Lifetime of Phosphorescence from Nanoparticles Yields Accurate Measurement of Concentration of Oxygen in Microdroplets, Allowing One To Monitor the Metabolism of Bacteria

A method to monitor the level of oxygen in microdroplets is presented. Optical sensor nanoparticles are dispersed in the aqueous phase of the microfluidic droplets for culturing bacteria. The oxygen sensor nanoparticles consist of phosphorescent indicator dye embedded in poly(styrene-block-vinylpyrrolidone) nanobeads. The nanoparticles are excitable by red light and emit in the near-infrared spectra region which minimizes background fluorescence from biological matter. The biocompatibility of the nanoparticles was proven. Nanoparticles sensors were read out by adapted miniaturized oxygen meters. The instruments can be easily integrated into the microfluidic system by placing it next to the tubing and measuring through the tubing wall. The phosphorescence lifetime-based measurement circumvents the drawbacks of intensity-based measurements and enables the determination of the absolute oxygen concentration in individual moving droplets. The technique can also be used for monitoring the growth of bacteria in microdroplets. We demonstrate simultaneous measurement of concentration of oxygen and optical density (OD) from micro cultures of E. coli and M. smegmatis.