AuCu Nanoparticles

Magnetic resonance imaging (MRI) maps information about tissues spatially and functionally. Protons (hydrogen nuclei) are widely used in MRI because of their abundance in water molecules. Water comprises ~80% of most soft tissue. The contrast of proton MRI depends largely on the density of the nucleus (proton spins), the relaxation times of the nuclear magnetization (T, longitudinal, and T, transverse), the magnetic environment of the tissues, and the blood flow to the tissues. Insufficient contrast between normal and diseased tissues requires the development of contrast agents. Most contrast agents affect the T and T relaxation times of the surrounding nuclei, mainly the protons of water. T* is the spin–spin relaxation time composed of variations from molecular interactions and intrinsic magnetic heterogeneities of tissues in the magnetic field (1). Cross-linked iron oxide nanoparticles and other iron oxide formulations primarily affect T and lead to decreased signals. On the other hand, paramagnetic T agents such as gadolinium (Gd) and manganese (Mn) accelerate T relaxation and lead to brighter contrast images (2). Gold nanoparticles have been used as X-ray and optical contrast agents in small animals with little toxicity (3). Su et al. (4) have developed hollow AuCu nanoparticles (nanoshells and nanocapsules) as bimetallic MRI contrast agents with enhancing effects in T- and T-weighted imaging. A Cu ion has an electron configuration of d with two unpaired electrons and less magnetic moment than Gd, with seven unpaired electrons; however, hollow AuCu nanoparticles are made of large numbers of paramagnetic Cu ions to provide a superparamagnetic effect. The large surface area of the porous nanoparticles also provides an effective interaction of Cu ions with water molecules.