Research Article  1 and  2 Relaxivities of Dendrons Based on a OEG-DTPA Architecture: Effect of Gd 3+ Placement and Dendron Functionalization Peter Fransen, 1,2 Daniel Pulido, 2,3 Lorena Simón-Gracia, 1,2 Ana Paula Candiota, 2,4,5 Carles Arús, 2,4,5 Fernando Albericio, 1,2,6,7 and Miriam Royo 2,3 1 Institute for Research in Biomedicine, Baldiri Reixac 10, 08028 Barcelona, Spain 2 Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Spain 3 Combinatorial Chemistry Unit, Barcelona Science Park, Baldiri Reixac 10, 08028 Barcelona, Spain 4 Departament de Bioqu´ ımica i Biologia Molecular, Universitat Aut` onoma de Barcelona, Unitat de Bioci` encies, Edifci C, 08193 Cerdanyola del Vall` es, Spain 5 Institut de Biotecnologia i de Biomedicina, Universitat Aut` onoma de Barcelona, 08193 Cerdanyola del Vall` es, Spain 6 Department of Organic Chemistry, University of Barcelona, Mart´ ı i Franqu` es 1-11, 08028 Barcelona, Spain 7 School of Chemistry & Physics, University of KwaZulua-Natal, Durban 4001, South Africa Correspondence should be addressed to Fernando Albericio; albericio@irbbarcelona.org and Miriam Royo; mroyo@pcb.ub.cat Received 23 December 2014; Revised 23 February 2015; Accepted 23 February 2015 Academic Editor: Paresh Chandra Ray Copyright © 2015 Peter Fransen et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In magnetic resonance imaging, contrast agents are employed to enhance the signal intensity. However, current commercial contrast agents are hindered by a low relaxivity constant. Dendrimers can be employed to create higher molecular weight contrast agents which have an increased relaxivity due to a lower molecular rotation. In this study, dendrimers containing DTPA derivatives as cores and/or branching units were used to chelate gadolinium ions. Locating the gadolinium ions inside the dendrimers results in higher relaxivity constants, possibly because the paramagnetic center is closer to the rotational axis of the macromolecule. Te highest gain in relaxivity was produced by decorating the dendron surface with peptide sequences, which could be explained by the presence of more second-sphere water molecules attracted by the peptides. Tese fndings could contribute to the development of more efective contrast agents, either by placing the paramagnetic gadolinium ion in a strategic position or through functionalization of the dendron surface. 1. Introduction Magnetic resonance imaging (MRI) is a widely used diag- nostic tool to study the anatomy and function of the human body in both disease and health. Advantages of this imaging technique are that MRI is noninvasive, does not involve radi- ation, and has excellent spatial resolution [1]. However, one of the drawbacks of MRI is its relatively low sensitivity. For this reason, there is an increasing demand for more efective and specifc contrast agents that help to increase the contrast between pathological and healthy tissues. Tis can be enhanced by using contrast agents based on Gd 3+ that change the longitudinal relaxation rate ( 1 ) or by contrast agents based on superparamagnetic iron particles [2, 3] that change the transverse relaxation rate ( 2 ). Te main mode of imaging currently employed is positive mode imaging, based on  1 relaxivity. For bimodal imaging using both positive and negative modes (based on  2 relaxivity), versatile contrast agents, which can enhance both relaxivity constants, are required. Such bimodal contrast agents would provide more clinical information, by combining the unique strengths of each technology and, at the same time, by reducing the adverse efects of administration of multiple agents [4]. Te efciency of both kinds of contrast agents is expressed in relaxivity constants  1 or  2 , depending on the type of relaxivity employed. Tese constants describe Hindawi Publishing Corporation Journal of Nanotechnology Volume 2015, Article ID 848020, 8 pages http://dx.doi.org/10.1155/2015/848020