A Neu5Acα2-3LacNAc 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. Neu5Acα2-3LacNAc is a ligand for selectins, Siglecs, and avian-type influenza hemagglutinin, which preferentially recognizes α2-3-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.