A.V. Mokrushina et al. / Electrochemistry Communications 29 (2013) 78–80 79(30% solution) were obtained at the highest purity from Reachim(Moscow, Russia) and used as received. Silica beads as templates were synthesized following proceduresinspired from the Stöber sol–gel process based on the ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane in ahydroalcoholic medium. A cyanide-free gold plating bath purchasedfrom Metalor (ECF-63; gold concentration 10 g L −1) was used asreceived for the metal deposition.2.2. Instrumentation Electrochemical experiments were carried out in a three-compartment electrochemical cell, containing a platinum net aux-iliary electrode and an Ag|AgCl reference electrode in 1 M KCl usingPalmSens (Netherlands) as a potentiostat interfaced to an IBM PC. Microelectrodes were made on the basis of isolated gold wires(ø 125 μm, Good Fellow, UK). Prior to modiﬁcation with PrussianBlue, gold microelectrodes were cleaned by cycling in 1 M sulfuricacid in a potential range from − 100 to 1500 mV at a sweep rate of40 mV s − 1. Electrodeposition of Prussian Blue was achieved in cyclic voltam-metric conditions with switching potentials from 0.4 to 0.6 V (cathodic)and from 0.7 to 0.8 V (anodic) at a sweep rate of 40 mV s −1 fromequimolar solution of K3[Fe(CN)6] and FeCl3. A supporting electrolytewas 0.1 mol L −1 HCl and 0.1 mol L −1 KCl. After deposition, PrussianBlue ﬁlms were activated in the latter by cycling from −0.05 to0.35 V at a rate of 40 mV s−1 until a stable voltammogram wasobtained. The PB surface coverage was calculated by integration ofcyclic voltammograms of its activation. Generation of macroporous microelectrodes was achieved follow-ing the template approach. An aqueous suspension of silica beads Fig. 1. Electron microscopy image of a microelectrode: a) global view of the isolatedwith 295 nm diameter was slowly evaporated at the surface of the gold wire (white spot) and b) zoom of the porous gold surface.microelectrodes. The resulting colloidal crystal served as a templatefor the subsequent electrodeposition of gold following the literatureprocedure . ratios are comparable to the ratio of active surface area (Fig. 2). At higher concentrations, commonly used for Prussian Blue deposition,3. Results and discussion a decrease in relative gain in sensitivity is observed, despite the improved amount of deposited electrocatalyst. At the maximum con-3.1. Macroporous gold microelectrodes centration, the sensitivity of the resulting sensor is similar to what would be obtained for a ﬂat microelectrode modiﬁed with Prussian Fig. 1a presents the electron microscopy image of a microelec- Blue. This can be explained in terms of a complete ﬁlling of the goldtrode. White spot in it corresponds to the gold wire. Electrodeposition pores with Prussian Blue. In this case analyte molecules will not beof gold through the colloidal crystal template leads, after dissolution able to penetrate the internal structure of the pores. Therefore onlyof the template, to surfaces with a very regular pore size . the outer electrode surface is accessible, and the resulting porousFig. 1b illustrates the porosity of such a modiﬁed microelectrode. Tomeasure the active area of gold electrodes, cyclic voltammograms inthe range from − 0.1 to 1.5 V have been recorded (sweep rate40 mV s −1; 1 M H2SO4). Integration of the charge associated with 8the gold oxide reduction peak leads to the active surface area .For our macroporous electrodes the ratio of active to geometric sur- porous-to-smooth ratioface area has been at the level of 3 (Fig. 2). 63.2. Prussian Blue deposition onto macroporous gold microelectrodes 4 Deposition of Prussian Blue on macroporous microelectrodes hasbeen carried out in cyclic voltammetric regime similarly to . How-ever, when the cathodic switching potential was of 0.4 V, the surfacecoverage of the porous microelectrodes was lower than for ﬂat ones. 2Choosing potentials above 0.5 V, it became possible to achieve similarand even higher PB surface coverages. For the following experimentsthe cathodic switching potential of 0.6 V has been chosen. 0 Concentration of ferric salts in the growth solutions also affected 0 5 10 15 20surface coverage and the resulting sensitivity of the microsensor. [FeFe(CN)6], mMAt 1 mM concentrations of Fe 3+ and [Fe(CN)6] 3−, both (equimolarprecursor concentrations are chosen in favor of one-to-one complex Fig. 2. Ratios of macroporous-to-conventional ultramicroelectrodes in terms of (▲) surfaceFe III[Fe III(CN)6] formation ), surface coverage and sensitivity area, (■) Prussian Blue surface coverage and (●) sensitivity in hydrogen peroxide detection.
80 A.V. Mokrushina et al. / Electrochemistry Communications 29 (2013) 78–80microelectrode is expected to display similar sensitivity as a conven- Discussing the achieved sensitivity of approximately 9 A M−1 cm−2,tional microelectrode. we consider its limit of 1 AM−1 cm−2 calculated for moderate stirring In order to elaborate sensors on the basis of porous structures . However, the same authors achieved the value of 2.3 AM−1 cm−2with improved sensitivity it is necessary to cover the inner surface for peroxidase modiﬁed gold electrodes . Since miniaturization pro-of the pores with the electrocatalyst, but preserving some empty vides increase in sensitivity (the PB modiﬁed disk Ø 125 μm is 3.7 timesspace for fast diffusion of analyte. Therefore low surface coverages of higher sensitive than the Ø 2 mm one ), the obtained sensitivity ofГ≈5–8 nmol cm−2 (found to be optimal for ﬂat systems ) seem to 9 AM−1 cm−2 is realistic.be preferable. This corresponds to a ﬁlm thickness of approximately50 nm , which ﬁts quite well with the diameter of the pores 4. Conclusions(200 nm), and thus leads to optimal analytical performance. Knowing limited operational stability of Prussian Blue in highly We conclude that macroporous systems can improve signiﬁcantlyconcentrated hydrogen peroxide solutions, it is interesting, whether the sensitivity for hydrogen peroxide reduction, even for very efﬁ-macroporous surface would provide improved stability of the corre- cient electrocatalysts such as Prussian Blue. A compromise has to besponding modiﬁed electrodes. Indeed, the inactivation constant in made when modifying porous structures with catalyst. Total ﬁlling1 mM H2O2 for macroporous modiﬁed electrode was 4–5 times of the pores with electrocatalyst is not efﬁcient, because diffusionlower as compared to the ﬂat microelectrode. of analyte into the pores will become too difﬁcult. In this case the microelectrode behaves like a ﬂat one. However when the pores are covered only with a thin layer (Г ≈ 5–8 nmol cm −2), it is possible3.3. Analytical characteristics of Prussian Blue modiﬁed macroporous to observe a dramatic increase in sensitivity. Using such optimizedmicroelectrodes Prussian Blue modiﬁed porous microelectrodes, a record sensitivity of 9 A M −1 cm −2 has been achieved, which should be of interest for The analytical performance of Prussian Blue modiﬁed macroporous clinical diagnostics and for electrocatalytic applications in general.and ﬂat microelectrodes are displayed in the calibration graph ofFig. 3. In both cases the plotted current densities are calculated with re- Acknowledgmentsspect to the geometric electrode area in order to illustrate the perfor-mance of the resulting sensors. The conventional microelectrode The ﬁnancial support of Russian Federation Ministry of Education andmodiﬁed with electrocatalyst displays a sensitivity (determined as Science through contracts no 14.740.11.1374 and no 11.519.11.2041 isthe slope of the low-concentration part of the calibration graph) of greatly acknowledged. We also gratefully acknowledge the Agence1.7 ± 0.3 A M−1 cm−2, which is in a good agreement with previous Nationale de la Recherche for the ﬁnancial support of this study throughwork . the ANR project HOPE (BLAN07-3-187142). The macroporous Prussian Blue modiﬁed microelectrode is charac-terized by a remarkably improved sensitivity of 8.8±0.7 AM−1 cm−2. ReferencesTo our knowledge, such sensitivity has not been reported so far, and  Y. Wang, J. Huang, C. Zhang, J. Wei, X. Zhou, Electroanalytical 10 (1998) 776.hence can be considered as a record one. The calibration graph (Fig. 3)  W.B. Nowall, W.G. Kuhr, Electroanalytical 9 (1997) 102.displays two linear slopes: the higher one at low H2O2 concentrations  A. Schwake, B. Ross, K. Cammann, Sensors and Actuators B: Chemical 46 (1998)prolonged over 2.5 orders of magnitude of hydrogen peroxide content 242.  B. Strausak, W. Schoch, Method and composition for disinfecting water, Patent EPand the lower one at higher concentrations. Flat Prussian Blue modiﬁed 0136973 A1, 1985electrodes are usually characterized by 3–4 orders of magnitude of line-  P.A. MacCarthy, A.M. Shah, Coronary Artery Disease 14 (2003) 109.arity [13–15]. Nano-electrode arrays based on Prussian Blue provided  R. Rodrigo, G. Rivera, Free Radical Biology & Medicine 33 (2002) 409.  R.S. Sohal, R.J. Mockett, W.C. Orr, Free Radical Biology & Medicine 33 (2002) 575.linear calibration range prolonged over 7 orders of magnitude of H2O2  T.T.C. Yang, S. Devaraj, I. Jialal, Journal of Clinical Ligand Assay 24 (2001) 13.concentration .  M.A. Yorek, Free Radical Research 37 (2003) 471.  G.G. Guilbault, G.J. Lubrano, Analytica Chimica Acta 64 (1973) 439.  G.G. Guilbault, G.J. Lubrano, D.N. Gray, Analytical Chemistry 45 (1973) 2255.  A.A. Karyakin, O.V. Gitelmacher, E.E. Karyakina, Analytical Chemistry 67 (1995) 2419. 1  A.A. Karyakin, E.E. Karyakina, Sensors and Actuators B: Chemical 57 (1999) 268. 10  A.A. Karyakin, E.E. Karyakina, L. Gorton, Analytical Chemistry 72 (2000) 1720.  A.A. Karyakin, Electroanalytical 13 (2001) 813.  A.A. Karyakin, E.A. Puganova, I.A. Budashov, I.N. Kurochkin, E.E. Karyakina, V.A. Levchenko, V.N. Matveyenko, S.D. Varfolomeyev, Analytical Chemistry 76 (2004) 474. 0 10  A.A. Karyakin, E.A. Puganova, I.A. Bolshakov, E.E. Karyakina, Angewandte Chemie International Edition 46 (2007) 7678.  A.A. Karyakin, E.A. Kuritsyna, E.E. Karyakina, V.L. Sukhanov, Electrochimica Acta 54 (2009) 5048. j,mA cm-2 -1  R. Szamocki, A. Velichko, C. Holzapfel, F. Mucklich, S. Ravaine, P. Garrigue, N. Sojic, 10 R. Hempelmann, A. Kuhn, Analytical Chemistry 79 (2007) 533.  S. Ben-Ali, D.A. Cook, S.A.G. Evans, A. Thienpont, P.N. Bartlett, A. Kuhn, Electrochemistry Communications 5 (2003) 747.  S. Ben-Ali, D.A. Cook, P.N. Bartlett, A. Kuhn, Journal of Electroanalytical Chemistry -2 579 (2005) 181. 10 0.1  R. Szamocki, S. Reculusa, S. Ravaine, P.N. Bartlett, A. Kuhn, R. Hempelmann, Angewandte Chemie. International Edition 45 (2006) 1317.  S.A.G. Evans, J.M. Elliott, L.M. Andrews, P.N. Bartlett, P.J. Doyle, G. Denuault, -3 Analytical Chemistry 74 (2002) 1322. 10 0.0  K. Itaya, T. Ataka, S. Toshima, JACS 104 (1982) 4767. 0 2 4 6 8 10  P.N. Bartlett, P.R. Birkin, M.A. Ghanem, Chemical Communications (2000) 1671–1672. 1 10 100 1000  A. Hamelin, Double-layer properties at sp and sd metal single-crystal electrodes, in: B.E. Conway, R.E. White, J.O.M. Bokris (Eds.), Mod. Aspect. Electroc., 16, 1985, p. 1. [H2O2], µM  T. Ruzgas, E. Csoregi, J. Emneus, L. Gorton, G. Marko-Varga, Analytica Chimica Acta 330 (1996) 123.  G. Presnova, V. Grigorenko, A. Egorov, T. Ruzgas, A. Lindgren, L. Gorton, T. Borchers,Fig. 3. Calibration curves for conventional (○) and macroporous (●) hydrogen peroxide Faraday Discussions 116 (2000) 281.sensors in a batch regime under stirring: 0.00 V; 0.05 M phosphate buffer, pH 7.0,containing 0.1 M KCl. Inset: low concentration range part of the calibration graph in linearcoordinates.