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One-step synthesis of gold nanoparticles using azacryptand and their applications in SERS and catalysis
Journal of Colloid and Interface Science 316 (2007) 476–481 www.elsevier.com/locate/jcis
One-step synthesis of gold nanoparticles using azacryptand and their applications in SERS and catalysis Kang Yeol Lee, Jaeyoung Hwang, Young Wook Lee, Jineun Kim, Sang Woo Han ∗ Department of Chemistry, Research Institute of Natural Science, Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju 660-701, Korea Received 11 June 2007; accepted 31 July 2007 Available online 6 August 2007
Abstract A new aqueous-phase method for the preparation of stable gold nanoparticles by using 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (azacryptand) as both reductant and stabilizer is reported. Reduction of HAuCl4 with azacryptand at room temperature yields nano-sized particles within a short time. The obtained gold nanoparticles have been characterized by UV–vis spectroscopy, transmission electron microscopy, and X-ray diffraction. Comparison of FT-IR spectra of azacryptand before and after reaction revealed that azacryptand molecules reduce gold ions as the amino moieties in the molecules are oxidized to imino groups. The prepared gold nanoparticles show efficient surface-enhanced Raman scattering properties and can effectively catalyze reduction of 4-nitrophenol by sodium borohydride in aqueous solution. © 2007 Elsevier Inc. All rights reserved. Keywords: Gold nanoparticles; Azacryptand; Interface; SERS; Catalysis
1. Introduction Metal nanostructured materials have been the subject of much scientific research due to their characteristic properties that are distinctly different from their bulk counterparts, and considerable attention from both fundamental and applied research has been paid to the synthesis and characterization of these materials [1]. Particular interest has been focused on the noble metal nanoparticles because they are technologically important in many fields such as catalysis [2], optics [3], biological assays [4], and surface-enhanced Raman scattering (SERS) [5,6]. Among the known metal nanoparticles, gold nanoparticles (AuNPs) are extensively studied because of their characteristic optical, spectroscopic, and catalytic properties. Preparation of uniform AuNPs becomes a very important issue in applications because physical and chemical properties of particles highly depend on their size and shape [1]. In general, AuNPs have been prepared chemically through the reduction of hydrogen tetrachloroaurate (HAuCl4 ) by suit* Corresponding author.
E-mail address: [email protected] (S.W. Han). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.07.076
able reducing agents in the presence of stabilizers. The type and molar ratio of reductant to gold ion, type of stabilizer, reaction time, and temperature are critical experimental variables for controlling the size and shape of the synthesized particles [6–8]. Most widely used synthetic protocol for the preparation of AuNPs is citrate reduction method [9]. Waterdispersible AuNPs with average diameter of 1.6–140 nm can be successfully prepared by this method [8,10]. Recently, amines have been used in the synthesis of AuNPs as both reductants and capping species of nanoparticles [8,11–17]. A variety of amines have been examined including primary amines [8, 11–13], amino acids [14,15], secondary and tertiary amines [8,11], and polymers containing amine moieties [8,16,17]. Since amines are present in most of biological and environmental systems, nanoparticles prepared with amines should be attractive materials for biological applications [11]. However, reaction time required for the formation of nanoparticles by using amines is relatively long and applications with prepared particles have been little reported. In this work, we report on the simple and fast aqueousphase method for preparing stable AuNPs by reduction of HAuCl4 with 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (azacryptand, Fig. 1) at room temperature. The prepared
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2.3. Measurements
Fig. 1. Molecular structure of azacryptand.
nanoparticles have been characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The SERS activity of the particles was tested by using various analytes and compared with that exhibited by common colloidal substrate for SERS measurements, i.e., citrate-reduced AuNPs. The catalytic activity of the nanoparticles in the reduction of 4-nitrophenol was also studied. Azacryptand has been used as a host molecule to selectively bind and isolate one desired ion from its surrounding environment [18]. To our knowledge, using the azacryptand as both reductant and stabilizer in the preparation of nanoparticles has not been reported yet. In fact, this molecule has been chosen because of its unusual structure. The azacryptand has 8 amine moieties per molecule. It can be assumed that this large amount of amine units should enhance the reduction function of the molecule. Indeed, AuNPs were rapidly prepared by using azacryptand and the resulting particles could be immediately applied to SERS spectroscopy and catalysis.
The extinction spectra were recorded with a SINCO S-3100 UV–vis spectrophotometer. TEM images were obtained with a JEOL JEM-2010 transmission electron microscope operating at 200 kV. High-resolution TEM characterizations were performed with a FEI Technai G2 F30 Super-Twin transmission electron microscope operating at 300 kV. XRD patterns were obtained with a Bruker AXS D8 DISCOVER diffractometer using CuKα (0.1542 nm) radiation. Infrared spectra were measured by using a Shimadzu FTIR-8400S spectrometer. Raman spectra were obtained using a Jobin Yvon/HORIBA LabRAM spectrometer equipped with an integral microscope (Olympus BX 41). The 632.8 nm line of an air-cooled He/Ne laser was used as an excitation source. Raman scattering was detected with 180◦ geometry using a Peltier cooled 1024 × 256 pixel CCD detector. The Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer. 2.4. Catalytic reduction of 4-nitrophenol In a typical experiment, 2.765 mL of water, 0.0750 mL of aqueous NaBH4 (2.00 × 10−1 M), and 0.150 mL of 1.00 × 10−3 M 4-nitrophenol were mixed in a standard quartz cuvette. Then, 0.010 mL of the azacryptand- or citrate-reduced gold hydrosol (the amount of metal was same in each sol) was introduced into the solution and time-dependent absorption spectra were recorded every 30 s in the range of 190–1000 nm at 25 ◦ C.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Particle synthesis and characterization
All chemicals, unless specified, were laboratory reagent grade, and triply distilled water, of resistivity greater than 18.0 M cm, was used when preparing aqueous solutions. Azacryptand was prepared by the condensation of tris(2aminoethyl)amine with glyoxal followed by reduction with NaBH4 [18,19].
AuNPs were prepared by simple mixing of chloroauric acid with azacryptand in aqueous solution at room temperature. This procedure spontaneously resulted in the formation of AuNPs without the addition of an extra reducing agent. The azacryptand simultaneously acts both as protective agent and as reductant, thereby significantly simplifying the process for preparing the nanoparticles. The AuNPs exhibit strong surface plasmon resonance (SPR) absorption that is dependent on the size and shape of particles. For spherical AuNPs, the SPR band maximum generally falls between about 520 and 530 nm [21]. Fig. 2 shows a typical spectral evolution of the SPR band for an aqueous solution of equimolar concentration of azacryptand and chloroauric acid (an equivalent ratio of amine to Au3+ , N/Au = 8). The general trend observed from Fig. 2 is the gradual increase of the SPR band at ∼532 nm. The absorption maximum plateaus after ∼7 min (see inset of Fig. 2). The spectral evolution was also accompanied by a distinct color change from pale-yellow to wine-red. These observations indicate that AuNPs can be formed by this simple process and the reaction time required for the synthesis of particles is relatively short. The prepared gold hydrosol is stable for several months. Very high concentration of azacryptand (N/Au = 80) results in the rapid reduction of the gold salt; however, under this condition the colloids formed were found to sediment after 12 h.
2.2. Particle synthesis The azacryptand-reduced AuNPs were prepared as following procedure. A 0.10 mL aqueous solution of HAuCl4 (0.024 M) was added rapidly into a vial containing aqueous solution of azacryptand (1.0 × 10−3 M, 2.4 mL). The concentrations of HAuCl4 and azacryptand are same in the final reaction mixture, i.e., 9.6 × 10−4 M. The solution in vial was kept at 25 ◦ C. The citrate-reduced AuNPs were prepared by following the literature with a difference only in the molar ratio of HAuCl4 to sodium citrate [20]. Namely, 40 mg of HAuCl4 was initially dissolved in 90 mL of water, and the solution was heated to boiling. A total of 10.2 mL of aqueous solution of sodium citrate (40 mM) was then added to the HAuCl4 solution under vigorous stirring, and boiling was continued for ca. 15 min.
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Fig. 4. Powder XRD pattern of the azacryptand-reduced AuNPs.
Fig. 2. Variation of UV–vis absorption spectra of the solution containing azacryptand and HAuCl4 as a function of reaction time (time interval = 60 s). Inset shows that SPR absorption maximum plateaus after ∼7 min.
Fig. 3. (a) TEM image of the azacryptand-reduced AuNPs. (b) High-resolution TEM image of an azacryptand-reduced AuNP. (c) Enlarged image of the square region in (b). A d-spacing between adjacent lattice planes of 2.36 Å corresponds to the (111) planes of fcc gold.
The actual structure and size distribution of particles can be determined by TEM. Fig. 3a shows a typical TEM image of the samples. The image reveals that the prepared particles have, in general, a spherical shape with average size of 20.9 ± 3.3 nm. Close inspection of the image also shows that some of the particles have polyhedral structures. For more insight into the fine structure of the AuNPs prepared in this work, a structural analysis of the particles was also performed by high-resolution TEM characterization. Fig. 3b shows the high-resolution TEM image of a nanoparticle obtained by the azacryptand. It was found that the majority of particles were predominantly faceted into crystallites. Fig. 3c gives the enlarged image of Fig. 3b to exemplify the perfect atomic arrangement in the particles. Atomic planes from the gold face-centered cubic (fcc) lattice are clearly visible; a d-spacing of 2.36 Å for adjacent lattice planes corresponds to the (111) planes of fcc gold [6,22]. The crystalline nature of the AuNPs was further confirmed by the XRD measurements. Fig. 4 shows the XRD pattern of the prepared sample. Five peaks were observed which can be assigned to the (111), (200), (220), (311), and (222) diffraction peaks of fcc gold metal [23], indicating that the sample is composed of pure crystalline gold. In the previous studies on the synthesis of AuNPs with amines, the reaction time required for the formation of particles was, in general, relatively long. For instance, in the case
of polymeric amines such as polyethyleneimine (PEI), complete reduction of gold ion occurred over a period of several days [8,16]. Furthermore, high amine to Au3+ ratio (N/Au), e.g., from several tens to hundred, was required to ensure stability of the system [8,11,16]. In this study, stable aqueous suspension of AuNPs can be rapidly prepared with small amount of amine. Even if 0.5 equivalent azacryptand (N/Au = 4) was used, AuNPs could be readily synthesized. This result can be attributed to unique structural characteristic of the azacryptand molecule. Azacryptand has two tertiary amines and six secondary amines per molecule. This high local concentration of secondary and tertiary amine units can enhance the reduction function of the molecule. In fact, Richardson et al. found that for short alkyl chains the tertiary amines are the most effective for reducing Au3+ , followed by secondary amines. When providing necessary coordination required for stabilization of the nanocrystals as they are formed, secondary amines are more effective [8]. A still remaining question is how gold cations were reduced by azacryptand. Kuo et al. reported that Au3+ was reduced by polymeric amine such as the branched PEI [16]. It has been postulated that the amino groups on the polymer are oxidized to imino moieties, and this is the source of electrons by which the reduction of Au3+ could take place. To investigate the changes in the molecule accompanying the reduction of HAuCl4 , FT-IR spectra of azacryptand before and after reaction were examined. Fig. 5a shows the FT-IR spectrum of pristine azacryptand. The characteristic bands of the molecule appear in the spectrum: CH2 scissoring + NH in-plane bending (1479, 1466 cm−1 ), NH in-plane bending (1439 cm−1 ), CH2 wagging (1335 cm−1 ), CH2 twisting (1291, 1273, 1221, 1149 cm−1 ), and C–N stretching (1111, 1068 cm−1 ) [24]. The broad band observed in the region of 1650 cm−1 may be attributed to NH deformation mode of azacryptand and bending vibration of residual water. Fig. 5b shows the spectrum of the azacryptand-reduced AuNPs. As shown in the figure, spectral features are noticeably changed after the reaction. The relative intensities of bands assigned to the C–N stretching, NH bending, and CH2 deformation (wagging and twisting) modes of azacryptand were considerably decreased and the expected concurrent appearance of new bands related to the formation of imine was clearly observed: C=N stretching (1645, 1541, 1514 cm−1 ) and CH in-plane bend-
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3.2. SERS measurements
Fig. 5. FT-IR spectra of (a) pristine azacryptand and (b) the azacryptand-reduced AuNPs.
ing (1381, 1252 cm−1 ) [25–27]. These observations indicate that azacryptand molecules reduce gold precursors as the amino moieties in the molecules are oxidized to imino groups.
For investigating the application aspect of the prepared particles for SERS spectroscopy, we have examined SERS efficiency of the azacryptand-reduced AuNPs. SERS spectra of various adsorbates were measured with the azacryptandreduced AuNPs and compared them with those obtained with common colloidal substrate for SERS measurements, i.e., citrate-reduced AuNPs. The citrate-reduced AuNPs were prepared by following the previously reported procedure and have similar average size (19.6 ± 3.7 nm) with the azacryptandreduced AuNPs. Fig. 6 shows the SERS spectra of (a) 5 × 10−5 M benzenethiol (BT), (b) 5 × 10−5 M 4-nitrobenzenethiol (4-NBT), (c) 5 × 10−5 M 4-aminobenzenethiol (4-ABT), (d) 5 × 10−5 M 2-mercaptopyridine (2-MP), and (e) 5 × 10−7 M rhodamine 6G (R6G) molecules obtained with both azacryptand-reduced (upper trace) and citrate-reduced (lower trace) AuNPs in aqueous solutions. The concentration of nanoparticles was adjusted to same value for all the analyte solutions. It is noticeable that azacryptand-reduced AuNPs give stronger signals; SERS intensities obtained with the azacryptand-reduced particles are at least ∼2× greater than those with the citrate-reduced particles. One of the possible reasons for the observed difference in SERS activities between azacryptand- and citrate-reduced particles is different surface chemistry of the particles. Since the azacryptand- and citratereduced particles are stabilized with azacryptand and citrate, respectively, net surface charge and molecular nature of the cap-
Fig. 6. SERS spectra of (a) 5 × 10−5 M BT, (b) 5 × 10−5 M 4-NBT, (c) 5 × 10−5 M 4-ABT, (d) 5 × 10−5 M 2-MP, and (e) 5 × 10−7 M R6G molecules obtained with azacryptand-reduced (upper trace) and citrate-reduced (lower trace) AuNPs in aqueous solutions. (f) UV–vis absorption spectra of azacryptand-reduced (- - -) and citrate-reduced (—) AuNPs. The vertical line represents the excitation wavelength for SERS measurement at 632.8 nm.
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ping group are different. This different environment can thus affect the access of the adsorbates to the surface of nanoparticles. However, this effect is not believed to be a dominant factor responsible for the higher SERS activity of azacryptand-reduced particles because the azacryptand-reduced nanoparticles show higher SERS efficiencies for all the adsorbates tested which have different polarities. The higher scattering intensity of azacryptand-reduced particles could be caused by the larger amount of overlap between SPR band and the excitation source. It is widely accepted that the SERS effect is the result of enhancement of localized electromagnetic field due to SPR [5]. For the azacryptandreduced particles the SPR band is located at slightly longer wavelength and rather broader than the SPR band for the citratereduced particles, result in larger SPR absorbance at the excitation wavelength (see Fig. 6f). The observed different UV– vis spectral features may be ascribed to the fact that different stabilizers are present on the surface of nanoparticles and some amount of polyhedral particles exist in the sample synthesized by azacryptand (Fig. 3a). In fact, most of the particles in the sample prepared by citrate-reduction have spherical shape [20]. Moreover, the presence of polyhedral nanoparticles is sure to contribute to the enhanced SERS activity of the azacryptand-reduced AuNPs because of their structural characteristics. Large enhancement of local electromagnetic field is generally observed near sharp surface features such as edges and corners [28–30]. These active regions can serve as hot sites for surface plasmon enhancement [31,32]. Polyhedral particles have more well-defined edges and corners, and have generally sharper surface features than do spherical ones. As a result, SERS signals of the adsorbates on the active sites of polyhedral particles can be largely enhanced and the contribution from these molecules can be another feasible source of the higher SERS activity of the sample prepared by azacryptand. 3.3. Catalysis
Fig. 7. Successive UV–vis absorption spectra (30 s interval) of the reduction of 4-NP by NaBH4 in the presence of (a) azacryptand-reduced and (b) citrate-reduced AuNPs. The insets show the plot indicating the variation of ln A vs time.
Metal nanoparticles have been widely used as efficient catalysts in transformations of organic substances because of their large surface-to-volume ratio and different electronic properties compared to corresponding bulk metals [2,33]. To study the catalytic activity of the azacryptand-reduced AuNPs, we have chosen the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4 ) as a model reaction because this reaction is rapid and can be easily characterized. The catalytic reduction of several aromatic nitro compounds to the corresponding amino derivatives using NaBH4 and metal nanoparticles has been reported [34–37]. Nitro compounds are inert to NaBH4 if it is used alone. However, the metal nanoparticles effectively catalyzed the reduction of nitro compounds by acting as an electron relay system; electron transfer took place between nitro compounds and NaBH4 through the metal particles. The reaction can be readily monitored with UV–vis spectroscopy. Fig. 7a shows successive UV–vis absorption spectra of the reduction of 4-NP by the azacryptand-reduced AuNPs. In NaBH4 medium the peak corresponding to 4-NP at 317 nm was red-shifted to 400 nm because of the formation of 4-nitrophenolate. In the
absence of nanoparticles, the peak due to 4-nitrophenolate remains unaltered. The addition of AuNPs to the solution causes the disappearance of the 400 nm peak with the concomitant appearance of a new peak at 300 nm. This peak has been attributed to 4-aminophenol [34–37]. This result shows that the azacryptand-reduced AuNPs catalyzes the reduction reaction. Since the concentration of BH− 4 used for the reaction largely exceeds the concentration of 4-NP and the AuNPs, pseudo-firstorder kinetics with respect to 4-NP could be used in this case to estimate the rate constant. A good linear correlation with time, that is, ln A vs time plot, was obtained (inset of Fig. 7a). Here, A stands for the absorbance at 400 nm at any time. The pseudo-first-order rate constant determined from this plots is 1.0 × 10−2 s−1 . For comparison, same experiment was also preformed with the citrate-reduced AuNPs. The result is shown in Fig. 7b. The estimated rate constant from ln A vs time plot (inset of Fig. 7b) is 2.0 × 10−3 s−1 . This indicates that reaction rate of the azacryptand-reduced AuNPs is 5 times greater than that of the citrate-reduced particles. The low reduction rate of the citrate-
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reduced AuNPs can be ascribed to the negatively charged surface of anionic stabilizer, i.e., citrate, which hindered the incorporation of phenolate ion onto the surface of gold nanoparticles [34]. The rate constant value of the azacryptand-reduced AuNPs is also higher than those obtained with other previously prepared AuNPs such as poly(amidoamine) dendrimerstabilized AuNPs [35] and spongy AuNPs [37]. The value is comparable to the reported one for AuNPs supported by poly(propyleneimine) dendrimer which effects the diffusion of 4-NP to the surface of catalyst particles and thus enforce the catalytic properties [35]. These indicate that the AuNPs prepared by azacryptand can effectively catalyze the reduction process. 4. Conclusions The present work demonstrated that crystalline gold nanoparticles have been prepared using azacryptand as a dual reductant/stabilizer. The advantages of the azacryptand-reduced nanoparticles are seen in fast and simple preparation at room temperature and their immediate applicability for SERS spectroscopy and catalysis. The prepared particles give stronger SERS signals for various analytes than do conventional citratereduced particles and also show enhanced catalytic activity in the reduction reaction of nitro compound. These characteristics of the new colloid can allow them to be highly promising substrate for a range of different analytical and catalytic applications. Acknowledgments This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF2006-311-C00355), by a grant from the MOST/KOSEF to the Environmental Biotechnology National Core Research Center (grant #: R15-2003-012-01001-0), and by Technology Development Program of the Ministry of Agriculture and Forestry, Republic of Korea. References [1] J.H. Fendler (Ed.), Nanoparticles and Nanostructured Films, VCH, Weinheim, 1998. [2] R. Narayana, M.A. El-Sayed, Nano Lett. 4 (2004) 1343. [3] P.V. Kamat, J. Phys. Chem. B 106 (2002) 7729. [4] T.A. Taton, C.A. Mirkin, R.L. Letsinger, Science 289 (2000) 1757. [5] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241.
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One-step synthesis of gold nanoparticles using azacryptand and their applications in SERS and catalysis Kang Yeol Lee, Jaeyoung Hwang, Young Wook Lee, Jineun Kim, Sang Woo Han ∗ Department of Chemistry, Research Institute of Natural Science, Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju 660-701, Korea Received 11 June 2007; accepted 31 July 2007 Available online 6 August 2007
Abstract A new aqueous-phase method for the preparation of stable gold nanoparticles by using 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (azacryptand) as both reductant and stabilizer is reported. Reduction of HAuCl4 with azacryptand at room temperature yields nano-sized particles within a short time. The obtained gold nanoparticles have been characterized by UV–vis spectroscopy, transmission electron microscopy, and X-ray diffraction. Comparison of FT-IR spectra of azacryptand before and after reaction revealed that azacryptand molecules reduce gold ions as the amino moieties in the molecules are oxidized to imino groups. The prepared gold nanoparticles show efficient surface-enhanced Raman scattering properties and can effectively catalyze reduction of 4-nitrophenol by sodium borohydride in aqueous solution. © 2007 Elsevier Inc. All rights reserved. Keywords: Gold nanoparticles; Azacryptand; Interface; SERS; Catalysis
1. Introduction Metal nanostructured materials have been the subject of much scientific research due to their characteristic properties that are distinctly different from their bulk counterparts, and considerable attention from both fundamental and applied research has been paid to the synthesis and characterization of these materials [1]. Particular interest has been focused on the noble metal nanoparticles because they are technologically important in many fields such as catalysis [2], optics [3], biological assays [4], and surface-enhanced Raman scattering (SERS) [5,6]. Among the known metal nanoparticles, gold nanoparticles (AuNPs) are extensively studied because of their characteristic optical, spectroscopic, and catalytic properties. Preparation of uniform AuNPs becomes a very important issue in applications because physical and chemical properties of particles highly depend on their size and shape [1]. In general, AuNPs have been prepared chemically through the reduction of hydrogen tetrachloroaurate (HAuCl4 ) by suit* Corresponding author.
E-mail address: [email protected] (S.W. Han). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.07.076
able reducing agents in the presence of stabilizers. The type and molar ratio of reductant to gold ion, type of stabilizer, reaction time, and temperature are critical experimental variables for controlling the size and shape of the synthesized particles [6–8]. Most widely used synthetic protocol for the preparation of AuNPs is citrate reduction method [9]. Waterdispersible AuNPs with average diameter of 1.6–140 nm can be successfully prepared by this method [8,10]. Recently, amines have been used in the synthesis of AuNPs as both reductants and capping species of nanoparticles [8,11–17]. A variety of amines have been examined including primary amines [8, 11–13], amino acids [14,15], secondary and tertiary amines [8,11], and polymers containing amine moieties [8,16,17]. Since amines are present in most of biological and environmental systems, nanoparticles prepared with amines should be attractive materials for biological applications [11]. However, reaction time required for the formation of nanoparticles by using amines is relatively long and applications with prepared particles have been little reported. In this work, we report on the simple and fast aqueousphase method for preparing stable AuNPs by reduction of HAuCl4 with 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (azacryptand, Fig. 1) at room temperature. The prepared
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2.3. Measurements
Fig. 1. Molecular structure of azacryptand.
nanoparticles have been characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). The SERS activity of the particles was tested by using various analytes and compared with that exhibited by common colloidal substrate for SERS measurements, i.e., citrate-reduced AuNPs. The catalytic activity of the nanoparticles in the reduction of 4-nitrophenol was also studied. Azacryptand has been used as a host molecule to selectively bind and isolate one desired ion from its surrounding environment [18]. To our knowledge, using the azacryptand as both reductant and stabilizer in the preparation of nanoparticles has not been reported yet. In fact, this molecule has been chosen because of its unusual structure. The azacryptand has 8 amine moieties per molecule. It can be assumed that this large amount of amine units should enhance the reduction function of the molecule. Indeed, AuNPs were rapidly prepared by using azacryptand and the resulting particles could be immediately applied to SERS spectroscopy and catalysis.
The extinction spectra were recorded with a SINCO S-3100 UV–vis spectrophotometer. TEM images were obtained with a JEOL JEM-2010 transmission electron microscope operating at 200 kV. High-resolution TEM characterizations were performed with a FEI Technai G2 F30 Super-Twin transmission electron microscope operating at 300 kV. XRD patterns were obtained with a Bruker AXS D8 DISCOVER diffractometer using CuKα (0.1542 nm) radiation. Infrared spectra were measured by using a Shimadzu FTIR-8400S spectrometer. Raman spectra were obtained using a Jobin Yvon/HORIBA LabRAM spectrometer equipped with an integral microscope (Olympus BX 41). The 632.8 nm line of an air-cooled He/Ne laser was used as an excitation source. Raman scattering was detected with 180◦ geometry using a Peltier cooled 1024 × 256 pixel CCD detector. The Raman band of a silicon wafer at 520 cm−1 was used to calibrate the spectrometer. 2.4. Catalytic reduction of 4-nitrophenol In a typical experiment, 2.765 mL of water, 0.0750 mL of aqueous NaBH4 (2.00 × 10−1 M), and 0.150 mL of 1.00 × 10−3 M 4-nitrophenol were mixed in a standard quartz cuvette. Then, 0.010 mL of the azacryptand- or citrate-reduced gold hydrosol (the amount of metal was same in each sol) was introduced into the solution and time-dependent absorption spectra were recorded every 30 s in the range of 190–1000 nm at 25 ◦ C.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Particle synthesis and characterization
All chemicals, unless specified, were laboratory reagent grade, and triply distilled water, of resistivity greater than 18.0 M cm, was used when preparing aqueous solutions. Azacryptand was prepared by the condensation of tris(2aminoethyl)amine with glyoxal followed by reduction with NaBH4 [18,19].
AuNPs were prepared by simple mixing of chloroauric acid with azacryptand in aqueous solution at room temperature. This procedure spontaneously resulted in the formation of AuNPs without the addition of an extra reducing agent. The azacryptand simultaneously acts both as protective agent and as reductant, thereby significantly simplifying the process for preparing the nanoparticles. The AuNPs exhibit strong surface plasmon resonance (SPR) absorption that is dependent on the size and shape of particles. For spherical AuNPs, the SPR band maximum generally falls between about 520 and 530 nm [21]. Fig. 2 shows a typical spectral evolution of the SPR band for an aqueous solution of equimolar concentration of azacryptand and chloroauric acid (an equivalent ratio of amine to Au3+ , N/Au = 8). The general trend observed from Fig. 2 is the gradual increase of the SPR band at ∼532 nm. The absorption maximum plateaus after ∼7 min (see inset of Fig. 2). The spectral evolution was also accompanied by a distinct color change from pale-yellow to wine-red. These observations indicate that AuNPs can be formed by this simple process and the reaction time required for the synthesis of particles is relatively short. The prepared gold hydrosol is stable for several months. Very high concentration of azacryptand (N/Au = 80) results in the rapid reduction of the gold salt; however, under this condition the colloids formed were found to sediment after 12 h.
2.2. Particle synthesis The azacryptand-reduced AuNPs were prepared as following procedure. A 0.10 mL aqueous solution of HAuCl4 (0.024 M) was added rapidly into a vial containing aqueous solution of azacryptand (1.0 × 10−3 M, 2.4 mL). The concentrations of HAuCl4 and azacryptand are same in the final reaction mixture, i.e., 9.6 × 10−4 M. The solution in vial was kept at 25 ◦ C. The citrate-reduced AuNPs were prepared by following the literature with a difference only in the molar ratio of HAuCl4 to sodium citrate [20]. Namely, 40 mg of HAuCl4 was initially dissolved in 90 mL of water, and the solution was heated to boiling. A total of 10.2 mL of aqueous solution of sodium citrate (40 mM) was then added to the HAuCl4 solution under vigorous stirring, and boiling was continued for ca. 15 min.
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Fig. 4. Powder XRD pattern of the azacryptand-reduced AuNPs.
Fig. 2. Variation of UV–vis absorption spectra of the solution containing azacryptand and HAuCl4 as a function of reaction time (time interval = 60 s). Inset shows that SPR absorption maximum plateaus after ∼7 min.
Fig. 3. (a) TEM image of the azacryptand-reduced AuNPs. (b) High-resolution TEM image of an azacryptand-reduced AuNP. (c) Enlarged image of the square region in (b). A d-spacing between adjacent lattice planes of 2.36 Å corresponds to the (111) planes of fcc gold.
The actual structure and size distribution of particles can be determined by TEM. Fig. 3a shows a typical TEM image of the samples. The image reveals that the prepared particles have, in general, a spherical shape with average size of 20.9 ± 3.3 nm. Close inspection of the image also shows that some of the particles have polyhedral structures. For more insight into the fine structure of the AuNPs prepared in this work, a structural analysis of the particles was also performed by high-resolution TEM characterization. Fig. 3b shows the high-resolution TEM image of a nanoparticle obtained by the azacryptand. It was found that the majority of particles were predominantly faceted into crystallites. Fig. 3c gives the enlarged image of Fig. 3b to exemplify the perfect atomic arrangement in the particles. Atomic planes from the gold face-centered cubic (fcc) lattice are clearly visible; a d-spacing of 2.36 Å for adjacent lattice planes corresponds to the (111) planes of fcc gold [6,22]. The crystalline nature of the AuNPs was further confirmed by the XRD measurements. Fig. 4 shows the XRD pattern of the prepared sample. Five peaks were observed which can be assigned to the (111), (200), (220), (311), and (222) diffraction peaks of fcc gold metal [23], indicating that the sample is composed of pure crystalline gold. In the previous studies on the synthesis of AuNPs with amines, the reaction time required for the formation of particles was, in general, relatively long. For instance, in the case
of polymeric amines such as polyethyleneimine (PEI), complete reduction of gold ion occurred over a period of several days [8,16]. Furthermore, high amine to Au3+ ratio (N/Au), e.g., from several tens to hundred, was required to ensure stability of the system [8,11,16]. In this study, stable aqueous suspension of AuNPs can be rapidly prepared with small amount of amine. Even if 0.5 equivalent azacryptand (N/Au = 4) was used, AuNPs could be readily synthesized. This result can be attributed to unique structural characteristic of the azacryptand molecule. Azacryptand has two tertiary amines and six secondary amines per molecule. This high local concentration of secondary and tertiary amine units can enhance the reduction function of the molecule. In fact, Richardson et al. found that for short alkyl chains the tertiary amines are the most effective for reducing Au3+ , followed by secondary amines. When providing necessary coordination required for stabilization of the nanocrystals as they are formed, secondary amines are more effective [8]. A still remaining question is how gold cations were reduced by azacryptand. Kuo et al. reported that Au3+ was reduced by polymeric amine such as the branched PEI [16]. It has been postulated that the amino groups on the polymer are oxidized to imino moieties, and this is the source of electrons by which the reduction of Au3+ could take place. To investigate the changes in the molecule accompanying the reduction of HAuCl4 , FT-IR spectra of azacryptand before and after reaction were examined. Fig. 5a shows the FT-IR spectrum of pristine azacryptand. The characteristic bands of the molecule appear in the spectrum: CH2 scissoring + NH in-plane bending (1479, 1466 cm−1 ), NH in-plane bending (1439 cm−1 ), CH2 wagging (1335 cm−1 ), CH2 twisting (1291, 1273, 1221, 1149 cm−1 ), and C–N stretching (1111, 1068 cm−1 ) [24]. The broad band observed in the region of 1650 cm−1 may be attributed to NH deformation mode of azacryptand and bending vibration of residual water. Fig. 5b shows the spectrum of the azacryptand-reduced AuNPs. As shown in the figure, spectral features are noticeably changed after the reaction. The relative intensities of bands assigned to the C–N stretching, NH bending, and CH2 deformation (wagging and twisting) modes of azacryptand were considerably decreased and the expected concurrent appearance of new bands related to the formation of imine was clearly observed: C=N stretching (1645, 1541, 1514 cm−1 ) and CH in-plane bend-
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3.2. SERS measurements
Fig. 5. FT-IR spectra of (a) pristine azacryptand and (b) the azacryptand-reduced AuNPs.
ing (1381, 1252 cm−1 ) [25–27]. These observations indicate that azacryptand molecules reduce gold precursors as the amino moieties in the molecules are oxidized to imino groups.
For investigating the application aspect of the prepared particles for SERS spectroscopy, we have examined SERS efficiency of the azacryptand-reduced AuNPs. SERS spectra of various adsorbates were measured with the azacryptandreduced AuNPs and compared them with those obtained with common colloidal substrate for SERS measurements, i.e., citrate-reduced AuNPs. The citrate-reduced AuNPs were prepared by following the previously reported procedure and have similar average size (19.6 ± 3.7 nm) with the azacryptandreduced AuNPs. Fig. 6 shows the SERS spectra of (a) 5 × 10−5 M benzenethiol (BT), (b) 5 × 10−5 M 4-nitrobenzenethiol (4-NBT), (c) 5 × 10−5 M 4-aminobenzenethiol (4-ABT), (d) 5 × 10−5 M 2-mercaptopyridine (2-MP), and (e) 5 × 10−7 M rhodamine 6G (R6G) molecules obtained with both azacryptand-reduced (upper trace) and citrate-reduced (lower trace) AuNPs in aqueous solutions. The concentration of nanoparticles was adjusted to same value for all the analyte solutions. It is noticeable that azacryptand-reduced AuNPs give stronger signals; SERS intensities obtained with the azacryptand-reduced particles are at least ∼2× greater than those with the citrate-reduced particles. One of the possible reasons for the observed difference in SERS activities between azacryptand- and citrate-reduced particles is different surface chemistry of the particles. Since the azacryptand- and citratereduced particles are stabilized with azacryptand and citrate, respectively, net surface charge and molecular nature of the cap-
Fig. 6. SERS spectra of (a) 5 × 10−5 M BT, (b) 5 × 10−5 M 4-NBT, (c) 5 × 10−5 M 4-ABT, (d) 5 × 10−5 M 2-MP, and (e) 5 × 10−7 M R6G molecules obtained with azacryptand-reduced (upper trace) and citrate-reduced (lower trace) AuNPs in aqueous solutions. (f) UV–vis absorption spectra of azacryptand-reduced (- - -) and citrate-reduced (—) AuNPs. The vertical line represents the excitation wavelength for SERS measurement at 632.8 nm.
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ping group are different. This different environment can thus affect the access of the adsorbates to the surface of nanoparticles. However, this effect is not believed to be a dominant factor responsible for the higher SERS activity of azacryptand-reduced particles because the azacryptand-reduced nanoparticles show higher SERS efficiencies for all the adsorbates tested which have different polarities. The higher scattering intensity of azacryptand-reduced particles could be caused by the larger amount of overlap between SPR band and the excitation source. It is widely accepted that the SERS effect is the result of enhancement of localized electromagnetic field due to SPR [5]. For the azacryptandreduced particles the SPR band is located at slightly longer wavelength and rather broader than the SPR band for the citratereduced particles, result in larger SPR absorbance at the excitation wavelength (see Fig. 6f). The observed different UV– vis spectral features may be ascribed to the fact that different stabilizers are present on the surface of nanoparticles and some amount of polyhedral particles exist in the sample synthesized by azacryptand (Fig. 3a). In fact, most of the particles in the sample prepared by citrate-reduction have spherical shape [20]. Moreover, the presence of polyhedral nanoparticles is sure to contribute to the enhanced SERS activity of the azacryptand-reduced AuNPs because of their structural characteristics. Large enhancement of local electromagnetic field is generally observed near sharp surface features such as edges and corners [28–30]. These active regions can serve as hot sites for surface plasmon enhancement [31,32]. Polyhedral particles have more well-defined edges and corners, and have generally sharper surface features than do spherical ones. As a result, SERS signals of the adsorbates on the active sites of polyhedral particles can be largely enhanced and the contribution from these molecules can be another feasible source of the higher SERS activity of the sample prepared by azacryptand. 3.3. Catalysis
Fig. 7. Successive UV–vis absorption spectra (30 s interval) of the reduction of 4-NP by NaBH4 in the presence of (a) azacryptand-reduced and (b) citrate-reduced AuNPs. The insets show the plot indicating the variation of ln A vs time.
Metal nanoparticles have been widely used as efficient catalysts in transformations of organic substances because of their large surface-to-volume ratio and different electronic properties compared to corresponding bulk metals [2,33]. To study the catalytic activity of the azacryptand-reduced AuNPs, we have chosen the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4 ) as a model reaction because this reaction is rapid and can be easily characterized. The catalytic reduction of several aromatic nitro compounds to the corresponding amino derivatives using NaBH4 and metal nanoparticles has been reported [34–37]. Nitro compounds are inert to NaBH4 if it is used alone. However, the metal nanoparticles effectively catalyzed the reduction of nitro compounds by acting as an electron relay system; electron transfer took place between nitro compounds and NaBH4 through the metal particles. The reaction can be readily monitored with UV–vis spectroscopy. Fig. 7a shows successive UV–vis absorption spectra of the reduction of 4-NP by the azacryptand-reduced AuNPs. In NaBH4 medium the peak corresponding to 4-NP at 317 nm was red-shifted to 400 nm because of the formation of 4-nitrophenolate. In the
absence of nanoparticles, the peak due to 4-nitrophenolate remains unaltered. The addition of AuNPs to the solution causes the disappearance of the 400 nm peak with the concomitant appearance of a new peak at 300 nm. This peak has been attributed to 4-aminophenol [34–37]. This result shows that the azacryptand-reduced AuNPs catalyzes the reduction reaction. Since the concentration of BH− 4 used for the reaction largely exceeds the concentration of 4-NP and the AuNPs, pseudo-firstorder kinetics with respect to 4-NP could be used in this case to estimate the rate constant. A good linear correlation with time, that is, ln A vs time plot, was obtained (inset of Fig. 7a). Here, A stands for the absorbance at 400 nm at any time. The pseudo-first-order rate constant determined from this plots is 1.0 × 10−2 s−1 . For comparison, same experiment was also preformed with the citrate-reduced AuNPs. The result is shown in Fig. 7b. The estimated rate constant from ln A vs time plot (inset of Fig. 7b) is 2.0 × 10−3 s−1 . This indicates that reaction rate of the azacryptand-reduced AuNPs is 5 times greater than that of the citrate-reduced particles. The low reduction rate of the citrate-
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reduced AuNPs can be ascribed to the negatively charged surface of anionic stabilizer, i.e., citrate, which hindered the incorporation of phenolate ion onto the surface of gold nanoparticles [34]. The rate constant value of the azacryptand-reduced AuNPs is also higher than those obtained with other previously prepared AuNPs such as poly(amidoamine) dendrimerstabilized AuNPs [35] and spongy AuNPs [37]. The value is comparable to the reported one for AuNPs supported by poly(propyleneimine) dendrimer which effects the diffusion of 4-NP to the surface of catalyst particles and thus enforce the catalytic properties [35]. These indicate that the AuNPs prepared by azacryptand can effectively catalyze the reduction process. 4. Conclusions The present work demonstrated that crystalline gold nanoparticles have been prepared using azacryptand as a dual reductant/stabilizer. The advantages of the azacryptand-reduced nanoparticles are seen in fast and simple preparation at room temperature and their immediate applicability for SERS spectroscopy and catalysis. The prepared particles give stronger SERS signals for various analytes than do conventional citratereduced particles and also show enhanced catalytic activity in the reduction reaction of nitro compound. These characteristics of the new colloid can allow them to be highly promising substrate for a range of different analytical and catalytic applications. Acknowledgments This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF2006-311-C00355), by a grant from the MOST/KOSEF to the Environmental Biotechnology National Core Research Center (grant #: R15-2003-012-01001-0), and by Technology Development Program of the Ministry of Agriculture and Forestry, Republic of Korea. References [1] J.H. Fendler (Ed.), Nanoparticles and Nanostructured Films, VCH, Weinheim, 1998. [2] R. Narayana, M.A. El-Sayed, Nano Lett. 4 (2004) 1343. [3] P.V. Kamat, J. Phys. Chem. B 106 (2002) 7729. [4] T.A. Taton, C.A. Mirkin, R.L. Letsinger, Science 289 (2000) 1757. [5] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241.
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