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PreprintCC BYChemRxiv

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

  • Rebecca McGonigle1
  • Christian Homann2
  • Ciera Connelly3,4
  • Marta Gajewska5
  • Niko Hildebrandt6,7
  • Robert Pal8
  • Shiao Chow3,4
  • Ute Resch-Genger2
  • Lewis E. MacKenzie1
8 affiliations
  1. 1Department of Pure and Applied Chemistry, Technology and Innovation Centre, 99 George Street, Glasgow, UK, G1 1RD
  2. 2Department of Analytical Chemistry (Dpt.1, Federal Institute for Materials Research and Testing (BAM, Richard-Willstätter-Straße 11, Berlin, Germany, 12489
  3. 3Pure and Applied Chemistry
  4. 4University of Strathclyde
  5. 5Academic Centre for Materials and Nanotechnology, AGH University of Krakow, Mickiewicza 30, Krakow, Poland, 30-059
  6. 6Department of Engineering Physics
  7. 7McMaster University
  8. 8Chemistry, Durham University, South Road, Durham, United Kingdom, DH1 3LE
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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 alters 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, 4 F 9/2 → 4 I 15/2 Er 3+ transition) in DI H 2 O. Dispersal in D 2 O increased emission across wavebands, with green emission ( 2 H 11/2 , 4 S 3/2 → 4 I 15/2 Er 3+ transition) increasing by 248 - 365% whereas red emission only increased between 38-64%. From our results, we infer that PEI may 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 H 2 O and 16% in D 2 O, respectively. These findings highlight that polymer MW should be considered when optimising UCNP synthesis.

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Cite

BibTeXRIS
BibTeX
@misc{mcgonigle2026polyethyleneimine,
  title = {{Polyethyleneimine (PEI) molecular weight alters the photophysical and FRET properties of water dispersible NaYF4:Yb,Er upconversion nanoparticles}},
  author = {Rebecca McGonigle and Christian Homann and Ciera Connelly and Marta Gajewska and Niko Hildebrandt and Robert Pal and Shiao Chow and Ute Resch-Genger and Lewis E. MacKenzie},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15004510/v2},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15004510/v2},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - Polyethyleneimine (PEI) molecular weight alters the photophysical and FRET properties of water dispersible NaYF4:Yb,Er upconversion nanoparticles
AU  - Rebecca McGonigle
AU  - Christian Homann
AU  - Ciera Connelly
AU  - Marta Gajewska
AU  - Niko Hildebrandt
AU  - Robert Pal
AU  - Shiao Chow
AU  - Ute Resch-Genger
AU  - Lewis E. MacKenzie
PY  - 2026
DA  - 2026/10/02
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15004510/v2
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15004510/v2
AB  - 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 alters 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, 4 F 9/2 → 4 I 15/2 Er 3+ transition) in DI H 2 O. Dispersal in D 2 O increased emission across wavebands, with green emission ( 2 H 11/2 , 4 S 3/2 → 4 I 15/2 Er 3+ transition) increasing by 248 - 365% whereas red emission only increased between 38-64%. From our results, we infer that PEI may 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 H 2 O and 16% in D 2 O, respectively. These findings highlight that polymer MW should be considered when optimising UCNP synthesis.
ER  -