Rhodamine based derivative and its zinc complex: synthesis and recognition behavior toward Hg(II)

A simple fluorescent probe, which contains rhodamine and aminoquinoline moieties, was designed and prepared for selective detection of Hg²⁺ in acetonitrile. RbQ exhibited high selectivity and sensitivity toward Hg²⁺ over other common metal ions. The recognition of RbQ toward Hg²⁺ can be detected by fluorescence spectra, absorption spectra, and even by naked eyes. The binding ratio of the RbQ–Hg²⁺ complex was found to be 1:1 according to Job plot experiment, and the limit of detection was 1.05×10⁻⁷ M. Moreover, the prepared complex RbQ–Zn²⁺ (RbQZ) could detect Hg²⁺ in a ratiometric way and showed lower limit of detection (2.95×10⁻⁸ M) than RbQ in the same condition. Finally, we also demonstrated that the aminoquinoline–zinc complex could be served as a new and effective FRET donor for rhodamine derivatives.     

A highly selective and sensitive fluorescein-based chemodosimeter for Hg2+ ions in aqueous media

A new fluorescein-based chemodosimeter (II) for Hg²⁺ ion was designed and synthesized, and it displayed excellent selective and sensitive toward Hg²⁺ ion over other commonly metal ions in aqueous media. II was a colorless, non-fluorescent compound. Upon addition of Hg²⁺ to the solution of II, the thiosemicarbazide moiety of II would undergo an irreversible desulfurization reaction to form its corresponding oxadiazole (IV), a colorful and fluorescent product. During this process, the spirocyclic ring of II was opened, causing instantaneous development of visible color and strong fluorescence emission in the range of 500-600 nm. Based on the above mechanism, a fluorogenic Hg²⁺-selective chemodosimeter was developed. The fluorescence increase is linearly with Hg²⁺ concentration up to 1.0 μmol L⁻¹ with a detection limit of 8.5 × 10⁻¹⁰ mol L⁻¹ (3σ). Compared with the rhodamine-type chemodosimeter, II is more stable in aqueous media and exhibits higher sensitivity toward Hg²⁺. The findings suggest that II will serve as a practical chemodosimeter for rapid detection of Hg²⁺ concentrations in realistic media.     

Azo 8-hydroxyquinoline benzoate as selective chromogenic chemosensor for Hg and Cu

Azo 8-hydroxyquinoline benzoate (2) was synthesized and studied to detect metal ions. Distinct color change was found for compound 2 in the presence of transition metal ions Hg²⁺ or Cu²⁺ in CH₃CN, respectively, which makes it possible for distinguishing Hg²⁺ and Cu²⁺ from other metal ions by the ‘naked eye’.     

Unexpected highly selective fluorescence ‘turn-on’ and ratiometric detection of Hg based on fluorescein platform

Methylated fluorescein 1 was explored for fluorescence ‘turn-on’ and ratiometric detection of Hg²⁺ in THF and CH₂Cl₂/MeOH (v/v=9:1), respectively, with unexpected high selectivity. In the presence of Hg²⁺, characteristic structured absorption band of 1 diminished and a new sharp band appeared at 445 nm. Meanwhile a blue shifted and enhanced emission was observed. The ratio of the fluorescence intensity at 559 and 478 nm increased linearly with [Hg²⁺], and solution color changing from yellow to cyan under irradiation at 365 nm in CH₂Cl₂/MeOH. Job plot indicated a 1:1 stoichiometry for 1-Hg²⁺ complex in solution. ¹H NMR titration and IR spectra suggested the coordination of carbonyl group in xanthene moiety to Hg²⁺, affording its spectral behavior. Compound 2 bearing two triazolyl amino esters in place of methyl group showed quite similar behavior to Hg²⁺, which indicated that substituents did not interfere with the specific binding behavior of fluorescein platform. Our work presents a new way to explore xanthene dyes as new chemosensors by modulating electron density on the xanthene ring through non-covalent interactions with carbonyl group.     

A two-photon excited luminescence of water-soluble rhodamine-platinum(II) complex: fluorescent probe specific for Hg2+ detection in live cell

The first example of cyclometalated platinum(II)-containing rhodamine probe (1) with two-photon induced luminescent properties was synthesized and investigated for mercury detection. A highly selective color change of 1, from light yellow to pink, is observed only in the presence of Hg²⁺ due to the formation of 1,3,4-oxadiazole ring in 2. This selectivity of Hg²⁺ with color changes can be observed easily by the naked-eye. Meanwhile, a remarkable turn-on and selective 20-fold fluorescent enhancement of 1 upon binding with Hg²⁺ over the other tested metal ions was observed. The water-soluble probe 1 was successfully applied in the visualizing of the site of Hg²⁺ accumulation as well as estimating of trace amounts of mercury ions in live HeLa cells by two-photon microscopy.     

Highly selective detection of Hg ion by push–pull-type purine nucleoside-based fluorescent sensor

Highly selective detection of Hg²⁺ ion has been achieved using the push–pull-type purine nucleoside-based fluorescent sensor L1. The sensor L1 incorporating aza-18-crown-6 at C6 position of purine nucleoside, is highly sensitive and selective toward Hg²⁺ ion in CH₃CN–H₂O mixture (92/8, v/v). The detection limit for the fluorescent sensor L1 toward Hg²⁺ ion is 7.8 × 10⁻⁸.     

New photochromic chemosensors for Hg and F

Two new chemosensors (1a and 1b) based on photochromic dithienylcyclopentene were designed and synthesized, and their spectral behaviors toward various metal ions and anions were investigated in detail. Compounds show excellent optical properties and distinguish Hg²⁺ and F⁻ in CH₃CN. Job’s plot reveals that the presence of Hg²⁺ induces the formation of a 1:1 complex between 1a or 1b and Hg²⁺. From the spectral responses and ¹H NMR analysis, the deprotonation of the thioamide protons is proposed to explain the sensing mechanism for 1a and 1b toward F⁻. It is found that 1a and 1b exhibit ring-opening and ring-closing photoisomerization with UV-vis light irradiation. Furthermore, their photochromic properties can be modulated by Hg²⁺ and F⁻ ions. Moreover, 1a and 1b in photostationary states become promising sensors for Hg²⁺ and F⁻ with high selectivity.     

A logic gate-based fluorogenic probe for Hg detection and its applications in cellular imaging

A new colorimetric and fluorogenic probe (RN₃) based on rhodamine-B has been successfully designed and synthesized. It displays a selective response to Hg²⁺ in the aqueous buffer solution over the other competing metals. Upon addition of Hg²⁺, the solution of RN₃ exhibits a ‘naked eye’ observable color change from colorless to red and an intensive fluorescence with about 105-fold enhancement. The changes in the color and fluorescence are ascribed to the ring-opening of spirolactam in rhodamine fluorophore, which is induced by a binding of the constructed receptor to Hg²⁺ with the association and dissociation constants of 0.22 × 10⁵ M⁻¹ and 25.2 μM, respectively. The Job's plot experiment determines a 1:1 binding stoichiometry between RN₃ and Hg²⁺. The resultant “turn-on” fluorescence in buffer solution, allows the application of a method to determine Hg²⁺ levels in the range of 4.0–15.0 μM, with the limit of detection (LOD) calculated at 60.7 nM (3σ/slope). In addition, the fluorescence ‘turn-off’ and color ‘fading-out’ happen to the mixture of RN₃-Hg²⁺ by further addition of I⁻ or S²⁻. The reversible switching cycles of fluorescence intensity upon alternate additions of Hg²⁺ and S²⁻ demonstrate that RN₃ can perform as an INHIBIT logic gate. Furthermore, the potential of RN₃ as a fluorescent probe has been demonstrated for cellular imaging.     

A specific and ratiometric chemosensor for Hg based on triazole coupled ortho-methoxyphenylazocalix[4]arene

Calix[4]arene 3, which contains two distal triazole groups on the lower rim and two distal o-methoxyphenylazo groups on the upper rim, was synthesized and found to be a specific and ratiometric sensor for Hg²⁺ in a polar protic solvent. A series of o-methoxyphenylazo derivatives (3, 4, 5, 7, and 9) were synthesized, which proved that the lower-rim triazoles and the hydroxyl azophenol(s) were the major ligands for metal ion binding. Though analogues 4 and 10 showed some sensitivity for Hg²⁺, compound 3 was the only ratiometric chemosensor for Hg²⁺ among the series of azocalix[4]arenes synthesized in this work. The formation of 3·Hg²⁺ complex was supported by UV/vis and NMR titration studies and Mass spectrometry. Based on the symmetrical features of NMR spectra of 3·Hg²⁺, the complex is believed to be symmetrical with respect to the calix[4]arene cavity. Furthermore, the complex was determined to be 1:1 binding stoichiometry by Job’s plot, and the association constant was determined to be 4.02×10³ M⁻¹ using Benesi-Hildebrand plot.     

A squaraine-based chemosensor for Hg and Pb

A novel unsymmetrical cationic squaraine dye (USqH⁺), absorbing in the near-infrared region (600-850 nm), was synthesized by reacting a semi-squaric acid derivative 3-[4-(N,N-dioctylamino)phenyl]-4-hydroxycyclobutene-1,2-dione with the squarylium dye, bis(3-methylbenzothiazol-2-ylidene)squaraine. Addition of micromolar amounts of Hg²⁺ and Pb²⁺ to solutions of USqH⁺ in dichloromethane brings about significant changes in its absorption spectrum, resulting in a change in colour of the solution from green to pink. The absorption spectrum of the dye is relatively insensitive to other environmentally relevant metal ions, making it possible to visually detect Hg²⁺ and Pb²⁺ in the presence of other metal ions. Molecular modeling of the complexes at the density functional theory (DFT) and semiempirical PM3 levels strongly suggests that the oxygen atoms are the most likely choice for cation binding. The calculations also indicate a high affinity of Hg²⁺ and Pb²⁺ toward the formation of 2:1 complexes, which is in good agreement with the experimental results.     

Rhodamine-labelled new architecture for dual sensing of Co and Hg ions

A new rhodamine-based chemosensor 1 has been designed and synthesized. The receptor selectively recognizes Co²⁺ and Hg²⁺ ions in CH₃CN/water (4:1, v/v; 10 μM tris HCl buffer, pH 6.8) by showing different extents of change in emission. The disappearance of colour of mercury-ensemble of 1 followed by appearance of distinct bluish colour under UV illumination upon addition of l-cysteine distinguishes Hg²⁺ from Co²⁺ ions. The receptor shows in vitro detection of both the ions in human cervical cancer (HeLa) cells.     

Hg- and Cu-selective fluoroionophoric behaviors of a dioxocyclam derivative bearing anthrylacetamide moieties

New dioxocyclam derivatives bearing two anthracene fluorophores were prepared, and their fluoroionophoric properties toward transition metal ions were investigated. Chemosensor 2 having anthrylacetamide moieties exhibited pronounced Hg²⁺- and Cu²⁺-selective fluoroionophoric properties in aqueous acetonitrile solution over other representative transition metal ions, as well as alkali and alkaline earth metal ions. Chemosensor 2 also exhibited Hg²⁺ and Cu²⁺ selectivity under competitive conditions in the presence of physiologically and environmentally important metal ions. The detection limits for the sensing of Hg²⁺ and Cu²⁺ ions were 7.8 × 10⁻⁶ and 1.5 × 10⁻⁶ M, respectively, in aqueous 95% acetonitrile solution.     

Linear Schiff-base fluorescence probe with aggregation-induced emission characteristics for Al detection and its application in live cell imaging

A simple Schiff-base derivative with salicylaldehyde moieties as fluorescent probe 1 was reported by aggregation-induced emission (AIE) characterization for the detection of metal ions. Spectral analysis revealed that probe 1 was highly selective and sensitive to Al³⁺. The probe 1 was also subject to minimal interference from other common competitive metal ions. The detection limit of Al³⁺ was 0.4 μM, which is considerably lower than the World Health Organization standard (7.41 μM), and the acceptable level of Al³⁺ (1.85 μM) in drinking water. The Job's plot and the results of ¹H-NMR and FT-IR analyses indicated that the binding stoichiometry ratio of probe 1 to Al³⁺ was 1:2. Probe 1 demonstrated a fluorescence-enhanced response upon binding with Al³⁺ based on AIE characterization. This response was due to the restricted molecular rotation and increased rigidity of the molecular assembly. Probe 1 exhibited good biocompatibility, and Al³⁺ was detected in live cells. Therefore, probe 1 is a promising fluorescence probe for Al³⁺ detection in the environment.     

Exploiting the deprotonation mechanism for the design of ratiometric and colorimetric Zn fluorescent chemosensor with a large red-shift in emission

The design, synthesis, and photophysical evaluation of a new naphthalimide-based fluorescent chemosensor, N-butyl-4-[di-(2-picolyl)amino]-5-(2-picolyl)amino-1,8-naphthalimide (1), were described for the detection of Zn²⁺ in aqueous acetonitrile solution at pH 7.0. Probe 1 showed absorption at 451 nm and a strong fluorescence emission at 537 nm (Φ_F=0.33). The capture of Zn²⁺ by the receptor resulted in the deprotonation of the secondary amine conjugated to 1,8-naphthalimide so that the electron-donating ability of the N atom would be greatly enhanced; thus probe 1 showed a 56 nm red-shift in absorption (507 nm) and fluorescence spectra (593 nm, Φ_F=0.14), respectively, from which one could sense Zn²⁺ ratiometrically and colorimetrically. The deprotonated complex, [(1-H)/Zn]⁺, was calculated at m/z 619.1800 and measured at m/z 618.9890. In contrast to these results, the emission of 1 was thoroughly quenched by Cu²⁺, Co²⁺, and Ni²⁺. The addition of other metal ions such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Al³⁺, Cd²⁺, Hg²⁺, Ag⁺, and Pb²⁺ produced a nominal change in the optical properties of 1 due to their low affinity to probe 1. This means that probe 1 has a very high fluorescent imaging selectivity to Zn²⁺ among metal ions.     

A triphenylamine based multi-analyte chemosensor for Hg and Cu ions in MeCN/H2O

A novel triphenylamine based oxidative chemosensor TOC was synthesized. The chromogenic and fluorogenic behaviors of TOC towards Hg²⁺ and Cu²⁺ ions in a binary mixture of MeCN/H₂O (9/1) were dramatically different. TOC displays colorimetric ‘naked eye’ recognition of Hg²⁺ and fluorogenic ‘turn on’ response towards Cu²⁺ via a unique cyclization reaction using two different detection modes. Moreover, TOCAZOL obtained from the oxidative cyclization reaction of TOC with Cu(ClO₄)₂ can be used as a selective fluorescent sensor toward Hg²⁺ ion.     

A highly selective ratiometric chemosensor for Hg based on the anthraquinone derivative with urea groups

Anthraquinone derivatives with different substituents 1-3 were synthesized by introducing the urea group. Their cations' binding properties were investigated by UV-vis absorption spectroscopy. Compared with 1 and 2, 3 with electron-withdrawing group (-NO₂) showed a remarkable absorption change for Hg²⁺ over the other metal ions. The anthraquinone moiety and the N-H fragment of the urea moiety played key roles in sensing Hg²⁺. The different acidity of N-H fragments of the urea moiety, caused by electron push-pull properties of the substituents on the phenyl para position, is the main reason for recognition.     

Luminophore-immobilized mesoporous silica for selective Hg sensing

Novel mesoporous silica-immobilized rhodamine (MSIR) and silica particle-immobilized rhodamine (SPIR) anchored by a tren (N(CH₂CH₂NH₂)₃) were synthesized. The binding and adsorption abilities of both MSIR and SPIR for metal cations were investigated with fluorophotometry and ion chromatography, respectively. Both MSIR and SPIR show selectivity for Hg²⁺ ion over other metal cations because the Hg²⁺ ion selectively induces a ring opening of the rhodamine fluorophores. The sensitivity of the MSIR for Hg²⁺ ion is greater than that of the SPIR and the MSIR adsorbs 70% of Hg²⁺ ion while the SPIR does only 40%. The MSIR can be also easily recovered by treatment of a solution of TBA⁺OH⁻. For the application of Hg²⁺ detection in the environmental field, the MSIR-coated glass plate is also developed and exhibits an excellent function in visual and fluorescence changes with Hg²⁺ ion.     

Photoactive chemosensors 4: a Cu protein cavity mimicking fluorescent chemosensor for selective Cu recognition

Fluorescent chemosensor 3 can sense Cu²⁺ ions (1-8 μM) even in the presence of elevated levels of Ni²⁺, Cd²⁺, Zn²⁺, Hg²⁺, Ag⁺ and Pb²⁺ (5000 μM). 3 can also analyze for Ag⁺ ions (50-500 μM) in the presence of Ni²⁺, Cd²⁺, Zn²⁺, Hg²⁺ and Pb²⁺ (5000 μM) but Cu²⁺ strongly interferes.     

A chemosensor showing discriminating fluorescent response for highly selective and nanomolar detection of Cu and Zn and its application in molecular logic gate

A fluorescent based receptor (4Z)-4-(4-diethylamino)-2-hydroxybenzylidene amino)-1,2dihydro-1,5-dimethyl-2-phenylpyrazol-3-one (receptor 3) was developed for the highly selective and sensitive detection of Cu²⁺ and Zn²⁺ in semi-aqueous system. The fluorescence of receptor 3 was enhanced and quenched, respectively, with the addition of Zn²⁺ and Cu²⁺ ions over other surveyed cations. The receptor formed host-guest complexes in 1:1 stoichiometry with the detection limit of 5 nM and 15 nM for Cu²⁺ and Zn²⁺ ions, respectively. Further, we have effectively utilized the two metal ions (Cu²⁺ and Zn²⁺) as chemical inputs for the manufacture of INHIBIT type logic gate at molecular level using the fluorescence responses of receptor 3 at 450 nm.     

New fluoro- and chromogenic chemosensors for the dual-channel detection of Hg and F

Two novel thioxanthone-based compounds 1a and 1b which possess thiosemicarbazone as binding site were developed for the dual-channel detection of Hg²⁺ and F⁻. Upon treatment with Hg²⁺ and F⁻ separately, the probes’ fluorescence is effectively quenched, meanwhile 1a and 1b show evident naked-eye color variations after addition of F⁻: colorless to yellow. They both exhibit specific sensitivity and selectivity for Hg²⁺ or F⁻ over other examined ions in tetrahydrofuran (THF) solution.