Poly(allylamine)-copper(II) coordination complex grafted on core@shell upconversion nanoparticles for ultrafast and sensitive determination of the phytohormone salicylic acid in plant extracts.

Salicylic acid (SA) is a phenolic phytohormone with critical roles in plant growth regulation and resistance to biotic and abiotic stress. Since low SA concentrations can modulate many plant biochemical responses, innovative analytical tools are required to deeply understand its activity and to control its exogenous application in modern agricultural systems. Herein, a NIR-activated composite based on NaYF:Yb,Er@NaYF core@shell upconversion nanoparticles decorated with the poly(allylamine)-Cu(II) complex [UCNPs-PAAm-Cu(II)] was developed to sensitively determine the SA molecule in plant-derived samples. Accordingly, the PAAm-Cu(II) complex grafted on the UCNPs induces a strategic charge transfer band which triggers a quenching process through a resonance energy transfer (RET) mechanism. Such process is gradually deactivated upon the addition of SA and the consequent formation of the SA-Cu(II) complex, allowing a luminescence recovery in the 1-800 nM linear range. This mechanism is promoted by the strong stability of the SA-Cu(II) complex (log  = 19.01) which is over twelve orders of magnitude stronger than the PAAm-Cu counterpart. Furthermore, the equilibrium and kinetic studies on the involved mononuclear Cu complexes formation permitted instantaneous analytical responses and excellent selectivity against other representative phytohormones and metallic cations. The reliability of this method was demonstrated by determining the SA content of some edible fruits and vegetables comprising apple, lemon, kiwi, tomato, and cucumber, whose concentrations ranged from 0.30 to 2.99 μg g, with percent recoveries between 94.6 to 102.3%. Thereby, the reported nanocomplex can help to understand the SA activity in plants with significant applications in crop yield improvement and food quality assessment.