  We report on a novel flow-based method for high-precision chemical detection
that integrates quantum sensing with droplet microfluidics. We deploy
nanodiamond particles hosting fluorescent nitrogen vacancy defects as quantum
sensors in flowing, monodisperse, picoliter-volume microdroplets containing
analyte molecules. ND motion within these microcompartments facilitates close
sensor-analyte interaction and mitigates particle heterogeneity. Microdroplet
flow rates are rapid (upto 4cm/s) and with minimal drift. Pairing this
controlled flow with microwave control of NV electronic spins, we introduce a
new noise-suppressed mode of Optically Detected Magnetic Resonance that is
sensitive to chemical analytes while resilient against experimental variations,
achieving detection of analyte-induced signals at an unprecedented level of a
few hundredths of a percent of the ND fluorescence. We demonstrate its
application to detecting paramagnetic ions in droplets with simultaneously low
limit-of-detection and low analyte volumes, in a manner significantly better
than existing technologies. This is combined with exceptional measurement
stability over >103s and across hundreds of thousands of droplets, while
utilizing minimal sensor volumes and incurring low ND costs (<$0.70 for an hour
of operation). Additionally, we demonstrate using these droplets as
micro-confinement chambers by co-encapsulating ND quantum sensors with
analytes, including single cells. This versatility suggests wide-ranging
applications, like single-cell metabolomics and real-time intracellular
measurements in bioreactors. Our work paves the way for portable,
high-sensitivity, amplification-free, chemical assays with high throughput;
introduces a new chemical imaging tool for probing chemical reactions in
microenvironments; and establishes the foundation for developing movable,
arrayed quantum sensors through droplet microfluidics.
