Sodium‐Ion‐Selective Two‐Photon Fluorescent Probe for In Vivo Imaging

Abstract

Sodium ion plays critical roles in many physiological and pathological processes such as nerve conduction, muscle and heart contraction; it modulates the electrolyte level, cation transport, and cell volume. [1] To understand these functions, a number of one-photon (OP) fluorescent probes have been developed, with SBFI and Sodium Green (SG) being commercially available (Figure 1). [2] A disadvantage of these probes is that they are bulky and suffer from difficulties in loading into live cells. The second problem, which is common for most OP probes, is their short excitation wavelength ( 500 μm), localized excitation, and prolonged observation time. [3] Using TPM, one can image intact tissue for a long period of time with minimum interference from tissue preparation artifacts that can extend more than 70 μm into the tissue slice. However, there is no report on the TP probe for Na+ that is capable of imaging [Na + ]i deep inside living tissues. To address both of these problems, we designed a two photon (TP) probe (ANa1) derived from 2acetyl-6-(dimethylamino)naphthalene (acedan) as the TP fluorophore and 1,7-diaza-15-crown-5 as the Na + ion receptor by considering the following requirements (Figure 1); (i) small MW for cell permeability; (ii) high selectivity for Na + ions; (iii) significant TP cross section for bright TPM image; (iv) large spectral shifts in different environments for discrimination between the cytosolic and membrane-bound probes; and (v) high photostability. Herein, we report that ANa1 is capable of detecting intracellular free Na + ions in live cells and tissues at >100 μm depth for a long period of