Letter pubs.acs.org/ac
Transition Metal Hexacyanoferrates in Electrocatalysis of H2O2 Reduction: An Exclusive Property of Prussian Blue Natalya A. Sitnikova, Maria A. Komkova, Irina V. Khomyakova, Elena E. Karyakina, and Arkady A. Karyakin* Department of Chemistry, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia ABSTRACT: The ability of Prussian Blue, ferric hexacyanoferrate (FeHCF), to sensitively and selectively detect hydrogen peroxide by its reduction in the presence of oxygen is of high importance for analytical chemistry. Success with Prussian Blue (PB) provided an appearance of contradictory reports concerning electrocatalysis of the other transition metal hexacyanoferrates (HCFs) in H2O2 reduction. Investigating thermodynamics of the catalyzed reactions as well as electrochemical properties of the hexacyanoferrates, we are able to conclude that the noniron hexacyanoferrates themselves are completely electrocatalytically inactive, except for a minor electrocatalysis in the opposite reaction, hydrogen peroxide oxidation, registered for NiHCF. Concerning the most important reaction, H2O2 reduction, the observed electrocatalytic activity (by the way, 100 times decreased compared to PB) is due to the presence of FeHCF (Prussian Blue) as defects in the structure of noniron hexacyanoferrates. This finding, considering other unique properties of transition metal HCFs, will provide a systematic search for sensing materials with improved analytical performance characteristics.
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oxygen.13 The latter allows low-potential H2O2 detection by its reduction with advantageous analytical characteristics,13−15 particularly with sensitivity of 1−2 A M−1cm−2. Moreover, on the basis of Prussian Blue-based nanoelectrode arrays,16 the electrochemical sensor characterized by the linear calibration range prolonged over 7 orders of magnitude of analyte concentration, denoted as a sensor with record performance characteristics,17 has been elaborated. Success with ferric ferrocyanide (Prussian Blue) electrocatalysis provided an interest of this property for other transition metal hexacyanoferrates (HCFs). Metal hexacyanometallates belong to a set of Prussian Blue analogues with a generic formula AxM′y[M″(CN)6]z·nH2O, where A, alkali metal cation; M′, nitrogen coordinated transition metal cation; M″, carbon coordinated metal cation (in the case of Prussian Blue, iron(III) hexacyanoferrate (II) A = K+; M′, M″ = Fe). Indeed, there was a report for similarly high catalytic activity for chromium hexacyanoferrate.18 However, as was mentioned already in a review,13 there were no evidence for formation of chromium hexacyanoferrate as an insoluble material. More promising candidates for synthesis of electrocatalytic materials are metal ions catalyzing Fenton reaction: H2O2 decomposition. Similarly to Fe2+ (the corresponding hexacyanoferrate is Prussian Blue), catalysis in this reaction is mentioned for copper, cobalt, and nickel ions. All of these metal ions are known to form electroactive hexacyanoferrates.
lectrocatalysts for hydrogen peroxide (H2O2) oxidation and reduction attract a particular interest due to the requirements of modern analytical science. H2O2 is an important analyte because of its excessive use in industry and atomic power stations, that dramatically affect the environment,1,2 as well as disinfecting agent for water pools, food, and beverage packages.3,4 H2O2 also plays an important role in the greatly expanding area of clinical diagnostics. First, it is a valuable marker for oxidative stress, recognized as one of the major risk factors in progression of pathophysiological complications in diabetes, atherosclerosis, renal disease, cancer, and aging.5−9 Second, hydrogen peroxide is also a side product of the reaction of oxidases, a family of enzymes used in the majority of analytical kits. As shown already almost 40 years ago, the detection of H2O2 provides the highest sensitivity for the corresponding biosensors.10,11 However, in clinical diagnostics, as well in food quality control, the required selectivity of the resulting (bio)sensors can be achieved only in the case of low-potential hydrogen peroxide detection, i.e., due to its reduction. Since 1994, when for Prussian Blue modified electrodes the selective hydrogen peroxide detection by its electroreduction in the presence of oxygen was shown,12 transition metal hexacyanoferrates (HCFs) have attracted important attention particularly for oxidase-based biosensors. The highest electrocatalytic activity toward H2O2 reduction is attributed to Prussian Blue, ferric ferrocyanide.13 Compared to platinum, which is the most widely used material for detection of hydrogen peroxide, Prussian Blue modified electrodes are (i) 3 orders of magnitude more active in H2O2 reduction and oxidation in neutral media and (ii) 3 orders of magnitude more selective for hydrogen peroxide reduction in the presence of © 2014 American Chemical Society
Received: February 12, 2014 Accepted: April 11, 2014 Published: April 11, 2014 4131
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Letter
0.7−0.8 to 0.4 V at a sweep rate of 0.04 V/s. The growing solution contained 0.5−4 mM of K3[Fe(CN)6] and 0.5−4 mM of FeCl3. A solution of 0.1 M HCl and 0.1 M KCl was used as supporting electrolyte. After deposition, Prussian Blue films were electrochemically activated in the same supporting electrolyte by cycling in the range from 0.35 to −0.05 V at a rate of 0.02−0.04 V/s until a stable voltammogram was obtained. Then, the electrodes were annealed at 100 °C for an hour. Electrodeposition of nickel, cobalt, and copper hexacyanoferrates from noncolloid solutions became possible due to stabilization of the diluted solution of Ni2+, Co2+, or Cu2+ and [Fe(CN)6]3− in aqueous media with the excessive amount of supporting electrolyte (KCl).26 The growing solution contained 0.2−1 mM equimolar solutions of precursors (Ni2+, Co2+, or Cu2+ and [Fe(CN)6]3−). The deposition was carried out by cycling the applied potential from 0.0 to 0.7−0.8 V in the case of NiHCF, 0.85 V in the case of CuHCF or 1.0 V in the case of CoHCF at a sweep rate 0.1 V s−1. After deposition, the resulting films were electrochemically activated in 0.1 M HCl with 0.1 M KCl supporting electrolyte by cycling the applied potential in the range from −0.05 to 1.0 V at a sweep rate of 0.04−0.1 V s−1 in order to saturate the films with potassium cations. Then, the electrodes were annealed at 100 °C during 1 h.
However, activities of Cu, Co, and Ni hexacyanoferrates in hydrogen peroxide reduction are orders of magnitude less as compared to Prussian Blue.13,19−22 The groundlessness of the only exceptional report on CoHCF with extra-high sensitivity23 (reported, by the way, by the same authors as for the CrHCF paper18) was already made in ref 13. According to our experience, the highest electrocatalytic activity toward H2O2 reduction is peculiar to NiHCF.24 Moreover, in contrast to Prussian Blue, the electrocatalytic mechanism for Ni, Co, and Cu hexacyanoferrates in hydrogen peroxide reduction is not evident. Indeed, Prussian Blue demonstrates true redox catalysis with the half-wave potential of the H2O2 reduction almost coinciding with the Prussian Blue|Prussian White redox potential.25 On the contrary, other mentioned hexacyanoferrates display their redox activity at much more anodic potentials, whereas hydrogen peroxide reduction occurs in the potential region similar to Prussian Blue. Taking into account that electrocatalytic activities of Ni, Co, and Cu hexacyanoferrates are on average 2 orders of magnitude less compared to Prussian Blue, perhaps only impurities of the latter are responsible for electrocatalysis and other transition metal hexacyanoferrates are idle? With this Article, we are trying to confirm this conclusion.
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EXPERIMENTAL SECTION Materials. All experiments were carried out with distilled and deionized water (Milli Q-plus-Millipore system). FeCl3· 6H2O, NiCl2·6H2O, CoCl2·6H2O, CuCl2, and K3[Fe(CN)6] were used as precursors for the synthesis of metal hexacyanoferrates. All inorganic salts and hydrogen peroxide (30% solution) were obtained at the highest purity from Reachim (Moscow, Russia) and used as received. Glassy carbon disk electrodes (2 mm in diameter) used as working electrodes were made by pressing glassy carbon rods (SU-2500) in Teflon. Prior to use, glassy carbon electrodes were mechanically polished in a water suspension of alumina powder with particle size of 0.3 μm and then with 0.05 μm (Al2O3, Sigma) until a mirror finish was observed. Instrumentation. Deposition of transition metal hexacyanoferrates as well as their characterization by cyclic voltammetry were made in a three-compartment electrochemical cell containing a platinum net auxiliary electrode and Ag|AgCl reference electrode in 1 M KCl. Cell construction allowed deaeration of the working electrode space. The Metrohm-μAutolab III or PalmSens interfaces were used. Catalytic performances were investigated in a flow-through mode. The setup consisted of a Cole Parmer (Vernon Hills, IL) peristaltic pump (7519-10), homemade flow-through wall-jet cell with 0.5 mm nozzle positioned at 1.5 mm distance from the surface of disk electrode, the above reference, and a homemade injector. Flow rates used were in the range from 0.5 to 1 mL per min. Working electrode potential was varied from −0.2 to 0.6 V. The pH dependencies were studied keeping constant concentration of KCl (0.1 M). Carrier solutions were buffered with HCl (0.1 M, 1 mM), 0.05 M citric acid (pH 4.0 and pH 6.0), 0.05 M phosphate (pH 6.0 and pH 7.0), and 0.05 M borate (pH 8.0 and pH 9.0). The concentration of hydrogen peroxide in stock solutions was controlled by optical density at 230 nm with an LKB-Ultraspec UII spectrophotometer (Broma, Sweden). Electrodeposition of Transition Metal Hexacyanoferrates. Electrodeposition of Prussian Blue was carried out in cyclic voltammetric conditions by cycling in the range from
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RESULTS AND DISCUSSION While Prussian Blue (ferric hexacyanoferrate, FeHCF) serves as a superior electrocatalyst for hydrogen peroxide reduction (and oxidation), there is rather contradictory and nonsatisfactory information concerning other transition metal hexacyanoferrates. According to our own experience with the iron triad mate (cobalt and nickel) HCFs as well as with CuHCF, their electrocatalytic activity is negligible (at least 2 orders of magnitude decreased) compared to Prussian Blue (PB). This is surprising because of two facts. First, nickel, cobalt, and cupric hexacyanoferrates are isostructural to Prussian Blue.27 Second, Ni2+, Co2+, and Cu2+ ions are similarly to Fe2+ known to be catalysts for H2O2 decomposition according the so-called Fenton reaction. Moreover, as mentioned, there is an additional not obvious observation, which is convenient to illustrate with hexacyanoferrate of nickel, possessing from our point of view the highest electrocatalytic activity toward H2O2 reduction among noniron HCFs. Whereas for Prussian Blue the half wave potential for H2O2 reduction almost coincides with the Prussian Blue| Prussian White redox potential,25 in the case of NiHCF, the potential windows for hexacyanoferrate redox activity and electrocatalytic activity in H2O2 reduction are clearly separated (Figure 1). To discuss this phenomenon, it is necessary to consider first the thermodynamics of hydrogen oxide reactions. One can assume that reduction of hydrogen peroxide at NiHCF modified electrodes starts at much more negative potentials compared to the hexacyanoferrate redox activity window due to thermodynamics of H2O2 reduction to water. However, the equilibrium H2O2|H2O potential is equal to 1.77 V (vs NHE, pH 1),28 which gives 1.2 V vs Ag|AgCl at pH 6. Hence, in the potential window of NiHCF redox activity, hydrogen peroxide reduction has to reach its current plateau region. What also can hinder the observation of NiHCF electrocatalysis in H2O2 reduction, is the reaction of hydrogen peroxide oxidation into oxygen. Surprisingly, O 2 |H 2 O 2 equilibrium potential is much lower than of hydrogen peroxide 4132
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Figure 2. Hydrodynamic voltammograms of 1 mM H2O2 reduction on MHCF-modified electrodes (M = Ni2+ (filled), Co2+ (open), or Cu2+(asterisks)): (●, ○, ∗: pH 1.0; ■, □: pH 3.0; ▲: pH 4.0; ▼: pH 6.0); wall-jet cell with flow rate of 0.8 mL/min; 0.05 M PBS with 0.1 M KCl; cyclic voltammogram of CoHCF modified electrode: 0.1 M KCl + 0.1 M HCl, 20 mV s−1.
Figure 1. Cyclic voltammogram of NiHCF modified electrode, 0.1 M KCl + 0.1 M HCl, 20 mV s−1; hydrodynamic voltammogram of 1 mM H2O2 reduction on NiHCF electrode, 0.05 M PBS, pH 6.0 with 0.1 M KCl, wall-jet cell, flow rate of 0.8 mL min−1. Inset: differential pulse voltammogram of NiHCF, amplitude of 50 mV, step height of 5 mV, and step width of 50 ms.
voltammogram is independent of the pH in the range from 1.0 to 4.0. A significant decrease of the cathodic current for NiHCF observed in neutral pH can be attributed to inhibition effect of the phosphate ions (data not shown). Cobalt hexacyanoferrate is able to display electroctrocatalytic activity similar to NiHCF (Figure 2). It is not stable enough in neutral solutions; however, in acidic media, the corresponding voltammograms of H2O2 reduction are also pH independent (Figure 2). Moreover, as seen for CoHCF, the potential window for electrocatalytic reaction is also separated from the redox activity window (Figure 2). Cupric hexacyanoferrate is less active and stable, but what is important is that its electrocatalytic activity is observed in the same range as for nickel and cobalt hexacyanoferrates (Figure 2). Summarizing data in Figure 2, we note that hydrodynamic voltammograms of H2O2 reduction catalyzed by noniron hexacyanoferrates are completely independent of the solution pH and, hence, are determined by the properties of the electrocatalysts. The fact that electrocatalytic potential window is separated from the redox activity window confirms the above conclusion concerning electrocatalytic properties of the noniron hexacyanoferrates in H2O2 reduction: they are due to the presence of FeHCF (Prussian Blue) as defects. The only electrocatalytic activity of the noniron hexacyanoferrates is recorded for NiHCF in hydrogen peroxide oxidation. As solution pH is increased, oxidation of H2O2 becomes plausible at more and more cathodic potentials. When it becomes possible thermodynamically to observe this reaction in a redox activity window of NiHCF, it appears in hydrodynamic voltammograms (Figure 3). As expected, the anodic current is increased with the rise of solution pH (Figure 3). We note that no electrocatalytic activity is recorded for hexacyanoferrate of cobalt (Figure 3).
reduction and equal to 0.682 V (vs NHE, pH 1)28 or 0.108 V vs Ag|AgCl at pH 6. Hydrogen peroxide oxidation rather actively occurs on various electrode surfaces, including inert carbon ones. However, we note that, when the enzyme peroxidase is used as an electrocatalyst, the valuable currents of H2O2 reduction can be observed starting from 0.6 V, SCE.29 This means that in NiHCF redox activity potential window there is no thermodynamic constraints to observe electrocatalysis in H2O2 reduction. Considering the above discussion, there is only one possible way to explain a separation of potential windows for redox and catalytic activities in the case of NiHCF. Concerning the electrocatalysis in H2O2 reduction, we note that, first, it is negligible compared to Prussian Blue.24 Second, it occurs at rather cathodic potentials, where Prussian Blue occurs in its reduced, Prussian White form.24 Hence, perhaps the traces of FeHCF (PB) occurring as defects in NiHCF structure provide its electrocatalytic activity? Indeed, the inset in Figure 1 shows that in the differential pulse voltammogram of NiHCF film there is a peak current far from NiHCF redox activity. This peak perfectly merges the redox activity window of Prussian Blue and hence can be attributed only to FeHCF structures in nickel hexacyanoferrate film. This brings us to the most unexpected conclusion that the electrocatalytic activity of the most active electrocatalyst in H2O2 reduction among noniron hexacyanoferrates is due to Prussian Blue type defects in its structure. To confirm the above conclusion, we decided to investigate pH dependence of the noniron hexacyanoferrate electrocatalysis. Indeed, whereas redox reactions of transition metal hexacyanoferrates are pH independent (charge compensation occurs due to alkali metal ion or anion transfer30,31), the equilibrium potentials of hydrogen oxide reactions obviously undergo Nernstian’s pH behavior. Accordingly, pH dependence of the electrocatalytic reactions is able to point to the limiting step of electrocatalysis. Figure 2 displays hydrodynamic voltammograms of H2O2 reduction for NiHCF, CoHCF, and CuHCF modified electrodes. As seen, the highest electrocatalytic activity is displayed for hexacyanoferrate of nickel, and the shape of the
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CONCLUSION Despite with this paper we are unable to discover the mechanism of the transition metal hexacyanoferrate electrocatalysis, the achieved result, that noniron HCFs are completely inactive in H2O2 reduction, is important not only from the fundamental point of view. Indeed, considering hexacyanoferrate-based cathode for hydrogen peroxide as an oxidant, it is 4133
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Figure 3. Hydrodynamic voltammograms in 1 mM H2O2 of MHCFmodified electrodes (M = Ni2+ (filled) or Co2+ (open)): (■: pH 7.0; ▲: pH 8.0; ●, ○: pH 9.0); wall-jet cell with flow rate of 0.8 mL/min; 0.05 M PBS with 0.1 M KCl.
impossible to expect an open circuit potential higher than Prussian Blue|Prussian White redox couple. For analytical application, a search for new sensing materials has to be carried out considering noniron hexacyanoferrates as suitable matrixes for entrapment of clusters of superior electrocatalyst (for instance, Prussian Blue). Such attempt was made by layer-bylayer deposition of Fe and Ni hexacyanoferrates resulting in a sensor with dramatically improved operational stability.24 However, a systematic search based on the currently obtained knowledge will result in sensors and biosensors with much more superior characteristics.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The support of Russian Scientific Foundation is appreciated. REFERENCES
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