-, a new highly selective and sensitive sensor 2-(3-(benzo[d]thiazol-2-yl)-4-hydroxybenzylidene)-1H-indene-1,3(2H)-dione (BID) was created by conjugating a benzothiazole moiety with 1H-indene-1,3(2H)-dione. The donor and acceptor components of this hybrid receptor were covalently connected through a double bond. The nucleophilic addition of a cyanide anion to the BID inhibits the intramolecular charge transfer (ICT) transition, resulting in spectral and colour alterations in the receptor. When the solvent polarity was increased from n-hexane to methanol, this molecule exhibited a bathochromic shift in the emission wavelength (610 to 632 nm), suggesting the presence of a solvatochromic action. The sensor BID has shown strong specificity towards CN- by interrupting its internal charge transfer (ICT), resulting in a significant change in the UV-vis spectrum and a notable blue shift in the fluorescence emission spectrum. The cyanide anion (CN-) is responsible for the optical alterations observed by BID, as opposed to the other anions examined. The detection limit was 5.97 nM, significantly less than the WHO's permitted amount of CN- in drinking water. The experimental findings indicate that BID's fluorescence response to CN- is pH insensitive throughout a wide pH range of 6.0 to 12.0. The interaction mechanism between the BID and CN- ions has been studied by HRMS, 1H-NMR titration experiments, FT-IR, and DFT, which confirmed the nucleophilic addition of CN- on vinylidene and subsequent disturbance of ICT. Additionally, we demonstrated the real-time detection application of CN- in environmental water samples and live-cell imaging.

Discovery of a highly selective and ultra-sensitive colorimetric fluorescent probe for malononitrile and its applications in living cells and zebrafish

A selective and ultra-sensitive colorimetric fluorescent probe was discovered to detect malononitrile in living cells and zebrafish.

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⁻).