Novel inorganic ion exchange materials based on silicates; synthesis, structure and analytical applications of magneso-silicate and magnesium alumino-silicate sorbents

Two novel inorganic ion exchange materials magneso-silicate and magnesium alumino-silicate have been synthesized under identical conditions. The structure of these materials was established by chemical analysis, X-ray diffraction, thermogravemetric and differential thermal analyses, Fourier transform infrared spectroscopy and X-ray fluorescence analysis. Magneso-silicate and magnesium alumino-silicate were found to have the formulas MgSi(5.59)O(12.18).5.93H(2)O and MgAl(2.32)Si(5.2)O(14.88).18.23H(2)O, respectively. The structure of both sorbents was arranged and predict according to the ChemDraw Ultra program. The ion exchange capacities of these materials for some radionuclides and heavy metals Cs(+), Co(2+), Cd(2+), Zn(2+) and Cu(2+) were investigated and the data obtained showed that magnesium alumino-silicate has a higher capacity for these cations compared to magneso-silicate. Distribution coefficients in nitric acid medium have been evaluated to explore the separation potentiality of magneso-silicate and magnesium alumino-silicate for Cs(+), Co(2+), Cd(2+), Cu(2+), Zn(2+) and Fe(3+) ions. Sorption isotherms for all cations were investigated and the data showed the applicability of Freundlich isotherm for all cases.     

Ion-exchange of Pb2+, Cu2+, Zn2+, Cd2+, and Ni2+ ions from aqueous solution by Lewatit CNP 80

Removal of trace amounts of heavy metals can be achieved by means of selective ion-exchange processes. The newly developed resins offered a high resin capacity and faster sorption kinetics for the metal ions such as Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) ions. In the present study, the removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) ions from aqueous solutions was investigated. Experimental investigations were undertaken using the ion-exchange resin Lewatit CNP 80 (weakly acidic) and were compared with Lewatit TP 207 (weakly acidic and chelating). The optimum pH range for the ion-exchange of the above mentioned metal ions on Lewatit CNP 80 and Lewatit TP 207 were 7.0-9.0 and 4.5-5.5, respectively. The influence of pH, contact time, metal concentration and amount of ion-exchanger on the removal process was investigated. For investigations of the exchange equilibrium, different amounts of resin were contacted with a fixed volume of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) ion containing solution. The obtained sorption affinity sequence in the presented work was Ni(2+)>Cu(2+)>Cd(2+)>Zn(2+)>Pb(2+). The metal ion concentrations were measured by AAS methods. The distribution coefficient values for metal ions of 10(-3)M initial concentration at 0.1mol/L ionic strength show that the Lewatit CNP 80 was more selective for Ni(2+), Cu(2+) than it was for Cd(2+), Zn(2+) and Pb(2+). Langmuir isotherm was applicable to the ion-exchange process and its contents were calculated. The uptake of metal ions by the ion-exchange resins was reversible and thus has good potential for the removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) from aqueous solutions. The amount of sorbed metal ion per gram dry were calculated as 4.1, 4.6, 4.7, 4.8, and 4.7mequiv./g dry resin for Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+), respectively. Selectivity increased in the series: Cd(2+)>Pb(2+)>Cu(2+)>Ni(2+)>Zn(2+). The results obtained showed that Lewatit CNP 80 weakly acidic resin had shown better performance than Lewatit TP 207 resin for the removal of metals. The change of the ionic strength of the solution exerts a slight influence on the removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+). The presence of low ionic strength or low concentration of NaNO(3) does not have a significant effect on the ion-exchange of these metals by the resins. We conclude that Lewatit CNP 80 can be used for the efficient removal of Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+) from aqueous solutions.     

Synthesis and metal ion binding properties of thiaaza crown macrocycles

Two new macrocyclic ligands (3) and (4), containing nitrogen-sulfur donor atoms, were designed and synthesized in a multi-step reaction sequence. The macrocycles (3) and (4) were used in solvent extraction of metal picrates such as Ag(+), Hg(2+), Cd(2+), Zn(2+), Cu(2+), Ni(2+), Mn(2+), Co(2+), and Pb(2+) from aqueous phase to the organic phase. The metal picrate extractions were investigated at 25+/-0.1 degrees C by using UV-visible spectrometry. The extractability and selectivity of the mentioned metal picrates were evaluated according to the organic solvents. The values of the extraction constants (logK(ex)) and the complex compositions were determined for the extracted complexes.     

Adsorption of divalent heavy metal ions from water using carbon nanotube sheets

Removal of some divalent heavy metal ions (Cu(2+), Zn(2+), Pb(2+), Cd(2+), Co(2+)) from aqueous solutions using carbon nanotube (CNT) sheets was performed. CNT sheets were synthesized by chemical vapor deposition of cyclohexanol and ferrocene in nitrogen atmosphere at 750°C, and oxidized with concentrated nitric acid at room temperature and then employed as adsorbent for water treatment. Langmuir and Freundlich isotherms were used to describe the adsorption behavior of heavy metal ions by oxidized CNT sheets. The obtained results demonstrated that the oxidized CNT sheets can be used as an effective adsorbent for heavy metal ions removal from water. It was found out that kinetics of adsorption varies with initial concentration of heavy metal ions. Preference of adsorption onto the oxidized CNT sheets can be ordered as Pb(2+)>Cd(2+)>Co(2+)>Zn(2+)>Cu(2+). Using the oxidized CNT sheets, waste water treatment without CNT leakage into water is economically feasible. Therefore, CNT sheets have good potential application in environmental protection.     

A new Schiff's base ligand immobilized agarose membrane optical sensor for selective monitoring of mercury ion

A highly selective optical sensor was developed for the Hg(2+) determination by chemical immobilization of 2-[(2-sulfanylphenyl)ethanimidoyl]phenol (L), on an agarose membrane. Spectrophotometric studies of complex formation between the Schiff's base ligand L and Hg(2+), Sr(2+), Mn(2+), Cu(2+), Al(3+), Cd(2+), Zn(2+), Co(2+) and Ag(+) metal ions in methanol solution indicated a substantially larger stability constant for the mercury ion complex. Consequently, the Schiff's base L was used as an appropriate ionophore for the preparation of a selective Hg(2+) optical sensor, by its immobilization on a transparent agarose film. A distinct color change, from yellow to green-blue, was observed by contacting the sensing membrane with Hg(2+) ions at pH 4.5. The effects of pH, ionophore concentration, ionic strength and reaction time on the immobilization of L were studied. A linear relationship was observed between the membrane absorbance at 650 nm and Hg(2+) concentrations in a range from 1×10(-2) to 1×10(-5) mol L(-1) with a detection limit (3σ) of 1×10(-6) mol L(-1). No significant interference from 100 times concentrations of a number of potentially interfering ions was detected for the mercury ion determination. The optical sensor was successfully applied to the determination of mercury in amalgam alloy and spiked water samples.     

Simultaneous multi-element detection of metal ions bound to a Datura innoxia material

An on-line detection scheme has been developed for the determination of metal ion affinities for binding to a plant-based substrate. This involves monitoring the effluent of a column packed with cell-wall fragments from the plant Datura innoxia for 27 different elements simultaneously by coupling the column to an ICP emission spectrometer. Previously accepted procedures for removing native metal ions from biological materials by washing the material with a pH 2 solution were found to be insufficient for this material. Measurable amounts of Na, Mg, Al, Ca, Mn, Fe, Ni, Cr, Zn, Cd, Pb, Ba, Sr, and Si were all detected in an effluent from the introduction of 1.0M HCl following washing the material in a pH 2 solution. Metal ion breakthrough curves for Cd(2+), Zn(2+), Ni(2+), Cu(2+), and Pb(2+) were found to exhibit an affinity order of Pb(2+)>Cu(2+)>Zn(2+) congruent with Cd(2+)>Ni(2+) for an equimolar mixture of these metal ions. This configuration also enabled the displacement of metal ions to be detected as the breakthrough curve for a subsequent metal ion was monitored. Comparison of Ni and Zn binding indicates a simple ion exchange model is insufficient to explain sequential binding of these metal ions.     

Natural and artificial radioactivity measurements in Eastern Black Sea region of Turkey

Sorption potential of waste short hemp fibers for Pb(2+), Cd(2+) and Zn(2+) ions from aqueous media was explored. In order to assess the influence of hemp fiber chemical composition on their heavy metals sorption potential, lignin and hemicelluloses were removed selectively by chemical modification. The degree of fiber swelling and water retention value were determined in order to evaluate the change in accessibility of the cell wall components to aqueous solutions due to the fiber modification. The effects of initial ion concentration, contact time and cosorption were studied in batch sorption experiments. The obtained results show that when the content of either lignin or hemicelluloses is progressively reduced by chemical treatment, the sorption properties of hemp fibers are improved. Short hemp fibers are capable of sorbing metal ions (Pb(2+), Cd(2+) and Zn(2+)) from single as well as from ternary metal ion solutions. The maximum total uptake capacities for Pb(2+), Cd(2+) and Zn(2+) ions from single solutions are the same, i.e. 0.078mmol/g, and from ternary mixture 0.074, 0.035 and 0.035mmol/g, respectively.     

The study of various parameters affecting the ion exchange of Cu2+, Zn2+, Ni2+, Cd2+, and Pb2+ from aqueous solution on Dowex 50W synthetic resin

A gel resin containing sulfonate groups (Dowex 50W) was investigated for its sorption properties towards copper, zinc, nickel, cadmium and lead metal ions. The use of selective ion exchange to recover metals from aqueous solution has been studied. The ion exchange behavior of five metals on Dowex 50W, depending on pH, temperature, and contact time and adsorbate amount was studied. Experimental measurements have been made on the batch sorption of toxic metals from aqueous solutions using cation exchanger Dowex 50W. The maximum recoveries (about 97%) Cu(2+), Zn(2+), Ni(2+), Cd(2+) and (about 80%) Pb(2+) were found at pH ranges 8-9. The amount of sorbed metal ion was calculated as 4.1, 4.6, 4.7, 4.8, and 4.7mequiv./gram dry resin for Pb(2+), Cu(2+), Zn(2+), Cd(2+), and Ni(2+), respectively. The precision of the method was examined at under optimum conditions. Selectivity increased in the series: Pb>Cd>Cu>Zn>Ni. It has been observed that, selectivity of the -SO(3)H group of the resin increases with atomic number, valance, degree of ionization of the exchanged metals. The equilibrium ion exchange capacity of resin for metal ions was measured and explored by using Freundlich and Langmuir isotherms. Langmuir type sorption isotherm was suitable for equilibrium studies.     

Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products from recycled coal fly ash

The removal performance and the selectivity sequence of mixed metal ions (Co(2+), Cr(3+), Cu(2+), Zn(2+) and Ni(2+)) in aqueous solution were investigated by adsorption process on pure and chamfered-edge zeolite 4A prepared from coal fly ash (CFA), commercial grade zeolite 4A and the residual products recycled from CFA. The pure zeolite 4A (prepared from CFA) was synthesized under a novel temperature step-change method with reduced synthesis time. Batch method was employed to study the influential parameters such as initial metal ions concentration, adsorbent dose, contact time and initial pH of the solution on the adsorption process. The experimental data were well fitted by the pseudo-second-order kinetics model (for Co(2+), Cr(3+), Cu(2+) and Zn(2+) ions) and the pseudo-first-order kinetics model (for Ni(2+) ions). The equilibrium data were well fitted by the Langmuir model and showed the affinity order: Cu(2+) > Cr(3+) > Zn(2+) > Co(2+) > Ni(2+) (CFA prepared and commercial grade zeolite 4A). The adsorption process was found to be pH and concentration dependent. The sorption rate and sorption capacity of metal ions could be significantly improved by increasing pH value. The removal mechanism of metal ions was by adsorption and ion exchange processes. Compared to commercial grade zeolite 4A, the CFA prepared adsorbents could be alternative materials for the treatment of wastewater.     

Poly(vinyl) chloride membrane copper-selective electrode based on 1-phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione

1-Phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione (H(2)L) was used as an effective ionophore for copper-selective poly(vinyl) chloride (PVC) membrane electrodes. Optimization of the composition of the membrane and of the conditions of the analysis was performed, and under the optimized conditions the electrode has a detection limit of 6.30×10(-7) M Cu(II) at pH 4.0 with response time 10s and displays a linear EMF versus log[Cu(2+)] response over the concentration range 2.0×10(-6) to 5.0×10(-3) M Cu(II) with a Nernstian slope of 28.80±0.11 mV/decade over the pH range of 3.0-8.0. The sensor is stable for 9 weeks and exhibits good selectivity with respect to alkali, alkali earth and transition metal ions (e.g. Na(+), K(+), Ba(2+), Ca(2+), Zn(2+), Cd(2+), Co(2+), Mn(2+), Ni(2+), Fe(2+), Al(3+)) in the 3.0-8.0 pH range. It was successfully applied for the direct determination of copper(II) in zinc, aluminum and nickel based alloys, in soils polluted by oil, and as an indicator electrode for potentiometric titration of copper ions with EDTA.     

Heavy metals binding properties of esterified lemon

Sorption of Cd(2+), Cr(3+), Cu(2+), Ni(2+), Pb(2+) and Zn(2+) onto a carboxyl groups-rich material prepared from lemon was investigated in batch systems. The results revealed that the sorption is highly pH dependent. Sorption kinetic data indicated that the equilibrium was achieved in the range of 30-240 min for different metal ions and sorption kinetics followed the pseudo-second-order model for all metals studied. Relative sorption rate of various metal cations was found to be in the general order of Ni(2+)>Cd(2+)>Cu(2+)>Pb(2+)>Zn(2+)>Cr(3+). The binding characteristics of the sorbent for heavy metal ions were analyzed under various conditions and isotherm data was accurately fitted to the Langmuir equation. The metal binding capacity order calculated from Langmuir isotherm was Pb(2+)>Cu(2+)>Ni(2+)>Cd(2+)>Zn(2+)>Cr(3+). The mean free energy of metal sorption process calculated from Dubinin-Radushkevich parameter and the Polanyi potential was found to be in the range of 8-11 kJ mol(-1) for the metals studied showing that the main mechanism governing the sorption process seems to be ion exchange. The basic thermodynamic parameters of metals ion sorption process were calculated by using the Langmuir constants obtained from equilibration study. The DeltaG degrees and DeltaH degrees values for metals ion sorption on the lemon sorbent showed the process to be spontaneous and exothermic in nature. Relatively low DeltaH degrees values revealed that physical adsorption significantly contributed to the mechanism.     

Fluorescence sensor array for metal ion detection based on various coordination chemistries: general performance and potential application

A sensor array containing 9 cross-reactive sensing fluorescent elements with different affinity and selectivity to 10 metal cations (Ca(2+), Mg(2+), Cd(2+), Hg(2+), Co(2+), Zn(2+), Cu(2+), Ni(2+), Al(3+), Ga(3+)) is described. The discriminatory capacity of the array was tested at different ranges of pH and at different cation concentrations using linear discriminant analysis (LDA). Qualitative identification of cations can be determined with over 96% of accuracy in a concentration range covering 3 orders of a magnitude (5-5000 microM). Quantitative analysis can be achieved with over 90% accuracy in the concentration range between 10 and 5000 microM. The array performance was also tested in identification of nine different mineral water brands utilizing their various electrolyte compositions and their Ca(2+), Mg(2+), and Zn(2+) levels. LDA cross-validation routine shows 100% correct classification for all trials. Preliminary results suggest that similar arrays could be used in testing of the consistency of the purification and manufacturing process of purified and mineral waters.     

Luminescent, freestanding composite films of Au15 for specific metal ion sensing

A highly luminescent freestanding film composed of the quantum cluster, Au(15), was prepared. We studied the utility of the material for specific metal ion detection. The sensitivity of the red emission of the cluster in the composite to Cu(2+) has been used to make a freestanding metal ion sensor, similar to pH paper. The luminescence of the film was stable when exposed to several other metal ions such as Hg(2+), As(3+), and As(5+). The composite film exhibited visual sensitivity to Cu(2+) up to 1 ppm, which is below the permissible limit (1.3 ppm) in drinking water set by the U.S. environmental protection agency (EPA). The specificity of the film for Cu(2+) sensing may be due to the reduction of Cu(2+) to Cu(1+)/Cu(0) by the glutathione ligand or the Au(15) core. Extended stability of the luminescence of the film makes it useful for practical applications.     

Heavy metal removal from aqueous solutions by activated phosphate rock

The use of natural adsorbent such as phosphate rock to replace expensive imported synthetic adsorbent is particularly appropriate for developing countries such as Tunisia. In this study, the removal characteristics of lead, cadmium, copper and zinc ions from aqueous solution by activated phosphate rock were investigated under various operating variables like contact time, solution pH, initial metal concentration and temperature. The kinetic and the sorption process of these metal ions were compared for phosphate rock (PR) and activated phosphate rock (APR). To accomplish this objective we have: (a) characterized both (PR) and (APR) using different techniques (XRD, IR) and analyses (EDAX, BET-N(2)); and, (b) qualified and quantified the interaction of Pb(2+), Cd(2+), Cu(2+) and Zn(2+) with these sorbents through batch experiments. Initial uptake of these metal ions increases with time up to 1h for (PR) and 2h for (APR), after then, it reaches equilibrium. The maximum sorption obtained for (PR) and (APR) is between pH 2 and 3 for Pb(2+) and 4 and 6 for Cd(2+), Cu(2+) and Zn(2+). The effect of temperature has been carried out at 10, 20 and 40 degrees C. The data obtained from sorption isotherms of metal ions at different temperatures fit to linear form of Langmuir sorption equation. The heat of sorption (DeltaH degrees), free energy (DeltaG degrees) and change in entropy (DeltaS degrees) were calculated. They show that sorption of Pb(2+), Cd(2+), Cu(2+) and Zn(2+) on (PR) and (APR) an endothermic process. These findings are significant for future using of (APR) for the removal of heavy metal ions from wastewater under realistic competitive conditions in terms of initial heavy metals, concentrations and pH.     

DNA-capped mesoporous silica nanoparticles as an ion-responsive release system to determine the presence of mercury in aqueous solutions

We have developed DNA-functionalized silica nanoparticles for the rapid, sensitive, and selective detection of mercuric ion (Hg(2+)) in aqueous solution. Two DNA strands were designed to cap the pore of dye-trapped silica nanoparticles. In the presence of ppb level Hg(2+), the two DNA strands are dehybridized to uncap the pore, releasing the dye cargo with detectable enhancements of fluorescence signal. This method enables rapid (less than 20 min) and sensitive (limit of detection, LOD, 4 ppb) detection, and it was also able to discriminate Hg(2+) from twelve other environmentally relevant metal ions. The superior properties of the as-designed DNA-functionalized silica nanoparticles can be attributed to the large loading capacity and highly ordered pore structure of mesoporous silica nanoparticles, as well as the selective binding of thymine-rich DNA with Hg(2+) . Our design serves as a new prototype for metal-ion sensing systems, and it also has promising potential for detection of various targets in stimulus-release systems.     

Impedimetric immobilized DNA-based sensor for simultaneous detection of Pb2+, Ag+, and Hg2+

An unlabeled immobilized DNA-based sensor was reported for simultaneous detection of Pb(2+), Ag(+), and Hg(2+) by electrochemical impedance spectroscopy (EIS) with [Fe(CN)(6)](4-/3-) as redox probe, which consisted of three interaction sections: Pb(2+) interaction with G-rich DNA strands to form G-quadruplex, Ag(+) interaction with C-C mismatch to form C-Ag(+)-C complex, and Hg(2+) interaction with T-T mismatch to form T-Hg(2+)-T complex. Circular dichroism (CD) and UV-vis spectra indicated that the interactions between DNA and Pb(2+), Ag(+), or Hg(2+) occurred. Upon DNA interaction with Pb(2+), Ag(+), and Hg(2+), respectively, a decreased charge transfer resistance (R(CT)) was obtained. Taking advantage of the R(CT) difference (ΔR(CT)), Pb(2+), Ag(+), and Hg(2+) were selectively detected with the detection limit of 10 pM, 10 nM, and 0.1 nM, respectively. To simultaneously (or parallel) detect the three metal ions coexisting in a sample, EDTA was applied to mask Pb(2+) and Hg(2+) for detecting Ag(+); cysteine was applied to mask Ag(+) and Hg(2+) for detecting Pb(2+), and the mixture of G-rich and C-rich DNA strands were applied to mask Pb(2+) and Ag(+) for detecting Hg(2+). Finally, the simple and cost-effective sensor could be successfully applied for simultaneously detecting Pb(2+), Ag(+), and Hg(2+) in calf serum and lake water.     

Manipulation of intraparticle charge delocalization by selective complexation of transition-metal ions with histidine moieties

Ruthenium nanoparticles were cofunctionalized with pyrene and histidine moieties through Ru═carbene π bonds. The selective complexation of the histidine moiety with transition-metal ions led to a marked diminishment of the emission peak at 490 nm which arose from the nanoparticle-bridged pyrene moieties that behaved analogously to pyrene dimers with a conjugated spacer. This is accounted for by the polarization of the core electrons by the added positive charge that impacted the intraparticle charge delocalization between the particle-bound pyrene moieties. This electronic interaction was likely facilitated by the π interactions between the metal ions and the imidazole ring as well as by the conjugated molecular backbone that linked the imidazole ring to the nanoparticle cores. Within the present experimental context, of all the metal ions tested, the impacts were much more drastic with Pb(2+), Co(2+), and Hg(2+) than with Li(+), K(+), Rb(+), Mg(2+), Ca(2+), and Zn(2+) ions, with the most sensitive variation observed with Pb(2+). This is ascribed to the enhanced π interactions of the histidine moiety with the Pb(2+), Co(2+), and Hg(2+) ions because of their capability of donating d electrons, a behavior consistent with prior studies based on conventional histidine-metal ion complexes.     

Sorption properties of the activated carbon-zeolite composite prepared from coal fly ash for Ni(2+), Cu(2+), Cd(2+) and Pb(2+)

Composite materials of activated carbon and zeolite have been prepared successfully by activating coal fly ash (CFA) by fusion with NaOH at 750 degrees C in N(2) followed by hydrothermal treatments under various conditions. Uptake experiments for Ni(2+), Cu(2+), Cd(2+) and Pb(2+) were performed with the materials thus obtained from CFA. Of the various composite materials, that were obtained by hydrothermal treatment with NaOH solution (ca. 4M) at 80 degrees C (a composite of activated carbon and zeolite X/faujasite) proved to be the most suitable for the uptake of toxic metal ions. The relative selectivity of the present sorbents for the various ions was Pb(2+)>Cu(2+)>Cd(2+)>Ni(2+), with equilibrium uptake capacities of 2.65, 1.72, 1.44 and 1.20mmol/g, respectively. The sorption isotherm was a good fit to the Langmuir isotherm and the sorption is thought to progress mainly by ion exchange with Na(+). The overall reaction is pseudo-second order with rate constants of 0.14, 0.17, 0.21 and 0.20Lg/mmol min for the uptake of Pb(2+), Cu(2+), Cd(2+) and Ni(2+), respectively.     

Efficiency of a zeolitized pumice waste as a low-cost heavy metals adsorbent

The unextracted residue obtained after a countercurrent two-step extractive process of silica from pumice lapillus, at 100 degrees C and room pressure, has been found mainly crystallized to the pseudo-cubic form typical of zeolite P. This residue could be active as a low-cost agent for the removal of heavy metals from wastewater. In this paper the removal capacity of six metallic cations (i.e. Cu(2+), Ni(2+), Zn(2+), Cd(2+), Pb(2+) and Cr(3+)) was studied in a stirred batch reactor. Results obtained showed that the removal of metal ions (100-500mgg(-1)) from wastewater is achieved in a short time and the concentration lowered under the legal limits. The adsorption mechanism mainly involves an ionic exchange between sodium ions from the solid phase and heavy metals in solution. However, if wastewater was accompanied by free acidity, it first should be neutralized to pH 4-5 to prevent zeolite destruction.     

Luminescent CdS quantum dots as selective ion probes

Water-soluble luminescent CdS quantum dots (QDs) capped by polyphosphate, L-cysteine, and thioglycerol were synthesized in aqueous solution. The ligands were found to have a profound effect on the luminesence response of CdS QDs to physiologically important metal cations. Polyphosphate-capped CdS QDs were sensitive to nearly all mono- and divalent cations, showing no ion selectivity. Conversely, thioglycerol-capped CdS QDs were sensitive to only copper and iron ions. Similar concentrations of physiologically relevant cations, such as zinc, sodium, potassium, calcium, and magnesium ions did not affect the luminescence of thioglycerol-capped CdS QDs. On the other hand, L-cysteine-capped CdS QDs were sensitive to zinc ions and insensitive to other physiologically important cations, such as copper, calcium, and magnium ions. To demonstrate the detection capability of these new ion probes, L-cysteine and thioglycerol-capped CdS QDs were used to detect zinc and copper ions in physiological buffer samples. The detection limits were 0.8 microM for zinc (II) and 0.1 microM for copper (II) ions. The emission enhancement of the QDs by zinc (II) is attributed to activation of surface states, whereas the effective reduction of copper (II) to copper (I) may explain the emission decrease of the thioglycerol-capped CdS QDs when charged with copper ions. Unlike organic fluorescent dyes, the thioglycerol-capped luminescent CdS QDs discriminate between copper and zinc ions and are therefore suitable for the analysis of copper ions in biological samples in the presence of physiological concentrations of zinc ions. The interference of iron ions with zinc and copper ion detection is attributed to an inner filter effect, which is eliminated by adding fluoride ions to form the colorless complex FeF6(3-). To the best of our knowledge, this is first use of luminescent semiconductor quantum dots as selective ion probes in aqueous samples.