Small-Volume pH Sensing with a Capillary Optode Utilizing Dye-Loaded Porous Nanocapsules in a Hydrogel Matrix


To develop simple optical pH sensors with adequate sensitivity, short response time, and extended life time for the analysis of very small sample volumes, high concentrations of pH sensitive indicator dyes were encapsulated into ∼200nm diameter nanocapsules (NCs) with ∼1nm thick porous walls. The capsules were immobilized into high-porosity polyvinyl alcohol (PVA) gels for robust and versatile sensor platform design (double immobilization). The NCs-loaded gels were molded into sensing cylinders and secured in glass capillaries, which served as sampling devices and flow-through optical detector cells (capillary optodes). The color of the sensing cylinder was measured as a function of the sample pH using an optical fiber-based UV-VIS spectrophotometer. To optimize the optical properties of the sensing gel the influence of the size of the NCs, the dye concentration in the NCs and the NCs concentration within the PVA gel were studied. The NCs-based optodes allowed the measurement of the pH within 2minutes in as little as ∼30μL volume of sample, with ∼±0.03pH unit uncertainty and less than 0.001 pH/hour drift.

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