Polydopamine/graphene/MnO2 composite-based electrochemical sensor for in situ determination of free tryptophan in plants

Highlights: • An electrochemical sensor detects tryptophan (Trp) in vivo and in vitro. • Trp selectively adsorbed by polydopamine is efficiently electro-oxidized by RGO-MnO₂. • Linear regression models at pH 4–7 are suitable for the variable pH of plants. • The in situ detection of free Trp levels is achieved in tomato fruit and juice. • The Trp levels determined in vivo and in vitro are correlated. The in vivo detection of small active molecules in plant tissues is essential for the development of precision agriculture. Tryptophan (Trp) is an important precursor material for auxin biosynthesis in plants, and the detection of Trp levels in plants is critical for regulating the plant growth process. In this study, an electrochemical plant sensor was fabricated by electrochemically depositing a polydopamine (PDA)/reduced graphene oxide (RGO)-MnO₂ nanocomposite onto a glassy carbon electrode (GCE). PDA/RGO-MnO₂/GCE exhibited high electrocatalytic activity for the oxidation of Trp owing to the combined selectivity of PDA and catalytic activity of RGO-MnO₂. To address the pH variability of plants, a reliable Trp detection program was proposed for selecting an appropriate quantitative detection model for the pH of the plant or plant tissue of interest. Therefore, a series of linear regression curves was constructed in the pH range of 4.0–7.0 using the PDA/RGO-MnO₂/GCE-based sensor. In this pH range, the linear detection range of Trp was 1–300 μM, the sensitivity was 0.39–1.66 μA μM⁻¹, and the detection limit was 0.22–0.39 μM. Moreover, the practical applicability of the PDA/RGO-MnO₂/GCE-based sensor was successfully demonstrated by determining Trp in tomato fruit and juice. This sensor stably and reliably detected Trp levels in tomatoes in vitro and in vivo, demonstrating the feasibility of this research strategy for the development of electrochemical sensors for measurements in various plant tissues.