A 6′-sialyl-LacNAc trisaccharide (6SLN; Neu5Acα2-6Galβ1-4GlcNAc) equipped with a maleimide-functionalized linker through a short propyl spacer. The maleimide group reacts selectively with free thiols, such as cysteine residues on proteins and peptides or thiol-modified substrates and surfaces, to form stable thioether conjugates. 6SLN 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. 6SLN is also a substrate for sialidases, enabling its use in enzyme activity assays and in studies of sialic acid turnover.