The present invention relates to a colloidal dispersion of amine-terminated silica particles having a narrowly controlled size range in an aqueous phase for use in diagnostic imaging, drug delivery and gene therapy, as well as methods for preparing surface-modified silica particles suitable for use in an aqueous colloidal carrier medium, for preparing a diagnostic or therapeutic agent for targeted delivery to specific anatomical structures of a patient, and for performing a diagnostic or therapeutic procedure by administration to a patient of at least one diagnostic or therapeutic agent coupled with a colloidal dispersion.