Abstract
The hydrazone functional group is widely applied in several fields. The versatility and large use of this chemotype are attributed to its easy and straightforward synthesis and unique structural characteristics which is useful for different chemical and biological purposes. Recently hydrazone scaffold has been widely adopted in the design of small-molecule fluorescent and colorimetric chemosensors for detecting metals and anions because of its corresponding non-covalent interactions. This chapter provides an overview of hydrazone-based fluorescent and colorimetric chemosensors for anions and metals of biological interest, with their representative rational designs in the last 15 years. We hope this chapter inspires the development of novel and powerful fluorescent and colorimetric chemosensors for a broad range of applications.
Keywords
- hydrazone
- cyanide
- acetate
- fluoride
- zinc
- copper
- aluminum
- magnesium
- mercury
- coumarin
- fluorescein
- rhodamine
- Schiff base
1. Introduction
Hydrazone-based molecular structures are ubiquitous in many research fields, such as medicinal chemistry [1], organic synthesis [2], supramolecular chemistry [3], metal-organic coordination [4], dyes [5], fluorescent sensors, and molecular machines [6], besides others applications [7]. Over the last decades, the popularity of hydrazone group has increased due to its easy and direct-obtaining synthesis, stability toward hydrolysis in comparison with imines, modularity, and mainly, functional diversity of C═N▬N useful in several fields (Figure 1). In terms of structure, hydrazones are considered as azomethine compounds; however they are distinguished from imines and oximes, for example, by the presence of additional linked nitrogen atom [8]. Hydrazone backbone has an imine carbon that has an electrophile character, two nucleophilic nitrogen in both imine and amine groups and a possible isomerization of C═N double bond typically from the conjugation of imine and the acid N▬H. These structural properties play a crucial role to determine the specificity of applications which hydrazone group can be involved [6, 9].
The main synthesis of hydrazones is carried out from acid-catalyzed condensation between hydrazines (R1NHNH2) and activated carbonyl aldehydes or ketones, generally in alcoholic media. Other forms to obtain hydrazones are from Japp-Klingemann reaction (i.e., aryl diazonium salts coupling with β-keto esters or acids) and coupling between aryl halides and non-substituted hydrazones [9].
1.1 Hydrazone-based compounds as fluorescent chemosensors
Most hydrazone derivative fluorescent chemosensors were designed combining fluorophores or aromatic structures with this functional group. The wide range of chemical reactivity of hydrazones allows their application in the detection of anions, cations, and other species [10].
Some hydrazone-based chemosensors have weak fluorescence because of quenching effects such as E/Z double bond isomerization in the excited state; photoinduced electron transfer (PET) process (excited electron is transferred from donor to acceptor; generating a charge separation, i.e., redox reaction takes place in excited state); [11] excited state intramolecular proton transfer (ESIPT) process (photoexcited molecule relax their energy through tautomerization by transfer a proton); and others [12, 13]. The main objective for this class of chemosensors is inhibiting the quenching effects after interaction with some analytes promoting a fluorescence state. Other possible mechanisms are based on nucleophilic addition or induced N▬H and O▬H deprotonation. These mechanisms will be detailed after.
A quick literature survey using Scopus database has shown few reviews on the chemistry of hydrazones, most of them focusing on medicinal chemistry [14] or organic synthesis [15]. Only one review on hydrazone compounds describes some examples of hydrazone-based fluorescent chemosensor, which covered some results reported before 2014 [6].
This chapter book aims to present the progress of fluorescent and colorimetric chemosensor based on hydrazone scaffold, as reported in the literature in the period of 2006 until 2019. We hope that this chapter book helps in the design and development of new and selective fluorescent and colorimetric chemosensors for a broad range of applications.
2. Fluorescent chemosensors for anions
Anions, such as cyanide (CN−), fluoride (F−), chlorine (Cl−), and acetate (AcO−), play an important role in many environmental, clinical, chemical, and biological processes. Due to these important roles, anion recognition is an area with growing interest in supramolecular chemistry, and considerable efforts has been focused on the design of receptors (compounds) that are able to recognize anions. The detection and quantification of anions is a challenge, especially in biological systems. Some aspects as microenvironmental sensitivity, specificity, basicity, and nucleophilicity are among the main complicating factors in the detection of anions. One way to solve these problems is to develop chemosensors with high specificity for individual anions [16, 17]. Among different types of anions, fluoride and cyanide aroused great interest. The optimum concentration of F− anions in the human body is a positive aspect to our health and can prevent dental caries and osteoporosis; however the excess of F− may cause dental or osseous fluorosis, thyroid and liver damage, and bone diseases [18, 19, 20]. Additionally, F− is known as a test index for residues of some nerve agents, being also associated with certain drugs, Alzheimer disease, and drinking water. The level of F− recommended in potable water by the US Environmental Protection Agency (EPA) is about 2 ppm [18, 19, 20, 21].
Commonly involved in chemicals and industrial processes, cyanide is highly toxic and its exposure to live organisms and environment is extremely detrimental. Moreover, cyanide anion has a strong affinity with cytochrome a3 which can lead to cell death because of respiratory arrest [22]. According to the World Health Organization (WHO), the permissible level of cyanide in drinking water is 1.9 × 10−6 M [18]. Therefore, considering the notorious toxicity of CN− and F−, the development of sensitive sensors for the accurate detection and quantification of anions is of great importance.
2.1 Hydrazone derivatives as chemosensors for CN−
The main mechanisms of fluorescent sensing CN− in hydrazone derivatives are based on nucleophilic addition to polarized C═N [23, 24] and C═O [25] bonds, which leads to disruption of C═N and C═O double bond to C▬NH or C▬OH forms, deprotonation of NH or OH by means of acid-base reactions [26, 27, 28], and the displacement of fluorescent hydrazones from hydrazone-copper complexes.
2.1.1 Nucleophilic addition of CN−
Two highly selective CN− chemosensors
Exploring the same principle of nucleophilic attack, a highly selective and sensitive naphthalene-acylhydrazone chemosensor for CN− in aqueous media was designed. By this time, the mechanism proposed was the nucleophilic attack on the carbonyl group instead of the imine one, according to 1H NMR, 13C NMR, ESI-MS, and DFT calculations data (Figure 2C). Among several anions tested, only CN− could induce a remarkable color change from colorless to yellow and increase of fluorescence emission in DMSO/H2O solution. Moreover, the detection limits were 5.0 × 10−7 M and 2.0 × 10−9 M of CN− for color and fluorescence changes respectively, far lower than the WHO guideline of 1.9 × 10−6 M [25].
2.1.2 Deprotonation mechanism
Cyanide anion is a Lewis base and can form hydrogen bonds with hydrogen bond donors as hydroxyl and amines usually followed by deprotonation.
The highly selective and sensitive chemosensor (
Another deprotonation mechanism was reported in the design of a two-dimensional carbazole-based chromophore
Like compound
2.2 Hydrazone derivatives as chemosensors for F−
2.2.1 Hydrogen bond interactions and deprotonation
Fluoride is a weak Lewis base and can form hydrogen bonds with hydrogen bond donors. This coordination usually promotes deprotonation.
A Ru-bpy-based quinone hydrazone was designed as chromo-fluorogenic hybrid chemosensor (
A thiocarbonohydrazone anion chemosensor
Still exploring the N▬H acidity of hydrazones, a new series of diketopyrrolopyrrole (DPP) derivatives
The visual naked-eye color change was observed under natural light for DPPPH (
2.3 Hydrazone derivatives as chemosensor for AcO−
Acetate (AcO−) and dicarboxylate are essential components in several metabolic processes in living organisms. Without them, many enzymes and antibodies are unable to function properly. In this sense, the synthesis of chemosensors that can recognize AcO−, mainly via hydrogen bond interaction, is of great importance for biological systems [32].
An interesting naked-eye selective colorimetric sensor for AcO− based on 1,10-phenanthroline-2,9-dicarboxyaldehyde-di-(p-nitrophenylhydrazone) (
Similarly a fluorescent and naked-eye colorimetric chemosensor (
2.4 Hydrazone derivatives as chemosensors for multiple anions
In contrast to previously described, where receptors had a certain degree of selectivity toward single anion, several hydrazone-based chemosensors have been published exhibiting a response to more than one anion species, and some interesting examples will be described below [34, 35, 36].
A tripodal benzaldehyde-phenylhydrazone (
A Schiff-base thiophene-based hydrazone (
Four furan/thiophene-based fluorescent hydrazones
3. Fluorescent chemosensors for metal ions
Metal ions such as Cu2+, Zn2+, Fe3+, Al3+, Hg2+, Mg2+, etc. play an important role in many biological and environmental processes, and excessive or insufficient amounts may lead to diseases [37]. As an example, copper (Cu2+) is the third most abundant transition metal in the human body and plays essential roles in several environmental, chemical, and physiological systems. In living organisms, Cu2+ acts as a key catalytic center in many enzymes and as cofactor in a variety of metalloproteins [38]. Its insufficient concentration may affect the development of bone and brain tissues as well as the nervous and immune system, whereas excessive intake may lead to serious problems including cirrhosis and neurological diseases such as Alzheimer’s and Wilson’s diseases and prion disorders [39]. The extreme toxicity of heavy metal ions such as Pb2+ and Hg2+, even in small amounts, remains a danger to human health and the environment, but they have been widely used in industrial processes [40]. Therefore, the development of sensitive sensors for the accurate detection and quantification of these ions is of great importance.
3.1 Hydrazone derivatives as chemosensors for Cu2+
Fluorescent and colorimetric hydrazone-based chemosensors for Cu2+ attract interest and are mainly based on coordination mechanism, often quenching the fluorescence emission due to PET mechanism [41].
Coumarins are widely associated with hydrazones for sensing Cu2+ and the on–off fluorescent chemosensor (
Differently from turn-off PET mechanism, off-on (turn-on) chemosensors for detecting Cu2+ often present a FRET mechanism (described by energy transfer between two light-sensitive molecules or chromophores, where a donor chromophore in its electronic excited state may transfer energy to an acceptor chromophore through nonradiative dipole-dipole coupling). An example is the hybrid coumarin-rhodamine hydrazone chemosensor
A highly selective and sensitive naked-eye colorimetric chemosensor for Cu2+ in aqueous solution was designed and developed based on hydrazone framework. In the UV-vis spectroscopic studies, compound
Trying to understand its sensing mechanism, the stoichiometry of the
The selectivity of
Using a strategy of intraligand charge transfer transition (ILCT) turn-on mechanism, a small chromo-fluorogenic chemosensor (
The binding sense mechanism was explained by 1H NMR titration in DMSO, in which the peak attributed to acid O▬H proton of
3.2 Hydrazone derivatives as chemosensors for Zn2+
Zinc (Zn2+) is the second most abundant transition metal (after Fe3+) in the human body and is considered essential for living organisms. Zn2+ exerts influence on many cellular processes, including proliferation, differentiation, apoptosis, transcription, neural signal transmission, and microtubule polymerization. Therefore, significant changes in Zn2+ concentration may be related to many diseases, including Alzheimer’s and Parkinson’s diseases, diabetes, and prostate cancer [46, 47]. Chemosensors for Zn2+ are mainly based on the coordination mechanism; however, these probes often still lack specificity.
Aroylhydrazone derivatives (
A fluorescein-coumarin conjugate (
The hydrazone-based fluorescent chemosensor
The free ligand (
3.3 Hydrazone derivatives as chemosensors for Hg2+
Mercuric ion (Hg2+) is considered highly dangerous, because it is known as one of the most toxic metal ions and is generated by many sources such as mercury lamps, gold production, electronic equipment, paints, and batteries [51]. Mercuric ion can cause serious detrimental effects to living organisms, resulting in hepatitis, uremia, digestive diseases, and fatal damage to the central nervous system, and its accumulation can lead to various cognitive and motor disorders, such as Minamata disease [52]. Due to its high toxicity, considerable attention has been devoted to the development of new sensors for Hg2+ detection. Hydrazone-based fluorescence chemosensors for Hg2+ are mainly based on coordination mechanism.
One example is the 3,4-ethylenedioxythiophene (EDOT) rhodamine-hydrazine-based compound
The previous exposed FRET mechanism has been used in the design of selective turn-on fluorescent chemosensor for Hg2+ based on bis-hydrazone derivative from 2,5-furancarboxaldehyde. In this case, furan ring is the donor, and rhodamine B is the acceptor chromophore. Free ligand has the spirolactam moiety which in turn inhibits the charge transfer between these chromophores. When Hg2+ binds to
3.4 Hydrazone derivatives as chemosensors for Al3+
Aluminum (Al3+) is the most abundant (8.3% by weight) metallic element and, after oxygen and silicon, is the third most abundant of all elements in the earth. Aluminum is widely used in the environment around us in modern society, such as in water treatment, food packing, medicines, etc. However, the excess of this metal can result in health problems such as Alzheimer’s and Parkinson’s diseases [55]. Moreover, it is believed that around 40% of the world’s acid solid are caused by aluminum toxicity, which is harmful to plants’ performance [56]. Thus, the detection of aluminum is essential in controlling its effects on environment and on human health. Hydrazone-based chemosensors for aluminum ion (Al3+) are mainly based on coordination with fluorescence turn-on response as a result of restricted molecular motion through inhibiting ESIPT or PET effects.
A Schiff-base 7-methoxychromone-3-carbaldehyde-(pyridylformyl) hydrazone was reported as turn-on fluorescent and colorimetric chemosensor for Al3+. This chemosensor (
Metal complexation to Schiff base produces a less efficient electron donor character, interrupting the PET process and, in some cases, improving the fluorescence emission, which is known as CHEF effect [57]. The selective coordination between
The simple and selective fluorescent naphthalene-hydrazone chemosensor (
3.5 Hydrazone derivatives as chemosensors for multiple metals
Although several chemosensors have relatively high degree of selectivity as previously exposed, some chemosensors have been reported to recognize more than one metal ion.
Following a previous described strategy, probes based on the opening of spirolactam ring upon metal coordination were designed as single molecule multianalyte (Cu2+ and Hg2+) chemosensors. Compound
With a similar structure to compound
The UV-vis absorption and fluorescence spectroscopy (CH3CN/0.02 M HEPES buffer at pH 7.3) indicated the binding behavior of chemosensor
Finally, the sensing mechanism of
4. Conclusion
As exposed in this chapter, hydrazone derivatives have been extensively employed as fluorescent and colorimetric chemosensors targeting important biological analytes such as inorganic anions and metal cations. Thus, it is clear that hydrazone scaffold is of great importance in the design of optical sensors. Here we have demonstrated just some representative examples of hydrazone and their ability as chemosensor for CN−, F−, AcO−, multiple anions, Cu2+, Zn2+, Hg2+, Al3+, and multiple metals. Furthermore, we hope that this book chapter with discussions about the sensing mechanisms (PET, FRET, ESIPT, etc.) could be an important tool and contribute to the development of new rational research projects with the hydrazine scaffold for biological and environmental monitoring of metals and anions.
Acknowledgments
This work was supported by Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
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