Aruna
,
Bhawna
Rani
,
Suman
Swami
,
Arunava
Agarwala
*,
Debasis
Behera
and
Rahul
Shrivastava
*
Department of Chemistry, Manipal University Jaipur, VPO-Dehmi-Kalan, Off Jaipur-Ajmer Express Way, Jaipur, Rajasthan, India 303007. E-mail: rahul.shrivastava@jaipur.manipal.edu; arunava.agarwala@jaipur.manipal.edu
First published on 27th September 2019
2,3-Diaminomaleonitrile (DAMN) has proved to be a valuable organic π-conjugated molecule having many applications in the area of chemosensors for sensing of ionic and neutral species because of its ability to act as a building block for well-defined molecular architectures and scaffolds for preorganised arrays of functionality. In this article, we discussed the utilization of 2,3-diaminomaleonitrile (DAMN) for the design and development of chemosensor molecules and their application in the area of metal ion, anion and reactive oxygen species sensing. Along with these, we present different examples of DAMN based chemosensors for multiple ion sensing. We also discuss the ion sensing mechanism and potential uses in other related areas of research.
Diaminomaleonitrile (DAMN) is a unique organic π-conjugated molecule in which electronic donor parts (–NH2 groups) and acceptor parts (–CN group) are linked by a single and double bond.19–21 The DAMN has tremendous potential to act as building block in designing of sensor molecules due to its fascinating electronic properties,22,23 which can be easily fine-tuned for sensible sensor design through functionalization by suitable functional group. In last few years, many excellent examples of DAMN based sensors have been published which proved its utility and versatility in research area of designing of molecular sensors. Reviews on different chemical reactions of DAMN as building block for synthesis of heterocyclic entities are previously reported in literature24,25 but to the best of our knowledge, reviews on DAMN based sensor molecules for sensing of ions and neutral guest species are still not available in literature. This review will focus on the recent development of DAMN based sensor molecules beginning with short discussion about the discovery and development of chemical structure of DAMN. Examples presented here are mainly grouped according to sensing properties. Finally, sensing mechanism and potential use in other related area of research will be discussed.
DAMN is proven as versatile scaffold for designing of sensor molecules for detection of anions due to its unique structural features. In this direction, two diaminomalenonitrile (DAMN) based colorimetric sensor molecules 2-(((E)-(9H-fluoren-2-yl)methylene)amino)-3-aminomaleonitrile (1) and (E)-2-(amino(((9-ethyl-9H-carbazol-2-yl)methylene)amino)methylene)maleonitrile (2) were developed in which DAMN was linked with fluorene and carbazole moiety respectively through imine linkage.34 Fluorene and its derivatives are electron rich fluorophore in which two benzene rings connected by a five-membered ring therefore provide extended π-conjugated orbitals. The fascinating electronic properties, high quantum yield and high sensitivity make fluorene an effective signalling unit for the development of chemosensors. The synthesized chemosensors (1 & 2) are π-conjugated organic molecules with electronic donor (NH2) and acceptor (–CN) unit linked by single and double bond leads significant electronics properties due to an intramolecular charge transfer (ICT). The synthesized sensor molecules displayed selective sensing of F− and CN− ions in presence of other anions through colorimetric changes and fluorescence responses (Fig. 3 and 4). The sensor (1) exhibited emission peak at 460 nm upon excitation at 380 nm. However, presence of CN− ions in solution of sensor (1) shifted the emission peak to 480 nm. The observed 20 nm red shift was explained by enhancement of intramolecular charge transfer from –NH− group to nitrile group, facilitated by hydrogen bond interaction of CN− ions with amino group (Fig. 3). Other synthesized sensor molecule (2) exhibited peak at 400 nm in UV-visible spectrum which assigned for the charge transfer from NH2 to CN− of diaminomaleonitrile (DAMN) (Fig. 4). Addition of fluoride ion to sensor (2) solution induced shift of absorption peak from 400 nm to 470 nm along with appearance of instance colour of reddish green. The sensing mechanism was further confirmed the by 1H NMR studies and DFT calculations.
In this direction, Son et al. have developed extended π-conjugated system (3) in which both ends of dioctyl fluorene–dibenzene were linked with two DAMN units through imine linkage (Fig. 5).35 The incorporation of two DAMN made this sensor (3) very efficient for detection of CN− ion in aqueous medium in nano-molar concentration which is much lower than the concentration level recommended by World Health Organization guidelines. The sensor (3) exhibited intense colour change from yellow to red along with fluorescence turn-on response upon interaction with CN− ion. Additionally, it was observed that in presence of CN− ion, absorption peak at 406 nm in UV-visible spectrum was shifted to 478 nm with isosbestic point at 446 nm. The shift in absorption maxima was explained by the deprotonation of NH2 of DAMN coupled with intermolecular charge transfer led to enhancement of donor-to-acceptor ICT transition (Fig. 5). They also examined the sensitivity of sensor (3) towards other anions such as H2PO4−, NO3−, AcO−, F−, Cl−, Br− and I− but none of these ions showed any responses in absorption, emission spectrum and colorimetric sensing.
Another DAMN derived biocompatible duel channel probe (4) was synthesized from hydroxyl benzothiazole moiety and 2,3-diaminomaleonitrile for selective detection of toxic CN− ions in 50% aqueous DMF.36 The merits of the probe (4) were; (i) reversibility; (ii) non-cytotoxicity; (iii) excellent cell viability; (iv) 0.16 μM detection limit and (v) imaging of CN− in live fibroblast L929 cells. The interaction of CN− ion with probe (4) resulted in disappearance of absorption peak at 370 mm with evolution of 445 nm peak with isosbestic point at 390 nm in UV-visible spectrum. Along with this, colorimetric change from yellow to red was observed upon addition of CN− ion into solution of probe (4). These observations were explained by the electronic push–pull effect from amino group of DAMN to nitrile groups resulted from interaction of anion with N–H bond which led polarization of N–H bond and then deprotonation process. In fluorescence studies, probe (4) exhibited weak emission peak at 517 nm upon excitation at 450 nm which was explained by the fact that photo-induced electron transfer process generated from amino group suppressed the emission response. The observed weak emission peak at 517 nm was significantly intensified in response to CN− ion probably due to the abstraction of proton of amino group by CN− ion which resulted strong intramolecular charge transfer process (ICT) from amine to nitrile units of DAMN. The strong ICT prevent photo-induced electron transfer process resulted in significant fluorescence enhancement (Fig. 6). Additionally, fluorescence turn-on behaviour of probe (4) upon interaction with CN− ion can also supported from suppression of ESIPT process (excited state intramolecular proton transfer) because of deprotonation of benzothiazole–phenol unit.
Indole and its derivative are interesting heterocyclic scaffolds attracted immense interest of medicinal and organic chemists due to its important application in medicinal and biological field. The structure of indole comprises with pyrrole and benzene rings makes it attractive fluorophore for designing of sensible sensor molecules. Additionally, the presence of N–H group in indole moiety can be utilized for sensing of anions through hydrogen bond interaction. To utilize these properties, a duel channel chemosensor (5) comprising two indole moieties was synthesized from condensation reaction of diaminomaleonitrile with indole-3-carboxaldehyde for sensing of fluoride ion.37 Reported sensor 5 selectively sensed fluoride ion without interferences of other anions. The significant changes in emission and absorption spectrum upon addition of fluoride was explained on the basis of hydrogen bond interaction between F− ion and N–H group of indole moiety. The sensing studies of chemosensor (5) has been validated by 1H NMR titration and spectrophotometric titrations.
Apart from DAMN based organic molecules, DAMN based metal complexes also showed interesting sensing properties. Two such complexes uranyl-N,N′-bis(5-tert-butyl-2-hydroxybenzylidene)-1,2-dicyano-1,2-ethenediamine (6) and uranyl-N,N′-bis(3,5-ditertbutyl-2-hydroxybenzylidene)-1,2-dicyano-1,2-ethenediamine (7) were developed by Cort and his group by reaction of salycilaldehydes derivatives, 1,2-diaminomaleonitrile and uranyl acetate for selective sensing of halide ion in chloroform and dichloromethane.38 Sensor 6 and 7 showed selectivity trends for halide ions in order of F− > Cl− > Br− which is in good agreement of hardness of anions. Among 6 and 7, compound 7 showed change in colour upon addition of fluoride and chloride ion but it exhibits fluorescence “turn-on” response only with fluoride ion in both CH2Cl2 and CHCl3. The weak sensing response of 6 towards halide ion was explained from its higher stability of the dimer formation as compare to 7. Further, they calculated distances of U⋯X (X = F−, Cl−, Br−) from DFT calculation for providing support to metal center and bound halide ions interactions.
To develop sensor molecules for metal ions, aza-crown ether moieties have been introduced into DAMN scaffold, as aza-crown ethers have excellent capability for binding to specific metal ions with stable complexation because of the nitrogen/oxygen are situated at interior of the ring to coordinate with target cation. The selectivity of these molecules depends on size of cavity as well as number of oxygen and nitrogen atoms. Thus, Zhou and his group introduced aza-crown ether moiety into diaminomaleonitrile through condensation reaction between diaminomaleonitrile with 4-(1,4,7,10-tetraoxa-13-aza cyclopentadecyl)benzaldehyde or 4-(1,4,7,10,13-pentaoxa-16-aza cycloctadecyl)benzaldehyde in benzene with aim to combine two binding sites in single sensor molecules.40 The sensor molecules (8 & 9) having different cavity size of aza-crown ether moieties were examined for sensing of metal ions. It was found that sensor 8 having smaller cavity size as compare to 9 showed different colorimetric responses for Hg2+ and Cu2+ ions. For example, 8–Hg2+ turned to pale yellow whereas 8–Cu2+ turned to colorless within a wide pH range of 5.5–10.5. In UV-visible spectrum, sensor 8 showed absorption peak at 420 nm which was decreased and appearance of a peak at 336 nm with isosbestic point at 366 nm upon stepwise addition of Cu2+ ion. The mole fraction at 0.5 in Job's plot revealed 1:1 binding stoichiometry between Cu2+ and 8 and stability constant was found to be 1.04 × 104 M−1. In case of sensor 9 with larger cavity as compare to 8 showed different sensing behavior for Cu2+ ion. They found that upon addition of Cu2+ to 9, absorptions peaks centered at 300 and 420 nm were decreased with isosbestic points at 316 nm and 373 nm while addition of more than 0.8 equivalents of Cu2+ ion, new absorption peak at 260 nm appeared with new isosbestic point at 297 nm. It was also observed that sensor 9 showed much lower detection limit for Cu2+ ion in comparison of 8. These differences in selectivity for Cu2+ ion was observed due to consequences of difference in cavity size of aza-crown ethers. The sensitivity of 8 was explained by the fact that Cu2+ ion coordination with the donor nitrogen atom of aniline group of DAMN while larger cavity of aza-crown ether and DAMN in 9 have competition in sensing of Cu2+ ion which was further supported by theoretical calculations.
Most of chemosensor molecules showed fluorescence turn-off responses for sensing of Cu2+ ion due to the paramagnetic nature of Cu2+ ion which may sometime reduce the sensitivity and selectivity of sensor molecules. In comparison to this, ratiometric and turn-on fluorescent sensors are much better options for sensing of Cu2+ ion due to better sensitivity and selectivity. In this context, DAMN based turn-on fluorescent sensor for Cu2+ ion using naphthalimide moiety was reported.41 Naphthalimide moiety is exceptional fluorogenic in nature widely used in designing of chemosensor molecules. The emission and absorption properties of naphthalimide derivatives largely depend upon substituents on naphthalimide skeleton and can be fine-tuned as per need. The reported DAMN conjugated naphthalimide sensor (10) showed two different emission channel upon addition of different concentrations of Cu2+ ions (Fig. 7). For instance, addition of less than 1.0 equivalent of Cu2+ ion, both absorbances at 325 and 405 nm decreased while addition of Cu2+ ion beyond 1.0 eq. induced increase of the absorption peak at 325 nm and a monotonic decrease at 405 nm. In fluorescence titration experiments, lower than 1.0 equivalent of Cu2+ ion, the intensity of emission peak at 522 nm was increased while further addition of Cu2+ ion, peak at 522 nm disappeared and a new emission peak gradually appeared at 425 nm. In the sensing process the first equivalent of Cu2+ ion coordinate with nitrogen atoms of imine and amine group and block the photo-induced electron transfer which results in significant fluorescence enhancement at 522 nm. Further, binding of second equivalent of Cu2+ ion with nitrile group of DAMN enhance the electron-withdrawing ability of imine group which results shift of emission peak from 522 to 425 nm. These observations were obtained only in acetonitrile solvent system whereas in other solvents like DMF, THF, water and DMSO only fluorescence enhancement at 522 nm was observed which clearly indicated that binding ability of nitrile group to Cu2+ was weak in nature and was replaced by the solvents having strong binding ability. In the continuation of this work, an interesting DAMN based fluorescent turn-on sensor for Cu2+ ion using indoline moiety (11) was reported with excellent detection limit of 6.18 × 10−8 mol L−1.42 The synthesized sensor 11 displayed large Stokes shift of 146 nm which can reduce the impact of automatic fluorescence and self-extinguishing thus suitable for wide applications. The fluorescence turn-on response of 11 was described by prevention of the photo-induced electron transfer (PET) process from indole moiety (donor) to DAMN unit (receptor).
To improve the sensitivity of sensor, 3-hydroxy-naphthalimide was used to construct effective naphthalimide based sensor molecules in which oxygen atom of hydroxyl group also participate in binding process.42 The resulting sensor (12) was examined for sensing of different metal ions and result showed selective colorimetric response from yellow to pink only for Cu2+ ion with detection limit of 1.6 μM. For instance, 12 showed strong absorbance at 457 nm which was decreased on addition of Cu2+ ion whereas new peak at 565 nm was evolved and enhancement of absorbance at 565 nm was more than 50 fold. The selectivity of the receptor was also checked by mixing other interfering metal ions with Cu2+ ion, even in the complex mixture sensor 12 selectively detected copper ion. The nature of the interaction between the 12 and Cu2+ ion was established through various analysis. The Job's plot indicated the formation 1:1 complex and DynaFit curve fitting program also indicated formation of 1:1 complex with association constant (Kasso) 5.9 × 104 M−1. An m/z peak at 449.3 in mass spectra indicated formation of a 12–Cu2+ complex in which Cu2+ ion binded with oxygen atom of hydroxyl group and nitrogen atom of amino group (Fig. 8). The use of the receptor in practical application was demonstrated on simulated semiconductor wastewater by extracting the Cu2+ ion in DMSO–ethyl acetate (containing the receptor) medium at 4.8 pH. After extraction the image of the ethyl acetate phase was captured using a smartphone and the image was analysed using RGB Grabber (Shunamicode) application in smartphone. A calibration curve made by red/green ratio as function of Cu2+ ion gave satisfactory Cu2+ concentration result in simulated semiconductor wastewater.
In another report benzimidazole derivative was coupled with two molecules of DAMN as optical properties of benzimidazole is highly sensitives towards metal ions interaction even in micro to nano level concentration.43 Sensor 13 demonstrated selective turn on fluorescent sensing behaviour for Cu2+ ion with 0.49 μM detection limit which was much lower concentration than permissible limit of Cu2+ ion in drinking water allowed by United States Environmental Protection Agency.44 The emission spectrum of 13 showed weak emission peak at 405 nm which enhanced to 155-fold with large Stokes shift of 60 nm on addition of Cu2+ ion. The most important aspect of sensor 13 is that it can selectively detect Cu2+ ion in presence other interfering ions in aqueous medium in wide pH range. The binding mode of sensor with Cu2+ ion was determined using Job's plot, mass-spectrometric methods as well as DFT calculation. All these methods indicated 1:2 stoichiometry ratio of 13–Cu2+ complex (Fig. 9). The m/z peak at 752.5849 in mass spectrometric analysis further supported proposed 1:2 13–Cu2+ ion complex. DFT analysis showed HOMO (−6.54 eV) and LUMO (−3.01 eV) band gap as 3.53 eV, whereas complexation with Cu2+ ion reduced the band gap to 3.24 eV. The fluorescence images of cell (HepG2) was carried out in presence of the 13, indicated the permeability and detection ability of the sensor in live cells.
Coumarin derivatives have high fluorescence quantum yield, large stoke shift and high photo-stability thus extensively utilized in designing of fluorescent sensor molecules. A coumarin–DAMN based fluorescent sensor with longer emission wave length was developed in which 7-diethylamino group (electron donor) and DAMN (strong electron acceptor) were introduced into coumarin scaffold to generate an extensive donor–acceptor system which enhanced ICT process.45 Although, synthesized sensor (14) exhibited weak fluorescence due to the quenching effect of nitrile group as well as its flexible structure, the intensity of emission peak at 620 nm was significantly increased up to 35-fold upon interaction with Zn2+ ion along with blue shift of 50 nm. The enhancement of fluorescence intensity was observed due to reduced ICT process and inhibition of conformational change upon combination with Zn2+ ion. The Job's plot revealed the 1:1 binding between 14 and Zn2+ ion which was supported by mass spectrometry. In addition, sensor (14) was used for Zn2+ ion imaging in HepG2 cells over Cd2+ ion which usually responses together with Zn2+ ion.
In continuation of our research efforts on developing sensor molecules,46–51 we synthesized hybrid chemosensors by combining diaminomaleonitrile, 4-formyl-1-substituted phenyl-pyrazole-3-carboxylate and 2-hydroxy-1-naphthaldehyde molecules.52 These three moieties were joined together by imine linkages and were capable of sensing Mn2+ and Zn2+ ion in complex mixture through colorimetric and fluorometric response. On addition Mn2+ and Zn2+ salt solutions the pale yellow coloured solution of the sensor (15–17) was immediately changed to dark brown-red and bright yellow colour respectively. The evaluation of interference by different metal ions like Al3+, Cr3+, Cu2+, Fe3+, Hg2+, Co2+, Pb2+, Cd2+ and Ni2+ indicated the sensor can selectively and efficiently detect Mn2+ and Zn2+ ions in mixture. The immediate colorimetric response made the molecule for possible colorimetric probe for Mn2+ or Zn2+ ions. The UV-vis spectroscopic study of the probe molecule showed three absorptions peaks at 328, 406 and 426 nm with highest absorbance (λmax) at 406 nm. On gradual addition of Mn2+ ion solution to probe molecule the 406 nm peak was completely disappeared and evolution of a new peak at 576 nm was observed. The evolution of new peak was complete with addition of 1.0 equivalent Mn2+ ion. Whereas the gradual addition of Zn2+ ion solution to probe solution resulted in formation of a new peak at 462 nm and the formation was complete with addition of 2 equivalents of Zn2+ ion. The Job's method and Benesi–Hildebrand plots indicated the 1:1 and 1:2 binding modes for Mn2+ and Zn2+ respectively. The binding constants for Mn2+ and Zn2+ were found to be 1.22 × 104 M−1 and 3.31 × 108 M−2 respectively. The practical applicability of the sensor was tested on tap water samples spiked with standard Zn2+ and Mn2+ ions and the results clearly indicated that the sensor can detect the presence of these ions even from aqueous medium. The fluorescent investigation on the sensor showed an emission peak at 507 nm upon excitation at 406 nm and the emission peak intensity decreased by 90% with addition of Mn2+ ion whereas the intensity of the 507 nm peak increased by 50% on addition of Zn2+ ion. The quenching of fluorescent intensity is due to binding of Mn2+ ion with the naphthalenic OH group and the imine nitrogen moiety, which causes internal charge transfer (ICT). The Stern–Volmer equation showed linear relationship with the quencher (Mn2+ ion) concentration, indicating quenching by the Mn2+ ion only. All these studies indicated potential application for detection for detecting Zn2+ and Mn2+ ions in water samples and which can become alternative to expensive techniques like AAS.
The existence, location and concentration of Fe3+ ions are extremely vital for execution of numerous biological functions in living organism whereas excess and deficiency lead various biological disorders in human body. In view of the paradox of Fe3+ ion, carbazole-diaminomaleonitrile derived fluorescence turn-off sensor (18) was developed for Fe3+ ion in which carbazole moiety was used a signalling unit.53 A strong fluorescence emission peak was appeared between 380–420 nm under excitation at 280 nm due to highly conjugated structure of sensor in which two carbazole unit connected on both end of DAMN through imine bond. The addition of Fe3+ ion in the solution of sensor induced significant fluorescence quenching while other metal ions like K+, Mn2+, Ag+, Co2+, Zn2+, Ni2+, Pb2+, Cd2+, Mg2+, Sr2+, Ca2+, Hg2+, Ba2+, Cu2+, Fe2+, Cr3+ and Al3+ did not influence the emission intensity of sensor. The nitrogen atoms of 18 were used for coordinating Fe3+ ion which induced internal charge transfer process from nitrogen of carbazole unit to Fe3+ ion causes significant fluorescence quenching (Fig. 10). Additionally, 18 showed fast response time with detection limit of 3.75 × 10−8 M for Fe3+ ion which make it useful for real time applications.
Mercury exists in nature mainly in organic, inorganic and elemental forms. All these forms have strong affinity toward carboxyl, thiol and phosphate functional groups present in human body which causes serious toxic effects. DAMN moiety was also used in development of sensor molecules for sensing of mercury. In this context, a highly efficient reversible chromo-fluorescent molecular probe (19) was synthesized by conjugating triphenyl amine and DAMN where triphenyl amine act as signalling unit and DAMN act as a binding unit.54 The synthesized receptors (19) exhibited absorption peak centred at 447 nm which shifted to 508 nm upon addition of Hg2+ with appearance of instant orange colour from light yellow in acetonitrile:water (1:1) solution. The observed red shift (60 nm) in the absorption spectra is probably due to the enhancement of intermolecular charge transfer after complexation of the Hg2+ ion with receptor. In addition to this Job's continuous variation method indicated 1:2 binding stoichiometry between sensor (19) and Hg2+ ions (Fig. 11). The detection limit was found to be 5.2 μM at neutral pH in 50% aqueous acetonitrile solution.
Sekar et al. fabricated colorimetric test strips based on DAMN derived sensor (20) for convenient and efficient sensing of Hg2+ ions. These test strips showed visible color changes for sensing of Hg2+ ion in acetonitrile. The chemosensor (20) used in test strips was synthesized by the combination of diaminomaleonitrile and 3-hydroxy-1,4-dioxo-1,4-dihydronaphthalene-2-carbaldehyde.55 The colorimetric sensing abilities of sensor (20) for Hg2+ ion was again evaluated by UV-visible spectroscopy. It was observed that absorption bands of 20 at 335 nm and 481 nm were disappeared and new absorption bands at 421 nm and 595 nm were appeared on addition of Hg2+ ion. It was found that absorption peak at 595 nm reached its saturation point upon addition of 1.0 equivalent of Hg2+ ion indicating the formation of 1:1 binding stoichiometry. The Job's plot, ESI-mass spectrometry analysis 1H NMR titration and DFT calculations further confirmed the 1:1 binding mode [Fig. 12].
Huo et al. reported five diaminomaleonitrile based Schiff base compounds (21–25). These Schiff bases were prepared by condensation of substituted-benzaldehyde (electron donor) and diaminomaleonitrile (electron acceptor). These compounds showed selective and efficient sensing of Hg2+ ion in ethanolic aqueous medium with micro molar detection limit.56 These sensor molecules exhibited visual colour changes upon interaction with Hg2+ ion over other tested metal ions (K+, Ca2+, Ba2+, Co2+, Cd2+, Mg2+, Ag+, Na+, Fe3+, Cu2+, Ni2+, Al3+, Zn2+, Cr3+ and Pd2+) in aqueous medium. For example, addition of 1.0 equivalent of Hg2+ ion changed the colourless solutions of 21–24 to yellow whereas pale yellow colour solution of 25 changed to orange red.
The sensor molecules (21–25) showed maximum absorbance peaks (λmax) centred at 362, 364, 365, 377 and 421 nm respectively. Addition of Hg2+ ion to solution of 24 caused a 28 nm red-shift (from 377 to 405 nm) and almost similar shifts in absorption peaks were observed with other sensor molecules (21–23 & 25). The binding mode of the sensors and Hg2+ ion was determined by Jos's plot method, indicated 2:1 binding stoichiometry which were confirmed by Benesi–Hildebrand equations (Fig. 13). The binding constants (Kasso) was found in the range of 102 to 103 M−1. Crystals of sensor molecules (23 and 24) and its complex with Hg2+ were grown and single crystal X-ray diffraction study revealed that 23 crystallized in the monoclinic crystal system with P21/c space group and the 24–Hg2+ complex crystallized in the monoclinic crystal system with C2/c space group.
Salen type ligands and its metal complexes have attracted immense attention in various fields like organic light-emitting diodes, catalysts, magnetic materials, supramolecular materials and cell imaging. These ligands and their complexes are easy to synthesis, reasonably stable and rich in photophysical properties. Furthermore, luminescent Salen ligands having π-conjugated tetradentate [O^N^N^O] chelating system are capable to coordinate with different metal ions, thus widely used in detection of wide-range of metal ions. Xiang group has recently prepared two Salen type ligands (26 and 27) by condensation reaction of diaminomaleonitrile and 4-(diethylamino)salicylaldehyde.57 Due to presence of diaminomaleonitrile moiety, it was possible to synthesize both cis and trans isomers. These isomers have pH sensitive groups such as hydroxyl, dimethylamine and –CN and ease of acid catalysed hydrolysis of these two ligands were exploited to get wide range pH probe using these compounds. These Salen pH probes were able to detect pH ranging from 1–12 in both colorimetric and fluorometric modes. It is important to note that addition of OH− ions (higher pH) the phenolic –OH group gets deprotonated and colour of the probe changes from pink to blue whereas addition of H+ ions (lower pH), colour of the probe solution changes from pink to purple due to protonation of imine nitrogen. Further addition of acid, probe gets hydrolysed to constituent diaminomaleonitrile and 4-(diethylamino)salicylaldehyde with disappearance of colour. All these protonated and deprotonated species were confirmed by 1H NMR measurements in which OH signal (δ = 11.71 ppm) disappeared upon addition of base and NCH signal shifted from δ = 8.43 ppm to 9.60 with addition of acid.
Among these two isomers cis isomer can act as tetradentate ligand whereas the trans form can act as bidentate ligand. It was observed that cis isomer can bind with various metal ions like Cu2+, Co2+ and Fe3+ and noticeable change in colour was observed by naked eye and fluorescence quenching response. In contrast to the cis isomer the trans isomer was able to detect Cu2+ ion selectively in ppb level by quenching the fluorescent intensity at 610 nm by 96% upon addition of 1.0 equivalent of Cu2+ ion and 100% quenching upon addition of 4.0 equivalents of Cu2+ ions. The investigation of Cu2+ ion detection mechanism indicated formation of a 1:1 trans ligand–Cu2+ complex, which then hydrolysed to diaminomaleonitrile and 4-(diethylamino)salicylaldehyde. The practical applicability of the trans isomer was checked by making the paper strip with the ligand and the dipping the strips in aqueous Cu2+ ion solution, acid and base solution. The obvious colour changes were observed as in solution.
In an another report, a multifunctional chemosensor based on julolidine and diaminomaleonitrile moieties was designed for detection of both Cu2+ and F− ion.60 The chemosensor (29) displayed absorption peak at 450 nm which was gradually decreased and simultaneously new peak at 375 nm was appeared with change in color from yellow to colorless on addition of Cu2+ ions. The sensing mechanism revealed the involvement of OH and NH2 groups of the receptor in Cu2+ ion binding led decrease intermolecular charge transfer transition (Fig. 15). It was found that sensor (29) can sense Cu2+ ion in much lower concentration than recommended by World Health Organization (WHO) in drinking water (30 mM) over a wide pH range of 4 to 12. Moreover, sensor (29) was also examined for sensing of different anions in similar condition because receptor having both phenolic and imine groups which can sense anions through hydrogen bonding. It was observed that upon addition of 600 equivalents of F− ion, 29 showed significant spectral changes consistent with color changes from yellow to orange while other anions exhibited no change in absorption spectrum.
A duel colorimetric sensor 2-(3-nitro-2-oxo-2H-chromen-4-ylamino)-3-aminomaleonitrile (30) was developed for the detection of Al3+ and F− ions with detection limit of 38.2 μM for Al3+ ion.61 The sensor showed absorption peaks at 250, 334, and 432 nm which were decreased and a new peak at 302 nm was appeared on stepwise addition of Al3+ ion with significant colorimetric changes over other competitive ions like Ga3+ and In3+ in aqueous medium. The color changes might be attributed to binding of Al3+ ion with NH and NH2 groups of the receptor which result weakened ICT transition of sensor 30 (Fig. 16). Interestingly, it was found that sensor can be recycled and reused after treatment with ethylenediaminetetraacetic acid (EDTA). On the other hand, it was found that 30 showed selective colorimetric sensing towards F− ion because of fluoride ion induced deprotonation process which led decrease in the intramolecular charge transfer.
A ninhydrin functionalized diaminomalenonitrile based sensor (31) was developed by condensation reaction between 2,3-diaminomalenonitrile and ninhydrin.62 Interestingly, sensor (31) exhibited blue color on simultaneous addition of Hg2+ and CH3COO−/F− ions whereas addition of either Hg2+ or CH3COO−/F− showed purple colour. These interesting switching behavior of sensor 31 was specific in terms of theses ion pairs as no other ion pairs exhibited similar behavior. These unique colorimetric behavior was further proven by UV-visible spectroscopy in which receptor 31 exhibited absorption peaks at 535 nm and 310 nm due to intramolecular charge transfer (ICT) band and sharp π–π transition band respectively. The addition of 2.0 equivalent of HgCl2 led no change in absorption spectrum as well as no visible color change. Interestingly, purple color of the receptor changed to blue as 2.0 equivalent of CH3COO−/F− was added to the same solution. Along with this, in UV-vis spectrum of receptor, peak at 535 nm shifted to 590 nm while peak at 310 nm remained almost unaffected. Other anions like H2PO4−, Cl−, Br−, C6H5COO− were not able to produce similar synergistic behavior with Hg2+ in terms of UV-vis spectral pattern or colorimetric responses.
Some DAMN based chemosensor molecules were designed in which sensing of both metal and anion were carried out through metal ion displacement approach. For example, a quinoline-based chemosensor (32) was developed for sequential sensing of Cu2+ and CN− ions.63 The sensor (32) exhibited remarkable colorimetric change with significant shift in absorption maxima from 390 nm to 537 nm on treatment with Cu2+ ion in aqueous solution. Therefore, sensor molecule was used for preparing test strips for monitoring of Cu2+ ion. In addition, 32–Cu2+ adduct was used for sequential monitoring of cyanide anion with dramatic colour change based on copper ion displacement mechanism as shown in Fig. 17.
By using similar approach, Singaravadivel et al. utilized phenothiazine moiety as fluorophore for synthesis of diaminomalenonitrile based chromo-fluorescent probe [3,3′-(((1Z,1′Z)-(10-hexyl-10H-phenothiazine-3,7-diyl)bis(methanylylidene))bis(azanylylidene))bis(2-aminomaleonitrile)] (33) for sensing of toxic Hg2+ and S2− ions.64 Synthesized probe (33) exhibited emission peak at 575 nm upon excitation at 450 nm in ethanol/water solvent system. Addition of Hg2+ ion lead quenching of emission peak at 575 nm along with visible colorimetric change from yellow to brown whereas other ions such as Mn2+, Cu2+, Pb2+, Cd2+, Co2+, Zn2+, Fe3+, Ag+, Mg2+, Al3+, Cr3+, Ni2+, Na+ and K+, did not lead any change in the emission peak at 575 nm. This quenching behavior was ascribed by inhabitation of internal charge transfer process from phenothiazine moiety to DAMN unit after binding with Hg2+ ion. Further, it was observed that quenched fluorescence can be restored by adding 1.0 equivalent of Na2S solution. This is explained by the fact that mercury ions have high affinity for sulfur hence S2− removed Hg2+ ion from 32–Hg2+ complex with regaining of emission peak at 575 nm (Fig. 18). It was also demonstrated that probe can be successfully used for selective imaging of Hg2+ and S2− in living cells.
Generally, DAMN based sensors have been developed by conjugating of electron-withdrawing groups (acceptor) with electron-donating group (donor) via imine linkage. An intramolecular charge transfer (ICT) occurs from DAMN (donor) to acceptor upon excitation by light which make sensor non-fluorescent. The electron-donating DAMN binds with cation which inhibit intramolecular charge transfer (ICT) due to the loses of electron-donating ability of DAMN and make the sensor cation adduct fluorescent. In an another approach, the fluorescence intensity can be enhanced by breaking the imine linkage between DAMN and acceptor moiety by the specific analyte and the recovered parent acceptor moiety (fluorophore dye) rejuvenate its original intense emission peaks as depicted in Scheme 1.
By using virtue of this approach, different research groups have developed interesting sensor molecules for detection of hypochlorite ion by conjugating DAMN (donor) with different acceptors (fluorophores) like benzoxazole, carbazole, quinolone, phenanthroline, naphthalenone, pyrene, carbazole, coumarin derivatives, xanthene dyes and phenanthroimidazole moiety as shown in Table 1. Analysis of LOD values of sensors reported in Table 1 indicated that most of these sensors can detect ClO− in the range of 10−6 to 10−8 M and DAMN attached to extended conjugated system exhibited lower detection limit. It was also observed that most of these sensor molecules exhibited low cytotoxic effect on cells thus can be used in imaging of living cells. In most cases the mechanism involves the attachment of ClO− ion to the carbon atom of imine group followed by nucleophilic attack of H2O to form corresponding aldehyde and DAMN as shown in Scheme 2. The sensing mechanism was confirmed by NMR titration experiments, mass spectrum analysis and DFT calculations by most of research groups.80,81,87
Entry | Sensor | Detection limit | References |
---|---|---|---|
1 | 2.00 × 10−4 M | Yuan et al.80 | |
2 | 1.07 × 10−6 M | Goswami et al.81 | |
3 | 3.30 × 10−6 M | Goswami et al.82 | |
4 | 2.83 × 10−6 M | Yang et al.83 | |
5 | 1.40 × 10−8 M | Zhao et al.84 | |
6 | 1.50 × 10−7 M | Ning et al.85 | |
7 | 2.88 × 10−8 M | Zhang et al.86 | |
8 | 7.87 × 10−7 M | Das et al.87 | |
9 | 8.00 × 10−8 M | Chen et al.88 | |
10 | — | Feng et al.89 |
Footnote |
† Dedicated to my mother “Smt. Abha Shrivastava”. |
This journal is © The Royal Society of Chemistry 2019 |