Abstract
Cubic (α-phase) NaYF4:Yb,Er upconversion nanoparticles (UCNPs) are uniquely suited to biophotonics and biosensing applications due to their near-infrared excitation and visible red emission (λex approx. 660 nm), enabling detection via thick overlying tissue with no bio-autofluorescence. However, UCNP synthesis typically requires high temperatures in combination with either high pressure reaction vessels or an inert atmosphere. Here, we report synthesis of α-phase NaYF4:Yb,Er,Mn UCNPs via the considerably more convenient PVP40-mediated route; a strategy that requires modest temperatures and relatively short reaction time (160°C, 2 h) in open air, with Mn2+ co-doping serving to greatly enhance red emission. The optimal Mn2+ co-doping level was found to be 35 mol %, which decreased the average maximum UCNP Feret diameter from 42 ± 11 to 36 ± 15 nm; reduced the crystal lattice parameter, a, from 5.52 to 5.45 Å; and greatly enhanced UCNP red/green emission ratio in EtOH by a factor of 5.6. The PVP40 coating enabled dispersal in water and organic solvents and can be exploited for further surface modification (e.g. silica shell formation). We anticipate that this straightforward UCNP synthesis method for producing strongly red-emitting UCNPs will be particularly beneficial for deep tissue biophotonics and biosensing applications.
Highlights
This article has been edited by the Royal Society of Chemistry, including the commissioning, peer review process and editorial aspects up to the point of acceptance
The PVP40 coating enabled dispersal in water and organic solvents and can be exploited for further surface modification. We anticipate that this straightforward upconversion nanoparticles (UCNPs) synthesis method for producing strongly red-emitting UCNPs will be beneficial for deep tissue biophotonics and biosensing applications
For biomedical applications involving imaging through tissue, red-emitting cubic (α-phase) NaYF4:Yb,Er UCNPs are ideal because the excitation and emission of such UCNPs lies within the ‘near-infrared biological window’, where optical absorption and scattering by biological tissues is minimal [2]
Summary
Yttrium(III) oxide [Y2O3] (99.99%); ytterbium(III) oxide [Yb2O3] (99.9%); erbium oxide [Er2O3] (99.9%); PVP 40 000 (PVP40) [(C6H9NO)x]; ammonium fluoride [NH4F] (greater than 99.99%); and sodium chloride (greater than 99.5% BioXtra) were purchased from Merck Life Science UK Ltd. Mixture A: Y2O3, Yb2O3, Er2O3 and Mn(NO3)2·4H2O stocks in 10% HNO3 were combined in various amounts as per electronic supplementary material, table S1 to create a transparent solution containing a total of 1 mmol of Ln3+ and Mn2+ ions. This solution was vigorously stirred for 1 min before heating at 120°C to evaporate the aqueous content, resulting in either a transparent residue (0 mol% Mn2+) or a brown residue (with Mn2+); the solution was removed from the heat when the residue stopped bubbling. Two more wash steps were conducted (with centrifugation at 7000 RCF for 30 min), with final suspension of UCNPs in 5 ml of EtOH
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