Improving planar optode lifetime: encapsulation and optical release of pH-sensing optodes and assessment of their lifetime in situ

Purpose Planar fluorescent optodes are important tools for probing key parameters, such as the soil pH, O 2 level, and CO 2 level, associated with the biology and biochemistry process in the rhizosphere. In this paper, we address an important limitation of existing optodes, their short lifetime due to the material degradation when buried under the soil. We report an approach based on encapsulating the optodes in black-dyed paraffin wax that enables long-term measurements in various soil conditions. Materials and methods Commercially available planar pH-sensing optodes were encapsulated in black-dyed paraffin wax with an average thickness of 700 microns. The sensors are only exposed to the environment at defined times using a controlled light burst. A 3D-printed holder containing multiple microchannels was designed to support the optodes and help wick the melted paraffin during release. The performance of the encapsulated optodes was compared to that of non-encapsulated ones over different durations in different soil types with varying pH levels. Results and discussion The lifetime of encapsulated optodes in the soil was significantly extended compared to non-encapsulated ones. For a 30-day study, we observed a 15-fold extension of the sensor’s lifetime in the soil. The encapsulation method effectively protected the optodes from degradation, maintaining their response and accuracy. In a separate 7-day in situ study, we showed that this encapsulation approach enabled the optodes to function consistently and reliably for soil types ranging in pH from 6.22 to 7.91. Conclusions Our novel encapsulation method significantly extends the lifetime of planar optodes in the soil. This approach enables long-term monitoring of the rhizosphere, providing non-invasive, real-time measurements at high spatial resolution.