Concentrating and Recycling Energy in Lanthanide Codopants for Efficient and Spectrally Pure Emission: The Case of NaYF₄:Er³⁺/Tm³⁺ Upconverting Nanocrystals

In lanthanide-doped materials, energy transfer (ET) between codopant ions can populate or depopulate excited states, giving rise to spectrally pure luminescence that is valuable for the multicolor imaging and simultaneous tracking of multiple biological species. Here, we use the case study of NaYF₄ nanocrystals codoped with Er³⁺ and Tm³⁺ to theoretically investigate the ET mechanisms that selectively enhance and suppress visible upconversion luminescence under near-infrared excitation. Using an experimentally validated population balance model and using a path-tracing algorithm to objectively identify transitions with the most significant contributions, we isolated a network of six pathways that combine to divert energy away from the green-emitting manifolds and concentrate it in the Tm³⁺:³F₄ manifold, which then participates in energy transfer upconversion (ETU) to populate the red-emitting Er³⁺:⁴F_9/2 manifold. We conclude that the strength of this ETU process is a function of the strong coupling of the Tm³⁺:³F₄ manifold and its ground state, the near-optimum band alignment of Er³⁺ and Tm³⁺ manifolds, and the concentration of population in Tm³⁺:³F₄. These factors, along with the ability to recycle energy not utilized for red emission, also contribute to the enhanced quantum yield of NaYF₄:Er³⁺/Tm³⁺. We generalize a scheme for applying these energy concentration and recycling pathways to other combinations of lanthanide dopants. Ultimately, these ET pathways and others elucidated by our theoretical modeling will enable the programming of physical properties in lanthanide-doped materials for a variety of applications that demand strong and precisely defined optical transitions

Controlled Synthesis and Single-Particle Imaging of Bright, Sub-10 nm Lanthanide-Doped Upconverting Nanocrystals

Phosphorescent nanocrystals that upconvert near-infrared light to emit at higher energies in the visible have shown promise as photostable, nonblinking, and background-free probes for biological imaging. However, synthetic control over upconverting nanocrystal size has been difficult, particularly for the brightest system, Yb³⁺- and Er³⁺-doped β-phase NaYF₄, for which there have been no reports of methods capable of producing sub-10 nm nanocrystals. Here we describe conditions for the controlled synthesis of protein-sized β-phase NaYF₄: 20% Yb³⁺, 2% Er³⁺ nanocrystals, from 4.5 to 15 nm in diameter. The size of the nanocrystals was modulated by varying the concentration of basic surfactants, Y³⁺:F^– ratio, and reaction temperature, variables that also affected their crystalline phase. Increased reaction times favor formation of the desired β-phase nanocrystals while having only a modest effect on nanocrystal size. Core/shell β-phase NaYF₄: 20% Yb³⁺, 2% Er³⁺/NaYF₄ nanoparticles less than 10 nm in total diameter exhibit higher luminescence quantum yields than comparable >25 nm diameter core nanoparticles. Single-particle imaging of 9 nm core/shell nanoparticles also demonstrates that they exhibit no measurable photobleaching or blinking. These results establish that small lanthanide-doped upconverting nanoparticles can be synthesized without sacrificing brightness or stability, and these sub-10 nm nanoparticles are ideally suited for single-particle imaging

Combinatorial Discovery of Lanthanide-Doped Nanocrystals with Spectrally Pure Upconverted Emission

Nanoparticles doped with lanthanide ions exhibit stable and visible luminescence under near-infrared excitation via a process known as upconversion, enabling long-duration, low-background biological imaging. However, the complex, overlapping emission spectra of lanthanide ions can hinder the quantitative imaging of samples labeled with multiple upconverting probes. Here, we use combinatorial screening of multiply doped NaYF₄ nanocrystals to identify a series of doubly and triply doped upconverting nanoparticles that exhibit narrow, spectrally pure emission spectra at various visible wavelengths. We then developed a comprehensive kinetic model validated by our extensive experimental data set. Applying this model, we elucidated the energy transfer mechanisms giving rise to spectrally pure emission. These mechanisms suggest design rules for electronic level structures that yield robust color tuning in lanthanide-doped upconverting nanoparticles. The resulting materials will be useful for background-free multicolor imaging and tracking of biological processes

Harnessing Chemical Raman Enhancement for Understanding Organic Adsorbate Binding on Metal Surfaces

Surface-enhanced Raman spectroscopy (SERS) is a known approach for detecting trace amounts of molecular species. Whereas SERS measurements have focused on enhancing the signal for sensing trace amounts of a chemical moiety, understanding how the substrate alters molecular Raman spectra can enable optical probing of analyte binding chemistry. Here we examine binding of trans-1,2-two­(4-pyridyl) ethylene (BPE) to Au surfaces and understand variations in experimental data that arise from differences in how the molecule binds to the substrate. Monitoring differences in the SERS as a function of incubation time, a period of several hours in our case, reveals that the number of BPE molecules that chemically binds with the Au substrate increases with time. In addition, we introduce a direct method of accessing relative chemical enhancement from experiments that is in quantitative agreement with theory. The ability to probe optically specific details of metal/molecule interfaces opens up possibilities for using SERS in chemical analysis