Thermosensitive polymer nanospheres based on N,N-diethylacrylamide and 2-hydroxyethyl methacrylate (HEMA) have been prepared, characterized, and conjugated with biotin. The thermosensitivity of poly(N,N-diethylacrylamide) was enhanced by the incorporation of HEMA up to about 40 mol %. Atomic force microscopic images show that these particles can be closely packed even without the surface charges as in the latex particles. Biotinylation reduces the thermosensitivity of the copolymer nanospheres. The biotinylated hydrogel nanospheres showed a reduction in size upon binding with streptavidin, indicating the formation of a less hydrophilic conjugate. No aggregation of the biotinylated particles due to the cross-linking effect of streptavidin was observed. This size change could be reversed by the addition of free biotin to the system. The interaction is specific, and no such changes were observed when streptavidin was replaced by bovine serum albumin.