R. Gopal & R.K. Swarnkar Laser Spectroscopy & Nanomaterials Lab. Centre of Advanced Studies Department of Physics University of Allahabad, INDIA STUDY OF SURFACE ENHANCED RAMAN SCATTERING OF ALIZARIN AND CRYSTAL VIOLET DYES

Out line Objective Introduction Experimental technique Result & Discussion Conclusion

Objective  Study of SERS of Alizarin dye on Ag colloidal NPs synthesized by PLA.  Study of SERS of Crystal Violet on Cu 2 Cu NPs synthesized by PLA.  Study the effect of halide ions on SERS of these dyes.

 Surface enhanced Raman scattering (SERS) revolutionalised the Raman spectroscopy in the last decade.  SERS is based on the marked enhancement of Raman signals of certain molecules when they are placed in the proximity of certain nano-structured metallic surfaces.  Two types of mechanisms (Electromagnetic and Chemical Enhancement) are currently used to explain the SERS phenomenon.  PLA in liquid medium is an excellent technique to synthesize colloidal NPs with desired shape and size and surface free from chemical contamination, which is essential requirement for surface application. Introduction

Experimental Techniques

Synthesis of Nanoparticles by PLA To synthesize Ag (Cu 2 Cu) NPs by PLA, a high purity silver (copper) slice is placed on the bottom of glass vessel containing 20 ml of distilled water. It is irradiated with focused output of 1064 nm of pulsed Nd:YAG laser (Spectra Physics Inc. USA) operating at fixed energy for 30 minutes. This results a yellow color in Ag (light green in Copper) colloidal solution.

Characterizations of Nanoparticles The synthesized nanoparticles are characterized by following techniques i.UV-Visible absorption spectroscopy: Perkin Elmer Lambda 35 double beam spectrophotometer ii.Transmission electron microscopy (TEM): Technai G20–stwin electron microscope iii.X-ray diffraction (XRD) Rikagu, D-Max X-ray diffractometer

MONOCROMATOR PMT Ar + L A S E R nm Sample holder SERS Experimental Set-up

Result & Discussion

Ag Nanoparticles UV-visible absorption spectrum: The absorption band at ~398 nm is due to SPR of Ag nanoparticles, confirms the formation of pure Ag nanoparticles. TEM image of Ag sample synthesized at 40 mJ/pulse laser energy confirms formation of nanoparticles with average particle size 2-3 nm. The inset shows SAED pattern, indicates formation of crystalline nanoparticles

Cu 2 Cu Nanoparticles TEM image of sample synthesized at 40 mJ/pulse laser energy confirms formation of nanoparticles with average particle size 5-8 nm. UV-visible absorption spectra: The absorption band at ~620 nm are due to SPR of Cu nanoparticles, while absorption in the range nm are due to interband transition of Cu 2 O.

The XRD spectrum of the sample synthesized at 40 mJ/pulse laser energy shows diffraction peaks corresponding to Cu 2 O and Cu, confirms the formation of Cu 2 Cu nanostructures.

SERS of Alizarin dye using Ag Nanoparticles Sample C: Alizarin+ Colloidal Ag NPs+ 1.0 ml solution of 1M NaCl Sample B: Alizarin+ Colloidal Ag NPs+ 0.5 ml solution of 1M NaCl Sample A: Alizarin + Colloidal Ag NPs Alizarin

Observed WavenumberAssignment 910  (C–H) 1203  (CC)/  (CH)/  (CCC) 1611  (C=O)  The enhancement of the Raman signal with addition of Ag NPs and increasing NaCl conc. can be fairly observed from the SERS spectra.  This fact clearly indicates the role of NaCl in signal enhancement by enhancing the rate of aggregation of colloidal Ag NPs. Observed wavenumbers (cm -1 ) in Alizarin & assignments J. Raman Spectrosc. 2006; 37: 993–1002 J. Raman Spectrosc. 2004; 35: 921–927

SERS of Crystal Violet (CV) dye with Cu 2 Cu NPs A4: CV+ Colloidal NPs synthesized at 60 mJ/pulse energy A3: CV+ Colloidal NPs synthesized at 50 mJ/pulse energy A2: CV+ Colloidal NPs synthesized at 40 mJ/pulse energy A1: CV+ Colloidal NPs synthesized at 30 mJ/pulse energy A0: Diluted pure CV Dye

 It is evident from the SERS spectra that with increase in ablation energy from 30 mJ/pulse (A1) to 40 mJ/pulse (A2), the Raman signals enhance and peaks get resolved.  As the ablation energy increased to 50 mJ/pulse (A3), the SERS signals resolved dramatically. (the peaks appear at 494, 518, 608, 656, 788, 824, 884, 908, 1021, 1399, 1460, 1555, 1598, 1621, 1664, 1682, 1754, 1790, 1826, 1868 and1898 cm -1 ).  This enhancement in Raman signals may be due to increase in NPs concentration.  Further, the SERS signals decreased with the addition of NPs synthesized at 60 mJ/pulse (A4). ( Peaks appear at 632, 674, 854, 914, 974, 1082, 1154, 1268, 1346, 1454, 1514, and 1796 cm -1 ). The decrease in SERS signals is possibly due to increase in size of the NPs.

 The ratios of peaks (λ 280 /λ 574 ) have highest value for the sample A3, which shows well resolved SERS spectrum and this confirm the strong interaction between CV and nanoparticles.  The value of ratio (λ 280 /λ 574 ) decreases drastically for A4, which shows weak interaction of CV with nanoparticles and thus indicating the possible cause of decrease in SERS signal. Role of NPs in Enhancement of Raman Signal

B3: CV+ Colloidal NPs synthesized at 50 mJ/pulse energy+ 0.5ml KI (0.2M) B2: CV+ Colloidal NPs synthesized at 50 mJ/pulse energy+ 0.5ml KBr (0.2M) B1: CV+ Colloidal NPs synthesized at 50 mJ/pulse energy+ 0.5ml KCl (0.2M) Effect of aggregating agent on SERS of CV due to colloidal copper/copper oxide NPs synthesized at 50 mJ/pulse with halide ions (Cl -, Br -, and I - ).

 It is evident from these spectra that all three halide ions Cl -, Br -, and I -, enhance the Raman signals of the CV dramatically and Peaks are well resolved.  Sample B1 have well resolved peaks at 433, 630, 677, 765, 829, 898, 927, 1020, 1283, 1421, 1595, 1799, and 1857 cm -1.  Sample B2 shows peaks at 445, 596, 642, 672, 706, 788, 840, 921, 908, 1002, 1061, 1211, 1247, 1404, 1520, 1613, 1706, 1851, and 1927 cm -1, while the sample B3 has peaks at 451, 555, 596, 636, 672, 718, 979, 1055, 1078, 1119, 1212, 1276, 1368, 1403, 1573, 1752, and 1857 cm -1.  The data show the Raman peaks move towards red with the addition of halide ions from Cl - to Br - to I -.

The role of Halide ions in the Enhancement of Raman Signal Peak corresponding to 347 nm appears only with the addition of I -, this confirm the strong interaction between CV and I – ion, which shows well resolved SERS spectrum

Conclusions  SERS study of Alizarin Red is done using colloidal Ag NPs as substrate and NaCl as aggregating agent with enhancement with NPs synthesized at 50mj energy.  Raman signal enhancement of Crystal violet using colloidal Cu 2 NPs as substrate is studied. It has been found that the NPs synthesized at 50 mJ/pulse energy not only enhance Raman signals but also show better resolved Raman signals.  The introduction of Cl -, Br -, and I - ions in SERS samples shows considerable enhancement in Raman signals but at different spectral range.

Acknowledgements Authors are thankful to Prof. B.R. Mehta, I.I.T. Delhi, New Delhi for providing TEM facility and DRDO, New Delhi for research grant. I am also thankful to DST New Delhi for travel support to attend this symposium.

Chemical Structure of Alizarin

Chemical Structure of Crystal violet

 Another factor in SERS experiments is the metal solution interface, since it affects the adherence of the adsorbate on the surface.

nanoparticles Metal Target Metal Plasma Distilled Water One Laser Pulse Metal Target Metal Cluster Distilled water Metal Target Step IStep IIStep III Mechanism of the nanoparticles synthesis by laser ablation: i.Front part of focused laser beam interacts with metal target and evaporates metallic vapors from its surface. These vapors are excited by back part of the laser pulse and creation of hot plasma plume expanding perpendicular to the target surface. ii.High-temperature and high-pressure plasma above the target after one laser pulse cools down through adiabatic expansion against liquid media and results in the formation of metal clusters iii. Further cooling of these clusters form metal or metal oxide nanoparticles depending upon chemical interaction of metal with liquid environment.

I. Electromagnetic (EM) enhancement The collective excitation of the electron cloud of a conductor is called a plasmon; if the excitation is confined to the near surface region it is called a surface plasmon. EM enhancement is a consequence of the interaction of incident electric field (from incident radiation) with the electrons in the metal surface, which causes excitation of surface plasmons and, as a result, enhancement of electric field at metal surface (i.e. SERS). metal Molecule Plasmons Incident light SERS Signal

II. Chemical Enhancement (CE) CE results from increase in molecular polarizability, due to the charge transfer between metal and sample molecule. When molecules are adsorbed to the surface, their electronic states can interact with the states in the metal and produce new transitions which causes enhancement of Raman signal. HOMO: Highest occupied molecular orbit LUMO: Lowest unoccupied molecular orbit

Observed wavenumbers (cm -1 ) in CV & assignments Observed Wave- number AssignmentObserved Wavenumber Assignment 450C + - Ph and ring skeletal out-of- plane vibration 764C – H out-of plane bending 555ring skeletal vibrations of radical orientation 787C – H out-of-plane bending 631ring skeletal vibrations of radical orientation 824C – H out-of-plane bending 636ring skeletal vibrations of radical orientation 1346N – Ph stretching vibrations 908ring skeletal vibrations of radical orientation 1382N – Ph stretching vibrations 914ring skeletal vibrations of radical orientation 1399N – Ph stretching vibrations 979ring skeletal vibrations of radical orientation 1598ring C-C stretching 986ring skeletal vibrations of radical orientation 1621ring C-C stretching 718C – H out-of-plane bending

UV-visible absorption spectrum shows red shift due to aggregation of Ag NPs