Synthesis and Characterization of Starch Derivative – Silver

Nanoparticle Glyconanospheres

Hu Chen1 and Chin Wee Shong

2

Department of Chemistry, National University of Singapore,

3, Science Drive 3, Lower Kent Ridge Road, Singapore 117543

ABSTRACT

Silver nanoparticles (NPs) were synthesized by reduction using sodium borohydride,

with cysteine as capping agent. The product was characterized with TEM, UV and IR

spectroscopy. The silver NPs were then aggregated with starch modified with

chloroacetic acid to generate glyconanospheres – nanosphere aggregated with layers of

starch. The product was analyzed with TEM and IR. The result proved that the synthetic

route was feasible and reproducible results were obtained. Several experimental

parameters, such as pH during silver NP synthesis, the role of unmodified starch during

aggregation processes, etc. can be further studied to increase the stability of the particles and may simplify the reaction procedure. The product was speculated for uses in field

such as bioimaging, especially in surface enhanced Raman spectroscopy (SERS).

INTRODUCTION

Due to the high surface energy of nanometer-sized particles, to cap the surface of such

particles using appropriate molecules is vital in maintaining stability of such molecules

[1, 2]. In this experiment, a new method of forming silver NPs and subsequent aggregates

of the silver NPs termed glyconanospheres [3] is devised based on electrostatic polyionic

layer interactions [4]. The silver particles are synthesized with cysteine as capping agent,

which can be protonated to carry a layer of positive charge. Starch is then modified with

chloroacetic acid which converts some –OH group to –CH2COOH group. Under

appropriate pH conditions, high negative charge density will then build up on the

modified starch, and electrostatical aggregation between cysteine capped silver NPs and

modified starch can occur, forming the glyconanospheres.

MATERIALS AND METHODS

AgNO3, NaBH4, L-cysteine, starch, chloroacetic acid and sodium citrate were all

purchased from Alfa Aesar. Distilled water was used for all purposes during the

synthesis. Varian 3100 FTIR spectrometer was used for IR spectrometry, Shimadzu

UV3600 spectrophotometer was utilized for UV spectroscopy, and JOEL 2100

transmission electron microscope was used for TEM.

EXPERIMENTAL PROCEDURE

1 Student 2 Associate Professor

Ag NPs were synthesized by slowly introducing 5 ml of 0.200M NaBH4 solution into

15 ml mixture of 0.010M AgNO3 and cysteine. The amounts of cysteine used for each

batches of synthesis were:

Table 1: amount of cysteine added during synthesis Ratio (cysteine:AgNO3), mol / mol 1:1 2:1 5:1 10:1

Amount of cysteine added / g 0.0182 0.0364 0.0910 0.182

The reaction was allowed to proceed for 20 to 30 minutes, which colour changed

rapidly to dark red or black, depending on the cysteine-to-silver ratio. The product was

separated by precipitation with 1× volume of acetone and centrifugation at 4000 ppm for

1 minute. Progress of reaction was tracked by UV spectroscopy, and the products were

characterized by both IR spectroscopy and TEM.

Starch modification followed the method suggested by R. Rebizak, et. al [5]; 100 mg

starch was mixed with 200 mg chloroacetic acid in 6N NaOH solution and heated for ½

hour. The fibre was precipitated out by 2× volume of methanol followed by centrifuging

at 4000 ppm for 1 minute. The Ag NPs were dispersed in water and mixed with starch,

the latter resuspended in pH 4.5 HCl acid-citrate buffer solution to maintain the surface

charge density. The final product was collected by centrifuging the mixture at 4000 ppm

for 1 minute as well. The product was characterized by IR spectroscopy and TEM.

RESULTS AND DISCUSSION

Above UV spectra [figure 1, left] showed that as ratio of cysteine used decreased, the

surface plasmon resonance (SPR) peak increased in sharpness, which was a sign for the

formation of well-dispersed nanoparticles [5]. Hence, formation of Ag NPs of very small

dimensions was best carried out at low cysteine ratio. Based on experiment results, the

ratio of 10:1:1 gave finely dispersed Ag NPs of very small dimensions (see TEM image

in figure 3 below).

On the other hand, at higher concentrations, the Ag NPs formed were observed to be

unstable, generally degrading after 1 hour of preparation. The hypothesis was that at

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

300 500 700

10:10:1,

t=30 min

10:1:1,

t=20 min

10:5:1,

t=20 min

0

0.2

0.4

0.6

0.8

300 400 500 600 700 800

t=5 min

t=10 min

t=20 min

t=30 min

Figure 1: UV spectra of Silver nanoparticles. Left: different ratios of NaBH4:

Cysteine: AgNO3 used during synthesis; Right: NaOH solution introduced, with

reagent ratio 10:2:1.

higher cysteine concentrations, the solution

particles. This was confirmed by carrying out the reaction with 5

introduced into the reaction mixture

found to be more stable,

sharp SPR peak near 500nm

For the silver NPs, The IR spectra obtained [

bond was formed, marked by the

is due to the thiol group. This peak was not present

NPs, which was due to the formation of sulfur

bonds.

The TEM image obtained

synthesized are of fine size of 3~ 5nm and most of them are regularly shaped

some irregularities were present as well

peaks in UV spectrum of samples.

for synthesized glyconanospheres showed

produced, with average size of 100 nm in diameter

starch was used instead, the product particles vary drastically in size from 1.0

50nm in diameter (TEM image not shown)

-10

10

30

50

70

90

300 800

Figure 2: IR spectra of

Figure 3: TEM images of

Cysteine-capped silver NPs

– modified starch Glyconanospheres

higher cysteine concentrations, the solution was acidic, resulted in digestion of the silver

s confirmed by carrying out the reaction with 5 ml 1.0N NaOH

the reaction mixture of ratio 10:2:1, the product silver NPs

found to be more stable, and apparently smaller NPs were generated

500nm [figure 1, right].

silver NPs, The IR spectra obtained [figure 2] indicated clearly that thiol

s formed, marked by the disappearance of 2500 cm-1 peak in free cysteine

. This peak was not present in the IR spectrum obtained for silver

s due to the formation of sulfur-silver (S-Ag) bond replacing the thiol S

The TEM image obtained for Ag NPs [figure 3, left] showed that the particles

synthesized are of fine size of 3~ 5nm and most of them are regularly shaped

were present as well. This confirmed the observation of sharp SPR

peaks in UV spectrum of samples. TEM images [figure 3, right; other images not shown

or synthesized glyconanospheres showed that regularly shaped gly

e size of 100 nm in diameter. On the other hand, when unmodified

starch was used instead, the product particles vary drastically in size from 1.0

(TEM image not shown).

1300 1800 2300 2800 3300

Figure 2: IR spectra of Silver NP compared with cysteine.

Figure 3: TEM images of Ag NPs and glyconanospheres

capped silver NPs, bar showing 20 nm. Right: Single Silver

modified starch Glyconanospheres, bar showing 50 nm.

digestion of the silver

ml 1.0N NaOH solution

silver NPs formed were

re generated, judging from the

clearly that thiol-silver

in free cysteine which

in the IR spectrum obtained for silver

Ag) bond replacing the thiol S-H

showed that the particles

synthesized are of fine size of 3~ 5nm and most of them are regularly shaped, though

This confirmed the observation of sharp SPR

; other images not shown]

that regularly shaped glyconanospheres are

the other hand, when unmodified

starch was used instead, the product particles vary drastically in size from 1.0 µm to

3300 3800

silver

cysteine

Ag NPs and glyconanospheres. Left:

le Silver

IR spectra for both silver glyconanospheres synthesized with modified starch and

unmodified starch were obtained [figure 4]. Modified starch was not used as one of the

samples since it aggregated when dried and was unable to grind into fine powder for KBr

pellets. The only conclusion can be drawn was, therefore, that with unmodified starch,

the degree of binding was not significant since peak shifts were not observed.

CONCLUSION

In conclusion, a reproducible method for synthesizing cysteine-coated silver NPs and

glyconanosphere formation using above NP with modified starch is confirmed. The

product generated is of 50nm in diameter and is relatively stable.

It is speculated that these glyconanospheres, because of the starch wrapping, would be

bio-compatible and is possible for applications in fields such as biological imaging. One

possible application is in SERS, which showed promising analytical power of detecting

single molecules [6]. The glyconanospheres could enhance the Raman spectrum of target

molecules, as the silver NP aggregates can act both as the substrate and the surface.

ACKNOWLEDGEMENT

Thanks are due to Assoc. Professor Chin Wee Shong and Ms. Xu Hairuo for their

guidance and support throughout this project.

REFERENCES

[1] (a) G. B. Sergeev, Nanochemistry, Elsevier, 2006; (b) R. Levy, N. T. K. Thanh, et. al,

J. Am. Chem. Soc., 2004, 126, 10076-10084

[2] Y. Chen, T. Ji, and Z. Rosenzweig, Nano Letters, 2003 Vol. 3, No. 5, 581-584

[3] Gero Decher, Yuri Lvov and Johannes Schmitt, Thin Solid Films, 244 (1994) 772-777

[4] Richard Rebizak, Michel Schaefer, and Edith Dellacherie, Bioconjugate Chem, 1997,

8, 605-610

[5] Mie, G., Ann. Physik, 25, 377 (1908)

[6] (a) S. Pande, S. K. Ghosh, et. al, J. Phys. Chem. C, 2007, 111, 10806-10813; (b)

Kwan Kim and J. K. Yoon, J. Phys. Chem. B 2005, 109, 20731-20736

-5

0

5

10

15

20

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Ag + Mod.

Starch

Ag+starch

starch

Figure 4: IR spectra for glyconanospheres prepared with modified and

unmodified starch. Free starch is included as reference.