Influence of Morpholine Substitution on DNBS-Based 1,8-Naphthalimide Fluorescent Probes for H2S Detection

A series of morpholine-appended 1,8-naphthalimide probes (S1–S5) was developed to investigate the influence of the morpholine moiety on H2S detection. All probes exhibited characteristic absorption and emission features and responded to H2S with fluorescence enhancement, although the intensity varied markedly across the series. S2 displayed the highest signal enhancement, while S5 showed minimal response, highlighting the critical role of a two-carbon spacer between the morpholine group and the fluorophore for optimal sensing. Kinetic analysis revealed that S1–S4 followed similar reaction profiles, whereas S5 reacted faster but produced a weaker signal. S2 maintained reliable performance across pH 4–9 and in DMSO-containing media and demonstrated excellent selectivity over common biothiols and other potentially interfering species. These findings provide a clear structure–activity relationship for morpholine-based fluorescent probes and inform the rational design of highly selective H2S sensors.