Characterization and electrocatalytic properties of Prussian blueelectrochemically deposited on nano-Au/PAMAM
dendrimer-modified gold electrode
Nian Bing Li a,b, Jun Hui Park a, Kyungsoon Park a, Seong Jung Kwon a,Hyunkyung Shin a, Juhyoun Kwak a,∗
a Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Koreab School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China
Received 10 August 2007; received in revised form 5 December 2007; accepted 9 January 2008Available online 17 January 2008
bstract
Gold electrode was modified with 3-mercaptopropionic acid (MPA) and further reacted with poly(amidoamine) (PAMAM) dendrimer (gen-ration 4.0) then attached the nano-Au to obtain films on which Prussian blue (PB) was electrochemically deposited to afford much wider pHdaptive range, much better electrochemical stability and excellent electrochemical response. The microstructure and electrochemical behavior ofu/MPA/PAMAM/nano-Au/PB electrode were investigated by scanning electron microscopy (SEM) and cyclic voltammetry. The electrochemical
esponse of the Au/MPA/PAMAM/nano-Au/PB-modified electrode for the electrocatalytic reduction of hydrogen peroxide was investigated, and itas found that the sensitivity as well as the corresponding detection limits were improved as compared to the voltammetric response of a Au/PB-odified electrode and Au/MPA/PAMAM/PB electrode. Based on this, a new electrochemical sensor for determination of hydrogen peroxide has
een developed.2008 Elsevier B.V. All rights reserved.
Prussian blue (PB) is a prototype of metal hexacyanoferratesith well-known electrochromic (Kulesza et al., 2001), elec-
rochemical (Itaya et al., 1986), photophysical (Kaneko et al.,985), and magnetic properties (Mingotaud et al., 1999) andotential analytical applications (Karyakin, 2001). Due to itsxcellent electrocatalysis, PB is widely used as an electron-ransfer mediator in the amperometric biosensors (Karyakin etl., 2000, 2002; Katz and Willner, 2004; Derwinska et al., 2003).ecently, PB has been defined as an “artificial peroxidase”ecause of its analogy with the biological family of peroxidasenzymes, responsible in nature for reduction hydrogen peroxide
Karyakin et al., 2000). The main problem of the chemicallyodified electrodes reported to date, however, relies on the fact
hat the electrocatalytic film of PB is stable only at low pH val-
es (Scharf and Grabner, 1996; Moscone et al., 2001a,b), andherefore, its integrity and activity are seriously compromised byulk and local changes in pH that often appear as a consequencef electron-transfer events in the interfacial region (Bustos et al.,005). The pH stability seems to be dependent on the differentodes of deposition of PB layer (Ricci and Palleschi, 2005). An
ncreased stability of the PB layer at alkaline pH was observedy adopting a chemical deposition method which was a usefullternative to the most used electrochemical approach for theodification of the electrode surface with PB (Moscone et al.,
001a,b; Ricci et al., 2003b). The greatly enhanced stability ofB layer, which deposited on screen-printed electrodes (SPEs),ade possible the practical application of H2O2 sensor even at
lkaline pH and with the coupling to an oxidase enzyme (suchs glucose oxidase and choline oxidase) having an optimum pHf 8.0 (Ricci et al., 2003a). Other methods, ranging from the use
f protective polymers to the use of additives in the depositionuffer, have been proposed to increase the operational stabilityf PB. In recent years, conducting and non-conducting polymersuch as poly(o-diaminobenzene) (Lukachova et al., 2002, 2003),
oly(o-phenylendiamine) (Garjonyte and Malinauskas, 1999,000), and poly(vinylpyrrolidone) (Uemura and Kitagawa,003; Xian et al., 2005) were used to improve the performance ofB layer by providing a better stability. Electropolymerization of,4′-bis(butylsulfanyl)-2,2′-bithiophene (Lupu et al., 2002) and-aminophenol (Pan et al., 2004) on a PB-modified platinumlectrode resulted in an enhanced reproducibility and stabilityf the sensor produced. In addition, the additive, tetrabutylam-onium toluene 4-sulfonate (TTS) presented in the working
olution helped stabilize hexacyanoferrate-modified electrodesLin and Shih, 1999). And it was demonstrated that TTS usedoth in the carrier stream of flow injection analysis or in theuffer used for the electrodeposition of PB markedly improvedhe stability of the PB layer (de Mattos et al., 2003; Haghighi etl., 2004). Furthermore, an ionic conductor such as Nafion haslso provided a higher stability of the PB film (Karyakin et al.,995).
Dendrimers are highly branched and monodisperse macro-olecules with a well-defined three-dimensional and globular
tructure (Grayson and Frechet, 2001). They have receivedxtensive attention due to their potential applications in the fieldsf chemical and biomedical sensors (Vogtle et al., 2000; Cagint al., 2000), microelectronic and biomimetic systems (Tullynd Frechet, 2001), adhesion, coating, and membrane chem-stry (Tsukruk, 1998), and nanotechnology (Emmrich et al.,002). Dendrimer molecules possess three basic architecturalomponents: an initiator core (e.g., ethylenediamine), interiorayers often called “generations”, which comprise repeatingnits attached to the initiator core, and the shell which generallyonsists of functionalized groups attached to the outermost inte-ior layer. Although most of the work with dendrimers has beenarried out in solution, these compounds have also been usedo modify electrode surface and some recent reports indicatehat these materials are capable of increasing the concentrationf hydrophobic molecules at the electrode–solution interface,mproving in this way the sensitivity as well as the selectivity ofertain specific electrochemical reactions. Among various den-rimers, poly(amidoamine) (PAMAM) is the most frequentlytudied. Godinez and co-researcher modified gold bead elec-rode with PB containing starburst PAMAM dendrimer to afford
ixed and stable electrocatalytic layers which not only showedn improved surface coverage of PB on the dendrimer-modifiedlectrode but also showed an enhanced stability at neutral pH val-es (Bustos et al., 2005). They also compared the preparation anderformance of covalently modified gold electrodes with variousenerations of PAMAM dendrimers loaded with PB, and devel-ped an amperometric sensors of H2O2 using the dendrimer–PB-odified electrodes (Bustos et al., 2006). Recently, Wu et al.
2007) improved the selectivity and stability of amperometricetection of hydrogen peroxide using PAMAM/PB supramolec-lar complex membrane as a catalytic layer.
To the best of our knowledge, no study using nano-u immobilized on PAMAM dendrimer to load with PB
as been reported. In this paper, we report the preparationnd characterization PAMAM/nano-Au/PB films anchored on-mercaptopropionic acid-modified electrode and some prelimi-ary results on the electrocatalytic activity of hydrogen peroxide.
Fb5t
ctronics 23 (2008) 1519–1526
. Experimental
.1. Reagents
Amine terminated G4 poly(amidoamine) dendrimerPAMAM), 3-mercaptopropionic acid (MPA), and KCl wereurchased from Aldrich. N-(3-dimethylaminopropyl)-N′-thylcarbodiimide hydrochloride (EDC), gold colloid (20 nm)nd FeCl3 were obtained from Sigma. K3Fe(CN)6 was obtainedrom Mallinckrodt. Hydrogen peroxide was obtained fromunsei Chemical Co. Ltd. All chemicals used were of analytical-eagent grade, and water (>18 M� cm) was obtained from a
illipore Milli-Q purification system.
.2. Apparatus
The cyclic voltammetry (CV) and electrochemicalmpedance measurements were performed with an Auto-ab potentiostat 10 (Ecochemie). A three-electrode system usedn the measurements consists of a gold electrode or a modifiedold electrode as the working electrode, platinum wire as theounter electrode, and an Ag/AgCl electrode as the referencelectrode. All potentials are given with respect to the Ag/AgCllectrode. Scanning electron microscopy (SEM) images werebtained using a Philips XL 30S FEG operated at 10 and 3 kV.
.3. Electrode preparation
Gold electrodes were prepared by electron beam evap-ration of 40 nm of Ti followed by 150 nm of Au ontoi(1 0 0) wafers. The electrode was cleaned in piranha solu-
ion, rinsed with water, and then dried with nitrogen gas. Theretreated electrode was immersed in 1.0 M ethanol solutionf MPA (75/25% ethanol/water) for 12 h at room temper-ture and then washed thoroughly in 75/25% ethanol/watero remove the non-chemisorbed materials. Subsequently, theu/MPA-modified electrode was immersed in 1.0 mg mL−1
AMAM dendrimer solution in presence of 5 mM EDC for2 h period at room temperature. Thus surface anchoring ofendrimers G4 PAMAM on the thiol-modified gold electrodesas carried out by means of peptidic bond formation using
raditional peptide chemistry protocols. After rinsed with dis-illed water, the Au/MPA/PAMAM membrane electrode wasransferred into gold colloid solution for 12 h at 4 ◦C. Then,he Au/MPA/PAMAM/nano-Au-modified electrode was fabri-ated. The schematic illustration of the stepwise self-assemblyrocedure is shown in Fig. 1 (Scheme 1). In order to com-are with the electrochemical behavior for different electrodes,hree modified electrodes, that is, Au/PB, Au/MPA/PAMAM/PBnd Au/MPA/PAMAM/nano-Au/PB electrodes were preparedy the electrochemical deposition of PB. The electrodepositionf PB were achieved by immersing the preprocessed electrode incarefully deoxygenated (20 min) solution containing 2.5 mM
eCl3, 2.5 mM K3Fe(CN)6, 0.1 M KCl and 0.1 M HCl, followedy a cyclic scan in a potential range of −0.5 to +0.65 V at0 mV/s for 15 cycles. After deposition, the electrodes werehoroughly washed with double-distilled water, then transferred
N.B. Li et al. / Biosensors and Bioelectronics 23 (2008) 1519–1526 1521
Fig. 1. Schematic illustration of the stepwise self-assembly fabrication process (Sche(b) and Au/MPA/PAMAM/nano-Au electrode (c)).
ia−w
3
3e
tAswt
tFdisotbtqfietdcsitmacalPgPcn
Scheme 1. Schematic representation of the modification process.
nto a supporting electrolyte solution (0.1 M KCl + 0.1 M HCl)nd electrochemically activated by cycling between +350 and50 mV (25 cycles) at a rate of 50 mV/s. Finally the electrodesere taken out and rinsed with double-distilled water again.
. Results and discussions
.1. Electrochemical characteristics of the modifiedlectrodes
The cyclic voltammograms of the bare gold elec-rode, Au/MPA electrode, Au/MPA/PAMAM electrode, and
u/MPA/PAMAM/nano-Au electrode in 2.5 mM K3Fe(CN)6
olution containing 0.125 M KCl and 0.05 M PBS (pH 7.4)ere obtained. Impedance measurements were performed in
he frequency range from 0.05 to 105 Hz at the formal poten-
ae2s
me 1) and SEM images of Au/MPA electrode (a), Au/MPA/PAMAM electrode
ial of 0.22 V in the same solution. It is clearly shown that thee(CN)6
3−/Fe(CN)64− redox couple can give a pair of well-
efined reversible peaks at the bare gold electrode. From thempedance plot, the bare gold electrode exhibited an almosttraight line that is characteristic of a diffusional limiting stepf the electrochemical process. However, it can be seen thathe peak current was decreased and the �Ep, the differenceetween the anodic peak potential and the cathodic peak poten-ial, was increased for the Au/MPA electrode. Since a largeuantity of negative charges from –COO− groups on MPAlm perturbed the interfacial electron-transfer rates between thelectrode and the electrolyte solution, the interfacial electron-ransfer resistance Ret corresponding to the respective semicircleiameters increased from 30 � to 164.6 �. When the PAMAMompounds are further incorporated on the Au/MPA electrodeurface, the peak currents on the Au/MPA/PAMAM electrodencreased obviously, this also reflect the electrostatic adsorp-ion between the positively charged surface confined PAMAM
olecule under such pH conditions (Manriquez et al., 2003),nd the negatively charged Fe(CN)6
3− probe. The positivelyharged surface confined PAMAM molecule would attract neg-tive redox marker, thus Ret decreased to 17.6 �, even muchower than bare Au electrode. These results show that dendrimerAMAM G4 was successfully attached to the MPA-modifiedold electrode surfaces. After nano-Au was attached on theAMAM-modified electrode, the peak current of the redoxouple of Fe(CN)6
3−/Fe(CN)64− increased. The reason is that
anometer-sized gold colloids play an important role similar to
conducting wire or electron-conducting tunnel, which makes itasier for the electrons transfer to take place (Szymanska et al.,007). Therefore, after absorption of nano-Au to the electrodeurface, the Ret obviously decreased again (Ret = 8.2 �).
1 ioelectronics 23 (2008) 1519–1526
maspiMtmnnsa
3e
ds0ttriHcarerocF
Pwisltnfitcp
3A
Pipoiw
Fig. 2. Cyclic voltammograms of Au/MPA/PAMAM/nano-Au electrode (a),Au/MPA/PAMAM electrode (b) and Au electrode (c) in a solution containing2i
t
522 N.B. Li et al. / Biosensors and B
SEM was also applied to confirm the each step of the electrodeodification process. Fig. 1a shows an image of MPA film self-
ssembled on a bare gold electrode surface. Although there weretill a small amount of drawbacks on the film, a layer of closedacked monolayer-film was obtained. Fig. 1b presents the SEMmage of PAMAM dendrimers, which anchored on the surface of
PA monolayer. It indicates that dendrimer PAMAM moleculesend to form a densely packed film on MPA surface in order to
aintain lower surface tension. The SEM image in Fig. 1c ofano-Au anchored on the surface of PAMAM film shows goldanoparticles are obviously formed on the PAMAM film. Ashown in this SEM image, the size of the gold nanoparticles isbout 20 nm.
.2. Electrodeposition and morphology of PB at threelectrodes
Fig. 2 displays typical cyclic voltammograms (CVs) obtaineduring the electrodeposition of PB on different electrodes in aolution of 2.5 mM FeCl3, 2.5 mM K3Fe(CN)6, 0.1 M KCl and.1 M HCl. For electrodeposition PB at PAMAM-modified elec-rode and the nano-Au/PAMAM electrode, it can be seen thatwo redox peaks grow with the successive scans in the potentialange of −0.5 to +0.65 V. The continuously increasing currentndicates that PB is accumulating in the modified electrodes.owever, for electrodeposition PB at gold electrode, only one
ouple of redox peaks located could be observed (Zhang etl., 2003, 2004). After rinsing these electrodes and transfer-ing them to the electrolyte (0.1 M KCl and 0.1 M HCl) andlectrochemically activated by cyclical scan in the potentialange of +350 to −50 mV at a rate of 50 mV/s for 25 cycles,nly a couple of redox peak appeared about at +0.17 V, whichorresponding to the electrochemical reactions of high spine3+/Fe2+.
The morphology of PB film on the Au electrode, Au/MPA/AMAM electrode and Au/MPA/PAMAM/nano-Au electrodeas characterized by SEM and the images of SEM were showed
n Fig. 3. It is very clear from Fig. 4a that bare gold electrodeurface is coated with uniform PB film along with tiny, irregu-arly shaped crystals of PB material. As shown in Fig. 3b, theendency to form larger agglomerates has been much more pro-ounced for the Au/MPA/PAMAM/PB film rather than Au/PBlm. From the image in Fig. 3c, on the nano-Au-modified elec-
rode surface was completely coated by a homogeneous andompact agglomerates PB grains. Compared with Fig. 3b, thearticle size of PB is smaller.
.3. Electrochemical characteristics of theu/MPA/PAMAM/nano-Au/PB electrode
The cyclic voltammograms of the Au/PB, Au/MPA/AMAM/PB and Au/MPA/PAMAM/nano-Au/PB electrodesn 0.1M HCl + 0.1 M KCl solution showed that a pair of redox
eaks can be observed clearly. For three electrodes, it can bebserved that the values of Epa and Epc shift slightly to the pos-tive and negative directions, respectively, and �Ep increasesith the increase of scan rate. That is to say, the redox peak poten-
ffpt
.5 mM FeCl3, 2.5 mM K3Fe(CN)6, 0.1 M KCl, 0.1 M HCl. The cyclical scann a potential range of −0.5 to +0.65 V at 50 mV/s for 15 cycles.
ials of both electrodes are scan rate dependent. However, the
ormal potentials E0 is almost independent of the scan rate. Theurther experimental results show that the anodic and cathodiceak currents for the three electrodes are well linearly propor-ional to scan rate, suggesting that the electrochemical behavior
N.B. Li et al. / Biosensors and Bioele
Fe
op
saftp
t
E
UwAeAAwes
hoAvoePptaTsw1htepiiAe
fu
Γ
wtAcacatote
ig. 3. SEM morphology of PB film on Au electrode (a), Au/MPA/PAMAMlectrode (b) and Au/MPA/PAMAM/nano-Au electrode (c).
f PB in the three electrodes is a typical surface-controlledrocess.
Laviron (1979) derived general expressions for the case ofurface-confined electroactive species. From this theory, the
pparent charge transfer rate constant (ks) for electron trans-er between the electrode and surface deposited layer as well ashe transfer coefficient (α) by measuring the variations of theeak potentials with scan rate (v) can be determined according
gini
ctronics 23 (2008) 1519–1526 1523
o the following equation:
p = E0 + RT
αnF
[ln
[RTks
αnF
]− ln v
](1)
sing the treatment proposed by Laviron, the α and ksere estimated for the three electrodes. The α for Au/PB,u/MPA/PAMAM/PB and Au/MPA/PAMAM/nano-Au/PB
lectrodes were 0.64, 0.59 and 0.38, respectively. The ks foru/PB, Au/MPA/PAMAM/PB and Au/MPA/PAMAM/nano-u/PB electrodes were 2.59, 1.89, and 0.41 s−1, respectively,hich decreased gradually. This is probably due to the large
lectron transfer distance between the PB molecule and theurface of the gold electrode (Finklea and Hanshew, 1992).
The effect of pH value on the electrochemical behavior of PBas been investigated. Fig. 4 shows the cyclic voltammogramsf Au/PB, Au/MPA/PAMAM/PB and Au/MPA/PAMAM/nano-u/PB electrodes in 0.1 M PBS and 0.1 M KCl with different pHalues. As can be observed from Fig. 4A, for pH ≤ 5.65, a pairf sharp redox peak can be observed obviously on the Au/PBlectrode. As the pH value increases to 6.77, the redox peak ofB comes to disappear. This implies that the electrochemicalroperties of PB at the PG electrode surface depend strongly onhe pH value of the solution and the Au/PB electrode is not stablet near neutral solution pH, as predicted by Ricci et al. (2003a).he reason for this behavior is probably to be ascribed to thetrong interaction between ferric ions and hydroxyl ions (OH−)hich forms Fe(OH)3 at higher pH (Feldman and Murray,987), thus leading to the destruction of the Fe CN Fe bond,ence solubilizing PB (Karyakin et al., 1999). However, forhe Au/MPA/PAMAM/PB and Au/MPA/PAMAM/nano-Au/PBlectrodes, as the pH value increases to 8.09, both well-definedeaks can be observed obviously. As the pH value furtherncreases to 9.55, the peaks are still observed. All these resultsndicate that Au/MPA/PAMAM and Au/MPA/PAMAM/nano-u electrodes greatly enhance the pH adaptive range of PB,
ven to alkaline solution.The surface coverages of PB on the three electrodes in dif-
erent pH value were estimated from the cyclic voltammogramsing the following equation:
= Q
nFA(2)
here Q is the charge in coulombs, n is the number of elec-rons involved in the process, F is the Faraday constant, and
is the geometric area of the working electrode in squareentimeters. The curves of Γ (using the anodic peak locatedround 0.2 V) versus pH that are presented in Fig. 4D. Itan be seen that the pH below 5.65, PB was preferentiallydsorbed on the Au/MPA/PAMAM/nano-Au electrode, then onhe PAMAM electrode, since the Γ value calculated is the largestn the Au/MPA/PAMAM/nano-Au electrode and about twoimes larger than that those obtained in the Au/MPA/PAMAMlectrode, four times larger than that those obtained in bare
old electrodes. The fact that Γ for the PAMAM electrodes larger than that for bare gold electrode is fully consistentot only with the dendrimer–PB interaction but also with thedea of a PB–dendrimer intermolecular interaction that pref-
1524 N.B. Li et al. / Biosensors and Bioelectronics 23 (2008) 1519–1526
rentially takes place within the PAMAM host. The Γ at theu/MPA/PAMAM/nano-Au electrode is the largest among the
hree electrodes at the same condition. One of the reasons maye that the nano-particulates have higher surface-area and themount of the PB electrodeposited on the modified surface isarger than the other electrodes. Another reason is that the PB
olecular can enter the cavity of the PAMAM and electrode-osited in the PAMAM host. From the inspection of the shape of
he curves presented in Fig. 4D, it can be readily seen that the PBlm on Au quickly loses stability at pH conditions above 5.65,ut the PB electroactivity on the PAMAM decreases graduallyfter the pH above 5.65. However, the electroactivity of PB on
ttgc
PA/PAMAM/nano-Au/PB (C) electrodes in 0.1 M KCl and 0.1 M PBS withes (D) of PB surface coverage, Γ , vs. pH of Au (1), Au/MPA/PAMAM (2) andKCl.
he Au/MPA/PAMAM/nano-Au-modified electrode almost isame as that in acidic solution even the pH has reached neuterondition. After the pH above 7.36, the PB electroactivity onhe Au/MPA/PAMAM/nano-Au-modified electrode decreasesrastically. Considering that the pKa values of PAMAM in solu-ion are 9.52, based on the protonated state of PAMAM atH < 9.52 and the consequent existence of local acidic condi-ions at the electrode–solution interface, the stability of PB on
he Au/MPA/PAMAM and Au/MPA/PAMAM/nano-Au elec-rodes improved. Since the nano-Au attached with the aminesroups of the PAMAM, the amount of amine groups whichan be protonated decreased, therefore, the stability of PB on
ioelectronics 23 (2008) 1519–1526 1525
tr
FH00titpcolAtrAa
PttFpcefArwfiktodo1A8ia
3A
m
Fig. 5. (A) Chronoamperograms obtained at the Au/MPA/PAMAM/nano-Au/PB electrode in absence (a) and presence of H2O2 of (b) 6.0 × 10−6 M,(c) 4.0 × 10−5 M, (d) 2.0 × 10−4 M, (e) 4.0 × 10−4 M, (f) 1.0 × 10−3 M, (g)2.0 × 10−3 M, (h) 4.0 × 10−3 M, (i) 6.0 × 10−3 M, and (j) 1.0 × 10−2 M at−0.2 V in 0.1 M HCl solution containing 0.1 M KCl. (B) Calibration plots forHe
rft0tiwt
ttvAtbtwciAA
TA
E
AAA
N.B. Li et al. / Biosensors and B
he Au/MPA/PAMAM/nano-Au-modified electrode decreasedapidly when the pH was above 7.36.
In our experimental, an interesting result was obtained.irst, the CVs of the three electrodes were obtained in 0.1 MCl + 0.1 M KCl solution in the potential range of −0.4 to.7 V with scan rate of 50 mV/s. Then, after finished the CVs in.1 M PBS (pH 9.55) containing 0.1 KCl with the same poten-ial range and scan rate, washed with water and then immersedn 0.1 M HCl + 0.1 M KCl solution, the CVs of the three elec-rodes were recorded again. From the CVs, a couple of redoxeaks can be seen, although the curves were not good, espe-ially for Au/PB electrode. The surface coverage values of PBn the three electrodes with different conditions were calcu-ated and the results showed that the surface coverage of PB onu, Au/MPA/PAMAM and Au/MPA/PAMAM/nano-Au elec-
rode remained 16.1%, 36.7% and 53.6% of the first values,espectively. It also indicated that the PB deposited on theu/MPA/PAMAM/nano-Au electrode has highest stability even
t alkaline solution.The dependence of the anodic peak current at Au/MPA/
AMAM/nano-Au/PB, Au/MPA/PAMAM/PB and Au/PB elec-rode on the placed time was investigated and the results showedhat the three electrodes have different electrochemical stability.or the Au/MPA/PAMAM/nano-Au/PB electrode, the anodiceak current decreased slowly in the first 2 h, but the anodic peakurrent decreased quickly for Au/MPA/PAMAM/PB and Au/PBlectrode in the same time. After 2 h, the anodic peak currentor Au/MPA/PAMAM/nano-Au/PB, Au/MPA/PAMAM/PB andu/PB electrode remained 91%, 62% and 50% of the first values,
espectively. For the Au/MPA/PAMAM/nano-Au/PB electrode,hen the anodic peak current decreased about 83% of therst value (it took about 6 h), the peak current then almostept constant. However, for the Au/MPA/PAMAM/PB elec-rode, after about 14 h the peak current kept constant (46%f the first value). For the Au/PB electrode, the peak currentecreased with the time increase. Only about 20 h later (24%f the first value), the peak current decreased slowly. After5 days, the anodic peak current for Au/MPA/PAMAM/nano-u/PB, Au/MPA/PAMAM/PB and Au/PB electrode remained0%, 41% and 12% of the first values, respectively. These resultsndicated that the Au/MPA/PAMAM/nano-Au/PB electrode hasgood electrochemical stability.
.4. Electrocatalytic behavior of the
u/MPA/PAMAM/nano-Au/PB electrode towards H2O2
In order to check the electrocatalytic activity of the threeodified electrodes towards H2O2 reduction, their voltammetric
iTc
able 1nalytical characteristics for the determination of H2O2 by different modified electro
2O2 at different electrodes. (1) Au/PB electrode, (2) Au/MPA/PAMAM/PBlectrode and (3) Au/MPA/PAMAM/nano-Au/PB electrode.
esponses were recorded in the absence and presence of dif-erent concentration of H2O2. The cyclic voltammograms ofhe Au/MPA/PAMAM/nano-Au/PB electrode in 0.1 M KCl and.1 M HCl with and without hydrogen peroxide shown that withhe gradual addition of H2O2, the reduction peak current for PBncreased and the oxidation peak current decreased gradually,hich indicated the catalytic properties of modified electrode to
he reduction of H2O2.The electrocatalytic reduction of H2O2 at the three elec-
rodes was studied by chronoamperometry. Fig. 5 shows theypical chronoamperometric response curves at 0.2 V for thearious concentrations of H2O2 with Au/MPA/PAMAM/nano-u/PB as working electrode. As Fig. 5 shows, it can be seen
hat an increase in concentration of H2O2 was accompaniedy an increase in reduction currents. Under the same condi-ion the reduction currents of H2O2 on the other electrodesere measured and the calibration graphs of �i against con-
entration of H2O2 were constructed. The results are listedn Table 1. Compared with Au/PB-modified electrode andu/MPA/PAMAM/PB electrode, the Au/MPA/PAMAM/nano-u/PB electrode has a highest sensitivity.From the results obtained in the chronoamperometric stud-
es, the diffusion coefficient was calculated for H2O2 reduction.he relationship between diffusion coefficient and bulk con-entration can be described by the Cottrell equation (Bard and
here D and C are the diffusion coefficient (cm2 s−1) and theulk concentration (mol cm3), respectively. From the Cottrellquation, it can be seen that the plot of i vs. t−1/2 is linear,nd from the slope, the value of D can be obtained. Theiffusion coefficients for H2O2 on the Au/MPA/PAMAM/nano-u/PB electrode, Au/MPA/PAMAM/PB electrode andu/PB electrode were 1.94 × 10−5, 2.74 × 10−6 and 2.08 ×0−6 cm2 s−1, respectively. The calculated value of D on theu/MPA/PAMAM/PB electrode and Au/PB electrode is close
o the value reported elsewhere (Yu et al., 2007), but the Dbtained on Au/MPA/PAMAM/nano-Au/PB electrode is largerhan that of the other electrodes.
. Conclusions
A new nano-Au-modified electrode, which was supportedy amino groups of G4 PAMAM dendrimer monolayer andhe PAMAM dendrimer membrane was successfully anchoredn the MPA-modified gold electrode have been fabricated. PBas efficiently electrodeposited on Au/MPA/PAMAM/nano-u-modified electrode and the microstructure and electro-
hemical behavior of Au/MPA/PAMAM/nano-Au/PB electrodeere investigated by cyclic voltammetry, and SEM. Com-ared with Au/PB and Au/MPA/PAMAM/PB electrode,he Au/MPA/PAMAM/nano-Au/PB electrode showed muchider pH adaptive range, much better electrochemical sta-ility and larger response current to the reduction of2O2.
cknowledgements
This work was supported by the Korea Science and Engi-eering Foundation (KOSEF) grant funded by the Koreaovernment (MOST) through the Bioelectronics ProgramM10536000001-06N3600-00110), the Basic Research Pro-ram (R01-2005-000-10503-0), and the National R&D Projector Nano Science and Technology.
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