An ionophore-based persistent luminescent 'Glow Sensor' for sodium detection

Abstract

Optical sensors have numerous positive attributes such as low invasiveness, miniaturizability, biocompatibility, and ease of signal transduction. Recently, there has been a strong research focus on using phosphorescent readout mechanisms, specifically from long-lifetime phosphorescent or 'persistent luminescence' particles, for in vitro and in vivo sensors. Persistent luminescence readouts can avoid cellular autofluorescence during biological monitoring, leading to an improved signal-to-noise ratio over a more traditional fluorescence readout. In this study, we show for the first time an ionophore-based optical bulk optode sensor that utilizes persistent luminescence microparticles for ion detection. To achieve this, we combined long-lifetime strontium aluminate-based 'glow-in-the-dark' microparticles with a non-fluorescent pH-responsive dye in a hydrophobic plasticized polymer membrane along with traditional ionophore-based optical sensor components to create a phosphorescent 'Glow Sensor'. The non-fluorescent pH indicator dye gates the strontium aluminate luminescence signal so that it decreases in magnitude with increasing sodium concentration. We characterized the Glow Sensor in terms of emission lifetime, dynamic range, response time, reversibility, selectivity, and stability.

Fig. 1. Glow Sensor mechanism. A charge balancing additive holds the Blueberry dye in a protonated state in the absence of sodium. Sodium from the test sample is extracted into the sensor core where it binds with the ionophore. Charged sodium ions force the deprotonation of the Blueberry dye to maintain electroneutrality in the organic phase. When deprotonated, the Blueberry dye absorbs photon emission from the persistent luminescence microparticles, minimizing the observed phosphorescence.

Fig. 2. Signal analysis for Glow Sensor spots. The Blueberry dye in the sensor turns from clear to blue upon deprotonation, increasing the absorbance of the glow from the persistence luminescence microparticles, thereby decreasing the amount of measured luminescence in basic solution. Due to the presence of sodium ionophore, the addition of sodium causes the same deprotonation of the Blueberry dye. (A) Phosphorescent decay curves (n = 3) from a single sensor spot averaged together at increasing sodium concentrations. Dotted lines show area of integration used to calculate sensor response. (B) Dose/response curve showing average response to sodium of the four individual sensor spots. This shows that the sensor phosphorescence decreases as a function of sodium concentration. (B, inset) Images of the phosphorescent spots under acidic (bright) and basic (dim) conditions.

Fig. 3. Full Glow Sensor characterization. (A) Sensor signal after addition of 100 mM sodium. Response time of the sensor is 9.6 min (T₉₅). (B) Reversibility of the sensor analyzed by exposing sensor to alternating solutions of 0 mM and 100 mM sodium. (C) The sensor is highly selective against the potentially interfering ions Li⁺ and K⁺. (D) The sensor response to sodium is stable over 14 days.