Upconverting nanoparticles
Bloembergen described the system as having excited-state emissions with energy differences much greater than kBT, in contrast to the anti-Stokes shift.
One of the first examples of efficient lanthanide doping, the Yb/Er-doped fluoride lattice, was achieved in 1972 by Menyuk et al.[9] Photon upconversion belongs to a larger class of processes by which light incident on a material induces anti-Stokes emission.
Photon upconversion is distinctly characterized by emission-excitation differences of 10–100 kBT[8] and an observable fluorescence lifetime after the excitation source has been switched off.
Since ESA is a process where two photons must be absorbed at a single lattice site, coherent pumping and high intensity are much more important (but not necessarily required) than for ETU.
[14] Recently, moving forward in the challenge of designing particles with tunable emissions, important progress in synthesis of high-quality nano-structured crystals has enabled new pathways for photon upconversion.
[18][19][20] The mechanism for photon upconversion in lanthanide-doped nanoparticles is essentially the same as in bulk material,[21] but some surface and size-related effects have been shown to have important consequences.
In recent decades, researchers have developed innovative solutions to synthesize upconversion nanocrystals with greatly improved efficiency.
[34] The host lattice provides structure for both the activator and sensitizer ions and acts as a medium that conducts energy transfer.
Low phonon energies in the host lattice prevent this loss, improving the conversion efficiency of incorporated activator ions.
Generally, UCNPs contain some combination of rare-earth elements (Y, Sc, and the lanthanides), such as Er3+, Tm3+, and Ho3+ ions, since they have several levels that follow this "ladder" pattern especially well.
[21] Lanthanide dopants are used as activator ions because they have multiple 4f excitation levels and completely filled 5s and 5p shells, which shield their characteristic 4f electrons, thus producing sharp f-f transition bands.
This ion provides a much larger absorption cross-section for incoming near-IR radiation, while only displaying a single excited 4f state.
[34] This method allows precise control over shape and size (monodisperse), but at the cost of long synthesis times and the inability to observe growth in real-time.
More specialized techniques include sol-gel processing (hydrolysis and polycondensation of metal alkoxides), and combustion (flame) synthesis, which are rapid, non-solution phase pathways.
Efforts to develop water-soluble and "green" total syntheses are also being explored, with the first of these methods implementing polyethylenimine (PEI)-coated nanoparticles.
[21][37] Growth is guided by precursor decomposition kinetics and Oswald ripening, allowing for fine control over particle size, shape and structure by temperature and reactant addition and identity.
It has been shown that the introduction of an inert shell of a crystalline material around each doped NP serves as an effective way to isolate the core from the surrounding and surface deactivators,[42] thus increasing upconverting efficiency.
The protocol for direct exchange is simple, generally involving mixing for an extended period of time, but the work-up can be tedious, conditions must be optimized for each system, and aggregation may occur.
The hydrophobic tails of the amphiphiles are inserted in between the oleate ligands on the surface of the NP, leaving the hydrophilic heads to face outwards.
For example, UCNPs can be laminated onto the back sides of semiconductors as a film, to collect low energy light and upconvert it.
[54][55] Lanthanide-doped upconversion nanocrystals have indeed emerged as promising alternatives to traditional super-resolution imaging probes like organic dyes and quantum dots, primarily due to their high photostability and unique nonlinear optical processes.
By using a doughnut-shaped depletion laser, the point spread function (PSF) is effectively compressed, overcoming the diffraction barrier and enabling super-resolution imaging.
The unique properties of upconversion nanocrystals allow for the realization of super-resolution imaging with sub-70-nm spatial resolution, achieved through simple scanning confocal microscopy without the need for complex computational analysis.
[60][61][62] In contrast to organic molecules and quantum dots, lanthanide ions exhibit complex excited states and significantly longer luminescence lifetimes.
[63][64] Membrane ion channels play a crucial role in various biological systems by facilitating the propagation and integration of electrical signals.
This capability is particularly valuable for in vivo applications, where the low attenuation of near-infrared (NIR) light in biological tissues enables precise and minimally invasive control of ion channel activity.
Furthermore, upconversion optogenetics has shown promise in suppressing seizures by inhibiting excitatory cells in the hippocampus and in eliciting memory recall.
This modulation converts MIR radiation to the visible (VIS) and near-infrared (NIR) regions, allowing for real-time detection and imaging using silicon photodetectors.
Core-shell UCNPs were used to initiate the photocleavage of a ruthenium complex using an intensity of NIR light that is completely safe in biomedical use.
The color produced from some overlapped ink depends on the power density of the NIR excitation, which enables the incorporation of additional security features.
