Synthesis of naphthalimide-based fluorescent probes via an ICT strategy for monitoring pH fluctuations

In this study, three molecular fluorescent probes (NIA-1, NIA-2, and NIA-3) were developed using naphthalimide as the fluorophore, combined with hydroxyl-, benzothiazole-, and benzimidazole-based recognition groups, respectively. The pKa values of these probes were determined to be 5.30, 4.90, and 7.70, and each exhibited a strong linear fluorescence response within specific pH ranges. Within narrow pH intervals, all three probes displayed a pronounced and reversible shift in their maximum fluorescence emission wavelengths, although their overall spectral profiles remained similar before and after protonation/deprotonation. Among the three, NIA-2 exhibited the narrowest pH-responsive range, spanning only 0.30 pH units. Distinct fluorescence color changes in the probe solutions were readily observed during acid-base transitions. Furthermore, these probes demonstrated strong potential for intracellular pH sensing and could be adapted into pH test papers for convenient and visual pH determination.