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Poly(acrylic acid) Hydrogel Microspheres for a Metal-Ion Sensor.

Monodispersed cross-linked poly(acrylic acid) (PAA) droplets (PAA X-droplets), prepared using the microfluidic method with in situ ultraviolet curing, were used as small spherical sensors to simultaneously detect both Ca2+ and Mg2+ in human saliva and serum. The PAA X-droplet treated with KOH (PAAKOH X-droplet) was used as a reference droplet because of its highly swollen state. The PAAKOH X-droplets shrunk in response to the presence of divalent metal ions (Ms) by forming a bridged structure of COO-M-OOC. The sizes of the PAAKOH X-droplets were precisely and dynamically monitored in the poly(dimethylsiloxane) (PDMS) channel with passing time when the aqueous metal-ion solutions were flowing at a controlled flow rate. The sizes of the PAAKOH X-droplets continuously decreased to the saturated constant size. The saturated size of the PAAKOH X-droplet did not change; however, the speed of size reduction increased with an increase in the concentration of the divalent metal ion. The saturated size was studied using the saturated diameter ratio (Rsat-dia) with respect to the initial diameter of the PAAKOH X-droplet before the metal-ion treatment, and the speed of the size reduction was investigated using the inverse time to reach half the saturated diameter reduction (T1/2-1). Ca2+ and Mg2+ exhibited Rsat-dia values of 75.9 and 83.6%, respectively, when the flow rate was 5 μL min-1, regardless of the metal concentration. The T1/2-1s for the Ca2+ and Mg2+ linearly increased with an increase in their concentrations. The Rsat-dia of the aqueous Ca2+/Mg2+ mixture solution had a linear relationship with φ [= CCa/(CCa + CMg), where CCa and CMg are the molar concentrations of Ca2+ and Mg2+, respectively]. The T1/2-1 of the aqueous Ca2+, Mg2+ mixture solution was calculated by adding the individual T1/2-1s of pure aqueous Ca2+ and Mg2+ solutions. Using the Rsat-dia and T1/2-1 of the Ca2+/Mg2+ mixture aqueous solution, the individual CCa and CMg in the mixture solution were successfully calculated. This method was applied to the human saliva and serum in which the major metal ions are Ca2+ and Mg2+, and other metal ions existed in undetectable amounts by the PAAKOH X-droplets. This method is simple, cost-effective, and highly accurate and solves the hurdles of separating the interference effect of a Mg2+ ion when a Ca2+ ion is measured in biofluids.