Short Communication

Determination of Cesium on Nickel Hexacyanoferrate-AluminumOxide Modified Electrodes

Luis Delgado, Hermes Carrero,* Lenys Fernandez

Departamento de Qu�mica, Universidad Simon Bol�var, Apartado 89000 Caracas 1080-A. Venezuela*e-mail: hcarrero@usb.ve

Received: February 19, 2009Accepted: August 16, 2009

AbstractThis report investigates the electrochemical behavior of hexacyanoferrate casted on Ni-Al2O3 modified electrodes andthe preconcentration and detection of cesium ions on such films. It also studies the morphology and the compositionof these surfaces. The film was grown on a glassy carbon (GC) surface. Five consecutive voltammetric cycles appliedwithin 0.0 V and �1.6 V at a scan rate of 10 mV/s were enough to cast the film. Scanning electron microscopy (SEM)analyses showed a homogeneous, porous but broken surface of the film. Its composition was studied by X-raydiffraction (XRD). The presence of NiHCFe was confirmed by Fourier transformed infrared spectroscopy (FT-IR).The Csþ preconcentration from diluted solutions was accomplished in 90 s, under a negative potential of �0.20 Vapplied to the modified working electrode. The detection of cesium has a good sensitivity and a wide linear interval(10�8 and 10�12 mol L�1). Even so, the limit of detection calculated was extremely low (2� 10�16 mol L�1), cesiumconcentrations lower than 10�12 mol L�1 gave signals with no analytical significance. However, to our knowledge, thisis the lowest level of cesium ever detected by an electroanalytical technique.

Keywords: Ni-Al2O3 film, Preconcentration, Electrodeposits, Cesium, Hexacyanoferrate, Electroactivity, Thin films

DOI: 10.1002/elan.200900112

Modified electrodes have been used extensively to precon-centrate and, consequently, improve the limit of detection(LOD) in electroanalytical procedures. The modificationmay be accomplished through electrodeposition; the filmobtained could be used to preconcentrate the analyte or tosupport other electroactive species. One of these deposits isthe Ni-Al2O3 composite, which is easily produced on GCelectrodes. There are many ways to generate the compositeon a GC surface; several of these methods and thecharacteristics of such deposits are well known. For instance,electrodeposits of Ni-Al2O3 have been prepared from NiCl2

or NiSO4 solutions containing suspended Al2O3 particles; afilm of those particles is easily prepared in organic mediafrom an Al sacrificial electrode [1]. The interaction of Al2O3

and Al with Ni, O2 and water in open air and wetenvironments is well characterized [2, 3]. The O2 causesthe oxidation of Al to various oxidation estates, but no NiOis formed. Additionally, the hydration of Al/Al2O3 films hasbeen studied by secondary ion mass spectrometry (SIMS)[4]. According to this study, when the oxide film is underwater, the hydration increases the OH� concentration in theoxide film, leading to a faster transport. If a potential isapplied to the film in the anodic polarization step, the OH� isgenerated through oxygen transformation; meanwhile, inthe cathodic polarization, the OH� is also present due to H2

evolution. The behavior of electroactive molecules confinedin Al2O3 films has been studied through different techniques[5 – 8]. Additionally, metal hexacyanoferrates are known to

interchange Csþ from aqueous solutions. Csþ can berecovered from radioactive wastes by co-precipitationwith those complexes [9, 10]; they can adsorb almost 100%of the Csþ from simulated radioactive wastes [10]. The aboveinformation suggests that Csþ could be preconcentrated on aNiHCFe-Al2O3 modified electrode. The objective of theexperiments discussed herein was to preconcentrate Csþ ona modified electrode and then to quantify it by differentialpulse voltammetry; hoping to decrease the LOD usuallyreported for that metal. AGC electrode was treated to cast aNiHCFe-Al2O3 film. The electrochemical behavior of theelectroactive molecule and the composition and morphol-ogy of the film was examined. Nitrate solutions were used inthe process because NO�

3 is easier to remove from the filmthan Cl� or SO2�

4 , which may hamper Fe(CN)3�6 uptake. The

Al2O3 increased the electrode surface area, thereby incor-porating the maximum amount of HCFe. Additionally,nickel was included to stabilize the hydrated Al2O3 and topromote the in situ formation of NiHCFe.

Figure 1 shows the first cyclic voltammogram (CV) of theelectrodeposition of Ni-Al2O3 on the GC electrodes. Apotential interval from 0.0 to �1.6 V was applied at a scanrate of 10 mV/s. The Al3þ and Ni2þ nitrate (1 : 2) solutionwithout the supporting electrolyte formed the electrolyticmedia. The inset on Figure 1 contains the first cycle on thegrowing of the Al2O3 registered from a 0.1 mol L�1 Al(NO3)3

solution, under similar conditions. Five voltammetric cycleswere sufficient to produce the electrodeposit. The Ni co-

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deposition added strength to the film, and a smooth andstable surface was obtained. According to the cell con-ditions, the reduction of Ni2þ occurred at�1.00 V, while theH2 evolution provides the alkaline environment on theelectrode surface. This could promote the chemical forma-tion of Al2O3 with various degrees of hydration that mayagglomerate on the electrode [11, 12]. The cyclic voltam-metry of the NiHCFe-Al2O3/GC electrodes at different scanrates (not shown) revealed that the peak current and thescan rate did not correlate in a full linear fashion, especiallyat low scan rate. The uncompensated resistance within thefilm may play a role on such behavior. The split of the peakcurrent usually observed on the NiHCFe CVs has beenascribed to the presence or absence of Kþ as the counter-ionof the metal-complex [13, 14].

The XRD analysis on the electrodeposited Ni-Al2O3 filmrevealed the presence of NiAl and Al2O3; Ni, Al, or NiOwere not observed. However, the continuous potentialsweeps, applied to stabilize the voltammetric behavior of theelectroactive species, might oxidize the NiAl. The resultantAl3þmay have generated different forms of Al2O3, while theNi2þ instantaneously reacted with the HCFe to produce themetal complex (NiHCFe), which was responsible for the Csþ

uptake. Such spontaneous reactions occurred only whenHCFe was in its reduced form, according to electrochemicalquartz crystal microbalance studies [15]. On the other hand,the SEM analysis presented on Figure 2, with a magnifica-tion of 1000X (Fig. 2a) and 10000X (Fig. 2b), showed ahomogeneous surface morphology with microcracks, similarto the metal/Al2O3 surfaces usually available in the liter-ature [4, 6, 16]. EDX analysis (not shown) of such filmsrevealed the elemental composition of HCFe-Al2O3, NiAl-Al2O3 and NiHCFe-Al2O3 films after immersion into

Fig. 1. The first cyclic voltammogram on the growth of Ni/Al2O3 electrodeposits on glassy carbon electrode. The electrodeposits wereobtained from solutions containing 0.1 mol L�1 of Al(NO3)3 and Ni(NO3)2 at a scan rate of 10 mV s�1. The inset shows a voltammogramon the growth of hydrated aluminum oxide under the same conditions but from a solution containing only the aluminum salt. Referenceelectrode: Ag/AgCl in saturated KCl.

Fig. 2. SEM images of the surface morphology of Ni-Al2O3 films.a) 1000 times. b) 10000 times. All deposits were grown voltam-metrically at a scan rate of 10 mV s�1.

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Electroanalysis 2009, 21, No. 24, 2713 – 2717 www.electroanalysis.wiley-vch.de � 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

1� 10�4 mol L�1 of Csþ solution. Al to Ni ratios of 5/2 wereobtained. The analysis showed a peak at 4.3 keV thatcorresponds to the Csþ adsorbed on the modified film.Figure 3 shows the FT-IR spectra of NiHCFe-Al2O3 films.The bands characteristic of the NiHCFe were observed. Thebroad band between 3700 and 2500 cm�1 is characteristic ofinterstitial molecules and OH– groups. The 2100 and2050 cm�1 bands were assigned to Fe-CN-Ni tensions [13]from the NiHCFe, and the 1612 cm�1 band was due to thed(OH) flexion. Less significant vibrational bands wereobserved in the low frequency zone. Some of these bandswere tension, v(FeII-C) at 600 cm�1 and v(FeIII-C) at544 cm�1, and flexions, d(FeII-CN) at 488 cm�1 and d(FeIII-CN) at 425 cm�1.

The NiHCFe-Al2O3 film was dipped into a low concen-tration solution of Csþ for 90 s, at �0.20 V, to promote thecation interchange on the composite film. According toKertezs and co-workers [17], a general equation for the Csþ

uptake and desorption on the film may be written as

CsxNiIIy jFeII(CN)6 j>Csx�1NiII

y jFeIII(CN)6 jþ e�þCsþ

where the equation, xþ 2y¼ 4, should be used to maintainthe neutrality.

The elemental composition of a film, after it wasimmersed in a cesium solution, included a significantamount of the preconcentrated cation. The Csþwas believedto selectively [17] interchange on the surface by removingthe Kþ, forming CsNiFe(CN)4�

6 [10], which is stable on themodified electrode. The great increase in the electrodesurface due to the high porosity of the hydrated aluminumoxide allowed incorporating a greater amount of HCFe and,consequently, a greater amount of cesium. The applicationof a negative potential (�0.20 V) during the accumulationprocess provided a negative charge density to the electrodesurface and reduced the NiHCFeIII to NiHCFeII, whichfavored the interchange of Kþ by Csþ. This procedure issimilar to the separation technique known as electricallyswitched ion exchange (ESIX) [18]. Once the preconcen-tration was accomplished, the electrode was placed into a

KNO3 solution to assess the Csþ influence on the voltam-metric behavior of NiHCFe. Figure 4 presents the voltam-metric behavior of the complex on the film (Fig. 4a), theinfluence of Csþ on the NiHFe signal (Fig. 4b) and thecorrelation between the registered signal and the Csþ

concentration (Fig. 4c). The CVs on Figure 4a were record-ed before (1) and after (2) the addition of the cation to thesolution media. The influence of Csþ is evident: the peakarea decreased. The CV transformation is not new; it isusually observed in voltammetric studies on NiHCFe filmsand cesium solutions [14, 19, 20]. Amos and co-workers [20]suggested that the area and shape modification of theNiHCFe voltammetric wave in cesium solutions is promotedby the alkaline ion, which partially deactivate the electro-active surface species. It is possible that the modification ofthe wave observed on a NiHCFe-Al2O3/GC electrode maybe due to a similar process, i.e., a partial deactivation of theactive sites promoted by the cesium ions. The peak area andshape modification of the voltammogram, which is aconsequence of the current or charge variation, was alwaysthe same if the cesium concentration was higher thanapproximately 10�8 mol L�1. A higher concentration mayhave saturated the interchange sites of the film. However, ifthe cation concentration was lower than that value, therewould be no saturation; therefore, the charge variation on aCsþ unsaturated film depended on the Csþ concentration (ifthe accumulation time is constant, 90 s), and a linearcorrelation between the charge and the concentration couldbe obtained. The use of low concentrations of the cation toavoid the film saturation allowed the analysis in an extendedlinear concentration range. Figure 4b shows the variation inthe cathodic peak of the differential step voltammograms,registered with NiHCFe modified electrodes immersed intoa 0.1 mol L�1 solution of KNO3. The signals described alinear relationship with the log of the Csþ concentration.Figure 4c shows a plot that includes the variation of thebackground corrected charge against the negative log of theCsþ concentration (� log [Csþ] or pCs). Every point of thecurve was obtained from the signal of a modified electrodecarrying the Csþ, accumulated during 90 s, from a specific

Fig. 3. FT-IR spectrum of a NiHCFe-Al2O3 film.

2715Determination of Cesium

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concentration solution. The linear equation can be writtenas [20], DQ¼m log jCsþ jþ b, where DQ is the differencebetween the cathodic peak area of the blank (Qb) and thecathodic peak area when the metal ion is present (QM); mand b are the slope and intercept of the curve, respectively.The background was the signal obtained from a NiHCFe-Al2O3/GC electrode free of cesium in 0.1 mol L�1 of KNO3

solution. This should be the higher cathodic peak areameasured because every increment of Csþ proportionallylowered the peak area. The curve then represented the (DQ/mC) versus pCs; if the curve is extended to the lower pCsvalues (not shown), a constant signal could be obtained. Thisbehavior responded to the intake of Csþ by the NiHCFe-Al2O3/GC film; it is reasonable to assume that when theNiHCFe is saturated with Csþ, no additional modification ofthe signal would be observed. As a result, the variation onthe peak current area may be used to quantify the concen-tration of the alkaline metal. The curve (Fig. 4c) shows acorrelation coefficient of 0.963 and a sensitivity of 0.052�0.003 mC pCs�1 with a wide linear interval (10�11 to10�8 mol L�1). According to the linear equation on Fig-ure 4c, when DQ¼ 0, which is the value of the blank, thepCs¼ 16.46. The limit of detection (LOD), calculated asthree times the standard deviation of the blank signal (3Sbl),was 2� 10�16 mol L�1. This is an approximated value; theintrinsic differences on the assembly of each film, may affectthe cation accumulation, and consequently, the results,especially at very low concentrations. The cesium solutionswith a concentration lower than 10�12 mol L�1 gave resultswith a very high variation, most of them with no analyticalsignificance. It is possible that the time period (90 s) used toachieve the accumulation process was not sufficient to raisethe signals. However, increasing the time of exposure mayaffect the mechanical stability of the film. As a result, thecapture of the cesium ions on the active sites of the film maycompete with desorption of them throughout the accumu-lation process. Therefore, the reported value may be thelowest limit of detection for cesium ever reported for anelectroanalytical technique; nevertheless, its actual quan-tification is confined to values that are more than tenthousand times higher than the LOD. According to theliterature information the electroanalytical methods pro-posed for cesium quantification have, by far, a higher LODor quantification limit. For instance, electrodes modifiedwith polymeric structures incorporating zeolites [21] orethylene glycol [22], electrodes coated with calix crownionophores [23 – 26], or even those based on hexacyanofer-rate films, such as Prussian blue [27] or nickel hexacyano-ferrate on nickel electrodes [20], reported LODs between10�5 and 10�8 mol L�1. Clearly, the method consideredherein greatly improves the cesium analytical conditions.

Hexacyanoferrate was properly confined on Ni-Al2O3

electrodeposits grown on GC electrodes by voltammetricsweeps. The prepared composites were mechanically stableand after few voltammetric cycles also became electro-chemically stable. The electrochemical behavior of theHCFe on Ni-Al2O3 appeared to be similar to the behaviorobserved for the NiII-HCFe complex; moreover, the FT-IR

Fig. 4. (a) Cyclic voltammograms of NiHCFe on Al2O3 with (1)and without (2) cesium incorporated into the film. Scan rate100 mV s�1. Supporting electrolyte: 0.1 mol L�1 of KNO3.Reference electrode: Ag/AgCl in saturated KCl. (b) The effectof Csþ on NiHCFe-Al2O3/GC modified electrodes assessed bydifferential step voltammetry in 0.1 mol L� KNO3 solution. Thesignal on top is the voltammogram with no Csþ incorporated.Next, down the arrow, every voltammogram contains Csþ

incorporated from 1.40� 10�11, 6.67� 10�10, 6.58� 10�9 and1.24� 10�8 mol L�1 of cesium solution respectively. (c) The plot(DQ/mC vs. pCs) of the calibration curve.

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results suggested that there was an in situ generation of suchcomplex. More importantly, this composite, NiHCFe-Al2O3,could be used to preconcentrate and quantify Csþ. Lowconcentrations of the alkaline metal (<10�8 mol L�1) toavoid the film saturation, allowed the analysis of cesium inan extended linear range. The detection has a goodsensitivity and a very low LOD (2� 10�16 mol L�1). How-ever, Csþ concentrations lower than 10�12 mol L�1 gavesignals with no analytical significance. Even so, this is thelowest level of Csþ ever detected by an electroanalyticaltechnique.

Experimental

The Al(NO3)3 · 9H2O (99%) and the Ni(NO3)2 · 6H2O werepurchased from Hopkins & William. CsNO3 (99%) waspurchased from Riedel-De Haen. All other chemicals werereagent grade. The solutions were prepared with distilledand demineralized water, filtered through a Milli-Q Re-agent Water System to a resistance higher than 15 MW cm.All electrochemical experiments were performed with aPrinceton Applied Research Galvanostat/Potentiostat(PAR) model 273A. The one compartment cell used todevelop the experiments housed a modified GC (0.34 cm2)as a working electrode, a Pt wire coil as the auxiliaryelectrode and an Ag/AgCl (KCl saturated) electrode as thereference. The electrolyte solution was purged and blan-keted with N2. The composition and morphology of the filmwere studied with a Philips SEM model XL-30 equippedwith an EDX spectrometer model DX4. To get informationon the inner crystal conformation, a Philips XRD systemwas used. The NiHCFe structural conformation on theAl2O3 was studied with a Brucker FT-IR model Tensor 27.The GC electrodes were ground using SiC-paper 2000 andpolished on a wet microcloth with intercalated ultrasonica-tion. The film was dried under the irradiation of a tungstenlamp for 20 min. After that, the electrode was placed in a4 mL volume of 0.01 mol L�1 HCFe solution for 20 min toincorporate the electroactive material. Once the HCFe-Ni-Al2O3 was formed, an equilibration step was applied to theworking electrode through voltammetric scans from þ0.80to þ0.10 V. The sweeps started at the most positivepotential. Increments on the peak current were observedon every scan, which became smaller during the wholevoltammetric process. More than five consecutive cycles butless than ten were enough to stabilize the current. Thenumber of cycles applied depended on the film preparation.This step allowed the oxidization of the Ni and madeavailable the in situ formation of the NiHCFe. The Csþ

preconcentration was then accomplished by dipping themodified electrode, for 90 s, into solutions containing 1.0�10�4 to 1.0� 10�11 mol L�1 of CsNO3. A negative potential(�0.20 V) was applied to the electrode to improve the Csþ

uptake.

Acknowledgements

The authors gratefully acknowledge the Fondo Nacional deCiencia, Innovacion y Tecnolog�a (FONACIT, Grant N8 S1-99000442) and the Universidad Simon Bol�var (USB) fromCaracas, Venezuela, for the financial support.

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2717Determination of Cesium

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