A medium-controlled fluorescence dual-responsive probe for Cu2+ and Hg2+ in aqueous solutions

In this study, we report a histidine-based fluorescence probe for Cu(2+) and Hg(2+), in which the amino group and imino group were modified by two common protective groups, 9-fluorenylmethoxycarbonyl and trityl group, respectively. In a water/methanol mixed solution, the probe displayed a selective fluorescence "turn-off" response to Cu(2+) when the ratio of CH(3)OH/H(2)O was higher than 1:1. Specifically, when the solvent is changed to 1:1 methanol/water, the 304 nm fluorescence peak is enhanced, while the 317 nm peak is weakened, upon addition of either Cu(2+) or Hg(2+) ions. The mechanism for such distinct responses of the probe to Cu(2+) and Hg(2+) was further clarified by using NMR and molecular simulation. The experiment results indicated that the polarity of solvent could influence the coordination mode of 1 with Cu(2+) and Hg(2+), and control the fluorescence response as a "turn-off" or ratiometric probe.     

2,1,3-Benzoxadiazole-based selective chromogenic chemosensor for rapid naked-eye detection of Hg2+ and Cu2+

We report a simple twisted intramolecular charge transfer (TICT) chromogenic chemosensor for rapid and selective detection of Hg(2+) and Cu(2+). The sensor was composed of an electron-acceptor 4-fluoro moiety and an electron-donor 7-mercapto-2,1,3-benzoxadiazole species where the S together with the 1-N provided the soft binding unit. Upon Hg(2+) and Cu(2+) complexation, remarkable but different absorbance spectra shifts were obtained in CH(3)CN-H(2)O mixed buffer solution at pH 7.6, which can be easily used for naked-eye detection. The sensor formed a stable 2:1 complex with Cu(2+), and both 2:1 and 3:1 complexes with Hg(2+). While alkali-, alkaline earth- and other heavy and transition metal ions such as Na(+), Mg(2+), Mn(2+), Co(2+), Ni(2+), Ag(+), Zn(2+), Pb(2+) and Cd(2+) did not cause any significant spectral changes of the sensor. This finding is not only a supplement to the detecting methods for Hg(2+) and Cu(2+), but also adds new merits to the chemistry of 4,7-substituted 2,1,3-benzoxadiazoles.     

Single sensor for two metal ions: colorimetric recognition of Cu2+ and fluorescent recognition of Hg2+

The first novel rhodamine B based sensor, rhodamine B hydrazide methyl 5-formyl-1H-pyrrole-2-carboxylate Schiff base (2) capable of detecting both Cu(2+) and Hg(2+) using two different detection modes has been designed and synthesized. The metal ion induced optical changes of 2 were investigated in MeOH:H(2)O (3:1) HEPES buffered solution at pH 7.4. Sensor 2 exhibits selective colorimetric recognition of Cu(2+) and fluorogenic recognition of Hg(2+) with UV-vis and fluorescence spectroscopy, respectively. Moreover, both of the Cu(2+) and Hg(2+) recognition processes are proven to be hardly influenced by other coexisting metal ions.     

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.     

Bis(N-methylindolyl)methane-based chemical probes for Hg2+ and Cu2+ and molecular IMPLICATION gate operating in fluorescence mode

Bis(N-methylindolyl)methane derivatives behave as chemosensors for Hg(2+) and Cu(2+) ions relying on the absorption as well as emission changes. Rarely reported IMPLICATION logic gate operating in the fluorescence mode has been developed with respect to the emission band of one of the derivatives as output, with Hg(2+) and I(-) as inputs.     

Indole-based chemosensor for Hg2+ and Cu2+ ions: applications in molecular switches and live cell imaging

An indole based "ratiometric" and "turn-off" tris(N-methylindolyl)methane based chemosensor depicting a contrasting fluorescent behavior towards Hg(2+) and Cu(2+) ions, exhibited NOR and YES logic functions, and also imaged intracellular Hg(2+) in cervix cancer (HeLa) cells.     

Dual-mode unsymmetrical squaraine-based sensor for selective detection of Hg2+ in aqueous media

A novel chemosensor based on unsymmetrical squaraine dye (USQ-1) for the selective detection of Hg(2+) in aqueous media is described. USQ-1 in combination with metal ions shows dual chromogenic and "turn-on" fluorogenic response selectivity toward Hg(2+) as compared to Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ba(2+), Al(3+), Cu(2+), Cd(2+), Mn(2+), Fe(3+), Ag(+), Pb(2+), Zn(2+), Ni(2+) and Co(2+) due to the Hg(2+)-induced deaggregation of the dye molecule. A recognition mechanism based on the binding mode is proposed based on the absorption and fluorescence changes, (1)H NMR titration experiments, ESI-MS study, and theoretical calculations.     

Exploring the emissive properties of new azacrown compounds bearing aryl, furyl, or thienyl moieties: a special case of chelation enhancement of fluorescence upon interaction with Ca(2+), Cu(2+), or Ni(2+)

Three new compounds bearing furyl, aryl, or thienyl moieties linked to an imidazo-crown ether system (1, 2, and 3) were synthesized and fully characterized by elemental analysis, infrared, UV-vis absorption, and emission spectroscopy, X-ray crystal diffraction, and MALDI-TOF-MS spectrometry. The interaction toward metal ions (Ca(2+), Cu(2+), Ni(2+), and Hg(2+)) and F(-) has been explored in solution by absorption and fluorescence spectroscopy. Mononuclear and binuclear metal complexes using Cu(2+) or Hg(2+) as metal centers have been synthesized and characterized. Compounds 2 and 3 show a noticeable enhancement of the fluorescence intensity in the presence of Ca(2+) and Cu(2+) ions. Moreover compound 3 presents a dual sensory detection way by modification of the fluorimetric and colorimetric properties in the presence of Cu(2+) or Hg(2+). EPR studies in frozen solution and in microcrystalline state of the dinuclear Cu(II)3 complex revealed the presence of an unique Cu(2+) type.     

Chromofluorescent indicator for intracellular Zn2+/Hg2+ dynamic exchange

The fluorescence of NABQ increases remarkably in the presence of Zn(2+) and is quenched by Hg(2+). As shown by confocal imaging, NABQ-Zn(2+) can penetrate cells, where the bound Zn(2+) is exchanged for Hg(2+). This results in the concomitant export of Hg(2+) from the cells, showing that NABQ can act as a Zn(2+) carrier and as a Hg(2+) extracting agent in living cells.     

A chelation enhanced selective fluorescence sensing of Hg2+ by a simple quinoline substituted tripodal amide receptor

Two tris(2-aminoethyl)amine (tren) based tripodal amide fluoroionophores, 1 and 2, functionalized with quinoline (chelating fluorophore) and naphthalene (non-chelating fluorophore) respectively, are synthesized in good yields. Fluoroionophore 1 shows a selective UV-Vis spectral shift in the case of Hg(2+) in acetonitrile among different metal ions like Li(+), Na(+), Ca(2+), Mg(2+), Cr(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), Pb(2+), and Ag(+). On the other hand, fluoroionophore 2 shows no selectivity towards any of the above metal ions in the UV-Vis study. Furthermore, 1 shows a selective chelation induced fluorescence enhancement in the presence of Hg(2+) whereas 2 shows the enhancement of fluorescence with most of the metal ions via a photoinduced charge transfer mechanism. The naked eye detection of Hg(2+) in an acetonitrile solution of 1 shows a greenish fluorescence upon UV light irradiation. The isolated Hg(2+) complex of 1, 3, shows a similar UV-Vis and fluorescence spectral output as observed from in situ spectroscopic studies of 1 in the presence of Hg(2+). Infra-red (IR) and (1)H- NMR studies also reveal the interaction of Hg(2+) with the quinoline nitrogen atoms as well as with the amide functionality.     

Hg2+-sensing system based on structures of complexes

The first example of a new Hg(2+)-sensing system based on the structures of complexes is reported. The system uses a combination of a new chiral bidentate ligand and CD spectroscopy. Significant CD spectral changes are observed when Hg(2+) is added, whereas no CD spectral changes are observed in the cases of Li(+), Na(+), K(+), Mg(2+), Ca(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Rb(+), Ag(+), Cd(2+), La(3+), Gd(3+), and Pb(2+).     

Dansyl-anthracene dyads for ratiometric fluorescence recognition of Cu2+

Dansyl-anthracene dyads 1 and 2 in CH(3)CN-H(2)O (7:3) selectively recognize Cu(2+) ions amongst alkali, alkaline earth and other heavy metal ions using both absorbance and fluorescence spectroscopy. In absorbance, the addition of Cu(2+) to the solution of dyads 1 or 2 results in appearance of broad absorption band from 200 nm to 725 nm for dyad 1 and from 200 nm to 520 nm for dyad 2. This is associated with color change from colorless to blue (for 1) and fluorescent green (for 2). This bathochromic shift of the spectrum could be assigned to internal charge transfer from sulfonamide nitrogen to anthracene moiety. In fluorescence, under similar conditions dyads 1 and 2 on addition of Cu(2+) selectively quench fluorescence due to dansyl moiety between 520-570 nm (for 1)/555-650 nm (for 2) with simultaneous fluorescence enhancement at 470 nm and 505 nm for dyads 1 and 2, respectively. Hence these dyads provide opportunity for ratiometric analysis of 1-50 μM Cu(2+). The other metal ions viz. Fe(3+), Co(2+), Ni(2+), Cd(2+), Zn(2+), Hg(2+), Ag(+), Pb(2+), Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ba(2+) do not interfere in the estimation of Cu(2+) except Cr(3+) in case of dyad 1. The coordination of dimethylamino group of dansyl unit with Cu(2+) causes quenching of fluorescence due to dansyl moiety between 520-600 nm and also restricts the photoinduced electron transfer from dimethylamino to anthracene moiety to release fluorescence between 450-510 nm. This simultaneous quenching and release of fluorescence respectively due to dansyl and anthracene moieties emulates into Cu(2+) induced ratiometric change.     

Highly sensitive, selective, and rapid fluorescence Hg2+ sensor based on DNA duplexes of poly(dT) and graphene oxide

Coupling T base with Hg(2+) to form stable T-Hg(2+)-T complexes represents a new direction in detection of Hg(2+). Here a graphene oxide (GO)-based fluorescence Hg(2+) analysis using DNA duplexes of poly(dT) that allows rapid, sensitive, and selective detection is first reported. The Hg(2+)-induced T(15)-(Hg(2+))(n)-T(15) duplexes make T(15) unable to hybridize with its complementary A(15) labelled with 6'-carboxyfluorescein (FAM-A(15)), which has low fluorescence in the presence of GO. On the contrary, when T(15) hybridizes with FAM-A(15) to form double-stranded DNA because of the absence of Hg(2+), the fluorescence largely remains in the presence of GO. A linear range from 10 nM to 2.0 μM (R(2) = 0.9963) and a detection limit of 0.5 nM for Hg(2+) were obtained under optimal experimental conditions. Other metal ions, such as Al(3+), Ag(+), Ca(2+), Ba(2+), Mg(2+), Zn(2+), Mn(2+), Co(2+), Pb(2+), Ni(2+), Cu(2+), Cd(2+), Cr(3+), Fe(2+), and Fe(3+), had no significant effect on Hg(2+) detection. Moreover, the sensing system was used for the determination of Hg(2+) in river water samples with satisfactory results.     

Development and applications of fluorescent indicators for Mg2+ and Zn2+

In a study of the spectroscopic behavior of two Schiff base derivatives, salicylaldehyde salicylhydrazone (1) and salicylaldehyde benzoylhydrazone (2), Schiff base 1 has high selectivity for Zn(2+) ion not only in abiotic systems but also in living cells. The ion selectivity of 1 for Zn(2+) can be switched for Mg(2+) by swapping the solvent from ethanol-water to DMF (N,N-dimethylformamide)-water mixtures. Imine 2 is a good fluorescent probe for Zn(2+) in ethanol-water media. Many other ions tested, such as Li(+), Na(+), Al(3+), K(+), Ca(2+), Cr(3+), Mn(2+), Fe(3+), Co(2+), Ni(2+), Cu(2+), Ag(+), Cd(2+), Sn(2+), Ba(2+), Hg(2+), and Pb(2+), failed to induce any spectral change in various solvents. The selectivity mechanism of 1 and 2 for metal ions is based on a combinational effect of proton transfer (ESPT), C═N isomerization, and chelation-enhanced fluorescence (CHEF). The coordination modes of the complexes were investigated.     

Gold nanoparticles generated through "green route" bind Hg2+ with a concomitant blue shift in plasmon absorption peak

We discuss here a quick, simple, economic and ecofriendly method through a completely green route for the selective detection of Hg(2+) in aqueous samples. Here we exploited the ability of chitosan to generate gold nanoparticles and subsequently to act as a stabilizer for the formed nanoparticles. When chitosan stabilized gold nanoparticles (CH-Au NPs) are interacted with Hg(2+) a blue shift for its localized surface plasmon resonance absorbance (LSPR) band is observed. The blue shift is reasoned to be due to the formation of a thin layer of mercury over gold. A concentration as low as 0.01 ppm to a maximum of 100 ppm Hg(2+) can be detected based on this blue shift of the CH-Au NPs. While all other reported methods demand complex reaction steps and costly chemicals, the method we reported here is a simple, rapid and selective approach for the detection of Hg(2+). Our results also show that the CH-Au NPs have excellent selectivity to Hg(2+) over common cations namely, Pb(2+), Cd(2+), Mn(2+), Fe(2+), Ag(1+), Ce(4+), Ni(2+), and Cu(2+).     

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.     

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.     

A squaraine-based colorimetric and "turn on" fluorescent sensor for selective detection of Hg2+ in an aqueous medium

A novel squaraine-based chemosensor SQ-1 has been synthesized, and its sensing behavior toward various metal ions was investigated by UV-vis and fluorescence spectroscopies. In AcOH-H(2)O (40:60, v/v) solution, Hg(2+) ions coordinate with SQ-1 causing a deaggregation which induces a visual color and absorption spectral changes as well as strong fluorescence. In contrast, the addition of other metals (e.g., Pb(2+), Cd(2+), Cu(2+), Zn(2+), Al(3+), Ni(2+), Co(2+), Fe(3+), Ca(2+), K(+), Mg(2+), Na(+), and Ag(+)) does not induce these changes at all. Thus SQ-1 is a specific Hg(2+) sensing agent due to the inducing deaggregation of the dye molecule by Hg(2+).     

Logic gates for multiplexed analysis of Hg2+ and Ag+

The design of devices with multiple functions, simple handling procedures and sufficient sensitivity has drawn great interests in the field of analysis. Metal-nucleotide based pairs, such as T-Hg(2+)-T and C-Ag(+)-C complexes accompanied by SYBR Green I (SG), are used to selectively bind duplex-strand DNA by observing a bright fluorescence signal in this work, thus yielding a simple method for the rapid detection of Hg(2+) and Ag(+) without a complex labeling process. Based on this principle, 'OR' and 'AND' logic gates for the multiplexed analysis of Hg(2+) and Ag(+) were developed, and their practical application for the detection of Hg(2+) and Ag(+) in drinking water was reported.     

A 1,8-naphthyridine-based fluorescent chemodosimeter for the rapid detection of Zn2+ and Cu2+

A novel fluorescent chemodosimeter based on 1,8-naphthyridine exhibits high selectivity to Zn(2+) and Cu(2+). When 1-(7-acetamino-1,8-naphthyridyl)-2-(6-diacetaminopyridyl)ethene was mixed with CuCl2, hydrolysis of the acetamino group catalyzed by Cu(2+) complex was first observed. Resulting from coordination and hydrolysis catalyzed by the corresponding complex of Zn(2+) or Cu(2+), the highly effective fluorescent detection of Zn(2+) and Cu(2+) is realized with Zn(2+)-selective dual-emission and Cu(2+)-selective ON-OFF behavior.