Fluorescent chemosensor for Zn(II) ion by ratiometric displacement of Cd(II) ion: A spectroscopic study and DFT calculation

Isatin-Schiff base functionalized graphene oxide as a highly selective turn-on fluorescent probe for the detection of Pd(II) via photoinduced electron transfer pathway

We investigated the use of isatin-Schiff base functionalized graphene oxide (ISBGO) as a selective fluorescent chemosensor for the detection of palladium ions. Selectivity tests indicated that over 23 metal ions tested, ISBGO (λex = 340 nm, λem = 504 nm) showed the highest affinity for Pd(II), displaying a 10.1-fold enhancement. Also, interference tests proved that in the presence of both Pd(II) and other metal ions, there was still high fluorescence intensity and no considerable quenching occurred. According to DFT and TD-DFT calculations, photo-induced electron transfer (PET) is responsible for the turn-on response produced by the chemosensor. Coordination of Pd(II) with ISBGO in fact blocks PET from imine nitrogen of 3-iminoindolin-2-one moiety to the benzene ring, which in turn leads to a turn-on response. In addition, Job's plot analysis and Benesi-Hildebrand approach suggest that ISBGO preferably forms a 1:1 complex with Pd(II) with an association constant of 1.020 × 105 M-1. Moreover, FT-IR spectroscopy and DFT study showed that amide oxygen and imine nitrogen of 3-iminoindolin-2-one moiety acted as binding sites of ISBGO. High sensitivity, fast response, great degree of sensitivity, short life time, low detection limit of 32 nM combined with high association constant (Kf) of 1.020 × 105 M-1 and increased fluorescence quantum yield (Φf) of roughly 1.5-fold in the presence of Pd (II), highlight the role of ISBGO as an excellent probe for sensing Pd(II) in aqueous solution.

Solvent Effects on the Photophysical Properties of a Donor-acceptor Based Schiff Base

In this work, a donor-acceptor substituted aromatic system ((E)-N-((E)-3-(4 (dimethylamino)phenyl) allylidene)-4-(trifluoromethyl) benzenamine (DPATB) has been synthesized and its detailed photophysics of intramolecular charge transfer process have been explored on the basis of steady state absorption, fluorescence and time resolved spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring from the donor group to the acceptor moiety in the excited state. Effect on addition of acid and pH on steady state spectral properties further reveals excited state charge transfer character. Quantum chemical calculations were performed in order to study the conformation and polarity of DPATB at their ground as well as excited electronic states. The HOMO and LUMO molecular orbital pictures are obtained at DFT level using B3LYP functional and 6-311 + g(d,p) basis set which clearly support excited state intramolecular charge transfer process. The molecular electrostatic potential maps for the optimized ground state, donor twisted and acceptor twisted geometry shed insight on the electrostatic potential and charge distribution in a system which gives information about the reacting site of the probe and nature of the reaction. In this work, detailed photophysics of excited state intramolecular charge transfer process in donor-acceptor system (DPATB) was evaluated using steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring in DPATB. Molecular orbital pictures as obtained from DFT based computational analysis reveals a significant change in the distribution of electron density upon transition from HOMO to LUMO which confirms an ICT process occurring from the donor group to the acceptor moiety in the excited state.

A simple sensitive ratiometric fluorescent probe for the detection of mercury ions in living cells and zebrafish

Mercury, as a highly toxic heavy metal, can cause very serious harm to human health and even death in severe cases. Therefore, we synthesized a novel ratiometric fluorescent probe for detecting mercury ions, with mercaptoethanol as the recognition receptor. Probe CMER could determine mercury ions in 0-1.6 μM and the detection limit is 7.6 nM. Moreover, CMER manifested a fast response for Hg²⁺ (within 5 s) and simultaneously observed that the color changed from light yellow to orange by naked eye. In addition to these preeminent spectral properties, the probe also had satisfactory bioimaging results in RAW 264.7 macrophage cells and zebrafish.

A single molecular sensor for selective and differential colorimetric/ratiometric detection of Cu2+ and Pd2+ in 100% aqueous solution

A novel salicylaldehyde bis-Schiff-base probe decorated with imidazolium ionic liquid moieties at both ends (SAS-IMIs) was designed and facilely synthesized as a colorimetric/ratiometric sensor to visually detect Cu²⁺ and Pd²⁺ in pure aqueous media. Due to the positively charged characters of its head and tail, the SAS-IMIs exhibited high water solubility with a potential advantage in minimizing the self-aggregation. More importantly, simply by varying the solution pH, colorimetric/ratiometric sensing detection of individual metal ion (Cu²⁺ or Pd²⁺) was realized without any mutual interference. Subsequently, sensitive, selective, and differential detections for Cu²⁺ (LOD: 0.080 μM) and Pd²⁺ (LOD: 0.076 μM) in 100% aqueous solutions were achieved, which proved to be applicable for real water samples. Results from density functional theory (DFT) calculations unveiled the Cu²⁺/Pd²⁺-binding properties of SAS-IMIs, which were in accordance with the experimental observations. Furthermore, a SAS-IMIs-based solid phase sensor was fabricated, which manifested satisfactory detection abilities for Cu²⁺ and Pd²⁺.

J-type aggregation and thermochromic behavior of a schiff base in solution: Role of keto-enol tautomerization

In present work, we have been investigated the thermochromism, keto-enol tautomerization, J-type aggregation and aggregation induced enhanced fluorescence (AIEF) in a single schiff base molecule (HNBzP) along with excited state intramolecular proton transfer (ESIPT) process. It is observed that keto-enol tautomerization plays an important role on thermochromism, J-type aggregation and aggregation induced enhanced fluorescence (AIEF) process in solution. From UV-Vis and fluorescence studies, J-type aggregation is observed and shows the aggregation induce enhanced fluorescence (AIEF) behaviour in solusion. The excited state intramolecular proton transfer (ESIPT) process is investigated in different solvents. Optimized geometry and electronic properties are calculated by DFT method for better understanding at molecular level.

Synthesis of novel 1,10-phenanthroline derivatives and it used as probes for sensitive detection of Zn2+ and Cd2+ metal ions - Spectroscopic and theoretical approach

A novel class of unexpected 1,10-phenanthrolinederivatives were synthesized from 2,3-dihydroacridin-4(1H)-ones with 3-aminonaphthalen-2-carboxylic acid in presence of phosphorus oxychloride at 130°C and simple perceptive emission intensity increasing assay was developed effectively to detect the very low concentrations of Zn²⁺ and Cd²⁺ ions. Emission intensity of compounds 3(a-c) directly related to the concentrations of Zn²⁺ and Cd²⁺ ions was due to metal chelating enhanced fluorescence (CHEF) effect and also its further validated by fluorescence lifetime measurement. Furthermore, the sensing mechanism for compounds 3(a-c) of Zn²⁺ and Cd²⁺ were sustained by theoretical calculations. Computational analysis results reveals that compounds 3(a-c) are more interested in Zn²⁺ ions than that of Cd²⁺ ions, while, compound 3c is more interested with Zn²⁺ and Cd²⁺ ions than those of the rest of the compounds. In addition, this proposed detection analysis has the direct application for monitoring Zn²⁺ and Cd²⁺ concentrations in tap and drinking water samples.

A triazole Schiff base-based selective and sensitive fluorescent probe for Zn²⁺: A combined experimental and theoretical study

A triazole-Schiff base, 4-(5-Chloro-2-hydroxybenzylideneamino)-1H-1,2,4-triazole-5(4H)-thione (HL), exhibits the high selectivity and sensitivity for Zn(2+) in the fluorescence spectrometry over other common metal ions, especially Cd(2+) in DMSO:H2O (1:9, v/v) solution. A 1:1 binding ratio of Zn(2+)/L for the complex has been obtained by Uv-Vis titration experiments and Job's plot with the detection limit of 51 nmol/L. The coordination mode of the complex in solution was further confirmed by density functional theory (DFT) calculations. Time-dependent density functional theory (TD-DFT) calculations indicate that a chelation-enhanced fluorescence (CHEF) effect occurs in the process of detecting Zn ion.

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.

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.

Optical chemosensors based on transmetalation of salen-based Schiff base complexes

We report our systematic studies of novel, simple, selective, and sensitive optical (both colorimetric and fluorescent) chemosensors for detecting Al(3+) based on transmetalation reactions (metal displacement or exchange reactions) of a series of K(I), Ca(II), Zn(II), Cu(II), and Pt(II) complexes containing different ligands of salen-based Schiff bases. Both the chemical structure of the salen ligand and the identity of the central metal ion have a tremendous impact on the sensing performance, which is mainly determined by the stability constant of the complex. Moreover, the selectivities of the salen-complex-based chemosensors are much better than those of the corresponding free salen ligands because of the shielding function of the filled-in metal ion in the complex. Therefore, the present work potentially provides a new and simple way to design optical probes via complex-based transmetalation reactions.

Al³⁺-induced far-red fluorescence enhancement of conjugated polymer nanoparticles and its application in live cell imaging

Fluorescent nanoparticles (NPs) for Al(3+) sensing with high selectivity were developed from a type of carbazole-based conjugated polymer with a two-dimensional donor-π bridge-acceptor (D-π-A) structure. These NPs are characterized by their small particle diameter (∼18 nm), far-red fluorescence emission (centered ∼710 nm), and Al(3+)-induced fluorescence enhancement with high selectivity owing to an Al(3+)-triggered inhibition on the intramolecular charge transfer (ICT) processes between the conjugated backbone and the pendant acceptors. This type of nanoparticle is easily suspended in aqueous solutions, indicating their practical applicability in physiological media, and their ability for intracellular Al(3+) sensing was confirmed. As compared to other types of conjugated polymer based probes showing metal ion mediated fluorescence quenching, these as-prepared NPs possess analyte-enhanced fluorescence emission, which is analytically favored in terms of sensitivity and selectivity. Fluorescence emission with wavelengths in the biological window of maximum optical transparency (∼700 to 1000 nm) is expected to impart a salient advantage for biological detection applications to these as-prepared probes. The superior features of merit of this new type of fluorescent probe, together with the validation of practicability for intracellular Al(3+) ion sensing, are indicative of their potential for application in fluorescence-based imaging and sensing, such as investigations on Al(3+)-related physiological and pathological processes.

Theoretical studies on the structural and spectroscopic properties of an iminocoumarin-based probe and its metal complexation: an implication for a fluorescence probe

To understand the sensing behaviors of molecular fluorescent probes, an N,N-di(picolyl)aminoethyl-iminocoumarin probe (L) and its complexation with metal(II) ions (ML, M = Mg, Ca, Zn, Cd and Hg) were examined by relativistic density functional theory (DFT). Four stable conformational isomers (labeled as g1, g2, a1 and a2) for each of them have been optimized, except for CaL having only three without the g2 isomer. All of these structures have been confirmed by frequency calculations. In the aqueous solution, the a2 isomer of the L probe was calculated to be the most stable, while the g1 isomer turns out to be energetically favorable upon binding with metal ions. At these isomeric geometries, the experimentally obtained absorption was well reproduced by calculations of time-dependent DFT (TD-DFT) and a conductor-like polarized continuum model (CPCM). A slight red-shifting from L (508 nm) to ML (516-528 nm) was found. This is due to the metal affinity that stabilizes the LUMOs of ML greater than the HOMOs. Singlet excited-state structures of L and ML (M = Zn, Cd and Hg) were fully optimized using the TD-DFT approach, giving more relaxed geometries than their respective ground-state ones. Their fluorescent emissions in the aqueous solution were calculated to be 543 and 551-560 nm, respectively, agreeing with experimental values of 543 nm for L and 558 nm for ZnL. The present study also presents theoretical support for a sensing mechanism of photo-induced charge transfer of the L probe that was proposed in the previous experiment.

Proton transfer assisted charge transfer phenomena in photochromic Schiff bases and effect of -NEt2 groups to the anil Schiff bases

Photochromic Schiff bases 5-diethylamino-2-[(4-diethylamino-benzylidene)-hydrazonomethyl]-phenol (DDBHP) and N,N'-bis(4-N,N-diethylaminosalisalidene) hydrazine (DEASH) with both the proton and charge transfer moieties have been synthesized, and their photophysical properties such as excited state intramolecular charge transfer (ICT) and proton transfer (ESIPT) processes have been reported on the basis of steady-state and time-resolved spectral measurement in various solvents. The ground-state six-membered intramolecular hydrogen bonding network at the proton transfer site accelerates the ESIPT process for these compounds. Both the compounds show large Stokes-shifted emission bands for proton transfer and charge transfer processes. The hydrogen bonding solvents play a crucial role in these photophysical processes. Excited-state dipole moment of DDBHP and DEASH calculated by the solvatochromic method supports the polar character of the charge transfer excited state. Introduction of -NEt(2) groups to the reported salicylaldehyde azine (SAA) Schiff base results an increase in fluorescence lifetime from femtosecond to picosecond time scale for the proton transfer process.