Development of a Microfluidic Droptode Sensor for Detection of Heparin in Citrated Human Plasma

Heparin, a large, negatively charged macromolecule, is an anticoagulant widely used in extracorporeal procedures. Methods to quantitate the amount of heparin in blood are therefore essential to ensure correct dosing, thus avoiding unwanted side effects such as thrombosis and hemorrhage. Electrochemical and optical methods have been developed for the detection of heparin in plasma and whole blood; however, deterioration of sensor response from matrix fouling and optical interference, respectively, are limitations of these technologies. Furthermore, methods that require minimal sample input are also desirable so as to limit consumption of blood during these procedures. Motivated by needs for a new, low volume and non-fouling clinical sensor, we developed a droplet microfluidic form of an ionophore-based optode. This method involves sub-nanoliter aqueous sample aliquots spaced by oil segments containing analyte-selective ionophore and pH-sensitive chromoionophore. Partitioning of the analyte from the aqueous droplet to the oil segment, facilitated by interactions with the ionophore, results in fluorescence changes that track with analyte concentration. Fluorescence measurements are recorded from sub-nanoliter oil segments adjacent to aqueous droplets, therefore components of the sample do not interfere with the measurement. Furthermore, reagent-conservative oil segments are continuously generated at tens of Hz in the droplet microfluidic device, eliminating the need to regenerate the oil phase for reuse. Here we report the application of this droplet optode (e.g. droptode) technology for the quantification of heparin in citrated human plasma samples over relevant clinical ranges.