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  • 1. Electrochemistry Communications 29 (2013) 78–80 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage: hydrogen peroxide sensing based on Prussian Blue modifiedmacroporous microelectrodesAnna V. Mokrushina a, Matthias Heim b, Elena E. Karyakina a, Alexander Kuhn b, Arkady A. Karyakin a,⁎a Chemistry faculty of M.V. Lomonosov Moscow State University, 119991, Moscow, Russiab Univ. Bordeaux, ISM, UMR 5255, Site ENSCBP, F-33607 Pessac, Francea r t i c l e i n f o a b s t r a c tArticle history: We report on the successful modification of macroporous microelectrodes with the electrocatalyst PrussianReceived 6 December 2012 Blue, resulting in improved hydrogen peroxide sensing. When the pores of the electrode surface areReceived in revised form 9 January 2013 completely filled with the electrocatalyst the resulting sensor shows a behavior similar to the one obtainedAccepted 9 January 2013 for a non-porous microelectrode. However, significantly improved analytical performance is obtainedAvailable online 27 January 2013 when the porous structure is only partly filled with the catalyst, allowing diffusional access of the analyteKeywords: to the inside of the pores. The resulting sensor is characterized by both improved operational stability andMacroporous microelectrode record sensitivity of 8.8 ± 0.7 A M−1 cm −2 for H2O2 detection.Prussian Blue © 2013 Elsevier B.V. All rights reserved.Hydrogen peroxide1. Introduction characteristics, namely a linear calibration range of over seven orders of magnitude in analyte (H2O2) concentration [16,17]. Sensors for hydrogen peroxide (H2O2) attract a particular attention The particular interest in microelectrode based sensors is due to twoin electroanalytical science nowadays. H2O2 is an important analyte be- main reasons. First, if the electrochemical reaction is limited by masscause of its excessive use in industry and atomic power stations, which transport, microelectrodes allow to improve signal-to-noise ratio anddramatically affects the environment [1,2], as well as disinfecting agent hence to achieve better analytical performance characteristics. Thisfor water pools, food and beverage packages [3,4]. Therefore it is impor- is also valid for Prussian Blue modified electrodes [18]. Second, it istant to measure its residual concentration. However, H2O2 plays also an possible to implant such miniaturized electrodes. However, on theimportant role in the greatly expanding area of clinical diagnostic. First, other hand microelectrodes obviously generate only low currents,it is a valuable marker for oxidative stress, recognized as one of the which may constitute a major problem with respect to their handling.major risk factors in progression of pathophysiological complications One way to overcome this problem is to increase artificially thein diabetes, atherosclerosis, renal disease, cancer and aging [5–9]. active surface area of such electrodes. This can be achieved either bySecond, hydrogen peroxide is also a side product of the reaction of generating a high surface roughness, or alternatively by using porousoxidases, a family of enzymes used in the majority of analytical kits. microelectrodes [19]. Such porous surfaces are especially interestingAs shown already almost 40 years ago, the detection of H2O2 provides for redox couples with a sluggish electron transfer kinetics likethe highest sensitivity for the corresponding biosensors [10,11]. bioelectrochemical reactions [20,21] or the aforementioned transfor- We already reported on Prussian Blue (PB) as the most advanta- mation of H2O2 [22]. In these cases, the immobilization of a redox cat-geous hydrogen peroxide transducer [12–14]. Comparing with the alyst or a mediator inside the pores allows to drastically improve thewidely used platinum, Prussian Blue modified electrodes are: (i) three faradaic currents [23]. Here we report on the possibility to modifyorders of magnitude more active in H2O2 reduction and oxidation in macroporous gold microelectrodes with Prussian Blue as a catalyst,neutral media, and (ii) three orders of magnitude more selective for resulting in electrochemical sensors with record sensitivities of uphydrogen peroxide reduction in the presence of oxygen [15]. The to 9 A M −1 cm −2.attractive performance of the electrochemically deposited PrussianBlue allowed to denote it as an artificial equivalent of the enzyme per-oxidase [13,14]. Moreover, nano-structuring of Prussian Blue resulted 2. Experimentalin the elaboration of electrochemical sensors with record performance 2.1. Materials ⁎ Corresponding author. Experiments were carried out with water from a Millipore MilliQ E-mail address: (A.A. Karyakin). system. All inorganic salts, organic solvents and hydrogen peroxide1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
  • 2. 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 [19]. 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 [25]. 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 [26].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 [18]. 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 [24]), surface coverage and sensitivity area, (■) Prussian Blue surface coverage and (●) sensitivity in hydrogen peroxide detection.
  • 3. 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 [27]. 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 [28]. 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 [18]), the obtained sensitivity ofГ≈5–8 nmol cm−2 (found to be optimal for flat systems [13]) seem to 9 AM−1 cm−2 is preferable. This corresponds to a film thickness of approximately50 nm [17], 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 [18]. 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 [1] 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) [2] W.B. Nowall, W.G. 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