Curcumin–Cu(II) Ensemble-Based Fluorescence “Turn-On” Mode Sensing the Plant Defensive Hormone Salicylic Acid <i>In Situ</i> and <i>In Vivo</i>

Abstract

Salicylic acid (SA), a crucial, plant-derived signal molecule, is capable of launching global transcriptional reprogramming to assist plants in obtaining the systemic acquired resistance (SAR) mechanism. Thus, the accurate detection of SA will not only significantly contribute to the understanding of the plant SAR but also contribute to crop protection and to the security of the agricultural production and food supply. However, detection of SA using fluorescent probes is a great challenge for scientists, because SA analogues can significantly interfere with the detection results. Herein, we first reported using a simple, natural curcumin-Cu²⁺ ensemble to selectively and sensitively monitor SA <i>in situ</i> and <i>in vivo</i>, directed by a fluorescence "turn-on" mode. A binary combination curcumin-Cu²⁺ was first fabricated with a fluorescence "turn-off" pattern caused by the paramagnetic nature of Cu²⁺. Subsequently, a fluorescence "turn-on" response was performed for detecting SA accompanied by the formation of the ternary complex curcumin-Cu²⁺-SA due to the high affinity of SA toward Cu²⁺, which reduced the fluorescent impact caused by the paramagnetism of Cu²⁺. Further study revealed that the rationally designed hybrid probe could monitor SA in living cell lines. We anticipate that this finding can inspire the discovery of a high-performance SA probe.