Ethylene is not only a ubiquitous building block in industry but also a hormone central to plant development. Formed in plants from 1-aminocyclopropanecarboxylic acid, ethylene binds to CuI proteins and triggers ripening, blossoming and senescence — processes we must time properly so fruits and flowers get to us in good shape. Despite the importance of these processes, we lack a good ethylene sensor to monitor the developmental state of our produce, a lot of which unnecessarily spoils during supply. This challenge has now been addressed by Timothy Swager and colleagues, who describe in ACS Central Science a sensitive system that quantifies ethylene by catalytically converting it to acetaldehyde.
The quest for greater sensitivity saw Swager’s team replace their reversible binding strategy in favour of subjecting ethylene to catalytic Wacker oxidation, a Pd-catalyzed reaction that consumes O2 to afford acetaldehyde. The team once more prepared a chemiresistor circuit but this time decorated their nanotubes with 4-pyridyl groups and replaced the CuI complex with a catalytic mixture of [PdCl2(NCPh)2], nBu4N[NO2] and BnOH. This mixture differs from the typical Wacker oxidation system, which instead features AgNO2 as the source of nitrite. “Here we use a new variant of the reaction optimized for our non-volatile solvent,” notes Swager. In the resting state, the nanotubes are p-type semiconductors that feature PdII sites bound to 4-pyridyl groups. Each PdII site can bind ethylene and H2O to generate acetaldehyde and convert into a Pd0 complex. This electron-rich site now engages in electron-transfer with the nanotubes to attenuate their p-type character and give a measurable decrease in conductance. The Pd0 state can re-enter the cycle on aerobic oxidation, which curiously proceeds even in the absence of CuII, an ingredient in the classic Wacker system and a typical O2 reduction electrocatalyst. Thus, although the new sensor generates acetaldehyde, it is not this product but rather its mechanism of formation that is quantified. The system consumes only negligible amounts of ethylene, so as not to perturb the environment around the plant or to generate too much toxic acetaldehyde.
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