A Neu5Acα2-6LacNAc trisaccharide equipped with a free thiol through a hydrophilic PEG3 spacer. The thiol reacts with maleimides to form stable thioether conjugates or with other thiols to form disulfide bonds; it is also compatible with thiol-gold chemistry for surface immobilization. Neu5Acα2-6LacNAc is a ligand for Siglecs and for influenza hemagglutinin, particularly the human-adapted strains that preferentially recognize α2-6-linked sialic acids. This probe can be used as a soluble inhibitor or immobilized on plates or sensors to investigate binding, and is a substrate for sialidases.