Polyethyleneimine (PEI) molecular weight alters the photophysical and FRET properties of water dispersible NaYF4:Yb,Er upconversion nanoparticles.

Abstract

Upconversion nanoparticles (UCNPs) are optically active materials with near-infrared excitation and tuneable anti-Stokes emission, typically resulting in visible emission. A route to routine and reliable synthesis of water-dispersible UCNPs for biomedical imaging applications is the polyethyleneimine (PEI)-assisted solvothermal autoclave synthesis; however, the properties of the resulting UCNPs remain understudied. Herein, we report how PEI molecular weight used in synthesis alters the properties of core-only, red-emitting, water-dispersible PEI-UCNPs. In terms of brightness, 1.2 kDa PEI-UCNPs exhibited 5 times more intense upconversion luminescence (UCL) than 25 kDa PEI-UCNPs and a 29% longer UCL lifetime (LT) for the red emission band (∼640-690 nm, 4F9/2 → 4I15/2 Er3+ transition) in DI H2O. Dispersal in D2O increased emission across wavebands, with green emission (2H11/2, 4S3/2 → 4I15/2 Er3+ transition) increasing by 248-365%, whereas red emission only increased by 38-64%. From our results, we infer that PEI may potentially quench the UCL via high-energy NH vibrations. We also examined Förster Resonance Energy Transfer (FRET) between PEI-UCNPs and an electrostatically adsorbed fluorescent model dye, AlexaFluor647® (AF647). 25 kDa PEI-UCNPs achieved highest LT-estimated FRET efficiencies of ∼11% in H2O and 16% in D2O, respectively. These findings highlight that polymer MW should be considered when optimising UCNP synthesis.

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