Quantitative determination of metal ions and anions in aqueoussolution by using pH-responsive redox-active receptors

摘要

N N N N Fe Fe Fe Fe L1 500 480 460 440 420 400 380 360 4 5 6 7 8 9 10 11 12 pH E1/2 / mV Ni Cu Pb Cd Zn L Quantitative determination of metal ions and anions in aqueous solution by using pH-responsive redox-active receptors Miguel E. Padilla-Tosta, Ram�on Mart�amp;yacute;nez-M�a�nez,* Teresa Pardo, Juan Soto and Mar�amp;yacute;a Jos�e L. Tendero Departamento de Qu�amp;yacute;mica, Universidad Polit�ecnica de Valencia, Camino de Vera s/n, 46071 Valencia, Spain The redox-active polyazacyloalkane 1,4,8,11-ferrocenylmethyl- 1,4,8,11-tetraazacyclotetradecane L1 is a versatile receptor for the quantitative electrochemical determination of metal ions (Cu2+, Zn2+, Cd2+) and ATP in aqueous solution.In recent years a number of work has been devoted to the development of new receptors containing binding sites and redox-active groups.1 Two properties are common to those new molecules; their electroactive character and the ability to bind substrates. Some of those receptors bearing different redoxactive groups (able to be reduced or oxidized), can electrochemically recognize alkali-metal and alkaline-earth-metal cations,2 anions3 and neutral species.4 Additionally it has recently been reported that transition-metal ions can also adequately be recognised by using ferrocene-functionalised polyamines as receptors.5,6 That approach leads to pHresponsive redox-active molecules where the shift of the oxidation potential in the presence of transition-metal ions is a function of the pH.5 Additionally the functionalisation of polyaza molecules with redox groups are good candidates to electrochemically recognize anion coordination based on the property of those systems to act as polybases.6 To further advance the use of redox-active receptors as practical electrochemical sensors we have performed electrochemical studies, using the redox-functionalised polyazacycloalkane 1,4,8,11-ferrocenylmethyl-1,4,8,11-tetraazacyclotetradecane L1,7 with the aim of quantitatively determining the concentration of cations and anions in solution.Fig. 1 shows the variation of the oxidation potential (E1/2, from rotating disk electrode techniques) vs. pH for the systems L1ndash;H+ndash;M2+ (M = Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, ligand-tometal ratio = 1 : 1) in thfndash;water (70 : 30 v/v) mixtures. The oxidation potential of L1 shifts to more anodic potentials when the pH is reduced as a consequence of the successive protonation of the amino groups. In the presence of stoichimetric amounts (M2+/L1 = 1) of metal ions different curves are found. Some of them are quite different (for Cu2+, Zn2+, Cd2+) and some quite similar (for Pb2+ and Ni2+) to that of the free receptor (see Fig. 1). We have also found that for ligand-tometal ratios 1, curves between that of L1 and that with M2+/L1 = 1 are obtained. At a fixed pH, E1/2 for a determined M2+/L1 ratio is E1/2 = xLE1/2 L + xMLE1/2 ML where xL and xML are the molar fractions of the free receptor and the metal complex, respectively; whereas E1/2 L and E1/2 ML are the oxidation potential of L1 and the oxidation potential of a 1 : 1 ligand-to-metal molar ratio.Knowing the amount of receptor used in the electrochemical experiments it is possible to determine the concentration of metal ions in solution. In a typical experiment we measured E1/2 for an unknown M2+/L1 ratio at at least five different pH values and have determined an average concentration of the metal and the standard deviation.Working with L1 concentrations in the range (4ndash;5) 3 1024 mol dm23 we have been able to determine metal ion concentrations from 5 3 1025 to 4 3 1024 mol dm23. Probably lower limits of detection could be reached by using lower concentrations of receptor L1. Table 1 shows the concentration of Cu2+, Zn2+ and Cd2+ found and compared with the concentration by standard atomic absorption analysis. We have also developed studies on the quantitative determination of two metal ions showing different L1ndash;H+ndash;M2+ curves to that of L1 and have found that the simultaneous determination of Cu2+, Cd2+ and Cu2+, Zn2+ in mixtures containing both ions is possible if L1 = Cu2+ + Zn2+ or L1 = Cu2+ + Cd2+, whereas the method seems not to be accurate enough if the total concentration of the metals is unknown. Additionally the existence of some metal ions, as Ni2+ and Pb2+, with L1ndash;H+ndash; M2+ curves close to that of L1 suggests that the concentration of Cu2+ (which probably has a higher stability constant with L1 than Ni2+ or Pb2+) could be determined in the presence of those metal ions.Data in Table 1 confirm that Cu2+ can be selectively determined in the presence of Ni2+ or Pb2+. Even with large amounts of Pb2+ copper(ii) can be quite accurately determined {Cu2+ = 4.9 3 1025, 2.33 3 1 1024 and 3.32 3 1024 mol dm23 found in the presence of Pb2+ = 2.23 3 1023 for copper concentrations of 1.33 31024, 2.66 31024 and 3.33 Fig. 1 Plot of E1/2 vs. pH for L1ndash;H+ and L1ndash;H+ndash;M2+ (M2+ = Ni2+, Cu2+, Zn2+, Cd2+, Pb2+) Chem. Commun., 1997 8879 8 7 6 5 4 3 380 400 420 440 460 480 pH L ATP E1/2 / mV 3 1024 mol dm23, respectively, from standard atomic absorption analysis}. Finally Fig. 2 displays the E1/2ndash;pH curve obtained for receptor L1 with ATP. The presence of the anion, as expected, does not change significantly the E1/2ndash;pH curve of L1 when the pH is basic, but if the pH is reduced a steady shift to more cathodic potentials is observed.Based on that difference between the two L1ndash;H+ and L1ndash;H+ndash;ATP systems and following a similar method to that used above we have determined ATP concentrations in aqueous solution (see Table 2). The results, which are as acurate as when metal ions are determined, although there is a small difference between L1ndash;H+ and L1ndash;H+ndash; ATP curves, suggest that the method may prove useful in the determination of ATP or other anions using similar systems containing larger numbers of amino groups. To our knowledge this is a new method for the quantitative determination of metal ions and anions in aqueous solution by using pH-responsive redox-active receptors. As an example molecule L1 has proved to be a versatile receptor which can be used to quantitatively determine metal ions (Cu2+, Zn2+ or Cd2+; mixtures of Cu2+, Cd2+ and Cu2+, Zn2+; Cu2+ selectively in the presence of Ni2+ or Pb2+) and anions (ATP).The development of new sensors for quantitative recognition is now being carried out.We should like to thank the DGICYT (proyecto PB95- 1121-C02-02) for support. Footnote * E-mail: rmaez@qim.upr.es References 1 P. D. Beer, Chem. Soc. Rev., 1989, 18, 409. 2 P. D. Beer, J. P. Danks, D. Hesek and J. McAleer, J. Chem. Soc., Chem. Commun., 1993, 1735; A. Kaifer, D. A. Gustowski, L. Echegoyen, V. J. Gatto, R. A. Goli, A. M. Rios and G. W. Gokel, J. Am. Chem. Soc., 1995, 107, 1958. 3 P.D. Beer, Chem. Commun., 1996, 689; C. Dusemund, K. R. A. S. Sandanayake and S. Sinkai, J. Chem. Soc., Chem. Commun., 1995, 333; P. D. Beer, M. G. B. Drew, D. Hesek and R. Jagessar, J. Chem. Soc., Chem. Commun., 1995, 1187. 4 P. D. Beer, Z. Chen, M. G. B. Drew and P. A. Gale, J. Chem. Soc., Chem. Commun., 1995, 1851; A. Ori and S. Sinkai, J. Chem. Soc., Chem. Commun., 1995, 1771. 5 M. J. L. Tendero, A. Benito, R. Mart�amp;yacute;nez-M�a�nez, J. Soto, J. Pay�a, A. J. Edwards and P. R. Raithby, J.Chem. Soc., Dalton Trans., 1996, 343; M. J. L. Tendero, A. Benito, R. Mart�amp;yacute;nez-M�a�nez, J. Soto, E. Garc�amp;yacute;a- Espa�na, J. A. Ramirez, M. I. Burguete and S. V. Luis, J. Chem. Soc., Dalton Trans., 1996, 2923. 6 P. D. Beer, Z. Chen, M. G. B. Drew, J. Kingston, M. Ogden and P. Spencer, J. Chem. Soc., Chem. Commun., 1993, 1046; P. D. Beer, Z. Chen, M. G. B. Drew, A. O. M. Johnson, D. K. Smith and P. Spencer, Inorg. Chim. Acta, 1996, 246, 143. 7 P. D. Beer, J. E. Nation, S.L. W. McWhinnie, M. E. Harman, M. B. Hursthouse, M. I. Ogden and A. H. White, J. Chem. Soc., Dalton Trans., 1991, 2485. Received in Cambridge, UK, 29th January 1997; Com. 7/00674H Table 1 Comparison between the determination of the concentration of metal ions by using electrochemical (L1 as receptor) and s105 Cu2+ 105Zn2+ 105Cd2+ 8.8(6)a 9.1b 5.2(9) 5.9 10.6(7) 11.1 17(1) 18.7 27(1) 28.4 19(1) 22.4 29(1) 28.0 18(1) 17.9 31(2) 33.6 105 Cu2+ + Cd2+a 105 Cu2+ + Zn2+d 105 Cu2+e 105 Cu2+f 13(2), 27(3) 12.9, 26.6 14(1), 26(1) 13.4, 26.1 11(4) 9.6 11(1) 9.7 19(2), 20(3) 20.1, 19.5 20(1), 20(1) 20.1, 19.7 25(4) 19.3 20(1) 19.4 28(1), 12(2) 26.9, 12.9 28(1), 12(1) 26.7, 13.2 30(3) 28.9 28(1) 28.9 a Concentration (mol dm23).Values in parentheses are the standard deviations in the last significant digit. b Metal concentration (mol dm23) found using standard atomic absorption methods. c Determination under the condition L1 = Cu2+ + Cd2+. d Determination under the condition L1 = Cu2+ + Zn2+. e Cu2+ determined in the presence of Ni2+, Ni2+ = 20 3 1025 mol dm23. f Cu2+ determined in the presence of Pb2+, Pb2+ = 20 3 1025 mol dm23. Fig. 2 Plot of E1/2 vs. pH for L1ndash;H+ and L1ndash;H+ndash;ATP Table 2 Determination of the concentration of ATP by using electrochemical methods and L1 as receptora 105 ATP 17(2)b 16.4 21(1) 18.9 40(2) 41.1 a Data for the ATP determination have been taken from pH 3.5 to 5. b Concentration (mol dm23). Values in parentheses are the standard deviations in the last significant digit. 888 Chem. Commun., 1997