A very rare fluorescent chemosensor of zinc(II) exhibiting AIEE, ESIPT and TICT: Spectroscopic, crystallographic and theoretical exploration

The basic systems in this study are HL (1; 1:2 condensation product of 2,6-diformyl-4-ethylphenol and o-anisidine) and its ZnII and CdII complexes of composition [ZnII(LH)Cl2]·CH3OH (2) and [CdII(LH)Cl2] (3), all of which are synthesized and characterized by CHN elemental analyses, single crystal X-ray crystallography, powder X-ray diffraction (PXRD) and fourier transform infrared (FT-IR) spectrum. It has been established from the following experimental and theoretical studies that 1 is a fluorescent turn on sensor of ZnII ion and it exhibits all of excited state intramolecular proton transfer (ESIPT), photoinduced electron transfer (PET), twisted intramolecular charge transfer (TICT) and aggregation induced enhanced emission (AIEE): (i) Detailed absorption and emission (steady state / time resolved) studies in various single solvents, in solvent mixtures, with pH variation, with various single metal ions, with mixtures of metal ions, on varying temperature and on varying viscosity; (ii) dynamic light scattering (DLS) and scanning electron microscopy (SEM) in solvent mixtures; (iii) density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations in ground and excites states of 1-3. It is shown that 1 can be efficaciously applied in inkless writing with the "write - erase - write" facility. The mechanisms/reasons of the observed properties have been addressed. The difference in fluorescence of ZnII and CdII complexes, unusual case of crystal structures of probe and complexes with ZnII and CdII, unusual features in the structures of 2 and 3 as well as a structure-property correlation have been discussed.

Solvent-regulated fluorescence off-on signaling of Ni(II) and Zn(II) with the formation of two mononuclear complexes with an ATP detection ability by Zn(II) assembly

The current study shows that Schiff base HL, (Z)-2,4-dibromo-6-(((piperidin-2-ylmethyl)imino)methyl)phenol, can be used successfully as a selective chemosensor for Zn(II) and Ni(II) among several competing cations in purely aqueous and semi-aqueous media. Under UV light in methanol-water (9 : 1) HEPES buffer, the receptor gives its response by changing its color to cyan color in the presence of Zn(II) and to bluish cyan color in the presence of Ni(II). Surprisingly, the chemosensor can only reliably identify Zn(II) in a hundred percent aqueous medium by changing its color to light yellow. UV and fluorescence studies in both aqueous and semi-aqueous media are used to further investigate this Zn(II) and Ni(II) recognition phenomenon. The high values of the host-guest binding constants, obtained by electronic and fluorescence titration, ensure that a strong bond exists between HL and Ni(II)/Zn(II). As anticipated, two highly luminescent mononuclear, crystalline compounds, complexes 1 and 2, have been developed by a separate reaction of HL and Zn(II)/Ni(II), and the high luminous properties are due to the occurrence of Chelation Enhanced Fluorescence (CHEF). According to the single crystal structure, the asymmetric units of both complexes consist of two deprotonated chemosensor units and one Zn(II)/Ni(II), leading to the formation of an octahedral complex. For Ni(II) and Zn(II) sensing, the predicted LOD is in the nanomolar range. Both complexes 1 and 2 are fluorescence active and studies to check their ATP detection ability, but intriguingly, only complex 2 is capable of detecting ATP in a fully aqueous solution. Finally, the live cell imaging study validates the two sensors' biosensing functionality.

A new diformyl phenol based chemosensor selectively detects Zn2+ and Co2+ in the nanomolar range in 100% aqueous medium and HCT live cells

A new diformyl phenol based chemosensor that can sense Zn2+ and Co2+ in the nanomolar range in 100% aqueous solution and in HCT cells was explored.

On the supramolecular properties of neutral, anionic and cationic cadmium complexes harvested from dithiolate–polyamine binary ligand systems

Three Cd(ii) complexes [Cd(i-mnt)(DMSO)₂]_n (1), {[Cd(i-mnt)pn]·2H₂O} (2), and [Cd(i-mnt)₃][Cd(tren)₂]₂ (3) harvested from 1,1-dicyanoethylene-2,2-dithiolate (i-mnt²⁻) and polyamine ligand systems have been designed, synthesized and structurally characterized.

Fluorescein derived Schiff base as fluorimetric zinc (II) sensor via 'turn on' response and its application in live cell imaging

A novel Schiff base L composed of fluorescein hydrazine and a phenol functionalized moiety has been designed and prepared via cost-effective condensation reaction. The L is utilized for selective sensing of Zn²⁺ over other environmental and biological relevant metal ions in aqueous alcoholic solution under physiological pH range. The binding of Zn²⁺ to the receptor L is found to causes ~23 fold fluorescence enhancement of L. The 1:1 binding mode of the metal complex is established by combined UV-Vis, fluorescence, and HRMS (high-resolution mass spectroscopy) spectroscopic methods. The binding constant (K_a) for complexation and the limit of detection (LOD) of Zn²⁺ is calculated to be 2.86 × 10⁴ M^-1 and 1.59 μM, respectively. Further photophysical investigations including steady-state, time-resolved fluorescence analysis and spectral investigations including NMR (nuclear magnetic resonance), IR (infrared spectroscopy) suggest introduction of CHEF (chelation enhance fluorescence) with the suppression of CN isomerization and PET (photo-induced electron transfer) mechanism for the strong fluorescent response towards Zn²⁺. Finally, the sensor L is successfully employed to monitor a real-time detection of Zn²⁺ by means of TLC (thin layer chromatography) based paper strip. The L is used in the cell imaging study using African green monkey kidney cells (Vero cells) for the determination of exogenous Zn²⁺ by Immunofluorescence Assay (IFA) process.

A unique benzimidazole-naphthalene hybrid molecule for independent detection of Zn2+ and N3- ions: Experimental and theoretical investigations

Single crystal X-ray structurally characterized benzimidazole-naphthalene hybrid (NABI) functions as a unique dual analyte sensor that can detect Zn²⁺ cation and N₃^- anion independently. The NABI forms chelate with Zn²⁺ to inhibit internal charge transfer (ICT) and CHN isomerisation resulting chelation enhanced fluorescence (CHEF). On the other hand, the sensing of N₃^- is based on formation of supramolecular H-bonded rigid assembly. The association constant of NABI for Zn²⁺ and N₃^- ions are 19 × 10⁴ M^-1 and 11 × 10² M^-1, respectively. Corresponding limit of detections (LOD) are 6.85 × 10^-8 and 1.82 × 10^-7 M, respectively. NABI efficiently detects intracellular Zn²⁺ and N₃^- ions with no cytotoxicity on J774A.1cells under fluorescence microscope. DFT studies unlock underlying spectroscopic properties of free NABI and Zn²⁺/N₃^- bound forms.

Design, Synthesis and Photochemical Properties of a Phenalenone-Based pH Sensor: Switchable pH Sensing in Four Detectable Channels

The synthesis and pH-sensing property of a novel phenalenone-based compound, 9-(4-hydroxyphenylamino)-1-oxo-phenalenone (HPAP), is reported. The newly synthesized compound is capable of functioning as a pH sensor in the region of pH 7 to 12. The sensor can be used as a colorimetric indicator in the transition from pH 10 to pH 11. The sensor is able to function in four detectable channels. All four channels (UV, emission, colorimetric/visible and photoluminescence) have been shown to be reversible, thus implying the reuse of this single-molecule sensor and indicator for several experiments. Mechanistic investigations have been performed by UV, NMR and DFT studies which indicate that a photoinduced electron transfer (PET) based mechanism could be operative. Straightforward and cost-effective application of the sensor in thin-layer chromatography has also been established.

Screening of biologically important Zn2+ by a chemosensor with fluorescent turn on-off mechanism

Reported herein the synthesis, characterization and biologically important zinc ion binding propensity of a weakly fluorescent chemosensor, 4-methyl-2,6-bis((E)-(2-(4-phenylthiazol-2-yl)hydrazono)methyl)phenol (1). ¹H NMR spectroscopic titration experiment reveals the binding knack of 1 to the essential Zn²⁺. The photo-physical studies of 1 exhibit an enhancement in the fluorescence by several folds upon binding with the zinc ions attributed to PET-off process, with a binding constant value of 5.22×10³M^-1. 1 exhibits an excellent detection range for Zn²⁺ with lower detection limit value of 2.31×10^-8M. The selectivity of 1 was studied with various mono and divalent metal cations and it was observed that most cations either quenches the fluorescence or remains unchanged except for Cd²⁺, which shows a slight enhancement in fluorescence intensity of 1. The ratiometric displacement of Cd²⁺ ions by Zn²⁺ ions shows an excellent selectivity towards in-situ detection of Zn²⁺ ions. Photo-physical studies also support the reversible binding of 1 to Zn²⁺ ions having on and off mechanism in presence of EDTA. Such recognition of the biologically important zinc ions finds potential application in live cell imaging.

A novel chemosensor based on rhodamine and azobenzene moieties for selective detection of Al3+ ions

Both rhodamine and azobenzene moieties have been conjugated to prepare a novel chemosensor for the detection of Al³⁺ through CHEF-PET and the spirolactam ring opening mechanism.

Combined experimental and theoretical studies on selective sensing of zinc and pyrophosphate ions by rational design of compartmental chemosensor probe: Dual sensing behaviour via secondary recognition approach and cell imaging studies

A compartmental chemosensor probe HL was designed and synthesized for dual sensing of zinc ions and PPi via secondary recognition approach.

Highly photostable zinc selective molecular marker bearing flexible pivotal unit: opto-fluorescence enhancement effect and imaging applications in living systems

Novel molecular probes for imaging zinc in biological systems are gaining interest as they help in understanding the role of zinc in regulating various bio-events. In this regard, a new C2-symmetric molecular system has been developed and successfully applied as light-up material for signaling divalent zinc with green emission. The fluorescence enhancement was highly zinc specific and this newly developed probe bears a submicromolar detection capability. While probe and the ensemble -Zn(2+) exhibited remarkably high photostability, light-triggered fluorescence enhancement was observed in the case of -Zn(2+). The nature of the -Zn(2+) complex and the associated spectral shift are further supported by theoretical calculations. As the present probe absorbs in the visible region and emits in the green, it was preferred as a potential material for imaging zinc in biological systems including animal and plant cells such as pollen grains and fish egg cells. Such fluorescence imaging of zinc revealed the efficacy of the probe in detection and localization of zinc in various biological systems.

Deciphering the CHEF-PET-ESIPT liaison mechanism in a Zn2+ chemosensor and its applications in cell imaging study

Proper fusion of fluorescence mechanisms in a single fluorophore unit is highly desirable to obtain better sensitivity, as well as selectivity towards a particular metal ion.

A thiosemicarbazone based chemo and fluorogenic sensor for Zn2+with CHEF and ESIPT behaviour: computational studies and cell imaging application

A novel diformyl-p-cresol (DFC)–thiosemicarbazide (TS) – based turn-on dual sensor that enables to determine, for the first time, the stepwise formation constants.

1,8-Naphthyridine-based molecular clips for off–on fluorescence sensing of Zn2+ in living cells

New 1,8-naphthyridine-based clip-like receptor as “off–on” fluorescent probe was designed and synthesised for selectively sensing Zn²⁺ in living cells.

A novel polynorbornene-based chemosensor for the fluorescence sensing of Zn2+ and Cd2+ and subsequent detection of pyrophosphate in aqueous solutions

A novel 8-hydroxyquinoline anchored polynorbornene for the fluorescence sensing of Zn²⁺/Cd²⁺ and subsequent detection of pyrophosphate in aqueous solutions and its biological application is introduced.

Fluorescent metal ion chemosensors via cation exchange reactions of complexes, quantum dots, and metal–organic frameworks

Overview of a new paradigm in the design of fluorescent chemosensors for detecting metal ions via cation exchange reactions of complexes, quantum dots, and metal–organic frameworks.

Rhodamine-based molecular clips for highly selective recognition of Al3+ ions: synthesis, crystal structure and spectroscopic properties

A novel fluorescent chemosensor based on a rhodamine derivative (L) was designed, synthesized, and used as a selective Al⁺³ ion sensor.