Wavelength modulated surface enhanced (resonance) Raman scattering for background-free detection Bavishna B. Praveen, a Christian Steuwe, b Michael Mazilu, a Kishan Dholakia * a and Sumeet Mahajan * bc Spectra in surface-enhanced Raman scattering (SERS) are always accompanied by a continuum emission called the ‘background’ which complicates analysis and is especially problematic for quanti- fication and automation. Here, we implement a wavelength modu- lation technique to eliminate the background in SERS and its resonant version, surface-enhanced resonance Raman scattering (SERRS). This is demonstrated on various nanostructured substrates used for SER(R)S. An enhancement in the signal to noise ratio for the Raman bands of the probe molecules is also observed. This technique helps to improve the analytical ability of SERS by alleviating the problem due to the accompanying background and thus making observations substrate independent. Introduction Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Resonance Raman Spectroscopy (SERRS) greatly improve the sensitivity of conventional Raman spectroscopy and even enable detection right down to the single molecule level. 1,2 However, in both of these spectroscopic techniques a broad continuum called the ‘background’ accompanies the enhanced Raman signal, whose origin is the topic of lively debate. 3–5 The SER(R)S enhancement is crucially dependent on parameters such as size, morphology of metallic nanomaterials as well as the coupling between them. The signals are extremely sensitive to even minute changes in any of these parameters. 6,7 Fluctuation in the ‘background’ is one such variation observed in SERS spectra which affects its reproducibility and utility for quantication and automation. In contrast to the case of conventional Raman spectroscopy where uorescence oen contaminates the spectra, it is not possible to choose higher excitation wavelengths in the near-infrared (NIR) region of the spectrum to completely eliminate the ‘background’ in the case of SER(R)S, since plasmonic resonances occur over the entire spectrum (from UV to NIR for different materials, shapes and sizes). Consequently a SERS background has been observed even with NIR excitation. 8 This makes it important to use background elimination techniques in conjunction with SER(R) S to enhance the reproducibility and Signal to Noise Ratio (SNR) of the SER(R)S signal. Wavelength Modulated Raman Spectroscopy (WMRS) is a background suppression technique that has the potential to be combined with SER(R)S. In WMRS, a series of Raman spectra are recorded whilst slightly shiing the excitation wavelength (<1 nm). In standard Raman spectroscopy the peaks shi with a shi in the excitation wavelength, however, uorescence is insensitive to small changes in the excitation wavelength and hence remains constant. The modulated Raman information can be recovered using multivariate techniques. 9–12 Thus WMRS provides a reliable hands-free technique for elimination of the uorescent signal in standard Raman spectra. Techniques such as time-resolved Raman spectroscopy (TRRS) 13 and polarisation modulation Raman spectroscopy (PMRS) 14 depend on the temporal and polarisation properties, which are poorly under- stood in the case of SER(R)S, for uorescence rejection in Raman spectra. WMRS does not depend on the polarisation or temporal properties of the background 15,16 and moreover, the ‘SERS background’ is not entirely composed of uorescence. In contrast to other background suppression techniques, 13,14,17 WMRS is a facile method as it does not require any complex modication of the Raman apparatus. Only the Raman laser in the conventional Raman set-up needs to be modulated. Whilst standard mathematical techniques including the rst derivative method may be used in background subtraction procedures, these can introduce artefacts in the nal processed SER(R)S spectra. Hence, usually manual adjustment methods are used since the shape of the background can be complex. This severely a SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, Scotland, KY16 9SS, UK. E-mail: kd1@st-andrews.ac.uk; Tel: +44 (0) 1334 463184 b Department of Physics, Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, CB3 0HE, UK c Institute of Life Sciences (IfLS) & Department of Chemistry, University of Southampton, Building 85 Higheld campus, SO17 1BJ, UK. E-mail: S.Mahajan@ soton.ac.uk; Tel: +44 (0)2380597747 Cite this: Analyst, 2013, 138, 2816 Received 7th January 2013 Accepted 7th March 2013 DOI: 10.1039/c3an00043e www.rsc.org/analyst 2816 | Analyst, 2013, 138, 2816–2820 This journal is ª The Royal Society of Chemistry 2013 Analyst COMMUNICATION Published on 07 March 2013. Downloaded by University of Southampton on 25/06/2013 14:28:45. View Article Online View Journal | View Issue