Water-soluble fluorescent Ag nanoclusters (see figure) are prepared by using star-branched polyglycerol-block-poly(acrylic acid) (PG-b-PAA) templates. The dimensions of the clusters are conveniently controlled by the synthetic conditions. The "cage effect" associated with densely-grafted PAA and the strong binding of protonated acrylic acid groups with Ag+ are two essential factors for the successful formation of the fluorescent nanoclusters, which exhibit remarkable photostability.