Surface Enhanced Raman Spectroscopy of Volatile ...

Surface Enhanced Raman Spectroscopy of Volatile Organic molecules on the surface of Zinc Nanoparticles Produced by Laser Ablation Subhash C. Singh, Raj K. Swarnkar, Preyas Ankit, Mahesh C. Chattopadhyaya, and R. Gopal Citation: AIP Conf. Proc. 1075, 67 (2008); doi: 10.1063/1.3046230 View online: http://dx.doi.org/10.1063/1.3046230 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1075&Issue=1 Published by the AIP Publishing LLC.

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Surface Enhanced Raman Spectroscopy of Volatile Organic molecules on the surface of Zinc Nanoparticles Produced by Laser Ablation Subhash C.Singh^ Raj K. Swamka/*, Preyas Ankit^ Mahesh C.Chattopadhyaya'' and R.Gopaf "Laser and Spectroscopy Laboratory, Physics Department, University ofAllahabad, Allahabad-211 002, India Chemical Sensors Laboratory, Department of Chemistry, University of Allahabad, Allahabad- 211 002, India SubhashJaserlab ®yahoo, co. in; spectra2 @ rediffmail. com Abstract: Surface enhanced Raman spectroscopy have provided potential tool for the detection of organic /biological molecules within the cell of living organism. This technique is suitable for in vivo as well as in vitro detection upto single molecular level. In the present work we have studied SERS activity of zinc nanoparticles on CCLt and CHCI3 molecules. Colloidal solution of zinc nanoparticles was found as a suitable substrate for Raman signal enhancement. The applied technique may be useful for the sensing of organic/biological molecules as a trace in solid as well as in liquid media. Keywords: Surface enhanced Raman, Zn Nanoparticles, Colloidal solution. Organic molecules PACS: 61.46.Hk; 52.38.-r; 52.38 Mf; 33.20 Fb

INTRODUCTION It is a matter of great debate as to how the local electromagnetic environment enhances the substrate-adsorbate complex's spectral response, since the discovery of Surface Enhanced Raman Spectroscopy [1]. It is proved that plasmon resonance of metallic substrates is responsible for electromagnetic contribution to SERS. Lack of reliable detecting instrumentation and powerful light sources was the main drawback in the study of metal surface property dependence local field. Roughened metallic surfaces were initially used for the enhancement of Raman signal, but now different types of metallic Nanoarchitectures such as Nanoparticles, Nanowires, Nanosheets, Nanorings etc. are available for enhancement of electromagnetic signal upto several modes. Following are some of the key features which are essential for the enhancement of Raman signals [2]. SERS occurs when molecules are brought to the surface of metals in a variety of morphologies. The smooth surface is not active for the enhancement. • Large enhancements are observed from silver, gold and copper. If the metal nanoparticles are used in the system, the particle size for enhancement of Raman to happen ranges from 20nm~300nm. • Molecules adsorbed in the first layer on the surface shows the largest enhancements. CPl 075, Perspectives in Vibrational Spectroscopy: ICOPVS 2008; edited by V. K. Vaidyan and V. S. Jayakumar ® 2008 American Institute of Physics 978-0-7354-0606-3/08/$23.00

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However, the enhancement also has long-range effects of about tens of nanometers. • The excitation profile (scattering intensity vs. exciting frequency) deviates from the fourth-power dependence of normal Raman scattering. As Copper, silver and gold colloidal solutions, Nanorods and films are widely applicable for enhancement of Raman signals [3-5] in recent days but Zn/ZnO nanomaterials have also shown considerable enhancement for the same. Gu et.al [6] have studied SERS on the inhibition mechanism for the imidazole on zinc surface while, Ruan et.al [7] have used ZnO nanostructured film. SERS activity of colloidal solution of zinc nanoparticles surface for CCU and CHCI3 molecules is main theme of the present investigation. One thousand parts of substrate is sufficient for the enhancement of the local electromagnetic signal of analyte molecules.

EXPERIMENTAL DETAILS Synthesis of zinc nanoparticles has been described in detailed elsewhere [8]. Zinc metal plate, placed on the bottom of glass vessel containing 10 mM, 40 ml aqueous solution of SDS, is allowed to irradiate with focused output of 1064 run of Nd: YAG laser (Spectra Physics, Quanta Ray, USA) operating at 35 mJ/ pulse energy, 10 Hz frequency and 10 ns pulsed width. Ablation is done for 30 min., and a pale yellowish colored colloidal solution is obtained, which is found stable for several months. EM-CM, 12 (Philips), microscope is used for TEM image recording and UNICAM 5625, UV/VIS Spectrophotometer for absorption. Pure CCI4 and CHCI3 (HPLC grade) is taken in the 4 ml quartz cuvette and 8100 |J,L colloidal solution of zinc nanoparticles are added to study the SERS activity of the Nanoparticles. 514.5 nm (19393.18 cm'^) line of Ar"^ laser is used to excite the analyte molecules adsorbed on the substrate surface and 0.5 M monochromator (Acton Research Corp., Spectra Pro, USA) with PMT detector is used for recording the SERS spectra using Spectra Sense software. Laser light is passed through the prism followed by circular aperture to eliminate plasma lines. Grams-32 software is used for spectral analysis and post processing.

RESULTS AND DISCUSSION

Wavdenth (nm)

Figure 1. UV-visible absorption spectrum of colloidal solution of Zn Nanoparticles

UV-visible absorption spectrum of produced colloidal solution shown in Figure 1 having surface plasmon peak at 232.4 nm, falls close to the characteristic wavelength of Zn Nanoparticles. As Nanoparticles are synthesized in the aqueous solution of SDS above CMC (8.3 mM), synthesis of zinc nanoparticles dominates over ZnO [9]. TEM image of as synthesized nanoparticles is displayed in Figure 2, which represents monodispersed 13.5 nm average, sized particles of metallic zinc. 68


Addition of few \iL colloidal solutions of zinc nanoparticles in the volatile organic compounds causes adsorption of these molecules on the surface of nanoparticles. Fig. 3 (a) shows normal Raman spectrum of pure chloroform, while 3(b) and 3(c) presents enhanced spectra after addition of 8 and 20 |a,L of colloidal solution of zinc 100 nm Nanoparticles respectively. There is no Figure 2. TEM images of Zinc significant difference between the spectra Nanoparticles enhanced due to addition of 8 and 20 |a,L solution, but it causes slight shift in Raman frequency. It is confirmed from figure 3 that strong enhancement in the Raman signal occurred after addition of very small amount of zinc NPs. Maximum enhancement is observed for the Raman peak at 580 cm"\ Besides this there is also significant enhancement in the Raman signal centered at 680, 1100 and 2728 cm"^ peaks. From this observation it is observed that very small amount of zinc nanoparticles causes strong enhancement in the Raman signal. SERS of chloroform with the addition of 40, 60, 80 and 100 |a,L of zinc nanoparticles are displayed in Fig. 4. Normal Raman and SERS of CCLt with the addition of 40 |a,L solution of colloidal zinc nanoparticles are presented in Fig. 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 -2500

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Raman Shift (cm-1)

Raman Shift (cm-1)

Figure 3. (a) Raman spectrum of pure CHCI3 (b) SERS for 8 and (c) 20 |xL colloidal solution of zinc NPs respectively

Figure 4. (a) SERS for the addition of (a) 40 (b) 60 (c) 80 and (d) 100 |xL colloidal solution of zinc NPs into chloroform

Enhancemient Mechanism Surface Enhanced Raman Spectroscopy (SERS) is a Raman Spectroscopic (RS) technique that provides greatly enhanced Raman signal from Raman-active analyte molecules that have been adsorbed onto certain specially prepared metal surfaces. Increase in the intensity of Raman signal have been regularly observed in the order

400

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Ram an Shift (cm"^)

Figure 5. (a) Raman (b) SERS of the CCI4 molecule on the surface of zinc NPs

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of lO'^-lO^, and can be as high as 10^ and 10^"* for some systems. The importance of SERS is that it is both surface selective and highly sensitive whereas RS is neither. RS is ineffective for surface studies because the photons of the incident laser light simply propagate through the bulk and the signal from the bulk overwhelms any Raman signal from the analytes at the surface. There are two primary mechanisms of enhancement described in the literature: an electromagnetic and a chemical enhancement. The electromagnetic effect is dominant, the chemical effect contributing enhancement only in the order of one or two of magnitude. The electromagnetic enhancement (EME) is dependent on the presence of the metal surface's roughness features, while the chemical enhancement (CE) involves changes to the adsorbate electronic states due to chemisorption of the analyte. The structural and molecular identification power of RS can be used for numerous interfacial systems, including electrochemical, modeled and actual biological systems, catalytic, in-situ and ambient analyses and other adsorbate-surface interactions. Due to the sensitivity of SERS, detection of trace molecules can also be accomplished. SERS is observed primarily for analytes adsorbed onto coinage (Au, Ag, Cu) or alkali (Li, Na, K) metal surfaces, with the excitation wavelength near or in the visible region. Theoretically, any metal would be capable of exhibiting SE, but the coinage and alkali metals satisfy calculable requirements and provide the strongest enhancement. Metals such as Pd or Pt exhibit enhancements of about 10^-10^ for excitation in the near ultraviolet.

ACKNOWLEDGEMENT The Authors are thankful to DRDO, New Delhi for financial support and Prof. O.N. Srivastava, Banaras Hindu University, Varanasi for TEM facility.

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