Figure 1.

Nanosensors for detecting lithium can be monitored with both photoacoustic (a) and fluorescent (b) imaging techniques. Both approaches use multiwavelength ratiometric imaging to generate a response that changes with lithium concentration and minimizes nonspecific changes. In photoacoustic monitoring, two wavelengths are used to interrogate the chromoionophore embedded in the sensors, and the photoacoustic waves from each wavelength change as lithium concentration changes. In fluorescent imaging, a near-IR fluorophore is added to the sensors. The intensity of FRET from the chromoionophore to the near IR dye changes with lithium concentration, whereas directly exciting the near-IR dye does not change intensity—serving as a sensing reference. The fundamental mechanism of the lithium response (c) is lithium extraction by an ionophore (L) into the core of the nanosensor, which deprotonates a chromoionophore (CH+), changing the optical properties of the nanosensor. An additive (R-) balances the charge inside the sensor.