Cyanides being highly poisonous to living beings and pollutants to our environment are among the most important anions studied over the years. As cyanide usage continues to sky-rocket, it is extremely important and high time that chemists devised methods for their detection to ensure harmless usage and safer working conditions for people coming into contact with cyanide and its compounds, day in day out. In this book, an attempt has been made to provide an in-depth commentary of literature for the synthesis of fluorescent dyes and mechanisms for the molecular recognition and detection of cyanide ions. It also covers some current entropy on colorimetric and fluorescent organic chemical probes for the detection and quantification of cyanide anions via fluorogenic and chromogenic procedures.
- fluorescent dyes
- molecular recognition
- chromogenic procedures
- cyanide detection
- sensing mechanisms
The design of protocol for selective optical signaling probes for anions has received much attention over the years as a result of the significant roles they play in biological and environmental procedures . The recognition of cyanide has become an area of increasing significance in supramolecular chemistry as a result of the vital role it plays in environmental, clinical, chemical, and biological applications, and the fact that much attention has been given to the preparation of artificial probes that have the capability of uniquely recognizing and sensing anion species [2, 3]. Cyanide is famous for being one of the most toxic materials and is very dangerous to the environment and human health . As a result of the extreme toxicity of cyanide ions in physiological [4, 5, 6] and environmental  systems, many investigators have designed optical probes [8, 9] for the sensitive and bias detection of cyanide. Till date, many strategies have been designed and developed for the detection of cyanide, including the formation of cyanide complexes with transition metal ions [10, 11, 12, 13, 14], boron derivatives [15, 16], CdSe quantum dots [17, 18], the displacement approach , hydrogen-bond interactions [20, 21, 22], deprotonation , and luminescence lifetime measurement . For the interferences of competing anions to be curtailed in the sensing of cyanide, the nucleophilicity of the cyanide ion has been utilized, which includes its nucleophilic reactions with oxazine [25, 26, 27], pyrylium , squaraine , acyltriazene , acridinium , salicylaldehyde [31, 32, 33], trifluoroacetophenone [34, 35, 36, 37, 38], trifluoroacetamide derivatives [39, 40, 41, 42, 43], and other highly electrophilic carbonyl groups or imine [22, 44, 45, 46, 47, 48].
A lot of chemosensors for cyanide ion have been developed , chromogenic and fluorogenic probes for the detection of cyanide by the naked eye have attracted much interest as a result of the facile, fast usage, and their high sensitivity. As it is well-known, the probes are normally designed by the combination of a luminophore and an anion binding unit. Mostly, the anion binder is basically composed of H-bonding donors . Herein, concise literature reports have been made on some strategies employed in the sensing of cyanide ions, dating from 2008 to 2017.
2. Sensing mechanisms and synthesis of cyanide sensors (CS)
The official methods of determining cyanides include titration [51, 52], spectrophotometry [65, 66], potentiometry with cyanide-selective electrodes [51, 53], flow injection (FI)-amperometry . Analysis of cyanide in various matrices including water, soil, air, exhaled breath, food, and biological fluids (blood, urine, saliva, etc.), have been reviewed in official documents [55, 56], books  and journal articles [58, 59, 60, 61]. Quiet recently, Xu et al.  and Zelder and Mannel-Croise  have respectively written reviews on optical sensors and colorimetric measurement of cyanide. Herein, different sensing strategies have been discussed.
2.1. Cyanide sensing via aggregation induced emission (AIE)
This uncommon fluorescence phenomenon was perceived by Luo et al.  in 2001 via a solution of 1-methyl 1,2,3,4,5-pentaphenylsilole, and the term aggregation-induced emission was given to it. Tang et al. gave an explanation on the AIE phenomenon through a series of experimental analyses. They realized that the main cause of the AIE phenomenon was due to restriction of intramolecular rotation in the aggregates.
Sun et al.  have prepared a turn-on fluorescent probe
Another AIE probe
2.2. Cyanide sensing via the chemodosimeter approach
The special nucleophilic character of cyanide has been utilized for the preparation of different chemodosimetric sensors for cyanide, mostly in aqueous solutions.
In 2009, Kim and Kim  prepared, through the condensation reaction of nitromethane and a coumarinyl aldehyde, a new fluorescent chemodosimeter
Hu et al.  have successfully synthesized and reported a 1,3-indanedione-based chemodosimeter that could be employed in sensing cyanide ions via both aggregation-induced emission enhancement (AIEE) and intramolecular charge transfer (ICT) in 90% aqueous medium. They prepared a solution of the chemodosimeter (1.0 × 10−5 M) in aqueous solution (THF:H2O = 1:9 [v/v], containing 10 × 10−3 M HEPES, pH = 7.3). In the aqueous medium, the chemodosimeter showed a strong ICT absorption band at 425 nm, and upon adding CN−, the ICT band was said to have disappeared and the color of the solution changed from yellow to colorless.
2.3. Cyanide sensing via the excited state intra- and inter-molecular proton transfer (ESIPT)
In 2017, Huo and co-workers  reported the synthesis of a novel isophorone-based red-emitting fluorescent probe
Shymaprosad Goswami and co-researchers  have reported an ESIPT exhibiting benzothiazole receptor possessing two aldehyde groups; one ortho and the other para to an OH group. The ortho aldehyde group being very reactive, was reported to have undergone a nucleophilic reaction with CN− selectively, thereby hampering an ESIPT. The investigators confirmed the process via DFT and TD-DFT computations. The affinity of the benzothiazole receptor toward different competing ions was investigated using UV-vis absorption and emission spectrometry in aqueous acetonitrile solution. The probe showed a green emission at 521 nm, a peculiar benzothiazolyl phenol ring emission. Upon adding CN−, the emission was reported to have drastically decreased, followed by an increase at 436 nm. This suggested that, as thought by the authors, a chemical reaction between the cyanide and the aldehyde group has interrupted the conjugation and thereby hampering the ESIPT process leading to a color change from green to blue. In the UV-vis absorption study, they found that, only CN− had induced the perturbation of the electronic behavior of benzothiazole receptor.
2.4. Cyanide sensing via the excimer/exciplex form
Wang et al.  have successfully designed and reported the synthesis of a novel probe
Shahid et al.  have prepared and described a new simple organic scaffold based on acenaphthene. The fluorogenic and chromogenic properties of the probe were investigated for signaling metal cations and anions in H2O/CH3CN (8:2, v/v) solvent mixture. The authors employed a metal chelate based sensing strategy of copper complexes for fluorescent sensing of cyanide.
2.5. Cyanide sensing via the Förster/fluorescence resonance energy transfer (FRET)
Goswami et al.  investigated a chemosensor
In 2009, Chung and co-workers  successfully developed a cyanide sensor for fluorescence study. In the fluorescence study, different anions, such as CN−, SCN−, AcO−, F−, Cl−, Br−, I−, H2PO4−, HSO4−, NO3−, and ClO4− were evaluated at pH 7.4 (0.02 M pH 7.4 HEPES). Using 100 equiv. of each of these anions, and 6 mM of the probe in the presence of Cu2+ (1 equiv.), only CN− was observed to have shown a large fluorescence enhancement.
2.6. Cyanide sensing via H-bonding
In 2015, our group  designed and reported the synthesis and application of chemosensor
A group of researchers  reported the synthesis of two receptors of specific signaling of cyanide ions in sodium cyanide solution. The authors associated the visual detection of CN− via color changes, with the formation of hydrogen bonded adducts. They found the probes to have limited solubility in water, and therefore employed mixed solvent, such as CH3CN/HEPES buffer (1:1, v/v), for the sensing studies. The fluorogenic and visually detectable chromogenic changes of the receptors were verified using aqueous solutions of the sodium salt of all the employed common anionic analytes such as F−, Cl−, Br−, I−, CN−, SCN−, CH3COO−, H2PO4−, P2O73−, HSO4−, NO3−, and NO2− present in excess (0.1 mM). For the contending anions, no spectral changes in their spectral patterns was observed by the investigators. However, the researchers observed changes in spectral pattern, naked-eye color, and fluorescence, only in the presence of added CN−. Interference studies conducted by them revealed that, the spectral response for CN− remained unaffected in the presence of 10 equiv of all interfering anions.
2.7. Cyanide sensing via the inter- or intra- molecular charge transfer (ICT)
In 2017, Hao et al.  designed and synthesized a probe
Mashraqui and co-workers  have developed a novel chemodosimeter that has the structural capabilities to convert the CN-binding event into an enhanced ICT process, inducing absorbance red shifts and a high fluorescence turn-on response. The use of the probe toward sensing different anions was investigated by the group via optical spectral analysis. The group realized that, the absorption spectra of the chemosensor (28 μM) in DMSO-H2O (7:3, v/v) in tris-HCl buffer pH 7.0, was insensitive to each of the competing anions (F−, AcO−, SCN−, HSO4−, NO3−, Br−, Cl−, I−, and H2PO4−) up to 75 mM. On the contrary, the concentration of cyanide (7.6 mM) which was at a 10-fold lower that of the interfering anions, was noticed to have elicited a monumental interaction, which was followed by an instant color change from colorless to deep yellow, an event that allowed selective visual detection of cyanide by the naked eye.
2.8. Cyanide sensing via the nucleophilic approach
Kwon et al.  successfully introduced a fluorescent chemodosimeter
Li et al.  reported the development of selective and sensitive red-emitting fluorogenic and colorimetric dual-channel sensor for detection of cyanide. The group realized that, adding cyanide ion to the probe led to the display of huge blue-shift in both fluorescence (130 nm) and absorption (100 nm) spectra. The authors found that, the probe could be capable of selective signaling of cyanide by the naked-eye. They therefore concluded that, the mechanism for the detection of cyanide was due to the nucleophilic attack of cyanide toward the benzothiazole group of probe, which could block conjugation between benzothiazole unit and the naphthopyran moiety, resulting in both color and spectral changes.
λab = 500 nm, λem = 500 nm.
2.9. Cyanide sensing via the photoinduced electron transfer (PET)
Qu et al.  described the synthesis of a fluorescent and colorimetric chemosensor
A group of researchers  successfully prepared a Co(II)-salen based fluorescent sensor that is applicable for selective recognition of cyanide anions in 1:2 binding stoichiometry. The scientists related the fluorescence enhancement of the solution of the probe, upon the addition of cyanide, to an interruption of photoinduced electron transfer from the coumarin fluorophore of the sensor to the cobalt(II) ion. In order to address the origin of the fluorescence enhancement of the sensor by the coordination of cyanide anions, the authors measured the HOMO and LUMO energy levels of the cobalt-salen complex of the chemosensor in the absence and the presence of cyanide anions via cyclic voltammetric and UV-vis spectroscopic measurements.
In summary, this chapter is limited to literature reports that have been published from 2008 to 2017. Some papers that have been published pre-2008 may have been used to illustrate important points. Some of the schemes for the synthetic pathways of the reported literature have not been illustrated in this chapter due to the limited space available to the authors. There are also a few papers that have been published within the period captured herein but could not be included. The omission of such literature does not in any way connote that such papers are of lesser importance.
We appreciate, with kind regards, the support and sponsorship of this endeavor by Gazi University and TUBITAK for the grant (Grant numbers; 111 T106, 113Z704, 114Z980, and 215Z567).
Conflict of interest
There is no conflict of interest, whatsoever, in publishing this piece.