The present invention relates to a
nanoparticle comprising a specific binding reactant, said
nanoparticle being useful for determining an
analyte to which
analyte or complex comprising said
analyte said binding reactant is specific. Characteristic for the
nanoparticle is that the
diameter of said nanoparticle is less than 200 nm, said nanoparticle is coated with multiple said specific binding reactants to the extent that the
affinity constant of said nanoparticle towards said analyte essentially exceeds that of free said binding reactant towards said analyte and / or the association rate constant between said nanoparticle and said analyte essentially exceeds the association rate constant between free said binding reactant and said analyte; and said nanoparticle comprises a detectable feature. The invention also relates to biochemical assays using said nanoparticle. The
assay further relates to a proximity based homogenous
assay comprising a first group labeled with an energy donating compound (donor) and a second group labeled with an energy accepting compound (
acceptor), wherein the donor is luminescent and has a long
excited state lifetime and the
acceptor is luminescent having a short or long
excited state lifetime or the
acceptor is non-luminescent, and the increase or decrease, respectively, in the
energy transfer from the donor to the acceptor resulting from shortening or lengthening, respectively, of the distance between said groups, is measured. Characteristic for the
assay is that the donor is a nanoparticle.