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In the design of novel fluorescent chemosensors, investigation of new sensing mechanisms between recognition and signal reporting units is of increasing interest. In recent years, a smart chemosensor probe containing a 1,8-naphthalimide moiety could be developed as a fluorescent and colorimetric sensor for toxic anions, metal ions, biomolecules, nitroaromatics, and acids and be further applied to monitor the relevant biological applications. In this field, several problems and challenges still exist. This critical review is mainly focused on various sensing mechanisms that have emerged in the past few years, such as Photo-Induced Electron Transfer (PET), Intramolecular Charge Transfer (ICT), Fluorescence Resonance Energy Transfer (FRET), Excited-State Intramolecular Proton Transfer (ESIPT), hydrogen bonding and displacement approach. The review concludes with some current and future perspectives, including the use of the naphthalimides for sensing anions, metal ions, biomolecules, nitroaromatics and acids and their potential uses in various fields.

Fluorescent determinations of analytes have proven to be a powerful method due to their simplicity, low cost, detection limit, rapid photoluminescence response, and applicability to bioimaging. Fluorescence imaging as a powerful tool for monitoring biomolecules within the living systems. Schiff base has been extensively used as strongly absorbing and colorful chromophores in the design of chemosensors. In recent years, Schiff base based fluorescent probes have been developed for the detection of various toxic analytes and imaging of various analytes in biological systems. This review gives an overview of the important fluorescent sensors which are based on Schiff base, their approaches for molecular recognition, and their potential application in bioimaging studies.

Among contrasting anions, CN− ion is extremely toxic and dangerous pollutant. Consequently, there is considerable interest in the colorimetric and fluorescent detection of CN− ions at trace level by using simple methods In this critical review begins with a brief discussion of the source, applications, toxicity of cyanide and mechanism of CN− ion sensor. Further, the fluorescent and colorimetric CN− ion sensors are classified according to their receptors into several categories like azine, azo, benzimidazole, benzothiazole, bisthiazole, BODIFY, coumarin, nanoparticles, hemicyanine, imine, indoline, malononitrile, naphthalene, naphthalimide, phenazine, pyrene, quinoline, rhodamine and thiazolidine functionalized sensing system in the year 2016.

Colorimetric and fluorometric detection of copper ion (Cu2+) has attracted much attention due to the biological, environmental and medical significance of the Cu2+ ion. Colorimetric and fluorescent detection of metal ions have proven to be powerful methods due to their simplicity, cost effectiveness, high detection limit and applicability to bioimaging. This review will consolidate selective and sensitive Cu2+ ion detection achieved by various receptor types in the years between 2013 to 2015.

Fluoride ion (F-) contamination can be accumulated along the water and the food chain and cause serious risk to public health. It is of the greatest importance that selects the suitable chromophores and fluorophores for the design and synthesis of outstanding selective, sensitive chromogenic and fluorogenic probes for detection of fluoride ion. In this review is mainly focused on the current progress of fluoride ion detection according to their receptors into several categories like anthracene, azo, benzothiazole, BODIPY, calixarene, coumarin, imidazole, diketopyrrolopyrrole, hydrazone, imidazole, naphthalene, naphthalimide, quantum dots, Schiff base and urea group sensing in the year 2015-2019.

A novel chemosensor based on azo linked Schiff base (receptor 1 ) was synthesized by simple condensation method. The sensor studies of receptor 1 disclose the selectivity toward fluoride and cyanide ion with a color change among the other common anions in aqueous medium. The binding affinity of receptor 1 was also enlightened by 1 H NMR titration and DFT calculation.

A new dual chemosensor (Z)-2-(anthracen-9-ylmethylene)-N-phenylhydrazinecarbothioamide R1 was synthesized by incorporating a thiourea derivative as a binding unit and an anthracene group as a fluorescence signalling unit. R1 selectively binds with Cu2+ and Hg2+ ions in aqueous medium. R1 exhibits highly selective recognition towards F− ion and picric acid in organic medium. Fluorescence quenching was observed in case of R1 with Cu2+ and Hg2+ ions over commonly coexistent metal ions. Furthermore, fluorescence imaging experiments of Cu2+ and Hg2+ ions in living RAW 264.7 cells demonstrate its value of practical applications in biological systems.

A series of novel azo linked polycyclic aromatic hydrocarbons-based sensors (R1–R3) were designed and synthesized in single step. R1–R3 was further developed for the selective and sensitive detecti...

The azo linked thiourea type Schiff base has been synthesized and characterized which is found to sense F−, AcO− and Cd2+ ions in aqueous medium over other anions and cations. R1 gave rise to significant changes in the absorption spectrum and 1H NMR spectroscopic studies upon the titration with F−, AcO−. R1 detects anions and metal ions at nanomolar level. F− ion detection property of R1 in aqueous medium was also extended to the real samples like toothpaste and mouthwash.

Heterocyclic ring based R 1 –R 3 have been synthesized from the simple condensation method. R 1 –R 3 exhibit highly selective and sensitive recognition towards transition metal ions in an aqueous medium via visual color change and were further confirmed by UV–vis and fluorescent spectroscopic methods. Fluorescent turn on and turn off behavior was observed for receptors tested with transition metal ions. The interaction of transition metal ions and receptors R 1 –R 3 was confirmed to adopt 1:1 binding stoichiometry. Micromolar detection limit was found for R 1 –R 3 with metal ions. DFT theoretical calculations were employed to understand the sensing mechanism of the sensors towards the metal ions. R 1 and R 2 were also successfully demonstrated as a fluorescent probe for detecting Cu 2+ ions in living cells.

A simple pyrene-linked thiourea 1 has been synthesized and shown to act as a fluorescent and visual sensor for the detection of Cu2+ and Hg2+ ions. The sensor showed effective detection of Cu2+ and Hg2+ at a low detection limit. Receptor 1 has been successfully applied in the fluorescence imaging of Cu2+ and Hg2+ ions in living cells. Receptor 1 also selectively detects picric acid via both color change and fluorescence change.

N -Phenylthiosemicarbazones (R 1 –R 4 ) have been synthesized and characterized by various spectroscopic techniques. The sensors (R 1 –R 4 ) exhibited highly selective and sensitive recognition toward Cu 2+ and F − ions over other cations and anions via visual color change. Sensors showed a significant fluorescent enhancement toward Cu 2+ in aqueous medium and found a sub-nanomolar detection limit with good quantum yield. R 1 displayed high selectivity to Cu 2+ over other interference ions with 89-fold fluorescence intensity enhancement upon the addition of 2 equiv. of Cu 2+ . In addition, the turn-on fluorescent changes upon the addition of Cu 2+ , is also applied in cell imaging.

Pyrrole based Schiff bases act as a highly sensitive probe for metal ions in aqueous medium. Both receptors R 1 and R 2 are sensitive towards Fe 3+ , Cu 2+ , Hg 2+ and Cr 3+ among the other metal ions. The sensing ability of the receptors are investigated via colorimetric, optical and emission spectroscopic studies. The binding stoichiometries of R 1 and R 2 with metal ions have been determined as 2:1 by using Job’s plot. The colorimetric receptors exhibited high sensitivity with a low detection limit of μM levels. In the presence of metal ions both receptors shows fluorescence quenching. This might be due to the photo induced electron transfer mechanism. The quenching constant was further determined using Stern–Volmer plot.

Thiophene-based diimine (R1) and monoimine (R2) were synthesized in a single step, and their cation binding affinity was tested using colorimetric and UV–vis spectral studies. R1 selectively shows a colorimetric turn-on response for Pb2+, Hg2+ ions and colorimetric turn-off with Sn2+ ions, and R2 shows visual response for Cu2+ and Hg2+ over other examined metal ions in aqueous medium. R1 forms 1:1 complex with Pb2+, Hg2+, and Sn2+ and exhibits fluorescence quenching, whereas R2 shows 2:1 complex with Hg2+, Cu2+ and shows fluorescence enhancement. The structural and electronic properties of the sensors and their metal complexes were also investigated using Density Functional Theory calculations. R2 was also successfully demonstrated as a fluorescent probe for detecting Cu2+ ions in living cells. Copyright © 2014 John Wiley & Sons, Ltd.

Highly efficient fluorescent chemosensors for metal ions have been synthesized by using thiosemicarbazide and aromatic aldehydes. Detection of transition metal ions was performed via UV–vis and fluorescence spectroscopic methods. This is the first report on thiosemicarbazone based sensor capable of detecting transition metal ions in aqueous medium. The binding constant, stoichiometry of the complex were confirmed by using B–H plot and Job's plot method. The fluorescence enhancement of thiosemicarbazones on binding with Hg 2+ , Zn 2+ , Co 2+ , Ni 2+ and Sn 2+ ions is due to the inhibition of photoinduced electron transfer mechanism whereas, quenching of fluorescence is attributed to the photoinduced electron transfer mechanism in case of Cu 2+ and Mn 2+ ions.

A highly sensitive and selective detection of Hg 2+ ion with simple salophen probe was developed. In DMSO: water (40:60, v/v) solution, Hg 2+ ions coordinate with imine and shows color turn-off from yellow to colorless. Receptor 1 showed its ability for sensing Hg 2+ cations sensitively through three channels: colorimetric, UV–vis and fluorescence spectroscopy. Hg 2+ ions coordinate to the imine (Receptor 1 ) through NONO binding site forming 1:1 complex. It exhibits fluorescent 'Turn-on' behavior based on solvent polarity. The detection limit of our receptor with mercury is 1 μg L −1 .

The cation sensing property of highly sensitive chromogenic receptor N, N'-bis (salicylidine)-o-phenylene diamine (receptor 1) was studied by visual observation, UV-vis spectroscopy and fluorescence spectroscopy. The proposed study has been targeted to sense the first transition series metal cations like Fe3+, Co2+, Ni2+ and Cu2+. Binding affinity toward Cu2+ is found to be of higher magnitude compared to the other three cations mentioned. Receptor 1 on binding with Fe3+, Co2+ Ni2+ and Cu2+ ions shows fluorescence enhancement which is due to the inhibition of PET mechanism.

Herein, we report the highly selective binding of Zn2+ ion by the salicylaldimine based Schiff base chromogenic receptor 1 [(N,N′-bis (salicylidine)-o-phenylenediamine]. Receptor 1 senses Zn2+ ion in aqueous medium by colorimetric and fluorescent response in the presence of other metal ions like Pb2+, Hg2+, Sn2+, Cd2+. Receptor 1 on binding with Zn2+ ion exhibits fluorescence enhancement which is due to the inhibition of the (ESIPT) mechanism.

A structurally simple (Z)-2-(naphthalen-2-ylmethylene)-N-phenylhydrazinecarbothioamide (R1) was used as a colorimetric and fluorescent sensor for both F− and Cu2+/Hg2+ ions. R1 selectively recognised F− ions as indicated by colour change from colourless to green. Fluorescence spectral data reveal that R1 is an excellent fluorescence chemosensor for Cu2+ ions. Finally, R1 was successfully applied to the bioimaging of Cu2+ ions in RAW 264.7 macrophage cells.

Thiophene-based diimine (R1) and monoimine (R2) were synthesized in a single step, and their cation binding affinity was tested using colorimetric and UV-vis spectral studies. R1 selectively shows a colorimetric turn-on response for Pb(2+), Hg(2+) ions and colorimetric turn-off with Sn(2+) ions, and R2 shows visual response for Cu(2+) and Hg(2+) over other examined metal ions in aqueous medium. R1 forms 1:1 complex with Pb(2+), Hg(2+), and Sn(2+) and exhibits fluorescence quenching, whereas R2 shows 2:1 complex with Hg(2+), Cu(2+) and shows fluorescence enhancement. The structural and electronic properties of the sensors and their metal complexes were also investigated using Density Functional Theory calculations. R2 was also successfully demonstrated as a fluorescent probe for detecting Cu(2+) ions in living cells.

Highly sensitive and selective fluorescent cation sensors 1–3, based on heterocyclic derivatives, have been synthesized from the condensation reaction between their corresponding heterocyclic aldehydes and thiosemicarbazide. All of the receptors 1–3 can act as optical sensors for the detection of Cu2+ and Hg2+ ions, and as fluorescent sensors for the Cr3+ ion, via a chelation enhanced fluorescence effect in an aqueous environment. The structures of the receptors 1–3 were well characterized by IR, 1H NMR, 13C NMR and HRMS techniques and their cation sensing abilities were studied by UV-vis and fluorescence spectroscopy.