A Facile Seed-Mediated Route to Au Nanoplates for SERS Application

Wei-Hao Lin Department of Materials Science and Engineering

National Chiao Tung University Hsinchu, Taiwan 30010, Republic of China

Yung-Jung Hsu*

Department of Materials Science and Engineering National Chiao Tung University

Hsinchu, Taiwan 30010, Republic of China *E-mail: yhsu@mail.nctu.edu.tw

Abstract— We have successfully synthesized Au nanoplates using a seed-mediated approach. Without the size-selection process, the yield of nanoplates in the as-obtained product was as high as 80 %. The nanoplate samples were characterized with TEM, SAED, XRD, UV–VIS-NIR and SERS spectroscopy. The results of SERS analyses demonstrate the potential of the anisotropically-shaped Au nanoplates as an active platform for Raman-sensitive analyte molecules.

Keywords- Au; nanoplates; PVP; SERS

I. INTRODUCTION Shape control for nanocrystals has been the focus of

intensive investigation in the last two decades since specific crystallographic planes may induce unique optical, electronic, physical and chemical properties. For noble metal nanocrystals such as Au, the anisotropic prototypes were found to show promising potentials in many practical applications including optics [1], catalysis [2], and medicals [3]. Besides, the surface plasmon resonance (SPR) absorption of Au significantly shifts with its shape, size, degree of aggregation as well as the dielectric constant of the surrounding solvent, which could be further applied in relevant sensing aspects. Fabrication of Au nanocrystals with controllable morphology thus draws extensive research attention in recent years.

Among the various anisotropic nanocrystals of Au, nanoplates are the most promising due to the sharp corners and edges that are ought to be active for surface enhanced Raman scattering (SERS) signals. Although the optical properties of Au nanoplates have been well studied, their characteristics in SERS are not widely investigated. There are a variety of synthetic approaches to synthesize nanoplates of Au. For example, Jena and Raj reported the controlled synthesis of Au nanoplates by using 5-hydroxytryptamine as the structure-directing reagent [4]. Goy-Lo´pez et al demonstrated that Au nanoplates could be obtained and stabilized by Tetronic T904 in the presence of star-shaped PEO-PPO block copolymer [5]. In addition, the addition of halide ions in the crystal growth process of Au was found to play a crucial role in the formation of Au nanoplates [6].

The preparation of Au nanoplates was usually accomplished with a post size-selection operation, which may further hinder the applicability of the synthetic route. Therefore, creation of a more facile, effective approach from which one could obtain Au nanoplates in high yield is crucial

to their practical application. We developed here a seed-mediated method to obtain Au nanoplates in high yield without the size-selection process. The as-synthesized Au nanoplates are single crystalline with preferential growth directions perpendicular to the comprising top and bottom surfaces of 111 planes. We analyzed various aspects of synthetic approach, studied the optical properties of products, and successfully used them as active SERS substrate for methylene blue analyte.

II. EXPERIMENTAL SECTION

A. Chemicals All chemicals, including hydrogen tetrachloroaurate

trihydrate (HAuCl4.3H2O, 99.9% purity) and poly(N-vinylpyrrolidone) (PVP, MW=10000 and 29000), were obtained from Aldrich and used without further purification. Water used in all reactions was deionized with a resistance of 18.2ΩM.

B. Synthesis of seeds and nanoplates Aqueous solutions of PVP-1 (MW=10000, 22.48 wt%),

PVP-2 ((MW=29000, 22.48 wt%) and HAuCl4 (0.01 M) were prepared in advance. To obtain Au seeds with size of 5 nm, 8 mL of PVP-1, 0.065 mL of HAuCl4 and 1.935 mL of DI water were mixed in a sealed vial and heated at 130°C in oil bath for 3 hr. To synthesize Au nanoplates, 0.1 mL of Au seed solution and 10 mL of PVP-2 were added into HAuCl4 solution with different amounts. The mixture was then heated at 25°C for 24 hr to result in the formation of Au nanoplates.

C. Preparation of SERS substrate Au nanoplate suspensions with appropriate amounts

were dropped on Si substrate, and dried using hot-plate to remove the solvent. The substrate was then immersed in 10-5 M methylene blue (MB) solution, and washed with DI water to remove the un-adsorbed MB molecules.

D. Characterizations The morphology of the products was investigated with a

transmission electron microscope (JEOL 2100) operated at

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2010 International Conference on Biology, Environment and Chemistry IPCBEE vol.1 (2011) © (2011) IACSIT Press, Singapore

200 kV. The crystal structures of the samples were examined with X-ray diffractometer (Bruker D2 phaser). The light source of XRD was Cu Kα radiation with applied voltage of 30 kV and current of 10 mA. The absorption spectra were measured by a UV-VIS-NIR spectrophotometer (JASCO V-670). The SERS spectra were obtained with a Raman spectroscope (HORIBA JOBIN YVON) using the He-Ne laser of 532 nm as the excitation source.

III. RESULTS AND DISSCUSSION

A. Morphology It is well known that PVP played a critical role in the

formation of Au nanoplates. Therefore, we chose PVP as the stabilizer and reducing agent. It is also known that the reducing rate of function group of alcohol decreases if its alkyl chain length increases. As a consequence, polymers with hydroxyl functional groups such as PVP can be utilized as an ideal reductant for kinetically controlled growth of Au [7]. On the other hand, there exists a strong interaction between PVP molecules and the 111 facets of Au. This interaction could effectively retard the growth of Au along its [111] direction, promoting crystal growth along other directions to lead to the formation of Au nanoplates.

In addition to the use of PVP, the molar ratio (MR) of

PVP to HAuCl4 is also determinant. If the amount of PVP is relatively too high, PVP may cover most of the Au seeds plus the reducing power is strong, which would in turn lead to the growth of pseudospherical nanocrystals. On the other hand, if HAuCl4 is too much in amount, the yield of Au nanoplates becomes low because PVP cannot effectively cover the (111) facets of Au seeds. To find an appropriate MR of PVP/HAuCl4 is thus important to obtain Au nanoplates in high yield. In this work, we briefly studied the effect of PVP/HAuCl4 MR on the yield of Au nanoplate

product by adding HAuCl4 solutions with different amounts. Figure 1 displays the representative TEM images for Au nanoplates obtained with four different MRs of PVP/HAuCl4. Triangular and hexagonal nanoplates in a high yield of about 80% could be obtained with the MR of PVP/HAuCl4 of 10. The edge length of these nanoplates is about 400 nm, and the thickness is around 20 nm. If the MR of PVP/HAuCl4 was changed to 5 and 20, the yield of nanoplates decreased accompanied by an increase in the number of faceted nanocrystals. As the MR of PVP/HAuCl4 was increased to 40, the morphology of product was dominated by faceted nanocrystals, and nanoplates were rarely observed. In a short summary, an optimal MR of PVP/HAuCl4 of 10 was found in this system, above or below which pseudospherical nanocrystal population starts to dominate the morphology of product.

B. Crystallographic structures Figure 2(A) shows the selected area electron diffraction

(SAED) pattern of the present Au nanoplates. The SAED result indicates that the as-obtained nanoplates are single crystalline with the comprising top and bottom surfaces of 111 planes. In addition, a set of forbidden reflections of 1/3422 and 2/3422 was found, which is characteristic of the facet-centered cubic single crystalline structure typically observed in nanoplates. The existence of forbidden reflections also implies the formation of twins and stacking faults that are parallel to the (111) surface [8]. These structural defects are usually related to thin nanostructures covered by the top and bottom planes of 111.

TABLE 1. RATIOS OF I(200)/I(111) FOR THE FOUR AU NANOPLATE

SAMPLES.

Figure 1. TEM image of Au nanoplates obtained with different MRs of

PVP/HAuCl4: (A) MR=5, (B) MR=10, (C) MR=20, (D) MR=20.

Figure 2. (A) SAED pattern of an individual hexagonal Au nanoplate, (B)

XRD patterns of Au nanoplates obtained with different MRs of PVP/HAuCl4: (a)5, (b)10, (c)20, (d)40 .

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MR of PVP / HAuCl4

Intensity of XRD I(200) I(111) I(200)/I(111)

5 28.3 1000 0.0283

10 14.1 1000 0.0141

20 39.7 1000 0.0397

40 55.2 1000 0.0552

The X-ray diffraction (XRD) patterns of the four Au

nanoplate samples were compared in Figure 2(B). As expected, the diffraction peaks correspond well to the facet-centered cubic crystal structure of Au (JCPDS 89-3697). More importantly, an abnormal strong diffraction peak of (111) relative to (200) was recorded. Table 1 lists the ratios of intensity of (200) to (111) for the four Au nanoplate samples. Note that for bulk Au, the ratio of I(200)/I(111) is about 0.53 [9]. Among the four Au nanoplate products, a lowest I(200)/I(111) ratio of 0.0141 was observed for the sample obtained with MR of PVP/HAuCl4 of 10. This result reflects the fact that (111) facets comprised the top surfaces of Au nanoplates, which is consistent with the deduction from SAED analysis. From SAED and XRD data, we concluded that Au nanoplates were grown with preferential growth directions perpendicular to the comprising top and bottom surfaces of 111 planes.

C. UV-VIS –NIR spectra Figure 3 displays the UV-VIS-NIR absorption spectra of

the four Au nanoplate products. The absorption spectrum of the sample with MR of PVP/HAuCl4 of 10 exhibits a major absorption band at 1200 nm and a minor shoulder at 700 nm. The broad absorption band at around 1200 nm was contributed by nanoplates with edge length of several hundred nm [10]. The absorption shoulder at about 700 nm however originated from the pseudospherical nanocrystals of Au. For the samples with MRs of PVP/HAuCl4 of 20 and 40, the peak at 700 nm was predominant over the band at 1200 nm, again consistent with the TEM, SAED and XRD results that nanocrystal population starts to dominate the morphology of products.

D. SERS Since its discovery in 1974, SERS serves as a sensitive

platform for detecting molecular species adsorbed on the surfaces containing noble metal nanocrystals such as Au, Ag and Cu [11]. The fingerprint property and relatively high sensitivity of SERS spectroscopy make the technique to be widely applied in the detection of various chemical molecules. In recent years, the investigation of SERS using Au nanoparticles has been extensively pursued because million-fold enhancement in sensitivity could be acquired. The enhancement in detection sensitivity can be attributed to two mechanisms. The first one comes from the electromagnetic (EM) effect, while the second one is related to the charge transfer (CT) between metal and the targeted molecules [12]. In the case of Au nanocrystals, it is believed that EM is the main contribution for signal enhancement.

Since Au nanoplates obtained in this work were anisotropically shaped, their specific facets may focalize the EM field, making them an ideal platform for SERS ultradetection of analytes. In this work, we chose MB as the probe molecule to evaluate the SERS activity for the as-synthesized Au nanoplates. Figure 4 shows the SERS spectra of MB adsorbed on the substrates containing the four Au nanoplate samples acquired under the excitation of 532 nm laser. The characteristic peaks at around 1623 , 1393 and 446 cm-1 were related to the adsorbed MB molecules and could be assigned to the C–C ring stretching, C–N symmetric stretching, and the C–N–C skeletal bending modes, respectively [13]. Notably, the SERS signals could only be registered when MB was adsorbed on the substrates containing nanoplates. This demonstration reveals that the present Au nanoplates can serve as efficient SERS enhancers toward relevant Raman-sensitive analytes.

Figure 3. UV-VIS-NIR spectra of Au nanoplates obtained with

different MRs of PVP/HAuCl4: (a)5, (b)10, (c)20, (d)40.

Figure 4. SERS spectra of Au nanoplates obtained with different MRs of PVP/HAuCl4: (a)5, (b)10, (c)20, (d)40. The result of pure MB

aquelous solution (10-3 M) was also included in (e) for comparison.

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IV. CONCLUSIONS In conclusion, a seed-mediated approach was

successfully developed to synthesize Au nanoplates without the size-selection process. The yield of Au nanoplates depending on the MR of PVP/HAuCl4 was as high as 80 %. The SAED and XRD analyses reveal the preferential growth directions perpendicular to the comprising top and bottom surfaces of 111 planes. The UV-VIS-NIR absorption spectra verify the high purity of nanoplate structures in the as-obtained product. Furthermore, the results of SERS analyses demonstrate the potential of the anisotropically-shaped Au nanoplates as an active platform for Raman-sensitive analyte molecules.

ACKNOWLEDGMENT This work was financially supported by the National

Science Council of the Republic of China (Taiwan) under grants NSC-98-2113-M-009-015-MY2.

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