Synthesis and Characterization of Gold Nanoparticles Coated with Ultrathin and Chemically Inert Dielectric Shells for SHINERS Applications JIAN-FENG LI, SONG-BO LI, JASON R. ANEMA, ZHI-LIN YANG, YI-FAN HUANG, YONG DING, YUAN-FEI WU, XIAO-SHUN ZHOU, DE-YIN WU, BIN REN, ZHONG-LIN WANG, and ZHONG-QUN TIAN* State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China (J.-F.L., S.-B.L., J.R.A., Z.-L.Y., Y.-F.H., Y.-F.W., X.-S.Z., D.-Y.W., B.R., Z.-Q.T.); and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (Y.D., Z.-L.W.) We very recently reported a new spectroscopic application for expanding the versatility of surface Raman called ‘‘shell-isolated nanoparticle- enhanced Raman spectroscopy’’ or ‘‘SHINERS’’. The most important and most difficult part of the SHINERS experiment is the effective transfer of the strong electromagnetic field from a gold core through the isolating silica or alumina shell to the probed surface. For this it is essential that the chemically inert dielectric shell be ultrathin (2–5 nm) yet pinhole-free. Herein we describe experimental and theoretical aspects of our SHINERS method in more detail. We provide a protocol for the synthesis and characterization of optimized shell-isolated nanoparticles (SHINs), and we examine the advantages of SHINERS nanoparticles over bare gold nanoparticles. We also present high-quality Raman spectra obtained from gold and platinum single-crystal surfaces in an electrochemical environ- ment by our SHINERS technique. SHINERS is a simple and cost-effective approach that expands the flexibility of surface-enhanced Raman scattering (SERS) for an unprecedented diversity of applications in materials and surface sciences. Index Headings: Core-shell nanoparticles; Gold nanoparticles; Au@SiO 2 ; Au@Al 2 O 3 ; Three dimensional finite-difference time-domain; 3D-FDTD; Au(111); Pt(111). INTRODUCTION For applied spectroscopy, detection sensitivity, resolution, and versatility are the three most important criteria for assessing different techniques that may be used to provide rich and meaningful information. In surface analysis and trace analysis, detection sensitivity is the most important issue because signal is acquired from an extremely small quantity of the analyte/probe species. Surface-enhanced Raman scattering (SERS) was discovered largely through the pioneering work of the Van Duyne and Fleischmann groups in the mid-1970s, 1,2 and it had a great impact on spectroscopy and surface science because the intrinsically low detection sensitivity of Raman was no longer a fatal disadvantage. 3–9 This surface plasmon (SP) resonance based vibrational spectroscopy 10–12 can provide nondestructive and ultra- sensitive detection down to the single-molecule level, 13–19 and it is comparable to fluorescence in this regard. 20 However, SERS traveled a tortuous path while developing into a powerful diagnostic technique, and its lack of versatility was a large contributing factor. Only three coinage metals (Au, Ag, and Cu) and some alkali metals can provide average enhancement factors as high as 10 6 , and this has severely limited the breadth of its practical applications. Furthermore, the coinage metal surface structure or colloid size must be on the scale of tens of nanometers. 10,11,21 As a consequence, the atomically flat surfaces of various single crystals, which are commonly used in surface science and semiconductor technologies, were almost completely excluded. It was therefore quite desirable to overcome the limitations of substrate materials and morphology. It is well known that SERS is a nanoscale phenomenon: the enhancement depends critically on the size, shape, and spacing of the particles or surface structures used to support SP resonance. 10–12,16 By taking advantage of the long-range electromagnetic (EM) field enhancement effect, several groups including ours have developed methods of ‘‘borrowing’’ SERS activity from the coinage metals for use by other materials over the last three decades. 21–26 For example, we coated SERS- active nanoparticles (NPs) and electrode surface nanostructures with ultrathin shells (one to five atomic layers) of various metals such as the group VIIIB transition metals. Probe molecules were then adsorbed on the surface of the shell. The weak (or even null) enhancement of the shell is augmented by the long-range SP resonance based field enhancement effect of the underlying coinage metal, and average enhancement factors of up to 10 4 or 10 5 can be obtained. 9,21 However, the enhancement is dependent upon shell thickness and it is very difficult to coat many other materials (such as insulators, certain oxides, and biological membranes) onto coinage metal NPs or surface nanostructures as uniform ultrathin shells. During the past three years we have proposed and developed a new operation mode in which the transition metal shell is replaced by a chemically inert electrical insulator. 26,27 A Au core is used to generate a large field enhancement, and the inert shell is used to put some distance between the core and the surface under study. The main virtue of this shell-isolated mode is an increased detection sensitivity that is applicable to surfaces having virtually any composition and any morphol- ogy. We call our new technique ‘‘shell-isolated nanoparticle- enhanced Raman spectroscopy’’ or ‘‘SHINERS’’. 26 In this paper we will describe our SHINERS method in more detail. We will provide a protocol for the synthesis and characterization of our SHINERS NPs because it is essential that the chemically inert dielectric shell be ultrathin (2–5 nm) yet pinhole-free. We will then show that SHINERS is widely applicable in the surface and materials sciences. Received 24 September 2010; accepted 8 March 2011. * Author to whom correspondence should be sent. E-mail: zqtian@xmu. edu.cn. DOI: 10.1366/10-06140 620 Volume 65, Number 6, 2011 APPLIED SPECTROSCOPY 0003-7028/11/6506-0620$2.00/0 Ó 2011 Society for Applied Spectroscopy