Thin and Robust Encapsulation of Silver and Gold Nanoparticles with Dithiocarbamate-anchored Polyelectrolytes
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Thin and Robust Encapsulation of Silver and Gold Nanoparticles with Dithiocarbamateanchored Polyelectrolytes Chih-Yu Jao1, Kai Chen1, Yong-Woo Lee2, and Hans D. Robinson1 1 Virginia Tech, Department of Physics, Blacksburg, VA, 24061, U.S.A. 2 Virginia Tech, Department of Biomedical Sciences and Pathobiology, Blacksburg, VA, 24061, U.S.A. ABSTRACT A robust, stable and thin primary amine functionalization is applied to gold and silver nanoparticles from poly(allylamine hydrochloride) (PAH) by converting a fraction of the amine groups in the polymer to dithiocarbamate (DTC) ligands, which absorb strongly onto noble metal surfaces. We observe marked improvements in the properties of gold nanospheres with a DTCanchored rather than physisorbed PAH cap. The same level of improvement is not seen in silver nanoparticles, although it is clear from a distinct change in the plasmon spectrum in silver nanocubes that the DTC ligand does interact with the silver surface. In spite of their amine functionalization, both silver and gold particles show low cytotoxicity, possibly due to absorption of serum proteins forming a protective coating on the positively charged particle surface. INTRODUCTION Noble metal nanoparticles are used in biological systems for applications such as imaging and heat delivery. Compatibility with the environment requires the particles to be encapsulated with a biologically inert capping layer. This is typically done either with thiol-containing small molecules that bind directly to the surface, or with a polymer layer that is physisorbed onto it. We present a technique that combines the best of both approaches, leading to a thin, robustly anchored, amine-functionalized surface in one simple step. The surface can then be further modified to render the particle surface biocompatible, with low cell cytotoxicity. In our technique, some fraction of the amine groups is polycations such as poly(allylamine hydrochloride) (PAH) is transformed into dithiocarbamate (DTC) groups. DTCs have recently emerged as a superior alternative to thiols for binding to noble metal surfaces, where it creates a stronger bond and is applicable to a wider range of materials [1,2]. The modified polymers are absorbed onto the particles where they form a thin, dense layer, where each polymer chain is covalently bound to the surface in multiple locations. The resulting functional layers are sufficiently robust to allow further functionalization of the surface amines and formation of a polyelectrolyte multilayer on the particles [3]. The thickness of the DTC-anchored functional layer (1-2 nm) is only a third of the corresponding physisorbed polyamine layer, while at the same time conferring greater colloidal stability to the particles. This combination of properties makes this an ideal technique for use in plasmonic applications, where access of the surface plasmon modes to the environment surrounding the particle is desired. Extending the technique to silver nanoparticles, including spheres, nanoprisms and nanocubes, we find that the conce
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