Smart Fluorescent Hydrogel Glucose Biosensing Microdroplets with Dual-Mode Fluorescence Quenching and Size Reduction

Fluorescent hydrogel glucose biosensor (FHGB) microdroplets were fabricated using a microfluidic method with glass capillaries having a coaxial flow-focusing geometry with cross-linked poly(acrylic acid) (PAAc) immobilized with carbon dots (CDs), glucose oxidase (GOx), and horseradish peroxidase (HRP) after the conversion of poly(acrylamide) to PAAc. The prepared FHGB droplets showed a dual response to glucose of CD fluorescence quenching and droplet size reduction upon bienzymatic reaction with glucose; the reaction of GOx and HRP with glucose produced gluconic acid and -OH radicals, which caused CD fluorescence quenching and size reduction, respectively. These small FHGB droplets showed good sensitivity (linear range of ∼30 mM with the limit of detection of 0.052 mM), high selectivity (against the main ingredients of human blood serum), and long stability (due to dry state during storage). These FHGB droplets were tested with human blood serum, and they maintained sensing performance for a long time in the dry state. Thus, the FHGB droplets demonstrate a new method to detect glucose levels with small sample amounts by the dual-mode response. They can be further applied as implanted continuous-detection biosensor droplets because of their biocompatibility.