Stable, Hydrophilic Nitrilotriacetic Acid-Capped Gold Monolayer Protected Clusters
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Stable, Hydrophilic Nitrilotriacetic Acid-Capped Gold Monolayer Protected Clusters Ndabenhle M. Sosibo1,2, Robert T. Tshikhudo2, and Neerish Revaprasadu1 1 Department of Chemistry, University of Zululand, Private Bag X1001, KwaDlangezwa, 3886, South Africa 2 Advanced Materials Division, Mintek, 200 Hans Strijdom Drive, Randburg, 2125, South Africa ABSTRACT A one step synthesis and functionalization of robust, hydrophilic monolayer protected clusters of gold (Au-MPCs) containing a nitrilotriacetic acid (NTA) is described. The 14 nm Au-MPCs were prepared by the attachment of two bifunctional thioalkylated-poly(ethylene glycol) ligands on the nanoparticles, one containing a nitrilotriacetic acid (NTA) terminal group, while the second hydroxyl (OH) terminated ligand was used as a co-stabilizer to promote the stability of the MPCs. The resulting PEGylated NTA functionalized Au MPCs, which are characterized by TEM, UV-vis and agarose gel electrophoresis are attractive probes for many target species e.g. hexahistidine-tagged proteins. Importantly, the NTA functionality on MPCs ligand shell can be varied. INTRODUCTION Techniques for the preparation of hydrophobic and hydrophilic monolayer protected clusters (MPCs) of gold and silver have been in steady development over the years [1-4]. The capability of gold MPCs to undergo place-exchange reactions compliments their potential applications as molecular tools of choice [5,6]. This feature allows for the incorporation of specialized biological recognition motifs on the MPCs surface rendering them impressive tools in biological areas such as biodiagnostics, labeling and targeted drug delivery [7-9]. One of the main attractions of the use of MPCs in biological applications is their size comparison with the biological molecules such as nucleic acids, proteins, peptides and many others; the second attribute is their strong absorption and scattering of light in the form of surface plasmon resonance (SPR) exhibited by these materials, making them responsive bio-detectors [10]. This unique feature of noble metal nanoparticles is due to the intrinsic oscillation of the conduction band electrons stimulated by interaction with an electromagnetic radiation (UV-Vis). The electric field of the electromagnetic radiation induces a dipole in the nanoparticle; this provokes a restorative force that compensates for such, resulting in a unique resonance wavelength [11]. The general synthesis of MPCs has in the past inherited a major shortcoming in the form of water insolubility due to the synthesis design methods [1]. Poly(ethylene glycol) (PEG) based ligands have been shown to impart the necessary stability, water solubility and specificity when attached onto the gold MPCs surface [12]. PEG considerably improves the ionic strength of the MPCs allowing for their dissolution in buffer solutions without any major aggregation. Profound studies have been conducted on the specific ligand design and function of thioalkylated derivatives of PEG [13]. Here we report the synthesis of stable
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