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RESEARCH PRODUCT

Au@MnO nanoflowers: hybrid nanocomposites for selective dual functionalization and imaging.

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Recently, the development of hybrid nanostructures consisting of various materials has attracted considerable interest. The assembly of different nanomaterials with specific optical, magnetic, or electronic properties to multicomponent composites can change and even enhance the properties of the individual constituents. Specifically tuning the structure and interface interactions within the nanocomposites has resulted in novel platforms of materials that may lead the way to various future technologies, such as synchronous biolabeling, protein separation and detection, heterogeneous catalysis, and multimodal imaging in biomedicine. Of the various kinds of nanomaterials, gold nanorods show an unusually high polarizability at optical frequencies arising from the excitation of localized surface-plasmon resonances (LSPRs). Furthermore, gold nanorods have promising therapeutic properties as hyperthermal agents because the local temperature around the gold nanorods can be increased by laser illumination through the tunable surface plasmon bands in the near infrared (NIR) region. Using NIR radiation for hyperthermal therapy is beneficial because of the low absorption and low scattering by blood and tissue in this spectral range. Magnetic nanoparticles constitute another major class of nanomaterials that have attracted much research effort over the past decades. In particular, exchange-coupled magnetic nanocomposites, such as antiferromagnetic/ferromagnetic core–shell nanoparticles, such as MnO/Mn3O4, have magnetic properties that are quite different from those of the individual components. Concerning biomedical applications, superparamagnetic nanoparticles are attractive as contrast agents for magnetic resonance imaging (MRI). The majority of nanoparticles that have been investigated in this field comprise iron oxides (Fe3O4, g-Fe2O3), which are known to shorten the transverse (or spin–spin) relaxation time T2. [11] Recently, manganese oxide nanoparticles (MnO NPs) have been shown to be interesting candidates as contrast agents for shortening of the longitudinal (or spin-lattice) relaxation time T1. [12] Consequently, a nanoparticulate system containing both an optically active plasmonic gold unit and a magnetically active MnO component would be advantageous for simultaneous optical and MRI detection. Although considerable research efforts have been put into the chemical design of suitable surface ligands, one of the major obstacles for biocompatible applications remains the lack of surface addressability. Therefore, a nanocomposite made up of individually addressable Au and MnO domains offers two functional surfaces for the attachment of different kinds of molecules, thus increasing both diagnostic and therapeutic potential. Furthermore, the size of either of the two components can be varied to optimize the magnetic and optical properties. Herein we present the successful synthesis of Au@MnO nanocomposites consisting of both paramagnetic MnO NPs and Au crystallites followed by separate surface functionalization of both domains with fluorescent ligands. Scheme 1 depicts a functionalized Au@MnO nanoflower with selective attachment of catechol anchors to the metal oxide petals and thiol anchors to the gold core. The nanoflowers were synthesized by decomposition of manganese acetylacetonate [Mn(acac)2] in diphenyl ether in the presence of preformed Au NPs (“seeds”), with oleic acid and oleylamine as surfactants, following a similar procedure for the preparation of Au@Fe3O4 heteroparticles by Sun et al. [15] The [*] T. D. Schladt, Dr. M. I. Shukoor, K. Schneider, Dr. M. N. Tahir, O. K hler, Prof. Dr. W. Tremel Institut f r Anorganische Chemie und Analytische Chemie Johannes-Gutenberg-Universit t Duesbergweg 10–14, 55099 Mainz (Germany) Fax: (+49)6131-39-25605 E-mail: tremel@uni-mainz.de