In vivo dynamics of indole- and phenol-derived plant hormones: Long-term, continuous, and minimally invasive phytohormone sensor

Specific phytohormone combinations regulate plant growth and responses to environmental stimuli. Monitoring their distribution is key for understanding signaling cross-talk and detecting plant stress early. However, typical means of monitoring these chemicals are often laborious, destructive, or limited to model plants. In this study, we present an amperometric and minimally invasive sensing platform that can be attached to plant leaves for the simultaneous detection of two key phytohormones, auxin [indole-3-acetic acid (IAA)] and salicylic acid (SA). The platform incorporates magnetized microneedles coated with superparamagnetic Fe₃O₄ intercalated into a scaffold of multiwalled carbon nanotubes (MWCNTs). It achieves detection limits of 1.41 μM (IAA) and 1.15 μM (SA) with a strong correlation (R² ≥ 0.7) to ultrahigh-performance liquid chromatography-tandem mass spectrometry measurements. Furthermore, implementing cyclical amperometric cleaning extends the sensor lifespan by preventing electrode passivation. Last, the sensor's capability to monitor the real-time plant responses to several stressors is validated, showcasing its potential for phytodiagnostics and precision farming.