(130d) Thermodynamic Control of Response in Ionophore-Based Optical Nanosensors

Optical nanosensors are used to detect a wide range of ions and molecules by changing their fluorescent properties in response to the local analyte concentration. Optical nanosensors have key attributes such as biocompatibility and high spatial and temporal resolution that will allow them to be put toward clinical use and answer scientific questions in a way not possible with conventional electrochemical sensors. However, optical nanosensors have narrower dynamic ranges than electrochemical sensors, and thus practical methods to adjust the sensor response characteristics are needed to match the sensor linear range with the expected analyte fluctuation for an application. Adjusting the dynamic range includes shifting the upper and lower limits and expanding the span of the range. In ionophore-based optical sensors (used to detect ions), the dynamic range is determined by three simultaneous equilibrium expressions: extraction of the analyte from aqueous media into the organic core of the sensor, analyte binding to an ionophore, and acid dissociation of a pH indicator. In this work, we determine the effects of adding a fourth equilibrium by including either a second pH indicator or second ionophore to the typical formulation. The conventional ionophore-based optical sensor response equation is expanded to include a fourth equilibria and used to predict dynamic range control and extension at different sensor compositions. We show that the thermodynamics of chromoionophore acid-dissociation and analyte/ionophore binding are fundamentally different when two pH indicators are loaded within the same sensor matrix, effectively coupling their equilibria, as opposed to being housed in separate sensor nanoparticles. We demonstrate that these two methods of dual-component sensor design give finer control over the response range of ionophore-based optical sensors with than previous methods and can extend the linear range span up to 47% over what is possible with a single-chromoionophore nanosensor. This work shows, as a proof of concept, novel tools for range control and extension that can be applied toward designing an optical ion sensor with a dynamic range matching electrochemical sensors.