Rational design of fused-ring-modified rhodamine chemosensors for salicylic acid detection: its mechanistic insights and biological application.

Salicylic acid (SA) functions as a critical phytohormone coordinating developmental regulation and defense responses in plants. Understanding SA's regulatory roles in both homeostasis and stress adaptation necessitates advanced monitoring platforms. We designed six rhodamine probes (R1-R6) containing spirolactam or spirohydrazone bridges to systematically evaluate five-membered spiro structures for SA detection. Furthermore, through Fourier infrared experiments (FTIR) and density functional theory (DFT) calculations, we performed molecular orbital analysis to disclose the SA-responsive mechanism underlying the rhodamine ring-opening process induced by SA. Comparative analysis revealed that spirohydrazone-modified probes displayed enhanced fluorescence performance and improved molecular recognition specificity for SA. The optimized probe R2, incorporating a quinoline moiety, achieved exceptional sensing performance through synergistic hydrogen bonding and C-H…π interactions, demonstrating high selectivity, rapid response kinetics (< 30 s), and excellent sensitivity (LOD = 0.87 μM). Overall, this study successfully visualized endogenous SA distribution in living tomato root systems, establishing a novel design framework for acylhydrazone-based rhodamine sensors and elucidating the SA response mechanism through molecular dynamics simulations.