Synthesis, characterization and SERS activity of Au–Ag nanorods

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Spectrochimica Acta Part A 70 (2008) 780–784

Synthesis, characterization and SERS activity of Au–Ag nanorods Daizy Philip a,∗ , K.G. Gopchandran b , C. Unni b , K.M. Nissamudeen b a b

Department of Physics, Mar Ivanios College, Thiruvananthapuram 695015, India Department of Optoelectronics, University of Kerala, Kariavattom 695581, India

Received 5 July 2007; received in revised form 31 August 2007; accepted 17 September 2007

Abstract The formation mechanism and morphology of Au–Ag bimetallic colloidal nanoparticles depend on the composition. Ag coated Au colloidal nanoparticles have been prepared by deposition of Ag through chemical reduction on performed Au colloid. The composition of the Au100−x –Agx particles was varied from x = 0 to 50. The obtained colloids were characterized by UV–vis spectroscopy and transmission electron microscopy (TEM). The Au80 –Ag20 colloid consists of alloy nanorods with dimension of 25 nm × 100 nm. The activity of these nanorods in surface enhanced Raman spectroscopy (SERS) was checked by using sodium salicylate as an adsorbate probe. Intense SERS bands are observed indicating its usefulness as a SERS substrate in near infrared (NIR) laser excitation. © 2007 Elsevier B.V. All rights reserved. Keywords: Gold nanoparticles; Mixed Au–Ag nanoparticles; Au–Ag alloy nanorods; UV–vis spectra; TEM; SERS; NIR laser excitation; Sodium salicylate

1. Introduction The wet chemical synthesis of inorganic nanoparticles of various shapes made from metals, semiconductor, magnetic materials or insulators has made significant progress in recent years [1,2]. The unique optical, electromagnetic and thermodynamic properties of metal nanoparticles have made them useful in a variety of applications including biomedical sensors, probing devices, drug administration, optical data storage and electronic and semiconductor devices [2–4]. However, for metal nanoparticles to be applied effectively to technological advances something must be known of their material properties, which can vary significantly from that of their bulk counterparts. New lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes and dielectric environments [5–7]. Among the different methods [8–12], the procedure developed by Jana et al. [10] has attracted a great deal of attention since it provides an easy way to obtain well controlled gold nanorods. Nanocomposites, i.e. alloy and core–hell particles, is an attractive subject because of their composition dependent optical, cat-

∗

Corresponding author. E-mail address: [email protected] (D. Philip).

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.09.016

alytic and surface enhanced Raman scattering (SERS) properties [13–25]. Au–Ag alloy [20] and Ag–clad Au spherical nanoparticles have been used in the SERS studies and it is reported [22] that the SERS behaviour depends upon Ag:Au ratio. The dependence of the SERS intensity on the number of gold nanorods and the nanorods spacing on silica surface [23] were investigated using a near infrared (NIR) excitation source (1064 nm). Silver colloids usually have a higher SERS activity for excitation wavelengths in the visible region whereas Au ones are more active in the red but their activity highly decreases in the NIR region. The preparation of Ag–Au mixed colloids allows for the combination of the SERS activity of both metals in a broader interval of the electromagnetic spectrum. On the addition of analyte, the aggregated colloids show new absorption bands which appear towards the red region: 500–600 nm for Ag colloids and 700–900 nm for Au colloids. The new absorption bands are due to the plasmon excitation in aggregated particles that govern the SERS excitation profile [20]. Enhancement is large when the exciting radiation is coincident with the plasmon absorbance [22]. To achieve this in Au colloids at NIR laser excitation, Au–Ag alloy nanorods can be used. Also, the surface plasmon resonance (SPR) properties of Au–Ag alloy nanoparticles are continuously tunable because of the possibility of composition changes. So far, there is no report on Au–Ag alloy nanorods. In this work we report the synthesis of alloy nanorods of Au–Ag

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by a seed mediated method and its usefulness as a SERS substrate. 2. Experimental AgNO3 , HAuCl4 ·3H2 O, trisodium citrate and sodium salicylate were of highest purity available from Sigma–Aldrich. The aqueous solutions were prepared using deionized water. All glass materials were cleaned by using aqua regia and rinsed several times with deionized water. The initial Au colloid was prepared according to Grabar et al. [26] with slight modification. 500 mL of 1.4 × 10−3 M solution of HAuCl4 ·3H2 O was brought to boil. To the boiling solution 50 mL of 1% sodium citrate was added drop by drop while stirring. Boiling was continued for 30 min and then left to cool to room temperature. The resulting solution was made up to 500 mL. This colloid is wine red in colour and has absorption maximum at 526 nm. 2.1. Preparation of Au–Ag mixed colloids 6 × 10−4 M solution of AgNO3 was prepared. Aliquots of this solution 20, 10, 6.7 and 5 mL were added drop by drop to 20 mL of preformed citrate reduced Au colloid with continuous stirring to get 1:1, 2:1, 3:1 and 4:1 mixed colloid. Afterward, the corresponding volume of the sodium citrate aqueous solution in relation to the AgNO3 volume was added to induce the chemical reduction of Ag and its deposition on Au particles [20]. The mixture was kept boiling for 30 min. The resulting mixed colloids, 1:1, 2:1, 3:1 and 4:1, correspond to Au50 –Ag50 , Au67 –Ag33 , Au75 –Ag25 and Au80 –Ag20 , respectively. Of these four preparations, Au80 –Ag20 does not show any precipitation and is very stable. The other three colloids precipitated within 1 month after the preparation. The UV–vis absorption spectra were recorded on a Jasco V-550 UV–vis spectrophotometer in a 1 cm optical path quartz cuvette. The size of particles was analyzed by TEM using a JEOL JEM 1011 transmission electron microscope operating at 80 kV.

Fig. 1. Photograph of pattern formed on a Whatman filter paper.

The samples for TEM were prepared on a foamvar coated copper grid by depositing and drying 10 ␮L of the colloid. SERS spectra were recorded using a Bruker RFS 100/S model spectrophotometer connected to a liquid nitrogen cooled Ge detector with sample placed in a 1 cm path length glass cuvette. The 1064 nm line provided by a Nd:YAG laser was used for excitation in the NIR with 150 mW power output. Samples for SERS measurements were prepared by adding 5 mL of 10−2 M aqueous solution of sodium salicylate to 5 mL of the colloid. 3. Results and discussion A novel evolution of pattern as shown in Fig. 1 is observed within 20 s when a drop of the initial gold colloid is deposited on a Whatman filter paper. At first water spreads out and a small purple coloured sphere is formed at the centre which increases in size with time and finally a ring is formed around it. This pattern evolution may be due to the property of Au colloid to self assemble into a dense layer [27] that excludes water. The

Fig. 2. Photograph of the colloids: (a) Au50 –Ag50 , (b) Au67 –Ag33 , (c) Au75 –Ag25 , (d) Au80 –Ag20 and (e) Au.

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Fig. 3. UV–vis spectrum of the colloids: (a) Au50 –Ag50 , (b) Au67 –Ag33 , (c) Au75 –Ag25 , (d) Au80 –Ag20 and (e) Au.

colour change of the Au100−x –Agx colloid on varying the mole fraction is evident from the photograph shown in Fig. 2. Fig. 3 shows the UV–vis spectra of the initial Au colloid and the mixed Au–Ag colloids in the molar ratio 1:1, 2:1, 3:1 and 4:1 corresponding to Au50 –Ag50, Au67 –Ag33 , Au75 –Ag25 and Au8 –Ag20 , respectively. The 1:1, 2:1 and 3:1 composites show two distinct SPR bands indicating individual existence of Ag and Au nanoparticles having core–shell structure [14,16,20]. The relative intensities of the SPR band maxima depend on the molar ratio. Table 1 gives the dependence of SPR maximum on Au mole fraction. Fig. 4 shows a linear variation for SPR with mole fraction up to 0.75 for Au100−x –Agx colloids. As Au mole fraction increases the SPR of Au is red shifted and that of Ag is blue shifted. A single plasmon band observed at 513 nm for the composite colloid of molar ratio 4:1. Similar results were reported [13,16,20,25,27,28] in mixed metallic colloids. The appearance of only one absorption band indicates that homogeneous mixed colloidal particles of both metals are formed without significant formation of independent particles. In other words, Au–Ag alloy nanoparticles are formed. Fig. 5 shows TEM of initial Au colloid with nanoparticles of size ∼30 nm. The TEM displays a more uniform distribution of sizes and shapes and no rod-shaped particles are seen. TEM image obtained for Au80 –Ag20 colloid is shown in Fig. 6. It has a novel feature compared to the previous reports [13,14,16,20] on alloy nanoparticles. Large rod-shaped particles having average

Fig. 4. Dependence of plasmon absorbance maximum of (a) Au and (b) Ag on the mole fraction of gold.

Fig. 5. TEM image of Au colloid.

Table 1 Dependence of SPR maximum on mole fraction Mole fraction

0.50 0.67 0.75 0.80 1.00

SPR maximum Au (nm)

Ag (nm)

493 500 504 513 526

405 387 375 – –

Fig. 6. TEM image of Au80 –Ag20 colloid.

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of deposited Ag. The rod-shaped alloy nanoparticles are characterized by optical absorption spectroscopy and TEM. The large aspect ratio ∼4 of the particles is favourable for the use of these alloy nanoparticles as a substrate for SERS in the NIR laser excitation. The enhancement is evidenced using sodium salicylate as a molecular probe. Acknowledgements This research has been sponsored by UGC under the Research Award scheme granted to Daizy Philip. Excellent technical support for electron microscope was provided by Sanjai D, RGCB, Thiruvananthapuram. References

Fig. 7. SERS spectrum of sodium salicylate in Au80 –Ag20 colloid.

size of about 25 nm × 100 nm with aspect ratio ∼4 and SPR maximum at 513 nm which is lower that that of the initial Au colloid is observed. The absence of the band arising from the longitudinal plasmon mode [23,29–32] is due to the high aspect ratio. The aspect ratio ∼4 for the rod-shaped alloy nanoparticles is favourable for SERS activity using the NIR laser excitation as the longitudinal plasmon mode can be partially excited [33]. We have checked the SERS activity of the Au, Au50 –Ag50 , Au67 –Ag33 , Au75 –Ag25 and Au80 –Ag20 colloids using sodium salicylate as molecular probe. SERS spectrum could be obtained only for the Au80 –Ag20 colloid and the enhancement observed may be attributed to the unique rod-like morphology exhibited by it compared to other mixtures. The morphology change of Au in the alloy form is supposed to increase its plasmon resonance towards NIR region on the addition of the analyte which leads to electromagnetic field enhancement. SERS spectrum of sodium salicylate (10−2 M aqueous solution) in Au80 –Ag20 colloid is shown in Fig. 7. The intense band observed at 1370 cm−1 is due to the carbonyl C–O stretching, ν(C–O)c , mode [34,35]. Bands appearing below 250 cm−1 correspond to the metal–oxygen stretching, νAg–Au· · ·O, mode [34–37]. The other prominent bands at 1233, 1003 and 448 cm−1 are assigned to the ν(C–O)h , δCH and δCC modes, respectively. The γCH and νCC modes [34,37] appear at 849 cm−1 and 1580 cm−1 , respectively. However, the intensity distribution is different from that observed for SERS of sodium salicylate in Ag colloid [34]. This is due to the surface selectivity of SERS. The results show that the use of Au–Ag alloy nanorods as a substrate can expand the capabilities of SERS using NIR laser excitation. 4. Conclusion The formation mechanism and morphology of colloidal nanoparticles of Ag coated Au depend on the relative amount

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