ANALYST, AUGUST 1992, VOL. 117 1247 Surface Modification With Macrocycle-containing Redox-active Polymers: Towards the Design of Novel Spectroelectrochemical Group IA/IIA Metal Cation Sensors* Paul D. Beer Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX I 3QR, UK Oldrich Kocian Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham, B15 ZTT, UK Roger J. Mortimer and Christopher Ridgway Department of Chemistry, L oug h boro ug h University of Tech nolog y, L oug h boroug h, L eicesters hire LEI1 3TU, UK The application of transition metal-polypyridyl complexes to chemical sensor technology is demonstrated. Using fluorescence emission spectroscopy the recognition of Na+ and Mg2+ metal cations by vinyl linked benzo- and aza- crown ether-bipyridyl ruthenium(ii) complexes is shown.Such complexes can be electropolymerized onto platinum and optically transparent conducting glass electrodes. Although electrochemical recognition by such modified electrodes is not observed, such systems show promise as novel spectrochemical sensors. Keywords: Modified electrode; sensor; spectrochemical; ruthenium tris-bip yridyl; crown ether Since their discovery 25 years ago1 crown ethers,2 as a new generation of complexing agents, have found many applica- tions in analytical chemistry.3 It is well known that crown ethers can bind Group IA and IIA metal cations, selectivity being determined by the compatibility of the cation radius and the size of the crown ether macrocyclic cavity. This effect has been exploited in the construction of ion-selective elec- trodes4.5 and recently in the development of selective electro- chemical recognition systems697 based on, for example, ferrocenyl-substituted crown ethers.Our aim is to extend such electrochemical recognition to the development of surface- modified electrode sensor systems. Variation of redox poten- tial with metal cation identity would allow development of such systems as amperometric sensors for the non-electro- active Group IA and IIA metal cations in flow injection analysis .* The surface modification approach described here utilizes the electropolymerization technique pioneered by Abruna et aZ.9 for metal complexes of vinyl-substituted bipyridyl (bipy) ligands. Experimental Synthesis The new trans vinyl-linked benzo-crown ether-, aza-crown ether- and bismethoxyphenyl-bipyridyl ligands L1 and L2 were prepared10 giving excellent yields (70-95%) via mono- or dilithiation of 4,4'-dimethyl-2,2'-bipyridine, addition of the appropriate 4-formyl substituted compound to give the alcohols L3 and L4, followed by dehydration.The structures of all these new compounds were characterized on the basis of spectroscopic and analytical evidence. The tris(1igand) ruthenium( 11) complexes [ RuL1-2,4a,b3] [PI?&, [R~(bipy)~][PF~]~ and the corresponding monoligand ruthenium(I1) complexes [ R~L~-~v~~*~(bipy)~][PF& were obtained by refluxing the appropriate ligand in dimethyl- formamide with RuC13.3H20 and [RuCl2(bipy)2]-2H20, respectively, followed by purification on Sephadex LH-20 and precipitation of the complexes on addition of ammonium hexafluorophosphate.10 * Presented at the Meeting on Analytical Applications of Chemi- cally Modified Electrodes, Bristol, UK, January 7-8. 1992. N T'R N T R L1a-C L2S-c L3" L4- c02 0 > a; R = * o d o J ,OMe c; R = *OM(?1248 ANALYST, AUGUST 1992, VOL. 117 Electrochemistry Electrochemical studies were performed using an EG & G Princeton Applied Research Model 273 potentiostat. A three-electrode system was employed with platinum flag (1 cm2 surface area) or indium-tin oxide optically transparent glass (30 G? 'per square' from Balzers, cut to a 9 X 50 mm surface area) working electrodes. The reference electrode [a Radiometer sodium chloride saturated calomel electrode (SSCE)] and counter electrode (platinum-mesh) were each separated from the working electrode compartment of the electrochemical cell by glass frits.Measurements were carried out in de-oxygenated acetonitrile (freshly distilled over calcium hydride) solutions containing a 0.1 mol dm-3 solution of the supporting electrolyte. Fluorescence Emission and Visible Absorption Spectrometry A Perkin-Elmer Model 3000 fluorescence spectrophotometer controlled by an Elonex 386 computer was used for recording fluorescence emission spectra. An interference filter provided a 450 nm excitation wavelength, with an absorption filter being used to prevent radiation below 490 nm from reaching the emission monochromator. The Perkin-Elmer front surface accessory (5212-3130) was used for measurements on the polymer films.A Hewlett-Packard HP 8451A diode-array spectro- photometer controlled by a Vectra QS/16S computer was employed for recording the visible absorption spectra. All measurements were conducted at 25 "C using a 1 x 1 cm rectangular quartz cuvette and de-oxygenated solutions. Results and Discussion Electropolymerization Following the earlier precedent,g it was anticipated that sequential potential scanning of solutions of the [Ru- L1-23][PF6]2 and [R~L~-~(bipy)~][pF~]~ complexes to the series of ligand-centred reductions would activate the vinylic lin- kages and initiate electropolymerization. All [RuLI3][PF6l2 complexes were indeed electropolymerized onto platinum electrodes to form smooth, adherent orange films, as exempli- fied by Fig. 1, which shows the steady increase in current attributable to the combined electroactivity of the polymeric film and that of the inward-diffusing complex.Complexes [ RuL23][ PF& were also electropolymerized although less efficiently (lower rate of current increase on sequential scanning), owing to steric crowding of the 4,4' substituents, and complexes [ R~L~-~(bipy)2][PF6]2 only exhibited solution redox processes analogous to the prototype [R~(bipy)3][PF6]~. 0 -0.4 -0.8 -1.2 -1.6 E N versus SSCE Fig. 1 Sequential cyclic voltammo rams for an acetonitrile solution containing 1.33 mmol dm-3 [RULla3f[PF& in 0.1 mol dm-3 Bu4NBF4 at 100 mV s-1; arrows indicate current increase Electrochemical Recognition Analysis of heteropolymetallic ruthenium(I1)-sodium com- plexes (isolated from the reaction of crown ether-containing ruthenium(I1) complexes and sodium hexafluorophosphate) and 13C nuclear magnetic resonance (NMR) titration studies have established10 that each crown ether moiety in the plexes is able to bind one metal cation.In order to discover whether the polymer films were electrochemically responsive to such binding, cyclic voltammograms of the modified electrodes were recorded in supporting electrolytes with a variety of cations. For illustration, a set of cyclic voltammo- grams for a poly[RuL1a3]2+ modified electrode in an aceto- nitrile solution containing 0.1 rnol dm-3 tetrabutylammonium tetrafluoroborate is shown in Fig. 2. The dependence of peak currents on scan rates was indicative of a combination of diffusional (peak current proportional to square root of scan rate) and surface control (peak current proportional to scan rate) for the RU'~"" redox response.Table 1 shows that Ru""" redox potentials, Epoly, for benzo-crown ether-containing poly[ RuL1a3]2+ and bismethoxyphenyl-containing poly- [RuL1c3]2+ modified electrodes are the same for a given supporting electrolyte , indicating that crown ether binding of metal cations in the former does not perturb the redox response of the Ru1I1"I wave. Unfortunately, although more likely to show electrochemical recognition due to the prox- imity of the crown ether macrocycle binding sites, it was not possible to monitor the ligand-based redox responses in the metal cation supporting electrolytes owing to low solvation energies of Group IA and IIA metal cations in acetonitrile, resulting in electroreduction to the metals at relatively positive potentials.11.12 [ R u L ~ ~ , ~ ~ ~ , ~ ~ ] [ P F ~ ] ~ and [ R u L ~ ~ , ~ ~ ~ , ~ (bipy) 21 fPF61 2 corn- Fluorescence Emission Spectrometry Analysis by fluorescence emission spectrometry has the advantage of high sensitivity, which allows the measurement of low analyte concentrations.That [R~(bipy)~][PF6]~ and related systems exhibit metal-to-ligand charge transfer (MLCT) emission maximal3 offers an alternative method for probing the binding of Group IA and IIA metal cations. The fluorescence emission spectra of all complexes in acetonitrile were fairly broad and featureless and of approximately the AE I 2.00 1 .oo 0 EN versus SSCE Fig. 2 Cyclic voltammograms, at A 20, B 50, C 100, D 200, and E 500 mV s-1 for a poly[RuL1a3]2+ modified electrode in a pure 0.1 mol dm-3 Bu4NBF4-acetonitrile solution after transfer from an electropolymerization solution containing 1.33 mmol dm-3 [RuLla3][PF6I2 in 0.1 rnol dm-3 Bu4NBF4-acetonitrileANALYST, AUGUST 1992, VOL.117 1249 Table 1 Ru””” redox potentials, Epoly (volts) versus SSCE, for polymer modified electrodes with variation in the supporting electrolyte. Values quoted were (f0.02) at 100 mV s-l in acetonitrile solutions containing 0.1 mol dm-3 supporting electrolyte. Redox potentials were calculated as EPoly = (& + Ep:,)/2 where E,,= = cathodic peak potential, EP,, = anodic peak potential Polymer Bu4NBF4 Bu4NC104 NaC104 Mg(C104)2 POly( RuLla3)2+ +1.20 +1.15 +1.10 +1.05 POly( RUL’C3)2+ +1.24 +1.15 +1.10 +1.07 Table 2 Metal-to-ligand charge transfer fluorescence emission wavelengths, h,,,(nm) for O.OOOO1 mol dm-3 solutions of complexes in pure acetonitrile and in acetonitrile with addition to individual solutions of excess amounts of each of the salts indicated Pure 605 606 605 672 659 670 673 696 69 1 686 Bu4NC104 605 608 606 672 659 670 673 696 69 1 686 NaC104 605 608 607 669 658 668 673 682 677 670 Mg(C104)2 605 608 607 665 657 666 673 694 689 683 same width.Since the spectra are all similar in shape and width, the emission maxima were used to assess the effect of ligand variation and metal-cation binding on excited-state energies. The emission maxima for the complexes of the alcohols [ R u L ~ ” ~ ] [ P F ~ ] ~ and [R~L~~(bbipy)~][PF~]~, shown in Table 2, are as for the prototype, [R~(bipy)~][PF&. In contrast, the presence of the vinyl linkage in the analogous complexes [ R u L ~ ~ ~ ] [ P F ~ ] ~ and [R~L~~(bipy)2][PF& causes a red shift owing to conjugation to the electron-donating benzo-crown ether.Addition of excess amounts of the salts, shown in Table 2, then provides evidence for the spectrochemical recognition of Group IA and IIA metal cations by the benzo-crown ether and aza-crown ether complexes. The MLCT emission maxima for these complexes are shifted to significantly lower wavelengths in the presence of Na+ or Mg2+. The largest shift was obtained with [R~L~~(bipy)z][PF&; addition of an excess amount of sodium ions giving a 16 nm blue shift.No corresponding shifts are observed for the bismethoxyphenyl-containing ‘model’ complex [ R u L ~ ~ ~ ] [ P F ~ ] ~ or the prototype [Ru(bipy)3][PF6]2, suggesting that metal cation coordination, causing lowering of the electron-donating strength of the crown ethers, is responsible for the effect. That the vinyl linkage is necessary for such spectrochemical recognition is shown by the data for the complexes of the alcohols [ R u L ~ ~ ~ ] [ P F ~ ] ~ and Spectrochemical recognition was also observed for polymer films prepared by the electropolymerization technique. For such measurements optically transparent conducting glass was chosen as the electrode material to allow visible absorption spectra to be recorded additionally. Poly[ RuL*a3]2+ films exhibited an MLCT absorption band (h,,, = 470 nm) corresponding to that of the monomeric precursor [ R u L ~ ~ ~ ] [ P F ~ ] ~ (h,,, = 475 nm).As for the monomeric complex, the MLCT emission maximum of the polymer was shifted to lower wavelengths in the presence of Mg*+, as shown in Fig. 3. The emission intensity also increases owing to the extra rigidity of the MLCT chromophore on metal-cation binding. Furthermore, the observation of a low wavelength [RUL4a(bipy)2] [PF612* E w o.2 I 0 1 500 550 600 650 700 750 Wavelengthhm Fig. 3 A, Fluorescence emission spectrum of a film of poly- [RuLla3I2+ coated onto an optically transparent conducting glass electrode and immersed in acetonitrile. B, Spectrum after addition of excess of Mg2+ to 10 mmol dm-3 [as Mg(C104)2-acetonitrile solution] shoulder in Fig.3, in addition to the main peak, is evidence that both saturated and vinyl linkages between the bipyridyls and the benzo-crown ethers are present in the polymer. Confirmation of spectrochemical recognition and indeed the retention of the benzo-crown ether after electropolymeriza- tion came from the visible absorption and fluorescence emission spectra of the bismethoxyphenyl-containing ‘model’ polymer. Poly[RuL2c3]2+ films displayed no shifts in the presence of Group IA and IIA metal cations. In conclusion these results have shown that the novel crown ether polymer film materials prepared by electropolymeriza- tion represent a new class of spectrochemical sensing devices for Group IA and IIA metal cations. 0. K. thanks the Science and Engineering Research Council (Molecular Sensors Initiative) for a postdoctoral research fellowship (GWE 71624). 1 2 3 4 5 6 7 8 9 10 11 12 13 References Pedersen, C. J., J. Am. Chem. SOC., 1967, 89,7017. Vogtle, F., Supramolecular Chemistry, Wiley, New York, 1991, ch. 2, pp. 27-83. Blasius, E., and Janzen, K.-P., Top. Curr. Chem., 1981, 98, 163. Covington, A. K., Grey, H., Kelly, P. M., Kinnear, K. I., and Lockhart, J. C., Analyst, 1988, 113, 895. Moody, G. J., Saad, B. B., and Thomas, J. D. R., Analyst, 1989, 114, 15. Beer, P. D., in Adv. Inorg. Chem., ed. Sykes, A. G., Academic Press, New York, 1992, vol. 39, in the press. Beer, P. D., Chem. SOC. Rev., 1989, 18,409. Thomsen, K. N., and Baldwin, R. P., Electroanalysis, 1990, 2, 263. Abrufia, H. D., Denisevich, P., Umaiia, M., Meyer, T. J., and Murray, R. W., J. Am. Chem. SOC., 1981, 103, 1. Beer, P. D., Kocian, O., Mortimer, R. J., and Ridgway, C., J. Chem. SOC., Chem. Commun., 1991, 1460. Kolthoff, I. M., and Coetzee, J. F., J. Am. Chem. SOC., 1957, 79, 870. Kolthoff, I. M., and Coetzee. J. F., J. Am. Chem. SOC., 1957, 79, 1852. Juris, A., Balzani, V., Barigelletti, F., Campagna, S., Belser, P., and von Zelewsky, A., Coord. Chem. Rev., 1988, 84,85. Paper 2/01 201 D Received March 5, 1992 Accepted April 23, 1992