De

Ma

Qb

c

S

a

ARRAA

KDFDU

1

tIieghpfa[

[d

0d

Electrochimica Acta 56 (2011) 7712– 7717

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

etection of dopamine in non-treated urine samples using glassy carbonlectrodes modified with PAMAM dendrimer-Pt composites

.G. Garcíaa,b, G.M.E. Armendáriza, Luis A. Godíneza,1, J. Torresa, S. Sepúlveda-Guzmánc, E. Bustosa,∗,1

Laboratory of Bioelectrochemistry, Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C., Parque Tecnológico, Querétaro, Sanfandila, Pedro Escobedo 76703,uerétaro, MexicoDepartment of Chemistry, Universidad de Guanajuato, Cerro de la Venada S/N Col. Pueblito de Rocha, 36040 Guanajuato, Gto, MexicoCentro de Innovación, Investigación y Desarrollo en Ingeniería y Tecnología, Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Av. Universidad,an Nicolás de los Garza, Nuevo León, 66451 Nuevo León, Mexico

r t i c l e i n f o

rticle history:eceived 11 February 2011eceived in revised form 29 May 2011ccepted 10 June 2011vailable online 17 June 2011

eywords:opamineIAENs-Pt

a b s t r a c t

Composites of hydroxyl-terminated PAMAM dendrimers, generation 4.0 (64 peripheral OH groups) con-taining Pt nanoparticles were synthesized at different reaction times using a microwave reactor. Thesynthetic procedure resulted in dendrimer encapsulated nanoparticles of Pt (DENs-Pt) of 1.53 ± 0.17 nmdiameter that was calculated from transmission electron microscopy, and the Pt nanoparticles had singlecrystal plane in (1 1 1) orientation determinate by selective area diffraction.

Each composite was electrochemically immobilized on a pre-functionalized glassy carbon (GC) elec-trode that was incorporated as a flow injection amperometric (FIA) detector, for the selective detectionand quantification of dopamine (DA) in untreated urine samples.

Comparison of the analytical performance of the novel electrochemical detector revealed that the

rine DENs-Pt modified GC electrode with the composite synthesized for 30 min in the microwave reactor,

showed the best response for the detection of DA in samples of non-treated urine, being the detectionand quantification limits smaller (19 and 9 ppb, respectively) than those corresponding to the naked aGC electrode (846 and 423 ppb, respectively) using the FIA detector. In addition, it was found that thiselectroanalytical approach suffers minimal matrix effects that arise in the analysis of DA in untreatedsamples of urine.

. Introduction

Dopamine (DA) is a neurotransmitter of paramount impor-ance for the proper functioning of the central nervous system.ts determination and quantification is usually carried out employ-ng a high performance liquid chromatography (HPLC), apparatusquipped with amperometric detection that commonly uses alassy carbon electrode. The analysis of real samples in most cases,owever, is long and complicated since the sample needs to bere-treated in order to avoid passivation of the electrode sur-ace. In this context, modified electrodes constitute an attractivepproach to quantify analytes of biological importance, such as DA1].

On the other hand, metallic nanoparticles such as Ag [2], Au3–5], Pt [6] and Pd [7], have attracted great interest for theesign and construction of electrochemical sensors due to their

∗ Corresponding author. Tel.: +52 442 211 6059; fax: +52 442 211 6001.E-mail address: ebustos@cideteq.mx (E. Bustos).

1 ISE Member.

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.06.035

© 2011 Elsevier Ltd. All rights reserved.

unique chemical and physical properties. In this context, severalreports in the literature have described novel sensing approachesthat improve the analytical capabilities of the most commonlyused techniques for the determination of biologically importantmolecules [8–13].

In order to promote a convenient interphasial environmentto confine metallic nanoparticles for electrochemical detection, apolymeric permeable thin layer is desired and in this context, den-drimer materials constitute a good choice. Dendritic materials areramified polymers which present a structure similar to covalentmicelles that exhibit defined internal cavities capable of concen-trating organic or inorganic compounds of nanometric dimensionssuch as nanoparticles [1].

Crooks and Balogh for instance, reported the preparation of Cunanoaggregates inside of PAMAM dendrimer molecules. The firststep exploited the interaction of Cu2+ ions with the internal ter-tiary amines of the dendrimer molecules, followed by the chemical

reduction of the metallic ions to form the nanoparticles [14–18].Different technologies using UV, laser, or X ray irradiation havedemonstrated to be effective routes for the synthesis of Au–Agnanoparticles inside PAMAM dendrimers [19–21].

imica Acta 56 (2011) 7712– 7717 7713

eopoatr

iweets

2

2d

(rda(aoriwt(

vssdttst8a

2

asdttrt

tva

iip

M.G. García et al. / Electroch

Chemical synthesis by microwaves on the other hand, hasxperienced an exponential growth in industry and in research lab-ratories. The irradiation of a reaction medium with microwavesrovides efficient heating that is broadly utilized in chemistry lab-ratories since 1986 [22–26]. The microwave reactor is simplend efficient, and the synthesis of Au nanoparticles supported inhird generation propylenimine dendrimers has been, for instanceeported [27,28].

Based on this information, the synthesis of Pt nanoparticlesnside generation 4.0 PAMAM dendrimers (–OH terminal groups)

as carried out using microwaves. These composites were thenlectrochemically deposited on pre-treated glassy carbon (GC)lectrodes as it has been reported before [1] to be used as elec-rochemical sensors to detect and quantify DA in non-treated urineamples utilizing a FIA system.

. Experimental

.1. Synthesis of composites of PAMAMendrimers-G4.0-OH-metallic nanoparticles

The synthesis of dendrimer encapsulated Pt nanoparticlesDENs-Pt) in aqueous solution was carried out using a microwaveeactor (MW, CEM brand). In this way, the preparation of theendrimer-ion complex was performed using a 2 mM NH3Pt(Cl)4s metallic ion source mixed with a 50 �M PAMAM dendrimer G4.064 hydroxyl terminal groups) solution in the presence of MW radi-tion (200 W, 338 K, continuous stirring). The reaction was carriedut at different times, 5, 15, 30, 45, 60 and 90 min, to find the besteaction time for the preparation of DENs-Pt. The resulting compos-tes were purified by dialysis for 72 h using a cellulose membrane

ith a molecular mass cut-off of 12,000 (Sigma Diagnostics, Inc.),o obtain the DENs-Pt solution free of impurities and metallic ionsFig. 1).

Characterization of the DENs-Pt species was carried out as pre-iously reported [1] using UV–vis (Agilent Model 8453 UV–vispectrophotometer with a diode-array detector, 2 nm resolution)pectra recorded at 298 K. Less than 0.03% stray radiation was stan-ard. In addition, DENs-Pt nanoparticles were characterized usingransmission electron microscopy (TEM) and selective area diffrac-ion (SAD) placing a drop with of about 10 �L of the compositeample on a carbon coated copper grid previously dried at roomemperature. TEM and SAD measurements were made on a TITAN0–300 transmission electron microscope operated at an acceler-ting voltage of 300 kV.

.2. Construction of the DENs-Pt modified glassy carbon electrode

The GC electrode was selected as support for its electrocatalyticctivity. Due to the wide variety of functional groups over the GCurface however (such as quinone, hydroquinone, phthalic anhy-ride and carboxylic acids), previous to its chemical modification,he electrode was sequentially polished with Al2O3 of different par-icle size (1, 0.3 and 0.05 �m) over a Buehler cloth, followed byinsing with Milli-Q water and sonication for 5 min to eliminatehe residual polishing material [1].

As we have previously reported, the electrochemical area ofhe electrodes used in this study was determined using the cyclicoltammetry response of a 1 mM Cl4Ru(NH3)6 solution in 0.1 M KClt different scan rates and the Randles–Sëvcik equation [29].

Once the GC was cleaned and characterized in terms of area,t was electrochemically treated in a 0.5 M H2SO4 solution apply-ng 1.6 V vs. Ag/AgCl for 1 h to promote phthalic anhydride as thereferential functional group over the GC electrode [1]. After this

Fig. 1. Schematic representation of the construction of modified GC-DENs-Pt elec-trodes using microwave (MW) synthesis.

treatment, the electrode was rinsed with Milli-Q water and modi-fied with the DENs-Pt composites as follows.

The modification protocol consisted on immersing the electrodein a mixture of 50 �L of 2 mM DENs-Pt solution in 5 mL of 0.1 M NaFas supporting electrolyte and applying 1.6 V vs. Ag/AgCl for 1 h aspreviously reported [1]. The electrochemical area of the modifiedGC electrodes was determined in the same way as that describedbefore for the naked GC.

2.3. Electrochemical detection of dopamine using the DENs-Ptmodified GC electrodes

The electrochemical determination of DA employing clean andmodified GC electrodes was carried out using cyclic voltammetry ina standard electrochemical cell containing a 1 mM buffer solutionof phosphates/methanol/acetonitrile in a 90:5:5, v/v relationshipas supporting electrolyte (pH 3.6). A BAS Epsilon potentiostat wasused for all experiments.

The detection (D.L.) and quantification (Q.L.) limits for DAwere calculated using the following equations: D.L. = 10�/m, and

Q.L. = 5�/m, where � represents the standard deviation and m isthe slope of the calibration curve, that reflects the sensitivity of theelectrochemical determination of DA.

7714 M.G. García et al. / Electrochimica Acta 56 (2011) 7712– 7717

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

11001000900800700600500400300200

λ / nm

AB

CD

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

λ / nm

Ab

sorb

ance

/ a.u

.

AB

CD

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

λ / nm

AB

CD

FDr

wse

3

3

usadttd

uiptttb

satphitasdaeat

iTer

Fig. 3. Absorbance vs. reaction time graph of the DENs-Pt composites, along with aphotograph of each synthesized solution. The absorbance values were obtained at300 nm.

ig. 2. UV–vis spectra of PAMAM dendrimer G4.0-64OH (A) with NH3Pt(Cl)4 (B) and

ENs-Pt before (C) and after (D) the dialysis treatment, using MW synthesis witheaction time of 45 min.

Additional experiments using FIA (flow injection amperometry)ere performed for dopamine determination in non-treated urine

amples, using clean and DENs-Pt modified GC electrodes. In thesexperiments, the flow rate used was 0.5 mL min−1.

. Results and discussion

.1. Spectroscopic characterization of DENs-Pt

The different synthesized solutions of DENs-Pt were analyzedsing spectrophotometry as described in Section 2. As expected, theolution that only contains metallic ions showed the characteristicqua complex signal. For the spectrum corresponding to PAMAMendrimers and metallic ions a signal at 260 nm was observed dueo the n–�* electronic transition (Fig. 2B) that is characteristic ofhe coupling of the free electrons of the tertiary amines of the den-rimers and the metallic ions [1,30].

Reduction of the PAMAM-dendrimer complex was carried outsing MW energy as described in Section 2. DENs-Pt synthesized

n this way after 45 min of MW reaction, showed a characteristiclasmon signal with a well defined peak at 250 nm (Fig. 2C) due tohe presence of reduced Pt nanoparticles. This signal corresponds tohe d–d* electronic transition between the metal complex and theertiary amines of PAMAM dendrimers and as expected, showed aetter definition after the dialysis treatment (Fig. 2D) [1].

In order to find the best synthetic conditions for DENs-Pt synthe-is using MWs, the mixture was tested at different reaction timesnd each resulting solution was analyzed using UV–vis spectropho-ometry. In this way, the spectra showed the characteristic DENs-Ptlasmon peak at 250 nm after 5 min (Fig. 3) of reaction time and theighest absorbance was observed at 15 min of synthesis maintain-

ng approximately the same absorbance value up to 30 min. Afterhis reaction time, the absorbance value decreased until its dis-ppearance after 90 min when a black precipitate was observed inolution. This precipitate can be a consequence of cluster formationue to the agglomeration of platinum particles out of dendrimers,s has been previously reported in the literature [31]. From thesexperiments, 30 min was the time selected to synthesize and char-cterize the DENs-Pt composites, which were later used to modifyhe GC electrodes to detect DA in non-treated urine samples.

In this way, the shape and dimensions of the Pt nanoparticles

n the DENs-Pt solution synthesized for 30 min was analyzed byEM (see Fig. 4A). Inspection of this figure shows that dendrimerncapsulated Pt nanoparticles are nearly spherical in shape (at thisesolution) with a high degree of monodispersity. In fact, analysis

Fig. 4. TEM (A) and SAD (B) images of DENs-Pt solutions after the dialysis treatmentusing the 30 min MW synthesis.

M.G. García et al. / Electrochimica Acta 56 (2011) 7712– 7717 7715

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 5 15 30 45 60 90

i / μ

A

Reaction Time / min

Fig. 5. Anodic current (i) of DA vs. reaction time for the GC modified electrodespbr

oandnap(i

3

wdtprDe

moftl(e

narcTcn0d

3

no

-10

-8

-6

-4

-2

0

2

4

6

8

10

0.3 0.32 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 0.5 0.52 0.54 0.56 0.58 0.6E / V vs . Ag/AgCl

j / μ

A c

m-2

C

BA

Fig. 6. Cyclic voltammograms of naked (A) and modified GC with PAMAM den-

of the naked GC electrode (Fig. 8A). In fact, experiments carried outwith FIA revealed that it is possible to detect 19 ppb and quantify9 ppb of DA using the modified GC electrode in a non-treated urinesample without noticeable matrix effects.

0

10

20

30

40

50

60

70

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5E / V vs. Ag/A gCl

j / μ

A c

m-2

B

A

repared from the different DENS-Pt composite solutions. 1 ppm of DA in 1 mMuffer solution of phosphates with pH 3.6/MeOH/MeCN (90:5:5, v/v), with a scanate of 100 mV s−1 and 297 K.

f nearly 100 randomly selected particles indicates that their aver-ge diameter is 1.53 ± 0.17 nm. The formation of monodispersedanoparticles on the other hand, suggest the effectiveness of theendrimer molecules as both; nanoreactors for the preparation ofanoparticles and nanoporous stabilizers for the prevention of theggregation of the metallic units. The presence of Pt in the com-osite solution was also evidenced from selective area diffractionSAD) experiments (see Fig. 4B) that revealed a single crystal planen (1 1 1) orientation.

.2. Construction of DENs-Pt modified glassy carbon electrode

The different solutions of DENs-Pt synthesized at different timesere used to modify a series of pre-functionalized GC electrodes asescribed in Section 2. Once each electrode was modified, its elec-rochemical response was obtained using cyclic voltammetry in theresence of 1 ppm of DA in a phosphate buffer solution (i = 0.1). Theelevant signals reveal that there is a larger current density for theENs-Pt modified GC electrodes (Fig. 5) probably reflecting largerlectroactive areas, metallic electroactivity and kinetic limitations.

This last assumption is consistent with the observation that allodified electrodes showed a non diffusional controlled response

f current vs. the square root of the scan rate (a slope differentrom 1). The larger electroactive area of DENs-Pt modified elec-rodes on the other hand, is supported from the observation of aarger anodic current of DA on the DENs-Pt modified electrodesFig. 6C), when compared to that observed for the PAMAM modifiedlectrode (Fig. 6B) and clean GC substrate (Fig. 6A).

For the different MW reaction time experiments, a pattern couldot be observed but the modified GC electrode synthesized using

reaction time of 30 min showed the largest current, probablyeflecting the largest electroactive area and/or the best electro-atalytic activity associated to the presence of Pt nanoparticles.o quantify this observation, the current obtained for the electro-hemical detection of 1 ppm of DA with the three electrodes wasormalized to the electroactive area to obtain current densities of.55, 1.66 and 8.52 �A cm−2 for clean and modified GC with PAMAMendrimers and DENs-Pt composites, respectively (Fig. 6).

.3. Determination of the hydrodynamic diagram

Before constructing the calibration curves for DA using theaked and the DENs-Pt modified GC electrodes, it was necessary tobtain a hydrodynamic diagram (Fig. 7) to verify the applied poten-

drimers (B) and DENs-Pt (C) synthesized for 30 min at 297 K. These voltammogramswere obtained in the presence of 1 ppm of DA in 1 mM buffer solution of phosphateswith pH 3.6/MeOH/MeCN (90:5:5, v/v), with a scan rate of 100 mV s−1 and 297 K.

tial to be used for the amperometric determination of a 17 ppmsolution of DA using FIA. These experiments were carried out usinga microcell with laminar flow.

In this test, both, the naked and DENs-Pt modified electrode syn-thesized with a 30 min reaction time, showed the starting growthof the electron transfer reaction of DA at 300 mV vs. Ag/AgCl.This behavior is consistent with the cyclic voltammetry responseobserved before. In the curves of Fig. 7, the mass transfer limitingcurrent reaction should reflect the dopamine oxidation reaction,DA − 2e− → DOQ + 2H+, where DOQ is the o-quinone form of DA [1].

3.4. Calibration curves for dopamine

A flow injection cell (using a peristaltic pump to maintain a con-stant laminar flow) was employed to construct calibration curvesof DA in urine without previous sample treatment using the cleanand the DENs-Pt modified GC electrode synthesized using 30 minof reaction time.

For these experiments, the concentrations of DA ranged from 1to 11 ppm, and as the data in Table 1 shows, it is clear that in thisconcentration window the DENs-Pt modified GC electrode shows afar larger sensitivity (see the slope in Fig. 8B) when compared to that

Fig. 7. Hydrodynamic diagram for the electro-oxidation of 17 ppm DA in 1 mMbuffer solution of phosphates with pH 3.6/MeOH/MeCN (90:5:5, v/v), and flow rateof 0.5 mL s−1 using naked (A) and modified (B) GC with DENs-Pt synthesized for30 min at 297 K.

7716 M.G. García et al. / Electrochimica Acta 56 (2011) 7712– 7717

Table 1Analytic parameters for the electro-oxidation of DA in urine samples with a stationary system using cyclic voltammetry (scan rate of 50 mV s−1) and a flow injection systemusing an applied potential of 900 mV vs. Ag/AgCl and a flow rate of 0.5 mL min−1 in the presence of naked and DENs-Pt modified GC electrodes.

Time of synthesis/min Equation m R2 D.L./ppb Q.L./ppb

GC i = 1 × 10−7 [DA] + 1 × 10−6 1 × 10−7 0.9990 845.69 422.85GC-DENs-Pt i = 3 × 10−6 [DA] + 6 × 10−6 3 × 10−6 0.9860 18.19 9.09

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8 9 10 11

j / μμ

A c

m-2

[Dop amine] / pp m

B

A

Fig. 8. Calibration curves for DA detection with naked (A) and DENs-Pt modified GCevw

3e

etot(n

moal

Fwmwv

lectrode (B), using amperometric measurements at an applied potential of 900 mVs. Ag/AgCl and a flow rate of 0.5 mL min−1 in 1 mM buffer solution of phosphatesith pH 3.2/MeOH/MeCN (90:5:5, v/v) at 298 K.

.5. Reproducibility and stability of the DENs-Pt modifiedlectrode for the detection of DA

To verify the reproducibility and stability of the modified GClectrode for the determination of DA in the flow cell with a non-reated urine sample (1:3, v/v, urine: water) successive injectionsf a 10 ppm standard DA solution were carried out (Fig. 9A). Inspec-ion of Fig. 9 shows that when the concentration of DA is doubled20 ppb) the electrochemical response is also doubled withoutoticeable matrix interferences of the real sample (Fig. 9B).

In this way, the stability of the modified electrode for the deter-

ination of DA constitutes a good alternative for the determination

f dopamine in biological samples with complex matrix effects suchs urine samples. In addition, this determination eliminates prob-ems in the pre-treatment of sample, as the use and cleaning of a

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8

j / μμ

A c

m-2

Number of I njection

B

A

ig. 9. Amperometric response of 10 ppb (A) and 20 ppb (B) of DA diluted in urineith MilliQ water without pre-treatment. The electrochemical measurements wereade with DENs-Pt modified GC electrode in 1 mM buffer solution of phosphatesith pH 3.2/MeOH/MeCN (90:5:5, v/v) using a constant applied potential of 900 mV

s. Ag/AgCl, and a flow rate of 0.5 mL min−1 at 298 K.

[[

[[

[[[[

[[[[[[

chromatographic column, the use of a high-pressure pump, as wellas the excessive use of solvents and time required for analysis.

4. Conclusions

GC electrodes modified with composites of DENs-Pt synthe-sized with microwaves for 30 min showed good performance indetecting DA in both synthetic and urine samples. Compared withthe stationary system using cyclic voltammetry analysis, the FIAmethod offers detection capabilities that appear to be more sen-sible, selective, stable and durable without the requirement forsample pre-treatment.

The DENs-Pt synthesized using a microwave reactor offers agood alternative for chemical synthesis and subsequent electrodemodification as it has been reported in the literature. In addition,this modified electrode can be incorporated as a FIA detector forthe electro-oxidation of DA in a urine sample without matrix andpassivation effects, showing good stability after several injectionsof the same concentration of DA in the FIA system. Also the D.L. andQ.L. are smaller than those showed by the naked GC electrode (19and 9 ppb, respectively).

Acknowledgments

The authors would like to thank Ciencia Básica 2007 – ConsejoNacional de Ciencia y Tecnología (CONACyT), No. 84955 and FondoMixto (FOMIX) – Veracruz – CONACyT, No. 9631 for their financialsupport of this research. The authors also want to thank to RoyRajan, Peace Corps volunteer at CIDETEQ for his English revisionof this manuscript. M.G. García is also grateful to CONACyT andUniversidad de Guanajuato for her internship in CIDETEQ to carryout this research.

References

[1] E. Bustos, M.G. García, B.R. Díaz-Sánchez, E. Juaristi, Th. Chapman, L.A. Godínez,Talanta 72 (2007) 1586.

[2] A.J. Haes, R.P. Van Duyne, J. Am. Chem. Soc. 124 (2002) 10596.[3] L. Wang, E.K. Wang, Electrochem. Commun. 6 (2004) 225.[4] S. Xu, X. Han, Biosens. Bioelectron. 19 (2004) 1117.[5] J. Liu, Y. Lu, J. Am. Chem. Soc. 125 (2003) 6642.[6] M. Huang, Y. Shao, X. Sun, H. Chen, B. Liu, S. Dong, Langmuir 21 (2005) 323.[7] K.R. Gopidas, J.K. Whitesecl, M.A. Fox, Nano Lett. 3 (2003) 1757.[8] A. Gole, C. Dash, V. Ramakrishnan, S.R. Sainkar, A.B. Mandale, M. Rao, M. Sastry,

Langmuir 17 (2001) 1674.[9] Y. Xiao, H.X. Ju, H.Y. Chen, Anal. Chim. Acta 391 (1999) 73.10] J. Jia, B. Wang, A. Wu, G. Cheng, Z. Li, S. Dong, Anal. Chem. 74 (2002) 2217.11] Y. Zhou, R. Yuan, Y. Chai, D. Tang, Y. Zhang, N. Wang, X. Li, Q. Zhu, Electrochem.

Commun. 7 (2005) 355.12] K. Di Gleria, H.A.O. Hill, C.J. McNeil, M.J. Green, Anal. Chem. 58 (1986) 1203.13] T. Nakamura, Y. Tsukahara, T. Yamauchi, T. Sakata, H. Mori, Y. Wada, Chem. Lett.

36 (2007) 154.14] M. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877.15] L. Balogh, D.A. Tomalia, J. Am. Chem. Soc. 120 (1998) 7355.16] M.E. Garcia, L.A. Baker, R.M. Crooks, Anal. Chem. 71 (1999) 256.17] K. Esumi, A. Kameo, A. Suzuki, K. Torigoe, Colloids Surf. A: Physicochem. Eng.

Asp. 189 (2001) 155.18] M. Zhao, R.M. Crooks, Angew. Chem., Int. Ed. 38 (1999) 364.

19] B.I. Lemon, R.M. Crooks, J. Am. Chem. Soc. 112 (2000) 12886.20] X. Sun, X. Jiang, S. Dong, E. Wang, Macromol. Rapid Commun. 24 (2003) 1024.21] X. Sun, S. Dong, E. Wang, Macromolecules 37 (2004) 7105.22] K. Esumi, A. Suzuki, N. Aihara, K. Usui, K. Torigoe, Langmuir 14 (1998) 157.23] M. Mandal, S.K. Ghosh, S. Kundu, K. Esumi, T. Pal, Langmuir 18 (2002) 7792.

imica

[[[

[[[

[tivity. A Tour Through the Periodic Table of the Elements, John Wiley & Sons,West Sussex, England, 1997.

M.G. García et al. / Electroch

24] K. Hayakawa, T. Yoshimura, K. Esumi, Langmuir 19 (2003) 5517.25] M.F. Ottaviani, R. Valluzzi, L. Balogh, Macromolecules 35 (2002) 5105.26] R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, Tetra-

hedron Lett. 27 (1986) 279.27] X. Xu, W. Yang, J. Liu, L. Lin, Adv. Mater. 12 (2000) 195.28] S. Xuping, L. Yonglan, Mater. Lett. 59 (2005) 4048.29] A.J. Bard, L.R. Fulker, Electrochemical Methods: Fundamentals and Applications,

John Willey & Sons, New York, USA, 2001.

[

Acta 56 (2011) 7712– 7717 7717

30] D.T. Richens, The Chemistry for Aqueous Ions: Synthesis, Structure and Reac-

31] R.M. Crooks, B.I. Lemon III, L. Sun, L.K. Yeung, M. Zhao (Eds.), Dendrimer-Encapsulated Metals and Semiconductors: Synthesis, Characterization andApplications, Springer, New York, USA, 2001.