SERS enhancement dependence on the diameter of Au nanoparticles

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1 Journal of Physics: Conference Series PAPER OPEN ACCESS SERS enhancement dependence on the diameter of Au nanoparticles To cite this article: Yifei Yang 2017 J. Phys.: Conf. Ser View the article online for updates and enhancements. Related content - Surface-Enhanced Raman Scattering in Multilayered Au Nanoparticles in Anodic Porous Alumina Matrix Toshiaki Kondo, Kazuyuki Nishio and Hideki Masuda - Metal-carbon nanoclusters for SERS A Kucherik, S Kutrovskaya, A Osipov et al. - SINGLE BACTERIUM DETECTION USING SERS S.A. Gonchukov, T.V. Baikova, M.V. Alushin et al. This content was downloaded from IP address on 25/11/2018 at 16:53

2 SERS enhancement dependence on the diameter of Au nanoparticles Yifei Yang Shanghai Jiaotong University Abstract Series of Au colloidal solutions with different diameters were synthesized by the chemical reaction method. The influence of Au nanoparticles with different size on SERS of R6G was investigated. Experiments indicate that the enhancement factor grows in direct proportion to size of Au nanoparticles within limit. 1. Introduction Surface-Enhanced Raman Scattering (SERS) has been a useful tool to study the vibrational mode and structure information for molecules. It is called fingerprint of molecules and its sensitivity has reached to a single molecule detecting level [1 3]. The enhancement factor (Ef) is an important parameter of SERS spectra which depends on the metal surface roughness, the size of metal particles used for the SERS substrate, and the wavelength chosen to stimulate SRES. Only suitable surface roughness or size of metal particles could produce stronger Raman signal [4 5]. Yabing Du studied the enhancement factor of different surface roughness for Cu and Ag substrate, respectively, and result indicated that the maximum Ef in visible excitation can be obtained for silver with an average surface roughness of 100 nm [6] and for copper with 50 nm surface roughness in red-visible excitation [7].The construction of substrate can also have an influence in the effect of SERS. Conventional methods make use of metal colloidal,metal electrode and island film to prepare SERS substrates. In recent years, a few kinds of metal substrates embellished by organic molecule and oxide has been fabricated to get a more stable and effective SERS system. Sanchez-Cortes research group synthesized metal nanoparticles embellished by calixarene molecules with different substituent group, and used Ag nanoparticles embellished as SERS substrate to detect four PAHs such as purine, benzanthracene, benzophenanthrene and coronene. The minimum concentrate which can be detected has reached mol/l. Li fabricated Au nanoparticles substrate with thickness-adjusted SiO2 shell. Zhang realized quantitative determination of nitrite ion in food sample with Au@SiO2 particles. Although these metal substrates have more effective enhancement, they are usually complex to synthesize. Compared with other SRES substrates, metal colloidal is easy to fabricated and has a comparable enhancement factor. The size of particles in metal colloidal is easy to control. There are many reports that metal nanoparticles with only one diameter are used as SERS substrate. But to our knowledge,the study of substrates for Au nanoparticles with different diameters is rare to be seen. The research may help us to learn more about the physical mechanism of SERS.In this paper a series of Au nanoparticles with different diameters was synthesized by the chemical reaction method. The influence of diameter of Au nanopartcles was investigated by detecting SRES spectra of R6G. 2.Experimental 2.1 synthesis of colloidal Au nanoparticles with different diameters Colloidal Au nanoparticles were prepared using Frens method[8].by changing the amount of reductive agent,the size of Au nanoparticles can be controlled.the standard procedure is as Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 follows:50 ml HAuCl4 aqueous solution ( g/ml) was heated to boiling. x ml C6H5Na3O7 ( g/ml) was added by a micro-pipette. Boiling was continued for 15 min. A purple-red Au colloidal solution was obtained after cooling at ambient. The analytical grade HAuCl4 and C6H5Na3O7 were purchased from Shanghai chemical reagent corporation and Sinopharm Chemical Reagent Co. Ltd. in China, respectively. Five samples were obtained by adding 5 different volumes of C6H5Na3O7,which are shown in table 1. TEM images of Au nanoparticles are shown in Fig 1,which were made in Tecnai G2 SpiritBiotwin. The average diameters obtained are 70nm,60nm,48nm,24nm,12nm, which reduce with the increasing concentrate of reductive agent. When the size of Au nanoparticles grows up, the shape are irregular and the ellipticity become larger. simple a b c d e x Diameter 70nm 60nm 48nm 24nm 14nm Table.1. amount of adding reductive agent The linear absorption spectra of solutions are shown in Fig. 2, which were obtained using a UV vis NIR spectrophotometer (type: Varian Cary 5000). For the five Au colloids prepared by chemical methods, the surface plasma resonance absorption peaks are located between 519nm and 539nm.And absorption peak shifts to longer wavelength as the diameter increases. The relation between absorption peak and diameter of Au nanoparticles is fitted linearly as shown in Fig.3. (a) (b) (c) (d) (e) Fig.1.TEM images of Au nanoparticles fabricated by adding (a)0.21ml (b)0.3ml (c)0.5ml (d)0.75ml (e)1.5ml C6H5Na3O7 solutions 2

4 Fig.2.The linear absorption spectra of solutions Fig.3.The relation between absorption peak and diameter of Au nanoparticles 2.2 detection of SERS spectra 5 Au colloidal samples with different average diameters (14nm 24nm 48nm 60nm 70nm)were prepared. Every sample was added with equal volume of R6G aqueous solution and dispersed by ultrasonic agitation to ensure a homogeneous distribution of Au nanoparticles. The concentrate of R6G is 10-4 M. After 4 hours,load the samples with capillary tube and detect the SERS spectra. The Raman spectra was made in Dispersive Raman Microscope Senterra R200-L. Fig.4.SERS spectra of R6G solution(10-4 M)doped Fig.5. Normal raman spectra of with Au nanoparticles R6G solution (10-3 M) 3

5 (a) Fig.6.enhancement factor of mode (a)1361cm -1 and (b)1509cm Result and discussion The result of SERS detection is shown in Fig.4. To the pure R6G aqueous solution, intensity of ramanshift is nearly hard to recognize because of the low concentrate(10-4m) while there are obvious raman peaks for the samples doped Au nanoparticles with different diameters. To calculate the enhancement factor exactly, we also make a detection of pure solution with a higher concentrate which is shown in Fig.5. The relation between raman peaks and molecule vibration mode is shown in table.2. The raman peak 1308cm belongs to the flexural vibration of Cx-H while 1361cm -1, 1509cm -1 and 1647cm -1 belong to stretching vibration of Cx-Cx. Formula used to calculate enhancement factor is as follows: Ef=(Is/In) (Cn/Cs) In is the normal raman intensity of peak while Is is the SERS intensity. Cn and Cs are the concentrate of molecule for normal and SERS spectra, respectively. We calculate the Ef of mode 1361 and 1509,respectively. The result is shown in Fig.6.On the whole,ef increases as the diameter of Au nanoparticles becomes larger for both 1361 and As we known, the physical motion of SERS is attribute to surface plasma resonance of metal and local field enhancement. After the SERS of single molecule was successfully observed, a new mechanism for SERS, so-called hot spot or hot site, had been put forward [1]. Hot spot is a very special place where a huge enhancement up to 1014 could be produced. Combining the experiments and related theoretic analysis, it was understood that the hot spot is the one between two very closely placed solid metal nano-particles. The theoretic calculation showed that the enhancement could reach up to 1010 or more, when a molecule is placed at a gap with 1 nm distance surface-to-surface between two silver nano-particles. And this enhancement would also dramatically decrease with the increase of gap width, for instance, when the distance surface-to-surface increases from 1 to 5.5 nm, the enhancement decreases more than three orders of magnitude [9]. When R6g solution is added,the distance of Au nanoparticles will reduce to form Au nanaoparticle aggregates after absorbing the R6G molecules. Au particles in these aggregates could become hot-spot if they were close enough.the intensity of electric field excited from hot spot is larger than that from only one nanoparticle. Compared with small Au nanoparticles in colloidal, the Au colloidal with large nanoparticles is more unstable dispersion system. So there may be more hot spots in the solution with larger nanoparticles. Besides this, the local field excited by periodic arrangement Au nanopartcles will become larger as the increasing of particle size in a certain range[10]. This two point may cause for the influence of Au nanoparticle size on SERS. On the other hand, from the result,the Ef of different raman peaks is different. We speculate there is chemical enhancement besides electric enhancement. The charge transfer model is selective to the vibration mode of molecules. But it is difficult to obtain how much share from the chemical enhancement at this moment, because the experimental data obtained up to now are not enough to analyze this problem. (b) Reference [1] S. Nie, S.R. Emory, Probing single molecules and single nanoparticles by surfaceenhanced Raman scattering, Science 275 (1997)

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