Bacteria DOI: 10.1002/smll.200701164 Functional Fe 3 O 4 /TiO 2 Core/Shell Magnetic Nanoparticles as Photokilling Agents for Pathogenic Bacteria Wei-Jen Chen, Pei-Jane Tsai, and Yu-Chie Chen* A photokilling approach for pathogenic bacteria is demonstrated using a new type of magnetic nanoprobe as the photokilling agent. In addition to their magnetic property, the nanoprobes have other features including a photocatalytic property and the capacity to target bacteria. The nanoprobes comprise iron oxide/titania (Fe 3 O 4 @TiO 2 ) core/shell magnetic nanoparti- cles. As dopamine molecules can self-assemble onto the surface of the titania substrate, dopamine is used as the linker to immobilize succinic anhydride onto the surfaces of the Fe 3 O 4 @TiO 2 nanoparticles. This is followed by the immobilization of IgG via amide bonding. We demonstrate that the IgG–Fe 3 O 4 @TiO 2 magnetic nanoparticles not only have the capacity to target several pathogenic bacteria, but they also can effectively inhibit the cell growth of the bacteria targeted by the nanoparticles under irradiation of a low-power UV lamp within a short period. Staphylococcus saprophyticus, Streptococcus pyogenes, and antibiotic-resistant bacterial strains, such as multiantibiotic-resistant S. pyogenes and methicillin-resistant Staphylo- coccus aureus (MRSA), are used to demonstrate the feasibility of this approach. 1. Introduction Titania materials are widely used in various research fields due to their several unique features. [1–9] For example, titania beads have been known as effective adsorbents specific for phos- phorylated peptides, [1–4] and nanocrystalline TiO 2 electrodes have been applied in the research of solar cells. [5] The ability of titania in photocatalytic reduction is applied in metal reduction to remove heavy metals from wastewater. [6] Furthermore, titania materials also have antimicrobial activities. Previous studies have demonstrated that titania materials can inhibit the cell growth of microorganisms via photochemical reactions. [7–9] However, these materials have no selectivity for specific microorganisms. Additionally, gold nanoparticles [10,11] and magnetic nanoparticles [12–15] have been used as either antibiotic agents or bacterial capture probes. WehavedemonstratedthatimmunoglobulinG (IgG)-bound magnetic nanoparticles can recognize several pathogenic bacterial strains, including Staphylococcus aureus, Staphylo- coccus saprophyticus, and Streptococcus pyogenes, using pseudo-immune interactions. [16] The recognition by nanop- robes of pathogenic bacteria is based on pseudo-immune interactions between the Fc sites of IgG molecules and the binding proteins on the surfaces of these bacteria. IgG-bound magnetic nanoparticles have broadband affinity for patho- genic bacteria, which have binding affinities with the Fc site of IgG molecules. Herein, we further extend the application of this type of affinity magnetic probe to inhibit the cell growth of bacteria by giving the probes a photocatalytic feature. That is, we immobilize a titania shell on the surface of magnetic nanoparticles prior to binding with IgG. Titania is known to Magnetic Nanoparticles as Photokilling Agents for Bacteria [  ] W.-J. Chen, Prof. Y.-C. Chen Department of Applied Chemistry National Chiao Tung University Hsinchu 300 (Taiwan) Fax: (þ886) 3-5131527 E-mail: yuchie@mail.nctu.edu.tw Prof. Y.-C. Chen Institute of Molecular Science National Chiao Tung University Hsinchu 300 (Taiwan) Prof. P.-J. Tsai National Laboratory Animal Center National Applied Research Laboratories Taipei 115 (Taiwan) Keywords:  bacteria  core/shell materials  magnetic properties  nanoparticles  photochemistry small 2008, 4, No. 4, 485–491 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 485