The invention provides methods and materials for use in the detection of influenza viruses which utilise a nanoparticle, for example gold nanoparticle, probe comprising a plurality of glycoconjugate ligands, each glyconjugate ligand (GL) having a plurality of sialic-acid containing recognition group (Y) coupled to the nanoparticle via a multivalent core (X), wherein the multivalent core (X) is a trivalent core, whereby there are 3 recognition groups per ligand, wherein the recognition groups on the bioconjugate specifically bind to the hemagglutinin on the target influenza virus. The probes may include further ligands bound to the nanoparticle which do not bind specifically to an influenza virus—for example polyethylene glycol groups. These can modulate density of the glycoconjugate ligand on the surface of the nanoparticle. Binding of probes is detected by a plasmonic signal which is specific to the influenza virus.