The present invention provides metallic nanocluster (NC) - antibody (Ab) conjugate, wherein the NC has an average size lower than 3 nm and the conjugate: (a) comprises one single NC, (b) the antibody maintains the binding ability; and (c) has a catalytic activity selected from peroxidase-like and photocatalytic activity.

The invention also provides a process for the preparation of the conjugate as well as compositions, kits and uses in therapy and diagnostics.

Advantageously, the conjugate of the invention shows improved catalytic activity, providing a remarkable improved immunoassay’s sensitivity.