Polyelectrolyte Biosensor Droplets Prepared from Uniform-Sized Liquid-Crystal Shell Templates

Poly(acrylic acid) (PAA) droplets immobilized with 3-aminophenylboronic acid (APBA) (PAAAPBA) were developed for nonenzymatic glucose sensing by monitoring their size changes in glucose solutions. Their sensing performance was compared with that of glucose oxidase-immobilized PAA (PAAGOx) droplets. The PAA-based biosensor droplets were fabricated via simple hatching from uniform-sized shell templates composed of solid-state liquid crystals with uniform pores without using any production instruments. The solid-state liquid crystal shell templates are initially fabricated using glass capillary microfluidic devices; however, once prepared, the subsequent hatching process to generate PAA droplets can be performed on demand without the need for complex microfluidic set-ups. APBA was immobilized onto the PAA droplets via direct N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide coupling. The NaCl-treated PAAAPBA (PAAAPBA/Na) droplets rapidly shrank to the saturated size within 30 min, and the shrinkage ratio exhibited a strong dependence on the glucose concentration (Cg). The NaOH-treated PAAGOx (PAAGOx/Na) droplets exhibited continuous size reduction to a nearly identical size regardless of Cg; however, the size reduction rate was strongly dependent on Cg. The PAAAPBA/Na droplets were used as a glucose biosensor by measuring their diameter ratio, achieving a detection limit of 0.624 mM and a linear range of 1-12 mM. Conversely, the PAAGOx/Na droplets were used as a glucose biosensor by measuring their size reduction rate, yielding a detection limit of 0.733 mM and a linear range of 1-10 mM. Overall, the PAAAPBA/Na droplets offer superior accuracy and convenience compared to the PAAGOx/Na droplets because their size rapidly decreases to a glucose-dependent value, whereas the PAAGOx/Na droplet size requires continuous temporal monitoring to determine the reduction rate. Furthermore, the nonenzymatic PAAAPBA/Na droplet biosensor demonstrates good selectivity, stability, and sensitivity in detecting glucose in human blood and serum.