In situ Surface-Enhanced Infrared Absorption Spectroscopy for the Analysis of the Adsorption and Desorption Process of Au Nanoparticles on the SiO 2 /Si Surface Dominik Enders, ²,‡ Tadaaki Nagao,* ,²,‡ Tomonobu Nakayama, ²,‡ and Masakazu Aono ²,‡ Nano System Functionality Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Nanoscale Quantum Conductor Array Project, ICORP, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed NoVember 6, 2006. In Final Form: February 14, 2007 The adsorption and desorption of Au nanoparticles (AuNP) in colloidal D 2 O suspension on the (3-aminopropyl)- triethoxysilane treated SiO 2 /Si surface was investigated by in situ attenuated total reflection surface enhanced infrared absorption (ATR-SEIRA) spectroscopy with a liquid flow cell. With increasing surface density of AuNP, the absorption of the vibrational modes of D 2 O and of the citrate molecules covering the AuNP increases due to SEIRA. Repulsive electrostatic Coulomb forces between the AuNP lead to the saturation of the AuNP surface density at submonolayer coverage. We show that the adsorption kinetics can be investigated by monitoring in situ the molecular vibrational modes of D 2 O and the citrate molecules. Furthermore, we clarify that the adsorption process can be described very well by a diffusion-limited first-order Langmuir kinetics model. When exposing a saturated AuNP submonolayer to 2-aminoethanethiol (AET)/D 2 O solution, the AuNP are removed from the surface and the IR absorption of the D 2 O vibrational modes become weaker again. Taking into account the time dependencies of the OD and the CH peaks, we propose a microscopic model where the AET molecules quickly adsorb on the AuNP by replacing most of the precovering citrate molecules exposed to the AET solution. As this takes place, the AuNP agglomeratesas we could detect with scanning electron microscopysand are finally removed from the surface. 1. Introduction The discovery of surface-enhanced infrared absorption (SEIRA) 1-4 as the analogon of surface-enhanced Raman scat- tering (SERS) 5,6 has gained much interest in the field of biospectroscopy in the recent years. Especially in the field of optical biosensing, SEIRA has recently been investigated with increasing attention 7-11 because of the advantages of SEIRA spectroscopy over the other spectroscopic techniques. For example, the intensity change in surface plasmon resonance (SPR) is mainly correlated with the macroscopic dielectric constant change and does not linearly correlate with the density of adsorbates. However, the SEIRA intensity comes from the molecular vibrations of the individual molecules and is therefore linearly correlated with the surface coverage by the adsorbates. This means the suitability of SEIRA spectroscopy for quantitative analysis. In addition, the use of an interferometric spectrometers which has become standard in the field of IR spectroscopy nowadayssis of great advantage (e.g., for time-resolved mea- surements). However, still more knowledge is needed to enable SEIRA spectroscopy to become a routine application in biology and medicine. For example, the morphology control and reproducibility of the metal island film is still on a poor level, but at the same time this is a very significant factor for the strength of enhancement, also because of interactions between adsorbate vibrations and surface plasmons in the IR range. 4,12-14 One approach to make the film preparation more reproducible and also less time-consuming and inexpensive was the change from films prepared by physical vapor deposition 12-25 to wet chemically prepared films. 11,26-31 Especially the use of wet chemically prepared Au nanoparticles on Si has become a promising and commonly used system. 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