A New Strategy for Immobilization of Electroactive Species on the Surface of Solid Electrode
Yinling Wang &Lin Liu &Dandan Zhang &Shudong Xu &Maoguo Li
Published online:8October 2010#Springer 2010
Abstract This letter reports on a new method for immo-bilization of electroactive species on a solid electrode surface bad on the composite film of naphthol green B (NGB)and Co –Al layered double hydroxides (LDHs).After the oxidation potential of Co(III)/(II)couple was dramatically enhanced by NGB,the functionalized LDHs exhibited a remarkable electrocatalytic activity toward the oxidation of hydrazine in a weak acidic medium.The amperometric respon to hydrazine showed a linear range of 0.17μM –400mM,with the calculated detection limit of 0.1μM at a signal-to-noi ratio of three and the nsitivity of the amperometric hydrazine nsor was found to be 35.26μA μM −1.The modified electrode displayed an acceptable reproducibility and good stability.The new strategy is expected to have wide applications for changing of the potential of redox couple in metal oxides,hydrox-ides,double hydroxides,and so on.
Keywords Layered double hydroxides .Electroactive specie .Immobilization .Hydrazine .Electrocatalytic activity
Introduction
It is significant to immobilize the electroactive species on the surface of solid electrode stably for fabrication of chemically modified electrodes (CMEs)[1],becau of the wide applications of CMEs in electrocatalysis,electro-chemistry,and electroanalysis [2].Though many organic and inorganic electroactive materials have been immobi-lized on the surface of electrodes,only a few soluble electroactive ions or molecules have been immobilized successfully becau it is difficult to prevent the electro-active species from dissolving into bulk solution.For example,to immobilized hexacyanoferrate ions ([Fe (CN)6]3−)on the surface of solid electrode stably,poly-pyrrole film was ud [3]or metal hexacyanoferrate nanostructures were synthesized [4];however,there still exist some problems such as complicated operation and ion leakage.
Recently,increasing attention has been paid to layered double hydroxides (LDHs)for their wide applications [5].Previous efforts have demonstrated that LDHs are attractive materials for electrochemical bionsors design.Owing to their good biocompatibility [6],inten adsorbability [7],and high catalytic activity [8],veral enzymes,such as polyphenol oxida [9,10],horradish peroxida [11],and gluco oxida [12],have been immobilized success-fully on the surface of tho low-cost synthetic anionic clays for fabrication of bionsors.However,most of LDHs have no e
lectroactivity,which limits their wide applications in electrochemistry.Our literature arch revealed that only LDHs containing Ni and Co have electroactivity directly in a strong alkaline medium (0.1M NaOH,for example)[13].The previous reports have shown that Ni-bad LDHs suited to perform electrocatalysis of oxidizable substrates
gazedY .Wang :L.Liu :D.Zhang :S.Xu :M.Li (*)Anhui Key Laboratory of Chemo-Bionsing,College of Chemistry and Materials Science,Anhui Normal University,Beijing East Road No.1,Wuhu 241000,China
e-mail:limaoguo@mail.ahnu.edu M.Li
e-mail:chmlim@nus.edu.sg
Electrocatal (2010)1:230–234DOI 10.1007/s12678-010-0030-1
宋耀如
[14].However,Co-bad LDH-modified electrodes do not displayed electrocatalytic activity toward oxidizable small biomolecules due to the lower redox potential of the Co (III)/Co(II)couple[14].
In this letter,we prent,for the first time,the enhancing electroactivity of Co/Al LDHs by a novel method and explore an application of the LDH-modified electrodes for the electrocatalytic oxidation i齐心战疫
n a weak acidic solution by chon hydrazine as model.This strategy for immobilization of electroactive species on the surface of solid electrode is expected for the development of electrochemical bionsors and electronic devices.
Experimental
Synthesis of Carbonate Anion-intercalated Co–Al LDHs Carbonate anion-intercalated cobalt-bad LDHs(Co–Al LDHs)were synthesized by a method involving constant pH coprecipitation of the chon M II and M III cations with alkaline solutions[15].Typically,the mixed basic solution containing0.3M NaOH and0.026M Na2CO3was quickly added into150ml of mixed salt solution containing6mM CoCl2·6H2O and3mM AlCl3·6H2O under vigorous stirring,maintaining the pH clo to10.86,followed by continuous magnetic stirring for20min.The resulting pink slurry was transferred from the colloid mill to a flask (250ml)and aged at60°C for2h.The final solid product was collected by means of centrifugation,washed four times with deionized water and anhydrous ethanol,respec-tively,to remove any possible remnants,and finally air-dried at room temperature.
Electrode Modification
The colloidal suspension(2mgml−1)was prepared by dispersing LDHs in deionized water with stirring
about 12h.After10μl of the as-prepared suspension was spread on the surface of the pretreated glassy carbon electrode (GCE),the electrode was dried in air.Then,the resulting electrode was immerd in NGB solution(0.5mM)for 25min.In such a way,a modified electrode(NGB/Co–Al-LDH/GCE)was fabricated.
Apparatus and Procedures
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The electrochemical measurements were carried out with CHI 660C electrochemical workstation(Shanghai Chenhua, China).A conventional three-electrode system was employed comprising a GCE or modified electrode as working electrode,a platinum wire as counter electrode,and an Ag/ AgCl(3M KCl)as reference electrode.All electrochemical experiments were performed in solutions deaerated by pure nitrogen at room temperature.
X-ray powder diffraction(XRD)data were recorded by a Shimadzu XRD–6000X-ray diffractometer bad on Cu Kαradiation(l=0.15406nm).The2θangle of the diffractometer was stepped from5°to70°at a scan rate of0.05°s−1.The field-emission scanning electron micros-copy(FESEM)was obtained using an S-4800FESEM (operating at5.0kV).
Results and Discussion
Figure1A shows the XRD pattern of the Co–Al LDH sample that was prepared by constant pH coprecipitation.All the diffraction peaks are consistent with tho of well-known LDH materials in CO32−form[16,17].No other crystalline pha was discerned,indicating the high purity of the product.The003reflection was located on the2θangle of~11.8°,indicating a basal interlayer spacing of~0.76
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nm, Fig.1XRD pattern(A)and FESEM(B)of Co–Al–CO3LDHs
quite similar to tho reported elwhere [17].Figure 1B shows SEM image of a typical LDH sample.As can be en,the sample consisted of nanoflakes with a lateral size of 100–300nm and a thickness of less than 10nm.
The electrochemical properties of modified electrodes were studied by cyclic voltammetry (CV).In a 0.1M pH 3.5Britton –Robinson (B –R)buffer solution,no peaks can be obrved for both bare GCE (curve a in Fig.2)and Co –Al LDH-modified electrode (Co –Al-LDH/GCE)(curve b).However,the NGB/Co –Al-LDH/GCE gave a pair of stable redox peaks with the apparent formal peak potential (E p )of 0.482V (curve c),which was much higher than that of a Co –Al LDH-modified Au electrode in 0.1M NaOH (about 0.06V)[13].For comparison,Zn –Al –CO 3LDHs were also prepared according to our previous work [18]and ud for fabrication of the NGB-modified electrode (NGB/Zn –Al –LDH/GCE).As can be en from Fig.3A ,only a pair of weak redox peaks due to redox of the electroactive site (Fe III /Fe II )of NGB displayed in the cyclic voltammo-gram (curve a)of NGB/Zn –Al-LDH/GCE,quite similar to the electrochemical behavior of NGB at a bare GCE (curve b in Fig.3B )in B –R buffer solution [19].Figure 3B shows the cyclic voltammograms of a GCE in B –R solution containing different electroactive species.In 0.1M pH 3.5B –R buffer solution containing both Co 2+and NGB (curve c),the GCE gave a pair of well defined redox peaks,indicating some new electroa
ctive compound is produced in the mixed solution.Furthermore,the E p (about 0.475V)is much clod to the E p of NGB/Co –Al-LDH/GCE.Bad on the results,the chemical reaction [Fe(III)NGB →Co(II)NGB]might take place during the NGB ud for modification of Co –Al-LDH/GCE.To obtain more eviden-ces that Co(II)combines with NGB,two non-electroactive compounds,1,10-phenanthroline (Phen)and 2,2′-bipyridine
(Bpy),were ud for fabrication of Phen/Co –Al-LDH/GCE and Bpy/Co –Al-LDH/GCE respectively.The cyclic vol-tammograms illustrated in Fig.4indicate that a couple of redox peak at eit
her Phen/Co –Al-LDH/GCE (curve b)or Bpy/Co –Al-LDH/GCE (curve c)was obrved.So the electrochemical reaction on the NGB/Co –Al-LDH/GCE would be:
Co III ðÞNGB þe À!Co II ðÞNGB
Figure 5gives the typical cyclic voltammograms of the NGB/Co –Al-LDH/GCE in pH 3.5B –R solution with scan rates of 20–250mVs −1.Peak currents varied linearly with the scan rates,as shown in the int of Fig.5,suggesting a surface-controlled electrode process.According to the slope of the I p –v curve and Laviron ’s equation [20],I p =n 2F 2vA Γ/4RT ,the average surface coverage (Γ)of
electroactive
Fig.2Cyclic voltammograms of (a )bare GCE,(b )Co –Al-LDH/GCE and (c )NGB/Co –Al-LDH/GCE in 0.1M pH 3.5B –R buffer solution at scan rate of 100mVs −
1
Fig.3A Cyclic voltammograms of (a )NGB/Zn –Al-LDH/GCE and (b )NGB/Co –Al-LDH/GCE in 0.1M pH 3.5B –R buffer solution at scan rate of 100mVs −1.B Cyclic voltammograms of bare GCE in the same buffer containing (a )0.5mM CoCl 2,(b )0.5mM NGB and (c )0.5mM CoCl 2+0.5mM NGB at sc
an rate of 100mVs −1
species on the surface of modified electrode was5.43×
10−9molcm−2.On the other hand,theΔE p(define asΔE p=
E pa−E pc)incread with the increa of scan rate.Herein,the electron transfer rate constant was estimated to be2.1s−1at
scan rate of200mVs−1according to Laviron[21],
suggesting a fast electron transfer between the electroactive
species and the GC electrode.
Becau the marked enhancement of redox potential of the
Co(III)/Co(II)couple can be obtained at the NGB/Co–Al-
LDH/GCE,the modified electrode may display electro-
catalytic activity toward oxidizable small molecules.In this
ca,hydrazine,an important compound ud in wide fields
[22],was chon as a model for exploring the electro-
catalytic activity of the modified electrode.Compared with the CV curves obtained from NGB/Co–Al-LDH/GCE without hydrazine(Fig.6a),an increa in the oxidation peak obtained from modified electrode at about+0.519V was obrved,coupled with a small decrea of reduction peak(Fig.6b).Figure7shows a typical amperometric respon of the NGB/Co–Al-LDH/GCE on the successive addition of hydrazine(1–4nmol)into continuously stirred 6.0ml0.1M pH3.5B–R solution at an applied potential of 0.52V,which resulted in a remarkable increa in the oxidation current.The time required to reach the95% steady-state respon was within2s,indicating a quick respon process.The amperometric respon to hydrazine showed a linear range of0.17μM–400mM(int of Fig.7), with the calculated detection limit of0.1μM at a
signal-to-Fig.6Cyclic voltammograms of NGB/Co–Al-LDH/GCE(a)without hydrazine and(b)with2.5μM hydrazine in0.1M pH3.5B–R buffer solution at scan rate of100mVs−
1 Fig.5Cyclic voltammograms of NGB/Co–Al-LDH/GCE in the same
buffer at different scan rates.The lected scan rates are(a)20,(b)40,
(c)60,(d)80,(e)100,(f)120,(g)150(h)200and(i)250mVs−1,
respectively.Int:Plots of anodic(a)and cathodic(b)peak currents
vs.scan
rates
Fig.4Cyclic voltammograms of(a)Co–Al-LDH/GCE,(b)Phen/Co–
Al-LDH/GCE and(c)Bpy/Co–Al-LDH/GCE in0.1M pH3.5B–R
buffer solution at scan rate of100mVs−
1
noi ratio of three and the nsitivity of the amperometric hydrazine nsor was found to be35.26μAμM−1.The detection limit is lower and the nsitivity is higher than tho of other previously reported amperometric hydrazine nsors[22–24].
千的英语怎么读The reproducibility of NGB/Co–Al-LDH/GCE electrode was investigated by comparing the amperometric current respon to hydrazine at eight modified electrodes prepared independently,and the relative standard deviation(RSD) was2.5%at a hydrazine concentration of0.10mM,which indicated that the modified electrode displayed an accept-able reproducibility.A continuous measurement of CV was carried out for90min in a0.10mM hydrazine solution.It was found that the peak current for hydrazine oxidation retained96.2%of its initial value and no obvious potential shift was obrved.The result shows that the modified electrode has good durability.If the modified electrode was stored in air at room temperature for4weeks,amperometric current respon to2.0μM of hydrazine changed some-what;only5.3%leakage was found,which indicated that the NGB/Co–Al-LDHs on the surface of GC electrode was quite stable.
Conclusions
We have demonstrated here that the oxidation potential of the Co(III)/Co(II)couple in Co–Al LDHs can be dramat-ically enhanced via immobilization of NGB on them.At the same time,the modified Co–Al LDHs have good electro-activity in a weak acidic or neutral solution;no longer always need a strong alkaline medium.The composite film on the surface of GCE has displayed high electrocatalytic activity toward oxidation of hydrazine along with good stability and excellent nsitivity.All the results may lead to the potential applications of Co–Al LDHs in electro-catalysis,electroanalysis,and electronic devices.The new strategy for immobilization electroactive species on surface of solid electrode is expected to have wide application for changing of the potential of redox couple in metal oxides, hydroxides,and double hydroxides using the appropriate ligands such as bipyridine,phenanthroline,and so on.Acknowledgements This work was supported by Natural Science Foundation of China(20801001and20875001)and the Science Foundation of Education Committee of Anhui Province (KJ2007B098).We also deeply appreciate the support of the foundation for doctor science rearch of Anhui Normal University. References
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