Low energy near-infrared (NIR) light excitation allows for deeper penetration into biological tissues than traditional bioimaging probes that require visible/UV excitation. Upconverting nanoparticles (UCNPs) have superior photostability and are unrestricted by autofluorescence and cross-talked emissions commonly observed in conventional bioprobes. UCNPs efficiently convert NIR photons into higher energy visible emissions demonstrating their applicability for the fields of biological imaging and sensing. Sensitizers such as Ytterbium (Yb3+) ions are commonly doped for lanthanide-based UCNPs. However, Yb3+-sensitized upconversion processes suffer from 980 nm absorption overlap with water --the most abundant NIR absorber in the body-- making these processes unsuitable for biological applications. Neodymium (Nd3+) substitution for Yb3+ shifts absorption to 808 nm, which effectively minimizes signal attenuation and reduces detrimental overheating in biological tissue. Previous reports attempted to integrate Nd3+ either by direct doping into the core NP or introducing a shell, but were limited to 1-20 mol% Nd. Herein we report the synthesis of NaYF4: Yb/Gd/Er and NaYF4: Yb/Gd/Tm cores via thermal decomposition. Shells with varying doping concentrations of Nd3+ (0-50 mol%) were introduced through self-focusing by Ostwald ripening. Larger shell thicknesses facilitated by our highly tunable epitaxial growth method ensure spatially confined Nd3+. Remarkably higher concentration doping of Nd3+ within a concentric shell was achieved. Nd3+ concentration in the shell was successfully tuned to achieve maximum upconversion efficiency. The optimal doping concentration of Nd3+ sensitizers was found to be 50 mol% Nd for both Er3+ and Tm3+ activated systems. Upconversion emission improved 12-fold as Nd3+ doping concentration in the shell increased. Cell cytotoxicity studies show PEGylated UCNPs are tolerable and further demonstrate the potential of our UCNPs for bioimaging.

Tuning upconversion in Nd(III)-sensitized core-shell nanoparticles for excitation with biobenign wavelength

254th National Meeting (2017)

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