Enhanced Contrast Agent for X-Ray Imaging Using Gold–Silicon Core–Shell Nanostructures

Gold nanoparticles (AuNPs) have been targeted as novel contrast agent for computerized tomography (CT). However, AuNPs suffer from low-contrast factor in the X-ray regime. Functionalization of AuNPs with folic acid or sugar-based molecules to induce selective uptake have displayed contrast enhancement with improved image brightness and CT signal intensity. However, it was not clear what the basic mechanism for the contrast enhancement was and whether it was related to the uptake enhancement or to a fundamental electromagnetic interaction effect. In this work, we conducted near-field Mie as well as finite-difference time-domain (FDTD) field distribution of the scattering to discern the effect of a thin dielectric coating layer on the contrast functionality of AuNPs. Our results show that upon the incorporation of the dielectric shell (thin film or nanoparticle layer), the cross section of X-ray scattering is enhanced, with silicon being more effective than silica coating, with multiresonance spectral response. The directionality and range and strength of the near field increase for silicon coating (high electron density or high k material in the visible). The effect may be understood in terms of several features. Even though the refractive indices of all materials in the X-ray regime are ∼ 1.0, the wavelength dependence of their approach may exhibit sizeable differences The enhancement is understood in terms of high densities of polarization charge especially in silicon, which allows multipole resonances. The multiplicity of resonances leads to enhanced scattering and directionality (angular distribution) with reduced range. A silicon-coating layer on AuNP may not only alleviate the contrast limitation, but it may afford synergistic integration of luminescence and scattering functionalities in the visible and X-ray regimes.