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Short aspect ratio gold nanorods prepared using gamma radiation in the presence of cetyltrimethyl ammonium bromide (CTAB) as a directing agent
ARTICLE IN PRESS Radiation Physics and Chemistry 79 (2010) 441–445
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Short aspect ratio gold nanorods prepared using gamma radiation in the presence of cetyltrimethyl ammonium bromide (CTAB) as a directing agent Jayashree Biswal a, S.P. Ramnani a,n, R. Tewari b, G.K. Dey b, S. Sabharwal a a b
Radiation Technology Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Material Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
a r t i c l e in fo
abstract
Article history: Received 13 August 2009 Accepted 4 November 2009
The synthesis of short aspect ratio gold nanorods using gamma radiation method by incorporating cetyltrimethyl ammonium bromide (CTAB) as a directing agent is reported in this communication. The radiolysis of Au + , in the presence of 2.5 nm Au seeds and 0.1 mol dm 3 isopropanol, results in the formation of Au spheres as evident from surface plasmon resonance band at 527 nm. However, by carrying out radiolysis at lower radiation dose rate, short aspect gold nanorods having surface plasmon bands at 513 and 670 nm have been prepared. The formation of rods at low radiation dose rate was observed to be governed by the kinetics of particle growth. The TEM of as-synthesized nanoparticles confirmed the formation of uniform sized nanorods having an aspect of 2.4. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Gold nanorods Gamma radiation Nanoparticles Anisotropic growth
1. Introduction Fine metal particles with nanometer scale dimensions are of current interest due to their unusual properties (El-Sayed, 2001; Feldheim and Foss, 2002; Daniel and Astruc, 2004). These nanoparticles show properties that are different from their corresponding bulk materials. For example, gold nanospheres appear red and not yellow when suspended in transparent media (El-Sayed, 2001; Feldheim and Foss, 2002; Daniel and Astruc, 2004). These nanoparticles have been investigated for potential applications in optics (Huynh et al., 2002), electronics (Gudiksen et al., 2002), magnetics (Thomas, 1988), catalyst (El-Sayed, 2001), chemical sensing (Kong et al., 2000) and biomedicine (Nicewarner-pena et al., 2001). The applications of nanomaterials depend strongly on the particle size, the interparticle distance and the shape of the nanoparticles. Therefore controlled preparation of nanomaterials is very important and significant. Many physical and chemical approaches have been developed to obtain special shaped nanomaterials such as nanorods, ellipsoids, cubes, etc. The physical methods produce nanomaterials from bulk materials with the help of exact and sophisticated equipments, while the chemical approach prepares nanomaterials from the atoms and molecules through simple chemical routes. In the past decade, many research groups have dedicated themselves to the preparation of metal nanomaterials following chemical approach. Chen et al. (2006) have reviewed in detail the account
n
Corresponding author. Tel.: + 91 22 25590175; fax: +91 22 25505050. E-mail address: [email protected] (S.P. Ramnani).
0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.11.004
of chemical approach for preparing special shaped nanomaterials. Usually, chemical approaches can be classified as (i) self structure confinement approach, (ii) hard or soft template approach, (iii) physical chemistry approach and (iv) soft solution approach. Among these the soft solution method, which is based on the reduction in the solution by mild reducing agents in the presence of additives like polymers (Sun et al., 2003; Yin et al., 2003), surfactants (Pileni, 2003; Jana et al., 2001; Kim et al., 2002; Johnson et al., 2002; Gai and Harmer, 2002) and dendrimers (Zhao et al., 1998; Ottaviani et al., 1997; Ahmadi et al., 1996; Won et al., 2002), is widely used. In the case of special shaped metal nanoparticles, these additives adsorb on the precursor metal ions first and then create anisotropic confinements by selective passivation of certain facets to induce and maintain anisotropic crystal growth. This approach enables to generate special shaped nanostructures with highly crystalline and well-controlled composition in a high yield. The ability of ionizing radiation to bring about ionization and excitation in the medium through which they travel results in the formation of reactive species, which can be utilized to reduce metal ions into metal atoms to generate metal nanoparticles. For example, water, upon exposure to ionizing radiation (radiolysis), generates reactive species such as hydrated electron (e aq ) and hydrogen atom (Hd). These radicals can easily reduce metal ions down to zerovalent state (Belloni et al., 1998; Temgire and Joshi, 2004). The difference between gamma radiation method and soft solution method is that in the former the reducing species are generated in situ, whereas in later the reducing agent is incorporated into the system from an external source. Further in radiation method, reduction is carried out at room temperature,
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so that additives sensitive to heat, such as polymeric systems that are not suitable for chemical reduction under heating may also be used in the radiolytic method. A particular advantage of radiolysis method is that the reduction rate is controlled by selected dose rate, which offers a wide range of conditions from slow to quasiinstantaneous atom production unlike chemical method where the local concentration of reducing species is very high and cannot be controlled very easily. In this communication, we report a gamma-radiation based method to synthesize short aspect ratio gold nanorods using CTAB as surfactant to induce anisotropy. The effect of absorbed radiation dose and dose rate in controlling the growth of particles has been also investigated. To the best of our knowledge, the use of gamma radiation to generate anisotropic gold particles is being reported for the first time in the literature.
2. Experimental 2.1. Chemicals and irradiation Aqueous solutions were prepared using nano pure water (resistivity= 18 MO). All the chemicals used were of highest purity. Hydrogen tetrachloroaurate trihydrate (99.99%) from MV laboratories, CTAB ( 499%) from Fluka, sodium borohydride from S. D. Fine Chemical Ltd., Mumbai, and AgNO3 from M/s Sarabhai Chemical, Vadodara, were used as received. Irradiations were carried out in 60Co gamma chamber having a radiation dose rate of 3.4 kGy h 1 determined using Fricke dosimetry (McLaughlin et al., 1980). Variation in the radiation dose rate was achieved by enclosing the solution in a lead attenuator during the irradiation. 2.2. Synthesis of Au seeds The Au seeds were prepared by a method similar to that described in the literature (Johnson et al., 2002). In a typical protocol, 4 ml of 1 10 3 mol dm 3 Au3 + solution, 5 ml of 0.2 mol dm 3 CTAB solution and 1 ml of 0.01 mol dm 3 ice cold sodium borohydride were mixed in this order. The solution was shaken vigorously for 2 min and kept aside for 2 h before use. The color of the solution changed from light yellow to light brown after the addition of sodium borohydride. The resultant Au seeds have particle size in the range 3.5–4.0 nm as determined by TEM analysis (Johnson et al., 2002).
in the wavelength region 250–900 nm. Transmission electron microscopy (TEM) was performed on a Model JEOL 2000 FX transmission electron microscope with an accelerating voltage of 160 kV. The solution of nanorods was placed on a carbon coated copper grid allowing the water to evaporate at room temperature.
3. Results and discussion 3.1. Irradiation at high dose rate The chemical method for synthesis of gold nanorods involves the reduction of Au3 + by weak reducing agent such as ascorbic acid in the presence of Au seeds and Ag + , the surfactant CTAB is used as a structure directing agent (Jing et al., 2006; Sau and Murphy, 2004; Murphy et al., 2005). In the present method reducing species, which are generated in situ by gamma radiolysis, are used for the reduction of Au + to metallic Au in aqueous medium. Aqueous solution containing 400 10 6 mol dm 3 of Au1 + , 0.2 mol dm 3 of isopropanol, 6 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed solution was irradiated with gamma rays (dose rate 3.4 kGy h 1). Under these conditions most of the energy is absorbed by water resulting in the formation of highly reactive species such as e aq , 0 Hd and OHd. Among these radicals e aq (E H2O/eaq = 2.87 VNHE) and H (E0 H + /H= 2.3 VNHE) are highly reducing in nature. Both these radicals bring down the reduction of Au1 + to Au0. However, in the presence of isopropyl alcohol the OH radical is scavenged to isopropyl radical. The isopropyl radical thus generated is reducing in nature (E0 = 1.5 VNHE) and is capable of reducing Au1 + to Au0 as shown below: OHU þ ðCH3 Þ2 CH OH-ðCH3 Þ2 CU OH þ H2 O
ð1Þ
Au þ þ ðCH3 Þ2 CU OH-Au0 þ ðCH3 Þ2 C ¼ O þ H þ
ð2Þ
The Au atoms thus generated upon radiolysis undergo coalescence resuting in the formation of Au nanoparticles stabilized by CTAB. The absorption spectra of irradiated solution at various times of irradiation are shown in Fig. 1. As can be seen from Fig. 1, only a single peak at 527 nm, which is a characteristic
1.8 1.6
2.3. Synthesis of Au1 +
c
The aqueous solution of 400 10 6 mol dm 3 HAuCl4 containing 0.2 mol dm 3 isopropanol and 0.1 mol dm 3 CTAB was irradiated in the gamma chamber (dose rate 3.4 kGy h 1) till the solution turned colorless. When the aqueous solution of Au3 + containing isopropanol is irradiated with gamma rays, most of the energy is absorbed by water that results in the formation of highly reactive hydrated electron (e aq ), H and OH radicals. The OH and H radicals react with isopropanol to generate isopropyl radical. Both of these radicals, namely e aq and isopropyl radicals, are highly reducing in nature that convert Au3 + into Au2 + and Au2 + thus formed disproportionates into Au3 + and Au1 + . The solution was irradiated till it become colorless. This solution was used as a precursor for gold in the synthesis of gold nanorods. 2.4. Characterizing equipments The UV–vis absorption spectra of gold nanoparticles were recorded on a Shimatzu Model 4600 recording spectrophotometer
Absorbance
1.4 1.2
b
1.0 0.8 a
0.6 0.4 0.2
300
450
600 Wavelength nm
750
Fig. 1. Absorption spectra of irradiated solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 0.85, (b) 1.7 and (c) 2.6 kGy. Dose rate= 3.4 kGy h 1.
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surface plasmon resonance band of Au spheres, is observed. The intensity of this band increases with increase in radiation dose. Under similar conditions, in the synthesis of Au nanoparticles (Sau and Murphy, 2004) by chemical method employing ascorbic acid as a reducing agent, two absorption bands, one at 522 nm and the other at higher wavelength region (650–800 nm), were observed which were attributed to the formation of gold nanorods. The position of second absorption band at higher wavelength region, also known as longitudinal surface plasmon band, varies with the length of the rod. It is red shifted with increase in the length of the rod (Murphy et al., 2005). The appearance of single absorption band at 527 nm after the radiolysis indicates the formation of a sphere. Although, there was an indication of formation of second absorption band during the radiolysis as evident from the appearance of shoulder at 590 nm; however, it did not grow with increasing radiation dose and finally only a single absorption peak at 527 nm was observed. It is known from the literature that during the synthesis of gold nanorods using chemical method the kinetics of the addition of Au + to the growing seeds governs the shape and size of the nanoparticle (Murphy et al., 2005). The radiolytic method offers wide ranges of dose rates and therefore the rate of generation of reducing species can easily be governed. The radiolysis was carried out at lower irradiation doses and the results obtained are described below. 3.2. Irradiation at lower radiation doses To investigate the effect of radiation dose rate on the formation of Au nanoparticles, irradiation of aqueous solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 6 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed solution at various dose rates was carried out. The absorption spectra of solution irradiated at a dose rate of 1.75 kGy h 1 at various intervals of irradiation are shown in Fig. 2. At lower absorption dose two absorption maxima at 523 and 610 nm were observed (Fig. 2a) that disappeared with further increase in radiation dose and finally only a single maxima at 528 nm was observed. The dose rate in this experiment was half as that of one used in the experiment carried out at the dose rate of 3.4 kGy h 1. The appearance of two absorption maxima at 522
and 610 nm in the beginning (Fig. 2a) indicated the formation of rods; however with increase in radiation dose the absorption maxima at 610 nm disappears and finally only a single peak at 527 nm is observed indicating that the net result is the formation of spheres. When the radiolysis of the above solution was carried out at a very low dose rate of 0.8 kGy h 1, the absorption spectra of solutions irradiated to various absorbed doses showed two absorption maxima as shown in Fig. 3, indicating the formation of Au nanorods. The first absorption maxima which is centered at 513 nm did not shift with increasing radiation dose; however, the second one red shifted from 643 to 670 nm with increase in radiation dose as shown in Fig. 3. This shift of absorption maxima by 27 nm with increasing radiation dose indicated the growth of nanorods. 3.3. Characterization by TEM The formation of nanorods was confirmed from transmission electron microscopy results. For this the solution after irradiation was centrifuged at 11000 rpm for 10 min. The supernatant solution was discarded and the residue was further suspended in water and centrifuged again. The procedure was repeated several times till the solution was free from CTAB. A drop of this solution was placed on a carbon coated copper grid and it was allowed to dry under ambient conditions and observed under the electron microscope. The TEM picture (Fig. 4) showed uniform sized nanorods without any spheres. The aspect ratio of nanorods was estimated to be 2.4 by measuring the length and breadth of 50 particles using ocular lens. 3.4. Suggested mechanism In chemical method of synthesis of gold nanorods (Sau and Murphy, 2004; Murphy et al., 2005), it has been postulated that presence of Ag + in the growth solution is a must for getting nanorods in high yield. It was found to be true for the radiation method of synthesis of nanorods also. Keeping all other parameters same, when irradiation was carried out in absence of Ag + , the absorption spectra at various irradiation doses are shown in Fig. 5. Appearance of only single absorption maxima at
e
1.6
e
d 1.4
d
1.75
c
Absorbance
1.0 a 0.8 0.6
1.25 b 1.00 a 0.75
0.4
0.50
0.2
0.25
300
400
500 600 700 Wavelength nm
c
1.50
b
1.2 Absorbance
443
800
Fig. 2. Absorption spectra of irradiated solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 1.75, (b) 2.6 (c) 3.0, (d) 3.5 and (e) 4.5 kGy. Dose rate= 1.7 kGy h 1.
300
400
500 600 700 Wavelength nm
800
Fig. 3. Absorption spectra of irradiated solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 1.2, (b) 1.6 (c) 2.0, (d) 2.3 and (e)2.6 kGy. Dose rate= 0.8 kGy h 1.
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0.5
Absorbance
0.4
0.3
0.2
0.1
300 Fig. 4. TEM of gold nanorods synthesized by radiolytic method. Bar length = 50 nm.
500 600 700 Wavelength nm
800
Fig. 6. Absorption spectra of irradiated solution containing 100 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 0.85, (b) 1.06 and (c) 1.2 kGy. Dose rate= 0.8 kGy h 1.
1.1 1.0
b
0.9 0.8 Absorbance
400
0.7 0.6
a
0.5 0.4 0.3 0.2 0.1 300
400
500 600 700 Wavelength nm
800
Fig. 5. Absorption spectra of irradiated solution in the absence of Ag + (a) 1.7 and (b) 6.8 kGy. All other conditions are same as mentioned in Fig. 3.
527 nm clearly indicates that only spheres were formed thus confirming that Ag + are essential for the synthesis of nanorods by radiolytic method also. The role played by Ag + 1 in the synthesis of Au nanorod is not yet fully understood. In the chemical method for the synthesis of Au nanorods (Nishioka et al., 2007) it is suggested that in the presence of CTAB, Ag + 1 gets precipitated as AgBr and gets preferentially deposited on certain facets of Au seeds that results in the growth of seed in one direction resulting in the formation of nanorod. The concentration of Ag + 1 in the solution also plays an important role in the synthesis of Au nanorods. We have carried out radiolysis at various concentrations of Ag + 1 keeping Au + 1 concentration constant (400 mol dm 3). The best and reproducible results were obtained at Ag + 1 concentration of 6 10 5 mol dm 3 and the same was used throughout this study. Thus in the radiolytic method nanorod formation occurs when the concentration ratio of Au:Ag is 7. Similar results were also obtained in the seed mediated method for the synthesis of short aspect ratio Au nanorods (Sau and Murphy, 2004) in aqueous medium using ascorbic acid as reducing agent.
Ascorbic acid, which is a mild reducing agent, is used in the chemical method of synthesis of nanorods. When ascorbic acid was added to the aqueous solution containing Au3 + , Au seeds and surfactant (CTAB), Au3 + is reduced to Au + . The ascorbic acid cannot reduce Au3 + down to Au0 due to its low reducing power. However in the presence of Au seed, Au + gets adsorbed on the surface of seed and the adsorbed Au + can be reduced by ascorbic acid to atomic Au. This results in the growth of seed; however in the presence of CTAB, which preferentially binds to the growing seed along the long axis forming a bilayer (Murphy et al., 2005), the addition of Au + takes place along the short axis that results in the growth in one direction. This anisotropic addition of Au results in the formation of nanorod. In the present method as mentioned earlier in the text, the radicals namely e aq and isopropyl radical generated during radiolysis are highly reducing in nature and can bring the reduction of Au + to atomic Au in the solution unlike chemical method where ascorbic acid cannot reduce Au + to Au. Therefore, in the chemical method when seeds are not added to the growth solution, no nanorods formation occurs. But in the present radiolytic method when seeds were not added to the solution, the reduction of Au + to Au still occurs as the solution after irradiation turned pink. The irradiated solution showed only one absorption maxima at 527 nm indicating the formation of spheres. Thus, in radiation method the growth of particles takes place by the addition of Au atoms formed in the solution upon radiolysis of the growing seed. When aqueous solution is subjected to gamma radiolysis, there is a steady-state concentration of radicals in the solution. The steady-state concentration of radicals depends on the radiation dose rate. In the present case all these reducing radicals, namely e aq and isopropyl radical, react with Au + in the solution. Therefore, the rate of formation of Au in the solution is equal to the product of steady-state radical concentration of reducing radicals and the concentration of Au + . At high radiation dose rate (3.4 kGy h 1) no nanorods are formed but only spheres are obtained as evident from single absorption maxima at 527 nm (Fig. 1). It appears that when the formation rate of Au in the solution is high, the rate of addition of Au to growing particle is high and under these conditions addition takes place isotropically which results in the formation of spheres. When dose rate is reduced by a factor of 4, the steady-state
ARTICLE IN PRESS J. Biswal et al. / Radiation Physics and Chemistry 79 (2010) 441–445
this communication produces short aspect ratio nanorods, further work is being carried out to produce high aspect ratio rods using an iterative method. The results of all these studies shall be communicated in future.
e 2.5 d
Absorbance
2.0
1.5
c
4. Conclusions
b
Gamma radiolysis method is very effective for the synthesis of gold nanorods of short aspect ratio in a single step. By proper choice of radiation dose rate and precursor concentration the gold nanorods can be synthesized in aqueous media. The method is clean as no external reducing agent is employed. As these short aspect ratio nanorods have both the absorption maxima in the visible region of electromagnetic spectrum, they may find potential application as chemical sensors.
1.0 a 0.5
300
400
445
500 600 700 Wavelength nm
800
Fig. 7. Absorption spectra of irradiated solution containing 1 10 3 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 1.28, (b) 2.12 and (c) 3.0, (d) 3.4 and (e) 4.25 kGy. Dose rate =0.8 kGy h 1.
concentration of Au in the solution is also correspondingly reduced and in turn the rate of addition of Au to the growing particle is reduced by a factor of 4. The results show that under slow kinetics conditions addition of Au to the growing particles takes place in anisotropic manner leading to the formation of rod. The evidence for the role of kinetics in governing the final shape of particle is further substantiated by the experimental observation where the concentration of Au + 1 used for the synthesis is reduced by a factor of 4. When aqueous solution containing 100 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 6 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed solution was irradiated with gamma rays at 0.8 kGy h 1 dose rate, the absorption spectra obtained at various irradiation times are shown in Fig. 6. Under these conditions the formation of gold nanorods takes place as evident from the appearance of two absorption maxima at 513 and 750 nm (Fig. 6c). The second absorption maxima, which was centered at 670 nm when the concentration of Au + was 400 10 6 mol dm 3 (Fig. 3), was red shifted to 750 nm at lower Au + concentration of 100 10 6 mol dm 3. It is reported that the longitudinal plasmon band is red shifted as the length of nanorods increases. At lower precursor concentration, the length of rod increases indicating that the addition of Au takes place in a more directional manner, i.e. on the tip of the growing rod. Under this condition the steady-state concentration of Au in the bulk is 4 times lower compared to the condition where Au + concentration is 400 10 6 mol dm 3. This clearly indicates that the formation of nanorod is controlled by kinetics, i.e. lower the steady-state concentration of Au in the bulk, lower is its addition rate to the growing rod and in turn higher is the probability of its addition in a particular direction, which results in the formation of nanorod. This hypothesis was further confirmed when synthesis of gold nanorod was carried out at high concentration of Au + (1 10 3 mol dm 3) at the same dose rate of 0.8 kGy h 1. Under these conditions no nanorods formation was observed as absorption spectra shows only single absorption maxima at 530 nm whose intensity increases with increase in radiation dose as shown in Fig. 7. Thus by proper choice of Au + concentration and radiation dose rate, nanorods of gold can easily be generated in the aqueous medium using gamma radiolysis technique. Although the present method described in
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Short aspect ratio gold nanorods prepared using gamma radiation in the presence of cetyltrimethyl ammonium bromide (CTAB) as a directing agent Jayashree Biswal a, S.P. Ramnani a,n, R. Tewari b, G.K. Dey b, S. Sabharwal a a b
Radiation Technology Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Material Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
a r t i c l e in fo
abstract
Article history: Received 13 August 2009 Accepted 4 November 2009
The synthesis of short aspect ratio gold nanorods using gamma radiation method by incorporating cetyltrimethyl ammonium bromide (CTAB) as a directing agent is reported in this communication. The radiolysis of Au + , in the presence of 2.5 nm Au seeds and 0.1 mol dm 3 isopropanol, results in the formation of Au spheres as evident from surface plasmon resonance band at 527 nm. However, by carrying out radiolysis at lower radiation dose rate, short aspect gold nanorods having surface plasmon bands at 513 and 670 nm have been prepared. The formation of rods at low radiation dose rate was observed to be governed by the kinetics of particle growth. The TEM of as-synthesized nanoparticles confirmed the formation of uniform sized nanorods having an aspect of 2.4. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Gold nanorods Gamma radiation Nanoparticles Anisotropic growth
1. Introduction Fine metal particles with nanometer scale dimensions are of current interest due to their unusual properties (El-Sayed, 2001; Feldheim and Foss, 2002; Daniel and Astruc, 2004). These nanoparticles show properties that are different from their corresponding bulk materials. For example, gold nanospheres appear red and not yellow when suspended in transparent media (El-Sayed, 2001; Feldheim and Foss, 2002; Daniel and Astruc, 2004). These nanoparticles have been investigated for potential applications in optics (Huynh et al., 2002), electronics (Gudiksen et al., 2002), magnetics (Thomas, 1988), catalyst (El-Sayed, 2001), chemical sensing (Kong et al., 2000) and biomedicine (Nicewarner-pena et al., 2001). The applications of nanomaterials depend strongly on the particle size, the interparticle distance and the shape of the nanoparticles. Therefore controlled preparation of nanomaterials is very important and significant. Many physical and chemical approaches have been developed to obtain special shaped nanomaterials such as nanorods, ellipsoids, cubes, etc. The physical methods produce nanomaterials from bulk materials with the help of exact and sophisticated equipments, while the chemical approach prepares nanomaterials from the atoms and molecules through simple chemical routes. In the past decade, many research groups have dedicated themselves to the preparation of metal nanomaterials following chemical approach. Chen et al. (2006) have reviewed in detail the account
n
Corresponding author. Tel.: + 91 22 25590175; fax: +91 22 25505050. E-mail address: [email protected] (S.P. Ramnani).
0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.11.004
of chemical approach for preparing special shaped nanomaterials. Usually, chemical approaches can be classified as (i) self structure confinement approach, (ii) hard or soft template approach, (iii) physical chemistry approach and (iv) soft solution approach. Among these the soft solution method, which is based on the reduction in the solution by mild reducing agents in the presence of additives like polymers (Sun et al., 2003; Yin et al., 2003), surfactants (Pileni, 2003; Jana et al., 2001; Kim et al., 2002; Johnson et al., 2002; Gai and Harmer, 2002) and dendrimers (Zhao et al., 1998; Ottaviani et al., 1997; Ahmadi et al., 1996; Won et al., 2002), is widely used. In the case of special shaped metal nanoparticles, these additives adsorb on the precursor metal ions first and then create anisotropic confinements by selective passivation of certain facets to induce and maintain anisotropic crystal growth. This approach enables to generate special shaped nanostructures with highly crystalline and well-controlled composition in a high yield. The ability of ionizing radiation to bring about ionization and excitation in the medium through which they travel results in the formation of reactive species, which can be utilized to reduce metal ions into metal atoms to generate metal nanoparticles. For example, water, upon exposure to ionizing radiation (radiolysis), generates reactive species such as hydrated electron (e aq ) and hydrogen atom (Hd). These radicals can easily reduce metal ions down to zerovalent state (Belloni et al., 1998; Temgire and Joshi, 2004). The difference between gamma radiation method and soft solution method is that in the former the reducing species are generated in situ, whereas in later the reducing agent is incorporated into the system from an external source. Further in radiation method, reduction is carried out at room temperature,
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so that additives sensitive to heat, such as polymeric systems that are not suitable for chemical reduction under heating may also be used in the radiolytic method. A particular advantage of radiolysis method is that the reduction rate is controlled by selected dose rate, which offers a wide range of conditions from slow to quasiinstantaneous atom production unlike chemical method where the local concentration of reducing species is very high and cannot be controlled very easily. In this communication, we report a gamma-radiation based method to synthesize short aspect ratio gold nanorods using CTAB as surfactant to induce anisotropy. The effect of absorbed radiation dose and dose rate in controlling the growth of particles has been also investigated. To the best of our knowledge, the use of gamma radiation to generate anisotropic gold particles is being reported for the first time in the literature.
2. Experimental 2.1. Chemicals and irradiation Aqueous solutions were prepared using nano pure water (resistivity= 18 MO). All the chemicals used were of highest purity. Hydrogen tetrachloroaurate trihydrate (99.99%) from MV laboratories, CTAB ( 499%) from Fluka, sodium borohydride from S. D. Fine Chemical Ltd., Mumbai, and AgNO3 from M/s Sarabhai Chemical, Vadodara, were used as received. Irradiations were carried out in 60Co gamma chamber having a radiation dose rate of 3.4 kGy h 1 determined using Fricke dosimetry (McLaughlin et al., 1980). Variation in the radiation dose rate was achieved by enclosing the solution in a lead attenuator during the irradiation. 2.2. Synthesis of Au seeds The Au seeds were prepared by a method similar to that described in the literature (Johnson et al., 2002). In a typical protocol, 4 ml of 1 10 3 mol dm 3 Au3 + solution, 5 ml of 0.2 mol dm 3 CTAB solution and 1 ml of 0.01 mol dm 3 ice cold sodium borohydride were mixed in this order. The solution was shaken vigorously for 2 min and kept aside for 2 h before use. The color of the solution changed from light yellow to light brown after the addition of sodium borohydride. The resultant Au seeds have particle size in the range 3.5–4.0 nm as determined by TEM analysis (Johnson et al., 2002).
in the wavelength region 250–900 nm. Transmission electron microscopy (TEM) was performed on a Model JEOL 2000 FX transmission electron microscope with an accelerating voltage of 160 kV. The solution of nanorods was placed on a carbon coated copper grid allowing the water to evaporate at room temperature.
3. Results and discussion 3.1. Irradiation at high dose rate The chemical method for synthesis of gold nanorods involves the reduction of Au3 + by weak reducing agent such as ascorbic acid in the presence of Au seeds and Ag + , the surfactant CTAB is used as a structure directing agent (Jing et al., 2006; Sau and Murphy, 2004; Murphy et al., 2005). In the present method reducing species, which are generated in situ by gamma radiolysis, are used for the reduction of Au + to metallic Au in aqueous medium. Aqueous solution containing 400 10 6 mol dm 3 of Au1 + , 0.2 mol dm 3 of isopropanol, 6 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed solution was irradiated with gamma rays (dose rate 3.4 kGy h 1). Under these conditions most of the energy is absorbed by water resulting in the formation of highly reactive species such as e aq , 0 Hd and OHd. Among these radicals e aq (E H2O/eaq = 2.87 VNHE) and H (E0 H + /H= 2.3 VNHE) are highly reducing in nature. Both these radicals bring down the reduction of Au1 + to Au0. However, in the presence of isopropyl alcohol the OH radical is scavenged to isopropyl radical. The isopropyl radical thus generated is reducing in nature (E0 = 1.5 VNHE) and is capable of reducing Au1 + to Au0 as shown below: OHU þ ðCH3 Þ2 CH OH-ðCH3 Þ2 CU OH þ H2 O
ð1Þ
Au þ þ ðCH3 Þ2 CU OH-Au0 þ ðCH3 Þ2 C ¼ O þ H þ
ð2Þ
The Au atoms thus generated upon radiolysis undergo coalescence resuting in the formation of Au nanoparticles stabilized by CTAB. The absorption spectra of irradiated solution at various times of irradiation are shown in Fig. 1. As can be seen from Fig. 1, only a single peak at 527 nm, which is a characteristic
1.8 1.6
2.3. Synthesis of Au1 +
c
The aqueous solution of 400 10 6 mol dm 3 HAuCl4 containing 0.2 mol dm 3 isopropanol and 0.1 mol dm 3 CTAB was irradiated in the gamma chamber (dose rate 3.4 kGy h 1) till the solution turned colorless. When the aqueous solution of Au3 + containing isopropanol is irradiated with gamma rays, most of the energy is absorbed by water that results in the formation of highly reactive hydrated electron (e aq ), H and OH radicals. The OH and H radicals react with isopropanol to generate isopropyl radical. Both of these radicals, namely e aq and isopropyl radicals, are highly reducing in nature that convert Au3 + into Au2 + and Au2 + thus formed disproportionates into Au3 + and Au1 + . The solution was irradiated till it become colorless. This solution was used as a precursor for gold in the synthesis of gold nanorods. 2.4. Characterizing equipments The UV–vis absorption spectra of gold nanoparticles were recorded on a Shimatzu Model 4600 recording spectrophotometer
Absorbance
1.4 1.2
b
1.0 0.8 a
0.6 0.4 0.2
300
450
600 Wavelength nm
750
Fig. 1. Absorption spectra of irradiated solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 0.85, (b) 1.7 and (c) 2.6 kGy. Dose rate= 3.4 kGy h 1.
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surface plasmon resonance band of Au spheres, is observed. The intensity of this band increases with increase in radiation dose. Under similar conditions, in the synthesis of Au nanoparticles (Sau and Murphy, 2004) by chemical method employing ascorbic acid as a reducing agent, two absorption bands, one at 522 nm and the other at higher wavelength region (650–800 nm), were observed which were attributed to the formation of gold nanorods. The position of second absorption band at higher wavelength region, also known as longitudinal surface plasmon band, varies with the length of the rod. It is red shifted with increase in the length of the rod (Murphy et al., 2005). The appearance of single absorption band at 527 nm after the radiolysis indicates the formation of a sphere. Although, there was an indication of formation of second absorption band during the radiolysis as evident from the appearance of shoulder at 590 nm; however, it did not grow with increasing radiation dose and finally only a single absorption peak at 527 nm was observed. It is known from the literature that during the synthesis of gold nanorods using chemical method the kinetics of the addition of Au + to the growing seeds governs the shape and size of the nanoparticle (Murphy et al., 2005). The radiolytic method offers wide ranges of dose rates and therefore the rate of generation of reducing species can easily be governed. The radiolysis was carried out at lower irradiation doses and the results obtained are described below. 3.2. Irradiation at lower radiation doses To investigate the effect of radiation dose rate on the formation of Au nanoparticles, irradiation of aqueous solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 6 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed solution at various dose rates was carried out. The absorption spectra of solution irradiated at a dose rate of 1.75 kGy h 1 at various intervals of irradiation are shown in Fig. 2. At lower absorption dose two absorption maxima at 523 and 610 nm were observed (Fig. 2a) that disappeared with further increase in radiation dose and finally only a single maxima at 528 nm was observed. The dose rate in this experiment was half as that of one used in the experiment carried out at the dose rate of 3.4 kGy h 1. The appearance of two absorption maxima at 522
and 610 nm in the beginning (Fig. 2a) indicated the formation of rods; however with increase in radiation dose the absorption maxima at 610 nm disappears and finally only a single peak at 527 nm is observed indicating that the net result is the formation of spheres. When the radiolysis of the above solution was carried out at a very low dose rate of 0.8 kGy h 1, the absorption spectra of solutions irradiated to various absorbed doses showed two absorption maxima as shown in Fig. 3, indicating the formation of Au nanorods. The first absorption maxima which is centered at 513 nm did not shift with increasing radiation dose; however, the second one red shifted from 643 to 670 nm with increase in radiation dose as shown in Fig. 3. This shift of absorption maxima by 27 nm with increasing radiation dose indicated the growth of nanorods. 3.3. Characterization by TEM The formation of nanorods was confirmed from transmission electron microscopy results. For this the solution after irradiation was centrifuged at 11000 rpm for 10 min. The supernatant solution was discarded and the residue was further suspended in water and centrifuged again. The procedure was repeated several times till the solution was free from CTAB. A drop of this solution was placed on a carbon coated copper grid and it was allowed to dry under ambient conditions and observed under the electron microscope. The TEM picture (Fig. 4) showed uniform sized nanorods without any spheres. The aspect ratio of nanorods was estimated to be 2.4 by measuring the length and breadth of 50 particles using ocular lens. 3.4. Suggested mechanism In chemical method of synthesis of gold nanorods (Sau and Murphy, 2004; Murphy et al., 2005), it has been postulated that presence of Ag + in the growth solution is a must for getting nanorods in high yield. It was found to be true for the radiation method of synthesis of nanorods also. Keeping all other parameters same, when irradiation was carried out in absence of Ag + , the absorption spectra at various irradiation doses are shown in Fig. 5. Appearance of only single absorption maxima at
e
1.6
e
d 1.4
d
1.75
c
Absorbance
1.0 a 0.8 0.6
1.25 b 1.00 a 0.75
0.4
0.50
0.2
0.25
300
400
500 600 700 Wavelength nm
c
1.50
b
1.2 Absorbance
443
800
Fig. 2. Absorption spectra of irradiated solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 1.75, (b) 2.6 (c) 3.0, (d) 3.5 and (e) 4.5 kGy. Dose rate= 1.7 kGy h 1.
300
400
500 600 700 Wavelength nm
800
Fig. 3. Absorption spectra of irradiated solution containing 400 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 1.2, (b) 1.6 (c) 2.0, (d) 2.3 and (e)2.6 kGy. Dose rate= 0.8 kGy h 1.
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0.5
Absorbance
0.4
0.3
0.2
0.1
300 Fig. 4. TEM of gold nanorods synthesized by radiolytic method. Bar length = 50 nm.
500 600 700 Wavelength nm
800
Fig. 6. Absorption spectra of irradiated solution containing 100 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 0.85, (b) 1.06 and (c) 1.2 kGy. Dose rate= 0.8 kGy h 1.
1.1 1.0
b
0.9 0.8 Absorbance
400
0.7 0.6
a
0.5 0.4 0.3 0.2 0.1 300
400
500 600 700 Wavelength nm
800
Fig. 5. Absorption spectra of irradiated solution in the absence of Ag + (a) 1.7 and (b) 6.8 kGy. All other conditions are same as mentioned in Fig. 3.
527 nm clearly indicates that only spheres were formed thus confirming that Ag + are essential for the synthesis of nanorods by radiolytic method also. The role played by Ag + 1 in the synthesis of Au nanorod is not yet fully understood. In the chemical method for the synthesis of Au nanorods (Nishioka et al., 2007) it is suggested that in the presence of CTAB, Ag + 1 gets precipitated as AgBr and gets preferentially deposited on certain facets of Au seeds that results in the growth of seed in one direction resulting in the formation of nanorod. The concentration of Ag + 1 in the solution also plays an important role in the synthesis of Au nanorods. We have carried out radiolysis at various concentrations of Ag + 1 keeping Au + 1 concentration constant (400 mol dm 3). The best and reproducible results were obtained at Ag + 1 concentration of 6 10 5 mol dm 3 and the same was used throughout this study. Thus in the radiolytic method nanorod formation occurs when the concentration ratio of Au:Ag is 7. Similar results were also obtained in the seed mediated method for the synthesis of short aspect ratio Au nanorods (Sau and Murphy, 2004) in aqueous medium using ascorbic acid as reducing agent.
Ascorbic acid, which is a mild reducing agent, is used in the chemical method of synthesis of nanorods. When ascorbic acid was added to the aqueous solution containing Au3 + , Au seeds and surfactant (CTAB), Au3 + is reduced to Au + . The ascorbic acid cannot reduce Au3 + down to Au0 due to its low reducing power. However in the presence of Au seed, Au + gets adsorbed on the surface of seed and the adsorbed Au + can be reduced by ascorbic acid to atomic Au. This results in the growth of seed; however in the presence of CTAB, which preferentially binds to the growing seed along the long axis forming a bilayer (Murphy et al., 2005), the addition of Au + takes place along the short axis that results in the growth in one direction. This anisotropic addition of Au results in the formation of nanorod. In the present method as mentioned earlier in the text, the radicals namely e aq and isopropyl radical generated during radiolysis are highly reducing in nature and can bring the reduction of Au + to atomic Au in the solution unlike chemical method where ascorbic acid cannot reduce Au + to Au. Therefore, in the chemical method when seeds are not added to the growth solution, no nanorods formation occurs. But in the present radiolytic method when seeds were not added to the solution, the reduction of Au + to Au still occurs as the solution after irradiation turned pink. The irradiated solution showed only one absorption maxima at 527 nm indicating the formation of spheres. Thus, in radiation method the growth of particles takes place by the addition of Au atoms formed in the solution upon radiolysis of the growing seed. When aqueous solution is subjected to gamma radiolysis, there is a steady-state concentration of radicals in the solution. The steady-state concentration of radicals depends on the radiation dose rate. In the present case all these reducing radicals, namely e aq and isopropyl radical, react with Au + in the solution. Therefore, the rate of formation of Au in the solution is equal to the product of steady-state radical concentration of reducing radicals and the concentration of Au + . At high radiation dose rate (3.4 kGy h 1) no nanorods are formed but only spheres are obtained as evident from single absorption maxima at 527 nm (Fig. 1). It appears that when the formation rate of Au in the solution is high, the rate of addition of Au to growing particle is high and under these conditions addition takes place isotropically which results in the formation of spheres. When dose rate is reduced by a factor of 4, the steady-state
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this communication produces short aspect ratio nanorods, further work is being carried out to produce high aspect ratio rods using an iterative method. The results of all these studies shall be communicated in future.
e 2.5 d
Absorbance
2.0
1.5
c
4. Conclusions
b
Gamma radiolysis method is very effective for the synthesis of gold nanorods of short aspect ratio in a single step. By proper choice of radiation dose rate and precursor concentration the gold nanorods can be synthesized in aqueous media. The method is clean as no external reducing agent is employed. As these short aspect ratio nanorods have both the absorption maxima in the visible region of electromagnetic spectrum, they may find potential application as chemical sensors.
1.0 a 0.5
300
400
445
500 600 700 Wavelength nm
800
Fig. 7. Absorption spectra of irradiated solution containing 1 10 3 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 8 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed at various absorbed doses (a) 1.28, (b) 2.12 and (c) 3.0, (d) 3.4 and (e) 4.25 kGy. Dose rate =0.8 kGy h 1.
concentration of Au in the solution is also correspondingly reduced and in turn the rate of addition of Au to the growing particle is reduced by a factor of 4. The results show that under slow kinetics conditions addition of Au to the growing particles takes place in anisotropic manner leading to the formation of rod. The evidence for the role of kinetics in governing the final shape of particle is further substantiated by the experimental observation where the concentration of Au + 1 used for the synthesis is reduced by a factor of 4. When aqueous solution containing 100 10 6 mol dm 3 of Au + 1, 0.2 mol dm 3 of isopropanol, 6 10 5 mol dm 3 of AgNO3, 0.1 mol dm 3 of CTAB and 1.2 10 6 mol dm 3 of seed solution was irradiated with gamma rays at 0.8 kGy h 1 dose rate, the absorption spectra obtained at various irradiation times are shown in Fig. 6. Under these conditions the formation of gold nanorods takes place as evident from the appearance of two absorption maxima at 513 and 750 nm (Fig. 6c). The second absorption maxima, which was centered at 670 nm when the concentration of Au + was 400 10 6 mol dm 3 (Fig. 3), was red shifted to 750 nm at lower Au + concentration of 100 10 6 mol dm 3. It is reported that the longitudinal plasmon band is red shifted as the length of nanorods increases. At lower precursor concentration, the length of rod increases indicating that the addition of Au takes place in a more directional manner, i.e. on the tip of the growing rod. Under this condition the steady-state concentration of Au in the bulk is 4 times lower compared to the condition where Au + concentration is 400 10 6 mol dm 3. This clearly indicates that the formation of nanorod is controlled by kinetics, i.e. lower the steady-state concentration of Au in the bulk, lower is its addition rate to the growing rod and in turn higher is the probability of its addition in a particular direction, which results in the formation of nanorod. This hypothesis was further confirmed when synthesis of gold nanorod was carried out at high concentration of Au + (1 10 3 mol dm 3) at the same dose rate of 0.8 kGy h 1. Under these conditions no nanorods formation was observed as absorption spectra shows only single absorption maxima at 530 nm whose intensity increases with increase in radiation dose as shown in Fig. 7. Thus by proper choice of Au + concentration and radiation dose rate, nanorods of gold can easily be generated in the aqueous medium using gamma radiolysis technique. Although the present method described in
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