Focus Article Aptamers: turning the spotlight on cells Dalia L ´ opez-Col ´ on, Elizabeth Jim ´ enez, Mingxu You, Basri Gulbakan and Weihong Tan ∗ This article is a review of the development and application of aptamer probes for cell imaging. Aptamers selected against whole cells have been modified with different fluorescent dyes and nanomaterials, such as gold nanoparticles, quantum dots, and superparamagnetic iron oxide, for their use as imaging probes of live cells. These probes have been successfully used for cell imaging both in vitro and in vivo by optical imaging, magnetic resonance imaging (MRI), computed tomography (CT), and positron-emission tomography (PET). In this article, we discuss the development of different aptamer-based probes currently available for imaging of live cells and their applications in the biomedical field.  2011 John Wiley & Sons, Inc. WIREs Nanomed Nanobiotechnol 2 0 1 1 3 328–340 DOI: 10.1002/wnan.133 INTRODUCTION S ince their discovery in 1989 by Ellington and Gold, 1,2 aptamers have been selected for a large array of targets ranging from small molecules to whole organisms. Aptamers are single-stranded nucleic acids (DNA or RNA) selected by a molecular selection process called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). 3,4 This process has been applied to whole cells to generate panels of aptamers capable of differentiating between different cell types. 5 Aptamers selected against cellular targets have potential as diagnostic and therapeutic agents, as they have several desirable properties as compared to antibodies. Aptamers are inexpensive, easily synthesized and chemically modified, and stable during long-term storage, and they exhibit low immunogenicity and better tissue penetration than antibodies. 6,7 Furthermore, because aptamers can be selected without prior knowledge of the target molecule present in the cell membrane, new molecular signatures can be identified and explored as new targets for diagnosis and treatment of diseases. Aptamers were first introduced as imaging probes for in vivo studies in 1997, when a group from NeXstar Pharmaceuticals used a technetium-99m ( 99m Tc)-labeled aptamer selected against human neu- trophil elastase to image inflammation. 8 This aptamer ∗ Correspondence to: tan@chem.ufl.edu Department of Chemistry, University of Florida, Gainesville, FL, USA DOI: 10.1002/wnan.133 probe showed a higher signal-to-background ratio (S/B) than its antibody counterpart, demonstrating the potential application of aptamers as imaging probes for in vivo studies. Since then, aptamers have been coupled to different reporter molecules and contrast agents to develop probes for biomedical imaging using optical imaging methods, magnetic resonance imaging (MRI), positron-emission tomography (PET), single photon emission computed tomography (SPECT), and computed tomography (CT). FLUORESCENCE IMAGING Fluorescence imaging is one of the most rapidly adapted imaging technologies in the medicinal and biological sciences. With the development of more sophisticated and powerful microscopic and macro- scopic imaging systems, as well as the development of better probes, fluorescence remains a popular imag- ing technique in the biomedical field. In fluorescence imaging, photons emitted from an external source are absorbed by a fluorophore and re-emitted at a longer wavelength. Today’s fluorescence imaging sys- tems offer different resolutions and depth penetrations ranging from micrometers to centimeters. 9,10 The Use of Fluorescently Labeled Aptamers for In Vitro Imaging Since aptamers are chemically synthesized, they can be directly modified at either the 5 ′ - or 3 ′ -end with any fluorescent dye using simple phosphoramidite 328  2011 John Wiley & Sons, Inc. Volume 3, May/June 2011