Fluorescent and colorimetric probes for mercury(II): tunable structures of electron donor and π-conjugated bridge

A new series of intramolecular-charge-transfer (ICT) molecules (compounds 1, 2, and 3) were synthesized by attaching various electron-donating thiophenes groups to a triphenylamine backbone with an aldehyde group as the electron acceptor. Based on the protection reaction between ethanethiol and aldehyde, the corresponding dithioacetals (compounds S1, S2, and S3) were prepared to serve as novel colorimetric and fluorescent chemosensors for Hg(2+) ions. Also, compound S1 was further utilized to construct the chemical-reaction-based conjugated polymer probe (PS1) towards Hg(2+) ions. In the presence of as little as 10 nM Hg(2+), compound PS1 displayed an apparent change in the fluorescent intensity. The sensing processes were revealed to be mediated by ICT, as confirmed by time-dependent DFT calculations. Furthermore, compound S1 was successfully applied to microscopic imaging for the detection of Hg(2+) in HeLa cells with ratiometric fluorescent methods.     

Rhodamine-based chemosensing monolayers on glass as a facile fluorescent "turn-on" sensing film for selective detection of Pb2+

Rhodamine-based chemosensors 1 and 2 were synthesized and self-assembled onto glass surfaces for the selective fluorescent sensing of Pb(2+). The immobilized chemosensors showed fluorescent responses that were turned-on with Pb(2+) in CH(3)CN, selectively over various metal ions. The Pb(2+)-selective fluorescent switch of the immobilized chemosensors was also reversible, allowing for repeated use for Pb(2+) detection.     

Facile synthesis of rhodamine-based highly sensitive and fast responsive colorimetric and off-on fluorescent reversible chemosensors for Hg2+: preparation of a fluorescent thin film sensor

Fluorescence-active chemosensors (L1-L4), comprising a rhodamine scaffold and a pseudo azacrown cation-binding subunit, have been proposed and characterized as a fluorescent chemosensor for Hg(2+). An on-off type fluorescent enhancement was observed by the formation of the ring-opened amide form of the rhodamine moiety, which was induced by the interactions between Hg(2+) and the chemosensor. Upon the addition of Hg(2+), an overall emission change of 350-fold was observed, and the selectivity was calculated to be 300 times higher than Cu(2+) for receptors L2-L4. A polymeric thin film can be obtained by doping poly(methyl methacrylate) or PMMA with chemosensor L2. Such a thin film sensor can be used to detect Hg(2+) with high sensitivity and can be recovered using diluted NaOH.     

Visual detection of lead(II) using a label-free DNA-based sensor and its immobilization within a monolithic hydrogel

Lead is highly toxic and its detection has attracted a lot of research interests. In recent years, DNA has been used for Pb(2+) recognition and many fluorescent sensors with low to sub-nM detection limits have been reported. These figures of merit were typically measured using a spectrophotometer that can detect nM DNA with a high signal-to-noise ratio. For visual detection, however, μM DNA or dye was required, making it difficult to detect low nM targets. We recently achieved a visual sensitivity of 10 nM Hg(2+) by immobilizing a DNA probe in a hydrogel. This was made possible because the gel was able to actively adsorb Hg(2+). In this work, we aim to test whether this method can be extended to the detection of Pb(2+). First, a new Pb(2+) sensor was designed based on a guanine-rich DNA and DNA binding dyes such as thiazole orange and SYBR Green I. The free DNA showed a detection limit of 8 nM Pb(2+) using 40 nM DNA. For visual detection in solution with 1 μM of the DNA probe, however, ～300 nM Pb(2+) was required. After immobilization in a monolithic polyacrylamide hydrogel, even 20 nM Pb(2+) could be visually detected with a sample volume of 50 mL. Therefore, sensitive detection without signal amplification was achieved. Finally, we demonstrated simultaneous detection of both Hg(2+) and Pb(2+) in the same water sample with shape encoded hydrogel sensors.     

Colorimetric assay for mercury (II) based on mercury-specific deoxyribonucleic acid-functionalized gold nanoparticles

A colorimetric nanoprobe-mercury-specific DNA-functionalized gold nanoparticles (Au-MSD) was developed for sensing Hg(2+). The new mercury-sensing concept relies on measuring changes in the inhibition of "non-crosslinking" aggregation of Au-MSD-induced by the folding of mercury-specific DNA strand through the thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination. In the absence of Hg(2+), a high concentration of MgCl(2) (50 mM) results in a rapid aggregation of Au-MSD because of the removal of charge repulsion. When Hg(2+) is present, the particles remain stable due to the folding of MSD functionalized on the particle surface. The assay enables the colorimetric detection of Hg(2+) in the concentration range of 0.1-10 μM Hg(2+) ions with a detection limit of 60 nM, and allows for the selective discrimination of Hg(2+) ions from the other competitive metal ions. Toward the goal for practical applications, the sensor was further evaluated by monitoring Hg(2+) in fish tissue samples.     

Fluorescence "turn on" chemosensors for Ag+ and Hg2+ based on tetraphenylethylene motif featuring adenine and thymine moieties

Two new tetraphenylethylene (TPE) compounds 1 and 2 bearing adenine and thymine moieties, respectively, were found to be fluorescence "turn on" chemosensors for Ag(+) and Hg(2+) by making use of the AIE feature of TPE motif and the specific binding of adenine/thymine with Ag(+)/Hg(2+).     

Dual-emission quantum dots nanocomposites bearing an internal standard and visual detection for Hg2+

A novel dual-fluorescence quantum dots (QD) nanocomposite with tuning emission wavelength and fluorescence intensity was synthesized, in which CdS and CdTe were the internal standard and probe, respectively. This nanocomposite exhibited good photobleaching and pH stability, and exhibited selective sensing for Hg(2+) with a detection limit (3SD/k) of 5.6 nM. Based on the blue background emitted by the internal standard CdS, a novel visual fluorescence detection method has been established, and can be used for the qualitative and semi-quantitative colorimetric analysis of Hg(2+).     

A highly selective turn-on colorimetric, red fluorescent sensor for detecting mobile zinc in living cells

We describe ZRL1, a turn-on colorimetric and red fluorescent zinc ion sensor. The Zn(2+)-promoted ring opening of the rhodamine spirolactam ring in ZRL1 evokes a 220-fold fluorescence turn-on response. In aqueous media, ZRL1 turn-on luminescence is highly selective for Zn(2+) ions, with no significant response to other competitive cations, including Na(+), K(+), Ca(2+), Mg(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Cd(2+), or Hg(2+). In addition to these characteristics, preliminary results indicate that ZRL1 can be delivered to living cells and can be used to monitor changes in intracellular Zn(2+) levels.     

Novel hemicyanine dye as colorimetric and fluorometric dual-modal chemosensor for mercury in water

A novel water soluble Hg(2+)-selective chemosensor 1 with hemicyanine as fluorescent reporting group and NO(2)Se(2) chelating unit as ion binding site was reported. Chemosensor 1 shows a specific Hg(2+) selectivity and discrimination between Hg(2+) and chemically similar ions in conjunction with a visible colorimetric change from red to colorless, potentially leading to both "naked-eye" and fluorometric detection of Hg(2+) cations.     

Reversible "off-on" fluorescent chemosensor for Hg2+ based on rhodamine derivative

A novel and simple fluorescent chemosensor based on rhodamine was designed and synthesized to detect Hg(2+) with high selectivity. The structure of chemosensor 1 was characterized by IR, (1)H NMR, and HRMS spectroscopies. Chemosensor 1 exhibited distinct fluorescent and colorimetric changes toward Hg(2+) in an ethanol/water (80/20, v/v) solution, which resulted in the formation of 1/Hg(2+) complex with the Hg(2+)-induced ring opening of the spirolactam ring in rhodamine. The reversibility of chemosensor 1 was verified through its spectral response toward Hg(2+) ions and TBAI (tetrabutylammonium iodide) titration experiments.     

A highly selective redox, chromogenic, and fluorescent chemosensor for Hg2+ in aqueous solution based on ferrocene-glycine bioconjugates

The synthesis, electrochemical, optical, and metal-cation-sensing properties of ferrocene-glycine conjugates C(30)H(38)O(8)N(8)Fe (2) and C(20)H(24)O(4)N(4)Fe (3) have been documented. Both compounds 2 and 3 behave as very selective redox (ΔE(1/2) = 217 mV for 2 and ΔE(1/2) = 160 mV for 3), chromogenic, and fluorescent chemosensors for Hg(2+) cations in an aqueous environment. The considerable changes in their absorption spectra are accompanied by the appearance of a new low-energy peak at 630 nm (2, ε = 1600 M(-1) cm(-1); 3, ε = 822 M(-1) cm(-1)). This is also accompanied by a strong color change from yellow to purple, which allows a prospective for the "naked eye" detection of Hg(2+) cations. These chemosensors present immense brightness and fluorescence enhancement (chelation-enhanced fluorescence = 91 for 2 and 42 for 3) following Hg(2+) coordination within the limit of detection for Hg(2+) at 7.5 parts per billion.     

Highly sensitive and selective colorimetric and off-on fluorescent probe for Cu(2+) based on rhodamine derivative

A new probe for Cu(2+) based on the Cu(2+)- induced reversible ring-opening mechanism of the rhodamine spirolactam was described. It displayed a highly selective and sensitive "turn-on" fluorescent and colorimetric response toward Cu(2+).     

Ratiometric sensing of Hg2+ based on the calix[4]arene of partial cone conformation possessing a dansyl moiety

A new fluorescent chemosensor based on the calix[4]arene of partial cone conformation possessing a dansyl moiety has been synthesized. The chemosensor demonstrates selective optical recognition of Hg(2+) and Cu(2+) in two contrasting modes. The receptor exhibited ratiometric sensing of Hg(2+) and "ON-OFF" type of fluorescence behavior in the presence of Cu(2+). The compound behaves as a fluorescent molecular switch upon chemical inputs of Hg(2+) and Cu(2+) ions.     

A new "turn-on" chemodosimeter for Hg2+: ICT fluorophore formation via Hg(2+)-induced carbaldehyde recovery from 1,3-dithiane

A novel sensitive and specific Hg(2+) chemodosimeter, derived from 1',3'-dithiane-substituted 2,1,3-benzoxadiazole, displays "turn-on" fluorescent and colorimetric responses via an Hg(2+)-triggered aldehyde recovery reaction. Its potential to monitor Hg(2+) in living organisms has been demonstrated using zebrafish larvae.     

A rhodamine-cyclen conjugate as a highly sensitive and selective fluorescent chemosensor for Hg(II)

A rhodamine-cyclen conjugate (1) behaves as a highly sensitive and selective fluorescent chemosensor for Hg(2+). The high emission selectivity is due to the formation of 1-Hg(2+) 1:2 complex leading to spirocycle opening of 1.     

Two selective fluorescent chemosensors for cadmium ions in 99% aqueous solution: the end group effect on the selectivity, DFT calculations and biological applications

Two fluorescent chemosensors for cadmium ions, 2-(2-formylquinolin-8-yloxy)-N,N-diisopropylacetamide (FQDIPA) and 2-(2-formylquinolin-8-yloxy)-N,N-diphenylacetamide (FQDPA), were first assessed in 99% aqueous solutions. The sensor FQDIPA with an end group of an aliphatic amine can recognize Cd(2+) from other metal ions more selectively and sensitively than FQDPA with that of aromatic amines, which was further demonstrated by DFT calculations that were comparable to the experimental results. It is indeed the distinction between the end groups of these chemosensors that results in the variation of the energy difference between the LUMOs and HOMOs and the interaction energies of FQDIPA·Cd(2+) and FQDPA·Cd(2+). Furthermore, the living cell image experiment could also indicate that the FQDIPA is more suitable than the FQDPA in the practical applications in biological systems.     

Visual and on-site detection of mercury(II) ions on lateral flow strips using DNA-functionalized gold nanoparticles

A test strip for detection of Hg(2+) in aqueous solution based on the DNA-functionalized gold nanoparticles (DNA-AuNPs) was developed and evaluated. When Hg(2+) ions were introduced, the biotinylated DNA(2) hybridized with thiolated DNA(1) functionalized on the AuNPs (DNA(1)-AuNPs) to form mismatch complexes through thymine-Hg(2+)-thymine (T-Hg(2+)-T) coordination. The formed mismatch complexes and excess DNA(1)-AuNPs could be captured on the test line formed by streptavidin and the control line formed by DNA(3)-BSA, respectively. Two red lines appeared due to the accumulation of AuNPs, enabling visual detection of Hg(2+) with a detection limit of about 6 nM. The assay results can be obtained within 5 min. The results show that the test strip has excellent sensitivity and selectivity for detection of Hg(2+); thus it holds a great potential for rapid, on-site and real time detection of Hg(2+).     

Rhodamine-based highly sensitive colorimetric off-on fluorescent chemosensor for Hg2+ in aqueous solution and for live cell imaging

A novel rhodamine-based highly sensitive and selective colorimetric off-on fluorescent chemosensor for Hg(2+) ions is designed and prepared by using the well-known thiospirolactam rhodamine chromophore and furfural hydrazone as signal-reporting groups. The photophysical characterization and Hg(2+)-binding properties of sensor RS1 in neutral N, N-dimethylformamide (DMF) aqueous solution are also investigated. The signal change of the chemosensor is based on a specific metal ion induced reversible ring-opening mechanism of the rhodamine spirolactam. The response of the chemosensor for Hg(2+) ions is instantaneous and reversible. And it successfully exhibits a remarkably "turn on" response toward Hg(2+) over other metal ions (even those that exist in high concentration). Moreover, this sensor is applied for in vivo imaging in Rat Schwann cells to confirm that RS1 can be used as a fluorescent probe for monitoring Hg(2+) in living cells with satisfying results, which further demonstrates its value of practical applications in environmental and biological systems.     

Selective colorimetric sensing of Cu2+ using triazolyl monoazo derivative

Although the high sensitivity, high selectivity and fast response make emission (fluorescence) based technique as one of the most promising tool for developing the chemosensors for metal ions, the past few years have witnessed a demand for the absorption based chemosensors for paramagnetic heavy metal ions, especially Cu(2+). Being paramagnetic, Cu(2+) leads to the low signal outputs ("turn-off") caused by decreased emission which may sometimes give false positive response, rendering the emission based technique less reliable for analytical purposes. Herein, we report synthesis and characterization of a hetarylazo derivative, characterized by a strong charge-transfer band which gets attenuated convincingly in the presence of Cu(2+) leading to distinct naked-eye color change (yellow to purple), and to a lesser extent in the presence of Cd(2+), Zn(2+), Co(2+), Pb(2+), Fe(2+), Ni(2+), Fe(3+) and Hg(2+) for which the naked eye sensitivity was comparatively (w.r.t. Cu(2+)) much less. No response was observed for the other metal ions including Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ba(2+), Mn(2+), Ag(+), Zn(2+), Cd(2+), Pb(2+), and lanthanides Ce(3+), La(3+), Pr(3+), Eu(3+), Nd(3+), Lu(3+), Yb(3+), Tb(3+), Sm(3+), Gd(3+). The proposed sensing mechanism has been ascribed to the stabilization of LUMO after complexation with Cu(2+) and a 1:1 stoichiometry has been deduced.     

Thiacalix[4]arene based reconfigurable molecular switches: set-reset memorized sequential device

The fluorescent chemosensors 3, 5 and 7 based on thiacalix[4]arene bearing naphthyl groups have been designed and synthesized. The optical chemosensor 3 based on a thiacalix[4]arene of cone conformation behaves as "turn-on" optical chemosensor for Fe(3+) and F(-) ions. However, chemosensors 5 and 7 based on a thiacalix[4]arene of 1,3-alternate conformation demonstrate "turn-on" optical behaviour for Hg(2+), F(-) ions (with receptor 5 as turn-on for K(+) ions also) and "turn-off" behaviour for Fe(3+) ions. The simultaneous presence of Fe(3+) and Hg(2+) or K(+) or F(-) ions results in formulation of reversible "on-off" switches. Various molecular logic gates developed in response to molecular switching between these chemical inputs have been integrated into sequential logic circuits with memory function in a feedback loop which mimics "set-reset" molecular level information processing device.