Highly Durable Double Sol–Gel Layer Ratiometric Fluorescent pH Optode Based on the Combination of Two Types of Quantum Dots and Absorbing pH Indicators

Long-term stable ratiometric fluorescent optical pH sensors (optodes) based on double sol–gel silica layers are prepared with the first layer embedding two types of quantum dots (QDs) and the second layer embedding light-absorbing pH indicators. The sensors are fabricated by a simple general sol–gel spin-coating method. The resulting double-layer pH optodes are designed as having long Stokes shift as well as ratiometric fluorescence emission response to pH in aqueous solutions of varying pH values. This optode has high durability against continuous light exposure, even under severe acidic condition (1 M HCl), and the storage stability is over a period of more than 6 months. These results indicate that the double-layer ratiometric fluorescence-based pH optode allows for long-term pH sensing. When two pH indicators of different pK_a values with an optimized mixing ratio are embedded into the second layer, a double-layer pH optode with reproducible linear response in a wide pH range of over 6 pH units (from pH 4 to 10) can be designed and fabricated.