Sub-20 nm Core-Shell-Shell Nanoparticles for Bright Upconversion and Enhanced Förster Resonant Energy Transfer

Abstract

Upconverting nanoparticles provide valuable benefits as optical probes for bioimaging and Förster resonant energy transfer (FRET) due to their high signal-to-noise ratio, photostability, and biocompatibility; yet, making nanoparticles small yields a significant decay in brightness due to increased surface quenching. Approaches to improve the brightness of UCNPs exist but often require increased nanoparticle size. Here we present a unique core-shell-shell nanoparticle architecture for small (sub-20 nm), bright upconversion with several key features: (1) maximal sensitizer concentration in the core for high near-infrared absorption, (2) efficient energy transfer between core and interior shell for strong emission, and (3) emitter localization near the nanoparticle surface for efficient FRET. This architecture consists of β-NaYbF₄ (core) @NaY_0.8-xEr_xGd_0.2F₄ (interior shell) @NaY_0.8Gd_0.2F₄ (exterior shell), where sensitizer and emitter ions are partitioned into core and interior shell, respectively. Emitter concentration is varied (x = 1, 2, 5, 10, 20, 50, and 80%) to investigate influence on single particle brightness, upconversion quantum yield, decay lifetimes, and FRET coupling. We compare these seven samples with the field-standard core-shell architecture of β-NaY_0.58Gd_0.2Yb_0.2Er_0.02F₄ (core) @NaY_0.8Gd_0.2F₄ (shell), with sensitizer and emitter ions codoped in the core. At a single particle level, the core-shell-shell design was up to 2-fold brighter than the standard core-shell design. Further, by coupling a fluorescent dye to the surface of the two different architectures, we demonstrated up to 8-fold improved emission enhancement with the core-shell-shell compared to the core-shell design. We show how, given proper consideration for emitter concentration, we can design a unique nanoparticle architecture to yield comparable or improved brightness and FRET coupling within a small volume.

Figure 1.

Schematics, micrographs, and size information on UCNPs. (a) Schematic of the core–shell–shell (CSS) UCNP and TEM micrographs of (b) the starting core and (c) the final CSS UCNPs. (d) Schematic of the core–shell (CS) UCNP and TEM micrographs of (e) the starting core and (f) the final CS UCNPs. All scale bars are 20 nm. (g) Average diameters of the 8 samples compared in this work: 7 CSS samples of different Er³⁺ doping and 1 CS structure. Core, core–shell, and core–shell–shell diameters shown as blue, green, and gray, respectively, for the CSS structure; core and core–shell diameters shown as teal and gray, respectively, for the CS structure. Error bars represent the standard deviation of the size measurement (N ≥ 700 nanoparticles).

Figure 2.

Ensemble and single particle upconversion characterization of CSS and CS structures. (a) Digital images showing upconversion luminescence of UCNPs suspended in hexanes when illuminated with a 980 nm diode laser. (b) Representative upconversion spectra, normalized to the same peak at 540 nm, comparing the CSS structure doped with 2% Er³⁺ and 20% Er³⁺ and the CS structure. Here, green emission is colored in green and similarly red emission is colored in red. Emission spectra collected under 980 nm illumination at 70 W/cm² for UCNPs suspended in hexanes. (c) Single particle measurements for CSS: 20% Er³⁺ (top) and CS (bottom) structures collected using a scanning confocal microscope with a Nikon 60× oil objective (NA 1.49) and a 976 nm fiber coupled laser at 500 kW/cm². Scale bars are 2 μm for both confocal images. Corresponding colocalization of particles using scanning electron microscopy (SEM) on right. Scale is identical for all SEM images. (d) Average single particle brightness for all 8 samples; note that 1 and 2% Er³⁺ doped CSS samples were not bright enough to be measured. Error bars represent the standard deviation of the measurement (N ≥ 450 nanoparticles). (e) Single particle brightness comparison of CS structure and CSS: 20% Er³⁺.

Figure 3.

Upconversion quantum yield and lifetime characterization of CSS and CS samples. (a) Total, red, and green upconversion quantum yield data for CSS structure and CS structure taken under 980 nm illumination at 70 W/cm². 50 and 80% Er³⁺ doped CSS data shown in inset for visibility. Note that CSS sample data is plotted on a different y-axis than CS data. (b) Comparison of decay lifetimes for Yb³⁺²F_5/2 emission after 980 nm excitation (note that these values are scaled by a factor of 1/10 so all lifetimes can be plotted on the same scale), red Er³⁺⁴F_9/2 emission after 649 nm excitation, and green Er³⁺⁴S_3/2 emission after 520 nm excitation.

Figure 4.

ATTO 542 dye emission enhancement. (a) Schematic energy diagram showing energy transfer from Er³⁺ to lowest unoccupied molecular orbital (LUMO) of dye, leading to dye emission. Normalized emission intensity of (b) CSS: 20% Er³⁺ and (c) CS structures at the three concentrations of dye investigated. Time traces of spectra shown prior to adding dye, immediately after adding dye, 5 s after, 1 min after, and 2 h after. Included gamma (γ) values report the integrated emission enhancement after adding the dye. (d) Summary of the integrated emission enhancement (γ) for all samples at the three dye concentrations.