Optical Investigation of Gold Shell Enhanced 25 nm Diameter Upconverted Fluorescence Emission AUTHORS : KORY GREEN, JANINA WIRTH AND SHUANG FANG LIM - DEPARTMENT OF PHYSICS, NORTH CAROLINA STATE UNIVERSITY, RALEIGH, NC 27695, USA PRESENTATION : ERIC CALKINS

Motivations  Develop bright, non-toxic fluorophore for medical imaging  Improve UCNP brightness through field enhancement Fig 1: In vivo Single Photon Emission Tomography (SPECT)/Optical imaging study after intravenous injection of 153 Sm-UCNPs. [1]

Background  Organic fluorophores require high energy excitation  Quantum dots extremely bright, but toxic  Non-toxic UCNP’s relatively dim, enhance using plasmonic properties Fig 2: The molar extinction coefficients of HbO 2 and Hb [2]

UCNP Composition  Core - NaYF 4 :Yb (20%), Er (2%) (~25nm)  Shell – Silica (~12nm)  Gold shell (~12nm [3] )  Capping – Thioglycolic Acid Fig 3. TEM images of (a) bare 25 nm NaYF 4 :Yb,Er nanoparticles, (b) with 12 nm thick silica amine and (c) with 3–5 nm gold seed coated nanoparticles, (d) 12 nm thick gold shell coated nanoparticles. Scale bar corresponds to 20 nm. Insets in (a)–(d) show selected cropped images. We note that TEM has a far greater depth of field compared to optical microscopy (or secondary electron microscopy). Hence, gold nanoparticles on the silica coated UCNP, on the top and bottom planes, are both visible at the same time.

Up-conversion Process  Multi-photon absorption in NIR  FRET/UET  SPR tuned to excitation of doped core  Anti-Stokes Emission (green and red) Fig 4: Energy level diagram of transitions contributing to green and red emission. Diagonal black dashed lines represent non-radiative relaxation.

Characterize Particle  Used AFM to find single particles vs clusters Fig 5: Wide field upconverted fluorescence image on an indexed quartz substrate (a) with zoomed region of interest ((a), bottom) and correlated AFM image (b) including smaller scans, showing a single bare 25 nm diameter UCNP (P1) and aggregates of UCNP (A1, A2). Fig 6:. AFM line profile (a) with corresponding height image of a single UCNP (P1) (b). The features in the background are probably cyclohexane residues (non-fluorescent) from the immobilization procedure

Results: Time-Resolved Decay Figure 7. Time-resolved decay of bare 25nm core at λ=545 nm (a), and λ=650 nm (b), for P1.

Results: Emission Figure 8. Upconverted emission at 975 nm excitation of upconverting core with no gold shell (dashed line) compared to that with a gold shell (bold line).  5-fold 545nm  2-fold 650nm  Increased Green/Red ratio

Rate Equations for Decay Fitting  N – Electron Population  W – Rate of transfer between Yb3+ and Er3+ ions  R – Radiative transitions  β – Non-radiative

Results: Time-Resolved Decay Figure 9. Time-resolved decay from 975nm pulse illumination at λ=545 nm (a) and λ=650 nm (b) of single UCNPs with a gold shell (bold line) and silica shell coating only (dashed line) with corresponding fit using the rate equation model (open triangles and squares).

Conclusion  Successfully created gold-coated UCNP  Field enhancement demonstrated by increased emission and decreased decay time

Possible future developments  Refine manufacture of particles for consistent distribution of gold shell.  For bio use, make much smaller ~10nm  Testing bio-conjucation of thin shell with targeting antibody  In-vitro/in-vivo studies

Questions?

References 1. Min, Yuanzeng, et al. "Recent advance of biological molecular imaging based on lanthanide- doped upconversion-luminescent nanomaterials." Nanomaterials 4.1 (2014): Molar extinction coefficients of oxy and deoxyhemoglobin compiled by Scott Prahl. URL: URL: 3. Li, Ling, et al. "Enhancement of single particle rare earth doped NaYF4: Yb, Er emission with a gold shell." Nanotechnology 26.2 (2014): Green, Kory, Janina Wirth, and Shuang Fang Lim. "Optical investigation of gold shell enhanced 25 nm diameter upconverted fluorescence emission."Nanotechnology (2016):