The cellular tracking, detection and sensing of protein or antibody movement are important aspects to advance our understanding of biomolecular interactions and activity. Antibodies modified with fluorescent dyes are also valuable tools, especially in immunology research. We describe here a proof-of-principle study of a new water-soluble coumarin probe with a maleimide thiol-reacting unit to fluorescently tag biomolecules. Highlights include: (1) a convenient water-based preparation of N-substituted maleimides, (2) a one-pot preparation of activated maleimido-esters, and (3) a bio-conjugation protocol for the selenol-promoted reduction of native disulfide bonds and the 'site-specific' labelling of antibodies with no significant loss of activity.