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

초록

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 (PAA(KOH) X-droplet) was used as a reference droplet because of its highly swollen state. The PAA(KOH) 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 PAA(KOH) 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 PAA(KOH) X-droplets continuously decreased to the saturated constant size. The saturated size of the PAA(KOH) 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 PAA(KOH) 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 (T-1/2(-1)). Ca2+ and Mg2+ exhibited Rsat-dia values of 75.9 and 83.6%, respectively, when the flow rate was 5 mu L min(-1), regardless of the metal concentration. The T(1/2)(-1)s 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 phi [= C-Ca/(C-Ca + C-Mg), where C-Ca and C-Mg are the molar concentrations of Ca2+ and Mg2+, respectively]. The T-1/2(-1) of the aqueous Ca2+, Mg2+ mixture solution was calculated by adding the individual T(1/2)(-1)s of pure aqueous Ca2+ and Mg2+ solutions. Using the Rsat-dia and T-1/2(-1) of the Ca2+/Mg2+ mixture aqueous solution, the individual C-Ca and C-Mg 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 PAA(KOH) 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.

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