bioorthogonal chemical reactions , those that do not interact or interfere with biology , have allowed for exploration of numerous biological processes that were previously difficult to study . 
 the reaction of azides with strained alkynes , such as cyclooctynes , readily forms a triazole product without the need for a toxic catalyst . 
 here we describe a biarylazacyclooctynone ( barac ) that has exceptional reaction kinetics and whose synthesis is designed to be both modular and scalable . 
 we employed barac for live cell fluorescence imaging of azide - labeled glycans . 
 the high signal - to - background ratio obtained using nanomolar concentrations of barac obviated the need for washing steps . 
 thus , barac is a promising reagent for in vivo imaging .