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Electrochemistry Communications 8 (2006) 1035–1040

Self-assembly of gold nanoparticles forthe voltammetric sensing of epinephrine

Liang Wang a,*, Junyue Bai a,b, Pengfei Huang a,b, Hongjing Wang a,Liying Zhang a, Yuqing Zhao a

a College of Biological Science, Dalian Nationalities University, Dalian, Liaoning 116600, PR Chinab School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian, 116024, PR China

Received 19 March 2006; received in revised form 7 April 2006; accepted 18 April 2006Available online 23 May 2006

Abstract

A novel method for fabricating gold nanoparticles combining the dithiothreitol (DTT) and dodecanethiol (DDT) mixed self-assem-bled approach is proposed. The mixed self-assembled monolayers were first formed by the assembly of DTT and DDT from solutiononto gold electrode. When these thiol-rich surfaces are exposed to Au colloid, the sulfurs form strong bonds to gold nanoparticles,anchoring the clusters to the electrode surface and a new nano-Au surface was obtained. In order to evaluate the electrochemical char-acteristics of nano-Au electrode, FeðCNÞ3�6 was chosen as molecule probe and cyclic voltammetry (CV) was used. In addition, the func-tional nano-Au electrode was applied to the voltammetric sensing of epinephrine (EP). The nano-Au electrode is demonstrated topromote the electrochemical response of EP by cyclic voltammetry. The electrode reaction of EP is significantly improved at thenano-Au electrode which results in a large increase in the voltammetric peak current with a detection limit of 6.0 · 10�8 M and the dif-fusion coefficient D is as 7.40 · 10�5 cm2 s�1. The nano-Au electrode shows high electrocatalytic activity and excellent sensitivityproperty.� 2006 Elsevier B.V. All rights reserved.

Keywords: Gold nanoparticle; Dithiothreitol; Dodecanethiol; Epinephrine; Voltammetric sensing

1. Introduction

The application of nanotechnology in electrochemistryhas generated a frenzy of excitement [1–3]. One of the rea-sons for the considerable current interest in nanoparticlesis because such materials frequently display unusualphysical (structural, electronic, magnetic and optical) andchemical (catalytic) properties. In recent years, gold nano-particles (GNPs) have been extensively studied in electro-chemistry for its special physico-chemical characteristics[4–10]. The use of GNPs superstructures for the creationof electrochemical devices is an extremely promising pros-pect. The superstructures give rise to porous, high-surface-

1388-2481/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2006.04.012

* Corresponding author. Tel.: +86 411 87615619; fax: +86 41187656278.

E-mail address: wangliang101@hotmail.com (L. Wang).

area electrodes, where the local microenvironment can becontrolled by the crosslinking elements and may lead to spe-cific and selective interactions with substrates [11]. Variousmethodologies have been used for the tailoring of GNPson electrode surfaces, which include the anchoring by elec-trostatic interaction, covalent linkage, electrochemicaldeposition, etc., [12–15]. Electrostatic assembly utilizes thenegatively charged citrate surface of the particles, whilecovalent attachment relies on the reaction of the Au surfacewith thiols or disulfides to form Au0AS bonds [11]. TheGNPs can be immobilized as an organized mono- ormultilayer on solid support with the help of short/long chainmolecules having suitable functional groups like ANH2,ASH at both ends [16]. The fabrication of sensors basedon self-assembly GNPs nanostructure is of recent techno-logical interest [17–20]. Arrays of GNPs have been utilizedfor electrochemical sensors as they exhibit excellent catalytic

Au

Au

S

OH

SH

S

HS

HO

OH HO

S

HS

HO

Au

HO

S

OH

SH

Au

OH

S

DTT

DDT

S S

Au

S

OH

S

S

S

HO

Au

OH HO

S

S

HO

HO

S

OH

S

Au

OH

S

AuAuAuAu

S S

S

OH

SH

S

OH

S

HS

HO

HO

S

OH

SH

OH

S

OHHO

Au

SH

Scheme 1. Schematic representation of the fabrication of the nano-Auself-assembly.

1036 L. Wang et al. / Electrochemistry Communications 8 (2006) 1035–1040

activity towards various reactions [21]. In these, the GNPsfunction as an ‘‘electron antennae’’, efficiently funnelingelectrons between the electrode and electrolyte [12,22].

Herein a novel fabrication procedure for self-assemblyGNPs nanostructure based on mixed self-assembled mono-layers (SAMs) was investigated. Mixed self-assembledmonolayers composed of alkanethiols of different chainlengths offer a controllable route for constructing moietiescontaining different chemical functionalities [23,24]. MixedSAMs are model systems for studies of adsorption, wet-ting, and heterogeneous electron transfer and have led toimportant applications in the development of biosensors[25] such as heterogeneous DNA/RNA sensors [26] andpreparation of biologically important surfaces [27]. Theshorter bisthiol were preferred in order to increase the rateof electron transfer and can provide superior conductiveproperties when used in electroanalysis. Working on theprinciple, dithiothreitol (DTT) was used as the crosslinkingagent to fabricate GNPs on the surface of the electrode.STM measurements indicate that the structure of DTTmonolayers on Au(1 11) is disordered and the majority ofDTT molecules bind to the gold surface via two AuASbonds [28]. As DTT bears two ASH groups, there mustbe some free ASH groups left among the DTT moleculeschemically adsorbed at gold [29]. In the presence ofdodecanethiol (DDT), some AuARASAAu of the DTTlayer would turn into HSARASAAu and the mixed self-assembled monolayers can provide a more compact andorder platform to fabricate GNPs on the electrode surface.The film formed by this technique has the advantages ofhigh organization and uniformity, which could provide adesirable microenvironment to assemble GNPs and facili-tate the concentration of the analyte from the bulk solutionto the electrode surface. All of these features of the GNPssuperstructures together with their porosity suggest thatGNPs-functionalized gold electrode could acts as sensitiveand rapid sensing devices. The proposed GNPs modifiedelectrode has been applied to investigate the electrochemi-cal behavior of epinephrine (EP) and the electrode is dem-onstrated to promote the electrochemical response of EPby cyclic voltammetry (CV).

2. Experimental

2.1. Chemicals and instrumentation

DL-Dithiothreitol (DTT), dodecanethiol (DDT) and epi-nephrine were purchased from Sigma and they were used asreceived. All other chemicals were of analytical grade andwere used without further purification. A 0.1 M phosphatebuffer solution was used to control the pH. All solutionswere prepared with deionized water treated in a Milliporewater purification system (Millipore Corp.). All experi-ments were carried out at room temperature (approx.20 �C).

Voltammetric measurements were performed with aCHI 440 electrochemical analyzer (CH Instruments, Chen-

hua Co. Shanghai, China). A conventional three-electrodecell was used, including a saturated calomel electrode(SCE) as reference electrode, a platinum wire counter elec-trode and a bare or modified gold working electrode. ThepH values were measured with a PB-10 pH meter (Sato-rius). Unless otherwise stated, the electrolyte solutions werethoroughly degassed with N2 and kept under a N2 blanket.

2.2. Preparation of Au colloids

The 2.6 nm diameter colloidal Au was prepared accord-ing to the literature [30]. Typically, One milliliter of 1%

400 600 8000.0

0.4

0.8

Ab

sorb

ance

Wavelength / nm

Fig. 1. Optical spectra of citrate stabilized colloidal Au.

0.8 0.4 0.0 -0.4-8

0

8

c

b

a

i p /

μ A

E/ V vs. SCE

Fig. 2. Cyclic voltammograms of 1.0 mM FeðCNÞ3�=4�6 at a bare gold

electrode (a), DTT + DDT/Au (c) and nano-Au electrode (b) in 0.1 MKCl, respectively. Scan rate, 100 mV s�1.

L. Wang et al. / Electrochemistry Communications 8 (2006) 1035–1040 1037

HAuCl4 was added to 90 ml of water at room temperature(20–23 �C). After 1 min of stirring, 2 ml of 38.8 mMsodium citrate was added. One minute later, 1 ml of freshlyprepared 0.075% NaHB4 in 38.8 mM sodium citrate wasadded and the colloidal solution was stirred for another5 min and stored in a dark bottle at 4 �C.

2.3. Immobilization of Au colloidal particles by self-

assembled technology

The working electrode was a Au disk electrode with adiameter of 2 mm. Prior to each measurement, the elec-trode was polished with diamond pastes and an aluminaslurry down to 0.05 lm on a polishing cloth (Buehler,Lake Bluff, IL), followed by sonicating in water and eth-anol. Then, the Au electrode was electrochemicallycleaned by cycling the electrode potential between 1.6and �0.4 V (vs. SCE) in 0.5 M H2SO4 until a stable vol-tammogram was obtained. After it was washed with son-ication and dried with nitrogen gas, the electrode wasimmersed in a aqueous solution of 0.05 M DTT for 5 h.Upon removal from the deposition solution, the substratewas thoroughly rinsed with ethanol and water to removethe physically adsorbed species. The resulting modifiedelectrode was dried in air and is defined as DTT/Au.The mixed SAMs modified gold electrode were preparedthrough immersing the DTT/Au electrode in an ethanolsolution containing 0.05 M DDT for 5 min and is definedas DTT + DDT/Au. The DTT + DDT/Au electrode wassubsequently soaked in the Au colloidal solution for12 h at 4 �C. The resulting electrode was washed withcopious amount of water and subjected to electrochemicalexperiments. Hereafter the Au nanoparticle-immobilizedelectrode (GNPs/DTT + DDT/Au) will be referred asthe nano-Au electrode. The whole process is shown inScheme 1).

3. Results and discussion

3.1. The absorption spectrum of colloidal Au

Fig. 1 depicts the absorption spectrum of colloidal Au.It has been shown that Au nanoparticles of diameter2.6 nm would give an absorption maximum at around514 nm whereas the particles of diameter 20–40 nm wouldexhibit an absorption band between 530 and 540 nm [31].The Au nanoparticles of diameter 64 nm would give anabsorption band at around 545 nm whereas the 120 nmparticles would exhibit an absorption band at 620 nm[31]. As the absorption band of our nanoparticles showsabsorption maximum at around 514 nm, it is consideredthat the nanoparticles would have a diameter around2.6 nm. The aggregated nanoparticles are expected to givebroad absorption band at around 700 nm [32]. In the pres-ent investigation, because the optical spectra obtained forthe nanoparticles shows only one absorption band it is con-sidered that the nanoparticles are not aggregated.

3.2. Characterization of nano-Au electrode with cyclic

voltammetry

Cyclic voltammetry of electroactive species FeðCNÞ3�=4�6

is a valuable tool for testing the kinetic of the barrier of theinterface. The extent of kinetic hindrance to the electron-transfer process increases with the increasing thicknessand the decreasing defect density of the barrier. Fig. 2shows the CV responses of 1.0 mM FeðCNÞ3�=4�

6 at bareAu, DTT + DDT/Au, and nano-Au electrode, respec-tively. As can be seen, FeðCNÞ3�=4�

6 shows a couple ofwell-defined redox waves at bare Au (Fig. 2, curve a) witha peak-to-peak separation (DEp) of 60 mV at 100 mV s�1.After modifying the electrode with DTT + DDT SAMs,a obvious decrease in redox peak current and increase inpeak-to-peak separation are observed (Fig. 2. curve c),indicating that the DTT + DDT SAMs act as the inertelectron and mass transfer blocking layer and thus hindersthe diffusion of ferricyanide toward the electrode surface.On the contrary, when GNPs was attached to the electrodesurface, the voltammetric response of ferricyanide isrestored close to that obtained at the bare Au (Fig. 2, curveb). This demonstrated that GNPs has been successfullyassembled on Au surface and provide the necessary con-duction pathways, besides acting like nanoscale electrodesin promoting the electron transfer between the analyteand the electrode surface.

1038 L. Wang et al. / Electrochemistry Communications 8 (2006) 1035–1040

3.3. The electrochemical response of EP at the nano-Au

electrode

Fig. 3 shows the cyclic voltammograms at bare Au,DTT + DDT/Au, and nano-Au electrode in presence of0.1 mM EP in phosphate buffer of pH 7.0, respectively.As can be seen, at the bare gold electrode, the cyclic vol-tammogram of EP demonstrated very poor and irreversiblewaves with the anodic peak potential (Epa) at 0.341 V andthe cathodic peak (Epc) at �0.238 V, showing that the elec-tron transfer for the oxidation process of EP is sluggish(Fig. 3, curve a). On the other hand, the DTT + DDT/Au electrode did not show any oxidation peak, just a littlereduction peak was observed at �0.326 V in the potentialwindow used in this investigation (Fig. 3, curve b). It is wellunderstood that the SAMs on the electrode surface havedefinite kinetic barrier for the electron transfer betweenthe electrode surface and electroactive species in solution[16]. The extent of this barrier depends on the compactness,thickness and nature of the monolayer; for a fixed thick-ness, the extent of barrier decreases with increasing defectsites of the self-assembly. In the present case, the self-assembly of DTT + DDT SAMs on the electrode surfaceexerts some barrier for the electron transfer process. TheDTT + DDT SAMs could not completely block the accessof EP to the electrode surface, as it has a less compactstructure with free sites. Consequently, the oxidation peakof EP disappeared and the reduction peak of EP shifts tomore negative potential.

Fig. 3 curve c illustrates the CV obtained for the oxida-tion of EP at the nano-Au electrode. Interestingly, a well-defined redox wave of EP was observed at the nano-Auelectrode. For instance, the oxidation peak appeared at0.187 V at the nano-Au electrode, which is 154 mV lessnegative than that at the bare Au electrode; the reductionpeak, appeared at �0.223 V at the electrode, is 15 mV lesspositive than that at the bare Au electrode and a newreduction peak appeared at 105 V. The fact implied thatthe immobilized Au nanoparticles efficiently catalyze theoxidation of EP. The sharp oxidation peak and reductionpeak associated with an enhancement in the peak current

0.4 0.0 -0.4

-4

-2

0

2

c

bai p

/ μ

A

E/ V vs. SCE

Fig. 3. Cyclic voltammograms of 0.1 mM EP at a bare gold electrode (a),DTT + DDT/Au (b) and nano-Au electrode (c) in phosphate buffer (pH7.0), respectively. Scan rate, 100 mV s�1.

reflects a fast electron transfer at the nano-Au electrode.It has been reported that nanometer-sized Au particlesexhibit excellent electrocatalytic activity [33,34]. Theimportant attribute of the nano-sized catalysts is the highsurface area and interface-dominated properties that differsfrom the atomic, molecular and bulk counterparts [35]. Inthe present investigation the facilitated oxidation of EP atthe nano-Au electrode is believed to be due to the excellentcatalytic activity of nano-sized Au particles. Control exper-iment has also been carried out with the electrodes of DTT/Au, DDT/Au, GNPs/DTT/Au and GNPs/DDT/Au,respectively. The results more confirmed the nano-sizedAu showed an excellent electrocatalytical effect on theoxidation of EP.

According to Ref. [36], the peak A, appeared at 187 mV,can be interpreted as the oxidation of epinephrine to theopen-chain quinone. Quinone, the first product in the elec-trochemical oxidation, is known to be proceeded deproto-nation in the solution of pH > 3. Peak B, appeared at105 mV, is the rereduction of this quinone, while peak C,appeared at �223 mV, is the reduction of the cyclizedproduct adrenochrome to leucoadrenochrome. Two con-tinuous cyclic voltammograms were shown in Fig. 4. PeakD, appeared at �192 mV, is corresponded to the reoxida-tion of leucoadrenochrome to adrenochrome. The proba-ble schemes of the reactions may be expressed as below.

The oxidative peak current of EP (appeared at 187 mV)has obviously changed between the first scan and the sec-ond scan (Fig. 4). The reduction of oxidative peak currentof the second scan was caused by the new oxidative reac-tion of the reoxidation of leucoadrenochrome to adreno-chrome. From second scan, the peaks shape andmagnitude did not change in subsequent scans. But during

0.4 0.0 -0.4

-4

-2

0

2

i p /

μ A

D

CB

A

E / V vs. SCE

Fig. 4. The two cycles of continuous cyclic voltammograms of 0.1 mM EPat nano-Au electrode in phosphate buffer (pH 7.0). Scan rate, 100 mV s�1.

0.3 0.4 0.5

-2.0

-1.6

-1.2

Q /

μ C

t 1/2 / s1/2

Fig. 7. Chronocoulometric dependence of charge on the square root oftime for 0.1 mM EP. Potential step: 0.0–0.5 V.

L. Wang et al. / Electrochemistry Communications 8 (2006) 1035–1040 1039

the experiment, the shape and magnitude of the peakswhich were observed in the every first scan have a verygood stability and reproducibility. Due to the higher cur-rent and very good stability of the first scan, the anodicpeak current of the first scan of EP was chosen for the elec-trochemical detection of EP.

3.4. Effects of pH on the peak current and peak potential of

EP

The effects of the pH value of phosphate buffer on theanodic peak potential and peak current were investigated,and the results are shown in Fig. 5. It can be seen that asthe solution pH increase, the anodic peak potential shiftsto the negative and the potential of Epa vs. pH in phosphatebuffer has a good linear relation in the range of pH 5.16–9.03. The linear regression equation Epa/V = 0.5884 �0.0572 pH (correlation coefficient r = 0.9958) wasobtained, which showed that the uptake of electrons isaccompanied by an equal number of protons. Fig. 5 alsoshows that the anodic peak current increased with increas-ing the pH up to 7.0, after that the peak current decreased.As well known, pH 7.0 is close to the physiological pH

0.4 0.0 -0.4

-4

-2

0

2

i p /

μ A

ea

E / V vs. SCE

4 8

2.4

3.2

p H

i pa /

μ A

0.1

0.2

0.3

Ep

a / V

Fig. 5. Cyclic voltammograms at nano-Au electrode for 0.1 mM EP in0.1 M phosphate buffer solutions at pH values of (a) 5.16, (b) 6.01, (c)7.00, (d) 8.06 and (e) 9.03. The inset shows effects of pH on the anodicpeak potential and anodic peak current in phosphate buffer solution. Scanrate 100 mV s�1.

0.4 0.0 -0.4

-8

-4

0

4

j

ai p / μ

A

E / V vs. SCE

Fig. 6. Cyclic voltammograms of 0.1 mM EP at nano-Au electrode inphosphate buffer (pH 7.0) at different scan rates (a) 20 mV s�1; (b)50 mV s�1; (c) 80 mV s�1; (d) 100 mV s�1; (e) 120 mV s�1; (f) 150 mV s�1;(g) 180 mV s�1; (h) 200 mV s�1; (i) 250 mV s�1; (j) 300 mV s�1.

value, thus, it was chosen for the electrochemical detectionof EP.

3.5. Effect of scan rate on the peak current and determination

of the diffusion coefficient (D)

As shown in Fig. 6, the anodic peak current (ipa) of EPincreased with increasing scan rate on nano-Au electrode inpH 7.0 phosphate buffer and exhibited a linear relation tothe square root of the scan rate, m1/2, with the linear regres-sion equation ipa/lA = 0.9971 + 0.2038 m1/2/(mV s�1)1/2

(correlation coefficient, r = 0.9951). The result indicatesthat the electron transfer reaction is controlled by thediffusion of EP.

The diffusion coefficient D of EP was determined bychronocoulometric method. In order to obtain the effectivesurface area of nano-Au electrode, K3Fe(CN)6 was used asa probe. CV experiments of 1 mM K3Fe(CN)6 at a nano-Au electrode were performed. For a reversible process,

ip ¼ ð2:69� 105Þn3=2AD1=2m1=2C0

for K3Fe(CN)6, n = 1, D = 7.6 · 10�6 cm2 s�1, then theeffective surface area of the nano-Au electrode could becalculated and 0.031 cm2 was obtained.

According to the formula given by Anson [37]:

Q ¼ 2nFACðDtÞ1=2

p1=2þ Qdl

where Qdl is double-layer charge (integration of chargingcurrent). From the slope of the linear relationship betweenQ and t1/2, D can be determined if C (concentration), A

(surface area of the electrode), and n (electron transfernumber) are known. Here n = 2. From the slope of the plotbetween Q and t1/2 (Fig. 7), D was calculated as7.40 · 10�5 cm2 s�1, which is higher than that of SAMsmodified gold electrode [36,38,39].

3.6. Calibration plot

Under the optimum analytical conditions, the linearrange of EP at the nano-Au electrode was studied usingcyclic voltammetry. The anodic peak current was linear

1040 L. Wang et al. / Electrochemistry Communications 8 (2006) 1035–1040

to EP concentration in the range of 1.0 · 10�7–1.0 · 10�6 M and 1.0 · 10�5–2.0 · 10�4 M. The linearregression equation was ipa/lA = 0.1618 + 0.3261 C/lMand ipa/lA = 0.5660 + 0.0233 C/lM, with correlation coef-ficients of 0.9963 and 0.9969, respectively. The detectionlimit was 6.0 · 10�8 M. The nano-Au electrode can be usedfor the determination of EP in practical injection.

4. Conclusion

A novel mixed self-assembled DTT-DDT-Au colloidmodified gold electrode was fabricated successfully. Thenano-Au electrode was used for the voltammetric sensingof epinephrine. The present investigation shows a surpris-ingly high electrocatalytic activity of nanosized Au parti-cles immobilized on the mixed self-assembled monolayers.The diffusion coefficient D of EP at the nano-Au electrodewas determined by chronocoulometric method. From theanalysis of currents and concentrations, the good linearrelationship between them can be used for the sensitivedetermination of EP. The potential application of themixed self-assembled nano-sized Au for the fabrication avoltammetric EP sensor is demonstrated for the first time.

Acknowledgement

This project was supported by the Doctor Foundationof Dalian nationalities University (20056101).

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