RESEARCH PAPER

Importance of poly(ethylene glycol) conformationfor the synthesis of silver nanoparticles in aqueous solution

Sunghyun Nam • Dharnidhar V. Parikh •

Brian D. Condon • Qi Zhao •

Megumi Yoshioka-Tarver

Received: 28 July 2010 / Accepted: 14 February 2011

� Springer Science+Business Media B.V. (outside the USA) 2011

Abstract Silver nanoparticles (NPs) were prepared

using silver nitrate (AgNO3) as a precursor in an

aqueous solution of poly(ethylene glycol) (PEG),

which acted as both a reducing and stabilizing agent.

The UV/Vis spectra showed that PEG 100 (100 kg/

mol) has a remarkable capability to produce silver

NPs at 80 �C, but the production of silver NPs by

both PEG 2 (2 kg/mol) and PEG 35 (35 kg/mol) was

negligible. This difference was explained by the

conformation of PEG in the reaction solution: the

entangled conformation for PEG 100 and the single-

coiled conformation for PEG 2 and PEG 35, which

were confirmed by pulse-field-gradient 1H NMR and

viscosity measurements. In an aqueous solution, the

entangled conformation of PEG 100 facilitated the

reduction reaction by caging silver ions and effec-

tively prevented the agglomeration of formed NPs.

The reaction in an aqueous PEG 100 solution was

observed to be stable under the conditions of a

prolonged reaction time or an increased temperature,

while no reduction reaction occurred in the PEG 2

solution. The synthesis of silver NPs by PEG 100 was

well controlled to produce fine silver NPs with

3.68 ± 1.03 nm in diameter, the size of which

remained relatively constant throughout the reaction.

Keywords Silver nanoparticle � Poly(ethylene

glycol) � Chain conformation � PFG 1H NMR �Self-diffusion coefficient � Viscosity �Antimicrobial properties

Introduction

The synthesis of silver nanoparticles (NPs) has been

extensively explored over the past few decades due to

the unique chemical, physical, and optical properties of

silver NPs. Particularly, the effective and stable

antimicrobial properties of silver NPs and the intrinsic

low toxicity of silver to human cells (Gosheger et al.

2004; Hollinger 1996) have attracted considerable

interest from the textile, medical, and personal-

hygiene industries (Rai et al. 2009). The antimicrobial

applications of silver NPs that have been investigated

by researchers include loading them onto textile fabrics

and wound dressings (Lee et al. 2003; Gorensek and

Recelj 2007; Maneerung et al. 2008; Ilic et al. 2009)

and incorporating them into polymers to be spun into

fibers and fabricated into composites (Yeo and Jeong

2003; Kim et al. 2004; Hong et al. 2006; Son et al.

S. Nam (&) � D. V. Parikh � B. D. Condon �M. Yoshioka-Tarver

USDA ARS Southern Regional Research Center,

1100 Robert E. Lee Blvd, New Orleans, LA 70124, USA

e-mail: sunghyun.nam@ars.usda.gov; sn314@yahoo.com

Q. Zhao

Department of Chemistry, Tulane University,

6400 Freret Street, New Orleans, LA 70118, USA

123

J Nanopart Res

DOI 10.1007/s11051-011-0297-z

2006). The antimicrobial efficiency of silver NPs

results from their high surface area to volume ratio.

With a decrease in particle size, the amount of silver

atoms exposed to the surrounding medium signifi-

cantly increases, which enhances the release of silver

ions with reduced reaction time.

Generally, silver NPs are prepared by reducing the

silver ions of a precursor in a solution and preventing

particle growth by using stabilizing agents such as

surfactants and polymers. The reduction reaction has

been carried out by irradiation or the use of reducing

agents such as hydrazine, sodium borohydride, and

N,N-dimethylformamide (Zhang et al. 1996; Pastor-

iza-Santos and Liz-Marzan 2002b; Gao et al. 2006;

Ilic et al. 2009). As an environmentally benign

reducing agent, ethylene glycol was found to reduce

silver ions through a polyol process at high temper-

atures (Fievet et al. 1989). The synthetic method

using ethylene glycol and polyvinylpyrrolidone

(PVP) stabilizer was constructed by several research-

ers, producing various shapes of silver NPs (Caro-

tenuto et al. 2000; Silvert et al. 1996; Slistan-Grijalva

et al. 2005; Sun and Xia 2002). Depending on the

reduction method, the reaction condition, and the

kind of stabilizer, the size and morphology of silver

NPs are changed (Henglein and Giersig 1999;

Pastoriza-Santos and Liz-Marzan 2002a; Shin et al.

2004; Silvert et al. 1997; Sun et al. 2003).

Recently, low-molecular-weight poly(ethylene

glycol)s (PEGs) were shown to act as both a reducing

and stabilizing agent. Luo et al. (2005) conducted the

synthesis of silver NPs in PEG melts (0.2–6 kg/mol)

and found that the reducing reactivity of PEG is

strongly dependent on its molecular weight. PEG

2 kg/mol was found to be the most effective,

producing spherical silver NPs (*10 nm) at the mild

temperature of 80 �C, at which ethylene glycol was

inactive. When the temperature was increased to

120 �C, these nanoparticles grew to large non-spher-

ical particles (*80 nm). Popa et al. (2007) also

observed similar temperature effect for PEG 0.2 kg/

mol. The formation of spherical silver NPs

(15–30 nm) was observed at as low as 30 �C, but

large triangular and pentagonal prism particles were

observed at 90–120 �C. On the other hand, Bo et al.

(2009) reported the capability of PEG 2 kg/mol to

produce silver NPs (8–10 nm) in water and the good

antibacterial activities of the obtained colloidal

solution against Gram-positive and Gram-negative

bacteria. However, the complete reduction of silver

ions was not proven in their study to support that the

antibacterial properties were solely caused by silver

NPs.

The ‘‘one-pot’’ synthetic method of using low-

molecular-weight PEGs without other chemicals is

facile and environmentally friendly. However, it is

still a challenge to control the reaction in bulk PEG.

For example, at low temperatures, small particles

were produced, but an incomplete reduction reaction

was caused. The elevated temperatures to facilitate

the reaction resulted in the formation of large non-

spherical particles (Luo et al. 2005). In addition, the

silver NPs formed in bulk PEG need to be washed

with organic solvents multiple times to be recovered,

which will eventually result in environmental issues

for industrial production. To improve upon the

‘‘green’’ method, we used water as a solvent. In an

aqueous PEG solution, however, the reduction reac-

tion may not be favored because silver ions and PEG

hydroxyl groups are stabilized by water molecules.

We attempted to enhance the reducing reactivity of

PEG and the stabilization of silver NPs in water by

studying the effect of PEG conformation.

The conformation of polymer in a solution

changes according to its critical concentration, c*,

above which polymer chains interpenetrate to form a

network, and below which these chains separate to

behave as single coils. For high-molecular-weight

polymer, the c* is very small because the c* depends

on the chain length (N) as c* * N-4/5 (de Gennes

1979). According to the theory proposed by de

Gennes (1976a, b), the entangled conformation in a

good solvent can be determined by the dependence of

the self-diffusion coefficient (D) of the polymer

chains on the concentration (c) as D * c-1.75. Below

c*, the polymer chains are less dependent on the

concentration. In this study, the conformations of

different molecular weight PEGs (2, 35, and 100 kg/

mol) in an aqueous solution were determined by the

self-diffusion coefficient measurement using pulsed-

field-gradient 1H NMR and viscosity measurement.

In the aqueous solutions of these PEGs, which act as

both reducing and stabilizing agents, the formations

of silver NPs were monitored by color change, UV/

Vis spectroscopy, and transmission electron micros-

copy (TEM).

J Nanopart Res

123

Experimental

Materials and synthesis of silver NPs

Silver nitrate (AgNO3, 99.9%) was purchased from

J. T. Baker and used as a precursor in the formation

of silver NPs. PEGs (HO–[CH2–CH2–O]n–H) with

different molecular weights (M = 2, 35, and 100 kg/

mol denoted as PEG 2, PEG 35, and PEG 100,

respectively) were purchased from Aldrich and used

as both reducing and stabilizing agents. All chemicals

were used without further purification. Deionized

(DI) water was used as the solvent.

The preparation of silver NPs was performed by

the addition of an aqueous AgNO3 solution to an

aqueous PEG solution. In a typical procedure, 2 g of

PEG was dissolved in 98 g of DI water to prepare a

2 wt% solution. The aqueous PEG solution was

filtered and heated to 80 �C. 0.05 g of AgNO3 was

dissolved in 25 mL of DI water at room temperature.

The aqueous AgNO3 solution was then added drop-

wise with a flow rate of 1 mL/min to the aqueous

PEG solution while the temperature was kept con-

stant at 80 �C under magnetic stirring. After the

AgNO3 solution had been added completely, the

reaction was allowed to proceed under constant

stirring at 80 �C for 2 h. The reaction temperatures

and reaction times varied as described in the ‘‘Results

and discussion’’ section.

Characterization

The self-diffusion coefficient, D, of PEG in D2O at

80 �C was measured by pulsed-field-gradient (PFG)

NMR using a Bruker Avance-300 spectrometer

equipped with a 5 mm Z-gradient probe. PEG

concentration varied from 0.05 to 40 wt%. To

suppress the convection currents generated at high

temperatures, the double-stimulated-echo PFG NMR

pulse sequence with symmetrical bipolar-gradient

pairs (Zhao 2004) was used (Fig. 1). Field gradient

calibration was accomplished using the self-diffusion

coefficient of pure water at 25 �C (2.299 9

10-9 m2 s-1) (Mills 1973). The relationship between

the attenuation of the echo amplitude, I(g), and PFG

parameters is given by:

IðgÞ ¼ Ið0Þ exp �kDf g ð1Þ

where k is equal to (cgd)2(T ? 4d/3 ? 3s/2). c is the

gyromagnetic ratio of the nucleus under observation

(1H for this study). The gradient strength (g) was

incremented 16 times from 2.7 to 50.8 G/cm, result-

ing in attenuation of the PEG resonance to approx-

imately 5% of its original intensity. I(0) is the echo

amplitude when g is equal to zero. The gradient pulse

duration (d/2) varied from 950 ls to 2 ms, depending

on the measured D and the used effective diffusion

time (T ? 4d/3 ? 3s/2). A gradient recovery time (s/2)

was 0.5–1 ms. Diffusion time was set to the range

from 250 ms to 2 s to be long enough to measure

D independently on the diffusion time. Homospoil

gradients (600 ls) were applied during the z-storage

period (T/2), and eddy current settling durations (Te)

was introduced to destroy the signals from unwanted

coherence paths. The recycle delay time was 3 s, and

16 dummy scans were applied before the first data

were collected. All spectra were measured with 32

accumulations. Prior to the acquisition of NMR

signal, the sample was equilibrated for 20 min at

80 �C.

The viscosity of aqueous PEG solution was

measured with a viscometer (LVDV-II ? PRO,

Brookfield) equipped with an UL adaptor using a

cylindrical geometry. The diameter of the stainless

steel spindle was 25. 2 mm, and the gap between the

spindle and wall was 2.4 mm. The measurements of

the sample volume of 16 mL were carried out at

80 �C to enable direct comparison with the NMR

data. The viscosities of the samples were found to be

constant for shear rates from 12 to 73 s-1, and a zero-

shear viscosity for each sample was therefore

determined.

2τ

2δ

2τ

2T

2τ

2τ

2τ

2τ

eT2T

2δ

2δ

2δ

2δ

2δ

2δ

2δ

2τ

2τ

1H:

Gradient:

FID

g

π/2 π π/2 π/2 π π π/2 π/2 π π/2 π/2Fig. 1 Double-stimulated-

echo PFG NMR pulse

sequence with symmetrical

bipolar-gradient pairs

J Nanopart Res

123

The UV/Vis spectra of the silver colloidal solu-

tions were recorded in a wavelength range of 200 to

800 nm using a UV/Vis/NIR spectrometer (Cary 500,

Varian) in a quartz curette with a 1 cm optical path

length. Photographs of the colloidal solutions were

taken with a digital camera (NV3, Samsung). The

size and dispersion of the silver NPs were examined

with a transmission electron microscope (TEM,

JEOL 2010) operating at 200 kV. For TEM sample

preparation, a drop of the silver colloidal solution was

placed on a carbon-film-coated copper grid and was

dried overnight. The average particle size and the size

distribution of silver NPs were obtained by image

analysis of TEM micrographs measuring more than

500 particles.

Results and discussion

Synthesis of silver NPs with different molecular

weight PEGs

Figure 2 shows the photographs and UV/Vis spectra

of the silver colloidal solutions prepared by PEGs

with different molecular weights. The initial colorless

solution turned bright yellow for PEG 100 after the

reaction. This color change signifies the formation of

silver NPs, which is attributed to the plasmon

resonance induced by the interaction between the

incoming light and the conduction electrons of a

small metal particle (Kreibig and Vollmer 1995). The

solutions for PEG 2 and PEG 35, however, showed

negligible changes in color, indicating the formation

of very low concentrations of silver NPs. During the

reaction, the color change for PEG 100 started to

occur shortly after the introduction of silver nitrate,

but those for PEG 2 and PEG 35 took considerably

longer. This observation suggests that the reduction

reaction is facilitated in PEG 100 solution. Corre-

sponding to the color changes, the UV/Vis spectrum

for PEG 100 exhibits a strong symmetric plasmon

absorption peak at 420 nm, while the spectra for PEG

2 and PEG 35 weakly appear at about 400 and

450 nm, respectively.

The reduction of silver ions by PEG has been

proposed to occur through the oxidation of the

hydroxyl terminal groups to aldehyde groups (Aners-

son et al. 2002; Liz-Marzan and Lado-Tourino 1996).

Table 1 presents the OH/Ag? molar ratios for the

solutions containing different molecular weight PEGs.

Notably, PEG 2, which has an almost seven-fold

excess of reducing hydroxyl sites compared to the

number of silver ions, exhibited much lower plasmon

absorption than PEG 100, which has a much smaller

number of reducing sites. The limited reducing

capability of a short PEG in an aqueous silver nitrate

solution was also reported by Kim et al. (2009), who

found no difference in reducing reactivity between

PEG (1.5 kg/mol) coupled to 2-chlorotrityl resin and

its corresponding dimethoxy-terminated PEG. It is

also notable that PEG 35, which exhibited comparable

absorption intensity with PEG 2, has a similar OH/

Ag? molar ratio with PEG 100. These results suggest

that the silver NP formation in the PEG aqueous

0

0.1

0.2

0.3

0.4

0.5

200 300 400 500 600 700 800

Ab

sorb

ance

Wavelength (nm)

b

c

a b c

PEG 2 (a)

×× 0.2

a

PEG 35 (b)PEG 100 (c)

Fig. 2 Photographs and UV/Vis spectra of the silver colloidal

solutions prepared from (a) PEG 2, (b) PEG 35, and (c) PEG

100. The reduction reaction was conducted in a 2 wt% aqueous

PEG solution at 80 �C for 2 h. The peak of the PEG 100

spectrum was reduced five times from its actual size

Table 1 Molar ratio of hydroxyl groups to silver ions for

PEGs with different molecular weights used in the synthesis of

silver NPs

PEG

(kg/mol)

OH groups

(mol)

Ag?

(mol)

OH/Ag?

(mol/mol)

2 2.00 9 10-3 2.94 9 10-4 6.79 9 100

35 1.14 9 10-4 2.94 9 10-4 3.88 9 10-1

100 4.00 9 10-5 2.94 9 10-4 1.36 9 10-1

J Nanopart Res

123

solution is not simply governed by the molecular

weight of PEG. In an aqueous solution, silver ions and

PEG hydroxyl groups are stabilized by the interac-

tions with water molecules such as coordinative and

hydrogen bonds, which resulted in hindering the

reduction reaction. The conformation of PEG chains,

therefore, may play an important role in the synthesis

of silver NPs by creating the steric environment

favorable for the reduction reaction and the stabiliza-

tion of NPs in water.

Role of PEG chain conformation

The conformation of PEGs with three different

molecular weights in the reaction solution was

investigated by measuring the self-diffusion coeffi-

cient and viscosity. Figure 3 shows the typical echo

attenuation plots, ln [I(g)/I(0)] versus k, for PEG 2

and PEG 35. Straight lines, signifying single-expo-

nential decays, were exhibited by PEG 2 and PEG 35

solutions at all concentrations used for this study, and

no deviations from Eq. 1 were observed. PEG 100,

however, showed a different decay behavior depend-

ing on the concentration. As can be seen in Fig. 4, the

logarithm of signal amplitude for a 0.05 wt% solution

linearly attenuates with an increase of k, while the

logarithm for a 2 wt% solution obviously deviated

from the single-exponential decay. This non-linear

decay at the increased concentration indicates that

polymer chains experience more than one diffusion

process: the cooperative diffusion associated with

entanglements and the self-diffusion of the individual

chains. Since the corporative diffusion time is

significantly increased by polymer size (de Gennes

1976a, b), the presence of these two diffusion

processes is discernable in long polymer chains.

The concentration dependence of the self-diffusion

coefficient for PEG 2 and PEG 35 is shown in Fig. 5.

The onset of the steeper decrease of self-diffusion

coefficient, interpreted to be c*, is not distinctive, but

the ranges of c* for PEG 2 and PEG 35 can be

estimated to be 15–25 and 3–5 wt%, respectively.

Above these c*s, the self-diffusion coefficients

approach the theoretical prediction, D * c-1.75, for

the chain entanglements. This scaling constant has

also been validated for PEG in other solvents

(Sundukov et al. 1985). Figure 6 shows the concen-

tration dependence of the viscosity for aqueous PEG

solutions measured at 80 �C. In agreement with the

self-diffusion data, the slope changes occur at about

20 wt% for PEG 2 and at about 5 wt% for PEG 35.

Below these concentrations, the viscosities for PEG 2

and PEG 35 became less dependent on the concen-

tration. For high-molecular-weight PEG 100, the

viscosity of the 2 wt% solution falls in the steeper

region of the curve, indicating the presence of chain

entanglements. At 2 wt%, therefore, the viscosity of

PEG 100 is higher than those of PEG 2 and PEG35.

Based on the results of PEG solution dynamics, PEG

100 chains are long enough to entangle one another at

the 2 wt%, as indicated by the constrained dynamics,

whereas at the same concentration, PEG 2 and PEG

35 chains are too short to form entanglements,

moving independently.

The possible interactions of PEG and silver ions in

an aqueous solution depending on the conformation

of PEG are shown in Fig. 7. For separated PEG 2 or

PEG 35 chains, silver ions and PEG hydroxyl groups

are exposed to interact with water molecules. The

entangled PEG 100 chains cage the silver ions to

increase the interaction of silver ions with the PEG

for the reduction reaction. According to the mecha-

nism of the formation of polymer-stabilized silver

NPs proposed by Shin et al. (2004), the silver atoms

aggregate at a close range to form primary NPs, and

these primary NPs coalesce with surrounding primary

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 2 4 6 8 10

ln[I

(g) /

I(0)

]

k /1010m-2s

PEG 2

PEG 35

Fig. 3 Typical echo attenuation plots for PEG 2 and PEG 35

measured at 80 �C. The solid lines are fits to Eq. 1, from which

self-diffusion coefficients of 1.08 9 10-10 m2/s for PEG 2

(25 wt%) and 3.53 9 10-11 m2/s for PEG 35 (5 wt%) were

determined

J Nanopart Res

123

NPs to form final NPs. In the entangled network of

PEG 100, the local aggregation of silver atoms is

expected to readily occur, while the particle agglom-

eration over a long distance is expected to be

prevented. Figure 8 shows a TEM image of a typical

sample of silver NPs obtained in an aqueous PEG 100

solution. It can be seen that silver NPs were

efficiently produced by this method. Most particles

are nearly spherical and monodispersed, indicating

that particles are well stabilized in water. The inset

shows a close up of one single particle. The several

boundaries presented in a particle show that a final

particle was formed by the coalescence of small

nanoparticles. The average diameter of silver NPs

was 3.68 nm with a standard deviation of 1.03 nm.

The stable reaction in an aqueous PEG 100

solution was demonstrated by increasing the reaction

temperature. Figure 9 shows the UV/Vis spectra of

silver colloidal solutions prepared with PEG 2 and

0.05 wt% 2 wt%-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 1 2 3 4ln

[I (g

)/I (

0)]

k /1010m-2s

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 5 10 15

ln[I

(g)/

I (0)

]

k /1010m-2s

(a) (b) Fig. 4 Echo attenuation

plots for PEG 100 at

a 0.05 wt% and b 2 wt% at

80 �C. The diffusion times

for both samples were 1 s

-11.5

-11

-10.5

-10

-9.5

-9

-1 0 1 2 3

Lo

g10

(D/m

2 sec-1

)

Log10

(c /wt%)

PEG 2

PEG 35

D∼c-1.75

Fig. 5 Double-logarithmic plots of self-diffusion coefficient

versus concentration for PEG 2 and PEG 35 at 80 �C. In each

case, the echo attenuation data obeyed the Eq. 1 and yielded a

single self-diffusion coefficient. The dotted lines represent the

de Gennes scaling law, D * c-1.75

2 wt%

-3.2

-2.8

-2.4

-2

-1.6

0 0.5 1 1.5 2

Lo

g10

(

/Pa⋅

s)

Log10

(c /wt%)

PEG 2

PEG 35

PEG 100

hh

Fig. 6 Double-logarithmic plots of zero-shear viscosity versus

concentration for PEG 2, PEG 35, and PEG 100 at 80 �C

Ag+

OH

Ag+

O

(a) PEG2 and PEG 35 (b) PEG 100

Fig. 7 Schematic of interactions of PEG and silver ion in an

aqueous solution

J Nanopart Res

123

PEG 100 at 80 and 90 �C. For PEG 2, when the

temperature was elevated to 90 �C, the plasmon

absorption peak that weakly appeared at 80 �C

disappeared. Instead a peak at 300 nm resulting from

the coordinative interaction of silver ions with water

molecules emerged. This same peak developed in an

aqueous silver nitrate solution without PEG. This

result indicates that the PEG 2 lost the interaction

with silver ions and thus produced no silver NPs at

90 �C. For PEG 100, however, the plasmon absorp-

tion significantly increased at 90 �C. PEG 100 was

able to maintain the interaction with silver ions at an

increased temperature to induce the reduction of

silver ions and provided effective stabilization of the

formed silver NPs in water. A slight blue-shift of

the plasmon peak observed at 90 �C is caused from

the incomplete reduction of silver ions on the surface

of particle (Slistan-Grijalva et al. 2005).

Silver NP formation by PEG 100

The formation of silver NPs in an aqueous PEG 100

solution was further investigated by changing the

reaction condition. Figure 10 shows the effect of

reaction temperature on the particle size and distri-

bution. When the temperature was lowered to 70 �C,

PEG 100 was also able to produce well-dispersed

silver NPs, although its plasmon absorption intensity

(figure is not shown here) was smaller than that

obtained at 80 �C. The average size (3.28 ± 1.02 nm)

of these NPs was not significantly different compared

with that of the NPs prepared at 80 �C. The reducing

reactivity of PEG 100, therefore, was enhanced by an

increase in temperature to increase the formation of

NPs. At 90 �C, the average particle size significantly

increased to 5.17 ± 1.29 nm, indicating that the

thermally induced particle growth had occurred. The

formation of silver NPs in an aqueous PEG 100

solution was monitored during the reaction time in

Fig. 11. The plasmon absorption peak developed

shortly after AgNO3 was introduced to the aqueous

PEG solution. The intensity of the peak then gradually

0

200

400

600

800

1000

1200

1 2 3 4 5 6 7 8 9 10

No

. of

par

ticl

es

Particle diameter (nm)

20 nm

Fig. 8 TEM micrograph and histogram of silver NPs prepared

in an aqueous PEG 100 solution at 80 �C for 2 h. The average

diameter of silver NPs is 3.68 ± 1.03 nm

0

0.5

1.0

1.5

2.0

2.5

200 300 400 500 600 700 800

Ab

sorb

ance

Wavelength (nm)

9080

0

0.05

0.1

0.15

0.2

200 300 400 500 600 700 800

Ab

sorb

ance

Wavelength (nm)

9080

Reaction temp (°C)Reaction temp (°C)

PEG 100 PEG 2

(b)(a)Fig. 9 UV/Vis spectra of

silver colloidal solutions

prepared at 80 and 90 �C

with a PEG 2 and b PEG

100

J Nanopart Res

123

increased with the reaction time. At the initial stage,

the peak was blue-shifted, but as the reaction time was

increased toward 3 h, the peak positions remained

constant. This stability differentiates this reaction

from other solution-phase synthetic methods wherein

a red shift or blue shift was observed when the

reaction time was extended. Pastoriza-Santos and

Liz-Marzan (2002a) observed a red shift when the

reduction reaction, which was performed in N,N-

dimethylformamide containing PVP at reflux, was not

controlled. As a result, particle agglomeration and

large polydispersity in particle size were observed.

Slistan-Grijalva et al. (2005, 2008) reported a blue

shift resulting from the incomplete reduction of silver

ions when reducing silver ions with ethylene glycol.

On the other hand, Bo et al. (2009), who performed

the reduction reaction in an aqueous PEG 2 solution,

reported that the plasmon absorption peak red-shifted

and then disappeared when the reaction time was

extended over 2 h. As can be seen in Fig. 12, the

reaction in an aqueous PEG 100 solution was stable,

so that the size of particles remained relatively

constant throughout the reaction.

Conclusions

This study has demonstrated that the conformation of

PEG, which acts as both a reducing and stabilizing

agent, significantly influences the formation of silver

NPs in water. Silver NPs (3.68 ± 1.03 nm) were

readily produced by the entangled PEG 100, but very

low concentrations of silver NPs were obtained by the

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 100

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10

20 nm 20 nm(a) 70°C (b) 90°C

No

. of

par

ticl

es

Particle diameter (nm)

No

. of

par

ticl

es

Particle diameter (nm)

Fig. 10 TEM micrographs

and histograms of silver

NPs prepared in an aqueous

PEG 100 solution at

a 70 �C and b 90 �C for

2 h. The average sizes of

silver NPs were determined

to be 3.28 ± 1.02 nm at

70 �C and 5.17 ± 1.29 nm

at 90 �C

0

0.5

1

1.5

2

2.5

200 300 400 500 600 700 800

Ab

sorb

ance

Wavelength (nm)

3

2

1

0.5

0

Reaction time (h)

3 h

0 h

Fig. 11 UV/Vis spectra of silver colloidal solutions prepared

by PEG 100 at different reaction times. Zero reaction time

means when the addition of AgNO3 was finished

J Nanopart Res

123

single-coiled PEG 2 and PEG 35. In an aqueous

solution, where silver ions and PEG are stabilized by

water molecules, the entangled conformation cages

silver ions to increase the interaction of silver ions with

PEG and facilitate the reduction reaction. The formed

NPs were effectively stabilized in the entangled

network. The reaction in an aqueous PEG solution

was stable to maintain the formation of uniform and

monodispersed particles at an increased reaction

temperature or reaction time. Such stable reaction

could not be achieved in the PEG 2 solution. The

obtained silver NPs are small enough that they are

expected to be feasibly incorporated into the micro-

structure of natural fibers as well as into polymers for

providing good antimicrobial properties. A further

study on increasing the reducing sites by blending low-

molecular-weight PEGs with PEG 100 is underway.

Acknowledgments We are grateful to Dr. Matthew Tarr at

the University of New Orleans for the access to the UV/Vis/

NIR spectrometer and to Dr. Jibao He at Tulane University for

his assistance with TEM. We also thank Dr. Ryan Slopek at the

Southern Regional Research Center, Dr. Marcus Foston at

Georgia Institute of Technology, and Cara Cotter at the

University of New Orleans for their reviews.

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0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3

Par

ticl

e d

iam

eter

(n

m)

Reaction time (h)

(a) 0 h (b) 1 h

(c) 3 h

Fig. 12 TEM micrographs

and the average diameters

of silver NPs prepared in an

aqueous PEG 100 solution

with different reaction

times. The scale bar is

20 nm

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