Carbon-Pt Nanoparticles Modi“ed TiO Nanotubes for...

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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 6668ndash6675 2011

Carbon-Pt Nanoparticles Modified TiO2 Nanotubes forSimultaneous Detection of Dopamine and Uric Acid

Sara Mahshid12 Shenglian Luo1lowast Lixia Yang1 Sahar Sadat Mahshid12Masoud Askari2 Abolghasem Dolati2 and Qingyun Cai1lowast

1State Key Laboratory of ChemoBiosensing and Chemometrics Department of Chemistry Hunan University Changsha 410082 China2Materials Science and Engineering Department Sharif University of Technology PO Box 11155-9466 Tehran Iran

The present work describes sensing application of modified TiO2 nanotubes having carbon-Ptnanoparticles for simultaneous detection of dopamine and uric acid The TiO2 nanotubes electrodewas prepared using anodizing method followed by electrodeposition of Pt nanoparticles onto thetubes Carbon was deposited by decomposition of polyethylene glycol in a tube furnace to improvethe conductivity The C-Pt-TiO2 nanotubes modified electrode was characterized by cyclic voltam-metry and differential pulse voltammetry methods The modified electrode displayed high sensitivitytowards the oxidation of dopamine and uric acid in a phosphate buffer solution (pH 700) Theelectro-oxidation currents of dopamine and uric acid were linearly related to the concentration overa wide range of 35times10minus8 M to 1times10minus5 M and 1times10minus7 M to 3times10minus5 M respectively The limitof detection was determined as 2times10minus10 M for dopamine at signal-to-noise ratio of 3 The inter-ference of uric acid was also investigated Electro-oxidation currents of dopamine in the presenceof fix amount of uric acid represented a linear behaviour towards successive addition of dopaminein range of 1times 10minus7 M to 1times 10minus5 M Furthermore in a solution containing dopamine uric acidand ascorbic acid the overlapped oxidation peaks of dopamine and ascorbic acid could be easilyseparated by using C-Pt-TiO2 nanotubes modified electrode

Keywords TiO2 Nanotubes Pt Nanoparticles Carbon Dopamine Biosensor

1 INTRODUCTION

Dopamine (4-(2-aminoethyl) benzene-12-diol C8H11

NO2 is a catecholamine neurotransmitter present in awide variety of animals including both vertebrates andinvertebrates Molecular studies revealed the existence offive different dopamine receptor subtypes in mammalianspecies12

Dopamine (DA) is produced in several areas of the brainand is also a neurohormone released by the hypothala-mus So it is an important neurotransmitter molecule whichis widely distributed in the central nervous system formessage transfer1ndash8 It influences a variety of motivatedbehaviours neuronal plasticity and plays a critical role inlearning and memory2 Low levels of DA may cause seri-ous neurological problems such as Parkinsonrsquos disease andschizophrenia278 In the extra-cellular fluid of the centralnervous system the basal DA concentration is very low(001ndash1 M)9 Electrochemical methods have been alwayspowerful tools for detecting low level of DA in solution

lowastAuthors to whom correspondence should be addressed

However there are some major problems and difficultiesin the determination of DA The most important one isthe presence of ascorbic acid (AA) and uric acid (UA)with similar oxidation potentials and concentrations muchhigher than DA910 Considering the above conditions itseems that for detection of DA in the presence of UA andAA a sensitive and accurate electrode should be requiredIt has been known that the bare electrode is not sen-

sitive enough for simultaneous detection of DA UA andAA Therefore there are several works on using modifiermaterials such as carbon nanotubes11ndash13 metal oxides14ndash16

graphene17 polymers1018 and metal nanoparticles198 toimprove the sensitivity as well as selectivity and stabilityof the bare electrode Among them metal nanoparticlesmodified electrodes have attracted much attention due totheir high surface area effective mass transport and catal-ysis However the metal nanoparticles at the electrodesurface are not conductive enough so that the electrodesurface could be modified with some conductive stabiliz-ing materials such as ligands carbon nanotubes and con-ductive polymers20 Therefore a mix structure of metal

6668 J Nanosci Nanotechnol 2011 Vol 11 No 8 1533-48802011116668008 doi101166jnn20114201


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Mahshid et al Carbon-Pt Nanoparticles Modified TiO2 Nanotubes for Simultaneous Detection of Dopamine and Uric Acid

nanoparticles and carbon can result into a more effectivestructure on the electrode surface According to the recentpublications the most widely used electrodes are GCE(glassy carbon electrode)19ndash22 and Metal electrodes such asAu and Pt16

Titanium dioxide (TiO2) as a semiconductor metal oxidehas attracted great attention in the past decade Nanotubu-lar structures of TiO2 with high surface area have shownpotential use in several applications such as biocompat-ible materials photo-electrochemical solar cells opticalcoatings photocatalyts and different type of sensors23ndash28

These properties are much more improved when the TiO2

nanotubular structure is modified with modifiers such asmetal nanoparticles andor carbon nanotubes There areseveral reports of using such architectures in electrochem-ical biosensors29ndash31 However to the authorrsquos knowledgethis is the first report on using the vertically orientedanodized TiO2 nanotubes (NTs) as the base electrode fordetection of DAIn this work a modified TiO2 nanotubes electrode was

prepared by electrochemical deposition of Pt nanoparti-cles and then vapor deposition of Carbon After check-ing the conductivity C-Pt-TiO2 NTs modified electrodewas used for sensitive detection of DA individuallyand in the presence of UA and AA by using dif-ferential pulse voltammetry method Comparing to theother electrodes this new electrode offers a very goodselectivity of these three compounds without using anyother modifiers which is an opportunity for an easierfabrication12181932

2 EXPERIMENTAL DETAILS

21 Reagents and Apparatus

Titanium foil (998 pure 0127 mm thick) was purchasedfrom Aldrich (Milwaukee WI) Dopamine hydrochloride(puritygt 990) Uric Acid (puritygt 990) L-AscorbicAcid (puritygt 997) and Polyethylene glycol 6000 wereused as received without any further purification Aqueoussolutions were prepared with double distilled water andother chemicals were of analytical gradeA ZAHNER IM6ex (Germany) working station was

used for electrodeposition of metal nanoparticles in threeelectrode configuration in which a Pt sheet (Aldrich999 purity 1 mm diameter) was used as counter elec-trode and SCE (Saturated Calomel Electrode) as referenceelectrode Electrochemical investigations were carried outin a CHI 660D (CH Instruments Inc Austin TX) elec-trochemical working station in accordance to the previ-ous configuration The schematic of detection setup hasbeen shown in Figure 1 The final topography of the elec-trode was studied using a scanning electron microscope(HITACHI S-4800) An Energy Dispersive X-ray (EDX)spectrometer fitted to the scanning electron microscopewas used for elemental analysis

CHI 660Dworkstation

Counterelectrode

Workingelectrode

Referenceelectrode

Glassycell

Processor

Fig 1 Schematic of electrochemical detection setup

22 Fabrication of C-Pt-TiO2 Nanotube Electrode

First a pure titanium foil was cut into 1times 3 cm piecesand cleaned in HF solution The cleaned Ti sheetswere immediately anodized in an electrolyte containing01 M NaF and 05 M NaHSO4 at the constant volt-age of 15 V for almost 2 h Anodizing cell was a two-electrode configuration with Pt and Ti sheets as cathodeand anode respectively After the anodization sampleswere rinsed with distilled water several times and dried inthe airSecond Pt nanoparticles were electrodeposited onto

titania nanotubes using pulse electrodeposition method atthe voltage of minus35 V for 60 s in an electrolyte contain-ing 2 mM H2PtCl6 A three-electrode configuration withTiO2 nanotubesTi as working electrode was used (hav-ing effective surface area of 1 cm2) The Pt-modified TiO2

nanotube electrodes were then checked for the conduc-tivity in a solution of 25 mM Fe(CN)4minus6 Fe(CN)3minus6 and01 M KCl using cyclic voltametry method at 100 mV sminus1On the Pt-modified TiO2 nanotubes electrode carbonwas deposited by carbonizing polyethylene glycol 6000at 600 C for 5 hours according to the papers3334 Inthis condition carbon were deposited through decom-position of polyethylene glycol in a vacuum tube fur-nace purged with N2 gas The final electrode was thenchecked for conductivity in the same solution as mentionedbefore

23 Detection of DA and UA

The supporting electrolyte in all electrochemical investi-gations was 01 M phosphate buffer solution (PBS) withpH 7 Cyclic voltammetry (CV) method with voltagesweep rate of 100 mV sminus1 was utilized for individualdetection of DA or UA from 1 mM solution The sensitiv-ity of the electrode towards DA and UA was then exam-ined using differential pulse voltammetry (DPV) betweenminus01 V and 04 V with pulse amplitude of 50 mV and20 mV sminus1 scan rate The oxidation current of each suc-cessive addition was measured in comparison to the DPV

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Carbon-Pt Nanoparticles Modified TiO2 Nanotubes for Simultaneous Detection of Dopamine and Uric Acid Mahshid et al

curve obtained in PBS Solutions of DA UA and AA wereprepared daily by using double distilled water and thendirectly applied for the detection All the experiments wereconducted at room temperature

3 RESULTS AND DISCUSSION

31 Characterization of C-Pt-TiO2 NTs

Scanning electron microscopy (SEM) was used to inves-tigate the microstructure of the prepared C-Pt-TiO2 NTselectrode as shown in Figure 2 Here Pt nanoparticleswith less than 10 nm in diameter have been uniformlydeposited onto the TiO2 NTs (Fig 2(A)) The presenceof uniformly dispersed Pt nanoparticles is essential for ahigh catalytic activity Figure 2(B) shows the top view ofC-Pt-TiO2 NTs electrode It seems that carbon has suc-cessfully deposited from inside of the tubes by decompo-sition of EG at 600 C The corresponding EDX spectrumin Figure 2(C) shows the presence of Pt and C in theelectrode structure with atomic percent of 270 and 637respectively It was expected that C-Pt-TiO2 NTs modifiedelectrode would act effectively as a sensor for detection ofelectrochemical reactions It is due to the large surface areaof the tubular structure which is filled with carbon from

Element

C K

Atomic

637

O K

Ti K

Pt M

Totals

(A) (B)

(C)

3877

5216

270

10000

Fig 2 SEM micrographs of C-Pt-TiO2 NTs electrode showing the morphology of (A) Pt-TiO2 NTs prepared at minus35 V for 60 s The Pt nanoparticlesare less than 5 nm in size (B) Carbon deposited on the Pt-TiO2 NTs electrode (C) EDS spectrum of the C-Pt-TiO2 NTs modified electrode

inside of the tubes and covered with Pt nanoparticles onedge and surface of the tubes Such architecture facilitatesthe electron transfer due to the enhanced conductivity ofcarbon and also improves the catalytic activity due to thepresence of Pt nanoparticlesFigure 3 illustrates the results of cyclic voltametry

on (a) bare TiO2 NTs (b) Pt-TiO2 and (c) C-Pt-TiO2

nanotube in the presence of redox solution of 25 mMFe(CN)4minus6 Fe(CN)3minus6 and 01 M KCl It could be seen thatthere was no obvious current response on the bare TiO2

NTs electrode indicating that the TiO2 NTs alone pos-sessed low conductivity to be used as an electrode withgood performance However the Pt-TiO2 NTs modifiedelectrode had strong electrocatalytic activity toward theredox reaction of Fe2+Fe3+ This is due to the effectivepresence of Pt nanoparticles onto the tubes which have sig-nificantly accelerated the redox reaction of Fe2+Fe3+ Ahuge increase of current could be observed after depositionof carbon onto the electrode surface In this case not onlythe anodic current increased but also the anodic voltageshifted to less positives suggesting the catalytic character-istic of the C-Pt-TiO2 NTs electrode and good character-istic of reversible reactions It means that the C-Pt-TiO2

NTs electrode could effectively work in electro-activatedreactions

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Fig 3 Cyclic voltammograms of (a) bare TiO2 NTs (b) Pt-TiO2 NTsand (c) C-Pt-TiO2 NTs in 25 mM Fe(CN)4minus6 Fe(CN)3minus6 solution with scanrate of 100 mV sminus1

32 Electro-Oxidation of DA and UA onthe C-Pt-TiO2 Nanotube Electrode

Figure 4(A) shows the cyclic voltammograms obtained for10times 10minus3 M DA in 01 M pH 70 PBS at three differ-ent electrodes (a) bare TiO2 NTs (b) Pt-TiO2 NTs and(c) C-Pt-TiO2 NTs Obviously there was no electrochem-ical response to oxidation of DA on the surface of bareTiO2 NTs electrode However a pair of wide redox peakswas observed after modification of the electrode with Ptnanoparticles representing the electro-catalytic activity ofPt towards DA oxidation In this reaction the oxidationand reduction peaks (Ep) appeared at 026 V and 004 Vrespectively with Ep = 022 V and oxidation current peakof 03 mA This enhanced current in oxidation reaction ofDA can be attributed to the catalytic activity of Pt Theperformance was much more improved by modification ofthe electrode with carbon Since carbon could be depositedfrom inside of the tubes and increased the conductivityof the electrode accordingly Curve (c) in Figure 4(A)shows the electrochemical response to the catalytic reac-tion of DA on the surface of C-Pt-TiO2 NT electrodeObviously two pairs of well-defined redox peaks couldbe observed with the oxidation and reduction potentials atminus024 V 019 V and 01 V minus031 V respectively Thesefindings confirm the results proposed by Zhao et al wheretwo pairs of redox peaks appeared from cyclic voltamme-try of DA in range of minus06 V to 04 V35 Furthermorethe oxidation current peak (Ip) had sharply increased to16 mA which was 5 times that of Pt-TiO2 NTs elec-trode The peak potential differences (Ep) were estimatedto be 007 and 009 V for the first and second redoxpeaks respectively This indicated that the electrochem-ical oxidation of DA on the surface of C-Pt-TiO2 NTselectrode was almost reversible with sharp oxidation andreduction peaks It is reported that DA oxidation under-goes a two-electron reaction at the surface of modifiedelectrode As shown in Eqs (1) to (3) dopaminequinonea product of two electrons oxidation of DA (Eq (1))undergoes follow-up ring closure reaction (Eq (2)) lead-ing to leucodopaminechrome which in turn is oxidized to

Fig 4 Cyclic voltammograms obtained for (A) 1times10minus3 M DA and (B)1times10minus3 M UA in 01 M pH 700 PBS with a scan rate of 100 mV sminus1(a) Bare TiO2 NTs (b) Pt-TiO2 NTs and (c) C-Pt-TiO2 NTs electrodes

dopaminechrome (Eq (3))3835 Zhao et al has claimedthat the right pair of redox peaks is corresponding to theredox process of Eq (1) and the left one is correspondingto Eq (3)35

Cyclic voltammograms obtained for 10times 10minus3 M UAin 01 M pH 70 PBS at (a) bare TiO2 NTs (b) Pt-TiO2

NTs and (c) C-Pt-TiO2 NTs are shown in Figure 4(B)Similar to DA no catalytic activity towards UA could beobserved on the bare TiO2 NTs electrode However theoxidation peak appeared in case of Pt-TiO2 NTs and C-Pt-TiO2 NTs modified electrodes at 045 V and 032 Vrespectively It seemed that the significant role of carbontowards UA oxidation caused a sharp rise in oxidationcurrent up to 094 mA While comparing to DA a small



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cathodic peak appeared at 022 V representing a semi-reversible behaviour in UA redox reaction As a whole itcould be concluded that the C-Pt-TiO2 NTs modified elec-trode had a significant electrocatalytic activity toward theoxidation reaction of DA and UAThe modified electrode has been also examined in dif-

ferent concentration of DA Figure 5 shows the cyclicvoltammetry at C-Pt-TiO2 NTs electrode in the presence of1times10minus3 M 05times10minus3 M and 025times10minus3 M DA in 01 MpH 70 PBS It was found that the oxidation peak currenthad decreased to fewer amounts as the DA concentrationdecreased in the solution

33 Sensitive Detection of DA and UA byDifferential Pulse Voltammetry

In order to achieve a sensitive detection of DA and UAat C-Pt-TiO2 NT electrode differential pulse voltammetry(DPV) was used to record anodic peak current of the reac-tion This method offers much higher current sensitivityand better peak separation than cyclic voltammetryFigure 6(A)(1ndash11) shows the DPV results of DA at C-

Pt-TiO2 NTs electrode in 01 M pH 70 PBS containedvarious concentrations of DA In this examination the bestparameters obtained were pulse amplitude of 50 mV pulsewidth of 02 s scan rate of 20 mV sminus1 Under theseoptimised condition the oxidation peak of DA occurredat 0115 V for all the concentrations of DA in range of35times10minus8 M to 50times10minus6 M As shown in Figure 6(B)the electrocatalytic oxidation currents of DA were lin-early related to the accumulated concentrations over thisrange with slope of 2648 AMminus1 and coefficient deter-mination of 0997 suggesting a very good sensitivityin comparison with other modified electrodes36 Further-more another series of DPVs were carried out in con-centration range of 05times 10minus10 M to 20times 10minus10 M DAin 01 M pH 70 PBS to determine the detection limit(Fig 6(C)) Considering such a low concentration of DAeach experiment was repeated for three times to makesure the reliability of obtained current responses Under

Fig 5 Cyclic voltammograms at C-Pt-TiO2 NTs in the presence of(a) 1times10minus3 M DA (b) 05times10minus3 M DA and (c) 025times10minus3 M DA witha scan rate of 100 mV sminus1 in 01 M pH 700 PBS

5

10

15

20

ndash005 0 005 01 015 02 025 03i (

microA)

E(V vs SCE)

(C)

2010ndash10 M DA1510ndash10 M DA0510ndash10 M DA0 M DA

Fig 6 (A) Differential pulse voltammograms of DA at C-Pt-TiO2 nano-tube in 01 M pH 700 PBS with scan rate 20 mV sminus1 and pulse amplitude50 mV The total concentration of DA in each step (1ndash11) 0 35times10minus860times 10minus8 30times 10minus7 55times 10minus7 80times 10minus7 10times 10minus6 15times 10minus620times 10minus6 30times 10minus6 and 50times 10minus6 M (B) The calibration plot ofcurrent response versus accumulated concentration of DA in each step(C) Limit of detection of DA in concentration range of 05times10minus10 M to20times10minus10 M DA in 01 M pH 70 PBS obtained by DPVs method

this condition the limit of detection (LOD) found to be2times10minus10 M for SN= 3 which was much lower than theprevious reports 111214193637 Table I shows a compari-son between the previous LOD results and the present oneSimilar studies were carried out for the sensitive detec-tion of UA in concentration range of 10times 10minus7 M to20times 10minus5 M (Fig 7(A)) Under the same conditions asDA the oxidation peak of UA occurred at 0255 V Asshown in Figure 7(B) the calibration curve indicated alinear relation between oxidation current of UA and accu-mulated concentration with coefficient determination of0996 Using DPV technique under optimum conditionsas the above the relative standard deviation (RSD) for 8successive measurements in 50times 10minus6 M DA and 50times10minus6 M UA were 320 and 324 respectively indi-cating a very good reproducibility for the C-Pt-TiO2 NTsmodified electrode

34 Simultaneous Detection of DA and UA withC-Pt-TiO2 Electrodes

In order to study the sensitivity for the simultaneous deter-mination of DA and UA DPV was applied to measure the



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Table I Comparing the estimated LODs for sensitive detection of DA obtained from different modified electrodes

Electrode Limit of detection (M) Year of publication Reference

Carbon-Pt NPsDagger modified TNTslowast 2times10minus10 Present studyCu NPs-Polypyrrole modified GCElowastlowast 85times10minus10 2010 S Ulubay Z Dursun15

DHBPDlowastlowastlowast-TiO2 NPs modified CPEdagger 314times10minus8 2010 M Mazloum Ardakani9

CNTdaggerdagger paste electrode modified with Melanic polymer 11times10minus8 2010 M D Rubianes et al7

Single-wall CNT 1times10minus8 2009 S Alwarappan11

Multi-wall CNTPEIdaggerdaggerdagger modified GCE 92times10minus7 2008 M C Rodriacuteguez36

Choline-Au NPs modified GCE 12times10minus7 2007 P Wang et al30

DaggerNanoparticles lowastTiO2 Nanotubes lowastlowastGlassy carbon electrode lowastlowastlowastDihydroxybenzylidene-1 4-phenylenediamine daggerCarbon paste electrode daggerdaggerCarbon nanotubesdaggerdaggerdaggerpolyethylenimine

anodic peak current on the C-Pt-TiO2 NTs electrodes Theelectro-oxidation processes of DA and UA in the mixturewas investigated when the concentration of one specieschanged whereas the other was kept constantFigure 8(A) shows the DPV obtained for the different

concentrations of DA in the presence of a large excess ofUA ie 50times 10minus5 M in pH 70 PBS at C-Pt-TiO2 NTsmodified electrode The oxidation peaks for DA and UAobtained at 0115 V and 0255 V respectively indicatingthe same oxidation potential as their individual detectionin Figure 6(A) (for DA) and Figure 7(A) (for UA) Itcould be seen that the location of the peak current for UAwas almost constant during the oxidation of DA Howeverthere was considerable deviation in oxidation peak currentof UA as DA was added continuously to the solution Itis believed that this was due to the effect of DA oxida-tion peak as it happened before for that of UA and shiftedthe baseline to higher cathodic currents Furthermore thecalibration curve in Figure 8(B) represented a linear rela-tion between the oxidation peak current of DA and itsaccumulated concentration in range of 10times 10minus7 M to70times10minus6 M with coefficient determination of 0995In a Similar experiment voltammetric determination of

UA was carried out in the presence of 50times10minus5 M DA in

Fig 7 (A) Differential pulse voltammograms of UA at C-Pt-TiO2 nano-tube in 01 M pH 700 PBS with scan rate 20 mV sminus1 and pulse amplitude50 mV The total concentration of UA in each step (1ndash10) 0 10times10minus750times 10minus7 20times 10minus6 50times 10minus6 60times 10minus6 80times 10minus6 10times 10minus520times10minus5 and 30times10minus5 M (B) The calibration plot of current responseversus accumulated concentration of UA in each step

pH 70 PBS As shown in Figure 9(A) oxidation currentof DA remained almost constant as the concentration ofUA was increasing It was also found that the peak cur-rent of UA was proportional to concentration of UA inrange of 10times 10minus7 M to 60times 10minus5 M with coefficientdetermination of 0996 (Fig 9(B)) From these results itcould be concluded that the responses to DA and UA atthe C-Pt-TiO2 NTs electrode were relatively independent

35 Differential Pulse Voltametry of DA and UA inthe Presence of AA

It is known that simultaneous determination of DAAA and UA has been always a great problem due totheir similar oxidation potentials which caused overlappedpeaks for the most common electrodes Especially in caseof AA and DA the oxidation peak potentials were veryclose to each other that even using modified electrodeswere sometimes useless Therefore the oxidation of DAand UA was examined in the presence of AA at C-Pt-TiO2 NTs modified electrode using DPV technique underthe same conditions as per the previous experiments Inorder to compare the DPV results of oxidation of DA UAand AA individually and simultaneously a series of exper-iments obtained in 50times 10minus5 M DA 50times 10minus5 M UA

Fig 8 (A) Differential pulse voltammograms of DA in the presence of50times10minus5 M UA at C-Pt-TiO2 NTs in 01 M pH 700 PBS with scan rate20 mV sminus1 and pulse amplitude 50 mV The total concentration of DA ineach step (1ndash11) 0 10times10minus7 30times10minus7 50times10minus7 80times10minus7 10times10minus6 15times10minus6 30times10minus6 50times10minus6 70times10minus6 and 10times10minus5 M (B)The calibration plot of current response versus accumulated concentrationof DA in each step



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Fig 9 (A) Differential pulse voltammograms of UA in the presence of10times10minus5 M DA at C-Pt-TiO2 NTs in 01 M pH 700 PBS with scan rate20 mV sminus1 and pulse amplitude 50 mV The total concentration of UA ineach step (1ndash11) 0 30times10minus7 50times10minus7 10times10minus6 30times10minus6 60times10minus6 15times10minus5 30times10minus5 40times10minus5 50times10minus5 and 60times10minus5 M (B)The calibration plot of current response versus accumulated concentrationof UA in each step

and 50times10minus5 M AA in 01 M pH 700 PBS (Fig 10(A))At the same concentration the oxidation current responseof DA was much higher than the othersrsquo oxidation currentresponse It suggests that that the C-Pt-TiO2 NTs modified

Fig 10 Differential pulse voltammograms obtained (A) in the individ-ual presence of 50times 10minus5 M DA 50times 10minus5 M UA and 50times 10minus5 MAA (B) in the simultaneous presence of (a) 50times 10minus5 M DA+ 50times10minus5 M UA+ 50times 10minus5 M AA at C-Pt-TiO2 NTs electrode in 01 MpH 700 PBS with scan rate 20 mV sminus1 and pulse amplitude 50 mV (b)The same conditions but 10times 10minus5 M DA+ 10times 10minus5 M UA+ 10times10minus5 M AA

electrode had more sensitivity to DA detection comparingto other two species Furthermore the oxidation peaks ofDA UA and AA in individual oxidation reactions wereobtained at 0115 V 0255 V and 01 V respectively How-ever an obvious shift in oxidation potential of AA and UAcould be observed as they coexist with DA This couldbe more obvious as the concentration of these speciesincreased Figure 10(B) shows the DPV of simultaneousdetection of (a) 50times10minus5 M (DA+UA+AA) and (b)10times 10minus5 M (DA+UA+AA) in 01 M pH 700 PBSat C-Pt-TiO2 NTs modified electrode Although the oxida-tion peak of DA and AA were very close to each otherthe C-Pt-TiO2 NTs modified electrode could successfullyseparate their signals However the oxidation of UA wasshifted to more positive potential

4 CONCLUSIONS

In the present study the C-Pt-TiO2 NTs modified electrodewas used for sensitive detection of DA for the first timeTaking advantage of extended surface area of TiO2 nano-tubes which was loaded with C and Pt the modified elec-trode represented high sensitivity towards detection of DAand UA individually and simultaneously Electrocatalyticactivities of the bare TiO2 nanotube as well as its con-ductivity were improved by Carbon-Pt modification UsingDPV method the detection limit of DA was determinedas 2times 10minus10 M C-Pt-TiO2 NTs modified electrode notonly improved the sensitivity towards simultaneous detec-tion of DA and UA but also could directly distinguish theoxidation response of DA UA and AA in mixture solu-tion Comparing to the other electrodes this new electrodeoffers a very good selectivity of these three compoundswithout using any other modifiers

Acknowledgment Funding for this work by the pro-gram for Changjiang Scholars and Innovative ResearchTeam in University (PCSIRT) National Basic ResearchProgram of China under Grants No 2009CB421601 andthe National Science Fund for Distinguished Young Schol-ars under grant No 50725825 is gratefully acknowledged

References and Notes

1 J N Oak J Oldenhof and H H M Van Tol Euro J Pharmacol405 303 (2000)

2 J Njagi M M Chernov J C Leiter and S Andreescu Anal Chem82 989 (2010)

3 A Ciszewski and G Milczarek Anal Chem 71 1055 (1999)4 T Yamada D-Y Jung R Sawada A Matsuoka R Nakaoka and

T Tsuchiya J Nanosci Nanotechnol 8 3973 (2008)5 L Niu M Shao Sh Wang L Lu H Gao and J Wang J Mater

Sci 43 1510 (2008)6 A Salimi K Abdi and Gh R Khayatian Microchim Acta 144 161

(2004)7 S Y Ly Bioelectrochem 68 227 (2006)8 Y Wang X Zhang Y Chen H Xu Y Tan and Sh Wang Am J

Biomed Sci 2 209 (2010)



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9 A J Downard A D Roddick and A M Bond Anal Chim Acta317 303 (1995)

10 K Pihel Q D Walker and R M Wightman Anal Chem 68 2084(1996)

11 S Alwarappan Sh Prabhulkar A Durygin and C-Zh LiJ Nanosci Nanotechnol 9 2991 (2009)

12 M D Rubianes A S Arribas E Bermeo M ChicharroA Zapardiel and G Rivas Sens Actuators B 144 274 (2010)

13 Z Wang J Liu Q l Liang Y Wang and G Luo Analyst 127 653(2002)

14 M Mazloum-Ardakani H Rajabi H Beitollahi B F MirjaliliA Akbari and N Taghavinia Int J Electrochem Sci 5 147 (2010)

15 S Yuan and Sh Hu Electrochim Acta 49 4287 (2004)16 H P Wu T L Cheng and W L Tseng Langmuir 23 7880 (2007)17 Y Wang Y Li L Tang J Lu and J Li Electrochem Commun

11 889 (2009)18 P Y Chen P C Nien and K C Ho Procedia Chem 1 285 (2009)19 J Mathiyarasu S Senthilkumar K L N Phani and V Yegnaraman

J Nanosci Nanotechnol 7 2206 (2007)20 S Ulubay and Z Dursun Talanta 80 1461 (2010)21 K Wu J Fei and Sh Hu Anal Biochem 318 100 (2003)22 X Kang Zh Mai X Zou P Cai and J Mo J Nanosci Nanotech-

nol 7 1618 (2007)23 A Ghicov and P Schmuki Chem Commun 20 2791 (2009)24 P Xiao B B Garcia Q Guo D Liu and G Cao Electrochem

Commun 9 2441 (2007)

25 G K Mor C A Grimes M Paulose and N Mukherjee J NanosciNanotechnol 4 733 (2004)

26 E Sennik Z Ccedilolak N Kılınccedil and Z Z Oumlztuumlrk Int J Hyd Energy35 4420 (2010)

27 K Shankar J I Basham N K Allam O K Varghese G K MorX Feng M Paulose J A Seabold K S Choi and C A GrimesJ Phys Chem C 113 6327 (2009)

28 G K Mor O K Varghese M Paulose K Shankar and C AGrimes Solar Energy Mater Solar Cells 90 2011 (2006)

29 Q Kang L Yang and Q Cai Bioelectrochem 74 62(2008)

30 X Pang D He and Q Cai Sens Actuators B 137 134(2009)

31 H Tao X Wang X Wang Y Hu Y Ma Y Lu and Z HuJ Nanosci Nanotechnol 10 860 (2010)

32 X Cao L Luo Y Ding X Zou and R Bian Sens Actuators B129 941 (2008)

33 L Yang Y Xiao G Zeng Sh Luo Sh Kuang and Q Cai EnergyFuels 23 3134 (2009)

34 L Yang Sh Luo Sh Liu and Q Cai J Phys Chem C 112 8939(2008)

35 Y Zhao Y Gao D Zhan H Liu Q Zhao Y Kou Y Shao M LiQ Zhuang and Z Zhu Talanta 66 51 (2005)

36 M C Rodriacuteguez M D Rubianes and G A Rivas J NanosciNanotechnol 8 6003 (2008)

37 P Wang Y Li X Huang and L Wang Talanta 73 431 (2007)

Received 11 December 2010 Accepted 27 January 2011


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nanoparticles and carbon can result into a more effectivestructure on the electrode surface According to the recentpublications the most widely used electrodes are GCE(glassy carbon electrode)19ndash22 and Metal electrodes such asAu and Pt16

Titanium dioxide (TiO2) as a semiconductor metal oxidehas attracted great attention in the past decade Nanotubu-lar structures of TiO2 with high surface area have shownpotential use in several applications such as biocompat-ible materials photo-electrochemical solar cells opticalcoatings photocatalyts and different type of sensors23ndash28

These properties are much more improved when the TiO2

nanotubular structure is modified with modifiers such asmetal nanoparticles andor carbon nanotubes There areseveral reports of using such architectures in electrochem-ical biosensors29ndash31 However to the authorrsquos knowledgethis is the first report on using the vertically orientedanodized TiO2 nanotubes (NTs) as the base electrode fordetection of DAIn this work a modified TiO2 nanotubes electrode was

prepared by electrochemical deposition of Pt nanoparti-cles and then vapor deposition of Carbon After check-ing the conductivity C-Pt-TiO2 NTs modified electrodewas used for sensitive detection of DA individuallyand in the presence of UA and AA by using dif-ferential pulse voltammetry method Comparing to theother electrodes this new electrode offers a very goodselectivity of these three compounds without using anyother modifiers which is an opportunity for an easierfabrication12181932

2 EXPERIMENTAL DETAILS

21 Reagents and Apparatus

Titanium foil (998 pure 0127 mm thick) was purchasedfrom Aldrich (Milwaukee WI) Dopamine hydrochloride(puritygt 990) Uric Acid (puritygt 990) L-AscorbicAcid (puritygt 997) and Polyethylene glycol 6000 wereused as received without any further purification Aqueoussolutions were prepared with double distilled water andother chemicals were of analytical gradeA ZAHNER IM6ex (Germany) working station was

used for electrodeposition of metal nanoparticles in threeelectrode configuration in which a Pt sheet (Aldrich999 purity 1 mm diameter) was used as counter elec-trode and SCE (Saturated Calomel Electrode) as referenceelectrode Electrochemical investigations were carried outin a CHI 660D (CH Instruments Inc Austin TX) elec-trochemical working station in accordance to the previ-ous configuration The schematic of detection setup hasbeen shown in Figure 1 The final topography of the elec-trode was studied using a scanning electron microscope(HITACHI S-4800) An Energy Dispersive X-ray (EDX)spectrometer fitted to the scanning electron microscopewas used for elemental analysis

CHI 660Dworkstation

Counterelectrode

Workingelectrode

Referenceelectrode

Glassycell

Processor

Fig 1 Schematic of electrochemical detection setup

22 Fabrication of C-Pt-TiO2 Nanotube Electrode

First a pure titanium foil was cut into 1times 3 cm piecesand cleaned in HF solution The cleaned Ti sheetswere immediately anodized in an electrolyte containing01 M NaF and 05 M NaHSO4 at the constant volt-age of 15 V for almost 2 h Anodizing cell was a two-electrode configuration with Pt and Ti sheets as cathodeand anode respectively After the anodization sampleswere rinsed with distilled water several times and dried inthe airSecond Pt nanoparticles were electrodeposited onto

titania nanotubes using pulse electrodeposition method atthe voltage of minus35 V for 60 s in an electrolyte contain-ing 2 mM H2PtCl6 A three-electrode configuration withTiO2 nanotubesTi as working electrode was used (hav-ing effective surface area of 1 cm2) The Pt-modified TiO2

nanotube electrodes were then checked for the conduc-tivity in a solution of 25 mM Fe(CN)4minus6 Fe(CN)3minus6 and01 M KCl using cyclic voltametry method at 100 mV sminus1On the Pt-modified TiO2 nanotubes electrode carbonwas deposited by carbonizing polyethylene glycol 6000at 600 C for 5 hours according to the papers3334 Inthis condition carbon were deposited through decom-position of polyethylene glycol in a vacuum tube fur-nace purged with N2 gas The final electrode was thenchecked for conductivity in the same solution as mentionedbefore

23 Detection of DA and UA

The supporting electrolyte in all electrochemical investi-gations was 01 M phosphate buffer solution (PBS) withpH 7 Cyclic voltammetry (CV) method with voltagesweep rate of 100 mV sminus1 was utilized for individualdetection of DA or UA from 1 mM solution The sensitiv-ity of the electrode towards DA and UA was then exam-ined using differential pulse voltammetry (DPV) betweenminus01 V and 04 V with pulse amplitude of 50 mV and20 mV sminus1 scan rate The oxidation current of each suc-cessive addition was measured in comparison to the DPV



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






Element

C K

Atomic

637

O K

Ti K

Pt M

Totals

(A) (B)

(C)

3877

5216

270

10000









IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE











IP 13713212369Sun 23 Oct 2011 024843

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ARTIC

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5

10

15

20

ndash005 0 005 01 015 02 025 03i (

microA)

E(V vs SCE)

(C)








IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

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IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

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4 CONCLUSIONS










Biomed Sci 2 209 (2010)



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)


















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






Element

C K

Atomic

637

O K

Ti K

Pt M

Totals

(A) (B)

(C)

3877

5216

270

10000









IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE











IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE








5

10

15

20

ndash005 0 005 01 015 02 025 03i (

microA)

E(V vs SCE)

(C)








IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE





















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






4 CONCLUSIONS










Biomed Sci 2 209 (2010)



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)


















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE











IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE








5

10

15

20

ndash005 0 005 01 015 02 025 03i (

microA)

E(V vs SCE)

(C)








IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE





















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






4 CONCLUSIONS










Biomed Sci 2 209 (2010)



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)


















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE








5

10

15

20

ndash005 0 005 01 015 02 025 03i (

microA)

E(V vs SCE)

(C)








IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE





















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






4 CONCLUSIONS










Biomed Sci 2 209 (2010)



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)


















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE





















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






4 CONCLUSIONS










Biomed Sci 2 209 (2010)



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)


















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE






4 CONCLUSIONS










Biomed Sci 2 209 (2010)



IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)


















IP 13713212369Sun 23 Oct 2011 024843

RESEARCH

ARTIC

LE












Commun 9 2441 (2007)

















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