BCN, an alternative to DIBO and DBCO, reacts with azide to form 1,2,3-triazole by copper free 1,3-dipolar Huisgen cycloaddition. MTS-BCN is a fast-reacting and highly selective thiol-reactive label that permits one to attach BCN to a thiol group. This copper free bioorthogonal reaction allows staining the surface of live cells or in cell extracts when there are concerns about native protein function loss with copper.