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Electrochimica Acta 105 (2013) 439– 446

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

lectrochemical synthesis of dysprosium hexacyanoferrate microtars incorporated multi walled carbon nanotubes and itslectrocatalytic applications

uniyandi Rajkumar, Balamurugan Devadas, Shen-Ming Chen ∗

lectroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1,ection 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC

a r t i c l e i n f o

rticle history:eceived 20 December 2012eceived in revised form 21 April 2013ccepted 30 April 2013vailable online 13 May 2013

eywords:lectrochemical synthesisysprosium hexacaynoferrateicro starsopamineric acid

a b s t r a c t

Herein, we report template free, surfactant less, fabrication of dysprosium hexacyanoferrate (DyHCF)micro stars by simple electrochemical deposition process. The electrochemical measurements and surfacemorphology of the as prepared composite electrode are studied using cyclic voltammetry (CV), electro-chemical impedance spectroscopy (EIS) and field emission scanning electron microscopy (FESEM). Thesynthesized DyHCF micro stars are characterized by Fourier transmitted infra-red spectroscopy (FTIR), X-ray diffraction (XRD) and ultra violet spectroscopy, respectively. The size of the micro stars are controlledby, controlling the number of electrodepositing cycles. The amount of multi walled carbon nanotubes(fMWCNTs) loading are optimized by EIS analysis. The presence of the fMWCNTs in the film enhancesthe surface coverage concentration and also increases the electron transfer rate constant, of the DyHCFmicro stars. This DyHCF incorporated multi walled carbon tube modified electrode (DyHCF/fMWCNTs)exhibits a prominent electrocatalytic activity toward the selective detection of DA and UA in presence

elective determination of AA. The peaks of the AA, UA, and DA get separated well in the DyHCF–fMWCNTs modified electrodewith potential value of 250 mV (between AA and DA) and 320 mV (between AA and UA). The modifiedelectrode shows the linear range of 3–137 �M for UA and DA. The proposed film also successfully used forthe selective detection of DA and UA in the presence of AA in human urine samples with a linear range of3–289 �M. Well separated peak for the detection of UA and DA in urine has proven this DyHCF/fMWCNTs

ucces

modified electrode as a s

. Introduction

In recent years, synthesis and investigation of the electrochem-cal behavior of metal hexacyanoferrate (MHCF) have attractedonsiderable attention due to their ability to mediate numberf electrocatalytic processes. In particular, high stability of theetal cyanide framework in MHCFs and the possibility for cation

xchange between a solution and the cages of framework has madehese metal complexes based research much more interesting1]. To date, many researchers have investigated several MHCFsnd their potential application in the field of electrochemistry2,3]. In the course of the developments MHCF have been mades nanoparticles and electrochemically characterized by severalesearchers [4]. Owing to the large surface area, good conductivity

nd outstanding catalytic activity, MHCF nanoparticles have beenidely used for the development of electrochemical sensors [5,6]

nd biosensors applications [7,8]. In the series of different MHCF

∗ Corresponding author. Tel.: +886 2270 17147; fax: +886 2270 25238.E-mail address: smchen78@ms15.hinet.net (S.-M. Chen).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.04.172

sful biosensor.© 2013 Elsevier Ltd. All rights reserved.

developments, rare earth metal dysprosium hexacyanoferrate(DyHCF) also prepared and applied for the catalytic purposes[9–12]. These preparation methods are chemical and do not showany specific structure to the DyHCF.

On the other hand, there is a constant requirement to develop asensor for the biologically important compounds dopamine (DA),uric acid (UA) and ascorbic acid (AA) which usually coexist togetherand considered as important molecules for physiological processesin human metabolism. UA and DA deficiencies result in severaldiseases and disorders [13–16]. The first species plays an impor-tant role in human brain and a loss of DA-containing neurons mayresult in some serious diseases such as Parkinson. The main diffi-culty with the electrochemical detection of DA in brain fluids is thecoexistence of many interfering compounds. Among these, ascorbicacid (AA) and UA are of particular importance; these compoundsmay be present in relatively high concentration (100–500 mM forAA and 1–50 mM for UA), while baseline dopamine level is of the

order of 50 nM [17]. In clinical point of view, detection of UA inurine also shares a similar importance and this again has interfer-ence from AA. Thus, researchers are keen on developing biosensorfor the selective detection of dopamine (DA) and uric acid (UA). But,

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in scan rates between 0.01 and 0.1 V/s. This indicated that theelectron transfer process occurring at DyHCF/fMWCNTs compos-

40 M. Rajkumar et al. / Electroc

etermination of DA and UA on solid electrodes is challenging oneue to the overlapping oxidation peak potentials.

There are few film modified electrodes have been reportedor the selective determination of these compounds inhe presence of ascorbic acid such as 2,2-bis(3-amino-4-ydroxyphenyl)hexafluoropropane modified glassy carbonlectrode [18], graphite oxide bulk modified carbon paste elec-rode [19], polymerized luminol film modified glassy carbonlectrode [20], and bio sensing properties of titanate-nanotubelms [21] new polymeric composite film [22]. These electro-hemical sensors satisfied many of the requirements such aspecificity, speed of response, sensitivity and simplicity of prepa-ation. However, the utilities of solid-electrode-based sensors areften hampered by not having sufficient selectivity. In particular,omplexity of real biological systems may result in overlappingoltammetric signals.

Here we report a simple electrochemical method to prepareyHCF and achieved specific micro star structure. This redoxomplex with the micro star structure was employed for theelective electrocatalytic determination of UA and DA by mak-ng DyHCF–fMWCNTs composite. This composite was preparedn GCE and ITO electrodes by simple two steps process elec-rodeposition of DyHCF followed by drop casting of fMWCNTs.s prepared electrode was characterized using surface analysis

echnique FESEM along with electrochemical techniques CV andIS. DyHCF/fMWCNTs modified electrode successfully separatedA and DA without AA interference and quantified in real sys-

em in urine sample using differential pulse voltammetric (DPV)ethod.

. Experimental

.1. Apparatus

Electrochemical measurements like cyclic voltammogram (CV)nd differential pulse voltammogram were performed by using

CHI 1205 A electrochemical analyzer. A conventional three-lectrode cell was used at room temperature with glassy carbonlectrode (GCE) (surface area = 0.07 cm2) as the working electrode,g/AgCl (saturated KCl) electrode as reference electrode and a plat-

num wire as counter electrode. The potentials mentioned in allxperimental results were referred to standard Ag/AgCl (saturatedCl) reference electrode. Surface morphology of the film was stud-

ed by FESEM (Hitachi, Japan). Electrochemical impedance studiesEIS) were performed by using ZAHNER impedance analyzer (ZAH-ER Elektrik GmbH & Co. KG, Germany).

.2. Materials

Dysprosium(III) chloride hexahydrate, multi walled carbonanotubes, ascorbic acid (AA), dopamine (DA) and uric acid (UA)ere purchased from Sigma–Aldrich. Potassium hexacyanofer-

ate(III) was purchased from Wako Pure Chemical Industries andA, DA and UA solutions were freshly prepared every day. The otherhemicals (Merck) were used in this investigation with analyticalrade (99%). All the solutions were prepared using doubly distilledater. Electrocatalytic studies were carried out in 0.2 M KCl solu-

ion. Pure nitrogen gas was purged through all the experimentalolutions for removing dissolved oxygen.

.3. Preparation of fMWCNTs and electrochemical fabrication ofyHCF/fMWCNTs composite modified electrode

There was an important challenge in the preparation ofMWCNTs. Because of its hydrophobic nature, it was difficult toisperse it in any aqueous solution to get a homogenous mixture.

Acta 105 (2013) 439– 446

Briefly, the hydrophobic nature of the MWCNTs was converted into hydrophilic nature by following the previous studies [23,24].The pretreatment and functionalization of MWCNTs was done bysuspending 150 mg of MWCNTs in mixture of concentrated sul-furic acid–nitric acid (3:1, v/v) and sonicated for 2 h. After thata nanotube mat was obtained and was filtered using a 0.45 mmhydrophilized PTFE membrane and washed with deionized wateruntil the pH becomes 7 and kept for drying under vacuum. 10 mgof thus obtained fMWCNTs was dissolved in 10 ml water and ultra-sonicated for 6 h to get a uniform dispersion. This process not onlyconverts fMWCNTs to hydrophilic nature but this helps to break-down larger bundles of the fMWCNTs into smaller ones.

Prior to the electrodeposition process, the bare glassy carbonelectrode was initially polished with 0.05 �M alumina powderusing BAS polishing kit and ultrasonically cleaned in water for aminute. The electrode was then washed with double distilled waterand utilized for further electrodeposition. The DyHCF particles areelectrochemically deposited on the GCE from 0.2 M KCl solutioncontaining 1 × 10−2 M DyCl3·6H2O and 1 × 10−2 M K3 Fe(CN)6, witha repetitive potential scan between 0.8 and −0.2 V (at the scanrate of 100 mV/s) for twenty cycles [25]. Then 5 �l of as preparedfMWCNTs was drop casted on the DyHCF modified electrode toform DyHCF/fMWCNTs composite film. Then the DyHCF/fMWCNTsmodified GCE was rinsed with deionized water and applied for thefurther electrochemical studies.

3. Results and discussion

3.1. Electrochemical characterization of DyHCF/fMWCNTsmodified electrode

Fig. 1A shows the cyclic voltammogram (CV) of DyHCF microstars deposition process. Here the CV shows the characteristiccurrent features of a cathodic peak corresponding to the reduc-tion of Fe(CN)6

3− to Fe(CN)64− appears at 0.223 V and the anodic

peak corresponding to the oxidation of Fe(CN)64− to Fe(CN)6

3−

appears at 0.313 V. On scanning the potential back to 0.8 V, itclearly shows the peaks currents corresponds to the reaction ofFe(CN)6

3−/4− redox couple decrease gradually with the increaseof the scan cycles, which validates the formation of DyHCF filmon the electrode surface. The Dy3+ ions reacted with Fe(CN)6

4−

to form DyHCF film on the electrode surface. Here the equivalentmolar concentrations (10−2 M) of dysprosium and hexacyanofer-rate have been taken for the balanced deposition of DyHCF film.These observations clearly suggest that the successive formation ofDyHCF film on the GCE surface. Furthermore 5 �l of fMWCNTs wasdrop casted on DyHCF modified GCE. The formation of DyHCF onthe electrode surface can be expressed by the following reactionmechanism.

Fe(CN)63− + e− → Fe(CN)6

4− (1)

Fe(CN)64− + Dy3+ + K+ → KDyFe(CN)6 (2)

In the next step, the prepared DyHCF/fMWCNTs electrode wastransferred into pH 7 PBS for scan rate studies. Fig. 1B shows thecyclic voltammograms obtained at DyHCF/fMWCNTs compositeelectrode in N2 saturated PBS solution (pH 7) at different scanrate studies. Both Ipa and Ipc increased linearly with increase

ite film is a surface confined process. The peak currents (Ipa andIpc) vs. scan rates plot is shown in Fig. 1B, inset. Both Ipa and Ipcexhibited linear relationship with scan rates, R2 = 0.991 and 0.999,respectively.

M. Rajkumar et al. / Electrochimica Acta 105 (2013) 439– 446 441

Fig. 1. (A) Cyclic voltammogram of electro deposition of DyHCF film on glassycarbon electrode surface from 0.2 M KCl containing (1 × 10−2 M) DyCl3·6H2O and(1 × 10−2 M) K3 Fe(CN)6 and potential scan between 0.8 and −0.2 for 20 cycles atthe scan rate of 100 mV/s. (B) Different scan of the DyHCF/MWCNTs composite filmm0a

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Fig. 2. (A). EIS of DyHCF/GCE modified by different concentration of MWCNTsin 0.2 M KCl containing 5 mM [Fe(CN)6]3−/4− . Inset shows the Randles circuit

odified electrode in 0.2 M KCl solution at the scan rate varies from 0.01, 0.02, 0.03,.04, 0.05, 0.06, 0.07, 0.08, 0.09, and 0.1 V/s. Inset shows a current vs. scan rate plott 0.2 M KCl solution.

.2. Investigation of electrochemical behavior of various filmodified electrodes using EIS studies

EIS is an exact method to elucidate the electrochemical prop-rties of the proposed film. The EIS analysis has been studiedy analyzing the Nyquist plots of the corresponding films. Herehe respective semicircle parameters correspond to the electronransfer resistance (Ret), solution resistance (Rs) and double layerapacity (Cdl) of the films. The plot of the real component (Zre)nd the imaginary component Zim (imaginary) resulted in theormation of a semi-circular Nyquist plot. From the shape of anmpedance spectrum, the electron-transfer kinetics and diffusionharacteristics can be extracted. The respective semicircle param-ters correspond to the electron transfer resistance (Ret) and theouble layer capacity (Cdl) nature of the modified electrode. Themount of MWCNTs loading concentration is optimized by the EIS

tudies. Fig. 2A shows the Nyquist plots of different concentrationsf fMWCNTs with DyHCF film modified electrodes concentrationanging from 2 to 6 �l in 0.2 M KCl containing 5 mM [Fe(CN)6]3−/4−

nd the inset shows the Randles equivalent circuit model for the

for the above-mentioned electrodes. (B) EIS of bare GCE, MWCNTs, DyHCF andDyHCF/MWCNTs/GCE in 5 mM [Fe(CN)6]3−/4− in 0.2 M KCl. Inset of (B) shows theEIS of Bare GCE and DyHCF/MWCNT/GCE in 5 mM [Fe(CN)6]3−/4− in 0.2 M KCl.

proposed film. As can be seen in Fig. 2A comparing with differ-ent concentrations of fMWCNTs with DyHCF film, 5 �l of fMWCNTsexhibits a small semicircle when compared with other concentra-tions. This result illustrates that the 5 �l of fMWCNTs drop castedon the DyHCF film possesses the lower electron transfer resis-tance comparing with the other concentrations which enhances theelectron-transfer kinetics process as a faster one and more suitablefor the electrocatalytic activities. Therefore, in all our experiments5 �l of fMWCNTs was used for the composite film electrode, unlessotherwise it is specified.

After confirming the MWCNTs concentration, we againemployed the EIS method for the individual films. Fig. 2B representsthe Nyquist plots for bare, fMWCNTs, DyHCF and DyHCF/fMWCNTsmodified GCE in 0.2 M KCl containing 5 mM [Fe(CN)6]3−/4− andthe inset in Fig. 2B shows the Nyquist plot for only bare andDyHCF/fMWCNTs modified electrode. Whereas, in Fig. 2B showsDyHCF/fMWCNTs modified GCE exhibits a very small semicircleregion indicating a very low impedence of the film due to thehigh conductivity nature of fMWCNTs. This results shows thatthe small semicircle region for the DyHCF/fMWCNTs compositefilm possesses a very good electrochemical activity compare withbare, only fMWCNTs and only DyHCF modified GCE. Therefore, thecomoposite film could be efficiently used for the various types

of electrocatalytic reactions. A simplified randles circuit model(Fig. 2B, inset) has been used to fit the impedance spectra. Therandles circuit model well suites with the impedance spectro-scopic results and the fit model error for the film was found as

442 M. Rajkumar et al. / Electrochimica Acta 105 (2013) 439– 446

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.3%. Finally the electrochemical impedance spectroscopic analysislearly illustrates that the electrochemical behavior of the proposedyHCF/fMWCNTs composite film is excellent.

.3. Characterization of DyHCF micro stars

In order to verify the composition of the synthesized DyHCFicro stars samples, it is necessary to characterize the specifiedyHCF samples by suitable techniques, such as FTIR, XRD, and UVxperiments. Fig. 3A is a typical FTIR spectrum of DyHCF samples.rom the spectrum, a broad band at around 3369 cm−1 and a sharpeak at 3573 cm−1 revealed that there are two kinds of H2O in thetructure of DyHCF. One is the interstitial or zeolitic water and thether is the water coordinated to Dy. The strong and sharp peakocated at 2078 cm−1 is corresponding to the stretching vibrationf C N group in the DyHCF. According to the earlier reported lec-ure by Goubard and Tabuteau, the peaks at 1652 and 1596 cm−1 aressigned to the binding vibration of crystal water contained in thetructure of DyHCF. Two metal–carbon peaks at 595 and 580 cm−1

re assigned to ı FeC N and ı Fe C [26]. These results indicate thathere is only one stoichiometric compound existing in the DyHCFystem. This conclusion is also verified by the results of XRD of sam-les in Fig. 3B. The major peaks (2�) appear at 21.6◦, 30.28◦, 36.3◦,5.5◦ and 56.6◦. All the peaks in the XRD pattern can be indexed

n an orthorhombic (Cmcm) space group with unit-cell volume ingreement with ionic radii values. These crystal structure dates andhe lattice parameters are almost the same as the previous report

bout the structural characterization of DyHCF [27]. UV adsorptionpectrum also further studied for the as prepared DyHCF sampless shown in Fig. 3C. As a result of the f–f electron transitions DyHCFons in the UV intensively absorb UV light in the range from 300

tion spectrum of the as prepared DyHCF micro stars.

to 350 nm. These results clearly indicate the characteristics peaksand the absorption spectrum of the as prepared DyHCF. A sharpand broad absorption appears at 317 nm and 410 nm which maybe attributed to the different crystal field interactions caused byligands bonded to DyHCF ions.

3.4. Morphological and EDX studies

The morphology and size of the DyHCF and DyHCF/fMWCNTsfilms on the ITO substrate were examined by FESEM and shown inFig. 4. Electrodeposition cycles were limited and examined to knowthe formation and growth of the micro stars. When the cyclingnumber constrained to 5 cycles (Fig. 4A) the particles formed ina smaller size without any specific morphology. Upon increase ofcycles to 10 (Fig. 4B), the particles started to have Carambolalikelike structure and gave a perfect star shape at 15 cycle (Fig. 4C).Further increase to 20 cycles leads to formation of closely packedmicro stars with increased size (Fig. 4D). On the other hand, theDyHCF/fMWCNTs on the ITO substrate show that the fMWCNTsare homogenously dispersed throughout the micro stars andthe distance between the micro stars is almost uniform. TheseDyHCF/fMWCNTs have a size distribution of 5–7 �m (Fig. 4E).The immobilized DyHCF/fMWCNTs composite film on the surfaceof ITO remains stable and there is no distortion in shape. Finallyfrom these results, it is evident that the size and morphologyfor the formation of DyHCF micro stars were electrochemically

controlled by the number of cycles and also the prepared DyHCFwere uniformly incorporated on the f MWCNTs. To confirm thepresence of dysprosium, EDX spectral study was performed forthe film (Fig. 4E) which shows the presence of 38% of Dy along

M. Rajkumar et al. / Electrochimica Acta 105 (2013) 439– 446 443

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ig. 4. FESEM images of DyHCF micro particles prepared at different cycles (A) 5 cyf DyHCF modfied ITO.

ith ITO electrode components Si and In. These results prove theresence and formation of DyHCF on the electrode surface.

.5. Selective detection of DA and UA in presence of AA

DyHCF/fMWCNTs modified GCE could be directly employedor the selective detection of DA and UA in presence of AA.ig. 5 represents the comparison study for the selective detec-ion of these compounds. In Fig. 5, the CV curve a representshe DyHCF/fMWCNTs film response, curve b represents the onlyyHCF and curve c represents the bare GCE response for the

elective determination of DA and UA. Comparing the results ofyHCF/fMWCNTs with only DyHCF film, the DyHCF/fMWCNTs

hows well defined obvious electrocatalytic peaks for the selec-ive detection of DA and UA in presence of AA. At the same, only

MWCNTs modification (figure not shown) gives separate peaks

hich were not as obvious as found using DyHCF/fMWCNTs film. AtyHCF/fMWCNTs the peak current has been enhanced by 2 times

han that of bare GCE and DyHCF/GCE. This enormous increase

B) 10 cycles, (C) 15 cycles, (D) 20 cycles, (E) DyHCF/fMWCNTs and (F) EDX spectra

in peak current shows the high conducting nature and increasedactive surface area of the DyHCF/fMWCNTs composite modifiedelectrode. As evident from the EIS results in Section 3.2, DyHCFforms a conducting composite with fMWCNTs and facilitates a fastelectron transfer and also may be due to the formation of DyHCFmicro stars which acts as electroactive centers thereby increasingthe efficiency of the electrode for the detection and determinationof DA and UA. Finally, this CV results clearly depict the capabilityof the proposed DyHCF/fMWCNTs film for the detection of DA andUA in presence of AA. Scheme 1 could explain the fabrication andelectron mediating properties of DyHCF/fMWCNTs composite filmtoward the selective oxidation of DA, UA in presence of AA.

In the next step, differential pulse voltammetry (DPV) has beenemployed for the selective detection of these compounds. Fig. 6shows the DPV curves of DA and UA for various concentrations

in the presence of higher concentrations of AA (0.5 mM) at theDyHCF/fMWCNTs composite film modified GCE. Here the voltam-metric response of DA and UA in the presence of AA shows twowell-defined voltammetric potential peaks at 0.36 V and 0.53 V,

444 M. Rajkumar et al. / Electrochimica Acta 105 (2013) 439– 446

Fig. 5. Cyclic voltammogram response of different electrodes for the selectivedD

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Fig. 6. DPVs of DyHCF/MWCNTs composite film modified electrode for the selec-

(0.5 mM) in human urine samples. The dopamine injection solu-

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etermination of AA (137 �M) in the presence of DA and UA (137 �M) at (a)yHCF/fMWCNTs, (b) only DyHCF and (C) bare GCE.

espectively and enough separation occurred between the AA–DA0.25 V) and AA–UA (0.32 V) oxidation peak potentials at theyHCF/fMWCNTs composite film modified GCE. Further the oxida-

ion peak currents of DA and UA increases linearly in conjunctionith increasing concentrations in the range of 3–137 �M. This

esults shows that the DyHCF/fMWCNTs composite film modifiedCE possess the specific electrocatalytic activity, which could beonsidered as the main reason for the successful anodic peak sep-ration between the DA and UA in presence of AA. The inset ofig. 6 shows the current vs. concentration plot for DA and UA inhe presence of higher concentrations of AA. Based on this calibra-

ion plot, the linear regression equation for DA has been expresseds I (�A) = 0.0318 C (�M) + 1.5671, R2 = 0.9346 and for UA has beenxpressed as I (�A) = 0.0556 C (�M) + 0.8554, R2 = 0.9392. The result

cheme 1. Schematic representation of the fabrication and selective electro catalytic oxlectrode.

tive determination of DA and UA in presence of AA. DA and UA were in the linearrange of (3–137 �M). Inset shows a current vs. concentration plot of DA and UAdetermination.

shows that the proposed DyHCF/fMWCNTs composite film mod-ified GCE possess the capability for the selective detection anddetermination of DA and UA in presence of AA without any foulingeffect.

3.6. Selective determination DA and UA in presence of AA inhuman urine samples

The analytical performance of the DyHCF/fMWCNTs compositefilm modified GCE has been evaluated for the selective detec-tion of DA and UA in presence higher concentrations of AA

tion (40 mg ml−1) and ascorbic acid (500 mg) tablets are utilizedfor the real sample analysis. Before the examination, freshly col-lected human urine sample (pH 6.7) were filtered several times

idation of DA, UA in presence of AA by DyHCF/fMWCNTs composite film modified

M. Rajkumar et al. / Electrochimica Acta 105 (2013) 439– 446 445

Table 1Comparison chart for the selective determination of DA and UA in various metal hexacyanoferrates and other modified electrodes based literature reports.

Electrochemical method Modification electrode Analyte Sample Linear range (�M) Reference

DPV CNT/ruthenium oxidehexacyanoferrate

Human urine UA 0.9–250 [28]

DPV Ruthenium oxidehexacyanoferrate/rutheniumhexacyanoferrate

Dopadic ampoule DA 0.5–25 [29]

CV Silver hexacyanoferratenanoparticles

HumanUrine

DA UA 2.4–130 2.0–150 [30]

DPV Carbon nano fiber Not reported DA UA 0.04–5.6 0.8–16.8 [31]DPV Poly vinyl alcohol Human

UrineDA UA 2.0–70 2.0–50 [32]

Amperometry (i–t) Nickel Not reported UA 0.1–18 [33]

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hexacyanoferrate/MWCNTsDPV Dysprosium

hexacyanoferrate/fMWCNTsInjection

sing Whatman filter paper (grade 1). Filtered human urine sam-les were diluted in pH 7 PBS in the ratio of 1:50. Fig. 7 shows thePV response of DyHCF/fMWCNTs composite film modified GCE for

he detection of DA and UA in the presence of AA in human urineamples. Here also we can observe the well separated peaks forA and UA in presence of AA with enough peak separation around.25 V for AA–DA and 0.43 V for AA–UA and the linear range wasound to be 3–289 �M, respectively. The inset shows the currents. concentration plot of DA and UA in human urine samples. Theinear range dependence of DPV response on concentration of DA isxpressed as I (�A) = 0.0462 C (�M) + 2.3468, R2 = 0.936 and for UAas been found as I (�A) = 0.0755 C (�M) + 2.232, R2 = 0.918. Basedn these results, it clearly validates the capability of the proposedlectrode for the detection of DA and UA (real samples) in humanrine.

.7. Repeatability, reproducibility and stability studies

The repeatability of the DyHCF/fMWCNTs composite film modi-ed film for the detection of DA and UA in the presence AA has beenvaluated by the DPV studies. DPVs were recorded in pH 7 PBS at

he scan rate of 100 mV/s in the presence of DA and UA at 135 �Mespectively. The fabricated sensors shows good repeatability with

relative standard deviation (RSD) of 4.7% for (n = 10). Moreover,t exhibits a good reproducibility with an and RSD value of found

ig. 7. DPVs of DyHCF/MWCNTs composite film modified electrode for the selectiveetermination of DA and UA in presence of AA in human urine samples. DA and UAere in the linear range of (3–289 �M). Inset shows a current vs. concentration plot

f DA and UA determination.

on and human urine DA UA 3–289 3–289 This work

as 3.8% for 5 successive individual measurements. The repeatabil-ity and the reproducibility values confirm that the proposed filmwas suitable for the detection and determination DA and UA in thepresence of AA. Next the stability of the film has been examined bystoring it at the room temperature in the open air condition (fig-ure not shown). After four weeks, it showed a stable behavior onlywith a gradual decrease (12%) from the initial current values. Theseresults validating that, the modified electrode has the good repeata-bility and reproducibility for DA and UA detection in presence ofhigher concentrations of AA (see Table 1).

4. Conclusions

In this report, DyHCF micro stars have been synthesized bya facile electrochemical deposition method. It is found that thesize and morphology of DyHCF Microstars were controlled by thenumber of electrodepositing cycles. The DyHCF/fMWCNTs modi-fied GCE possess the high electro active surface area which is wellsuited for the selective determination of DA and UA. Finally, theDyHCF/fMWCNTs modified GCEs also applied for the detection ofDA and UA (real samples) in human urine samples and remark-ably suppressed the interference effect. Thus, the DyHCF/fMWCNTsmodified GCE electrode showed higher stability, reproducilibityand exhibits promising electrocatalytic activity and rapid responsetoward selective determination of DA and UA in both lab and realsample analysis.

Acknowledgments

This work was supported by grants from National Science Coun-cil (NSC) of Taiwan (ROC). The authors would like to thank Mr.Baskar Selvaraj and Mr. Thiyagarajan Natarajan for their valuablehelp throughout this project and also extend our thanks to Ms.Doffyand Dr. Krishna Kumar for providing characterization facilities.

References

[1] S.J. Reddy, A. Dostal, F. Scholz, Solid state electrochemical studies of mixednickel-iron hexacyanoferrates with the help of abrasive stripping voltammetry,Journal of Electroanalytical Chemistry 403 (1996) 209.

[2] U. Schrèoder, F. Scholz, The solid-state electrochemistry of metal octa-cyanomolybdates, octacyanotungstates, and hexacyanoferrates explained onthe basis of dissolution and reprecipitation reactions, lattice structures, andcrystallinities, Inorganic Chemistry 39 (2000) 1006.

[3] P. Wu, S. Lu, C. Cai, Electrochemical preparation and characterization of asamarium hexacyanoferrate modified electrode, Journal of ElectroanalyticalChemistry 569 (2004) 143.

[4] P.G. Fenga, N.R. Stradiotto, M.I. Pividori, Silver nanocomposite electrode modi-

fied with hexacyanoferrate. Preparation, characterization and electrochemicalbehaviour towards substituted anilines, Electroanalysis 23 (2011) 1100.

[5] H. Yu, S.W. Song, Y.Y. Lian, Z.Y. Liu, G.C. Qi, Electrochemical preparation ofcopper hexacyanoferrate nanoparticles under the synergic action of EDTA andHAuCl4, Journal of Electroanalytical Chemistry 650 (2010) 82.

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46 M. Rajkumar et al. / Electroc

[6] H. Razmi, A. Taghvimi, Tin hexacyanoferrate nanoparticles based electrochem-ical sensor for selective and high sensitive determination of H2O2 in acidicmedia, International Journal of Electrochemical Science 6 (2010) 751.

[7] M. Yang, J. Jiang, Y. Yang, X. Chen, G. Shen, R. Yu, Carbon nanotube/cobalt hexa-cyanoferrate nanoparticle-biopolymer system for the fabrication of biosensors,Biosensors and Bioelectronics 21 (2006) 1791.

[8] A.P. Baioni, M. Vidotti, P.A. Fiorito, S.I.C.D. Torresi, Copper hexacyanoferratenanoparticles modified electrodes: a versatile tool for biosensors, Journal ofElectroanalytical Chemistry 622 (2008) 219.

[9] H. Aono, E. Traversa, M. Sakamoto, Y. Sadaoka, Crystallographic charac-terization and NO2 gas sensing property of LnFeO3 prepared by thermaldecomposition of LnFe hexacyanocomplexes, Ln[Fe(CN)6]·nH2O, Ln = La, Nd,Sm, Gd, and Dy, Sensors and Actuators B 94 (2003) 132.

10] V. Kavecansky, J. Kovac, M. Zentkova, I. Zezula, Structure and magnetism ofrare-earth ferricyanides and the role of 4f electrons, Czechoslovak Journal ofPhysics 52 (2002) 333.

11] F. Hulliger, H. Vetsch, The ferrocyanides CsLnFe(CN)6·nH2O(Ln = La, . . ., Lu, Y),European Journal of Solid State and Inorganic Chemistry 31 (1994) 363.

12] D.F. Mullica, H.O. Perkins, E.L. Sappenfielf, D.A. Grossie, Synthesis and struc-tural study of samarium hexacyanoferrate(III) tetrahydrate, SmFe(CN)6·4H2O,Journal of Solid State Chemistry 74 (1988) 9.

13] C. Martin, Dopamine detection in brain, Chemistry in Britain 34 (1998) 40.14] R.M. Wiightman, L.J. May, A.C. Michael, Detection of dopamine dynamics in the

brain, Analytical Chemistry 60 (1988) 769.15] A. Heinz, H. Przuntek, G. Winterer, A. Pietzcker, Clinical aspects and follow-

up of dopamine-induced psychoses in continuous dopaminergic therapy andtheir implications for the dopamine hypothesis of schizophrenic symptoms,Nervenarzt 66 (1995) 662.

16] V.S.E. Dutt, H.A. Mottola, Determination of uric acid at the microgram level bya kinetic procedure based on a pseudo-induction period, Analytical Chemistry46 (1974) 1777.

17] R.D. O’Neill, Microvoltammetric techniques and sensors for monitoring neuro-chemical dynamics in vivo. A review, Analyst 119 (1994) 767.

18] G. Milczarek, A. Ciszewski, 2,2-Bis(3-amino-4-hydroxyphenyl)hexa-fluoropropane modified glassy carbon electrodes as selective and sensitivevoltammetric sensors. Selective detection of dopamine and uric acid,Electroanalysis 16 (2004) 1977–1983.

19] T. Thomas, R.J. Mascarenhas, C. Nethravathi, M. Rajamathi, B.E. Kumara Swamy,Graphite oxide bulk modified carbon paste electrode for the selective detection

of dopamine: a voltammetric study, Journal of Electroanalytical Chemistry 659(2011) 113.

20] S. Ashok Kumar, H.W. Cheng, M. Chen, Selective detection of uric acid in thepresence of ascorbic acid and dopamine using polymerized luminol film mod-ified glassy carbon electrode, Electroanalysis 20 (2009) 2281.

[

Acta 105 (2013) 439– 446

21] Y. Liu, L. Xu, Electrochemical sensor for tryptophan determination based oncopper–cobalt hexacyanoferrate film modified graphite electrode, Sensors 7(2007) 2446.

22] Y. Xiao, C. Guo, C.M. Li, Y. Li, J. Zhang, R. Xue, S. Zhang, Highly sensitive andselective method to detect dopamine in the presence of ascorbic acid by a newpolymeric composite film, Analytical Biochemistry 371 (2007) 229.

23] K. Jiang, S. Schadler, W. Siegel, X. Zhang, C.H. Zhang, M. Terronesd, Proteinimmobilization on carbon nanotubes via a two-step process of diimide-activated amidation, Journal of Materials Chemistry 14 (2004) 37.

24] K.C. Lin, T.H. Tsai, S.M. Chen, Performing enzyme-free H2O2 biosensor andsimultaneous determination for AA, DA, and UA by MWCNT–PEDOT film,Biosensors and Bioelectronics 26 (2010) 608–614.

25] P. Wu, Y. Shi, C.X. Cai, Electrochemical preparation and characterization ofdysprosium hexacyanoferrate modified electrode, Journal of Solid State Elec-trochemistry 10 (2006) 270.

26] B.I. Swanson, J.J. Rafalko, Perturbation of intramolecular vibrations by stronginterionic forces. Vibrational spectra and assignments for hexacyanoferrate(4−) ion and cesium magnesium hexacyanoferrate (Cs 2MgFe(CN)6), InorganicChemistry 15 (1976) 249–253.

27] F. Goubard, Tabuteau, Synthesis, spectroscopic, thermal, and structural char-acterization of complex ferrocyanides KLnFe(II)(CN)6·3.5H2O (Ln = Gd–Ho),Structural Chemistry 14 (2003) 257–262.

28] J.B. Raoof, R. Ojani, M. Baghayeri, A selective sensor based on aglassy carbon electrode modified with carbon nanotubes and rutheniumoxide/hexacyanoferrate film for simultaneous determination of ascorbic acid,epinephrine and uric acid, Analytical Methods 3 (2011) 2367–2373.

29] R.K. Shervedani, H.A. Alinajafi-Najafabadi, Electrochemical determination ofdopamine on a glassy carbon electrode modified by using nanostructureruthenium oxide hexacyanoferrate/ruthenium hexacyanoferrate thin film,International Journal of Electrochemistry 5 (2011) 603135.

30] M. Noroozifara, M. Khorasani-Motlagh, A. Taheri, Preparation of silverhexacyanoferrate nanoparticles and its application for the simultaneousdetermination of ascorbic acid, dopamine and uric acid, Talanta 80 (2010)1657–1664.

31] Y. Liu, J. Huang, H. Hou, T. You, Simultaneous determination of dopamine, ascor-bic acid and uric acid with electrospun carbon nanofibers modified electrode,Electrochemistry Communications 10 (2008) 1431–1434.

32] Y. Li, X. Lin, Simultaneous electroanalysis of dopamine, ascorbic acid and uricacid by poly (vinyl alcohol) covalently modified glassy carbon electrode, Sen-

sors and Actuators B 115 (2006) 134–139.

33] B. Fang, Y. Feng, G.F. Wang, C. Zhang, A. Gu, M. Liu, A uric acid sensor basedon electrodeposition of nickel hexacyanoferrate nanoparticles on an electrodemodified with multi-walled carbon nanotubes, Microchimica Acta 173 (2011)27–32.