Coordination of mercury(II) to gold nanoparticle associated nitrotriazole towards sensitive colorimetric detection of mercuric ion with a tunable dynamic range

A simple colorimetric assay with high sensitivity, excellent selectivity and a tunable dynamic range is reported for detecting trace amounts of mercuric ion in aqueous solution based on the coordination of Hg(2+) to the gold nanoparticle (AuNP)-associated 3-nitro-1H-1,2,4-triazole (NTA). The NTA can stabilize the AuNPs against tris-induced aggregation through capping the AuNPs. In the presence of Hg(2+), the NTA is released from the AuNP surface via the formation of a NTA-Hg(2+) coordination complex, leading to the aggregation of AuNPs in tris. This detection strategy is unique in terms of high sensitivity and excellent selectivity, a tunable dynamic range, and simplicity of probe preparation. Low detection limits of 7 nM (1.4 ppb) and 50 nM (10 ppb) can be achieved by spectrophotometer and by direct visualization, respectively, under the optimized conditions. No noticeable colour changes are observed towards other metal ions (Ag(+), Zn(2+), Ni(2+), Cr(3+), Mg(2+), Cu(2+), Co(2+), Cd(2+), Pb(2+), Fe(2+)) at concentrations up to 100 μM without the need of any other masking agents. In addition, the dynamic range of the assay can be easily tuned by adjusting the amount of NTA in the NTA-AuNP probes. More importantly, the NTA-AuNP probes can be simply prepared by mixing NTA with as-synthesized citrate-capped AuNPs. This not only avoids complicated surface modifications and tedious separation processes, but also is cost-effective.     

Colorimetric detection of Cd2+ using gold nanoparticles cofunctionalized with 6-mercaptonicotinic acid and L-cysteine

We have developed a colorimetric assay for the highly sensitive and selective detection of Cd(2+) using gold nanoparticles (AuNPs) cofunctionalized with 6-mercaptonicotinic acid (MNA) and L-Cysteine (L-Cys) through the formation of an Au-S bond. In the presence of Cd(2+), the aggregation of functionalized AuNPs occurred by means of a metal-ligand interaction that led to visible color changes. Most importantly, cofunctionalized AuNPs had better responses for Cd(2+) than that functionalized by either MNA or L-Cys. Cd(2+) could be detected by the colorimetric response of AuNPs that could be detected by the naked eye or a UV-vis spectrophotometer. The absorbance ratio (A(620)/A(523)) was linear with the Cd(2+) concentration in the range of 2.0 × 10(-7) to 1.7 × 10(-6) M. Under optimum conditions (2.0 × 10(-5) M MNA, 2.0 × 10(-6) M L-Cys and 0.020 M NaCl at pH 10.0), the detection limit (3σ) of Cd(2+) could be as low as 1.0 × 10(-7) M. Interference experiments showed that Pb(2+) and Cu(2+) caused a slight interference for Cd(2+) determination while other metal ions caused no interference. The proposed method was successfully applied to determine the concentration of Cd(2+) in environmental samples (lake water).     

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+).     

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.     

Chemometric analysis of voltammetric data on metal ion binding by selenocystine

The behavior of selenocystine (SeCyst) alone or in the presence of various metal ions (Bi(3+), Cd(2+), Co(2+), Cu(2+), Cr(3+), Ni(2+), Pb(2+), and Zn(2+)) was studied using differential pulse voltammetry (DPV) over a wide pH range. Voltammetric data matrices were analyzed using chemometric tools recently developed for nonlinear data: pHfit and Gaussian Peak Adjustment (GPA). Under the experimental conditions tested, no evidence was found for the formation of metal complexes with Bi(3+), Cu(2+), Cr(3+), and Pb(2+). In contrast, SeCyst formed electroinactive complexes with Co(2+) and Ni(2+) and kinetically inert but electroactive complexes with Cd(2+) and Zn(2+). Titrations with Cd(2+), Co(2+), Ni(2+), and Zn(2+) produced data that were reasonably consistent with the formation of stable 1:1 M(SeCyst) complexes.     

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.     

Incorporation of triazole into a quinoline-rhodamine conjugate imparts iron(iii) selective complexation permitting detection at nanomolar levels

Two new rhodamine based probes 1 and 2 for the detection of Fe(3+) were synthesized and their selectivity towards Fe(3+) ions in the presence of other competitive metal ions tested. The probe 1 formed a coloured complex with Fe(3+) as well as Cu(2+) ions and revealed the lack of adequate number of coordination sites for selective complexation with Fe(3+). Incorporation of a triazole unit to the chelating moiety of 1 resulted in the probe 2, that displayed Fe(3+) selective complex formation even in the presence of other competitive metal ions like Li(+), Na(+), K(+), Cu(2+), Mg(2+), Ca(2+), Sr(2+), Cr(3+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Zn(2+), Cd(2+), Hg(2+) and Pb(2+). The observed limit of detection of Fe(3+) ions (5 × 10(-8) M) confirmed the very high sensitivity of 2. The excellent stability of 2 in physiological pH conditions, non-interference of amino acids, blood serum and bovine serum albumin (BSA) in the detection process, and the remarkable selectivity for Fe(3+) ions permitted the use of 2 in the imaging of live fibroblast cells treated with Fe(3+) ions.     

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+).     

N-1-(2-mercaptoethyl)thymine modification of gold nanoparticles: a highly selective and sensitive colorimetric chemosensor for Hg2+

An approach for mercury ions (Hg(2+)) sensing based on the Hg(2+)-induced aggregation of thymine (T)-SH-functionalized gold nanoparticles (AuNPs) has been reported. The T-SH ligands that we synthesized can easily be coupled to the surface of AuNPs through the Au-S bond and can recognize Hg(2+) with high selectivity by forming a T-Hg-T complex with strong affinity. For the T-SH-functionalized AuNPs (T-S-AuNPs) sensor, upon addition of Hg(2+), the formation of the T-Hg-T complex induces aggregation of T-S-AuNPs and results in a significant change of color and UV-Vis absorption spectra. Thus, our method can be used for the rapid, easy and reliable screening of Hg(2+) in aqueous solution, with high sensitivity (2.8 nM) and selectivity over competing analytes. The developed method is successfully applied to the sensing of Hg(2+) in real environmental samples.     

Studies on metallochromic indicators-III Hematoxylin and hematein

The use of hematoxylin and hematein as metallochromic indicators in direct EDTA titration of Zr(4+), Th(4+), Bi(3+), VO(+), Ga(3+), In(3+), Al(3+), Pb(+), Zn(2+), Mn(2+), Cd(2+), Cu(2+), Ni(2+), Co(2+), Mg(2+), and a few rare earths is described. Aluminium is titrated directly in presence of acetate buffer, lactic or glycoliic acid being used as auxiliary complexing agent. Mixtures of two metal ions can be titrated if one is Bi(3+) and the other Al(3+), Pb(2+), Zn(2+), Cu(2+), Cd(2+), La(3+), Ce(3+), Pr(3+), Nd(3+), Sm(3+), Gd(3+) or Er(3+). Aluminium alloys can be analysed via EDTA titrations, with these indicators.     

Highly sensitive and fast responsive fluorescence turn-on chemodosimeter for Cu2+ and its application in live cell imaging

A rhodamine B derivative 4 containing a highly electron-rich S atom has been synthesized as a fluorescence turn-on chemodosimeter for Cu(2+). Following Cu(2+)-promoted ring-opening, redox and hydrolysis reactions, comparable amplifications of absorption and fluorescence signals were observed upon addition of Cu(2+); this suggests that chemodosimeter 4 effectively avoided the fluorescence quenching caused by the paramagnetic nature of Cu(2+). Importantly, 4 can selectively recognize Cu(2+) in aqueous media in the presence of other trace metal ions in organisms (such as Fe(3+), Fe(2+), Cu(+), Zn(2+), Cr(3+), Mn(2+), Co(2+), and Ni(2+)), abundant cellular cations (such as Na(+), K(+), Mg(2+), and Ca(2+)), and the prevalent toxic metal ions in the environment (such as Pb(2+) and Cd(2+)) with high sensitivity (detection limit < or =10 ppb) and a rapid response time (< or =1 min). Moreover, by virtue of the chemodosimeter as fluorescent probe for Cu(2+), confocal and two-photon microscopy experiments revealed a significant increase of intracellular Cu(2+) concentration and the subcellular distribution of Cu(2+), which was internalized into the living HeLa cells upon incubation in growth medium supplemented with 50 muM CuCl(2) for 20 h.     

Label-free colorimetric sensing of cobalt(II) based on inducing aggregation of thiosulfate stabilized gold nanoparticles in the presence of ethylenediamine

As a sensitive and selective analytical technique, gold nanoparticles-based colorimetric sensing was characterized by its simplicity and cost-effectiveness. Specific methods have been extensively developed for different targets in diverse samples. In this study, a label-free method for sensing Co(2+) in aqueous solutions was described. The target was achieved by the induced aggregation of thiosulfate (S(2)O(3)(2-)) stabilized gold nanoparticles (AuNPs) in the presence of ethylenediamine (en). Co(2+) first reacted with en and formed complexes of Co(en)(3)(2+) in aqueous solutions, which was followed by the oxidation of Co(en)(3)(2+) to Co(en)(3)(3+) by dissolved oxygen. Co(en)(3)(3+) then attacked S(2)O(3)(2-) ligands adsorbed on the AuNPs' surfaces, forming positively charged (en)(2)CoS(2)O(3)(+) on the AuNPs' surfaces, which reduced the surface charges of AuNPs and induced the aggregation of AuNPs. The process was accompanied by a red-shift in the adsorption spectrum and a visible colour change from wine red to blue. Potential effects of relevant experimental conditions, including pH, concentrations of S(2)O(3)(2-) and en, and incubation time were evaluated for optimization of the method. The proposed method is sensitive (LOD = 0.0 4 μM or 2.36 ppb) and selective (by at least 100-fold over other metal ions except for Cu(2+)) toward Co(2+) with a linear range from 0.1 to 0.7 μM. The cost-effective method allows rapid and simple determination of the concentrations of Co(2+) ions in drinking water.     

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.     

A highly selective on/off fluorescence sensor for cadmium(II)

A polypyridyl ligand, 2,3,6,7,10,11-hexakis(2-pyridyl)dipyrazino[2,3-f:2',3'-h]quinoxaline (HPDQ), was found to have excellent fluorescent selectivity for Cd(2+) over many other metal ions (K(+), Na(+), Ca(2+), Mg(2+), Mn(2+), Fe(2+), Ni(2+), Co(2+), Cu(2+), Ag(+), Hg(2+), Zn(2+), and Cr(3+)) based on the intramolecular charge-transfer mechanism, which makes HPDQ a potential fluorescence sensor or probe for Cd(2+). An obvious color change between HPDQ and HPDQ + Cd(2+) can be visually observed by the naked eye. The structure of the complex HPDQ-Cd has been characterized by X-ray crystallography. Density functional theory calculation results on the HPDQ and HPDQ-Cd complexes could explain the experimental results.     

Separation study of cadmium through an emulsion liquid membrane

A study of the transport of Cd(2+) ions through a tri-ndashoctylamine(TOA)-sorbital monooleate (Span 80)-oxylene liquid membrane has been performed with varying concentrations of HCl, KI, TOA, Span 80 and NaOH in the feed, membrane and stripping solutions. Maximum transport was observed at 0.01 M KI, 0.025 M HCl, 0.015 M TOA, 3% (w/v) Span 80 and 0.025 M NaOH. With this system, cadmium could be completely separated from Zn(2+), Fe(2+), Co(2+), Ni(2+), Cr(3+) and Mn(2+). The transport mechanism of this metal ions through the membrane has been discussed.     

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.     

Separation study of cadmium through an emulsion liquid membrane using triisooctylamine as mobile carrier

A study of the transport of Cd(2+) ions through a triisooctylamine (TIOA)-sorbitan monooleate (Span 80)-dimethylbenzene liquid membrane has been performed with varying concentrations of HCl, KI, TIOA, Span 80 and NaOH in the feed, membrane and stripping solutions. Maximum transport was observed with 0.025 M HCl, 0.01 M KI, 0.02 M TIOA, 3% (w/v) Span 80 and 0.05 M NaOH. With this system cadmium could be completely separated with Cu(2+), Zn(2+), Fe(2+), Co(2+), Ni(2+), Mn(2+), Cr(3+) and Al(3+). The transport mechanism of this metal ions through the membrane has been discussed.     

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.     

Colorimetric detection of Fe3+ ions using pyrophosphate functionalized gold nanoparticles

A sensitive, selective colorimetric Fe(3+) detection method has been developed by using pyrophosphate functionalized gold nanoparticles (P(2)O(7)(4-)-AuNPs). Gold nanoparticles were prepared by reducing HAuCl(4) with sodium borohydride, in the presence of Na(4)P(2)O(7). IR spectra suggested that pyrophosphates were capped on the surface of the gold nanoparticles. Aggregation of P(2)O(7)(4-)-AuNPs was induced immediately in the presence of Fe(3+) ions, yielding a color change from pink to violet. This Fe(3+)-induced aggregation of P(2)O(7)(4-)-AuNPs was monitored using first the naked eye and then UV-vis spectroscopy with a detection limit of 5.6 μM. The P(2)O(7)(4-)-AuNPs bound by Fe(3+) showed excellent selectivity compared to other metal ions (Ca(2+), Cd(2+), Co(2+), Fe(2+), Hg(2+), K(+), Mg(2+), Mn(2+), Na(+), Ni(2+), Pb(2+), and Zn(2+)). The best detection of Fe(3+) was achieved in a pH range from 3 to 9. In addition, the P(2)O(7)(4-)-AuNPs were also used to detect Fe(3+) in lake water samples, with low interference.