Nanoparticle Assembly via Hydrogen-Bonding: IRS, TEM and AFM Characterizations
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Nanoparticle Assembly via Hydrogen-Bonding: IRS, TEM and AFM Characterizations Li Han, Mathew M. Maye, Chuan-Jian Zhong Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902. ABSTRACT This paper reports results of the characterizations of nanoparticle assembly formed via spontaneous core-shell and shell-shell reactivities at thiolate-capped gold nanoparticles. Gold nanoparticles of two different core sizes and thiols with carboxylic acid terminals are exploited as a model system. The reactivities involve covalent Au-thiolate bonding and noncovalent hydrogen-bonding with anisotropic linking character. We employed infrared reflection spectroscopy (IRS), atomic force microscopy (AFM) and transmission electron microscopy (TEM) for the characterizations. While IRS provides structural assessment, TEM and AFM imaging measurements probe the morphological properties. INTRODUCTION The study of nanostructured materials is emerging as a research area of tremendous interest, largely because of their potential functional properties in molecular recognition, chemical and biological sensors, microelectronics, optic devices, magnetic materials and catalysis [1]. Nanoconstruction via molecular linking strategies [1b] using metallic nanocrystal cores with organic shell encapsulation as building blocks has recently been demonstrated. Excellent examples include stepwise layer-by-layer assembling [2-4] and DNA-based linking [5] . Because hydrogen-bonding is one of the common non-covalent interactions in chemical and biological systems, we view it as an ideal model towards building biomimetic functional nanomaterials from the core-shell nanoparticles [6-7]. We have recently demonstrated [7] that hydrogen-bonding of carboxylic acid functionalized shells at gold nanocrystals of different size and shape exhibits intriguing anisotropic reactivity in terms of cis- or trans-configuration. (Scheme 1). An understanding of how structures and morphologies of such core-shell architecture correlate with size and shape has important implications to the ultimate exploration of nanostructured molecular recognition. We have employed IRS, TEM, and AFM to address relevant issues.
Scheme 1. Schematic illustration of two possible shell-shell linking via cis- and trans-configurations of the head-to-head hydrogen-bonding at carboxylic acid groups. The Cα-Cβ bond is in the same side of the C=O group for cis configuration, whereas the Cα-Cβ bond is in the opposite side of the C=O group for trans configuration. C4.5.1
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EXPERIMENT Synthesis. Gold nanoparticles of ~2 nm core size (Au2-nm, 1.9 ±0.7 nm) encapsulated with decanethiolate (DT) monolayer shells were synthesized by standard two-phase method [89] . Particles of ~5 nm core size (Au5-nm, 5.2 ±0.3 nm) were derived from the Au2-nm by thermally-activated processing route [10]. Preparation. As detailed in a previous report [7], DT-encapsulated Au2-nm and Au5-nm particles and COOH-terminated thiol, i.e., 11-mercaptoundecanoic acid (MUA), were used as the networking b
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