Fig. 1. Molecular-assisted surface electrochemical tuning (MASET).

a Schematic illustration of electrochemically controlled energy transfer (ET) from UCNPs to viologen molecules (R represents a functional group). An applied electric field induces electrochemical conversion of viologen molecules. For a given set of nanocrystals and viologen molecules, no energy transfer occurs upon NIR laser excitation (λ_exc), owing to lack of spectral overlap between nanocrystal emission and dye absorption. Applying an external electric field drives the oxidised state (Oxi.) of viologen molecules to the reduced state (red.) through a redox process, resulting in selective quenching of luminescence owing to spectral resonance between nanocrystals and molecules. A reverse electric field oxidises the reduced viologen molecules, leading to a recovery of upconversion emission. b Simplified energy level scheme of lanthanide-activated UCNPs and schematic energy level diagram of viologen molecules in oxidised (Black) and reduced (Blue) forms, illustrating electrically controlled energy transfer from UCNPs to dye molecules. Solid and dashed lines represent electronic transitions.