Published: June 06, 2011 r2011 American Chemical Society 5873 dx.doi.org/10.1021/ac2005839 | Anal. Chem. 2011, 83, 5873–5880 ARTICLE pubs.acs.org/ac Effect of Oxidation on Surface-Enhanced Raman Scattering Activity of Silver Nanoparticles: A Quantitative Correlation Yun Han, † Robert Lupitskyy, ‡ Tseng-Ming Chou, † Christopher M. Stafford, § Henry Du,* ,† and Svetlana Sukhishvili* ,‡ † Department of Chemical Engineering and Materials Science and ‡ Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030, United States § Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States b S Supporting Information S urface-enhanced Raman scattering (SERS) is an attractive analytical technique for label-free detection and identification of chemical and biological species at trace concentrations. 1À3 While signi ficant advances have been made in engineering various SERS- active metal (mainly Ag and Au) nanostructures, 4À13 SERS is far from being adopted as a routine analytical technique. This is mainly due to the strong dependence of SERS enhancement on structural, morphological features of metal nanostructures 13À20 as well as on surface chemistry and details of molecular binding. Signi ficant challenges remain, in particular, when Ag, often a metal of choice because of its inherently higher SERS activity compared to its Au counterpart, is used in preparation of SERS-active nanostructures. Silver is prone to oxidation even under ambient conditions, and this results in signi ficant changes in the chemical and plasmonic properties of Ag nanoparticles (Ag NPs). 21À25 For example, exposure to air or oxygen has been reported to cause a red shift of Ag NP plasmonic bands 21,23,24 Another example is an interesting observation of the dependence of oxidation of Ag NPs on the NP size. 22,26 In this report, we aim to identify and quantify surface oxidation species using X-ray photoelectron spectroscopy (XPS) and to demonstrate their effect on SERS activity of Ag NPs. The effects of oxidation on SERS activity of Ag nanostructures are far from being understood, and experimental reports are both rare and fragmented. In one earlier study of cryogenically deposited (32 K) porous silver film, Raman signals from ethylene and pyridine adsorbed at porous silver film were quenched upon exposure to small amounts of oxygen (approximately 0.1 monolayer). 27 This effect was hypothetically attributed to dis- sociative adsorption of oxygen at SERS active sites of silver surface and changes in the chemical component of the SERS enhancement. Recently, we have demonstrated that exposure of SERS substrates with immobilized Ag NPs synthesized in Ar to the lab ambient environment was sufficient to result in 5 orders of magnitude penalty in the detection limit for nitroaromatic molecules in aqueous solutions. 28 Qi et al. 26 reported similar observations regarding the instability of Ag NPs in air. While oxidation of Ag nanostructures has previously been shown to decrease their SERS activity, here we have revealed the chemical identity of oxidized surface species (different forms of Ag oxides), quantified their amounts, and for the first time established quantitative correlations between the degree of Ag NP oxidation and SERS enhancement factors (EFs). Our studies Received: March 5, 2011 Accepted: June 6, 2011 ABSTRACT: We quantitatively studied, using X-ray photoelectron spectroscopy (XPS), oxidation of substrate-immobilized silver nano- particles (Ag NPs) in a wide range of conditions, including exposure to ambient air and controlled ozone environment under UV irradia- tion, and we correlated the degree of silver oxidation with surface- enhanced Raman scattering (SERS) enhancement factors (EFs). The SERS activity of pristine and oxidized Ag NPs was assessed by use of trans-1,2-bis(4-pyridyl)ethylene (BPE) and sodium thiocynate as model analytes at the excitation wavelength of 532 nm. Our study showed that the exposure of Ag NPs to parts per million (ppm) level concentrations of ozone led to the formation of Ag 2 O and orders of magnitude reduction in SERS EFs. Such an adverse effect was also notable upon exposure of Ag NPs under ambient conditions where ozone existed at parts per billion (ppb) level. The correlated XPS and SERS studies suggested that formation of just a submonolayer of Ag 2 O was sufficient to decrease markedly the SERS EF of Ag NPs. In addition, studies of changes in plasmon absorption bands pointed to the chemical enhancement as a major reason for deterioration of SERS signals when substrates were pre-exposed to ambient air, and to a combination of changes in chemical and electromagnetic enhancements in the case of substrate pre-exposure to elevated ozone concentrations. Finally, we also found UV irradiation and ozone had a synergistic effect on silver oxidation and thus a detrimental effect on SERS enhancement of Ag NPs and that such oxidation effects were analyte-dependent, as a result of inherent differences in chemical enhancements and molecular binding affinities for various analytes.