Communications to the Editor Bull. Korean Chem. Soc. 2009, Vol. 30, No. 5 999 Influence of Surface Functionalities of Self-Assembled Monolayers on the Adsorption of Gold Nanoparticles Ji Eun Kim, Hyung Jun Kim, and Sangwoon Yoon * Department of Chemistry, Center for Photofunctional Energy Materials, Dankook University, Gyeonggi 448-701, Korea * E-mail: sangwoon@dankook.ac.kr Received December 31, 2008, Accepted March 8, 2009 Key Words: Gold nanoparticles, Self-assembled monolayers, SERS, Adsorption Figure 1. FE-SEM images of AuNPs adsorbed on the SAMs with a terminal group indicated in each figure. The measured contact angles of each SAMs are also included in the lower right corner of each image for comparison with the adsorption density. Surface-enhanced Raman scattering (SERS) has drawn a lot of attention due to extremely high sensitivity, which provides a wide range of applications including sensors, 1 single-molecule detection, 2,3 and cellular imaging. 4 Surface plasmon resonances of noble metal nanoparticles produce intense electromagnetic fields particularly at the interstitial sites of the nanostructures, enhancing the Raman scattering of molecules residing at the place. 5 Therefore, arranging nano- structures in close distances in a controlled fashion is a key to producing SERS. In this respect, a nanostructure that exploits self-assembled monolayers (SAMs) is one of the most attractive SERS-active platforms. 6 SAMs are produced by the spontaneous assembly of thiolate molecules on gold surfaces. 7 Intermolecular van der Waals forces make SAMs a rugged and highly ordered structure. The surface properties of SAMs can be easily modified by changing the terminal groups of thiolate molecules. Adsorption of gold nanoparticles (AuNPs) on the surfaces of SAMs creates SERS hot spots between the nanoparticles and the gold surfaces. 8 The SERS behavior of such structures can be controlled at the molecular level simply by changing the molecules constituting the SAMs. 9 For the fabrication and application of those AuNP-SAMs- Au structures, it is required to understand the nature of interactions between AuNPs and the surfaces of SAMs. The affinity of a few functional groups such as ‒SH and ‒NH2 toward AuNPs has been reported. 10,11 However, no systematic studies have been done on the reactivity of various other functional groups. Here we explore the nature of interactions between the citrate-stabilized AuNPs and the surfaces of SAMs made up of ‒CH3, ‒OCH3, ‒NH2, ‒NO2, ‒OH, or ‒COOH functional groups. The SERS activity of the resulting struc- tures is also compared. For the experiments, we produce SAMs of phenyl rings with different functional groups on the surface by immersing cleaned gold substrates in a 10 mM ethanol solution of p-HSC6H4X (X= CH3, OCH3, NH2, NO2, OH, COOH) for 24 hours. The contact angle measurements (SEO Co., Phoenix 450) characterize the surface properties of SAMs. AuNPs are prepared by reduction of HAuCl4 using sodium citrate as reported previously. 12 High resolution transmission electron microscopy (JEOL, JEM3010) determines the size of AuNPs (29 nm). The AuNP solutions are used as prepared to maintain the acidic condition (pH 3.0) without aggregation. To investigate the interactions of AuNPs with the surface functionality of SAMs, we immerse the SAMs on gold substrates into the AuNP solutions for 12 hours and measure the adsorption density using field-emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F). Extending the immersion time to 24 hours did not change the results. Raman spectra of the SAM molecules interlaid between AuNPs and gold substrates are acquired by a Raman microscope (Kaiser, Raman MicroProbe). Diode laser (λ = 785 nm, 200 mW) is focused on the sample through a 100× objective and the resulting Raman scattering is collected by the same objective and delivered to a holographic spectrometer (f/1.8). All presented spectra are the average of 3 spectra each of which is obtained at an exposure time of 3 s. Figure 1 shows that AuNPs are densely adsorbed onto the ‒CH3, ‒OCH3, and ‒NH2 surfaces whereas the surface density of AuNPs on ‒NO2, ‒OH, and ‒COOH is rather low. We believe that the electrostatic and van der Waals interactions play a major role in the adsorption of citrate-stabilized AuNPs. The interplay among AuNPs, organic functional groups, and solvent also counts. Notably, ‒CH3 and ‒OCH3 are hydro- phobic while ‒OH and ‒COOH are strongly hydrophilic, as indicated by the measured contact angles presented in Fig. 1. Hydrophobic surfaces (‒CH 3 and ‒OCH 3 ) favor the interactions with citrate-capped AuNPs rather than water solvent, leading to the dense adsorption. In contrast, hydrophilic ‒OH and ‒COOH surfaces prefer solvation by water, deterring AuNPs from approaching the surfaces, resulting in the sparse adsorp- tion.