Disclosed herein are conjugates comprising a biomolecule linked to a label that have biological activity and are useful in a wide variety of biological applications. For example, provided herein are polymerase-nanoparticle conjugates including a polymerase linked to a nanoparticle, wherein the conjugate has polymerase activity. Such conjugates can exhibit reduced aggregation and improved stochiometries wherein the average biomolecule:nanoparticle ratio approaches or equals 1:1. Also disclosed herein are improved methods for preparing such conjugates, and methods and systems for using such conjugates in biological applications such as nucleotide incorporation, primer extension and single molecule sequencing.