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Recent Advances in Metal Nanoparticle - Attached Electrodes

Munetaka Oyama , Akrajas Ali Umar , and Jingdong Zhang

297

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

8

8.1

Introduction

Metal nanoparticle s ( NP s) have attracted vast attention during the past decade

on the basis of their unique optical, electronic, magnetic, and catalytic properties.

In the case of gold NPs (AuNPs), their assembly, supermolecular chemistry,

quantum - size related properties, and applications to biology, catalysis, and nano-

technology have been summarized by Daniel and Astruc [1] .

Within the ﬁ eld of electrochemistry, and in particular of electroanalysis, metal

NPs have been actively utilized in the modiﬁ cation of electrodes as functional

nanomaterials, and this subject has been extensively reviewed [2 – 4] .

In order to utilize metal NPs as unique devices in functional electrodes,

they must normally be attached onto the electrode surfaces. Consequently, a

wide variety of methods is available for preparing NP - attached functional elec-

trodes with respect to the metal NPs used, the nature of the substrate electrode

materials, and the method of attachment. One well - known method of modiﬁ cation

is that of electrochemical deposition , which involves the electrochemical reduction

of metal ions onto the surfaces of a conducting material, such that the nanoparti-

cles become attached to the surfaces. This procedure is used in the case of

AuNPs [5, 6] .

A second, “ chemical, ” method is also available for preparing metal NPs. For

this, bottom - up - type preparation strategies for metal NPs in solution are well

known and well established, again as reported for AuNPs [7 – 9] . In terms of regu-

larity and uniformity of NP size, the chemical approach is generally superior to

electrochemical deposition, although certain difﬁ culties may be encountered when

attaching metal NPs which have been formed in solution onto the electrode sur-

faces. It is possible that the more interesting characteristics of metal NPs might

be diminished when they aggregate to form larger clusters, and consequently it is

necessary to either ﬁ x or attach the NPs while retaining a moderate degree of NP

dispersion. The development of such attachment methods with appropriate

298 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

degrees of dispersion are, therefore, very important if the best use is to be made

of the characteristics of metal NPs for electrode modiﬁ cation. But this situation is

also be true for the preparation of nanocomposites, as are used in electronic,

sensing, and optical devices.

When attaching AuNPs to the electrode surfaces, a number of peculiar binding

molecules that are suitable for connecting AuNPs with substrates have been

used. For example, when connecting AuNPs with a glass surface, both 3 -

mercaptopropyl - trimethoxysilane ( MPTMS ) and 3 - aminopropyl - trimethoxysilane

( APTMS ) have been adopted; this process utilizes not only the bonding ability of

the silanol group towards the glass surfaces but also the afﬁ nity of the – SH or

– NH

2

group for AuNPs [10 – 12] . The AuNP - attached surfaces thus formed were

successfully applied to surface - enhanced Raman spectroscopic measurements.

Although today, the use of these peculiar binder molecules has become standard

strategy when attaching AuNPs to electrode surfaces [13, 14] , it is to be expected

that the characteristics of metal NPs – notably their conductivity and catalytic

ability – might be greatly affected by the chemical reagents that surround or bind

them. Hence, although the use of these bridging reagents is clearly effective in

some respects (e.g., size - uniformity and rigid - ﬁ xation of metal NPs), they may

interfere in the process of utilizing the characteristics of the metal NPs.

If it were possible to attach metal NPs to a conducting substrate without the use

of a binder molecule, it might be expected that novel conducting nanodispersed

materials with unique electrochemical properties involving both the metal NPs

and the conducting substrates, could be fabricated. Consequently, our group has

recently developed a seed - mediated growth method for surface modiﬁ cation with

metal NPs, notably on the surfaces of indium tin oxide ( ITO ).

In this chapter, we summarize the development and applications of seed -

mediated growth methods for the modiﬁ cation of electrode surfaces. Examples

include the wet chemical preparation of advanced functional nanomaterials

composed of metal NPs and a conducting substrate.

8.2

Seed - Mediated Growth Method for the Attachment and Growth of AuNPs on ITO

During the early stages of the studies, a seed - mediated growth method was used

which originally had been developed by Murphy and coworkers to synthesize Au

nanorods in aqueous solution, by the chemical reduction of HAuCl

4

[15, 16] . This

approach was applied to the formation of AuNPs on the ITO surfaces [17] .

In the actual procedure, a piece of ITO - coated glass was washed ﬁ rst by sonica-

tion in acetone, and then in pure water. The washed ITO substrate was dried in

air and prepared for immersion in a seed solution produced by adding 0.5 ml of

a cooled pure aqueous solution of 0.1 M NaBH

4

to 19.5 ml of a pure aqueous solu-

tion of 0.25 m M HAuCl

4

and 0.25 m M trisodium citrate, with stirring. When using

this procedure, it has been reported that Au nanoseed particles of 3.5 nm were

formed in the seed solution [15] .

8.2 Seed-Mediated Growth Method of AuNPs on ITO 299

The ITO substrate was then immersed in the seed solution so as to attach the

Au nanoseed particles onto the surface. Typically, immersion in the seed solution

was maintained for 2 h, and without any particular treatments. The ITO substrate

was then removed from the seed solution, and the surface washed carefully by

ﬂ ushing pure water over the surface several times. Any water retained on the

surface was removed with tissue paper, simply by touching the edges of the glass.

After blowing the surface of the ITO with N

2

gas until dry, the substrate was

immersed in the growth solution. This was prepared by adding 2.5 ml of a pure

aqueous solution of 0.01 M HAuCl

4

, 0.5 ml of a pure aqueous solution of 0.1 M

ascorbic acid, and 0.5 ml of a pure aqueous solution of 0.1 M NaOH into 90 ml of

a pure aqueous solution of 0.1 M cetyltrimethylammonium bromide ( CTAB ).

Again, the ITO substrate was retained in the growth solution typically for a period

of 24 h in order to promote the growth of AuNPs from the attached Au nanoseed

particles. Finally, the sample was washed by ﬂ ushing with pure water.

The real surface nanoscale images were recorded using ﬁ eld emission scanning

electron microscopy ( FE - SEM ). Figure 8.1 a shows the FE - SEM image observed

Figure 8.1 FE - SEM images of the ITO

surfaces. (a) After immersion of the ITO into

the seed solution for 2 h. The white circles

indicate areas of attachment of small seed

particles; (b) After subsequent immersion of

the ITO in the growth solution for 24 h; (c) A

low - magniﬁ cation image of the ITO surface of

panel (b); (d) The ITO substrate was modiﬁ ed

initially with MPTMS, after which seed -

mediated growth was performed in the same

manner as for (b). Reproduced with

Chemical Society.

300 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

after seeding, while the images in Figures 8.1 b and c were observed after the seed -

mediated growth treatment. In the later images, the AuNPs grown on ITO were

seen as white particles in the images against a background of ITO crystals. The

AuNPs clearly grew to approximately 50 – 70 nm on the ITO surface from the

Au nanoseed particles attached on ITO (Figure 8.1 a), following immersion in

the growth solution (i.e., by applying the seed - mediated growth method), while

retaining a moderate dispersion [17] .

The initial attachment of nanoseed particles would be expected to be promoted

via the physisorption of ultrasmall NPs which had been dispersed in the seed

solution by simply dipping the substrates [18] . The second process should involve

crystal growth via the chemical reduction of metal ions in the growth solution,

which is inferred to proceed gradually around the nanoseed particles on the

substrates.

As shown in Figure 8.1 d, the FE - SEM image of AuNPs on the ITO surfaces

premodiﬁ ed with MPTMS revealed signiﬁ cant differences in the morphology of

the grown AuNPs [17] . Because the crystal - like structures of AuNPs were observed

in the absence of MPTMS (Figures 8.1 b and c), the seed - mediated growth method

was proposed as a unique means of attaching Au nanocrystals onto the ITO

surfaces, without the use of binder molecules.

8.3

Electrochemical Applications of AuNP - Attached ITO

The AuNP - directly attached ITO (AuNP/ITO) electrodes were then applied to

electrochemical measurements. As inferred by the fact that as - prepared AuNPs

on the ITO surface (Figure 8.1 c) displayed a different morphology than the

MPTMS - linked AuNPs modiﬁ ed on ITO (Figure 8.1 d), the AuNP/ITO electrodes

provided attractive electrochemical and electrocatalytic properties to promote het-

erogeneous electron transfer kinetics [19, 20] . In particular, the electrochemical

impedance measurements performed to evaluate the effects of AuNPs on the

interfacial electron transfer property revealed that the presence of a MPTMS layer

caused a signiﬁ cant increase in charge transfer resistance. The relatively low

charge transfer resistances of the AuNP/ITO electrodes were clariﬁ ed from the

impedance results, with the typical charge transfer resistance of the AuNP/ITO

electrode being approximately one - third that of the AuNP/MPTMS/ITO electrode

(Figure 8.2 ) [20] .

The AuNP/ITO electrodes were used for observing the electro - oxidation of uric

acid, ascorbic acid, dopamine, norepinephrine (noradrenaline), and epinephrine

(adrenaline) [20] . Whilst the AuNP/MPTMS/ITO electrodes caused a depression

in selectivity when determining epinephrine in the presence of ascorbic acid, due

to the existence of the thiol monolayer, the AuNP/ITO electrodes were able to

improve the detection sensitivity while retaining the selectivity [20] .

The AuNP/ITO electrodes also provided a biocompatible matrix for the

immobilization of hemoglobin ( Hb ). By using electrochemical impedance

8.3 Electrochemical Applications of AuNP-Attached ITO 301

Figure 8.2 Electrochemical impedance spectra of ITO (

䊉

),

MPTMS/ITO (

䊏

), Au/MPTMS/ITO ( 䉱 ) and Au/ITO (

䉲

)

electrodes in 0.1 M phosphate - buffered saline (pH 7.4)

containing 1 m M K

3

Fe(CN)

6

and 1 m M K

4

Fe(CN)

6

. Electrode

potential = 0.22 V; Frequency range = 100 kHz to 100 mHz;

Voltage amplitude = 5 mV. Reproduced with permission from

measurements, the modiﬁ cation of AuNPs, followed by the immobilization of Hb

on the electrode surfaces, was characterized with the [Fe(CN)

6

]

3 −

/[Fe(CN)

6

]

4 −

redox

probe [21] . Due to the promoted electron transfer of Hb by AuNPs, the Hb -

immobilized AuNP/ITO electrodes exhibited an effective catalytic response to the

reduction of H

2

O

2

with good reproducibility and stability. A linear relationship

existed between the catalytic current and the H

2

O

2

concentration in the range of

1 × 1 0

− 5

to 7 × 1 0

− 3

M , with a detection limit [ signal - to - noise ratio ( SNR ) = 3] of

4.5 × 1 0

− 6

M [21] .

A mediator - free H

2

O

2

sensor was also developed along with the application

of AuNP/ITO electrodes for the immobilization of myoglobin ( Mb ) [22] . The

Mb - modiﬁ ed AuNP/ITO electrodes showed good reproducibility and stability in

pH 7.0 buffer, this being based on the catalytic activity of Mb immobilized on

AuNP/ITO towards the reduction of H

2

O

2

. A linear relationship existed between

the catalytic current and the H

2

O

2

concentration in the range of 2.5 × 1 0

− 6

to

5 × 1 0

− 4

M , with a detection limit (SNR = 3) of 4.8 × 1 0

− 7

M ). In this case, the direct

electron transfer of Mb was recorded as a stable and well - behaved voltammetric

response, with the Mb - immobilized AuNP/ITO electrodes in buffer solutions [22] .

The effects of capping reagents on electron transfer reactions on the AuNP/ITO

electrodes were also investigated using cyclic voltammography [23] . In addition to

the conventional preparation, with CTAB present in the growth solution, the pres-

ence of sodium dodecyl sulfate ( SDS ) was investigated to determine whether the

fabrication of AuNP/ITO was also possible. Systematic cyclic voltammography

was then carried out using the AuNP/ITO electrodes prepared with different

302 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

surfactants – namely cationic CTAB and anionic SDS – for the oxidation of anionic

[Fe(CN)

6

]

4 −

and reduction of cationic [Ru(NH

3

)

6

]

3+

. The results showed that the

electrochemical responses were signiﬁ cantly improved on both the AuNP/ITO

electrodes, but did not depend on the charges of the capping surfactants and the

redox species. Thus, the capping of CTAB and SDS on AuNP/ITO was concluded

to be quite weak, so that the electron - transfer reactions of both [Fe(CN)

6

]

4 −

and

[Ru(NH

3

)

6

]

3+

were not disturbed [23] .

The electrocatalytic activity of a three - dimensional ( 3 - D ) monolayer of 3 -

mercaptopropionic acid ( MPA ) assembled on AuNPs on the surface of ITO was

investigated by using the weak capping of CTAB on the grown AuNPs [24] . The

electrochemical behavior of nicotinamide adenine dinucleotide hydrate ( NADH )

on the 3 - D MPA monolayer assembled on the AuNP/ITO electrodes indicated that

the MPA layer promoted the electron transfer, which was similar to that of a two -

dimensional ( 2 - D ) MPA monolayer assembled on planar gold electrodes. However,

for the electro - oxidation of ascorbic acid and dopamine, the 3 - D MPA monolayer

showed an obvious electrocatalytic promotion, while the 2 - D MPA monolayer

exhibited a blocking effect. The catalytic activity of the 3 - D MPA monolayer would

be an interesting feature of the proposed AuNP/ITO electrodes.

8.4

Improved Methods for Attachment and Growth of AuNPs on ITO

A number of reﬁ ned methods for improving or perturbing the seeding processes,

namely the attachment of Au nanoseed particles, have been proposed by the

present authors ’ group in order to recognize the signiﬁ cance of the seeding

treatment.

The ﬁ rst approach involved an in situ reduction method, rather than seeding in

the Au colloid solution. Here, it was shown that, when the seeding procedure was

modiﬁ ed to be carried out by the impregnation reduction of AuCl

4

−

in the presence

of the ITO substrate, the density of AuNPs grown directly on the surface was

greatly improved [25] . Figure 8.3 shows the FE - SEM images of the AuNP - attached

ITO surfaces prepared using the in situ reduction method. The initial attachments

of the seed particles were recognized on the ITO crystals (Figure 8.3 a), after which

the AuNP growth could be seen with time on the nanostructured ITO surface.

After a 24 - h growth period, the smaller AuNPs were found to be attached densely

on the ITO surfaces (Figure 8.3 c). Repeating the growth treatment was also effec-

tive in increasing the size of the AuNPs (Figure 8.3 d), although an increasing time

for the repeated treatment led to a decrease in the monodispersion properties of

the AuNPs on the electrode surface. In other words, the particle size difference

became larger, indicating that the AuNPs had not grown uniformly.

The facile heterogeneous electron transfer kinetics resulting from the deposition

and growth of the AuNP - arrays was then monitored by using the [Fe(CN)

6

]

3 −

/

[Fe(CN)

6

]

4 −

redox probe for cyclic voltammetry measurements. In addition, the

8.4 Improved Methods for Attachment and Growth of AuNPs on ITO 303

Figure 8.3 FE - SEM images of AuNP - attached ITO surfaces.

For seeding, the ITO substrate was immersed into the

solution containing AuCl

4

−

and citrate ions before the

reduction by NaBH

4

. (a) Observed just after seeding;

(b, c) After growth treatment for (b) 15 min and (c) 24 h;

(d) After repeating 24 - h growth three times. Reproduced with

AuNP/ITO electrodes thus prepared exhibited a high catalytic activity for the

electro - oxidation of nitric oxide ( NO ), which in turn led to electroanalytical applica-

tions for NO sensing [25] .

For the second approach, an attempt was made to grow high - density AuNPs on

the ITO surfaces by using a “ touch ” seeding technique rather than the “ normal ”

seeding in the seed - mediated growth procedure [26] . This approach provided a

simple and useful strategy to promote the growth of AuNPs on ITO surfaces by

simply touching the surface that had already been covered with a drop of Au

nanoseed solution with tissue paper.

Subsequent FE - SEM characterization of the growth of AuNPs on two different

structured ITOs – namely, rough and smooth structures (Figure 8.4 ) – has con-

ﬁ rmed that this approach was commonly effective and prospective for fostering

the growth of high - density AuNPs with a relatively small size (ca. 10 – 30 nm) of

spherical particle [26] .

Based on optical absorption measurements of the AuNP - densely attached ITO

samples, it was conﬁ rmed that the modiﬁ ed ITO substrates could be used for

304 8 Recent Advances in Metal Nanoparticle-Attached Electrodes

Figure 8.4 FE - SEM images of AuNP - attached ITO surfaces

prepared using the touch seed - mediated growth method.

(a, c) High - magniﬁ cation and (b, d) low - magniﬁ cation images

of the surfaces of (a, b) rough ITO and (c, d) smooth ITO.

American Chemical Society.

photoelectrochemical applications, an example being the optically transparent

electrode [26] . In addition, the densely attached AuNP/ITO electrodes thus fabri-

cated were applied to observe the electrochemical responses of hydroquinone

and p - aminophenol in phosphate buffered solutions [27] . Although the electron -

transfer reactions of these compounds were sluggish on bare ITO electrodes,

improved electrochemical responses could be remarkably observed on the AuNP/

ITO electrodes fabricated with the “ touch ” seed - mediated growth method [27] .

The dense attachment of AuNPs could be also carried out using a cast

seed - mediated growth method [28] . Cast seeding which involved three cycles of

dropping the seed solution containing Au nanoseed particles and evaporation at

30 ° C, followed by treatment in the growth solution containing HAuCl

4

, CTAB,

and ascorbic acid, was found to be suitable for preparing the AuNP - attached ITO

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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