Ratiometric fluorescence-based and chromogenic sensors for the detection of fluoride ions and their application in real samples.

This review focuses on the results of synthetic ratiometric fluorescent and colorimetric probes, which have been applied to qualitatively and quantitatively detect fluoride anions in cells, living organisms, and real samples. Primary attention is given to progress made in the working mechanism and applications of these probes to detect fluoride ions in living systems. In addition, design strategies and detection limit for these probes are discussed. This review aims to deliver a comprehensive compilation of the examples reported from 2005 to 2021 on the developments of ratiometric chromogenic and fluorogenic chemosensors for fluoride anions. A total of 20 different ratiometric/colorimetric sensors have been selected for the novelty in their design, sensitivity, detection limit, dynamic range, and speed of detection based on the three fundamental principles of F- ion detection, namely Si-O bond cleavage; excimer emission; and intramolecular charge transfer emission through the B-F monomer, B-F-B bridged dimers, and deprotonation of the amide N-H. Special emphasis has been given to categorize the fluorophores that work in aqueous media, and possible strategies that might be adopted to design green sensors are discussed. Finally, a tabular summary of the comparative studies of all the sensors based on their sensitivity, detection limit, working solvent, and applications is provided. This extensive review may expedite improvements in the development of advanced fluorescent probes for vast and stimulating applications in the future.

Interaction of water with a benzimidazole derivative: fluorescence and colorimetric recognition of trace level water involving intra-molecular charge transfer process.

Benzimidazole-derived ICT-based probe, DFPBEN is developed for trace level determination of water. In presence of water, the naked eye color of DFPBEN changes from red to yellow, while it turns to green from red under UV light. Upon addition of water, DFPBEN shows a ratiometric absorbance change in methanol.

Tailoring Ligand Environment toward Development of Colorimetric and Fluorescence Indicator for Biological Mn(II) Imaging.

Mn(2+) ion plays an essential role in all forms of life. Paramagnetic nature of Mn(2+) and its close resemblance with Ca(2+) and Mg(2+) are two key limiting factors responsible for the least development of fluorescence probes suitable for bioimaging. In literature we have found only a few Mn(2+) selective fluorescent sensor and their applications. These probes are mainly based on linear polydentate and macrocyclic ligands. Systematic tuning of ligand environment allows colorimetric and fluorescence recognition of traces Mn(2+) in real sample and fluorescence indicator in living RAW264.7 cells. Two probes, one based on fluorescein (FHDB) and the other based on rhodamine (RDDB) showed turn-on response toward Mn(2+) in DMSO and acetonitrile, respectively. Colorimetric detection of Mn(2+) ion is also possible in the presence of other metal ions. The new sensing probe RDDB shows higher sensitivity as well as faster response compared to the reported systems. The detection limit of RDDB is 5 × 10(-8) M and FHDB is 1 × 10(-7) M. DFT studies strongly support the experimental facts.

Hydrazine selective dual signaling chemodosimetric probe in physiological conditions and its application in live cells.

A rhodamine-cyanobenzene conjugate, (E)-4-((2-(3',6'-bis(diethylamino)-3-oxospiro[isoindoline-1,9'-xanthene]-2-yl)ethylimino)methyl)benzonitrile (1), which structure has been elucidated by single crystal X-ray diffraction, was synthesized for selective fluorescent "turn-on" and colorimetric recognition of hydrazine at physiological pH 7.4. It was established that 1 detects hydrazine up to 58 nM. The probe is useful for the detection of intracellular hydrazine in the human breast cancer cells MCF-7 using a fluorescence microscope. Spirolactam ring opening of 1, followed by its hydrolysis, was established as a probable mechanism for the selective sensing of hydrazine.

Pyridine Based Fluorescence Probe: Simultaneous Detection and Removal of Arsenate from Real Samples with Living Cell Imaging Properties.

Pyridine based fluorescence probe, DFPPIC and its functionalized Merrifield polymer has been synthesized, characterized and used as an arsenate selective fluorescence sensor. Arsenate induced fluorescence enhancement is attributed to inter-molecular H-bonding assisted CHEF process. The detection limit for arsenate is 0.001 µM, much below the WHO recommended tolerance level in drinking water. DFPPIC can detect intracellular arsenate in drinking water of Purbasthali, West Bengal, India efficiently. Graphical Abstract DFPPIC and its Merrifield conjugate polymer are used for selective determination and removal of arsenate from real drinking water samples of Purbasthali, a highly arsenic contaminated region of West Bengal, India. DFPPIC is very promising to imaging arsenate in living cells.

An INHIBIT logic gate from a thiophene derivative using iron and zinc ions as the input: tuning the efficiency on moving from naphthalene to anthracene to pyrene for the green luminescent detection of the intracellular iron.

A pyrene-thiophene conjugate has been used for the efficient detection of intracellular irons through the generation of bright green luminescence. Amongst three different probes being reported, it is observed that the efficiency for the naked eye green luminescent detection of iron increases on moving from naphthalene to anthracene to pyrene. The developed technique may be very useful to monitor iron and zinc supplementation as single micronutrients in living systems through the construction of an INHIBIT logic gate.

FRET based tri-color emissive rhodamine-pyrene conjugate as an Al3+ selective colorimetric and fluorescence sensor for living cell imaging.

A rhodamine-pyrene hybrid molecule acts as a colorimetric and fluorimetric sensor for Al(3+) through time dependent PET-CHEF and FRET processes associated with tri-color emission. Intracellular Al(3+) has been visualized through time dependent blue-green-red emission. The lowest limit of detection for Al(3+) is 0.02 µM.

Selective sensing of Hg²âº using rhodamine-thiophene conjugate: red light emission and visual detection of intracellular Hg²âº at nanomolar level.

Rhodamine-thiophene conjugate (L) has been synthesized and characterized by (1)H NMR, FTIR and mass spectra. L shows a large enhancement in emission intensity in presence of Hg(2+). Moreover, naked eye color of L becomes intense red in presence of Hg(2+). The lowest detection limit for Hg(2+) is 1 × 10(-9)M in HEPES buffer (0.1M in EtOH/water, 1/1, v/v, pH 7.4). Hg(2+) induced chelation enhanced fluorescence (CHEF) is associated with spirolacram ring opening of the rhodamine unit. Trace level intracellular Hg(2+) is visualized under fluorescence microscope.

Fluorescence sensing of arsenate at nanomolar level in a greener way: naphthalene based probe for living cell imaging.

Naphthalene-salisaldehyde conjugate (NAPSAL) is established as a novel arsenate (H2AsO4(-)) selective 'turn-on' fluorescence probe. It can detect as low as 5 × 10(-9) M H2AsO4(-) in HEPES buffered EtOH : water (0.1 M, 1 : 9, v/v, pH 7.4). Trace level H2AsO4(-) in drinking water samples is measured using standard addition method. Intracellular arsenate in Candida albicans, grown in arsenic contaminated water of Purbasthali has successfully been detected under fluorescence microscope.

A coumarin-based "turn-on" fluorescent sensor for the determination of Al3+: single crystal X-ray structure and cell staining properties.

An efficient Al(3+) receptor, 6-(2-hydroxybenzylideneamino)-2H-chromen-2-one (HBC), has been synthesized by condensing salicylaldehyde with 6-aminocoumarin. The molecular structure of HBC has been determined by a single crystal X-ray analysis. It was established that in the presence of Al(3+), HBC shows 25 fold enhancement of fluorescence intensity which might be attributed to the chelation-enhanced fluorescence (CHEF) process. HBC binds Al(NO3)3 in a 1 : 1 stoichiometry with a binding constant (K) of 7.9 × 10(4) M(-1). Fe(3+) and Mn(2+) quench the emission intensity of the [HBC + Al(3+)] system to an insignificant extent at a concentration 10 times higher compared to that of Al(3+). HBC is highly efficient in the detection of intracellular Al(3+) under a fluorescence microscope.

A naphthalene-thiophene hybrid molecule as a fluorescent AND logic gate with Zn2+ and OAc- ions as inputs: cell imaging and computational studies.

A naphthalene-thiophene hybrid molecule (Z)-1-((thiophen-2-ylmethylamino)methylene)naphthalen-2(1H)-one () was prepared by condensation of 2-thiophenemethylamine and 2-hydroxy-1-naphthaldehyde. According to FTIR, (1)H NMR spectrometry and single crystal X-ray analysis, exists in the cis-keto-amine tautomeric form. behaves like a molecular AND type binary logic gate with two inputs viz. Zn(2+) and OAc(-) ions whereby the fluorescence of the system turns on. The structures of and its zinc acetate complex were also optimized by DFT calculations. binds Zn(2+) in a 1 : 1 ratio with an association constant K(a) = 2.05 × 10(4) M(-1), and detects Zn(2+) as low as 3 × 10(-8) M. is useful for the detection of intracellular Zn(2+) under a fluorescence microscope.

Rhodamine-based fluorescent probe for Al3+ through time-dependent PET-CHEF-FRET processes and its cell staining application.

Rhodamine-diformyl p-cresol conjugate (L) has been developed as a novel Al(3+)-selective fluorometric and colorimetric sensor based on the FRET mechanism for the first time. L can selectively detect Al(3+) through time-dependent PET-CHEF and FRET processes. This phenomenon is nicely reflected from (1)H NMR, fluorescence lifetime, and fluorescence cell imaging studies. The probe can detect Al(3+) as low as 5 × 10(-9) M in HEPES-buffered EtOH:water (0.1 M, 4:1, v/v, pH 7.4). The probe shows pH-dependent emission properties viz. an intense red emission (585 nm) at acidic pH and an intense green fluorescence (535 nm) at basic pH. Thus, L can also be used as a pH sensor via tunable wavelength.

Ratiometric fluorescence sensing and intracellular imaging of Al3+ ions driven by an intramolecular excimer formation of a pyrimidine-pyrene scaffold.

An Al(3+) selective ratiometric fluorescent probe (L) has been synthesized by condensation of 4,5-diaminopyrimidine with 1-pyrenecarboxaldehyde. The structure of L has been confirmed by single crystal X-ray analysis. In the presence of Al(3+), L exhibits a considerable excimer emission at 445 nm along with the decrease of its monomer emission at 368 nm in DMSO-H2O (4 : 1, v/v). The lowest detection limit for Al(3+) is 0.24 µM. Furthermore, L can permeate through the cell membrane and detects intracellular Al(3+) ions under a fluorescence microscope.

A rhodamine derivative as a "lock" and SCN- as a "key": visible light excitable SCN- sensing in living cells.

A visible light excitable rhodamine based probe operates as a SCN(-) selective fluorescent "turn-on" sensor for living cell imaging with a detection limit of 0.01 µM, which is much lower than the normal SCN(-) level in the human body. A "lock" and "key" model has been proposed to explain the fluorescence enhancement of the rhodamine probe in the presence of SCN(-).

Antipyrine based arsenate selective fluorescent probe for living cell imaging.

Condensation of salicylaldehyde and 4-aminoantipyrine has yielded a new fluorescent probe (APSAL) capable of detecting intracellular arsenate at the micromolar level for the first time. The structure of the probe has been established by different spectroscopic techniques and confirmed from X-ray crystallography. Common anions, viz., F(-), Cl(-), Br(-), I(-), N(3)(-), NCO(-), NO(2)(-), NO(3)(-), SCN(-), CN(-), CH(3)COO(-), SO(4)(2-), ClO(4)(-), and HPO(4)(2-) do not interfere. The binding constant of APSAL for H(2)AsO(4)(-) has been determined using the Benesi-Hildebrand equation as 8.9 × 10(3) M(-1). Fluorescence quantum yield of APSAL (0.016) increases more than 12 times upon binding arsenate ion.

Cd(II)-triggered excimer-monomer conversion of a pyrene derivative: time dependent red-shift of monomer emission with cell staining application.

An efficient fluorescent probe (E)-N1-((E)-2-((pyren-7-yl)methyleneamino)ethyl)-N2-((pyren-7-yl)methylene)ethane-1,2-diamine (L) has been synthesized by a facile one-step condensation reaction. L can selectively detect Cd(2+) in presence of other common metal ions in 0.1 M HEPES buffered DMSO-water (4 : 1, v/v) medium. The detection limit of Cd(2+) is 1.8 × 10(-8) M. Cd(2+) can effectively convert the excimer emission of L into its monomer emission which in turn exhibits a time-dependent red-shift.

Nickel(II)-induced excimer formation of a naphthalene-based fluorescent probe for living cell imaging.

Ni(2+)-induced intramolecular excimer formation of a naphthalene-based novel fluorescent probe, 1-[(naphthalen-3-yl)methylthio]-2-[(naphthalen-6-yl)methylthio]ethane (L), has been investigated for the first time and nicely demonstrated by excitation spectra, a fluorescence lifetime experiment, and (1)H NMR titration. The addition of Ni(2+) to a solution of L (DMSO:water = 1:1, v/v; λ(em) = 345 nm, λ(ex) = 280 nm) quenched its monomer emission, with subsequent enhancement of the excimer intensity (at 430 nm) with an isoemissive point at 381 nm. The fluorescence lifetime of free L (0.3912 ns) is much lower than that of the nickel(2+) complex (1.1329 ns). L could detect Ni(2+) as low as 1 × 10(-6) M with a fairly strong binding constant, 2.0 × 10(4) M(-1). Ni(2+)-contaminated living cells of plant origin could be imaged using a fluorescence microscope.

A naphthalene exciplex based Al3+ selective on-type fluorescent probe for living cells at the physiological pH range: experimental and computational studies.

2-((Naphthalen-6-yl)methylthio)ethanol (HL) was prepared by one pot synthesis using 2-mercaptoethanol and 2-bromomethylnaphthalene. It was found to be a highly selective fluorescent sensor for Al(3+) in the physiological pH (pH 7.0-8.0). It could sense Al(3+) bound to cells through fluorescence microscopy. Metal ions like Mn(2+), Fe(3+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Ag(+), Cd(2+), Hg(2+), Cr(3+) and Pb(2+) did not interfere. No interference was also observed with anions like Cl(-), Br(-), F(-), SO(4)(2-), NO(3)(-), CO(3)(2-), HPO(4)(2-) and SCN(-). Experimentally observed structural and spectroscopic features of HL and its Al(3+) complex have been substantiated by computational calculations using density functional theory (DFT) and time dependent density functional theory (TDDFT).

Thiophene anchored coumarin derivative as a turn-on fluorescent probe for Cr3+: cell imaging and speciation studies.

A thiophene-coumarin hybrid molecule, (6E)-6-((thiophen-2-yl)methyleneamino)-2H-chromen-2-one (TMC) has been prepared and its single crystal X-ray structure is reported. TMC can selectively detect Cr(3+) in presence of other common cations. Both TMC and its Cr(3+) complex are well characterized by different spectroscopic techniques like (1)H NMR, QTOF-MS ES(+), FTIR and elemental analysis as well. TMC exhibits fluorescence enhancement upon binding Cr(3+) in CH(3)CN-HEPES buffer (0.02 M, pH 7.4) (4:6, v/v) medium. Detection limit of the method is 1 × 10(-6)M. Binding constant is estimated with the Benesi-Hildebrand method and the value 8 × 10(4) indicates a fairly strong interaction between TMC and Cr(3+). Speciation studies have been performed in a fast and environment friendly way using least sample volume, less hazardous chemicals and solvents. Cr(3+) assisted restricted rotation around the imine bond and inhibited photo-induced electron transfer from the N,S-donor sites to the coumarin unit are responsible for fluorescence enhancement. TMC is capable to detect intracellular Cr(3+) in living cells.

Highly selective organic fluorescent probe for azide ion: formation of a "molecular ring".

An indole based naphthalene derivative is reported as a highly selective fluorescent probe for azide ion in aqueous ethanol. The probe is applied for cell imaging of the N(3)(-) ion in contaminated living cells. Both experimental and theoretical studies have been performed to figure out the plausible mechanism of fluorescence enhancement of the probe upon binding with N(3)(-).