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

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 ~1200 Eu3+ 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 Eu3+-bound water exchange peak and broadening of the bulk water signal. This observation was traced to catalysis of proton exchange from the Eu3+-bound water molecule by excess positively charged amino groups on the surface of the nanoparticles. Copyright © 2012 John Wiley & Sons, Ltd.