A hydroxylamine electrochemical sensor

Electrochimica Acta 55 (2010) 2835–2840

Contents lists available at ScienceDirect

Electrochimica

Acta

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c t

a

A hydroxylamine electrochemical sensor based on electrodeposition of porous ZnO nano?lms onto carbon nanotubes ?lms modi?ed electrode

Cuihong Zhang,Guangfeng Wang,Min Liu,Yuehua Feng,Zhidan Zhang,Bin Fang ?

College of Chemistry and Materials Science,Anhui Key Laboratory of Chem-Biosensing,Beijing East Road No.1,Anhui Normal University,Anhui,Wuhu 241000,PR China

a r t i c l e i n f o Article history:

Received 29October 2009Received in revised form 20December 2009

Accepted 22December 2009

Available online 11 January 2010Keywords:MWCNTs ZnO

Modi?ed electrode Electrocatalysis Hydroxylamine

a b s t r a c t

A novel route (electrodeposition)for the fabrication of porous ZnO nano?lms attached multi-walled car-bon nanotubes (MWCNTs)modi?ed glassy carbon electrodes (GCEs)was proposed.The morphological characterization of ZnO/MWCNT ?lms was examined by scanning electron microscopy (SEM)and X-ray powder diffraction (XRD).The performances of the ZnO/MWCNTs/GCE were characterized with cyclic voltammetry (CV),Nyquist plot (EIS)and typical amperometric response (i –t ).The potential utility of electrodes constructed was demonstrated by applying them to the analytical determination of hydroxy-lamine concentration.An optimized limit of detection of 0.12?M was obtained at a signal-to-noise ratio of 3and with a fast response time (within 3s).Additionally,the ZnO/MWCNTs/GCE exhibited a wide linear range from 0.4to 1.9×104?M and higher sensitivity.The ease of fabrication,high stability,and low cost of the modi?ed electrode are the promising features of the proposed sensor.

? 2010 Elsevier Ltd. All rights reserved.

1.Introduction

Hydroxylamine,NH 2OH (abbreviated as HA),a derivative of ammonium,is an intermediate in two important microbial pro-cesses of the nitrogen cycle:it is formed during nitri?cation as well as during anaerobic ammonium oxidation [1,2].Although it is a well-known mutagen,moderately toxic and harmful to human,animals,and even plants [3],which has been known to cause both reversible and irreversible physiological changes [4],it is available commercially and frequently used industrially widely in pharma-ceutical intermediates and ?nal drug substances synthesis,nuclear fuel reprocessing and the manufacturing of semiconductors [5].In recent years,chemists became aware of the potentials of hydroxy-lamine as a result of two major accidents,one occurred in the USA in February 1999,which killed ?ve people,and the other occurred in Japan in June 2000,which killed four people [6,7].Therefore,from the industrial,environmental and health viewpoints,development of a sensitive analytical method for the determination of low levels of hydroxylamine is of signi?cant importance.

The reported methods of the hydroxylamine determination include spectrophotometry [8],high performance liquid chro-matography [9],gas chromatography [10],potentiometry [11],polarography [12]and biamperometry [13].However,the pro-

?Corresponding author.Tel.:+865533869302;fax:+865533869303.E-mail address:binfang 47@https://www.360docs.net/doc/1316525796.html, (B.Fang).cesses involved in many of these methods are extremely complex,and the linear ranges are relatively narrow and have low precision.Fortunately,electrochemical techniques offer the opportunity for portable,cheap and rapid methodologies.However,hydroxylamine cannot be electrooxidized at bare carbon electrodes.One promis-ing approach is the use of chemically modi?ed electrodes (CMEs)containing speci?cally selected redox mediators immobilized on conventional electrode materials.Recently,various chemically modi?ed electrodes (CMEs)have been prepared and applied in the determination of hydroxylamine [14–18],which can signi?-cantly lower the overpotentials and increase the oxidation current response.

In recent years,the II–VI semiconductor zinc oxide (ZnO)nanostructures have drawn many attentions in the application of ef?cient amperometric sensors with many extraordinary proper-ties,including nontoxicity,biological compatibility,chemical and photochemical stability,high electrochemical activities and easy preparation,and so on [19–24].For example,the use of ZnO nanos-tructures to fabricate electrochemical sensor have been reported in the literature [25,26,46–48].Among various fabrication strategies of nano-or microscaled ZnO,such as precipitation [27],thermal decomposition [28]and electrodeposition [29],the one-step elec-trochemical deposition method by treatment of the reactant in different solvents seems to be the simplest and most effective way to prepare nicely crystallized ZnO at relatively low temper-atures,exempted from further calcination.However,as for the electrodeposition,the template strategy (anodic alumina mem-brane or porous polycarbonate membrane)was often used and

0013-4686/$–see front matter ? 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2009.12.068

2836 C.Zhang et al./Electrochimica Acta55 (2010) 2835–2840

it is dif?cult to dissolve template completely and undissolved ?lm will in?uence the capability of biosensor,so there are still few studies concerning the electrodeposition of nanostructure ZnO applied to biosensor[30].In order to overcome these draw-backs,a new strategy was introduced by Hrapovic et al.[31] who deposited the Pt nanoparticles onto CNT?lm successfully. Because CNTs have the high accessible surface area,low electrical resistance,extremely high mechanical strength and stiffness,out-standing charge-transport characteristics,high chemical stability, and doughty sorption[32–37],there have been a series of investi-gations to use CNTs for biosensor interface fabrication[38,39].

In this work,we present,for the?rst time,a novel hydroxy-lamine sensor fabricated based on electrodeposition of the loose and porous ZnO nano?lms onto multi-walled carbon nanotubes ?lms using potentiostatic electrodeposition technique.The advan-tages of this architecture are as follows:(1)the MWCNTs have strong physical adsorption ability.What’s more,–COOH and–OH are existent in hydrophilic carboxyl MWCNTs,the H of the–COOH and–OH may form oxygen–hydrogen bonds with O from ZnO. The porous ZnO?lms can be adsorbed on the surface of the GCE by MWCNT?lms very?rmly due to the strong physical adsorp-tion ability and oxygen–hydrogen bonds;(2)MWCNTs not only help to moor the ZnO?lms onto the electrode surface,but also to increase the electron transfer rate;(3)in this paper,the ZnO we electrodeposited is a low density,loose and porous material with good dispersion and uniformity,and higher accessible surface area, which is favorable to a catalytic application.

2.Experimental

2.1.Materials

Hydrophilic multi-walled carbon nanotubes(MWCNTs)were purchased from Chengdu Institute of Organic Chemistry(China). Zn(NO3)2·6H2O(99.5%)was obtained from Shantou Xilong Chem-ical Factory(China).All other chemicals were obtained from Chemical Reagent Company of Shanghai(China),such as hydroxy-lamine,KCl,NaH2PO4,Na2HPO4.All chemicals were of analytical grade and used as received without further puri?cation.Dou-bly distilled water was used throughout this research.Phosphate buffer solutions(PBS,0.1M)were prepared by varying the ratio of NaH2PO4to Na2HPO4and adding0.1M KCl.

2.2.Measurements and apparatus

The electrochemical experiments were performed on a CHI660B electrochemical workstation(Shanghai Chenhua,China).All exper-iments were carried out by a three-electrode system with a bare glassy carbon electrode(GCE)or with modi?ed electrodes as the working electrode,a platinum wire as the auxiliary electrode,and a saturated calomel electrode(SCE)as the reference electrode.Scan-ning electron microscopy(SEM)was performed with a Hitachi S-4300(Japan);X-ray diffraction(XRD)studies were done with an X-ray diffractometer(Shimadzu,Japan)using a Cu K?source (l=0.154060nm)at40kV,30mA in the range of20?<2?<70?at a scan rate of6.0?/min.

2.3.Fabrication of the modi?ed electrode

Prior to the modi?cation,the GCE was polished to a mirror-like surface with alumina slurry followed by rinsing thoroughly with doubly distilled water.The electrode was successively sonicated in nitric acid(1:1),anhydrous ethanol and doubly distilled water (each for5min),and then allowed to dry at room temperature.

3.0mg MWCNTs were dispersed in1mL of dimethylformamide (DMF)solution by using a60min sonication.Then,the mixture

was

Scheme1.The process to prepare ZnO/MWCNTs/GCE.

dropped onto the GCE surface and dried in room temperature to obtain the MWCNTs modi?ed GCE.The MWCNTs/GCE was used as the working electrode in electrodeposition.Electrodeposition[40] of the porous ZnO thin?lms was performed in electrolytic solu-tion containing0.1mol/L KCl,0.05mol/L Zn(NO3)2at?1.2V,room temperature.Porous ZnO nanostructures were obtained on the sur-face of the MWCNT?lms after30min in the solution.The process to prepare ZnO/MWCNTs/GCE illustrated in Scheme1.COOH?and OH?are existent in hydrophilic carboxyl MWCNTs,the H of the COOH?and OH?may form oxygen–hydrogen bonds with O from ZnO.And the MWCNTs have strong physical adsorption ability,so the porous ZnO?lms can be adsorbed on the surface of a GCE by MWCNT?lms very?rmly.At the end of the growth duration,the substrate was removed from the solution and immediately rinsed in?owing deionized water to eliminate any residual salt from the surface and dried at room temperature,and the ZnO/MWCNTs/GCE was obtained.When not in use,the electrode was stored at4?C in

a refrigerator.

3.Results and discussion

3.1.Characterization of the ZnO/MWCNT?lms

Fig.1(A)and(B)shows typical SEM images of the MWCNTs and of the?lms consisting of ZnO and MWCNTs.The pristine MWC-NTs are curved and twisted with each other and have very much smooth surfaces(Fig.1(A)).The SEM image shown in Fig.1(B)con-?rmed the co-existence of MWCNTs and porous ZnO thin?lms, and the ZnO is a low density,loose and porous material that is favorable to a catalytic application.ZnO?lms strongly adhere onto the network structure provided by MWCNTs.The strong physical adsorption ability of MWCNTs and oxygen–hydrogen bonds which are important to tightly attach the ZnO?lms on the surface of the electrode.And there are a few MWCNTs unable to be covered with ZnO?lms,which can increase the electron transfer rate.

The phase of ZnO/MWCNTs was examined by XRD,as shown in Fig.2.The peaks tagged by ring assign to the pure ZnO in the com-posite which agree well with the values on the standard card(JCPDS Card No.36-1451).The peaks(002)of nanocomposite(26?)which re?ect the existence of MWCNTs is similar to that of the pure MWC-NTs[41],which indicates that electrodeposition ZnO thin?lms onto the surface of MWCNTs did not lead to the distortion of MWCNT shells.

3.2.Electrochemical behavior of the ZnO/MWCNTs/GCE

Fig.3exhibited the EIS of the change of the electrode sur-face status with the fabricated procedure,shown as Nyquist plot (Z im vs.Z re).The semicircular portion at higher frequencies

C.Zhang et al./Electrochimica Acta 55 (2010) 2835–2840

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Fig.1.SEM image of pure MWCNTs (A)and the ZnO/MWCNT ?lms

(B).

Fig.2.XRD pattern of ZnO/MWCNT ?lms.

represents the electron transfer-limited process,and the electron transfer resistance (R et )is equal to the semicircle diameter.Curve a in Fig.3shows EIS of the bare GCE.There was a very small semicir-cle domain (150±5 )as reported in the literature [42],implying a very low electron transfer resistance to the redox-probe dissolved in the electrolyte solution.In curve b,the EIS of the ZnO ?lms modi?ed electrode displayed a much higher interfacial

electron

Fig.3.EIS of (a)bare GCE,(b)ZnO/GCE and (c)ZnO/MWCNTs/GCE in the presence of 5mmol L ?1Fe(CN)63?/4?with 0.1mol L ?1KCl as the supporting electrolyte.The electrode potential was 0.2V vs.SCE.transfer resistance (2400±50 ),indicating that the ZnO ?lm lay-ers obstructed the electron transfer of the electrochemical probe of [Fe(CN)6]3?/4?.The ZnO/MWNTs/GCE shows a lower interfacial electron transfer resistance (470±10 )than that of the ZnO/GCE,in despite of higher than bare GCE.These facts suggest that the electron transfer is faster at the ZnO/MWCNTs/GCE in comparison with that at the ZnO/GCE,which facilitates the arrival of the electro-chemical probe on the surface of the electrode.This illustrates that MWCNTs not only help to moor the ZnO ?lms onto the electrode surface,but also to increase the electron transfer rate.3.3.Electrocatalytic activity of hydroxylamine on the ZnO/MWCNTs/GCE

In order to investigate the electrocatalytic activity of hydroxy-lamine on the ZnO/MWCNTs/GCE,the response of such electrodes toward the analytical determination of hydroxylamine was explored.The CVs of (a)bare GCE and (b)MWCNTs/GCE in the presence of 1.0×10?3M hydroxylamine are shown in Fig.4.It can be seen that hydroxylamine cannot be electrooxidized on a bare GCE (curve a).Under the same experimental conditions,the oxida-tion of hydroxylamine on MWCNTs/GCE demonstrated increased background current,but no de?ned anodic peaks were observed (curve b).The anodic peak obtained for ZnO/GCE at about 0.83V (curve c),and the current response increased more than that for bare GCE and MWCNTs/GCE.What’s more,the experimental results suggest that in contrast to the bare GCE,ZnO/GCE,and

MWC-

Fig. 4.CVs of the (a)bare GCE,(b)MWCNTs/GCE,(c)ZnO/GCE and (d)ZnO/MWCNTs/GCE with 1.0×10?3M hydroxylamine in N 2-saturated 0.1M PBS (pH 7.4)at 100mV s ?1.

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55 (2010) 2835–2840

Scheme 2.Schematic of hydroxylamine detection by electrochemical method using ZnO/MWCNTs/GCE.

NTs/GCE,a well-de?ned oxidation peak in the ZnO/MWCNTs/GCE (curve d)with 1.0×10?3M hydroxylamine in 0.1M PBS is observed with an E pa (peak potential)of 0.80V and I pa (peak current)of ?1.278×10?4A.The oxidation process begins around 0.2V and electrochemical response is irreversible,as no cathodic current is observed during the reverse sweep.As faster electron transfer leads to a sharper and more well-de?ned peak,the substantial increase in the peak current re?ects a faster electron transfer reaction.There-fore,hydroxylamine is effectively detected by oxidation on the ZnO/MWCNTs/GCE.According to a previous report [43–45],the possible electrochemical reaction for the hydrazine is proposed to be 2NH 2OH +4OH ?→N 2O +5H 2O +4e ?.The schematic diagram as shown in Scheme 2.

Fig.5(A)shows a typical amperometric response of the ZnO/MWCNTs/GCE upon the successive addition of hydroxylamine (4.0×10?7–1.9×10?2M)into continuously stirred 0.1M PBS (pH 7.4)at an applied potential of 0.80V;rotatingrate,4000rpm.The amperometric sensor exhibited a rapid and sensitive response to the change of hydroxylamine concentration,and an obvious increase in the oxidation current upon successive addition of hydroxylamine was obtained.The modi?ed electrode achieved 95%of the steady-state current within 3s.

As can be seen in Fig.5inset,the corresponding calibration curve of the fabricated amperometric hydroxylamine sensor exhibits a linear response in the concentration range from 0.4to 1.9×104?M,which is wider than that of Alizarin red S [43]and rutin

multi-wall

Fig.5.(A)Amperometric response of the ZnO/MWCNTs/GCE with successive addi-tion of hydroxylamine into 0.1M PBS buffer solution (pH 7.4);rotating rate,4000rpm (a →b,4.0×10?7M steps;b →c,8.0×10?5M steps;c →d,3.3×10?4M steps;d →e,6.7×10?4M steps;e →,1.3×10?3M steps).The applied potential was 0.8V.Plot of 1/i vs.1/C ,exhibiting a linear relationship for the steady-state current and hydrazine concentration (inset

B).

Fig.6.Effect of pH on the ZnO/MWCNTs/GCE in PBS with different values of pH 7.0,7.5,8.0,8.5,9.0,9.5for determining 3.0×10?4M hydroxylamine.Scan rate:100mVs ?1.

carbon nanotubes [44]and so on [14,17,45](Table 1).The correla-tion coef?cient of the line R 2=0.99924,and the linear equation is y =?2.52194?0.00745x .Based on signal-to-noise ratio (S/N)of 3,it has a very low detection limit,which was estimated to be 0.12?M.3.4.Effect of pH and scan rate on the oxidation of hydroxylamine The pH of the solution is important to obtain the ef?cient electrocatalytic oxidation of hydroxylamine.Fig.6shows the dependence of the solution pH (from 7.0to 9.5)on the injection of 3.0×10?4M hydroxylamine due to ZnO is not stable in acidic solu-tion.The results indicate that the anodic peak current increased remarkably with pH value increasing until it reached 8.5and then increased indistinctively with pH value increasing.Since the physi-ological pH value is about 7.4,we chose it as the support electrolyte in the electrochemical detection of hydroxylamine.

The scan rate effect on the cyclic voltammetric response of the ZnO/MWCNTs modi?ed electrode in 0.1M PBS containing 0.1M KCl (pH 7.4)with 5.0×10?4M hydroxylamine is shown in Fig.7.

As

Fig.7.Cyclic voltammograms of ZnO/MWCNTs/GCE at different scan rates in 0.1M PBS containing 0.1M KCl (pH 7.4)with 5.0×10?4M hydroxylamine.Scan rate:10,50,100,150,200,250mV/s from (a)to (f).Inset:the plot of peak current vs.scan rate.

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Fig.8.Amperometric responses of the different electrodepositing times of porous ZnO nano?lms:(a)20min;(b)30min;(c)50min.Operating potential:?1.2V vs. SCE.

the scan rate increases,the oxidation peak current(I pa)increases.

A good linear relationship between I pa and scan rate was obtained over the range of10–250mV/s.The linear regression equation is I pa (?A)=27.9587+0.6055V(V s?1)with a correlation coef?cient(R2) is0.9986.Hence,the peak currents responses of the hydroxylamine on the surface of ZnO/MWCNTs modi?ed electrode is controlled by adsorption.

3.5.Effect of electrodepositing time on the oxidation of hydroxylamine

The electrode response characteristics toward hydroxylamine can be in?uenced by the time of electrodepositing porous ZnO nano?lms.From Fig.8,the current–time curves of the different electrodepositing time of ZnO(a:20min;b:30min;c:50min)for successive addition of1.0×10?4M hydroxylamine in PBS(pH7.4). Obviously,from electrodes(a)to(b),the sensitivity increased with the time of electrodeposition.However,the sensitivity of electrode (c)was depressed,this was because too much ZnO nano?lms cover-ing onto MCWNTs,which reduced the electronic transfer properties of MCWNTs.So,30min was selected as the optimal time of elec-trodepositing.

3.6.Repeatability and stability of the ZnO/MWCNTs/GCE

The modi?ed electrode exhibited high stability.The relative standard deviation(RSD)of8successive scans was3.2%for50.0?M hydroxylamine.Only1.8%loss of current was found even after three weeks,with storage in air at4?C or in phosphate buffer when not in uses.The reproducibility of8independently fabricated elec-trodes showed a satisfactory value of4.2%(RSD).The electrode response was very stable and showed excellent anti-fouling

prop-Fig.9.Interference test protocols of the hydroxylamine biosensor to the heavy metal ions interferences in the absence of hydroxylamine((a and e)1.0×10?4M hydroxylamine;(b)5.0×10?3M Hg2+;(c)5.0×10?3M Cd2+;5.0×10?3M Cu2+).

erties against the oxidation products,which can be related to the MWCNTs binder.

3.7.Interference study

The effect of various substances,as the potential interference compounds,was studied on the determination of hydroxylamine under1.0×10?4M hydroxylamine in PBS(pH7.4).No interference was observed with common cations and anions(500-fold quanti-ties of K+,Na+,Ca2+,Mg2+,Cl?,Br?,I?,SO42?and NO3?ions),and for 50-fold quantities of glucose,fructose,and lactose in determination of1.0×10?4M of hydroxylamine concentration,the deviation of the determination was within3.2%.Fig.9presents the results of the heavy metal ions(Hg2+,Cd2+and Cu2+)interferences test carried out by amperometric measurements with the fabricated biosensor, for50-fold Hg2+,Cd2+and Cu2+in determination of1.0×10?4M of hydroxylamine concentration,the deviation of the determination was within1.8%.

3.8.Application to simulant sample analysis

In order to evaluate the validity of the proposed method for the determination of hydroxylamine,?ve distilled water samples were spiked with different concentrations of hydroxylamine and were analyzed under optimized conditions using the above technique. The data given in Table2shows that the recoveries are appropriate in despite of some deviation.The possible reason of deviation is random error in experiments.Overall the results are satisfactory, showing that the proposed methods could be ef?ciently used for the determination of hydroxylamine.

Table1

Comparison of the responses of some amperometric hydroxylamine sensors constructed based on different modi?ed electrode materials.

Electrode materials Response time(s)Detection limit(?M)Linear range(?M)Sensitivity(?A?M?1)Ref.

Alizarin red S–7.210–800–[43] Rutin multi-wall carbon nanotubes–– 1.0–33.80.0288

––33.8–81.70.025[44] Gold nanoparticle–polypyrrole nanowrie0.211–5000.0639[45]

500–18,0000.0104

Hybrid copper–cobalt hexacyanoferrate?lm0.21 4.6–1800–[17] Coumestan derivative10.7560–1000–[14] This work<30.120.4–19,0000.0075–

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Table2

Results of the recovery test.

Sample(distilled water)Hydroxylamine added(?M)Hydroxylamine found(?M)a Recovery(%)

11010.1±0.4101 25049.8±0.299.6 3100103±0.5103 4150150±0.4100 5200198±0.399

Quantitative determination of hydroxylamine from distilled water sources spiked with known quantity of hydroxylamine.

a Average of three determinations.

4.Conclusions

In this work,we have successfully fabricated an electrochemical hydroxylamine sensor based on electrodeposition of the loose and porous ZnO?lms onto carbon nanotubes?lms.An elegant chemi-cal architecture to construct electrodes using MWCNTs as a binder to stabilize the ZnO?lms onto the surface of a GCE was presented. The ZnO is a low density,loose and porous material with higher accessible surface area,which is favorable to a catalytic application. And MWCNTs not only help to bind the ZnO?lms onto the elec-trode surface because of the great sorption and oxygen–hydrogen bonds,but also to increase the electron transfer rate.The response of such electrodes toward the analytical determination of hydroxy-lamine was studied.A lower detection limit of0.12?M,wide linear range,high sensitivity,good stability and so on,which are provid-ing an opportunity for ZnO/MWCNTs/GCE as a promising method to detect hydroxylamine ef?ciently.

Acknowledgements

We appreciated the support of the National Natural Science Foundation of China(No.20675001),Science Foundation of Of?ce of Anhui Province(No.KJ2009B013z)and the Program for Innova-tive Research Team in Anhui Normal University.

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