Mn²⁺ Dopant‐Controlled Synthesis of NaYF₄:Yb/Er Upconversion Nanoparticles for in vivo Imaging and Drug Delivery

Abstract

Pure dark red emission (650-670 nm) of NaYF(4):Yb/Er upconversion nanoparticles (UCNPs) is achieved by manganese ions (Mn(2+)) doping. In addition, the Mn(2+)-doping can also control the crystalline phase and size of the resulting UCNPs simultaneously. Drug delivery studies suggest the promise of these UCNPs as drug carriers for intracellular drug delivery and eventually as a multifunctional nanoplatform for simultaneous diagnosis and therapy.

Transition Metal‐Involved Photon Upconversion

“…Intense pure red emission is also obtained in sub‐10 nm NaMnF 3 :Yb 3+ ,Er 3+ nanocrystals 19. Analogous tuned behaviour of NaYF 4 :Yb 3+ ,Er 3+ nanoparticles by Mn 2+ doping was observed in previous research,55, 58, 59 one of which is illustrated in Figure 17 . It is attributed to the non‐radiative energy transfer from the 2 H 9/2 and 4 S 3/2 levels of Er 3+ to the 4 T 1 level of Mn 2+ , followed by back energy transfer to the 4 F 9/2 level of Er 3+ , as discussed above.…”

“…As observed above, Mn 2+ emission is rarely observed when codoped with Ln 3+ in an Yb 3+ –Mn 2+ UC system owing to the complex forward and backward energy transfer between Ln 3+ and Mn 2+ ions,55, 57, 60, 61 which have abundant energy levels above and below the 4 T 1 emitting state of Mn 2+ . There are two approaches to obtain additional Mn 2+ emission besides the Ln 3+ emission in these triply codoped systems.…”

“…However, Mn 2+ substituted for Y 3+ in NaYF 4 can result in an extra F ‐ ion on the grain surface and induces transient electric dipoles with their negative poles pointing outward. This effect would substantially hinder the diffusion of the F ‐ ions required for crystal growth from the solution to the grain surface owing to charge repulsion, resulting in retardation of NaYF 4 nanocrystal growth 55, 56. Similar phase transformation phenomena were observed in NaLnF 4 :Yb/Er (Ln = Lu, Yb, Gd) UC nanoparticles,57 with simultaneous phase/size control and significantly enhanced UC intensity via Mn 2+ doping.…”

“…For instance, Mn 2+ ‐doping affects the growth dynamics and provides simultaneous control of the crystalline phase and the size of the resulting NaYF 4 :Yb/Er UC nanoparticles synthesised using a modified liquid‐solid solution solvothermal strategy 55. Without Mn 2+ doping, the resulting product is a mixture of cubic and hexagonal phases.…”

“…Results show that the signals from such deep tissue are obvious for the Mn 2+ doped samples. The Mn 2+ doping also benefit the magnetic resonance imaging as a second imaging capacity 55. Additionally, significant tuning of the output colour by adjusting the contents of Mn 2+ is only obtained in cubic NaYF 4 nanoparticles and not in hexagonal microtubes 58.…”