Modification of Self-assembled Monolayers on Gold Nanoparticles by Oxygen Plasma and

UV-induced Graft Polymerization and Quantitative Analysis

Ko-Shao Chen1＊, Tsui-Shan Hung1, Shu-Chuan Liao1 , Tzu-Jung Wang1, Hon-Ru Lin2, Hsin- Jung Lin1, Yuan-An Ku3

1Department of Materials Engineering, Tatung University, Taipei, Taiwan

2 Department of Chemical Engineering, Southern Taiwan University, Tainan, Taiwan

3Department of Raw Materials and Yarn Formation, Taiwan Textile Research Institute, Taipei, Taiwan

E-mail: kschen@ttu.edu.tw Tel: +886-2-25867150.

*Corresponding Author.

ABSTRACT

In the past, thermosensitive gold nanoparticles have been synthesize successfully by O2

plasma modification and UV-induced grafting polymerization of poly(N-isopropylacrylamide)

(PNIPAAm) after self-assembled monolayers (SAMs) by 11-Mercaptoundecanoic acid (MUA)

including thiol group. But there were difficulties in quantitative analysis of modified nanoparticles.

Quartz crystal microbalances (QCM) are transducers for chemical and biochemical sensing in

general. The oscillation frequency of QCM will be affected when the adsorbed mass on the surface,

in view of this characteristics, it can be used as mass sensor applications. On the gold film electrode

of a QCM was modified by plasma-treatment and then UV-induced graft polymerization with

functional monomers after SAMs has been carried out. MUA including thiol group was employed

for the deposition of SAMs from ethanol solutions onto gold surfaces. O2 plasma treatments can be

employed to generate sufficient amounts of peroxides and hydroperoxides on the SAMs for the

subsequent UV-induced graft polymerization. The monomers used for graft polymerization were

water-soluble NIPAAm. Contact angle and oscillation frequency of QCM measurements indicate

that the SAM-modified Au surfaces could be an effective method making hydrophilic surface and

reducing the frequency of QCM through the plasma modification and graft copolymerization with

an appropriate monomer.

Keywords: Quartz crystal microbalances (QCM), PNIPAAm, plasma

1 INTRODUCTION

Nano-Au exhibit surface plasmon absorption in the visible region [1]. Based on the optical

properties, chromatic sensors using Nano-Au have been widely investigated. But the intrinsic lack

of stability of naked nanoparticles and their tendency to aggregate are highly undesirable, because

of a deleterious impact on the optical, electrical, catalytic, and magnetic properties of the

nanoparticles [2-4].

There is thus a need to build up a coalescence barrier (steric or else) around the nanoparticles [5].

Another problem related to the gold nano particle is their recovery after use, which is more

acute when the stabilization is more effective. It is the reason why attention was paid to polymeric

stabilizers, whose solvation is stimuli-dependent, such as poly-(N-isopropylacrylamide) (PNIPAM),

which is a water-soluble polymer with a lower critical solution temperature (LCST). The LCST

behavior of PNIPAM thermoresponsive polymers are water-soluble below their phase-transition

temperatures, and are insoluble above them. A reversible phase transition of thermoresponsive

polymers is achieved by controlling the solution temperature. The LCST behavior of PNIPAM is

commonly accounted by a balance between hydrogen bonding and the hydrophobic effect.

PNIPAM has a hydrophilic amide group and a hydrophobic isopropyl group. Below the LCST,

aqueous solutions are stabilized by hydrogen bonding between the amide groups and the water, and

by ice-like structures

that water molecules form around the hydrophobic groups. As the temperature is increased, the

hydrogen bonding weakens and the attractions between hydrophobic groups increase leading to the

eventual shrink of the PNIPAM chains above the LCST [6-10]. Several applications take advantage

of this temperature-sensitive water solubility, such as controlled drug delivery [11,12], separation

[13] and catalysis[14].

In recent years, polymerization of thermosensitive polymer PNIPAAm on Nano-Au was

carried out by two steps. Polymerization of thermosensitive polymer was done first and then using

graft thiol on it, finally using SAM with nano-Au particles[15,16]. But the reaction steps were too

unstable and take long time. In this study, the novel method is modified Nano-Au with MUA by the

SAM and O2 plasma pretreatment of the surface on modified Nano-Au(MUA). After O2 plasma

pretreatment, there are carboxyl group and peroxides on the Nano-Au(MUA), it could be used in

photo-induced grafting thermosensitive polymer. The abstraction of hydrogen atoms from the

surface results in the formation of free radicals on the polymer chains. Subsequently exposure of the

activated surface to air causes oxygen to be incorporated on to the polymer surfaces, leading to

surface oxidation and the formation of peroxides and hydroperoxides species. The peroxides species

formed will subsequently initiate the surface free radical coupling reaction with thermosensitive

polymer grafted by UV-light. This novel method can polymerize the thermosensitive polymer on

nanoparticles stablely and fast.

In a previous work, thermosensitive gold nanoparticles have been synthesize successfully by

O2 plasma modification and UV-induced graft polymerization of poly(N-isopropylacrylamide)

(PNIPAAm) after self-assembled monolayers (SAMs) by 11-Mercaptoundecanoic acid (MUA)

including thiol group. In general, there were difficulties in quantitative analysis of modified

nanoparticles. Recently other surface sensitive quantitative techniques such as surface plasmon

resonance (SPR), quartz crystal microbalance (QCM), and ellipsometry have obtained increasing

attention to analysis of surface modification. The relationship between the frequency change of

QCM and adsorption of mass in air phase is known as the Sauerbrey's equation:

ΔF ＝ −2.3×106 F2ΔM/A

where ΔF is the frequency shift, F (Hz) the fundamental frequency, ΔM the change of mass

adsorption, and A (m2) the electrode area. These methods are applicable for direct quantitative

determination of surface deposited without complex procedures and are hence highly attractive in

low equipment [17].

This work describes the further development of the previous work on coatings. In particular,

to develop a simple quantitative analysis of gold nano particles by Scanning electron microscopy

(SEM), Contact angle analysis and investigated by a quartz crystal microbalance (QCM) are used as

tools to follow the assembly process.

This study should provide fundamental information that can lead to the further development of gold

nano particles derivatives for advanced applications in nanotechnology and biotechnology.

2 EXPERIMENT

2.1 Materials and Reagents

The AT-cut QCM devices with basic resonant frequency of 10 MHz with gold electrodes on

both sides were purchased from Mercury Electronics Ind. Co., Ltd., Taiwan. The surface of

electrode was cleaned with alcohol for 15 min.

Hydrogen Tetracholoroaurated (III) Tetrahydrate, Ammonium Peroxodisulfate and

N-Isopropylacrylamide were purchased from Wako Pure Chemical Industry, Japan. The

11-Mercapto-undecanoic acid was obtained from Sigma-ALDRICH Inc, USA. All the above

reagents were used as received without further purification.

2.2 Preparation of Gold-Thersmosensitive thin film

The QCM having PNIPAAm grafts on the surface were showed in Figure 1. 0.15 mM

11-mercaptoundecanoic acid (MUA) including thiol group was used to modify the surface of gold.

This modified QCM were initiated by O2 plasma pretreatment (40 mtorr, 100W). Subsequently use

photo-induced grafting polymerization on the QCM with NIPAAm solution (10mmol) and

Ammonium peroxodisulfate (APS, 0.1mmol). Irradiation with a high-pressure mercury lamp

(1000W) as carried out at room temperature.

2.3 Frequency measurement of QCM

The QCM-FIA apparatus (ANT Technology Co., Ltd.) was used. The conduits of QCM were

inserted into the channel of the detector. The frequency variations were continuously recorded using

a universal frequency counter. The data (the resonant frequency) was displayed on the main display

screen and could be read directly by a computer. The reported standard deviation (S.D.) of

frequency shift was ±1.5 Hz.

2.4 Characterization

2.4.1 The hydrophilic property change on the surface of substrate

The hydrophilicity on the surface of substance was observed by water contact angle (θH2O) for

each treatment. Distilled water was used as a standard testing agent and contact angle meter,

Gonio-meter type G-1 made by Erma Optical Works Co. Ltd., was employed. The measured value

was the average of five to ten values of different positions.

2.4.2 Electron spectroscopy for chemical analysis (ESCA)

The elemental composition at the polymer surface was determined by ESCA spectra (PHI

590AM, Perkin-Elmer) using Mg Kα exciton radiation. Typical operating conditions were X-ray

gun, 15 kV, 250 W, and the 10−10 Torr pressure of the sample chamber.

2.4.3 Surface morphology observation of by SEM

The gold film samples were coated with gold prior to being observed by scanning electron

microscope (SEM), JEQL JSM-6300, was used to observe the micro morphologies, structure of

original and grafted PNIPAAm. First of all, the samples were placed on an aluminum holder with

carbon glue and then coated with a thin layer of gold by sputtering for 60 seconds to improve the

electrical conductivity.

2.4.4 The thickness of thin film measurement

The thickness of the plasma deposited film was measured by a stylus profiler (Veeco

Metrology TEKTAK 150).

3 RESULTS AND DISCUSSION

3.1 The hydrophilic property change on the surface of substrate

The hydrophilicity on the surface of substrate was observed by water contact angle (θH2O) at

each treatment conditions. Contact angle has been defined as the angle between the solid surface

and the tangent of the liquid–vapor interface of a water drop. For a water droplet on a sample

surface, the contact angle approaches zero (the drop spreads out over the surface) for a hydrophilic

surface and increases for increasingly hydrophobic surface. The variation of wettabilities was

caused by the change of surface chemical structure. The smaller the theta (θ) is, the better

hydrophilic property will be. Table 1 illustrates that the surface of gold coated on glasses had varied

water angles under the control and after different treatments. It was found that the water contact

angle of self-assembled monolayers of mua were same as the control gold film, after O2 plasma

modification films were more hydrophilic (θH2O＜10°) because increase the radicals and peroxides.

The film thickness after self-assembled monolayers was 35.5nm and plasma modification reaction

not only formed a hydrophilicity surface of gold but also made the material surface structure

become activity. The active groups to combine with oxygen and moisture in the atmosphere, and

then formed the polarity base of the peroxide that can form free radicals for surface graft

polymerization.

Table 1 showed that the thickness of grafting NIPAAm monomer onto the plasma

modification films results 703.9 nm. The increase in thickness behavior of substrate through grafted

of PNIPAAm. The changes in water contact angle after grafting of NIPAAm reflect the changes in

density.

3.2 The frequency change after each surface treatment

The typical frequency change after each treatment on QCM surface is shown in Figure 2 The

frequency begins to change from control and decreases subsequently at self-assembled monolayers

(ΔF＝－232). The frequency change was caused by the mass absorbed on the surface of QCM at

each treatment. Although the mass absorption after each treatment was very small, the frequency

change was still detectable due to the high sensitivity of QCM. Fig.2 also shows clearly that the

frequency was decreased after grafting PNIPAAm. After the surface modification on QCM, the

frequency change was 13687±81 Hz. The results of frequency change provide further evidence of

successful grafting PNIPAAm on the QCM surface. It indicates that the sensor developed in this

study is a promising technique to detect the grafting quantitative.

3.3 Electron spectroscopy for chemical analysis (ESCA)

As indicated above, thermosensitive QCM surface using SAM modification, O2 plasma

pretreatment finally photo-induced grafting PNIPAAm polymerization. Figure 3(a)–(c) shows

respectively the XPS wide scan spectra, for the nano-Au(mua), nano-Au(mua)-PNIPAAm and

PNIPAAm. At the nano-Au(mua) surface shows four main peak components, associated with the

gold, sulfur, carbon and oxygen species. After O2 plasma pretreatment and subsequent graft

thermosensitive polymer, the intensity of the oxygen component and carbon increases significantly

besides occur the nitrogen component. The phenomenon is consistent with the photo graft

PNIPAAm. The O2 plasma causes the breakage of C–H bonds at the surface of nano-Au that

modified with MUA.

3.4 Surface morphology observed

Figure 4 showed a morphological examination was carried out by SEM in order to evaluate

the effect of the surface modification. Figure 4 (a) showed the gold coatings were observed to be

smooth and after O2 plasma treatment by MUA modification, subsequently UV induced

grafting(Figure 4 (c)) were observed to be rough surface; SEM examinations confirmed that this

was due to a rough surface morphology with many small grainy deposition on the surface, the

grains are uniform and denseness.

4 CONCLUSIONS

In this study, PNIPAAm was successfully grafted on the QCM surface through a series of

surface treatments. The frequency change of QCM can be used to detect the modification between

gold and polymer. The result indicates the frequency change of thermosensitive QCM was 13687±

81 Hz via plasma deposition after mua SAM and subsequent grafting polymerization PNIPAAm.

These methods are applicable for direct quantitative determination of surface deposited without

complex procedures and are hence highly attractive in low equipment.

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List of figure:

Figure 1 Reaction steps for the preparation of thermosensitive QCM.

Figure 2 Frequency change resulting of QCM with different surface modification

process.

Figure 3 SEM micrographs of (a)gold films and that were (b) after MUA modification, (c) O2

plasma treatment and (d) grafting PNIPAAm

Figure 4 XPS (ESCA) wide-scan spectra of: (a) the nano-Au(mua) (b) the

nano-Au(mua)-PNIPAAm and(c) the PNIPAAm.

Table captions:

Table 1: Graft density, Wettability and thickness of surface modified glasses evaluated by water

contact angles and stylus profiler.

HS-(CH2)10-COOH

MUA Self-Assembled Monolayer(SAM)

S COOH

COOHS

Plasma

O2

S COOH

COOHS

O-OH

OOH

NIPAAm Monomer

UV induced Graft Polymerization

S COOH

PNIPAAmS

PNIPAAm

OOHHS-(CH2)10-COOH

MUA Self-Assembled Monolayer(SAM)

S COOH

COOHS

Plasma

O2

S COOH

COOHS

O-OH

OOH

NIPAAm Monomer

UV induced Graft Polymerization

S COOH

PNIPAAmS

PNIPAAm

OOH

Figure 1

Table 1

Density Film Thickness Water contact

(mg/cm2) (nm) angel (°) Gold film 253.7 20.3 31.85 Gold film-MUA 254.2 35.5 36.40 Gold film-MUA- 254.0 25.3 ＞10 O2 Plasma Gold film-MUA-O2 283.7 703.9 40.95 Plasma-grafted PNIPAAm

9986000

9988000

9990000

9992000

9994000

9996000

9998000

10000000

Freq

uenc

y(Hz

)

Gold film-MUA-O2

Plasma-grafted

Gold film-MUA- O2 Plasma

Gold film-MUA

Gold Film

Figure 2

(a) (b)

(c) (d)

Figure 3

(a)

0 200 400 600 800 1000 1200 14000

20000

40000

60000

80000

100000

Coun

ts/s

Binding Energy (eV)

Nano-Au(mua)

0

20000

40000

60000

80000

100000

Au

S

C

O

0 200 400 600 800 1000 1200 140

Nano-Au(mua)

Coun

ts/s

Binding Energy (eV)

Au

S

C

O

0

(b)

0 200 400 600 800 1000 1200 14000

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0 200 400 600 800 1000 1200 14000

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PNIPAAmC

N

O

0 200 400 600 800 1000 1200 14000

5000

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ts/s

Binding Energy (eV)

PNIPAAmC

N

O

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ts/s

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Nano-Au(mua)-g-PNIPAAm

0 200 400 600 800 1000 1200 14000

5000

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Au

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0 200 400 600 800 1000 1200 14000

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Au

C

N

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Nano-Au(mua)-g-PNIPAAm

Coun

ts/s

Bindin

Au

C

N

O

S

g Energy (eV)

(c)

Figure 4