The present invention relates to antibody-based probes (including single domain antibody fragment, scFv molecules, antibodies, antibody fragments, diabodies, and the epitope-binding domains thereof) that are capable of immunospecifically and selectively binding to a phosphoserine-containing epitope of Tau, such as, for example, Tau-phospho-serine 396/404 peptide. Such imaging ligands are useful to detect pathological Tau protein conformer if present in a biological sample, especially in conjunction with the diagnosis of Alzheimer's disease or other tauopathy, and thus provide a diagnostic for Alzheimer's disease and other Tau pathologies. The scFv molecules of the present invention have utility as diagnostic markers for, Alzheimer's disease and related tauopathies and as pharmaceutical compositions for the treatment of such conditions.