Conjugation approaches for construction of aptamer-modified nanoparticles for application in imaging

The development of imaging probes based on advances in nanotechnology aims to substantially improve specificity and sensitivity of diagnostic imaging through non-invasive and quantitative detection of specific biomolecules in living subjects. A promising class of molecular imaging probes consists of nanoparticles (NPs) functionalized with a certain targeting agent. Such targeting agents can, for instance, be selected to recognize disease-related biomarkers located on the cell surface. Among the possible agents that direct the NPs to the target site, aptamers, being single-stranded DNA or RNA molecules that can be designed to bind preselected targets such as proteins and peptides with high affinity and specificity, play an increasingly important role. Indeed, aptamers selected against proteins or whole cells have been conjugated to a variety of nanomaterials (NMs) such as Au NPs, quantum dots (QD), and superparamagnetic iron oxide NPs (SPIONs). These probes have successfully been 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). This review presents an overview of the commonly used techniques involved in conjugating aptamer to NPs and their application as probes in cellular or in vivo imaging.