CONFERENCE PROCEEDINGS International Student Conference “Science and Progress” German-Russian Interdisciplinary Science Center St. Petersburg – Peterhof November, 14-18 2011 Organizing committee Prof. Dr. S.F. Bureiko, Dean of Faculty Physics, SPSU Prof. Dr. A.M. Shikin, Coordinator of G-RISC, SPSU E.I. Spirin, Dean-assistant of Faculty of Physics, SPSU E.V. Serova, Head of Academic Mobility Department, SPSU Dr. A.G. Rybkin, G-RISC office, SPSU A.A. Popova, G-RISC office, SPSU Program Committee Prof. Dr. E. Rühl, Coordinator of G-RISC, FU Berlin Prof. Dr. C. Laubschat, Faculty of Physics, TU Dresden Prof. Dr. A.M. Shikin, Coordinator of G-RISC, SPSU Prof. Dr. Yu.S. Tver’yanovich, Faculty of Chemistry, SPSU Prof. Dr. V.N. Troyan, Faculty of Physics, SPSU Contacts Faculty of Physics Saint-Petersburg State University Ulyanovskaya ul. 3, Peterhof, St. Petersburg, Russia 198504 Tel. +7 (812) 428-46-56, Fax. +7 (812) 428-46-55 E-mail: [email protected] Website: www.phys.spbu.ru/grisc 3 Heads of sections A. Chemistry – Prof. Dr. Yu.S. Tver’yanovich, Faculty of Chemistry, SPSU B. Geo- and Astrophysics – Prof. Dr. V.N. Troyan, Faculty of Physics, SPSU, Dr. V.G. Nagnibeda, Faculty of Mathematics and Mechanics, SPSU C. Mathematics and Mechanics – Prof. Dr. V. Reitmann, Faculty of Mathematics and Mechanics, SPSU D. Solid State Physics – Prof. Dr. A.P. Baraban, Faculty of Physics, SPSU E. Applied Physics – Prof. Dr. A.S. Chirtsov, Faculty of Physics, SPSU F. Optics and Spectroscopy – Prof. Dr. Yu.V. Chizhov, Prof. Dr. N.A. Timofeev, Faculty of Physics, SPSU G. Theoretical, Mathematical – Prof. Dr. Yu.M. Pis’mak, and Computational Physics Faculty of Physics, SPSU H. Biophysics – Prof. Dr. N.V. Tsvetkov, Faculty of Physics, SPSU I. Resonance Phenomena – Prof. Dr. V.I. Chizhik, in Condenced Matter Faculty of Physics, SPSU 4 A. Chemistry Solid-contact ion-selective electrodes with ion-to-electron transducer layer composed of nanostructured materials Ivanova Nataliya [email protected] Scientific supervisor: Prof. Dr. Mikhelson K.N., Department of Physical Chemistry, Chemical Faculty, Saint-Petersburg State University Introduction Ion-selective electrodes (ISEs): potentiometric sensors of ions, comprise rou- tinely used analytical tool for essay of various analytes of clinical, industrial, and environmental relevance. However ISEs of the conventional design, containing internal aqueous solution and internal reference electrode, don’t fit modern re- quirements. The so-called solid-contact ISEs (SC-ISEs) – without internal filling – would allow for easier miniaturization, for planar technology of manufacturing, and, eventually, for better quality combined with lower production cost. The fun- damental problem with SC-ISEs refers to ion-to-electron conductivity transduction at a stable and reproducible potential difference at the interface between the ioni- cally conducting sensor layer (membrane) and electronically conducting substrate. This problem has been successfully solved for electrodes with glass and crystalline membranes. Between a glass membrane containing an alkali metal oxide and the lead they place a tin alloy doped with the respective alkali metal, thus utilizing the first-kind electrode concept. The second-kind electrode concept is implemented in SC-ISEs with crystalline membranes normally containing Ag2S: the inner side of the membrane is vacuum-sputtered with Ag, and then the lead is soldered to this silver layer. None of these approaches can be used for SC-ISEs with solvent polymeric membranes containing ionophores: neutral or charged species selectively binding ions, and in this way ensuring a selective potentiometric response of the sensor. Conducting polymers appear the most promising ion-to-electron transducers for this kind of sensors [1]. So far, however, the long-term stability and the piece-to-piece reproducibility of the potentials of ionophore-based SC-ISEs does not fit practical needs, and remains well below that of conventional ISEs. In this work we try graphenes, nanostructured polymeric composite of Cu(I), and also hexacyanoferrates as active components of ion-to-electron transducer layer in SC-ISEs. Materials and methods The electronically conducting substrate was always graphite encapsulated in a PVC body. The transducer layers were formed by drop-casting solutions of the respective materials on top of the substrate. As active components of ion-to-electron transducer layer we used nanostructured materials such as electroactive conjugated polymer: polymeric complex of Cu(I) with bichynolyl-containing polyamidoacid 6 (as 4.5 % solution in N-metylpyrrolidone – N-MP) – PAC-2, graphemes (as 2.5% solution in dimethylformamide - DMFA), both kindly provided by the Institute of Macromolecular compounds RAS, St.Petersburg, and fullerene – C60 (kindly provided by St.PSU). Optionally, the layers were doped with dispersed carbon black (CB) and/or RedOx pair: K3Fe(CN)6/ K4Fe(CN)6. For the transducer layer compositions see Table 1. Table 1. Electrode Composition of the transducer layer 1 2,5 % solution of graphenes in DMFA 2 4,5 % solution of polymeric complex of copper Cu(I) PAC-2 in N-MP 3 No transducer layer (the so-called coated-wire electrode - CWE) 200 µl saturated solution of salts mixture K [Fe(CN) ]+ 4 3 6 +K4[Fe(CN)6]·3H2O+300 mg PVC+1,7 ml THF 5 Same as 4, but dispersed in an ultrasonic bath 6 Conventional ISEs with inner solution KCl 10-2 M based on valinomycin 300 mg PVC+1,7 ml THF+150µl saturated solution of salts mixture 7 K3[Fe(CN)6]+ K4[Fe(CN)6]·3H2O, prepared by 1 M aqueous solution KCl 300 mg PVC+150µl DOP+1,7 ml THF+150µl saturated solution of 8 salts mixture K3[Fe(CN)6]+ K4[Fe(CN)6]·3H2O, prepared by 1 M aque- ous solution KCl Dry mixture of salts K [Fe(CN) ]+ K [Fe(CN) ]·3H O with suspension 9 3 6 4 6 2 of carbon black in the ratio 1:1 100 µl saturated solution of salts mixture K3[Fe(CN)6]+ 10 K4[Fe(CN)6]·3H2O+ 100µl 2,5 % solution of graphenes in DMFA+300 mg PVC+1,7 ml THF 100 µl saturated solution of salts mixture K3[Fe(CN)6]+ 11 K4[Fe(CN)6]·3H2O+ 100µl 2,5 % solution of graphenes in DMFA+600 mf mixture of PVC:carbon black=1:1+ 2 ml THF 300 mg PVC+1,7 ml THF+100µl saturated solution of salts mix- 12 ture K3[Fe(CN)6]+ K4[Fe(CN)6]·3H2O+200 mg fullerenes- C60 K+-selective membranes contained poly(vinylchloride) (PVC) and bis(2- ethylhexyl)phthalate (DOP) (1:3) doped with 0.03 M potassium tetrakis(p-Cl- phenyl)borate (KClTPB). The membranes were formed by drop-casting the membrane cocktail: the aforementioned substances dissolved in tetrahydrofuran (THF) on top of the transducer layer. Conventional ISE with internal aqueous 7 solution (0.01 M KCl) and Ag/AgCl internal reference electrode was used for the back-to-back comparison with SC-ISEs. Zero-current potentiometric measurements were accompanied by chronopoten- tiometry (ChP) and electrochemical impedance spectroscopy (EIS). The reference electrode was always saturated Ag/AgCl electrode, as counter electrode for ChP and EIS measurements we used glassy carbon rod. Results and discussion Electrodes 1 and 2 showed steady positive drift of the potentials, while elec- trodes 4, 5, 8, 7 and 12 showed negative drift. Only electrodes 3, 9, 10 and 11 showed relatively stable potential readings over time, although worse than con- ventional ISE 6, see Fig. 1. Fig. 1. Drift of the potential in control 0.1 M KCl solution for SC-ISEs 3, 9, 10, 11 and conventional ISE 6. Electrode 10, although showing a relatively stable potential in the control so- lution 0.1 M KCl, slowly by slowly lost response in solutions below 10-3 M - see Fig. 2, while electrodes 1, 2, 3, 6, 7, 8 and 9 retained Nernstian response down to 10-5 M KCl – see Fig. 3. Fig. 2. Behaviour of electrode 9 retaining Nernstain response down to 10-5 M KCl over 6 months of observation. 8 Fig. 3. Behavior of electrode 10 over time: gradual degradation of the response. The potential drift may be caused by slow Red-Ox reaction responsible for the ion-to-electron conductivity transduction. This transduction can be studied by im- pedance and chronopotentiometric measurements. For selected electrodes, appeared most promising we carried out EIS ansd ChP measurements. SC-ISEs 1, 2, and 12 showed only bulk impedance, while SC-ISE 11 showed depressed semicircle, most likely a superposition of a bulk and a slow charge-transfer process, Fig. 4. Fig. 4. EIS curves for SC-ISEs 1, 2, 11, 12 – curves 1, 2, 3 and 4, respectively. Chronopotentiometric data support the latter conclusion: po- larization then plotted vs. square root of time is almost linear for ISEs 1, 2 and significantly non- linear for ISEs 11, 12 (Fig. 5) suggesting slow charge-transfer in the latter electrodes. Indeed, SC- ISEs 12, and especially 11 show relatively large charge-transfer resistance. Besides insufficient stability of the ion-to-electron transduc- tion, the additional reason for the Fig. 5. Chronopotentiometric measurements: degradation in sensor response can polarization vs. square root of time. 9 be the existence of water layer between the membrane and the electrode substrate, which behaves unintentionally as an extremely non-buffered electrolyte reservoir [2]. Indeed, some SC-ISEs, in particularly electrode 9 exhibit large potential drifts when 0.1 M KCl is replaced with 0.1 M NaCl, suggesting the existence of thin water layer between the membrane and the transducer layer, see Fig. 6. Fig. 6. Response of SC-K +-SEs upon replacement of 0.1 M KCl with 0.1 M NaCl and back. Conclusions The insufficient stability of SC-ISEs under study is caused, most likely, both by relatively slow RedOx process at the transducer layer, and the existence of water layer beneath the membrane.