Nanoparticles for molecular imaging

Imaging techniques have been instrumental in the visualization of fundamental biological processes, identification and diagnosis of diseased states and the development of structure-function relationships at the cellular, tissue and anatomical levels. Together with the advancements made in imaging techniques, complementary chemical compounds, also known as imaging probes or contrast agents, are developed to improve the visibility of the image by enhancing sensitivity, and for the identification and quantitation of specific molecular species or structures. Extensive studies have been conducted to explore the use of inorganic nanoparticles which exhibit magnetic and optical properties unique to the nano regime so as to enhance the signals sensitivity for magnetic resonance and fluorescent imaging. These physical properties are tailored by controlling the size, shape and surface properties of nanoparticles. In addition, surface modification of nanoparticles is often required to improve its stability, compatibility and functionality. Surfactants, surface-active agents, have been used to engineer the surface characteristics of nanoparticles to improved particle stability and functionality. Surfactants enhance nanoparticle stability through the reduction of surface energy, and by acting as a barrier to agglomeration through either steric hindrance or repulsive electrostatic forces. Coupling of nanoparticles with biomolecules such as antibodies or tumor targeting peptides are enabled by the presence of functional groups (e.g., carboxyl or amine groups) on surfactants. This paper provides an overview of the chemistry underlying the synthesis and surface modification of nanomaterials together with a discussion on how the physical properties (e.g., magnetic, absorption and luminescent) can be controlled. The applications of these nanoparticles for magnetic resonance, fluorescent and photoacoustic imaging techniques that do not rely on ionizing radiation are also covered in this review.