Highly reproducible, stable and multiply regenerated surface-enhanced Raman scattering substrate for biomedical applications Agnieszka Kami nska, * a Igor Dzie ˛ cielewski, b Jan L. Weyher, b Jacek Waluk, a Sylwester Gawinkowski, a Volodymyr Sashuk, a Marcin Fia1kowski, a Marta Sawicka, b Tadeusz Suski, b Sylwester Porowski b and Robert Ho1yst * a Received 4th October 2010, Accepted 2nd April 2011 DOI: 10.1039/c0jm03336g We fabricated a Surface Enhanced Raman Scattering (SERS)-active surface based on photo-etched and Au-coated GaN. The highest enhancement factor (EF) in SERS and high reproducibility of spectra were obtained from surfaces covered with bunched nanopillars which were produced by relatively long defect-selective photo-etching. The surfaces exhibited SERS enhancements of the order of 2.8  10 6 for malachite green isothiocyanate (MGITC) and 2  10 6 for p-mercaptobenzoic acid (PMBA). These SERS enhancement factors were comparable to those of conventional SERS substrates, while the EF for MGITC was two orders of magnitude larger than the corresponding one reported for the SERS platform made on porous GaN. The standard deviation of the relative intensity of the 1180 cm 1 mode of MGITC was less than 5% for 100 randomly distributed locations across a single platform and less than 10% between different platforms. The SERS signal of MGITC at our GaN/Au surface (kept under ambient conditions) was extremely stable. We could not detect any peak shift or appreciable change of intensity even after three months. We used these surfaces to detect biological molecules such as amino acids and bovine serum albumin (BSA) at low concentration and with short detection time. We developed simple and effective cleaning procedures for our substrates. After cleaning, the same substrate could be used multiple times retaining the SERS activity. We are not aware of any other multiply regenerated SERS substrate which provides simultaneously such high stability with high enhancement, good uniformity, and high reproducibility. Introduction Surface-Enhanced Raman Spectroscopy (SERS) can become a powerful technique for detection of biomolecules. 1 Typically, SERS can enhance Raman signals of adsorbed non-resonant molecules by 5 to 6 orders of magnitude, 2 however, significantly greater enhancements, up to 10 14 compared with a normal Raman signal have been reported. 3 The largest enhancements of the signal are believed to come from ‘‘hot spots’’, spatially localized surface plasmon resonances where the electric field becomes huge. 4–6 However, a standard application of SERS in biological and biomedical tests is strongly hampered by the lack of substrates which would satisfy the following requirements: high enhancement factor, high stability upon exposure to air, high reproducibility of SERS signals, both across a single and between different substrates, and the possibility to clean these surfaces for multiple usage without a destruction of hot spots. Here we present such SERS substrates obtained by defect- selective etching of GaN surfaces in a newly developed KOH– K 2 S 2 O 8 etching system, 7,8 together with simple cleaning proce- dures allowing us to use the surfaces several times without affecting the SERS spectra. The SERS technique has been largely confined to specialist research laboratories despite the high level of research interest during almost 40 years since its discovery. One major obstacle in transferring this technique into industry has been the challenge of fabricating low cost, stable, reproducible and sensitive SERS substrates. Since the initial discovery of the SERS effect on roughened silver electrode surfaces 9 researchers have explored numerous other promising substrates that could be used as effi- cient SERS-platforms. The substrates include electrochemically roughened electrodes, colloidal particle arrays, 10 nanowire bundles, nanoprisms, nanoshells, and nanospheres. 11 Although considerable progress has been made towards improving and optimizing SERS substrates for biomedical applications, 12 the fabrication of a reproducible SERS platform still remains a challenging task. In this respect a promising approach was proposed, namely the use of porous GaN covered by gold and silver layers. 13 a Institute of Physical Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. E-mail: akamin@ichf.edu.pl; holyst@ ptys.ichf.edu.pl; Fax: +48 22 343 3333; Tel: +48 22 343 3112 b Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland 8662 | J. Mater. Chem., 2011, 21, 8662–8669 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. 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