Nanoparticle-based PARACEST agents: the quenching effect of silica nanoparticles on the CEST signal from surface-conjugated chelates

Contrast Media Mol Imaging. 2012 Jan-Feb;7(1):19-25. doi: 10.1002/cmmi.459.


Silica nanoparticles of average diameter 53 ± 3 nm were prepared using standard water-in-oil microemulsion methods. After conversion of the surface Si-OH groups to amino groups for further conjugation, the PARACEST agent, EuDOTA-(gly)₄ (-) was coupled to the amines via one or more side-chain carboxyl groups in an attempt to trap water molecules in the inner-sphere of the complex. Fluorescence and ICP analyses showed that approximately 1200 Eu(3+) complexes were attached to each silica nanoparticle, leaving behind excess protonated amino groups. CEST spectra of the modified silica nanoparticles showed that attachment of the EuDOTA-(gly)₄ (-) to the surface of the nanoparticles did not result in a decrease in water exchange kinetics as anticipated, but rather resulted in a complete elimination of the normal Eu(3+) -bound water exchange peak and broadening of the bulk water signal. This observation was traced to catalysis of proton exchange from the Eu(3+) -bound water molecule by excess positively charged amino groups on the surface of the nanoparticles.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Chelating Agents / chemistry*
  • Chelating Agents / metabolism
  • Computer Simulation
  • Contrast Media / chemistry*
  • Contrast Media / metabolism
  • Coordination Complexes / chemistry*
  • Coordination Complexes / metabolism
  • Emulsions
  • Europium / chemistry
  • Europium / metabolism
  • Magnetic Resonance Imaging / methods*
  • Microscopy, Electron, Transmission
  • Models, Chemical
  • Molecular Structure
  • Molecular Weight
  • Nanoconjugates / chemistry*
  • Nanoparticles / chemistry*
  • Protons
  • Silicon Dioxide / chemistry*
  • Silicon Dioxide / metabolism
  • Water / metabolism


  • Chelating Agents
  • Contrast Media
  • Coordination Complexes
  • Emulsions
  • Nanoconjugates
  • Protons
  • Water
  • Europium
  • Silicon Dioxide