- Email: [email protected]
Rapid Synthesis of Size-controlled Gold Nanoparticles by Complex Intramolecular Photoreduction
Available online at www.sciencedirect.com
CHEM. RES. CHINESE U. 2007, 23 ( 5 ) , 500-504 Article ID 1005-9040( 2007) -05-500-05
,
+ ScienceDirect
Rapid Synthesis of Size-controlled Gold Nanoparticles by Complex Intramolecular Photoreduction* DONG Shou-an' * , YANG Sheng-chun and TANG Chun Kunming Institute of Precious Metals, Kunming 650221 , P . R. China Received Dec. 12, 2006 A rapid synthesis of size-controlled gold nanoparticles was proposed. The method is based on the sensitive intramolecular photoreduction reaction of Fe ( ) -EDTA complex in chloroacetic acid-sodium acetate buffer solution,
n
where Fe( ) -EDTA complex generated by photo-promotion acts as a reductant of AuCl; ions. Gold nanoparticles formed were stabilized by EDTA ligand or other protective agents added. As a result, well-dispersed gold nanoparticles with an average diameter range of 6.7 to 50.9 nm were obtained. According to the characterizations by the UV spectrum and TEM , the intramolecular charge transfer of the excited states of complex Fe( I5 ) -EDTA and the mechanism of forming gold nanoparticles were discussed in detail. Keywords Gold nanoparticle ; Synthesis; Intramolecular photoreduction ; Fe( ) -EDTA complex
m
Introduction Gold nanoparticles or monolayer-protected clusters by small organic molecules or macromolecules have attracted considerable attention because of their unique properties of physical chemistry and potential applications in many fields, such as optics, electronics, catalysis and the preparation of biosensors""]. The properties of gold nanoparticles are dependent on the size, shape and size distribution. However, the preparation of gold particles with monodisperse and narrower size distributions is still a formidable task in the nano-chemical material field. In recent years, numerous liquid phase chemistry methods have been developed to prepare gold particles with different sizes and shapes, including the use of chemical reductants in both aqueous and nonaqueous ~olvents[~-'~] , electrochemical methods"*-201 , sonochemical methodst2' 9221 , photochemical method^[^^-^'] and so on. Among them, the photochemical reduction method for size-controlled synthesis of gold nanoparticles has some important advantages. They are as follows: ( 1 ) controlled reduction of metal ions can be carried out without using excess reducing agent and no adsorbed contamination on the product occurs in the preparation process; ( 2 ) the radiation is absorbed regardless of the presence of light-absorbing solutes and products and the reduction reaction can take place uniformly in the solution; ( 3 ) from the point of view of practical applications, the photochemical method can be a cost-effective and convenient technique. However, to date, it has been well-known that all photochemical
preparations of gold nanoparticles were carried out by the direct photo-reduction of gold compound in s o h tion. A novel method of synthesizing gold nanoparticles by complex intramolecular photoreduction to generate reducing agent is described. It is based on a sensitive UV photochemical reduction reaction of complex Fe( ) -EDTA in chloroacetic acid-sodium acetate buffer solution. Owing to the lower reduction potential of system Fe ( JII ) -EDTA/Fe ( II ) -EDTA and higher oxidation potential of Au( )/Au ( 0 ) couple[329331 ,a rapid redox reaction can occur between Fe ( JJ ) -EDTA complex generated by photo-promotion and Au( JII) ion in the solution. As a result, better dispersed gold nanoparticles formed were stabilized by the complexant. In the presence of foreign protective agents, such as citrate, poly( ethylene glycol) (PEG) or poly( vinylpyrrolidone) ( PVP) , gold nanoparticles with different sizes can also be prepared. According to the characterizations by the UV spectrum and TEM image, the intramolecular charge transfer of complex Fe ( a ) -EDTA and the mechanism of forming gold nanoparticles were discussed in detail. Compared with conventional chemical preparation of gold nanoparticles , the method proposed is simple, rapid and convenient. Additionally, it is easy to develop a method of preparing gold/Fe,O, catalyst in situ by forming smaller gold nanoparticles.
m
Experimental 1 Materials HAuC1, was prepared by dissolving pure gold (99. 99% purify) in aqua regia and removing HNO, in
* Supported by the Natural Science Foundation of Yunnan Province, China( No. 2000E0008Z) * * To whom correspondence should be addressed. E-mail : dsaw@ xinhuanet. corn
No. 5
DONG Shou-an et al.
the mixture solution with HC1. The concentration of Au( lU ) ions in the solution is 6.00 mg/mL. A 2 x moVL Fe ( III )-EDTA complex buffer solution ( p H = 4 ) was prepared by dissolving a certain amount of Fe,(SO,), and EDTA in 0.02 moVL (ClCH,COOH-NaAc) solution and was stored in the dark. The working solutions of HAuCl, and F e ( EDTA complex were obtained by diluting the solutions to 10 times volumes with water, respectively. All chemical reagents used were of analytical reagent grade and purchased from Shanghai Reagent Ltd. Co. Double distilled water was used for all the experiments. All glassware was rigorously cleaned with chromic acid solution and was rinsed in turn with distilled water before use.
m)-
2 Preparation of Gold Nanoparticles Working solution of the complex Fe( JlI ) -EDTA of 20 mL and 2 mL of 0.60 mg/mL Au( fl) solution were added to a volumetric flask of 25 mL and diluted to the mark with water. The concentration of Au ( lJ.l ) ions was 2.44 x moVL: After shaking vigorously, the mixture solution was transferred to a 250-mL triangle quartz container and was irradiated with 254 nm UVlight ( Transilluminator light source, 48 W , Ultra-lum Co. ) in an exposed surface area of 50 cm2 and a distance of 4 cm between the solution and light source. The solution was lightly shaken in the irradiation process. The UV-Vis spectrum curves of the solution with irradiation were recorded on a Lambda-900UV/Vis/Infrared spectrophotometer( Perkin-Elmer) , using a 1-mL quartz cuvette. After complete reaction, the electron micrographs of the gold nanoparticles were taken with a JEM-2000EX TEM operated at 200 kV. TEM specimens were prepared by placing microdrops of gold sol on a carbon film supported by copper grids. Approximately 200 nanoparticles from TEM image of each sample were measured for particle sizes and size distribution. For the syntheses of gold nanoparticles in the presence of citrate or soluble polymers, such as PEG and PVP, similar procedure was used. The concentration of the protective agent in the mixture solution was 2.0 mL 10 mg/mL of citrate , 1 mL of PEG400 and 1. 0 mL of 80 mg/mL PVP, respectively.
Results and Discussion 1 Photochemical Reduction of Fe ( III ) -EDTA Complex The timer-resolved UV absorption spectrum of
501
Fe( ) -EDTA complex with 254 nm UV irradiation is shown in Fig. 1. Before irradiation, Fe ( III )-EDTA complex exhibits a UV absorption maximum peak at 256.77 nm. The absorption intensity of the peak gradually decreased with increased irradiation time. Finall y , the absorption curve coincided with the UV absorption curve of the pre-prepared Fe( II ) -EDTA complex. 1.5 R
5
.$ 1.0 a,
H
%
2
4
0.5
0.0 200
Fig. 1
250
300 350 Wavelengthhm
400
UV absorption spectra of Fe( HI) -EDTA complex at different irradiation time Dash line is the curve for Fe ( II )-EDTA complex. a. 0 ; b. 30; c. 60; d. 90; e. 120; f. 150; g. 180; h. 210; i. 240; j . 270. t/s:
Obviously, Fe( JJI ) -EDTA complex molecules absorbed photon energy and became excited states, after undergoing the local charge transfer, and were reduced to Fe( fl ) -EDTA complex molecules. A similar change in the absorption spectrum was also observed under 300 or 365 nm UV irradiation. However, the rate of the photochemical reduction distinctly decreased because of lower radiation energy. In order to obtain rapid photochemical reduction reaction, the UV irradiation with 254 nm wavelength was used for all experiments.
2 Preparation and Characterization of Gold Particles When HAuCl, -Fe ( ) -EDTA complex mixture solution was irradiated by UV light, the generated
m
Fe( fl ) -EDTA complex which has a stronger reducing ability reduced Au ( JlI ) ions rapidly to Au ( 0 ) . The UV-Vis absorption spectrum curves shown in Fig. 2( A ) indicate that after 60 s irradiation, the surface plasmon resonance ( SPR) absorption band of gold nanoparticles appeared at 536 nm with the disappearance of the ligand-to-metal charge transfer ( LMCT) band of the AuC14- at ca. 310 nm. After 90 s irradiation, the band shifted to 531 nm and its intensity did not change with further irradiation. This is an indication of the complete reduction reaction of Au ( JlI ) ions. The reaction of forming gold nanoparticles was so fast that no induction period was observed in the photochemical reaction process. Finally , nearly colorless mixture solution turned to mauve.
CHEM. RES. CHINESE U.
502
1.0 (A)
?
G
8
.e
0.5
P
2 0.0 300
500 700 Wavelengthhm
Fig. 2 W-Vis absorption spectra(A ) and TEM image( B ) of Au nanoparticles U S :a. 0; b. 30; C . 60; d. 90.
In the complex buffer solution, the formation of gold nanoparticles depends on two factors. They are the rate of the photochemical reaction of complex Fe ( B ) EDTA and the rate of the chemical reaction between Au( B ) ion and Fe( fl ) -EDTA complex generated by photo-promotion. On the basis of the lower reduction potential of Fe ( ) -EDTA/Fe ( II ) -EDTA couple and higher oxidation potential of Au ( )/Au ( 0 ) , a rapid redox reaction can occur between Fe ( II ) -EDTA generated by photo-promotion and Au ( ID ) ions. Therefore, the rate of the nucleation of gold was predominated by the former factor. After forming gold nanoparticles , they were stabilized by free EDTA in the buffer solution. The image of nanoparticles characterized by TEM is shown in Fig. 2 ( B ) . The average diameter is 29.7 nm. The gold particles colloidal solution lacked longtime stability. The gold particles deposited from the solution after three days. In order to improve dispersity and obtain smaller gold nanoparticles , we utilized citrate as the protective agent, because this reagent has been widely used for the preparation of gold nanoparticles with different size^[^-'^]. In th e presence of citrate, the evolution of UV-Vis spectrum of the mixture solution with irradiation time is given in Fig. 3. It can be seen from Fig. 3 that
Vol. 23
after 30 s irradiation, the SPR band appeared at 530 nm. With increasing irradiations, the band shifted gradually in short wavelength direction and the mixture solution turned rose-red. Finally, the time required for a complete photochemical reaction only was 90 s , A mBx was at 518.9 nm. TEM image of obtained gold nanoparticles is shown in Fig. 4, having an average diameter of 6. 7 nm and narrower size distributions. By varying the molar ratio of citrate to Au ( III ) or by adding 4. 0 mL 10 mg/mL of citrate to the mixture solution, it is possible to obtain gold nanoparticles of less than 5 nm diameter.
m
2.0
i
1.5
9 0
ki 1.0
2 3 c
0.5 0.0 200
400 600 Wavclengthhm
800
Fig. 3 UV-Vis absorption spectra with irradiation in the presence of citrate U s : a . 0 ; b. 30;
c.
60; d. 90.
2
4
6 8 1 Diameterhm
0
Fig. 4 TEM image( A ) and size distribution( B ) of Au nanoparticles in the presence of citrate
In order to further verify the adaptability of synthesizing gold nanoparticles by this photochemical method, the experiments were also carried out in the presence of some soluble polymer, such as PEG or PVP. The results show that the better dispersed gold nanoparticles with 46. 0 and 50.9 nm in average diameter could be obtained in the presence of the above-mentioned corresponding polymer.
3 Mechanisms of the Photochemical Reduction and Forming Gold Nanoparticles
m
When Fe ( ) -EDTA molecule with six coordination position is excited by UV light, according to the energy level of the molecular orbits ( MOs) and the types of photochemical reaction in transition metal organic complex[341, it is assumed that the electron to be excited generally belongs to a weaker binding orbital energy state, and therefore T bond electrons at 0 = C-0-Fe section in the complex molecule may have a higher excited probability. Thereupon, an intramolecular electron transfer can happen from a antibonding T * orbital to Fe ( III ) d orbits. This is a reverse process of electron transfer from metal ( M ) to ligand ( L ) , that is generally known as T * -d transition or CTLM. Simultaneously, Fe ( III ) center ion was reduced to Fe( fl ) ion. The intramolecular charge transfer of Fe (
)-
No. 5
DONG Shou-an et al. -
0
503
3
11\'
I -
AuCI;
H'
Au(0)
+
CICH;COOH(or H20)
Scheme 1 Illustration of intramolecular photoreduction mechanism
EDTA complex is illustrated in Scheme 1 and the process has been confirmed by the evolution of UV absorption spectrum of Fe( ) -EDTA complex( Fig. 1 ) . After generating Fe( Iz )-EDTA complex, a rapid redox reaction can occur between this complex and Au( ions because of their potential difference of electrode c o ~ p l e ' ~. ~In , ~ a~ ]single complex solution
were stabilized by free EDTA. In the presence of citrate the mechanism for forming smaller gold nanoparticles is quite complicated, because the free radical reactions can also lead to nucleation besides the nucleation by generating the complex reaction. On the basis of our very recent study and test ~erification'~'] , the following reactions could occur :
m
m)
system, gold particles coalesced from Au ( 0 ) atoms hu
H,O-H' H, C-C
+'OH H, C-COO
00-
I HO-C-COO' I
+ ' H ( 'OH)-
HO-C-COO-
I
H,C-COO-
I I
---+
HO-C
I
I
I
-
+Au(
m)*
I I
-----+O
H,C-COOH, C-C
I O=C I
I I
+ CO,
*
3HO-C'
I
+ Au( 0 )
I
H,C-COO H,C-COO-
H, C-COOH, C-COOHO-C'
+H,(H,O)
=C
I I
+Hi
H, C-COO-
00
H,C-COO-
(2)
(3)
H,C-COOH, C-COO
H,C-COOH, C-COO 3HO-C
-
H, C-COO H, C-COO-
H, C-COO
HO-C-COO'
(1)
+ Au( f l ) - - - t A u (
0)
+ HCOOH + C H 2 0 + CO2
(4)
CHEM. RES. CHINESE U.
5 04
Cao Y. W. C. , Jin R. , Mirkin C. A. , Science. 2002, 297,
Precisely these free radical reactions and indirect photochemical reduction made the nucleation rate far and away larger than nanocrystal growth rate, so that smaller particles with narrow size distrihution were formed and stabilized by citrate protective agent. However, for the mechanism of stabilizing gold nanoparticles in the presence of PEG or PVP, the steric hindrance effect of the polymer macromolecule with an amphiphilic structure is still a main factor. The difference is that the protective action of PEG is mainly ascribed to forming a pseudocrown ether structure”‘” , whereas for PVP the affinity between nanoparticle and both the C-N bond and carbonyl group on pyrrole ring is one of the important reasons of stabilizing action‘16’.
1536 Maxwell D. J. , Taylor J. R. , Nie S. , J. Am. Chem. Soc. ,
2002, 124, 9606 E’rens C:. , Nature: Physical Science, 1973, 241 , 20 Jana N. R . , Gearheart L. , Murphy C. , J. Chem. Mazer. , 2001, 1 3 , 2313 Hrown K. R. , Walter D. 6. , Natan M. , J. Chem. Mater. , 2000, 1 2 , 306 Zhu T. , Vasilev K. , Kreiter M. , et al. , Langmuir, 2003, 1 9 , 9518 Pei L. H. , Mori K. , Adachi M. , fangmuir, 2004, 2 0 , 7837 16n .I. , Zhou W . , Charles J . 0. , Materials Ixtters, 2001, 4 9 , 282 Esuini K.
m
m
References Conipton 1). , Cornish I.. , van iier Lingen E. , Cold Hu.11. , 2003,36, 10.51 [ 2 ] Gittins D. I. , Rethrll D. , Schiffrin D. J. , et al. , Nuture, 2000, 408, 67 [ 3 1 Haruta M . , Cold Bull. , 2004, 37( l / 2 ) , 27 4 ] Hutching 6. .I. , (;old Hull. , 2004, 37( 1/2) , 3 [ 5 ] Taton T. A. , Mirkin C. A. , Letsinger R. 1.. , Science, 2000, 289, 1757 [ 6 1 Park S . J. , Taton T. A. , Mirkin, C. A. , Science, 2002, 2Y5,
[ I ]
1503
, Suzuki A. , Aihara N . ,
et al.
, Langmuir, 2000,
16, 2604, 2978
‘reranishi T. , Kiyokawa I. , Mikio M. , Adu. Mater. , 1998, 1 0 ( 8 ) , 596 Jana N. R. , Gearheart I,. , Murphy C. J. , J. Phys. Chem. B ,
Conclusions This article describes a new method of rapidly synthesizing gold nanoparticles by intramolecular photoreduction of complex Fe( ) -EDTA to generate complex Fe( II ) -EDTA as the reductant. The results indicate the following: ( I ) gold nanoparticles formed can be stabilized by free EDTA in single complex system; ( 2 ) in the presence of foreign protective agent added, such as citrate , PEG or PVP, gold nanoparticles with an average diameter of 6 . 7 to 50.9 nm were generated in the complex solution, indicating very good adaptability of this indirect photochemical reduction method ; ( 3 ) the mechanisms of electron transfer for the excited states of complex Fe ( ) -EDTA and forming gold nanoparticles were discussed in detail. Obviously, complex Fe ( ) -EDTA is only a medium of photoelectron transition in the process of forming gold nanoparticles.
Vol. 23
1221
2001, 105, 4065 van der Zande B. M. I. , Koper G. J. M. , Lekkerkrrker H. N. W . , J . Phys. Chem. B , 1999, 103, 5154 van der Zande B. M . I. , Bohmer M. R . , Fokkin L. 6. J . , et al. , Langmuir, 2000, 16, 451 Chang S. S. , Shih C. W. , Chert C. D. , et al. , Lnngmuir, 1999, I S , 701 Caruso K. A. , Ashokkumar M . , Criesrr F. , Langmuir, 2002, 18, 7831 Su C. H . , Wu P. L. Yeh C. S. , J. I’hys. Chem. R , 2003,
“23 1
107, 14240 Han M. Y. , Quek C. H. , f.angmuir, 2000, 1 6 , 362
r241
Kim F. , Song J. H. , Yang P. I). , J . iim. Chem. Soc. , 2002, 124, 14316 Mandal M. , Ghosh S. K. , Kundu S. , et al. , hng-muir, 2002, 18, 7792
Mallik K. , Mandal M.
,
Pradlian N. , et al.
,
Nano Letters,
2001, 1 , 3 1 9 Mallick K. , Wang Z. L. , Pal T. , J. I’hotochem.
Photohio. A : Chern. , 2001, 140, 75 Dong S. A. , Tang C. , Trans. Nonferrous Met. Soc. China, 2005, 15, 1 Yang S. C. , Dong S. A. , Tang C. , et al. , Acta Chimica Sinic a , 2005, 63(10), 873 1301 Dong S. A. , Tang C. , Zhou H. , et al. , Gold Hull. , 2004,
37, 187 Wu H. C. , Dong S. A. , Dong Y . N. , et al. , Chern. J. Chinese Universities, 2007 , 28 ( 1 ) , 10 Dong S. A. , Wu C. R. , Precious Metals, 2003, 24, 30 Dong S. A. , Proceeding oJ IPM1’99, 1999, 85 Rohatgi-Mukherjee K. K. , Fundamentals of Photochemistry, Johu Wiley & Sons, New Delhi, 1978
CHEM. RES. CHINESE U. 2007, 23 ( 5 ) , 500-504 Article ID 1005-9040( 2007) -05-500-05
,
+ ScienceDirect
Rapid Synthesis of Size-controlled Gold Nanoparticles by Complex Intramolecular Photoreduction* DONG Shou-an' * , YANG Sheng-chun and TANG Chun Kunming Institute of Precious Metals, Kunming 650221 , P . R. China Received Dec. 12, 2006 A rapid synthesis of size-controlled gold nanoparticles was proposed. The method is based on the sensitive intramolecular photoreduction reaction of Fe ( ) -EDTA complex in chloroacetic acid-sodium acetate buffer solution,
n
where Fe( ) -EDTA complex generated by photo-promotion acts as a reductant of AuCl; ions. Gold nanoparticles formed were stabilized by EDTA ligand or other protective agents added. As a result, well-dispersed gold nanoparticles with an average diameter range of 6.7 to 50.9 nm were obtained. According to the characterizations by the UV spectrum and TEM , the intramolecular charge transfer of the excited states of complex Fe( I5 ) -EDTA and the mechanism of forming gold nanoparticles were discussed in detail. Keywords Gold nanoparticle ; Synthesis; Intramolecular photoreduction ; Fe( ) -EDTA complex
m
Introduction Gold nanoparticles or monolayer-protected clusters by small organic molecules or macromolecules have attracted considerable attention because of their unique properties of physical chemistry and potential applications in many fields, such as optics, electronics, catalysis and the preparation of biosensors""]. The properties of gold nanoparticles are dependent on the size, shape and size distribution. However, the preparation of gold particles with monodisperse and narrower size distributions is still a formidable task in the nano-chemical material field. In recent years, numerous liquid phase chemistry methods have been developed to prepare gold particles with different sizes and shapes, including the use of chemical reductants in both aqueous and nonaqueous ~olvents[~-'~] , electrochemical methods"*-201 , sonochemical methodst2' 9221 , photochemical method^[^^-^'] and so on. Among them, the photochemical reduction method for size-controlled synthesis of gold nanoparticles has some important advantages. They are as follows: ( 1 ) controlled reduction of metal ions can be carried out without using excess reducing agent and no adsorbed contamination on the product occurs in the preparation process; ( 2 ) the radiation is absorbed regardless of the presence of light-absorbing solutes and products and the reduction reaction can take place uniformly in the solution; ( 3 ) from the point of view of practical applications, the photochemical method can be a cost-effective and convenient technique. However, to date, it has been well-known that all photochemical
preparations of gold nanoparticles were carried out by the direct photo-reduction of gold compound in s o h tion. A novel method of synthesizing gold nanoparticles by complex intramolecular photoreduction to generate reducing agent is described. It is based on a sensitive UV photochemical reduction reaction of complex Fe( ) -EDTA in chloroacetic acid-sodium acetate buffer solution. Owing to the lower reduction potential of system Fe ( JII ) -EDTA/Fe ( II ) -EDTA and higher oxidation potential of Au( )/Au ( 0 ) couple[329331 ,a rapid redox reaction can occur between Fe ( JJ ) -EDTA complex generated by photo-promotion and Au( JII) ion in the solution. As a result, better dispersed gold nanoparticles formed were stabilized by the complexant. In the presence of foreign protective agents, such as citrate, poly( ethylene glycol) (PEG) or poly( vinylpyrrolidone) ( PVP) , gold nanoparticles with different sizes can also be prepared. According to the characterizations by the UV spectrum and TEM image, the intramolecular charge transfer of complex Fe ( a ) -EDTA and the mechanism of forming gold nanoparticles were discussed in detail. Compared with conventional chemical preparation of gold nanoparticles , the method proposed is simple, rapid and convenient. Additionally, it is easy to develop a method of preparing gold/Fe,O, catalyst in situ by forming smaller gold nanoparticles.
m
Experimental 1 Materials HAuC1, was prepared by dissolving pure gold (99. 99% purify) in aqua regia and removing HNO, in
* Supported by the Natural Science Foundation of Yunnan Province, China( No. 2000E0008Z) * * To whom correspondence should be addressed. E-mail : dsaw@ xinhuanet. corn
No. 5
DONG Shou-an et al.
the mixture solution with HC1. The concentration of Au( lU ) ions in the solution is 6.00 mg/mL. A 2 x moVL Fe ( III )-EDTA complex buffer solution ( p H = 4 ) was prepared by dissolving a certain amount of Fe,(SO,), and EDTA in 0.02 moVL (ClCH,COOH-NaAc) solution and was stored in the dark. The working solutions of HAuCl, and F e ( EDTA complex were obtained by diluting the solutions to 10 times volumes with water, respectively. All chemical reagents used were of analytical reagent grade and purchased from Shanghai Reagent Ltd. Co. Double distilled water was used for all the experiments. All glassware was rigorously cleaned with chromic acid solution and was rinsed in turn with distilled water before use.
m)-
2 Preparation of Gold Nanoparticles Working solution of the complex Fe( JlI ) -EDTA of 20 mL and 2 mL of 0.60 mg/mL Au( fl) solution were added to a volumetric flask of 25 mL and diluted to the mark with water. The concentration of Au ( lJ.l ) ions was 2.44 x moVL: After shaking vigorously, the mixture solution was transferred to a 250-mL triangle quartz container and was irradiated with 254 nm UVlight ( Transilluminator light source, 48 W , Ultra-lum Co. ) in an exposed surface area of 50 cm2 and a distance of 4 cm between the solution and light source. The solution was lightly shaken in the irradiation process. The UV-Vis spectrum curves of the solution with irradiation were recorded on a Lambda-900UV/Vis/Infrared spectrophotometer( Perkin-Elmer) , using a 1-mL quartz cuvette. After complete reaction, the electron micrographs of the gold nanoparticles were taken with a JEM-2000EX TEM operated at 200 kV. TEM specimens were prepared by placing microdrops of gold sol on a carbon film supported by copper grids. Approximately 200 nanoparticles from TEM image of each sample were measured for particle sizes and size distribution. For the syntheses of gold nanoparticles in the presence of citrate or soluble polymers, such as PEG and PVP, similar procedure was used. The concentration of the protective agent in the mixture solution was 2.0 mL 10 mg/mL of citrate , 1 mL of PEG400 and 1. 0 mL of 80 mg/mL PVP, respectively.
Results and Discussion 1 Photochemical Reduction of Fe ( III ) -EDTA Complex The timer-resolved UV absorption spectrum of
501
Fe( ) -EDTA complex with 254 nm UV irradiation is shown in Fig. 1. Before irradiation, Fe ( III )-EDTA complex exhibits a UV absorption maximum peak at 256.77 nm. The absorption intensity of the peak gradually decreased with increased irradiation time. Finall y , the absorption curve coincided with the UV absorption curve of the pre-prepared Fe( II ) -EDTA complex. 1.5 R
5
.$ 1.0 a,
H
%
2
4
0.5
0.0 200
Fig. 1
250
300 350 Wavelengthhm
400
UV absorption spectra of Fe( HI) -EDTA complex at different irradiation time Dash line is the curve for Fe ( II )-EDTA complex. a. 0 ; b. 30; c. 60; d. 90; e. 120; f. 150; g. 180; h. 210; i. 240; j . 270. t/s:
Obviously, Fe( JJI ) -EDTA complex molecules absorbed photon energy and became excited states, after undergoing the local charge transfer, and were reduced to Fe( fl ) -EDTA complex molecules. A similar change in the absorption spectrum was also observed under 300 or 365 nm UV irradiation. However, the rate of the photochemical reduction distinctly decreased because of lower radiation energy. In order to obtain rapid photochemical reduction reaction, the UV irradiation with 254 nm wavelength was used for all experiments.
2 Preparation and Characterization of Gold Particles When HAuCl, -Fe ( ) -EDTA complex mixture solution was irradiated by UV light, the generated
m
Fe( fl ) -EDTA complex which has a stronger reducing ability reduced Au ( JlI ) ions rapidly to Au ( 0 ) . The UV-Vis absorption spectrum curves shown in Fig. 2( A ) indicate that after 60 s irradiation, the surface plasmon resonance ( SPR) absorption band of gold nanoparticles appeared at 536 nm with the disappearance of the ligand-to-metal charge transfer ( LMCT) band of the AuC14- at ca. 310 nm. After 90 s irradiation, the band shifted to 531 nm and its intensity did not change with further irradiation. This is an indication of the complete reduction reaction of Au ( JlI ) ions. The reaction of forming gold nanoparticles was so fast that no induction period was observed in the photochemical reaction process. Finally , nearly colorless mixture solution turned to mauve.
CHEM. RES. CHINESE U.
502
1.0 (A)
?
G
8
.e
0.5
P
2 0.0 300
500 700 Wavelengthhm
Fig. 2 W-Vis absorption spectra(A ) and TEM image( B ) of Au nanoparticles U S :a. 0; b. 30; C . 60; d. 90.
In the complex buffer solution, the formation of gold nanoparticles depends on two factors. They are the rate of the photochemical reaction of complex Fe ( B ) EDTA and the rate of the chemical reaction between Au( B ) ion and Fe( fl ) -EDTA complex generated by photo-promotion. On the basis of the lower reduction potential of Fe ( ) -EDTA/Fe ( II ) -EDTA couple and higher oxidation potential of Au ( )/Au ( 0 ) , a rapid redox reaction can occur between Fe ( II ) -EDTA generated by photo-promotion and Au ( ID ) ions. Therefore, the rate of the nucleation of gold was predominated by the former factor. After forming gold nanoparticles , they were stabilized by free EDTA in the buffer solution. The image of nanoparticles characterized by TEM is shown in Fig. 2 ( B ) . The average diameter is 29.7 nm. The gold particles colloidal solution lacked longtime stability. The gold particles deposited from the solution after three days. In order to improve dispersity and obtain smaller gold nanoparticles , we utilized citrate as the protective agent, because this reagent has been widely used for the preparation of gold nanoparticles with different size^[^-'^]. In th e presence of citrate, the evolution of UV-Vis spectrum of the mixture solution with irradiation time is given in Fig. 3. It can be seen from Fig. 3 that
Vol. 23
after 30 s irradiation, the SPR band appeared at 530 nm. With increasing irradiations, the band shifted gradually in short wavelength direction and the mixture solution turned rose-red. Finally, the time required for a complete photochemical reaction only was 90 s , A mBx was at 518.9 nm. TEM image of obtained gold nanoparticles is shown in Fig. 4, having an average diameter of 6. 7 nm and narrower size distributions. By varying the molar ratio of citrate to Au ( III ) or by adding 4. 0 mL 10 mg/mL of citrate to the mixture solution, it is possible to obtain gold nanoparticles of less than 5 nm diameter.
m
2.0
i
1.5
9 0
ki 1.0
2 3 c
0.5 0.0 200
400 600 Wavclengthhm
800
Fig. 3 UV-Vis absorption spectra with irradiation in the presence of citrate U s : a . 0 ; b. 30;
c.
60; d. 90.
2
4
6 8 1 Diameterhm
0
Fig. 4 TEM image( A ) and size distribution( B ) of Au nanoparticles in the presence of citrate
In order to further verify the adaptability of synthesizing gold nanoparticles by this photochemical method, the experiments were also carried out in the presence of some soluble polymer, such as PEG or PVP. The results show that the better dispersed gold nanoparticles with 46. 0 and 50.9 nm in average diameter could be obtained in the presence of the above-mentioned corresponding polymer.
3 Mechanisms of the Photochemical Reduction and Forming Gold Nanoparticles
m
When Fe ( ) -EDTA molecule with six coordination position is excited by UV light, according to the energy level of the molecular orbits ( MOs) and the types of photochemical reaction in transition metal organic complex[341, it is assumed that the electron to be excited generally belongs to a weaker binding orbital energy state, and therefore T bond electrons at 0 = C-0-Fe section in the complex molecule may have a higher excited probability. Thereupon, an intramolecular electron transfer can happen from a antibonding T * orbital to Fe ( III ) d orbits. This is a reverse process of electron transfer from metal ( M ) to ligand ( L ) , that is generally known as T * -d transition or CTLM. Simultaneously, Fe ( III ) center ion was reduced to Fe( fl ) ion. The intramolecular charge transfer of Fe (
)-
No. 5
DONG Shou-an et al. -
0
503
3
11\'
I -
AuCI;
H'
Au(0)
+
CICH;COOH(or H20)
Scheme 1 Illustration of intramolecular photoreduction mechanism
EDTA complex is illustrated in Scheme 1 and the process has been confirmed by the evolution of UV absorption spectrum of Fe( ) -EDTA complex( Fig. 1 ) . After generating Fe( Iz )-EDTA complex, a rapid redox reaction can occur between this complex and Au( ions because of their potential difference of electrode c o ~ p l e ' ~. ~In , ~ a~ ]single complex solution
were stabilized by free EDTA. In the presence of citrate the mechanism for forming smaller gold nanoparticles is quite complicated, because the free radical reactions can also lead to nucleation besides the nucleation by generating the complex reaction. On the basis of our very recent study and test ~erification'~'] , the following reactions could occur :
m
m)
system, gold particles coalesced from Au ( 0 ) atoms hu
H,O-H' H, C-C
+'OH H, C-COO
00-
I HO-C-COO' I
+ ' H ( 'OH)-
HO-C-COO-
I
H,C-COO-
I I
---+
HO-C
I
I
I
-
+Au(
m)*
I I
-----+O
H,C-COOH, C-C
I O=C I
I I
+ CO,
*
3HO-C'
I
+ Au( 0 )
I
H,C-COO H,C-COO-
H, C-COOH, C-COOHO-C'
+H,(H,O)
=C
I I
+Hi
H, C-COO-
00
H,C-COO-
(2)
(3)
H,C-COOH, C-COO
H,C-COOH, C-COO 3HO-C
-
H, C-COO H, C-COO-
H, C-COO
HO-C-COO'
(1)
+ Au( f l ) - - - t A u (
0)
+ HCOOH + C H 2 0 + CO2
(4)
CHEM. RES. CHINESE U.
5 04
Cao Y. W. C. , Jin R. , Mirkin C. A. , Science. 2002, 297,
Precisely these free radical reactions and indirect photochemical reduction made the nucleation rate far and away larger than nanocrystal growth rate, so that smaller particles with narrow size distrihution were formed and stabilized by citrate protective agent. However, for the mechanism of stabilizing gold nanoparticles in the presence of PEG or PVP, the steric hindrance effect of the polymer macromolecule with an amphiphilic structure is still a main factor. The difference is that the protective action of PEG is mainly ascribed to forming a pseudocrown ether structure”‘” , whereas for PVP the affinity between nanoparticle and both the C-N bond and carbonyl group on pyrrole ring is one of the important reasons of stabilizing action‘16’.
1536 Maxwell D. J. , Taylor J. R. , Nie S. , J. Am. Chem. Soc. ,
2002, 124, 9606 E’rens C:. , Nature: Physical Science, 1973, 241 , 20 Jana N. R . , Gearheart L. , Murphy C. , J. Chem. Mazer. , 2001, 1 3 , 2313 Hrown K. R. , Walter D. 6. , Natan M. , J. Chem. Mater. , 2000, 1 2 , 306 Zhu T. , Vasilev K. , Kreiter M. , et al. , Langmuir, 2003, 1 9 , 9518 Pei L. H. , Mori K. , Adachi M. , fangmuir, 2004, 2 0 , 7837 16n .I. , Zhou W . , Charles J . 0. , Materials Ixtters, 2001, 4 9 , 282 Esuini K.
m
m
References Conipton 1). , Cornish I.. , van iier Lingen E. , Cold Hu.11. , 2003,36, 10.51 [ 2 ] Gittins D. I. , Rethrll D. , Schiffrin D. J. , et al. , Nuture, 2000, 408, 67 [ 3 1 Haruta M . , Cold Bull. , 2004, 37( l / 2 ) , 27 4 ] Hutching 6. .I. , (;old Hull. , 2004, 37( 1/2) , 3 [ 5 ] Taton T. A. , Mirkin C. A. , Letsinger R. 1.. , Science, 2000, 289, 1757 [ 6 1 Park S . J. , Taton T. A. , Mirkin, C. A. , Science, 2002, 2Y5,
[ I ]
1503
, Suzuki A. , Aihara N . ,
et al.
, Langmuir, 2000,
16, 2604, 2978
‘reranishi T. , Kiyokawa I. , Mikio M. , Adu. Mater. , 1998, 1 0 ( 8 ) , 596 Jana N. R. , Gearheart I,. , Murphy C. J. , J. Phys. Chem. B ,
Conclusions This article describes a new method of rapidly synthesizing gold nanoparticles by intramolecular photoreduction of complex Fe( ) -EDTA to generate complex Fe( II ) -EDTA as the reductant. The results indicate the following: ( I ) gold nanoparticles formed can be stabilized by free EDTA in single complex system; ( 2 ) in the presence of foreign protective agent added, such as citrate , PEG or PVP, gold nanoparticles with an average diameter of 6 . 7 to 50.9 nm were generated in the complex solution, indicating very good adaptability of this indirect photochemical reduction method ; ( 3 ) the mechanisms of electron transfer for the excited states of complex Fe ( ) -EDTA and forming gold nanoparticles were discussed in detail. Obviously, complex Fe ( ) -EDTA is only a medium of photoelectron transition in the process of forming gold nanoparticles.
Vol. 23
1221
2001, 105, 4065 van der Zande B. M. I. , Koper G. J. M. , Lekkerkrrker H. N. W . , J . Phys. Chem. B , 1999, 103, 5154 van der Zande B. M . I. , Bohmer M. R . , Fokkin L. 6. J . , et al. , Langmuir, 2000, 16, 451 Chang S. S. , Shih C. W. , Chert C. D. , et al. , Lnngmuir, 1999, I S , 701 Caruso K. A. , Ashokkumar M . , Criesrr F. , Langmuir, 2002, 18, 7831 Su C. H . , Wu P. L. Yeh C. S. , J. I’hys. Chem. R , 2003,
“23 1
107, 14240 Han M. Y. , Quek C. H. , f.angmuir, 2000, 1 6 , 362
r241
Kim F. , Song J. H. , Yang P. I). , J . iim. Chem. Soc. , 2002, 124, 14316 Mandal M. , Ghosh S. K. , Kundu S. , et al. , hng-muir, 2002, 18, 7792
Mallik K. , Mandal M.
,
Pradlian N. , et al.
,
Nano Letters,
2001, 1 , 3 1 9 Mallick K. , Wang Z. L. , Pal T. , J. I’hotochem.
Photohio. A : Chern. , 2001, 140, 75 Dong S. A. , Tang C. , Trans. Nonferrous Met. Soc. China, 2005, 15, 1 Yang S. C. , Dong S. A. , Tang C. , et al. , Acta Chimica Sinic a , 2005, 63(10), 873 1301 Dong S. A. , Tang C. , Zhou H. , et al. , Gold Hull. , 2004,
37, 187 Wu H. C. , Dong S. A. , Dong Y . N. , et al. , Chern. J. Chinese Universities, 2007 , 28 ( 1 ) , 10 Dong S. A. , Wu C. R. , Precious Metals, 2003, 24, 30 Dong S. A. , Proceeding oJ IPM1’99, 1999, 85 Rohatgi-Mukherjee K. K. , Fundamentals of Photochemistry, Johu Wiley & Sons, New Delhi, 1978