Spectral characterization of a newly synthesized fluorescent semicarbazone derivative and its usage as a selective fiber optic sensor for copper(II)

In this work photoluminescent properties of highly Cu(2+) selective organic fluoroionophore, semicarbazone derivative; bis(naphtho[2,1-b]furan-2-yl)methanone semicarbazone (BNF) was investigated in different solvents (dichloromethane, tetrahydrofuran, toluene and ethanol) and in polymer matrices of polyvinylchloride (PVC) and ethyl cellulose (EC) by absorption and emission spectrometry. The BNF derivative displayed enhanced fluorescence emission quantum yield, Q(f)=6.1 x 10(-2) and molar extinction coefficient, epsilon=29,000+/-65 cm(-1)M(-1) in immobilized PVC matrix, compared to 2.6 x 10(-3) and 24,573+/-115 in ethanol solution. The offered sensor exhibited remarkable fluorescence intensity quenching upon exposure to Cu(2+) ions at pH 4.0 in the concentration range of 1.0 x 10(-9) to 3.0 x 10(-4)M [Cu(2+)] while the effects of the responding ions (Ca(2+), Hg(+), Pb(2+), Al(3+), Cr(3+), Mn(2+), Mg(2+), Sn(2+), Cd(2+), Co(2+) and Ni(2+)) were less pronounced.     

Evaluation of a carbon paste electrode modified with organofunctionalised SBA-15 nanostructured silica in the simultaneous determination of divalent lead, copper and mercury ions

The performance of a carbon paste electrode (CPE) modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol in the simultaneous determination of Pb(II), Cu(II) and Hg(II) ions in natural water and sugar cane spirit (cacha?a) is described. Pb(II), Cu(II) and Hg(II) were pre-concentrated on the surface of the modified electrode by complexing with 2-benzothiazolethiol and reduced at a negative potential (-0.80 V). Then the reduced products were oxidised by DPASV procedure. The fact that three stripping peaks appeared on the voltammograms at the potentials of -0.48 V (Pb2+), -0.03 V (Cu2+) and +0.36 V (Hg2+) in relation to the SCE, demonstrates the possibility of simultaneous determination of Pb2+, Cu2+ and Hg2+. The best results were obtained under the following optimised conditions: 100 mV pulse amplitude, 3 min accumulation time, 25 mV s(-1) scan rate in phosphate solution pH 3.0. Using such parameters, calibration graphs were linear in the concentration ranges of 3.00-70.0 x 10(-7) mol L(-1) (Pb2+), 8.00-100.0 x 10(-7) mol L(-1) (Cu2+) and 2.00-10.0 x 10(-6) mol L(-1) (Hg2+). Detection limits of 4.0 x 10(-8) mol L(-1) (Pb2+), 2.0 x 10(-7) mol L(-1) (Cu2+) and 4.0 x 10(-7) mol L(-1) (Hg2+) were obtained at the signal noise ratio (SNR) of 3. The results indicate that this electrode is sensitive and effective for simultaneous determination of Pb2+, Cu2+ and Hg2+ in the analysed samples.     

Nanoparticles as scaffolds for FRET-based ratiometric detection of mercury ions in water with QDs as donors

The quenching of quantum dots' emission by some analytes (Hg(2+), Pb(2+), etc.) has long been hindering the fabrication of QD-based 'turn-on' or ratiometric fluorescent sensors for these analytes. In this study, we demonstrate a facile solution for constructing a robust FRET-based ratiometric sensor for Hg(2+) detection in water with CdTe QDs as the donor. By using the reverse microemulsion approach, CdTe QDs were first embedded into nanosized silica particles, forming the QDs/silica cores, a positively charged ultrathin spacer layer was then deposited on each QDs/silica core, followed by the coating of a mercury ion probe on the particle surfaces. The resultant multilayered QDs/silica composite nanoparticles are dispersible in HEPES buffered water; and in the presence of mercury ions, the QDs inside the nanoparticles will not be quenched by mercury ions due to the existence of the positively charged spacer layer, but can transfer their excited energy to the acceptors (probe/Hg(2+) complex), thus achieving the FRET-based ratiometric sensing for mercury ions in totally aqueous media. With its detection limit of 260 nM, this QD-based sensor exhibits high selectivity toward mercury ion and can be used in a wide pH range. This strategy may be used to construct QDs-based ratiometric assays for other ions which quench the emission of QDs.     

Use of surface-modified CdTe quantum dots as fluorescent probes in sensing mercury (II)

Thioglycolic acid (TGA)-capped CdTe quantum dots (QDs) were synthesized in aqueous medium, and their interaction with metal cations was studied with UV-vis absorption, steady-state and time-resolved fluorescence spectra. The results demonstrated that Hg(II), Cu(II) and Ag(I) could effectively quench the QD emission based on different action mechanisms: Cu(II) and Ag(I) quenched CdTe QDs because they bound onto particle surface and facilitated non-radiative electron/hole recombination annihilation of QDs; electron transfer process between the capping ligands and Hg(II) was mainly responsible for the remarkable quenching effect of Hg(II). To prevent the approach of metal cations to QD core, the original TGA-capped CdTe QDs were further coated by denatured bovine serum albumin (dBSA). It was found that the dBSA-coated CdTe QDs could be quenched effectively by Hg(II), but Cu(II) and Ag(I) could hardly quench the QDs even at fairly higher concentration levels because the dBSA shell layer effectively prevented the binding of metal cations onto the QD core. On the basis of this fact, a simple, rapid and specific method for Hg(II) determination was proposed. Under optimal conditions, the quenched fluorescence intensity increased linearly with the concentration of Hg(II) ranging from 0.012 x 10(-6) to 1.5 x 10(-6) mol L(-1). The limit of detection for Hg(II) was 4.0 x 10(-9) mol L(-1). The developed method was successfully applied to the detection of trace Hg(II) in real samples.     

Determination of lead in phosphate ore and phosphogypsum

Laser induced breakdown spectroscopy (LIBS) is applied to analyze aqueous solutions of Li(+), Na(+), Ca(2+), Ba(2+), Pb(2+), Cd(2+), Hg(2+) and Er(3+) and suspensions of ErBa(2)Cu(3)O(x) particles (d=0.2 microm). An excimer (308 nm) pumped dye laser with laser pulse at 500 nm and pulse energy at 22+/-2 mJ is used to produce plasma in aqueous solution. Plasma emission lines of the elements are detected by a photodiode array detector. Detection limits of the metal ions are 500 mg/l for Cd(2+), 12.5 mg/l for Pb(2+), 6.8 mg/l for Ba(2+), 0.13 mg/l for Ca(2+), 13 microg/l for Li(+) and 7.5 microg/l for Na(+). No mercury and erbium emission can be detected, even at Hg(2+) and Er(3+) concentrations of up to the g/l range. On the other side, for Er in suspensions of ErBa(2)Cu(3)O(x) particles a more than 10(3) times higher sensitivity is found than for dissolved Er(3+). This result gives a possibility to analyze colloid-borne metal ions with an increased sensitivity.     

Selective colorimetric and fluorometric sensing of Cu(II) by iminocoumarin derivative in aqueous buffer

A new iminocoumarin based receptor L (C(27)H(26)N(4)OS) is synthesized with pyridyl and benzothiazolyl functionality. Synthesis of L is easy and it is isolated in good yield. L shows a selective and distinct color change from yellow to orange with Cu(2+) over Li(+), Na(+), Ca(2+), Mg(2+), Cr(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Pb(2+), and Ag(+) whereas a slight change in color is also observed in the case of Hg(2+) but L shows selective fluorescent quenching only in the presence of Cu(2+) in aqueous HEPES buffer (pH 7.0). The naked eye detection limit of Cu(2+) is determined at 2 μM whereas an emission experiment shows a lower detection limit at 200 nM. Selectivity studies of L in presence of 50 equivalents of other ion(s) by emission experiment show no interference toward the detection of 1 equivalent of Cu(2+). Both UV-Vis and fluorescence studies in the presence of Cu(2+)-salts of different counter anions with various sizes and shapes (Cl(-), ClO(4)(-), NO(3)(-), CF(3)SO(3)(-), SO(4)(2-) and BF(4)(-)) show almost similar spectral output in buffer media irrespective of the nature of the counter anions. The detailed UV-Vis and fluorescence titration experiments suggest the existence of both 1:1 and 2:1 (L:Cu(2+)) complexation stoichiometry and EPR study shows d(x(2)-y(2)) ground state of the Cu(II) centre in the complex. Furthermore the formation of a mononuclear [Cu(L)(CH(3)CN)].2ClO(4) complex and the flexible conformation of L in the solid state are confirmed by the single-crystal X-ray structural study.     

Bi-1,10-phenanthrolines and their mononuclear Ru(II) complexes

A series of four biphen (phen = 1,10-phenanthroline) ligands, 2,2'-biphen (1), 3,3'-biphen (2), 2,2'-dimethylene-3,3'-biphen (3), and 2,3'-dimethylene-3,2'-biphen (4), is prepared by coupling and Friedl?nder methodology. The corresponding mononuclear Ru(II) complexes, [Ru(1-4)(Mebpy)(2)](2+) where Mebpy = 4,4'-dimethyl-2,2'-bipyridine, are prepared. These complexes show long wavelength electronic absorptions at 441-452 nm and emissions at 622-641 nm. Metal-based oxidations occur in the range 1.18-1.21 V, and ligand-based reductions, at -1.20 to -1.30 V. The addition of Zn(2+), Cd(2+), or Hg(2+) ions results in a strong enhancement and red shift of the luminescence of complex Ru-3. Alkali and alkaline earth metal ions barely affect the luminescence of Ru-3 while transition metal ions such as Co(2+), Cu(2+), Ni(2+), and Mn(2+) lead to efficient quenching of the Ru-3 luminescence. The luminescence of Ru-2 and Ru-4 is quenched in the presence of Zn(2+) because of a conformationally induced reduction in electronic communication between the two phen halves of the ligand. The addition of Zn(2+) has only a slight effect on the luminescence of Ru-1 because of steric hindrance toward complexation.     

Ribonucleic acid as a novel ionophore for potentiometric membrane sensors of some transition metal ions

Ribonucleic acid (RNA) is used as a novel ionophore in plasticized poly(vinyl chloride) matrix membrane sensors for some transition metal ions. Membranes incorporating RNA and doped in Cu(2+), Cd(2+) and Fe(2+) display fast near-Nernstian and stable responses for these ions with cationic slopes of 31.1, 31.3 and 35.5 mV per decade, respectively, over the concentration range 10(-6)-10(-2) M and pH range 4-6.5. The cadmium RNA-based sensor shows no interference by Cu(2+), Fe(2+) Hg(2+) and Ag(+), which are known to interfere significantly with the solid-state CdS/Ag(2)S membrane electrode. The copper RNA-based sensor displays general potentiometric characteristics similar to those based on macrocyclic ionophores and organic ion exchangers and has the advantage of a better selectivity for Cu(2+) over some alkaline earth, divalent and transition metal ions. The iron RNA-based membrane sensor exhibits no interference by Hg(2+) and Zn(2+), which are known to interfere with other previously suggested sensors. The nature and composition of the RNA ionophore and its cadmium complex are examined using electrophoresis, Fourier-transform infrared analysis, elemental analysis and X-ray fluorescence techniques.     

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.     

Luminescent ruthenium(II) bipyridine-calix[4]arene complexes as receptors for lanthanide cations

The synthesis and photophysical properties of novel luminescent ruthenium(II) bipyridyl complexes containing one, two, or six lower rim acid-amide-modified calix[4]arene moieties covalently linked to the bipyridine groups are reported which are designed to coordinate and sense luminescent lanthanide ions. All the Ru-calixarene complexes synthesized in this work are able to coordinate Nd(3+), Eu(3+), and Tb(3+) ions with formation of adducts of variable stoichiometry. The absorbance changes allow the evaluation of association constants whose magnitudes depend on the nature of the complexes as well as on the nature of the lanthanide cation. Lanthanide cation complex formation affects the ruthenium luminescence which is strongly quenched by Nd(3+) ion, moderately quenched by the Eu(3+) ion, and poorly or moderately increased by the Tb(3+) ion. In the case of Nd(3+), the excitation spectra show that (i) the quenching of the Ru luminescence occurs via energy transfer and (ii) the electronic energy of the excited calixarene is not transferred to the Ru(bpy)(3) but to the neodymium cation. In the case of Tb(3+), the adduct's formation leads to an increase of the emission intensities and lifetimes. The reason for this behavior was ascribed to the electric field created around the Ru calix[4]arene complexes by the Tb(3+) ions by comparison with the Gd(3+) ion, which behaves identically and can affect ruthenium luminescence only by its charge. However, especially for compounds 1 and 3, it cannot be excluded that some contribution comes from the decrease of vibrational motions (and nonradiative processes) due to the rigidification of the structure upon Tb(3+) complexation. In the case of Eu(3+), compounds 1, 2, and 4 were quenched by the lanthanide addition but the quenching of the ruthenium luminescence is not accompanied by europium-sensitized emission which suggests that an electron-transfer mechanism is responsible for the quenching. On the contrary, compound 3 exhibits enhanced emission upon addition of Eu(3+) (as nitrate salt); it is suggested that the lack of quenching in the [3.2Eu(3+)] adduct is due to kinetic reasons because the electron-transfer quenching process is thermodynamically allowed.     

Terbium hybrid particles with spherical shape as luminescent probe for detection of Cu2+ and Fe3+ in water

A novel green emissive terbium inorganic-polymeric hybrid particle was designed and this material could detect cations in water. Polyvinyl alcohol as an amphiphilic surfactant rendered the powders dispersible in water with regular round shape (10-20 μm). Interestingly, we noticed that not only Cu(2+) (detection limit 10(-4)M) but also Fe(3+) (detection limit 10(-4) M) can give rise to emission quenching to this target material in comparison with K(+), Na(+), Fe(2+), Mn(2+), Pd(2+), Cd(2+) and Co(2+) (10(-3) mol L(-1)). We regarded that the coordination interactions between ligand and metal ions resulted in these quenching processes. Additionally, it was found that the sensing material can be repeatedly used at least 5 cycles. More importantly, this novel material demonstrated higher thermal-stability in aqueous media than pure silica hybrid material.     

Oxidative cyclization of thiosemicarbazone: an optical and turn-on fluorescent chemodosimeter for Cu(II)

A weakly fluorescent thiosemicabazone (L(1)H) was found to be a selective optical and "turn-on" fluorescent chemodosimeter for Cu(2+) ion in aqueous medium. A significant fluorescence enhancement along with change in color was only observed for Cu(2+) ion; among the other tested metal ions (viz. Na(+), K(+), Mg(2+), Ca(2+), Cr(3+), Zn(2+), Cd(2+), Hg(2+), Pb(2+), Ag(+), Ni(2+), Co(2+), Fe(3+) and Mn(2+)). The Cu(2+) selectivity resulted from an oxidative cyclization of the weak fluorescent L(1)H into highly fluorescent rigid 4,5-dihydro-5,5-dimethyl-4-(naphthalen-5-yl)-1,2,4-triazole-3-thione (L(2)). The signaling mechanism has been confirmed by independent synthesis with detail characterization of L(2).     

CdS/ZnS core-shell quantum dots capped with mercaptoacetic acid as fluorescent probes for Hg(II) ions

We have synthesised water soluble CdS/ZnS core-shell quantum dots (QDs) capped with mercaptoacetic acid (MAA). They were characterised by UV–vis absorption spectroscopy, fluorescence spectroscopy, FT-IR and transmission electron microscopy. Such QDs can be used as fluorescent probes for the determination of metal ions because they quench the fluorescence of the QDs. The QDs exhibit absorption and emission bands at 345?nm and 475?nm respectively, which is more longer wavelength compared to MAA-capped CdS QDs and obviously is the result of the larger particle size. The fluorescence intensity of CdS-based QDs is strongly enhanced by coating them with a shell of ZnS. In addition, such functionalised QDs are more sensitive to Hg(II) ions. Parameters such as pH, temperature and concentration of the QDs have been optimised. A high selectivity and sensitivity toward Hg(II) ions is obtained at pH 7.4 and a concentration of 12.0?mg of QDs per L. Under optimum conditions, the fluorescence intensity of CdS/ZnS QDs is linearly proportional to the concentration of Hg(II) in the range from 2.5 to 280?nM, with a detection limit of 2.2?nM. The effect of potentially interfering cations was examined and confirmed the high selectivity of this material.

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.     

Surface-modified CdS quantum dots as luminescent probes for sulfadiazine determination

A novel, sensitive and convenient determine technology based on the quenching of the fluorescence intensity of functionalized CdS quantum dots by sulfadiazine was proposed. Luminescent CdS semiconductor quantum dots (QDs) modified by thioglycollic acid (TGA) were synthesized with the microwave method. The modified CdS QDs are water-soluble, stable and highly luminescent. The possible mechanism for the reaction was also discussed. When sulfadiazine was added into the CdS QDs colloid solution, the surface of CdS QDs generates the electrostatic interaction in aqueous medium, which induces the quenching of fluorescence emission at 489 nm. Under optimum condition, the fluorescence intensity versus sulfadiazine concentration gave a linear response according Stern-Volmer equation with an excellent 0.9981 correlation coefficient. The linearity range of the calibration curve was 1.2 x 10(-5) to 2.13 x 10(-3) mol L(-1). The limit of detection (3delta) is 8.0 micromol L(-1). The relative standard deviation for five determinations of 0.13 x 10(-3)mol L(-1) sulfadiazine is 1.4%. The concentrations of sulfadiazine injections were determined by the proposed method with a satisfactory result.     

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.     

Selective fluorescent Hg(II) detection in aqueous solutions with a dye intermediate

A dye intermediate, 1-amino-8-naphthol-3,6-disulfonic acid sodium (ANDS) was first used to selectively recognize Hg(II) in aqueous solutions with its fluorescence being strong quenched. The fluorescence quenching of ANDS was attributed to the formation of an inclusion complex between Hg(II) and ANDS by 2:1 complex ratio (K=6.2 x 10(9)), which has been utilized as the basis of the fabrication of the Hg(II)-sensitive fluorescent chemosensor. The analytical performance characteristics of the proposed chemosensor were investigated. The sensor shows a linear response toward Hg(II) in the concentration range 2.9 x 10(-6) to 5.5 x 10(-5)M with a limit of detection of 5.3 x 10(-7)M, and a working pH range from 5.0 to 9.0. It shows excellent selectivity for Hg(II) over a large number of cations such as alkali, alkaline earth and transitional metal ions. The proposed method was utilized successfully for the detection of Hg(2+) in water samples.     

A BODIPY-based colorimetric and fluorometric chemosensor for Hg(II) ions and its application to living cell imaging

A new monostyryl boron dipyrromethene derivative (MS1) appended with two triazole units indicates the presence of Hg(2+) among other metal ions with high selectivity by color change and red emission. Upon Hg(2+) binding, the absorption band of MS1 is blue-shifted by 29 nm due to the inhibition of the intramolecular charge transfer from the nitrogen to the BODIPY, resulting in a color change from blue to purple. Significant fluorescence enhancement is observed with MS1 in the presence of Hg(2+); the metal ions Ag(+), Ca(2+), Cd(2+), Co(2+), Cu(2+), Fe(2+), Fe(3+), K(+), Mg(2+), Mn(2+), Ni(2+), Pb(2+), and Zn(2+) cause only minor changes in the fluorescence of the system. The apparent association constant (K(a)) of Hg(2+) binding in MS1 is found to be 1.864 × 10(5) M(-1). In addition, fluorescence microscopy experiments show that MS1 can be used as a fluorescent probe for detecting Hg(2+) in living cells.     

A new Schiff base system bearing two naphthalene groups as fluorescent chemodosimeter for Zn2+ ion and its logic gate behavior

1-((E)-(2-((2-nitrobenzyl)(2-((E)-(2-hydroxynaphthalen-1-yl)methyleneamino)ethyl)amino)ethylimino)methyl)naphthalen-2-ol (H(2)L), The new compound featuring two naphthalene units was synthesized and characterized. We find that H(2)L has high selectivity and sensitivity to detect Zn(2+) ion over other metal ions such as Na(+), Ag(+), Cd(2+), Co(2+), Cr(3+), Cu(2+), Hg(2+), Mn(2+), Ni(2+), Fe(3+), and the sensitivity is about 10(-7)M. The fluorescent changes of H(2)L upon the addition of cations Zn(2+) and triethylamine is utilized as an AND logic gate at the molecular level, using Zn(2+) and triethylamine as chemical inputs and the fluorescence intensity signal as output.     

Silver(I) ion selective PVC membrane based on bis-pyridine tetramide macrocycle

A PVC membrane based on bis-pyridine tetramide macrocycle exhibits a good response for silver ion over a wide concentration range (1.0x10(-1)-4.0x10(-5) mol l(-1)). The electrode has a relatively fast response time and can be used for more than 5 months without observing any divergence. High selectivity for Ag(+) ions over that of Na(+), K(+), Ca(2+), Sr(2+), Pb(2+) and Hg(2+) have been observed.