  Droplet-based high throughput biomolecular screening and combinatorial
synthesis entail a viable indexing strategy to be developed for the
identification of each micro-reactor. Here, we propose a novel indexing scheme
based on the generation of droplet sequences on demand to form unique encoding
droplet chains in fluidic networks. These codes are represented by multiunit
and multilevel droplets packages, with each code unit possessing several
distinct signal levels, potentially allowing large encoding capacity. For proof
of concept, we use magnetic nanoparticles as the encoding material and a giant
magnetoresistance (GMR) sensor-based active sorting system supplemented with an
optical detector to generate and decode the sequence of one exemplar sample
droplet reactor and a 4-unit quaternary magnetic code. The indexing capacity
offered by 4-unit multilevel codes with this indexing strategy is estimated to
exceed 104, which holds great promise for large-scale droplet-based screening
and synthesis.
